Unusual Case of Higher Cyclic Stability at a Wider Voltage Window in Sodium Vanadium Phosphate

NASICON-type Na3V2(PO4)3 is a promising cathode material for sodium-ion batteries (SIBs). However, large-scale synthesis of Na3V2(PO4)3 with a robust microstructure favoring enhanced sodium-ion storage, which is crucial for commercial usage as an electrode for SIBs, is still illusive. In this work, in situ carbon-coated Na3V2(PO4)3 (C-NVP) nanoparticles embedded in a three-dimensional mesoporous carbon matrix has been prepared by the scalable microwave-assisted sol–gel route. It delivers stable specific capacities of ∼112 and ∼102 mA h g–1 at 0.1 and 1 C-rates (1 C = 118 mA g–1), respectively, in the potential window of 2.3–3.9 V versus Na/Na+. In a wider potential window of 1.2–3.9 V, C-NVP shows reversible insertion/extraction of ∼2.4 moles of Na+ ions corresponding to a specific capacity of ∼143 mA h g–1, with 75% capacity retention after 500 cycles at 1.0 C-rate. We attribute such unusual stability at higher moles of Na+-ion insertion to the ability of nanocrystallites to freely expand against mesoporous carbon as Na3V2(PO4)3 converts to Na4V2(PO4)3. Moreover, a symmetric full cell using C-NVP as both cathode and anode shows excellent cyclability and rate performance, with a high specific capacity of 50 mA h g–1 at 2 A g–1 stable for >10,000 cycles, corresponding to specific energy and power density of 88 W h kg–1 and 3504 W kg–1, respectively. A proto-type pouch cell (symmetric full cell) delivers 7 mA h capacity at 0.1 A g–1. The scalable microwave-assisted sol–gel route provides a robust solution for the large-scale synthesis of C-NVP with superior sodium-ion storage performance.