Spin-regulated d–p hybridization enables high energy density and wide temperature operation of Na3V2(PO4)2O2F-type cathodes for sodium-ion batteries

Abstract

As a member of the sodium superionic conductor (NASICON) family, Na₃V₂(PO₄)₂O₂F (NVPOF) has attracted tremendous research interest owing to its high operating voltage and excellent structural stability. It is well established that NVPOF cathodes are electron-ion mixed conductors. However, improving electron or ionic conductivity via a single approach fails to effectively enhance the rapid sodium storage capability, which impedes their practical application in sodium-ion batteries. Herein, the electronic and ionic conductivities were enhanced through a transition-metal/fluorine dual-doping method. The introduction of transition metals with different spin states adjusted the spacing between the V 3d_xz–O 2p_x bond and the Fermi level, thereby improving the material's intrinsic conductivity. Meanwhile, the introduction of F atoms effectively optimized the diffusion kinetics of Na⁺. In particular, Na₃(VO)_1.9Fe_0.1(PO₄)₂F_1.1 (NVPOF–Fe) is obtained by dual-doping with high-spin Fe³⁺/F⁻. It has considerable specific capacity and power density at 100C (80 mA h g⁻¹, 245 Wh kg⁻¹) and retains 90.6% capacity after 2500 cycles at 20C. The assembled NVPOF–Fe‖hard-carbon full cell exhibits excellent capacity and cycling stability at −20 °C, 25 °C, and 45 °C, respectively. This work provides a new paradigm for the development of advanced sodium-ion battery cathodes.