Optimizing Lithium Ion Conduction through Crown Ether-Based

Sep 24, 2018 - †Department of Chemistry, Graduate School of Science, ‡Chirality Research Center (CResCent), and §Institute for Advanced Materials...
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Article Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

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Optimizing Lithium Ion Conduction through Crown Ether-Based Cylindrical Channels in [Ni(dmit)2]− Salts Katsuya Ichihashi,† Daisuke Konno,† Takuya Date,† Takumi Nishimura,† Kseniya Yu. Maryunina,†,‡ Katsuya Inoue,†,‡,§ Toshimi Nakaya,∥ Kazuhiro Toyoda,⊥ Yoko Tatewaki,# Tomoyuki Akutagawa,∇ Takayoshi Nakamura,○ and Sadafumi Nishihara*,†,‡,§

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Department of Chemistry, Graduate School of Science, ‡Chirality Research Center (CResCent), and §Institute for Advanced Materials Research, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan ∥ Department of Applied Chemistry, Graduate School of Engineering & Technical Center, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan ⊥ Faculty of Environmental Earth Science, Hokkaido University, N10W5, Kita-ku, Sapporo 060-0810, Japan # Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei 184-8588, Japan ∇ Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ○ Research Institute for Electronic Science, Hokkaido University, N20W10, Kita-ku, Sapporo 001-0020, Japan S Supporting Information *

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, Li 2 ([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, respectively1) 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.

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INTRODUCTION

used to accomplish chemical modification of the channels. For example, by controlling the hydrophilicity/hydrophobicity of an inner channel, it is possible to realize transmembrane transport of not only ions18−21 but also water molecules22,23 and neutral organic molecules.24,25 Moreover, the transport properties can be regulated by surface charge reversal by manipulating pH-26 or light-responsive27 functional groups present in the channel or charge transfer interactions.28 However, the optimization of ionic conductivity in the channel

Ion channels play a critical role in cell regulation and intercellular communication, exploiting processes such as ion transport, ion exchange, and ion storage.1−3 As these functions and processes can be applied in the fields of electronic devices,4−7 catalysis,8−10 separation technology,4,11,12 and so on, numerous artificial ion channels have been developed with great precision in recent years. The ion channels reported to date include examples based on cyclic polypeptides13,14 and porous coordination polymers,15−17 which have a vast network of metal ions and organic compounds. Artificial ion channels possess a significant advantage namely, the rational design of the constituent molecules can be © XXXX American Chemical Society

Received: July 17, 2018 Revised: September 24, 2018 Published: September 24, 2018 A

DOI: 10.1021/acs.chemmater.8b03027 Chem. Mater. XXXX, XXX, XXX−XXX

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SHELXS-97 and refined via the full-matrix least-squares method on F2 with SHELXL-97.44 All atoms in crystals 1 and 2 were refined anisotropically, except for the hydrogen atoms. The hydrogen atoms of crystalline water molecules could not be determined. The selected crystallographic data of 1 and 2 are listed in Table 1. Further details of

in the crystalline state is challenging because of crystal strain, polymorphism, and potentially limited stability.29 In this study, the molecular design approach was used to investigate the correlation between ionic channel conductivity and its pore size. To this end, supramolecular cations composed of crown ethers and Li+ were employed. These supramolecular complexes can be used in combination with [Ni(dmit)2]δ− (dmit2− = 2-thioxo-1,3-dithiole-4,5-dithiolate)30 to yield various multifunctional materials with electrical (0 < δ < 1) and magnetic (δ = 1, S = 1/2 spin) properties.31−33 In fact, we have successfully developed distinctive materials based on the supramolecular cations and [Ni(dmit)2]− molecules such as spin ladder,34−36 ferromagnetic,37−39 and ferroelectric materials.40−42 Furthermore, we reported that the Li0.6([15]crown-5)[Ni(dmit)2]2(H2O) salt43 exhibits both ionic conduction, mediated by the crown ether−Li+-unit based ion channel, and electronic conduction facilitated by stacks of [Ni(dmit)2]0.3− molecules. Here, two isomorphic salts were employed, namely 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). These salts were composed of a supramolecular cation and monovalent [Ni(dmit)2] molecules (Chart 1), which imparted magnetic properties

