Effect of the alkyl group on the synthesis and the electrochemical properties of N-alkyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquids

The Mass-Balancing between Positive and Negative Electrodes for Optimizing Energy Density of Supercapacitors

Supercapacitors (SCs) are some of the most promising energy storage devices, but their low energy density is one main weakness. Over the decades, superior electrode materials and suitable electrolytes have been widely developed to enhance the energy storage ability of SCs. Particularly, constructing asymmetric supercapacitors (ASCs) can extend their electrochemical stable voltage windows (ESVWs) and thus achieve high energy density. However, only full utilization of the electrochemical stable potential windows (ESPWs) of both positive and negative electrodes can endow the ASC devices with a maximum ESVW by using a suitable mass-ratio between two electrodes (the mass-balancing). Nevertheless, insufficient attention is directed to mass-balancing, and even numerous misunderstandings and misuses have appeared. Therefore, in this Perspective, we focus on the mass-balancing: summarize theoretic basis of the mass-balancing, derive relevant relation equations, analyze and discuss the change trends of the specific capacitance and energy density of ASCs with mass-ratios, and finally recommend some guidelines for the normative implementation of the mass-balancing. Especially, the issues related to pseudocapacitive materials, hybrid devices, and different open circuit potentials (OCPs) of the positive and negative electrodes in the mass-balancing are included and emphasized. These analyses and guidelines can be conducive to understanding and performing mass-balancing for developing high-performance SCs.

Influence of surface nanostructure-induced innermost ion structuring on capacitance of carbon/ionic liquid double layers

Ionic liquids have drawn great interest as electrolytes for energy storage applications in which they form characteristic electrical double layers at electrode interfaces. For ionic liquids at carbon electrode interfaces, their double layers are subject to nanoscale structuring of the electrode surface, involving altered ion structure and interactions that significantly influence the double layer capacitance. In this regard, we investigate the modulation of ionic liquid double layers by electrode surface roughness and the resulting effects on the ion structure, interaction, and capacitance. We performed fixed voltage molecular dynamics simulations to compute the differential capacitance profiles for the ionic liquids [BMIm+][TFSI-] and [BMIm+][FSI-] at model carbon electrode interfaces with the surface channel width at subnanometer and nanometer scales. We find that both [BMIm+][TFSI-] and [BMIm+][FSI-] exhibit enhanced differential capacitance for the electrode surface with a subnanometer channel width relative to the flat graphene surface, but the most pronounced enhancements for these two ionic liquids unexpectedly appear at different applied potential regimes. For [BMIm+][TFSI-], the nanostructured electrode shows significant enhancement of capacitance at high positive potential. For [BMIm+][FSI-], on the other hand, this enhancement is small at positive polarization but noticeable at low negative potential. We demonstrate that differences in these capacitance trends is due to differences in ion correlation that arise from a steric constraint of nanostructured electrode surface on the voltage-mediated restructuring of ions closest to the electrode interface. For example, the TFSI- and FSI- anions tend to structure with their charged and nonpolar groups in contact with the positive electrode surface when the constraint on these close-contact anions is relaxed. This anion structuring largely retains the cation association near the nanostructured electrode, resulting in only a slight increase in capacitance at positive polarization. Our simulations highlight the sensitive dependence of the innermost ion structure on the electrode surface nanostructure and applied voltage and the resulting influence on ion correlation and capacitance of ionic liquid double layers.

Physical and Electrochemical Analysis of N-Alkylpyrrolidinium-Substituted Boronium Ionic Liquids

In this work, a series of novel boronium-bis(trifluoromethylsulfonyl)imide [TFSI-] ionic liquids (IL) are introduced and investigated. The boronium cations were designed with specific structural motifs that delivered improved electrochemical and physical properties, as evaluated through cyclic voltammetry, broadband dielectric spectroscopy, densitometry, thermogravimetric analysis, and differential scanning calorimetry. Boronium cations, which were appended with N-alkylpyrrolidinium substituents, exhibited superior physicochemical properties, including high conductivity, low viscosity, and electrochemical windows surpassing 6 V. Remarkably, the boronium ionic liquid functionalized with both an ethyl-substituted pyrrolidinium and trimethylamine, [(1-e-pyrr)N111BH2][TFSI], exhibited a 6.3 V window, surpassing previously published boronium-, pyrrolidinium-, and imidazolium-based IL electrolytes. Favorable physical properties and straightforward tunability make boronium ionic liquids promising candidates to replace conventional organic electrolytes for electrochemical applications requiring high voltages.

