Dual-Functional Hydroxyethylidene Diphosphate Additive for Broadening the Operating Temperature Range and Optimizing the Reaction Kinetics of Electrolyte in Vanadium Redox Flow Battery

Vanadium redox flow battery (VRFB) as a promising large-scale energy storage technology is gradually entering the commercialization stage, the stability and reliability of VRFB are the foundation of its large-scale development. At this stage, VRFB is still constrained by the bottleneck of vanadium pentoxide (V2O5) precipitation in the cathode V(V) electrolytes under high-temperature conditions. This precipitation phenomenon severely restricts the energy density and cycle life of the VRFB. In this study, hydroxyethylidene diphosphate (HEDP) is introduced as a bifunctional additive to systematically explore its regulatory impacts on the thermal stability and electrochemical performance of the electrolyte. The results indicate that HEDP can effectively coordinate with V ions through its bifunctional phosphate groups and inhibit the aggregation and crystallization of V species. Meanwhile, the −OH groups in its molecular structure can promote the electrode–electrolyte contact to some extent, thereby simultaneously enhancing the thermal stability and electrochemical activity of the electrolyte. Notably, the unique bifunctional structure of HEDP realizes the synergistic effects of coordination stability and interface regulation at the molecular level. The battery incorporating HEDP exhibited approximately twice the capacity retention of the unmodified control battery following 50 charge–discharge cycles conducted at 50 °C and under a current density of 250 mA cm–2. Therefore, HEDP not only overcomes the limitations of single additives but also offers a novel collaborative strategy for optimizing the high-temperature stability and electrode kinetics of vanadium electrolytes synchronously, which lays a solid foundation for their future commercial applications.