# Study of the Cathode/Electrolyte Interface in an All-Sulfide-Solid-State Battery Using Lithium-Rich Transition Metal Sulfide

Mathieu Caspar, Yohan Biecher, Yann Tison, Frédéric Le Cras, Hervé Martinez

► To cite this version:

Mathieu Caspar, Yohan Biecher, Yann Tison, Frédéric Le Cras, Hervé Martinez. Study of the Cathode/Electrolyte Interface in an All-Sulfide-Solid-State Battery Using Lithium-Rich Transition Metal Sulfide. ACS Applied Materials & Interfaces, 2025, 17 (4), pp.7142-7150. 10.1021/acsami.4c18890. hal-04925458

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Distributed under a Creative Commons Attribution 4.0 International License
Study of the cathode/electrolyte interface in an
all-sulfide-solid-state battery using lithium-rich
transition metal sulfide

Mathieu Caspar,†,‡ Yohan Biecher,‡ Yann Tison,† Frédéric Le Cras,‡,¶ and Hervé Martinez*‚,†‚§

†Université de Pau et des Pays de l'Adour, E2S UPPA, CNRS, IPREM, Pau, 64000, France

‡Université Grenoble Alpes, CEA, LITEN, DEHT, Grenoble Cedex 9, 38045, France

¶Universite de Bordeaux, CNRS, Bordeaux INP, ICMCB UMR 5026, F-33600, Pessac, France

§Ecole Centrale Casablanca, Centre de Recherche Systèmes Complexes et Interactions,
Bouskoura Ville verte, Maroc

E-mail: herve.martinez@univ-pau.fr

Abstract

All-solid-state lithium batteries (ASSB) are amongst the most promising energy
storage technology, particularly for electric vehicles due to their enhanced safety. Per-
formances are however still limited by interfacial side reactions at electrode/electrolyte
interfaces, especially with sulfide electrolyte, to which are added issues of mechanical
nature. In this work, an ASSB composed of argyrodite (Li₅.₇PS₄.₇Cl₁.₃) electrolyte, a
lithium-rich sulfide cathode (Li₁.₂Ti₀.₈S₂) and a lithium metal anode is investigated.
The positive electrode/electrolyte interface is scrutinized during several battery cycles
using XPS via two complementary approaches (ex *situ* and *in situ*). We show that both titanium and sulfur species contribute to the electrochemical process without any detectable degradation of the electrolyte after 20 cycles and without the use of any protective coating. These results suggest that the electrochemical performances of such systems are not limited by the cathode/electrolyte reactivity, opening a promising route for the development of new cathode materials adapted to sulfide electrolytes.

Introduction

Following several decades of improvement, lithium-ion batteries are approaching their the-
oretical energy density limits. ¹,² Furthermore, for some their applications such as electrical
cars, the presence of a flammable liquid electrolyte contributes to thermal runaway and
represents a significant safety issue.³ In order to provide a response to both concerns, all-
solid-state batteries (ASSB) using a thermally stable solid electrolyte are developed.⁴ The
solid electrolyte can act as a separator that allows to reconsider the use of lithium metal as
an anode due to its expected capacity to block lithium dendrites growth.⁵

An ASSB has a structure derived from Li-ion batteries. The stack is made of (i) a com-
posite positive electrode (also called catholyte) composed of cathode active material, solid
electrolyte and conductive carbon additive, (ii) a separator layer made of solid electrolyte
and (iii) an anode which is in the best case a lithium metal foil. Until now, the develop-
ment of ASSB is hindered by two main bottlenecks. Firstly, the ionic conductivity of the
solid electrolyte which significantly reduce the number of viable materials. Secondly, the mi-
crostructure and chemical properties at the different solid/solid interface may significantly
impact the lithium transfer.⁶

Among the numerous solid electrolyte, argyrodite Li₅.₇PS₄.₇Cl₁.₃ displays a high lithium ion conductivity (6.4 mS.cm⁻¹) combined with a low density, can be processed and shaped at room temperature, and does not include any critical material.⁷,⁸ However, its electrochemical stability window is very narrow (1.8-2.5 V vs Li⁺/Li).⁹
As a result, when used with most of the cathode oxide materials such as $LiNi_xMn_xCo_{1-2x}O_2$ (NMC),¹⁰ $LiFePO_4$ or $Li_2MnO_4$¹¹ operating in the 3,5-4,5 V vs $Li^+/Li$ range, an interphase tends to form at the electrode/electrolyte interface which often leads to limited electrochemical performances. Development of a protective coating interposed between both materials is the most common countermeasure used to inhibit this reactivity.¹² This type of coating is challenging because it needs to provide sufficient ionic conductivity and mechanical stability and have a limited impact on the manufacturing cost of the cell.

