Studies On The Tin Oxide-Based Anode Materials For Lithium Ion Batteries

On the basis of reviewing the developments of lithium ion battery and relative materials in detail, with tin oxide-based anode materials as objects of the research, focusing studies on the specific capacity, cyclability and rate capability of materials, the tin oxide-based materials were prepared using different methods such as sol-gel, electrochemical deposition, mechanochemical, rheological phase reaction and homogeneous precipitation method, and characterized by various electrochemical methods in combination with powder X-ray diffraction (XRD), Scanning Electron Microscope (SEM), Infrared (IR) spectroscopy, Brunauer-Emmer-Teller (BET) surface area measurement, Electron Spin Resonance (ESR), inductively coupled plasma (ICP) spectroscopy, particle size analysis, and so on.Nanosized SnO₂ powders were successfully prepared by non-hydrolytic sol-gel approach combined with heat treatments using tin tetrachloride as starting material and ethylene glycol as solvent. The reaction mechanisms of the sol-gel process are proposed. Results indicate that the -OHCH₂CH₂OH- prevent Cl^- ion from access to tin Sn⁴+due to steric effect and hence increase the stability of the sol solution. Enthylene glycol functions not only as a complextion agent to form a polymer network but also as a "spacer" to modulate the distance between metal ions, preventing metal oxide particles from aggregation during desiccation of the sol. The effects of heat treatments on the evolution of structure and morphology of nanosized SnO₂ powders were studied. The powders with uniform size around 15-20 nm can be obtained at 500℃ for 4 hours.Results on the electrochemical properties nanosized SnO₂ powders show that heat treatment temperature, voltage windows, current density for charge and discharge, and particle size effect on their specific capacities, cyclability and rate capability. The powders obtained at 500℃ for 4 hours shows the best electrochemical peoperties: a reversible capacity of more than 868mAh/g was obtained and the rate of capacity fading was only 0.56% after 30 cycles at 0.1C rate in the potential rangeof 0-2.0V; and the reversible capacity was 498 mAh/g with a rate of capacity fading of 0.075% after 30 cycles at 0.5C rate in the potential range of 0-1.0V. The interface processes of lithium insertion into nanosized SnO2 electrode was studied by A.C. impedance and three equivalent circuits were obtained according to different A.C. impedance spectrum at different discharge states.The optimal elctrochemical deposition parameters to prepare nanosized SnO2 thin films were obtained. Electrochemical tests show that the sample heat-treated at 400 °C for 2 h can deliver a reversible capacity of 798mAh/g, 630mAh/g and 550mAh/g at 0.1C,1.0C and 2.0C rate respectively, and 97% of reversible capacity of 773mAh/g can be retained after 50 cycles at 0.1C rate. These results indicate that electrochemical deposition technique is a promising method to produce SnO2 films with high specific capacity, good cyclability and rate capability.The preparation and characterization of SnO2-based comopsite oxides derived by mechanochemical method and rheological phase reaction were carried out and comparatively studied. The reversible capacity was more than 570mAh/g and the capacity loss was only 0.22% per cycle after being cycled 20 times for the sample derived by rheological phase reaction, while that of the sample derived by mechanochemical method was 578 mAh/g and 0.31% respectively. These results show that SnO2-based comopsite oxides prepared by rheological phase reaction possess better elctrochemical properties due to their smaller particle size and more uniform distribution of particle size.In order to interpret the reason of the improvement of cyclability of SnO2 by doping with element of Al, P, B, from the point of view of the ratio of spectator atoms to tin atom (X:Sn), a model for the aggregation of tin in SnO2-based materials was proposed on the basis of a series of assumptions. We show that the ratio of X:Sn is close correlative to the cyclability of SnO2-based materials,i.e.,the larger the ratio of X:Sn is, the smaller the Sn cluster is formed during cycling, the less the capacity of the electrode materials is lost and the better cyclability of electrode materials have.SnO2-graphite composites were prepared by homogeneous precipitation method and the composition, structure and morphology aswell as the electrochemical properties were studied. Stuides suggest that SnO2 is uniformly distributed over the surface of graphite, which is because that the oxide anions in SnO2 could have bridged with carbon surface species to result in good spreading between SnC>2 and the graphite. The composites of 30%SnO2-70%graphite heat-treated at 600 °C for 4h have a reversible capacity of 520mAh/g and 88% of the reversible capacity can be retained after 20 cycles. 92.1% and 83.2% of the capacity discharged at 0.1 C can be obtained when diachrge at 1.0 C and 2.0 C respectively. SnO2-graphite composites have good reversible capacities, rate capability and cycle life. These results show that SnO2-graphite composites have good reversible capacities, rate capability and cyclability. The observation of specific capacities that are larger than the weighted sums of the capacities of SnO2 and graphite suggests the presence of synergistic interaction between the two constituents.A mechanism of Li storage in SnO2-graphite composite was proposed. Because the presence of SnO2 on the surface of graphite, a new active surface site is created, the activation energy of the Li-ion transport process is significantly reduced, so the transport process of Li-ion across the interface is accelerated, which lead to the improvement of cyclability and rate capability of SnO2-graphite composite.The kinetics behaviors of SnO2-based electrode are studied by means of linear sweep voltammetry and chronoamperometry measurements. It is found that the exchange current density (i0) and diffusion coefficient of lithium (Dy) increase with Li intercalation into SnO2-based electrode. Nanosized SnO2 films and powders have the largest DLi and i0, respectively. SnO2-based composite oxides derived by mechanochemical method have the least DLi and i0. Compared to the micron-sized SnO2 powders, the i0 and Dy of nanosized SnO2 powders or films and the SnO2-graphite composites increase significantly; and that of the SnO2-based composite oxides decrease obviously. The discharge capacities of SnO2-based electrodes decrease with the increase of currentrate, and the nanosized SnC>2 powders and SnCVbased composite oxides derived by mechanochemical method have the best and worst fast discharge capability, respectively at same current rate.