Preparation Of Polymer Electrolyte Based On P(VDF-HFP) And Its Electrochemical Study

Polymer lithium-ion batteries have now got much attention due to the advantagessuch as safety, high energy density and easy preparation. Electrolyte is the keycomponent of polymer lithium-ion batteries, so the research upon it is very important.The gel polymer electrolyte has higher conductivity than the solid polymer electrolyte,and shows better safety and thermal stability than the liquid electrolyte. As a result, itbecomes the hot researching issues. On account of high dielectric constant （ε＝8.4）,good mechanical performance, good chemical and thermal stabilities and lowcrystallinity, poly（vinylidene difluoride-co-hexafluropropylene）[P（VDF-HFP）] as oneof the conventional matrices of gel polymer electrolyte has been widely studiedrecently.In this work, we used P（VDF-HFP）（Knyar Flex2801, hexafluoropropylene12wt%） as matrix through three different methods to prepare the blended polymerelectrolyte, nanocomposite gel polymer electrolyte and porous gel polymer electrolyte.The main contents are as follows:1. A kind of polymer membrane based on P（VDF-HFP）/PMMA blends andplasticized with a room temperature ionic liquid1-butyl-3-methyl-imidiazoliumhexafluorophosphate （BMIPF6） was prepared. This polymer membrane shows a goodthermal stability. The crystallinity of the system decreased with the increasing amountof PMMA blended. When the mass ratio of PMMA to P（VDF-HFP） is1:1, its ionicconductivity closes to10^-3S·cm^-1at room temperature. Using this polymer electrolyteas a separator, the Li/LiFePO4cells show good cycling performance and ratecapability.2. A kind of nanocomposite polymer membrane （NCPM） based on P（VDF-HFP）was prepared by incorporating with SiO₂nanoparticles, which come from the in-situhydrolysis of tetraethylorthosilicate （TEOS）. It was plasticized with a roomtemperature ionic liquids1-ethyl-3-methylimidazolium tetrafluoroborate （EMIBF4）forming the nanocomposite polymer electrolyte （NCPE）. The effect of SiO₂nanoparticles on the electrochemical performances of the NCPE was studied. FTIRresults suggest that TEOS was decomposed and formed nano-SiO₂. SEM resultsinicate that nanoparticles are evenly distributed in the polymer matrix. TG resultsshow that the polymer electrolytes exhibit high decomposition temperature above320 oC, demonstrating good thermal stability. DSC results display that the crystallinity ofthe polymer electrolyte decreases with the increasing amount of nanopartilces. Whenit contains10wt%of SiO₂, the NCPE has the lowest crystallinity. From the cyclicvoltammogram （CV） and linear sweep voltammogram （LSV） results, we can see thatthe NCPEs exhibit the cathodic and anodic stabilities of <-1.0and4.6～5.2V （vs.Li/Li+）, respectively. When filled with10wt%of SiO₂, the NCPE shows the highestionic conductivity （1.77×10-3S·cm-1） at room temperature. The Li/LiFePO4cellsusing the NCPE as separator also show good cycling performance and rate capability.3. We used CaCO₃nanoparticles as the template and then moved the template byHCl to prepared a polymer membrane with a three-dimensionally macroporousstructure. After soaked in the liquid electrolyte （1mol/L LiPF6/EC-DMC （1:1, w/w））,the gel polymer electrolyte was obtained. CaCO₃nanoparticles were prepared by thereaction of CaCl2with Na2CO3solution using oleic acid as dispersant. The obtainedCaCO₃was modified by titanic acid ester coupling agent TC-114. FTIR, TG and XRDmeasurements proved that the CaCO₃is completely removed by immersing thepolymer membrane in HCl acid. Effect of the amount of CaCO₃on the porousstructure of the polymer membrane was studied. When the mass ratio of CaCO₃toP（VDF-HFP）) is3:1, the room temperature ionic conductivity and the uptake of liquidelectrolyte of the polymer electrolyte reaches the maximum. However, its electrolyteleakage is serious. The results from Li/LiMn2O4cells indicate that the polymerelectrolyte has the best cycling performance and rate capability when the mass ratio ofCaCO₃to P（VDF-HFP） is2:1.