The Investigation of Fe─F Bond Chemistry on Structural Stability for Highly Durable Layered Na2FePO4F Cathode

Abstract

Layered iron (Fe) -based fluorophosphates, Na₂FePO₄F (NFPF) stands for a cost-effective and voltage-advantageous cathode material for sodium-ion batteries. Nevertheless, the lack of stability imposes constraints on its development and the decay mechanism remains shrouded in ambiguity. Herein, this work proposes the breakup of Fe─F bond in octahedral dimer accountable for the dissolution of redox centers and the formation of electrochemically inert phase, ultimately leading to the deterioration of electrochemical stability. To verify and address this, Boron (B) atoms situated in interstitial positions of PO₄ tetrahedra appearing trigonal BO₃ can be specifically targeted to enhance bond covalency and tailor electronic rearrangements at Fe─F bonds, thus stabilizing the octahedral dimer structure. This also facilitates rapid Na⁺ diffusion dynamics and accelerated electronic conductivity. As expected, NFPF-B exhibits an ultra-high discharge specific capacity (118.34 mAh g⁻¹ at 0.1C) and excellent long-term durability (capacity retention of 91.9% after 1000 cycles). The stability of the octahedra dimer is underscored by minimal volume change (2.9%) within the two-stage biphase reaction of sodium storage mechanism. This work elucidates the enduring degradation mechanism of NFPF from octahedral dimers and offer theoretical guidance for Fe-based cathode materials with prolonged stability.