Faraday Discussions

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Stabilization of lithium metal in

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concentrated electrolytes: effects of

electrode potential and solid electrolyte
interphase formation†
Anusha Pradhan,a Shoma Nishimura,a Yasuyuki Kondo,a
Tomoaki Kaneko, b Yu Katayama, a Keitaro Sodeyama c

and Yuki Yamada *a

Received 26th February 2024, Accepted 16th April 2024

DOI: 10.1039/d4fd00038b

Lithium (Li) metal negative electrodes have attracted wide attention for high-energy-
density batteries. However, their low coulombic efficiency (CE) due to parasitic
electrolyte reduction has been an alarming concern. Concentrated electrolytes are
one of the promising concepts that can stabilize the Li metal/electrolyte interface,
thus increasing the CE; however, its mechanism has remained controversial. In this
work, we used a combination of LiN(SO2F)2 (LiFSI) and weakly solvating 1,2-
diethoxyethane (DEE) as a model electrolyte to study how its liquid structure changes
upon increasing salt concentration and how it is linked to the Li plating/stripping CE.
Based on previous works, we focused on the Li electrode potential (ELi with reference
to the redox potential of ferrocene) and solid-electrolyte-interphase (SEI) formation.
Although ELi shows a different trend with DEE compared to conventional 1,2-
dimethoxyethane (DME), which is accounted for by different ion-pair states of Li+ and
FSI−, the ELi-CE plots overlap for both electrolytes, suggesting that ELi is one of the
dominant factors of the CE. On the other hand, the extensive ion pairing results in the
upward shift of the FSI− reduction potential, as demonstrated both experimentally and
theoretically, which promotes the FSI−-derived inorganic SEI. Both ELi and SEI
contribute to increasing the Li plating/stripping CE.

a
SANKEN, Osaka University, 8-1, Mihogaoka, Ibaraki, Osaka 567-0047, Japan. E-mail: yamada@sanken.
osaka-u.ac.jp
b
Department of Computational Science and Technology, Research Organization for Information Science and
Technology (RIST), 1-18-16, Hamamatsucho, Minato-ku, Tokyo 105-0013, Japan
c
Center for Basic Research on Materials, National Institute for Materials Science (NIMS), 1-1, Namiki, Tsukuba,
Ibaraki 305-0044, Japan
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4fd00038b

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Introduction
Lithium (Li) metal negative electrodes have been broadly employed in advanced
rechargeable batteries. The basic reason for this is that Li (an alkali metal) bears
the lowest atomic number among all metal elements, and hence a high theoret-
ical capacity of 3860 mA h g−1 can be achieved with its plating and stripping
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reactions. At the same time, a high battery voltage can be attained due to its low
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electrode potential of −3.0 V vs. the standard hydrogen electrode (SHE). However,
Li metal shows a lower plating/stripping coulombic efficiency (CE), which has
hampered its practical applications.1,2 The poor CE is due to the strong reducing
ability of Li metal. In general, the electrode potential of Li is set far outside the
potential window of organic electrolytes, which accelerates the reductive
decomposition of the electrolytes. Practically, the reduction products accumulate
on the negative electrode to form an interphase known as the solid electrolyte
interphase (SEI).3 This SEI is Li+-conducting but electron-insulating and thus
kinetically retards further electrolyte decompositions by blocking direct contact
between the electrode and electrolyte.4,5 Hence, the nature of the SEI is an
important factor that dominates the CE of Li metal electrodes.
To minimize the reductive decomposition of the electrolytes, various electro-
lyte concepts have been proposed. The state-of-the-art concepts are (a) concen-
trated electrolytes,6–11 (b) localized concentrated electrolytes,12,13 (c) electrolyte
additives,14,15 (d) weakly solvating electrolytes,16–18 and (e) liqueed gas electro-
lytes,19,20 etc. Among them, concentrated electrolytes are one of the most funda-
mental concepts that have suggested the importance of liquid structures, from
which various electrolyte design concepts have been developed. We reported in
2014 that concentrated electrolytes with extensive ion pairing promote the pref-
erential reduction of salt anions, thus leading to an anion-derived SEI, which may
contribute to stabilization of the Li metal/electrolyte interface.9,21 This mecha-
nism is widely accepted and applied to various electrolyte systems, including
aqueous electrolytes.22,23 On the other hand, we discovered in 2022 that the
extensive ion pairing induced by, for example, increasing salt concentration can
remarkably upshi the Li electrode potential, ELi, which decreases the ELi–
potential-window gap, thus suppressing electrolyte reduction and leading to
a higher CE of Li plating/stripping.24 As a result, there are several factors to be
discussed for the stabilization mechanism of Li metal electrodes in concentrated
electrolytes, namely (i) liquid structure, (ii) ELi, and (iii) SEI formation.
Here, we have chosen a combination of LiN(SO2F)2 (LiFSI) and 1,2-diethoxy-
ethane (DEE) as a model electrolyte to discuss how each factor contributes to
increasing the CE of Li plating/stripping. Compared to conventional 1,2-dime-
thoxyethane (DME), DEE is known as a weakly Li+-solvating solvent due to the
steric hindrance effect of the bulkier ethyl groups.17,25 The LiFSI/DEE system
exhibits a high CE of Li plating/stripping even at a low 1 mol dm−3 (M) concen-
tration and a further increased CE at a higher salt concentration.25 Here we
compared LiFSI/DEE with LiFSI/DME to highlight the effects of Li+-solvation
ability as well as salt concentration on (i) liquid structure, (ii) ELi, and (iii) SEI
formation. Based on our previous work that highlights ELi,24 we rst evaluated the
ELi in LiFSI/DEE with reference to the ferrocene redox potential (Fc/Fc+) and
discussed its shi based on the liquid structures. Next, we evaluated the CE of Li

