Study On Preparation And Perforance Of PVDF-HFP-based Gel Polymer Electrolyte For Lithium-Ion Batteries

Lithium-ion batteries are widely used because of their high energy density and stability.However,safety accidents caused by leakage of the liquid electrolyte used in lithium-ion batteries continue to occur,so the development of batteries that do not contain liquid electrolyte has become a research hotspot.This led to research on solid electrolytes.However,due to the problems of low ionic conductivity and large interface impedance in solid electrolytes,gel polymer electrolytes have become transition materials for solid electrolytes due to their non-liquid electrolyte,room temperature ionic conductivity similar to liquid electrolytes and excellent workability.Polyvinylidene fluoride-hexafluoropropylene(PVDF-HFP)copolymer was used as the polymer matrix material to prepare the gel polymer membrane.PVDF-HFP is a corrosion-resistant and high-dielectric material,which can promote the dissociation of lithium salt and has good chemical stability when applied to gel polymer electrolytes.The main problems currently existing in PVDF-HFP-based gel polymer electrolytes are poor mechanical strength and ionic conductivity that cannot reach commercial levels.Therefore,this study used a phase inversion method to prepare a porous polymer to improve its ability to absorb electrolyte and thus its ionic conductivity.The separator was further modified by adding pore formers and additives to improve its ionic conductivity,mechanical strength,and thermal stability.The main research contents are as follows:(1)Determined the optimal solvent-solute ratio.Using PVDF-HFP and N,N-dimethylformamide(DMF)solvents with a mass ratio of 1:5 to prepare the membrane casting solution to prepare the diaphragm.The membrane obtained at this concentration has the best overall mechanical strength,porosity and ionic conductivity.Use this ratio to carry out the full text experiment.(2)Used polyvinylpyrrolidone-K30(PVP-K30)as an additive to modify the diaphragm.The use of PVP increases the number of large pores in the diaphragm,shortens the transmission path of lithium ions,reduces the crystallinity of the diaphragm,and increases the mechanical strength of the diaphragm.When the amount of PVP added is 10%,the prepared gel polymer membrane has the largest ionic conductivity of 0.874×10-3 S·cm-1,and the mechanical strength reaches 24.76 MPa.(3)Added octavinyl-polyhedral oligomeric silsesquioxane(OVAPOSS)for modification.The gel polymer separator with PVP added was doped with OVAPOSS to further modify it to improve its temperature resistance and ionic conductivity,and enhance its safety and electrochemical performance.The addition of OVAPOSS effectively reduced the viscosity of the casting liquid,and the porosity of the obtained diaphragm was greatly improved.Moreover,as an organic-inorganic composite material,OVAPOSS had good compatibility with the diaphragm.By comparing the shrinkage of the diaphragm at different temperatures,it is determined that the addition of OVAPOSS can greatly improve the temperature resistance of the diaphragm.The addition of OVAPOSS greatly improved the temperature resistance of the diaphragm.The shrinkage rate was below 5%at 160?,and it still had a certain degree of toughness and mechanical strength.When the addition amount of OVAPOSS was 5%,the diaphragm had the best performance,the ion conductivity reached 1.62×10-3 S·cm-1,and the electrochemical window was above 5.5 V.The Li2CoO2/Li half-cell prepared by using this separator had a specific capacity of 141 mAh·g-1,and the efficiency remained above 99%within 100 cycles,and the capacity retention rate reached 99.5%.It had excellent cycle stability.