Investigations On The Preparation And Performance Of Gel Polymer Electrolyte For Lithium Ion Battery

Lithium ion battery has been widely used in portable electronic devices because of itsadvantages including high energy density and long cycle life. However, safety problem existsin current commercial lithium ion battery due to the using of combustible liquid organicelectrolyte, especially in large scale application for electrical vehicles. Therefore, polymerlithium-ion batteries with gel polymer electrolyte （GPE） have attracted intensive attention dueto their excellent thermal and mechanical stability, flexibility and safety.In order to improve the ionic conductivity of GPE and the compatibility of GPE withelectrode, several kinds of polymers were synthesized and the performances of thesynthesized polymers and corresponding membranes and GPEs were studied withthermogravimetric analysis （TG）, Fourier transform infrared analysis （FTIR）, scanningelectron microscope （SEM）, linear sweep voltammetry （LSV）, electrochemical impedancespectroscopy （EIS）, and charge-discharge test. Following results were obtained:（1）Polyethylene（PE）-supported polyvinylidene fuoride （PVDF）-cellulose acetate butyrate（CAB） blending polymer membrane was prepared by co-solvents method. The effects ofco-solvent systems and polymer blending ratios on the performance of the corresponding GPEwere studied. It is found that the GPE with PVDF:CAB=2:1（in weight） has the largest ionicconductivity (2.48×10^-3S·cm^-1), is stable at the potential lower than5.0V on steel electrode.The LiCoO2/graphite battery using this GPE exhibits good performance, especially at elevatedtemperature.（2）P（AN-co-MMA） were prepared by solution polymerization with different mole ratios ofmonomers, acrylonitrile（AN） and methyl methacrylate（MMA）. It is found that the GPEusing the PE-supported copolymer with AN to MMA=4:1（mole） exhibits the highest ionicconductivity of2.06×10³S·cm^-1at room temperature. The copolymer is stable up to270℃.The PE-supported copolymer membrane shows a cross-linked porous structure and has150%（wt） of electrolyte uptake. The electrochemical window of the GPE on steel electrode is5.5V（vs.Li/Li⁺）. With the application of the PE-supported GPE in lithium ion battery, the batteryshows its good rate performance and cyclic stability. （3） The nano-Al₂O₃was doped in copolymer P（AN-co-MMA） and PE-supportedP（AN-co-MMA）/nano-Al₂O₃microporous composite polymer membrane was prepared. Theeffect of the amounts of nano-Al₂O₃on GPE was investigated. It is found that the nano-Al₂O₃significantly affects the GPE performances. Compared to the GPE without any nano-Al₂O₃,the GPE with10wt.%nano-Al₂O₃shows a improved performances. The ionic conductivity isimproved from2.0×10³S·cm^-1to3.2×10³S·cm^-1, the oxidation decomposition potential onsteel electrode is enhanced from5.5V to5.7V （vs. Li/Li+） and its interfacial resistance onlithium is reduced from520Ω·cm2to160Ω·cm2.（4） P（AN-co-VAc） was synthesized by emulsion polymerization with different mole ratios ofacrylonitrile （AN） to vinyl acetate （VAc）. Self-supported P（AN-co-VAc） membrane wasprepared and coated with polymethyl methacrylate （PMMA） to prepare a new type of gelpolymer electrolyte matrix, P（AN-co-VAc）/PMMA. It is found that when the ratio of AN toVAc is7:3, the mechanic tensile strength of the prepared polymer film reaches15Mpa, whichcan meet the application requirments of the battery. Compared with P（AN-co-VAc）,P（AN-co-VAc）/PMMA is better as the matrix for gel polymer electrolyte for lithium ionbattery application. Its ionic conductivity is improved from1.4×10³S·cm^-1to1.88×10³S·cm^-1, its oxidation decomposition potential on steel electrode is enhanced from4.8V to5.2V （vs. Li/Li⁺）. With the application of the P（AN-co-VAc）/PMMA GPE in lithium ionbattery, the battery shows its good cyclic stability.