Reduced graphene oxide wrapped Na₃(VO)₂(PO₄)₂F microcubes for high-performance sodium-ion batteries

Na₃(VO)₂(PO₄)₂F (NVOPF) has been recognized as a promising cathode of sodium-ion batteries (SIBs) owing to its high working potential and high energy density, but its practical application is hindered by the poor electronic conductivity and difficulty in structure regulation. Herein, we adopt the phytic acid (PA) as a new phosphorus source to chelate VO²⁺ and Na⁺/F^- ions under hydrothermal condition, leading to the successful fabrication of cuboid-like NVOPF with average length of 1.96 μm, while introduction of citric acid (CA) results in the formation of truncated cube-like NVOPF with uniform and smaller average length of 0.81 μm, which exhibits enhanced reaction kinetics and superior sodium storage performance. To overcome the poor conductivity of NVOPF, reduced graphene oxide (rGO) nanosheets are introduced during hydrothermal synthesis, leading to the formation of rGO-wrapped NVOPF microcubes (NVOPF/rGO) with tunable rGO contents. The optimized NVOPF/rGO demonstrates significantly improved electronic conductivity and excellent sodium storage performance, displaying a theoretically high reversible capacity of 130.3 mAh g^-1 at 0.2C and outstanding cycling stability with a capacity retention of 100.0 mAh g^-1 after 3500 cycles at 10C (corresponding a decay rate of 0.0043% per cycle). Operando synchrotron analysis reveals that Na₃(VO)₂(PO₄)₂F undergoes complete solid solution reaction with V⁴⁺/V⁵⁺ redox couples upon sodiation/desodiation, and the Na₃(VO)₂(PO₄)₂F displays highly reversible lattice parameters with a small overall volume change of 2.28%, which almost completely recovers with a negligibly variation of 0.038%, indicating the robust structure stability of NVOPF. Moreover, the full cell with NVOPF/rGO cathode and hard carbon anode also demonstrates superior cycling stability for practical application.