Advanced Batteries and Fuel

Cells Technology
2. Li-ion batteries

1
 Introduction to Li-ion batteries

• Rechargeable Li-ion batteries have gained considerable

interest in recent years in terms of

 High specific energy

 High cell voltage

 Good capacity retention

 Negligibly small self-discharge

2
Comparison of the characteristics and performance commonly used
rechargeable batteries

3
 It is desirable that the energy delivered by a battery during its discharge should be
as high as possible.

 The energy output of a battery is dependent on the equivalent weight of active

material present in it.

𝟐𝟔.𝟖
 Specific capacity (i.e capacity per gram) of active material = 𝑬𝒒𝒖𝒊𝒗𝒂𝒍𝒆𝒏𝒕 𝒘𝒆𝒊𝒈𝒉𝒕

Ah.g-1

 Lithium metal being the third lightest element

 Li based materials with low molecular weight can effectively produce batteries
with high capacity.

 Li ion batteries employs non aqueous electrolyte that offer high operating voltage
(> 4 V) in comparison to other batteries with aqueous electrolyte (1-2 V).

 Thus, low weight, compact Li ion batteries established a strong market place for
portable electronic devices and could find central application if Li ion batteries in
electric vehicles become reality. 4
 Basics of Lithium ion batteries
• Li ion battery consists of three main components, positive and negative

electrode separated by a separator dipped in electrolyte.

• Negative electrode (anode) is normally an electron donor group which is

electropositive in nature like lithium metal.

• Positive electrode (cathode) is normally an electron acceptor which is

strongly electronegative (e.g. LiMO2 (M = Co, Ni, Mn, etc) compounds).

• During discharge process, the negative electrode (anode)

electrochemically oxidised and releases electron.

• This electron moves through outer circuit to the positive electrode which

accepts electron.
5
The schematic of Li ion
battery operation is
explained using LiCoO2
as positive electrode
(cathode) and graphite
(carbon) as negative
electrode (anode).

6
• During charging, Li+ moves from LiCoO2 to carbon
through the electrolyte which causes oxidation of Co3+
to Co4+ .
• The reverse happens during discharging; Li+ moves
from carbon to LiCoO2.
• Role of electrolyte is to act as a medium for the
transfer of ions between the two electrodes.
• In general, lithium salt dissolved in organic solvent is
used as electrolyte in lithium ion batteries.
7
 Main requirements for the electrolyte

 It should be a good Li-ion conductor and electronic insulator

 Stability over the operating voltage window

 Chemical compatibility with cell components and electrodes

 Thermal stability

 There should not be any charge accumulation and

concentration polarization.

8
• Material that undergoes chemical reaction producing current
during battery operation is known as active mass or active
material.

• In batteries the electrode itself takes part in chemical reaction

apart from being a charge transfer media.

• Consequently, the chemistry associated with electrode-electrolyte

interface and bulk of electrode are the main factors that
determine the battery performance.

• Thus, performance of lithium ion battery crucially depends on the

nature of electrode material used.

9
 Parameters that are used to validate the quality of
electrode material are
 Cell voltage

 Conductivity

 Coulombic efficiency

 Specific capacity

 Capacity retention (stability/cycle life)

 Gravimetric and volumetric energy density

 Power density

 Cost, toxicity and safety issues

10
 Cell voltage
• Cell voltage is represented by open circuit voltage (voltage between the
two terminals when no external current flows) or closed circuit voltage
(voltage between the two terminals when it is connected with external
circuit).
• Open circuit voltage Voc is calculated from the chemical potential of the
negative (µN Li) and positive electrode (µP Li) as:
µ𝑵 𝑳𝒊−µ𝑷 𝑳𝒊
• 𝑽𝒐𝒄 = Where, F is Faraday constant 96485 JK-1.
𝑭

• Thus, chemical potential of positive electrode should be high and that of

negative electrode should be low in order to achieve high cell voltage.
• In addition for the electrode to be thermodynamically stable, redox
energies of the electrode should lie within the band gap of electrolyte
material Eg. 11
 Conductivity

• Electrode material should be capable of conducting

electrons as well as lithium ions for better battery
performance.
• However, many Li ion insertion materials are semi
conductors by nature. Tuy nhiên, nhiều vật liệu chèn ion Li về bản chất là chất bán dẫnTuy nhiên, nhiều vật liệu chèn ion Li về bản chất là chất bán dẫn

• Some of them are even highly insulating (conductivity <10-

9 S.cm-1).
• Conductivity of the electrode is usually improved by
mixing the electrode material with conducting carbon.
12
 Coulombic efficiency (CE)

 CE is the ratio of number of charges that enter the

battery during charging to the number of charges that
can be extracted during discharge.
 Secondary reactions associated with electrolyte side
reactions, structural instability of the electrode
material and impurity in electrolyte etc., cause loss of
electron during charging and reduce the CE.
 Electrode material with more than 95% of CE is
considered as better electrode.
13
 Specific capacity

 𝑺𝒑𝒆𝒄𝒊𝒇𝒊𝒄 𝒄𝒂𝒑𝒂𝒄𝒊𝒕𝒚 𝒊. 𝒆 𝒄𝒂𝒑𝒂𝒄𝒊𝒕𝒚 𝒑𝒆𝒓 𝒈𝒓𝒂𝒎 𝒐𝒇 𝒂𝒄𝒕𝒊𝒗𝒆 𝒎𝒂𝒕𝒆𝒓𝒊𝒂𝒍 =

𝑭 𝒙 𝜟𝑿
Ah.g-1
𝑴𝒐𝒍𝒆𝒄𝒖𝒍𝒂𝒓 𝒘𝒆𝒊𝒈𝒉𝒕

 Where F is the Faraday constant 26.8 Ah

𝟗𝟔,𝟒𝟖𝟓 𝑨.𝑺𝒆𝒄
 𝑭 = 𝟗𝟔, 𝟒𝟖𝟓 𝑪𝒐𝒖𝒍𝒐𝒎𝒃𝒔 = = 26.8 Ah
𝟑𝟔𝟎𝟎 𝑺𝒆𝒄

 ΔX is the amount of reversible lithium

 Materials with low molecular weight and higher lithium

reactivity would deliver high capacity.

