Structure and Ion Dynamics in Imidazolium-Based Protic Organic Ionic Plastic Crystals

The effects of defects on the transport mechanisms of lithium ions in organic ionic plastic crystals

Organic ionic plastic crystals (OIPCs) consist of molecular ions of which interactions are strong enough to maintain crystalline order but are weak enough to allow the rotations of the molecular ions at sufficiently high temperatures. When defects such as Schottky vacancies and grain boundaries are introduced into OIPCs, the defects facilitate the transport of dopants such as Li+ ions, for which OIPCs are considered as strong candidates for solid electrolytes. The transport mechanism of dopant ions in OIPCs with defects, however, remains elusive at a molecular level partly because it is hard in experiments to track the dopant ions and control the types of defects systematically. In this work, we perform molecular dynamics simulations for 1,3-dimethylimidazolium hexafluorophosphate ([MMIM][PF6]) OIPCs with Li+ ions doped and show that the transport mechanism of Li+ ions depends on the types and concentrations of defects. A high concentration of Schottky vacancies enhance the overall ion conduction, but decrease the transference number. The transference numbers of Li+ ions in [MMIM][PF6] with grain boundaries are similar to that in [MMIM][PF6] with 0.78 mol% point vacancies. We also find that the transport of ions in OIPCs is strongly heterogeneous and the time scales of the dynamic heterogeneity of the ions are sensitive to the types of defects.

Entropy-Driven Ultrafast Ion Conduction Via Confining Organic Plastic Crystals in Ordered Nanochannels of Covalent Organic Frameworks

Low conductivity over a wide temperature region due to ultra-slow ion migration dynamics is a key issue in the field of solid-state electrolytes (SSE), which needs to be solved and improved. Covalent organic frameworks (COFs), a rapidly growing class of porous crystalline materials, emerge as a new research hotspot in the field of SSEs. This is due to their homogeneously dispersed sites and well-defined pathways for ion diffusion, demonstrating great advantages over conventional non-porous solids. Herein, a composite solid electrolyte by confining organic ionic plastic crystal (OIPC) in the 1D ordered nanochannels of COFs as the host matrix for solid-state lithium-ion conduction, is reported. Due to the loss of coupling between PBu₄ ⁺ cations and TFSI^- anions, the cation-anion interaction is weakened; and thus, the lithium-ion transportation is facilitated. As a result, the COF-confining OIPC SSEs show ultra-high lithium-ion conductivity of 0.048 S cm^-1 at 30 °C and 0.021 S cm^-1 at the extremely low temperature of -30 °C. The dynamic origin of this fast ion conduction is characterized by differential scanning calorimetry (DSC), X-ray photoelectron spectroscopy (XPS), and variable temperature solid-state nuclear magnetic resonance (NMR) spectroscopy.

Effect of Lithium-Ion on the Structural Organization of Monocationic and Dicationic Ionic Liquids

In recent times, ionic liquid-based (ILs) electrolytic system has emerged as suitable alternative to the conventional organic solvent-based electrolytic system. However, since, anion of ILs is known to form aggregates in the presence of lithium-ions (Li⁺), and this can influence the transport properties of Li⁺ ion in a significant manner, it is, therefore, important to understand how lithium-ions influence the structure and dynamics of ILs. With this objective, in the present study, intermolecular interaction, structural organization, and dynamics of monocationic ILs (MILs) and dicationic IL (DIL) have been studied in the absence and presence of lithium salt. Specifically, for this purpose, two MILs, 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide ([C₃C₁im][NTf₂]), 1-hexyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide ([C₆C₁im][NTf₂]), and a DIL, 1,6-bis(3-methylimidazolium-1-yl)hexane bis(trifluoromethylsulfonyl)amide ([C₆(mim)₂][NTf₂]₂) have been chosen in such a way that either the alkyl chain of MILs becomes equal or half of the spacer chain length of DIL. To understand the effect of the addition of lithium-ion on the structural organization of MILs and DIL, steady-state absorption and fluorescence spectroscopies, time-resolved fluorescence anisotropy and nuclear magnetic resonance (NMR) techniques have been used. Structural organization in the apolar and polar domains of ILs has been probed by following the rotational diffusion of suitably chosen solute in the concerned media through time-resolved fluorescence anisotropy (TRFA) measurements. TRFA studies have revealed that with the addition of Li⁺ ion, coordination between the Li⁺ ions and anions of MILs and DILs takes place in the ionic region leading to a change in the structural organization of the apolar regions of the respective medium. In fact, upon adding lithium-ions, a reduction in the packing of alkyl chains has also been observed for the MILs. However, not much change in the structural organization of the apolar region of the DIL has been observed when Li⁺ ion is added to it. In the presence of Li⁺ ions, a similar trend in the change of structural organization of polar regions for both MILs and DIL has been observed. Further, measurements of the self-diffusion coefficient through NMR have also supported the observation that Li⁺ ion also perturbs the nanostructural organization of the MIL in a significant manner than that it does for the DIL. The behavior of DIL in the presence of Li⁺ ion, as revealed by the present study, has been rationalized by considering the folded arrangement of DIL in the fluid-structure. Essentially, all of these investigations have suggested that the addition of lithium-ion significantly alters the microscopic behavior of MILs in comparison to that of DIL. The outcome of this study is expected to be helpful in realizing the potentials of these media as electrolytes in battery applications.