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plating/stripping in LiFSI/DEE compared to LiFSI/DME and discussed its rela-
tionship with ELi. In addition, with an eye to the SEI formation, we also investi-
gated the reduction potential of the electrolyte with reference to Fc/Fc+ and
discussed its variation in salt concentrations based on density functional theory-
based molecular dynamics (DFT-MD) simulations.
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Results and discussion

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Li electrode potential (ELi)

ELi was evaluated with reference to the Fc/Fc+ redox potential on a Pt electrode as
an internal standard of electrode potentials recommended by IUPAC.26,27 For this,
a three-electrode cell with Pt as the working electrode and Li metal as the counter
and reference electrodes was used (Fig. 1a). Fc (1 mM) was introduced to the
LiFSI/DEE electrolyte. Fig. 1b shows cyclic voltammetry (CV) proles of the Fc/Fc+
redox reaction at various LiFSI salt concentrations (units of mol kg−1are hereaer
denoted as m). The CV proles are close to that of a fully reversible system (a peak
separation of 59 mV at 25 °C) except for the lowest concentration of 0.12 m that
shows high electrolyte resistance. The CV proles show that the Fc/Fc+ redox
potential with reference to Li/Li+ was shied to lower potentials at higher salt
concentrations. If it is supposed that the electrode potential of Fc/Fc+ is constant
and independent of the electrolyte used, then the different CV redox potentials
result from the Li reference electrode, suggesting that ELi (with reference to Fc/
Fc+) changes depending on the electrolyte used. We extracted the Fc/Fc+ redox
potential (with reference to Li/Li+) from the half-wave potential at the centre point
between the oxidation and reduction peaks in the CV and then converted it to ELi
(with reference to Fc/Fc+) by just adding a negative sign (e.g., a Fc/Fc+ potential of
3 V vs. Li/Li+ corresponds to ELi = −3 V vs. Fc/Fc+). For accurate assessment of ELi,
we prepared two or three cells for each electrolyte and obtained average ELi values
(Fig. S1 and Table S1†). Fig. 1c and Table S2† summarize the relationship between
ELi and salt concentration. As a comparison, the data for LiFSI/DME reproduced

Fig. 1 Evaluation of ELi with reference to the Fc/Fc+ redox potential. (a) Schematic of
a three-electrode cell with Pt as a working electrode (WE) and Li metal as counter/
reference electrodes (CE/RE). (b) Cyclic voltammograms (scan rate: 5 mV s−1) of the Fc/
Fc+ redox reaction in LiFSI/DEE at various LiFSI concentrations (mol kg−1 = m) of 0.12,
0.60, 1.3, 2.0, 2.8, 4.7, 7.1, 8.5, and 10 m. 0.12 m and 10 m correspond to 0.1 M and 5 M,
respectively. (c) ELi at various LiFSI concentrations evaluated from the cyclic voltammo-
grams. Average ELi values obtained with two or three cells for each concentration are
plotted with error bars (standard deviations). The data for LiFSI/DME are reproduced from
ref. 24 as a comparison.

