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are sensitive to the starting configuration of the inter- face system. In addition, it is important to understand that the structural-relaxation method only optimizes the atomic coordinates locally at the interface and can- not account for any activated process, such as atomic diffusion or the nucleation of new solids. Interfaces for LiFePO4 (FePO4)/Li3PS4 (ref.89), Li7P3S11/Li, Li10GeP2S12/Li, β-Li3PS4/Li (ref.90) and NaCoO2/Na3PS4 (ref.91) have been modelled using ab initio molecular dynamics. This type of simulation of the interface has a high computational cost and typically only captures the dynamics of the system at elevated temperatures and very small time scales (<1 ns). Hence, it should always be combined with a thermodynamic assessment of the possible reaction products. In the following sections, we relate results obtained using these computational methods to experimental observations in interface systems involving various classes of SEs. Sulfides Sulfides, especially thio-phosphates based on the Li–P–S system, have emerged as leading SE candidates because of their high ionic conductivities. In addition, their solution processability and ability to deform under cold pressing provide sulfides with an advantage for cell manufac- turing compared with oxides. Examples of sulfide SEs with high ionic conductivity include the thio-LISICON conductor Li3.25Ge0.25P0.75S4 (2.2 mS cm−1)30, LGPS (12 mS cm−1)32, Li7P3S11 glass-ceramic (17 mS cm−1)36 and nanoporousβ-Li3PS4 (0.16mScm−1)92. Narrow stability windows of sulfides Despite the high ionic conductivity of sulfide SEs, their lack of interfacial stability in SSBs remains a pressing issue. Although electrochemical stability windows from 0 V to more than 4 V (versus Li metal) have been claimed in many studies based on cyclic voltammetry (CV) measure- ments30,32,36,92,93, DFT calculations predicted a propensity for S2− to oxidize at approximately 2–2.5 V (refs64–66). Furthermore, SSBs employing sulfide SEs often exhibit a large first-cycle capacity loss and subsequent capacity fade of approximately 1–2% per cycle60,94. Such poor capacity retention can be partly attributed to the high and growing interfacial resistance between the sulfide SE and electrode (or carbon), which has been observed in both theoretical modelling and care- fully designed electrochemical measurements. Using electrochemical impedance spectroscopy, the variation of the resistance of a β-Li3PS4-based solid-state cell as a function of the open-circuit voltage has been separated into different origins60 (fig. 3a). It has been demonstrated that a large and irreversible interfacial resistance built up at the cathode/sulfide SE interface upon the first charge, with the most drastic increase occurring between 3.2 and 3.4 V. This high interfacial resistance at the cathode/ sulfide SE interface can be understood by considering the narrow DFT-calculated electrochemical stability windows of sulfides between 1.5 and 2.5 V (refs64–66,95,96), above which the oxidation decomposition of sulfides would occur. For example, LGPS is predicted to have an electrochemical stability window of 1.7–2.1 V (ref.65) or2.1–2.3V(ref.64),bothofwhicharemuchnarrower than the stability limits claimed from CV measurements. The pitfalls of CV measurements are discussed in detail in a later section. The discrepancy between the CV measurements and ab initio predictions was reconciled by adding carbon to LGPS to increase the active area (the contact area between LGPS and an electron con- ductor) for the charge-transfer reaction97, thus increasing the extent of the decomposition reaction. The CV result of a Li|LGPS|LGPS+C|Pt cell between 1.0 and 3.5 V is presented in fig. 3b, which clearly shows the oxidation of LGPS starting at approximately 2.1 V (refs97,98). Using the same method, a reduction potential at 1.7 V was also observed for LGPS97. These measured oxidation and reduction limits are in excellent agreement with the DFT-predicted values, contrary to previous experimental reports32. In a different attempt, a Na SE was mixed with carbon to determine its electrochemical stability window by slow galvanostatic charging and discharging in a liq- uid cell and monitoring the voltage–capacity profiles50. The resulting windows of 0.9–2.5 V and 1.25–2.35 V for Na3PS4 and Na3PSe4, respectively, are in reasonably good agreement with the theoretical predictions (1.55–2.25 V for Na3PS4 and 1.80–2.15 V for Na3PSe4)50. Electronically conductive additives such as carbon have an important role in the SE decomposition97: the SE decomposition at high voltage is a pure electrochem- ical process, as it can occur at the SE/carbon interface, where the SE provides the Li-ion path and carbon pro- vides the electron path. At this interface, electrochemical oxidation of the sulfides occurs instead of reduction, as would be expected from a purely chemical reaction with carbon. These insights further highlight a serious prob- lem associated with SSBs: although adding conductive additives, such as carbon, to the cathode composite is common, decomposition of the SE will occur wherever the SE contacts the electron path (current collector, con- ductive additive). Even though this degradation may not be immediately visible in the short-term performance of the cell, as this interface is not along the Li-ion or electron-transport path to the cathode particles, con- tinued degradation of the SE from this interface will ultimately impair the Li-ion conductivity and lead to performance decay, as observed with long-term cycling of sulfide-based cells69,70. Severe oxidation of β-Li3PS4 on the current collector has also been observed experimen- tally68. This problem can only be resolved by creating a passivating interface between the SE and electron path or by minimizing the addition of conductive additive to the cathode composite. The narrow stability window of LGPS has been fur- ther confirmed by the low voltage of a battery made solely from LGPS as both the active electrode materials and SE98. In line with the predicted low oxidation voltage for sulfides, operando XPS measurements indicated the onset of Li2S–P2S5 oxidation at approximately 2.7 V (ref.99). Sn-substituted LGPS, Li10SnP2S12 (refs34,100), has a simi- lar narrow predicted stability window (1.78–2.02 V)43. Indeed, an electrochemical stability window of 1.5–2.5 V for Li10SnP2S12 was determined from CV measurements with a three-electrode setup, where a lithium counter electrode was not used to avoid side reactions101. Reviews Nature reviews | MaterialsPDF Image | Understanding interface stability in solid-state batteries
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