Bioderived Ionic Liquids with Alkaline Metal Ions for Transient Ionics

Choline lactate, an ionic liquid composed of bioderived materials, offers an opportunity to develop biodegradable electrochemical devices. Although ionic liquids possess large potential windows, high conductivity, and are nonvolatile, they do not exhibit electrochemical characteristics such as intercalation pseudocapacitance, redox pseudocapacitance, and electrochromism. Herein, bioderived ionic liquids are developed, including metal ions, Li, Na, and Ca, to yield ionic liquid with electrochemical behavior. Differential scanning calorimetry results reveal that the ionic liquids remained in liquid state from 230.42 to 373.15 K. The conductivities of the ionic liquids with metal are lower than those of the pristine ionic liquid, whereas the capacitance change negligibly. A protocol of the Organization for Economic Co-operation and Development 301C modified MITI test (I) confirms that the pristine ionic liquid and ionic liquids with metal are readily biodegradable. Additionally, an ionic gel comprising the ionic liquid and poly(vinyl alcohol) is biodegradable. An electrochromic device is developed using an ionic liquid containing Li ions. The device successfully changes color at -2.5 V, demonstrating the intercalation of Li ions into the WO3 crystal. The results suggest that the electrochemically active ionic liquids have potential for the development of environmentally benign devices, sustainable electronics, and bioresorbable/implantable devices.

Temperature-Dependent Electrochemical Stability Window of Bis(trifluoromethanesulfonyl)imide and Bis(fluorosulfonyl)imide Anion Based Ionic Liquids

The electrochemical stability of 22 commercially available hydrophobic ionic liquids was measured at different temperatures (288.15, 298.15, 313.15, 333.15 and 358.15 K), to systematically investigate ionic liquids towards electrolytes for supercapacitors in harsh weather conditions. Bis(trifluoromethanesulfonyl)imide and bis(fluorosulfonyl)imide anions in combination with 1-Butyl-1-methylpyrrolidinium, 1-Ethyl-3-methylimidazolium, N-Ethyl-N, N-dimethyl-N(2methoxyethyl)ammonium, 1-Methyl-1-(2-methoxyethyl)pyrrolidinium, N-Pentyl-N-methylpyrrolidinium, N, N-Diethyl-N-methyl-N-propylammonium, N, N-Dimethyl-N-ethyl-N-benzyl ammonium, N, N-Dimethyl-N-Ethyl-N-phenylethylammonium, N-Butyl-N-methylpiperidinium, 1-Methyl-1-propylpiperidinium, N-Tributyl-N-methylammonium, N-Trimethyl-N-butylammonium, N-Trimethyl-N-butylammonium, N-Trimethyl-N-propylammonium, N-Propyl-N-methylpyrrolidinium cations were selected for the study. Linear regression with a numerical model was used in combination with voltammetry experiments to deduce the temperature sensitivity of both anodic and cathodic potential limits (defining the electrochemical stability window), in addition to extrapolating results to 283.15 and 363.15 K. We evaluated the influence of the cations, anions, and the presence of functional groups on the observed electrochemical stability window which ranged from 4.1 to 6.1 V.

Synthesis, Physical Properties and Electrochemical Applications of Two Ionic Liquids Containing the Asymmetric (Fluoromethylsulfonyl)(Trifluoromethylsulfonyl)imide Anion

