Ion transport in a solvent-free crosslinked polymer electrolyte

In addition to improved energy density due to compatibility with lithium metal electrodes, solid electrolyte-based systems have the added benefit of increased mechanical strength and flexibility compared to their liquid-containing counterparts. To this end, a series of crosslinked solid polymer electrolytes prepared via solvent-free polymerization reactions of epoxide and amine functional groups is presented here. The impact of salt loading on ion transport in this crosslinked material is investigated by determining the lithium ion diffusion coefficient, the relative proportion of contact ion pairs and ion aggregates with respect to free ions and the ionic conductivity as a function of the molar ratio of lithium to ethylene oxide (r). The material with r = 0.07 was found to have the highest ionic conductivity despite having a lower lithium transport number and lithium ion diffusion coefficient than samples with lower r values. It was determined that the ionic conductivity of the crosslinked electrolyte was influenced by a combination of salt loading, ion pairing and ion mobility. The electrochemical and mechanical stability of the material with r = 0.07 were evaluated and compared with a traditional PEO-LiTFSI electrolyte. Although similar electrochemical stability was observed, the crosslinked electrolyte had significantly improved lithium ion transport properties and higher mechanical strength.