Understanding the Role of Oxygen Substitution in Lithium Conduction and Air/Interfacial Stability of LGPS-Structured Oxy-Sulfide Electrolytes

The development of stable and high-performance solid electrolytes is critical for the commercialization of solid-state batteries (SSBs). This study explores materials with the composition Li_9.6P₃S_12-<i>k</i>O_<i>k</i> (0 ≤ <i>k</i> ≤ 1.0) by partially substituting sulfur with oxygen in Li_9.6P₃S₁₂ to enhance electrochemical and ambient stability while maintaining high ionic conductivity. The synthesized compounds Li_9.6P₃S_11.1O_0.9, Li_9.6P₃S_10.5O_1.5, and Li_9.6P₃S_9.9O_2.1 having 7.5, 12.5, and 17.5% oxygen content, named LPSO-7.5, LPSO-12.5, and LPSO-17.5, respectively, exhibit an LGPS-type structure with reduced secondary phases compared to previous studies. The highest ionic conductivity of 0.75 mS cm^-1 was observed for LPSO-7.5 at 510 MPa, while increased oxygen content led to lower conductivity due to structural distortions and reduced lithium-ion mobility. Electrochemical impedance spectroscopy (EIS) and symmetric Li/SE/Li cells confirmed improved electrochemical performance with lithium metal for LPSO-7.5. Full-cell tests with the LiNi_0.8Co_0.15Al_0.05O₂ (NCA) cathode and graphite or lithium anodes showed that LPSO-7.5 demonstrated the highest capacity retention and the lowest polarization. Additionally, H₂S gas evolution tests confirmed improved air stability with an increasing oxygen content. The results indicate that moderate oxygen substitution optimally balances conductivity, air stability, and interfacial compatibility. LPSO-7.5 emerges as a promising solid electrolyte candidate for next-generation SSBs with enhanced cycle life and reduced degradation.