Modulating the electron spin states of Na₂FePO₄F cathode via high entropy strategy for enhanced sodium storage and ultra-high cycling stability

Despite its promising two-dimensional transport channels for sodium ions, the layered iron-based fluorophosphate Na₂FePO₄F (NFPF) demonstrates inadequate cycling stability and rate capability, primarily due to its suboptimal intrinsic electronic conductivity and structural stability. In this investigation, a high-entropy strategy is utilized to modify the electron spin states of NFPF by partially substituting the Fe site with a combination of Cu, Mg, Ti, and Al elements. This high-entropy substitution approach has been demonstrated to markedly improve the kinetics of sodium ion diffusion and diminish charge transfer impedance during sodium ion insertion and extraction as well as enhance structural integrity. Moreover, investigations utilizing X-ray absorption near edge structure (XANES) reveal that the Fe-O/F bond lengths in HE-NFPF are shorter relative to those in NFPF. Additionally, temperature-dependent magnetic susceptibility assessments indicate that HE-NFPF presents a reduced effective magnetic moment, suggesting an augmentation of stability within the Fe-O/F octahedral crystal field of the HE-NFPF electrode material. Consequently, the HE-NFPF electrode displays superior rate capability and enhanced cycling stability with specific capacities reaching 118.4 mAh g⁻¹ at 0.1C and 70 mAh g⁻¹ at 20C. After 1000 cycles at a rate of 10C, the specific capacity retains 73.5 % of its initial value.