In 1991, Iijima, a Japanese professor and Nobel laureate, discovered nano-scale carbon multilayer tubes – carbon nanotubes in the product of evaporating graphite electrodes with vacuum arcs. The discovery of carbon nanotubes has attracted extensive attention from researchers in the field of lithium batteries. Studies have shown that when carbon nanotubes are used as anode materials in lithium-ion batteries, their charge and discharge capacity can exceed the theoretical capacity of graphite lithium intercalation compounds. The same nanosized metal oxide also exhibits a higher theoretical capacity than existing carbon materials (372 mA·h/g). Therefore, the application of nanosized metal oxide electrode materials in lithium-ion batteries has become a research hotspot. The researchers began to prepare metal oxide nanomaterials with special morphologies. These nanomaterials with special morphology not only have a large specific surface area, increase the contact between the active material and the electrolyte, increase the diffusion rate of lithium ions, and improve the electrochemical performance of the material at high rates of charge and discharge, but also can effectively Slow down the expansion and pulverization of electrode materials during the cycle, control capacity decay, and improve cycle stability. Among many nanomaterials with special morphology, hollow structures have obvious advantages, they can effectively alleviate the volume effect of metal oxides in the process of lithium ion deintercalation.
The research group of Professor Lou Xiongwen of the National University of Singapore has carried out a lot of work on the construction of metal oxide hollow nanostructures, and has achieved fruitful results. For example, they used Cu2O nanocubes as templates and prepared SnO2 nano-hollow cubes by adding SnCl4 to etch the template. Using it as a negative electrode material for Li-ion batteries, SnO2 nano-hollow cubes still have a capacity of 570 mA·h/g at a current density of 156 mA/g, even in the low voltage range (0.01-2.0 V) for 40 cycles. The high specific capacity is attributed to the hollow structure of the nanomaterials, which effectively shortens the transport distance of Li ions, relieves the stress generated during the charging and discharging process, and improves its structural stability. In addition, they also prepared Fe2O3 hollow nanospheres by quasi-emulsion template method. Compared with solid Fe2O3 nanospheres, the Fe2O3 hollow nanospheres exhibited enhanced performance, and the specific capacity of Fe2O3 hollow nanospheres remained 710 mA•h/g after 100 cycles at a current density of 200 mA/g. Clearly, the hollow nanostructures exhibit more superior performance.
Similarly, they used copper nanowires as templates to prepare Fe2O3 nanotubes by adding FeCI3 and then etching the template. As shown in Figure 2, the diameter of the nanotube is about 60 nm, and many tiny nanoparticles can be observed on the surface, which is a typical nanotube-like structure composed of nanoparticles. Electrochemical performance tests show that the electrode material can maintain 100% of the initial capacity after 50 cycles at a current density of 500 mA/g, showing particularly excellent cycle stability. The well-designed porous nanotubes are beneficial to the transport of lithium ions and electrons, and can effectively suppress the volume effect caused by lithium deintercalation, thereby achieving ideal electrochemical performance.
Limin Qi et al. used one-dimensional SiO2 mesoporous materials as templates to synthesize SnO2 nanotubes. As shown in Figure 3 , the diameter of the nanotube is 150-250 nm, and the tube wall thickness is 15-20 nm. The electrochemical performance test shows that the electrode material exhibits a much higher initial capacity (1849mA·h/g and 1724mA·h) than SnO2 nanoparticles in the voltage range of 0.05~1.5V at a current density of 100mA/g. /g). The well-designed porous nanotube has a high specific surface area, which is beneficial to the transport of lithium ions and electrons, and can effectively suppress the volume effect caused by lithium deintercalation, thereby achieving ideal electrochemical performance.
In addition to the excellent electrochemical performance of the hollow structure, the disadvantage of metal oxides can also be overcome by forming a special structure through the composite of two oxides. The composite electrode material can not only effectively suppress the volume effect caused by lithium deintercalation due to its special structure, but also perfectly exhibit the excellent electrochemical performance of the composite components due to the synergistic effect of the two.
Hong Jin Fan et al. combined chemical vapor deposition and hydrothermal process to prepare Fe2O3/SnO2 heteronanostructures, which can controllably tune the composition of the composites. As shown in Figure 4, the composite has a dendritic hetero-nanostructure with six-order symmetry. Electrochemical performance tests show that the composite exhibits lower first-time irreversible capacity and enhanced cycle life relative to the two single oxides, and the improved performance stems from the synergistic effect between the two oxides and the dendritic structure larger specific surface area.
The V2O5/SnO2 core-shell nanowires were prepared by a combined chemical vapor deposition and high temperature pyrolysis method by Pooi See Lee et al. As shown in Figure 5, the composite has a core-shell structure with a diameter of about 100 nm. Electrochemical performance tests show that the composite exhibits high power density (60kW/kg) and high energy density (282W•h/kg). Such excellent performance is attributed to the core-shell nanowires. The thin V2O5 shell structure facilitates the deintercalation of lithium ions, while the SnO2 core provides a fast conduction path for electron transfer.
In summary, the construction of nanostructures with special morphologies and the preparation of heterogeneous nanostructures with different components play an important role in improving the performance of lithium-ion battery electrode materials. Therefore, the construction of nanomaterials with the above characteristics is of great practical significance for improving the performance of Li-ion batteries.