| Through the modification of ionic liquid (IL), polymer matrix and chemical crosslinking structure, several novel gel polymer electrolytes (GPE) were desighed. The GPE were prepared by introducing IL and LiClO4 into polymer matrix of nitrile rubber (NBR) or its derivatives. The effect of IL, polymer matrix and crosslinking on the properties of GPE was studied.From the point of modification towards IL, epoxy resin was used to react with carboxyl functionalized IL, and the modified IL was blended with NBR and LiClO4 to prepare GPE samples. Fourier transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance (1H-NMR) spectroscopy results confirmed the ring-open addition reaction between epoxy resin and IL with high conversion rate. The GPE samples with modified IL as plasticizer showed higher ionic conductivity than the ones with non-modified IL or without IL from alternating current (AC) impedance analysis. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) confirmed that the epoxy resin modified IL could contribute to the dissolution and dissociation of LiClO4.From the point of modification towards polymer matrix, poly(ethylene glycol) monomethylether (mPEG) of different molecular weight were attached to carboxylated butadiene-acrylonitrile rubber (XNBR) as side chains through esterification and comb-like copolymers XNBR-mPEG were obtained. After blending IL, LiClO4 and benzoyl peroxide into XNBR-mPEG or XNBR, chemically crosslinked GPE were prepared by thermal curing. The grafting of mPEG was confirmed by FTIR, and the grafting content of mPEG was calculated from 1H NMR results. The room temperature ionic conductivity of GPE samples increased with raising the content of LiClO4-IL, and the GPE based on the copolymer with more grafted mPEG showed higher ionic conductivity. A maximum value of over 10-3 S/cm was obtained by AC impedance analysis. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) results indicated that the crystallization of mPEG section in copolymer was inhibited, and the glass transition temperature and GPE modulus was reduced with more grafted mPEG. Linear sweep voltammetry (LSV) analysis indicated a high electrochemical stability up to over 5 V, and cyclic voltammogram (CV) analysis indicated that the unit cell of LiFePO4/GPE/Li could be cycled at room temperature.From the point of crosslinking, epoxycyclohexyl polyhedral oligomeric silsesquioxane (POSS) was used as crosslinking agent. After blending amine terminated butadiene-acrylonitrile copolymer (ATBN) blended with IL, LiClO4 and POSS, inorganic-organic hybrid GPE was obtained by thermal curing. FTIR was applied to analyze the crosslinking reaction and the interaction between IL and LiClO4 and such interaction could contribute to the dissolution and dissociation of LiClO4. XRD and FESEM results indicated that LiClO4 crystals were reduced with increasing IL content and mainly dispersed in IL phase. The ionic conductivity of GPE samples increased first and then decreased with raising IL content. The GPE samples crosslinked by less POSS showed higher ionic conductivity (as to 10-4 S/cm order of magnitude), but lower gel content and modulus. Thermogravimetric analysis (TGA) indicated the GPE could be stable until over 200°C. CV analysis indicated an electrochemical stability up to more than 4 V and a reversible electrode process in the unit cell of Al/GPE/Li at room temperature. |
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