Synthesis of an original fluorinated triethylene glycol methacrylate monomer and its radical copolymerisation with vinylidene fluoride. Its application as a gel polymer electrolyte for Li-ion batteries
The synthesis and characterisation of novel poly[VDF-g-oligo(EO)] graft copolymers (where VDF and EO stand for vinylidene fluoride and ethylene oxide, respectively) are presented. First, 2-trifluoromethyl oligo(EO) acrylate, MAFTEG, was prepared by esterification of triethylene glycol monomethyl ether (TEG) with 2-trifluoromethacrylic acid (MAF) catalyzed by methanesulfonic acid, in 50% yield. Then, various radical copolymerisations of the MAFTEG with VDF from different feeds (VDF ranging from 85 to 95 mol%) led to random poly[VDF-co-MAFTEG] copolymers that bore oligo(OE) side-chains in satisfactory yields (60–70%). These original graft copolymers were characterised by 1H and 19F NMR spectroscopy. Their molar masses reached ca. 7000 g mol−1 and their thermal properties were investigated while their glass transition temperatures ranged between −31 and −19 °C. Their thermogravimetric analyses in air showed decomposition temperatures from 235 to 325 °C with 10% weight loss (Td,10%). Gel polymer electrolytes were achieved at room temperature by blending an ionic liquid electrolyte (RTIL), poly[VDF-g-oligo(EO)] graft copolymers, and silica nanoparticles, the ionic liquid being made of 1-propyl-1-methyl pyrrolidinium bis(fluorosulfonyl)imide (PyrFSI) by dissolving lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). These novel copolymers are of potential interest as gel polymer electrolytes in lithium ion batteries, showing competitive ambient conductivities of 0.2 mS cm−1 that increased up to 0.5 mS cm−1 with the content of silica nanoparticles at 20 wt%. In addition, the electrolyte gel appeared to be electrochemically stable in a wide range of potentials varying from 1.5 V to 4.1 V vs. Li+/Li, compatible with 4 V class lithium ion batteries.