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

Ionic liquids (ILs) have received considerable attentions for application as potential electrolytes in various electrochemical devices in recent years, which is mainly attributable to their superior properties over conventional volatile electrolytes. In spite of much effort being expended in trying to develop ionic liquid electrolytes, there is no clear correlation between their chemical structures and properties so far, moreover, every electrochemical device requires different characteristics from the ionic liquids. Design and development of new series of ionic liquid electrolytes is very important for further understanding the structure-property relationship of ionic liquids, and grasping the performances of lithium secondary batteries using ionic liquids as electrolytes.In this thesis, imidazolium-based ionic liquid electrolytes have been firstly investigated. Subsequently, two new series of ionic liquid electrolytes, i.e. asymmetrical sulfonium-based ionic liquids and dicationic ionic liquids have been developed. Finally, the potential applications of the above-mentioned ionic liquids as electrolyte bases or additive in lithium secondary batteries have been explored.Several ionic liquids containing 1-alkyl-3-methylimidazolium cations and BF4- or TFSI- anion (RMeImBF4 and RMeImTFSI,wherein R = C2H5 (Et), n-C3H7 (Pr),n-C4H9 (Bu)) as potential electrolytes have been prepared and fundamental electrochemical properties of the neat ionic liquids and those mixed with EC-DMC-DEC (1:1:1, mass ratio) have been analyzed. The Arrhenius equation is approximately fit for the relationship between conductivity and temperature for neat ionic liquids within low temperature range (25⁵0℃). The VTF interpretation describes the conductivity temperature dependence for the ionic liquids containing BF4- anion more accurately than those containing TFSI- anion within wider temperature range. The potential windows are approximately 4.0 V for all these ionic liquids. Conductivities of the mixed electrolytes show a maximum value as the solution concentrations increase.A new series of ionic liquids based on asymmetric sulfonium cations ([R1R2RS]+, wherein R1, R2 =CH3, R = C2H5, n-C4H9, n-C6H13, n-C8H17, corresponding to S₁₁₂+, S₁₁₄+, S116+, S118+, respectively) with TFSI- or PF₆^- anion as potential electrolytes using cheap sulfide as a starting material, have been developed and investigated. Among these ionic liquids, S₁₁₂TFSI and S₁₁₄TFSI are liquids at or below room temperature. Three types of phase transition behavior and two types of decomposition behavior are observed for these ionic liquids on heating. [R1R2RS]+PF₆^- show much higher solubility in water compared with PF₆^--type ionic liquids with imidazolium cation in which there have only alkyl groups. In addition, The Arrhenius equation approximately describes the relationship between conductivity and temperature for S₁₁₂TFSI and S₁₁₄TFSI in the high temperature region studied (40⁸0℃). The room temperature electrochemical windows are approximately 4.1V for S₁₁₂TFSI and S₁₁₄TFSI.An extensive new family of ionic liquids containing aliphatic tetraalkylammonium dications with alkyl linkage chains and TFSI- anion (Cn(N₁₁m₂)_2-TFSI₂ and Cn(N₂₂₂)_2-TFSI₂, wherein N11m2 represents dimethylalkylammonium, N₂₂₂ represents triethylammonium, n represents spacer length and n = 2,3,6,9,12) as potential electrolytes have been developed and studied. These ammonium dicationic ionic liquids exhibit five types of phase transition behavior on heating and one stage decomposition behavior. The spacer length and the head group overall length are responsible for the solid/liquid transformation temperature, and the spacer length is more effective in lowering the solid/liquid transformation temperature than the head group overall length. The interesting findings are that several ammonium dicationic ionic liquids, C₆(N₁₁₈)_2-TFSI₂, C₉(N₂₂₂)_2-TFSI₂ and C₁₂(N₂₂₂)_2-TFSI₂ show the lowest solid-liquid transformation temperatures among analogues with alkyl links, and belong to the greatest thermal stable ionic liquids. The Arrhenius equation approximately describes the relationship between conductivity and temperature for C₉(N₂₂₂)_2-TFSI₂ and C₁₂(N₂₂₂)_2-TFSI₂ in the low temperature region studied (25⁶0℃), and for C₆(N₁₁₈)_2-TFSI₂ in the wider region (25⁸0℃). The room temperature electrochemical windows are about 4.3V for C9(N₂₂₂)_2-TFSI₂ and C12(N₂₂₂)_2-TFSI₂, and 4.7V for C6(N118)2-TFSI₂. On the basis of symmetrical dicationic ionic liquids, asymmetrical dicationic ionic liquids based on the combination of imidazolium and aliphatic ammonium cations with TFSI anion (MICnN₁₁₁-TFSI₂, wherein MI represents N-methyl imidazolium, N₁₁₁ represents trimethylammonium, n represents spacer length and n = 2, 5) have also been developed. MIC₂N₁₁₁-TFSI₂ shows multiphase transition characteristics.MIC₅N₁₁₁-TFSI₂ has one of the lowest solid-liquid transformation temperatures among analogues, and belongs to the greatest thermal stable ionic liquids. In addition, the Arrhenius equation approximately describes the relationship between conductivity and temperature for MIC₅N₁₁₁-TFSI₂ in the low temperature region studied (25⁶0℃), and its room temperature electrochemical window is about 3.7V. S₁₁₂TFSI and S₁₁₄TFSI have been used as electrolyte bases in lithium secondary batteries. The influence of the amount of vinlene carbonate (VC) as an additive on cell performance is investigated. Cycling experiments at 0.1C rate between 2.5-4.2V reveals that Li/LiMn₂O₄ cell performance is improved when VC is used in small amounts, and the best performance is obtained when 10wt% VC is added to 0.4mol L^-1 LiTFSI/S₁₁₄TFSI. The discharge capacities of Li/LiMn₂O₄ cell containing 0.4mol L^-1 LiTFSI/S₁₁₄TFSI+10wt%VC at the initial and 50th cycle are 169.0 and 129.8 mAh g^-1, respectively, and the coulombic efficiency at entire cycles is more than 90%, which could be equivalent to those of the cells using conventional electrolytes (1mol L^-1 LiPF6/EC:DMC) or 0.4mol L^-1 LiTFSI/EtMeImTFSI+10wt%VC. However, the cell rate performance is poor. Simple experiment reveals that the cells containing S₁₁₂TFSI and S₁₁₄TFSI-based electrolytes might be safe when being in contact with the small fire.MIC₅N₁₁₁-TFSI₂ has been used as a conventional electrolyte (1mol L^-1 LiPF₆/EC:DMC) additive in lithium secondary batteries. Cycling tests at 1C rate between 2.5-4.2V reveal that for the Li/LiMn₂O₄ cell, no deterioration of performance is observed when MIC₅N₁₁₁-TFSI₂ is added to the conventional electrolyte, moreover, the cell performance is slightly improved when the amount of MIC₅N₁₁₁-TFSI₂ is 5wt%. On the other hand, in the case of C/LiMn₂O₄ cell, when the amount of MIC₅N₁₁₁-TFSI₂ is 2.5wt%, the cell performance is similar to that of the cell using conventional electrolyte, and the performance tends to worsen as the MIC₅N₁₁₁-TFSI₂ amount increases. However, in the whole, the performance is obviously superior to that of the cell employing the EtMeImTFSI-added electrolyte.