In an attempt to develop “coating-free” ASSB using argyrodite electrolyte, cathode materials with lower operating voltage but higher specific capacity should be used in order to provide sufficient energy density without major electrolyte degradation. In this regard, the Li-rich layered sulfide material $Li_{1.2}Ti_{0.8}S_2$ (LTS) developed by Flamary-Mespoulie *et al.*¹³ could be a relevant option. Its lamellar structure is built upon of $TiS_6$ and $LiS_6$ octahedra arranged in a honeycomb pattern with $Li^+$ in the interlayer spaces. In liquid electrolyte-based batteries, this material exhibits a high reversible capacity of 250 mAh.g⁻¹ after 20 cycles at C/30 (theoretical specific capacity of 293 mAh.g⁻¹) with a mean working potential of 2.4 V vs $Li^+/Li$. This is due to contributions of both cations ($Ti^{4+}/Ti^{3+}$) and anions ($S_2^−/S^{2−}$) to the electrochemical process.

Theoretically, a LTS‖Argyrodite‖$Li^0$ ASSB cell should deliver a gravimetric energy density comparable to those provided by oxide materials such as $LiCoO_2$ or $LiNi_{1/3}Mn_{1/3}Co_{1/3}O_2$.¹⁴ Argyrodite is known to form an interphase with $Li^0$ which is believed to increase progressively the ohmic resistance of the cell which is an issue that still needs to be fixed.¹⁵ However, very few studies have been conveyed on the interface between transition metal sulfides and sulfide argyrodite. In a previous work [ref 1er article], we have shown that a reactivity occurs upon contact between grinded LTS and argyrodite with partial reduction of argyrodite into $Li_xP$ ($0 ≤ x ≤ 3$), $Li_2S$ and $LiCl$ and partial oxidation of $Ti^{3+}$ of LTS into $Ti^{4+}$. We are now extending these investigations towards an operating “all-sulfide-solid-state-cell” using LTS as a cathode.
In this study, we have assessed the viability of the LTS||Argyrodite||Li⁰ ASSB. The main focus was probing the evolution of the cathode/electrolyte interface to check out for potential undesired interfacial reaction along the cycling process. To perform this, surface scientific techniques such as XPS are particularly relevant.¹⁰,¹⁶,¹⁷ We used two complementary approaches in this work: the *ex situ* approach consisting in performing cycling and characterization operations in different systems and the *in situ* approach consisting in realizing both operations in the same experimental setup.

# Experimental section

## Materials

Li₁.₂Ti₀.₈S₂ (LTS) has been synthesized by solid-state reaction of LiTiS₂, TiS₂ (Sigma-Aldrich, 99.9 %) and Li₂S (Sigma-Aldrich, 99.8 %) as described in a previous work [ref 1er article]. LiTiS₂ has been synthesized by lithiation of TiS₂ using n-butyllithium (Sigma-Aldrich, 1.6 M in hexane) as described in Flamary-Mespoulie *et al*.¹³

Argyrodite Li₅.₇PS₄.₇Cl₁.₃ has been synthesized by solid-state reaction using Li₂S, LiCl (Sigma-Aldrich, 99%) and P₂S₅ (Sigma-Aldrich, 99 %) as precursors using the method detailed in a previous work [ref 1er article].

Commercials carbon black (Ketjenblack EC600JD) and lithium foil (Goodfellow, 99.9 %, 0.2 mm thick) are used respectively as conductive additive and as anode.

## Cell assembly and electrochemical characterization

Batteries used in this study are elaborated using a SPHERE ASC-AD™ cell as both a mold (8 mm diameter) and a full-cell casing (Polyethylenimine body and stainless steel current collectors). First, 80 mg of argyrodite is placed in the cell and pelletized using a uniaxial hydraulic press at 1T (≈ 200 MPa).

For the positive electrode; LTS, Argyrodite and Carbon black are mixed in quantities cor-
responding to the 30.3/66.1/3.6 mass ratio. This mixing operation is performed using a planetary mixer (SpeedMixder™ DAC400.1 FVZ) during 3 min at 2500 rpm. 10 mg of this mixture is then placed on top of the argyrodite pellet and the whole is compressed at 2T (≈ 400 MPa).