14
 Capacity retention/stability/cycle life

 Capacity retention is derived from the number of

cycles that the cell undergoes charge-discharge
processes.
 Although factors like electrolyte stability,
temperature, etc, influence the capacity degradation
phase stability of the electrode is the prime
component in determining the cycle life of a
cell/battery.

15
 Gravimetric and volumetric energy density

 Energy density is the energy per unit weight

(Gravimetric Wh.kg-1) or unit volume (volumetric
Wh.L-1).

 Gravimetric energy density = (Specific capacity/kg) x

Cell voltage

 Volumetric energy density = (specific capacity/litre)

x Cell voltage

16
 Power density

 Power density (W/kg or W/litre) is the power of the battery per

unit weight that represents the speed at which the energy can be
delivered to the load.

 Power density = Current (A/kg or A/litre) x Voltage (V) = Energy

density/Time.

 Power of the battery depends on the cell impedance, lithium

diffusion through the electrode, electrolyte and other
components.

 In many cases lithium ion diffusion through the electrode material

is the limiting factor and determines the power of the battery.
17
 Electrode materials for Li-ion batteries
compounds

 Cpds that accommodate Li+ either in vacant sites or having open

channels (1D, 2D or 3D) for Li-ion migration are capable of reversible Li
insertion.

 Transition metal cpds with vacant sites to accommodate Li have been

studied as Li insertion material.

 They have the ability to exist in various oxidation states and hence Li
exchange could get compensated with electron flow in the outer circuit.
chứa
 Other materials that do not have vacant sites for accommodating Li have
also been found to be potential candidates for the Li ion batteries.
cơ chế
 They react with Li by a different mechanism such as conversion, alloying
etc; that is different from that of insertion mechanism.
18
Nature 2001, 414 (6861), 359-367 19
 PTC
• Most commercial, cylindrical Li-ion cell design are equipped with a
positive thermal coefficient (PTC) current limiting switch to provide
hazard protection.

20
Mechanism of Li-ion Battery

21
 Voltage vs. Capacity for positive and negative-
electrode materials

Nature 2001, 414 (6861), 359-367 22

 Positive electrodes (cathodes)

 Electrode materials that exhibit a potential of > 2.25 V vs.

Li metal are considered as positive electrode materials.
 The main Li ion battery cathode materials are layered cpds
such as
 Li transition metal oxides (LiMO2)
 Spinels like LiMn2O4
 Olivines such as LiFePO4
 Cpds crystallizing in NASICON related structure
 Other transition metal oxides like MnO2, V2O5 etc.

23
 Layered LiMO2 (M = V, Cr, Mn, Fe, Co, Ni, etc)
• Li-ion intercalation in layered cpds was first studied with dichalcogenides in
1970’s.

• For example in TiS2, it has a layered structure and it reversibly intercalate Li ion
to form LixTiS2.

• Similarly other metal sulphides and selenides have also been studied as
intercalation materials.

• A cell voltage of ~2 V is obtained for LixTiS2 vs. Li cells.

• For improving the cell voltage, oxides are preferred compared to sulphides and
selenides (large free energy for the reaction; xLi + MXn LixMXn; when X=
Oxygen, and hence higher cell voltage).

• Metal oxides with layered structure have been studied extensively as electrode
materials for Li-ion batteries.
24
• Reversible Li reactivity in LiCoO2 was first discussed

by John B Goodenough in the year 1980.

khai thác

• LiCoO2 still remains as the most exploited

commercial cathode for Li ion batteries due to

its high stability during electrochemical cycling

superior capacity

ease of preparation in bulk quantities

25
• Structure of LiCoO2 as a representative example
of layered LiMO2

• It crystallizes in α-NaFeO2 structure with

hexagonally close packed oxygen array.

• Li+ and M3+ occupy octahedral interstitial sites

such that Li+ and M3+ ions occupy alternate
(111) planes of rock salt structure to form O-Li-
O-M-O chains (lithium layers separated by MO2
layers).

• This layered arrangement provides 2D channels

Crystal structure of an ideal
for Li diffusion. Thus, Li from the interconnected layered LiCoO2 showing the Li
ions between CoO2 layers of
site can reversibly insert from/into the structure edge shared CoO6 octahedra
(blue)
with the concurrent oxidation/reduction of
cobalt ion. 26
Commercial positive electrode-LiCoO2

27
 Specific capacity of LiCoO2 should be ~280 mAh.g-1 if all the 1Li+/Co could be
removed.

 However, practically only ~0.5 Li could be removed in the voltage window of 3 to

4.2 V thus reducing the capacity of LiCoO2 to 140 mAh.g-1.

 Charging of LiCoO2 to higher voltages is limited by oxygen evolution.

 This is because Co4+/Co3+ redox couple in LiCoO2 is pinned at the top of the O-2p
band and any further removal of Li oxidizes O2- ions at the surface into peroxide
(O2)2- ions evolving oxygen according to the reaction:

(O2)2- O2- + ½ O2 , causing safety concerns.

 Further, cobalt is toxic and expensive

 This has pushed the research towards nickel- and manganese-based lithium
oxides as alternative cathodes.

28
• LiNiO2 and LiMnO2 have gained great interest due to

 its more abundance

 less cost

 less toxicity

• But, stoichiometric LiNiO2 could not be synthesized and it always exists as Li1-

yNi1+yO2.

• Even small amount of Ni2+ in the sample causes cation exchange due to similar
ionic radii for Ni2+ (0.69 Å) and Li+ (0.76 Å).

