Enhanced Air and Electrochemical Stability of Li₇P_2.9Ge_0.05S_10.75O_0.1 Electrolytes 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 S2–/O2– cosubstitution strategy increases the σLi+ 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 Li7P2.9Ge0.05S10.75O0.1 SSEs 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 L7P3S11 after GeO2 substitution. Accordingly, the LiNbO3@NCA/Li7P2.9Ge0.05S10.75O0.1/Li–In cell performed remarkably; first discharge capacity, 158.9 mAh g–1; capacity retention, 89%; and Coulombic efficiency, ∼100% after 50 cycles @ 0.064 mA cm–2 and even at 0.3 mA cm–2 versus the first discharge capacity and retention (129.4 mAh g–1 and 75.73%) after 70 cycles @ RT. These remarkable results could be attributable to the excellent σLi+, chemical/electrochemical stability toward LiNbO3@NCA, and meager interfacial resistance, essential for the practical application of sulfide-based batteries.