Enhanced Air and Electrochemical Stability of Li₇P_2.9Ge_0.05S_10.75O_0.1Electrolytes with High Ionic Conductivity for Thiophosphate-Based All-Solid-State Batteries

Sulfide solid electrolytes (SSEs) show tremendous potential to realize high-energy-density secondary batteries and offer distinguishing safety features over the traditional liquid-electrolyte-based system. However, their installation is hindered by the air sensitivity and substandard interfacial compatibility with Li-metal anodes. Herein, an aliovalent P⁵⁺/Ge⁴⁺and isovalent S^2-/O^2-cosubstitution strategy increases the σ_Li+to 4.77 mS cm^-1, which is associated with the lowest activation energy (18.66 kJ mol^-1). Impressively, with limited substitution of P/Ge and S/O in Li₇P₃S₁₁, the derived electrolytes largely suppressed the structural hydrolysis in the air. Furthermore, the Li//Li cell with novel Li₇P_2.9Ge_0.05S_10.75O_0.1SSEs realized Li plating/stripping over 100 h at 0.1 mA cm^-2/0.1 mAh cm^-2@ RT, with the lowest overpotential at ∼5 mV. Next, ex situ X-ray photoelectron spectroscopy (XPS) quantified the electrochemical decomposition of the Li₇P₃S₁₁/LiNbO₃@NCA interface during cell operation. XPS results confirmed better thermodynamic stability between LiNbO₃@NCA and L₇P₃S₁₁after GeO₂substitution. Accordingly, the LiNbO₃@NCA/Li₇P_2.9Ge_0.05S_10.75O_0.1/Li-In cell performed remarkably; first discharge capacity, 158.9 mAh g^-1capacity retention, 89%; and Coulombic efficiency, ∼100% after 50 cycles @ 0.064 mA cm^-2and even at 0.3 mA cm^-2versus the first discharge capacity and retention (129.4 mAh g^-1and 75.73%) after 70 cycles @ RT. These remarkable results could be attributable to the excellent σ_Li+, chemical/electrochemical stability toward LiNbO₃@NCA, and meager interfacial resistance, essential for the practical application of sulfide-based batteries.