Experimental Assessment of the Practical Oxidative Stability of Lithium Thiophosphate Solid Electrolytes

All-solid-state batteries are often expected to replace conventional lithium-ion batteries in the future. However, the practical electrochemical and cycling stability of the best-conducting solid electrolytes, i.e. lithium thiophosphates, is still a critical issue that prevents long-term stable high-energy cells. In this study, we apply a stepwise cyclic voltammetry approach to obtain information on the practical oxidative stability limit of Li10GeP2S12, two different Li2S–P2S5 glasses, as well as the argyrodite Li6PS5Cl solid electrolytes. We employ indium metal and carbon black as the counter and working electrodes, respectively, the latter to increase the interfacial contact area to the electrolyte as compared to the commonly used planar steel electrodes. Using a stepwise increase in the reversal potentials, the onset potential of oxidative decomposition at the electrode–electrolyte interface at 25 °C is identified. X-ray photoelectron spectroscopy is used to investigate the oxidation of sulfur(-II) in the thiophosphate polyanions to sulfur(0) as the dominant redox process in all electrolytes tested. Our results suggest that in later cycles the crystalline solid electrolyte itself is not the major redox active phase, but rather that only after the formation of such electrolyte decomposition products is significant redox behavior observed. Indeed, the redox behavior of the decomposition products is an additional contributor to the overall cell capacity of solid-state batteries. The stepwise cyclic voltammetry approach presented here shows that the practical oxidative stability at 25 °C of thiophosphate solid electrolytes against carbon is kinetically higher than predicted by thermodynamic calculations and that the decomposition products dominate the redox behavior of cathode composites. The method serves as an efficient guideline for the determination of practical, kinetic stability limits of solid electrolytes with respect to the employed electrode materials.