Graphene‐Based Sulfur Cathodes and Dual Salt‐Based Sparingly Solvating Electrolytes: A Perfect Marriage for High Performing, Safe, and Long Cycle Life Lithium‐Sulfur Prototype Batteries

Abstract

The growing requirements for electrified applications entail exploring alternative battery systems. Lithium‐sulfur batteries (LSBs) have emerged as a promising, cost‐effective, and sustainable solution; however, their practical commercialization is impeded by several intrinsic challenges. With the aim of surpassing these challenges, the implementation of a holistic LSB concept is proposed. To this end, the effectiveness of coupling a high‐performing 2D graphene‐based sulfur cathode with a well‐suited sparingly solvating electrolyte (SSE) is reported. The incorporation of bis(fluorosulfonyl)imide (LiFSI) salt to tune sulfolane and 1,1,2,2‐tetrafluoroethyl‐2,2,3,3‐tetrafluoropropylether based SSE enables the formation of a robust and compact lithium fluoride‐rich solid electrolyte interphase. Consequently, the lithium compatibility is improved, achieving a high Coulombic efficiency (CE) of 98.8% in the Li||Cu cells and enabling thin and dense lithium depositions. When combined with a high‐performing 2D graphene‐based sulfur cathode, a symbiotic effect is shown, leading to high discharge capacities, remarkable rate capability (2.5 mAh cm⁻² at C/2), enhanced cell stability, and wide temperature applicability. Furthermore, the scalability of this strategy is successfully demonstrated by assembling high‐performing monolayer prototype cells with a total capacity of 93 mAh, notable capacity retention of 70% after 100 cycles, and a high average CE of 99%.

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RESEARCH ARTICLE
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Graphene-Based Sulfur Cathodes and Dual Salt-Based
Sparingly Solvating Electrolytes: A Perfect Marriage for High
Performing, Safe, and Long Cycle Life Lithium-Sulfur
Prototype Batteries
Julen Castillo, Asier Soria-Fernández, Sergio Rodriguez-Peña, Jokin Rikarte,
Adrián Robles-Fernández, Itziar Aldalur, Rosalía Cid, Jose Antonio González-Marcos,
Javier Carrasco, Michel Armand, Alexander Santiago,* and Daniel Carriazo
The growing requirements for electrified applications entail
exploring alternative battery systems. Lithium-sulfur batteries
(LSBs) have emerged as a promising, cost-effective, and sustainable
solution; however, their practical commercialization is impeded
by several intrinsic challenges. With the aim of surpassing these challenges,
the implementation of a holistic LSB concept is proposed. To this end,
the effectiveness of coupling a high-performing 2D graphene-based sulfur
cathode with a well-suited sparingly solvating electrolyte (SSE) is reported.
The incorporation of bis(fluorosulfonyl)imide (LiFSI) salt to tune sulfolane
and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropylether based SSE enables
the formation of a robust and compact lithium fluoride-rich solid electrolyte
interphase. Consequently, the lithium compatibility is improved, achieving
a high Coulombic efficiency (CE) of 98.8% in the Li||Cu cells and enabling
thin and dense lithium depositions. When combined with a high-performing
2D graphene-based sulfur cathode, a symbiotic effect is shown, leading to
high discharge capacities, remarkable rate capability (2.5 mAh cm−2at C/2),
enhanced cell stability, and wide temperature applicability. Furthermore, the
scalability of this strategy is successfully demonstrated by assembling high-
performing monolayer prototype cells with a total capacity of 93 mAh, notable
capacity retention of 70% after 100 cycles, and a high average CE of 99%.
J. Castillo, A. Soria-Fernández, S. Rodriguez-Peña, J. Rikarte,
A. Robles-Fernández, I. Aldalur, R. Cid, J. Carrasco, M. Armand,
A. Santiago, D. Carriazo
Centre for Cooperative Research on Alternative Energies (CIC
energiGUNE)
Basque Research and Technology Alliance (BRTA)
Vitoria-Gasteiz 01510, Spain
E-mail: asantiago@cicenergigune.com
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/aenm.202302378
© 2023 The Authors. Advanced Energy Materials published by
Wiley-VCH GmbH. This is an open access article under the terms of the
Creative Commons Attribution-NonCommercial-NoDerivs License,
which permits use and distribution in any medium, provided the original
work is properly cited, the use is non-commercial and no modifications
or adaptations are made.
