| The development of electric vehicles in the world has been paid close attention to face the severe challenges of energy crisis and environmental pollution. In the current development of the anode materials, SnO2is regarded as a potential next-generation anode material of power lithium ion batteries for electric vehicles, due to its advantages of low lithium inserted potential, high capacity and environmental friendliness. However, its commercial application has been limited by its poor cycle life and high current charge-discharge performance. So quantum chemical calculations for lithium intercalation behaviors were carried out in this paper, which can be more intuitive understanding the internal of dramatic volume expansion in SnO2electrodes. Based on the calculated conclusions, doped SnO2materials, special morphology SnO2materials and special morphology SnO2-based composite materials were prepared to improve the electrochemical properties. The impact mechanisms were studied to further guide the design and synthesis of high performance SnO2-based materials.The geometric and electronic structures of crystalline LixSn formed during lithiation of SnO2anode material were investigated by first-principle calculations. SnO2material is reduced to the elemental Sn, in processes of the phase transformation of crystalline Sn to crystalline LixSn with increased concentration of lithium intercalation, and the results of quantum chemical calculation showed that crystalline LixSn exhibits metallic conductivity. With the concentration of lithium insertion increasing, the electrons near the Fermi level in DOS, N(EF), display fluctuant values, declaring that the electric conductivity is different for various Li intercalation compounds. This phenomenon results from the differences of hybrid orbital caused by diverse crystal structures. The value of N at the Fermi level (EF) of Sn, Li2Sn5, LiSn and Li22Sn5are1.01,5.18,3.71and24.26states/eV, respectively. The high conductivity of lithium intercalation phases is beneficial to the formation of stabilized Solid Electrolyte Interphase (SEI) films and electron transfer, resulting in improved cycling performance. However, the volume expansion coefficient of Li22Sn5is maximum (343%) by calculating the geometry of LixSn. Thus, high conductivity and large volume expansion of LixSn alloy compounds is a pair of conflicting constraints for improving their cycling performance.Based on the analysis conclusion of lithium intercalation behavior, taking into account the electronic conductivity, the impacts of Ni and W dopants on the electrochemical properties of Ni-doped SnO2and W-doped SnO2were researched. SnO2grains were decreased and the conductivities of SnO2were improved after doping with Ni, W, and the effect of W-doped SnO2is more obvious than that of Ni-doped SnO2. At the early stage of the cycle, the impact was less obvious when the integrity of electrode was maintained well; when the electrode has more cracks and severe pulverization due to huge volume expansion at later period of the cycle, high conductivity electrodes are better able to maintain their electronic conduction, thus extending the cycle life. The portion reversible phenomenon of intermediate Li2O catalytic decomposition reaction was confirmed by Mott-Schottky testing investigating SnO2state during lithium de-intercalation cycling.Hydrothermal synthesis process of special morphology SnO2microspheres was optimized to prepare SnO2solid microspheres, SnO2hollow microspheres and partial core-shell structure SnO2hollow microspheres. The electrochemical properties of these morphology SnO2microspheres were studied to reveal the factors that affect the cycle performance of the electrodes. The study found that the three prepared SnO2microspheres exhibit different cycle performances. The initial discharge capacity is842.1mA h g1for SnO2solid microspheres,996.5mA h g1for SnO2hollow microspheres, and688.8mA h g1for partial core-shell structure SnO2hollow microspheres. The retained specific capacity for each sample is386.5mA h g1up to48cycles for SnO2solid microspheres,406.5mA h g1up to64cycles for SnO2hollow microspheres, and374.2mA h g1up to100cycles for partial core-shell structure SnO2hollow microspheres. These results suggested that the hollow structured materials have advantages for the anode cycle performance. Partial core-shell structure SnO2hollow microspheres exhibited the most excellent cycle performance. The superior stability of partial core-shell structure SnO2hollow microspheres can be attributed to the formation of smaller SnO2hollow microspheres and incomplete core-shell structure. The interior microcavities of core-shell structured materials are capable of accommodating large volume change. Simultaneously, the mutual support between core and shell heightens structural stability during Li insertion-extraction. In addition, the charging voltage platform of sample with heat treatment is higher (about0.05V) than that without heat treatment; the discharge voltage of sample with heat treatment is lower than that without heat treatment. The increased charging voltage and reduced discharge voltage expanded effective voltage range of lithium de-intercalation, facilitated the lithium de-intercalation reaction of electrode material and increased the lithium de-intercalation capacity of the electrode.Different SnO2-based hollow core-shell structure composite materials were prepared via hydrothermal-impregnation method and hydrothermal-polymerization method based on SnO2hollow microspheres. In the case of hollow SnO2/B2O3core-shell composites, the cycle performance has been greatly improved, especially for B2.1wt%electrode, still maintaining622.7mA h g1of discharge capacity at the160th cycle; at a rate of5C (3900mA g1), the specific capacity of B2.1wt%electrode is above528.6mA h g1. Apart from the inner hollow space that is able to mitigate the enormous volume change to some degree, the much improved cycle performance can be attributed to the inactive B2O3buffer layer, accommodating the enormous volume change during continuous cycling and keeping the nanoparticles from agglomeration during the charge-discharge process. The enhanced rate performance is ascribed to the electron-deficient nature of boron, which reduced the Rct, indicating enhanced ionic conductivity in the nanocomposite. The retained specific capacity is448.4mA h g1in the100th cycle for the hollow SnO2/21wt%PPy core-shell nanocomposite anode;647.8mA h g1in the100th cycle for the hollow SnO2/17wt%rGO/21wt%PPy ternary core-shell nanocomposite anode. At a current density of3900mA g1(5C), the ternary composite material still exhibited a reversible capacity above117.6mA h g1. The diffusion coefficient of Li+ions (DLi) was calculated from a linear relationship between Ip and v1/2according to CV curves. The Li+diffusion coefficients in hollow SnO2/20wt%B2O3core-shell composites, hollow SnO2/21wt%PPy core-shell nanocomposites and hollow SnO2/17wt%rGO/21wt%PPy ternary core-shell nanocomposites were calculated to be4.5×108cm2s1,7.4×109cm2s1and1.8×10-8cm2s1, which were all larger than that of SnO2hollow microspheres (1.2×109cm2s1). These data explain the reason of excellent rate capability and electrochemical performance, providing guidance and reference for SnO2practical application as anode materials. |
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