Electrochemical Redox Behavior of Li Ion Conducting Sulfide Solid

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Electrochemical Redox Behavior of Liion Conducting Sulfide Solid Electrolytes Tushar Swamy, Xinwei Chen, and Yet-Ming Chiang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03420 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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Chemistry of Materials

Electrochemical Redox Behavior of Li-ion Conducting Sulfide Solid Electrolytes Tushar Swamy#, Xinwei Chen&,†, Yet-Ming Chiang&,* #Department

of Mechanical Engineering, Massachusetts Institute of Technology Cambridge, MA 02139 USA &Department of Material Science and Engineering, Massachusetts Institute of Technology Cambridge, MA 02139 USA ABSTRACT: The lithium-phosphorus-sulfide family of solid electrolytes has attracted much attention for applications in solidstate batteries, as it exhibits some of the highest lithium ion conductivities of solid electrolytes to date. Here, we systematically assess the stability of the β-Li3PS4 exemplar over a wide electrochemical window, from zero to 5 volts vs Li/Li+, that encompasses the potentials of all negative electrodes and most positive electrodes of interest for high energy density lithium batteries. Using a unique cell construction in which the solid electrolyte is fabricated as an electro-active electrode by adding carbon as an electronic conductor, redox activity is amplified. the interphase (SEI)-forming solid-state reactions at high potential are found to be irreversible and passivating, whereas those occurring at low potential are reversible. This contrasts with the irreversible anode SEI formed at low potentials in lithium-ion and lithium metal batteries, and the general absence of an SEI at cathode potentials. Using elemental sulfur and phosphorus as redox-active internal standards, we show that the redox behavior of upon decomposition is essentially a superposition of that of sulfur and phosphorus species formed at the interphase.

INTRODUCTION Lithium-ion conducting solid electrolytes are promising alternatives to conventional liquid electrolytes for two main reasons: greater safety, and the potential to enable the rechargeable use of metal electrodes. Improvements in the conductivity of sulfide-based (Li2S-P2S5) glassy and crystalline solid-state electrolytes (SSEs) have prompted further investigation into their bulk and interfacial properties.1–5 Within this compositional family, β-lithium thiophosphate (β-Li3PS4) is a useful model material owing to its reasonably high ionic conductivity (~0.2 mS⋅cm-1 at 298 K), negligible grain boundary resistance, and scalable synthesis route.3,6–8 Density functional theory (DFT) calculations have predicted a relatively narrow electrochemical stability voltage window for sulfide SSEs (1.71-2.31 V vs Li/Li+ for Li3PS4),9 and oxidative and reductive decomposition of sulfide SSEs has been experimentally observed.10–12 When in contact with 4 V cathodes, sulfide SSEs oxidize to form a high impedance interface leading to poor rate capability.1,13–18 Whether the interfacial impedance is due to the passivating nature of the oxidation products or due to the formation of an interfacial space charge layer is unclear.10,11,19 While metal-oxide SSE coatings of cathode particles lower the interfacial impedance by shielding the sulfide SSE from the high cathode potential, rapid capacity fade of the cell has not been avoided.13,14,16–18,20–24 Furthermore, DFT calculations predict the formation of elemental S upon Li3PS4 oxidation,9 but experimental investigations by Koerver et al.10 and Hakari et al.11 report the presence and absence of S,

