processes

Review
Review of Thermal Runaway Monitoring, Warning and
Protection Technologies for Lithium-Ion Batteries
Sumiao Yin 1 , Jianghong Liu 1, * and Beihua Cong 2, *

1 Ocean Science and Engineering College, Shanghai Maritime University, Shanghai 201306, China
2 Shanghai Institute of Disaster Prevention and Relief, Tongji University, Shanghai 200092, China
* Correspondence: liujh@shmtu.edu.cn (J.L.); bhcong@tongji.edu.cn (B.C.)

Abstract: Due to their high energy density, long calendar life, and environmental protection, lithium-
ion batteries have found widespread use in a variety of areas of human life, including portable
electronic devices, electric vehicles, and electric ships, among others. However, there are safety
issues with lithium-ion batteries themselves that must be emphasized. The safety of lithium-ion
batteries is receiving increasing amounts of attention as incidents such as fires and explosions caused
by thermal runaway have caused significant property damage and fatalities. Thermal runaway
can easily occur when lithium-ion batteries experience issues such as electrical abuse and thermal
abuse. This study compares various monitoring, warning, and protection techniques, summarizes
the current safety warning techniques for thermal runaway of lithium-ion batteries, and combines
the knowledge related to thermal runaway. It also analyzes and forecasts the future trends of battery
thermal runaway monitoring, warning, and protection.

Keywords: lithium-ion batteries; thermal runaway; monitoring and warning; protection

1. Introduction
Since the 20th century, there has been a growing trend toward using clean, non-
polluting, affordable, and easily accessible electric energy instead of highly polluting and
expensive traditional energy sources, as the global community has become increasingly
Citation: Yin, S.; Liu, J.; Cong, B.
concerned with environmental and energy issues. Due to their advantages over other
Review of Thermal Runaway
high-energy secondary batteries, such as high energy density, low self-discharge, good
Monitoring, Warning and Protection
cycling performance, and environmental friendliness, lithium-ion batteries are widely used
Technologies for Lithium-Ion
Batteries. Processes 2023, 11, 2345.
in the power supply of portable electronic communication devices such as cell phones and
https://doi.org/10.3390/pr11082345
laptops, as well as in the energy supply of new electric vehicles. These advancements have
significantly altered most industries, including land and marine transportation, and will
Academic Editor: Gang Wang soon become the norm. This will play a significant role in the renewable technology field
Received: 6 April 2023 and in the development of the next-generation power system. The suppression of thermal
Revised: 7 June 2023 runaway events has emerged as a key issue in improving the safety of lithium-ion batteries;
Accepted: 12 June 2023 however, due to the nature of the safety-critical applications of the batteries themselves,
Published: 4 August 2023 the development of methods to detect and evaluate the safety of lithium-ion batteries has
recently become a major challenge for industry and academia [1].
A lithium-ion battery may experience mechanical abuse if the external stress causes
the battery casing to deform or become punctured by some sharp items (such as nails).
Copyright: © 2023 by the authors. Additionally, due to overcharge, overdischarge, and external short circuit, lithium-ion
Licensee MDPI, Basel, Switzerland. batteries may not adhere to their fundamental electrical properties. This could result in
This article is an open access article
electrical abuse of lithium-ion batteries. Additionally, lithium-ion batteries may overheat
distributed under the terms and
as a result of internal electrochemical side reactions or external heat sources heating up,
conditions of the Creative Commons
leading to thermal abuse of lithium-ion batteries. All of these elements have the potential
Attribution (CC BY) license (https://
to cause a succession of exothermic reactions in lithium-ion batteries within minutes, which
creativecommons.org/licenses/by/
would cause a sharp rise in the internal temperature of battery. This would then likely
4.0/).

Processes 2023, 11, 2345. https://doi.org/10.3390/pr11082345 https://www.mdpi.com/journal/processes

Processes 2023, 11, 2345 2 of 33

result in a thermal runaway event, which would typically result in smoke, fire, or possibly
an explosion [2–4].
Thermal runaway events represent a serious hazard to human life and property in
the form of smoke, fires, and explosions [5,6]. However, large-scale distributed energy
management systems around the world are currently working toward achieving good
safety and reliability. Therefore, a number of international organizations and committees,
including the International Organization for Standardization (ISO) and the International
Electrotechnical Commission (IEC), have created and published some authoritative test
specifications to gauge the safety of lithium-ion batteries in an effort to enhance their
performance and safety as well as allay concerns about thermal runaway [7,8]. To ensure
the quality and safety of lithium-ion batteries used, these specifications mandate that
lithium-ion batteries must pass a number of safety verification tests, such as overcharge,
overdischarge, overheat, mechanical shock, and other tests. They also stipulate that the test
cannot expose the battery to any secondary hazards (leakage, fire, explosion, etc.).
Lithium-ion batteries could still experience thermal runaway issues even if they
entirely complied with all specifications and testing criteria since actual operating circum-
stances and surroundings can be more challenging and harsher than many test situations.
Abuse from a variety of sources is frequently inevitable. The lithium-ion battery pack
of a Tesla Model X, for instance, deformed and ruptured after colliding with a concrete
bulkhead on U.S. Highway 101 in the San Francisco Bay Area in early 2018. A new electric
vehicle in Fujian, China, unexpectedly caught fire in August 2020 while charging at a
charging station. A 13-ton Megapack huge lithium-ion battery that was being tested for the
Australian battery project caught fire in July 2021.
The development and acceptance of electric vehicles, electric ships, and lithium-ion
batteries has all been significantly hampered by the growing serious safety issue with
lithium-ion batteries. Consequently, research on lithium-ion battery thermal runaway
characterization, particularly for equipment using lithium-ion batteries as a power source,
can prevent casualties and property damage caused by a lack of timely warning of thermal
runaway. This is essential for addressing lithium-ion battery safety issues and advancing
the sustainable development of industry.
There are two main areas that can be improved upon to increase the safety perfor-
mance of lithium-ion batteries and decrease the likelihood of dangerous accidents. In
order to increase the safety performance of the battery from the battery’s own perspective,
one method is to improve the production process within the lithium-ion battery itself by
adding electrolyte additives, improving electrode materials, and improving the separator
preparation process; an alternative method is to take changes in voltage, internal resis-
tance, temperature, and other parameters accompanying the process of thermal runaway of
lithium-ion batteries into consideration from the perspective of monitoring and warning to
achieve the purpose of reducing casualties. As a result, this article provides a thorough as-
sessment of the literature on current thermal runaway monitoring, warning, and protection
techniques, as well as an analysis and forecast of the likely direction of future development
for battery monitoring, warning, and protection technologies.

2. Lithium-Ion Battery Thermal Runaway Monitoring Technology

The following elements are primarily monitored for determining the lithium-ion
battery’s operational parameters: battery voltage, operating current, internal resistance,
and ambient temperature. In order to determine whether thermal runaway occurs during
the charging and discharging cycle of the battery, the monitoring of voltage, temperature,
and internal resistance can be used.

2.1. Monitoring Techniques Based on External Battery Parameters

The most popular technique for keeping track of lithium-ion battery health is the
battery management system (BMS). It uses temperature and voltage sensors that are already
incorporated into the device as its primary measurement instruments. The system can
 2.1. Monitoring Techniques Based on External Battery Parameters
The most popular technique for keeping track of lithium-ion battery health is the bat-
Processes 2023, 11, 2345 3 of 33
tery management system (BMS). It uses temperature and voltage sensors that are already
incorporated into the device as its primary measurement instruments. The system can
measure the surface temperature and terminal voltage of each lithium-ion battery in real
timemeasure
if enoughthe surface
sensors aretemperature
installed [9].and
Onceterminal voltagesignal
an abnormal of each lithium-ion
is detected, thebattery
BMS canin real
time if enough sensors
immediately trigger an alarm. are installed [9]. Once an abnormal signal is detected, the BMS can
immediately trigger an alarm.
2.1.1. Voltage
2.1.1. Voltage
A multi-voltage sensor interleaved voltage topology measurement system was pre-
A multi-voltage sensor interleaved voltage topology measurement system was pre-
sented by Xia et al. [10] to precisely measure the terminal voltage of each lithium-ion bat-
sented by Xia et al. [10] to precisely measure the terminal voltage of each lithium-ion battery,
tery, as is shown in Figure 1. In order to monitor the voltage anomalies of each lithium-
as is shown in Figure 1. In order to monitor the voltage anomalies of each lithium-ion
ion battery in a series battery pack, the method first constructs a voltage sensor topology
battery in a series battery pack, the method first constructs a voltage sensor topology for
for redundant lithium-ion batteries and then uses clever algorithms, control circuits, and
redundant lithium-ion batteries and then uses clever algorithms, control circuits, and exact
exact voltage thresholds. The voltage sensor value will vary if a lithium-ion battery expe-
voltage thresholds. The voltage sensor value will vary if a lithium-ion battery experiences
riences thermal runaway as a result of overcharging or overdischarging. If the voltage
thermal runaway as a result of overcharging or overdischarging. If the voltage sensor value
sensor value is
is below thebelow the minimum
minimum threshold,
threshold, the lithium-ion
the lithium-ion battery isbattery is regarded
regarded as over- If
as overdischarged.
discharged. If not, it is deemed to be
not, it is deemed to be overcharged. overcharged.

(a) (b) (c)

(d) (e)
Figure 1. Schematics
Figure od thermal
1. Schematics runaway
od thermal monitoring
runaway and detecting
monitoring method
and detecting basedbased
method on multi-voltage
on multi-voltage
sensors. (a) The
sensors. prevailing
(a) The voltage
prevailing measurement
voltage measurement method;
method;(b)
(b)cell
cell level redundancy method;
level redundancy method;(c)(c)string
stringlevel
levelredundancy
redundancymethod;
method;(d)(d)voltage
voltagecalibration
calibrationmethod;
method; (e)
(e) fault
fault tolerant
tolerant voltage
voltage measure-
measurement
mentmethod
method[10].
[10].

Locating a defective
Locating cell within
a defective a battery
cell within packpack
a battery is a key advantage
is a key of multi-voltage
advantage of multi-voltage
sensor-based approaches
sensor-based approachesoverover
temperature-
temperature- or or
gas-based
gas-based monitoring
monitoringsystems,
systems,and
andnu-
numer-
merous
ous sensors can
can prevent
preventfalse
falsealarms
alarmsofofbattery
battery failure
failure caused
caused by by individual
individual sensor
sensor failures.
failures. However,
However, the is
the cost cost is typically
typically significant
significant becausebecause
so manyso many
voltagevoltage
sensorssensors are
are employed.
employed. Moreover, a significant amount of sensing data place a heavy computational
Moreover, a significant amount of sensing data place a heavy computational demand on
the digital signal processor of BMS. Large lithium-ion battery systems, such as those used
in electric vehicle battery packs, for instance, typically contain thousands of cells. When
each battery cell is fitted with a voltage sensor, the BMS receives a significant amount of
sensor data. The enormous volume of real-time data may consume a significant amount of
 demand on the digital signal processor of BMS. Large lithium-ion battery systems, s
as those used in electric vehicle battery packs, for instance, typically contain thousand
Processes 2023, 11, 2345 cells. When each battery cell is fitted with a voltage sensor, the BMS receives 4 of 33 a signifi
amount of sensor data. The enormous volume of real-time data may consume a signifi
amount of BMS processing resources and storage space, jeopardizing the ability and
BMS processingbustness of the
resources BMS
and to successfully
storage manage charging
space, jeopardizing the abilityand
anddischarging
robustness behavior.
of the
As a result, installing a single
BMS to successfully manage charging and discharging behavior.voltage sensor that is shared by a number of batt
cells installing
As a result, is standard practice
a single in thesensor
voltage industry.
thatFor instance,
is shared bythe Deluxe of
a number Tesla Model S batt
battery
pack contains 7104 cells total among 14 series modules, each
cells is standard practice in the industry. For instance, the Deluxe Tesla Model S battery of which contains 6 se
pack containscells
7104andcells74total
parallel
among cells. However,
14 series onlyeach
modules, 84 voltage
of whichsensors
containsare6needed instead of 7
series cells
because
and 74 parallel all parallelonly
cells. However, cells
84share a voltage
voltage sensors sensor. A battery
are needed insteadmonitoring board (BMB
of 7104 because
stalled
all parallel cells shareon one end ofsensor.
a voltage each module measures
A battery the temperature
monitoring board (BMB) of the module’s
installed on positive
negative terminals in addition to the battery voltage. A
one end of each module measures the temperature of the module’s positive and negativedaisy-chain cable assembly wi
10-pin Molex connector on top of the BMB connects all of
terminals in addition to the battery voltage. A daisy-chain cable assembly with a 10-pin the modules to a central B
boardon
Molex connector attop
the ofendtheofBMB
the main battery
connects all assembly, whichtothen
of the modules communicates
a central BMS board with other
at the end ofhicle
the components
main batteryvia the controller
assembly, whichareathennetwork (CAN) bus.
communicates with other vehicle
components via the controller area network (CAN) bus.
2.1.2. Temperature
2.1.2. Temperature The idea behind temperature sensors is to use voltage signals to monitor tempera
The ideasignals.
behindThermistors,
temperaturethermocouples,
sensors is to useanalog
voltage signals to monitor
temperature sensors,temperature
and digital tempera
signals. Thermistors,
sensors are thermocouples, analog temperature
examples of common temperaturesensors,
sensors.and digital temperature
However, the same drawbacks
sensors are examples
ply to all of of common temperature
these temperature sensors.
sensor types:However, the same
poor detection drawbacks
accuracy and sensitivit
apply to all of these temperature
environmental changes. sensor types: poor detection accuracy and sensitivity to
environmental changes.
Nascimento et al. [11] used fiber Bragg grating sensors and K-type thermocouple
Nascimento et al.the
monitor [11] used fiber
surface Bragg grating
temperatures of thesensors and K-type
top, middle, thermocouples
and bottom to
of a lithium-ion batt
monitor the surface temperatures
under normal of the operating
and abusive top, middle, and bottom
conditions of a lithium-ion
at various dischargebattery
multiplicities (
under normal C,and
2.67abusive
C, and operating conditions at
8.25 C), respectively, invarious
order todischarge
increase multiplicities
the detection (0.53 C, of sur
accuracy
2.67 C, and 8.25 C), respectively, in order to increase the detection accuracy
temperature and to improve the reliability of monitoring with surface temperature d of surface
temperatureas and to improve
depicted the reliability
in Figure of monitoring
2. The findings demonstratewith surface
that whiletemperature
both sensors data,
are capabl
as depicted in Figure 2. The findings demonstrate that while both sensors are
detecting the surface temperature of lithium-ion battery in real time, the fiber capable of Bragg g
detecting theing
surface
sensortemperature of lithium-ion
has a higher temperature battery in realand
sensitivity time,better
the fiber Bragg grating
resolution than that of th
sensor has a higher temperature sensitivity and better resolution than that of the K-type
type thermocouple. This enhances the accuracy and dependability of the BMS system
thermocouple. This enhances the accuracy and dependability of the BMS system to monitor
monitor the temperature of lithium-ion batteries.
the temperature of lithium-ion batteries.

