Elucidating Reversible Electrochemical Redox of Li6PS5CI 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 vertical bar Li6PS5Cl vertical bar 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.