respectively. Thus, the products of Li3PS4 oxidation remain to be verified. In Li/SSE/Li symmetric cells, the interphase formed upon sulfide SSE reduction has low impedance immediately after cell assembly,3,25 but the influence of cycling is not clear. Impedance growth may result as SSE reduction occurs over time, as is well-known for solid electrolyte interphases (SEIs) formed from liquid electrolyte reduction.26 Clearly, the electrochemical redox behavior of sulfide SSE, the decomposition products that form at high and low potentials, and the corresponding implications for battery performance, require further investigation. Here, we study the redox behavior of β-Li3PS4 (hereafter abbreviated as LPS) over a wide potential range of 0-5.0 V (vs Li/Li+), encompassing the potential range of interest for existing cathodes and anodes in rechargeable Li batteries. SEI’s are typically difficult to study since, by definition, they form only at interfaces. However, SEI can be prepared as a volume reaction product if the sample is fabricated as a nanoscale, mixed electronic-ionic conducting composite. Our laboratory previously synthesized volume samples of the SEI formed on graphite for spectroscopic study by discharging lithium into nanoscale graphite in a half-cell.27 Here, we likewise amplify the extent of reaction for easier study of the reaction products by preparing samples in which LPS particulates are intimately mixed with carbon black, forming a mixed-conducting composite in which electronic charge transfer can readily occur on a volume scale. This facilitates thermodynamically-preferred reactions of the electronically insulating LPS. Cyclic voltammetry (CV) is performed on lithium half-cells in

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which the positive electrodes are the above-described composites. X-ray photoelectron spectroscopy is performed on LPS electrochemically oxidized at 5.0 V in order to identify the reaction products. Finally, we introduce elemental sulfur and phosphorus in some samples as an internal standard, the redox behavior of which is compared to that of LPS. Previously, Han at al.28 used the same experimental configuration to corroborate the DFTcalculated electrochemical windows for Li10GeP2S12 (LGPS) and Li7La3Zr2O12 (LLZO) SSEs. In our study, we exclude specific cathode or anode compounds from the working electrode in order to focus on the potential-dependent reactivity of the LPS. We use carbon as the conductive additive, which in addition to providing electronic conductivity, may accelerate the observed reactions through chemical reactivity. The presence of carbon in LiCoO2 cathodes in contact with LGPS has been correlated with increased impedance.29,30 However, we do not observe any reaction products that can be traced to the carbon specifically.

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Figure 1. Schematic of the cell used for CV and potentiostatic experiments in which a 1mm layer of LPS was used as an SSE membrane, pellets of C, LPS+C, or LPS+C+S/P were used as the working electrodes, and a Li metal disc as the counter electrode.

Material characterization, The ionic conductivity of the as-synthesized LPS sample was previously measured to be 0.2 mS⋅cm-1 at room temperature using electrochemical impedance spectroscopy (EIS).31 Possible reactivity between the LPS and conductive carbon was investigated by conducting X-ray diffraction on the LPS samples before and after ball-milling with amorphous carbon black. The LPS diffraction spectra were unchanged, indicating an absence of reaction. Ex-situ XPS measurements were carried out using a Physical Electronics Versaprobe II X-ray Photoelectron Spectrometer. A monochromatic Al Kα X-ray source was used (200 μm beam diameter, 49.3 W of X-ray power). The pass energy of the analyzer was set to 23.5 eV and the chamber pressure was ~10-7 Pa during measurements. Two types of samples were measured: 1) The surface of a pellet of as-synthesized LPS powder, 2) The surface of a LPS pellet oxidized to 5.0 V w.r.t. Li/Li+. Samples were transferred to the XPS chamber using an anaerobic transfer vessel provided by the instrument manufacturer. XPS spectra were analyzed using CasaXPS software. The spectra were charge-corrected relative to the adventitious C 1s peak (284.8 eV). After applying a Shirley-type background correction, a least-squares regression peak model was developed using a Gaussian-Lorentzian (GL(30)) line shape. The 2p3/2 to 2p1/2 doublet peak area ratio was fixed at 0.5 according to the ratio of degeneracy, and the full width half maximum (FWHM) values were set equal, while being constrained to 164 eV), which are not present in the reference LPS spectrum in Figure 2a, suggest the formation of additional phases. A significant increase in the contribution from bridging sulfur bonds (−[S]n−) was observed (~30%), which is clear evidence for sulfur oxidation in LPS. Possible moieties that contain bridging sulfur bonds include: P2S84−, P2S74−, P2S62−.10,11,19 DFT calculations predict the formation of S and P2S5 upon LPS oxidation.9 Regarding experimental evidence for the formation of elemental sulfur, Hakari et al.11 have argued that the absence of a peak at 164 eV, the binding energy of elemental sulfur,32 indicates that elemental S does not form at the interface. In contrast, we believe that elemental sulfur may be treated as a sub-component of the bridged sulfur bonding environment ([S]8 or S−[S]6−S), and the observed increase in the XPS signal intensity above 164 eV is evidence for the formation of elemental S upon LPS oxidation. In case of P2S5, Koerver et al. found that the XPS peak corresponding to P2S5 did not align with the fitted components of the oxidized LPS XPS spectrum and thus they ruled out the formation of P2S5 as an LPS oxidation product.19 While our oxidized LPS XPS data agrees well with Koerver et al.’s data, we believe that the formation of P2S5 cannot be dismissed. This is because P2S5 contains both bridging and terminal sulfur bonds,12 both of which manifest in the XPS spectrum of oxidized LPS, as seen in Figure 2b. In addition to S and P2S5, a variety of species with an intermediate S oxidation state (between S2− and S0) may form as a result of LPS oxidation. Possible species include: S8, P2S5, P2S84−, P2S74−, P2S62−.10 The P 2p spectrum of as-synthesized LPS and LPS oxidized to 5.0 V while in contact with carbon black is shown in supplementary Figure S1. As in case of the S 2p spectrum, new signal at higher binding energies is a clear indication if LPS oxidation. Component analysis was not performed on the P 2p spectrum due to the unavailability of component level binding energy data. Cyclic Voltammetry for LPS in Contact with a Planar Electronic Conductor. LPS reduction reaction products at the Li metal interface (0 V w.r.t. Li/Li+) have been experimentally characterized in literature,11,12 and LPS oxidation reaction products (5.0 V w.r.t. Li/Li+) are described above. However, the electrochemical redox response of LPS over the range 0-5.0 V (vs Li/Li+) is unclear. We investigated this behavior by first conducting CV