DOI: 10.1002/aenm.202302378
1. Introduction
The increasing demand for new electri-
fied applications underscores the crucial
importance of the development of new
energy storage technologies.[1,2]So far,
this demand has been met by lithium-
ion batteries (LIBs), which have per-
vaded the rechargeable battery market
since their commercialization in the
1990s.[3,4]However, the growing energy
density requirements for some appli-
cations such as transportation, exceed
the capabilities of current commercial
LIBs.[5–7]This, together with the con-
cerns related to the supply chain, high
cost, and sustainability issues associated
with many materials used in conven-
tional LIBs like nickel or cobalt, fur-
ther emphasizes the need for the devel-
opment of new sustainable and poten-
tially cost-effective rechargeable battery
technologies.[8,9]In this context, lithium-
sulfur batteries (LSBs) are emerging as a
promising and viable alternative among
J. Castillo, S. Rodriguez-Peña, J. A. González-Marcos
University of the Basque Country (UPV/EHU)
Barrio Sarriena, s/n, Leioa 48940, Spain
J. Carrasco, D. Carriazo
IKERBASQUE, Basque Foundation for Science
Plaza Euskadi 5, Bilbao 48009, Spain
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the different next-generation battery technologies owing to the
excellent properties that their operating chemistry offers.[10]
Apart from the low cost, wide availability, and environmen-
tal friendliness of sulfur as active material, its outstandingly
high theoretical specific energy (up to 2600 Wh kg−1) makes
LSBs an ideal technology for weight-critical applications, such as
heavy automotive vehicles, aviation, high-altitude pseudo satel-
lites (HAPS), high-altitude long endurance (HALE) vehicles or
electric vertical take-off and landing (eVTOL) aircraft.[9,11–13]
Despite its potential benefits, the development of LSB
technology has encountered significant challenges since its
discovery in the 1960s, posing obstacles to its successful
commercialization.[14–16]Some of the main issues to be ad-
dressed include the insulating nature of both the initial prod-
uct (elemental sulfur, S8) and the end-products (Li2S2and Li2S),
the dissolution of the intermediate components (lithium polysul-
fides, LiPS) into the electrolyte that causes the loss of active mate-
rial and the poisoning of the lithium anode (commonly referred
as “shuttle effect”), and the substantial volume expansion experi-
enced by the positive electrode after complete lithiation of sulfur
to Li2S (up to 80%).[17–21]
Over the past decades, a tremendous research effort has been
carried out to address these challenges. For instance, academia
has put special interest in the screening and development of the
carbonaceous material used for the design of the sulfur cath-
ode, since it is of paramount importance to mitigate the above-
mentioned issues.[22–25]In this line, our group has reported the
outstanding sulfur utilization and partial retention of polysul-
fides provided by 2D graphene-based activated carbons, used
both as a carbon to maximize the performance of LSBs even at
prototype scale.[15,26]
Nevertheless, despite the multiple benefits of these carbon
materials, using them as the only improvement strategy is
not enough to solve other important operational problems that
emerge during LSB cycling, such as the degradation of the
lithium metal anode (LMA) and electrolyte depletion.[9,27]These
issues are usually masked under laboratory conditions, as the ac-
tive material loading and/or the electrolyte amount are not opti-
mized. However, when attempting to replicate industry-specific
conditions, such issues emerge, especially during the upscaling
process to prototype cells.[12,14,26–29]Regrettably, the prevailing
strategy has been to tackle the immediate problems using a sin-
gular approach, disregarding the broader spectrum of challenges
that demand resolution. In other words, rather than adopting the
industry’s imperative for a holistic research effort, the focus has
remained narrow and incomplete.
On the other hand, through the electrolyte engineering strat-
egy, great strides are currently being made in modifying and de-
veloping new electrolyte systems.[30–32]This essential component
can be envisaged as the lifeblood of the battery, playing a key role
in the correct operation and ensuring the long-term stability of
a battery due to its interaction with the LMA. Ether-based elec-
trolyte comprising lithium bis(trifluoromethanesulfonyl)imide
(LiTFSI) salt dissolved in a solvent mixture based on 1,2-
dimethoxyethane (DME) and 1,3-dioxolane (DOL) in an equal
volumetric ratio, and lithium nitrate (LiNO3) as solid electrolyte
interphase (SEI) promotor additive, has been widely employed
as the conventional electrolyte for LSBs.[32–34]Despite the good
properties in terms of ionic conductivity and high sulfur utiliza-
tion, it proved to be unable to guarantee long-term cycling, partic-
ularly under practical operating conditions.[15,27,34]Their use re-
sults in a battery system based on the dissolution-precipitation
mechanism, which leads to the dissolution of a significant
amount of LiPS, thereby facilitating their migration to the Li
anode and ultimately culminating in the aforementioned “shut-
tle effect”. This unwanted process aggravates the stability is-
sues of the electrolyte accelerating the LiNO3depletion by its
interaction with the dissolved LiPS. In summary, decreasing
the amount of electrolyte in this system is a real challenge, as
enough quantity is required to ensure the smooth occurrence of
dissolution/precipitation-based reaction pathways.[35,36]
In light of the operational issues of the conventional elec-
trolyte, in recent years sparingly solvating electrolytes (SSEs),
also known as localized or diluted high concentrated electrolytes
(LHCEs or DHCEs, respectively), have recently emerged as an
appealing alternative for their practical application in LSBs.[37–39]
These types of electrolytes not only reduce or inhibit the disso-
lution of LiPS by shifting the battery operation to a quasi-solid
state reaction mechanism system but also possess a unique sol-
vation structure and remarkable physicochemical properties that
enable the correct regulation of lithium deposition.[38]SSEs are
formed according to the highly concentrated electrolytes (HCEs)
concept, in which the increased amount of salt results in the re-
duction of free solvent molecules in the electrolyte and greatly
improves the stability against the LMA.[40,41]Nevertheless, the ap-
plication of HCE in batteries is hampered due to their high viscos-
ity, low ionic conductivity, and high cost associated with the high
amount of lithium salt employed.[42,43]In this sense, the incorpo-
ration of hydrofluoroethers (HFEs) as diluents, which are misci-
ble with the electrolyte solvents but do not dissolve lithium salt
due to their low donor number, overcome these limitations while
maintaining the excellent properties of HCEs.[44]As has been
widely reported in lithium-metal batteries (LMBs), SSEs demon-
strate remarkable compatibility with LMA. This is due to their
distinguished solvation structure, which promotes the formation
of a protective SEI layer that is primarily created from the de-
composition products of lithium salt anions rather than organic
solvents, owing to the higher presence of contact ion pairs (CIPs)
and cation-anion aggregates (AGGs).[45–47]This change results in
the formation of an inorganic-rich SEI layer that effectively pre-
vents parasitic reactions between the electrolyte and the LMA.
Therefore, the proper selection of the lithium salt anion is criti-
cal. As a result, LMBs have leveraged the unique solvation prop-
erties of these electrolytes by employing bis(fluorosulfonyl)imide
(LiFSI) as main salt, which leads to the formation of a lithium flu-
oride (LiF)-rich SEI protective layer,[6,48,49]or by utilizing LiNO3
to create an N-rich SEI layer.[50,51]The approach of the appro-
priate selection of lithium salt has led to substantial enhance-
ment in battery stability, even under realistic operating condi-
tions. However, in the case of LSBs, due to the inherent limita-
tions of a technology based on conversion reactions, the poten-
tial impact of lithium salt on SEI formation has not been fully
exploited and LiTFSI has commonly been used as the unique
lithium salt. Despite the good properties in terms of thermal
and chemical stability and high ionic conductivity that LiTFSI
presents, due to its highly stable anion molecule, the protective
layer formed is usually less functional than in the case of LiFSI or
LiNO3.
