Ultraefficient Li⁺/Mg²⁺ separation with MXene/CNT membranes under electric field assistance

Similar physicochemical properties render the separation of Li⁺/Mg²⁺ from salt lake brine difficult for two-dimensional (2D) membranes. Herein, ultraefficient Li⁺/Mg²⁺ separation was achieved under the influence of an electric field using MXene/carbon nanotube (CNT) membranes developed on tubular ceramic substrates via thermal crosslinking. Crosslinking reactions between CNTs and MXene yielded well–intercalated CNTs between MXene nanosheets, forming antiswelling MXene/CNT membranes (MCMs) (interlayer spacing: 8.04 Å) suitable for Li⁺/Mg²⁺ separation. At 2.8 V applied voltage, the MCMs exhibited Li⁺ flux of 0.0491 mol m⁻² h⁻¹ and high Li⁺/Mg²⁺ selectivity of 54.5, which outperform existing state–of–the–art 2D membranes. Specifically, under a positive electric field, Li⁺ with a small hydration diameter and low hydration energy permeated through the membrane nanochannels more easily than Mg²⁺, increasing and decreasing Li⁺ and Mg²⁺ flux, respectively. Additionally, Mg²⁺ retention was enhanced via the complexation of enriched Mg²⁺ with –OH and –COOH on the membrane surface. The MXene membrane surface was thus reconfigured to be positively charged, inducing effective Mg²⁺ rejection. This study offers a novel and practical approach for efficiently separating Li⁺/Mg²⁺ as well as monovalent/multivalent ions.