Figure 2. S 2p XPS spectrum and component models for (a) assynthesized LPS and (b) LPS subjected to 5.0 V w.r.t. Li/Li+ when interfaced with Super P® carbon black. New signal above 164 eV is a clear indication of LPS oxidation as terminal sulfur bonds transform into bridging sulfur bonds (−[S]n−). A variety of intermediate oxidized species between S2− and S0 may exist.

Terminal sulfur

(S 2p)

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A similar first-anodic-sweep, starting at the OCV, was carried out on a separate cell of identical configuration, again starting with “pristine” LPS, in order to examine LPS oxidation stability. As shown in Figure 5b, LPS oxidation commences virtually as soon as the WE voltage is increased above the OCV, but with a larger current of tens of microamperes, demonstrating a more facile oxidation than reduction reaction. Nonetheless, we observe redox activity in these cells for essentially any deviation from the open circuit potential. As we show below, the phases formed upon reduction and oxidation do correspond to those for which the electrochemical window in ref. 9 was calculated (P and Li2S upon reduction, and S and P2S5 upon oxidation). Thus, we consider our observations to be, qualitatively, of the same electrochemical window. While our experiments are conducted in the presence of carbon, we do not detect any unexpected phases. Han et al.2 performed CV experiments on LGPS+C on Pt and LGPS on Pt and likewise concluded that the redox peaks in the LGPS+C case are not induced by reactions between LGPS and C, but are due to the decomposition of LGPS itself. Electrochemical Oxidation of LPS. We next consider the phases formed upon oxidation of the LPS. In Figure 4, a relatively large oxidation current is observed in the first anodic sweep up to 5.0 V (peak 5), over which the integrated charge passed is ~0.3 mAh (~0.24 mAh/cm2, where the area is the macroscopic LPS area). Based on the above XPS analysis, the LPS oxidation reaction results in the formation of bridging sulfur moieties such as S8, P2S5, P2S84−, P2S74−, P2S62−.10 Among these, at least three phases, S, P2S5, and Li2P2S6, are known to have negligible electronic conductivity.10,33 Note that the magnitude of the anodic peak above 3 V (peak 5) is significantly diminished in subsequent cycles, suggesting that once these electronically-insulating oxidation products are formed, they passivate the LPS against further reaction. Interestingly, by the fourth cycle, good electrochemical reversibility is seen; the total charge passed over the entire 0-5.0 V range, is 0.53 and 0.52 mAh (~0.42 and 0.41 mAh/cm2) for the cathodic and anodic sweep, respectively. The LPS/C interphase formed upon oxidation, deduced from the CV results, is shown schematically in Figure 6. LPS decomposition forms two kinds of products. An electronically passivating but ionically conducting layer (in red) separates the redox-active LPS decomposition products (orange) from the LPS bulk. Redox-active components of the LPS/C interphase remain in contact with carbon, and are electrochemically active, including within the 1.71-2.31 V window. After the initial decomposition of LPS, it is these redox-active LPS decomposition products that undergo electrochemical redox and not LPS itself. The oxidized LPS/C interphase has an electrochemically inactive component that is analogous to the well-known SEI on graphite, formed upon reduction of non-aqueous liquid electrolytes.34 Our observations of an irreversible passivating interphase are consistent with the widespread observations of high interfacial impedance and low rate capability in LPS-based cells that use cathodes operating at > 4 V (vs. Li/Li+).13-18 Our conclusions do differ from those