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In this study, we aim to address the current challenges and
boost the commercialization of LSBs by implementing a compre-
hensive battery design approach. Our proposal involves integrat-
ing multiple strategies based on the holistic battery concept, en-
abling us to effectively overcome existing obstacles. To this end,
we propose the combination of sulfur cathodes constructed us-
ing a graphene-based activated carbon, which has been devel-
oped by our research group, with the utilization of SSEs, fur-
ther enhancing the battery’s performance. Two different SSEs
were developed during the study, both based on a combination
of sulfolane (SL) as solvent and 1,1,2,2-tetrafluoroethyl-2,2,3,3-
tetrafluoropropylether (TTE) as co-solvent. The first electrolyte,
named SSE_REF, was completed with LiTFSI as the unique
lithium salt while the second electrolyte (SSE_LiFSI) studies the
combination of LiTFSI and LiFSI considering that the latter salt
can act as LiF-rich SEI layer precursor. The proposed SSEs are
thoroughly characterized in terms of their physicochemical and
electrochemical properties. In addition, classical molecular dy-
namics (MD) simulations were performed to analyze the sol-
vation structure of these electrolytes. The flammability test was
evaluated as a key criterion for assessing the safety of the battery
system. Subsequently, the compatibility of these electrolytes with
the lithium anode has been extensively evaluated through various
tests such as Li/Cu cells to determine their Coulombic efficiency
(CE), galvanostatic cycling of Li||Li symmetric cells, and critical
current density (CCD) test. The electrochemical properties of the
electrolytes are further characterized in practical Li-S coin cells.
Finally, the feasibility of scaling the holistic approach is success-
fully demonstrated through the fabrication of 20 cm2pouch cells.
2. Results and Discussion
As previously mentioned, this study focuses on the implemen-
tation of a holistic approach to LSBs, achieved through the com-
bination of a previously reported high-performing 2D graphene-
based sulfur cathode[26]and a newly developed SSE that was fine-
tuned during the course of this work, and which will be referred
to as SSE_LiFSI for the remainder of this discussion. To evaluate
its performance, this new SSE was compared against both con-
ventional DME/DOL electrolyte and a reference SSE electrolyte
(SSE_REF). Figure 1acompiles the ionic conductivities of the
tested electrolytes at room temperature (i.e., 23 ±2°C). The con-
ventional DME/DOL electrolyte exhibits the highest ionic con-
ductivity of 6.47 mS cm−1, which is approximately three times
higher than that of the prepared SSEs. This difference can pri-
marily be attributed to the lower viscosity of the DME/DOL elec-
trolyte as well as the lower concentration of solvated salt by
the solvent molecules. Nevertheless, the two SSEs present suf-
ficiently high ionic conductivities of 1.21 and 2.08 mS cm−1for
SSE_REF and SSE_LiFSI, respectively, enough to ensure reliable
battery operation. It is worth noting a slight difference between
SSE_LiFSI and SSE_REF, which can be attributable to the in-
creased mobility of the smaller LiFSI molecules. Furthermore,
a wettability study conducted on the Celgard 2500 separator con-
firms the higher viscosity of SSEs. The conventional electrolyte,
formed by low viscosity and high volatility solvents, presents an
excellent wettability of the separator (contact angle of 31°)at-
tributed to its lower viscosity (Figure 1b–d). In contrast, both
SSEs exhibited lower wetting properties of the separator (67°and
54°for SSE_REF and SSE_LiFSI, respectively), primarily due to
the higher viscosity of SL employed for their preparation, as cor-
roborated by the viscosity measurements shown in Figure S1
(Supporting Information).
MD simulations were performed to gain atomistic-level in-
sights into the solvation structure of the prepared electrolytes
and the effect of the co-solvent incorporation on the microscopic
structure of the Li salt/SL mixtures. The snapshots illustrating
the chemical structure of the different electrolytes after reach-
ing thermodynamic equilibrium are presented in Figures 1e–g
and Figure S2 (Supporting Information). To compare the solva-
tion structures, radial distribution functions (g(r), represented by
solid lines) and coordination numbers (n(r), indicated by dash-
point lines) were calculated, as observed in Figures 1f,h.The
DME/DOL electrolyte predominantly exhibits a higher fraction
of solvent-separated ion pairs (SSIPs) structures, indicated by the
lower coordination number value of Li-OTFSI, where the coordi-
nation of Li+is mainly governed by DME and NO3−.Thesharp
radial distribution function peak observed for NO3−, compared
with the relatively low peak of DME, underscores that while NO3−
is consistently coordinated with the Li+, there are many DME
molecules not coordinated with it as illustrated in Figure 1e.In
Figure S3a (Supporting Information), an illustrative representa-
tion of Li+coordination is shown, highlighting the role of NO3−
as a connecting bridge between various Li+ions, forming the
aggregations depicted in Figure 1e. Conversely, in both SSEs, a
distinct Li-OSL peak at ≈2 Å, together with Li-OTFSI and Li-OFSI
peaks, are identified, indicating that the lithium salts (LiTFSI for
SSE_REF and both LiTFSI and LiFSI in the case of SSE_LiFSI)
are surrounded by the SL solvent molecules within the first coor-
dination shell. Examples of these can be found in Figures S3b,c
(Supporting Information). Similarly to NO3−in DME_DOL elec-
trolyte, TFSI, and FSI serve to connect different Li+and form
the aggregates observed in Figure 1g and Figure S2a (Supporting
Information). In contrast, the TTE molecules display negligible
coordination with Li+. Therefore, this clearly demonstrated that
both prepared SSEs exhibit localized concentrations of Li salt/SL
pairs. These are surrounded by TTE molecules that primarily act
as diluents without being involved in the solvation structures of
Li+.
