Thermal‐Induced Structure Evolution at the Interface between Cathode and Solid‐State Electrolyte

The interfaces between the electrode and solid‐state electrolyte play a decisive role in the performance of all‐solid‐state batteries. For example, the formation of the interphase between cathode and solid‐state electrolyte can affect interfacial impedance and thus the rate capability. Herein, the thermal stability at the solid–solid interface between LiMn 2 O 4 cathode and garnet electrolyte LLZTO via combined in situ techniques is studied, including in situ X‐Ray diffraction, in situ transmission electron microscopy, and in situ Raman. The dynamic process of interfacial reaction at different scales is elucidated. Starting from 300 °C, Mn ions from LiMn 2 O 4 would migrate into the solid‐state electrolyte, accompanied with the formation of LiMn 3 O 4 interphase. As the temperature increases to 500 °C, the LiMn 3 O 4 interphase transforms to MnO structure which hinders Li‐ion transportation and therefore increases the interfacial impedance. Although both LiMn 2 O 4 and LLZTO could withstand continuous heating, their interface is inherently thermally unstable at relatively low temperature, which requires special attention during thermal treatment for practical fabrication. Findings provide mechanistic insights into the interfacial reaction which serves as a guidance for the design and manufacturing of all‐solid‐state batteries.