experiments on cells in which the LPS layer was in contact with a carbon working electrode (containing no LPS), or a stainless steel current collector. This experiment limits the reactivity of the LPS to the planar interface in contact with the electronic conductor. Three different working electrodes were tested in the configuration in Fig. 1: 1) a Super P® carbon black pellet; 2) an ECP carbon black pellet, and 3) 416 stainless steel (polished with 1000 grit SiC abrasive paper). In all cases, the WE voltage was first swept from the as-assembled open circuit voltage (OCV) down to 0 V, then back up to 5.0 V, and ultimately back to the asassembled OCV again, constituting a full cycle between 0-5.0 V (vs Li/Li+). The CV results show several common redox peaks in all three cell configurations, labeled 1 through 4 in Figure 3. Similarities in the CV peak positions for the metal and carbon black electrodes suggest that the observed electrochemical activity is solely due to LPS redox rather than any reactivity with the WE. The differences in the peak currents between the three WEs are attributed to varying reaction rates. The same peaks are seen when the planar WE is replaced by a LPS + C electrode designed to enhance reactivity, as discussed below. Cyclic Voltammetry using LPS+C Working Electrodes. Next, we conducted CV experiments on cells in which the WEs were mixed conducting composites with 75 wt% LPS and 25 wt% Super P® carbon black (see Figure 1). The WE voltage was first swept to 0 V, following which the voltage was cyclically swept between 0-5.0 V (vs Li/Li+). The first four CV sweeps are shown in Figure 4. At 100 μV⋅s-1 sweep rate, the experimental time was ~27 hours per sweep. The redox peaks are labeled 1 through 5 in Figure 4. Significantly higher anodic and cathodic currents are observed compared to the experiments with planar WEs in Figure 3, which we attribute to the greater interfacial area available for reaction in the composite electrodes. The coexisting phases for LPS at 0, 1.71, 2.31, and 5 V, calculated by DFT, are shown at the top of Figure 4. The calculated electrochemical stability window,9 of 1.71-2.31 V, is shown by the orange lines. The starting OCV of the cell after assembly is ~2.4 V, indicated by the black diamond in Figure 4. During the first cathodic sweep starting from the OCV at 2.4 V (cycle 1 - black curve in Figure 4), a measurable reduction current is observed within the 1.71-2.31 V window where LPS stability has been proposed (peak 1). An expanded view of the cycle 1 cathodic sweep, Figure 5a, reveals a microampere scale reduction current beginning almost immediately as the WE voltage drops below OCV (2.41 V). The origin of the small peak at 2.04 V is unclear, but this peak contributes negligible charge to the total reduction current in this voltage window. The reduction current increases monotonically as the potential decreases in this first sweep. Figure 4 shows that in subsequent sweeps, the reduction current in this voltage window grows substantially (peak 1). However, decomposition of the LPS has already been induced in those subsequent sweeps; only the first cathodic sweep is a measure of the reactivity of the “pristine” LPS and can be used as a measure of the lower bound of the electrochemical stability window.


