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% yields. Then, various radical copolymerisations of such MAFTEG with VDF from different feeds (VDF ranging from 86 to 95 mol.%) led to random poly[VDF-co-MAFTEG] copolymers that bore oligo(OE) side-chains in satisfactory yields (67%). These original graft copolymers were characterised by 1H, 19F and 13C NMR spectroscopy. Their molar masses reached 7,000 g∙mol-1 and their thermal properties were investigated while their glass transition temperatures ranged between -31 and -19 °C. Their thermogravimetric analyses under 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 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) 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 V vs. Li+/Li, compatible with 4 V class lithium ion batteries.