Submicrometer Rod-Structured Na<sub>7</sub>V<sub>4</sub> (P<sub>2</sub>O<sub>7</sub>)<sub>4</sub>(PO<sub>4</sub>)/C as a Cathode Material for Sodium-Ion Batteries
Li-ming Zhang, Naiqing Ren, Shuo Wang, Wenjie Deng, Fei Chen, Zhaoyin Wen, Chunhua Chen
Abstract
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.