• Thus, always some amount of Ni presents in Li layer and poses difficulty in Li

diffusion during cycling.

• This type of exchange of sites between transition metal and Li is known as cation
exchange or cation mixing.

• It is one of the main factors in deteriorating the cell performance.

29
• Layered LiMnO2 is not thermodynamically stable and hard to prepare by
normal ceramic method or other high temperature synthesis.

• LiMnO2 has been synthesized via ion exchange from NaMnO2.

• But this layered arrangement of LiMnO2 gets transformed to spinel

phase on cycling due to the instability associated with Jahn Teller
distortion from low spin Mn3+ ion.

• Accordingly, the solid solutions of LiMO2 (M = Co, Ni and Mn), exhibiting

a capacity value higher than LiCoO2 and preclude the deficiencies of the
end members are studied.

• Among many such possible solid solutions LiNi0.8Co0.2O2,

LiNi1/3Mn1/3Co1/3O2 and LiNi1/2Mn1/2O2 are the most attractive.

30
• LiNi0.8Co0.2O2, a Ni-rich phase of LiNi1-xCoxO2 system crystallizes in R3m space

group with hexagonal ordering iso-structural to LiCoO2 and LiNiO2.

• Small amounts of Co in the framework of LiNi1-xCoxO2 reduce Jahn-Teller

distortion of Ni3+-ions and help minimizing structural strain associated with

distorted NiO6 octahedra.

• This decreases the electrode resistance and helps sustaining high capacity of

LiNi1-xCoxO2.

• Stronger Co-O bond helps stabilizing the structure during cycling with enhanced

thermal stability of the material.

• LiNi0.8Co0.2O2 can be charged to higher voltages without any oxygen loss and

hence the material reversibly intercalates ~0.65 Li with stable capacity of 180

mAh.g-1.

• These solid solutions mainly involve one electron redox process of Ni3+/Ni4+. 31
• However, with LiMnO2 based solid solutions like LiNixMn1-xO2, LiNixMnyCo1-x-yO2,
nickel presents in Ni2+ and involves 2e- redox process Ni2+/Ni4+.

• Mn4+ ions with very high activation barrier for cation movement through
tetrahedral site acts as a pillar to stabilize the structure during cycling. Hence,
these compounds provide stable capacity of ~200 mAh.g-1. However, presence of
Ni2+ poses problem in synthesizing the material without cation mixing.

• Several other oxides like LiVO2, LiCrO2, etc also exhibit layered structure but did
not show promising electrochemical activity due to spinel phase formation
during cycling.

• LiTiO2 and LiFeO2 are difficult to synthesize by direct high temperature synthesis.

• These cpds could be prepared starting from sodium precursors (NaTiO2 and
NaFeO2) followed by ion exchanging sodium with Li.

• But they are found to be unstable during cycling.

32
 Spinel cathodes
 Another closely related structure to α-NaFeO2 is
the spinel phases having general formula AB2X4.

 Spinel structure is thermodynamically stable.

 Among various spinels, LiMn2O4 is the most

studied oxide for Li ion battery electrodes due to
its

 low cost

 safety Structure of spinel LiMn2O4 (space

group: fd3m) showing the existing
 environmentally benign
3D channels for lithium diffusion
 Structure of LiMn2O4 crystallizing in spinel
structure with cubic symmetry (space group:
Fd3m)
33
 In LiMn2O4, Mn3+/Mn4+ and Li+ cations occupy octahedral (16d) and
tetrahedral sites (8a) in the cubic close packed arrangement of oxygen.

 Edge shared arrangement of MnO6 octahedra forms ordered Mn2O4 array

with 3D channels for Li movement.

 During initial stages of deinsertion, Li from one 8a site moves to another

8a site through an octahedral 16c site.

 Movement of Li+ through 16c site is unfavourable and requires higher

energy.

 Thus, Li deinsertion occurs at 4 V region and the material still maintains

the cubic symmetry.

 The process is reversible during discharge and stable capacity of 120

mAh.g-1 (~0.4 Li ) could be achieved.
34
• 16c site of LiMn2O4 is unoccupied and further
insertion of Li into the structure is possible during
discharge to form Li1+xMn2O4.

• This process occurs in the low voltage region of 3

V.

• Thus, LiMn2O4 is capable of inserting ~2 Li into

the structure. However, intercalating more of Li
increases the number of Jahn-Teller ion (Mn3+:
t2g3eg1) and leads to cooperative distortion of
MnO6 octahedra.
Voltage vs. Composition plot of LiMn2O4
• Thus, cycling up to 3 V transforms the spinel in the voltage window of 2- 4.3 V.
phase into tetragonal phase with large variation
in unit cell volume. Consequently, structural
integrity with cycling to lower voltage is too low
and hence capacity fade also increases.
35
• Capacity fade is reduced by safely cycling the LiMn2O4
electrode in the 4 V region.

• Longer cycling produces capacity fade mainly because of

 Mn dissolution

 instability associated with low spin Mn3+

 Jahn-Teller distortion

 incompatibility of manganese oxide with most of the

electrolytes

36
• Al2O3 coating, doping of metal ions like, Al3+ , Zn2+ etc and stabilizing Li excess phase were

employed to increase the structural stability of the LiMn2O4 cathode.

• Substitution of metal ions like Cr, Co, Ni, etc in the Mn site are also found to improve the

electrode properties.

• Among these classes, LiNi0.5Mn1.5O4 is the main alternative spinel that is free of Mn3+ ions and

mainly involves Ni2+/Ni4+ redox process.

• This spinel provides high voltage batteries (4.7 V whereas it is 4.2 V for LiMn2O4 with respect

to Li/Li+) and high energy density.

• Stable capacity of ~130 mAh.g-1 was obtained for several hundred cycles without much

capacity fade.

