Interfacial Observation between LiCoO2 Electrode and Li2S−P2S5 Solid Electrolytes of All-Solid-State Lithium Secondary Batteries Using Transmission Electron Microscopy

In all-solid-state lithium secondary batteries, both the electrode and electrolyte materials are solid. The electrode and solid electrolyte interface's structure and morphology affect a battery's electrochemical performance. Observation of the interface between LiCoO2 positive electrode and highly lithium-ion-conducting Li2S−P2S5 solid electrolyte was conducted using transmission electron microscopy. An interfacial layer was formed at the interface between LiCoO2 electrode and Li2S−P2S5 solid electrolyte after the battery's initial charge. Furthermore, mutual diffusions of Co, P, and S at the interface between LiCoO2 and Li2S−P2S5 were observed. The mutual diffusion and the formation of the interfacial layer were suppressed using LiCoO2 particles coated with Li2SiO3 thin film. Results showed that all-solid-state cells using Li2SiO3-coated LiCoO2 had better electrochemical performance than those using noncoated LiCoO2. The all-solid-state cells functioned at −30 °C. Moreover, the all-solid-state cell using Li2SiO3-coated LiCoO2 was charged and discharged under a high current density of 40 mA cm−2 at 100 °C.