Table 1. Crystallographic Data of Salts 1 and 2 Formula Molecular weight/g·mol−1 Crystal color and shape Temperature/K Crystal system Space group (no.) a/Å b/Å c/Å α/deg β/deg γ/deg Volume/Å3 Z Density (calc.)/g·cm−1 Data/parameters Abs coefficient/mm Goodness of fit R1, wR2 (I > σ(I))a R1, wR2 (all data)a

Chart 1. Chemical Structures of (a) Supramolecular Cation (n = 1, 2) and (b) [Ni(dmit)2]− Molecule a

Salt 2 C21H32LiNiO8.5S10 806.77 black plate 293(2) triclinic P1̅ (no. 2) 10.6327(4) 12.6385(5) 13.3500(6) 82.8866(13) 77.4852(12) 86.8376(12) 1737.14(12) 2 1.542 8259/579 1.201 1.077 0.0524, 0.1267 0.0770, 0.1468

R1 = ∑∥Fo| − |Fc∥/∑|Fo|, Rw = [∑w(Fo2 − Fc2)2]/∑[w(Fo2)2]1/2.

the crystal structure investigations can be obtained from The Cambridge Crystallographic Data Centre (CCDC) with reference numbers 1847588 and 1847591 for 1 and 2, respectively. Measurement of Magnetic Properties. The temperature dependence of the magnetic susceptibility was measured with a Quantum Design MPMS-5S superconducting quantum interference device (SQUID) magnetometer using polycrystalline samples contained in gelatin capsules. A direct current (DC) magnetic field of 5 kOe was applied in the temperature range from 2 to 300 K. Measurement of Electrical Properties. The temperature dependence of impedance spectra was measured using sample single crystals with thickness and area dimensions of d = 0.250 mm and S = 0.886 mm2, respectively, for 1 and d = 0.330 mm and S = 0.463 mm2, respectively, for 2. The impedance measurements were conducted by attaching two gold wires (25 μm in diameter) to one electrode on each side of the single-crystal samples using a gold paste supplied by Tokuriki Honten Co., Ltd. The measurements were carried out in a cryostat equipment under vacuum. The impedance spectra were recorded with an Agilent E4980A Precision LCR Meter. An alternating current (AC) electric field of 2 V was applied and the frequency was varied from 20 Hz to 2 MHz. The obtained data were analyzed using ZView (Scribner Associates, Inc.). The temperature dependence of DC conductivity was measured for a single-crystal sample of 1 with dimensions of d = 0.080 mm (thickness) and S = 0.214 mm2 (area). For the DC conductivity measurements, a single gold wire (10 μm in diameter) was attached to one electrode prepared on each side of the single-crystal sample using gold paste. The measurements were carried out in cryostat equipment under vacuum. Data were recorded with a Keithley 6517A electrometer by applying a DC electric field of 0.5 V. All measurements of electrical properties were repeated to ensure accuracy. Solid-State 7Li NMR Studies. Solid-state 7Li magic angle spinning (MAS) NMR spectra were recorded at 233.161 MHz on a Varian 600PS solid NMR spectrometer using a powdered sample of 1 with a 3.2 mm-diameter zirconia rotor at a spinning speed of 5 kHz,

instead of electronic conduction. The insulating properties of the salts were well-suited for detailed investigations of Li+ ionic conductivity in comparison to the Li0.6([15]crown-5)[Ni(dmit)2]2(H2O) salt. Furthermore, these salts are expected to find use as solid electrolytes, as well as in materials coupling Li+ ionic conductivity and [Ni(dmit)2]−-mediated magnetic properties.