The solvation structures of the developed SSEs were confirmed
by Raman spectroscopy. Figure 1i displays the Raman spectra
SO2scissoring vibration of sulfolane within the corresponding
spectra range. In pure sulfolane, this peak appears at 568 cm−1
and undergoes a shift toward higher wavenumber upon com-
plexation with Li+, as observed by increasing salt concentration
in the electrolyte, reflecting the reduction of the free SL solvent
molecules. Interestingly, this displacement is independent of the
type of lithium salt used in the electrolyte, being this trend simi-
larly observed in both developed electrolytes.
Additionally, it is worth focusing on the spectral region be-
tween 740 and 750 cm−1, which is associated with the CF3bend-
ing coupled with the S−N stretching vibration of the TFSI an-
ions. In the same region but at slightly lower Raman shifts (730–
740 cm−1), the vibrations related to the stretching of S─Nmoi-
eties of the FSI anions are found. These particular vibrational
modes are known for their sensitivity to variations in Li-ion co-
ordination (Figure 1j). When the compound is uncoordinated, as
in the case of low-concentration electrolytes, the Raman peak in
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Figure 1. Individual properties and electrolyte solvation structures: a) Ionic conductivity of the studied electrolytes measured at room temperature
(23 ±2°C). Wettability test on Celgard separator of b) DME/DOL, c) SSE_REF, and d) SSE_LiFSI. MD simulation snapshots and radial distribution
function of e) and f) DME/DOL and g) and h) SSE_LiFSI. Raman spectra of different configurations of electrolytes for varied i) sulfolane and j) LiTFSI
salt ranges.
this region shifts to shorter wavelengths, especially ≈740 cm−1.
However, as the salt concentration in the system increases, the
formation of CIPs or AGGs occurs appearing a new peak in the
spectra at ≈747 cm−1(see deconvoluted spectra of concentrated
electrolytes in Figure S4, Supporting Information), leading to a
shift of the band toward longer wavelengths. This shift from 740
to 742 cm−1of the main peak of LiTFSI could indicate the pos-
sible monodentate coordination between TFSI−and Li+. In the
case of the electrolytes containing FSI anion in their composition,
a new peak at 734 cm−1can be found.[52]The observed band shifts
in the analyzed spectra suggest that the developed electrolytes in
this work present the formation of CIPs and AGGs by the anions
of the salts.
Liquid electrolytes have long been criticized for their high
flammability, which would eventually lead to well-known safety
incidents. Consequently, ensuring fire safety is of paramount im-
portance in the development of secure battery systems, which are
closely intertwined with the combustion characteristics and ther-
mal stability of the employed electrolytes. To evaluate the safety
level of the developed electrolytes, a flammability test was con-
ducted. Figure 2a–c displays the digital photos captured during
the flammability test, where the commercial Celgard separator
was soaked in the respective electrolytes. Remarkably, the con-
ventional DME/DOL electrolyte presents highly flammable be-
havior, evident from its easy ignition and violent combustion, re-
sulting in complete combustion of the Celgard within just 1 s.
In contrast, neither of the two SSEs show any signs of catching
fire, evidence of their exceptional resistance to combustion. The
Celgard separator only experiences slight melting in some areas,
likely due to the higher temperature of the lighter flame exceed-
ing the melting point of the material. Additionally, as shown in
Figure S5 (Supporting Information), the flammability test of the
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Figure 2. Flammability test of the Celgard separator soaked in different electrolytes. The digital photos of the burning process of a) DME/DOL, b)
SSE_REF, and c) SSE_LiFSI.
electrolytes was conducted to quantify their flame-retardant rate
through the self-extinguishing time (SET) values. Once again,
the low thermal stability and susceptibility to combustion of the
conventional DME/DOL electrolyte are evidenced by its vigorous
flame, resulting in a SET value of 117 s g−1. Indeed, both SSEs
continue to demonstrate their non-flammable nature, reflecting
a negligible SET value. Hence, the replacement of the flammable
conventional electrolyte with the prepared SSEs will significantly
enhance the fire safety properties of the battery system.
In addition to the intrinsic properties, the compatibility of the
electrolyte with LMA is a key requirement for ensuring long-term
LMB cycling. Hence, a deep study of the reversibility and stabil-
ity of the LMA during the plating and stripping process in the
developed electrolytes was assessed using different Li||Cu and
Li||Li cell tests represented in Figures 3–5. First, the lithium plat-
ing and stripping cycling in Li||Cu cells at 0.5 mA cm−2was per-
formed in order to evaluate the CE obtained through this study
(Figure S6, Supporting Information). Figure 3a demonstrates the
excellent stability of the SSE_LiFSI electrolyte, evidenced by a
plating/stripping average CE value of 97% during the analyzed
cycles. Notably, there is a significant stability contrast when com-
pared to the SSE_REF electrolyte, which fails to yield an accept-
able CE value (<80%) due to different side reactions. In addi-
tion, as depicted in Figure 3b and Figures S7 and S8a (Support-
ing Information), SSE_LiFSI presents lower polarization voltage
compared to SSE_REF, without showing any cycling failure sign.
Therefore, the substantial stability improvement resulting from
the introduction of the LiFSI salt into the system for the SSE
preparation is highlighted. On the other hand, as expected, the
conventional DME/DOL electrolyte displays high CE values dur-
ing cycling and low polarization voltage (Figure S8b, Support-
ing Information), mainly attributed to the protective and denser
lithium deposition properties provided by the LiNO3additive.
