Complex Nature of Ionic Coordination in Magnesium Ionic Liquid-Based Electrolytes: Solvates with Mobile Mg²⁺ Cations

The Raman shifts of the TFSI⁻ expansion-contraction mode in <i>N</i>-butyl-<i>N</i>-methylpyrrolidinium bis­(trifluoromethanesulfonyl)­imide ionic liquid (IL) electrolytes were analyzed to compare the ionic coordination of magnesium with lithium and sodium. In the Mg²⁺-IL electrolytes, the TFSI^– anions are found in three different potential energy environments, while only two populations of TFSI^– are evident in the Na⁺- and Li⁺-IL electrolytes. For Mg²⁺, the high frequency peak component is associated with a TFSI^– that is in a bidentate coordination with a single metal cation and can therefore be considered a contact ion pair (CIP) solvate. The mid frequency component is attributed primarily to bridging aggregate (AGG) TFSI^– solvate or a weakly bound monodentate CIP TFSI^–. The low frequency peak is well-known to be associated with “free” TFSI^– anions. The average number of TFSI^– per Mg²⁺ cation (<i>n</i>) is 3 to 4. In comparison, the value of <i>n</i> is 4 at very low concentrations and decreases with increasing salt mole fraction to 2 for Li⁺ and Na⁺, where <i>n</i> of Na⁺ is larger than that of Li⁺ at any given concentration. The results imply the existence of anionic magnesium solvates of varying sizes. The identity of the Mg²⁺ charge-carrying species is complex due to the presence of bridging AGG solvates in solution. It is likely that there is a combination of single Mg²⁺ solvate species and larger complexes containing two or more cations. In comparison, the primary Li⁺ and Na⁺ charge-carrying species are likely [Li­(TFSI)₂]⁻ and [Na­(TFSI)₃]^2– in the concentration range successfully implemented in IL-based electrolyte batteries. These solvates result in Mg²⁺ cations that are mobile in the IL-based electrolytes as demonstrated by the reversible magnesiation/demagnesiation in V₂O₅ aerogel electrodes

Mechanisms of Magnesium Ion Transport in Pyrrolidinium Bis(trifluoromethanesulfonyl)imide-Based Ionic Liquid Electrolytes

Inert polar aprotic electrolytes based on pyrrolidinium bis­(trifluoromethanesulfonyl)­imide ionic liquids were investigated for Mg battery applications. On a molecular scale, there are two TFSI^– populations coordinating Mg²⁺ ions: one in a bidentate coordination to a single Mg²⁺ and one in a bridging geometry between two Mg²⁺ ions. On average, each Mg²⁺ cation is surrounded by three to four TFSI^– anions. The electrolytes, in general, remain amorphous far below ambient conditions, which results in a wide useable temperature range in practical devices. There is a change in the ratio of bidentate:bridging TFSI^– and in the conductivity, viscosity, and diffusion behavior at a salt mole fraction of 0.12–0.16. At concentrations above this threshold, there is a more dramatic decrease of the diffusion coefficients and the conductivity with increasing salt concentration due to slower exchange of the more strongly coordinated bidentate TFSI^–. The mechanism of ion transport likely proceeds via structural diffusion through exchange of the bridging and “free” TFSI^– anions within adjacent [Mg_<i>n</i>(TFSI)_<i>m</i>]^{(<i>m</i>−2<i>n</i>)–} clusters and exchange of bidentate anions via a bidentate to bridging mechanism. The vehicular mechanism likely makes only a small contribution. At concentrations above approximately 0.16 mole fraction, the structural diffusion is more closely related to the tightly bound bidentate anions

Influence of Anions on Proton-Conducting Membranes Based on Neutralized Nafion 117, Triethylammonium Methanesulfonate, and Triethylammonium Perfluorobutanesulfonate. 2. Electrical Properties

The electrical properties of a Nafion proton exchange membrane change dramatically when neutralized and then doped with a proton-conducting ionic liquid (PCIL). Broadband electric spectroscopy elucidates the molecular relaxation and polarization phenomena of neutralized Nafion (nN117) doped with triethylammonium methanesulfonate (TMS) and triethylammonium perfluorobutanesulfonate (TPFBu) ionic liquids. These data, coupled with those of part 1 suggest proton conduction mechanisms for both the pure PCILs and in PCIL-doped nN117. At 130 °C, the PCILs have conductivities of σ_TMS = 1.4 × 10^–2S/cm and σ_TPFBu = 9 × 10^–3S/cm, while correspondingly doped nN117 have conductivities of σ_NTMS = 6.1 × 10^–3 S/cm and σ_NTPFBu = 1.8 × 10^–3S/cm. The pure PCILs show three interfacial polarizations associated with proton transfer mechanisms above the melting point. PCIL-doped nN117 also has three interfacial polarizations that depend on the nanostructure characteristics of the PCIL sorbed within the nN117 polar domains. Below the PCIL melting point, doped nN117 has two dielectric relaxations, α and β, associated with dipolar relaxations involving both the sorbed PCILs and the ionomer matrix. The data indicate a long-range charge transfer process that occurs through proton exchange between cationic clusters. Segmental motion of the polymer chains and the molecular dimensions of the ionic liquid nanoaggregates mediate this charge transfer

Interplay between Structural and Dielectric Features of New Low k Hybrid Organic–Organometallic Supramolecular Ribbons

The synthesis and characterization of low k one-dimensional (1D) hybrid organic–organometallic supramolecular ribbons <b>3a</b>,<b>b</b>, through halogen-bond driven co-crystallization of <i>trans</i>-[Pt­(PCy₃)₂(CC-4-py)₂] (<b>1</b>) with 1,4-diiodotetrafluorobenzene (<b>2a</b>) and <i>trans</i>-1,2-bis-(2,3,5,6-tetrafluoro-4-iodophenyl)-ethylene (<b>2b</b>), are reported. The co-crystals <b>3a</b>,<b>b</b> have been obtained by isothermal evaporation of a chloroform solution containing the corresponding starting materials at room temperature. X-ray structure determinations show that noncovalent interactions other than halogen bonds help in the construction of the crystal packing; these interactions are stronger in <b>3b</b>, thus reducing the chain mobility with respect to <b>3a</b>. Accordingly, the broadband dielectric spectroscopic determinations, carried out from 10^–2 to 10⁷ Hz and at a temperature ranging from 25 to 155 °C, showed that both <b>3a</b> and <b>3b</b> materials exhibit a real component of dielectric permittivity (ε′) significantly lower than SiO₂. In particular in the case of <b>3b</b>, the rigidity of the 1D chain explains the observed ε″ and tan δ values. A permittivity value that is significantly lower than that of the silica reference, tan δ values lower than 0.02 in the entire investigated temperature range, and less than 0.004 at <i>T</i> < 100 °C make <b>3b</b> a very promising low k hybrid organic–organometallic material for application as dielectric films in next generation microelectronics