A Functional Air-Stable Li9.8GeP1.7Sb0.3S11.8I0.2 Superionic Conductor for High-Performance All-Solid-State Lithium Batteries

Solid-state electrolytes (SSEs) based on sulfides have become a subject of great interest due to their superior Li-ion conductivity, low grain boundary resistance, and adequate mechanical strength. However, they grapple with chemical instability toward moisture hypersensitivity, which decreases their ionic conductivity, leading to more processing requirements. Herein, a Li_9.8GeP_1.7Sb_0.3S_11.8I_0.2 (LGPSSI) superionic conductor is designed with a Li⁺ conductivity of 6.6 mS cm^-1 and superior air stability based on hard and soft acids and bases (HSAB) theory. The introduction of optimal antimony (Sb) and iodine (I) into the Li₁₀GeP₂S₁₂ (LGPS) structure facilitates fast Li-ion migration with low activation energy (E_a) of 20.33 kJ mol^-1. The higher air stability of LGPSSI is credited to the strategic substitution of soft acid Sb into (Ge/P)S₄ tetrahedral sites, examined by Raman and X-ray photoelectron spectroscopy techniques. Relatively lower acidity of Sb compared to phosphorus (P) realizes a stronger Sb-S bond, minimizing the evolution of toxic H₂S (0.1728 cm³ g^-1), which is ∼3 times lower than pristine LGPS when LGPSSI is exposed to moist air for 120 min. The NCA//Li-In full cell with a LGPSSI superionic conductor delivered the first discharge capacity of 209.1 mAh g^-1 with 86.94% Coulombic efficiency at 0.1 mA cm^-2. Furthermore, operating at a current density of 0.3 mA cm^-2, LiNbO₃@NCA/LGPSSI/Li-In cell demonstrated an exceptional reversible capacity of 117.70 mAh g^-1, retaining 92.64% of its original capacity over 100 cycles.