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from our previous publication are also shown.24 For both DEE and DME systems,
ELi is upshied concomitantly with increasing salt concentration, suggesting that
the reducing ability of the Li metal is weakened at high salt concentration.24,28 For
LiFSI/DEE, ELi was the highest (−2.95 V vs. Fc/Fc+) for the highest salt concen-
tration (10 m, corresponding to approximately 5 M), and it was the lowest (−3.42 V
vs. Fc/Fc+) for the lowest salt concentration (0.12 m, corresponding to 0.1 M).
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A comparison of DEE and DME systems enables us to discuss the results from
the viewpoint of the Li+-solvation abilities. It was reported that DEE has a weaker
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Li+-solvation ability than DME due to the steric hindrance effect of its bulkier
ethyl groups.17,25 In the low concentration region below 4 m, LiFSI/DEE showed
meaningfully higher ELi values than LiFSI/DME, suggesting that a weak solvation
environment of Li+ leads to a high ELi. Similar correlations are also reported for
dimethoxymethane (DMM), DME, and diglyme (G2), whose solvation abilities are
in the order of DMM < DME < G2 and ELi values are in the order of DMM > DME >
G2.24 To verify the effect of Li+-solvation abilities, we further measured ELi in 1.0 M
LiFSI/DEE : toluene, in which toluene with almost no Li+-solvation ability was
introduced at different molar ratios (DEE : toluene = 10 : 0, 7 : 3, and 4 : 6). As
shown in Fig. S2,† the three variations of 1.0 M LiFSI/DEE : toluene showed
different ELi values, even at the xed 1.0 M concentration. The highest ELi (−3.26 V
vs. Fc/Fc+) was achieved by introducing the largest amount of toluene, suggesting
that the weakly solvating environment of Li+ leads to a higher ELi.
On the other hand, increasing the salt concentration above 4 m, both LiFSI/
DEE and LiFSI/DME showed similar ELi values at similar molalities. Hence, ELi
in the concentrated region is not related to the Li+-solvation abilities of the solvent
molecules. In such a concentrated region, free solvent molecules that can solvate
Li+ are remarkably decreased in number, which induces ion pairing of Li+ and
FSI−. Hence, the commonly high ELi values are due to similar ion-pair states at
high concentrations, as discussed later.
In essence, the upward shi of ELi is achieved by (i) increasing salt concen-
trations or (ii) employing weakly Li+-solvating solvents. The resultant high ELi can
decrease the ELi–potential-window gap, which can weaken the driving force of
electrolyte reduction on Li metal. Therefore, concentrated electrolytes and weakly
solvating electrolytes are inherently less susceptible to its reduction in conjunc-
tion with Li metal. The inuence of ELi on the CE of Li plating/stripping will be
discussed in the following sections.

Liquid structure
Next, we discuss how ELi is related to the liquid structure of electrolytes. A theo-
retical consideration shows that ELi is linearly correlated with the Li+ chemical
potential in the electrolyte (mLi+),
mLiþ
ELi f
F
where F is the Faraday constant.24 It should also be noted that the Li+ chemical
potential in the SEI is cancelled out during the derivation of the equation; hence,
the SEI chemistry does not theoretically affect ELi.24 Basically, mLi+ indicates to
what extent the Li+ is stable in its environment. Hence, mLi+ should be closely
related to its coordination environment in the electrolyte. To this end, we studied
the liquid structure of LiFSI/DEE.

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The dissolution of LiFSI in DEE (an aprotic solvent) is illustrated as competitive
coordination of DEE and FSI− (both being Lewis bases) towards the Li+ (a Lewis
acid). There are several random and driven interactions of Li+–DEE and Li+–FSI−
taking place in the solution structure, which generates various LiFSI–DEE solvates,
such as (a) solvent-separated ion pairs (SSIPs), (b) contact ion pairs (CIPs, FSI−
coordinating to a single Li+ ion), and (c) aggregates (AGGs, FSI− coordinating to two
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or more Li+ ions) depending on the salt concentrations.29 Broadly, an increase in the
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salt concentration results in (i) a reduction in free solvent molecules and (ii) a surge
in the ionic association (CIPs and then to AGGs).29 Focusing on the coordination
environment of Li+, it is mainly coordinated by solvent molecules at low concen-
trations but is forced to be paired with FSI− at high concentrations. To identify such
Li+ coordination environments, we study the ion-pair states in LiFSI/DEE at different
concentrations using Raman spectroscopy (Fig. 2a). We focused on the S–N–S
vibration peak of FSI− in the range of 650–800 cm−1, and the wavenumber of this
peak is sensitive to the ion-pair states of Li+ and FSI−.29 A at prole was observed for
low concentrations (0.12 m and 0.60 m) because of the insufficient amount of FSI−
for detection. Over 1.3 m (corresponding to 1.0 M), the S–N–S vibration peak of FSI−
was observed, and it was shied to a higher wavenumber with increasing salt
concentration. This higher wavenumber shi has been attributed to more extensive
ion pairing of Li+ and FSI− from SSIPs to CIPs and AGGs.29 At low concentrations,
SSIPs and CIPs are dominant; Li+ is solvated by DEE molecules or partially coordi-
nated by an FSI− anion. Considering the low ELi (i.e., low mLi+) at low concentrations,
Li+ is highly stable in such solvent-dominant coordination environments. On the
other hand, with an increase in the LiFSI concentration, CIPs and AGGs become
dominant; Li+ is coordinated by more FSI− anions in the –Li+–FSI−–Li+–FSI−–Li+–
aggregate with partial DEE solvation. This situation results in high ELi (i.e., high mLi+),
thus Li+ being highly unstable in such anion-dominant coordination environments.
To theoretically support the spectroscopic analysis, DFT-MD was applied to the
dilute (LiFSI : DEE = 1 : 10 by mol, corresponding to 0.85 m) and concentrated
(LiFSI : DEE = 1 : 2 by mol, corresponding to 4.2 m) electrolytes. Fig. 3 shows the
supercells and representative coordination environments of Li+ in dilute and
concentrated electrolytes. In the dilute electrolyte (Fig. 3b), Li+ is mainly solvated
by DEE molecules, though an FSI− anion is partially coordinated to Li+,