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of Hakari et al.,11 that the high voltage LPS/C interphase is reversible.

Figure 3. CV data for three planar WEs in a Li/LPS/WE cell: (a) pressed Super P® carbon black (blue), (b) pressed ECP carbon black (green), and (c) grade 416 stainless steel (red), ground using a 1000 grit SiC abrasive paper. The voltage was swept between 0-5 V w.r.t. Li/Li+ at 100 μV⋅s-1. The right hand scale corresponds to the stainless steel current collector, and shows a lower absolute current than for the two carbon electrodes. All three cases demonstrate similar CV peak positions suggesting that the electrochemical activity is predominantly from LPS redox at the LPS/WE interface.

Figure 4. Cyclic voltammograms for the first four cycles of a Li/LPS/WE cell where the WE is a composite of LPS and Super P® carbon black. The voltage was swept between 0-5 V.0 w.r.t. Li/Li+ at 100 μV⋅s-1. The charge passed during the reduction/oxidation sweep in cycle 4 was 0.53/0.52 mAh, demonstrating the electrochemical reversibility of the LPS/C interphase across the 0-5.0 V range. Unlike



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liquid electrolytes which reduce/oxidize irreversibly, the LPS/C interphase displays reversible redox behavior in the operating voltage range of graphite anodes and sulfur cathodes, and its chemical composition varies during cycling. However, LPS irreversibly oxidizes in the voltage range of high-voltage cathodes resulting in a passivating, high impedance interphase. Figure 6. Schematic of the LPS/C interphase deduced from CV results. A passivating layer (red, electrically resistive, ionically conductive), separates the reversible LPS decomposition products (orange) from LPS.

Electrochemical Reduction of LPS. During the first cathodic sweep from the OCV down to 0 V (vs Li/Li+), and in each subsequent cathodic sweep after cycling to 5.0 V, a large, reversible, reduction current is observed (Figure 4 peak 1). The main difference in subsequent cycles is a shift of the reduction current during the cathodic sweep to higher potential. We hypothesized that the redox activity in the potential range between zero and the OCV at 2.41 V is primarily a superposition of the reactions due to sulfur and phosphorus species formed in the interphase. Accordingly, we prepared cells using working electrodes in which elemental sulfur or phosphorus were added as redox-active internal standards, and carried out cyclic voltammetry to test this hypothesis. Cyclic Voltammetry with S and P as internal redox standards. In Figure 7, CV results are shown for experiments in which the working electrode is a LPS+C+S composite pellet containing (1) 30 wt% and (2) 60 wt% sulfur, respectively. The first cycle for these two is plotted in Figure 7a along with cycle 2 from Figure 4, where the working electrode is LPS+C only. Five features are labeled in the cyclic voltammogram. For the cell with 30 wt% sulfur, a sweep from 1.0-3.5 V (red curve) shows clearly the reduction and oxidation peaks (peaks 1 and 2) associated with the formation of S0 and S2− species,23 respectively. Next, consider the first cycle results for the cell containing 60 wt% added sulfur in the working electrode, which is here swept over 0-5.0 V (green curve). The feature labeled “Peak 3” occurring below 1.5 V, which is also seen in Figure 4, is due to the reduction of phosphorus from P5+ (in assynthesized LPS) ultimately to P3− (in the Li3P phase), which is a known reduction product of LPS when in contact with Li metal.9,12 Further work is required to determine specifically the potential at which Li3P forms, since literature reports only confirm the presence of Li3P at 0 V vs Li/Li+.9,12 Peak 4, seen during the cathodic sweep from 0 V, is attributed to the subsequent oxidation of P3- to elemental P; it is clearly differentiated from the oxidation of sulfur (peak 2), which occurs at much higher potential (> 2.31 V vs Li/Li+) as shown by the red curve in Figure 7a. Upon further oxidation, a prominent broad feature labeled “Peak 5” appears in the green curve, which we attribute to the oxidation of P0 to P5+.

