Enabling durable sodium storage of Fe-based fluorophosphate cathode via anion substitution

Na₂FePO₄F is a promising sodium ion cathode due to its low cost, non-toxicity, and high stability. However, the sluggish Na⁺ diffusion kinetics and limited intrinsic electronic conductivity critically restrict its worldwide application. Herein, an anion-substitution strategy is proposed with SiO₄⁴⁻ as the dopant. SiO₄⁴⁻ substitution for PO₄³⁻ can apparently alter the localized electronic density and structural configuration in the lattice of Na₂FePO₄F, effectively elevating the charge transfer efficiency. As a result, the electrochemical reaction kinetics of Na₂FePO₄F is significantly enhanced, which is well demonstrated by a series of electrochemical characterizations. As-obtained Na_2.2Fe(PO₄)_0.8(SiO₄)_0.2F renders a specific capacity of 84.9 mA h g⁻¹ within the region of 2.5–4.0 V at 60 mA g⁻¹ (0.5 C), good rate capability, and a capacity retention of 70.0% after 1000 cycles at 1.24 A g⁻¹ (10 C). Furthermore, the stabilities of the cathode-electrolyte interface and structure are strengthened, which are verified by in situ EIS and ex situ XRD analysis. These findings highlight silicate anion substitution as a promising and cost-effective strategy for overcoming the limitations of Na₂FePO₄F, contributing to the development of sustainable energy storage solutions.