Multiscale strain alleviation of Ni-rich cathode guided by in situ environmental transmission electron microscopy during the solid-state synthesis

Ni-rich layered oxides are one of the most promising cathode materials for Li-ion batteries due to their high energy density. However, the chemomechanical breakdown and capacity degradation associated with the anisotropic lattice evolution during lithiation/delithiation hinders its practical application. Herein, by utilizing the in situ environmental transmission electron microscopy (ETEM), we provide a real time nanoscale characterization of high temperature solid-state synthesis of LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) cathode, and unprecedentedly reveal the strain/stress formation and morphological evolution mechanism of primary/secondary particles, as well as their influence on electrochemical performance. We show that stress inhomogeneity during solid-state synthesis will lead to both primary/secondary particle pulverization and new grain boundary initiation, which are detrimental to cathode cycling stability and rate performance. Aiming to alleviate this multiscale strain during solid-state synthesis, we introduced a calcination scheme that effectively relieves the stress during the synthesis, thus mitigating the primary/secondary particle crack and the detrimental grain boundaries formation, which in turn improves the cathode structural integrity and Li-ion transport kinetics for long-life and high-rate electrochemical performance. This work remarkably advances the fundamental understanding on mechanochemical properties of transition metal oxide cathode with solid-state synthesis and provides a unified guide for optimization the Ni-rich oxide cathode.