Elucidating Reversible Electrochemical Redox of Li6PS5Cl Solid Electrolyte

Sulfide-based solid electrolytes are promising candidates for all solid-state batteries (ASSBs) due to their high ionic conductivity and ease of processability. However, their narrow electrochemical stability window causes undesirable electrolyte decomposition. Existing literature on Li-ion ASSBs report an irreversible nature of such decompositions, while Li–S ASSBs show evidence of some reversibility. Here, we explain these observations by investigating the redox mechanism of argyrodite Li6PS5Cl at various chemical potentials. We found that Li–In | Li6PS5Cl | Li6PS5Cl–C half-cells can be cycled reversibly, delivering capacities of 965 mAh g–1 for the electrolyte itself. During charging, Li6PS5Cl forms oxidized products of sulfur (S) and phosphorus pentasulfide (P2S5), while during discharge, these products are first reduced to a Li3PS4 intermediate before forming lithium sulfide (Li2S) and lithium phosphide (Li3P). Finally, we quantified the relative contributions of the products toward cell impedance and proposed a strategy to reduce electrolyte decomposition and increase cell Coulombic efficiency.