A Novel Na₂Fe₂P₂O₇SO₄ Cathode Material: Study on the Mechanism of Double Anion Synergistic Coupling and Electrochemical Performance Enhancement

Polyanionic ca–thode systems offer long cycle life and tunable redox potential due to their three–dimensional rigid skeleton. Here, we design a new Na₂Fe₂P₂O₇SO₄ cathode with a sulfate–pyrophosphate double anion for the first time. [SO₄]²⁻ and [P₂O₇]⁴⁻ tetrahedra are coupled into the FeO₆/FeO₄ polyhedral skeleton, and in situ biomass carbon coating constructs a highly conductive network with continuous 3D Na⁺ migration channels. Using density functional theory and the climbing–image–driven elastic band method, the effects of different anion compositions on structural stability and sodium ion diffusion are evaluated. Our findings reveal that the composite anion displays the lowest migration energy barrier, thereby confirming the efficacy of the dual–anion synergy strategy in optimizing diffusion kinetics. The resulting Na₂Fe₂P₂O₇SO₄/C–BM–2 delivers an initial discharge capacity of 124.70 mAh g⁻¹ (0.05 C), a reversible capacity of 88.15 mAh g⁻¹ at 1 C, and retains 64.43% after 10 000 cycles at 20 C. At −25°C, capacity retention reaches 95.46% after 1000 cycles at 0.5 C. This dual–anion–regulated material provides an innovative design for high-energy, wide–temperature, long–life polyanion cathodes, particularly suitable for large–scale energy storage sodium–ion batteries requiring safety and durability.