• Other type of spinel phases such as LiFe2O4, LiCr2O4, LiCo2O4 are difficult to prepare by high

temperature chemical synthesis.

• On the other hand, LiV2O4 has been synthesized and found to behave like LiMn2O4.

• But the material is rarely used due to V dissolution and migration during cycling. 37
 LiFePO4 as cathode materials for Li-ion batteries
• Spinels and layered cpds are reported to show improved electrochemical
performance.

• However, they suffer from capacity fading on long cycling for instance, greater
than 1000 cycles.

• The stability of fully charged material is also poor.

• In this regard, an olivine type compound LiFePO4 with high safety and long
cyclability had been identified as the strongest candidate for Li-ion battery
application.

• LiFePO4 is a natural mineral known by the name Triphylite.

• Its crystal structure was first analysed by Yakubovich in the year 1977.

• The structure consists of corner shared FeO6 octahedra and edge shared LiO6
octahedra which are linked by PO4 tetrahedra.
38
Olivine structure of LiFePO4 that is built with FeO6 octahedra linked by
corners to PO4 tetrahedra.

39
• Li reactivity of LiFePO4 was first recognised by Padhi
et. al., in the year 1997.
• They showed that Li can be electrochemically
extracted from LiFePO4 thus leaving FePO4 with same
space group of LiFePO4.
• Cycling behaviour of LiFePO4 versus Li is shown in Fig.
• About 1 Li can be electrochemically extracted from
LiFePO4 which accounts to a capacity value of 170
mAh.g-1 which is close to the theoretical capacity
(complete removal of one Li).
Voltage vs. composition curve
• Li extraction and reinsertion proceeds via a two-phase of LiFePO4 cycled in the voltage
process and the ordered olivine framework maintains window of 2.75- 4 V.

during cycling.
• Hence, LiFePO4 is capable of cycling for thousands of
cycle without capacity loss. 40
LiFePO4

41
• Electronic conductivity of LiFePO4 is very low (10-9- 10-10 S.cm-1) due to strong
covalent nature of bonds.

• Lithium diffusion through these olivine structures is restricted to tunnels along

the ‘b’-axis and are 1D ionic conductors.

• Poor electronic conductivity and slow diffusion of Li requires charge-discharge

process at much slower rate to realize the full capacity.

• With increased specific current rate, insertion/extraction limited to 0.6 lithium.

• However, conductivity and hence rate capability of LiFePO4 is shown to be

improved by

 making small particles in the nano regime

 coating with conducting carbon

nội tại
 doping with other metal ions to achieve increased intrinsic electronic
conductivity.
42
• Another drawback of LiFePO4 is the low redox
potential (3.5 V) leading to lower energy density.

• Thus, LiNiPO4 (5.2 V), LiMnPO4 (4.1 V), LiCoPO4 (4.8

V), and other substituted phosphates are studied as
alternatives

• Nevertheless, they have not exceeded the

capability of LiFePO4 till date.
Tuy nhiên, chúng vẫn chưa vượt quá khả năng của LiFePO4 cho đến nay.

43
 Tavorites
• Research on electrode materials that exhibit higher redox voltages
regained interest towards fluoride based cpds.

• Thus, various fluorides (FeF3), fluorophosphates (LiFePO4F) and

oxyflourides have been analysed.

• Recently, a new family of fluorosulphates (LiMSO4F) have been

introduced by Tarascon et. al.

• These materials have tavorite structure similar to LiMgSO4 .

• It consists of slightly distorted MO4F2 octahedra linked together by

fluorine vertices.

• The resulting chains are bridged together by SO4 tetrahedra.

44
• The structure thus consists of 3D framework
for Li diffusion.

• Li ion diffusion through the material is

faster and stable capacity of >130 mAh.g-1
has been obtained with LiFeSO4F at higher
rates.
hòa tan
• However, LiFeSO4F is soluble in water and
unstable at high temperatures.

• This causes the need for special preparative

methods and improvement in electrode Tavorite structure of LiFeSO4F built from
FeO2F2 octahedra linked together by fluorine
optimization.
vertices forming a chain along the c-axis. The
• Further, it is difficult to synthesize LiMSO4F chains are bridged by SO4 tetrahedra and Li

phases in bulk quantities. resides in the tunnels

45
 NASICON type poly-anionic compounds
• Li rich Li2M2(XO4)3 (M= Ni, Co, Mn, Fe, Ti & V and X= P, S, As, Mo, W) and Li less LixM2(XO4)3
cpds were found to crystallize in NASICON type open framework structure.

• NASICON type structures have shared 3D O-X-O-M-O-X-O-framework that allows large

multiple spaces for varieties of guest species to accommodate.

• Li insertion property of NASICON type cpd was first studied on Fe2(MoO4)3.

• The material intercalate Li with the reduction of Fe3+ to Fe2+ and forms LixFe2(MoO4)3.

• The end member is found to have orthorhombic structure.

• Similarly, Fe2(WO4)3 and Fe2(SO4)3 have also been found to have reversible Li insertion
property.

• Fe2(SO4)3 crystallizes in two different structures namely, NASICON type structure and
monoclinic structure.

• Both forms could reversibly insert ~2 Li at 3.6 V.

46
• Similarly, Li3V2(PO4)3 exists in number of
phases.

• These structures are similar to NASICON

structure and shows intriguing
electrochemical properties.

• Almost 2 Li is inserted into rhombohedral

form in which ~1.3 Li is reversible.

• On the other hand, monoclinic form of

Li3V2(PO4)3 has potential application as it
could cycle 3 Li reversibly to give capacity
of 197 mAh.g-1.
NASICON-type structure of LiM2(PO4)3
47
 Other oxide cathodes
• Varieties of vanadium oxides such as orthorhombic V2O5,
amorphous V2O5.nH2O gels, V6O13 and VO2(B) etc. have
also been studied as cathode materials

• They show high capacity and cyclability.