(a) (b)
Figure 2. (a) Figure 2. (a) The
The location location
of fiber of fiber
Bragg Bragg
grating grating
optical optical
sensor and sensor
K-typeand K-type thermocouple;
thermocouple; and and
schematic[11].
(b) schematic separator separator [11].

Since then, a Since

networkthen, a network
of fiber of has
sensors fiber sensors
been hasbased
created been on
created based
the fiber on the
Bragg fiber Bragg g
grating
ing sensor
sensor enabling enabling
real-time, in situ,real-time, in situ, monitoring
and multipoint and multipoint monitoring
of the of the surface temp
surface temperature
distribution on a smartphone
ture distributionlithium-ion battery,lithium-ion
on a smartphone as shown inbattery,
Figure 3.asInshown
order to
in simulate
Figure 3. In orde
the lithium-ion batterythe
simulate response in the battery
lithium-ion dry, moderate,
response and
incold regions,
the dry, variousand
moderate, temperature
cold regions, var
and relative humidity conditions are taken into account. It was discovered that the surface
temperature of the smartphone’s lithium-ion battery was nearly twice as high as it would
be under normal circumstances due to the greater discharge rate and dry climate. The
temperature clearly decreased from the top to the bottom of the battery cell, which was
another notable tendency. The top of the cell, which is close to the electrodes, had the
 at the bottom of the middle being somewhat higher than the middle. The lowest t
ature was and
temperature found nearhumidity
relative the bottom of theare
conditions lithium-ion battery,Itcompared
taken into account. to the other
was discovered
that the
[12]. surface temperature of the smartphone’s lithium-ion battery was nearly twice as
high as it would be under normal circumstances due to the greater discharge rate and dry
Processes 2023, 11, 2345 5 of 33
climate. The temperature clearly decreased from the top to the bottom of the battery cell,
which was another notable tendency. The top of the cell, which is close to the electrodes,
had the highest temperature, which was followed by the middle top, with the temperature
athighest temperature,
the bottom which
of the middle was somewhat
being followed by the middle
higher than thetop, with the
middle. Thetemperature at the
lowest temper-
bottom
ature wasof the middle
found near thebeing somewhat
bottom higher than
of the lithium-ion the middle.
battery, Thetolowest
compared temperature
the other regions
was found near the bottom of the lithium-ion battery, compared to the other regions [12].
[12].

Figure 3. Separator of the thermal chamber and fiber Bragg grating locations utilized in the
setup for rechargeable smartphones [12].

Figure 3.K-type
Figure Separator
3. thermocouples
ofof
Separator the thermal
the thermal were
chamber
chamber andused
fiber
and by
Bragg
fiber Feng etlocations
grating
Bragg grating al. [13]utilized
locations to measure
inin
utilized the LIB
the the
LIB testtempera
test
setup for rechargeable smartphones [12].
the battery during thermal runaway. These thermocouples were positioned in the
setup for rechargeable smartphones [12].
side,K-type
and center of the battery’s two pockets as[13]
well as close the to the safety valve, as is
K-type thermocouples
thermocouples were
wereused
used bybyFeng
Fengetetal.al.[13] totomeasure
measurethe temperature
temperatureofof
in
the Figure
thebattery
battery 4. Ultimately,
during
during thermal
thermal the findings
runaway.
runaway. These
These revealed that
thermocouples
thermocouples the
were
were battery’s
positioned
positioned internal
inin
thethe temperatu
bottom,
bottom,
approximately
side,and
side, andcenter 870
centerofofthe °C, significantly
thebattery’s
battery’s twopockets
two higher
pocketsasas than
closetothe
wellasasclose
well tothe ambient
thesafety
safetyvalve, temperature.
valve,asasis is shown Based
shown
in Figure
incalculated 4. Ultimately,
Figure 4. Ultimately,
temperature the findings
the findings revealed
revealed
difference andthatthat
thethethe battery’s
battery’sdata,
recorded internal
internal temperature
temperature
it was discovered wasthat 97%
was
approximately 870 ◦ C, significantly higher than the ambient temperature. Based on the
approximately
time during870 the°C, significantly
test period, the higher than the ambient
temperature differencetemperature.
inside Based on the stayed b
the battery
calculated
calculated temperature
temperature difference
difference andand
thethe recorded
recorded data, data, it was
it was discovered
discovered that of
that 97% 97%
theof
°C, while
the during
time when
during thethermal
test period,runaway occurred,
the temperature the temperature
difference inside difference
the battery stayed reached its
below
time the test period, the temperature difference inside the battery stayed below 1
level,
1 ◦while
°C, approximately
C, while
whenwhen thermal
thermal 520
runaway °C.
runaway occurred,
occurred, thethe temperature
temperature difference
difference reached
reached itsits highest
highest
level,approximately
approximately520 ◦ C.
520°C.
level,

Figure 4. The position of the thermocouples [13].

Figure 4. The position of the thermocouples [13].
Figure 4. The position of the thermocouples [13].
At the moment, the most common method of monitoring battery temperature is to
At the moment, the most common method of monitoring battery temperature is to
install thermocouples on the surface of the battery, with three or more thermocouples in-
installAt the moment,
thermocouples onthe
the most
surfacecommon method
of the battery, with of monitoring
three battery temperatu
or more thermocouples
stalled at the top, middle, and bottom of the battery, respectively. According to research
installed
installthus at the top,
thermocouples middle, and bottom
on the surface of the battery, respectively.
of thetobattery, According to research
results far, three thermocouples are sufficient reflect thewith three or
real surface more thermocou
temperature
results thus far, three thermocouples are sufficient to reflect the real surface temperature
stalled at the top, middle, and bottom of the battery, respectively. According to r
of the battery. However, the internal temperature of the battery is more relevant to the
results thus far,
real condition of thethree thermocouples
battery. are sufficient
It is generally believed to reflect
that the battery the realissurface
temperature fairly temp
uniform when it is operating normally; nevertheless, in the event of a battery failure or a
nearby heat source that causes the battery to heat up quickly, the difference between the
surface and internal battery temperatures can be as high as 40–50 ◦ C [14–18]. Therefore,
 addition to the surface temperature measurements in the BMS, additional detection meth
ods are required to monitor the internal battery temperature.

2.2. Monitoring Technology Based on Internal Battery Parameters

Processes 2023, 11, 2345 6 of 33
It is more accurate to utilize the interior temperature as a parameter to determine th
state of the lithium-ion battery in a fully enclosed state rather than simply monitoring th
voltage and surface temperature for the lithium-ion battery. Nowadays, electrochemica
in addition to the surface temperature measurements in the BMS, additional detection
impedance
methodsspectroscopy
are required to(EIS) and
monitor theintegrated fibertemperature.
internal battery optic sensors are the two main tech
niques used to monitor the internal condition of lithium-ion batteries.
2.2. Monitoring Technology Based on Internal Battery Parameters
It is more
2.2.1. Embedded accurate
Fiber Opticto Sensor
utilize the interior temperature as a parameter to determine
the state of the lithium-ion battery in a fully enclosed state rather than simply monitoring
According
the voltage toandasurface
method Du et al.for
temperature [19]
theproposed
lithium-ionfor estimating
battery. Nowadays, theelectrochem-
internal core tem
peratureical of lithium-ion
impedance batteries
spectroscopy based
(EIS) and on fluorescence
integrated fiber optic lifetime
sensorsmeasurement,
are the two mainan appa
techniques used to monitor the internal condition of lithium-ion
ratus with a nickel-coated fluorescent fiber was made in order to reliably monitor the in batteries.
ternal 2.2.1.
core Embedded
temperature Fiberof lithium-ion
Optic Sensor batteries. A source driving circuit, an optical cou
pling system, a fluorescence
According to a method signal
Du et al.detecting and processing
[19] proposed for estimatingsystem,
the internala display system, an
core temper-
a fluorescence excitation
ature of lithium-ion source
batteries with
based on afluorescence
wavelength of 470
lifetime nm makeanup
measurement, the apparatu
apparatus
When withthe rare-earth
a nickel-coated material
fluorescent on thefiberfluorescent
was made inprobe order to isreliably
exposed to thetheUV
monitor source, it ex
internal
core temperature of lithium-ion batteries. A source driving
cites fluorescence and emits afterglow, the decaying life of which is a single-value functio circuit, an optical coupling
system, a fluorescence signal detecting and processing system, a display system, and a
of temperature, i.e., the higher the temperature, the shorter the decaying life. The fluore
fluorescence excitation source with a wavelength of 470 nm make up the apparatus. When
cent probe is nickel-coated
the rare-earth material on andtheburied
fluorescentinside
probetheis cell core.to the UV source, it excites
exposed
Scientists
fluorescence have
and discovered
emits afterglow, that thethe fiber life
decaying Bragg grating
of which sensor canfunction
is a single-value accurately
of trac
temperature, i.e., the higher the temperature, the shorter the
and identify internal strain and temperature variations in lithium-ion batteries. Howeve decaying life. The fluorescent
probe is nickel-coated and buried inside the cell core.
Nascimento et al. [20] found that the monitoring method based on a single Bragg fibe
Scientists have discovered that the fiber Bragg grating sensor can accurately track
could and
be flawed
identify because
internal strain the andtwotemperature
signals could interfere
variations with one
in lithium-ion another,
batteries. making it im
However,
possible to quickly
Nascimento and
et al. [20]precisely
found that identify the internal
the monitoring method based temperature
on a singleof a lithium-ion
Bragg fiber could batter
be flawed because the two signals could interfere with one
during a thermal runaway. This is precisely because they were able to monitor both sig another, making it impossible
to quickly and
nals. Therefore, precisely for
a method identify the internal
accurately temperature
measuring theof internal
a lithium-ion battery during
temperature of lithium
a thermal runaway. This is precisely because they were able to monitor both signals.
ion batteries was proposed by coupling a fiber Bragg grating to the Fabry–Perot cavitie
Therefore, a method for accurately measuring the internal temperature of lithium-ion
and then subtracting
batteries was proposed the internal
by coupling strain signal
a fiber Braggmonitored
grating to the by Fabry–Perot
the Fabry–Perot cavitiescavities
and from
the signal
then detected
subtractingby thethe fiberstrain
internal Bragg grating.
signal Thisbywas
monitored the carried
Fabry–Perot outcavities
basedfrom on the
the fact tha
signal detected by the fiber Bragg grating. This was carried
Fabry–Perot cavities are extremely sensitive to strain but not to temperature, as is show out based on the fact that
Fabry–Perot cavities are extremely sensitive to strain but not to temperature, as is shown
in Figure 5.
in Figure 5.

(a) (b)
Figure Figure
5. (a) Experimental separator
5. (a) Experimental separatorof
ofthe hybridsensor;
the hybrid sensor;
(b) (b) experimental
experimental setup [20].
setup [20].

This method requires certain damaging alterations to the lithium-ion battery, which
can weaken its structural integrity when subjected to harsh circumstances, making it mainly
Processes 2023, 11, x FOR PEER REVIEW 7 of 34

Processes 2023, 11, 2345 7 of 33

This method requires certain damaging alterations to the lithium-ion battery, which
can weaken its structural integrity when subjected to harsh circumstances, making it
mainly unsuitable for commercial and military applications. As a result, non-invasive
unsuitable for
monitoring commercial
methods and military
for lithium-ion applications.
batteries As a result,
are a prominent non-invasive
topic monitoring
in today’s study.
methods for lithium-ion batteries are a prominent topic in today’s study.
2.2.2. Electrochemical Impedance Spectroscopy (EIS)
2.2.2. Electrochemical Impedance Spectroscopy (EIS)
Researchers have developed an EIS-based technique for monitoring internal battery
Researchers have developed an EIS-based technique for monitoring internal battery
temperature because numerous studies have demonstrated a correlation between electro-
temperature because numerous studies have demonstrated a correlation between electro-
chemical impedance spectra and internal battery temperature. EIS is a popular monitoring
chemical impedance spectra and internal battery temperature. EIS is a popular monitoring
method that is used to describe the electrochemical activity of lithium-ion batteries with-
method that is used to describe the electrochemical activity of lithium-ion batteries without
out harming the battery itself. The basic idea behind this technology is that a lithium-ion
harming the battery itself. The basic idea behind this technology is that a lithium-ion
battery’s impedance reduces as its interior temperature rises. The ohmic impedance 𝑅 ,
battery’s impedance reduces as its interior temperature rises. The ohmic impedance Rb ,
the
the impedance
impedanceof oflithium
lithiumions
ionsthrough
throughthe thesolid
solidelectrolyte interface(SEI) 𝑅Rsei,, the
electrolyteinterface(SEI) the charge
charge
transfer impedance 𝑅
transfer impedance Rct , and the lithium ion diffusion impedance W are the four main
, and the lithium ion diffusion impedance W are the four main
components
components of of the
the electrochemical
electrochemical impedance
impedance spectrum
spectrum of
of aalithium-ion
lithium-ionbattery,
battery, which
which is
is
schematically represented in Figure
schematically represented in Figure 6 [21].6 [21].

Figure 6.
Figure Schematicseparator
6. Schematic separator of
of impedance
impedance spectrum
spectrum of
of lithium-ion
lithium-ion battery
battery [21].
[21].