Novel ionic liquid (IL) electrolytes based on the asymmetric (fluoromethylsulfonyl)(trifluoromethylsulfonyl)imide (FTFSI)− anion, combined with the N-trimethyl-N-butyl-ammonium (N1114)+ and N,N-diethyl-N-methyl-N(2-methoxyethyl)-ammonium (N122(2O1))+ cations, were successfully synthesized and investigated in terms of thermal, vibrational and electrochemical properties. Thermogravimetric measurements revealed that the ionic liquids are stable up to 300 °C (2% mass loss). Differential scanning calorimetry measurements evidenced no phase transition down to −90 °C, suggesting a transition towards a glass state at lower temperatures. Infrared spectroscopy measurements, for the first time performed on ILs containing FTFSI, could not detect any crystallization down to −140 °C. The frequency of the main absorption bands of the ILs are in good agreement with DFT calculations. The FTFSI ionic liquid electrolytes, containing 20% mol of LiTFSI, show no solid-liquid phase transition due to the asymmetry of the FTFSI− anion, increasing the −10 °C conductivity up to 10−4 S cm−1. These interesting ion transport properties remarkably extend the operative temperature range down to low temperatures. The FTFSI electrolytes exhibit remarkable electrochemical stability up to 4.8 V, this making them appealing for realizing safer and highly reliable lithium battery systems operating at high voltages.

Critical Review on Low-Temperature Li-Ion/Metal Batteries

With the highest energy density ever among all sorts of commercialized rechargeable batteries, Li-ion batteries (LIBs) have stimulated an upsurge utilization in 3C devices, electric vehicles, and stationary energy-storage systems. However, a high performance of commercial LIBs based on ethylene carbonate electrolytes and graphite anodes can only be achieved at above -20 °C, which restricts their applications in harsh environments. Here, a comprehensive research progress and in-depth understanding of the critical factors leading to the poor low-temperature performance of LIBs is provided; the distinctive challenges on the anodes, electrolytes, cathodes, and electrolyte-electrodes interphases are sorted out, with a special focus on Li-ion transport mechanism therein. Finally, promising strategies and solutions for improving low-temperature performance are highlighted to maximize the working-temperature range of the next-generation high-energy Li-ion/metal batteries.

Structure-Property Relationship of Polymerized Ionic Liquids for Solid-State Electrolyte Membranes

Eight new polymerized ammonium-based ionic liquids were prepared as thin membrane films and evaluated within the scope of their usage in lithium-ion batteries. The focus of this work is to get a better understanding of the influence of structural modifications of the monomers on the polymerized materials. Further, different concentrations of a lithium-ion conducting salt were applied in order to receive an optimized combination of monomer structure and lithium salt concentration. It was found that an increased side chain length of the studied ammonium-based polymerized ionic liquids leads to a reduction in glass transition temperatures and increased ionic conductivity values. As a result of the addition of conducting salt to the PIL membranes, the glass transition temperatures and the ionic conductivity values decreases. Nevertheless, PFG-NMR reveals a higher lithium-ion mobility for a sample with higher conducting salt content.

Long-Chain Ionic Liquids Based on Monoquaternary DABCO Cations and TFSI Anions: Towards Stable Electrolytes for Electrochemical Capacitors

The desirable properties of ionic liquids (ILs) enable their use in various branches of chemistry, through a wide range of applications, e. g. as organic electrolytes. In the present study, an efficient two-step method was developed for the synthesis of long-chain ionic liquids with alkyl derivatives of DABCO as cations and bis(trifluoromethane)sulfonimide as anions. ILs obtained with high yields (≥91 %) were solids with melting points that increased with the rise in the number of carbon atoms of the alkyl substituent in the bicyclic cation. The structure of the compounds was confirmed by spectroscopic methods and elemental analysis. All compounds were soluble in the main solvents except water and hexane. The solubility in organic solvents such as acetonitrile allowed the use of synthesized ILs in electrochemical capacitors. Electrochemical tests revealed that the ILs enhanced the conductivity of organic electrolytes. This phenomenon improved the cyclability and reduced the internal resistance of the electrochemical capacitors.

Insight into the role of ion-pairing in the adsorption of imidazolium derivative-based ionic liquids by activated carbon

