Ternary Electrolytes with Broad Ionic Liquid Content Range Enabled by Equimolar Phosphorylcholine Monomer:Lithium Salt Ratio

A series of ternary electrolytes was prepared via in situ UV-initiated photopolymerization of precursor solutions containing an equimolar ratio of the zwitterionic (ZI) monomer 2-methacryloyloxyethyl phosphorylcholine (MPC) and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), together with varying amounts (30–90 wt %) of the ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP TFSI). Maintaining an equimolar MPC:LiTFSI ratio enabled the incorporation of a wide range of IL contents in spite of the limited solubility of MPC in neat BMP TFSI, facilitating study of the effect of IL content variation on the thermal and electrochemical properties of the resulting polymerized ternary electrolytes. FTIR spectroscopy confirmed the successful polymerization of the MPC monomer, even in the most viscous, IL-lean composition studied (30 wt % BMP TFSI). Thermal characterization indicated good thermal stability of the ternary electrolytes up to around 300 °C across the composition range, and for the sample containing 30 wt % BMP TFSI, a glass transition was observed at a substantially lower temperature than that of the pure polymer. Room temperature total ionic conductivity of the ternary electrolytes was observed to increase monotonically with increasing IL content. Notably, for the ternary electrolyte compositions where it was feasible to create LiTFSI/BMP TFSI solutions with comparable moles of LiTFSI per liter BMP TFSI values, the ionic conductivity of each ternary electrolyte was found to be essentially unchanged compared to that of the corresponding polymer-free Li salt/IL solution. The approach described here to realize IL-containing ternary electrolyte compositions with otherwise inaccessible MPC/BMP TFSI or LiTFSI/BMP TFSI effective molarities may be highly beneficial for designing nonvolatile electrolytes that can improve the safety aspects of lithium-ion batteries.