Fig. 2 Spectroscopic analysis on the liquid structures of LiFSI/DEE. (a) Raman spectra of
LiFSI/DEE at various LiFSI concentrations of 0.12, 0.60, 1.3, 2.0, 2.8, 4.7, 7.1, 8.5, 10 m. The
Raman peak in the range of 650–800 cm−1 is derived from the S–N–S vibration of FSI−.
The wavenumber resolution was 1 cm−1. (b) Raman peak positions of FSI− in LiFSI/DEE and
LiFSI/DME at various LiFSI concentrations. The Raman peak position is an indicator of how
extensively FSI− is paired with Li+. The data for LiFSI/DME are reproduced from ref. 24 as
a comparison. (c) Raman peak positions of FSI− plotted versus ELi.

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Fig. 3 DFT-MD simulations on the liquid structures of (a–c) dilute (0.85 m, LiFSI : DEE = 1 :
10 by mol) and (d–f) concentrated (4.2 m, LiFSI : DEE = 1 : 2 by mol) electrolytes. (a and d)
Supercells used. (b and e) Representative local coordination states of Li+. (c and f) Pair
distribution functions (g(r)) from Li+ and integrated coordination numbers (N(r)) to Li+.
Atom colors: Li, green; C, brown; H, light pink; O, red; N, blue; S, yellow; F, purple. Li atoms
are magnified in size.

suggesting that the coordination states are SSIPs and CIPs. On the other hand, in
the concentrated electrolyte (Fig. 3e), Li+ is more coordinated by multiple FSI−
anions, suggesting the presence of AGGs. To quantitatively discuss the local
coordination environment of Li+, we analyzed the pair distribution functions, g(r),
in the DFT-MD (Fig. 3c and f). The integrals of the pair distribution function,
shown as N(r), give the coordination number of each atom to Li+. We found that,
in the dilute electrolyte, Li+ is coordinated by four O atoms of DEE and less than
one O atom of FSI− on average (Fig. 3c). In contrast, the average Li+ environment
in the concentrated electrolyte is that of Li+ coordinated by three O atoms of DEE
and one O atom of FSI− (Fig. 3f). All these results are consistent with the Raman
spectroscopic analysis (Fig. 2a).
Next, we compared the ion-pair states in the LiFSI/DEE and LiFSI/DME electro-
lytes to discuss the effects of the Li+-solvation abilities. Fig. 2b and Table S2† show
the Raman peak positions of the S–N–S vibration of FSI−, which is an indicator of
how extensively Li+ is paired with FSI− to form SSIPs, CIPs, or AGGs.24,29 Both
electrolytes showed higher wavenumber shis of the FSI− vibration with increasing
salt concentrations, suggesting progressive formation of ion pairs. However, when
compared at similar concentrations, LiFSI/DEE showed more extensive ion pairing
(i.e., a higher wavenumber above a resolution error of 1 cm−1) than LiFSI/DME,
except at a high concentration region of around 8 m. This indicates that the ion
pairing is promoted in a weakly Li+-solvating solvent, which may be a reason for the
higher ELi values for LiFSI/DEE than LiFSI/DME at similar concentrations. To
demonstrate this, we made a plot of ELi vs. the Raman peak position of FSI− for
LiFSI/DEE and LiFSI/DME (Fig. 2c).24 The plots of LiFSI/DEE and LiFSI/DME are
overlapping, and both represent a linear correlation. This suggests that the ion-pair

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state of Li , which can be controlled by modifying the salt concentrations or Li+-
+

solvation abilities, subdues mLi+, and hence ELi, in the various electrolyte systems.

