Lithium ion-selective membrane with 2D subnanometer channels

Abstract

In the last two years, the rapidly rising demand for lithium has exceeded supply, resulting in a sharp increase in the price of the metal. Conventional electric driven membrane processes can separate Li⁺ from divalent cations, but there is virtually no commercial membrane that can efficiently and selectively extract Li⁺ from a solution containing chemically similar ions such as Na⁺ and K⁺. Here, we show that the different movement behavior of Li⁺ ion within the sub-nanometre channel leads to Li⁺ ion-selectivity and high transport rate. Using inexpensive negatively charged 2D subnanometer hydrous phyllosilicate channels with interlayer space of 0.43 nm in a membrane-like morphology, we observed that for an interlayer spacing of below 1 nm, Li⁺ ions move along the length of the channel by jumping between its two walls. However, for above 1 nm spacing, the ions used only one channel wall to jump and travel. Molecular dynamic (MD) simulation also revealed that ions within the nanochannel exhibit acceleration-deceleration behavior. Experimental results showed that the nanochannels could selectively transport monovalent ions of Li⁺> Na⁺> and K⁺ while excluding other ions such as Cl^- and Ca²⁺, with the selectivity ratios of 1.26, 1.59 and 1.36 for Li⁺/Na⁺, Li⁺/K^+, and Na⁺/K⁺ respectively, which far exceed the mobility ratios in traditional porous ion exchange membranes. The findings of this work provide researchers with not only a new understanding of ions movement behavior within subnanometer confined areas but also make a platform for the future design of ion-selective membranes.

Keywords: Li ion selective membrane; Lithium extraction; Subnanometer channels; Two-dimensional materials; Vermiculite.