The internal
The internal resistance
resistance and
andcapacitance
capacitanceofoflithium-ion
lithium-ionbatteries,
batteries,asas
well
wellasas
their thermo-
their ther-
physical characteristics and thermal behavior models, have traditionally
mophysical characteristics and thermal behavior models, have traditionally been deter- been determined
using EIS-based
mined analysis analysis
using EIS-based [17,22,23].[17,22,23].
The relationship between EIS,
The relationship the state
between of health
EIS, (SOH)
the state of
and internal temperature has recently been thoroughly studied by researchers
health (SOH) and internal temperature has recently been thoroughly studied by research- [24–30].
The benefits of EIS over embedded sensors include not requiring physical insertion of
ers [24–30]. The benefits of EIS over embedded sensors include not requiring physical in-
thermocouples or other instruments, which could compromise the structural integrity of
sertion of thermocouples or other instruments, which could compromise the structural
the battery; being extremely sensitive and quickly reflecting the current condition of battery;
integrity of the battery; being extremely sensitive and quickly reflecting the current con-
and finally, being appropriate for batteries of any size and shape.
dition of battery; and finally, being appropriate for batteries of any size and shape.
Lithium-ion battery misuse and capacity deterioration can be tracked using the SOH
Lithium-ion battery misuse and capacity deterioration can be tracked using the SOH
measurement provided by EIS [31–35]. In order to compare the SOH of individual lithium-
measurement provided by EIS [31–35]. In order to compare the SOH of individual lithium-
ion batteries and battery modules under normal charge, discharge, and abuse from over-
ion batteries and battery modules under normal charge, discharge, and abuse from over-
charging, Love et al. [31,32] employed EIS. The findings of this study demonstrated that
charging, Love et al. [31,32] employed EIS. The findings of this study demonstrated that
the method can track the SOH of both individual batteries and battery modules, as well as
the method can track the SOH of both individual batteries and battery modules, as well
overcharged cells, suggesting that the method has promising future development.
as overcharged cells, suggesting that the method has promising future development.
Batteries made of various materials and forms have also benefited from the application
Batteries
of internal made of various
temperature materials
monitoring and forms
technology have
of EIS. also benefited
Spinner et al. [36]from
used the applica-
single-point
tion of internal temperature monitoring technology of EIS. Spinner
electrical impedance measurements to monitor the transient internal temperature ofet al. [36] used single-
com-
point electrical impedance measurements to monitor the transient internal
mercial 18650-type lithium-ion batteries. The results of the study showed a correlation temperature of
commercial 18650-type lithium-ion batteries. The results of the study showed
between the internal temperature of the battery and thermal runaway, and it was observed a correlation
between the internalresponse
that the impedance temperature of the to
is related battery andwhich
the SEI, thermalwillrunaway, and itlead
subsequently was observed
to battery
that the impedance response is related to the SEI, which will subsequently
deactivation once the SEI decomposes. A method to determine the internal temperature lead to battery
of a lithium-ion battery based on the electrochemical impedance intercept frequency was
proposed by Raijmakers et al. [29] after analyzing the variation of the EIS of the battery with
Processes 2023, 11, 2345 8 of 33

temperature and state of charge (SOC), defined as the frequency when voltage and current
are in phase. The mechanism is that the SOC of lithium-ion battery is unrelated to this
impedance intercept frequency, which only relates to its internal temperature. Therefore,
an accurate estimate of the internal temperature can be derived by EIS probing as long as
a stable SEI exists at the anode of the active lithium-ion battery. Using an electrochemi-
cal impedance spectrum that was produced from a 1 kHz sine wave stimulation signal,
Schwarz et al. [37] obtained the estimation of the interior core temperature of lithium-ion
battery. This method was used to construct and incorporate a device into the BMS. Lastly,
the results demonstrate that the interior core temperature of a lithium-ion battery may be
accurately measured in real time.
Monitoring a number of impedance spectrum characteristics enables the EIS-based
approach for monitoring and detecting the internal temperature of lithium-ion batteries.
The choice of the excitation source, including the mode and frequency of the excitation
signal, is crucial for correctly forecasting the internal temperature of lithium-ion batteries
and the ensuing thermal runaway [38]. Around 60–135 ◦ C is the temperature at which the
SEI layer begins to break down, at which point the lithium-ion battery becomes vulnerable
to thermal runaway.

2.3. Summary
In conclusion, Table 1 lists the benefits and drawbacks of two methods for detecting
external conditions, such as the monitoring of surface temperature and terminal voltage, as
well as two methods for detecting internal conditions, such as the use of an embedded fiber
optic sensor for direct temperature measurement and the use of an EIS analysis method for
indirect temperature estimation.

Table 1. Advantages and disadvantages of different thermal runaway monitoring methods.

Method Advantages Disadvantages

Monitor the voltage in real- time
Complex topology of voltage
Terminal voltage Capable of locating faulty battery
External parameters sensors, high cost
Easy to operate, low cost
Significant temperature difference
Monitor the surface temperature in real time from the internal temperature,
Surface temperature
Easy to operate, low cost Inaccurate for determining the
state of the battery
High cost
Monitor the internal core temperature of the
Embedded optical fiber sensors Higher requirements for
Internal parameters battery directly
battery packaging
Electrochemical impedance Predict the internal core temperature of the
Fail to monitor large-scale
spectroscopy battery without complex hardware
batteries quickly and effectively
analysis Online monitoring of battery status

3. Lithium-Ion Battery Thermal Runaway Warning Technology

3.1. Thermal Runaway Warning Technology Based on Lithium-Ion Battery Voltage
Voltage is a crucial signal for BMS monitoring, and the key to early warning is the
analysis of voltage change during thermal runaway. The time delay between voltage drop
and temperature rise, which is about 15 s, as shown in Figure 7, was demonstrated by
Feng et al. [13] using a large accelerated calorimeter on a large-capacity lithium-ion battery.
Because this time period is favorable for thermal runaway early warning, it can be used as
a signal for a thermal runaway warning system. Meanwhile, using the tiny current pulse
discharge approach, it has been discovered that the resistance of the battery gradually
increases as the battery temperature rises. By using the modest current pulse discharge
approach, it was discovered that the battery resistance rose as the battery temperature
rose. In-depth research on this phenomenon by Ren et al. [39] established a connection
between electrical signal changes brought on by internal short circuits and temperature
increases brought on by thermal runaway. When an irregular signal is discovered, the
 be used as a signal for a thermal runaway warning system. Meanwhile, using the tiny
current pulse discharge approach, it has been discovered that the resistance of the battery
gradually increases as the battery temperature rises. By using the modest current pulse
discharge approach, it was discovered that the battery resistance rose as the battery tem-
Processes 2023, 11, 2345 perature rose. In-depth research on this phenomenon by Ren et al. [39] established a9 con-of 33
nection between electrical signal changes brought on by internal short circuits and tem-
perature increases brought on by thermal runaway. When an irregular signal is discov-
ered, thesensors
voltage voltage sensors incorporated
incorporated intomay
into the BMS the promptly
BMS may raise
promptly raiseand
an alarm an do
alarm and job
a good do
a good job of monitoring the terminal voltage of
of monitoring the terminal voltage of battery [40]. battery [40].

Figure 7. The interval between the voltage drop and the temperature rise [13].
[13].

Mao
Mao etet al.
al. [41]
[41] conducted
conducted an an extensive
extensive and
and methodical
methodical study
study onon the
the electrochemical
electrochemical
and
and thermal behavior of lithium-ion batteries from overcharge to thermal runaway,
thermal behavior of lithium-ion batteries from overcharge to thermal runaway, andand
divided the overcharge behavior into four stages by examining the
divided the overcharge behavior into four stages by examining the voltage and voltage and temperature
tempera-
changes of lithium-ion
ture changes batteries
of lithium-ion during
batteries overcharge
during andand
overcharge thermal
thermalrunaway,
runaway,as shown
as shownin
Figure 8. This study was carried out in order to use battery voltage as a warning
in Figure 8. This study was carried out in order to use battery voltage◦ as a warning pa- parameter.
The voltage
rameter. Theincreases to 4.74 V, the
voltage increases temperature
to 4.74 gradually climbs
V, the temperature to 31climbs
gradually C, and
to the battery
31 °C, and
charge condition increases to 136% in the first stage. In the second stage, the
the battery charge condition increases to 136% in the first stage. In the second stage, battery keeps
the
growing as the voltage increases to Vmax (5.07 V) and the temperature quickly increases
battery keeps growing as the voltage increases to 𝑉 (5.07 V) and the temperature
to 39 ◦ C. The battery temperature rises to 89.5 ◦ C and the voltage drops to 4.47 V in the
quickly increases to 39 °C. The battery temperature rises to 89.5 °C and the voltage drops
third stage due to severe electrode damage; in the fourth stage, the voltage rises to 4.93 V
to 4.47 V in the third stage due to severe electrode damage; in the fourth stage, the voltage
and the temperature reaches 97.9 ◦ C; and finally, the battery ruptures at roughly 174%
rises to 4.93 V and the temperature reaches 97.9 °C; and finally, the battery ruptures at
SOC and subsequently catches fire at 174.33% SOC. They came to the conclusion that
Processes 2023, 11, x FOR PEER REVIEW 10 ofthe
34
roughly 174% SOC and subsequently catches fire at 174.33% SOC. They came to the con-
highest value of Vmax may be employed as a crucial point for warning in order to prevent
clusion that the highest value of 𝑉 may be employed as a crucial point for warning in
thermal runaway.
order to prevent thermal runaway.

Figure
Figure 8.
8. Lithium-ion
Lithium-ion battery
battery 0.5
0.5 C
C overcharge
overcharge test
test curve
curve [41].
[41].

In conclusion,
In conclusion, voltage
voltage can
can be
be used
used as
as aa warning
warning parameter
parameter for
for thermal
thermal runaway,
runaway, but
but
because it
because it is
isrelated
relatedtotothe
thenature
natureof of
thethe
battery body
battery andand
body changes therethere
changes are caused by other
are caused by
conditions as well as thermal runaway, it is less frequently used as a separate
other conditions as well as thermal runaway, it is less frequently used as a separate warn- warning
ing parameter. Voltage will, however, become more crucial in the assessment of battery
warning as battery packing, grouping, and other technologies advance continuously.

3.2. Thermal Runaway Warning Technology Based on Lithium-Ion Battery Temperature

 Processes 2023, 11, 2345 10 of 33

parameter. Voltage will, however, become more crucial in the assessment of battery warning
as battery packing, grouping, and other technologies advance continuously.

3.2. Thermal Runaway Warning Technology Based on Lithium-Ion Battery Temperature

Lithium-ion batteries can experience thermal runaway, which is characterized directly
by a significant rise in internal temperature and indirectly by a rise in surface temperature.
Thermal runaway is produced by the degradation of internal microstructure and a number
of side reactions.
A multi-stage warning system for 18650 lithium-ion batteries and battery packs was
created by Yang et al. [42]. The law of lithium-ion battery heat production was investigated
using charge/discharge cycle testing on 18650 lithium-ion batteries at various multipliers
and real-time thermocouple monitoring of the battery surface temperature. Battery capacity
started to degrade when battery temperature reached 55 ◦ C, and the trend of increase in
temperature was slower from 55 to 80 ◦ C, particularly in the stage of 70–80 ◦ C, according
to the analysis of the test findings. Finally, a three-stage warning system was decided on
with a first-level warning temperature of 55 ◦ C, a second-level warning temperature of
70 ◦ C, and a third-level warning temperature of 80 ◦ C, as is shown in Figure 9. The internal
temperature of the battery is actually close to 100 ◦ C when the surface temperature is 80 ◦ C,
at which point the SEI coating begins to decompose. When more heat is continuously
accumulated, the battery’s risk of thermal runaway increases significantly. The gadget
Processes 2023, 11, x FOR PEER REVIEW 11 of 34
demonstrates outstanding early warning performance for the unusually hot lithium-ion
battery and has the advantages of high efficiency, convenience, and quick response.

Figure
Figure 9. System
9. System determination
determination flowchart.
flowchart.

However, the biggest issue with using temperature as an early warning parameter is
However, the biggest issue with using temperature as an early warning parameter is
that the thermocouple or temperature sensor’s measurement of the battery’s internal and
that the thermocouple or temperature sensor’s measurement of the battery’s internal and
external temperature has a certain error, which will cause the battery to experience thermal
external temperature has a certain error, which will cause the battery to experience ther-
runaway before the set warning temperature and ultimately result in early warning failure.
mal runaway before the set warning temperature and ultimately result in early warning
In order to monitor the battery temperature, Li et al. [43] introduced a resistance
failure.
temperature detector (RTD) that was mounted behind the electrode current collector of
In order to monitor the battery temperature, Li et al. [43] introduced a resistance tem-
CR2032 coin cells, as is shown in Figure 10. The results revealed that the temperature
perature
measureddetector
in this(RTD) thatonwas
way was, mounted
average, behind
5.8 ◦ C higherthe electrode
than current
the external RTD,collector of
with a detec-
CR2032 coin cells, as is shown in Figure 10. The results revealed that the temperature
tion speed that was almost ten times faster, preventing thermal runaway events without
measured in this
interfering withwaythe was, on average,
operation 5.8 °C
of the LIBs. higher
Future than the external
temperature RTD,techniques
monitoring with a detec-
must
tion
bespeed that was to
more accurate almost ten the
increase times faster,rate
success preventing
of thermalthermal
runawayrunaway events without
warnings.
interfering with the operation of the LIBs. Future temperature monitoring techniques
must be more accurate to increase the success rate of thermal runaway warnings.
 perature detector (RTD) that was mounted behind the electrode current collector of
CR2032 coin cells, as is shown in Figure 10. The results revealed that the temperature
measured in this way was, on average, 5.8 °C higher than the external RTD, with a detec-
tion speed that was almost ten times faster, preventing thermal runaway events without
Processes 2023, 11, 2345 interfering with the operation of the LIBs. Future temperature monitoring techniques
11 of 33
must be more accurate to increase the success rate of thermal runaway warnings.

(a) (b)
Figure 10. (a) Schematic of customized RTD embedded LIB coin cell; (b) RTD embedded polylactic
Figure 10. (a) Schematic of customized RTD embedded LIB coin cell; (b) RTD embedded polylactic
acid spacer and CR2032 cell with internal RTD [43].
acid spacer and CR2032 cell with internal RTD [43].

Whentemperature
When using using temperature as a parameter
as a parameter forwarning,
for early early warning, the biggest
the biggest issue isissue
that is that
the thermocouple or temperature sensor’s ability to accurately measure both
the thermocouple or temperature sensor’s ability to accurately measure both the battery’s the battery’s
interiorinterior and exterior
and exterior temperatures
temperatures suffers suffers
from a from a certain
certain error.
error. This This the
causes causes the battery
battery to to
experience thermal runaway before the temperature is set to warn, which
experience thermal runaway before the temperature is set to warn, which ultimately results ultimately re-
in earlysults in early
warning warning failure.
failure.
In their research of battery early warning, Zhang et al. [44] discovered that a ternary
lithium battery’s surface temperature is only 56.3 ◦ C when it deforms and catches fire, indi-
cating that temperature is not an appropriate basis for detecting fires in lithium-ion batteries
and that a more effective method to track the battery’s actual temperature is required.
Grandjean et al. [45] performed a simulation study on the thermal characteristics of
LiFePO4 lithium-ion batteries with a capacity of 20 Ah and discovered that the temperature
difference between the internal temperature and the battery’s surface could reach 20 ◦ C in
the large multiplier discharge state. They came to the conclusion that it was challenging
to accurately reflect the lithium-ion battery’s true state by measuring the battery surface
temperature. Additionally, the conventional technique of employing thermocouples to
monitor the surface temperature of the battery in order to predict thermal runaway has
a certain time delay due to the presence of heat conduction. Researchers have suggested
that in order to provide early warning of thermal runaway in lithium-ion batteries, it is
necessary to detect the battery’s interior temperature more precisely.
Parhizi et al. [46] developed a temperature tracking model of the internal battery based
on heat conduction analysis and experimentally validated it using lithium-ion batteries
with two different cathode materials based on the thermal characteristics of lithium-ion
batteries and the kinetic characteristics of chemical reactions during thermal runaway. It is
not logical to merely use the surface temperature measurements to monitor the thermal
runaway of the lithium-ion battery because simulations and experiments have shown that
the maximum internal core temperature of the lithium-ion battery during thermal runaway
is nearly 500 ◦ C higher than the surface temperature. An internal battery failure symptom
that is frequently observed is an excessively high internal temperature of the battery. When
the battery’s internal temperature exceeds 90 ◦ C and 130 ◦ C, there is a risk of explosion
and the possibility of thermal runaway. As a result, it is important to keep an eye on the
battery’s interior temperature.
 battery. When the battery’s internal temperature exceeds 90 °C and 13
of explosion and the possibility of thermal runaway. As a result, it is im
eye on the battery’s interior temperature.
A lithium-ion battery internal state monitoring scheme based on
Processes 2023, 11, 2345 12 of 33
lapsible Bragg fiber sensor was proposed by Raghavan et al. [47] to add
of monitoring the internal core temperature of lithium-ion batteries as
A lithium-ion
11. When battery internal
the internal state
stress ormonitoring
temperature schemeofbased
theonbattery
an embedded col-
changes, the
lapsible Bragg fiber sensor was proposed by Raghavan et al. [47] to address the challenge of
tive indextheand
monitoring refracted
internal light ofwavelength
core temperature willaschange.
lithium-ion batteries is shown inBy measuring
Figure 11.
fracted light wavelength, the internal stress and temperature of the batt
When the internal stress or temperature of the battery changes, the Bragg fiber refractive
index and refracted light wavelength will change. By measuring the change of refracted
mined.
light wavelength, the internal stress and temperature of the battery are then determined.

