Sulfate-Pyrophosphate Cathodes with Dual-Anion Synergy for High-Performance Sodium Storage

Cathode materials for sodium-ion batteries (SIBs) require a balance between capacity, rate capability, and cycle life. This study employs an anhydrous precursor approach to investigate the novel polyanionic cathode, Na₂Fe₂P₂O₇SO₄ (NFPS). By regulating the carbon coating content derived from biomass, we achieve synergistic optimization of the electronic and ionic transport properties of the material. The resulting NFPS/C-F-1 cathode exhibits outstanding electrochemical performance across a wide temperature range, with a reversible discharge specific capacity of 63.81 mAh g^-1 at 10 C (30 °C) and a capacity retention of 91.34% after 400 cycles at 0.5 C at -25 °C. The anhydrous precursor prevents structural collapse during pyrolysis, which is a common issue with traditional hydrated precursors, which ensures high crystallinity and consistent batch results. Analysis using the electron localization function (ELF) and the bond-valence energy landscape (BVEL) confirms that the introduction of sulphate ions into the pyrophosphate framework creates a heterogeneous electron density field and three-dimensional diffusion pathways, which enhance electrochemical performance in synergy with the carbon coating strategy. This performance characteristic establishes a foundation for rational design of carbon coating thickness in applications.