Beyond conventional sodium-ion storage mechanisms: a combinational intercalation/conversion reaction mechanism in Ni-ion modified hydrated vanadate for high-rate sodium-ion storage

Exploration of advanced anode materials remains a great challenge in further promoting the performance of sodium-ion batteries. From the perspective of Na+ storage mechanisms, conversion/alloying-type anode materials typically offer high Na+ storage capacities, whereas the volume expansion during operation gives rise to unsatisfactory cycling stability. Intercalation-type anode materials with appropriate crystallographic structures have been identified to deliver decent cycling and rate performances. However, the deformations that the structures can withstand, as well as the limited numbers of available vacant sites in the crystal structures, significantly constrains the Na+ storage capacity. Herein, breaking from the conventional Na+ storage mechanisms, we reveal for the first time the combinational intercalation/conversion reaction mechanism upon Na+ storage in the Ni-ion modified hydrated vanadate (Ni0.24V2O5·nH2O). Based on in-situ/ex-situ characterizations and theoretical analysis, the conversion reaction of the interlayer Ni3+ is found to be triggered after the Na+ intercalation process, which not only contributes to high specific capacities but also leads to fast and stable solid-state Na+ diffusion. Paring Ni0.24V2O5·nH2O with a Zn/Mg dual-doped P2-Na0.67MnO2 cathode material, a high-performance Na-ion battery prototype full cell is fabricated. The unconventional Na-ion storage mechanism that endows the anode material with both high capacity and outstanding cyclic and rate performances has implications for further boosting the comprehensive performance of sodium-ion batteries.