Exothermal reaction and microstructure study of the charged LiNi0.5Co0.2Mn0.3O2 electrode layers using argyrodite Li7−xPS6−xClx solid electrolytes

Abstract

Sulfide-based solid electrolytes (SEs) are thermally stabler than liquid organic electrolytic solutions. However, sulfide-type all-solid-state batteries exhibit exothermic reactions during charge and discharge because sulfide-based SEs thermally react with the electrode active materials. Herein, we examined the exothermic behavior of a LiNi0.5Co0.2Mn0.3O2 (NCM523) positive electrode composite using argyrodite-type SEs. To clarify the exothermal mechanism of the NCM523 composites, the microstructural changes owing to heating were investigated via transmission electron microscopy (TEM). The charged NCM523 composites exhibited a conspicuous exothermic reaction at ∼200 °C. Ex situ TEM observations at 180 °C and 250 °C revealed that the SEs become considerably amorphized and nanocrystallites of Li2S and LiCl precipitate in the SE region at the SE/NCM523 interface. Energy-dispersive X-ray spectroscopy also revealed that oxygen disperses into the SE region. At 250–400 °C, furthermore, the charged NCM523 composites showed several exothermic reactions. Ex situ TEM observations at 400 °C revealed that nanocrytallites of transition metal sulfides, Li3PO4 and Li2SO4, are formed, which indicated that the SE thermally reacts with NCM523 and LiNbO3 coatings. These results suggest that the main exothermic reaction of the NCM523/argyrodite-type SE composites is attributable to the oxidative decomposition reaction of argyrodite-type SEs. To suppress exothermic reactions, it is necessary to prevent oxygen from dispersing into the SE. It is thus found that a coating material is one of the keys to improve the thermal stability of all-solid-state cells using sulfide-based SEs.