Li plating/stripping reaction
Having found the link between ELi and liquid structure, Li plating/stripping in
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LiFSI/DEE was studied in a Cu/Li half-cell conguration. The charge–discharge

voltage proles are presented in Fig. S3.† To accurately evaluate the CE of Li
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plating/stripping at various salt concentrations, we prepared three cells for each

concentration. Based on the charge–discharge proles, we evaluated the average
CE of Li plating/stripping (Fig. 4a, Tables S2 and S3†). Here we extracted the CE
values of the 2nd to 20th cycles for the average CE. The CE in the 1st cycle was
excluded from the average CE because it is mostly affected by the irreversible
capacity for SEI formation, which is not suitable for discussing the stability of Li
metal aer SEI formation. In both LiFSI/DEE and LiFSI/DME, the average CE
increased concomitantly with the increase in salt concentration (Fig. 4a). Such
trends have been widely reported in various electrolytes.24
Comparing LiFSI/DEE and LiFSI/DME, we found a signicant difference in CE
in the low-concentration region of <2 m. As shown in Fig. 4a, LiFSI/DEE showed

Fig. 4 Average CE of Li plating/stripping in LiFSI/DEE and LiFSI/DME plotted versus (a) LiFSI
concentrations and (b) ELi. The average CE was evaluated from the 2nd to 20th cycles in
three CujLi cells for each LiFSI/DEE electrolyte. The error bars are standard deviations. The
current density was 0.5 mA cm−2. Li plating on Cu was performed for 1 h, followed by Li
stripping up to 0.5 V. The data for LiFSI/DME, evaluated under a similar condition, are
reproduced from ref. 24 as a comparison. (c) Potential diagrams and liquid structures of
dilute (dissociated) and concentrated (ion-paired) electrolytes. The upward shift of ELi can
decrease the ELi–potential-window gap.

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a high CE of 93.8% at 1.3 m (corresponding to 1.0 M), while LiFSI/DME showed
a remarkably lower CE of 71.6% at a similar concentration of 1.2 m (corre-
sponding to 1.0 M). Such a difference was also reported previously, highlighting
the usefulness of weakly Li+-solvating solvents, but its mechanism was not clear.25
Inspired by our previous paper,24 here we focused on ELi as an inuential factor
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and replotted the average CE versus ELi in both electrolyte systems (Fig. 4b). We
found that the plots of LiFSI/DEE and LiFSI/DME are overlapping with each other;
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different electrolytes with similar ELi values result in similar average CE values. It
should also be noted that, even for weakly Li+-solvating DEE, further dilution from
1.3 m (1.0 M) to 0.12 m (0.1 M) LiFSI/DEE resulted in a much lowered CE of only
19.6%. This can also be accounted for by the ELi-CE correlation because ELi is
signicantly lower at 0.12 m (−3.44 V vs. Fc/Fc+) than at 1.3 m (−3.33 V vs. Fc/Fc+).
All the results suggest that ELi is one of the dominant factors of the CE for Li
plating/stripping.
Next, we discuss how ELi affects the Li metal CE or the stability of the Li metal/
electrolyte interface (Fig. 4c). ELi is usually far below the cathodic limit of the
potential window of organic electrolytes. However, when ELi is upshied by
forming extensive Li+–FSI− pairs in the electrolyte, the gap between ELi and the
potential window can be decreased.24 Since the ELi–potential-window gap corre-
sponds to the driving force for the reductive decomposition of the electrolyte (or
the reducing ability of Li), the decreased gap can prevent the unnecessary
reductive decomposition of the electrolyte, thus leading to a higher CE of Li
plating/stripping. This way, the Li loss is minimized in an electrolyte with high
ELi, which brilliantly paves the path for longer cycling life of Li metal batteries.
It is also worth noting that there is a change in trend in the ELi-CE correlation.
Below ELi = −3.33 V vs. Fc/Fc+, the CE is drastically increased with increasing ELi,
whereas above ELi = −3.33 V vs. Fc/Fc+, the CE is only gradually increased. As
a result, there should be at least two mechanisms that describe the ELi-CE
correlation. At present, however, it is an open question what the two mechanisms
are and why there is a trend change at the ELi value of −3.33 V vs. Fc/Fc+.

Electronic structure and SEI formation

Having established ELi as an inuential factor, we next discuss other factors,
namely (i) Li deposition morphology and (ii) SEI formation. The two factors have
been widely studied for various dilute and concentrated electrolytes, including the
specic cases of LiFSI/DEE and LiFSI/DME (both 1 M and 4 M).25 For (i), Li is
deposited in a rounded shape in both LiFSI/DEE and LiFSI/DME, and there is no
remarkable difference between low (1 M) and high (4 M) salt concentrations.25
Hence, (i) Li deposition morphology may not be a major factor that accounts for the
high CE in weakly solvating solvents as well as at high concentrations. As for (ii),
a widely accepted notion is that, in concentrated electrolytes, the Li-salt anion is
preferentially reduced over the solvent to generate an inorganic-rich SEI, which can
stabilize the Li metal/electrolyte interface to prevent unfavorable electrolyte
decomposition.9 We proposed this mechanism in 2014 based on the unique elec-
tronic structure at high concentrations, in which the lowest unoccupied molecular
orbital (LUMO) energy level of the Li-salt anion is shied downward to be more
susceptible to reduction.9,21 Hence, when discussing the concentration effect, we
need to consider the electronic structure and resulting SEI chemistry as well.

