Reversible V4+/V5+ redox in Na4VFe(PO4)3 cathode for high-power sodium-ion batteries

Delivery of high energy density in polyanion compound-Na 3 V 2 (PO 4 ) 3 based on three-electron redox is still challenging because of their controversial multi-electron reaction mechanism and unsatisfactory electrochemical reversibility. Herein, the reversible V 4+ /V 5+ redox ( ca. 4.0 V) could be successfully activated in Na 4 VFe(PO 4 ) 3 cathode material through a Na-rich strategy, demonstrating a reversible specific capacity of 154.7mAh g −1 at 0.1C and 90.3mAh g −1 after 800 cycles at 20C, with 80 % of the initial capacity retained. The entire electrochemical reaction undergoes a highly recoverable phase transition by in-situ X-ray diffraction. Impressively, the Na-rich strategy could not only allow Na2 site to be occupied to activate the V 4+ /V 5+ redox pair confirmed by X-ray absorption near-edge structure spectroscopy, but also enable Na + transportation across the generated vacancies rather than synergistic Na + diffusion. This work provides rational design strategy of multi-electron transfer reaction in high-energy sodium-ion batteries.