Design of metal salt/amide-based deep eutectic monomers toward sustainable production of ion-conductive polymers by radical polymerization

Abstract

Deep eutectic electrolytes are a recent focus in the development of energy-related fields, particularly in the area of Li batteries to combine with safer, sustainable, and cost-effective electrolytes. We herein describe a solvent-free synthetic route to ion-conductive polymers from metal salt/amide-based deep eutectic monomers (DEMs), which were prepared by mixing amido-containing solid monomers with metal salts under suitable feed ratios. N-Isopropyl acrylamide (NIPAM) and N-vinylacetamide (NVA) were selected as vinyl monomers bearing an amido functional group that can act as a hydrogen bond donor. Lithium chloride (LiCl), zinc chloride (ZnCl2), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), which can act as hydrogen bond acceptors, were employed as metal salts. The simple mixing of NIPAM with solid LiTFSI in a 5:1 M ratio at 60 °C for 0.5 h afforded liquid NIPAM/0.2LiTFSI, which exhibited the characteristic eutectic effect. By mixing NIPMA with ZnCl2 in a suitable feed ratio, NIPAM/0.25ZnCl2 was obtained as a white turbid liquid at room temperature. Similarly, NVA/0.2LiCl and NVA/0.2LiTFSI were obtained as transparent liquids from solid NVA. In addition, the thermally-induced free-radical polymerization of the metal salt/amide-based DEMs in the bulk and in solution resulted in the formation of the corresponding polymers. Poly(NVA/0.2LiTFSI) exhibited the highest ionic conductivity of 2.83 × 10−6 S/cm at 50 °C. Polymeric products were also obtained by photoinduced radical polymerization from liquid DEMs without the use of volatile organic solvents under an air environment. These results reveal that the DEMs developed in this study can provide a green and sustainable process for the radically-induced production of novel ion-conductive polymers in air.