Research On Mesoporous Si/mwnt Composite Anode Materials For Lithium-ion Battery

Commercial graphite anode material of lithium-ion battery has reached to the theoretical lithium storage capacity(372 mAh/g). It is not worth developing and applying for the graphite anode material. Therefore, it is a hotspot to develop new anode materials with high lithium storage capacity. As the material with highest lithium storage capacity(4200 mAh/g), silicon is a material which is used in the preparation of anode materials is restricted by its large volume effect and low efficiency in the charge and discharge process. In order to solve these problems, nanocrystallization and compound were used to improve the electrochemical performance of silicon-based anode materials. In this paper, silicon carbon nanocomposite had been prepared by using in situ grown carbon nanotubes on the surface of porous silicon which we had prepared before. In this way, lithium capacity storage of anode materials was enhanced and cycle stability of silicon materials was improved.Firstly, mesoporous silicon was prepared via magnesiothermic reduction by adopting diatomite as the silicon source under Ar atmosphere. Then, preparation conditions were optimized and characterization mesoporous silicon by Brunaure Emmett Teller(BET), X-ray Diffraction(XRD), Scanning Electron Microscope(SEM). Results showed that, there was no effect whether or not the diatomite was purified before the preparation experiment. 700℃ was the best temperature. It reduced the difficulty because of the vacuum condition was replaced by the Ar protective atmosphere. Results showed that, impurities on the surface of diatomite was completely removed and the pore structure reserved though XRD and SEM observation. After magnesiothermic reduction, specific surface area of porous silicon was improved from 52.9 m2/g to 364 m2/g and pore diameter was increased from 7.8 nm to 16.7 nm.Secondly, silicon and multi-walled carbon nanotube(MWNT) were successfully synthesized by a Chemical Vapor Deposition(CVD) process. SEM, XRD, Thermogravimetric(TG), Infrared Spectroscopy(IR) and BET were employed for characterizations as the morphology of composite material, structure, specific surface area. Results indicated that MWNTs wre synthesised in the surface of the porous silicon. The structure was not destroyed after ultrasonic, because of components combined closely. Specific surface area and pore diameter of composite material were up to 489.1 m2/g and 9.2 nm, respectively. TG analysis showed that carbon content of the composite material was 34 wt%. Results showed that optimum dosage of silicon and catalyst were 0.3 g and 0.15 g, respectively.Finally, electrochemical performance of silicon carbon composites were tested. Discharge and charge specific capacity of composite material were 1724.1 mAh/g and 1529.1 mAh/g in the first cycle process and coulomb efficiency was 88.7%. After 40 cycles processes, level of composite material circulation was decreased. Until 100 cycles processes, it also has 531.4 mAh/g. Composites cycle performance was excellent with high reversible specific capacity. Carbon nanotubes possess excellent conductivity and flexibility which could effectively maintain the conductive network of the composite we prepared, buffer the volume change of silicon and improve the cycle stability.It is expected to make a contribution for the preparation of lithium-ion battery anode materials with diatomite by this paper.