Crystallization behavior of the Li₂S-P₂S₅ glass electrolyte in the LiNi_1/3Mn_1/3Co_1/3O₂ positive electrode layer

Sulfide-based all-solid-state lithium batteries are a next-generation power source composed of the inorganic solid electrolytes which are incombustible and have high ionic conductivity. Positive electrode composites comprising LiNi_1/3Mn_1/3Co_1/3O₂ (NMC) and 75Li₂S·25P₂S₅ (LPS) glass electrolytes exhibit excellent charge-discharge cycle performance and are promising candidates for realizing all-solid-state batteries. The thermal stabilities of NMC-LPS composites have been investigated by transmission electron microscopy (TEM), which indicated that an exothermal reaction could be attributed to the crystallization of the LPS glass. To further understand the origin of the exothermic reaction, in this study, the precipitated crystalline phase of LPS glass in the NMC-LPS composite was examined. In situ TEM observations revealed that the β-Li₃PS₄ precipitated at approximately 200 °C, and then Li₄P₂S₆ and Li₂S precipitated at approximately 400 °C. Because the Li₄P₂S₆ and Li₂S crystalline phases do not precipitate in the single LPS glass, the interfacial contact between LPS and NMC has a significant influence on both the LPS crystallization behavior and the exothermal reaction in the NMC-LPS composites.