A 4 mm diameter lithium disk is punched out of the lithium foil and placed under of the argyrodite pellet. The cell is then tightly closed using a spring system provided by SPHERE which allows to apply a constant pressure of ≈ 40 MPa on the sample. Due to this pressure, a rest period of at least 1 hour is respected before cycling during which the lithium disk is creeping until it reach a diameter close to 8 mm.

Galvanostatic cycling is then performed on a BioLogic VMP-3. The C-rate is calculated taking into account the mass of LTS in the cell and its theoretical specific capacity assuming 1.2 lithium ions transferred.

## XPS characterization

We conducted XPS measurements in a Thermo Electron Escalab 250 Xi spectrometer equipped with a monochromatized Al Kα X-ray source (1486.6 eV). The spectrometer is directly connected to an Ar glovebox to limit the exposure of samples to sources of contamination. Core peak spectra were realized using a 400 µm X-ray spot and a constant pass energy of 20 eV and size under ultra-high vacuum ($<10^{-8}$ mbar). Scans with shorter acquisition time were performed before and after each spectrum to detect potential beam damage. At least three measurement points were probed for each sample to ensure a good repeatability. The Cl2p 3/2 core peak of argyrodite at 198.9 eV is used as an internal reference for energy calibration. Non-linear Shirley functions were used to model continuous background. Fitting were performed using a weighted least-square method (Casa XPS software) and a Voigt function line shape. Peaks were attributed to chemical environment using reference compounds analyzed in the same conditions. Quantification is based on relative sensitivity factors provided by
Thermo Scientific. 3 points per sample are probed and averaged to limit the effect of surface inhomogeneity.

## *In situ* characterization

The *in situ* characterization is performed using a homemade cell adapted from the standard Thermo sample holder that allows galvanostatic cycling in the analysis chamber of the machine. A schematic view of the cell is presented figure 1. At first, a battery stack is assembled by pelletizing 80 mg of argyrodite in a 10 mm diameter stainless steel dye at ≈ 200 MPa. Then 30 mg of positive electrode powder (prepared using the same protocol described previously) is placed on top of the argyrodite pellet and the whole is compressed at ≈ 400 MPa. A lithium disk (diameter 8 mm) is punched out of the lithium foil and sticked to the bottom of the argyrodite pellet.

The sample is then deposited on top of a PEEK block. A molybdenum grid is placed on

<figure><img src="image_2.png" /><figcaption>Figure 1: Side view (a) and top view (b) of the *in-situ* cell</figcaption></figure>

the sample. This grid allows to homogenize the current and the electric fields applied to the surface of the positive electrode. The cell is closed with a brass component that features a circular hole that permits an XPS beam to probe the surface of the positive electrode. The negative electrode of the sample is electrically connected to the sample holder copper body by a stainless steel spring that also allows to apply a pressure on the stack. This body is then connected to the sample manipulator of the machine. The positive electrode is electronically
connected to the sample manipulator via the brass component and a copper wire.

During XPS acquisition, the sample manipulator is connected to the ground of the machine which allows the neutralization of charges on the surface. In order to perform galvanostatic cycling, this sample manipulator is then connected to an external potentiostat (Biologic™ SP-50).

# Results and discussion

## LTS/argyrodite interface study during the first cycle

Full cells using LTS/argyrodite/carbon black (30.3/66.1/3.6) as a positive electrode, argyrodite as separator layer and lithium as negative electrode were assembled. The electrochemical behavior of the system has been scrutinized during one cycle at C/30 (≈ 66 µA/cm²) between 1.8 and 3 V. The charging curve of the first cycle (Figure 2-a) shows a two-step process with two plateaus at 2.2-2.4 V and at 2.7-2.8 V. According to a previous study of this material realized by Flamary-Mespoulie et al.,¹³ these two plateaus can be respectively assigned to Ti³⁺/Ti⁴⁺ and S₂⁻/S²⁻ redox couples. The discharge curve exhibits only one continuous slope from 2.7 to 2.1 V which suggest that both redox couples are active simultaneously, as it was already shown by Flamary-Mespoulie et al.¹³ The first charge represents an exchange of 210.6 mAh.g⁻¹ of LTS. Compared to the theoretical capacity of LTS (293 mAh.g⁻¹), the experimental charge seems incomplete. After this charge, the discharge capacity is only of 166.7 mAh.g⁻¹ which represents a significant irreversible loss of 43.9 mAh.g⁻¹.
<figure><img src="image_3.png" /><figcaption>Figure 2: (a) First cycle of LTS vs Li⁰ in all-solid-state battery (C/30). Circles indicate stop points used for *ex situ* XPS study. Ti2p (b), S2p (c) and P2p (d) core peak spectra of the positive electrode at pristine, charged and discharged states during the first cycle</figcaption></figure>

