5.3 Hierarchical 3D M
Figure 6.7. Cycling performance (a) and corresponding coulombic efficiency profiles (b) of the SnO2, SnO2-C, and Sn-C samples at a rate of C/5 between voltage limits of 0.02 and 3.0 V . 6.5.3 Hierarchical 3D Mixed Conducting Networks Hierarchical 3D mixed conducting network consists of transport channels for both electrons and Li ions, which has shown an exceeding electrochemical performance compared with other electrode materials through enhancing the electronic conductivity of the material. An example of this structure involves with the above anatase TiO2 sub-micrometer spheres (see Fig. 6.5), the performance of which becomes poor at high current rates. In order to obtain anode materials for high-energy and high-power lithium-ion batteries, an optimization has to be made on the structure design of electrode materials, and the introduction of hierarchical 3D mixed conducting networks on nano/micron scale can be an effective way to carry out such an optimization. For example, with said mesoporous TiO2 spheres as the starting material, TiO2:RuO2nanocomposite can be synthesized through RuO2 coating, and
the prepared nanocomposite shows superior high rate capability when used as anode material for lithium-ion batteries . As can be seen in Fig. 6.8, the nanoscopic network structure is composed of a dense net of metalized mesopores that allow both Li+ and e-to migrate; thereby, the effective diffusion length is reduced. Moreover, this network with mesh size of about 10 nm can be overlapped by another net with similar structure on the micron-scale formed by the composite of the mesoporous particles and the conductive admixture. Besides, RuO2 or LixRuO2 formed during Li insertion also allows for quick Li permeation, and this can be proved by the fact that at a very high charge/discharge rate of 30 C, the specific charge capacity of the composite is 91 mAh g-1, nine times larger than that of the original mesoporous TiO2 spheres (10 mAh g-1).