Enhancement of Cycle Stability and Durability in Secondary Zinc-Air Batteries By Optional Ion-Transporting Separator

Abstract

Highly durable secondary zinc air batteries were developed using polypropylene (PP) membrane (Celguard 5550) coated with polymerized ionic liquid as separators. The anionic exchange polymer was synthesized copolymerizing 1-[(4-ethenylphenyl)methyl]-3-butyl-imidazolium hydroxide (EBIH) and butyl methacrylate (BMA) monomers by free radical polymerization for both functionality and structural integrity. The ionic liquid induced copolymer was coated on a commercially available PP membrane. The coat allows anionic transfer through the separator and minimizes the migration of zincate ions, which causes the loss of active sites by the formation of zinc oxide on the surface of the catalyst layer. Electrochemical Impedance Spectroscopy (EIS) during the cycle process elucidated the premature failure of cells due to the zinc cross over for the untreated cell and revealed a substantial importance must be placed in zincate control. Scanning electron microscopy (SEM) revealed significant decrease in the porosity of the coated film without large cell performance sacrifice. Energy dispersive x-ray spectroscopy (EDS) data revealed the copolymer coated separator showed less zinc element in the cathode electrolyte, indicating lower zinc permeation through the membrane. Ion Coupled Plasma Optical Emission Spectroscopy (ICP-OES) analysis confirmed over 96% of zinc ion cross over was reduced. In our charge/discharge setup, the constructed cell with the ionic liquid induced copolymer casted separator exhibited drastically improved durability as the battery life increased more than 281% compared to the pure commercial PP membrane.