Solid polymer electrolytes based on poly(PEG-oxo-carbonate-co-cyclic carbonate-thioether) copolymers

Abstract

Poly(ethylene oxide) (PEO) is the more promising material for lithium-ion conducting solid polymer electrolyte. However, linear PEO solid polymer electrolytes (SPE)s have insufficient ionic conductivity and low lithium transference number. The incorporation of carbonate groups, as polar units, has been evaluated to improve the dissolution of the salt and reduce the tendency for ion aggregate formation. In this research work, we propose an original method for the synthesis of new CO2 sourced poly(PEG oxo carbonate), poly(cyclic carbonate-thioether) and their corresponding copolymer with different (oxo-carbonate/cyclic carbonate-thioether) ratios. The copolymerization has been performed by the polyaddition of bis(αCC)s; PEG 4000, as diol; and 2,2′-(Ethylenedioxy)diethanethiol, as dithiol, in DMF at room temperature in presence of DBU. As a result, copolymers with high molecular weight (20,980 g mol-1 to 90,400 g mol-1) and stable to temperatures up to 200 °C, were obtained. These polymers show low glass transition temperature (Tg = -48 °C) and semi crystalline phase; but the crystallinity domains are diminished by the increasing proportion of cyclic carbonate thioether in the copolymers. These decrease on crystallinity is evident on the detrimental effect on the mechanical properties were the Young Modulus decreases from 135.7 MPa to 41.9 MPa. The mixture of copolymers with 30 wt% of bis(trifluoromethylsulfonyl)amine lithium salt (LiTFSi) completely suppresses the crystallinity on the poly(PEG-oxo-carbonate-co-cyclic carbonate-thioether)s. FTIR spectra revealed the preference of coordination of the Li+ with the cyclic carbonate group rather than the linear carbonate group.