Polymer electrolytes based on dicationic polymeric ionic liquids: application in lithium metal batteries

Abstract

Polymeric ionic liquids (PILs) have stirred up great interest for their potential applications as electrolyte hosts in lithium metal batteries (LMBs) because of their desirable performance. In this work, PIL-based gel polymer electrolytes applied in lithium metal batteries (LMBs) at low–medium temperatures (25 °C, 30 °C and 40 °C) are first reported. A novel imidazolium-tetraalkylammonium-based dicationic polymeric ionic liquid, poly(N,N,N-trimethyl-N-(1-vinlyimidazolium-3-ethyl)-ammonium bis(trifluoromethanesulfonyl)imide) is successfully synthesized, and its structure and purity are confirmed by ¹H NMR, FTIR and elemental analysis. Subsequently, the ternary gel polymer electrolytes are prepared by blending the as-synthesized dicationic PIL as the polymer host with 1,2-dimethyl-3-ethoxyethyl imidazolium bis(trifluoromethanesulfonyl)imide (IM(2o2)11TFSI) ionic liquid and LiTFSI salt in different weight ratios. The PIL-LiTFSI-IM(2o2)11TFSI electrolytes reveal low glass transition temperatures around −54 °C and high thermal stability to about 330 °C. Moreover, the ternary gel polymer electrolytes show good ion conductivity around 10⁻⁴ S cm⁻¹ at low–medium temperatures, high electrochemical stability and good interfacial stability with lithium metal. Particularly, the Li/LiFePO₄ cells assembled with polymer electrolytes at a rate of 0.1 C are able to deliver discharge capacities of about 160 mA h g⁻¹, 140 mA h g⁻¹ and 120 mA h g⁻¹ at 40 °C, 30 °C and 25 °C, respectively, with excellent capacity retention, as well as exhibiting acceptable rate capability. These findings reveal that dicationic PIL-based electrolytes have great potential for use as safe electrolytes in LMBs.