Sustainable production of V3.5 + electrolyte for vanadium redox flow batteries via thermal reduction of ammonium metavanadate

Vanadium redox flow batteries (VRFB) is recognized as one of the most promising technologies for large-scale renewable energy storage, owing to its high safety, long cycle life and lack of cross-contamination. The vanadium electrolyte, which governs the overall energy storage capacity, performance and stability of VRFB, still faces challenges such as complex preparation process and high production cost. In this work, we present a short-route synthesis of V 3.5+ electrolytes via COG-assisted thermal reduction of NH 4 VO 3 under controlled atmospheric conditions. This approach enables rapid conversion of NH 4 VO 3 into mixed-valence vanadium oxide, followed by direct acid dissolution into the desired V 3.5+ electrolyte. Combined thermodynamic analysis and molecular dynamics simulations reveal a sequential mechanism involving deammoniation, dehydration, and reduction, with V 4 O 7 (V 3.5+ ) identified as the key intermediate phase. After optimized acid dissolution, the resulting electrolyte exhibits a V 3+ /V 4+ ratio of 1.01. Electrochemical evaluations confirm that the prepared electrolyte possesses reversibility, conductivity, and stability comparable to those of commercial counterparts, along with superior capacity retention over 400 charge–discharge cycles. A techno-economic analysis indicates that the proposed process increases the profit to 4.88 $ per kg of NH 4 VO 3 compared to conventional chemical reduction–electrolysis methods, while reducing energy consumption by 40.64 %. These findings underscore the potential of this route for scalable, industrial production of high-performance vanadium electrolytes.