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For LiFSI/DEE, the SEI on Li has been reported in detail at low (1 M) and high
(4 M) concentrations.25 A unique feature of this specic electrolyte is that, even at
the low (1 M) concentration, the SEI is derived primarily from LiFSI, thus being
rich in inorganic species with Li, F, O, and S elements.25 As a result, there is no
remarkable difference in SEI chemistries in 1 M and 4 M LiFSI/DEE. Hence, the
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observed gradual increase in the CE (93.8% to 96.3%, Fig. 4a) from 1.3 m (1.0 M)
to 10 m (over 4 M) LiFSI/DEE cannot be accounted for by the SEI chemistries. The
increased CE may result from the upshi of ELi that can decrease the ELi–
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potential-window gap.
To understand this similar SEI formation process in dilute and concentrated
LiFSI/DEE, DFT-MD was applied to the dilute (LiFSI : DEE = 1 : 10 by mol, cor-
responding to 0.85 m) and concentrated (LiFSI : DEE = 1 : 2 by mol, corre-
sponding to 4.2 m) electrolytes. Fig. 5a and b shows the projected density of states
(PDOS) of the equilibrium trajectories. The curves in blue and red denote the
density of electronic states of LiFSI and DEE, respectively. To discuss the elec-
trolyte reduction, we focus on the lowest edge of the conduction bands (i.e.,
unoccupied orbitals), which corresponds to the LUMO and directs the nature of
the reduction reactions. Notably, the LUMO structures of the PDOS proles are
not remarkably different in dilute and concentrated LiFSI/DEE; in both electro-
lytes, the LUMO is mainly composed of FSI−. This means that under a reducing
atmosphere (e.g., on Li metal), FSI− receives an electron and thus is reduced to
form an FSI−-derived SEI. The similar LUMO structures at low and high
concentrations result from two factors. First, there is only a small difference in the
ion-pair states of FSI− (SSIPs and CIPs in both cases) owing to the weakly solvating
nature of DEE (Fig. 3). Second, the inherently high unoccupied orbital level of
ethers (i.e., high reduction stability) makes the unoccupied orbital of FSI− the
lowest energy level at any concentration. This accounts for the similar SEI
chemistries observed for dilute and concentrated LiFSI/DEE. This theoretical
study indicates that weakly solvating solvents are useful in promoting the pref-
erential reduction of FSI− for inorganic-rich SEI formation as well as in increasing
ELi, both of which are benecial to stabilizing the Li metal/electrolyte interface.

Fig. 5 Theoretical and experimental investigations on the FSI− reduction potential. PDOS
profiles of (a) dilute (0.85 m, LiFSI : DEE = 1 : 10 by mol) and (b) concentrated (4.2 m, LiFSI :
DEE = 1 : 2 by mol) electrolytes. (c) LSV curves of a Pt electrode in 1.3 m and 4.7 m LiFSI/
DEE at 1 mV s−1. The onset and peak potentials of the reduction reactions are indicated.
The onset potential was defined at the cathodic current flow of 0.25 mA.

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Another notable feature of the PDOS proles is that the lowest edge of the
unoccupied orbitals of FSI− is shied downward at high concentrations, which is
also observed in other concentrated electrolytes.9,21 This downward shi is
qualitatively explained by the coordination of Li+ (a strong Lewis acid) to FSI−,
which results in partial electron donation from FSI− to Li+. This electronic feature
indicates that the reduction potential of FSI− is upshied at higher concentra-
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tions, thus promoting the reduction of FSI− to generate an inorganic SEI. To

Open Access Article. Published on 16 April 2024. Downloaded on 7/15/2025 2:58:48 AM.

experimentally identify the reduction potential, linear sweep voltammetry (LSV)

was performed for a Pt electrode in 1.3 m (1.0 M) and 4.7 m (3.0 M) LiFSI/DEE
using a three-electrode cell with Li metal counter and reference electrodes.
Fig. 5c shows LSV curves, in which the potential is shown with reference to Fc/Fc+.
Here we focus on the reduction onset potential, which results from the reduction
of FSI− to form an inorganic SEI. We found that the reduction onset potential of
FSI− was −0.74 V vs. Fc/Fc+ in 1.3 m LiFSI/DEE but was upshied to −0.44 V vs. Fc/
Fc+ in 4.7 m LiFSI/DEE. This FSI− reduction potential (well over 2 V vs. Li/Li+)
seems to be quite high but it was also observed in FSI-based ionic liquids with
LiFSI salt.30,31 Otherwise, focusing on the reduction current peak, it was also
shied from −1.66 V to −1.48 V vs. Fc/Fc+. This upshi in the FSI− reduction
potential is consistent with the PDOS proles.