As a complementary study to determine the CE of LMA in
Li||Cu cells, the modified Aurbach method was performed.[53]
This method differs from the previous one as it incorporates a
preconditioning step of the Cu substrate (see Experimental Sec-
tion). As can be observed in Figure S9 (Supporting Information),
the SSE_REF shows again the worst compatibility against lithium
with a low CE of 58.1%. This incompatibility is solved by the addi-
tion of LiFSI in SSE_LiFSI electrolyte, as presented in Figure 3c
where the results of the DME/DOL and SSE_LiFSI electrolytes
are displayed, demonstrating the suitable properties of this salt
in generating a robust SEI layer, resulting in a CE of 98.8%.
Notably, this CE value is even higher than that calculated for
the conventional DME/DOL-based electrolyte (95.2%). Regard-
ing the plating/stripping profiles, despite exhibiting a slightly
lower CE, the conventional electrolyte displays a lower overpo-
tential than SSE_LiFSI (Figure 3c). This can be attributed to the
higher ionic conductivity of the electrolyte and the formation of
an N-rich SEI protective layer facilitated by LiNO3in contrast
to the insulator LiF-rich SEI layer formed in the SSE_LiFSI sys-
tem. The validity of the aforementioned description is supported
by Figure 3d and Figure S10 (Supporting Information), which
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Figure 3. Electrochemical compatibility with LMA of the studied electrolytes. a) Coulombic efficiency of the cycling performance of Li||Cu cells employing
different electrolytes. b) Polarization of SSE_LiFSI during the Li plating and stripping in Li||Cu cells. c) Li metal CE of the different electrolytes using the
modified Aurbach method.[52]d) CE and nucleation overpotential of the first lithium deposition.
confirm the high compatibility of the SSE_LiFSI electrolyte with
lithium. The compatibility is found to be comparable to that of
the conventional electrolyte. The CE is nearly identical for both
DME_DOL and SSE_LiFSI electrolytes, although the former ex-
hibits lower nucleation overpotential, improving in both cases the
performance obtained with SSE_REF.
Continuing the study of the compatibility of the different elec-
trolytes with LMA, surface and cross-section morphologies of the
first lithium deposition on Cu were examined by scanning elec-
tron microscopy (SEM) analysis (Figure 4a; Figure S11, Support-
ing Information). Conventional DME/DOL electrolyte exhibits a
compact lithium deposition film thickness of 50 μmwithlarge
particle size (Figure S11a, Supporting Information), evidencing
good compatibility between the lithium and the electrolyte in
the first Li deposition. In contrast, in the case of the SSE_REF
electrolyte, the deposited lithium film presents a porous and
poorly dense shape with a loosely packed structure with an aver-
age thickness of ≈80 μm(FigureS11b, Supporting Information).
This morphology further highlights the unsuitable compatibility
of this electrolyte with Li. Interestingly, SSE_LiFSI yields a larger
particle size as well as a tightly denser and smoother lithium de-
position, leading to a compact and thin film of ≈25 μm(Figure
S11c, Supporting Information). This dense lithium deposition
reduces the available surface area for interacting with the elec-
trolyte, resulting in higher CE values as previously demonstrated.
It is worth noting that, even after 30 cycles, there is only a slight
increase in surface porosity, and the SSE_LiFSI still maintains
large and dense lithium depositions (as shown in Figure S12,
Supporting Information), in contrast to the conventional elec-
trolyte where more fibrillar and whisker-like structures begin to
emerge upon cycling.
XPS analyses were conducted to study the surface chemistry of
the lithium deposition using different electrolytes. The wide-scan
XPS spectra of the Li deposition surface on the Cu substrate with
different electrolytes can be found in Figure S13 (Supporting In-
formation), and their corresponding atomic content of different
elements is presented in Table S1 (Supporting Information). The
surface of Li0deposits with SSE_LiFSI presents a higher propor-
tion of LiF compound than when SSE_REF and DME/DOL are
employed, ascribed to the stronger interaction of LiFSI with Li0
due to the weaker S─F bond (Figure 4b). The XPS results, to-
gether with the SEM images, demonstrate that the addition of
a certain amount of LiFSI enables the formation of a more com-
pact and denser lithium deposition with an interface that is richer
in LiF (peak at 685 eV in F 1s core level). Moreover, it is worth
noting that a qualitative and quantitative difference in the O con-
tent within the SEI layer is observed between SSE_REF and the
other electrolytes. On the one hand, SSE_REF produces a SEI
with lower O content (almost 24%) compared to DME/DOL and
SSE_LiFSI (≈29% and 31%, respectively, see Figure S13 (Sup-
porting Information)). On the other hand, the additional O con-
tent in DME/DOL and SSE_LiFSI electrolytes seems to be of a
different nature, as suggested by the distinctive shape of the O
1s spectra (Figure S14a, Supporting Information). Notably, the
visible peak at 528.6 eV when using DME/DOL and particularly
SSE_LiFSI electrolytes indicates the presence of Li2O in the SEI,
whereas only a small amount is distinguished when SSE_REF is
employed. Alongside the increase in Li2O, the peak at ≈531.5 eV
(mainly comprising carbonates and hydroxides) shifts to slightly
lower binding energies and becomes the dominant component.
Furthermore, analysis of the Li 1s core level also reveals a quali-
tative difference between the case of SSE_REF and the other two
electrolytes. The deconvolution of Li 1s (Figure S14b, Supporting
Information) spectra demonstrates that the formation of LiOH
(component at 54.7 eV) is responsible for the increase and shift
of the peak at 531.5 eV when DME/DOL and SSE_LiFSI elec-
trolytes are used. Hence, the formation of Li2O and LiOH seems
to contribute to the development of a more benign interface when
employing SSE_LiFSI and DME/DOL electrolytes. Finally, the
appearance of a distinct peak corresponding to metallic lithium
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Figure 4. The morphologies, corresponding XPS surface analyses, and design principle of the first Li deposition behavior. a) SEM images of the lithium
deposition on Cu foil with three different electrolytes: DME/DOL, SSE_REF, and SSE_LiFSI electrolytes. b) High-resolution XPS spectra of F 1s of the
lithium deposition on Cu substrate with different electrolytes. c) Schematic illustration of the electrolyte structure and the correspondingly formed SEI
in DME/DOL, SSE_REF, and SSE_LiFSI electrolytes (where SSIP, CIP, and AGG refer to solvent-separated ion pair, contact ion pair and cation-anion
aggregates structure abbreviation, respectively).
