Fabrication Of Membrane Based Safe And Flexible Polymer Electrolytes For Lithium Ion Batteries

Lithium ion batteries(LIBs)are stand out from mamy electrochemical batteries due to their good cycle performance,high specific energy,long service life and so on.Till now the most commercialized electrolytes are carbonate liquid electrolyte,by which potential risks(combustion and explosion)caused would seriously lower the safety level.What's more,to prevent the liguid leakage,heavy metal shells are used,which increase the costs and reduce the specific energy of LIBs.These shortcomings limits LIBs' further application and development.Inorganic solid electrolytes(ISEs)are very brittle with poor mechanical deformation,breaking easily when processing and with large interface impedance between electrolyte and electrode material.So employment of the polymer electrolyte is the most effective solution to the above problems.The usage of polymer electrolyte can overcome the flammable,leakage and other safty issues of traditional liquid electrolytes and enable the shape diversity LIBs which are also urgently required for the development of bendable,implantable,and wearable electronic products.Conductivity of all solid state LIBs is pretty low with large bulk resistance;the existence of liquid plasticizer in the solid-like electrolyes(gel electrolytes)would pay for the increasing of ionic conductivity with mechanical properties.To solve these problems,grafting branching and crosslinking are employed for the Polymer structure modification.Moreover,to maintain the processing performance and mechanical properties of polymer based electrolytes,porous electrospun membranes serve as framework materials.This dissertation includes three major parts.The first part: The conductivity of PEO,that has been mostly studied,is rather low(aroud 10-7 S cm-1 at room temperature)due to its high crystallinity.So in this part,through structure modification,crystallinity of PEO based electrulytes are damaged and the room temperature(RT)ionic conductivity is improved on the country.Poly(ethyleneglycol)methylethermethacrylate(PEGMA)is grfted on to the 2-ethyl-2-(hydroxymethyl)-1,3-propanediol(TMP)to form three armed PEO(3PEG).The grafting and combing structure disorder the PEO chain,so that 3PEG is completely amorphous with RT ionic conductivity of 1.1?10-4 S cm-1.filling 3PEG into three porous electrospun matrices,including PVDF-HFP membrane,3PEG cladding PVDF-HFP membrane(3PEG/PHP)and poly(ethylene terephthalate)membrane(PET).The difference of each skeleton membrane and influence of skeleton membrane on overall performance of composite electrolytes are studied.The obtained composite electrolyte is deformable and ultrathin.Composite electrolye based on PET electrospun membrane perform best owe to its none shrinkage and good compatibility with 3PEG.The second part: Polymer ionic liquids(PIL)posess much superior performance.To further improve the the ionic conductivity of PILs,structure modification is imployed.Three armed imidazole based PILs(3P(MPBIm-X),X-=Br-,CF3SO3-(Tf-)and(CF3SO3)2N-(TFSI-))are synthesized by ATRP with 1-[(2-methacryloyloxy)propyl]-3-butylimidazolium bromide(MPBIm-Br)as monster and TMP as nucleus.The effects of anion type on the chemical,thermal and ionic conductivity of PILs have been investigated,respectively.Furthermore,to form safe and flexible electrolytes with superior performance,solid-like composite electrolyte(SLCE)is fabricated by filling ion gel,the mixture of 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide(EMIM-TFSI)incorporated into 3P(MPBIm-TFSI),in the micropores of PVDF-HFP electrolspun membrane.SLCE is nonflammable and showed negligible leakage property when subjected to load.At 50 °C,ionic conductivity of SLCE could reach 1 mS cm-1(1.2×10-3 S cm-1).The third part: Physically cross-linked electrolyte limits the maximum liquid content.So,on the premise of preventing the leakage of liquid electrolyte,chemical cross-linked ones are designed to further improve the RT ionic conductivity.To ensure the safety property of GPEs,PILs and IL based electrolyes are produced.Here in,1-[(2-methacryloyloxy)hexyl]-3-butylimidazolium bis(trifluoromethanesulphonyl)imide(MHBIm-TFSI),PEGDA?EMIM-TFSI?0.5 M LiTFSI and small amout of AIBN are mixed in different ratio to form precursor solution,then PVDF-HFP electrospun membranes are immersed into the homogeneous solution.After in situ polymerizatioin,GPEs with interpenetrating polymer networks(IPN-GPE)obtained.