D8-poly(methyl methacrylate)-poly[(oligo ethylene glycol) methyl ether methacrylate] tri-block-copolymer electrolytes: Morphology, conductivity and battery performance

Abstract

A series of deuterated tri-block copolymers with the general structure d8-PMMA-POEGMA-d8-PMMA, with variation in d8-PMMA chain length, were synthesized using sequential controlled radical polymerization (ATRP). Solid polymer electrolytes (SPEs) were produced by blending tri-block copolymers and lithium bis(trifluoro methylsulfonate) (LiTFSI). Small-angle neutron scattering (SANS) was used to study the bulk morphology of the deuterated tri-block copolymer electrolyte series at 25 °C, 60 °C and 95 °C. The lack of a second Tg in DSC analysis together with modelling with the random phase approximation model (RPA) confirmed that the electrolytes are in the mixed state, with negative Flory-Huggins interaction parameters. AC impedance spectroscopy was used to study the ionic conductivity of the SPE series in the temperature interval 30 °C–90 °C, and a battery device was constructed to evaluate a 25 wt% d8-PMMA electrolyte. The Li | SPE | LiFePO4 cell cycled at 60 °C, giving a discharge capacity of 120 mAh g−1, while cyclic voltammetry showed that the SPE was stable at 60 °C.