Preparation And Properties Of Polymer Electrolytes For Lithium Ion Batteries

Currently,most of the separators are polyolefin materials.Since the melting temperature is lower than 200?,too strong discharge current or heat may destroy the separator and cause a short circuit.While there is amounts of flammable organic solvent in the electrolyte,the short circuit may at last lead to a fire even an explosion.In order to improve the safety of traditional lithium ion battery,this paper adopts the polymer electrolyte instead of liquid one and the separator.By the addition of nanosilica liquid,the PI based were researched in depth and the PMMA based polymer electrolytes were studied.(1)Soluble polyimide was synthesized by 3,3',4,4'-diphenyl ether tetracarboxylic acid dianhydride and 1,4-bis(4-amino-2-trifluoromethylphenoxyl)benzene,nanosilica fluid containing imidazole with PEG chain(NSiF-Ims)was synthesized by nanosilica(LUDOX SM-30,30 wt.%),N-methylimidazole,(3-propylchloride)trimethoxy silane and poly(ethylene glycol)4-nonylphenyl 3-sulfopropyl ether potassium salt.The structure of the products were characterized by Fourier transform infrared spectroscopy(FTIR)and nuclear magnetic resonance spectrum(1H NMR).Thermogravimetric analysis(TGA)shows that the decomposition of the polyimide and the NSiF-Ims were respectively 562? and 358? which means that it will significantly improve the safety and life of cell to apply PI/NSiF-Ims based electrolyte into lithium ion batteries.(2)A homogenous polymer electrolyte system formed by addition of NSiF-Ims into blend of PI and LiClO4.The surface morphology was characterized to be homogenous with no phase splitting by SEM.XRD further confirmed that the addition of NSiF-Ims gave rise to compatibility of LiClO4 with PI.TGA confirmed that the decomposition temperature of PI/NSiF-Ims based composite polymer electrolyte got close to that of NSi F-Ims,up to 370?.DSC further indicated that the operating temperature reached 300? which is far higher than that of polyolefin separators.Compared with almost 50% shrinkage of Celgard2325,PI/NSiF-Ims based composite polymer electrolyte showed no heat shrinkage after heat treatment at 150? for 8h.The stress strain curves showed that the tensile strength of PI/NSiF-Ims based composite polymer electrolyte reached 8.5MPa and the elastic modulus reached 3232.4MPa at 25?.The Nyquist plot of AC impedance showed that the ionic conductivity of PI/NSiF-Ims based composite polymer electrolyte at 25? increased to 1.8×10-3 S/cm with 60%-NSiF-Ims and then declined.Linear scanning volt-ampere characteristic(LSV)curves indicated that the electrochemical windows of PI/NSiF-Ims based composite polymer electrolyte were all beyond the typical scope from 2.5V to 3.7V with the content of NSiF-Ims ranging from 30 wt.% to 70 wt.%.After PI/NSiF-Ims composite polymer electrolytes were assembled into Li/electrolyte/LiCoO2 non-blocking batteries,ion transference numbers were calculated by using AC impedance method and chronoamperometry.It turned out that the change trend was consistent with the ionic conductivity and the maximum of 0.23 met the requirements of the market application.Through the battery charge and discharge performance test,the results showed that the initial discharge capacity reached 130 mAh/g.(3)The monomer methyl methacrylate(MMA),polymerization initiator azodiisobutyronitrile(AIBN),NSi F-Ims and lithium perchlorate were mixed in proportion.Through thermal polymerization,the Poly(methyl methacrylate)(PMMA)based electrolytes were obtained.XRD showed that after the addition of NSiF-Ims,the crystallization of lithium perchlorate was eliminated and the compatibility with PMMA increased.The AC impedance Nyquist curves demonstrated that the addition of NSiF-Ims gave a rise to ionic conductivity,which increased to a maximum 10-4 S/cm until the NSi F-Ims content reached 2 wt.% at 25?.After the electrolytes were assembled into Li/electrolyte/LiCoO2 non-blocking batteries,ion transference numbers were obtained by AC impedance and chronoamperometry method.The results were consistent with the change trend of ionic conductivities and reached a peak value 0.29 when the content of NSi F-Ims was 2 wt.%.