Synthesis Of New Ionic Liquid Electrolytes And Their Applications In Lithium Secondary Batteries

Ionic liquids (ILs) have attracted great interests of many researchers due to their unique characteristics, including nonvolatility, nonflammability, good thermal stability, great chemical and electrochemical stability and high ionic conductivity. And they can be used as safety electrolytes for lithium secondary batteries. In this thesis, three serials of new asymmetrical trialkylsulfonium, guanidinium and functionalized guanidinium ILs have been prepared. And physical and electrochemical properties of these products, including melting point, thermal stability, viscosity, conductivity and electrochemical window have been investigated. Two guanidinium ILs and one ether-functionalized guanidinium IL have been used as new electrolytes for lithium secondary batteries.Nine new ILs based on small asymmetric trialkylsulfonium cations with TFSI- anion have been prepared. Physical and electrochemical properties of these products have been investigated. All the products were liquids at room temperature, and some of these hydrophobic ionic liquids showed low-viscosity and low-melting point characteristics. The viscosities of S223TFSI, S221TFSI and S123TFSI were 33, 36 and 39 mPa s at 25℃, respectively. The viscosities of these ILs were well fit by the Arrhenius model over the temperature range studied (25-80℃). The thermal decomposition temperatures of these ILs slightly changed in the range of 275-315℃. Electrochemical and thermal stabilities of these ILs permitted them to become promising electrolytes used in electrochemical devices.Sixteen new guanidinium ILs based on small cations and TFSI- anion were prepared. Physical and electrochemical properties of these products have been investigated. Twelve products were liquids at room temperature. The thermal decomposition temperatures of these products slightly changed in the range of 380-425 oC. The viscosities of cg22TFSI, cg12TFSI and cg13TFSI were 45, 46 and 52 mPa s at 25℃, respectively. The viscosities and conductivities of the twelve ILs were well fit by the VTF model over the temperature range studied (25-80℃). Nine guanidinium ILs without cyclic structure had better electrochemical stability than three ILs with cyclic structure. 1g23TFSI and 1g33TFSI had the widest electrochemical windows (4.3 V at 25℃).Eight new functionalized guanidinium ILs based on small cations containing ether group (methoxyethyl group) or ester group (methyl acetate goup) and TFSI- anion have been synthesized. Physical and electrochemical properties of these products have been investigated. All the products were liquids at room temperature, and they had low melting points. The thermal decomposition temperatures of the 4 ILs with ester group were in the range of 300-345℃, which were obviously lower than the other 4 guanidinium ILs with ether group. The viscosities of cg1(2o1)TFSI and cg2(2o1)TFSI were 46 and 48 mPa s at 25℃, respectively. The viscosities and conductivities of these ILs were well fit by the VTF model over the temperature range studied (25-80℃). The ether and ester group could not remarkably affect the electrochemical stability, and 1g1ETFSI had the widest electrochemical window (4.4 V at 25℃).Two ILs based on guanidinium cations and TFSI- anion (1g13TFSI and 1g22TFSI)were chosen to be used in lithium secondary batteries as new electrolytes. The cathodic limiting potentials of the two ILs were 0.7 V versus Li/Li+. However, the lithium plating and striping on Ni electrode could been observed in the two IL electrolytes containing 0.3 mol kg-1 of LiTFSI without additive. Li/LiCoO2 cells using the two IL electrolytes without additive showed good capacity and cycle property at the current rate of 0.2 C. Discharge capacity for the two IL electrolytes decreased obviously with the increasing of the current rate from 0.2 C to 1.0 C.One IL based on ether-functionalized guanidinium cations and TFSI- anion (1g1(2o1)TFSI) were chosen to be used in lithium secondary batteries as new electrolyte. The viscosity and conductivity of IL electrolytes with different concentrations of LiTFSI have been investigated. Although the cathodic limiting potentials of this ILs were 0.7 V versus Li/Li+, the lithium plating and striping on Ni electrode could been observed in these IL electrolytes with different concentrations of LiTFSI. The interfaces between lithium metal and the IL electrolytes were also investigated by impedance spectroscopy method with lithium metal symmetrical cells. The concentrations of lithium salt had obvious effect to the capacity and cycle property of Li/LiCoO2 cells using the IL electrolytes at the current rate of 0.2 C. Discharge capacity for the two IL electrolytes decreased with the increasing of the current rate from 0.2 C to 1.5 C, and the electrolyte with 0.75 mol kg-1 of LiTFSI owned better rate performance.