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Salt 1 C24H40LiNiO11S10 890.81 black plate 173(2) triclinic P1̅ (no. 2) 11.5264(8) 12.7009(9) 13.7691(9) 82.6384(7) 88.3814(8) 69.7873(7) 1875.7(2) 2 1.577 6659/424 1.125 1.030 0.0223, 0.0613 0.0259, 0.0642

EXPERIMENTAL SECTION

Syntheses of Salts 1 and 2. Crystals of salt 1 were grown using the diffusion and evaporation method. A solution of TBA[Ni(dmit)2] (TBA = tetrabutylammonium, 50 mg, 0.10 mmol) in 60 mL of CH3CN was added dropwise to a 60 mL solution of CH3CN containing LiClO4 (60 mg, 0.56 mmol) and [18]crown-6 (600 mg, 2.27 mmol). The solution was left undisturbed in the dark at room temperature for 2 weeks to obtain black plate-shaped crystals. Crystals of salt 2 were grown using a method similar to that described for 1. A solution of TBA[Ni(dmit)2] (50 mg, 0.10 mmol) in 60 mL of CH3CN was added to a 60 mL solution of CH3CN containing LiClO4 (60 mg, 0.56 mmol) and [15]crown-5 (600 mg, 2.72 mmol). The solution was left undisturbed in the dark at room temperature for several days to obtain black plate-shaped crystals. The mass percentages of Li and Ni in 1 and 2 were determined by inductively coupled plasma-atomic emission spectrometry (ICPAES). Anal. Calc. for 1: Li, 0.78; Ni, 6.59. Found: Li, 0.78; Ni, 6.74. Anal. Calc. for 2: Li, 0.86; Ni, 7.28. Found: Li, 0.86; Ni, 7.21. X-ray Crystallography. Single-crystal X-ray diffraction data of crystals 1 and 2 were collected with Bruker diffractometers: a SMART APEX II ULTRA diffractometer at 173 K and D8-QUEST diffractometer at 293 K, respectively. The measurements were performed with a monochromatic Mo Kα radiation (λ = 0.71073 Å). The structures were solved via direct methods using SIR97 or B

DOI: 10.1021/acs.chemmater.8b03027 Chem. Mater. XXXX, XXX, XXX−XXX

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Figure 1. Crystal structures of salt 1 viewed along the (a) a-axis and (b) b-axis. (c) Arrangement of supramolecular cation units ([18]crown-6, Li+, and water molecules) that adopt an ion channel configuration.

Figure 2. Crystal structures of salt 2 viewed along the (a) a-axis and (b) c-axis. (c) Arrangement of supramolecular cation units ([15]crown-5, Li+, and water molecules) that adopt an ion channel configuration. 64 scans, and a relaxation interval of 2 s. A molar LiCl aqueous solution was used as the reference (0 ppm) for the chemical shift of 7 Li.

revealed that the elimination of all crystalline water molecules began simultaneously at around 333 K (Figure S5). After dehydration was complete, 1 was cooled to room temperature and then rehydrated. The resulting crystal structure was confirmed to be the same as that before dehydration. The [Ni(dmit)2]− molecule forms a dimer via π−π interaction, and the intradimer distance is estimated to be 3.455 Å. The dimer is aligned along the b-axis, and the distance of the nearest neighboring S (S···S contact) is 3.485 Å (Figure S3a). This value is smaller than twice the van der Waals radius of S (3.6 Å),46 suggesting that the [Ni(dmit)2]− molecules form a spin ladder structure. The ladder structures are inserted between the adjacent channel structures formed by the supramolecular cation units. The crystal structure of 2 is similar to that of 1, but the [15]crown-5 molecules are obtained as a disordered structure (Figure S4). The supramolecular cation unit (the pore/cavity diameter is 1.70−2.20 Å),1 Li2([15]crown-5)3(H2O)2, is stacked one-dimensionally along the c-axis (Figure 2). The main difference between the crystal structures of 1 and 2 is in the position of the Li+ ion included in the crown ether and the manner of coordination of the water molecules (Figure S2). In

