Electrochemical investigation of multi-electron reactions in NaVOPO4 cathode for sodium-ion batteries.

Realization of multi-electron reactions per transition metal is an effective strategy to increase the energy density of cathode materials, and comprehensively understanding of their underlying mechanism is necessary to further improve the electrochemical performance of the sodium-ion batteries. Here, we investigate and realize multi-electron reactions in NaVOPO 4 , which could deliver very high specific capacity. In addition, we explore its electrochemical mechanism in great detail by a series of advanced techniques, including XANES, in situ SXRD, 23Na, 31P, and 51V solid state NMR. The results demonstrate that V3−δ/V4+/V5+ redox couples can be realized in NaVOPO 4 , and a biphasic transition followed by a monophasic reaction process occurred during discharge process. Finally, the causes of battery failure are analyzed and corresponding strategies to improve electrochemical performance of NaVOPO 4 are discussed. [ABSTRACT FROM AUTHOR]