Investigation of Moisture-Induced Degradation Mechanisms Between Sulfide Solid Electrolytes and Cathode Materials and Reviving Strategies for Electrode Interface Stabilization

Inorganic sulfide-based solid-state electrolytes (SSEs) are critical for enhancing the ionic conductivity. However, these SSEs suffer from interfacial chemical instability, reacting with even trace amounts of moisture to produce toxic hydrogen sulfide (H_{<span style="font-size:10.8333px">2</span>}S) gas and other by-products. This sensitivity leads to compositional changes at SSE particle interfaces and significant performance degradation through additional parasitic reactions. Despite the critical impact of moisture-induced degradation at the electrode level, particularly with cathode materials like LiNi_xCo_yMn_zO_{<span style="font-size:10.8333px">2</span>} (NCM), systematic studies under practical conditions, especially those focusing on electrode-level interactions rather than SSEs themselves, remain scarce, leaving key mechanisms inadequately understood. In this study, we elucidate the moisture-induced degradation mechanisms of the sulfide electrolyte Li₆PS₅Cl (LPSCl) within NCM-SSE composites by employing synchrotron-based high-resolution imaging and advanced X-ray absorption techniques. Contrary to previous reports that emphasized gaseous by-products, our findings reveal that moisture exposure leads to the formation of solid-phase reductive products that attack active NCM materials at the electrode level, resulting in significant spinel-like phases on the surfaces of NCM particles. Quantitative depth profiling demonstrates severe degradation near the electrolyte layer, with the reduction of cathode particles confined to their surfaces, suggesting a limited overall impact. Furthermore, we show that subsequent vacuum heating effectively removes surface moisture and reductive by-products. By flowing oxygen gas at a constant rate during the vacuum annealing process, residual materials undergo oxidation and oxygen substitution occurs at sulfur sites in the LPSCl lattice, forming stable surface layers that restore lithium ion conductivity in the sulfide SSE. These mitigation strategies, including O_{<span style="font-size:10.8333px">2</span>} annealing with vacuum treatments, not only restore the ionic conductivity of the degraded SSE but also prevent the degradation of NCM when combined as a composite electrode. Our study provides a deeper understanding of the electrode-SSE interface chemistry and offers practical solutions for improving the durability and efficiency of solid-state energy storage systems.