The association of the cation and anion of ionic liquids (ILs) dominates the absorbability of ILs by activated carbon (AC). Nevertheless, the mechanism behind the role of ion-pairs is largely unknown. In this study, the adsorption of a series of imidazolium derivative-based ILs by AC was involved in response to the octanol-water partition coefficient (K_OW), hydrogen bonding acidity (α), ion-pair binding constants (K_IP), binding energy of ion-pairs (E_binding) and density functional theory (DFT) calculation of ILs. A significant positive correlation between lg K_OW and K_d and between K_IP and lg K_OW was observed (p < 0.05). However, both E_binding and α was inversely proportional to K_IP. Hence, the substitution of oxygen-containing functional groups, such as carboxyethyl, 1-methoxyethyl, and 1-(ethoxycarbonyl)methyl, on imidazolium ring enhanced the hydrogen bond interaction with water molecules and then weakened the binding of imidazolium cation and [NTf₂]^-, thereby reducing the adsorption of ILs to AC. DFT calculation further revealed that the polar substitution improved the electron density and electronegativity of imidazolium skeleton. By contrast, the ILs functionalized with non-polar groups (e.g., butyl, allyl, and benzyl) generally displayed high K_IP values and low α values. Consequently, the formation of hydrogen bond between the oxygen-containing functional groups of IL cation and water would facilitate the dissociation of IL ion-pairs and then decrease the adsorption of ILs on AC.

The behavior of conductivity dynamic modulus and its connection to thermodynamic bulk modulus in ionic liquids

In this paper, we investigate the interplay between the dynamics and thermodynamics of aprotic ionic liquids in the supercooled and normal liquid states. For this purpose, the conductivity dynamic modulus Mσp-T, being defined as the ratio of activation energy (Ep) and activation volume (VT), and its relation to bulk modulus BT under isobaric and isothermal conditions is examined. We found that both isobaric cooling and isothermal compression lead to an increase in Mσp-T. Specifically, Mσp-T(P)T rises linearly similar to BT. Consequently, a direct linear relationship between Mσp-T(P)T and BT is established under isothermal conditions.

The influence of alkyl chain branching on the properties of pyrrolidinium-based ionic electrolytes

Ionic liquids and plastic crystals based on pyrrolidinium cations are recognised for their advantageous properties such as high conductivity, low viscosity, and good electrochemical and thermal stability. The pyrrolidinium ring can be substituted with symmetric or asymmetric alkyl chain substituents to form a range of ionic liquids or plastic crystals depending on the anion. However, reports into the use of branched alkyl chains and how this influences the material properties are limited. Here, we report the synthesis of six salts - ionic liquids and organic ionic plastic crystals - where the typically used linear propyl chain substituent is replaced by the branched alternative, isopropyl, to form the cation [C(i3)mpyr]+, in combination with six different anions: dicyanamide, (fluorosulfonyl)(trifluoromethanesulfonyl)imide, bis(trifluoromethanesulfonyl)imide, bis(fluorosulfonyl)imide, tetrafluoroborate and hexafluorophosphate. The thermal and transport properties of these salts are compared to those of the analogous N-propyl-N-methylpyrrolidinium and N,N-diethylpyrrolidinium-based salts. Finally, a high lithium salt content ionic liquid electrolyte based on the bis(fluorosulfonyl)imide salt was developed. This electrolyte showed high coulombic efficiencies of lithium plating/stripping and high lithium ion transference number, making it a strong candidate for use in lithium metal batteries.

A fluorine-substituted pyrrolidinium-based ionic liquid for high-voltage Li-ion batteries

A fluorine-substituted ionic liquid based on a pyrrolidinium cation and a bis(fluorosulfonyl)imide anion was synthesized using a facile one-step reaction. The resulting ionic liquid is highly pure and when dissolved with LiFSI, the IL-based electrolyte showed good compatibility both in Li and graphite anodes, and superior voltage stability is demonstrated in a LiNi_0.5Mn_0.3Co_0.2O₂ cell.

Surface Science and Electrochemical Model Studies on the Interaction of Graphite and Li-Containing Ionic Liquids

The process of solid-electrolyte interphase (SEI) formation is systematically investigated along with its chemical composition on carbon electrodes in an ionic liquid-based, Li-containing electrolyte in a combined surface science and electrochemical model study using highly oriented pyrolytic graphite (HOPG) and binder-free graphite powder electrodes (Mage) as model systems. The chemical decomposition process is explored by deposition of Li on a pre-deposited multilayer film of 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][TFSI]) under ultrahigh vacuum conditions. Electrochemical SEI formation is induced by and monitored during potential cycling in [BMP][TFSI]+0.1 m LiTFSI. The chemical composition of the resulting layers is characterized by X-ray photoelectron spectroscopy (XPS), both at the surface and in deeper layers, closer to the electrode|SEI interface, after partial removal of the film by Ar⁺ ion sputtering. Clear differences between chemical and electrochemical SEI formation, and also between SEI formation on HOPG and Mage electrodes, are observed and discussed.

