| As a new type of promising anode materials for lithium ion batteries (LIBs), tin dioxide (SnO2) has attracted more and more attention due to its some advantages such as good cycling life and high reversible capacity. However, along with the continuous intercalation-extraction processes of lithium ions, anode material SnO2suffers from a huge volume change (e.g., deformation and crack of the electrode, collapse and pulverization of active materials), and the lithium storage capacity of which decays rapidly during the discharge-charge cycles. This limits its potential application for practical purpose.Recently, literature results show that, for a LIB anode material, its nanofabrication, the shape and size of crystallites and the structural parameters of hierarchical nanostructures should exert a great influence on its electrochemical properties. Therefore, the controlling synthesis of nanostructured SnO2is of crucial importance. And then, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, nitrogen adsorption-desorption, cyclic voltammetry and galvanostatic discharge-charge tests have been used to characterize the structural properties of as-prepared SnO2. In comparison with the bulk material, the acquired porous feature can increase the contact area between SnO2and electrolyte, shorten the diffusion path of lithium ions and then improve the electrochemical performance when applied as a LIB anode; while the hierarchical nanostructures can effectively buffer the huge volume expansion of SnO2during its discharge-charge processes, alleviate the pulverization and then enhance the lithium storage capability of active substances.This dissertation mainly deals with two aspects:the size-controlled synthesis of porous SnO2rods and the preparation of flowerlike SnO2hierarchical nanostructures, shown as below.(1) A facile two-step approach has been used for the synthesis of porous SnO2rods:the initial room-temperature precipitation of precursor SnC2O4and its subsequent thermal decomposition at550℃. As examples, the as-obtained porous SnO2microrods (Sample I, a length-10.0±3.5μm, a diameter-l.1±0.4μm) and submicrorods (Sample II, a length-5.8±1.9μm, a diameter-0.4±0.1μm) are the crystalline mixtures of major tetragonal and minor orthorhombic crystal phases, showing a tetragonal fraction of84.7and87.0%, respectively. When applied as a LIB anode, the porous SnO2submicrorods (specific surface area-13.6m2g-1) can deliver an initial discharge capacity of1730.7mAh g-1with a high Coulombic efficiency of61.6%and show the50th discharge capacity of662.8mAh g-1at160mA g-1within a narrow potential range of10.0mV and2.0V. Similarly, even the anode of porous SnO2microrods (specific surface area-11.8m2g-1) can still exhibit an initial discharge capacity of1661.1mAh g-1at160mA g-1with a Coulombic efficiency of60.9%. Regardless of the polymorphic nature, the acquired porosity may only alleviate the huge volume change of anodes for the first cycle, thus the structural parameters of average size and specific surface area can be feasibly associated with the enhanced lithium storage capability. Anyway, these indicate a facile oxalate precursor method for the controlling synthesis and high performance of rodlike SnO2as LIB anodes.(2) A facile hydrothermal method has been used to prepare flowerlike SnO2hierarchical nanostructures without the assistance of any templates or surfactants. All the nanostructures are composed of two-dimensional nanosheets with smooth surfaces, which are endowed with a specific surface area of46.7m2g-1. As lithium ion battery anodes, flowerlike SnO2hierarchical nanostructures present a high initial Coulombic efficiency of63.4%and give the40th reversible discharge capacity of771.9mAh g-1at a current density of160mA g-1. Therefore, the180℃heat-treatment of aqueous mixture of tin dichloride and sodium hydroxide facilitates the mass production of nanostructured SnO2for its potential application as LIB anodes. |
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