Submicrometer Rod-Structured Na₇V₄ (P₂O₇)₄(PO₄)/C as a Cathode Material for Sodium-Ion Batteries

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