Figure 11. Battery with fiber Bragg grating sensor [47].

Figure 11. Battery with fiber Bragg grating sensor [47].
3.3. Thermal Runaway Warning Technology Based on EIS
The EIS-based thermal runaway warning approach is also a frequently explored area
3.3. Thermal
of current Runaway
research Warning
since it has Technology
been demonstrated Based onstudies
by numerous EIS that there is a
relationship between electrochemical impedance spectra and internal battery temperature.
The AC impedance behavior of a commercial sealed lithium-ion battery at 10–40 ◦ C
and different SOC was examined by Suresh et al. [25]. The data were composed of two
capacitive parts in the frequency range of 100 Hz to 10 mHz and an inductive component
in the range of 100 kHz to 100 Hz. The impedance parameters were assessed after the data
were processed using an equivalent circuit and a nonlinear least-squares fitting method. The
findings demonstrate that the SEI layer impedance in lithium-ion batteries is temperature-
sensitive rather than cell-charge- or charge-transfer-resistance-sensitive. According to
impedance characteristics, a method developed by Schmidt et al. [16] can be used to
estimate the average internal temperature of a battery even when the temperature inside
the battery is not constant or changes abruptly. The results of the discussion on the
impact of SOC on internal temperature estimation at various frequencies revealed that SOC
considerably influences impedance temperature estimation results at low frequencies, while
having less of an impact at high frequencies. Based on the monotonic link between the
impedance phase shift and the internal cell temperature suggested by the prior study [27],
Zhu et al. [48] further developed an impedance-based temperature estimation method
taking into account the electrochemical imbalance generated by current excitation.
 Processes 2023, 11, 2345 13 of 33

Srinivasan et al. [24,49] proposed an early warning method for the thermal runaway of
lithium-ion batteries based on rapid monitoring of cell impedance. The Solartron analytical
electrochemical interface and frequency response analyzer were used to measure the cell
voltage, the phase-shift, and the amplitude |Z| at 5 Hz. The internal temperature of the
battery is monitored and the likelihood of thermal runaway of the battery is predicted using
the phase shift, which is marginally connected with the battery capacity and substantially
correlated with the internal temperature T. Figure 12 illustrates how the temperature varies
gradually before a lithium-ion battery thermal runaway, but the impedance phase shift will
appear abnormal, leading experts to assume that monitoring the internal impedance can
successfully produce a thermal runaway warning. It is not appropriate to use the internal
resistance of the battery as the determining factor of the thermal runaway of the battery
and should be used in conjunction with other parameters to determine whether the thermal
runaway of the battery is or is not occurring. This is because the sudden change in the
internal resistance of the battery is not always caused by thermal runaway of the battery,
but can also be caused by an external disturbance of the battery, such as poor contact. For
transient internal temperature monitoring of commercial 18650-type lithium-ion batteries,
Spinner et al. [36] used a single-point electrical impedance measurement technique based
on electrochemical impedance spectroscopy. This allowed for the attainment of impedance
spectra in the temperature range of −10 ◦ C to 95 ◦ C and 0% to 100% SOC charge, and it
demonstrated the correlation between the internal temperature of the battery and the imag-
inary part of the impedance. This investigation expands on findings that Srinivasan [24,49],
Processes 2023, 11, x FOR PEER REVIEW Schmidt [16], Love [31,32], and others previously published. It expands the capabilities14of of 34
the approach as a potential crucial tool for next-generation battery diagnostics by becoming
the first single-point impedance test for interior temperature monitoring up to 95 ◦ C.

(a) (b)
Figure 12. 12.
Figure (a) (a)
Graph ofofvoltage,
Graph voltage,impedance phaseand
impedance phase andtemperature
temperature change;
change; (b) impedance
(b) impedance phasephase
shiftshift
andand
surface temperature
surface temperature[24,49].
[24,49].

Carkhuff
Carkhuff etetal.
al.[50]
[50] constructed
constructed a tiny, low-power,
a tiny, multi-frequency
low-power, (1–1000 Hz)
multi-frequency impedance
(1–1000 Hz) im-
battery management system for numerous batteries of various
pedance battery management system for numerous batteries of various capacity using a capacity using a multi-
frequency impedance meter. Phase shift and amplitude monitoring capabilities of the
multi-frequency impedance meter. Phase shift and amplitude monitoring capabilities of
sensor enable simultaneous monitoring of the interior temperature of each cell. By monitor-
the ing
sensor enable simultaneous
and correcting mismatches and monitoring of theand
other electrical interior
thermaltemperature
irregularitiesofthat
each cell. By
occur
monitoring
in individual cells, the BMS maintains battery safety and efficiency without costing more that
and correcting mismatches and other electrical and thermal irregularities
occur in individual
money, taking upcells,
morethe BMSormaintains
space, using more battery
powersafety
than aand efficiency
typical without
BMS. The costing
interior
more money, taking
temperature up morebatteries
of lithium-ion space, ormay usingnow more power than
be measured usinga typical
a novel BMS. The interior
technique, ac-
cording toof
temperature Raijmakers
lithium-ion et al. [29]. Due
batteries may to now
the close relationship
be measured between
using zero-intercept
a novel technique, ac-
frequency
cording (ZIF) and et
to Raijmakers theal.
internal
[29]. Duebattery temperature
to the and the fact
close relationship that ZIF
between is unaffected fre-
zero-intercept
by SOC, the internal battery temperature is calculated using the
quency (ZIF) and the internal battery temperature and the fact that ZIF is unaffected ZIF method. ZIF is the by
frequency at which the imaginary part of the impedance in the electrochemical impedance
SOC, the internal battery temperature is calculated using the ZIF method. ZIF is the fre-
spectrum is zero. Then, a non-ZIF technique based on the internal battery temperature
quency at which the imaginary part of the impedance in the electrochemical impedance
spectrum is zero. Then, a non-ZIF technique based on the internal battery temperature
estimating method is provided [51], which can successfully prevent thermal runaway, in
order to avoid the interference of the internal battery current and increase the prediction
Processes 2023, 11, 2345 14 of 33

estimating method is provided [51], which can successfully prevent thermal runaway, in
order to avoid the interference of the internal battery current and increase the prediction
accuracy of the model. It is easy and practical to apply this impedance-based sensorless
temperature measurement in a variety of stationary, portable, and high-power equipment,
such as electric vehicles, electric ships, etc. Richardson et al. [30,52] looked into a novel
technique for calculating the interior temperature distribution of a cylindrical battery by
combining measurements of EIS and surface temperature. The model is effective enough
to be used in the BMS of an electric car and does not require knowledge of the thermal
characteristics of the cell, heat production rate, or thermal boundary conditions. Internal
thermocouple measurements, which have not previously been shown for impedance-based
temperature estimate, are used to validate the model. Then, using a thermal model and EIS
measurements, a technique to estimate the interior and surface temperatures of the battery
is suggested.
Based on earlier research, Lyu et al. [53] extended their proposal and confirmed the
characteristics of the dynamic impedance slope in the 30–90 Hz region that gradually
changed from negative to positive during overcharging. Additionally, using 70 Hz as
an example, the thermal runaway mishap can be avoided when the battery does not
release gas, bulge, or burn by monitoring the 70 Hz dynamic impedance and stopping the
charging when the impedance slope turns positive since there is enough time to take the
necessary precautions to stop the thermal runaway because of the warning period of 580 s
prior to the thermal runaway. This property, which enables the overcharge warning and
thermal runaway prediction of lithium-ion batteries, is simple to recognize and does not
necessitate the use of sophisticated mathematical models and parameters. Additionally,
a large-scale use of the early warning technique based on this slope shift in conjunction
Processes 2023, 11, x FOR PEER REVIEW 15 of 34
with an online dynamic impedance monitoring equipment is possible to prevent thermal
runaway incidents, as is shown in Figure 13.

Figure 13.Diagrams
Figure13. Diagramsof
ofEIS
EISmeasurement
measurementusing
usingcurrent-type
current-typeexcitation.
excitation.

In
Inconclusion,
conclusion,there
thereisiscurrently
currentlynono
standard way
standard forfor
way EIS-based internal
EIS-based battery
internal temper-
battery tem-
ature prediction, and each method will rely on intricate mathematical models. The method
perature prediction, and each method will rely on intricate mathematical models. The
described by Lyu that does not depend on mathematical models and parameters has a
method described by Lyu that does not depend on mathematical models and parameters
good chance of development since researchers in the field of EIS-based thermal runaway
has a good chance of development since researchers in the field of EIS-based thermal runa-
prediction think that the reliability of the warning is more significant than the accuracy.
way prediction think that the reliability of the warning is more significant than the accuracy.
3.4. Summary
3.4. Summary
The advantages and disadvantages of the early warning approaches based on battery
Thebattery
voltage, advantages and disadvantages
temperature, and EIS areof the earlyupwarning
summed in Tableapproaches
2. based on battery
voltage, battery temperature, and EIS are summed up in Table 2.

Table 2. Advantages and disadvantages of different thermal runaway warning methods.

Method Advantages Disadvantages

Voltage can be monitored in Voltage variations are complex and
real time poorly regulated
Based on battery
Faulty batteries can be located Thermal runaway warning is lag-
voltage
Predict the state of charge of ging behind
Processes 2023, 11, 2345 15 of 33

Table 2. Advantages and disadvantages of different thermal runaway warning methods.

Method Advantages Disadvantages

Voltage can be monitored in real time Voltage variations are complex and
Faulty batteries can be located poorly regulated
Based on battery voltage
Predict the state of charge of the battery Thermal runaway warning is lagging behind
in real time Influenced by other external factors
Surface temperature can be monitored in Large temperature difference from
real time internal temperature
Predict the state of health of the battery in Low accuracy of thermal runaway prediction
Based on battery temperature
real time Thermal runaway warning is lagging behind
Directly measure internal Used in combination with EIS technology or
battery temperature embedded fiber optic sensor technology
Influenced by other external factors
Sensitive to temperature changes
Complex calibration process as different
Independent of SOC
Based on EIS lithium-ion battery systems have different
The original structure of the battery will
parameters of impedance
not be destroyed
Relies on more complex mathematical models

4. Lithium-Ion Battery Thermal Runaway Protection Technology

Lithium-ion battery thermal runaway events can have serious consequences; therefore,
in order to avoid them, not only do we need to monitor their status in real time to detect
thermal runaway, but we also need to take certain precautions to stop it from spreading once
it does. Enhancing the safety of lithium-ion batteries at the cell level (internal protection)
and using cooling or barrier technologies throughout the battery (external protection) are
the two most typical ways to slow down the thermal runaway propagation process.

4.1. Lithium-Ion Battery Thermal Runaway Internal Protection Technology

In order to increase the safety of lithium-ion batteries themselves, a more rigorous
preparation procedure and better battery materials must be used in battery manufacturing.
This is due to the operating principle and thermal runaway mechanism of lithium-ion batteries.
The primary components of the cathode, anode, separator, and electrolyte make up the
internal structure of lithium-ion battery. The separator, one of the crucial four core materials,
serves as a migration channel for lithium ions, allowing the lithium ions in the electrolyte to
freely pass through the micro-pores during charging and discharging to ensure the normal
operation of the battery. It also serves to isolate the cathode and anode to prevent a short
circuit by contacting the two electrodes. The essential inner layer components, particularly
puncture resistance, self-shutdown, and high-temperature resistance of the separator, play
a significant role in safety performance of the battery and have an impact on the resistance,
capacity, and life of the battery [54–56].

4.1.1. Electrolyte
In lithium-ion batteries, the electrolyte serves as the medium for the transportation
of lithium ions, and it is often made up of high-purity organic solvents, lithium salts of
electrolytes, and associated additives. Lithium hexafluorophosphate (LiPF6 ) is commonly
used as a solute in the current electrolyte for lithium-ion batteries, while carbonate is
typically used as a solvent because both substances are flammable and combustible. In
order to increase the thermal robustness of lithium-ion batteries and lower the risk of
thermal runaway, electrolytes are typically adjusted in the ways listed below to address the
thermal runaway issue.
1. Improve the thermal stability of lithium salts
LiPF6 , lithium hexafluoroarsenate (LiAsF6 ), lithium perchlorate (LiCIO4 ), lithium per-
chlorate (LiBF4 ), and others are the most frequently utilized lithium salts in lithium-ion bat-
teries. The fact that these lithium salts are all very unstable to heat is one of the factors con-
tributing to the low safety of electrolytes. The severe toxicity of LiAsF6 restricts its use even
Processes 2023, 11, 2345 16 of 33

though the mixture of LiAsF6 and tetrahydrofuran (THF) has good electrochemical charac-
teristics and thermal stability. The thermal stability of some novel lithium salts, such as
lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulphonyl)imide
(LiTFSI), is hampered by their poor aluminum collector passivation [57]. Therefore, it is
essential to modify the lithium salts in order to enhance the performance of electrolyte.
First, it is possible to think about substituting the unstable lithium salts with oth-
ers, such as lithium bis(oxalate)borate (LiBOB) and lithium difluoro(oxalate) borate (LiD-
FOB), which have been shown to form passivation films on collector surfaces and may
be connected to the breakdown of B-O compounds [58,59]. Figure 14 illustrates the ultra-
concentrated lithium bis(fluorosul-fonyl)amide (LiFSA)electrolyte that Wang et al. [60]
Processes 2023, 11, x FOR PEER REVIEW 17 of 34
utilized for batteries. They discovered that it has a stable cycling performance, good
multiplicative performance, and low flammability.

