Solvate Ionic Liquids at Electrified Interfaces

Al(III) and Ga(III) triflate complexes as solvate ionic liquids: speciation and application as soluble and recyclable Lewis acidic catalysts

This work reports on the first solvate ionic liquids (SILs) based on aluminium(III) and gallium(III) triflates, M(OTf)3, and triglyme (G3). Liquid-phase speciation of these new SILs was studied by multinuclear NMR spectroscopy. Across the compositional range of G3 : M(OTf)3 mixtures, both metals were found to be in a hexacoordinate environment, with both G3 and [OTf]- ligands present in the first coordination sphere, and apparently exchanging through a dynamic equilibrium. The Lewis acidity was quantified by the Gutmann acceptor number (AN) and compared to the performance of SILs as Lewis acidic catalysts in model [3 + 3] cycloadditions. Despite saturated coordination, AN values were relatively high, reaching AN = ca. 71-83 for Al-SILs and ca. 80-93 for Ga-SILs, corroborating the labile nature of the metal-ligand bonding. In a model catalytic reaction, SILs were fully soluble in the reaction mixtures, in contrast to the corresponding triflate salts. The catalytic performance of SILs exceeded that of the corresponding triflate salts, and Ga-SILs were more active than Al-SILs, in agreement with AN measurements. In conclusion, the new Group 13 SILs can be considered as soluble and catalytically active forms of their corresponding metal triflates, with potential uses in catalysis.

Recent Advanced Applications of Ionic Liquid for Future Iontronics

Recently, electronic devices that make use of a state called the electric double layers (EDL) of ion have opened up a wide range of research opportunities, from novel physical phenomena in solid-state materials to next-generation low-power consumption devices. They are considered to be the future iontronics devices. EDLs behave as nanogap capacitors, resulting the high density of charge carriers is induced at semiconductor/electrolyte by applying only a few volts of the bias voltage. This enables the low-power operation of electronic devices as well as new functional devices. Furthermore, by controlling the motion of ions, ions can be used as semi-permanent charge to form electrets. In this article, we are going to introduce the recent advanced application of iontronics devices as well as energy harvesters making use of ion-based electrets, leading to the future iontronics research.

Applying Classical, Ab Initio, and Machine-Learning Molecular Dynamics Simulations to the Liquid Electrolyte for Rechargeable Batteries

Rechargeable batteries have become indispensable implements in our daily life and are considered a promising technology to construct sustainable energy systems in the future. The liquid electrolyte is one of the most important parts of a battery and is extremely critical in stabilizing the electrode-electrolyte interfaces and constructing safe and long-life-span batteries. Tremendous efforts have been devoted to developing new electrolyte solvents, salts, additives, and recipes, where molecular dynamics (MD) simulations play an increasingly important role in exploring electrolyte structures, physicochemical properties such as ionic conductivity, and interfacial reaction mechanisms. This review affords an overview of applying MD simulations in the study of liquid electrolytes for rechargeable batteries. First, the fundamentals and recent theoretical progress in three-class MD simulations are summarized, including classical, ab initio, and machine-learning MD simulations (section 2). Next, the application of MD simulations to the exploration of liquid electrolytes, including probing bulk and interfacial structures (section 3), deriving macroscopic properties such as ionic conductivity and dielectric constant of electrolytes (section 4), and revealing the electrode-electrolyte interfacial reaction mechanisms (section 5), are sequentially presented. Finally, a general conclusion and an insightful perspective on current challenges and future directions in applying MD simulations to liquid electrolytes are provided. Machine-learning technologies are highlighted to figure out these challenging issues facing MD simulations and electrolyte research and promote the rational design of advanced electrolytes for next-generation rechargeable batteries.

Enabling Magnesium Anodes by Tuning the Electrode/Electrolyte Interfacial Structure

A new deposition mechanism is presented in this study to achieve highly reversible plating and stripping of magnesium (Mg) anodes for Mg-ion batteries. It is known that the reduction of electrolyte anions such as bis(trifluoromethanesulfonyl)imide (TFSI-) causes Mg surface passivation, resulting in poor electrochemical performance for Mg-ion batteries. We reveal that the addition of sodium cations (Na+) in Mg-ion electrolytes can fundamentally alter the interfacial chemistry and structure at the Mg anode surface. The molecular dynamics simulation suggests that Na+ cations contribute to a significant population in the interfacial double layer so that TFSI- anions are excluded from the immediate interface adjacent to the Mg anode. As a result, the TFSI- decomposition is largely suppressed so does the formation of passivation layers at the Mg surface. This mechanism is supported by our electrochemical, microscopic, and spectroscopic analyses. The resultant Mg deposition demonstrates smooth surface morphology and lowered overpotential compared to the pure Mg(TFSI)2 electrolyte.