• Similarly, some Mn-based amorphous oxides also show Li

insertion property with huge capacity of ~300 mAh.g-1.

• However, main drawback of these oxide cathodes is that

they do not contain Li and requires Li containing anodes.

48
Cathode Materials
1D Olivine 2D Layered 3D Spinel

49
Cathodes

50
Charging-Behaviors

51
 Relative Abundance of Chemical Elements

Reference: http://en.wikipedia.org/wiki/Lithium 52
 Negative electrodes (Anodes)
• Ideally Li metal should be the right anode material for Li batteries since its low
molecular weight and higher specific capacity.

• Capacity of 3.861 Ah.g-1 could be attained using Li metal as anode.

• However, use of metallic Li as anode in rechargeable batteries cause difficulty in

terms of safety and reversibility.

• The morphology of Li deposited during charging process is different from that of

Li metal.

• The needle like deposit thus produced is known as dendrite and may become
electrically isolated from the Li metal due to non-uniform dissolution of Li at
different portions during continuous charge-discharge cycles leading to capacity
loss.

• These dendrites may pierce the separator and can cause internal short circuit.
53
54
• The problem of Li dendrite formation is overcome by the use of Li intercalation
cpds.

• The concept of “rocking chair battery” was first introduced by Scrosati et. al,
wherein, both the electrodes are Li intercalating materials and the Li shuttles
(move back and forth) between the electrodes.

• Other mechanisms, where Li ion does not intercalate but reversibly reacts with
materials by alloying or conversion reactions have also been developed.

• Some of the common anode materials are classified as follows:

 Carbon based materials

 Silicon based alloys

 Tin based anodes

 Titanium based materials and

 Conversion electrodes
55
 Carbon based materials
• Among the various alternates for using Li metal as
anode, carbon based materials remain the best
choice due to
 high reversibility
 inexpensive
 non toxic.
• Electrochemical reduction of lithium in carbon takes
place as follows:
6C+ Li+ + e- LiC6
56
• Li ion intercalates into the
vacant sites of carbon by
forming lithiated carbon and
deintercalate from the
lithiated carbon when
reverse polarisation is
applied.

• Typical cycling behaviour of

graphitic carbon during Li
deinsertion/insertion is Charge-discharge plot of Meso Carbon Micro
Beads (MCMB) vs. Li cell in the voltage window
shown in Fig.
of 0- 1.5 V at room temperature

57
• The extent of Li accommodation in the C depends on the

 crystallinity

 microstructure

 morphology of the carbonaceous host

• Varieties of carbons have been prepared by pyrolizing various carbon

precursors at high T.

• They are classified as graphitic and non graphitic carbon.

• Graphitic carbon comprised of orderly arranged graphene layers bonded

together by weak Van der Waals forces.

58
Charging/discharging behaviors of Graphitic Carbon in
the 1st Cycle

59
Commercial negative electrode-graphite

60
Natural graphite (ore)
MCMB: meso carbon
Artificial graphite
micro-beads: Graphitic C
from pitch

Li intercalates
through edge
planes only
61
 Graphite

Lithiation

Theoretical capacity = 1x96500/72 = 1340 C

= (1340/3.6) mAh = 372 mAh/g-G J.
(Li+ + e-) Electrochem. Soc., Vol. 140, 2490 (1993) 62
• Graphene layer is composed of sp2 carbon arranged in a
planar honeycomb like structure.

• Graphite is highly ordered in nature and has large number

of vacant sites to intercalate guest ions.

• Graphite can intercalate both anions and cations and is

known as redox amphoteric intercalation host.

• Intercalation of Li into graphitic carbon occurs in the

vacant space between the graphene layers by opening the
Van der Waals gap between the layers (staging).
63
• When graphite is polarized to negative potential, ethylene carbonate in the
electrolyte solution reductively decomposes to form stable surface film (SEI) on
the surface of graphite electrode.

• The SEI prevents further reduction and co-intercalation of electrolyte and allows
only Li ion diffusion.

• Therefore, SEI acts as a protective surface film making carbon electrode stable
even at potentials lower than 1 V.

• However, due to the usage of Li for SEI formation and other side reactions, first
cycle discharge capacity of graphite anode is always higher than the theoretical
capacity.

• The subsequent discharge recovers only 80-85% Li leading to huge irreversibility

in the first cycle.

• It is true with all other types of carbon also.

64
• Non-graphitic carbons have high disorder in the structure.

• They are further classified into graphitizing and non-graphitizing carbon.

• Graphitizing carbons get graphitized by heating to T >1000oC in inert atmosphere.

• If the precursor material has condensed aromatic ring structure, then heating
induces ordered graphitic arrangement and structure transforms from disordered
structure to ordered one.

• Non-graphitizing carbons are synthesized by pyrolysing organic polymers that

have cross linked structure.

• Thus, while heating to higher T, it cannot align in planar aromatic ring forms
(graphitic form).

• Non-graphitizing carbons are mechanically harder than graphitizing carbons and

are divided into soft and hard carbon.

65
• The Li intercalation mechanism with different carbon (graphitic, ordered,

disordered, soft, hard etc.) is different.

• Li intercalation in graphitic carbon occurs in steps and is represented as

stage formation.

• Thus, the potential varies according to different stages of Li intercalation

and a clear plateau is visible.

• However, in disordered carbons, surface groups and porosity play a major

role and lead to charge-discharge profiles different from that of graphite.

• The charge-discharge curve is rather continuous and complex.

• Capacity near to 372 mAh.g-1 is attained with soft C too but with lower rate

capability. 66
• Rate capability of carbon based anodes is improved by using materials

like MCMB (Meso Carbon Micro Beads), MCF (Micro Carbon Fibre) with

perfectly ordered structure and morphology.

• Still, first cycle irreversibility associated with SEI formation is high for

these carbons.

