Protic ionic liquids: preparation, characterization, and proton free energy level representation

We give a perspective on the relations between inorganic and organic cation ionic liquids (ILs), including members with melting points that overlap around the borderline 100 degrees C. We then present data on the synthesis and properties (melting, boiling, glass temperatures, etc.) of a large number of an intermediate group of liquids that cover the ground between equimolar molecular mixtures and ILs, depending on the energetics of transfer of a proton from one member of the pair to the other. These proton-transfer ILs have interesting properties, including the ability to serve as electrolytes in solvent-free fuel cell systems. We provide a basis for assessing their relation to aprotic ILs by means of a Gurney-type proton-transfer free energy level diagram, with approximate values of the energy levels based on free energy of formation and pK(a) data. The energy level scheme allows us to verify the relation between solvent-free acidic and basic electrolytes, and the familiar aqueous variety, and to identify neutral protic electrolytes that are unavailable in the case of aqueous systems.     

Landscapes and fragilities

The concept of fragility provides a possibility to rank different supercooled liquids on the basis of the temperature dependence of dynamic and/or thermodynamic quantities. We recall here the definitions of kinetic and thermodynamic fragility proposed in the last years and discuss their interrelations. At the same time we analyze some recently introduced models for the statistical properties of the potential energy landscape. Building on the Adam-Gibbs relation, which connects structural relaxation times to configurational entropy, we analyze the relation between statistical properties of the landscape and fragility. We call attention to the fact that the knowledge of number, energy depth, and shape of the basins of the potential energy landscape may not be sufficient for predicting fragility. Finally, we discuss two different possibilities for generating strong behavior.     

Solvate Ionic Liquids for Li,Na, K,and Mg Batteries

From the viewpoint of element strategy, non‐Li batteries with promising negative and positive electrodes have been widely studied to support a sustainable society. To develop non‐Li batteries having high energy density, research on electrolyte materials is pivotal. Solvate ionic liquids (SILs) are an emerging class of electrolytes possessing somewhat superior properties for battery applications compared to conventional ionic liquid electrolytes. In this account, we describe our recent efforts regarding SIL‐based electrolytes for Li, Na, K, and Mg batteries with respect to structural, physicochemical, and electrochemical characteristics. Systematic studies based on crystallography and Raman spectroscopy combined with thermal/electrochemical stability analysis showed that the balance of competitive cation?anion and cation?solvent interactions predominates the stability of the solvate cations. We also demonstrated battery applications of SILs as electrolytes for non‐Li batteries, particularly for Na batteries.     

Amide-Based Ionic Liquid Electrolytes for Alkali-Metal-Ion Rechargeable Batteries

Ionic liquids (ILs) have a wide variety of applications in energy storage and material production. ILs are composed of only cations and anions, without any molecular solvents, and are generally known as “designer liquids (solvents)” because their physicochemical properties can be tuned by the combination of ionic species. In recent several decades, research and development activities of rechargeable batteries have garnered considerable attention because certain groups of ILs exhibit high electrochemical stability and moderate ionic conductivity, rendering them suitable for application in high-voltage batteries. ILs with amide anions are representative electrolytes and are extensively researched by many research groups, including our group. This paper focuses on amide-based ILs as electrolytes for alkali-metal-ion rechargeable batteries, introducing their history, characteristics, and existing challenges to be addressed.     

Describing the unsuspected advantage of redox ionic liquids applied to electrochemical energy storage

Ionic liquids are a class of solvents widely studied in the literature for various applications. As a subclass of ionic liquids, redox ionic liquids can endow charge exchange properties (electrons transfer) to these electrolytes for electrochemical energy storage. In this review article, we propose to study this family of ionic liquids and suggest a chronological classification. We introduce five generations of redox ionic liquids with different basic compounds such as polyethylene glycol, ferrocene, different linker lengths, TFSI anion, and biredox ionic liquids. The versatility of the redox ionic liquids synthesis will be discussed as well as the fundamental and applied aspects of their use as electrolytes, which have high charge densities. The impact of the redox ionic liquids on the electrochemical mechanisms will be described. We also present how the redox shuttle effect, detrimental to supercapacitors, can be prevented while it can be used to improve lithium-ion batteries.     