(signal at 51.8 eV in Li 1s) with SSE_REF and DME/DOL elec-
trolytes may indicate a non-compact, inhomogeneous, or ex-
tremely thin SEI in those cases (thin enough in some parts to
allow collecting photoelectrons from the buried lithium metal
platted), while the dense and compact lithium deposits with
SSE_LIFSI are covered by an also compact and protective SEI.
This compact and chemically more benign interface could ex-
plain the good compatibility of the developed electrolyte with
lithium metal explained previously. Based on the results ob-
tained in SEM and XPS analyses, schematic depictions of the SEI
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Figure 5. Electrochemical performance of different electrolytes in Li||Li cells. a) current test of Li||Li cells using DME/DOL and SSE_LiFSI electrolytes at
different current densities from 0.1 to 1 mA cm−2and their corresponding zoomed profiles. The critical current density of Li||Li cells using the different
electrolytes: b) DME/DOL, c) SSE_REF, and d) SSE_LiFSI.
layer formation depending on the employed electrolyte after the
lithium plating are presented in Figure 4c. The addition of LiFSI
as an additive in the SSEs enhances the quality of the formed
SEI compared to the SSEs based solely on LiTFSI. As demon-
strated, this improvement is attributed to the higher decomposi-
tion of LiFSI, resulting in a greater proportion of LiF within the
SEI layer. Additionally, the morphology and compactness of the
lithium deposition are also influenced by the presence of LiFSI
in the SSE.
To complete the electrochemical characterization of the dif-
ferent electrolytes, their stability at different current densities
from 0.1 to 1 mA cm−2was tested in a Li||Li symmetric cell
(Figure 5a). This test serves as additional proof of the instabil-
ity of the SSE_REF electrolyte, as previously observed. This un-
satisfactory performance can be primarily attributed to irregular
and fibrillar lithium deposition. In contrast, both SSE_LiFSI and
DME/DOL electrolytes exhibit a flat and stable cycling behavior
with minimal polarization across all current steps. This further
endorses the influence of the denser and more homogeneous
lithium deposition evidenced in these two electrolytes. However,
it should be noted that DME/DOL electrolyte does exhibit some
erratic cycles, particularly at high current densities. This behav-
ior may be attributed to the increasing deposition porosity men-
tioned earlier. Noteworthy, when examining the voltage profiles
in detail, SSE_LiFSI consistently displays lower overpotential
throughout all current steps. This can be ascribed to its compact
and thinner deposition film behavior, as discussed previously.
As a complementary study to the plating/stripping analysis,
the critical current density (CCD) of the studied electrolytes was
evaluated. This parameter holds significant importance in the
application of electrolytes in LSBs, as it provides insights into
the rate-determining steps of lithium metal kinetics and ensures
the compatibility of the lithium metal interface with specific cur-
rent densities and capacities. Therefore, it is essential to deter-
mine the suitability of the electrolyte for high-power applications.
As shown in Figure 5b–d, the previously observed trend persists.
The SSE_LiFSI electrolyte, with its robust and compact deposi-
tion characteristic, exhibits a higher resistance to current den-
sity, even up to 4 mA cm−2, surpassing the performance of all
other studied electrolytes. At this current density of 4 mA cm−2,
some inhomogeneous deposition of Li starts to appear and, as
a consequence, the shape of the plateaus turns irregular. Con-
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Figure 6. Electrochemical performances of Li-S coin cells. a) Cycling performance of high sulfur loading LSBs using different electrolytes. b) Rate capa-
bilities of Li-S cells using DME/DOL and SSE_LiFSI electrolytes. c) Influence of cut-off voltage on the cycling performance of high sulfur loading LSBs
using SSE_LiFSI electrolyte. d) Cycling behavior of LSBs with SSE_LiFSI electrolyte at high temperature (60 °C).
versely, the DME/DOL electrolyte shows lower stability, experi-
encing short-circuit of the cell at a current density of 3 mA cm−2.
Despite the homogeneous lithium deposition presented in the
first cycles, this electrolyte shows increasing deposition poros-
ity as previously stated. Moreover, in contrast to the robust LiF-
rich SEI layer in the SSE_LiFSI system, the LiNO3present in
the SEI layer formed in the DME/DOL electrolyte is depleted
over cycling. This LiNO3consumption is higher at higher cur-
rent densities, leading to a weaker SEI layer with subsequent
short-circuit of the cell ascribed to Li dendrite formation. Finally,
the SSE_REF electrolyte shows the poorest performance, consis-
tent with the earlier observations, displaying the lowest robust-
ness against current due to the porous and fibrillar nature of its
depositions.
The electrochemical performance of the proposed electrolytes
was further assessed in LSBs using our recently developed high-
sulfur loading cathode made up of the ResFArGO 2D graphene-
based activated carbon (Figure 6a).[26]For all the coin cell cycling
experiments, a reduced amount of electrolyte of E/S (electrolyte
to sulfur) ratio of 7 μlmg
S−1was established, an adjusted value
for this cell configuration to facilitate its eventual upscaling pro-
cess. Notably, both SSEs demonstrate remarkably high capacities,
despite their lower conductivity when compared to the conven-
tional electrolyte. Particularly, it is noteworthy the performance
of SSE_LiFSI, which exhibits initial capacity values close to the
theoretical ones, suggesting a potential synergistic effect between
this type of electrolyte and graphene-based cathodes. This im-
pressive behavior can be attributed to the excellent compatibil-
ity between the sulfur cathode and SSEs , which is enabled by
the textural properties and surface chemistry of the ResFArGO
material. The open and 2D flat-shaped structure of both the Res-
FArGO carbonaceous material and the high-loading cathode de-
veloped shown in Figure S15 (Supporting Information), along
with the O-rich functional groups on its surface reported in our
previous work,[26]enhances the wettability of the SSEs despite its
higher viscosity (as shown Figure S1, Supporting Information)
and maximizes sulfur utilization during cycling. The validity of
this hypothesis was further reinforced by conducting a parallel
LSB cycling experiment without the inclusion of the ResFArGO
material, as shown in Figure S16 (Supporting Information). In
this case, the DME/DOL electrolyte, which does not encounter
wettability issues, demonstrated superior capacity compared to
the SSE results. Figure S17 (Supporting Information) provides
clear evidence that the presence of the ResFArGO material sig-
nificantly enhances the wettability of the SSEs, resulting in im-
proved cycling performance.
