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.