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-/Sx^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%. DFT calculations reveal that interfacial electronic coupling, rather than simple phase addition, is responsible for the enhanced adsorption and reduced reaction barriers.