To explain this behavior, three cells were stopped at various stages of the cycling: at the initial state (Open voltage of 1.7 V), after the first charge (voltage of 3.0 V) and after the first discharge (voltage of 1.7 V). The cells were then disassembled and the positive electrode was analyzed by XPS. In particular, the evolution of Ti2p, S2p and P2p spectra is monitored along the cycling process (Figure 2 b,c and d). Regarding the initial state, the Ti2p spectrum exhibits 4 doublets: o at 455.5-460.7 eV attributed to Ti³⁺ in a sulfide environment, at 456.6-
462.4 eV attributed to Ti⁴⁺ in a sulfide environment, at 457.4-463.1 eV attributed to Ti⁴⁺ in a mixed oxygen/sulfur environment and, lastly, at 458.7-465.0 eV attributed to Ti⁴⁺ in oxygen environment. Ti³⁺ and Ti⁴⁺ environments are characteristics of LTS and Ti⁴⁺ in oxygen or oxysulfide environments are oxidation products due to the very strong reactivity of titanium sulfides with O₂.¹⁸The phosphorus core peak spectrum is composed of 3 doublets at 132.0-132.9 eV, 132.8-133.6 eV and 134.2-135.1 eV attributed to phosphorus in a mixed Li/S environment, phosphorus in S environment and lithium phosphate species, respectively. The S2p spectrum is composed of three doublets : at 160.6-161.9 eV attributed to S²⁻ in a Ti/Li environment such as in LTS, at 161.6-162.8 eV attributed to S²⁻ in a Li/P environment such as in argyrodite. and at 163.0-164.1 eV. attributed to Sₓ species which are probably oxidation products.

In our previous work, [Intégrer ref ici] we studied LTS/argyrodite interface in depth by making LTS/argyrodite composite. We have shown in this previous study a spontaneous reaction between LTS and argyrodite leading to reduction of argyrodite into LiₓP species and oxidation of titanium of LTS. Here, this behaviour is not observed by XPS. The different preparation method used here (mixing vs milling) and the differences in the formulation may limit the formation of degradation products to concentrations below the detection limit of XPS (atomic concentration ≈ 0.01-0.1 % depending on the sensitivity factor).

In the charged state (3 V vs Li⁺/Li⁰), the intensity of the Ti³⁺ peak is strongly decreased on the Ti2p spectrum which demonstrates the participation of the Ti⁴⁺/Ti³⁺ couple to the electrochemical process. On the S2p spectrum, the only visual difference is the presence of SOₓ species at 167.1-168.3 eV. XPS studies of LTS cycling behavior conducted by Flamary-Mespoulie et al.¹³ have shown that S₂²⁻ species are expected at ≈ 161.9 eV. In our case, this peaks overlaps with the peak at 161.6-162.7 eV associated with Li/P environment of argyrodite. Thus, it is expected that the area of the peak at 161.6 eV should increase relatively to the area of the component at 160.6 eV characterizing S²⁻ in LTS. In this regard, the ratio between the components at 161.7 eV and at 160.5 eV is monitored along the first cycle
(Table 1) to probe for the presence of $S_2^{2-}$. This ratio increases slightly in the charged state compared to the initial value which may suggest the appearance of disulfur bonds. No significant difference can be observed between the P2p spectrum in charged state and the one in pristine state which suggest that the electrolyte remains stable during the delithiation of LTS.

<table><caption>Table 1: Evolution of the peak area ratio of peaks at 161.7 eV and at 160.5 eV on S2p spectra and of peaks at 456.5 eV and at 455.6 eV on Ti2p spectra (ex *situ* study)</caption><thead><tr><th></th><th>Pristine</th><th>Charged (3 V)</th><th>Discharged (1,8 V)</th></tr></thead><tbody><tr><td>S2p 161.7 eV/ S2p 160.5 eV</td><td>7.4</td><td>8.1</td><td>7.5</td></tr><tr><td>Ti<sup>4+</sup>/Ti<sup>3+</sup> (S environment)</td><td>2.0</td><td>28.4</td><td>2.6</td></tr></tbody></table>

In the discharged state (1.8 V vs Li⁺/Li⁰), the proportion of Ti³⁺ on the Ti2p spectrum gets close to its value in the pristine state (Table 1). The fact that the initial ratio of Ti³⁺/Ti⁴⁺ is not exactly reached at the end may explain why the observed discharge capacity is lower than the charge capacity. On the S2p spectrum, the proportion of SOₓ increases slightly and the area ratio between peaks at 161.6 eV and 160.6 eV gets close to the initial value (Table 1). Once again, there is no evolution of the P2p spectrum at the discharged state.

