Layered Si-C Nanocomposites Synthesized from Layered Silicate As High Performance Anodes for Lithium-Ion Batteries

1. Introduction Lithium Ion Batteries (LIB) are already widely used for small potable electronics (cellular phone, lab top computer, MP3 etc.) as a main energy part due to light weight, good energy storage rate, and long term stability (cyclaility and durability). Existing LIBs use graphite as an anode material. This carbon material has high cycle stability and electrical conductivity. However, the specific lithiation capacity (370 mA h g -1 ) of the graphite can’t satisfy the demands for higher energy density batteries. Silicon (Si) has emerged as a promising material to produce high capacity anodes for LIBs, due to its high theoretical capacity (~4200 mA h g -1 ) which is 10 times higher than that of graphite. However, Si anodes for LIBs have a disadvantage from the large volume change. The expanding to 3-4 times its original size often break the electrical contacts with current collector. Therefore, we focus on making ideal particle size and effective structure of Si. Layered Si particles are prepared by kanemite as a raw material, followed by magnesiothermic reduction. And then, the phenol-formaldehyde liquid resin pyrolyzed carbon coated on the surface of Si particles and inserted into layered Si inner space. In this research, This novel anode active materials exhibit a high reversible specific capacity and good capacity retention. 2. Materials and Method The natural clays with layered Na + silicates are used in constructing the structure of the silicon layers. A layered silicate kanemite (NaHSi 2 O 5 ㆍ3H 2 O) was prepared by dispersing δ-Na 2 SiO 5 (1.0g) in deionized water (100mL) with stirring for 1h. After that, the kanemite was filtrated, washed, and then dried. The kanemite powder (200mg) was mixed with Mg powder (200mg), and calcined for magnesiothermic reduction at 650°C under Ar atmosphere for 5h. 0.1 M of HCl solution remove redundant Mg, MgO and Mg 2 Si. We used HF and ethanol solution for remove remained SiO 2 in the layered silicon particles. 200 mg of synthesized layered silicon particles, 100ml of Phenol-Formaldehyde (PF) liquid and 1ml of HCl were dispersed in the mixture by ultrasonication for 2h. The mixed solution was filtrated with 0.2mm size of membrane filter. During the filtrate process, dispersed silicon thin plates are stacked like bricks. The filter and mixture become phenolic resin structure after heated at 128°C for 12h. Finally, the silicon-phenolic resin solid compound was calcined at 700°C for 5h in Ar atmosphere in order to be layered Si-C nanocomposites. For the characteristic comparison, we prepare commercial original Si nanoparticles (40~60nm). 3. Results and discussion As shown in Fig. 1a,b, the PF resin pyrolized carbon and carbon black showed constant reversible capacity of 170mA h g -1 and 150mA h g -1 respectively. The layered Si and commercial Si particle showed a first reversible capacity of almost 1100mAhg -1 and 1200mAhg -1 in Fig. 1c,d. however, the reversible capacity in both samples were less that 100mAhg -1 after 25 cycles. On the other hand, the reversible capacity on the layered Si-C nanocomposite is still as high as 792mA h g -1 after 25 cycles at a rate of 100mAg -1 as shown in fig. 1e. This means capacity retention is 90% because the first reversible capacity was 875 mA h g -1 . Fig. 1f result indicating that the layered Si-C nanocomposite has an excellent cycling performance. One possibility of the improved performance maybe accounted to the fact that the thin layer structures of nano silicon may have less volumetric changes and restrain the pulverization of the electrode materials during lithium ion insertion and extraction. Another reason for the enhanced behavior could be assigned to the carbon, which can improve the electrical conductivity of the composite and suppress the polarization. 4. Conclusions In summary, layered Si plates were prepared by the means of magnesiothermic reduction of Kanemite. And then, layered Si-C nanocomposites were successfully synthesized by dispersing layered silicon particles in a liquid phenol-formaldehyde followed by a subsequent pyrolysis process. This process enables a strong contact of the silicon layers and carbon sources. The layered Si-C nanocomposites exhibit an initial reversible capacity of 875mAhg -1 and a high retention of 90% after 25cycles at a current density of 100mAg -1 . The highly enhanced electrochemical performance of layered Si-C nanocomposite maybe attributed to the intercalated and coated carbon on the composites which suppress the volume change of Si upon cycling, prevent the pulverization of the electrode, and increase the electrical conductivity of nanocomposite. The extremely thin Si layer (<1nm) will also provide better strength than general Si particles from the volume change, as same function as the Si thin film. Figure 1