| The increasing energy crisis and the alarming effect on the environment aroused by fossil fuels consuming have been creating international vulnerability which will endanger social stability. Intensive studies have been carried out on how to convert electricity energy efficiently from alternative energy sources, such as solar, wind and wave energy.Batteries are considered to be the largest group of technologies and the most convenient form to store the electrical energy generated intermittent in time and dispersed in space.Among the batteries, lithium ion batteries have been widely used in portable electronic devices. Recent studies demonstrate that lithium ion batteries are potential power sources for electric vehicles(EVs), hybrid electric vehicles(HEVs) and stationary energy storage.Among the anodes, spinel Li4Ti5O12 showing excellent reversibility during dischargecharge processes for its intrinsic zero-strain property is highly expected to be a proper anode for high power devices. While considering the ratively lower theoretical capacity of 175 mAh g-1when discharged to 1 V(vs. Li/Li+), the rather low electronic and Li+conductivity, Li4Ti5O12 seems less competitive to other anodes.In this work, graphitic carbon coated Li4Ti5O12 and lithium rich phase Li2TiO3 coated Li4Ti5O12were successfully synthesized to improve the electronic and Li+conductivity at the voltage range of 3-1 V(vs. Li/Li+). To improve the capacity, traditionally fabricated Li4Ti5O12 electrodes were coated with glassy lithium phosphate layer and metal oxides layer, respectively. The capacity of Li4Ti5O12 was succefully extended to293 mAh g-1by discharging the batteries to 0.05 V(vs. Li/Li+). The batteries exhibited excellent rate performance and cycle stability. The studies were summarized as follows:(1) An economic and easy to scale up route was proposed to synthesize graphitized carbon coated Li4Ti5O12. The graphitized carbon coating layer on the starting material prevented the particles from growing larger and aggregation, leading to the grain size down to around 200 nm. The composite showed excellent rate performance even charging/discharging at high current rate of 12 C, which was attributed to the improved electronic conductivity as well as the reduced Li+diffusion length.(2) Li4Ti5O12/Li2TiO3 materials with surface enriched in lithium were successfully synthesized by a simple solid state process with different excess of lithium carbonate in the starting materials. The electrochemical impedance spectroscopy(EIS)of the platelets made of the as-prepared samples were also tested to calculate the conductivity of the lithium ions, which proved that it is the synergistic effect of Li4Ti5O12 and Li2TiO3that impove the lithium ion conductivity. The structure surprisingly improved the rate performance and cycle stability significantly. At the same time, Li2TiO3 was similarly synthesized, and detailed electrochemical information was studied at the aid of charge-discharge test, cyclic voltammograms and electrochemical impedance spectra test.(3) To study the compact of lithium ion conductive material on surface modification,Li4Ti5O12/Li3PO4 composites was synthesized by solid state process. The composites exhibiting excellent cycle stability, the possible reason of which was suggested to be the existence of Li3PO4 improving the compatibility between electrode and electrolyte.(4) To investigate the compact of glassy lithium phosphate, radio frequency magnetron sputtering technique was employed to deposit a thin glassy lithium phosphate layer on traditionally fabricated Li4Ti5O12 electrodes. The capacity was succefully extended to 293 mAh g-1by discharging the batteries to 0.05 V(vs. Li/Li+) with excellent rate performance and cycle stability. The glassy lithium phosphate coating layer successfully stabilized SEI film, formed a conducting network, and compensated the anisotropic surface lithium insertion.(5) To futher study the role of electrode modification. Metal oxides(ZnO、 Al2O3)coating layer was deposite by RF magnetron sputtering method. The coating layer impoved the cycle stability of the batteries by stabilizing the structure of the electrodes.What’s more, high rate performance was also achieved for ZnO coated electrodes.In conclusion, powder modification improve the rate performance and cycle stability at 3-1 V(vs. Li/Li+). Electrode modification would greatly promote the commercialization of Li4Ti5O12 at the range of 3-0.05 V(vs. Li/Li+). |
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