Fabrication By Electrodeposition of a New Tin Anode Reinforced with Carbon Nanotubes for Lithium Ion Batteries
Lithium ion batteries using graphite as the anode active material have higher energy densities than conventional rechargeable batteries, but obtaining higher capacity will require use of an alternative anode material. Tin has a higher capacity than graphite (991 mAh g-1 vs. 372 mAh g-1) and was used in the current research as an active material. However, tin slips down from the current collector during charging and discharging due to a large volume change, and the cycling characteristics deteriorate. Anchoring the tin layer to the underlying copper layer with a fibrous material should improve adhesion. We previously demonstrated the excellent charge/discharge characteristics of a new tin anode comprising an electroless tin plating film deposited on a Cu/CNT (carbon nanotube) composite plating film.1)
In this study, we created a new negative electrode structure incorporating CNTs by electrodeposition and explored the correlation between the structure and charge/discharge characteristics of the electrode.
A Cu/CNT composite plating bath (0.85 M CuSO4・5H2O + 0.55 M H2SO4 + CNTs + polyacrylic acid) and a tin plating bath (1 M K4P2O7 + 0.25 M Sn2P2O7 + additive) were prepared. Cu/CNT composite plating film was plated on a pure copper plate, then tin was electrodeposited on the Cu/CNT composite plating film, generating a new CNT-reinforced tin anode structure. For comparison, tin was electrodeposited directly on a pure copper plate. Electroplating was carried out under galvanostatic conditions at 25°C. The Cu/CNT composite plating film on a pure copper plate and a platinum plate were used as the cathode and anode, respectively, for tin plating.
The microstructure of the new tin anode was examined by field-emission scanning electron microscopy (FE-SEM). The phase structures of the deposits were analyzed by X-ray diffraction (XRD).
Electrochemical studies of the tin-based anodes were carried out using coin cells assembled in an Ar-filled glove box. Each coin cell consisted of lithium foil as the counter and reference electrodes and the tin-based anode as the working electrode. The electrolyte was 1 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 vol%). Cycling tests were performed in the range 0.02 - 1.5 V (vs. Li/Li+) at a constant temperature of 25°C.
Results and Discussion
Fig. 1 shows a surface SEM image of the new tin anode and demonstrates that tin particles had deposited on the CNTs. Fig. 2 shows a cross-sectional SEM image of the new tin anode; CNTs are clearly seen between the tin and copper layers, and no defects are observed.
The charge/discharge characteristics of the new tin anode will be discussed at the meeting.