Effect of electrolyte anions on the cycle life of a polymer electrode in aqueous batteries

Redox polymers are a class of high-capacity, low-cost electrode materials for electrochemical energy storage, but the mechanisms governing their cycling stability are not well understood. Here we investigate the effect of anions on the longevity of a p-dopable polymer through comparing two aqueous zinc-based electrolytes. Galvanostatic cycling studies reveal the polymer has better capacity retention in the presence of triflate anions than that with sulfate anions. Based on electrode microstructural analysis and evolution profiles of the cell stacking pressure, the origin of capacity decay is ascribed to mechanical fractures induced by volume change of the polymer active materials during repeated cycling. The volume change of the polymer with the triflate anion is 61% less than that with the sulfate anion, resulting in fewer cracks in the electrodes. The difference is related to the different anion solvation structures—the triflate anion has fewer solvated water molecules compared with the sulfate anion, leading to smaller volume expansion. This work highlights that anions with low solvation degree are preferable for long-term cycling.