Despite recent cost reductions in clean, sustainable energy sources like wind and solar, their intrinsic intermittency remains a crucial problem for year-round reliability, necessitating the use of grid-scale energy storage. Nonaqueous redox flow batteries (NRFB) offer the potential to meet this need, with benefits such as the ability to support both long and short-term storage, individually scalable energy and power ratings, wider electrochemical windows, and a safer profile than integrated systems like lithium-ion batteries. However, NRFB electrolytes currently have a poor energy density due to their low solubility in nonaqueous solvents such as acetonitrile. Synthetically exploring the vast chemical and parameter space of NRFB active materials, on the other hand, is not only expensive but also intractable. In our previous work, we elaborated on a highly stable bio-inspired octa-coordinated dianionic vanadium complex, vanadium<sup>IV</sup> bis-hydroxyiminodiacetate (VBH<sup>2-</sup>) which showed promising solubilities when paired with alkylammonium cations. In this study, we present a computational methodology, coupled with experimental confirmation, for predicting electrolyte solubility trends of the VBH active material that includes both the lattice and solvation free energies. We show that the previously overlooked lattice free energy has a significant role in determining electrolyte solubility, and that solvation free energies alone are insufficient, especially for ionic compounds. Modifications on the redox inactive alkylammonium cation by desymmetrization leading to short-chain, asymmetric cations resulted in a slight increase in solubility of the crystalline active material. The demonstrated agreement between the predicted trends and the solubilities of synthesized active materials highlights the potential of this approach in NRFB electrolyte development and optimization. This synergistic computational-experimental approach offers a cost-effective strategy for developing high-solubility active materials for NRFB systems with high energy density.

Tailoring cation structure for solubility improvement of anionic flow battery active-materials: A computational and experimental investigation

ACS Spring 2022

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