Alkali Salts And Ionic Liquids Based On Fluorosulfonyl Imide:Synthesis, Characterization, And Their Application In Li-ion Batteries

The commercial non-aqueous electrolyte is typically a mixture of carbonated solvents (such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and ethylene carbonate (EC)), LiPF6 and varieties of functional additives, which makes up the key material for Li-ion batteries (LIBs). Several crucial properties, such as the cycling life, resilience to elevated temperature and safety problems, are close relative to the electrolyte because of the volatile and inflammable organic solvents and the reactive LiPF6. When LIBs are under abusive conditions, the potential exists to heat a cell beyond its thermal stability limit, and then thermal runaway would be initiated, sometimes resulting in combustion and even explosion incidents. Meanwhile, HF and PF5 would be generated, which have been certified as the main factors to break down the cycling performance of LIBs, once the LiPF6-based electrolyte is thermal operated or contaminated by moisture or alcohol impurities. Therefore, the conventional non-aqueous electrolytes based on carbonates and LiPF6 have become the bottle-neck of rechargeable energy-storage devices with large energy density and high power source. A serial lithium salts and ionic liquids (ILs) based on the weakly-coordinating fluorosulfonylimide anions (n=0,1,2,4,6,8) were designed and prepared in this thesis. Their physicochemical, electrochemical properties and application in the field of LIBs electrolytes were also investigated extensively.In chapter 1, the development and status corresponding to LIBs, as well as their constituents and operation mechanism, were introduced. Afterward the electrolyte reviews, including the organic solvents, conductive salts and ionic liquids, were detained. And the influencing factors of the safty issues and capacity fading for LIBs were also analyzed.In chapter 2, the intermediates of HN(SO2C1)(SO2CnF2n+1) would be obtained by reaction of CnF2n+1SO2NH2, SOC12 and CISO3H in one pot, subsequently to derive HN(SO2F)(SO2CnF2n+1) by fluorination with SbF3. The alkali salts of potassium, rubidium and cesium were prepared according to the neutralization of HN(SO2F)(SO2CnF2n+1) with alkali carbonates. Meanwhile, the lithium and sodium salts should be received by exchanging KN(SO2F)(SO2CnF2n+1) with LiC104 or NaC104 in CH3CN solvent for the purpose of excluding the H2O contamination. All the products were selectively conformed by means of 1H NMR,19F NMR, ESI-MS, FT-IR and EA. And the thermal properties of the alkali salts were characterized using DSC and TG methods, with the results that Tm ranging form 94 to 198℃, and Td from 223 to 369℃, respectively.Eighty-five novel ILs comprised of the sulfonium cations ([SR1R2R3]+ or SR1; R1, R2, R3 =alkyl or CH3OCH2CH2) and [N(SO2F)(SO2CnF2n+1)]- anions were prepared in chapter 3, and conformed employing 1H NMR,19F NMR, ESI-MS and EA analytical methods. The sulfonium serial ILs generally showed low viscosity and high conductivity, and would decompose from 236 to 312℃. However, they were more weakly resistant toward reduction (ca.-2.50 V vs. Fc+/Fc) as compared with the tetraalkyl ammonium based salts. As introducing ether function to the sulfonium cations, the results indicated that it would tend to bring down the viscosity and glass transition temperature and increase the ionic conductivity, while their resistance to anodic oxidation and cathodic reduction was weakened simultaneously. A reverse Li+ deposit-dissolve process on the Ni electrode was detected for S222FSI-LiFSI (0.32 mol kg-1) electrolyte, indicating an effective SEI film would be formed on the Ni electrode to restrain the S222+ cation decomposing, which was absolutely different from S222TFSI-LiTFSI electrolyte.Solutions of Li[N(SO2F)(SO2CnF2n+1)] dissolved in PC solvent were studied intensively in chapter 4. Several regulations, that the viscosity of the electrolyte increasing, the conductivity decreasing, and the oxidation potential improving together with the HOMO values, were found as the fluoroalkyl chain was enlarged. And only glass transitions were observed in the temperature range of-150 to 30℃. Li[N(SO2F)(SO2CF3)] (LiFTFSI) exhibited severe corrosion to Al foil with an initial potential of 3.7 V (vs. Li+/Li) by cyclic voltammetry (CV) measurement. Li[N(SO2F)2] (LiFSI) and Li[N(SO2F)(SO2C2F5)] (LiFPFSI) were testified unstable when polarized at 4.5 V (vs. Li+/Li) for long time, whereas anions with longer fluoroalkyl chains, such as Li[N(SO2F)(SO2C4F9)] (LiFNFSI),Li[N(SO2F)(SO2C6F13)] (LiFHFSI) and Li[N(SO2F)(SO2C8F17)] (LiFOFSI), showed similar electrochemical response toward Al electrode to LiPF6. A serial of LiFNFSI-PC electrolytes with concentrations from 0.01 to 2.00 M were also investigated. Correlations of the concentration with viscosity (or conductivity) was perfectively fitted by Jones-Dole (or Casteel-Amis) function. And the large values of∧imp/∧diff implied a high dissociation degree for LiFNFSI in PC solution, which was beneficial for high ionic conductivity and Li+ transference number.In chapter 5, lithium bis(fluorosulfonyl)imide (LiFSI) has been studied in comparison with LiPF6 in EC/EMC (3:7, v/v) electrolyte, in terms of the physicochemical and electrochemical properties. It exhibited far superior stability towards hydrolysis than LiPF6. Solution comprised of LiFSI was less viscous, more conductive and provided with larger Li+ transference number than that containing LiPF6. The stability of LiFSI-based electrolyte on the Pt electrode was detected highly up to 5.6 V (vs. Li+/Li). And a non-corrosion performance toward Al in the high potential region (3.0～5.0 V vs. Li+/Li) had been confirmed for high purity LiFSI electrolytes using CV, SEM and chronoamperometry, whereas Al corrosion indeed occurred in the LiFSI-based electrolytes tainted with trace amounts of LiCl (50 ppm). With high purity, LiFSI outperformed LiPF6 in both Li/LiCoO2 and graphite/LiCoO2 cells. Furthermore, in contrary to LiPF6, graphite/LiCoO2 cell constituted of LiFSI-based electrolyte operated normally, even though 1000 ppm H2O was contained. Appropriate Na+ content in the LiFSI electrolyte hardly affected the cycling performance of the graphite/LiCoO2 cell, while K+ added with 0.2 M content almost destroyed the cell.A novel lithium salt, lithium (fluorosulfonyl)(nonafluorobutanesulfonyl)imde (LiFNFSI), was investigated as conducting salt for lithium-ion cells in chapter 6. The neat salt (Td:220℃) and the corresponding electrolyte showed better thermal stability than LiPF6. Contrary to the absolutely decomposition of LiPF6 in EC/EMC (3:7, v/v) solvent after aging at 85℃for 2 w, LiFNFSI electrolyte stayed colorless and transparent at the same condition. The electrolyte comprised of 1.0 M LiFNFSI in EC/EMC (3:7, v/v) showed high conductivity comparable to LiC1O4, good electrochemical stability, and would not corrode Al collector. At both room temperature (25℃) and elevated temperature (60℃), the graphite/LiCoO2 cells with LiFNFSI exhibited better cycling performances than those with LiPF6. These outstanding properties of LiFNFSI made it an attractive candidate to overcome the rapid capacity fading of LIBs at elevated temperatures.