Design And Preparation Of Cu-,Ni-based Compounds And Their Improved Electro-chemical Performances As Conversion-reaction Anodes For Lithium Ion Batteries

The research topic carried out in this dissertation is about 3d transition metal oxides and phosphides as anodes for lithium ion batteries. The main objective in this research is to develop methods for enhancing the initial coulombic efficiency and cycling performance of 3d transition metal oxides and phosphides. We choose CuO, Cu2O and Ni3P as research objects, and develop several methods to improve their electrochemical performances, such as preparation of nanostructured powders, films, composites, and surface modification.Different cupric oxide particles (leaf-shaped, shuttle-shaped, flower-like, dandelion-like and caddice clew-like) were self-assembled by a simple ammonia-evaporation method. The electrochemical performances of CuO are tightly related to their nanostructures and morphologies. The leaf-shaped and shuttle-shaped CuO exhibit high initial coulombic efficiencies but relatively low discharge capacities for their small specific surface. For the dandelion-like and caddice clew-like CuO, although the initial coulombic efficiencies are a little lower, the discharge capacities and cycling properties are much better than the leaf-shaped and shuttle-shaped CuO, especially at high rate. The clew-like CuO synthesized in the solution with pH value of 11.5 exhibits relative low initial coulombic efficiency of 62%, but high reversible capacity of 400 mAh g-1 and 375 mAh g-1 at rate of 0.1 C and 0.5 C, respectively. It is attributed to the hierarchical nanostructure, which provides higher specific surface area, leading to larger contact area for CuO/electrolyte and shorter diffusion length of Li+.CuO nanoflower-like and nanotube films were anodically grown on Cu substrate, and highly ordered porous Cu2O arrays were fabricated by electrodeposition through polystyrene sphere template, respectively. These nanostructured films can not only provide sufficient contact between active materials and electrolyte, but also improve the electrical contact between active materials and current collector, thus the cycling performance of CuO and Cu2O are enhanced. At a current density of 0.02 mA cm-2, the CuO nanoflower-like and nanotube films can exhibit reversible capacity of 530 mAh g-1 and 420 mAh g-1 after 50 cycles, respectively. But at a higher current density, the cycling performances of these CuO nanostructured films are still a little disappointing. For the electrodeposited porous Cu2O film, it exhibits reversible capacity of 336 mAh g-1 and 213 mAh g-1 after 50 cycles at rate of 0.1 C and 5 C, respectively.CuO mirosphere with needle-like surface morphology, core-shell Cu2O/Cu and Ni-coated CuO nanoflower-like film were prepared by surface modification of CuO and Cu2O, respectively. The initial coulombic efficiency of hierarchical CuO mircosphere with needle-like surface increases from 60%to 65%, and the reversible capacity at rate of 0.1 C and 1 C for 50 cycles sustains 62.4%and 56.4%, respectively, which is much higher than those of the CuO mircosphere without surface modification. Comparing to pure Cu2O, the core-shell Cu2O/Cu exhibits improved initial coulombic efficiency (from 69.5%to 78.8%) and better cycling performance (reversible capacity is 360 mAh g-1 at 0.1 C after 50 cycles). The CuO/Ni nanoflower-like film also delivers enhanced electrochemical performances. The initial coulombic efficiency increases from 57.0%to 72.1%, and the reversible capacity at 0.02 mA cm-2 for 50 cycles increases from 584 mAh g-1 to 530 mAh g-1. The improved electrochemical performances of these surface-modified materials are attributed to the improved electrochemical reactive interfaces, at which the charge transfer is accelerated and the reversibility of electrode reaction is facilitated.The electrochemical impedance response of the conversion reactions in hierarchical CuO electrode composed of 2D nanoplates is analyzed by a modified two-parallel diffusion path model. It is confirmed that the SEI film is formed at the potential range of 0.47-0.15 V during the discharge process, and it takes about eight cycles to form integrated and persistent SEI film on the surface of CuO electrode. The charge-transfer resistance is greatly influenced by the electrode surface morphology during cycling. More Li2O phases in the matrix lead to the lower diffusion coefficient of Li+. It is also suggested that Li+diffusion with short diffusion length is much easier than that with long diffusion length. Moreover, three proposals for optimizing the electrochemical performances of CuO electrode are summarized from the analysis of electrochemical impedance spectrums:(1) The discharge cutoff potential of CuO should be below 0.1 V to form integrated and persistent SEI film; (2) fabrication of porous, hollow, and hierarchical structures is favorable for enhancing the charge transfer on solid/electrolyte interfaces and reducing the corresponding impedance; (3) the 3D nanostructured electrode can abate the negative effect of low diffusion coefficient of Li+and make the electrode reaction kinetics enhanced over the 2D and ID nanostructures. i dense and porous films were prepared by electrodeposition. The triple-layer porous Ni3P/Ni film with a three-dimensional network nanostructure shows a significant improvement of electrochemical performances, especially the rate capability. The reversible capacity of the triple-layer Ni3P film is 557mAh g-1 and 243 mAh g-1 after 50 cycles at charge-discharge rate of 0.2 C and 5 C, respectively. The reversible capacity still sustains 44%as the discharge-charge rate rises even 25 times. The enhanced reversibility and rate capability is ascribed to the three-dimensional network nanostructure. Firstly, the larger active surface decreases the exchange current density and the impedance of Li+ migration through surface-passivating layer and further transfer into Ni3P; Secondly, the direct growth of Ni3P/Ni film on copper substrate leads to better electrical contact between active materials and cunent collector; Thirdly, the nickel nanoparticles dispersed in the films facilitate the decomposition of Li3P and promote the charge reaction to a higher extent.