Exploring Highly Reversible 1.5-Electron Reactions (V3+/V4+/V5+) in Na3VCr(PO4)3 Cathode for Sodium-Ion Batteries.

The development of highly reversible multielectron reaction per redox center in sodium super ionic conductor-structured cathode materials is desired to improve the energy density of sodium-ion batteries. Here, we investigated more than one-electron storage of Na in Na3VCr(PO4)3. Combining a series of advanced characterization techniques such as ex situ 51V solid-state nuclear magnetic resonance, X-ray absorption near-edge structure, and in situ X-ray diffraction, we reveal that V3+/V4+ and V4+/V5+ redox couples in the materials can be accessed, leading to a 1.5-electron reaction. It is also found that a light change on the local electronic and structural states or phase change could be observed after the first cycle, resulting in the fast capacity fade at room temperature. We also showed that the irreversibility of the phase changes could be largely suppressed at low temperature, thus leading to a much improved electrochemical performance.