Submicrometer\nRod-Structured Na₇V₄ (P₂O₇)₄(PO₄)/C as a\nCathode Material for Sodium-Ion Batteries

Abstract

A vanadium-based\nmixed polyanionic material Na₇V₄(P₂O₇)₄(PO₄) (NVPP)\nhas a high working potential of around 4.0 V vs Na⁺/Na\ndue to the presence of pyrophosphate with strong electronegativity.\nIn addition, NVPP has intrinsically long cycle life. A carbon-coated\nrod-structured NVPP powder is synthesized by a scalable solid-state\nreaction method with poly­(vinyl alcohol) as the carbon source. As\na cathode, the optimal NVPP sample can deliver discharge capacities\nof 92.1 and 86.4 mA h g^–1 at 0.2 and 10C, respectively,\nin the voltage range of 2.5–4.2 V. It also exhibits a good\ncycling stability with a capacity retention of 73% after 550 cycles\nat a 10C rate. Based on XRD analysis, the volume change of the NVPP\nis less than 4% during the charge and discharge processes. In addition,\nNVPP also accommodates more sodium ions in the lattice in a voltage\nwindow of 2.5–4.2 V vs Na⁺/Na.