Electrochemical Preparation of CNTs-Coated Cu Substrate for Si-O-C Composite Deposition and Characteristics of Si-O-C/CNTs/Cu As an Anode of Li Secondary Batteries

A silicon anode for Li secondary batteries has a critical problem with volume change up to 400 % during lithiation/delithiation process, causing cracks on surface of active material and formation of new solid electrolyte interphase (SEI) on the cracked surface. For this reason, stable cycle ability of battery is decreased. To solve the serious problem, in our previous studies, a novel Si-O-C composite prepared by electrodeposition from an organic solvent was reported for anode of Li secondary batteries [1-3]. In this research, CNTs were deposited on a Cu substrate by electrophoretic deposition (EPD) with cobalt (II) chloride at high voltage. The deposited CNTs on the Cu substrate is supposed to be as 3-dimension (3D) layer inside of the Si-O-C composite and it can reduce the internal resistance of the Si-O-C composite and increase the adhesion between Si-O-C with the Cu substrate because of its high electric conductivity. This research introduces the Si-O-C composite deposited on the CNTs/Cu substrate (Si-O-C/CNTs/Cu) with outstanding delithiation capacity compared to the Si-O-C composite on the conventional Cu substrate (Si-O-C/Cu composite).

Figure 1 shows the schematic figure of Si-O-C composite deposition process on the CNTs/Cu substrate. In Fig. 1a, 2D-surface morphology of the conventional Cu substrate is obviously indicated. However, it is observed that the CNTs were homogeneously deposited on the Cu substrate forming 3D-structured morphology as shown in Fig. 1b. The Si-O-C composite was deposited on the 3D-structured CNT layer with good uniformity in Fig. 1c. The amount of Si deposited on the CNTs/Cu substrate increases in 33% compared to that of conventional Cu substrate. It is supposed that the electrodeposited Si-O-C composite were filled inside of CNT layer because of increased adhesion between Si-O-C with Cu substrate (see schematic figure in Fig. 1d).

Electrochemical properties were examined with cycle ability for 100 cycles at 0.1 C-rate in Fig. 2. The discharge capacity of Si-O-C/CNTs/Cu delivered 2463.2 and 1340.6 mAh g^-1 _{of Si} at the 1st and 100th cycle, which was higher than that of Si-O-C/Cu (1340.9 and 840.0 mAh g^-1 _{of Si} at the 1st and the 100th cycle, respectively), as shown in Fig. 2. Coulombic efficiencies of the Si-O-C/CNTs/Cu and the Si-O-C/Cu anodes were compared. The coulombic efficiencies of both anodes at 1st cycle were decreased under the 24.1% because of formation of SEI layer on anode. However, after 2nd cycle the coulombic efficiencies were increased dramatically. Both anodes achieved high coulombic efficiencies over 86.9 and 99.7% at the 2nd and 100th of the cycle test. In addition, after 10th cycle, Si-O-C/CNTs/Cu and Si-O-C/Cu anode show the stable coulombic efficiencies over 99.8%. It is obviously indicates that the Si-O-C/CNTs/Cu was performed as anode with good cycle ability and high coulombic efficiencies for Li secondary batteries.