On the other hand, cyclability is another key parameter influ-
enced by the choice of electrolyte. As shown also in Figure 6a,
the SSE_LiFSI electrolyte demonstrates extraordinary LSB stabil-
ity maintaining a remarkably high and constant CE value of 99%
during the analyzed cycles, addressing a known weakness of the
technology. In contrast, the DME/DOL electrolyte experienced a
continuous CE decrease from cycle 30 and could only withstand
60 cycles due to the depletion of the LiNO3additive, which plays
a pivotal role in cell stability. The success of the anion-selection
strategy reflected in the SSE_LiFSI is worth mentioning, as it out-
standingly improves both the capacity and, especially, the stabil-
ity of the Li-S cell. Compared to SSE_REF electrolyte, SSE_LiFSI
presents a longer cycle life and improved CE values (96.7% vs
99%, respectively) that are linked with the results obtained above.
These results highlight the potential of the SSE_LiFSI electrolyte,
demonstrating the advantages of using LiFSI in the electrolyte
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composition while discarding SSE_REF due to its poor perfor-
mance in both lithium symmetric and full-cells.
To further explore the potential of the SSE_LiFSI electrolyte,
additional electrochemical characterizations were conducted, in-
cluding the rate capability study using medium sulfur loading
cathodes of 3 mgScm−2(Figure 6b; Figure S18, Supporting In-
formation). In comparison to the conventional electrolyte, it has
been observed that the performance of SSE_LiFSI is comparable
at low C-rates. However, at high C-rates, the SSE_LiFSI presents
superior sulfur utilization, achieving significantly higher capac-
ities of 829 mAh g−1at C/2, outperforming the conventional
electrolyte. This remarkable cycling rate performance can be
attributed to several factors, including the improved Li plat-
ing/stripping kinetics, suitable ionic conductivity, and favorable
wettability, thus allowing it to outperform the rate capability per-
formance of conventional DME/DOL electrolyte. Furthermore,
it is worth noting that the capacity of the cell using SSE_LiFSI
electrolyte is consistently recovered when returning to C/5, pre-
senting a stable CE during the entire test. In contrast, while
DME/DOL electrolyte can also recover capacity, it shows a clear
CE decay due to the consumption of LiNO3during cycling at high
rates. Moreover, the excellent compatibility of the SSE_LiFSI elec-
trolyte at remarkably high cycling rates is further evidenced in its
ability to support high sulfur loading long-term cycling at C/5
(Figure S19, Supporting Information). This electrolyte provides
remarkable initial high-capacity values of 1200 mAh g−1while
providing outstanding cell stability.
Indeed, while the SSE_LiFSI electrolyte has shown promising
results, a capacity fading process has been observed during long
cycling. To address this issue and further optimize the system,
the effect of cut-off voltage on cell performance was studied by an-
alyzing three different voltage ranges: 1.2–3.0, 1.4–2.7, and 1.7–
2.5 V. As illustrated in Figure 6c, a wider voltage range leads to
higher capacity values (i.e., higher sulfur utilization); however,
the capacity drop per cycle is notably more pronounced. Accord-
ingly, the results reflect that a voltage range of 1.7–2.5 V yields
the highest capacity retention (80%) after 100 cycles. In contrast,
cycling between 1.4 and 2.7 V results in 69% capacity retention,
while cycling between 1.2 and 3 V shows 61% retention. How-
ever, it should be noted that the 1.7–2.5 V voltage range may not
be wide enough, leading to a low and insufficient sulfur utiliza-
tion of ≈50% (Figure S20a–c, Supporting Information). There-
fore, a compromise between sulfur utilization and capacity re-
tention during cycling was sought, thus selecting 1.4–2.7 V as
the optimal working voltage range for this system.
The temperature range at which batteries can operate is a cru-
cial parameter for their commercialization. The excellent ther-
mal stability demonstrated by the SSE_LiFSI electrolyte prompts
the study of battery cycling at high temperatures (60 °C in this
study). As depicted in Figure 6d, the SSE_LiFSI electrolyte ex-
hibits remarkable cycling performance with high-capacity values,
evidencing its ability to ensure safe battery cycling even under
high-temperature conditions. However, it is worth noting that at
60 °C, the capacity drop is slightly higher, and the CE values are
lower compared to RT cycling. Figure S21 (Supporting Informa-
tion) provides the charge and discharge profiles when cycling at
60 °C. It is evidenced that RT cycling follows a quasi-solid re-
action pathway mechanism without clear plateaus, whereas at
60 °C the profiles resemble the dissolution-precipitation reac-
tion pathway typically observed in conventional DME/DOL-based
electrolytes, characterized by two distinct plateaus. This change
in the voltage profile suggests the presence of soluble polysul-
fides in the electrolyte due to the increased temperature. This hy-
pothesis has subsequently been confirmed through a polysulfide
generation test in the SSE_LiFSI electrolyte. To do this, S8and
Li2S were mixed in the electrolyte and stirred at the two cycling
temperatures (25 and 60 °C). As shown in Figure S22 (Supporting
Information), after several days of reaction, there is minimal col-
oration at 25 °C. However, at 60 °C, there is a noticeable change
in coloration, with the solution turning orange. These results cor-
roborate what was observed in the charge and discharge profiles,
demonstrating that increasing the temperature enhances the sol-
ubility of these compounds in the electrolyte, thereby modifying
the system’s conversion mechanism from quasi-solid at room
temperature to precipitation-dissolution at 60 °C. Despite these
effects, the SSE_LiFSI electrolyte demonstrates good properties
for ensuring safe cycling even at high temperatures, although
some modifications may be considered to address the new chal-
lenges posed.