This *ex situ* study of the first cycle shows that the electrochemical behavior observed in solid-state seems similar to that observed using liquid electrolyte based cells. [ref] Furthermore, no significant degradation of argyrodite has been observed during this first cycle, especially comparing with a similar study realized with an oxide cathode.¹⁰ However, the *ex situ* approach shows some limits. Several errors or misinterpretations can arise from the fact that different samples are used. Surface analysis techniques such as XPS are very sensitive to surface contamination that are obviously different from one sample to another. Furthermore, disassembling the cell is not an easy task to perform and contamination of the surface or short-circuit may happen.

In an attempt to lift the uncertainties raised by the *ex situ* method, we are developing an *in situ* approach allowing us to analyze by XPS the same point of a solid-state-battery at
different states of charge without manipulating the sample and under ultra high vacuum conditions so to limit as best as possible surface contamination between each measurements. A LTS/Argyrodite/Li⁰ stack has been placed in the *in situ* cell as described in the experimental section and entered in the analysis chamber of the XPS machine. The initial open circuit voltage of the cell after introduction in the UHV chamber was 2.2 V which is consistent with cells assembled in the *ex situ* study. A first XPS measurement has been conveyed after introduction (Figure 3 b-d). Ti2p, S2p and P2p spectra obtained are similar to the one reported in the *ex situ* study. After measurement, open circuit voltage remained constant (2.1 V). The cell was then charged up to 3 V vs Li⁺/Li⁰ at C/50 (≈ 45 µA/cm²). The charge curve is presented Figure 3 -a. It is possible to identify the two step process involving cations and anions on this curve. However, some voltages drops can be noticed on the cycling curve. These types of drops are often due to dendrite formation in the solid electrolyte, despite a lower current density applied compared to the *ex situ* study. It is possible that even using a metallic grid, the current is not uniformly distributed in the sample which favors the formation of high current points and dendrites. Furthermore, even after 50 h of charging, the voltage of the cell did not reach the cut-off voltage of 3 V and stayed on a plateau at 2.8 V. The OCV voltage after few hours of rest was 2.75 V which was consistent with an at least partially delithiated phase of LTS (see Supplementary Information [GITT]). XPS measurements have been carried on the same analysis point. On the Ti2p spectrum, the Ti³⁺ peak is significantly decreased. On the S2p spectrum, it is here clear that the area of the peak at 161.7 eV increased with respect to the peak at 161.0 eV (Table 2). There is no increase of the SOₓ peak unlike the *ex situ* study. As a result, it is now clear that these species were due to sample contamination either during the dismantling of the cell or during cycling (i.e. gas infiltration issues with the cell). Once again, the P2p spectrum did not significantly evolve after charging the battery.
<figure><img src="image_4.png" /><figcaption>Figure 3: (a) Charging curve of LTS vs Li⁰ in the *in situ* cell (C/50). Ti2p (b), S2p (c) and P2p (d) spectra performed on the same analysis point at pristine and charged state</figcaption></figure>

Both *ex situ* and *in situ* show that during the first cycle, the electrochemical behavior of LTS in all-solid-state cells is similar to the one in liquid cell with contribution of both Ti³⁺/Ti⁴⁺ and S₂⁻/S²⁻ redox couples. Argyrodite seems to remain stable during the process and do not contribute to the observed capacity.

<table><caption>Table 2: Evolution of the peak area ratio of peaks at 161.7 eV and at 160.5 eV on S2p spectra and of peaks at 456.5 eV and at 455.6 eV on Ti2p spectra (*in situ* study)</caption><thead><tr><th></th><th>Pristine</th><th>Charged (3 V)</th></tr></thead><tbody><tr><td>S2p 161.7 eV/ S2p 160.5 eV</td><td>4.0</td><td>12.0</td></tr><tr><td>Ti⁴⁺/Ti³⁺ (S environment)</td><td>2.3</td><td>10.8</td></tr></tbody></table>

If one compares these approaches, it is now clear that the *in situ* approach allows for more relevant XPS spectra compared to the *ex situ* analysis. However, it is harder to obtain good electrochemical performances with the *in situ* approach, since the geometry of the
battery stack has to be adapted to the sample holder. Our setup still requires optimization
in order to be more representative of a real battery. For instance, when attempting to realize
a discharge operation, the system behave like an infinite resistance for an unknown reason
which prevent us to monitor a full cell cycle (Supplementary [OCV]).

