| Recently,Lithium-ion batteries have been widely used in portable electronic devices such as cellular phones,camcorders,notebook computers due to their high energy density,high voltage and non-pollution. Meanwhile,they have great potential for using in electric vehicles. However,at present high rate ability and high reversible capacity of Lithium ion batteries are urgently needed to improve because they are the large obstacles for the application of Lithium ion batteries in electric vehicles.For the problems of larger capacity loss and lower reversible capacity of carbon materials using in lithium ion batteries during charge and discharge at high rate,this research firstly focused on the design and preparation of the carbon materials with different porosity and length/diameter aspect ratio,involving expended mesocarbon microbeads (EMCMB)and carbon nanotubes(CNTs).The morphology,structure and electrochemical properties regarding reversible capacity,coulombic efficiency,cycle performance,lithium storage mechanism and kinetics of the materials as negative electrode materials for lithium-ion batteries were systematically investigated by SEM,TEM,HREM,XRD,BET and a variety of electrochemical test techniques.On the basis of above investigation,the relationship between the morphology,structure and electrochemical properties at high rate was analyzed,and the effecting reasons of porosity and length/diameter ratio on the high-rate performance of carbon materials were elucidated,which lays a foundation for the design of negative materials with favorable high-rate performance for lithium ion batteries in the future.And then the novel tin and tin oxides/carbon composites were prepared using the carbon materials with both high electronic conductivity and high porosity as the matrix of tin and tin oxides.The morphological and structural characteristics of the tin and tin oxides/carbon composites were studied by FE-SEM,XRD,EDX,XPS measurements and the electrochemical properties of the composites were investigated via galvanostatic charge and discharge,cyclic voltammetry and electrochemical impedance spectroscopy tests.The relationship between the synthesis condition,morphology,structure of composites and their electrochemical properties was analyzed.Finally,the novel composites with higher reversible capacity and better cycle performance were successfully prepared through optimizing the synthesis condition.It had great academic and practical significances for the broadening of new carbon materials and the promotion of the development of negative materials with high capacity for lithium ion batteries.The results showed that the effect of porosity on the high rate performance of EMCMB was carried out through the variation of soakage amount of electrolyte in electrode materials,which changed the amount of the solid/liquid interface and significantly influenced the electrochemical activity of lithium ion at electrode materials/electrolyte interface;while the effect of length/diameter aspect ratio on the high rate performance of CNTs was achieved by changing the diffusion path and velocity of lithium ions in the materials.At optimized condition,EMCMB and CNTs with lower length/diameter ratio could deliver the reversible capacities of 260 and 170 mAh·g-1,respectively,and both showed excellent cycle performance at the higher current density of 0.8 mA·cm-2.When the novel tin and tin oxides/carbon composites were prepared using the carbon materials(EMCMB,EG and CNTs spheres)with both high electronic conductivity and high porosity as the matrix of tin and tin oxides,the type of matrix,synthesis method,synthesis condition and the content of tin in composites had significant influence on the morphology, structure and electrochemical properties of composites.At optimized condition,the tin and tin oxides/EMCMB composite had the reversible capacity of 401 mAh·g-1and capacity retention of 94%over 30 cycles,and also showed higher reversible capacity and stable cycle performance at higher current density.Similarly,the tin oxide/EG composite delivered the reversible capacity of 350 mAh·g-1and almost didn't show capacity loss over the same cycles.The tin dioxide/CNTs sphere composite prepared by in-situ method showed the reversible capacity as high as 669 mAh·g-1. Whereas the tin dioxide/CNTs sphere composite prepared by impregnation and carbonization treatment had the capacity retention of 98%over 30 cycles.All above results indicated that EMCMB,EG and CNTs spheres are the suitable matrix of tin and tin oxides because they not only could ensure the high electronic conductivity of composites,but also could efficiently buffer the specific volume change of tin during charge and discharge process.
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