Electrochemical performance of Si thin-film with buckypaper for flexible lithium-ion batteries

Abstract

There exists a high market demand for small, thin, lightweight, and bendable batteries to power flexible, portable electronic equipment. Therefore, it is necessary to develop materials capable of being used in flexible, lightweight, high-capacity, low-cost, and eco-friendly electronic devices. This study aims to replace heavy-duty metal current collectors with lighter, flexible, and highly conductive buckypaper (oxidized multi-walled carbon nanotubes (MWCNTs)) to improve the electrochemical performance of Si and increase the electrode energy density. We prepared chemically modified MWCNTs using a strong oxidizer (1:3 mixture of nitric and sulfuric acid) and extracted the MWCNT thin sheets (or buckypaper) to assemble a free, stand-alone electrode with no binder or current collector. Si was deposited on the buckypaper and a Cu foil by magnetron sputtering to produce Si/buckypaper and Si/Cu electrodes, respectively, and their electrochemical performances were compared. The morphology of the buckypapers and the Si thin film was studied using the field scanning electron microscopy, and the crystal structures of the deposited Si film were investigated by X-ray diffraction. The electrical conductivity of the electrodes was measured using the two probe technique measurement system. The electrochemical performance of the buckypaper, Si/buckypaper, and Si/Cu electrodes was examined by performing the galvanostatic charge-discharge tests and a cycle voltammetry test. The Si/buckypaper electrode showed high discharge and charge capacities of 89 mu Ah and 86 mu Ah, respectively, after 100 cycles corresponding to 97% Coulombic efficiency, which was 9 times higher than that of the Si/Cu electrode. The bending test revealed a battery performance similar to that of a conventional cell, and a sufficiently high electrical conductivity (104 S m-1) was obtained. Therefore, the heavy-metal current collector can be replaced with a light-weight (4 times lighter than Si/Cu electrode), thin, flexible MWCNT thin sheet. This provides an innovative solution for the nextgeneration thin, light-weight, and flexible lithium-ion batteries.