High-voltage Na₃V₂(PO₄)₂F₃ cathodes enabled by low-valence metal cations

Na₃V₂(PO₄)₂F₃ (NVPF) is regarded as a highly promising cathode material for sodium-ion batteries. Here, we propose a general strategy for modulating the local electronic structure of vanadium (V) by introducing low-valence metal ions, such as Cu²⁺, Cd²⁺, and Ag⁺. This approach microscopically shortens the length of the suspended V─F2 bonds within the NVPF framework, effectively mitigating the loss of fluorine and the formation of undesirable Na₃V₂(PO₄)₃ (NVP). Consequently, this intervention indirectly enhances the overall working voltage and energy density of the battery. Density functional theory (DFT) is used to verify and deeply investigate the intrinsic mechanism of fluorine stabilization in the NVPF system. The experimental results show that NVPF with 2.5% of doped Cu exhibits a higher mid-working voltage (3.69 volts), higher energy density (447.7 watt-hours per kilogram), and excellent cycling stability (83.3% capacity retention at 20 C after 10,000 cycles).