The demand for electrochemical energy storage (EES) devices, which can deliver high power with large charge storage capabilities are increasing day-by-day. Sodium based devices are an appropriate choice to meet the current necessities considering the abundance, cost and performance. NASICON-type Na3V2(PO4)3 is a promising cathode material for sodium-ion batteries (SIB). However, large-scale synthesis of Na3V2(PO4)3 with robust microstructure favoring enhanced sodium ion storage, which is crucial for commercial usage as electrode for SIBs, is still illusive. Sodium vanadium phosphate nanocomposite (C-NVP) has been engineered by making the NVP particles embedded within nitrogen doped mesoporous carbon matrix in-situly through scalable microwave-assisted sol-gel route. C-NVP has delivered 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 wider potential window of 1.2-3.9 V, C-NVP has showed 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. Moreover, a symmetric full cell using C-NVP as both cathode and anode has showed an excellent cyclability and rate performance. It exhibited high specific capacity of 50 mAh g-1 at 2 A g-1, corresponding to a specific energy and power density of 88 Wh kg-1 and 3504 W kg-1, respectively; which is stable for more than 10000 cycles. A proto-type symmetric pouch cell delivered 7 mAh nominal capacity at a nominal voltage of ~1.8 V.
Using C-NVP, a hybrid cell has been devised with activated carbon in asymmetric cell fashion and its electrochemical performance has been compared to symmetric cell. Asymmetric and symmetric cells have demonstrated cell level energy densities of 77 & 65 and 59 & 46 Wh kg-1 at 0.1 and 1 A g-1 respectively, which are much higher than many reported values. At 2 A g-1, both configurations delivered high power (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 suggests that dual mechanism of intercalation in C-NVP and surface/sub-surface charge storage in nitrogen-doped mesoporous carbon are playing major role. The obtained results are superior to different hybrid devices reported earlier which paves a way for demonstrating sodium-ion device for practical applications.
Sodium vanadium fluorophosphates [Na3V2(PO4)3-xF3x] are known for their highest specific energy (~500 Wh kg-1) amongst the class of polyanions for sodium ion battery (SIB) application. However, they cannot compete with lithium-ion counterparts in terms of specific energy. The need for improving the specific energy of sodium-ion cathodes is highly required in order to bring them into the market. Vanadium, as an interesting transition metal exhibiting varied oxidation states from V2+ toV5+ assists to utilize different vanadium redox couples in Na3V2(PO4)2F3 for improving its specific energy. Moreover, partially occupied sodium sites in Na3V2(PO4)2F3 structure can be completely filled to obtain the specific energies >600 Wh kg-1 which are comparable to that of lithium ion batteries (LIBs). An in-situ carbon coated Na3V2(PO4)F3 (C-NVPF) nanoparticles embedded in nitrogen doped mesoporous carbon has been synthesized through scalable microwave assisted sol-gel route. As prepared C-NVPF exhibited ~126 mAh g-1 with two Na+-ion cycling reversibly. An additional sodium insertion of ~1.05 and 1.3 moles into the structure have shown highest specific capacities of 195 and 210 mAh g-1 respectively. It has been realised that the nanocomposite microstructure enabled excess sodium-ion insertion reversibly by efficiently utilizing the V3+/V2+ redox couple. Structural stability at different levels of state of charge (SOC) have been understood through ex-situ X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopic (XAS) analysis. Increase in the electrochemical impedance have been noticed with extraction and insertion of sodium-ions into the C-NVPF structure and analyzed it to be due to the sodium-ion vacancy ordering during de-sodiation and reduced sodium-ion diffusivity during sodiation respectively. The structural and electrochemical studies on additional sodium inserted C-NVPF provides new insights for the development of high-energy SIBs. The scalable microwave assisted sol-gel route provides a robust solution for the large scale synthesis of C-NVP and C-NVPF with superior sodium-ion storage performance.