Research On Fabrication And Properties Of Tin-Based Anode Materials For Lithium-Ion Secondary Batteries

Recently,lithium-ion secondary batteries have been widely used in portable electronic devices due to their high energy density,high voltage and non-pollution. Meanwhile,they have great potential for using in electric vehicles(EVs).The development of anode materials with the high-capacity and good cycle stability to replace the conventional carbon-based materials has attracted more attention for the new generation of lithium-ion secondary batteries.In this dissertation,the research and development of anode materials of lithium-ion secondary batteries were reviewed. In order to improve the electrochemical performance of tin-based anode materials, tin-based anode materials were prepared and characterized by XRD,SEM and TEM measurements.The electrochemical performance of tin-based anode materials was evaluated by means of various techniques including galvanostatic method,cyclic voltammetry(CVs) and electrochemical impedance spectroscopy(EIS).The main content is presented as following:Because the limited inner space in lithium-ion secondary batteries,it is important to find the materials with a high capacity and a high tap density as anode materials. Based on the high crystal density of alloys,Sn-rich La-Co-Sn ternary alloys were prepared by arc melting.The relationship between the morphology,microstructure and electrochemical properties were analyzed.In particular,the effect of the crystallinity in Sn-rich La-Co-Sn ternary alloys on electrochemical properties was elucidated.On the basis of obtained results,all the as-cast La-Co-Sn ternary alloys have the same main phase of La₃Co₄Sn₁₃ and low electrochemical capacities due to its large particle sizes.Among these alloys,the as-cast LaCoSn₄ alloy exhibits the best electrochemical performance.The ball-milling process results in the reduced crystallinity,and the enhanced electrochemical capacities as compared to the as-cast alloy.In particular,the LaCoSn₄ alloy,obtained after ball-milling for 16 h,provides the higher discharge capacity of 500 mAh/g after 40 cycles.Both the charge-transfer resistance and the Warburg impedance of the ball-milled alloy are much lower than that of the as-cast alloy,which are beneficial to the great improvement of the electrochemical performance.Although further studies are required to understand and increase the coulombic efficiency in the initial cycle,La-Co-Sn ternary alloys with the high crystal density can be considered as promising anode materials for lithium-ion secondary batteries.Tin-based oxides are good candidates as new electrode materials for lithium-ion secondary batteries.Unfortunately,the large volume change generated during the lithium alloying processes causes serious mechanical damage to the electrode, resulting in a large capacity loss.It was reported that small and uniform distribution of electrode materials may minimize the dimensional changes of the active materials and provide the improved performance of the electrode.In this work,spinel Co₂SnO₄ nanocrystals were successfully synthesized by one-pot hydrothermal method.The hydrothermal conditions,such as alkaline concentration,reaction temperature,and duration time on the structures and morphologies of the resultant products were investigated.The well crystallinity and pure phase of Co₂SnO₄ nanocrystals can be obtained with 2.0M of NaOH solution at 240℃for 48h.The as-prepared Co₂SnO₄ nanocrystals exhibit good electrochemical performance with initial coulombic efficiency of 71%,high reversible capacity of 1088.8 mAh/g and relatively good capacity retention.It is the first report about spinel Co₂SnO₄ nanocrystals prepared by hydrothermal method as anode materials for lithium-ion secondary batteries.Although the significant improvement in the electrochemical performance is obtained for nanocrystalline active materials,the poor electronic conductivity of Co₂SnO₄ nanocrystals still is retained.To resolve the problem,Co₂SnO₄-MWCNTs nanocomposites were prepared in this work.In the nanocomposites,3D conducting networks of MWCNTs would enhance the electronic conductivity of the nanocomposites.Moreover,MWCNTs may also provide an elastic buffer for releasing the strain of Co₂SnO₄ nanocrystals during the electrochemical processes.On the basis of obtained results,it is demonstrated that the Co₂SnO₄-MWCNTs nanocomposite(5:1) has highly reversible capacity(about 900 mAh/g) and capacity retention of 91.1%after 50 cycles,and improved rate capability.These results suggest that carbon nanotubes are beneficial for increasing electronic conductivity,and decreasing charge-transfer resistance in Co₂SnO₄-MWCNTs nanocomposites.Taking into account of the large volume change of tin during the electrochemical cycle,the novel tin-highly porous carbon composites were prepared using highly porous carbon materials as the matrix of tin nanoparticles.The tin-highly porous carbon composite with optimized tin contents(45.6%) has the high reversible capacity of 503 mAh/g and good capacity retention of 83.1%after 60 cycles,and good high rate discharge ability.The results indicate that the highly porous carbon is the suitable matrix for supporting tin nanoparticles because the highly porous carbon can ensure the high electronic conductivity of composites and buffer the volume change of tin nanoparticles during the electrochemical processes.