Surface-grafted Ionic Liquids On Silica Nanoparticles And Their Application As Ion Conductors

Ionic liquids（ILs）have been widely applied in a variety of electrochemical devices including lithium ion batteries,fuel cells,solar cells and supercapacitors due to their good thermal stability,less volatility,non-flammability,high ionic conductivity and wide electrochemical window.While applying ILs as ion conductors,the accordingly assembled electrochemical devices exhibit the improved performance,high safety,high recation kinetics,good cyclability and long-term stability.Although ionic liquid electrolytes can improve the performance of lithium-ion batteries,the leakage of liquid electrolytes induced performance decay and safety issues are still of great concern.Thus,formation of solid electrolytes by immobilization of ILs has attracted great attention to address the problems arisen from liquid electrolytes.In this dissertation,we designed and synthesized a series of solid ion conductors through immobilization of ionic liquid moieties on to silica nanoparticles via simple silane chemistry.Further improvement of ion conductivity has been conducted by ion exchange of cations with lithium ion based salt（LiTFSI）to meet the requirement for lithium ion batteries.Moreover,a new kind of solid state lithium ion conductive membrance was developed by doping the synthesized ion conductors with SPEEK.In addition,such idea for design of ion conductors was also applied to proton conductive membranes which is expected to be suitable as proton exchange membranes for elevated temperature fuel cell applications.To this,ionic liquids functionalized triethoxysilane was in situ hydrolyzed in sulfonated poly（ether ether ketone）（SPEEK）solution,followed by solvent evaporation and thermal treatment.Such in situ hydrolysis process can guranteethe well distribution of ionic liquids moieties through the membrane,which in turn helps the formation of continuous ion transport channels in the membranes.The major conclusions can be drawn as following:1)The designed ion conductors were successfully synthesized through simple silane chemistry as confirmed by various characterization techniques including FT-IR,CHNS elemental analysis and XPS.All the synthesized ion conductors have good thermal stability.The ionic conductivity for all samples gradually increased with increase in temperature and the doped amount of LiTFSI.2)The casted composite membranes composed of SPEEK and synthesized solid ion conductors displayed high tensile strength and good thermal stability.SEM observation revealed that the composite membranes with different doping amount possessed discrepant surface morphology,and the immobilized ionic liquids also had good compatibility with SPEEK.The ionic conductivity of the membrance under anhydrous state was quite low which needs to be further improved for practical applations.With the doping amount of the synthesized ion conductors of 10 wt%,the ionic conductivity of the composite film at 120℃was only 7.80×10^-6S/cm.3)Composite membranes consisting of SPEEK and ionic liquids grafted silica nanoparticles（[CPIM]Br）showed good flexibility and high thermal stability.Through SEM test,it was found that when the doping amount of[CPIM]Br was low,the nano particles produced in the process of hydrolysis were about 100 nm,relatively evenly dispersed in the SPEEK.With the increase in the content to 76wt%,the nanoparticles arranged along the main chain of the SPEEK through electrostatic interactions and thus formation of nanowires of silica was observed.The diameter of the formed nanowires ranged from 200nm to 300nm.It was also found that the addition of[CPIM]Br can improve electrochemical properties of the composite membranes at high temperature without humidification.