Stability of highly soluble ferrocyanides at neutral pH for energy-dense flow batteries

Ferrocyanides and ferricyanides are among the most employed positive electrolyte materials in aqueous flow battery research, but the limited solubility of commonly available sodium and potassium salts is a critical factor limiting application at scale. Here, we systematically study the cation-dependent solubility of these materials and, importantly, the cation-dependent stability of the anion in aqueous solution. For Li4Fe(CN)6, we report a maximum solubility of 2.3 M and show stable cycling of a 2 M symmetric cell, corresponding to electrolyte capacities of 54 Ah L−1, over 200 days. We also demonstrate solubilities of 1.6 M for ammonium and calcium salts and investigate the pronounced anion decomposition observed in the presence of ammonium cations. Our observations lead to a discussion of challenges associated with the osmotic strength of concentrated electrolytes that employ ions with high charge states, such as ferrocyanide.