Figure
Figure 14.
14. Cycling
Cycling performance
performance and
and rate
rate performance
performance of
of cell
cell in
in LIFSA
LIFSA electrolyte
electrolyte [60].

The concentration
The concentration of of the
the lithium
lithium salts
salts can
can bebe changed
changed to to improve
improve performance,
performance, and and
it has been discovered that high concentration electrolyte (HCE)
it has been discovered that high concentration electrolyte (HCE) lessens the flammability lessens the flammability
of the
of theelectrolyte.
electrolyte.ThisThisis is a result
a result of of
thethe
highhigh concentration
concentration of lithium
of lithium salts,salts,
where where
the ma-the
majority
jority of solvent
of solvent molecules
molecules combine
combine withwith cations
cations totoform
forma asolventized
solventizedstructure
structurein in the
the
HCE system.
HCE system. ThisThis decreases
decreases the the amount
amount of of free
free solvent
solvent molecules
molecules and and creates
creates aa special
special
solventizedstructure,
solventized structure,which
whichsignificantly
significantly inhibits
inhibits thethe interfacial
interfacial reactions
reactions between
between the
the sol-
solvent and the electrode and lowers the flammability of the cell [61].
vent and the electrode and lowers the flammability of the cell [61]. As an illustration, by As an illustration, by
raising the concentration of LiPF6 in the electrolyte to 2.5 M, the electrolyte demonstrateda
raising the concentration of LiPF in the electrolyte to 2.5 M, the electrolyte demonstrated
anoticeably
noticeablylonger
longerignition
ignitiontime,
time,shorter
shorterself-extinguishing
self-extinguishing time time (SET),
(SET), and better cycling
and better cycling
performance in
performance in the
the cell
cell as
as aa result
result ofof the
the improved
improved shuttling
shuttling of of abundant
abundant lithium
lithium ions
ions
between the cathode and anode [62]. Aluminum collectors can develop
between the cathode and anode [62]. Aluminum collectors can develop a protective coat- a protective coating
of lithium
ing of lithiumfluoride and
fluoride andbebe protected
protectedfrom fromcorrosion
corrosionby byusing
usingaa highhigh concentration
concentration of
of
−1
LiTFSI electrolyte. According to Liang et al. [63], a high concentration
LiTFSI electrolyte. According to Liang et al. [63], a high concentration (2.3 mol kg ) of (2.3 mol kg ) of
LiTFSI can be employed as an electrolyte that is non-flammable
LiTFSI can be employed as an electrolyte that is non-flammable and has good thermal and has good thermal
stability. The
stability. The dissolution
dissolutionnumbers
numbersofofethylene
ethylenecarbonate
carbonate(EC) (EC) andand ethylene
ethylene glycol dimethyl
glycol dime-
ether (DME) dropped and increased, respectively, with increasing salt content, according
thyl ether (DME) dropped and increased, respectively, with increasing salt content, ac-
to the Raman spectroscopy results. It follows that the lithium ions attach to fewer EC
cording to the Raman spectroscopy results. It follows that the lithium ions attach to fewer
but more DME molecules in the high concentration electrolyte, enhancing the thermal
EC but more DME molecules in the high concentration electrolyte, enhancing the thermal
stability and nonflammability. As seen in Figure 15, this electrolyte not only exhibits out-
standing thermal stability but also electrochemical properties that are equivalent to those
of traditional carbonate-based electrolytes. High viscosity, poor wettability, and high cost
still prevent their use in commercial lithium-ion batteries, despite the fact that highly con-
 Processes 2023, 11, 2345 17 of 33

stability and nonflammability. As seen in Figure 15, this electrolyte not only exhibits
outstanding thermal stability but also electrochemical properties that are equivalent to
those of traditional carbonate-based electrolytes. High viscosity, poor wettability, and high
Processes 2023, 11, x FOR PEER REVIEW 18 of 34
cost still prevent their use in commercial lithium-ion batteries, despite the fact that highly
concentrated electrolytes can lower flammability and increase electrochemical performance.

Figure
Figure15.
15.Cycling
Cyclingperformance
performanceofofcell 2.3mol
cellinin2.3 molkg
kg−1LiTFSI
LiTFSI[63].
[63].

2.2. An
Anadditive
additivefor
forflame
flameretardancy
retardancy
The
Themost
mostflammable
flammablecomponent
componentofofthe theelectrolyte
electrolyteisisthe theorganic
organiccarbonate
carbonatesolvent,
solvent,
and
and the basic idea behind this technique is to incorporate flame-retardant chemicalsinto
the basic idea behind this technique is to incorporate flame-retardant chemicals into
organic
organicelectrolytes
electrolytestotoincrease
increasetheir theircapacity
capacitytotoneutralize
neutralizecombustion
combustionradicals radicalsand andthus
thus
reduce
reducetheir
theirflammability
flammability[64,65].
[64,65].Commonly
Commonlyused usedflame
flameretardants
retardantson onthe
themarket
marketinclude
include
phosphorus-based
phosphorus-basedadditives additivessuch suchasastributyl
tributylphosphate
phosphate(TBP), (TBP),triphenyl
triphenylphosphate
phosphate(TPP),(TPP),
triethyl
triethylphosphate
phosphate(TEP),
(TEP), etc.
etc. Liu et al. al. [64]
[64] created
createdaashell shelloutoutofofa poly(vinylidene
a poly(vinylidene fluo-
fluoride-
ride-hexafluoropropylene)
hexafluoropropylene) (PVDF-HFP) (PVDF-HFP) andand employed
employed TPPTPP as theas the
inner inner
core.core.
TheThe PVDF-
PVDF-HFP
HFP
shellshell
meltsmelts andTPP
and the the within
TPP within leaksthe
leaks into into the electrolyte
electrolyte when the when the battery
battery temperaturetempera-
is too
ture
high,is too
whichhigh,
canwhich
preventcanthe prevent
monomer the monomer
from burning. from Traditional
burning. Traditional
carbonic acid carbonic acid
electrolyte
electrolyte
(propylene(propylene
carbonate carbonate
(PC) as solvent (PC) as and solvent
LiPF6 and LiPF as
as lithium lithium
salt) was givensalt) was given a
a fluorinated
diethyl carbonate
fluorinated co-solventco-solvent
diethyl carbonate by Pham et byal.
Pham[65], etwhich could
al. [65], whichreduce
could the flammability
reduce the flam-of
electrolyte
mability by mixingby
of electrolyte fluorine
mixingand hydrogen
fluorine radicals during
and hydrogen radicalscombustion.
during combustion.Pires et al. [66]
Pires
etused triphosphite
al. [66] as a supplement
used triphosphite to the electrolyte.
as a supplement The cycling The
to the electrolyte. performance of the battery
cycling performance
ofwas
the found
batterytowasbe improved
found to beby partial fluorination
improved of alkyl phosphate,
by partial fluorination which alsowhich
of alkyl phosphate, raised
the nonflammability of the cathode and electrolyte and increased
also raised the nonflammability of the cathode and electrolyte and increased the safety of the safety of lithium-
ion batteries.
lithium-ion Jin et Jin
batteries. al. et
[67] usedused
al. [67] a synthetic
a synthetic bifunctional
bifunctional additive
additive asasa aflame-retardant
flame-retard-
film-former
ant film-former inin LiPF
LiPF 6 electrolyte
electrolyte totomeasure
measurethe theflame
flameretardancy
retardancy of of the additive.
additive.They They
measuredthe
measured theelectrolyte’s
electrolyte’s self-extinguishing
self-extinguishing time time to achieve
to achieve this.this. The results
The results showed showed
that
that dimethyl
dimethyl allyl phosphonate
allyl phosphonate enhanced enhanced the electrolyte’s
the electrolyte’s thermalthermal
stabilitystability
and hadand had a
a favor-
favorable electrochemistry with graphite anodes. Additionally,
able electrochemistry with graphite anodes. Additionally, as flame-retardant additives to as flame-retardant additives
to the
the electrolyte,
electrolyte, ZengZeng et [68]
et al. al. [68]
usedused large
large quantities
quantities of of LiFSI
LiFSI salts
salts andandphosphate
phosphateorganicorganic
solvents. During the combustion of the electrolyte, the solvent
solvents. During the combustion of the electrolyte, the solvent molecules and lithium ions molecules and lithium ions
formaasolventized
form solventizedshell,shell,retaining
retainingthe theelectrolyte’s
electrolyte’snonflammability
nonflammabilityand andenhancing
enhancingthe the
coulombic efficiency and cycling stability of the
coulombic efficiency and cycling stability of the lithium-ion battery. lithium-ion battery.
Additionally,itithas
Additionally, hasbeenbeen demonstrated
demonstrated that that
novelnovel electrolyte
electrolyte additives
additives are effec-
are effective
tive flame-retardants, even enhancing the electrochemical
flame-retardants, even enhancing the electrochemical performance of batteries under high performance of batteries un-
der high pressures. Due to their lower saturated vapor
pressures. Due to their lower saturated vapor pressure, which effectively prevents the pressure, which effectively pre-
vents the evaporation
evaporation of solvents fromof solvents from theand
the electrolyte electrolyte
reducesand reduces
the risk the risk of in
of combustion combus-
com-
tion in combustible solvents, it is thought that their excellent flame retardancy is firstly
bustible solvents, it is thought that their excellent flame retardancy is firstly caused by the
caused by the prevention of combustion propagation through the generation of reactive
prevention of combustion propagation through the generation of reactive radicals by P
radicals by P radical capture reactions [69]. Dagger et al. [70] investigated the flame-
radical capture reactions [69]. Dagger et al. [70] investigated the flame-retardant effect of
five flame-retardant additives (tris(2,2,2-trifluoroethyl)phosphate(TFP), tris(2,2,2-trifluo-
roethyl)phosphite (TTFPi), bis(2,2,2 trifluoroethyl)methylphospho-nate (TFMP), (eth-
oxy)pentafluorocyclotriphosphazene(PFPN) and (phenoxy)pentafluoro-cyclotriphos-
phazene(FPPN)) in electrolytes, and found that fluorinated cyclic phosphoramidites
Processes 2023, 11, 2345 18 of 33

retardant effect of five flame-retardant additives (tris(2,2,2-trifluoroethyl)phosphate(TFP),

tris(2,2,2-trifluoroethyl)phosphite (TTFPi), bis(2,2,2 trifluoroethyl)methylphospho-nate
(TFMP),
Processes 2023, 11, x FOR PEER REVIEW (ethoxy)pentafluorocyclotriphosphazene(PFPN) and (phenoxy)pentafluoro- 19 of 34
cyclotriphosphazene(FPPN)) in electrolytes, and found that fluorinated cyclic phospho-
ramidites (PFPN and FPPN) outperformed the others in terms of electrolyte safety and
electrochemical performance, as shown in Figure 16, and concluded that phosphorus-
synergistic. Furthermore,
halogen synergistic. nitrogen nitrogen
Furthermore, can produce
can aproduce
protective carbon layer
a protective during
carbon the
layer com-
during
bustion process process
the combustion by creating ammonia,
by creating thus limiting
ammonia, the oxygen
thus limiting supply. supply.
the oxygen

CV plots
Figure 16. CV plots of flame−retardantadditives
of electrolytes containing five flame−retardant additives[70].
[70].

3.
3. Non-flammableelectrolyte
Non-flammable electrolyte
flame-retardant ingredient just reduces the flammability
The flame-retardant flammability of of the
the electrolyte.
electrolyte. Thus,
Thus,
the essential
essential fixfixfor
forlithium-ion
lithium-ionbatterybattery thermal
thermal runaway
runaway is the use use
is the of non-flammable
of non-flammable or even
or
non-combustible electrolytes. Utilizing flame-retardant elements
even non-combustible electrolytes. Utilizing flame-retardant elements or completely or completely swapping
out volatileout
swapping solvents
volatile is asolvents
successful is method
a successful for creating
methodelectrolytes
for creating that are non-flammable.
electrolytes that are
When heated, typical carbonate-based solvents release
non-flammable. When heated, typical carbonate-based solvents release hydrogen hydrogen radicals. Theseradicals.
radicals
then interact
These radicalswith
thenoxygen
interacttowith
produce
oxygen oxygen radicals,
to produce whichradicals,
oxygen can spark the production
which can spark the of
further free of
production radicals
furtherand freeeventually
radicals and cause fires. Introducing
eventually cause fires. hydrogen
Introducingor oxygen
hydrogenradical
or
scavengers
oxygen is one
radical efficient technique
scavengers is one efficient to stop this freeto
technique radical chain
stop this reaction.
free Most scientists
radical chain reaction.
Most scientists concur that compounds containing fluorine or phosphorus can act asradical
concur that compounds containing fluorine or phosphorus can act as powerful pow-
scavengers
erful radicalwhen electrolytes
scavengers when break down. The
electrolytes electrolyte
break down. decomposition
The electrolyteproducts include
decomposition
fluorinated
products and phosphorus
include fluorinated radicals, which can
and phosphorus interactwhich
radicals, with canhydrogen
interact radicals to stop a
with hydrogen
previous chain reaction of radicals and prevent the burning of
radicals to stop a previous chain reaction of radicals and prevent the burning of the the electrolyte solvent [71,72].
elec-
The most prevalent
trolyte solvent [71,72]. type of flame-retardants, renowned for a wide range of uses, are
organic compounds that include phosphorus. They have low
The most prevalent type of flame-retardants, renowned for a wide range of uses, aretoxicity, low volatility, and
good thermal
organic compoundsstabilitythat[73].
includeIt has been demonstrated
phosphorus. They havethat lowatoxicity,
numberlow of nonflammable
volatility, and
electrolytes based on organic solvents, phosphonitrile, and
good thermal stability [73]. It has been demonstrated that a number of nonflammable fluorinated phosphoric elec-
acid
work well in lithium-ion batteries as well as sodium-ion and potassium-ion batteries [74,75].
trolytes based on organic solvents, phosphonitrile, and fluorinated phosphoric acid work
As flame-retardant electrolyte components, trimethyl phosphate (TMP), TEP, and tripropyl
well in lithium-ion batteries as well as sodium-ion and potassium-ion batteries [74,75]. As
phosphate (TPrP) have drawn a lot of interest. TEP and TMP decreased the SET time
flame-retardant electrolyte components, trimethyl phosphate (TMP), TEP, and tripropyl
at low concentrations, but Xu et al. [75] discovered that 40 vol% was still enough to
phosphate (TPrP) have drawn a lot of interest. TEP and TMP decreased the SET time at
achieve nonflammability. Alkyl phosphates, while possessing strong oxidative stability, are
low concentrations, but Xu et al. [75] discovered that 40 vol% was still enough to achieve
incompatible with graphite, which lowers cycle rates. For lithium-ion batteries with high
nonflammability. Alkyl phosphates, while possessing strong oxidative stability, are in-
flash points and non-flammable hydrofluoric ether as the electrolyte, Fang et al. [76] used
compatible with graphite, which lowers cycle rates. For lithium-ion batteries with high
a mixture of diethylene glycol diethyl ether and non-flammable methyl-nonafluorobutyl
flash points and non-flammable hydrofluoric ether as the electrolyte, Fang et al. [76] used
ether, which has a better electrochemical performance and is more safe compared to
a mixture of diethylene glycol diethyl ether and non-flammable methyl-nonafluorobutyl
conventional electrolytes. Chung et al. [77] created a novel electrolyte by replacing one
ether,
hydrogenwhich has in
atom a better electrochemical
the electrolyte with a performance
fluorinated methyland is moregroup. safe compared
This to con-
electrolyte not
ventional electrolytes. Chung et al. [77] created a novel electrolyte
only reduced flammability but also significantly improved battery cycling performance. As by replacing one hy-
drogen atom in the electrolyte with a fluorinated methyl group. This electrolyte not only
reduced flammability but also significantly improved battery cycling performance. As
shown in Figure 17, the complete cell performance improved with the addition of vinyl
carbonate (VC) additive and remained nonflammable.
Processes 2023, 11, 2345 19 of 33

Processes 2023, 11, x FOR PEER REVIEW 20 of 34

shown in Figure 17, the complete cell performance improved with the addition of vinyl
carbonate (VC) additive and remained nonflammable.

