Preparation And The Study Of The Properties For Gel Polymer Electrolytes And Ionic Liquids

People are studying new energy sources and energy storage devices asglobal energy resources become increasingly scarce. The car industry alsomakes the electric vehicles and hybrid vehicles as the future direction. Sothe rechargeable batteries are studied widely. Electrolyte is to conduct ionsin the battery. Traditional organic solvent is easy to volatilize so their long-term use can cause leaks, combustion or explosion; Solid electrolytes aresafer but their conductivities are usually too low to meet the applicationrequirements. Gel polymer electrolyte, composed of polymer and liquidelectrolyte, has high conductivity of liquid electrolyte and good processingproperties of polymers and also improves the safety of batteries. They havevery good application prospects. Compared with traditional organicelectrolyte, ionic liquids possess low vapor pressure, almost non-volatile,green, non-flammebility and they can significantly improving the safety ofbatteries. There are many studies about ionic liquids, but there has notreported the preparation of new ionic liquids by taking the segment structureof classic solid electrolyte polyethylene glycol into imidazole ring. These newionic liquids will maintain high conductivity of traditional ionic liquids, butincrease the solubility of ionic liquids on metal-salts. That will be of greatsiginificance for application of ionic liquids.We have prepared several gel polymer electrolytes such as PVDF/EC-LiClO₄, PVDF/PC-LiClO₄, PVDF/BMIMPF₆-LiClO₄using immersion methodbased on PVDF membrane with low crystallinity and high porosity. These gelpolymer electrolytes are studied using XRD, FTIR, SEM, TGA, DSC, CV andEIS. The Study has shown that:（1） The crystalline structure of PVDFchanged after immersing the liqiud solutions, and PVDF/BMIMPF₆-LiClO₄electrolyte formed new crystalline structure as the concentration of LiClO₄increasing.（2） The thermostability of PVDF/EC-LiClO₄and PVDF/PC-LiClO₄is not good because of the volatilization of EC and PC, butPVDF+LiClO₄/BMIMPF₆electrolyte shows sufficiently high thermal stabilityand there was no obvious weightlessness before271℃.（3） For the gelpolymer electrolytes of PVDF-EC/LiClO₄, PVDF-PC/LiClO₄, PVDF-BMIMPF₆/LiClO₄, their highest conductivity was2.5×10-3S/cm,1.92×10-3S/cm,8.474×10-4S/cm separately, and their electrochemical potential window was4.8V,5.2V,4.4V separately. In general, the gel polymerelectrolyte PVDF-BMIMPF₆/LiClO₄has the best comprehensive performanceand it seems to be an excellent candidate to use in lithium rechargeablebatteries.We have prepared new kinds of ionic liquids such as PEG（400）MIMBF₄,PEG（600）MIMBF₄, PEG（400）MIMPF₆. They were studied using FTIR,conductivity test and viscosity test. Their viscisity was0.954,5.66,1.8Pa·Sand their conductivity was0.251,0.105,0.121mS/cm seprately forPEG（400）MIMBF₄、PEG（400）MIMPF₆、PEG（600）MIMBF₄at25℃. Thenwe have prepared several ionic liquid electrolytes by dissolving LiSCN,NaSCN and KSCN in the above three kinds of ionic liquids according ton（EO）: n（SCN-）=80:1,60:1,20:1. These metal-salts can be well dissolved,and this shows that the introduction of polyethylene glycol segment canreally make the imidazolium ionic liquids possess better ability to dissolvemetal-salts. The ion-aggregation status was studied by FTIR with SCN-anion as the research object. Results show that the variation of ion-aggregation status with the concentration of thiocyanate is primarily relatedto the type of thiocyanate. The results have drawn as following.（1） For thesystem of ionic liquid-LiSCN, with the concentration of LiSCN increasing, therelative percentage content of free ion SCN-and the ion pairPEG（400/600）MIM+SCN-decreased, and that of ion pair Li⁺SCN^-and tripleincreased. The conductivity of the system first increased and then lightlydecreased with the concentration of LiSCN increasing.（2） For the system ofionic liquid-NaSCN, with the concentration of NaSCN increasing, the relativepercentage content of free ion SCN-and triple decreased, and that of ionpair Na+SCN-and PEG（400/600）MIM+SCN-increased. The conductivity ofthe system first increased and then fairly decreased with the concentration ofNaSCN increasing.（3） For the system of ionic liquid-KSCN, with theconcentration of KSCN increasing, the relative percentage content of freeion SCN-and triple lightly decreased, and that of ion pair K+SCN-decreasedbut that of PEG（400/600）MIM+SCN-increased. The conductivity of thesystem increased slowly with the concentration of KSCN increasing.We made some improvements according to the high viscosity and poorconductivity of the ionic liquids in chapter three. Ethylene glycol monomethylether monomer was introduced to the imidazole ring to prepare ionic liquidsof MEMIMBF₄and MEMIMPF₆. The results are as following. On the basis ofmaintaining good solubility of metal-salts, the viscosity of MEMIMBF₄andMEMIMPF₆was down to0.0592Pa.s and0.324Pa.S separately, and the conductivity increased to5.07mS/cm and2.5mS/cm separately. Severalliquid electrolytes were prepared by dissolving LiClO₄, LiBF₄and LiSO₃CF₃in the above two ionic liquids, and these lithium salts are easily dissolvedeven if the concentration was2.5M. This is very beneficial for application ofionic liquids. The highest conductivity of each kind of electrolyte was asfollowing: MEMIMBF₄-LiClO₄（0.5M）/7.53mS/cm, MEMIMBF₄-LiBF4（2.5M）/7.74mS/cm, MEMIMBF₄-LiSO₃CF₃（1.0M）/6.83mS/cm, MEMIMPF6-LiClO₄（2.5M）/4.46mS/cm, MEMIMPF6-LiBF4（2.5M）/2.99mS/cm, MEMIMPF₆-LiSO₃CF₃（2.5M）/4.34mS/cm.