Insights into Indigo K⁺ Association in a Half-Slurry Flow Battery

Potassium-ion batteries (PIBs) are promising energy storage devices owing to the abundance and low cost of potassium. However, the development of PIBs is still in its infancy owing to the poor kinetic diffusivity, limited capacity, and severe side reactions. It is imperative to explore new materials to address the issues of capacity, stability, and cycle life in PIBs. Here, we present a hybrid slurry/flow battery utilizing redox-active organic materials for K⁺ storage, wherein the indigo slurry serves as the anolyte and K₄Fe(CN)₆ solution serves as the catholyte. This battery presented a cycle life of 1000 cycles with a capacity of 18.4 Ah/L and capacity retention of ∼83%. Voltammetry measurements and density functional theory calculations indicated that the redox activity of indigo involves a one-step, double-electron process, instead of a two-step, single-electron process. This work provides new insights into the application of organic compounds as association materials for PIBs.