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

A Combined Theoretical and Experimental Study of the Influence of Different Anion Ratios on Lithium Ion Dynamics in Ionic Liquids

In this paper, we investigate via experimental and simulation techniques the transport properties, in terms of total ionic conductivity and ion diffusion coefficients, of ionic liquids doped with lithium salts. They are composed of two anions, bis­(fluorosulfonyl)­imide (FSI) and bis­(trifluoromethanesulfonyl)­imide (TFSI), and two cations, <i>N</i>-ethyl-<i>N</i>-methylimidazolium (emim) and lithium ions. The comparison of the experimental results with the simulations shows very good agreement over a wide temperature range and a broad range of compositions. The addition of TFSI gives rise to the formation of lithium dimers (Li⁺–TFSI^––Li⁺). A closer analysis of such dimers shows that involved lithium ions move nearly as fast as single lithium ions, although they have a different coordination and much slower TFSI exchange rates

Toward Na-ion BatteriesSynthesis and Characterization of a Novel High Capacity Na Ion Intercalation Material

The rapid growth of the worldwide demand of lithium for batteries (LIBs) can possibly lead to a shortage of its reserves. Sodium batteries represent a promising alternative because they enable much higher energy densities than other battery systems, with the exception of LIBs, and are not limited by sodium availability. Herein, we present a novel, Na⁺ ion intercalation material, Na_0.45Ni_0.22Co_0.11Mn_0.66O₂ (space group <i>P</i>6₃/<i>mmc</i>) synthesized in air by a coprecipitation method followed by a thermal treatment and a water-rinsing step. This material performs a specific capacity of 135 mA h g^–1 with a Coulombic efficiency exceeding 99.7%. Upon long-term cycling tests the material shows excellent capacity retention after more than 250 cycles. Such an overall performance, superior to that of presently known sodium-ion cathodes, represents a step further toward the realization of sustainable batteries for efficient stationary energy storage