Sustainable Production of Ion-Conductive Polyelectrolytes by Ultrafast Photopolymerization of Lithium, Sodium, and Potassium Salts/Amide-Based Deep Eutectic Monomers

Abstract

Herein, the photopolymerization of metal-salt/amide-based deep eutectic monomers (DEMs) derived from lithium, sodium, and potassium bis(trifluoromethanesulfonyl) imide (LiTFSI, NaTFSI, and KTFSI, respectively) is described. Three series of DEMs consisting of N-isopropyl acrylamide (NIPAM) and three different metal salts (LiTFSI, NaTFSI, and KTFSI) are tested at various molar ratios to identify suitable combinations. NIPAM/LiTFSI (1/0.2, 1/0.3, 1/0.4, and 1/0.5) and NIPAM/NaTFSI (1/0.2 and 1/0.3) are obtained as liquid DEMs by simple mixing under ambient conditions (≈25 °C in air), while NIPAM/KTFSI (1/0.1, 1/0.2, and 1/0.3) is obtained as a liquid DEM at 50 °C. The nature of the metal species and NIPAM/metal salt ratio affected the characteristic features of the DEMs and specific interactions. Ultrafast photopolymerization of NIPAM/metal salt DEMs is achieved using LED-UV light, with nearly complete monomer conversion attained within 10 s. The mechanical and thermal properties of the polymerized DEMs (PDEMs) depended substantially on the metal species and NIPAM/metal salt ratio. P(NIPAM/0.2LiTFSI) with 20 wt.% succinonitrile (SN) serving as a plastic crystal exhibited the highest ionic conductivity (1.05 × 10⁻⁴ S cm⁻¹ at 55 °C), and P(NIPAM/0.2NaTFSI) and P(NIPAM/0.2KTFSI) also exhibited improved ionic conductivities of 4.19 × 10⁻⁵ and 6.64 × 10⁻⁵ S cm⁻¹, respectively, at 55 °C with 20 wt.% SN.