Sodium-ion batteries (SIBs) are considered as most promising electrochemical energy storage system as an alternative to lithium-ion batteries (LIBs) due to 1) large abundance of sodium, 2) their uniform distribution across the world, and 3) similar electrochemical properties to that of LIBs. The development of high energy-high power SIB relies on the intrinsic properties of cathode and anode materials. Among different types of cathode materials, polyanions are a promising candidate for commercialization due to their excellent electrochemical properties and ease of synthesis. NASICON-type sodium vanadium phosphate, Na3V2(PO4)3, is a structurally and thermally stable polyanionic cathode material suitable for large-scale grid energy/electric vehicle applications. However, large-scale synthesis of Na3V2(PO4)3 with suitable microstructure for good storage and cyclic stability is still elusive.
In-situ prepared nitrogen-doped carbon-coated Na3V2(PO4)3 (NC-NVP) nanoparticles embedded in a 3-dimensional mesoporous carbon matrix has been prepared by scalable microwave-assisted sol-gel route. NC-NVP delivers stable specific capacities of ~112 and ~102 mAh g-1 at 0.1 and 1 C-rates (1C = 118 mA g-1), respectively, in the potential window of 2.3-3.9 V vs. Na/Na+. In a wider potential window of 1.2-3.9 V, NC-NVP shows reversible insertion/extraction of ~2.4 moles of Na+-ions corresponding to a specific capacity of ~143 mAh g-1, with 75% capacity retention after 500 cycles at 1.0 C-rate. We attribute such unusual stability at higher moles of Na+-ions insertion to the ability of nanocrystallites to freely expand against mesoporous carbon, as Na3V2(PO4)3 converts to Na4V2(PO4)3. The electrochemical performance of a hybrid cell fabricated using the NC-NVP as cathode and activated carbon (AC) as anode has been compared with a symmetric cell, where NC-NVP is used as both cathode and anode. The cell-level energy densities of 77 and 65 Wh kg-1 have been displayed by asymmetric and symmetric cells at 0.1 A g-1, respectively. Even at higher rates (2 A g-1), the asymmetric and symmetric configurations delivered high power of 3722 and 3750 W kg-1 within 1.4 and 1.5 minutes at retentions of 63 and 51 % after 14000 cycles, respectively. Excellent electrochemical performance is attributed to the nitrogen-doped mesoporous carbon matrix covering NVP particles which aid in charge storage at the surface or sub-surface layers at higher rates, as pseudocapacitive current by contributing to the overall capacitance. A proto-type pouch cell (symmetric full cell) delivers ~7 mAh capacity at 0.1 A g-1.
Replacement of one phosphate unit (PO4)3- in Na3V2(PO4)3 with a high electronegative element like fluorine can further increase the sodium ion intercalation/de-intercalation potential due to increase in V-F bond strength and the specific capacity by lowering of weight. So, NASICON type sodium vanadium fluorophosphate, Na3V2(PO4)2F3 (NC-NVPF), has been synthesized through a microwave-assisted sol-gel route and tested for SIB. The fabrication and testing of pouch-type full cells using NASICON type NC-NVP & NC-NVPF as cathodes and hard carbon as an anode are underway.