Optimizing Lithium Ion Conduction through Crown Ether-Based Cylindrical Channels in [Ni(dmit)2]− Salts

Abstract

The synthesis of artificial ion channels is one of the core areas of biomimetics and is aimed at achieving control over channel functionality by careful design and selection of the constituent components. However, the optimization of ionic conductivity in the channel in the crystalline state is challenging because of crystal strain, polymorphism, and potentially limited stability. In this study, the pore size of cylindrical channels was controlled with the aim of optimizing ionic conductivity. We prepared two isomorphic salts, Li2([18]crown-6)3[Ni(dmit)2]2(H2O)4 (1) and Li2([15]crown-5)3[Ni(dmit)2]2(H2O)2 (2), both of which possess ion channels formed by a one-dimensional array of crown ethers, Li+ ions, and crystalline water molecules. Meanwhile, [Ni(dmit)2]− (S = 1/2) molecules formed a ladder configuration with Jrung/kB = −631(5) K, Jleg/kB = −185(5) K for 1, and Jrung/kB = −517(4) K, Jleg/kB = −109(5) K for 2. For 1, the Li+ ionic conductivity at 293 K in the crystalline state was enhanced from 1.89(18) × 10–8 S·cm–1 to 2.46(6) × 10–7 S·cm–1 via dehydration. Furthermore, analysis of Li+ ionic conductivities of 2, which incorporated a crown ether with a smaller cavity (the cavity diameters of [18]crown-6 and [15]crown-5 are 2.60–3.20 Å and 1.70–2.20 Å, respectively(1)) at the same temperature both before and after dehydration revealed conductivities of 1.93(31) × 10–8 S·cm–1 and 7.01(21) × 10–7 S·cm–1, respectively. This molecular design approach can contribute to increasing the ionic conductivity as well as the development of all-solid-state lithium ion batteries and other electronic device fabrications.