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 NaV(PO)OF (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), we demonstrate broadening df 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 at 100 C; 103.3 mAh g after 1300 cycles at 20 C; 1 C = 130 mA g) and full cell (313.8 Wh kg@4063.5 W kg; 113.9 Wh kg@16,397.2 W kg). This study enlightens the valuable role of broadening df in fundamentally maximizing the polyanionic insertion-type performance.