Observation of electron-beam-induced phase evolution mimicking the effect of the charge-discharge cycle in Li-rich layered cathode materials used for Li ion batteries

Capacity loss and voltage fade upon electrochemical charge-discharge cycling observed in lithium-rich layered cathode oxides (Li[LixMnyTM1-x-y]O2, where TM = Ni, Co, or Fe) have recently been correlated with a gradual phase transformation featuring the formation of a surface reconstructed layer (SRL) that evolves from a thin (<2 nm), defect spinel layer upon the first charge to a relatively thick (∼5 nm), spinel or rock-salt layer upon continuous charge-discharge cycling. Here we report observations of an SRL and structural evolution of the SRL on the Li[Li0.2Ni0.2Mn0.6]O2 (LNMO) particles, which are identical to those reported due to the charge-discharge cycle but are a result of electron-beam irradiation during scanning transmission electron microscopy (STEM) imaging. Sensitivity of the lithium-rich layered oxides to high-energy electrons leads to the formation of a thin, defect spinel layer on surfaces of the particles upon exposure to a 200 kV electron beam for as little as 30 s under normal high-resolution STEM imaging conditions. Further electron irradiation produces a thicker layer of the spinel phase, ultimately producing a rock-salt layer at a higher electron exposure. Atomic-scale chemical mapping by energy dispersive X-ray spectroscopy in STEM indicates the electron-beam-induced SRL formation on LNMO is accomplished by migration of the transition metal ions to the Li sites without deconstruction of the lattice. This study provides insight into understanding the mechanism of forming the SRL and also possibly a means of studying structural evolution in the Li-rich layered oxides without involving electrochemistry.