Multiphase electrolyte enables extremely high capacity in aqueous organic flow battery

Aqueous organic flow battery is considered as a promising long‐duration energy storage technology but faces the capacity bottleneck due to the insufficient solubility of electroactive materials. Here, we develop a high‐capacity aqueous organic slurry flow battery (AOSFB) by introducing a multiphase electrolyte to break through the limitation of solubility. Three carbon additives are evaluated as electron conductors, among which Ketjen black (KB) shows superior dispersibility, thereby forming an effective percolation network. Rheological analysis reveals a liquid‐to‐gel transition with increasing carbon volume fraction, with 20 vol% KB achieving an optimal balance between conductivity and viscosity. This conductive network ensures uniform reaction distribution within the reactor, yielding a low overpotential. As a demonstration, an AOSFB employing 1.75 M 2‐quinoxaline carboxylic acid (56% beyond its solubility) achieves a record capacity of 88.4 Ah L −1 . The multiphase concept presents one universal strategy to decouple the capacity from the solubility constraint in flow battery systems.