Hexafluoro-, heptafluoro-, and octafluoro-salts, and [MₙF₅ₙ₊₁]⁻ (n=2, 3, 4) polyfluorometallates of singly charged metal cations, Li⁺–Cs⁺, Cu⁺, Ag⁺, In⁺ and Tl⁺

The AMF₆, A₂MF₇, A₃MF₈, AM₂F₁₁, AM₃F₁₆ and AM₄F₂₁ compounds (A = Li, Na, K, Rb, Cs, Cu, Ag, In, Tl; M = P, As, V, Rh, Ru, Au, Pt, Ir, Os, Re, Sb, Mo, W, Nb, Ta, Bi) are reviewed. Some of the structural data of the AM₆ compounds are based just on powder diffraction work from the middle of the last century. The crystal structure types of AMF₆ compounds have been re-classified in this review, based mainly on single crystal data. The crystal structure types of AMF₆ compounds can be classified into six main groups: LiSbF₆ type, NaSbF₆ type, structures of cubic APF₆ and AAsF₆ with orientational disorder of the anions, tetragonal KSbF₆ (T) types and similar structures, AgSbF₆ type and similar structures, and KOsF₆ type. Reported crystal structures of A₂MF₇, A₃MF₈, AM₂F₁₁, AM₃F₁₆ and AM₄F₂₁ compounds are limited. K₂WF₇ in the orthorhombic crystal system. Among the A₃MF₈ compounds the complete crystal structure has been determined only for Na₃TaF₈, which is monoclinic. The only known examples of crystal structures of AM₂F₁₁ compounds are ASb₂F₁₁1 (A = Ag, K, Cs). Crystals of KSb₂F₁₁ are orthorhombic and isostructural to AgSb₂F₁₁, while CsSb₂F₁₁ is monoclinic. CsSb₃F₁₆ is the only example of a structurally characterized AM₃F₁₆ compound. Its crystals are orthorhombic. For the rest of the known A₂MF₇, A₃MF₈, AM₂F₁₁, AM₃F₁₆ and AM₄F₂₁ compounds, only lattice parameters are known

Structural characteristics of alkylimidazolium-based salts containing fluoroanions

An overview of recent structural studies on alkylimidazolium-based salts containing fluoroanions is presented. Alkylimidazolium cations have been most extensively used for syntheses of ionic liquids (room temperature molten salts) because they usually exhibit low melting points, low viscosities and high conductivities. This review mainly focuses on structures of alkylimidazolium-based salts combined with a fluorocomplex anion ((FH)nF⁻, BF₄⁻, PF₆⁻, AsF₆⁻, SbF₆⁻, NbF₆⁻, TaF₆⁻), N(SO₂CF3)₂⁻ (TFSI⁻) and OSO₂CF₃⁻. The first part describes crystal structures of these salts and the second part describes computational, spectroscopic and diffraction studies on their liquid structures. Related studies on ionic liquids of non-alkylimidazolium cation and non-fluoroanion are also briefly summarized

Ionic liquids for electrochemical devices

A short review of ionic liquids (ILs) and their applications as electrolytes for electrochemical devices, such as electric double layer capacitors, fuel cells, lithium batteries, and solar cells, are presented here. The properties of ILs, such as non-volatility, non-flammability, wide liquid temperature ranges, and wide electrochemical windows, have the potential to be improved, including improvements in durability and safety, extending the operational temperature ranges and enabling improvements in power and energy densities of the devices

Electrochemically stable fluorohydrogenate ionic liquids based on quaternary phosphonium cations

Fluorohydrogenate ionic liquids of quaternary phosphonium cations, tri-n-butylmethylphosphonium (P4441) fluorohydrogenate, tetra-n-butylphosphonium (P4444) fluorohydrogenate, and tri-n-butyl-n-octylphosphonium (P4448) fluorohydrogenate, have been synthesized by the metatheses of anhydrous hydrogen fluoride and the corresponding phosphonium chloride precursors. All the obtained salts have melting points below room-temperature with a vacuum-stable composition of P₄₄₄ₘ(FH)₂.₃F (m = 1, 4, and 8) and were characterized by density, conductivity, and viscosity measurements. Linear sweep voltammetry with a glassy carbon working electrode shows that the P₄₄₄ₘ(FH)₂.₃Fs have wide electrochemical windows exceeding 5.2 V. In particular, P₄₄₄₁(FH)₂.₃F has an electrochemical window of 6.0 V, which is the widest among fluorohydrogenate ionic liquids reported to date. The thermal stability of these ionic liquids is also improved compared to the salts of N-heterocyclic ammonium cations

Coordination environment around the lithium cation in solid Li₂(EMIm)(N(SO₂CF₃)₂)₃ (EMIm=1-ethyl-3-methylimidazolium): Structural clue of ionic liquid electrolytes for lithium batteries

Crystal structure of Li₂(EMIm)(N(SO₂CF₃)₂)₃ (EMIm = 1-ethyl-3-methylimidazolium cation) has been determined by single-crystal X-ray diffraction as a structural clue of ionic liquid electrolytes for lithium batteries. Li₂(EMIm)(N(SO₂CF₃)₂)₃ crystallizes in the space group P2₁/c, a = 15.184(3)Å, b = 11.358(3)Å, c = 21.249(5)Å, β = 111.454(12)°, Z = 4, V = 3561.18(14)ų. Two of the three N(SO₂CF₃)₂ anions have cis-conformations and the third anion shows a trans-conformation. The asymmetric unit contains two crystallographically independent lithium ions and both of them are trigonal-bipyramidally coordinated by five oxygen atoms of N(SO₂CF₃)₂ anions, forming a two-dimensional network. EMIm cation occupies a space in the network, weakly interacting with the anions

