Chemically Stabilized Nasicon-Na1.5V0.5Nb1.5(PO4)3 Anode for Long-Life Sodium-Ion Batteries

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Sodium-ion batteries (SIBs) are considered as a potential solution for low cost and grid storage applications due to the more abundance of Na precursors compared to their Li analogues. For SIBs, several attractive candidates are available as cathodes such as layered oxides, Prussian blue analogues and polyanionic compounds but for the anodes hard is used as the primary choice for the fabrication of full SIBs.1-3 The NASICON (Natrium Super-Ionic CONductor) framework can be an efficient electrode material for SIBs due to its high structural and thermal stability along with superior sodium ion mobility. Recently, NASICON compounds are widely explored as cathode and solid-state electrolyte for SIBs and largely ignored as anodes due to their low capacity and high voltage. The general molecular formula for NASICON structure is NaxM2(PO4)3, where a maximum of four sodium can accommodate into the structure, which is built by lantern units, consisting of two MO6 and three PO4 corner shared with each other. We have studied the electrochemical (de)intercalation of sodium in empty NASION-Nb2(PO4)3 for the first time and showed its potential application in the anodes for SIBs.4 Nb2(PO4)3 is attracts lot of attention due to its high capacity (150 mAh g-1) and low voltage (1.4 V vs. Na+/Na0) which make it suitable candidate for SIBs anode. However, it suffers from rapid capacity decay due to the structural degradation, which is triggered by a lack of sufficient sodium ions at the Na (1) site (39% capacity retention after 200 cycles).To overcome this issue, we introduce extra sodium ions at Na(1) site via V3+ substitution gives the composition of Na1.5V0.5Nb1.5(PO4)3, which can act as the stabilizing agent to hold lantern units together during cycling.6 The Na-filled anode delivers reversible capacity of ~140 mAh g-1 at 1.2 V vs. Na+/Na0 through Nb5+/Nb4+/Nb3+ and V3+/V2+ redox activities, as predicted by density functional theory (DFT) calculations and confirmed by X-ray absorption spectroscopy (XAS). Additionally, the Na (de)intercalation mechanism changes from the multiple two-phase reactions (observed in the Nb2(PO4)3) to complete solid-solution formation in the sodium filled anode demonstrates the critical role of Na(1) site in the NASICON framework, in-line with DFT calculations, and enhances sodium diffusivity (≈ 10-9 cm2s-1), as deduced from in-situ X-ray diffraction (XRD) and galvanostatic intermittent titration technique (GITT) experiments. The anode retains 89% of its initial capacity retention at 5C after 500 cycles and extraordinary rate performances (105 mAh g-1 at 5C). When the NASICON-Na3V2(PO4)3 (NVP) cathode paired with the sodium filled anode, the full Na-ion cell delivers a remarkable energy density of 98 Wh kg-1 (based on the mass of anode and cathode) compared to the state-of-the-art NASICON-based Na-ion full cells and provides 80% capacity retention at 5C rate over 1000 long cycles. This work creates new opportunities for chemical and structural modifications that improve the electrochemical performance of NASICON anodes.