1D Nanostructured Na₇V₄(P₂O₇)₄(PO₄) as High-Potential and Superior-Performance Cathode Material for Sodium-Ion Batteries

Abstract

Tailoring materials into nanostructure offers unprecedented opportunities in the utilization of their functional properties. High-purity Na₇V₄(P₂O₇)₄(PO₄) with 1D nanostructure is prepared as a cathode material for rechargeable Na-ion batteries. An efficient synthetic approach is developed by carefully controlling the crystal growth in the molten sodium phosphate. Based on the XRD, XPS, TG, and morphological characterization, a molten-salt assisted mechanism for nanoarchitecture formation is revealed. The prepared Na₇V₄(P₂O₇)₄(PO₄) nanorod has rectangle sides and preferential [001] growth orientation. GITT evaluation indicates that the sodium de/intercalation of Na₇V₄(P₂O₇)₄(PO₄) nanorod involves V³⁺/V⁴⁺ redox reaction and Na₅V^3.5+₄(P₂O₇)₄(PO₄) as intermediate phase, which results in two pairs of potential plateaus at the equilibrium potentials of 3.8713 V (V³⁺/V^3.5+) and 3.8879 V (V^3.5+/V⁴⁺), respectively. The unique nanoarchitecture of the phase-pure Na₇V₄(P₂O₇)₄(PO₄) facilitates its reversible sodium de/intercalation, which is beneficial to the high-rate capability and the cycling stability. The Na₇V₄(P₂O₇)₄(PO₄) cathode delivers 80% of the capacity (obtained at C/20) at the 10 C rate and 95% of the initial capacity after 200 cycles. Therefore, it is feasible to design and fabricate an advanced rechargeable sodium-ion battery by employment of 1D nanostructured Na₇V₄(P₂O₇)₄(PO₄) as the cathode material