The Fabrication, Structure And Properties Of CuO/Cu Composites Anode Materials For Lithium Ion Batteries

On the basis of a brief summary of the investigation and application of lithium ion batteries (LIBs) and a review of the investigations and development of copper oxide (CuO), which is one of the promising anode materials for LIBs, the thesis aims to prepare CuO/Cu composites as anode materials by simple and economic methods of thermal oxidation and reduction. By means of the introduction of metallic Cu into CuO, the good electron conductivity and the excellent toughness of Cu plays effective roles in improving the electron conductivity and reducing the pulverization of the CuO anode, respectively, during cycling. Therefore, CuO/Cu anode materials with high capacity and favorable cycle stability are hopefully obtained. The effects of the fabrication technique and its parameters on the structure and electrochemical properties are investigated by XRD, SEM, EDS, TEM and HRTEM etc. and electrochemical testing of galvanostatic charge-discharge, cyclic voltammograms. The key factors that affect the cycle stability as well as the initial irreversible capacity of the composite anodes and the mechanism are also discussed. In addition, effects of high-energy ball-milling on the structure and electrochemical properties of nano-sized CuO and addition amount of PVDF binder on the electrochemical properties of nano-sized CuO are also studied.The results show that high-energy ball-milling reduces the particle size and results in a tendency of amorphous feature of the CuO particles, which all improve its electrochemical properties. The content of PVDF binder shows visible effect on the cycle stability and capacity of CuO anode. Among the three weight ratios of 8:1, 8:0.18 and 8:0.47 of CuO to PVDF, the first one possesses the best electrochemical properties.Commercial nano-CuO (ca.40-50 nm) are reduced partially in the atmosphere of H2 at the temperature ranging from 80 to 140℃, forming CuO/Cu composites. The content of Cu of the CuO/Cu composite slightly increases from 0.4 wt.% to 4.5 wt.% with the reduction temperature increasing from 80 to 100℃. Whereas the Cu content increases obviously to 38.4 wt.% when the reduction temperature is increased to 120℃and no Cu is found in the product when the reduction temperature is further increased to 120℃. The initial coulombic efficiency of the product reduced at 80℃shows correspond with the raw CuO (57-58%), and the efficiency increases slightly as the reduction temperature increases to 100℃. The initial charge capacity of the product obtained at 100℃shows the highest initial charge capacity among all the products, indicating that the suitable amount Cu favors the decomposition of Li2O and enhances the utilization efficiency of the CuO active material. The products reduced at 80 and 100℃show higher initial reversible capacity compared with the raw CuO, however, the improvement of the cycle stability is limited. The product reduced at 120℃exhibits good cycle performance, showing a capacity retention of 92% after 80 cycles. The inactive Cu with good toughness buffers the volume change of CuO caused by the lithium extraction and insertion during cycles and hence improves the cycle stability of the composite. However, as the reduction fraction of the active CuO, the capacity is unfavorably decreased.Commercial nano-Cu powders are mostly oxidized to CuO at the temperature rangeing from 250 to 500℃for 4 hours in air and the content of CuO was higher than 94 wt.% for each product. The nano-morphology of the raw Cu powder is preserved. About 3-4 wt.% Cu remains in the products which prepared by oxidation temperature from 250 to 450℃, whereas no Cu is found for the oxidation temperature of 500℃. The electrochemical study shows that the nano-sized products synthesized by the thermal oxidation method possess favorable cycle stability as anode materials for lithium ion battery. The product prepared at 450℃displays the best cycle stability and a favorable capacity. A capacity of 423 mAh/g is approached after an activation of 8 cycles, and the capacity after 80 cycles maintains as 377 mAh/g, showing a capacity retention of almost 90%. The study of the effect of Cu particle size and oxidation time on the electrochemical properties of oxidized products shows that either reducing Cu particle size or prolonging the oxidation time improves the cycle capability of the oxidized products effectively.