低聚离子液体的体相与界面及其电化学储能应用

近年来,随着单阳离子液体的发展,新型低聚物离子液体被合成并应用。这类离子液体可看作是由几个重复的单阳离子组合而成,可以通过改变阳离子带电基团、间隔连接的长度或种类、末端链的长度以及阴离子种类来获得更多不同的结构。因此,低聚离子液体有更复杂的微观结构和内部相互作用,决定了其多特征的物化性质和电化学特性,有望满足更多对溶剂性能有特定要求的应用。例如,与单阳离子液体相比,低聚离子液体具有更大的可调节性、更宽的液态温度范围、更高的热稳定性等优点,使其在电化学储能设备中得到越来越多的应用,如用作超级电容器和锂离子电池的电解液。在本综述中,我们系统地总结并详细解释了低聚离子液体的性质和结构（包括单个离子的结构和本体液内部的纳米组织）之间的关联,主要是双阳离子液体和三阳离子液体;概括了低聚离子液体作为超级电容器和锂离子电池的电解液的相关研究,重点阐述了由低聚离子液体和不同类型电极组成的双电层的结构和性能,以及与相应单阳离子液体电解液的比较结果;提供了降低低聚离子液体粘度和加速离子扩散的优化措施,提出了低聚离子液体电解液未来可能面临的主要问题和发展前景。     

Water-in-salt electrolytes: An interfacial perspective

Liquid electrolytes with high ionic conductivity, high transference number for the target ions, and excellent electrochemical, chemical, and thermal stability are essential for electrochemical energy storage devices. Water-in-salt (WIS) electrolytes, in which the salt–water ratio is larger than one, are gaining intensive attention in the electrochemical community. Here, we review the recent work on WIS electrolytes and the closely related water-in-ionic liquid electrolytes. We highlight the fact that many properties of these electrolytes, in bulk and at electrolyte–electrode interfaces, are underpinned by the physics and chemistry of the interfaces formed between water and ions (or aggregated water/ion clusters). Manipulating these interfaces by tailoring the selection of ions and water–ion ratio opens up new dimensions in the optimization of liquid electrolytes but also poses new challenges. We conclude the review by highlighting several directions for research on WIS electrolytes, in particular, the study of WIS electrolyte–electrode interfaces using surface force measurements.     

Ionic liquid electrolytes for dye-sensitized solar cells

The potential of room-temperature molten salts (ionic liquids) as solvents for electrolytes for dye-sensitized solar cells has been investigated during the last decade. The non-volatility, good solvent properties and high electrochemical stability of ionic liquids make them attractive solvents in contrast to volatile organic solvents. Despite this, the relatively high viscosity of ionic liquids leads to mass-transport limitations. Here we review recent developments in the application of different ionic liquids as solvents or components of liquid and quasi-solid electrolytes for dye-sensitized solar cells.     

Ionic Liquid/Poly(ionic liquid)-based Semi-solid State Electrolytes for Lithium-ion Batteries

Ionic liquids(ILs) have appeared as the most promising electrolytes for lithium-ion batteries, owing to their unique high ionic conductivity, chemical stability and thermal stability properties. Poly(ionic liquid)s(PILs) with both IL-like characteristic and polymer structure are emerging as an alternative of traditional electrolyte. In this review, recent progresses on the applications of IL/PIL-based semi-solid state electrolytes, including gel electrolytes, ionic plastic crystal electrolytes, hybrid electrolytes and single-ion conducting electrolytes for lithium-ion batteries are discussed.     

面向柔性超级电容器的凝胶电解质研究进展

柔性超级电容器卓越的功率密度和柔性应用能力与可穿戴设备对柔性电源的紧迫需求相吻合,同时具备较大的能量密度提升潜力,使其受到了广泛关注。将水、有机液体、离子液体和导电离子等溶入三维聚合物网络构建导电凝胶作为电解质,不仅简化了柔性超级电容器结构,还通过引入多样的交联方式和合成材料进一步提升其性能,已成为近年备受瞩目的研究方向。本文深入分析并总结了凝胶电解质应用于柔性超级电容器的独特优势及其关键性能优化的方法,包括：调控导电离子含量及传输路径以提高离子电导率;采用双重物理交联和模板化合成策略调节凝胶网络结构以改善机械性能;引入有机液体、离子液体等溶剂限制冰晶形成,从而拓宽工作温度范围。然而,凝胶电解质在柔性超级电容器应用中仍然面临一系列挑战,包括生物相容性不足、电极/电解质界面兼容性弱以及合成材料的环保性不佳。未来研究需进一步解决上述问题,以实现凝胶电解质在柔性超级电容器中的高效应用。     