Figure17.
Figure 17.Cycling
Cyclingperformance
performanceofofbattery LiPF
batteryininLiPF [77].
6 [77].

Due
Duetototheir
theirnon-flammability,
non-flammability,fluorinated
fluorinatedand
andphosphate-based
phosphate-basedelectrolytes
electrolytesare
areaa
possible route to safer battery electrolytes. However, in the creation and research of
possible route to safer battery electrolytes. However, in the creation and research of these these
electrolytes,
electrolytes,the
theenvironmental
environmental sustainability of fluorinated
sustainability compounds
of fluorinated and the
compounds andlong-term
the long-
stability of phosphate-based
term stability electrolytes
of phosphate-based must be
electrolytes taken
must be into
takenaccount.
into account.
4.4. Solid
Solidelectrolyte
electrolyte
Solidelectrolytes
Solid electrolyteshave have thethe benefits
benefits of non-flammability,
of non-flammability, minimal
minimal leakage,
leakage, and long
and long life
life over liquid electrolytes, which significantly raises the safety
over liquid electrolytes, which significantly raises the safety of lithium-ion batteries [78]. of lithium-ion batteries
[78].toDue
Due theirtogreat
theirelectrical
great electrical conductivity,
conductivity, sulfide sulfide electrolytes
electrolytes are currently
are currently a fre-
a frequently
quently discussed
discussed GeP2 SLi
issue. Li10issue. 12 , aGeP
new S three-dimensional
, a new three-dimensional framework framework
structure structure
publishedpub- by
lished by
Kamaya et Kamaya et al. [79],
al. [79], exhibits exhibits anhigh
an incredible incredible
lithiumhighioniclithium of 12 mS cm−of
ionic conductivity
conductivity 1 12
at
mS cm temperature.
ambient at ambient temperature.
Its conductivity Its conductivity
surpasses that surpasses
of liquidthat of liquid
organic organic electro-
electrolytes and is
the greatest
lytes and isever recordedever
the greatest for arecorded
solid electrolyte.
for a solid This new solid-state
electrolyte. This new battery electrolyte
solid-state batteryis
stable, non-volatile, non-explosive, and has excellent electrochemical
electrolyte is stable, non-volatile, non-explosive, and has excellent electrochemical fea- features such as high
conductivity
tures such asand highwide potential window.
conductivity and wideIt potential
is also simplewindow. to mold,
It is shape, and integrate
also simple to mold,
during
shape,manufacture.
and integrate during manufacture.
The
Themain
maindisadvantage
disadvantageofofan anall-solid
all-solidpolymer
polymerelectrolyte
electrolytewith withno noliquid
liquidcomponent
component
isisits
its low conductivityatatambient
low ionic conductivity ambienttemperature.
temperature. TheThe simplest
simplest andand mostmost practical
practical tech-
technique to increase the conductivity of polymer
nique to increase the conductivity of polymer electrolytes is electrolytes is to use non-aqueous
use non-aqueous phase phase
organic
organicsolvents
solventsasasplasticizers.
plasticizers. Gel Gelpolymer
polymer electrolytes
electrolytes are arethetheendendresult
resultofofthis
thisshift
shift
from traditional liquid electrolytes to all-solid electrolyte intermediates
from traditional liquid electrolytes to all-solid electrolyte intermediates [80]. Gel polymer [80]. Gel poly-
mer electrolytes
electrolytes typically
typically contain
contain plasticizers,
plasticizers, lithiumlithium salts,
salts, andand polymers
polymers with with a special
a special mi-
microporous
croporous structurestructure that
that allows
allows ions
ions to to
movemove between
between thethe liquid
liquid electrolyte
electrolyte andandthethemi-
microporous structure.Stephan
croporous structure. Stephanetetal. al. [81]
[81] reviewed
reviewed five five organic
organic polymers,
polymers, including
including
poly(ethylene
poly(ethylene oxide) (PEO), poly(vinylidene fluoride) (PVDF), poly(acrylonitrile)(PAN),
oxide) (PEO), poly(vinylidene fluoride) (PVDF), poly(acrylonitrile) (PAN),
poly(methyl
poly(methylmethacrylate)
methacrylate)(PMMA), (PMMA), and and PVDF-HFP,
PVDF-HFP,with withPEOPEObeing beingone oneofofthethemost
most
widely
widelystudied
studiedpolymer
polymerelectrolytes.
electrolytes.
However,
However,the thehigh
highcrystallinity
crystallinityofofPEO PEOresults
resultsininits itslow
lowionic
ionicconductivity
conductivityatatroom room
temperature, which impairs the stability of the electrochemical
temperature, which impairs the stability of the electrochemical performance and reduces performance and reduces
the
thelifetime
lifetimeofof thethecell.
cell.GelGel
polymer
polymer electrolytes,
electrolytes, which are the
which arecurrent
the currenttransition to all-solid
transition to all-
electrolytes, maintain the dual benefits of liquid electrolytes and all
solid electrolytes, maintain the dual benefits of liquid electrolytes and all solid electrolytes, solid electrolytes, such
as high ionic conductivity and good thermal stability. However, there are still many factors
such as high ionic conductivity and good thermal stability. However, there are still many
to take into account, including electrolyte compatibility, mechanical strength of the polymer
factors to take into account, including electrolyte compatibility, mechanical strength of the
matrix, and cycle life.
polymer matrix, and cycle life.

4.1.2. Separator
In addition to keeping the cathode and anode of the battery apart to prevent short
circuits by making contact with the two poles, the separator serves as a conduit for lithium
Processes 2023, 11, 2345 20 of 33

4.1.2. Separator
In addition to keeping the cathode and anode of the battery apart to prevent short
circuits by making contact with the two poles, the separator serves as a conduit for lithium
ions from the electrolyte, one of the essential elements of the lithium-ion battery. The
integrity of lithium-ion batteries during operation can be ensured by its good chemical char-
acteristics and mechanical strength, which also helps to prevent spontaneous combustion,
which has a significant impact on the safety of lithium-ion batteries [82].
Currently, polyolefin separator, which can be classified into polyethylene (PE) and
polypropylene (PP) microporous films based on the base material, is the industry standard
for lithium-ion battery separator. It is popular due to its high-cost performance, good
mechanical properties, and electrochemical stability. Higher standards for comprehensive
power lithium batteries are being put forth with the development of new electric vehicles,
including high-temperature resistance, high pore size uniformity, and high discharge rate.
Due to their high-temperature resistance line, electrolyte wetness, puncture resistance, and
subpar oxidation resistance, the present PP/PE polyolefin separators pose serious safety
risks and cannot be employed in power batteries [83].
There are now numerous techniques to increase separator safety, which are as follows.
1. High performance of polyolefin separator
Currently, dry and wet techniques are primarily used to create lithium-ion battery
separators. The production of separators using the dry process is solvent-free, pollution-
free, and beneficial to the environment. The separator experiences nearly minimal thermal
contraction during the transverse heating process, homogeneous microporous size distribu-
tion, high microporous conductivity, and other hot spots since only longitudinal stretching
takes place. The separator, on the other hand, is not stretched transversely during the
manufacturing process and is prone to cracking transversely when in use, which results in
a relatively high likelihood of internal micro-short circuits in mass-produced batteries and
low battery safety and reliability. Wet process separators have strong puncture resistance
and biaxial tensile strength, and the finished product can be quite thin. However, the wet
process uses a lot of solvents, which can lead to environmental pollution; in comparison
to the dry process, the equipment is complex, expensive, takes a long time to complete a
cycle, has high costs, and uses a significant amount of energy. In addition, the wet process
can only produce thin single-layer PE material films, with a melting point of only 130 ◦ C
and poor thermal stability [84–86].
The following characteristics are primarily expressed in polyolefin separator high
performance technology.
(1) Control of thermal shrinkage.
PE and PP, the basic materials for polyolefin separators, have melting values of
130–140 ◦ C and 150–160 ◦ C, respectively. These crystalline polymers are molded under
the combined action of a temperature field and tensile force field. The molecular chain
segments in the aggregated state structure of the polyolefin become unoriented when the
temperature to which the separator is exposed increases to the softening temperature,
which presents itself macroscopically as a drop in size known as thermal shrinkage. When
heated at 90 ◦ C for two hours, the separator of lithium-ion batteries shrinks transversely by
2.5% and longitudinally by 4%. Heat treatment can lessen thermal shrinkage by acceler-
ating the secondary crystallization of polymer, changing the molecular chain orientation
into a crystalline orientation, removing the internal tension of the separator, and improving
crystallinity. Li et al. [87] demonstrated that heat treatment at 90 ◦ C for 30 min could
enhance the lithium-ion battery’s overcharge safety performance, lengthen the internal
short-circuit failure time and increase the heat dissipation efficiency.
(2) High mechanical strength.
The mechanical strength of the separator includes tensile strength and puncture
strength, which, on the one hand, can increase the deformation ability of the battery when
subjected to external forces, and, on the other hand, can prevent the separator from being
Processes 2023, 11, 2345 21 of 33

pierced by cell burrs or dendrites [88]. According to research by Xu et al. [89] on the impact
of separators with various physical characteristics on the safety of lithium-ion batteries, the
higher the mechanical strength, the safer the battery is under the same thickness conditions.
A wet separator is highly favored in the field of power batteries due to its superior tensile
strength and puncture strength when compared to a dry separator for various preparation
methods and raw materials. This is because the molecular weight of the raw material is
higher than that of the latter.
(3) Self-shutdown function.
The lithium-ion battery separator has a very low hole closure temperature and high
film-breaking characteristics, which can cause the separator holes to close before the battery
temperature increases too much, ensuring that the ion channels in the battery are closed.
This property is known as the self-shutdown function. The melting point of raw materials
and the breaking temperature of separators are closely related, and the PP separator has
a greater breaking temperature than the PE separator. Because the existing dry separator
has inadequate transverse tensile strength, the wet separator has poor thermal stability
and cannot fulfill the needs of high-energy, high-power lithium-ion batteries. Due to this,
Celgard [56] proposed integrating the attributes of dry and wet microporous membrane
technology, using the qualities of good flexibility and low closed pore temperature and
polypropylene (PP) with high mechanical properties and high fusing temperature into a
lithium battery separator, successfully producing an “ABA” sandwich structure multi-layer
separator. This lithium-ion battery separator has a low closed cell temperature of 130 ◦ C and
a high fusing temperature of 160 ◦ C, which significantly improves the safety performance
of batteries. Hu et al. [90] used high-density polyethylene(HDPE) and PP as raw materials
using triple co-extrusion and bi-directional stretching technology successfully studied and
prepared a PP/PE/PP three-layer composite separator; the closed pore temperature of the
PE separator is lower than that of the PP separator. When the temperature reaches 130 ◦ C,
the PE layer melts and closes the pores of the separator, blocking the migration of lithium
ions in the electrolyte, while the PP layer remains intact to avoid internal short circuit.
2. Surface modification of polyolefin separator
The internal resistance of lithium-ion batteries can easily increase due to the high
crystallinity of the polyolefin separator, low surface energy, small polarity, poor affinity
with electrolytes, lack of wettability and liquid retention, and poor surface contact with
positive and negative electrode sheets. At the same time, severe shrinkage will occur
at temperatures above 120 ◦ C, which will easily lead to the two poles contacting and
forming a short circuit, which may cause safety accidents. In order to fully improve the
overall performance of polyolefin separators and guarantee the integrity of the separator in
the event of thermal runaway of lithium-ion batteries, polyolefin separator modification
technology combines the benefits of flexible organic materials and inorganic materials
containing multiple hydrophilic groups.
(1) Inorganic material coating.
Nano alumina (Al2 O3 ) is commonly used commercially to create ceramic coatings
for polyolefin separators because of its excellent thermal and chemical stability, low cost,
and ease of accessibility. In their study of Al2 O3 single-sided coated PE separators for
lithium-ion batteries, Lei et al. [91] found that the ceramic-coated separators had a lower
internal resistance of the cell and a discharge temperature rise of about 3 ◦ C. Al2 O3 was
coated on both sides of a PE separator by Yao et al. [92], and the results revealed that after
baking at 150 ◦ C for two hours, the thermal shrinkage of the ceramic-coated separator
was less than 5%, significantly enhancing the thermal safety performance of lithium-ion
batteries. Due to its low specific gravity and low hardness, boehmite (Al2 O3 -H2 O), a material
comparable to alumina, has also steadily made its way onto the market. By altering the
boehmite shape, An et al. [93] successfully created ceramic separators with moisture contents
below 600 ppm.
Processes 2023, 11, 2345 22 of 33

(2) Organic material coating.

