Broadening the Na⁺ diffusion degree of freedom to unlock a rapid sodium storage potential in fluorophosphate cathode

High safety and high energy-density sodium-ion batteries require the promising polyanionic insertion-type cathode possessing fast dis-/charging capability, yet persistent challenges remain in the kinetic optimization to accelerate their intrinsically low Na⁺ diffusivity. Exampled by the representative Na₃V₂(PO₄)O₂F (NVPOF) with considerable theoretical energy density, structural distortion results in a one-dimensional sluggish Na⁺ diffusion out of the two-dimensional Na⁺ pathway provided structurally. Previous endeavors with Na site or transition-metal site regulation successfully optimize the Na⁺ diffusion energy barrier of the available one-dimensional path. However, these substituted elements with non-equivalent valances or sizes further elevate the energy barrier of the other unavailable Na⁺ diffusion path. Herein, by defining the independently accessible Na⁺ diffusion pathways in the crystallographic structure as Na⁺ diffusion degree of freedom (df_[Na+]), we demonstrate broadening df_[Na+] to two in NVPOF by a mild perturb at the dangling site can fundamentally revise the Na⁺ diffusion behaviour. As demonstrated by in-situ synchrotron, various spectroscopic techniques, and density functional theory (DFT) modeling, this mild perturb equalizes the Na⁺ diffusion energy barriers along a and b directions and enables two-dimensional Na⁺ transportation. The as-prepared NVPOF depicts an altered solid-solution phase transition, higher disorder in the framework and dramatically enhanced Na⁺ diffusivity, which leads to unprecedentedly high sodium storage properties in half cell (68.6 mAh g⁻¹ at 100 C; 103.3 mAh g⁻¹ after 1300 cycles at 20 C; 1 C = 130 mA g⁻¹) and full cell (313.8 Wh kg⁻¹@4063.5 W kg⁻¹; 113.9 Wh kg⁻¹@16,397.2 W kg⁻¹). This study enlightens the valuable role of broadening df_[Na+] in fundamentally maximizing the polyanionic insertion-type performance.