Studies On Tin Oxides-based Composites As Anode Materials For Lithium Ion Batteries

With the development of the portable devices, electric vehicle (EV) and hybrid electric vehicle (HEV), there is a demand for higher gravimetric and volumetric capacity electrode materials for lithium ion batteries. Carbonaceous material is a major material for the negative electrode in the rechargeable lithium ion batteries, but it cannot meet the demand of the next generation lithium ion batteries due to the low theoretical capacity (372 mAh·g^-1). It is significant to develop new type negative electrode materials of high performance.The investigation and development of the lithium ion batteries and their electrode materials are systematically reviewed in this paper. Based on the specialities that SnO₂ has high theoretical capacity, but the formation of irrevisable Li₂O during the first charge cycle cases a high initial irreversible capacities of SnO₂, and the transition metal can catalyse the decomposition of Li₂O, this paper focuses on preparing SnO₂-Ni and SnO₂-Cu composites by mechanochemistry milling nano-sized SnO₂ with Ni and Cu, respectively, in the aims of reducing the initial irreversible capacity of SnO₂ and developing them to be used as negative electrode materials. SnO₂/C, SnO₂-Sn-C and SnO₂/C-CNTs composites were also synthesized in this paper by a sol-gel method by using SnCl₄ as precursor and citric acid as chelating reagent and carbon source. Different sintering atmospheres of air, N₂ and N₂+20%H₂ was applied and carbon nanotubes addition was used in the SnO₂/C-CNTs.The microstructure the synthesized SnO₂ based composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The relationships of the fabrication parameters, the microstructures and the electrochemical properties including the initial coulombic efficiency, cycle stability and specific capacity of the SnO₂ based composites electrode are systematically investigated. The electrochemical properties of the anodes were measured by galvanostatic charge-discharge tests and cyclic voltammetric (CV). The mechanism of the improvement of the charge and discharge properties of the prepared new SnO₂ based composites electrode is also discussed.The results show that electrochemical performance of SnO₂-Ni and SnO₂-Cu composites was greatly improved compared with the initial nano-sized SnO₂. Combining the optimization of the fabrication parameters of the milling time and theamount of the addition of Ni and Cu, the initial coulombic efficiency of SnO₂-Ni and SnO₂-Cu increases from 55% to nearly 80%.With the increase of the milling time, the contact of Ni and SnO₂ increased, causing an effective action of Ni on the decomposition of Li₂O, and increasing initial coulombic efficiency of SnO₂. After 20 hours of milling, Ni and SnO₂ with a mass ratio of 1 exhibit a highest value of initial coulombic efficiency. Further increasing the milling time cause a serious reaction of Ni and SnO₂ forming more amount of Ni-based Sn solution and reducing the fraction of SnO₂, then decreases the initial coulombic efficiency of SnO₂-Ni composites. Small amount of addition of Ni can obviously increase the initial coulombic efficiency of SnO₂-Ni, however, too amount of Ni also means low fraction of SnO₂, and also lead to a considerable amount of Ni-based Sn solution, which also decreases the initial coulombic efficiency of SnO₂-Ni electrode. The similar results were observed for the Cu-SnO₂ composites.With the increase of the milling time during the preparation process of SnO₂-Ni composite, Ni can effectively suppress the volume expansion of SnO₂ during cycling, favoring the cycle performance of SnO₂-Ni . For the SnO₂-Ni sample in a mess ratio of 1 prepared by 120 hours of milling, the capacity retention reached 60% after 50 cycles. Furthermore, with the increase of Ni content, the cycle performance of SnO₂-Ni was also improved. However, Ni is inactive to lithium ion, therefore the increase of content of Ni will cause the decrease of the specific capacity.The study of the microstructure and electrochemical properties of the SnO₂/C, SnO₂-Sn-C and SnO₂/C-CNTs composites prepared by sol-gol method shows that the non-active or low active phases of carbon can effectively suppress the volume expansion of SnO₂ during charge and discharge cycling, thus improving the cycle stability of SnO₂ anode. The SnO₂/C sample sintering in N₂ atmosphere was amorphous, and its cycle performance was better than that sintering under air atmosphere, which has a crystal structure for the SnO₂. The pH value of the precursor varying from 2 to 8 and the sintering time had no obvious effect on the cycle performance. Sintering temperature of 550 ℃ for the SnO₂/C-based composites is a recommended temperature for getting satisfactory cycle performance than that sintered at other temperature. The carbon content of SnO₂/C increases with increasing the content of citric acid, leading to an improvement of the cycle stability of the SnO₂/C anode due to carbon can reduce the volume expansion of SnO₂ during cycling.The addition of carbon nanotubes into the precursor during sol process was also studied in this thesis. The results show that the carbon nanotube can also accommodate the volume expansion of SnO₂ during cycling and the cycle performance of SnO₂ is greatly improved. Moreover, the carbon nanotube also improves the conductivity of the composite and thus increases the rate capacity of SnO₂.The specific capacity and coulombic efficiency of the SnO₂-Sn-C composites were high and the cycle performance was good comparing to the metallic tin. What's more, the charge and discharge mechanism of was studied in this work.