Preparation Of Gel Polymer Electrolyte Based On P(VDF-HFP) Plasticized With Ionic Liquid And Its Electrochemical Study

Polymer lithium- ion batteries(LiBs) are consisted of anode, cathode and electrolyte. They have so many advantages such as high energy density, good security performance, easy preparation and good cycling performance that they caused much attention. Although solid polymeric lithium- ion batteries(PLiBs) can solve the security issue of the conventional liquid lithium- ion batteries, the low ionic conductivity of solid polymer electrolyte limits the wide application of PLiBS in high power lithium- ion batteries. Gel polymer electrolyte(GPE) can be obtained by doping the solid polymer electrolyte with organic compounds. Especially, ionic liquids have been chosen to plasticize the polymer electrolyte to form the GPE with high ionic conductivity at room temperature and good security performance, which has attracted widespread attention. In this paper, we carried out the research of gel polymer electrolyte based on P(VDF-HFP), which is plasticized with ionic liquids. The research contents are as follows:1. Various P(VDF-HFP)-based GPEs plasticized with seven different imidazole-based ionic liquids(ILs) have been prepared by a simple solution casting process, in which the corresponding lithium salt is added to improve lithium ion source. Thermal decomposition temperature and crystallinity of the as-prepared GPEs decrease with doping of ILs, but the GPEs still show high thermal stability over 300 oC. Among the ILS plasticized, the GPE plasticized with 1-ethyl-3- methyl imidazolium bis(trifluoromethyl-sulfonyl) imide(EMImTFSI) has the higest ionic conductivity of 0.7×10-3 S cm-1 at room temperature, and exhibits a good electrochemical stability. The assemble Li/LiFePO4 cells using this GPE revealed good cycling performance and rate capability.2. In order to improve the electrochemical performances and simplize the fabrication process of the GPE based on P(VDF-HFP), MMA monomers were added to P(VDF-HFP) solution and polymerized in situ by atom transfer radical polymerization to form PMMA in one pot. The P(VDF-HFP)/PMMA blended GPE was prepared by polymerization induced phase separation. FTIR spectra indicated that MMA monomer was successfully polymerized into PMMA. SEM images showed that the as-prepared PMMA dispersed uniformly in the P(VDF-HFP) phase forming a semi- interpenetrating network structure, which appeared as a microstructure of bicontinuous phase. With addition of MMA component, the size of spherical P(VDF-FHP) crystallites was reduced in the polymer membrane. The pore size within the polymer membrane decreased whereas the number of inner pores increased. DSC results suggested that crystallinity of the GPE decreased with the amount of MMA monomers. When the mass ratio of P(VDF-HFP) to PMMA is 2:1, the resultant GPE has an ionic conductivity up to 0.89×10-3 S cm-1 at room temperature and an electrochemical window of 5.5 V. Taking this GPE as separator, the assembled Li/LiFePO4 cells could deliver an initial capacity of 157 mAh g-1 at the 0.1C rate, and remained146 mAh g-1 after 50 cycles. The capacity retention ratio is 93%. At the 1C rate, the cells discharge capacity is 129 mAh g-1 in the frist cycle and 112 mAh g-1 after 50 cycles.3. In order to further improve ion conductivity, mechanical properties, interfacial stability and lithium ion transference number of the GPE based on the P(VDF-HFP)/PMMA blends, SiO2 nanoparticles from in-situ hydrolysis were doped to the polymer matrix to produce a kind of nano-composite polymer electrolyte(NCPE). Effects of the amount of SiO2 on the film- forming performance, mo rphology, porous structure and electrochemical properties of the NCPE were studied. When the the amount of SiO2 doped is 5%, the obtaind NCPE has the highest room temperature ionic conductivity of 1.1×10-3 S·cm-1.The electrochemical window is still about 5.5 V while the stability of the interface between Li electrode and NCPE is improved. The Li/LiFePO4 module cells could deliver a specific capacity of 168 mAh g-1 at the 0.1C rate in the first cycle, and remained 96% of the initial capacity after 50 cycles. At the 1C rate, the discharge capacity is 135 mAh g-1 in the first cycle while the capacity retention ratio is 90% after 50 cycles.