Synthesis Of Nano-scaled Transition Metals By Electrochemical Milling And Study Of Electrochemica Performances Of The Transition Metal Oxides

The development of the Li-ion batteries impulses researchers to explore some new possible electrode materials, improve the performances of the Li-ion batteries and promote the development of Li-ion batteries'industrialization. The electrochemical performances of the Li-ion batteries depend on the cathode materials, the anode materials and the electrolyte. The conventional electrode materials in Li-ion batteries are insertion/extraction compounds such as LiCoO2, LiNiO2 and graphite. Some non-insertion/extraction compounds such as CoO, Co₃O₄ and LaCoO₃ show higher reversible capacities and perhaps to be new anode materials. In recent years, researchers'attentions have been attracted in nano-metallic materials due to their own performances. In this article, we explored some potential metal oxides on their electrochemical performances and mechanisms. At the same time, we prepared nano-metallic particles from Li-ion batteries via electrochemical reduction reaction.In the 1st chapter of this thesis, the author reviewed the advantages and disadvantages, the structure and the working principle, common electrode and electrolyte materials of lithium ion batteries, especially the common anode materials used in lithium ion batteries. Furthermore, the popular ways to synthesize nano-metallic materials were also presented and evaluated.Tarascon and other research groups have studied the electrochemical reaction mechanism of transition metal oxides electrode with metallic lithium, which showed that the final reduction product was metallic in nano-scaled. According to this idea, in the 2nd chapter of this thesis, the author put forward a novel synthesis way to prepare nano-metallic particles by the reduction reaction of metallic lithium and NiO electrode in lithium batteries, which was entitled electrochemical milling (ECM) route. Using X-ray absorption fine structure (XAFS), the author analysed the valence and coordination number changes of Ni atoms in the process of lithium-ion inserting, inferring the possible process of lithium-ion reducing transition metal oxides. The results showed that metallic nickel particles in nano-size were acquired from deoxidizing the NiO electrode by metallic lithium, illuminating that the ECM way is feasible to synthesize the metallic nickel particles. The author also found that the particle size of the acquired metallic nickel was influenced by the current density, discharging temperature and cycles. At the same time, in the 3rd chapter of this thesis, the author synthesized nano-transition-metal silver by the same method of electrochemical milling, and found that the effect of current density and discharge temperature on the morphology of silver was the same as that on nickel, illustrating that the method of preparing transition metal by electrochemical reduction was replicable,.Aiming at the pop research in metal oxides as anode materials, in the 4th chapter of this thesis, the author synthesized nano-rod structured Co₃O₄ by a post anneal-assisted hydrothermal method. Its electrochemical performances were also analyzed. The results showed that the Co₃O₄ electrode with an appropriate morphology and specific surface area presented a preferable electrochemical performance with the first discharge capacity of 954mAh/g, even after 35 cycles, this electrode retained a discharge capacity of 807mAh/g. By changing the sintering temperature and adding surfactants to the solution during the hydrothermal process, the author synthesized Co₃O₄ powder with different morphologies, such as nano-plates, nano-particles, discovering that there was much difference for the Co₃O₄ electrode with different morphologies. The excellent electrochemical performance of nano-rod Co₃O₄ was also interpretedFinally, in the 5th chapter, the author gave a general overview on the achievements and the deficiency in this thesis. Some prospects and suggestions of the possible future research directions were pointed out.