| Si anode materials have received extensive attention of the researchers due to its greatest theoretical specific capacity. Si materials that are used as lithium ion battery anode materials mainly face two problems:huge volume changing during charging/discharging and the poor electrical conductivity. Novel nanostructured (such as hollow nanostructure, core-shell nanostructure) Si anode materials can buffer the huge volume expansion in the process of charging and discharging; the electrical conductivity of Si anode materials can be improved by combining Si materials with carbon or mental.The organisms in nature have all kinds of microstructures and most of the plants contain silicon element. The silicon element mainly exists in the form of silica nanostructures that have various structures, such as sheet, columnar, fiber and so on. In this topic, the bamboo leaves and thatch are selected as the source of Si anode materials. After heat treatment for 3h under 700℃ and air atmosphere, we can get white biological nano-SiO2 from the HCl treated bamboo leaves and thatch. Thermogravimetric analysis shows that the biological nano-SiO2 quality percentage in the bamboo leaves and thatch are 14% and 6.3% respectively. Under argon atmosphere, the biological nano-SiO2 is first treated by magnesiothermic reduction for 3h at 700℃, then after magnesiothermic reduction the sample is treated orderly with HCl, HF, distilled water washing and alcohol washing. After all the steps above, we will get the biological nano-Si anode material. Electrochemical performance test results show that the first charge and discharge specific capacity of the bamboo leaves nano-Si anode is 1636mAh·g-1 and 2337mAh·g-1 respectively at the current density of 100mA·g-1, the initial coulombic efficiency is 70%, but the specific capacity is only 150mAh·g-1 after 100 cycles. Electrochemical performance test results exhibit that Si nanomaterials directly extracted from bamboo leaves is not ideal as lithium-ion battery anode. The first charge and discharge specific capacity of the thatch nano-Si anode is 2114mAh·g-1 and 2851mAh·g-1 respectively at the current density of 100mA·g-1, the initial coulombic efficiency is 74%, but the specific capacity reduces to 200mAh·g-1 after 22 cycles. Electrochemical performance test results exhibit that Si nanomaterials directly extracted from thatch is also not ideal as lithium-ion battery anode.The experimental results show that the cycle stability of biological nano-Si anode materials that directly extract from bamboo leaves and thatch is bad. After further research, we can keep part of the biological carbon during heat treatment on the bamboo. After carbonization at different temperature, we can first get the bamboo leaves nano-SiO2/C composite with different carbon contents. Then we can get the bamboo leaves nano-Si/C anode using magnesiothermic reduction method. Electrochemical performance test results show that the nano-Si/C anode with 500℃ heat treatment has the best electrochemical performance. The first discharge specific capacity is 1206mAh·g-1 at the current density of 100mA·g-1, the specific capacity is 350mAh·g-1 after 100 cycles. After keeping biological carbon, the cycle stability is improved obviously.Different nanostructured SiO2 materials can be prepared by sol-gel method. Firstly, mixing SiO2 sol solution with different concentration of dextrin, then the compound is dried and carbonized after stirring uniformly. After all the steps above, we will get the nano-SiO2/C composites that have different carbon contents. Finally, we can get the nano-Si/C anode using magnesiothermic reduction method. Electrochemical performance test results show that the nano-Si/C anode with 1.5g dextrin has the best electrochemical performance. The first charge and discharge specific capacity is 1176mAh·g-1 and 615mAh·g-1 at the current density of 100mA·g-1, the initial coulombic efficiency is 52%, the specific capacity is 300mAh·g-1 after 100 cycles. |
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