• Other carbon based materials with special structure like fullerenes, CNT,

graphene, graphene based composites have also been studied as

anodes.

• They showed improved capacity but with more of first cycle

irreversibility and hence are of not much implication.

67
 Silicon based alloys
• Most commercial Li-ion batteries are based on graphite anode electrode,

which has a maximum attainable capacity of 372 mAh/g.

• Hence, graphite fails to meet future demands of the optimal capacity of

1000 mAh/g.

• In this regard, silicon stands as one of the most outstanding options, with

a theoretical capacity close to 4000 mAh/g, more than 10 times higher

capacity than graphite.

• Therefore, the shift from graphite- to silicon-based electrode can

significantly increase the energy density of batteries.

68
• A variety of alloy anodes and intermetallic cpds of group III, IV and V (Al,

Sn, Ge, Si, Sb, etc) have been studied for battery application.

• Among these, Si is the most promising candidate due to

 highly abundant

 low molecular weight

 high capacity

 relatively low cost

 less toxic (environmentally friendly)

• Theoretical capacity of silicon is ~ 4000 mAh.g-1 corresponds to 4.4 Li/Si.

69
anode: Li-alloying anodes

990 mAh/g

LiC6 Li15Si4
372 mAh/g 3579 mAh/g
M. N. Obrovac et al., J. Electrochem. Soc., 154, A849-A855 (2007) 70
• However, large volume change of silicon during charging and discharging process
remains the major challenge in commercialization of silicon-based anode
batteries.

• The volume expansion associated with Li4.4Si and Si which are formed during the
discharge-charge process is about 300%.

• Thus, the electrode material swells and shrinks during charging and discharging
process.
tan rã,
• As a consequence, after a few cycle the structure of the electrode disintegrates,
loses contact between each particle and no longer insert Li+ ion.

• It leads to quick loss of capacity on number of cycles.

• Therefore, researchers in this area aim in designing electrode integrity and

increase battery life.

71
• Various strategies have been tried to overcome the volume expansion
problem of alloy based anodes.

• For example, by creating stable oxide/carbon/polymer matrix or by using

nanocrystalline materials, the volume expansion had been overcome for
certain extent.

• Nano structured electrode provides structural stability to tolerate the

huge volume expansion during cycling.

• Further, it facilitates ionic and electronic conduction.

• For example, nano-wires of Si are developed as Li-ion battery anode

material that exhibits 10 times higher capacity than traditional graphite
anode.

• They were prepared by template method and found to retain 90% of its
initial capacity even after 200 cycles. 72
 Volume change during cycling has also been solved by using composite of
silicon with various carbonaceous materials which accommodate the
stress associated with volume expansion.

 It prevents the Si anode from pulverizing and improves capacity

retention of the electrode.

 Various types of carbons such as soft carbon, hard carbon, graphite, CNT
and graphene, etc are used as carbon matrix for silicon anode.

 Recently, Graphene-silicon composite have been seriously pursued as

high capacity anode material.

 Graphene layers have huge surface area which possibly accommodates

the volume expansion exerted by silicon particles.
73
• Such composites are reported to exhibit a stable capacity of >2200
mAh.g-1 even after 50 cycles in contrast to about 1000 mAh.g-1 obtained
with graphitic carbon composites

• Volume expansion can also be reduced by using inactive-active

composites.

• Inactive metal like Fe, Co, Ni, Mn etc are made to form a composite with
active metals like Sn, Si, or Al.

• Such a combination of composite dissociate to form matrix of inactive

metal dispersed LixM (M= active material like Si).

• Inactive metal provides a stable matrix for volume expansion and

maintains the contact.

• Thus, stable capacity is preserved for long cycling.

74
 Tin based anodes
• Metallic Sn react with Li to form an alloy Li4.4Sn and exhibit a capacity of ~1000
mAh.g-1. However, formation of Li4.4Sn is associated with large volume expansion
as in Li4.4Si.

• To reduce the problem of volume expansion, nanostructuring of Sn, or by making

Sn-C composites, or inactive-active material matrix have been used and found to
be effective.

• For example, Cu-Sn alloy having a composition Cu6Sn5 which react with Li by
forming Li4.4Sn alloy in a stable Cu matrix has been reported.

• Cu is electrochemically inactive and forms a nanocrystalline matrix to

accommodate the volume expansion.

• Similarly, Ni and Co forms alloy with Sn (Ni3Sn2 and Co3Sn2) and reacts similarly.

• Sn-Ag alloy also was tried and showed excellent capacity retention with better
electronic conductivity.
75
• Stannic and stannous oxide also is capable of forming Li-Sn alloys as,

2Li+ SnO2 Sn + Li2O

Sn + xLi LixSn

• Formation of Li2O + Sn matrix is associated with first cycle irreversibility.

• The formed Li2O matrix restricts the volume expansion associated with
LixSn formation.

• Volume expansion can still be reduced by using nanostructured

SnO2/SnO materials.

• Varieties of SnO2 nanorods, nanocubes, nanotubes, nanoparticles, etc

have been prepared by different synthetic routes and the resulting
nanostructures showed better electrochemical activity than bulk SnO2.

76
Volume expansion issue Example: Expansion upon lithiation of Sn

electrolyte

77
 Detachment at active
layer/current collector interface.
 Finally, a drastic capacity fading
occur.
78
SEI (a passivating film on anode surface ) formation
 By the reaction of the anode surface with the Li-ion and decomposition of electrolyte
at low potentials ~1 V (lithium carbonate (Li2CO3), lithium alkyl carbonate (ROCO2Li),
lithium alkoxide (ROLi), LiF and polycarbonates).
 Electronically insulating and Li-ion conducting.
 Prevent electrolyte to directly contact the active materials.
 The film cracking occur during cycling.
 Thicker and thicker SEI formation from the re-expose Si surface.
 Since film formation consumes charge and materials, it strongly contributes to the
irreversible capacity.

