Impact Of Cation-π Interactions On The Cell Voltage Of Carbon Nanotube-based Li Batteries

Cation–π interactions play important roles in many processes such as molecular recognition, enzyme catalysis, stability of protein structures, drug design, and biomembranes. These interactions generally exist between cations and π-electron-rich carbon-based structures such as aromatic rings, graphene, carbon nanotubes, and fullerenes. It has been widely reported that cation–π interactions greatly affect the properties and applications of these π-electron-rich carbon-based materials, including surface- or interface-charged behaviors, wetting behavior, ion distribution, absorption of organic pollutants, and the adsorption and diffusion dynamics of cations.How to store electrical energy with a high charge and discharge rate, high capacity, high energy density, and high cyclability is one of the key challenges in this century. The development of nanotechnology, particularly the fabrication of nanostructures in the past decades, provides a new approach to improve the properties of energy storage materials. Recently, a CNT-based Li battery has been reported to have a much improved lithium capacity compared with graphite, and is believed to be one of the most promising electrochemical energy storage systems. We note that in CNT-based Li batteries, the main interaction between Li+ and CNT is cation–π interactions. However, the impact of cation–π interactions on CNT-based Li batteries remains unknown.In this paper, based on DFT calculations, we find that cation–π interactions are highly favorable to the cell voltage. In addition, when we consider both Li+–π and Li–π interactions, the cell voltage increases with the increase of the CNT diameter. Moreover, the cell voltage is larger when the Li+ ions adsorbs on the external wall. Compared with Li+ ions in the internal wall of(4,4) CNT, the cell voltage of Li+ ions on the external wall of(10,10) CNT is increased by 0.55 V, which is a 38% improvement. This indicates that by careful design we should be able to greatly improve the energy density of CNT-based Li battery.