Vanadium-site multivalent cation doping strategy of fluorophosphate cathode for low self-discharge sodium-ion batteries

Abstract

Na₃V₂O_2x(PO₄)₂F_3−2x (NVPOF) is considered one of the most promising cathode materials for sodium-ion batteries due to its favorable working potential and optimal theoretical specific capacity. However, its long-cycle and rate performance are significantly constrained by the low Na⁺ electronic conductivity of NVPOF. Furthermore, the prevalent self-discharge phenomenon restricts its applicability in practical applications. In this paper, the cathode material Na₃V_1.84Fe_0.16(PO₄)₂F₃ (x = 0.16) was synthesized by quantitatively introducing Fe³⁺ into the V-site of NVPOF. The introduction of Fe³⁺ significantly reduced the original bandgap and the energy barrier of NVPOF, as demonstrated through density functional theory calculations (DFT). When material x = 0.16 is employed as the cathode material for the sodium-ion battery, the Na⁺ diffusion coefficient is significantly enhanced, exhibiting a lower activation energy of 42.93 kJ mol⁻¹. Consequently, material x = 0.16 exhibits excellent electrochemical performance (rate capacity: 57.32 mA h g⁻¹ @10 C, cycling capacity: the specific capacity of 101.3 mA h g⁻¹ can be stably maintained after 1000 cycles at 1 C current density). It can also achieve a full charge state in only 2.39 min at a current density of 10 C while maintaining low energy loss across various stringent self-discharge tests. In addition, the sodium storage mechanism associated with the three-phase transition of Na_XV_1.84Fe_0.16(PO₄)₂F₃ (X = 1, 2, 3) was elucidated by a series of experiments. In conclusion, this study presents a novel approach to multifunctional advanced sodium-ion battery cathode materials.