79
 Titanium based anodes
• Transition metal oxides like Li4Ti5O12 and TiO2 have also been used as anodes for
Li-ion batteries.

• They intercalate Li at potential higher than (>1 V) that of Li-insertion in carbon

based anodes.

• This results in reduced voltage (<2.5 V) when combined with cathodes like
LiCoO2, LiMn2O4 or LiFePO4.

• However, they can be used in combination with high voltage cathodes such as
LiMn1.5Ni0.5O4 or LiMn0.5Ni0.5O2 phases to get ~ 3 V battery.

• Li4Ti5O12 crystallize in spinel phase and represented as Li[Li1/3Ti5/3]O4.

• Lithium occupies 8a position and 16d position is occupied by both Li and Ti.

• Li4Ti5O12 is capable of inserting about 3 Li to form Li7Ti5O12 thereby achieving a

capacity of 175 mAh.g-1.
80
• The cell voltage for lithium insertion/de-insertion is 1.5 V.

• Li7Ti5O12 crystallize in rock salt structure which is similar to that of

Li4Ti5O12 and does not cause expansion of lattice or volume
change.

• Therefore, no strain is created in the material during cycling.

• Hence, Li4Ti5O12 is represented as zero strain material and the

capacity of ~175 mAh.g-1 is maintained for thousands of cycles
without any capacity loss.

• However, electronic conductivity of Li4Ti5O12 is less, thereby

carbon coating and other doping methods were used in order to
increase the conductivity.
81
• Titania (Titanium (IV) oxide) is the well studied transition metal oxide in the
fields of catalysis, photocatalysis, Li-ion batteries, hydrogen storage etc.

• TiO2 exists in three different crystallographic forms namely anatase, rutile and
brookite.

• Anatase and rutile has tetragonal geometry whereas brookite crystallise in

orthorhombic symmetry.

• All these forms are electrochemically active and the Li insertion property
depends on the crystallography and microstructure of the material.

• Most electrochemically active form of TiO2 is anatase phase, it could reversibly

intercalate 0.6 Li and exhibit a stable capacity of ~170 mAh.g-1.

• However, the Li insertion potential is still higher (1.7 V) than Li4Ti5O12

necessitating the need for high voltage cathode.

82
 Conversion electrodes
• A group of transition metal cpds have been reported to deliver
stable capacities 2-3 times greater than that of conventional
graphite anode.
• They don’t have vacant sites to accommodate Li , as a result
excluding the possibility of intercalation mechanism.
• The actual mechanism involves electrochemical reduction of
transition metal compound (MxXy) to nanometer scale metal
particles and LinX.
• The nanocomposite (metal in LinX matrix) thus formed is highly
reactive and hence decompose back to Li and MaXy when reverse
polarisation is applied.
83
• The nanometric nature of the composite is shown to be
maintained for a number of charge/discharge cycles.

• Consequently, the reaction of Li with transition metal cpd is highly

reversible for a number of cycles.

• The mechanism is conventionally referred as “conversion

reaction” and can be represented as follows:

MxXy + (n.y) Li xM + yLinX; Where M = Transition metal

(Most of the transition metal cpds have been shown to react by
conversion mechanism except group 4 and 5), X= anion like O, S, P, N,
and even F/H, n = Oxidation state of X.
84
• The capacity of the material that reacts by
conversion mechanism is always higher

• Therefore, they are expected to show higher energy

density.

• Further, the potential at which the conversion

reaction takes place depends on the M and X.

• Thus, by varying M and X, operating voltage can

easily be tuned based on the requirement.
85
• The conversion reaction mechanism is complex involving electrochemical
reduction/oxidation of initial phase with Li to metallic nanoparticles,
electrolyte side reaction so as to form SEI and so on.

• As a result, repeated cycling leads to pulverisation and hence loss of

electrical contact.

• Further, the polarisation associated with charge/discharge processes is

also more thereby reducing the coulombic efficiency.

• Thus electrodes based on conversion mechanism still remains in

laboratory level and not reached commercialisation yet.

• Overall, there have been significant efforts to improve the performance

of anode materials but finding an alternative to carbon anode still
remains as a daunting job.
86
Anodes

87
 AC-impedance (AC-IR)

+Rsei

Low frequency

High frequency
Inject a series of
sine wave signal Transformed Response plot

88
Temp effect

89
 Failure Mechanism
 Internal short circuit
 Dendrite formation

 Failure of separator

 SEI degradation

 Impurities

 Cell deformation

 Overcharged
 Improper A/C ratio

 Non-uniformity of electrodes

 Non-uniformity of cells

 Improper charging process

 Thermal run away 90

 Potential Window

The most challenge for further

development of Li rechargeable
batteries for electric
vehicles is safety, which requires
development of a nonflammable
electrolyte with either a larger window
between its lowest unoccupied
molecular orbital (LUMO) and highest
occupied molecular orbital (HOMO) or
a constituent (or additive) that can
develop rapidly a solid/electrolyte-
interface (SEI) layer to prevent plating
of Li on a carbon anode during a fast
charge of the battery.

J.B. Goodenough and Y. Kim, Chem. Mater. (2009)

91
 Thermal Behavior of LiCoO2

Ref. Journal of The Electrochemical Society, 149 (7) A912-A919 (2002) 92

 Thermal Behavior of LiFePO4

Self-heating rate versus T for the three charged

DSC profiles of LiFePO4 charged to 3.8 V (4.2 V) cathode materials with EC/DEC solvent.

Ethylene carbonate (EC)

Diethyl carbonate (DEC)

Ref. Journal of Power Sources 108 (2002) 8–14, Electrochemistry Communications 2004, 6 (1), 39-43 93
 Overcharged Behavior of Cathode Materials

The oxygen will evolve during the

overcharging process:

3CoO2 Co3O4 + O2
The in situ XRD patterns of LiCoO2
collected during the 1st charge to 4.8 V at C/4.5
rate in the 003 to 104 region.