The interfacial characteristics and heat resistance of polyolefin separators can be
enhanced via organic coating, and polymer systems such as PVDF and its copolymers,
polyimide (PI), aramid, polyethylene terephthalate (PET), PEO, and cellulose have emerged
as new hot areas for the development of lithium-ion battery separators [94]. PVDF coating
is one of the commercial coating technologies that has matured and been accepted by many
lithium-ion battery companies. An et al. [95] demonstrated that a PVDF-coated separator
can increase electrolyte absorption/retention ability, improve interfacial bonding between
the electrode and separator, increase the electrode’s hardness, make the battery thinner and
stronger, and simultaneously increase battery safety. By effectively using the impregnation
approach, Xiong et al. [96] created an ethyl cellulose-coated polyolefin separator, and both
its closed pore temperature and thermal shrinkage rate dramatically decreased. In order
to address the safety issues brought on by separator shrinkage in lithium-ion batteries at
high temperatures, Song et al. [97] successfully prepared a polyimide-coated polyolefin
separator by impregnation. This separator has a significantly improved mechanical strength
and reduced high-temperature shrinkage to within 10% at 140 ◦ C. Aramid can endure
temperatures of up to 400 ◦ C thanks to its heat resistance and fire-retardant qualities. A high-
performance composite separator with aramid-coated coating has closed-hole qualities,
heat resistance, wettability, and the ability to absorb and hold liquid. The basic technology
is currently mastered by Teijin, Toray, and Sumitomo Chemical of Japan, and Shanghai
Research Institute of Chemical Industry Co. of China.
(3) Multi-layer coating.
Multi-layer coating entails coating the first layer (for instance, the Al2 O3 layer) first,
the second layer (for instance, the PVDF layer), on top of that, and then preparing the
multi-layer composite separator. By baking at 180 ◦ C and baking composite separators with
a PVDF/Al2 O3 /PE separator/Al2 O3 /PVDF multilayer structure, An et al. [98] created
composite separators that had a thermal shrinkage of only 7.8% and no melt collapse
at 200 ◦ C.
(4) Mixed coating.
In order to create a composite separator, organic and inorganic materials are mixed to
create a homogeneous slurry, which is then coated on top of the substrate for the separator.
When a PVDF/Al2 O3 hybrid slurry was applied to a PE separator and coated on both sides,
Jeong et al. [99] found that the electrical conductivity increased to 0.719 × 103 S cm−1 and
the composite separator’s shrinkage was reduced from 94% to 74%.
The separator is the fundamental component of a lithium-ion battery with the highest
technical barrier and the final localization, and it plays a significant role in the safety
and cost performance of lithium-ion batteries. From an economic standpoint, polyolefin
separator still holds a dominant position in the field of lithium-ion battery separator
research at this time, and the primary research focus is on enhancing the safety and high
performance of polyolefin separator. The development of the polymeric separator from a
simple structure to a high-order complex structure, to its high mechanical strength, high
heat resistance angle, and to protect the safety of the separator for lithium-ion batteries, is
influenced by the screening and compounding of separator raw material, post-treatment
process improvement, and surface modification. At this point, new substrate materials
for separators are also a hot topic in laboratory research, with some studies concentrating
on inorganic materials such as hydroxyapatite and sodium titanate to enhance the heat
resistance and flame retardancy of separators under ultra-high temperature conditions.
Traditional separator materials have also changed from polyolefins to organic materials
such as PVDF, PET, PI, and fibrin.

4.1.3. Electrodes
If appropriate changes are made to the electrode, a crucial component of lithium-ion
batteries, the risk of thermal runaway can be reduced. One of the main causes of the
thermal runaway process in lithium-ion batteries is the extra oxygen produced during
Processes 2023, 11, 2345 23 of 33

the cathodic reaction [100]. One of the main reasons for separator deformation is lithium
dendrites, which are created when lithium particles accumulate on the anode surface [101].
In order to raise the pyrolysis temperature, it is therefore effective to dope the cathode
material at the atomic level to increase the cathode’s thermal stability. Applying a protective
coating to the cathode surface is a popular technique that can successfully prevent the
cathode material from directly contacting the electrolyte, thus minimizing side reactions
that produce heat [102–104]. Additionally, cathode safety can be significantly increased by
altering the electrode with PTC materials [105,106].
The unequal anode current distribution caused by the instability and inhomogeneity
of SEI is the most fundamental reason of lithium dendrite formation on the anode surface in
terms of material attributes. Lithium dendrite formation can be reduced by enhancing the
cathode materials [107,108], such as silicon-based materials and graphite-carbon materials,
or by applying a protective coating to the anode surface. The unequal distribution of anode
current close to the anode causes lithium ions to be converted to lithium particles and
deposited on the anode surface to create lithium dendrites when lithium-ion batteries are
misused, such as through overcharging or overdischarging. The most typical remedy is to
add electrolyte additives to stabilize the contact between the anode and the electrolyte in
order to prevent the growth of lithium dendrites caused by uneven SEI production.

4.2. Lithium-Ion Battery Thermal Runaway External Protection Technology

A better cooling design to efficiently disperse heat and insulate all surrounding cells
from thermal propagation is only one example of a system-level protection solution. By
accelerating the cooling of the lithium-ion battery with air, liquids, and phase change mate-
rials, efficient heat dissipation can decrease the severity of thermal runaway. By obstructing
the heat transfer path to surrounding healthy cells, isolation of thermal propagation can
stop the thermal runaway process and avert a domino effect or chain reaction.

4.2.1. Battery Management Technology

The monitoring technology for lithium-ion batteries is currently developing and is
gaining popularity. Setting up a monitoring and alarm system is one of the more popular
ways to obtain thermal runaway warning. A thermal runaway monitoring and warning
system based on PLC was developed by Shao et al. [109] and is mostly used in electric
vehicles. Figure 18 shows the specific control flow chart, which can calculate the time
when the battery is about to undergo thermal runaway reaction based on the collected
temperature value and the rate of change. It can track the internal temperature change
of a single cell and set 80 ◦ C as the alarm signal for thermal runaway. The system can
assess the situation and sound an early alarm If it notices that the battery pack’s interior
temperature has reached 80 ◦ C. Additionally, the technology can forecast when a thermal
runaway would occur based on how quickly the battery pack is heating up, protecting both
drivers and passengers.
Attention is also being paid to a different approach to battery pack equalization. The
goal of battery pack equalization technology is to transfer and distribute electrical energy
evenly throughout each and every cell in the battery pack. Different battery health states
result from variations in each individual cell’s voltage, internal resistance, capacity, etc., or
from varied depths of charge and discharge. There are two types of equalization technology:
energy dissipation type and energy non-dissipation type. The energy non-dissipation kind,
which transfers power using capacitance or inductance to balance the power of each battery
pack, is more energy-efficient than the other two. Niu [110] employs an energy non-
dissipative equalization circuit based on the SOC equalization technique. The experiment
demonstrates that the equalization effect is effective, and the voltage is monitored by an
LTC6802 chip. Furthermore, the circuit operates steadily while taking real time and safety
into consideration, successfully preventing the growth of battery pack inconsistency and
extending the battery pack’s useful life. Speltino et al. [111] used the extended Kalman
filter SOC algorithm and validated its simulation results for constant-current charging and
Processes 2023, 11, 2345 24 of 33

periodic step-current discharging; the system is able to equalize the single cell charge state
during
Processes 2023, 11, x FOR PEER REVIEW bi-directional operation and continuously track the individual cell charge state. The
25 of 34
results show that the algorithm boosts the equalization effect and also fulfills the objective
of lowering the discrepancies between individual cells.

Figure 18. Early

Figure 18. Early warning
warning mechanism
mechanism control
control flow chart.
flow chart.

4.2.2.Attention
Cooling Technology
is also being paid to a different approach to battery pack equalization. The
goal Battery
of battery pack equalization
cooling technology
technologies include is to transfer
air cooling and distribute
technology, electrical
liquid cooling energy
technology
evenly
and phase throughout each andcooling
change material every cell in the battery pack. Different battery health states
technology.
resultResearch on thermal
from variations management
in each individual using gasvoltage,
cell’s media isinternal
currently mainly focused
resistance, capacity, onetc.,
cell
organization,
or from variedgas flow channel,
depths of chargegas and flow direction,
discharge. gas rate,
There andtypes
are two various optimization tech-
of equalization tech-
niques. The
nology: energyZ-type battery thermal
dissipation type and management system was developed
energy non-dissipation type. The by Sun et
energy al. [112],
non-dissi-
utilizing a tapered inlet and outlet duct design and an analytical
pation kind, which transfers power using capacitance or inductance to balance the power design of experiments
technique.
of This led
each battery pack,to the development
is more energy-efficientof Z-typethanflowthebattery
othermodules
two. Niu with improved
[110] employsther- an
mal performance.
energy non-dissipativeA J-type battery thermal
equalization circuitmanagement
based on thesystem, which has one
SOC equalization more outlet
technique. The
than the typical
experiment U-type andthat
demonstrates Z-type battery thermal
the equalization effectmanagement
is effective,systems,
and the was proposed
voltage is moni- by
Liu et by
tored al. [113]. The adjustable
an LTC6802 flexibility ofthe
chip. Furthermore, battery
circuit thermal
operates management
steadily whileis substantially
taking real
increased
time by installing
and safety a control valve
into consideration, at the outlet,
successfully which the
preventing cangrowth
be utilized to modify
of battery pack thein-
consistency and extending the battery pack’s useful life. Speltino et al. [111] used theand
opening degree of the two outlets under various operating conditions. The benefits ex-
drawbacks
tended Kalmanof U-,filter
Z-, and
SOCJ-type battery
algorithm andthermal
validatedmanagement
its simulation methods arefor
results listed in Table 3.
constant-cur-
The ideal cooling performance for the cubic arrangement of
rent charging and periodic step-current discharging; the system is able to equalize the battery module was found
the
by Wang
single celletcharge
al. [114]state
afterduring
studying the cooling performance
bi-directional operation and of the battery module
continuously trackinthevarious
indi-
configurations
vidual cell charge andstate.
taking cost
The and cooling
results show that effect
theinto account.
algorithm The locations
boosts of the cooling
the equalization effect
system’s inlets and outlets as well as the number of inlets
and also fulfills the objective of lowering the discrepancies between individual and outlets were allcells.
varied in
the designs that Peng et al. [115] for a variety of cell configurations. The findings indicated
that an
4.2.2. arrangement
Cooling Technologywith a smaller aspect ratio was more advantageous to enhance the
cooling system’s performance. The cooling capabilities of aligned, staggered, and crossed
Battery cooling technologies include air cooling technology, liquid cooling technol-
battery modules under various airflow rates were studied by Fan et al. [116]. The aligned
ogy and phase change material cooling technology.
design, followed by the staggered and then the cross arrangement, is determined to have
Research on thermal management using gas media is currently mainly focused on
the optimum cooling performance and temperature uniformity. By varying the diameter of
cell organization,
the input channelgas at aflow
fixedchannel,
flow rate, gasYang
flowet direction,
al. [117] gas
were rate, and
able various optimization
to rationally design the
techniques. The Z-type battery thermal management
layout of the cell module and ultimately produce a greater cooling effect. system was developed by Sun et al.
[112], utilizing a tapered inlet and outlet duct design and an analytical design of experi-
ments technique. This led to the development of Z-type flow battery modules with im-
proved thermal performance. A J-type battery thermal management system, which has
one more outlet than the typical U-type and Z-type battery thermal management systems,
was proposed by Liu et al. [113]. The adjustable flexibility of battery thermal management
is substantially increased by installing a control valve at the outlet, which can be utilized
to modify the opening degree of the two outlets under various operating conditions. The
benefits and drawbacks of U-, Z-, and J-type battery thermal management methods are
 advantageous
aligned,
by Fan etstaggered, to enhance
al. [116]. and the cooling
The crossed
aligned design,system’s
battery modules performance.
followedunder
by thevarious The cooling
airflow
staggered and thencapabilities
rates were studied
the cross of
ar-
aligned,
by
rangement, staggered,
Fan et al.is[116]. and crossed
The aligned
determined battery
design,
to have modules
the followed
optimum by under various
the staggered
cooling airflow
performance rates
and then were
and the studied
cross ar-
temperature
by Fan et al.By
rangement,
uniformity. is[116]. The the
determined
varying aligned
to design,
have
diameter the followed
input by
of optimum
the the staggered
cooling
channel at a fixedand
performance then
flowand the cross
et ar-
temperature
rate, Yang al.
rangement,
uniformity.
[117] were able is determined
By varying
to rationally todesign
have the
the diameter the optimum
of layout
the input cooling
channel
of the performance
at a fixed
cell module andflow and
rate,temperature
ultimately Yang et al.
produce
auniformity.
[117] By to
werecooling
greater able varying the diameter
rationally
effect. design theoflayout
the input channel
of the at a fixed
cell module andflow rate, Yang
ultimately et al.
produce
Processes 2023,
[117] 11, 2345 able to rationally design the layout of the cell module and ultimately produce
were 25 of 33
a greater cooling effect.
a greater
Table cooling
3. Battery effect.
thermal management system for different models.
Table 3. Battery thermal management system for different models.
Models
Table Illustrations
3. Battery thermal Table
management Advantages
system
3. Battery for different
thermal system for Disadvantages
models.
management different models.
Models Illustrations Advantages Disadvantages
Bad temperature field con-
Models Illustrations Small pressure drop, low en-
Advantages Disadvantages
U-type
Models Illustrations Advantages Bad sistency,temperature
higher maximum field con-Disadvantages
Small pressure drop, low en- Bad temperature field con-
ergy consumption
U-type Smallconsumption
pressure
Small drop,
pressure lowlow
drop, en- sistency,
temperature higherBad maximum temperature field consistency,
U-typeU-type ergy sistency, higher maximum
Good temperature
energy
ergy consumption consumption field con- temperature
High pressure drop,
higher high
maximumen- temperature
Z-type temperature
Good
sistency temperature field con- High ergy pressure drop,
consumption high en-
High pressure drop, high
Z-typeZ-type GoodGood temperature
temperature field field consistency
Z-type sistency
Small pressure drop, lowcon- en- ergy Highconsumption
pressure drop, energyhigh en-
consumption
sistency
Small Small pressure drop, low
pressure drop, low en- Complex structure,
ergy consumption ergy consumption highstructure, high
Complex
J-typeJ-type Small energy
pressure consumption
drop, low en-
ergy
Good Goodconsumption
temperature Complex structure,
field con- sealing requirements requirements sealing high
J-type ergy consumption temperature field consistency Complex structure, high
J-type Good
sistency temperature field con- sealing requirements
Good
sistency temperature field con- sealing requirements
Insistency
liquid cooling technology, according to Hirano et al. [118], a module with ten
In liquid cooling technology, according to Hirano et al. [118], a module with ten lith-
lithium-ion batteries was given a direct contact liquid cooling system. Since the liquid will
ium-ionIn liquid cooling
batteries wastechnology,
given a direct according
contactto Hirano
liquid et al. [118],
cooling system. a module
Since the with liquidten lith-will
In liquid cooling directly contact
technology, accordingthe battery,
to it must
Hirano et have higha electric
al. system.
[118], module resistance, be non-flammable, and be
ium-ion
directly batteries
contact thewas given ita must
battery, direct contact
have high liquid
electric cooling
resistance, be Since thewith
non-flammable, liquid ten will
lith-
and
ium-ion batteries was ecologically
givenit amust
direct acceptable.
contact In
liquid this study,
cooling hydrofluoroether
system. Since the liquidwereand perfluoroketone
will were tested.
directly
be contact
ecologically the battery,
acceptable.
The massesIn thisthen have
study, high
flowed electric
hydrofluoroether
back to resistance,
the moduleandbeperfluoroketone
non-flammable,
for cyclic operation and after being vaporized
directly
be contact
ecologically thethen
battery,
acceptable. it must
In this have thehigh electric resistance, be non-flammable, and
tested. The masses flowed
and cooled back instudy,
to
a heat hydrofluoroether
module
exchanger for cyclic
outside andtheperfluoroketone
operation module. after being were
vapor-
The liquid-cooled battery thermal
be ecologically
tested.
ized and The masses
cooled acceptable.
in athen
heat flowed In back
exchanger thisoutside
study,
to the hydrofluoroether
module
the module.for cyclic
The and perfluoroketone
operation
liquid-cooled after being
battery were
vapor-
thermal
management system was thoroughly examined by Smith et al. [119] from the perspective
tested.
ized andThe masses
cooled in athen
heat flowed back to the the
module for cyclic al.operation afterthebeing vapor-
management system ofexchanger
was thoroughly
the outside
entire vehicle examined module.
by Smith
and practicality. The liquid-cooled
etFor a[119]
module from battery
with thermal
perspective
eight square cells, a computational
ized and
management cooled in
systemanda heat
was exchanger
thoroughly outside
examined the module.
bywithSmith The
etAal.liquid-cooled
[119] from battery
athe thermal
of the entire vehicle practicality.
fluid dynamics For a module
model was created. eight square
cold cells,
plate with aperspective
computational
liquid channel was then inserted
management
of thedynamics
fluid system
entire vehicle model andwas thoroughly
practicality.
was created.the AForexamined
a module
cold plate by Smith
with
with aeightet square
liquid al.channel
[119]cells, froma the perspective
computational
underneath battery module, and to close the airwas gap then
between inserted the cells and the cold plate,
of thedynamics
fluid
underneath entire the
vehicle
model and
battery waspracticality.
created. A For aclose
cold module
plate with
with a eight
liquid square
channel cells, was a computational
then inserted
the soft material with strong thermal conductivity was linked. cold
module, and to the air gap between the cells and the Mondal et al. [120] created
fluid dynamics
underneath
plate, the soft model was
thematerial
battery module,
with created.
strong Atocold
andthermal close platethe with
air gap
conductivity a liquid
between
was channel
linked. theMondalwas and
cells then et theinserted
al. cold
[120]
cold plate modules with various structural forms and work fluids in the form of nanofluids,
underneath
plate,
created thecold the
softplatebattery
material
modules module,
withas strong
with and to
thermal close the
conductivityair gap
formswas betweenlinked. the cells
Mondal and the
et al. cold
[120]
such air,various
water structural
and a combination and work fluids
of ethylene glycol in theand form
water. ofWater and other liquid
plate, the
created
nanofluids, coldsoft
suchmaterial
plate asmodules withwith
air,work
water strong
andvarious
acan thermal
combination conductivity
structural of forms
ethylene was
and linked.
work
glycol Mondal
fluids
and water.inand theet form
Water al. [120] of function; however, an
fluids freeze in low-temperature situations ceaseand to
created
nanofluids,
other coldsuch
liquid plate
work modules
asfluids canwith
air,ethylene
water and
freeze various
a combination structural
in low-temperature forms
of ethylene and work
glycol andfluidswater. into the
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glycol-water solution hassituations
antifreeze and cease
qualities and function;
is employed in the Tesla Model
nanofluids,
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however, liquid ansuch
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can
glycol-water
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freeze asolution
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when tothe
liquids function;
in the
can
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however,
Tesla
changed Model an ethylene
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to achieve glycol-water
thermal
thecombined
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and qualities
at ratio
− 45 of
when ◦ and
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C.the Tang is
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glycol employed
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et al. ratio can
[121]reaches in the
be a mini-channel and a
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and etratio
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changed
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nel and to achieve
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ultimately created
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nel and
distinct a water cooling
water-cooling
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techniques for
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ultimately
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that a Three
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and morerise.
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evenly are change
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phase created, and it is (PCMs)
water-cooling
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et al. [122] used a cylindri-
contact
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For phase and
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while analyzing may
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theal.performances
[122] used a of stop the changed depending
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drical For temperature
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while cooling
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and
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graph- the
combined to investigate
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and Furthermore,
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the impacts
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of various
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and compounded
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effect. Furthermore, liquid cooling ofand
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and increase can uniformity
be combined of the to internal temperature,
Wu et al. [123] examined the features of battery pack temperature variation under high
temperature environments (25–40 ◦ C). The findings demonstrate that compared to the
single cooling technique, the composite thermal management system can still ensure that
the battery pack temperature is kept within the safe temperature range of 37–43 ◦ C while
the ambient temperature increases from 25 ◦ C to 40 ◦ C.

