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

Abstract 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 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 , and is opposite to the electric field direction when . By analyzing the speed difference and ionic current difference , 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 , 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.