Controllable phosphidation engineering of Co₂P₂O₇@CoP core-shell electrodes for enhanced sulfur-iodine flow batteries

Polysulfide/iodide redox flow batteries (SIFBs) are promising for large-scale energy storage, but their practical performance is limited by sluggish interfacial kinetics and severe shuttle of soluble intermediates. Here we report a core-shell Co₂P₂O₇@CoP catalytic electrode that addresses these two challenges through an integrated conductivity-adsorption strategy. In this architecture, the CoP shell serves as a highly conductive pathway for rapid charge transfer, while the Co₂P₂O₇ core provides polar P-O sites that strongly immobilize both polysulfide and polyiodide intermediates. This dual-function design enables simultaneous acceleration of the S^2–/S_x^2– and I^–/I^3– redox reactions and effective suppression of side reactions. As a result, the assembled SIFB delivers an initial energy efficiency of 84.53% at 20 mA·cm^–2 and maintains 64.61% after 50 cycles. Long-term operation at 10 mA·cm^–2 and 10% SOC remains stable for 400 cycles with energy efficiency above 70%. Density functional theory (DFT) calculations reveal that interfacial electronic coupling, rather than simple phase addition, is responsible for the enhanced adsorption and reduced reaction barriers.