Elucidating Reversible Electrochemical Redox of Li 6 PS 5 Cl 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 Li 6 PS 5 Cl at various chemical potentials. We found that Li–In | Li 6 PS 5 Cl | Li 6 PS 5 Cl–C half-cells can be cycled reversibly, delivering capacities of 965 mAh g –1 for the electrolyte itself. During charging, Li 6 PS 5 Cl forms oxidized products of sulfur (S) and phosphorus pentasulfide (P 2 S 5 ), while during discharge, these products are first reduced to a Li 3 PS 4 intermediate before forming lithium sulfide (Li 2 S) and lithium phosphide (Li 3 P). 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.