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.