Sodium Ion Storage in Na₄MnV(PO₄)₃@C Free‐Standing Electrode

Abstract To enhance the energy density of batteries and explore intrinsic charge storage mechanism of the active materials, it is important to reduce or eliminate the use of non‐active materials in electrodes, such as binder and conductive additives. Herein, free‐standing Na 4 MnV(PO 4 ) 3 @C (F‐NMVP@C) fiber membrane is fabricated and directly used as a sodium‐ion battery (SIB) cathode. In situ X‐ray diffraction reveals that the V 3+ /V 4+ redox reaction occurs through a solid‐solution reaction while a two‐phase Mn 2+ /Mn 3+ redox reaction is identified, and both are highly reversible. Meanwhile, ex situ electrochemical impedance spectroscopy reveals that both the ion diffusion coefficient and charge transfer resistance of F‐NMVP@C change reversibly during the Na + intercalation/de‐intercalation. Battery full cells are assembled based on the free‐standing F‐NMVP@C cathodes and F‐Sb@C anodes, which manifests a high energy density (293 Wh kg −1 ) and good cyclability (87.5% after 100 cycles at 1 C). The high‐performance free‐standing cathodes and anodes shed light on the development of flexible SIBs.