Ref. Journal of The Electrochemical Society, 153 (11) A2152-A2157 (2006) 94

 Band Structures of cathode materials

 LiMn2O4 and LiFePO4 are better compared to the layered

cobalt and nickel oxides as far as the structural, chemical
stability, lower oxygen loss and thereby better SAFETY
ISSUES are concerned at deep levels of charge/discharge.
 This is due to the fact that the t2g redox state of transition
metal in LiMn2O4 and LiFePO4 is well above the O:2p state. 95
 Solid Electrolyte Interface (SEI) on Anode

Example: Use EC as an electrolyte Bad SEI

Good SEI
(I) No additive to help SEI’s formation.
(II) Some additives perform good SEI formation. 96
Reference: S. S. Zhang, J. Power Sources 162 (2006) 1380.
Anodic SEI properties
depends on:
1. graphite material:
 surface area
 Surface morphology
 Surface heterogeneities
(basal plane and prismatic
surfaces, surface groups),
 Graphite crystallinity
2. Electrolyte composition

97
 The Aging Mechanism of SEI

Ref. Journal of Power Sources 2005, 147 (1-2), 269-281 98

 Thermal Stability of Anode Materials

Electrolyte salt effect

Particle size
DSC curves of the reactions occurring in a fully increased
lithiated MCMB graphite containing electrolyte or
not.

Ref. Journal of The Electrochemical Society 1998, 145 (2), 472-

477, Electrochimica Acta 54 (2009) 3339–3343
Particle size effect 99
Electrolyte & Salt Table

100
 Band Structure of Electrolytes

HOMO/LUMO energy level diagram for various solvents and salt

101
 Additives

Functions of additives:
 SEI forming improver
 Cathode protection agent
 LiPF6 salt stabilizer
 Safety protection agent
 Li deposition improver
 Ionic solvation enhancer
 Al corrosion inhibitor
 Wetting agent and viscosity diluter 102
 Separator
• A separator can block electrons between cathode & anode, but ions can transfer
cross it. It usually use high strength, thin & micro-porous materials.

By using DSC analysis, the

composition of separator can be
differentiate

 The thinner separator can increase the energy density but lower the
safety of batteries.
103
 Separators

Residual Al2O3
104
 FT-IR of Separator

Comparing with database, the material

of separator should be polyethylene (PE)

105
Main Thermal Reactions

Ref. Industrial Materials Magazine

2008, 264, 118-122, 34

106
 How to provide an intrinsic safe Battery?

 Design or selection of cathode material with intrinsic safety

 Surface coating on cathode

 Surface coating on anode

 Surface coating on separator

 SEI modified additives

 Fire retardant

 Overcharged Protector

107
Surface Coating on Cathode
Choices of coating: ZrO2, Al2O3, MgO,
AlPO4, TiO2, etc.

108
 Surface Coating on Anode

 DSC curves of fully lithiated electrodes

in the presence of electrolyte.
 Bare NG electrode (dotted line) and
NG electrode coated with 5Al2O3

Advanced Materials 2010, 22 (19), 2172-2176 Atomic Layer Deposition (ALD) cycles.
109
Surface Coating on Separator

Single layer PVDF separator: C1

Bilayer composite separator : C2

The separator enabled a high thermal

stability than commercial PP separator

Ref. Journal of Power Sources 196 (2011) 8125– 8128 110

 SEI Modified Additives

DSC curves of various investigated electrodes: Ethyltriacetoxysilane (ETAS); 1,3-

benzoldioxole (BDO); Tetra(ethylene glycol)dimethyl ether (TEGME); Vinylene
carbonate (VC)
Ref. Electrochimica Acta 49 (2004) 2351–2359 111
 Fire Retardant
• Phosphorus cpds such as Trimethyl phosphate (TMP),
Triphenylphosphate (TPP), Tributylphosphate (TBP), 4-isopropyl phenyl
diphenyl phosphate (IPPP), diphenyloctyl phosphate (DPOF), etc.

Without decreasing the electrochemical performance but increasing

the thermal stability. 112
 Overcharged Protector
Redox shuttle system

Charge-discharge curves of LiCoO2 coin-type cells

containing various redox shuttle

Ref. Journal of The Electrochemical Society, 146 (4) 1256-1261 (1999),

Journal of Power Sources 2010, 195 (15), 4957-4962. 113
 Lithium-ion Batteries (LIBs)

Higher demanding situation in safety requirements, environmental conditions,

use profile, or some combination of the preceding factors.
114
 Comparison and Trade-offs

Source : Batteries for Electric cars http://www.bcg.com/documents/file36615.pdf 115

 Summary: How to design or operate a cell in a safer
way?
 Materials

 Intrinsically safe (Cathode, anode, electrolyte and separator)

 Safety improved by surface coating

 Uniformity

 Cell

 A/C ratio in terms of capacity and rate capability

 A proper charging/discharging window

 Uniformity of fabricated electrodes

 Safety devices (PTC, CID)

 Pack

 Power management (Energy vs. Power vs. Safety)

 Thermal management 116

 Next-generation rechargeable batteries
• The current Li-ion batteries suffer from slow charging time, low
cycle life and low energy density, which can significantly impact
the overall performance of the device.

• Next-generation rechargeable batteries as energy storage systems

with superior characteristics of outmost importance.

• They have higher energy densities, light weight, better safety

characteristics, environmentally friendly, low cost, and longer
cycle life than the current battery systems.

• They include both advanced Li-ion batteries such as all-solid-state

Li-ion, Li-metal, Li-air, Li-sulfur and post-Li battery chemistries
such as Na-ion, multivalent metal-ion, metal-air, redox flow, etc.117