Extended cycling performances

After showing the stability of the electrode/electrolyte interface during one cycle, the study
has been pursued to a longer battery lifetime. An LTS/argyrodite/Li⁰ cell has been cycled
between 1.8-3 V during 20 cycles at C/30. The cell is then disassembled and the composite
positive electrode is analyzed using the same procedure as the previous *ex situ* study.

Cycling curves are presented figure 4-a. These curves show that, after the first charge, the
capacity remains stable at 160 mAh.g⁻¹ with a coulombic efficiency of 99.92 % at the 20th
cycle. By taking a closer look to the discharge curves and charge curves (Figure 4 b-c), a
decrease of the mean voltage is observed for both charge and discharge over the course of the
cycling process. This voltage decay has already been reported for Li-rich layered oxides [ref]
and several explanations have been proposed, most of which are based on the low stability of
superoxyde O-(O)ₙ⁻ species. Although the literature regarding Li-rich layered sulfide is not
as comprehensive as that dedicated to Li-rich oxides, one can infer that a progressive mod-
ification of the structure leads to stabilization of the inserted lithium within the host material.
<figure><img src="image_5.png" /><figcaption>Figure 4: (a) Charging curve of LTS vs $Li^0$ (20 cycles, 1.8-3 V, C/30). Ti2p (b). A voltage decay is observed during discharges (b) and charges (c). Ti2p (d), S2p (e) and P2p (f) spectra of the sample after 20 cycles are compared with spectra of pristine electrode</figcaption></figure>

Ti2p, S2p and P2p spectra of the electrode after 20 cycles are compared with respective spectra in the initial state (Figure 4 d-f). The Ti2p spectrum is similar to pristine state. The S2p spectrum shows almost no doublet at 160.5-161.7 V ($S^{2-}$ in Ti/Li environment) and a significant proportion of $SO_x$ species. A quantitative approach (Table 3) shows that the proportion of $Ti^{3+}$ in the discharged state apparently increases with the number of cycle. In parallel, the area of the peak of $S^{2-}$ in LTS (160.5 eV) compared to that of the peak at 161.7 eV is also decreasing as the battery ages. It is possible that this progressive modification of the environment of titanium and sulfur plays a role in the observed voltage decay. Finally,
the P2p spectrum after 20 cycles shows no difference with the pristine state. In other words, even after 20 cycles at C/30, no degradation of the electrolyte can be detected by XPS.

<table><caption>Table 3: Evolution of the peak area ratio of doublets at 161.7 eV and at 160.5 eV on S2p spectra and of peaks at 456.5 eV and at 455.6 eV on Ti2p spectra between initial state and after 1 then 20 cycles</caption><thead><tr><th></th><th>Pristine</th><th>1 cycle</th><th>20 cycles</th></tr></thead><tbody><tr><td>S2p 161.7 eV/ S2p 160.5 eV</td><td>4.0</td><td>7.5</td><td>337.7</td></tr><tr><td>Ti<sup>4+</sup>/Ti<sup>3+</sup> (S environment)</td><td>2.3</td><td>2.6</td><td>4.3</td></tr></tbody></table>

## Conclusion

In this work, we used the complementary *ex situ* and *in situ* approaches to demonstrate that no significant unwanted side reactions appears at the LTS/argyrodite interface during the first cycle. After several cycles, no major evolution of the electrolyte could be detected. Thus, the LTS may be a promising cathode material for developing "coating-free" ASSB using argyrodite.

Several issues still remain regarding the handling the lithium/argyrodite interface and the optimization of the assembling procedure so to increase the gravimetric capacity of the cell. We showed that the redox transition $S_2^2^- / S^2^-$ of LTS is reversible without degradation of the electrolyte. This may pave the way to the development of newall-sulfide-solid-state batteries based on lithium-rich transition metal sulfide.

## Acknowledgement

The authors thank the E2S-UPPA consortium and CEA Liten for the financial support of this work.
**Supporting Information Available**

Supplementary information are available on a separate document.

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