High Safe Polymer Electrolytes Based On Novel Imidazolium Cationic Polymeric Ionic Liquids: Application In Lithium Ion Batteries

Polymeric ionic liquids(PILs) have stirred great interest for their potential application as electrolyte hosts in lithium ion batteries(LIBs) because of their desirable performance. In this thesis, the synthetic routes and molecular structures of imidazolium-based PILs have been studied, subsequently, the PILs-based gel polymer electrolytes(GPEs) have been prepared, and their application performances in LIBs have also been compared. In addition, the physicochemical and electrochemical parameters of these PILs-based GPEs, including thermal stability, ionic conductivity, electrochemical stability and interfacial stability with lithium metal, have been systematically investigated.An imidazolium-based PIL, poly(1-ethyl-3-vinylimidazolium bis(trifluoromethanesulfonylimide)) is successfully synthesized via a new three-step process comprising a direct radical polymerization of the 1-vinylimidazole monomer, and subsequent quaternization reaction followed by an anion exchange procedure. Furthermore, polymer electrolytes are prepared by blending as-obtained PIL as the polymer host with 1,2-dimethyl-3-ethoxyethyl imidazolium bis(trifluoromethanesulfonyl) imide(IM(2o2)11TFSI) ionic liquid and LiTFSI salt. Electrochemical measurements demonstrate that, compared with polymer electrolytes containing the PIL host synthesized by the conventional route, the polymer electrolytes containing the PIL host obtained by new synthetic process exhibit significantly improved capacity and cycling performance, which is due to higher ionic liquid content.PIL-based gel polymer electrolytes applied in lithium metal batteries(LMBs) at low-medium temperatures(25℃, 30℃ and 40℃) are firstly reported. A novel imidazolium-tetraalkylammonium-based dicationic PIL, 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 this dicationic PIL as host with IM(2o2)11TFSI ionic liquid and Li TFSI salt in different weight ratios. The PIL-LiTFSI-IM(2o2)11TFSI electrolytes reveal low glass transition temperatures around-54℃ and high thermal stability to about 330 ℃. Moreover, the ternary gel polymer electrolytes show good ion conductivity around 10-4 S cm-1 at the low-medium temperatures, high electrochemical stability and good interfacial stability with lithium metal. Particularly, the Li/LiFePO4 cells assembled with the polymer electrolytes at 0.1C rate are able to deliver discharge capacities of about 160 mAh g-1, 140 mAh g-1and 120 mAh g-1 at 40℃, 30℃ and 25℃, respectively, with excellent capacity retention, as well as exhibit acceptable rate capability. These findings reveal that dicationic PIL-based electrolytes have great potential for use as safe electrolytes in LIBs.