Study On Tin And Silicon Based Anode Materials For High Performance Lithium-ion Battery

With the widespread use of mobile electronic devices and increasing demand for electric energy storage in the transportation and energy sectors, lithium ion batteries(LIBs) have become a major research and development focus in these years. The energy and power density of most commercially available LIBs are still severely limited by the electrode materials used. The current generation of LIBs uses graphite as the anode materials, with a theoretical capacity of 372 mAh/g. In comparison, tin and silicon have been considered as the leading anode materials for next generation LIBs, due to their favorable working voltage and unsurpassed theoretical specific capacity. However, overcoming the rapid storage capacity degradation of tin and silicon, due to their large volumetric changes(>200%) upon cycling, remains a major challenge to the successful implementation of such materials.In this paper, we used tin and silicon based nanoparticles as active materials to reduce the volumetric change. Then we took advantages of adding electrical conductive carbon matrix to greatly accommodate drastic volumetric variation, minimize agglomeration, and improve the electrochemical performances of tin and silicon based nanoparticles. As the structure of carbon matrix had a great influence on the electrochemical performances of tin and silicon based anode materials, we also tried some different structure carbon matrixes(such as 1-D profiled carbon fibers, 2-D carbon layer, 3-D graphene hydrogel) and analyzed the influences of structure on electrochemical properties. Then we studied the mechanism and methods to improve stability of electrochemical properties, which can provide references for further researches. The major contents and rusults were as follows:1. 1-D profiled carbon fiber supported Sn and SiOx anode materials were sytheized and analyzed. In the Sn/CF composites, most of the Sn particles, with an average diameter smaller than 1 μm, were imbedded in the surface grooves of profiled carbon fibers(CF). SiOx particles, with an average diameter about 20 nm, showed loose and homogenous distribution on the surface of CF to form core-shell SiOx/CF composites. The grooves provided enough capillary channels for rapid lithium-ion transport and enough inter-fiber space for the accommodation of large volume changes on lithium insertion and extraction. The loose network of active particles also provided space for the volume change. The Sn/CF composites delivered 740 mAh/g reversible capacities in the 160 th cycle(at 0.1C) and remained 350 mAh/g at 2C. The SiOx/CF composites delivered 481 mAh/g reversible capacities in the 160 th cycle(at 250 mA g-1) and remained 370 mAh/g at 500 mA/g.2. 2-D graphite(G) supported SnO2 anode materials were manufactured by coprecipitation method. Using graphite(the commercial anode materials) as the matrix to prepare the SnO2/G composites for LIBs was an effective way to prevent agglomeration and suppress the volume effects, especially with relatively loose SnO2 network. Based on the special synergistic effect between SnO2 and graphite, SnO2/G composites showed an initial specific capacity of 740 mAh/g, and 518 mAh/g could still be retained after 50 cycles.3. 2-D self-standing CNTs/graphene films(GP) were used as self-standing matrix to form Si/CNTs/GP composites, through ultra-sonication and vacuum filtration approach with vapor posttreatment. In Si/CNTs/GP composites, Si and CNTs were uniformly embedded between graphene sheets, resulting in a 2-D conductive network for electrons. The large interlayer space of graphene film provided open channels for Li+ diffusion, as well as accommodated the volumetric changes of Si nanoparticles during lithiation/de-lithiation. The free-standing composite electrodes showed high rate performances(600 mAh/g at 4 A/g) for LIBs.4. 3-D graphene hydrogel(GH) was used as matrix for SnO2 and Si@SiOx anode materials. The graphene hydrogel provided large specific surface areas for efficient loading(71 wt.%), uniformly distribution of nanoparticles, and accommodated large volume changes, while providing large amount of lithium-ion diffusion channels, fast electron transport kinetics, and excellent penetration of electrolyte solutions within their microstructure. SnO2-GH and Si@SiOx/GH composites showed outstanding rate capability and good cyclic stability. SnO2-GH showed 500 mAh/g at 5 A/g and 865 mAh/g at 0.05 A/g after rate cycle test. Si@SiOx/GH delivered 1020 mAh/g at 4 A/g and 1640 mAh/g at 0.1 A/g. This study demonstrated an exciting pathway to the rational design and fabrication of 3-D graphene matrix for application in LIBs and other electrochemical energy storage materials.