Influence of the Halogen in Argyrodite Electrolytes on the Electrochemical Performance of All-Solid-State Lithium Batteries

All-solid-state lithium batteries (ASSLBs) are considered as an alternative solution to lithium-ion batteries, because of their safety and high theoretical energy density. Argyrodite-based solid-electrolytes (SEs), Li₆PS₅X (X = Cl, Cl_0.5Br_0.5 or Br), are promising candidates for ASSLBs. Most of the previous reports have used Li₆PS₅Cl as the default SE composition. Here, the electrochemical behavior of three different argyrodites with Cl⁻ or Br⁻, or both, as the halogen is systematically studied. Using LiNi_0.6Co_0.2Mn_0.2O₂ as a model cathode, the behavior of these SEs in ASSLB cells is also studied. SEs containing Br show higher near-room-temperature ionic conductivity (>2 mS cm⁻¹) and the critical current density (≥1 mA cm⁻²) during Li plating/stripping, and are stable up to 5 V versus Li/Li⁺. Li₆PS₅Br gives the best electrochemical performance in terms of C-rate and long-term cycling among the three samples. Specifically, the cathode delivers an initial reversible capacity of 156 mAh g⁻¹, with ≈27% irreversible capacity loss and 90% capacity retention over 100 cycles, and >99% Coulombic efficiency. It delivers ≈56 mAh g⁻¹ at 10C, 36% of its initial capacity at 0.2C, whereas Li₆PS₅Cl and Li₆PS₅Cl_0.5Br_0.5 deliver only 20 and 46 mAh g⁻¹ at 10C.