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 J rung/k B = −631(5) K, J leg/k B = −185(5) K for 1, and J rung/k B = −517(4) K, J leg/k B = −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) 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.

Selective Ion Exchange in Supramolecular Channels in the Crystalline State

“…The K + ionic and electronic conductivities at 293 K of 1‐K were estimated to be 3.98(12)×10 −8 S cm −1 and 1.05(1)×10 −7 S cm −1 , respectively, and their activation energies during the heating process were 474 meV and 274 meV, respectively (Figure S9). The K + ionic conductivity of 1‐K is lower than the Li + ionic conductivity of 1 (2.46(6)×10 −7 S cm −1 ), which can be explained by considering the association constants of [18]crown‐6 to alkali metal ions …”

“…Herein, we focused on Li 2 ([18]crown‐6) 3 [Ni(dmit) 2 ] 2 (H 2 O) 4 ( 1 , dmit 2− =2‐thioxo‐1,3‐dithiole‐4,5‐dithiolate) to achieve selective ion transport and exchange in the crystalline state and to change the physical properties of the material through this exchange. Compound 1 has an ion channel in the crystal, which is formed by a one‐dimensional array of [18]crown‐6 molecules.…”

“…Figure shows the temperature dependence of the product of the molar magnetic susceptibility and the temperature ( χ m T ) for 1 and 1‐K . The measurements were carried out using polycrystalline samples under a direct current magnetic field of 5 kOe in the temperature range from 2 to 300 K. We previously reported that [Ni(dmit) 2 ] − molecules formed a ladder structure in 1 , and the magnetic exchange interactions for the ladder rung and leg were estimated to be J rung / k B =−631(5) K and J leg / k B =−185(5) K, respectively . It was suggested that in 1‐K , antiferromagnetic interactions dominate as in 1 , but the magnitude of the magnetic exchange interactions is smaller than in 1 .…”

“…Figure 4s hows the temperature dependence of the product of the molar magnetic susceptibility and the temperature (c m T)f or 1 and 1-K.T he measurements were carried out using polycrystalline samples under ad irect current magnetic field of 5kOe in the temperature range from 2t o 300 K. We previously reported that [Ni(dmit) 2 ] À molecules formed al adder structure in 1,a nd the magnetic exchange interactions for the ladder rung and leg were estimated to be J rung /k B = À631(5) Ka nd J leg /k B = À185(5) K, respectively. [12] It was suggested that in 1-K,a ntiferromagnetic interactions dominate as in 1,but the magnitude of the magnetic exchange interactions is smaller than in 1.A st he amount of Curie impurities increased upon ion exchange,the c m T curve above 20 Kw as reproduced by the S = 1/2 Heisenberg antiferromagnetic spin-ladder model, with the magnetic exchange interactions for the ladder rung and leg determined to be approximately J rung /k B %À43(3) Ka nd J leg /k B %À15(4) K. [17] Thenotable decrease in the magnetic interactions for 1-K was considered to be due to the expansion of the unit cell of 1 upon ion exchange of Li + with K + .T oc onfirm this hypothesis,L eBail analysis was performed by using the powder X-ray diffraction (PXRD) pattern of 1-K measured at the BL02B2 beamline,S Pring-8. [18] Theo btained PXRD pattern allowed the determination of the unit cell of 1-K on the basis of the lattice in which the b and c axes of 1 were doubled ( Figure S6).…”

“…Herein, we focused on Li 2 ([18]crown-6) 3 [Ni(dmit) 2 ] 2 -(H 2 O) 4 (1,d mit 2À = 2-thioxo-1,3-dithiole-4,5-dithiolate) [12] to achieve selective ion transport and exchange in the crystalline state and to change the physical properties of the material through this exchange.Compound 1 has an ion channel in the crystal, which is formed by ao ne-dimensional array of [18]crown-6 molecules.T his channel contains Li + ions and crystalline water molecules,w hich are located on either side of [18]crown-6 along the ion channel (Figure 1a,b). TheL i + ion conduction (transport) through the channel was determined by impedance and solid-state 7 Li magic angle spinning NMR measurements.I na ddition, the [Ni(dmit) 2 ] À molecule, which is present as acounteranion, has an S = 1/2 spin, and is au seful building block for constructing various molecular magnets.…”