The efficient separation of lithium and sodium ions in aqueous solution through positively charged nanochannels

Abstract

In this study, we employed molecular dynamics simulations to investigate the Li+/Na+ separation performance of positively charged nanochannels in aqueous solutions. For the first time, we uncover the physical mechanism underlying efficient Li+/Na+ separation through positively charged nanochannels at the atomic scale. It is demonstrated that the free energy required for Li+ to enter the nanochannel is significantly lower than that for Na+. This disparity primarily arises from two key factors: First, Na+ must shed more hydration water—including molecules from both its first and second hydration shells—when entering the nanochannel, resulting in a higher potential barrier. Second, the distinct hydration properties of Li+ and Na+ within the nanochannel cause Li+ to preferentially occupy regions near the channel axis, while Na+ tends to localize closer to the channel walls. This spatial distribution leads to stronger electrostatic interactions between Na+ and the positively charged channel, further impeding Na+ entry. As a result of these combined effects, the flow rate of Li+ through the nanochannel can exceed that of Na+ by more than 20 times—representing the highest Li+/Na+ separation factor reported in the field to date. This work provides a robust theoretical foundation for the rational design of nanochannels capable of precise ion separation.