Design, Synthesis And Electrochemical Properties Of Organosulfide Compounds As Cathode Materials And Safe Electrolytes For Secondary Lithium Batteries

Lithium ion battery (LIB) with light weight, high energy density and high cycle performance has been widely used in portable devices, such as laptops. However, the scale-up of the technology for electric vehicle/hybrid electric vehicle (EV/HEV) applications meets an obstacle. The tough problem is how to increase the specific energy and the application safety of the battery.Organodisulfide compounds have attracted enhancing research interests as positive active materials with large energy density, lightweight for high power LIBs. The disulfide bond (S-S), providing reversible two-electron reaction during oxidation and reduction (RS-SR+2e-=2RS-), is the key moiety contributing high specific capacity to the organodisulfide compounds in the batteries. The most effective approach for increasing practical specific capacity and cycling stability is that the S-S bond is combined with the side chain of some kind conducting polymer. And the modifications in the structure not only enable the catalysis effect of the backbone chain on the redox process of disulfides, it also brings about an effective inhibition of the loss of sulfur in the electrolyte during the charging and discharging.Among many battery components, the properties of the electrolyte can influence performance and safety tremendously. Carbonic acid esters, which are used as the solvents in current lithium ion battery electrolyte, can enhance the LIB’s charge and discharge capacity and circulation working life. Whereas their flash point are low and easy to burn, increasing the risk of fire even explosion. Hence, it is critical to explore novel fire-retardant solvents or electrolyte additives in commercial electrolytes of LIB. Moreover, how to balance performance and safety is another challenge for a reliable battery. Using an appropriate amount of the non-ionic flame retardants, or optimizing manipulation of the surface reaction between the electrode and the electrolyte are proper methods for pursuing an effective outcome in terms of both safety and performances.The main contents of the thesis are as follows:Chapter1reviews the recent research progress in organodisulfide compound as the cathode materials for LIBs and makes a general introduction of the development of safe electrolyte. On the basis of literature research, we put forward the design ideas and determine the research contents of this thesis.In chapter2, the synthesis, characterization and related electrochemical performances of novel phenylethynyl-containing organodisulfide as cathode material for secondary lithium batteries were discussed. The total synthesis for target compound TMSEDTTA was achieved in four steps with the yield of20%. The structure of TMSEDTTA was characterized by NMR, FT-IR, Raman, XPS and elemental analysis. After a series of attempts, the "Bunte Salt" was chosen as the intermediate to synthesize the S-S bond. Then, the electrochemical properties of the active material were investigated by cyclic voltammetry and charge-discharge technology. Tests indicated that the separation of the anodic and cathodic peak potentials, that corresponding with the electrochemical cleavage and recombination process of S-S bond for TMSEDTTA is0.4V. The result shows that redox couple of TMSEDTTA can be reversible. The discharging tests of Li/TMSEDTTA cell showed a discharge capacity of330mAh/g in the first cycle with a significant discharge platform at ca.2.6V.In chapter3, we mainly discussed the synthesis, characterization and related electrochemical properties of novel anthracene based organo-sulfide (C-S-C) polymers as LIB cathode. In the aspect of material synthesis, we obtained and characterized two types of target compounds containing the organo-sulfide structure for AYDT, DDTYA and their corresponding polymers PAYDT, PDDTYA. The structure of polymer PDDTYA was characterized by FT-IR, Raman, XPS and elemental analysis. In the TG test, the polymer PDDTYA showed good thermal stability. During the following electrochemical testing, the results indicated that the cathode material’s electrochemical cycle activity was enhanced greatly after polymerized. The initial discharge specific capacity of Li/PDDTYA half cell was253mAh/g. After25cycles, the discharge capacity remained156mAh/g.In chapter4, the new phosphamide compounds used as retardant additive in the electrolyte of LIBs were designed, synthesized and characterized. We mainly compared the fire retardant properties, thermal stability and electrochemical properties for the electrolytes with and without DEMEMPA additive. Under the synergy of P-N, the self-extinguishing time value fell43%for the electrolyte with addition of10%BMEMAP and the decomposition temperature significantly delayed. It was shown that adding10vol.%BMEMAP in1M LiPF6/EC+DMC (1:1, v/v) can provide a wide electrochemical window (5.2V) and high conductivity (9.6mS cm-1). The electrochemical tests revealed that the electrolytes with and without BMEMAP have similar electrochemical behavior with LiFePO4electrode and natural graphite.In chapter5, the phosphoate compounds containing vinyl double bond were synthesized as bi-functional additive of flame retardant and film former in commercial electrolytes. It was indicated that addition of10%BMEMAP can enhance the thermal stability and distinctly shorten the flame extinguishing time of the electrolyte. The electrolyte with10%BMEMAP could provide a wide electrochemical window (>4.5V) and high ion conductivity (9.3mS cm-1). In addition, the BMEMAP additive exhibited good electrochemical compatibility with LiFePO4electrode. The Li/MCMB charge-discharge tests indicated that the formed SEI layer ascribed to the decomposition of BMEMAP additive was stable and effective in preventing the co-intercalation of PC.In chapter6, the compounds containing P=N were synthesized as flame retardant additive in the commercial electrolytes. The flame retardant efficiency reached0.31with the commercial electrolytes adding5%VI-2. The mixed electrolytes had good electrochemical stability with the anodic decomposition potential above4.7V. The ion conductivity of electrolyte containing VI-2additive was closed to9.0mS cm-1, indicating the potential use in LIBs. In the Li/LiFePO4half cell, the initial discharge capacity of the electrolyte with5%VI-2was129mAh g-1, and the capacity loss reached8.0%compared with the blank electrolyte. In the cycle performance test, the coulomb efficiency remained at above96%.