Preparation And Electrochemical Performance Of Si@SiO_x/C Composite As An Anode For Lithium-ion Batteries

Silicon is considered as the most optimal anode material for next-generation lithium-ion batteries because of the highest theoretic capacity among known materials, low alloying/dealloying potentials and abundant resources. However, it experiences a large volume change of about 300% during cycling. The tension resulted from the volume variation has great damage to the structure of electrodes, causing rapid capacity degradation. As an alternative to pure silicon, nano-Si@SiO_x, in which nanosilicon is embedded in SiOx matrix, has been demonstrated to be effective to alleviate the destructive force from the volume change of silicon. Compared with pure silicon, nano-Si@SiO_x shows a relatively small volume change during alloying/dealloying. Besides, Li2 O and lithium silicates generated during the first lithiation of SiOx serve as buffer matrix of silicon in subsequent cycles. Therefore, it becomes a hot topic for researchers in recent years. However, the intrinsically low electrical conductivity and unstable electrode structure of Si@SiO_x severely limit its electrochemical performance as anodes in lithium-ion batteries. To resolve the two problems above, we conduct the following work.Firstly, the Si@SiO_x/C composite was prepared by combining chemical vapor deposition with disproportionation under the protection of argon, using commercial SiO as starting material. In order to obtain better cycle performance, Si@SiO_x/C/graphite composite with double buffer systems was prepared by dispersing the produced Si@SiO_x/C in graphite powder. The composition, morphology and structure of the synthesized composites were characterized by X-ray diffraction(XRD), elemental analysis(EA) and scanning electron microscope(SEM). The electrochemical behavior of the synthesized materials as anodes for lithium-ion batteries was evaluated by constant current(CC) technique, cyclic voltammogram(CV) and electrochemical impedance spectroscopy(EIS). In addition, the influence of sodium alginate, guar gum and polyacrylic acid/sodium carboxymethyl cellulose as binders on the electrochemical properties of Si@SiO_x/C/graphite was examined. The results indicated that the amorphous pyrolytic carbon was evenly coated on the surface of Si@SiO_x particles. At a current density of 100 mA g-1, Si@SiO_x/C/graphite composite presented an initial discharge capacity of 1250 mAh g-1 and an initial charge capacity of 820 mAh g-1 with a first coulombic efficiency of 65%. Si@SiO_x/C/graphite composite exhibited a stable discharge capacity about 742 mAh g-1 and the capacity retention was about 90% after 230 cycles. Excellent electrochemical performance was attributed to three reasons as follows.(1) Ball-milling reduced the particle size of the Si@SiO_x;(2) Rotary tube furnace was helpful for uniform coating of carbon on the surface of Si@SiO_x;(3) Graphite and pyrolytic carbon mitigated the stress of the electrodes together and effectively restrained the volume change of silicon. Compared with guar gum and polyacrylic acid/sodium carboxymethyl cellulose binders, Si@SiO_x/C/graphite composite exhibited a better cycling performance when using sodium alginate as binder, in which a reversible capacity of 416 mAh g-1 was obtained after 500 cycles at a current density of 400 mA g-1, indicating the sodium alginate was more suitable for the binder of silicon-based composite.Secondly, considering the relatively low reversible capacity of commercial SiO, this study designed a new method to produce nano-Si@SiO_x with higher capacity. We combined electrolysis with peeling technique to produce nano-Si@SiO_x from bulk SiO2. XRD, X-ray photoelectron spectra(XPS), SEM and high resolution transmission electron microscope(HRTEM) were employed to characterize the composition, morphology and structure of the prepared materials. Furthermore, we also examined the feasibility of the produced nano-Si@SiO_x as the precursor of high-performance electrode materials. The research showed that produced nano-Si@SiO_x exhibited flaky shape and nano-silicon was embedded in SiOx matrix. At a current density of 100 mA g-1, the nano-Si@SiO_x/graphite composite prepared by using nano-Si@SiO_x as the precursor exhibited a stable discharge capacity of about 1560 mAh g-1 after 70 cycles, which was apparently higher than Si@SiO_x/graphite composite prepared using commercial SiO as the precursor. This work provided a new method to produce nano-Si@SiO_x.Finally, we synthesized Si@SiO_x/C composite via mechanical milling and high-temperature treatment under the protection of argon, using the Si@SiO_x produced by electrolysis and peeling technique above and citric acid as starting materials. The electrochemical performance of the produced Si@SiO_x/C composite was further improved by optimizing pyrolytic temperature and material ratios. The research showed that the Si@SiO_x/C composite was composed of nanosized crystalline Si, amorphous SiOx and carbon, in which Si@SiO_x was evenly coated. Electrochemical measurement revealed that the Si@SiO_x/22.4 wt%C composite prepared at 1000 oC possessed the optimized electrochemical performance. At a current density of 100 mA g-1, it presented an initial charge capacity of 1128 mAh g-1, and a reversible capacity of 1055 mAh g-1 was presented after 150 cycles with a capacity retention of 93%. Compared with the previous reports, the cycle performance and specific capacity of the Si@SiO_x/C composite have been improved. The meliorative electrochemical performance was attributed to the uniform coating of pyrolytic carbon on surface of Si@SiO_x, which improved the conductivity of the electrodes. Meanwhile the pyrolytic carbon with porous structure acted as buffer media of the volume change of silicon.