| Due to its high energy density, high voltage and long cycle life, lithium-ion battery becomes the most popular secondary battery since it was firstly introduced in 1990. It is necessary to develop large scale and high power lithium-ion battery to satisfy the requirements of electric vehicles (EV) or plug-in hybrid electric vehicles (PHEV) because of the energy and environment problems in the new century. Development of new lithium-ion battery that has high energy density, low cost and good safety is one of the most popular research fields in the past several years. After 20 years'development, lithium-ion batteries have been widely used as power source of cell phones, laptops and digital cameras, as well as other potable electric devices. However, lithium-ion battery still cannot be used as the power source for electric vehicles (EV) or plug-in hybrid electric vehicles (PHEV) because of safety concerns, short cycle life and high cost. For example, the battery can only work less than 5 years at room temperature, which is much shorter for the requirements of electric vehicles, 10 to 15 years; the battery will cause safety problems such as explosion under abused conditions; and the battery decays quickly at elevated temperatures (≥50oC). In order to improve the stability and safety of lithium-ion battery, the main work of this dissertation is focused on preparation, properties and application of critical functional electrolyte components, including the interface film formation additives on anode/cathode of lithium-ion battery, lithium salt, flame retardant additives, and overcharge protection additives.In chapter 3, 1,3-propane sultone (PS) used as interface film formation additive on anode and cathode of lithium ion battery, as well as its functional mechanism, was investigated via electrochemical methods with Nuclear Magnetic Spectroscopy (NMR), X-ray Photoelectron Spectroscopy (XPS) and Density Functional Theory (DFT) computation. It is found that PS was electrochemical reduced on graphite prior to electrolyte solvents, participating in SEI film formation. The major product of PS reduction is lithium alkyl sulfonic compound. Simultaneously, the formation of inorganic components, such as LiF and LixPOyFz, on cathode surface was suppressed, and this results in the improvement of conductivity of interface film on cathode. On the other hand, the interface film is much stabler and contains much higher concentrations of organic lithium alkyl sulfonic compounds in the electrolyte with PS than of the electrolyte without PS. Therefore, the impedance of the electrode surface film formed in the electrolyte without PS is much higher than that of the electrolyte containing PS, which will lead to the significant degradation of capacity after storage at elevated temperature. In chapter 4, dimethyl-vinylene carbonate (DVC) used as interface film formation additive on anode and cathode of lithium ion battery, as well as its functional mechanism, was studied via electrochemical methods combination NMR, XPS as well as DFT computation suggest that DVC has much higher reactivity than the solvents of electrolyte. It could be electrochemically reduced on graphite electrode prior to the electrolyte solvent reduction, participating in SEI formation. The major decomposition component of DVC is poly-DVC or its derivate, which can significantly improve the stability of interface films.In chapter 5, a novel lithium salt, tetrafluorooxalatophosphate (LiPF4C2O4), was synthesized and purified. This salt dissolved in carbonate solvents has similar ionic conductivity with LiPF6/carbonates electrolyte in the range of -40~65oC. And the LiPF4C2O4/carbonates electrolyte also shows wide electrochemical window on aluminum and copper foil electrodes. The LiNi0.8Co0.2O2/MCMB cell with LiPF4C2O4/carbonates electrolyte shows slightly lower discharge capacity at the first cycle than that of LiPF6/carbonates electrolyte at room temperature, however, the charge-discharge coulombic efficiency is comparable. More importantly, the thermal stability of LiPF4C2O4/carbonates electrolyte is significantly improved compared to the LiPF6/carbonates electrolyte. No evidence shows decomposition of bulk LiPF4C2O4/carbonates electrolyte after storage at 85oC over 6 months, and the electrolyte remains colorless.In chapter 6, a class of non-flammable electrolytes was developed using DMMP as retardant solvents and LiBOB as SEI stabilizer. The electrolytes are totally non-flammable, and exhibit similar ionic conductivity with base electrolyte at low temperatures. More importantly, thermal stability of the electrolyte is significantly improved by introduction of DMMP and LiBOB into the electrolyte. The electrolyte can be stable at 85oC for over 2 months. Cells with the DMMP/LiBOB electrolyte show slightly lower discharge capacity at the first cycle compared to the base electrolyte, due to the reduction of oxalate species presented in the LiBOB salt, however, the charge-discharge coulombic efficiency is comparable. And the cell with the DMMP/LiBOB electrolyte shows much higher stability at elevated temperature.In chapter 7, the mechanism of cyclohexyl benzene as electrolyte additive for overcharge protection of lithium ion battery was investigated. It is found that the oxidation electro-polymerization potential of CHB is about 4.7 V, in the range of 4.2 V to 5.0 V. During overcharge process, the voltage profile of the cells with 5% CHB electrolyte rises up very slowly and then a charge platform is present when the voltage reaches 4.7 V, which is related to the polymerization of CHB on cathode electrode. SEM observations of electrode surface and separator of the cell after overcharge suggest that both the electrode and the separator are covered by polymer films. More importantly, CHB has only slight effects on the cycling performance, and discharge capacity of the cells compared to the cells with base electrolyte, but the safety performance is significantly improved by the use of CHB.
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