New Insights into Decoupled Cation and Anion Transport and Dynamic Heterogeneity in a Diethyl(methyl)(isobutyl)phosphonium Hexafluorophosphate Organic Ionic Plastic Crystal

Organic ionic plastic crystals (OIPCs) are an emerging family of materials with demonstrated applications in electrochemical devices such as lithium/sodium ion batteries, dye-sensitized solar cells, and hydrogen fuel cells. Herein, we present direct evidence of anion diffusion through a relatively static background of a cation lattice in an ionic plastic crystal compound, [P_122i4][PF₆], in an elevated temperature solid phase. We found all anions are diffusive, whereas only a small population of cations is diffusive. Two anion populations were identified with diffusion coefficients differing by 2 orders of magnitude. The slow-diffusing anion is attributed to the plastic crystal region where the cation forms a relative static background, allowing anions to diffuse possibly through a defect-assisted hopping mechanism.

Heterogeneous Rotational Dynamics of Imidazolium-Based Organic Ionic Plastic Crystals

Organic ionic plastic crystals (OIPCs) are a unique class of materials that undergo orientational and conformational motions while maintaining a long-range ordered lattice structure. OIPCs have attracted attention because the rotational motions were known to accelerate the diffusion of mobile ions such as lithium ions. However, only a small number of combinations of cations and anions lead to OIPCs because the rotational motion may be restricted by both the molecular structure and the crystal class. In this work, we perform molecular dynamics simulations to study the effects of the molecular structure and the crystal class on the rotational motion and the phase transitions. We investigate four imidazolium-based ionic crystals: (1) 1-methyl-3-methylimidazolium hexafluorophosphate ([MMIM][PF₆]), (2) 1-methyl-3-methylimidazolium chloride ([MMIM][Cl]), (3) monoclinic 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]), and (4) orthorhombic [BMIM][Cl] ionic crystals. We construct initial configurations of OIPCs by employing experimental crystalline structures. Then, we increase the temperature gradually and monitor the density and the radial distribution functions. We estimate the rotational van Hove correlation functions and find that molecules in plastic crystal phases undergo rotational hopping motions and OIPCs exhibit rotational dynamic heterogeneity significantly. The structure of anions and cations affect the phase transition of OIPCs. And the crystal class is also critical to the phase transition of OIPCs because the rotational motion of ions depends on the crystal class.

Crystal structure, magneto-structural correlation, thermal and electrical studies of an imidazolium halometallate molten salt: (trimim)[FeCl4]

A novel imidazolium halometallate molten salt with formula (trimim)[FeCl4] (trimim: 1,2,3-trimethylimidazolium) was synthetized and studied with structural and physico-chemical characterization. Variable-temperature synchrotron X-ray powder diffraction (SXPD) from 100 to 400 K revealed two structural transitions at 200 and 300 K. Three different crystal structures were determined combining single crystal X-ray diffraction (SCXD), neutron powder diffraction (NPD), and SXPD. From 100 to 200 K, the compound exhibits a monoclinic crystal structure with space group P21/c. At 200 K, the former crystal system and space group are retained, but a disorder in the organic cations is introduced. Above 300 K, the structure transits to the orthorhombic space group Pbcn, retaining the crystallinity up to 400 K. The study of the thermal expansion process in this temperature range showed anisotropically evolving cell parameters with an axial negative thermal expansion. Such an induction occurs immediately after the crystal phase transition due to the translational and reorientational dynamic displacements of the imidazolium cation within the crystal building. Electrochemical impedance spectroscopy (EIS) demonstrated that this motion implies a high and stable solid-state ionic conduction (range from 4 × 10-6 S cm-1 at room temperature to 5.5 × 10-5 S cm-1 at 400 K). In addition, magnetization and heat capacity measurements proved the presence of a three-dimensional antiferromagnetic ordering below 3 K. The magnetic structure, determined by neutron powder diffraction, corresponds to ferromagnetic chains along the a-axis, which are antiferromagnetically coupled to the nearest neighboring chains through an intricate network of superexchange pathways, in agreement with the magnetometry measurements.

Ion Vacancies and Transport in 1-Methylimidazolium Triflate Organic Ionic Plastic Crystal

Organic ionic plastic crystals (OIPCs) are an important family of materials that have shown exciting possibilities as solid electrolytes for lithium ion batteries and other electrochemical devices. In this study we demonstrate for the first time that, although the X-ray shows sharp diffraction peaks, both cation and anion clearly exhibit significant ion diffusion in solid phase I. Two phases with ion diffusivities differing by 2 orders of magnitude can be identified. The populations of the cation and anion in both phases are found to be unequal, hinting at the existence of (negatively charged) cation vacancies in the plastic crystal phase and a positively charged grain boundary phase. These interesting properties of ion vacancies and unequal populations of cation and anion are likely to be ubiquitous in other OIPCs, and it is of paramount importance to be aware of these features to correctly understand the structure-property relationships of this important material family.

Exploring high-performance integration in a plastic crystal/film with switching and semiconducting behavior

As a room-temperature plastic crystal, (N,N-dimethylpiperidinium)₃Bi₂Cl₉ can integrate semiconducting behavior and switchable properties into one single flexible material, making it a potential candidate in flexible multifunctional devices.