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

Hong Yu; Jinjin Wang; Hongbo Jing; Chao Wu; Erhai Hu; Shibo Xi; Xiaomei Wang; Zhiyu Fang; Xing-Long Wu; Qinghua Liang; Weihong Qi; Qingyu Yan; Hongqiang Wang; Cheng-Feng Du

doi:10.1016/j.scib.2025.06.005

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

Sci Bull (Beijing). 2025 Jun 9:S2095-9273(25)00611-5. doi: 10.1016/j.scib.2025.06.005. Online ahead of print.

Authors

Hong Yu¹, Jinjin Wang², Hongbo Jing², Chao Wu³, Erhai Hu⁴, Shibo Xi³, Xiaomei Wang², Zhiyu Fang², Xing-Long Wu⁵, Qinghua Liang⁶, Weihong Qi², Qingyu Yan⁴, Hongqiang Wang², Cheng-Feng Du⁷

Affiliations

¹ State Key Laboratory of Solidification Processing, Center of Advanced Lubrication and Seal Materials, Northwestern Polytechnical University, Xi'an, Shaanxi 710072, China. Electronic address: yh@nwpu.edu.cn.
² State Key Laboratory of Solidification Processing, Center of Advanced Lubrication and Seal Materials, Northwestern Polytechnical University, Xi'an, Shaanxi 710072, China.
³ Institute of Sustainability for Chemicals, Energy and Environment, Agency for Science, Technology and Research (A*STAR), Singapore, 627833, Singapore.
⁴ School of Materials Science and Engineering, Nanyang Technological University, Singapore, 639798, Singapore.
⁵ MOE Key Laboratory for UV Light-Emitting Materials and Technology, Northeast Normal University, Changchun 130024, China.
⁶ Key Laboratory of Rare Earth, Ganjiang Innovation Academy, Chinese Academy of Sciences, Ganzhou 341000, China.
⁷ State Key Laboratory of Solidification Processing, Center of Advanced Lubrication and Seal Materials, Northwestern Polytechnical University, Xi'an, Shaanxi 710072, China. Electronic address: cfdu@nwpu.edu.cn.

PMID: 40581503
DOI: 10.1016/j.scib.2025.06.005

Abstract

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.

Keywords: Fluorophosphate; Na(+) diffusion degree of freedom; Na(+) diffusivity; Polyanionic cathode materials; Sodium-ion batteries.