Folded graphene film-based electrodes for energy storage in rechargeable batteries

Rechargeable batteries are widely used today for portable electronics and grid-scale storage. Even though considerable efforts have been made to develop electrode materials and structures with superior performance, electrode configuration in most cases has remained the same. The electrode preparation process involves loading composite materials on the current metal collector resulting to the inactive components such as binders and conductive carbon which significantly lower the energy densities of the batteries.

Although several electrodes have been developed over the past few years, not all of them have achieved the desired superior performance and energy storage of the batteries. For example, freestanding 3D battery electrodes, constructed by integrating active materials into carbon-based materials matrix like graphene, are associated with high porosity, fast ion diffusion and poor volumetric performance due to the low tap density. Therefore, the development of new types of 3D battery electrodes to ensure good electron/ion transport, robustness and compactness among others is on the rise.

Researchers at the Center for Multidimensional Carbon Materials (an Institute for Basic Science Center) and the Ulsan National Institute of Science and Technology led by Professors Rod Ruoff and Soojin Park developed a folded graphene composite electrode with high areal energy density. The electrodes were made from graphene-based films and by a folding process that was facilitated by water. They used scanning electron microscopy to characterize the cross-sectional structures and surfaces of the films. Their research work is currently published in ACS Nano.

The research team observed that folding resulted to an improvement in the ionic and electronic transport kinetics and enhancement in the volumetric capacity of the electrode during cycling. This was attributed to the fact that folding ensured a continuous path for electron transport and introduced more channels for ion diffusion in the folded regions. Furthermore, these ‘fold electrodes’ exhibited stable cycling over 500 cycles at 1.70 mA cm_-2 and enhanced rate capability as compared to thick electrodes without folds but the same mass loading. The cycling stability depends on the structural stabilities of the particles and the electrodes.

According to the authors, the excellent performance obtained during the experiments was as a result of the fold structures that provided for built-in pores for electrolyte infiltration and accommodation of volume expansion and contraction, as well as the additional flow paths for electrons. The full assembly of a cell bearing a folded electrode could produce a relatively good areal capacity (up to 2.84 mAh cm^-2) even after 300 cycles. This results in an improvement in the energy densities. The difference in the morphological structures also resulted in the difference in the electrode swelling. For instance, lack of buffering spaces in a control electrode of closely stacked layers lead to a high swelling, in contrast to the folded electrode that showed limited volume change due to the folds and gaps present between the layers.

Additionally, using freestanding electrodes such as this can result in the realization of high energy densities without having to incorporate binders, conductive carbons, and current collectors. Therefore, the folded electrode is an excellent improvement of the conventional traditional thick graphene composite electrodes. Since the folding method is versatile and can be used in other electrodes materials irrespective of the type of the active materials, it will help in advancing the development of rechargeable batteries with superior performances and energy storage.