Nonaqueous Redox Flow Batteries: Opportunities and Challenges for Sustainable Energy Storage

Redox flow batteries (RFBs) have emerged as promising electrochemical energy storage technologies for long-duration stationary applications. Although aqueous RFBs represent the most commercially mature class, they are inherently limited by a narrow electrochemical stability window and high temperature sensitivity. Nonaqueous RFBs (NRFBs) expand the accessible voltage and temperature ranges, offering a promising route toward high-energy-density storage for long-duration and extreme-environment applications. However, their practical deployment remains impeded by several challenges, including high costs, insufficient power density, limited cycling stability, and safety concerns. This review offers a systematic synthesis of recent advances in key NRFB components, encompassing solvents, supporting electrolytes, membranes, electrodes, and redox-active molecules. We also highlight emerging methodologies expected to accelerate development, including machine-learning-enabled screening, advanced characterization, and life cycle assessment. Finally, this review compares aqueous and nonaqueous systems from commercialization and sustainability perspectives. Collectively, this review aims to advance design principles and optimization strategies for NRFBs, providing guidance for next-generation low-cost, high-energy-density flow battery technologies.