Heterogeneous conversion reaction of hexagonal NiO anode toward its reversible electrochemical cycling for Li-ion batteries

The chemical reaction of transition-metal oxides with Li ions can give much higher capacities than the intercalation of the commercial graphite in Li-ion batteries (LIBs). Yet, its lithiation process is usually oversimplified as a typical conversion reaction regardless of crystal structures of transition-metal oxides. Herein, we investigate the electrochemical performance of hexagonal NiO compared with the commonly used cubic NiO and other transition-metal oxides. Neither Li intercalation nor phase transformation is observed while hexagonal NiO starts to be lithiated, unlike other transition-metal oxides. Therein, the initial lithiation activates a conversion reaction confined to its surface producing ultrafine crystallites of ca. 5 nm. The unreacted NiO core does not maintain its single crystalline nature but forms the uniformly orientated crystallites. Further lithiation continues to reduce the size of the inner crystallites, accompanied by the formation of solid-electrolyte interface layers and loss of orientation. Complete lithiation yields ultrafine crystallites, and then this morphology is maintained during the following delithiation and subsequent cycles. This structural evolution inspires that the core-shell conversion reaction of transition-metal oxides enables their electrochemical stability to be extensively enhanced paving a novel way to properly manipulate the merit of conversion reaction in transition-metal oxides for the anode of LIBs.