| Spinel Li4Ti5O12 is developed as a promising alternative anode material not only because of its appealing advantages such as long cycle life, resulted from the negligible difference of lattice parameters during the lithiation/delithiation process, high safety owing to the high Li insertion potential at approximately 1.55 V(vs. Li/Li+), which avoids the formation of SEI, but also benefiting from its well-known advantages of good resistance to overcharge performance, high thermal stability and low toxicity. These characteristics determine its great research value and commercial application prospect. Despite many advantages associated with Li4Ti5O12, low electric conductivity leads to high polarization. What’s more, Li4Ti5O12 has a low energy density and the theoretical specific capacity of Li4Ti5O12 is only half of the graphite materials. This prohibits its use in large-scale applications.In order to overcome these problems, the Li4Ti5O12 anode material was simply synthesized by solid-state reaction with organic lithium salt(lithium acetate) and TiO2 as raw materials. Compared with inorganic lithium sources, organic lithium sources show lower decomposition temperature, indicating that the synthesis route using lithium acetate as lithium source consumes less energy which is beneficial to industrial production and worthy of further investigation. In addition, the effects of different temperature and atmosphere on the electrochemical performance were discussed in detail. The optimum synthetic route is identified under 800 ℃ and N2 atmosphere. The optimized sample with oxygen vacancies shows the initial discharge capacity of 170.7 mAh·g-1 with a capacity retention of 94.6 % after 100 cycles at 1 C. Moreover, the outstanding rate and cycling performance are further demonstrated with a discharge capacity of 143.0 mAh·g-1 and a capacity loss of 16.8 % after 100 cycles at 10 C.Carbon doping has been proved as an effective and feasible way to enhance the electric conductivity of Li4Ti5O12. However, previous studies mainly focused on the synthesis of Li4Ti5O12/C by adding extra carbon sources. In this paper, spinel Li4Ti5O12/C composites were obtained from C3H5O3 Li and TiO2 via one-step solid-state reaction without adding extra carbon sources. In addition, several influence factors on the crystal structures, microstructures and elechochemical properties of Li4Ti5O12/C composites including synthesis temperature and molar ratio of Li/Ti were investigated by thermogravimetric analysis, X-ray diffraction, scanning electron microscopy, cyclic voltammograms and charge-discharge test. As-prepared Li4Ti5O12/C composites at 800℃ with a molar ratio of 4:5 show significant improvement in discharge capacity and high rate cycling performance. At 1 C and 5 C, the initial discharge capacity is 168.0 and 151.0 mAh g-1 with capacity retention of 97.9 and 91.7% after 50 cycles, respectively. Even at 10 C, the discharge capacity is 140.0 mAh g-1 at 1st cycle and 127.3 mAh g-1 at 50 th cycle.Despite high theoretical capacity, SnO2 suffers the disadvantages of low electric conductivity and lithium-ion transfer coefficient. Here, SnO2-based materials comprised of core-shell structure SnO2@C microspheres as host materials mixing with Li4Ti5O12/C composites were prepared via a hydrothermal method and solid-sate reaction. The effects of Li4Ti5O12/C content on the physical and electrochemical performances of SnO2-based materials were systematically studied. The electrochemical impedance spectra of SnO2-based electrodes were performed, and the results demonstrate that the(SnO2/C)@C-5% composites show enhanced cycling performance and rate capability as compared to pure SnO2, which is due to the multi-functional roles of the electronic conductor C and lithium ionic conductor Li4Ti5O12. |
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