Crystal Orientation-Dependent Interface Compatibility in the Oxide Composite Cathode by in Situ Heating Transmission Electron Microscopy

Abstract

All-solid-state batteries (ASSBs) with oxide-based solid electrolytes are getting prominence as a forthcoming battery system capable of overcoming the drawbacks of current lithium-ion batteries by satisfying the expanding demand for high energy density and safety. However, the poor interfacial contact between cathode active materials and solid electrolytes, at which lithium ions diffuse and the charge transfer occurs, is a major concern for practical utilization. Although the co-sintering process at high temperatures is essential to achieve an intimate interface contact, excessive thermal energy reversely accelerates the interfacial degradation reaction, of which the mechanism has been still unexplored. Here, using an epitaxial model system in which the crystal orientations of the Li(Ni1/3Co1/3Mn1/3)O2 cathode and Li3xLa(2/3)-x⎕(1/3)-2xTiO3 solid electrolyte are controlled, we directly probe the interfacial reaction in real-time during heating by in situ heating transmission electron microscopy, and investigate the impact of the crystal orientation at the interface on the interfacial reaction mechanism and kinetics. In situ observation reveals that the interfacial reactions are highly dependent on the crystal orientation at the interface involving the onset temperature of the reaction, the diffusion behavior of lithium ions, the intermediate states, and the overall degradation mechanism between the active material and the solid electrolyte. The interfacial degradation during heating increases the charge transfer resistance between the cathode active material and the solid electrolyte, and the increasing tendency of the resistance is closely related to the crystal orientation-dependent interfacial degradation.