Preparation And Electrochemical Study Of The Lonic Liquid/Nanocomposite Polymer Electrolyte

The rechargeable polymer lithium battery has got much attention due to its advantages such as safety, high energy density and easy preparation. Ionic liquids as a kind of green chemical solvent have developed into the focus of investigation nowadays On account of the advantages such as high ionic conductivity, wide electrochemical stability, nonflammability, negligible vapor pressure and good thermal satbility. The polymer electrolyte incorporated with ionic liquids can solve the potential safety problem of the rechargable lithium batteries, which usually use the conventional organic solvents.In this study, the new high-temperature safety nanocomposite polymer electrolytes based on SiO2, poly (vinylidene fluoride-co-hexafluoropropane) (P(VDF-HFP)), and LiBF4 were prepared by incorporating the 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4).The main contents are as follows:Firstly, nanocomposite polymer electrolytes were prepared by the solvent casting method by incorporating EMIBF4 into the SiO2-P(VDF-HFP)-LiBF4 system. The resultant nanocomposite polymer electrolytes were studied by Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), thermogravimetry (TG), differential scanning calorimetry (DSC) and electrochemical impedance spectra (EIS), cyclic voltammetry (CV), linear sweep voltammetry(LSV). FTIR spectra indicated that there was an interaction between the polymer chain and nanoparticles and the polymer crystalline phase changed. The TG results showed that the polymer electrolytes have high decomposition temperature above290℃. The addition of SiO2 nanopartilces has no effect on the thermal stability of the polymer electrolyte. DSC results showed that the addition of SiO2 nanopartilces decreased the crytallinity of P(VDF-HFP) and thus the amorphous region increased, which would lead to a high ionic conductivity of the polymer electrolyte. From the CV and LSV results, we can see that the polymer electrolytes exhibit the cathodic and anodic stabilities of -0.5～1.0V and 4.05～5.66V (vs. Li/Li+), respectively. The nanocomposite polymer electrolyte possesses 0.47×10-3S/cm of ionic conductivity at room temperature when it contains 2 wt% of SiO2 nanoparticles. The charge-discharge results showed that the Li/LiFePO4 batteries using this type of polymer electrolyte delivered an initial specific capacites of 133,101,73 and 58 mAh/g at the rates of 0.1C,0.2C,0.5C and 1.0C, respectively, exhibiting good rate ability and good cycling performance at room temperature.Secondly, the nanocomposite (SiO2-P(VDF-HFP) polymer membranes were prepared by electrospinning method, the nanocomposite polymer electrolytes were obtained by plasticized the polymer film with EMIBF4. The results showed that the additions of DEC and y-BL reduced the thermal stability of the polymer electrolyte, but can improve the ionic conductivity and interface stability. CV and LSV results indicated that the cathodic and anodic stabilities of the polymer electrolyte were-1.0V and 5.0V (vs. Li/Li+) or so, respectively. The nanocomposite polymer electrolyte possesses 3.48×10-3 S/cm of ionic conductivity at room temperature when its content was 5%SiO2-lmol/L LiBF4/EMIBF4-DEC-γ-BL(2:1:1,v/v/v). The Li/LiFePO4 cell using this polymer electrolyte as separator delivered an initial specific capacity of 157, 138 and 99 mAh/g at the rates of 0.1 C,0.2C, and 0.5 C, respectively, and maintained about 97%,85%, and 84% of the initial capacity after 50 cycles, showing good rate ability and good cycling performance at room temperature.