4.2.3. Blocking Technology

The “Lithium-ion Batteries for Power Storage” GB/T 36276 [124], published in 2018,
explicitly states the requirements and test methodologies for thermal runaway propagation
of lithium-ion batteries in response to the risk of this occurrence. Since blocking technology
can prevent the diffusion of heat and high-temperature materials during thermal runaway,
it is vital to create thermal runaway propagation suppression technology for lithium-ion
batteries, which will reduce the development of thermal runaway events. Thermal runaway
Processes 2023, 11, 2345 26 of 33

propagation in the battery module is mostly caused by the heat transfer process in the
battery pack. There is a dearth of research on the use of barriers to prevent the spread of
thermal runaway, and there is no reliable screening technique for choosing barriers.
inserted insulating and thermal insulation plates between various layers of cells.
Larsson et al. [127] added liquid-cooled and insulated panels between the cells to use
numerical simulations to confine thermal runaway to several cells. Mehta et al. [128–130]
reduced the thermal impact of the thermally runaway cell on neighboring cells by using
an expansion material placed between the inner surface of the cell shell and the matching
electrode assembly to expand and absorb heat when the cell is heated. Zhang et al. [131]
compared the behavioral traits of soft pack type lithium-ion batteries during thermal
runaway propagation at various gaps and discovered that, as the distance increases, the
time needed for thermal runaway propagation adjacent to the battery becomes longer,
while the intensity of thermal runaway occurring in the battery is relatively weaker, which
is related to the deformation state during thermal runaway of lithium-ion battery.
In their study of a 10s4p structured 18650 lithium-ion battery pack, Wilke et al. [132,133]
induced thermal runaway in one of the cells after filling the pack’s gap with phase change
material. They discovered that the presence of the phase change material effectively reduced
the thermal impact of the thermally runaway cell on the neighboring cells. Even though
adding insulation materials with lower thermal conductivity between cells helps prevent
heat transmission, it considerably improves the circumstances for the cells to dissipate heat
and has a tendency to lead to heat accumulation. In order to prevent the heat accumulation
phenomenon, the phase change material can absorb a specific amount of heat under the
heat situation. Unfortunately, the latent heat of phase shift has a limit, making it challenging
to completely absorb the significant quantity of heat emitted by the thermally runaway
battery. Phase change materials are mostly utilized to lower the cyclic heat output of battery
because they also somewhat reduce the ability of the battery and environment to exchange
heat. To increase the safety of lithium-ion batteries in air travel, Yi et al. [134] conducted a
study on the usefulness of aerogel mats of various thicknesses in preventing the spread
of thermal runaway of lithium-ion batteries with various SOCs. The experiment’s results
demonstrate that lithium-ion battery thermal runaway could be effectively stopped using
aerogel mats of various thicknesses. Only the battery that was directly in contact with
the heating rod experienced thermal runaway over the course of the experiment, and the
temperature of the batteries next to it remained within a safe range. Although the thinner
aerogel felt can also stop the spread of thermal runaway, the temperature of the battery
next to it has already risen above the safe level, and an irreversible reaction has taken place
inside the battery.
The lithium-ion battery must be put out using fire extinguishing technology if the
thermal runaway of the battery is not promptly prevented and controlled. In a review of
the effectiveness of the most widely used fire extinguishing agents, Yuan et al. [135] intro-
duced a number of typical extinguishing agents and their fire extinguishing mechanisms,
summarized their fire extinguishing effects, and discovered that water-based extinguishing
agents performed the best overall. Cui et al. [136] provided a description of the fine water
mist fire extinguishing mechanism, followed by a discussion of the impact of internal and
external factors on the effectiveness of the fine water mist fire extinguishing, including
fine water mist characteristics, additives, obstacles, ventilation conditions, fuel type, and
flame scale. They also reviewed the research on the use of fine water mist technology in
battery fires. The future development direction and research ideas for fine water mist fire
extinguishing technology are then presented based on the current research trends, and the
development prospect of its use in the field of battery fires is anticipated. The readers are
referred to several review papers that were previously cited for a more thorough evaluation
and comparison of the available fire suppression techniques.
However, with lithium-ion batteries, not only may there be a fire, but there could also
be an explosion, and the fine water mist fire extinguishing system is significantly longer
than other gas fire extinguishing systems. A fire extinguishing time that is too long will also
Processes 2023, 11, 2345 27 of 33

influence the fire suppression effect, as would the combustion of lithium-ion batteries and
the explosion of the flame created by nearby quickly ignited flammable objects. Therefore,
the primary inhibitory mechanism for asphyxiation and cooling inert gas extinguishing
agent, such as carbon dioxide, heptafluoropropane, can better prevent the combustion and
explosion of the battery. When the carbon dioxide stored in liquid form in the gas tank
is sprayed out, it will quickly vaporize, producing a large amount of gas carbon dioxide,
reducing the concentration of oxygen in the combustion area, and reducing the amount
of oxygen in the combustion area. Since the vaporization process will absorb a lot of heat,
it will have a better cooling effect, reducing the risk of a lithium battery explosion, and it
will not pollute the environment. Additionally, heptafluoropropane, a better-performing
halon substitute, has the physical effects of reduced temperature, reduced oxygen concen-
tration, chemical inhibition, and superior environmental friendliness. Heptafluoropropane
extinguishing chemical excels at putting out liquid, electrical, solid surface, or fusible
solid fires by using total flooding due to its superior gas-phase electrical insulation, it can
take into account the security of crucial locations including computer rooms, dangerous
chemical storage rooms, communication equipment, and generator rooms. In comparison
to typical high-pressure fire extinguishing agents, heptafluoropropane has the benefit of
high insulation, and the process of burning lithium-ion battery fire will not cause harm
to other components. Inert gas extinguishing agents such as carbon dioxide and hep-
tafluoropropane are more suitable as extinguishing agents to prevent the combustion and
explosion of lithium-ion batteries when compared to fine water mist, dry powder, and
aerosol extinguishing agents. However, if a thermally runaway battery still has enough
energy to manufacture combustible material, it may still offer a risk of delayed explosion;
hence, more research on battery explosion suppression is required.

5. Conclusions and Outlook

The safety concerns brought on by lithium-ion battery thermal runaway occurrences
are receiving increasing amounts of attention as lithium-ion batteries grow in popularity,
having emerged as a frequently discussed topic in current research.
Lithium-ion battery thermal runaway monitoring and warning systems currently in
use rely on keeping an eye on specific characteristic defect signals, such as terminal volt-
age, temperature, internal resistance, etc. Enhancing the precision of voltage sensors and
temperature sensors in the BMS can enhance the accuracy of thermal runaway detection
for external monitoring techniques such as terminal voltage and surface temperature moni-
toring. Additionally, the sensor array architecture can be improved from both a hardware
and software standpoint to obtain the best intelligent monitoring with the fewest possible
sensors, which will also save expenses. With regard to the internal detection method,
the internal state warning method and BMS can be combined to create a more precise
thermal runaway warning model for lithium-ion batteries, and the detection resolution and
high-temperature resistance of the embedded sensors can be improved at the same time.
In order to prevent casualties and property damage, lithium-ion batteries must be
monitored for thermal runaway and given an early warning. To stop or slow the spread
of thermal runaway after it has occurred, some actions are still required. The study of
the evolution and mechanism of thermal runaway has recently gone more in-depth, but
there are still numerous issues with the monitoring and warning systems for lithium-ion
batteries. To effectively avoid the development of fire, the three methods of external
protection technology can be combined and used when thermal runaway occurs. First, by
using battery management technology to detect the early warning signal of the system,
the precise location where the abnormality occurs can be determined. Secondly, utilizing
blocking technology to keep the number of thermal runaway modules within a certain
range could be a useful approach. These modules are then cooled down by using air cooling,
liquid cooling, or phase change material cooling technology, which can successfully prevent
fire accidents and achieve safe thermal runaway response. Finally, it is possible to put
out the fire and minimize damage by combining various fire suppression techniques with
Processes 2023, 11, 2345 28 of 33

various application scenarios if all other last-resort prevention and control measures are
unsuccessful and the battery still experiences thermal runaway.
The most important issue is the enhancement of the safety performance of the battery
itself, in addition to the aforementioned methods of monitoring, warning, and protection.
High ionic conductivity, suitability for the majority of anode and cathode materials, high
lithium salt solubility, consistent electrochemical performance, low toxicity, and environ-
mental protection are all characteristics of a high-quality lithium-ion battery electrolyte.
The present electrolyte modification techniques still have a number of flaws, and the major-
ity of them cannot simultaneously satisfy the requirements listed above. Although there
are still some issues, the gel polymer electrolyte enhanced by solid electrolytes retains
the benefits of both liquid and solid electrolytes and has a promising future. Existing
commercial separators are not strong enough from a mechanical perspective to withstand
rigid collisions; therefore, new high-temperature-resistant, high-strength separators should
be actively developed with the goal of meeting the battery’s fundamental performance
requirements. The current excellent performance of the separator, as well as modification
technologies for the desired high mechanical qualities and thermal stability, provides the
opportunity. The optimum separator is crucial to the energy density, power density, cycle
life, and safety of the battery. A thorough understanding of the separator’s fundamental
properties, failure mechanisms, and other factors is necessary to ensure battery safety. At
the same time, new intelligent separator alternative materials, failure detection methods,
and other cutting-edge technologies are being actively developed to address the current
issue of mechanical and thermal failure of the separator and enhance battery safety. Con-
cerning the electrode, a key future research path is the hunt for a cathode battery that is
more heat-resistant and better able to prevent the growth of lithium dendrites.

Author Contributions: Conceptualization, S.Y. and B.C.; methodology, J.L.; formal analysis, S.Y.,
B.C. and J.L.; writing—original draft preparation, S.Y.; writing—review and editing, B.C. and J.L.;
visualization, B.C.; supervision, J.L. All authors have read and agreed to the published version of
the manuscript.
Funding: This research and the APC was funded by Project 22dz1201100 of Science and Technol-
ogy Innovation Action Plan supported by the Science and Technology Commission of Shanghai
Municipality and Project C2022362 supported by the Shanghai Municipal Education Commission.
Data Availability Statement: Not applicable.
Acknowledgments: The authors appreciate the support of this work by Project 22dz1201100 of
Science and Technology Innovation Action Plan supported by the Science and Technology Com-
mission of Shanghai Municipality and Project C2022362 supported by the Shanghai Municipal
Education Commission.
Conflicts of Interest: The authors declare that they have no known competing financial interest or
personal relationships that could have appeared to influence the work reported in this paper.

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