Asymmetric transport and anomalous diffusion of ions in charged narrow carbon nanotubes

In this letter, we study the transport behavior and anomalous diffusion of ions (Li+, Na+ and K+) inside narrow charged carbon nanotubes under an external electric field by using molecular dynamics simulations. Mean square displacement (MSD) with a power-law fitting parameter α, directional motion speed vu and ionic current I of ions are calculated. It is found that Na+ and K+ show a superdiffusion behavior with α > 1 and a subdiffusion behavior with α < 1 at different surface charge Q, while only the superdiffusion behavior is observed for Li+ in spite of the values of Q. It is found that the ions exhibit directional motion subject to the external electric field, where the direction is identical to the external electric field direction for surface charge Q > 0, and is opposite to the electric field direction when Q < 0. By analyzing the speed difference Δ|vu| and ionic current difference Δ|I|, the asymmetric profiles are observed for the positive and negative surface charges with the same magnitude |Q|. Meanwhile, by calculating the average number of hydrogen bonds of water molecules (Nh), it is found that the ions could enhance the interaction between water molecules and lead to an increase of hydrogen bonds. Furthermore, compared with Li+ and Na+, K+ exhibits an asymmetric radial distribution that implies its advantageous small solvated radius in the design of potassium-ion batteries. Our results provide a general picture for the mediation role of surface charge to the transport behaviors of ions in charged nanochannels.