Structural Design of Ionic Conduction Paths in Molecular Crystals for Selective and Enhanced Lithium Ion Conduction

Study of Lithium-Extraction Systems Based on Benzo-15-Crown-5 Ether and Alkylimidazolium-Based Ionic Liquid

The extraction of lithium from aqueous solutions of LiNTf₂ and LiCl salts using benzo-15-crown-5 ether (B15C5) as an extractant in [C8mim][NTf₂] ionic liquid was studied. The transition of the extractant into the aqueous phase and the distribution of Cl^- ions during lithium extraction from LiCl solutions were determined. LiNTf₂ complexes with B15C5 with different LiNTf₂:B15C5 ratios were isolated for the first time and characterized via X-ray diffraction and IR spectroscopy. Differences in the extraction process of LiCl and LiNTf₂ were determined via an infrared spectroscopic study of the extraction systems.

Incongruent Melting and Vitrification Behaviors of Anionic Coordination Polymers Incorporating Ionic Liquid Cations

Several meltable coordination polymers (CPs) that possess substantial advantages attributable to their high flexibility and processability have been developed recently; however, the melting mechanism and vitrification conditions of these materials are not yet fully understood. In this study, we synthesized meltable CPs [A][K(TCM)₂] (A = onium cation, TCM = C(CN)₃^-) incorporating ionic liquid components and investigated their crystal structures and melting behaviors in detail. These CPs feature two- or three-dimensional anionic [K(TCM)₂]_n^- frameworks incorporating onium cations. Each CP was found to undergo incongruent melting at a temperature between 73 and 192 °C to produce a heterogeneous mixture of the ionic liquid ([A][TCM]) and microcrystalline K[TCM]. Furthermore, they formed homogeneous liquids upon further heating to ∼240 °C. The melting points of these CPs were linearly correlated with those of their constituent ionic liquids. The vitrification of these materials upon rapid cooling from the molten state was further investigated. The cooling rates required for vitrification differed greatly between the CPs and were correlated with the cation flexibility.

Organic Crystalline Solid Electrolytes with High Mg-Ion Conductivity Composed of Nonflammable Ionic Liquid Analogs and Mg(TFSA)2

The development of solid electrolytes with Mg-ion conductivity at room temperature is an important issue to achieve all-solid magnesium batteries. We focus on organic ionic crystals with Mg-ion conduction paths in addition to nonflammable and nonvolatile features as an innovative candidate of solid electrolytes with Mg-ion conductivity. Herein, we show the development of novel organic ionic crystals, [N(CH₃)_4-n(CH₂CH₃)_n][Mg{N(SO₂CF₃)₂}₃] (n = 0 or 2), using analogs of ionic liquids, [N(CH₃)₄][N(SO₂CF₃)₂] (N₁₁₁₁TFSA) and [N(CH₃)₂(CH₂CH₃)₂][N(SO₂CF₃)₂] (N₁₁₂₂TFSA), and magnesium salt, Mg{N(SO₂CF₃)₂}₂ (Mg(TFSA)₂). We also report the crystal structures of the obtained crystals and the high Mg-ion conductivity of 10^-4 S cm^-1 under mild conditions of 80 °C in the solid state. These results indicate that organic ionic crystals with ion conduction paths have significant potential as safe solid electrolytes and provide insights into developing innovative Mg-ion conductors.

High Li-Ion Conductivity in Li{N(SO2F)2}(NCCH2CH2CN)2 Molecular Crystal

There is an urgent need to develop solid electrolytes based on organic molecular crystals for application in energy devices. However, the quest for molecular crystals with high Li-ion conductivity is still in its infancy. In this study, the high Li-ion conductivity of a Li{N(SO₂F)₂}(NCCH₂CH₂CN)₂ molecular crystal is reported. The crystal shows a Li-ion conductivity of 1 × 10^-4 S cm^-1 at 30 °C and 1 × 10^-5 S cm^-1 at -20 °C, with a low activation energy of 28 kJ mol^-1. The conductivity at 30 °C is one of the highest values attainable by molecular crystals, whereas that at -20 °C is approximately 2 orders of magnitude higher than previously reported values. Furthermore, the all-solid-state Li-battery fabricated using this solid electrolyte demonstrates stable cycling, thereby maintaining 90% of the initial capacity after 100 charge-discharge cycles. The finding of high Li-ion conductivity in molecular crystals paves the way for their application in all-solid-state Li-batteries.

Luminescent ionic liquid formed from a melted rhenium(v) cluster

Recently, non-crystalline coordination materials have been shown to represent a versatile class of functional materials. However, such materials incorporating metal complex clusters have remained largely unexplored. Herein, we demonstrate that a luminescent tetranuclear ReV cluster melts at 489 K, with the cluster structure being maintained in the corresponding supercooled ionic liquid phase.

Synthesis of an Adduct-Type Organic Ionic Crystal with Solid-State Ionic Conductivity from A Thiocyanate-Based Ionic Liquid and B(C6F5)3

We synthesized the novel adduct-type organic ionic crystal [C3mim][SCN·B(C6F5)3] (1) by the reaction of 1–methyl–3–propylimidazolium thiocyanate ([C3mim][SCN]), which is a room temperature ionic liquid, and B(C6F5)3, a bulky Lewis acid. The formation of a coordinative B–N bond between the SCN anion and the B(C6F5)3 in 1 was revealed by single-crystal X-ray diffractometry. We showed that 1 displays ionic conductivity in the crystalline state and that doping 1 with sodium thiocyanate and B(C6F5)3 results in a dramatic increase in ionic conductivity compared to that of 1.

Water-Induced Breaking of the Coulombic Ordering in a Room-Temperature Ionic Liquid Metal Complex

Control of ion arrangements in ionic liquids represents a major challenge owing to the presence of the predominant coulombic interactions between cationic and anionic ion species that forms the coulombic ordering. Here, water-induced ion rearrangement in a room-temperature ionic liquid (RT-IL) metal complex, (1-ethyl-3-methylimidazolium)₂ [MnN(CN)₄ ], is demonstrated through coordinative interactions between anions. Solidification occurred, which was associated with the formation of a "separated" structure consisting of cation columns and anionic cyanide-bridged one-dimensional coordination polymers. The energy diagram is in accord with the resultant RT-IL incorporating mononuclear [MnN(CN)₄ ]^2- molecules being a kinetic phase stabilized by inter-ion repulsions of the anionic divalent metal complex moieties. Water acts to decrease the coulombic interactions, including repulsion, giving rise to breaking of the coulombic ordering arising from coordination bond formation in the IL phase.

Construction of nanostructures for selective lithium ion conduction using self-assembled molecular arrays in supramolecular solids

In the development of innovative molecule-based materials, the identification of the structural features in supramolecular solids and the understanding of the correlation between structure and function are important factors. The author investigated the development of supramolecular solid electrolytes by constructing ion conduction paths using a supramolecular hierarchical structure in molecular crystals because the ion conduction path is an attractive key structure due to its ability to generate solid-state ion diffusivity. The obtained molecular crystals exhibited selective lithium ion diffusion via conduction paths consisting of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and small molecules such as ether or amine compounds. In the present review, the correlation between the crystal structure and ion conductivity of the obtained molecular crystals is addressed based on the systematic structural control of the ionic conduction paths through the modification of the component molecules. The relationship between the crystal structure and ion conductivity of the molecular crystals provides a guideline for the development of solid electrolytes based on supramolecular solids exhibiting rapid and selective lithium ion conduction.