One step in-situ synthesis of Na3V2(PO4)3/Na3V3(PO4)4 biphase coexisted cathode with high energy density by inducing of polyvinylpyrrolidone for sodium ion batteries

The low voltage platform (∼3.4 V) and energy density of Na3V2(PO4)3 are far from the requirement of practical applications. Herein, Na3V2(PO4)3/Na3V3(PO4)4 biphase coexisted cathode is synthesized by a sol-gel method in one step. It exhibits an extra stable and sustained high voltage plateau (∼3.9 V) derived from the Na3V3(PO4)4. Polyvinylpyrrolidone (PVP) acts as a structural guiding agent to induce the formation of Na3V3(PO4)4 and behaves as a chelating agent to facilitate the synthesis of Na3V2(PO4)3. Nitrogen atoms on five-membered rings in PVP are reductive to make the reaction of V5+/V3+ faster and more complete. Furthermore, PVP decomposes into clusters and free radicals to form uniform carbon with relatively ordered structure. The generated nitrogen-doped carbon coating formed by PVP possesses more beneficial defects to accelerate the transport of electrons, resulting in superior kinetic characteristics. Ex-situ X-ray photoelectron spectroscopy demonstrates the reversible V3+/V4+ redox pair features both at voltage of 3.4 and 3.9 V, further indicating the biphase coexistence of Na3V2(PO4)3/Na3V3(PO4)4. Correspondingly, Na3V2(PO4)3/Na3V3(PO4)4 biphase material submits a capacity of 119.17 mAh g−1 at 0.1C and delivers an impressive energy density of 471.19 Wh kg−1. This novel biphase cathode significantly improves the energy density of Na3V2(PO4)3 to promote its practical application.