Ca2+-Based Dual-Carbon Batteries in Ternary Ionic Liquid Electrolytes

Herein, we propose Ca²⁺-based dual-carbon batteries (DCBs) that undergo a simultaneous occurrence of reversible accommodations of Ca²⁺ in a graphite anode (mesocarbon microbeads) and of bis(trifluoromethanesulfonyl)imide (TFSI^-) in a graphite cathode (KS6L). For this purpose, we precisely tune electrolytes composed of Ca²⁺ complexed with a single tetraglyme molecule ([Ca:G₄]) in N-butyl-N-methylpyrrolidinium TFSI (Pyr₁₄TFSI) ionic liquid (IL). This ternary electrolyte is required for the enhancement of anodic stability that is needed to accomplish maximal TFSI^- intercalation into KS6L at a high potential. A solution of 0.5 M [Ca:G₄] in IL ([Ca:G₄]/IL) is found to be optimal for DCBs. First, the electrochemical properties and the structural evolution of each graphite in a half-cell configuration are described to demonstrate excellent electrochemical performance. Second, the negligible intercalation of Pyr₁₄⁺ into an MCMB anode is ascertained in 0.5 M [Ca:G₄]/IL. Finally, DCBs are constructed by coupling two electrodes to show high capacity (54.0 mA h g^-1 at 200 mA g^-1) and reasonable cyclability (capacity fading of 0.022 mA h g^-1 cycle^-1 at 200 mA g^-1 during 300 charge/discharge cycles). This work is the first to examine DCBs based on Ca²⁺ intercalation and helps pave the way for the development of a new type of next-generation batteries.

Redox-active glyme-Li tetrahalogenoferrate(iii) solvate ionic liquids for semi-liquid lithium secondary batteries

Solvate ionic liquids (SILs), comprising long-lived, Li solvate cations and counter anions, serve as highly Li-ion-conductive and non-flammable electrolytes for use in lithium secondary batteries. In this work, we synthesized a series of novel redox-active glyme(oligoether)–Li salt-based SILs, consisting of a symmetric ([Li(G3)]⁺) or asymmetric ([Li(G3Bu)]⁺) triglyme–Li salt complex and redox-active tetrahalogenoferrate ([FeX]⁻ (X = Br₄, Cl₃Br, Cl₄)), for use as the catholyte in semi-liquid lithium secondary batteries. The successful formation of stable molten complexes of [Li(G3/G3Bu)][FeX] was confirmed by Raman spectroscopy and thermogravimetry. The melting point (T_m) depended on both the molecular weights of the complex anions and the structures of the complex cations. [Li(G3)][FeCl₄] comprised complex cations with a symmetric structure, and the smallest complex anions showed the lowest T_m of 28.2 °C. The redox properties of the [FeX]⁻/[FeX]²⁻ couple strongly suggested the suitability of [Li(G3/G3Bu)][FeX] as a catholyte. The discharge capacities of semi-liquid lithium secondary batteries utilizing the [Li(G3/G3Bu)][FeX] catholyte depended on the structure of the SILs, and the cell with [Li(G3)][FeCl₄] showed the highest capacity with relatively good capacity retention. This study confirmed the feasibility of the glyme-based redox-active SILs as catholytes for scalable redox-flow type batteries.

Solvate ionic liquids (SILs), comprising long-lived, Li solvate cations and counter anions, serve as highly Li-ion-conductive and non-flammable electrolytes for use in lithium secondary batteries.

Understanding Electric Double-Layer Gating Based on Ionic Liquids: from Nanoscale to Macroscale

In electric double-layer transistors (EDLTs), it is well known that the EDL formed by ionic liquids (ILs) can induce an ultrahigh carrier density at the semiconductor surface, compared to solid dielectric. However, the mechanism of device performance is still not fully understood, especially at a molecular level. Here, we evaluate the gating performance of amorphous indium gallium zinc oxide (a-IGZO) transistor coupled with a series of imidazolium-based ILs, using an approach combining of molecular dynamics simulation and finite element modeling. Results reveal that the EDL with different ion structures could produce inhomogeneous electric fields at the solid-electrolyte interface, and the heterogeneity of electric field-induced charge distributions at semiconductor surface could reduce the electrical conductance of a-IGZO during gating process. Meanwhile, a resistance network analysis was adopted to bridge the nanoscopic data with the macroscopic transfer characteristics of IL-gated transistor, and showed that our theoretical results could well estimate the gating performance of practical devices. Thereby, our findings could provide both new concepts and modeling techniques for IL-gated transistors.