Enhanced Air and Electrochemical Stability of Li 7 P 2.9 Ge 0.05 S 10.75 O 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 P 5+ /Ge 4+ 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 Li 7 P 3 S 11 , the derived electrolytes largely suppressed the structural hydrolysis in the air. Furthermore, the Li//Li cell with novel Li 7 P 2.9 Ge 0.05 S 10.75 O 0.1 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 Li 7 P 3 S 11 /LiNbO 3 @NCA interface during cell operation. XPS results confirmed better thermodynamic stability between LiNbO 3 @NCA and L 7 P 3 S 11 after GeO 2 substitution. Accordingly, the LiNbO 3 @NCA/Li 7 P 2.9 Ge 0.05 S 10.75 O 0.1 /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 σ Li+ , chemical/electrochemical stability toward LiNbO 3 @NCA, and meager interfacial resistance, essential for the practical application of sulfide-based batteries.