Electrodeposited Cu/MWCNT composite-film: a potential current collector of silicon-based negative-electrodes for Li-Ion batteries†
With the aim of developing the potential high theoretical capacity of Si as a negative electrode material for Li-ion batteries, a new type of composite current collector in which multi-walled carbon nanotubes (MWCNTs) are immobilized on a Cu surface was developed using an electroplating technique. For the Si electrode with a flat-Cu substrate, voltage plateaus related to the stepwise electrochemical lithiation were observed below 0.27 V (vs. Li/Li+), whereas the Cu/MWCNT substrate distinctly decreased the overvoltage to enhance charge/discharge capacities to approximately 1.6 times that obtained in the flat-Cu system. Field-emission scanning microscopy revealed that MWCNTs immobilized on the Cu surface extended inside the active material layer. Adhesion strength between the substrate and electrode mixture layer was reinforced by MWCNTs to increase the reversibility of change in electrode thickness before and after cycling: the expansion ratio was 200% and 134% for flat-Cu and Cu/MWCNT systems, respectively. Electrochemical impedance analysis demonstrated that MWCNTs served as an electron conduction pathway inside the electrode. By controlling the upper cutoff voltage from 2.0 V to 0.5 V, synergetic effects including improved adhesion strength and a more developed conduction pathway became noticeable: a reversible capacity of 1100 mA h g−1 with 64% capacity retention was achieved even after the 100th cycle. The results indicate that the Cu/MWCNT is a promising current collector for expansion/contraction-type active materials for rechargeable batteries.