Modification Of Gel Polymer Electrolytes For Lithium-ion Batteries

Gel Polymer Electrolyte(GPE) has attracted extensive attention, as it owns advantages of high ion conductivity(of liquid electrolytes) and high safety(of solid electrolytes). At the same time, the GPE has bifunctionality of separator and electrolyte. However, the mechanical strength, thermal stability and electrochemical performances of GPE need to be improved. In order to develop GPE with excellent comprehensive performance, the following work has been done in the thesis. Firstly, the modification effect of PMMA(poly(methyl methacrylate)) on the properties of poly(vinylidene fluoride-co-hexafluoropropylene)(PVDF-HFP) was studied; Then, the doping effects of inorganic nanoparticles on the performance of PVDF-HFP/PMMA were investigated. Finally, lithium-ion transporting materials introduced to improve the performance of PVDF-HFP/PMMA-based GPE. The main results are as follows.(1) The modification effect of PMMA on the properties of PVDF-HFP was investigated. The results show that when the ratio of PMMA is 50 wt%, PVDF-HFP/PMMA exhibits best comprehensive performance. Its electrolyte uptake can reach about 160%. The ionic conductivity at room temperature is 1.35×10-3S/cm. The coin cell with this polymer electrolyte shows good cycle and rate performance. The initial capacity of the cells is 143.6m Ah/g, and maintains 80% of the initial capacity after 50 cycles at 0.2C-rate.(2) The doping effects of Ti O2, Sn O2 and Mo O3 nanoparticles on the performance of PVDF-HFP/PMMA were investigated. The results show that all three nanoparticles can obviously improve the performance. The SEM images exhibit that the PVDF-HFP/PMMA with 5 wt% Ti O2 has the high porosity with interactional network.The electrolyte uptake of the composite polymer reaches 267%. The ionic conductivity at room temperature is 2.49×10-3S/cm. The electrochemical stable window is 4.75 V. After Li Co O2/Li cell was assembled with the CPE, exhibits good performance with capacity of 80.3 m Ah/g at the high rate of 5C. 5 wt% Sn O2 doped composite polymer electrolyte has an irregular reticular structure. The electrolyte uptake of the composite polymer is 193%. The ionic conductivity is 1.52×10-3S/cm and the electrochemical stability window reaches 5.25 V. At 0.2 and 2C-rate, cells exhibit the capacities of 147.7 and 95.6 m Ah/g, respectively; 5 wt% Mo O3 doped composite polymer electrolyte has a higher porosity with small pores structure and the electrolyte uptake of composite polymer electrolyte is up to 232%. The ionic conductivity is 1.83×10-3S/cm and the electrochemical stability window reaches 5.5V. At 0.2 and 2C-rate, cells exhibit the capacities of 173.5, 100.7 m Ah/g, respectively.(3) Li Co O2 and Li Mn2O4 were introduced into GPEs to improve the performance. CPEs by doping the Li Co O2 and Li Mn2O4 materials. The doped Li Co O2(Li Mn2O4) effects the properties of CPEs. At room temperature, the ionic conductivity of CPEs with Li Co O2(Li Mn2O4) is 1.64×10-3S/cm(1.90×10-3S/cm). The electrochemical stable window of PVDF-HFP/PMMA/Li Mn2O4 can reach 5.25 V. Moreover, after Li Co O2/Li(Li Mn2O4/Li) cell was assembled with the CPE, the cell show good electrochemical performance.