Figure 5. Cyclic voltammogram on expanded scale for (a) the first cathodic sweep, and (b) first anodic sweep, starting from OCV, for a Li/LPS/WE cell where the WE is a composite of LPS and Super P® carbon black. The scan rate was 100 μV⋅s-1. LPS reduction in (a) and oxidation in (b) commence as soon as the WE voltage deviates from OCV.


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Figure 7. (a) CV sweeps for Li/LPS/LPC+C+S cells in which the working electrode contains (1) 30 wt% (red curve) and (2) 60 wt% sulfur (green curve), compared to the CV sweep for a reference LPS+C sample (blue curve). The reduction and oxidation peaks in the 30 wt% sulfur case correspond to the formation of S2− and S0 species, respectively, which correspond to peak 1 and peak 2 in the LPS+C CV profile. Phosphorus redox reactions can then be identified and correlated to the remaining CV peaks in the 60 wt% case, thus making possible the deconvolution of the CV redox activity of LPS/C interphase. (b) CV data for Li/LPS/LPC+C+P cell with 30 wt% phosphorus, overlaid on reference LPS CV sweeps, cycle 1 and 2 from Figure 4.



As a final test, phosphorus was added as an internal standard by preparing cells in which the working electrode is a LPS+C+P composite containing 30 wt% red phosphorus powder. In Figure 7b, the first cycle of this cell over 0-5.0 V (purple curve) is shown against cycles 1 and 2 from Figure 4. This cell was cycled in the same manner, and at the same sweep rate, as cycle 1 (dashed blue curve), beginning with a cathodic sweep from the OCV. In general, the redox features of this cell are both broadened and reduced in intensity compared to the cells containing added sulfur. During the cathodic sweep, there is reduced intensity in the low potential regime where reduction of both sulfur and phosphorus is expected. At least some of the deliberately added phosphorus may be reduced to P3- during the sweep to 0 V. During the subsequent anodic sweep, broad features are seen over a wide voltage range. Compared to the cycles from Figure 4 (blue curves), sulfur redox is diminished, but phosphorus oxidation in the voltage range for Peak 5 in increased, as expected for the added phosphorus. It does appear, however, that the addition of phosphorus diminishes overall reactivity.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y.-M Chiang)

Present Address †Current affiliation: Institute of Materials Research and Engineering, Singapore 138634, Singapore

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors gratefully acknowledge support from the US Department of Energy, Office of Basic Energy Science, through award number DE-SC0002633 (J. Vetrano, Program Manager). Equipment and support for XPS from the MIT Materials Research Laboratory (MRL) is gratefully acknowledged.

