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

Delivery of high energy density in polyanion compound-Na₃V₂(PO₄)₃ based on three-electron redox is still challenging because of their controversial multi-electron reaction mechanism and unsatisfactory electrochemical reversibility. Herein, the reversible V⁴⁺/V⁵⁺ redox (ca. 4.0 V) could be successfully activated in Na₄VFe(PO₄)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 V⁴⁺/V⁵⁺ 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.