Phase-Transitional Catalyst-Assisted Membraneless ZnI2 Battery

Aqueous zinc-based batteries have been recognised as a promising solution for large-scale energy storage because of their safety, low cost, and high theoretical capacity. However, further development has largely been limited by a need for a membrane to eliminate self-discharge, which obstructs ion transport. This project has demonstrated a novel membraneless aqueous zinc iodide battery, using N-methyl-N-butyl pyrrolidinium iodide (MBPI) to complex molecular iodine released during charging into a quasi-solid charging product, which phase-separates from the bulk electrolyte, thus eliminating self-discharge. Moreover, this mechanism achieves increased iodide utilisation because iodine is not complexed by aqueous iodide ions. Raman studies have confirmed that polyiodide concentration in the electrolyte is significantly reduced in the presence of MBPI, and the MBPI charging product contains iodine. Electrochemical studies of cells containing 4 M ZnI₂ electrolytes have demonstrated 65 cycles with a Coulombic efficiency of over 85% with electrolyte containing 0.3 M MBPI, compared to 0 cycles without MBPI; the battery was cycled at a 100-mAh capacity, achieving an energy density of 18.6 WhL^-1, which exceeds many existing zinc-halide membraneless designs. Also, the presence of MBPI in the electrolyte inhibited Zn dendritic growth due to a reduced concentration of shuttling polyiodides in the aqueous phase, which cause electrolyte polarisation and produce a non-uniform electric field, thus initiating Zn dendritic growth. With its high capacity and specific current compared to existing alternatives, this phase-transitional membraneless zinc-iodide battery is a promising design for future energy storage systems.