Study On Preparation And Modification For PVDF-HFP-based Gel Polymer Electrolyte

At present,the liquid electrolyte widely used in lithium-ion batteries is prone to fire and leakage problems,and there is a risk of lithium dendrite growth inside,which leads to safety risks,these problems restrict the development of lithium-ion batteries towards higher performance and energy density.Therefore,the use of flammable liquid electrolytes has become a major obstacle to overcome in the development of the next generation of high-performance lithium-ion batteries.In order to solve the above problems,solid electrolyte has become a hot spot.However,there are still many scientific and technical problems in all solid electrolyte that need to be broken through.Gel polymer electrolytes(GPEs),whose conductivity is between solid electrolyte and liquid electrolyte,has attracted much attention due to its many advantages.However,the ionic conductivity of GPEs still has a gap compared with that of liquid electrolyte,which cannot meet the needs of industrial development.In recent years,GPEs formed by polyvinylidene difluoroethylene hexafluoropropene copolymer(PVDF-HFP),as the intermediate product of the transition from liquid electrolyte to solid electrolyte,has attracted extensive attention because it owns the advantages of liquid electrolyte and characteristics of solid electrolyte.In this paper,a gel polymer electrolyte was prepared by forming an interpenetrating network structure with PVDF-HFP,blending with PVDF-HFP,and adding active filler in PVDF-HFP.The crystallinity,liquid absorption rate and ionic conductivity of these electrolytes were studied.A symmetric lithium battery was assembled for performance test,and the interface stability between the electrolyte and lithium metal was discussed.In addition,it was applied to the batteries using Li Fe PO₄ as cathode,and the battery performance were systematically analyzed.The main research work of this paper is as follows:(1)In order to improve PVDF-HFP based GPE,ultraviolet(UV)-curable semi-interpenetrating polymer network is designed and synthesized through UV-irradiation technique,and the as-prepared semi-interpenetrating matrix is constituted by pentaerythritol tetracrylate(PETEA)polymer network and poly(vinylidene fluoride-co-hexafluoropropylene)through UV-irradiation technique.The effects of different mass of PETEA monomers on the ionic conductivity were discussed.The PETEA monomer can be polymerized under ultraviolet light,and the crystallinity of the modified gel electrolyte is reduced,and the porosity is increased.Electrochemical analysis showed that the highest ionic conductivity was obtained when PETEA:PVDF-HFP=1:4.Compared with PVDF-HFP GPEs,PVDF-HFP/PEPEA GPEs owned wider(?4.8V)electrochemical window and better interfacial stability.At the same time,the lithium-ion battery test results show that the initial discharge specific capacity of Li Fe PO₄|PVDF HFP/PETEA|Li is up to 151 m Ah g^-1 at 0.5 C,and the battery has a good rate performance.(2)Composite gel polymer electrolyte PPL was prepared by the casting-solution method.The optimum mass fraction of LATP was investigated.The electrolyte containing 15 wt%LATP(PPL-15%)owns the highest liquid absorption rate and ion conductivity.And PPL-15%shows lower crystallinity and better mechanical properties.Electrochemical analysis showed that PPL-15%owns the highest electrochemical window and lithium-ion transfer number of 0.66.Compared with the battery of Li|PVDF-HFP|Li Fe PO₄ and the battery of Li|PVDF-HFP/PPC|Li Fe PO₄.At 0.1 C,the initial discharge specific capacity of the battery of Li|PPL|Li Fe PO₄ is 164 m Ah g^-1,at0.5 C,the capacity retention rate is up to 99%after 150 cycles and it owns better cycle stability than Li|PVDF-HFP|Li Fe PO₄ and Li|PVDF-HFP/PPC|Li Fe PO₄.The interface stability of Li|PPL|Li is improved obviously.(3)Composite gel polymer electrolyte with LLZTO nanoparticle was prepared by the casting-solution method.Two electrolytes PVDF-HFP and PL-10%were compared.PL-10%showed a lower crystallinity.Electrochemical analysis showed that PL-10%had the highest liquid absorption rate and ionic conductivity.Compared with the battery of Li|PVDF HFP|Li Fe PO₄,At 0.1 C,the initial discharge specific capacity of the battery of Li|PL|Li Fe PO₄ is 160 m Ah g^-1.After 100 cycles,the capacity retention rate of the battery of Li|PL|Li Fe PO₄ is 66%at 0.5 C,therefore Li|PVDF HFP|Li Fe PO₄ has better cycle stability.