Research On The Extraction Of Horsetail Biomass Silicon And Its Application As A Negative Electrode Material For Lithium-ion Batteries

With the rapid development of electric vehicles,consumer electronics and other fields,the conventional lithium-ion battery has been unable to meet the needs of the industry.At present,the commercial large-scale use of graphite anode materials only 372 mAh/g theoretical specific capacity,so the capacity as high as 3580 mAh/g silicon anode material has become a research hotspot.Traditional Si anode materials are mostly made from metallurgical Si.The preparation process is complex,energy consumption is high,and environment is polluted.In order to solve this problem,the Nano-mesoporous biomass SiO₂ was extracted from wild Equisetum horsetail with almost no cost.Then the mesoporous SiO₂ was reduced to mesoporous Si by magnesium thermal reduction.Finally,the Si/C composite structure was prepared by carbon-coated method and applied to the negative electrode of lithium ion batteries.In this paper,the effects of the morphology and structure of SiO₂,the structure of reduction product Si and the composite structure of Si/C on the electrochemical performance of lithium ion batteries were studied.The main results are as follows:（1）The stem of horsetail consists of protective tissue,mechanical tissue and parenchyma from surface to interior.The first two parts contain a lot of Si elements,and the third part contains almost no Si elements.Cylindrical SiO₂ fiber bundles with a diameter of 120 microns were obtained by HCl pickling and calcination at high temperature.The large fibers were closely arranged by SiO₂fibers with a diameter of 10 microns.The purity of SiO₂ was 99%by XRF and the maximum specific surface area was 300 m²/g by BET.The SAXS results show that the SiO₂ particles have multistage structure and the size of the primary particles is about 9 nm.（2）Fiber bundle mesoporous Si material was obtained from SiO₂ extracted by magnesium thermal reduction and acid pickling drying at 700℃.The results of SEM and SAXS measurements show that they have similar morphology and multi-level structure with SiO₂ precursors.BET test results show that the specific surface area of the reduction product reaches 353 m²/g,which is larger than that of the SiO₂ precursor.（3）Four organic compounds were used as precursors to prepare Si/C composite structure by pyrolysis at high temperature.Carbon coating improves the conductivity of the electrode material,reduces the volume expansion of Si material during charging and discharging,and maintains the stability of the battery.（4）The Si/C composite structure was used as the negative electrode of lithium-ion batteries.Under the condition of 500 mA/g current density,the first charge capacity was 2437.4 mAh/g,and the capacity decayed quickly in the previous several times.After 1000 cycles,the capacity was 650mAh/g.（5）In this thesis,mesoporous nanosilica with high purity was extracted by simple impurity removal and calcination process from low-cost wild plant Verbena officinalis,and then the Si/C composite structure was prepared by magnesium thermal reduction and carbon encapsulation.Compared with the traditional preparation method of Si negative electrode,the method adopted in this paper has the advantages of low cost,simple process and environmental friendliness.