Preparation methods of V3.5+ electrolyte and related capacity recovery strategies for vanadium flow batteries: a review

Abstract

Vanadium flow batteries (VFBs) represent a prominent large-scale long-duration energy storage technology, with vanadium electrolyte serving as a critical component that fundamentally governs battery performance. In practical implementations, mixed-valence V^3.5+ electrolyte is universally employed as the initial active solution in both half-cells. The preparation methodology and raw material selection for V^3.5+ electrolyte substantially influence both the economic viability and environmental sustainability of VFB systems. This review systematically examines established techniques for preparing V^3.5+ electrolyte, including the chemical reduction method, electrolysis method, and chemical reduction–electrolysis method, and further classifies them systematically, analyzes the basic principles, process architecture, key equipment, technical advantages and existing problems of each process, and discusses the potential of each method in sustainable application. In addition, this study also analyzed the capacity attenuation mechanism of vanadium batteries, and summarized the capacity recovery strategies. The reduction process of high valence vanadium ions involved in the capacity recovery process and the preparation of V^3.5+ electrolyte have high commonality in chemical principle and process technology. The review concludes with a forward-looking perspective on technological innovations in electrolyte synthesis, emphasizing sustainable production routes and performance optimization strategies for next-generation VFB applications.