Influence of the Sintering Temperature on LLZO-NCM Cathode Composites for Solid-State Batteries Studied by Transmission Electron Microscopy

Abstract

The high impedance of the solid electrolyte (SE)| cathode active material (CAM) interface often limits the charge transfer in oxide-based solid-state battery (SSB) cells. Lowering the interfacial impedance by maximizing the contact between SE and CAM can be realized by co-sintering the materials at elevated temperatures. In this work Li_6.25Al_0.25La₃Zr₂O₁₂ (LLZO) and LiNi_0.6Co_0.2Mn_0.2O₂ (NCM) composite cathodes co-sintered at different temperatures were investigated by employing various transmission electron microscopy (TEM) techniques to study the influence of the sintering temperature on the material’s integrity. A degradation of the layered NCM phase (𝑅3̅𝑚) to a NiO (rock salt) like structure (𝐹𝑚3̅𝑚) at the particle surfaces accompanied with an exchange of elements from LLZO and NCM are observed for a sintering temperature as low as 500 °C. This is significantly lower than previously reported and questions the viability of a LLZO-NCM cathode. In-depth research on concepts to stabilize the SE| CAM interfaces is recommended.

Keywords: Solid-state batteries, composite cathodes, SE, CAM interface, LLZO, NCM, co-sintering, layered phase, rock salt-like phase, transmission electron microscopy