Asymmetric Mass Transport in Polybromide Ionic Liquids and Its Impact on Dual-Plating Zinc Bromine Batteries

Zinc-halogen batteries (ZHBs) offer a safer, cost-effective alternative to lithium-ion batteries, leveraging abundant zinc resources and high energy density. Among ZHBs, dual-plating zinc bromine batteries (ZBBs) utilizing ionic liquid (IL)-forming bromine complexing agents (BCAs) exhibit enhanced performance by minimizing halogen crossover and enabling high conductivity via Grotthuss-type halide transport. In this study, the electrochemical impedance of the bromide redox reaction in the presence of 1-ethyl-1-methylpyrrolidinium bromide (MEPBr), an IL-forming BCA, was analyzed. Potentiodynamic operando impedance measurements revealed pronounced asymmetry in mass transport impedance between polybromide ionic liquid (PBIL) formation and dissolution. This asymmetry significantly influenced the potential and impedance trends during galvanostatic cycling of dual-plating ZBBs. During charging, facilitated Br^- transport lowered the positive electrode impedance, resulting in minimal positive electrode overpotential even at high current densities. In contrast, during discharging, PBIL dissolution at the positive electrode exhibited large overpotential at high current densities due to the relatively sluggish internal mass transport of Br_2n+1 ^-. Furthermore, similar asymmetry was observed across various IL-forming BCAs, indicating that the mass transport disparity is an intrinsic property of PBIL rather than limited to MEPBr. These findings provide new insights into PBIL mass transport dynamics and their impact on high-current-density operation in dual-plating ZBBs.