Hydroxyl Functionalized Pyridinium Ionic Liquids: Experimental and Theoretical Study on Physicochemical and Electrochemical Properties

Structurally modified hydroxyl functionalized pyridinium ionic liquids (ILs), liquid at room temperature, were synthesized and characterized. Alkylated N-(2-hydroxyethyl)-pyridinium ILs were prepared from alkylpyridines via corresponding bromide salts by N-alkylation (65–93%) and final anion exchange (75–96%). Pyridinium-alkylation strongly influenced the IL physicochemical and electrochemical properties. Experimental values for the ILs physicochemical properties (density, viscosity, conductivity, and thermal decomposition temperature), were in good agreement with corresponding predicted values obtained by theoretical calculations. The pyridinium ILs have electrochemical window of 3.0–5.4 V and were thermally stable up to 405°C. The IL viscosity and density were measured over a wide temperature range (25–80°C). Pyridine alkyl-substitution strongly affected the partial positive charge on the nitrogen atom of the pyridinium cations, as shown by charge distribution calculations. On-going studies on Mg complexes of the new ILs demonstrate promising properties for high current density electrodeposition of magnesium.

Carbonate Solvents and Ionic Liquid Mixtures as an Electrolyte to Improve Cell Safety in Sodium-Ion Batteries

Ionic liquid-based electrolytes proved to be effective in terms of alleviating the safety problems associated with lithium/sodium ion batteries, especially for large-scale applications, due to their superior thermal stability and nonflammability. The main disadvantage of ionic liquids is their relatively high viscosity. Adding a suitable amount of organic “thinning” solvents could be a potential solution for this problem: while the electrolyte viscosity is greatly reduced, the electrochemical properties and thermal stability remain almost as good as those of pure ionic liquid. In this study, electrolyte mixtures based on 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) (EMI-TFSI) and carbonate solvents (EC-PC) were prepared. The electrochemical compatibility in half-cell configuration with respect to sodium metal anode of various electrode materials, including SnS/C, hard carbon (HC), and Na0.44MnO2, was evaluated. Moreover, the thermal stability, the flammability, and the conduction mechanism of such electrolyte mixtures were also explored and discussed.

Correlating structure and transport behavior in Li+ and O2 containing pyrrolidinium ionic liquids

Using experiments and molecular simulations, we evaluate pyrrolidinium-based ionic liquid Li electrolytes and find that Li⁺ and O₂ transport can be enhanced by varying the pyrrolidinium structure and Li concentration.

Synthesis and physicochemical characterization of room temperature ionic liquids and their application in sodium ion batteries

Sodium ion batteries (SIBs) based on IL electrolytes have attracted great attention, particularly in large-scale energy storage systems for renewable energy due to the abundance of sodium and the excellent safety resulting from the use of non-flammable ionic liquid (IL) electrolytes. In this article, a series of 15 functionalized room temperature ionic liquids (RTILs) suitable as electrolytes is presented. Special emphasis was laid on the purity of the synthesized RTILs and a consistent and uniform characterization of their physicochemical properties. Evaluation of the viscosity, conductivity, and thermal and electrochemical stabilities resulted in clear structure-property relationships, rendering the ether functionalized RTILs most promising for application in SIBs. Electrochemical investigations of the ether functionalized IL electrolytes in SIB half cells (Na0.6Mn0.9Co0.1O2 as cathode material) proved their compatibility with a SIB system. Stable cycling performance was achieved with the piperidinium based RTIL IL 6 outperforming the organic electrolyte by far with a retention of 81% after 350 cycles. These results show the suitability of RTILs to enhance the performance of SIB systems and serve as a basis for the design of high performance IL electrolytes.