Redox-active electrolyte supercapacitors using electroactive ionic liquids

Here we present redox ionic liquid supercapacitors (RILSCs) which use electrolytes made from ionic liquids modified with an electroactive function to increase the energy density of activated carbon electrodes via faradaic reactions. More specifically, two different ionic liquids were made by modifying either the imidazolium cation or the bis(trifluoromethanesulfonyl)imide anion with ferrocene in order to determine the importance of the electroactive ion's polarity. The functionalization of an ionic liquid with ferrocene led to high concentrations of redox moieties in the electrolyte (2.4 M) and a large maximum operating voltage (2.5 V). An energy density of up to 13.2 Wh per kg (both electrodes) was obtained which represents an 83% increase vs. the unmodified ionic liquid. When the ionic liquid's anion is modified with ferrocene, the self-discharge at the positive electrode is fully suppressed due to the deposition of a film on the electrode. The results presented herein demonstrate that electroactive ionic liquids constitute a promising alternative to conventional solute in solvent electrolytes found in energy storage devices, and are particularly well-suited for redox-active electrolyte supercapacitors.     

全固态电池界面的研究进展

Owing to the serious energy crisis and environmental problems caused by fossil energy consumption, development of high-energy-density batteries is becoming increasingly significant to satisfy the rapidly growing social demands. Lithium-ion batteries have received widespread attention because of their high energy densities and environmental friendliness. At present, they are widely used in portable electronic devices and electric vehicles. However, security aspects need to be addressed urgently. Substantial advances in liquid electrolyte-based lithium-ion batteries have become a performance bottleneck in the recent years. Traditional lithium-ion batteries use organic liquids as electrolytes, but the flammability and corrosion of these electrolytes considerably limit their development. Continuous growth of lithium dendrites can pierce the separator, leading to electrolyte leakage and combustion, which is a serious safety hazard. Replacement of organic electrolytes with solid-state electrolytes is one of the promising solutions for the development of next-generation energy storage devices, because they have high energy densities and are safe. Solid electrolytes can remarkably alleviate the safety hazards involved in the use of traditional liquid-based lithium-ion batteries. In addition, the composite of solid-state electrolytes and lithium metal is expected to result in a higher energy density. However, due to the lack of fluidity of the solid electrolytes, problems such as limited solid-solid contact area and increased impedance at the interface when solid-state electrolytes are in contact with electrodes must be solved. The localized and buried interface is a major drawback that restricts the electrochemical performance and practical applications of the solid-state batteries. Fabrication of a stable interface between the electrodes and solid-state electrolyte is the main challenge in the development of solid-state lithium metal batteries. All these aspects are critical to the electrochemical performance and safety of the solid-state batteries. Current research mainly focuses on addressing the problems related to the solid-solid interface in solid-state batteries and improving the electrochemical performance of such batteries. In this review, we comprehensively summarize the challenges in the fabrication of solid-state batteries, including poor chemical and electrochemical compatibilities and mechanical instability. Research progress on the improvement strategies for interface problems and the advanced characterization methods for the interface problems are discussed in detail. Meanwhile, we also propose a prospect for the future development of solid-state batteries to guide the rational designing of next-generation high-energy solid-state batteries. There are many critical problems in solid-state batteries that must be fully understood. With further research, all-solid-state batteries are expected to replace the traditional liquid-based lithium-ion batteries and become an important system for a safe and reliable energy storage.     

Recent Progress in the Electrolytes of Aqueous Zinc-Ion Batteries

Rechargeable aqueous zinc-ion batteries (ZIBs) have garnered tremendous attention in the field of next energy storage devices due to their high safety, low cost, abundant resources, and eco-friendliness. As an important component of the zinc-ion battery, the electrolyte plays a vital role in the electrochemical properties, since it will provide a pathway for the migrations of the zinc ions between the cathode and anode, and determine the ionic conductivity, electrochemically stable potential window, and reaction mechanism. In this Minireview, a brief introduction of electrochemical principles of the aqueous ZIBs is discussed and the recent advances of various aqueous electrolytes for ZIBs, including liquid, gel, and multifunctional hydrogel electrolytes are also summarized. Furthermore, the remaining challenges and future directions of electrolytes in aqueous ZIBs are also discussed, which could provide clues for the following development.     

Recent advances in the use of ionic liquids for electrochemical sensing

Ionic Liquids are salts that are liquid at (or just above) room temperature. They possess several advantageous properties (e.g. high intrinsic conductivity, wide electrochemical windows, low volatility, high thermal stability and good solvating ability), which make them ideal as non-volatile electrolytes in electrochemical sensors. This mini-review article describes the recent uses of ionic liquids in electrochemical sensing applications (covering the last 3 years) in the context of voltammetric sensing at solid/liquid, liquid/liquid interfaces and carbon paste electrodes, as well as their use in gas sensing, ion-selective electrodes, and for detecting biological molecules, explosives and chemical warfare agents. A comment on the future direction and challenges in this field is also presented.     

