Synthesis And Application Of Functional Ionic Liquids For Lithium-based Electrochemical Energy Storage Systems

The safety of lithium secondary batteries(LSBs)has always been the focus of attention.Organic solvents are the main factors affecting the safety of LSBs.Using non-volatile and non-flammable ionic liquids instead of organic solvents in electrolytes can effectively improve the safety of LSBs.However,the compatibility of conventional ionic liquids with graphite or lithium metal anodes is poor,and the electrochemical performance needs to be improved.In this paper,novel functional ionic liquids were synthesized,and the effects of ionic liquids structures on their physical and chemical/electrochemical properties were studied.Meanwhile,functional ionic liquid based gel polymer electrolytes(FILGPEs)were prepared,and the mechanical properties,compatibility with cathodes and anodes,charge-discharge performance at room and elevated temperatures were studied.The morphology,thermodynamic properties,electrochemical window and lithium ion transport number(TLi+)of the FILGPEs are comprehensively characterized.The mechanism of the binder,the interface stability with high-voltage cathode and lithium metal anode were also investigated.The main contents and results are as follows:(1)In order to solve the problem that imidazolium cations can easily intercalate into graphite interlayers initiating reduction reactions,resulting in deterioration of battery performance and even incapability of charge-discharge cycle,an dual-imidazolium functional ionic liquid named Im2-TFSI2 was synthesized in this paper.The functional ionic liquids electrolyte named IM2,formed by adding 1M bis(trifluoromethane)sulfonimide lithium salt(LiTFSI)into Im2-TFI2,can be compatible with graphite anode without any additives.The results show that IM2 has a wide liquid range and non-flammability.Its glass transition temperature(Tg)is 214.6 K,thermal decomposition temperature(Td)reaches 710 K,ionic conductivity are 0.05 mS/cm at room temperature(R.T.)and 0.76 mS/cm at 343 K,electrochemical window(EW)is 0-5.4 V vs.Li/Li+.IM2 can generate stable solid electrolyte interphase(SEI)on graphite surface and achieve good compatibility with graphite anode at R.T.and elevated temperature.After 100 charge-discharge cycles at 0.1C current density,the capacity retention ratios of Li|IM2|graphite cells reach 94%at R.T.and 70%at 373 K.(2)In view of the fact that flammable organic solvents still exist in conventional gel polymer electrolytes,resulting in reduction of battery safety,we prepared functional ionic liquid based gel polymer electrolytes named Gel-IM2,which contains 74 wt.%(Im2-TFSI2+1 mol/kg LiTFSI)and 26 wt.%P(VDF-HFP).Gel-IM2 has good self-standing property and flexibility.Its Tg is 228.2K,crystallinity is 9.73%,Td is 681K,ionic conductivity are 0.091 mS/cm at R.T.and 1.284 mS/cm at 343K,TLi+ is 0.239,and EW can reaches 0-5.34 V vs.Li/Li+(3)Li|Gel-IM2|LiFePO4 cells show excellent capacity and cycle performance.At 0.1C current density,the discharge specific capacities reach 155 mAh/g and 166 mAh/g,while capacity retention ratios reach 99%and 88%,at R.T.and 343K,respectively.The performances of Li|Gel-IM2|Li-Rich and Li|Gel-IM2|Li cells show that Gel-IM2 is compatible with high voltage cathodes and lithium metal anodes as well.