Fabrication Of PVDF-HFP Based Nanofibrous Electrolyte Membranes Based On Electrospinning And Study On Their Electrochemical Performance

Lithium metal batteries（LMBs）have become one of the research hotspots of chemical power source due to their advantages of high energy density and long cycle life.However,the lithium dendrite will be easily formed during cycling process in liquid electrolytes,which leads to serious safety issues and thus hindering its practical application.In this dissertation,based on the properties of single ion conducting copolymer（SIPE）and silica-coated silver nanowires（AgNWs@SiO₂）,the poly（vinylidene fluoride-hexafluoropropylene）（PVDF-HFP）based nanofibrous electrolyte membranes are prepared by an electrospinning technology and their electrochemical performance of LMBs are also studied,which provide a new idea for the research of high-performance polymer electrolytes.Firstly,to fabricate the PVDF-HFP/AgNWs@SiO₂ composite polymer membranes（CPMs）by an electrospinning method,the different weight ratio of AgNWs@SiO₂ with respect to that of PVDF-HFP are dispersed in a solvent N,N-dimethylformamide（DMF）and acetone.CPMs absorb liquid electrolyte to form composite gel polymer electrolyte membranes（CGPEs）.On the one hand,not only the mechanical strength and thermal conductivity of CPMs can be improved by the addition of AgNWs@SiO₂,but also the ionic conductivity of CGPEs can be enhanced.On the other hand,the heat dissipation can be improved by the addition of AgNWs@SiO₂ in CGPEs and SiO₂ nanolayer coated on AgNWs offers electrical insulation during the charge-discharge process of the battery,which can improve the high-temperature electrochemical performance and safety of the battery.When 5 wt%of AgNWs@SiO₂ in CPM,the Li/GCPEs/LiFePO₄ cells can exhibit good cycle stability and excellent rate performance at a wide operating temperature range.Secondly,acetonitrile solutions with poly（ethylene oxide）（PEO）and lithium bis（trifluoromethanesulfonimide）（Li TFSI）are added into PVDF-HFP/AgNWs@SiO₂electrospun membrane to obtain PEO-based solid polymer electrolytes（SPEs）.The electrospun membrane not only shows a stable three-dimensional network framework for PEO-based electrolytes,but also offers a good channel for lithium ion（Li⁺）transport.The PVDF-HFP/AgNWs@SiO₂-PEO-Li TFSI SPEs exhibit excellent thermal stability,high mechanical strength and good cycle stability.Thirdly,PEGMA-UPy MA-SSPSILi single-ion conducting copolymer（SIPE）,which is composed of poly（ethylene glycol）methyl ether methacrylate（PEGMA）,2-（3-（6-Methyl-4-oxo-1,4-dihydropyrimidin-2-yl）ureido）ethyl methacrylate（UPy MA）,and lithium 4-styrenesulfonyl（phenylsulfonyl）imide（SSPSILi）,is synthesized by reversible addition-fragmentation chain transfer（RAFT）polymerization.The samples are dissolved via a various weight ratio of SIPE with respect to that of PVDF-HFP in a solvent acetone and DMF.The porous nanofiber structure SIPE/PVDF-HFP CPMs are prepared by an electrospinning technology.Meantime,the CPMs absorb～60μL PC solution to obtain SIPE/PVDF-HFP single ion conducting gel polymer electrolyte membranes（SIGPEs）.The hydrogen bonds and physical interactions can be formed between SIPE and PVDF-HFP polymer matrix so as to improve the mechanical strength of the CPMs.The-SO₂-N^--SO₂-anions endow lithium ions with good dissociation in SIGPEs so as to show high Li⁺transference number（t Li⁺）,which can significantly hinder the growth of lithium dendrite.When the content weight ratio of SIPE in CPM is up to 10 wt%,the SIGPE can exhibits a high t⁺_Li value（0.89）and high ionic conductivity of 2.78×10^-5 S cm^-1 at 30 ^oC.Meanwhile,the Li/SIGPE/LiFePO₄ cells also exhibit superior cycle stability and excellent rate performance.Finally,the SIPE/PVDF-HFP/AgNWs@SiO₂ CPMs are prepared by an electrospinning technology.The appropriate weight ratio of SIPE and AgNWs@SiO₂ in composite single ion gel polymer electrolyte membranes（CSIGPEs）is vital for obtaining high ionic conductivity and t Li⁺.When the SIPE weight ratio of in CSIGPEs is up to 10wt%,the addition of different proportions of AgNWs@SiO₂ can significantly improve the high-rate（1C）charge-discharge performance of Li/CSIGPEs/LiFePO4 cells at 60 ^oC.Therefore,the synergy of SIPE and AgNWs@SiO₂ makes Li/CSIGPEs/LiFePO4 cells have outstanding cycle stability and superior rate performance.