Study On The Preparation And Performances Of PVDF-HFP Based Composite Polymer Electrolytes For Lithium-ion Batteries

Due to the excellent safety performances, polymer electrolytes have become a hot topic in the lithium-ion batteries field since they had been investigated. However, some shortcomings, such as low ionic conductivity at room temperature (R.T.) and lithium ion transfer number, inferior mechanical and interfacial performances, have prevented them from the practical application in the lithium-ion batteries. To solve the above-mentioned problems, several specific methods are proposed in the paper to prepare the practical polymer electrolyte with excellent performances.After the performances of the gel PVDF-HFP polymer electrolytes prepared by the phase inversion, direct vacuum drying and steam bath methods being characterized respectively, the phase inversion method is considered as the optimal preparation process of the polymer electrolyte. The optimal process parameters are:nPVDF-HFP:nDMF-1:4, temperature of the membrane preparation40℃, temperature and time of the initial membrane preparation25℃and90min, respectively, the time of phase inversion process12h. The effect on the performances of the as-prepared polymer electrolytes using PVP, PEG-200and Urea as forming-agent was studied, and the results indicate that the best forming-agent is Urea and the optimal addition is10%of the mass of PVDF-HFP. The ionic conductivity at R.T. of the as-prepared polymer electrolyte with Urea is2.823mS·cm-1, but the mechanical strength is only18.84MPa.In the experiment, three kinds of molecular sieves, ZSM-5, MCM-41and SAPO-11, were added into the polymer matrix PVDF-HFP to improve the mechanical performances and ionic conductivity at R.T. of the as-prepared polymer electrolytes. The results of the performances characterization show that the mechanical strength of the polymer electrolyte doped with ZSM-5increases to23.78MPa, and the corresponding ionic conductivity at R.T. is3.078mS·cm-1. To improve the ionic conductivity at R.T. of the as-prepared polymer electrolytes, the surface of ZSM-5was modified by silane. The modified ZSM-5was then added into the polymer matrix PVDF-HFP to prepare the composite polymer electrolytes. The results of the characterization indicate that the performances of the composite polymer electrolytes are greatly improved, in which the ionic conductivity at R.T. and mechanical strength are3.851mS·cm-1and23.01MPa, respectively, but the performances of the interface between the electrolytes and the electrodes are not stable.To improve the interfacial stability between the as-prepared polymer electrolytes and the electrodes, the nano-La2O3was modified by vinyl silane and the modified nano-La2O3was then added into the polymer matrix PVDF-HFP to prepare composite polymer electrolytes with stable interface. The results of the characterization show that the composite polymer electrolytes have stable interface and the interfacial resistance can stay stabilized at560Ω after5days of storage, and the ionic conductivity at R.T. and electrochemical working window are3.546mS·cm-1and5.1V, respectively, the thermal decomposition temperature increases to350℃, but the lithium ion transfer number is only0.3157, which impairs the rate performances of the batteries assembled with the as-prepared composite polymer electrolytes.The single ionic conductor SiO2@Li+with core-shell structure was prepared by the hydrolytic condensation, free radical polymerization and ion exchange method. SiO2@Li+was then added into the polymer matrix PVDF-HFP to enhance the lithium ion transfer number of the batteries assembled with the as-prepared composite polymer electrolytes. The results of the characterization show that the lithium ion transfer number of the cell with the composite polymer electrolytes is up to0.4373, and the ionic conductivity at R.T. and electrochemical working window are3.885mS·cm-1and5.2V, respectively, the thermal decomposition temperature reaches up to440℃which illustrates excellent interfacial stability. The discharge specific capacity of the Li/PE/LiCoO2cell with the as-prepared composite polymer electrolytes is151.1mAh·g-1at0.1C, and142.6mAh·g-1at0.5C, the discharge specific capacity retention rate of the cell is97.43%and93.28%at0.2C and0.5C, respectively. The charge specific capacity of the Li/PE/Graphite cell with the as-prepared composite polymer electrolytes is351.8mAh·g-1at0.1C, and the charge specific capacity and coulombic efficiency of the cell are351.8mAh·g-1and100%after20cycles, respectively. The results suggest that the as-prepared composite polymer electrolytes are well matched with the anode LiCoO2and cathode graphite materials.In addition, the resistance of the batteries assembled with the composite polymer electrolytes doped with single ionic conductor SiO2@Li+is considered to be composed of bulk resistance Rb, charge transfer resistance Rct and interfacial reaction resistance Rsf by investigating the EIS of the cell under different discharge-charge states. The first cycle makes the largest contribution to the charge transfer resistance Rct in the Li/PE/LiCoO2system and to the interfacial reaction resistance Rsf in the Li/PE/Graphite system.