To validate the practical application of the SSE_LiFSI elec-
trolyte, a high sulfur loading (4 mgScm−2) prototype Li-S pouch
cell was assembled and tested. In this case, the same E/S ratio of
7μlmg
S−1established for the coin cells was employed due to the
acknowledged presence of more pronounced dead space in the
monolayer pouch cell configuration when compared to the mul-
tilayer pouch cell design. Figure 7ademonstrates that the per-
formance trend observed in the coin cell results is consistent in
the larger pouch cells. The conventional DME/DOL electrolyte
presents a limited cycling stability of only 20 cycles. Beyond this
point, the consumption of LiNO3results in a decrease in both ca-
pacity and CE due to its diminished ability to protect the lithium
anode until the cell eventually fails. Furthermore, as illustrated
in Figure 7b, an increased polarization of the cell is observed due
to the high LiPS solubility of this electrolyte. However, the stabil-
ity of the Li-S pouch cell using SSE_LiFSI electrolyte is signifi-
cantly improved due to the robust and effective LiF-rich protec-
tive SEI layer formed on the Li anode, enabling a minimum of
100 cycles (over five times higher compared to those of the con-
ventional electrolyte) while maintaining a stable CE high value of
99%. Notably, this CE value is slightly superior to that achieved
during coin cell cycling, which reached a value of 98%. This slight
improvement observed in the CE of the prototype cell may be
attributed to the application of external stacking pressure, a fac-
tor demonstrated to exert a remarkable and beneficial influence
on LMB performance by enhancing Li deposition behavior and
morphology.[54,55]In addition, the SSE_LiFSI electrolyte exhibits
excellent sulfur utilization, leading to a remarkably high capacity
of 93 mAh (4.6 mAh cm−2) at C/10, with an impressive capacity
retention of 70% after 100 cycles. Moreover, it presents smooth
one-single charge/discharge profiles (Figure 7c), which are char-
acteristic of quasi-solid reaction pathways associated with the use
of SSE. These results highlight the effectiveness of the holistic
Li-S battery approach and pave the way for the development of
high-performing LSBs suitable for practical applications.
Despite the progress made in this manuscript, there is still
room for improvement. Through the approach proposed during
this study, the scaling of the system has been achieved, obtaining
an average Coulombic Efficiency of 99.0% at moderate C-rates
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Figure 7. Electrochemical cycling performance of LSB pouch cells. a) Total and specific capacities of the pouch cells using DME/DOL and SSE_LiFSI
electrolytes. Pouch cell voltage profile comparison between b) DME/DOL and c) SSE_LiFSI electrolytes.
such as C/10 that must be enhanced with the aim of approach-
ing more realistic scenarios, close to their future industrial appli-
cation. Therefore, a further improvement in the stability of the
LMA is needed, either through an enhancement in its quality,
protection by means of an ex-situ SEI layer, or a combination of
both approaches.
3. Conclusion
This work demonstrates the successful implementation of a
holistic LSB approach by combining a high-performing 2D
graphene-based sulfur cathode with a well-suited SSE in the fi-
nal battery design. Due to the importance of anion selection for
the generation of a suitable SEI layer, LiFSI salt was included in
the SSE formulation resulting in the construction of a robust and
effective LiF-rich SEI protective layer on the LMA. Consequently,
the SSE_LiFSI electrolyte exhibits outstanding compatibility with
lithium metal, achieving a high CE of 98.8% in the Li||Cu test, fa-
cilitating a compact, thin, and dense lithium deposition, and of-
fering an excellent current density tolerance. Notably, SSE_LiFSI
electrolyte presents a non-flammable nature, ensuring the safety
of the battery system by mitigating the fire hazard compared to
the conventional electrolyte. In terms of LSB performance, the
holistic strategy demonstrates an unprecedented symbiotic ef-
fect, significantly improving cell stability and sulfur utilization
compared to the conventional electrolyte and SSE_REF. In ad-
dition, the scalability of this new battery system is successfully
demonstrated in a 20 cm2monolayer prototype cell, yielding out-
standing cycling stability and achieving a fivefold increase in cy-
cle life compared to the conventional electrolyte. The cell deliv-
ers a high total capacity of 94 mAh at C/10, with a remarkable
capacity retention of 70% after 100 cycles and a high average CE
of 99.0%. This study highlights the applicability of the holistic
battery concept as a cornerstone in paving the way toward the
commercialization of high energy density, safe, and long cycle
life LSBs.
Supporting Information
Supporting Information is available from the Wiley Online Library or from
the author.
Acknowledgements
J.C. is a beneficiary of the Predoctoral Program from the Education De-
partment of the Basque Government. The authors want to acknowledge
GRAPHENEA for supplying graphene oxide. Chunmei Li is acknowledged
for fruitful discussion. Hegoi Manzano at the University of the Basque
Country is thanked for providing technical advice and access to facilities
throughout computational work. This work was funded by the European
Union’s Horizon 2020 research and innovation program Graphene Flag-
ship Core Project 3 (GrapheneCore3) under grant agreement 881603.
Conflict of Interest
The authors declare no conflict of interest.
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Data Availability Statement
Research data are not shared.
Keywords
2D graphene-based sulfur cathodes, electrolyte additives, electrolyte engi-
neering, lithium metal anodes, lithium-sulfur batteries, pouch cells, solid
electrolyte interphases
Received: July 24, 2023
Revised: October 9, 2023
Published online: November 13, 2023
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