The construction of multiscale charge-transport pathway in Na₂FePO₄F towards high-rate sodium ion cathode

Na₂FePO₄F (NFPF) is an attractive sodium-ion cathode owing to its low cost, non-toxicity, and structural robustness, yet its practical deployment is often limited by sluggish Na⁺ transport and insufficient intrinsic electronic conductivity under high-rate operation. In this work, multiscale charge-transport pathways were constructed in NFPF through a progressive design: partial PO₄³⁻ substitution with BO₃³⁻ was employed to regulate the localized electronic structure/defect chemistry and accelerate intrinsic transport, and carbon nanotubes (CNTs) were subsequently incorporated to reinforce electrode-scale electronic percolation and stabilize interfacial charge-transfer processes. As a result, Na₂Fe(PO₄)_0.9(BO₃)_0.1F/CNTs delivers a high reversible capacity of 124.2 mA h g⁻¹ at 0.1 C and retains 85.32% after 500 cycles at 5C and 78.4% after 1000 cycles at 10 C. Scan-rate CV yields markedly increased apparent diffusion coefficients, consistent with GITT-derived enhancement over most of the voltage window, while DRT-resolved impedance analysis reveals strongly suppressed potential-dependent fluctuations of R_ct and R_CEI. DFT and EPR analysis further support band-gap narrowing from 3.226 to 2.076 eV and oxygen-vacancy-related defect states. These results highlight multiscale pathway construction via anion-site regulation coupled with conductive-network reinforcement as an effective route to improving the high-rate durability of fluorophosphate cathodes.