Discussion
An important attribute of concentrated electrolytes and weakly solvating electro-
lytes is the extensive ion pairing of Li+ and FSI−. This extensive ion pairing provides
two features to the concentrated electrolytes: (i) an upshi of ELi, resulting from
more unstable Li+ ions (i.e., high mLi+), and (ii) an upshi of FSI− reduction
potential, resulting from partial electron donation from FSI− to Li+. Feature (i)
weakens the reducing ability of Li metal, while feature (ii) promotes the formation
of an FSI−-derived inorganic SEI, both of which contribute to stabilizing the Li
metal/electrolyte interface and leading to higher Li plating/stripping CE. However,
there is a requirement to assess each contribution in a quantitative manner.
A thing to note is that the observed ELi-CE correlation may hold true in the
presence of a similar good SEI. In this study as well as the previous paper, we used
LiFSI in all electrolytes, in which LiFSI more or less contributes to the SEI formation
in various solvents and at various concentrations, thus highly stabilizing the Li
metal/electrolyte interface.24 On the other hand, when using LiPF6, although
a similar ELi-CE correlation was observed, the CE values were much lower than
those for LiFSI in the same solvents.24 This suggests that the SEI chemistry is
undoubtedly important to increase the CE. However, even in the FSI−-derived good
SEI, the interface is only kinetically stabilized outside the potential window. In this
situation, the upward shi of ELi plays a vital role in decreasing the ELi–potential-
window gap to weaken the driving force of electrolyte reduction.
To achieve 100% CE for Li plating/stripping, an ultimate goal is to enable
a greater upward shi of ELi inside the potential window of the electrolyte. This
situation causes no decomposition of Li salt or solvent, leading to SEI-free Li
metal electrodes. In this regard, a further question arises to the two upshis of ELi
and the FSI− reduction potential; which upshi is larger? To answer this ques-
tion, the overall potential diagram is presented in Fig. 6. Comparing 1.3 m (1.0 M)
and 4.7 m (3.0 M) LiFSI/DEE, ELi is upshied by +0.13 V, while the FSI− reduction

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Fig. 6 Potential diagram of dilute (1.3 m) and concentrated (4.7 m) LiFSI/DEE that high-
lights the upward shift of ELi and FSI− reduction potential.

potential is further upshied by +0.30 V for the onset and by +0.18 V for the peak.
This implies that we cannot shi ELi beyond the FSI− reduction potential; hence,
the reduction of FSI− is inevitable on Li metal. This difficulty lies essentially in the
fact that both (i) upshi of ELi and (ii) upshi of the FSI− reduction potential are
currently achieved by the same strategy of forming extensive ion pairs. To over-
come this situation, the upshi of ELi (i.e., higher mLi+) must be achieved inde-
pendently of ion pairing. This requires a full reconsideration of the design
concept of both anion and solvent but may be a promising step forward.

Conclusions
Using LiFSI/DEE as a model electrolyte, we studied the effects of salt concentra-
tion and solvation ability on the liquid structure, the shi of ELi, and the shi of
FSI− reduction potential, and discussed their contributions to stabilizing the Li
metal/electrolyte interface and increasing the Li plating/stripping CE. Generally,
increasing the salt concentrations or employing weakly Li+-solvating solvents
leads to more extensive ion pairing of Li+ and FSI− (i.e., from SSIPs to CIPs and
AGGs). This structural feature stabilizes the Li metal/electrolyte interface and thus
increases the Li plating/stripping CE in two ways. First, the extensive ion pairing
decreases the LUMO level of FSI− owing to the strong Lewis acidity of Li+, which
results in the upward shi of the FSI− reduction potential to promote inorganic
SEI formation. Second, increasing the extent of ion pairing can shi ELi upward
owing to the unstable Li+ (i.e., increased mLi+), which decreases the ELi–potential-
window gap and weakens the driving force of electrolyte reduction. As a result,
there is a clear correlation between ELi and CE in various solvents.
There are still many open questions as listed below.
(1) To what extent do the two factors (ELi and SEI) contribute to stabilizing the
Li metal/electrolyte interface?
(2) Why is the FSI−-derived SEI good? Is there any alternative for FSI−?
(3) How does the solvent reduction potential change at high concentrations?
(4) How is ELi or mLi+ theoretically described and linked to the liquid structure?
(5) Is it possible to independently shi ELi and the FSI− reduction potential
without controlling the ion-pair state?
(6) Can ELi be moved into the potential window to achieve an SEI-free Li metal
electrode?

