Molecular Design and Redox Behavior of Polyoxometalates: Structure–Property Insights for Flow Battery Materials

Redox flow batteries (RFBs) are a promising technology for large-scale battery energy storage systems (BESS), supporting electricity grid stability and reliability. Critically, flow battery design decouples energy storage and power generation, making them uniquely well-suited among battery technologies for long-duration energy storage. However, flow batteries generally have low energy density compared to other energy storage systems, such as conventional lithium-ion batteries. Further increasing the volumetric energy density and electrolyte stability of flow batteries is crucial to enabling their application for grid-scale BESS. Finding electrolytes that provide high energy density and superior rate capability remains a challenge in flow battery research. Polyoxometalates (POMs) are a promising class of inorganic clathrate materials with multielectron transfer capability and fast charge-transfer kinetics for high-energy-density electrolytes. POMs have been demonstrated in flow batteries in numerous studies and show promising potential with a high volumetric energy density. Yet, in-depth studies on core chemical properties, such as charge stored per atom and material stability, remain underexplored. Design principles for the structure–property relationships of such materials are lacking. This perspective examines the current state of development of POMs as active materials for RFBs, focusing on the structure–property relationships governing their reduction potential and performance. We highlight the need for further first-principles design to guide future POM electrolyte development.