Enhanced Thermal Stability and Electrochemical Performance of Positive Electrolytes in Vanadium Flow Batteries via a Bifunctional Additive System

As a promising large-scale electrochemical energy storage system, vanadium flow batteries suffer from the inadequate thermal stability of positive electrolytes at elevated temperatures. Conventional additives, while improving thermal stability, often compromise electrochemical properties. Herein, a novel dual-additive strategy that synergistically enhances both thermal stability and electrochemical activity was proposed. The bifunctional additive system—comprising methanesulfonic acid (MSA) and ammonium dihydrogen phosphate (ADP)—extends the onset time for V(V) electrolyte precipitation at 45 °C from 1 day to 10 days, through phosphate-mediated complexation and the strongly protonating environment provided by methanesulfonic acid. In terms of electrochemical performance, the bifunctional additive demonstrates the optimal redox reversibility, reduced charge transfer resistance, decreased polarization resistance, and increased diffusion coefficient, which may be attributed to the nitrogen/sulfur bifunctional modification on the working electrode induced by the additives. In single-cell tests at 100 mA·cm −2 , the electrolyte with the addition of 0.1 mol·l −1 MSA and 0.1 mol·l −1 ADP achieves the highest capacity retention of 73.9% after 50 cycles and an energy efficiency of 83.1%. This study paves a new paradigm for for simultaneously enhancing the stability and electrochemical performances of positive vanadium electrolytes.