Understanding Structural and Transport Properties of Dissolved Li2S8 in Ionic Liquid Electrolytes through Molecular Dynamics Simulations

Lithium-sulfur batteries with high energy density are considered as one of the most promising future energy storage devices. However, the parasitic lithium polysulfides shuttle phenomenon severely hinders the commercialization of such batteries. Ionic liquids have been found to suppress the lithium polysulfides solubility, diminishing the shuttle effect effectively. Herein, we performed classical molecular dynamics simulations to explore the microscopic mechanism and transport behaviors of typical Li₂S₈ species in ionic liquids and ionic liquid-based electrolyte systems. We found that the trifluoromethanesulfonate anions ([OTf]⁻) exhibit higher coordination strength with lithium ions compared with bis(trifluoromethanesulfonyl)imide anions ([TFSI]⁻) in static microstructures. However, the dynamical characteristics indicate that the presence of the [OTf]⁻ anions in ionic liquid electrolytes bring faster Li⁺ exchange rate and easier dissociation of Li⁺ solvation structures. Our simulation models offer a significant guidance to future studies on designing ionic liquid electrolytes for lithium-sulfur batteries.     

Recent progress in organic redox flow batteries:Active materials,electrolytes and membranes

Redox flow batteries(RFBs) have great potentials in the future applications of both large scale energy storage and powering the electrical vehicle. Critical challenges including low volumetric energy density,high cost and maintenance greatly impede the wide application of conventional RFBs based on inorganic materials. Redox-active organic molecules have shown promising prospect in the application of RFBs,benefited from their low cost, vast abundance, and high tunability of both potential and solubility. In this review, we discuss the advantages of redox active organic materials over their inorganic compart and the recent progress of organic based aqueous and non-aqueous RFBs. Design considerations in active materials, choice of electrolytes and membrane selection in both aqueous and non-aqueous RFBs are discussed.Finally, we discuss remaining critical challenges and suggest future directions for improving organic based RFBs.     

Recent Theoretical and Experimental Advancements of Aluminum-Sulfur Batteries

Aluminum-sulfur batteries (AlSBs) exhibit significant potential as energy storage systems due to their notable attributes, including a high energy density, cost-effectiveness, and abundant availability of aluminum and sulfur. In order to commercialize AlSBs, an understanding of their working principles is necessary. In this review, we examine the current advancements in cathodes, both in theory and practice, as well as the progress made in aqueous and nonaqueous electrolytes. We also explore the modifications made to separators and the theoretical understanding of problems associated with AlSBs. Furthermore, we discuss future research directions aimed at resolving these issues. Our aim is to summarize the current progress in AlSBs and, based on recent progress and understanding of the mechanism, help design a battery to overcome the challenges that such batteries have been facing.     

Electrolytes for Rechargeable Lithium–Air Batteries

Lithium–air batteries are promising devices for electrochemical energy storage because of their ultrahigh energy density. However, it is still challenging to achieve practical Li–air batteries because of their severe capacity fading and poor rate capability. Electrolytes are the prime suspects for cell failure. In this Review, we focus on the opportunities and challenges of electrolytes for rechargeable Li–air batteries. A detailed summary of the reaction mechanisms, internal compositions, instability factors, selection criteria, and design ideas of the considered electrolytes is provided to obtain appropriate strategies to meet the battery requirements. In particular, ionic liquid (IL) electrolytes and solid‐state electrolytes show exciting opportunities to control both the high energy density and safety.     

Room temperature ionic liquids and their mixtures—a review

Room temperature ionic liquids are salts that are liquid at room temperature and their use as catalysts and catalytic support has been studied extensively. They are also being considered as “green solvents” for various separation processes. Recent measurements reported on the properties of pure ionic liquids and their mixtures, including gas and liquid solubility in common organic solvents will be reviewed. While some property values are in good agreement, some show large differences. These values will be compared and reasons for the discrepancies will be conjectured. Since traditional approaches to predicting the properties of fluid liquids require extensive LLE and VLE measurements, alternative predictive methods need to be explored. The predictions of the properties of mixtures of ionic liquids using COSMOtherm, an approach based on unimolecular quantum chemical calculations of the individual molecules, will be presented.