Study On Lithium-ion Solid Polymer Electrolyte Based On Comb-like Polymer With A Novolac Main Chain

Compared with traditional liquid electrolyte,solid electrolytes avoid the use of volatile organic solvents and therefore is considered to be safer.In this thesis,novolac is grafted with thioether for the first time to synthesize comb-like polymer(TLN).The structure is characterized by gel permeation chromatography(GPC),nuclear magnetic resonance(NMR),Fourier transform infrared spectroscoy(FTIR)and differential scanning calorimetry(DSC).After addition of LiTFSI as the lithium salt to make electrolyte,the ion conductivity,electrochemical stability,lithium ion transference number,and cycling performance of lithium deposition/striping are studied using electrochemical workstation and battery test system.The results are as follows,(1)The thioether-labeled novolac(TLN)is successfully synthesized through the click chemistry between mercaptoethanol and vinyl-labeled novolac(VLN).The grafting ratio is found to be 29.2 mol%by NMR analsysis.The side chain enhances the flexibility of the polymer segment and reduces the glass transition temperature of novolac from 73℃ to 8℃.LiTFSI is then added to TLN by different mass fractions(10 wt%～40 wt%vs.TLN)to make polymer electrolyte.A decrease of Tg is found with the increase of lithium salt content.The lowest Tg is found to be-12℃ for TLN40 which shows the highest ionic conductivity by 7.0×10-6 S/cm at 60℃,and 1.6×10-4 S/cm at 100℃.Generally,with the increase of LiTFSI from 10 wt%to 40 wt%),the ionic conductivity of electrolyte increases,but the lithium ion transference number decreases from 0.64 to 0.49.The electrochemical stability of TLN is found to be 4.0 V for oxidative potential,and it shows a good cycling performance in the lithium deposition/stripping test at the current density by J=0.02 mA/cm2.(2)Composite solid polymer electrolyte is prepared by adding different mass fractions(10 wt%～40 wt%vs.TLN)of tantalum-doped lithium lanthanum zirconate(LLZTO)rare earth inorganic lithium ion conductor nanoparticles to TLN10 and TLN40.With the increase of LLZTO content,the Tg of the composite electrolyte decreases from 2℃ and-12℃ to-5℃ and-20℃,respectively.At 60℃,when the LLZTO content is 30 wt%,the ionic conductivity of TLN1030LLZTO and TLN40-30LLZTO reached 2.0×10-6 S/cm and 4.3×10-5 S/cm,respectively,and at 100℃ the ion conductivity increases further to 2.7×10-5 S/cm and 7.1×10-4 S/cm,respectively.At the same time,the ion transference number of the polymer electrolyte increases from 0.64 and 0.49 to 0.69 and 0.62,respectively,and the electrochemical stability increased from 4.0 V to 4.4 V.It shows good cycling performance in the lithium deposition/stripping experiment at J=0.02 mA/cm2.(3)Plasticizer succinonitrile(SN)and ethylene carbonate(EC)are added to the TLN4030LLZTO sample,respectively.When the added plasticizer is by 1 wt%,the Tg decreased from19.3℃ to-20.9℃ and-20.6℃,respectively.When the added amount is increased to 10 wt%,the Tg of TLN40-30LLZTO-10SN decreases to-23.4℃,and the TLN40-30LLZTO-10EC decreases to-22.1℃,at 60℃,the ionic conductivity is increased from 4.3×10-5 S/cm to 5.4×105 S/cm and 4.9×10-5 S/cm,respectively.And at 100℃,the ion conductivity increases from the 7.1×10-4 S/cm to 2.7×10-3 S/cm and 1.8×10-3 S/cm,respectively.The ion transference number is increased also from 0.62 to 0.78 and 0.76,respectively,and the electrochemical stability both rise to 4.5 V.