Electrochemical Study Of Poly(Vinylidene Fluoride-co-hexafluoropropane)-based Micro-porous Composite Polymer Electrolyte

Polymer lithium ion batteries (PLiB) have been widely used as the chemical power sources for many portable electronic devices on account of their advantages, such as high energy density, good safety and excellent cyclic property, etc. With development of electrode materials, polymer electrolytes play a key role in the PLiB. The gel polymer electrolytes (GPE) take the place of solid polymer electrolytes due to their high conductivity. However, it is difficult to obtain the GPE membranes with good physical properties because the polymer materials such as PEO, PMMA, PVC, and so on, dissolve in the solvent of electrolyte (i.e. carbonate ester) easily. A series of micro-porous composite polymer films (MCPFs) have been prepared by using poly(vinylidene fluoride-co-hexafluoropropane), which can't dissolve in the organic solvent of electrolyte such as carbonate ester, as the matrix for GPE, and inorganic oxides nanoparticles as fillers. The micro-porous composite polymer electrolytes (MCPE) will be achieved by impregnating the polymer films in non-aqueous electrolyte. The composite polymer films were characterized by the measurement of SEM, DSC, FTIR, and XRD. The electrochemical characteristics, for example, ionic conductivity, activation energy for ions transport and Li~+ ions transference numbers, were studied by the electrochemical method. Four types of lithium secondary batteries have been fabricated by sandwiching the MCPE membranes, or Celgard multi-layers membrane between LiCoO2 cathode and lithium anode. Their ac impedance and discharging property were studied in this article. First, a series of micro-porous composite polymer films (MCPFs) have been prepared by three kinds of processes. The influence of process on the microstructure of the films was studied by SEM technology. The MCPFs achieved by evaporation of solvent (i.e. Bellcore technology) have larger pores with 1-5μm in diameter, but this process makes the fabrication complicated and need long time to form film. However, the phase inversion process simplifies the formation of polymer film and shortens the time to obtain the MCPFs. The resulting MCPFs have the pores with 0.3-1.0μm in diameter. On account of the intense surface effect, the nanoparticles aggregate within the MCPFs prepared by these two above-mentioned processes. In order to inhibit the aggregation of nanoparticles, a new process, precipitation in situ, was used to prepare the MCPFs that have the average pores with 0.3μm in diameter. Within the polymer matrix, naoparticles disperse even and not more than 0.1μm in diameter. The MCPFs possess higher porosity because there are interfacial layers between the surface of nanoparticles and polymer matrix. Since lithium ions migrate quickly in these interfacial layers, the micro-porous composite polymer electrolytes (MCPE) obtained by impregnating the MCPFs in non-aqueous electrolyte have higher ionic conductivity up to the magnitude order of 10-3S/cm at temperature. When the MCPFs contain 10wt% of alumina, the ionic conductivity of the MCPE prepared by evaporation process is 1.95×10-3S/cm at temperature while that of the MCPE prepared by phase inversion process is 2.11×10-3S/cm. The MCPE prepared by the process of precipitation in situ possesses 2.40×10-3S/cm of ionic conductivity at room temperature when the MCPFs contain 8.5wt% of TiO2. Second, the effect of nanoparticles on the Li+ ions transference numbers of MCPE was studied in detail by electrochemical technology. It exhibits that Li+ ions transference numbers increase with the amount of nanoparticles in MCPFs firstly, and then decrease with excessive addition of nanoparticles into MCPFs. The interaction between nanoparticles and polymer chains was checked by the measurement of DSC, FTIR, and XRD. The results show that the crystallinity of MCPFs decreases with the amount of nanoparticles filling. On the other hand, since Al2O3 and TiO2 have the effect of Lewis acid, they can decrease the polarity of CF2 groups for P(VDF-HFP) copolymer chains. Therefore, it weakens the interaction between Li+ ions and polar F atoms. Moreover, the interaction of OH groups on the surface of nanoparticles with PF6-anions of electrolyte salt (LiPF6) promotes the disassociation of lithium salt, so Li+ ions transference numbers of the MCPE increase with addition of nanoparticles into polymer matrix. Nevertheless, the aggregation of nanoparticles lessens the number of the effective acid centers and OH groups on their surface. This results in the decrease of Li+ ions transference numbers of the MCPE. That Li+ ions transference numbers increase firstly and decrease then with the rising amount of nanoparticles can be testified by the measurement of the apparent activation energy for ions transport of the MCPE filled with various amounts of nanoparticles. When the MCPFs contain 10wt% of alumina, the MCPE prepared by evaporation process has 0.73 of Li+ ions transference numbers and 5.6kJ/mol of activation energy for ions transport. While Li+ ions transference numbers and activation energy for ions transport of the MCPE prepared by phase inversion process are 0.66 and 4.8 kJ/mol, respectively. The MCPE prepared by the process of precipitation in situ possesses 0.80 of Li+ ions transference numbers and 4.4kJ/mol of activation energy for ions transport when the MCPFs contain 8.5wt% of TiO2. Finally, a kind of solid polymer electrolytes, which can work at evalate temperature, was studied in this paper. It is fabricated by blending 1-methyl-3-ethyl imidazolium tetrafluoroborate(EMIBF4), LiBF4 and PVDF-HFP copolymer after evaporation of solvent, and can't nearly loss weight until 270oC. So it can promise the safety of polymer lithium secondary batteries operating at high temperature. The plasticized polymer electrolytes aretransparent elastic membranes when the mass ratio of EMIBF4 to PVDF-HFP is not more than 1.0, and has 0.55×10-3S/cm of the ionic conductivity at room temperature. Owing to the phase separation, they will become the non-opaque fragile membranes if the mass ratio is over this value. Addition of lithium salt into the platicized polymer electrolytes results in the decrease of the ionic conductivity because a complex forms between LiBF4 and EMIBF4. This phenomenon can be explained by measuring the activation energy for ions transport that increases with the rising of amount of lithium salt in polymer electrolytes. In addition, among the lithium secondary batteries constructed in laboratory, liquid lithium secondary batteries have the lowest inner resistance. The batteries using the MCPE that prepared by the process of precipitation in situ as membranes possess the lower inner resistance and good discharging property at small current density while the batteries using the MCPE prepared by evaporation as membrane have the highest inner resistance and good discharging property at larger current density. The interfacial electrochemical stability of polymer lithium secondary batteries is better than that of liquid lithium secondary batteries. Meanwhile, polymer lithium secondary batteries using the MCPE prepared by the process of precipitation in situ as membranes have the best stability among the sample batteries.