Aqueous electrolytes offer significant advantages over their organic counterparts, particularly for large-scale applications that prioritize non-flammability, scalability, and sustainability. However, the narrow electrochemical stability window of water imposes voltage limitations, resulting in low energy densities of aqueous batteries. Highly concentrated electrolytes have enabled high-voltage, mostly intercalation-type, aqueous battery chemistries, and over the past decade many additives, coatings, and highly soluble salts have been developed to manipulate the solution structure of the electrolyte and the interphasial chemistry at the electrode surfaces. However, the large amounts of specialized salts required in this approach, put into question whether GWh-scale storage is really the best application of this technology.Storage at such very large scale, however, is required for further integration of renewables and complete decarbonization of power grids. Flow batteries that decouple energy storage from power generation were developed to simplify scaling of very large batteries while maintaining cost and safety benefits of aqueous electrolytes. Here, the solubility of active materials governs the achievable energy density, which is typically an order of magnitude lower for aqueous flow batteries compared to state-of-the-art lithium-ion cells.Capitalizing on our experience with highly concentrated electrolytes and the impact of molecular symmetry and ion-pairing on electrolyte properties,1–5 we manipulate the solution structure of flow battery electrolytes to maximize active material concentration. We manipulate the hydrogen-bonding environment in the electrolyte and mediate anion-cation interactions via cation-engineering and additives, enabling more than four-fold higher concentrations for a series of metal-organic chelates and ferrocyanides than previously reported.6–8 We further developed a simple flame emission spectroscopy setup to track cation ratios in solution and study cation-dependent instabilities of presumably highly stable ferrocyanide anions.8,9 Achieving very high concentrations, however, presents a series of challenges for flow batteries. A two-molar lithium ferrocyanide electrolyte, for example, has an ionic strength comparable to a twenty-molar lithium chloride solution.8 Managing osmotic balance across the membrane thus becomes very challenging in a flow cell. Additionally, the high viscosity of concentrated electrolytes results in significant efficiency and power losses, suggesting that the “dissolve-as-much-as-possible” approach may not necessarily lead to better flow batteries.We thus studied the importance of energy density for real-world MWh-scale batteries by using satellite images to estimate the footprint of such large-scale deployments. We find that installations using lithium-ion batteries often have a comparable footprint to sodium-sulfur batteries and even demonstrator-stage flow batteries.10 This implies that low cell level energy density of flow batteries is not the key limitation of the technology, challenging the common narrative. For applications such as residential use, on the other hand, energy density certainly plays a critical role, and we discuss approaches that could enable much more compact systems.
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Open Access
Jan 8, 2024
Yohichi Kohzuki 
