Studies Of The Problems Related To The Development Of High Performance Lithium-ion Batteries

In comparison with conventional battery systems, Li-ion battery possesses intrinsic advantages of high specific energy and high power density and is now considered as the competitive power sources for various applications ranging from microelectronics, electric vehicles to space missions. However, the currently commercialized Li-ion batteries cannot satisfactorily meet the performance requirements of some important applications, especially in the safety and low temperature performances. This Ph. D work was oriented to deal with the basic and applied problems associated with high-rate or high-capacity applications of Li-ion batteries. The main results and new findings are summarized as follows:1, A new approach was proposed to modify the surface structure of natural graphite by use of polymer adsorption and thereby to improve the initial charge-discharge efficiency of natural graphite anode in Li-ion battery. The experimental result showed that the surface modification of graphite anode by polydimethylsiloxane could effectively suppress the initial irreversible reactions and accelerated the SEI film formation, and therefore greatly improved the first charge-discharge efficiency. In comparison with the bare graphite electrode, the modified graphite electrode exhibited a 13%-reduced irreversible capacity in the first charge-discharge cycle, while the reversible capacity almost remained unchanged. In addition, the surface modification also improved the cycling stability during subsequent cycles.2, In order to improve the safety of Li-ion battery during overcharge, an overcharge protection method based on internal electropolymerization reaction was proposed and the electropolymerization behaviors of biphenyl and 2,2-diphenylpropane were investigated as the electrolyte additives at theovercharged positive electrodes. The results demonstrated that biphenyl has suitable electropolymerization potential (4.5~4.8V, vs. Li/Li+) and required kinetic behaviors, and is eligible for use as the overcharge protection additive. The data from both simulated cells and practical batteries showed that biphenyl as an additive in electrolyte could electrochemically polymerize at 4.5-4.75V (vs. Li/Li+) to form a thin layer of conductive film on cathode surface. The surface film thus formed could increase the internal resistance, and at the same time the gases, released during ?the electrooxidative polymerization, could also increase the internal pressure of the batteries, which therefore enhanced the sensitivities of the electrical disconnecting devices. During prolonged overcharge, the growth of the conductive polymer film could create short-circuiting between the positive and negative electrodes and cause self-discharging the batteries to a safer state. Furthermore, no discernible detrimental effects were observed in normal charge-discharge properties and in the shelf-life behaviors of the battery with the electrolyte containing less than 5% biphenyl additive by weight.3 The solvent composition is a key factor in determining the low temperature performance of Li-ion batteries. In order to develop the highly conductive electrolyte at low temperature, the ionic conductivities of EC-based multi-component electrolytes with different solvent compositions were measured at wide temperature range of +40?0 and the influences of each solvent components on the conductivity were analyzed qualitatively by use of conductivity contour map method and discussed according to the phase changes of the multi-component system. It was found that the solvent component with high dielectric constant and low viscosity could only increase the room temperature conductivities of the mixed electrolyte systems. Nevertheless, the conductivities of the electrolyte solutions at low temperature were predominantly dependent on the eutectic points of these systems. It wasdemonstrated that the addition of EMC in the EC-based electrolytes could considerably lower down the temperature of eutectic points of the electrolyte systems, and the ternary solvent systems o...