Molecular Dynamics Simulation of Lithium Extraction Using Two-dimensional Metal-Organic Framework Membranes

Lithium is an important element in energy industry (e.g., lithium batteries). However, natural sources of Li are quite limited. This makes the extraction of Li from various sources an important issue. Particularly, the extraction of Li from saline water has attracted great interest recently. In this work, we investigate the extraction of Li⁺ from solutions containing Li⁺ and Mg²⁺ using two-dimensional Metal-Organic Frameworks (MOFs) through molecular dynamics simulations. By studying different MOFs, it is found that the pore size of MOFs plays an essential role in separating Li⁺ and Mg²⁺. For MOFs with pore sizes smaller than 5.32 Å, Li⁺ can be completely separated from Mg²⁺. However, the pressure is high and the flux of Li⁺ is relatively low. For MOFs with large pores (≥8.29 Å), the flux of Li⁺ is greatly enhanced, but the separation of Li⁺ and Mg²⁺ becomes challenging. MOFs with a pore-limited diameter (PLD) of approximately 6.5 Å demonstrate exceptional separation performance, achieving a separation ratio of 1 while exhibiting a high Li⁺ flux. Additionally, it is found that the shape of the MOFs' pores also influences the separation performance, affecting the distribution of water molecules surrounding the ions within the MOFs' pores. To understand the separation mechanism, the potential of mean force, water distribution, and ion hydration structures under different pressures in various MOFs are analyzed.