| Li-ion batteries have been considered as a preferred power source for a variety of electric storage applications in the development of sustainable new energy technologies. High Li-storage capacity and cost-effective electroactive materials, especially the cathodic materials, have been recognized to be a key factor for building next generation lithium batteries with enhanced energy densities. Presently, lithium battery technologies are now mostly based on intercalation chemistry, which allows no more than one Li ion per formula unit to insert reversibly into the cathodic host lattices and, therefore, provides insufficient utilization of the electrochemical capacity of the cathode materials. Electrochemical conversion reactions seem to provide a possible approach to realize the significant breakthrough in the storage capacity of the cathode materials for Li-ion batteries through full utilization of all the oxidation states of the multivalent transition-metal compounds. Moreover, the conversion materials have additional advantages of natural abundance, low-cost and environmental friendliness, particularly suitable for the high-capacity cathode materials of Li-ion batteries. In this thesis, we tried to explore such new cathode materials based on electrochemical conversion reactions, and optimize their kinetic performance to realize their potentially high redox capacity for reversible Li-storage. The main results and new findings in this work are summarized as follows:1. A new approach was proposed to improve the capacity and power capability of metal fluorides taking FeF3 as an example by downsizing the FeF3 particle size to create the favorable nanodomains for the phase-transformation reactions. We synthesized FeF3 nanocrystals by different chemical routes and investigated their electrochemical performance at room temperature, especially the high rate capability. The experimental results show that as-synthesized FeF3 nanocrystals with a quite uniformly distributed crystallite size of～5 to 15 nm can convert reversibly to LiF/Fe during the charge-discharge process with a considerably high rate capability. The FeF3 electrode can be well cycled at very high rates of 1000 mA g-1, giving a quite high capacity of～500 mAh g-1. Moreover, conversion reaction of FeF3 nanocrystals with Na+ can be achieved in Na-ion electrolyte, delivering a reversible capacity of 550 mAh g-1.2. Lithium-rich LiF/M (transition-metals) nanocomposites were designed and prepared as an applicable cathode material to match the lithium-deficient anode in commercial production of Li-ion batteries. Taking LiF-Fe nanocomposites as an example, we explored the feasibility of these nanocomposites to undergo conversion reaction reversely from initial discharged state to fully charged state. The structural characterizations revealed that the nanocomposite was composed of well-dispersed and intimately contacted LiF and Fe particles created by high-energy ball milling using TiN grinding nanoparticles, forming appropriate nanodomains for the reversible conversion reaction of LiF and Fe. It was found out that LiF-Fe nanocomposite with 30% TiN shows the best electrochemical charge-discharge performance with a very high capacity of 568 mAh g-1 at 20 mA g-1, approaching the theoretical capacity of LiF-Fe nanocomposite, and even gives a reversible capacity of～300 mAh g-1 at 500 mA g-1 with a good cycling stability and rate capability. Furthermore, NaF-Fe and NaF-Cu nanocomposites can also realize their conversion reaction upon Na+ uptake or removal, delivering a reversible capacity of～150 mAh g-1.3. The possibility of metal chlorides as the conversion cathode materials was first investigated. The approach for achieving conversion reaction of unsolvable chloride was proposed to downsize the active phase and create the favorable nanodomains for the phase-transformation reactions. For the soluble metal chloride, porous carbon were used to load and immobilize the active chloride for suppressing the diffusive loss of the reactants and intermediates by the strong adsorption of the micropores or mesopores. The experimental results indicate that AgCl nanocrystals can convert to Ag reversibly with nearly a one-electron transfer through electrochemical conversion reaction, delivering a reversible capacity close to its theoretical value (187 mAh g-1). The CuCl2 nanocomposite supported by ordered mesoporous carbon CMK-3 can realize two-electron transfer reversibly and maintain a high reversible capacity of 466 mAh g-1 after 20 cycles, showing a good cycling stability. The FeCl3 nanocomposite supported by high surface area active carbon can realize three-electron transfer reversibly through conversion reaction. It is also revealed that the conversion reactions of chlorides can proceed reversibly as long as cathode-active phase are well-dispersed in nanoscale and fixed stably by porous carbon to restrain their dissolution and diffusive loss from the electrodes.4. To test the possibility of using metal oxides as conversion cathodes, we designed and prepared a novel CuO/Li2O nanocomposite by mechanical ball-milling and examined their conversion reaction mechanisms of this material in charge-discharge process. The experimental results demonstrated that conversion reaction of CuO/Li2O nanocomposites can proceed reversibly and rapidly as long as the different phases of the cathode-active particles are well-dispersed and closely contacted to create electrochemically favorable nanodomains in the electrode. The CuO/Li2O nanocomposite can deliver a reversible high capacity of～500 mAh g-1 at 50 mAg-1 and show a strong power capability with a quite high rate output of about～400 mAh g-1 even at 500 mA g-1. The mechanistic studies revealed that the CuO/Li2O nanocomposite can realize nearly a three-electron transfer through electrochemical conversion of CU2O/CU to LiCuO2 and vice versa, involving lithium intercalation and deintercalation.
|
PDF Full Text Request