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) its 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 promising candidate for commercialization due to their good electrochemical properties and ease of synthesis. NASICON-type sodium vanadium phosphate, Na3V2(PO4)3 is a structurally and thermally stable polyanionic cathode material, which is suitable for large scale grid energy/electric vehicle applications. However, large scale synthesis of Na3V2(PO4)3 with suitable microstructure for large storage and cyclic stability is still illusive.
In-situ carbon coated Na3V2(PO4)3 (C-NVP) nanoparticles embedded in 3-dimensional mesoporous carbon matrix, has been prepared in large amount by scalable microwave-assisted sol-gel route. C-NVP delivers stable specific capacities of ~112 and ~102 mAh g-1 at 0.1 and 1C-rates (1C = 118 mA g-1) respectively, in the potential window of 2.3-3.9 V vs. Na/Na+. In wider potential window of 1.2-3.9 V, C-NVP shows reversible insertion/extraction of ~2.4 moles of Na+-ions corresponding to 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. Moreover, symmetric full cell using C-NVP as both cathode and anode shows an excellent cyclability and rate performance, with a high specific capacity of 50 mAh g-1 at 2 A g-1 stable for > 10000 cycles, corresponding to specific energy and power density of 88 Wh kg-1 and 3504 W kg-1, respectively. 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 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 (C-NVPF) will be synthesized through microwave assisted sol-gel route and tested for SIB. An ultra-low voltage anode material sodium titanate, Na2Ti3O7 (C-NTO) has been synthesized through solid state route with ex-situ carbon coating and its electrochemical testing in half-cell level was accomplished to further take it to full cell studies. The pouch type full cells will be fabricated using NASICON type C-NVP and C-NVPF as cathodes with C-NTO as anode and their electrochemical performance will be evaluated.