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RESULTS AND DISCUSSION Crystal Structures. The crystal structure of 1 features an ion channel with the supramolecular cation unit (the pore/ cavity diameter is 2.60−3.20 Å),1 which is composed of two Li+ ions, three [18]crown-6 molecules, and four water molecules, stacked one-dimensionally along the b-axis (Figures 1a and 1b). The Li+ ions are included in [18]crown-6 molecules on both sides of this unit and absent in the central [18]crown-6 molecule (Figure 1c). The distances between Li+ and each of the six O atoms are 1.990, 2.202, 2.409, 3.366, 3.558, and 4.163 Å (Figure S1). The fact that three of these values are smaller than 2.5 Å (the sum of the ionic radius of Li+45 and the van der Waals radius of O atom46) shows that Li+ forms coordination bonds with three of the O atoms. The water molecules coordinated to Li+ are located on either side of [18]crown-6 along the direction of the ion channel, and the distances between water molecules and Li+ are 1.897 and 1.911 Å (Figure S1). The thermogravimetric (TG) curve of 1 C

DOI: 10.1021/acs.chemmater.8b03027 Chem. Mater. XXXX, XXX, XXX−XXX

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Figure 3b shows the χmT−T plots of salt 2 for both the cooling and heating processes, from which the contributions of the temperature-independent diamagnetic component of −2.09(1) × 10−3 emu·mol−1 were subtracted. The χmT value also increased when the temperature increased above ca. 90 K. In the χmT curve for 2, a clear thermal-hysteresis loop is observed in the region from 180 to 280 K. Figure S7 shows the differential scanning calorimetry (DSC) curve obtained for 2, for which a first-order phase transition was observed as an endothermal peak at 277.5 K during the heating process, where ΔH = 580 J·mol−1. It should be noted that the temperature range of the measurement device affected the observed behavior in that the transition was not detected during the cooling process. Single-crystal X-ray analyses were carried out at 293 and 90 K in an attempt to explain the origin of this hysteresis. Although the crystal structure of the low-temperature (LT) phase could not completely be solved because of a crack in the crystal, the crystal parameters that could be obtained are as follows: triclinic, P1̅, a = 12.5507(9), b = 15.0444(11), c = 18.2992(13) Å, α = 76.921(2), β = 85.173(2), γ = 87.395(2) deg, V = 3351.1(4) Å3. The cell volume of the LT phase was twice as large as that of the hightemperature (HT) phase, which indicated the formation of a double lattice in the LT phase. From the roughly solved crystal structure of the LT phase, it was evident that the ion channels were oriented along the b + c-axis, with a length (26.1876 Å, Figure S8) that was twice that of the c-axis related to the length of the supramolecular cation unit in the HT phase (13.3500(6) Å). The χmT curve of 2 was also fitted to the S = 1/2 Heisenberg antiferromagnetic spin-ladder model47 in the temperature range from 2 to 170 K using a g value of 2.101 for [Ph(NH3)]([18]crown-6)[Ni(dmit)2].34 The magnetic behavior was consistent with the magnetic exchange interactions for the ladder rung and leg of Jrung/kB = −517(4) K, Jleg/kB = −109(5) K, and Curie impurity of 1.74(3) × 10−2 emu·K· mol−1. In addition, the spin gap was calculated to be Δ/kB = −420(7) K.48 The differences observed in the magnetic exchange interactions for 1 and 2 are in good agreement with their crystal structures. Li+ Ionic Conductivities. The frequency- and temperature-dependent impedance spectra were measured for a single crystal of salt 1 along the direction of the ion channel. The signals obtained for the heating process (288−348 K, with a temperature step size of 5 K) were presented in a compleximpedance-plane, Z″ versus Z′, known as the Nyquist plot (Figure 4a). The depressed semicircles observed in these measurements originate from a nonideal sample/electrode interface. The Nyquist plots exhibited a decreasing impedance Z with increasing temperature. All plots were assumed to be constructed by two-component semicircles; above 303 K, the two-component semicircles were clearly visible, whereas they were assumed to be present but not clearly visible below 303 K. We presumed that salt 1 was a mixed conductor with both ionic and electronic contributions to conductivity, and thus, the equivalent circuit shown in Figure S9 was used to analyze the obtained signals. This equivalent circuit consisted of the geometric constant phase element, CPEgeom, the lithium ionconducting component, Ri, the electrical double layer capacitance between the sample and the electrode, CPEDL, and the electron-conducting component, Re.50 The CPEDL was included in this equivalent circuit to account for the blocking of Li+ ion conduction by the Au electrode.50 This circuit

1, only three of the distances between Li+ and the O atoms are