Studies On The Functional Electrolyte For Lithium-ion Batteries

In order to solve the problem of fossil energy shortage and environmental pollution, it is necessary to develop new clean energy systems, such as wind energy, solar energy etc. which are usually intermittent and require electric energy storage to be coupled with. Lithium-ion batteries are considered as the most attractive energy storage system due to its high specific energy and long life. However, the safety concern of the lithium-ion batteries becomes a major challenge for large-scale energy storage applications. Highly flammable carbonate-based electrolytes are the major cause for the safety hazards of currently commercialized lithium-ion batteries. Therefore, the development of flame retardant or nonflammable electrolyte is a feasible strategy for improvement of the safety of the batteries. In addition, It is also needed to develop new electrolytes with wide electrochemical window and high-voltage tolerance, so as to widen the operating voltage range and therefore enhance the specific energy of the battery. This thesis was aimed to explore the flame-retardant, nonflammable, and high voltage electrolytes for lithium ion batteries. The main results and main conclusions are as follows:1. Phosphazene-containing (-P=N-) and fluoride-containing phosphate compounds were investigated as flame-retardant electrolyte additives for constructing safer lithium-ion batteries. Though the (EtO)3P=N-PO(OEt)2(PNP) additive has a low flame retardant efficiency, it shows a good electrochemical compatibility for both cathode and anode materials. The fluoride-containing phosphonate, CH3PO(OCH2CF3)2(TFMP) and CH3CH2PO(OCH2CF3)2(TFEP), show high flame retardant efficiency, which demonstrate a50%reduction in the SET of the electrolytes at5%content and a complete nonflammability at20%content. Both TFMP and TFEP show higher flame retardancy and electrochemical compatibility than dimethyl methylphosphonate (DMMP), a well-known electrolyte additive. showing promising prospects for battery applications.2. To develop completely nonflammabile electrolyte, we investigated the phosphate-based electrolytes as a safe alternative to flammable carbonate-base electrolyte and characterized their electrochemical compatibility with cathode and anode materials. The experimental results showed that the graphite electrode can not perform well in these phosphate-based electrolytes even in the presence of film-forming agent, but alloy anode materials can work well in these phosphate-based electrolytes as in the carbonate-based electrolytes. The electrochemical characterization revealed that these phosphate-based electrolytes have an ion conductivity of approximately6mS cm-1at room temperature and a wide electrochemical window of0-5V, possibly suitable for lithium-ion batteries. In addition, the phosphate-based electrolytes have a electrochemical compatibility with the silicon-based and antimony-based anode materials and could give similar performances such as the capacity, initial coulombic efficiency and cyclic stability to those measured in the carbonate-based electrolyte after the addition of film-forming additive, fluroethylenecarbonate (FEC). Also, the cathode materials such as LiFePO4and LiMn2O4etc can work very well in these electrolytes. The full cells of SiO|LiFePO4and Sb|LiFePO4using phosphate electrolytes exhibited considerably good charge/discharge behaviors and cycle stability. These results provide a feasible approach for the development of safer lithium-ion batteries.3. To seek for the suitable electrolyte for high voltage materials, we investigated the develop a new aromatic electrolyte additive, which forms a polymer film on the surface of LiNi0.5Mn1.5O4by in situ electropolymerzation and thus reduces the possibility of oxidation decomposition of the electrolyte by preventing the direct contact with electrolyte. The results indicated that the100th capacity retention of the Li/LiNi0.5Mn1.5O4half cell is improved to84.7%and86.2%respectively after the addition of0.1wt.%naphthalene and0.1wt.%triphenylmethane as compared to the blank electrolyte (73.5%).These data reveal that the aromatic compounds can be used as an additive for the high voltage batteries.4. A number of non-carbonate solvents were also investigated as possible alternatives to carbonate electrolytes. The solvents studied were tri(ethoxyethylene) borate (TEEB), methoxyethylene nitrate (MEN) and dibutyl sulfate (DBS). The results showed that all the electrolytes exhibited relatively wide liquid range and electrochemical window but their ion conductivities are too low for lithium-ion batteries. The DBS-based electrolyte shows a good cell performance on MCMB negative electrode after addition of5%FEC while the TEEB-based electrolyte can not provide well cycle performance even the existence of5%FEC. The MEN-based electrolyte could obtain a good cycle performance after the usage of1%FEC, but it appears a large irreversible capacity in the first discharge process caused by the reductive decomposition on MCMB at2.5V. LiFePO4and LiMn2O4half cells showed normal charge-discharge performance in these electrolytes except for DBS-based electrolyte on the LiMnO4. However, LiMn2O4half cells display relative low coulombic efficiency in all these novel electrolytes. Therefore, the three electrolytes can not be used as single solvents for lithium-ion battery, resulting from the low ion conductivities and poor electrochemical compatibility with both cathode and anode materials.