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

Abstract

Vanadium flow battery (VFB) represents 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 V3.5+ electrolyte is universally employed as the initial active solution in both half-cells. The preparation methodology and raw material selection for V3.5+ electrolyte substantially influence both the economic viability and environmental sustainability of VFB systems. This review systematically examines established techniques for preparing V3.5+ electrolyte, including chemical reduction method, electrolysis method, 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 battery, and summarized the capacity recovery strategy. The reduction process of high valence vanadium ions involved in the capacity recovery process and the preparation of V3.5+ 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.