Effects of HF content in the (FH)ₙF⁻anion on the formation of ionic plastic crystal phases of N-ethyl-N-methylpyrrolidinium and N,N-dimethylpyrrolidinium fluorohydrogenate salts

Fluorohydrogenate salts based on N-ethyl-N-methylpyrrolidinium (EMPyr(FH)ₙF) and N, N-dimethylpyrrolidinium (DMPyr(FH)ₙF) cations were synthesized, and the effects of the HF content n in EMPyr(FH)ₙF (1.0 ≤ n ≤ 2.3) and DMPyr(FH)ₙF (1.0 ≤ n ≤ 2.0) on their thermal and structural properties were discussed, focusing on the characterization of ionic plastic crystal (IPC) phases. Several solid phases (IPC (I) and IPC (II) phases, and crystal phases of EMPyr(FH)₁F, EMPyr(FH)₂F, and EMPyr(FH)₃F) are observed in the EMPyr(FH)ₙF system. The IPC (I) phase has an NaCl-type structure and is composed of EMPyr⁺ cations and (FH)ₙF⁻ (n = 1, 2, and 3) anions randomly occupying the anion positions in the lattice over a wide range of n values in (FH)ₙF⁻. The melting point of EMPyr(FH)ₙF in the range 1.8 ≤ n ≤ 2.3 is maximal at n = 2.0, whereas it increases with a decrease in n in the range 1.0 ≤ n ≤ 1.2. Furthermore, in the range 1.3 ≤ n ≤ 1.7, the solid phase is regarded as the IPC phase (IPC (II)), and their melting points are nearly constant (260–270 K). In the DMPyr(FH)ₙF system, the IPC (I′) phase and crystal phases of DMPyr(FH)1F and DMPyr(FH)₂F were observed. Although the IPC (I′) phase has an NaCl-type structure, similar to the IPC (I) phase of EMPyr(FH)ₙF, it has higher ordering compared to the IPC (I) phase. The melting point of DMPyr(FH)ₙF increases monotonously with decreasing n but disappears in the small n region where the salt decomposes below the melting point

Physical and Electrochemical Properties of 1-ethyl-3-methylimidazolium Ionic Liquids of Mixed Anions, (FH)ₙF⁻, BF₄⁻, and N(SO₂CF₃)₂⁻

Physical and electrochemical properties of 1-ethyl-3-methylimidazolium ionic liquids of mixed anions, (FH)₂.₃F⁻, BF₄⁻, and N(SO₃)₂, have been investigated. Molar volume shows almost linear behavior, whereas molar conductivity is decreased by mixing for the systems involving (FH)₂.₃F⁻ due to the enhancement of ion association in spite of the decrease in viscosity. The currents at the anode and cathode limits in the cyclic voltammogram of EMIm(FH)₂.₃F decreases with decrease in the molar ratio of (FH)ₙF⁻, suggesting the involvement of (FH)ₙF⁻ for both electrode reactions. Electrochemical stability of the BF₄-TFSA mixture is unchanged by mixing

Phase Behavior of 1-Dodecyl-3-methylimidazolium Fluorohydrogenate Salts (C₁₂MIm(FH)ₙF,n= 1.0–2.3) and Their Anisotropic Ionic Conductivity as Ionic Liquid Crystal Electrolytes

The effects of the HF composition, n, in 1-dodecyl-3-methylimidazolium fluorohydrogenate salts (C₁₂MIm(FH)ₙF, n = 1.0–2.3) on their physicochemical and structural properties have been investigated using infrared spectroscopy, thermal analysis, polarized optical microscopy, X-ray diffraction, and anisotropic ionic conductivity measurements. The phase diagram of C₁₂MIm(FH)ₙF (n vs transition temperature) suggests that C₁₂MIm(FH)ₙF is a mixed crystal system that has a boundary around n = 1.9. For all compositions, a liquid crystalline mesophase with a smectic A interdigitated bilayer structure is observed. The temperature range of the mesophase decreases with increasing n value (from 61.8 °C for C₁₂MIm(FH)₁.₀F to 37.0 °C for C₁₂MIm(FH)₂.₃F). The layer spacing of the smectic structure decreases with increasing n value or increasing temperature. Two structural types with different layer spacings are observed in the crystalline phase (type I, 1.0 ≤ n ≤ 1.9, and type II, 1.9 ≤ n ≤ 2.3). Ionic conductivities parallel and perpendicular to the smectic layers (σ|| and σ⊥) increase with increasing n value, whereas the anisotropy of the ionic conductivities (σ||/σ⊥) is independent of the n value, since the thickness of the insulating sheet formed by the dodecyl group remains nearly unchanged

Thermodynamics of the O₂/O²⁻ redox couple in molten (LiCl+KCl+Li₂O) systems

Activity coefficients of oxide ion were determined by the measurements of the standard formal potential of O₂/O²⁻ with a boron-doped diamond (BDD) electrode in molten (LiCl + KCl): LiCl:KCl = {58.5:41.5 (eutectic), 65:35, 70:30, 75:25} mol% at T = (673 to 803) K. The activity coefficient decreases with the increase of the LiCl content in the melt: (18.7 ± 1.5, 7.7 ± 1.0, 4.1 ± 0.4, and 1.6 ± 0.1), respectively, at T = 773 K in each melt. The result is explained by the attractive force between Li⁺ and O²⁻ which is stronger than that between K⁺ and O²⁻