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Experimental
Materials
LiFSI was provided by Nippon Shokubai. Ferrocene (Fc) and DEE were purchased
from Sigma Aldrich and Tokyo Chemical Industry, respectively. DEE was dried to
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a H2O content of 16 ppm (Karl Fischer titration) with activated molecular sieves.
Super dehydrated toluene (H2O content of 6 ppm) was purchased from Fujilm. All
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the electrolytes were prepared by adding LiFSI to the solvent in an argon-lled glove
box. For the evaluation of the Li electrode potential (ELi), 1 mM Fc was added to the
electrolyte.

Electrochemical measurement
All the electrochemical measurements were performed in an argon atmosphere. For
the ELi measurement, a three-electrode cell, with Pt as the working electrode and Li
metal as the counter and reference electrodes, was used. The temperature of the cell
was maintained at 25 °C for 2 hours using a thermostatic oven. The cell was then
subjected to cyclic voltammetry (CV) with a Celltest 1470E system from Solartron.
The redox potential of the Fc/Fc+ couple was measured with reference to the Li
reference electrode, and ELi of the various electrolytes was evaluated given that the
redox potential of Fc/Fc+ is constant.
Electrochemical Li plating/stripping tests were performed using CujLi coin cells
with various electrolytes (not containing Fc). The surface area of the Cu electrode
was 1.13 cm2. The coin-cell parts were purchased from Hohsen. Cu foil and Li foil
were purchased from Hohsen and Honjo Metal, respectively. A glass ber (GC-50,
Advantec) was used as the separator. The Li plating/stripping tests were conduct-
ed with a charge–discharge unit (TOSCAT-3100, Toyo System) at a constant current
density of 0.5 mA cm−2 for 1 h during Li plating on Cu and up to a cut-off voltage of
0.5 V during Li stripping. The average CE (2nd–20th cycle) was calculated, excluding
the rst cycle because it is mostly affected by the SEI-formation process.
The FSI− reduction potential was estimated using linear sweep voltammetry
(LSV) with a VMP3 system from Biologic, using a similar cell to that used for the
ELi measurement. A Pt plate was used as the working electrode and Li metal foil
was used as the counter and reference electrodes. Using the ELi value in each
electrolyte, the observed potential vs. Li/Li+ was converted to that vs. Fc/Fc+ to
show the LSV curves.

Material characterization
Raman spectroscopy was applied to understand the liquid structure of the elec-
trolytes with a laser excitation wavelength of 532 nm and a resolution of 0.8 cm−1
employing an NRS-5100 spectrometer from JASCO. The instrument was calibrated
with a standard Si peak with a wavelength value of 520 cm−1. The electrolytes were
properly sealed in quartz cells in an argon-lled glove box and were excited using
the 532 nm laser.

Computational details
DFT-MD simulations were performed using the CP2K code.32 DZVP-MOLOPT-SR-
GTH-type mixed Gaussian and plane-wave basis sets were used where the cutoff

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Faraday Discussions Paper
33
energy of the plane wave was chosen as 400 Ry. The PBE functional with a D3-
type semi-empirical van der Waals correction34 and GTH norm-conserving pseu-
dopotentials35 were employed. DFT-MD simulations were performed in the NVT
ensemble with a time step of 1 fs using a Nosé–Hoover chain thermostat36–38 with
a chain length of three. DEE and LiFSI molecules were randomly distributed in
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the cubic unit cell whose lattice constants were determined by the experimental
densities of the electrolytes. The number of molecules and atoms are summarized
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in Table S4.† Firstly, we roughly optimized the atomic positions using the Hell-
man–Feynman's force. Secondly, the liquid structures were annealed by the DFT-
MD simulations for 30 ps at 450 K. Using the annealed structures, we performed
DFT-MD simulation for 30 ps at 300 K. The trajectories of the last 20 ps were used
for the calculations of the pair distribution functions. Structures obtained at every
1 ps time step from 10 ps to 30 ps were chosen to calculate the projected density of
states (PDOS).

Author contributions
Y. Y. proposed and supervised the project. A. P., S. N., Y. Kondo, Y. Katayama, and
Y. Y. designed the experiments. A. P. and S. N. conducted the experiments. T. K.
and K. S. designed and conducted the theoretical calculations. All authors
contributed to the discussion. A. P., Y. Kondo, and Y. Y. wrote the manuscript.

Conflicts of interest
There are no conicts to declare.

Acknowledgements
This work was partially supported by JSPS KAKENHI Grant-in-Aid for Trans-
formative Research Areas (B) (23B207, 23H03824, 23H03827) and JST-GteX
(JPMJGX23S3).

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