REFERENCES (1) Takada, K. Progress and Prospective of Solid-State Lithium Batteries. Acta Mater. 2013, 61 (3), 759–770. (2) Seino, Y.; Ota, T.; Takada, K.; Hayashi, A.; Tatsumisago, M. A Sulphide Lithium Super Ion Conductor Is Superior to Liquid Ion Conductors for Use in Rechargeable Batteries. Energy Environ. Sci. 2014, 7 (2), 627. (3) Liu, Z.; Fu, W.; Payzant, E. A; Yu, X.; Wu, Z.; Dudney, N. J.; Kiggans, J.; Hong, K.; Rondinone, A. J.; Liang, C. Anomalous High Ionic Conductivity of Nanoporous Beta-Li3PS4. J. Am. Chem. Soc. 2013, 135 (3), 975–978. (4) Kamaya, N.; Homma, K.; Yamakawa, Y.; Hirayama, M.; Kanno, R.; Yonemura, M.; Kamiyama, T.; Kato, Y.; Hama, S.; Kawamoto, K.; Mitsui, A. A Lithium Superionic Conductor. Nat. Mater. 2011, 10 (9), 682–686. (5) Minami, K.; Hayashi, A.; Tatsumisago, M. Preparation and Characterization of Superionic Conducting Li7P3S11 Crystal from Glassy Liquids. J. Ceram. Soc. Japan 2010, 4 (118), 305–308. (6) Chen, Y.; Cai, L.; Liu, Z.; dela Cruz, C. R.; Liang, C.; An, K. Correlation of Anisotropy and Directional Conduction in β-Li3PS4 Fast Li+ Conductor. Appl. Phys. Lett. 2015, 107 (1), 13904-13909. (7) Gobet, M.; Greenbaum, S.; Sahu, G.; Liang, C. Structural Evolution and Li Dynamics in Nanophase Li3PS4 by Solid-State and Pulsed-Field Gradient NMR. Chem. Mater. 2014, 26 (11), 3558– 3564. (8) Homma, K.; Yonemura, M.; Kobayashi, T.; Nagao, M.; Hirayama, M.; Kanno, R. Crystal Structure and Phase Transitions of the Lithium Ionic Conductor Li3PS4. Solid State Ionics 2011, 182 (1), 53–58. (9) Zhu, Y.; He, X.; Mo, Y. First Principles Study on Electrochemical and Chemical Stability of the Solid ElectrolyteElectrode Interfaces in All-Solid-State Li-Ion Batteries. J. Mater. Chem. A 2016, 4 (9), 3253–3266. (10) Koerver, R.; Walther, F.; Aygun, I.; Sann, J.; Dietrich, C.; Zeier, W. G.; Janek, J. Redox-Active Cathode Interphases in SolidState Batteries. J. Mater. Chem. A 2017, 5 (43), 22750–22760. (11) Hakari, T.; Deguchi, M.; Mitsuhara, K.; Ohta, T.; Saito, K.; Orikasa, Y.; Uchimoto, Y.; Kowada, Y.; Hayashi, A.; Tatsumisago, M. Structural and Electronic-State Changes of a Sulfide Solid Electrolyte during the Li Deinsertion-Insertion Processes. Chem. Mater. 2017, 29 (11), 4768–4774. (12) Wenzel, S.; Weber, D. A.; Leichtweiss, T.; Busche, M. R.; Sann, J.; Janek, J. Interphase Formation and Degradation of Charge Transfer Kinetics between a Lithium Metal Anode and Highly Crystalline Li7P3S11 Solid Electrolyte. Solid State Ionics 2016, 286 (1), 24–33.

SUMMARY AND CONCLUSIONS A comprehensive evaluation of the redox stability of βLi3PS4 (LPS) solid electrolyte over the voltage range 0-5.0 V, encompassing the use of LPS with lithium metal electrodes and high voltage cathodes in all-solid state batteries, has been conducted. In order to accentuate reactivity of the LPS, electronic conductivity is provided by preparing working electrodes consisting of LPS-carbon black composites, in lithium metal half-cells in which the electrolyte layer is pure LPS. In addition, cells are tested in which elemental sulfur and phosphorus are added as internal redox standards to the composite working electrode. The electrochemical stability window of LPS appears to be narrower than computationally determined by DFT for decomposition to the same nominal phases, at least when carbon is present. Upon electrochemical oxidation, LPS decomposes irreversibly to form interphase rich in oxidized sulfur species. The irreversibility is attributed to the electronically insulating nature of these reaction products, and is analogous to the SEI formed on graphite in lithiumion batteries. Upon electrochemical reduction, LPS undergoes decomposition to species in which both sulfur and phosphorus are reduced, and the total extent of reaction is several times greater (as measured by charge flow in cyclic voltammetry) than that which occurs during oxidation. However, the interphase formed upon reduction appears to be reversible, in contrast to that formed upon oxidation. These results help to explain the higher impedance typically observed at the cathode interface in solid state batteries that use LPS-based electrolytes. The experimental methodology demonstrated here should be generally useful for characterizing solid electrolyte stability in other systems.

Supporting Information Phosphorus 2p spectrum of as-synthesized LPS, and LPS oxidized to 5.0 V, while in contact with carbon black.


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Electrochemical Redox Behavior of Li Ion Conducting Sulfide Solid

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