Energetics of metal ion adsorption on and diffusion through crown ethers: First principles study on two-dimensional electrolyte

Macrocyclic crown ethers (CEs) have tunable cavity sizes and site-selective binding with metal ions, making the CE-ion complex a promising candidate as a two-dimensional (2D) electrolyte. In this work, density functional theory method is used to determine the energetically stable structures of 12-crown-4 ether (CE4) and 15-crown-5 ether (CE5) complexed with four cations: Li+, Na+, Mg2 +, Ca2 +. In addition to the CE-ion binding energies, the diffusion barriers for ion transport through the CE cavities are calculated. Among the complexes investigated, CE5 presents the lowest energy barrier for ion diffusion. The barriers for Li+ travelling through a single CE5 and moving between two CE5s are 0.29 eV and 0.16 eV, respectively. Field-controlled modulation of the diffusion barrier is also demonstrated. By applying a 0.15 V/Å electric field perpendicular to the plane of the CE, the diffusion barrier of Li+ through one CE5 can be reduced from 0.29 to 0.20 eV to facilitate the ion transport.