Study On The Preparation And Performances Of Composite Porous Polymer Electrolyte For Lithium-ion Batteries

The use of polymer electrolytes is widely regarded as a promising approach for lithium-ion batteries, due to their lack of leakage, high flexibility within the cell geometry and high physical and chemical stability. Among of all the polymer electrolytes, the composite porous polymer electrolytes (CPPEs) shows simultaneously high ionic conductivity and good mechanical strength. So the composite porous polymer electrolytes in this paper were prepared by non-solvent evaporate method and supercritical CO2 (ScCO2) induced phase separation, respectively. The morphology, crystalline, thermal behavior, mechanical strength and electrochemical properties of these polymer membranes have been investigated by scanning electron microscopy (SEM), X-ray diffraction pattern (XRD), differential scanning calorimetry (DSC), Thermogravimetric Analysis (TGA), stess-strain test and electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). Effects of the different technological parameters during preparing membranes on the structure and properties of polymer membranes were studied, aiming at obtaining GPPEs with good electrochemical and mechanical properties for practical application.First of all, the GPPEs were prepared via non-solvent evaporate method. PVDF-HFP and PMMA were polymer matrixes of membranes. DMF was solvent, and EG was non-solvent. It has been investigated that the effects of drying temperature, weight ratio of EG, weight fraction of PMMA in blend membrane and TiO2 nanoparticles content on the physical and electrochemical properties. Firstly, PVDF-HFP polymer electrolyte was prepared, and the effects of drying temperature on the memebranes have been investigated. It is found that the membrane with good appearance can not be obtained when drying temperature was lower than 60?. With the increase of temperatue, porosity and ionic conductivity of polymer membranes decreased, but mechanical properties were enhanced. Secondly, the effects of weight ratio of EG on the memebranes were investigated. The porosity and ionic conductivity of polymer membranes were increased by increasing weight ratio of EG. The polymer electrolyte prepared with the highest weight ratio of EG exhibited the highest ionic conductivity of 0.75 mS/cm and the electrochemical window was above 6 V. Thirdly, the PVDF-HFP/PMMA blend membranes were prepared, and the effects of weight fraction of PMMA in blend membrane on the properties of memebranes were investigated. It is found that after blending PMMA, the room temperature ionic conductivity of the polymer electrolyte is significantly enhanced. The ionic conductivity of polymer electrolyte with 0 and 30% PMMA were 0.75 and 1.53 mS/cm. With the addition of PMMA, the pore density, crystalline and melting temperating of polymer blend membranes were depressed. However, their tensile strengths were increased. In the end, the TiO2 nanoparticles were introduced in the PVDF-HFP/PMMA blend membranes. It is found that the pore density and crystalline of membranes continuously decreased with increasing TiO2 content. The particle aggregation can be found after TiO2 content more than 2%, which caused the ionic conductivity of the polymer electrolyte depressed quickly and significantly. In particular, the GPPEs with 2% TiO2 showed excellent performance. The ionic conductivity can be up 1.68 mS/cm, as well as thermal decomposition temperature and electrochemical stability up to 350? and 6 V vs. Li/Li+, respectively. The LiCoO2/Li cells assembled using PVDF-HFP/PMMA (30%)/TiO2 (2%) exhibit excellent cyclic stability and C-rate performance. The specific capacities of 140.2,127.1,114.7 and 105.3 mAh/g were obtained when the charge/discharge rates at 0.2, 0.5,1 and 2 C, respectively. And the capacity retention was 90% after 50 cycles at 1.0 C charge/discharge rate.In this paper, the GPPEs were also prepared by ScCO2 induced phase separation. PVDF-HFP was polymer matrixes of membranes, and NMP was solvent. The effects of ScCO2 pressure and temperature, polymer concentration and TiO2 nanoparticles content on the physical and electrochemical properties of polymer membranes hvave been investigated. The results show that the membranes prepared by ScCO2 induced phase separation hold honeycomb macroporous structure, high porosity and ionic conductivity and low tensile strength. The porosity and ionic conductivity increased with increasing ScCO2 pressure, but decreased with increasing ScCO2 temperature and polymer concentration. However, the tensile strength of polymer membranes decreased with increasing ScCO2 pressure, and increased with increasing ScCO2 temperature and polymer concentration. With the incorporation of TiO2, the GPPEs exhibit improved electrochemical and mechanical properties. The crystalline, uptake and tensile strength increased firstly and then decreased with the increase of TiO2 content. When 1% TiO2 nanoparticles were introduced, GPPEs showed the optimum comprehensive properties. The ionic conductivity can be up 3.13 mS/cm, as well as thermostability and electrochemical stability were excellent. The specific capacity of assembled Li/LiCoO2 cells using PVDF-HFP/TiO2(1%) was 131.9 mAh/g at 1 C, and coulomb efficiency can maintain 99% during 50 cycles approximately.