Interface-Engineered Li7 La3 Zr2 O12 -Based Garnet Solid Electrolytes with Suppressed Li-Dendrite Formation and Enhanced Electrochemical Performance

High grain-boundary resistance, Li-dendrite formation, and electrode/Li interfacial resistance are three major issues facing garnet-based solid electrolytes. Herein, interfacial architecture engineering by incorporating 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide (BMP-TFSI) ionic liquid into a garnet oxide is proposed. The "soft" continuous BMP-TFSI coating with no added Li salt generates a conducting network facilitating Li⁺ transport and thus changes the ion conduction mode from point contacts to face contacts. The compacted microstructure suppresses Li-dendrite growth and shows good interfacial compatibility and interfacial wettability toward Li metal. Along with a broad electrochemical window larger than 5.5 V and an Li⁺ transference number that practically reaches unity, LiNi_0.8 Co_0.1 Mn_0.1 O₂ /Li and LiFePO₄ /Li solid-state batteries with the hybrid solid electrolyte exhibit superior cycling stability and low polarization, comparable to those with commercial liquid electrolytes, and excellent rate capability that is better than those of Li-salt-based ionic-liquid electrolytes.

Liquid-like Poly(ionic liquid) as Electrolyte for Thermally Stable Lithium-Ion Battery

A liquid-like poly(ionic liquid) (PIL) with a very low glass transition temperature of -51 °C and a thermal decomposition temperature of 202.7 °C was synthesized. A PIL-based electrolyte by mixing this poly(ionic liquid) with additives of 10 wt % propylene carbonate and 0.1 M LiClO₄ is proved to be an excellent electrolyte for lithium-ion battery. The obtained PIL-based electrolyte exhibits a high ionic conductivity of 8.3 × 10^-5 S cm^-1 at 25 °C and 2.0 × 10^-4 S cm^-1 at 60 °C and a wide electrochemical potential window up to 5.61 V at 25 °C and 4.14 V at 60 °C. The Li/LiFePO₄ batteries equipped with this PIL-based electrolyte achieve high capacity, outstanding cycling stability and rate capability at 25 °C, and even improved performance at high temperature like 60 °C. Such excellent performances of batteries are attributed to the formation of stable solid-electrolyte interface film at the lithium-electrolyte interface and the stability of electrolyte during cycling.

Variations and applications of the oxygen reduction reaction in ionic liquids

Increasing energy demands call for new energy storage technologies with high energy density to meet current and future needs. Metal-air batteries are especially attractive due to their superior specific energy, which is as much as 8 times that of today's best Li-ion batteries. However, the practical values achieved to date are far from the theoretical one and require further research to enhance the battery performance. The electrolyte plays an important role in the performance of the battery whose properties can be tuned by varying the chemical composition and through the use of additives. That is the case of ionic liquids which offer a wide variety of anion-cation combinations to realise maximum performance. This feature article overviews recent developments in ionic liquids as electrolytes for both magnesium-air and sodium-air batteries.

Thermal, electrochemical and radiolytic stabilities of ionic liquids

Ionic liquids show instability when exposed to high temperature, to high voltage as electrolytes, or under irradiation.

A Lithium-Ion Battery with Enhanced Safety Prepared using an Environmentally Friendly Process

A new lithium-ion battery chemistry is presented based on a conversion-alloying anode material, a carbon-coated Fe-doped ZnO (TMO-C), and a LiNi1/3 Mn1/3 Co1/3 O2 (NMC) cathode. Both electrodes were fabricated using an environmentally friendly cellulose-based binding agent. The performance of the new lithium-ion battery was evaluated with a conventional, carbonate-based electrolyte (ethylene carbonate:diethyl carbonate-1 m lithium hexafluorophosphate, EC:DEC 1 m LiPF6 ) and an ionic liquid (IL)-based electrolyte (N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide-0.2 m lithium bis(trifluoromethanesulfonyl)imide, Pyr14 TFSI 0.2 m LiTFSI), respectively. Galvanostatic charge/discharge tests revealed a reduced rate capability of the TMO-C/Pyr14 TFSI 0.2 m LiTFSI/NMC full-cell compared to the organic electrolyte, but the coulombic efficiency was significantly enhanced. Moreover, the IL-based electrolyte substantially improves the safety of the system due to a higher thermal stability of the formed anodic solid electrolyte interphase and the IL electrolyte itself. While the carbonate-based electrolyte shows sudden degradation reactions, the IL exhibits a slowly increasing heat flow, which does not constitute a serious safety risk.