Room Temperature Ionic Liquid Electrolyte Research Articles

Design of High-Entropy Electrolytes Enabled By the High-Throughput and Autonomous Procedure

Electrolytes in Li-ion batteries play a significant role as they influence different aspects directly related to the battery performance, such as safety, voltage window, electrochemical stability, and the formation of solid-electrolyte interphase (SEI). Conventionally, these electrolytes are composed of a lithium salt dissolved in an organic solvent such as ethylene carbonate and propylene carbonate. Regarding safety, these organic electrolytes can be replaced by room-temperature ionic liquids (RTILs), which present lower vapor pressure and non-flammability.1 Moreover, adding polymer to these liquid electrolytes to form polymer gel electrolytes has proven to be a good strategy to avoid dendritic growth and increase electrolyte stability.2 The properties of RTIL electrolytes containing Li salts have been studied in the last decade. Still, the scope has been limited primarily to simple RTILs consisting of one cationic and one anionic species. Inspired by the success of high-entropy alloys (HEAs) for forming stable solid solutions with excellent mechanical properties,3 the concept of high-entropy materials has been recently transferred to electrolytes, where the effect of mixing multiple Li salts is explored.4 However, the number of compositional combinations increases rapidly when multiple salts, cationic and anionic species are introduced, and the explorable compositional space increases even more when polymers are added to the mixture.Automating the electrolyte preparation and characterization can be the key feature to allow the exploration of all the possible electrolyte combinations. In this work, we explain the first preliminary results obtained from an autonomous preparation and characterization of high-entropy electrolytes composed of a mixture of two different lithium salts in ionic liquids consisting of multiple anionic and cationic species. In addition, the adaptation of the autonomous system of preparation and characterization of liquid electrolytes to polymer gel electrolytes is also proposed and explained. References. (1) Niu, H.; Wang, L.; Guan, P.; Zhang, N.; Yan, C.; Ding, M.; Guo, X.; Huang, T.; Hu, X. Recent Advances in Application of Ionic Liquids in Electrolyte of Lithium Ion Batteries. J. Energy Storage 2021, 40, 102659. https://doi.org/10.1016/j.est.2021.102659.(2) Chen, J.; Wu, J.; Wang, X.; Zhou, A.; Yang, Z. Research Progress and Application Prospect of Solid-State Electrolytes in Commercial Lithium-Ion Power Batteries. Energy Storage Mater. 2021, 35, 70–87. https://doi.org/10.1016/j.ensm.2020.11.017.(3) Yeh, J.-W.; Chen, S.-K.; Lin, S.-J.; Gan, J.-Y.; Chin, T.-S.; Shun, T.-T.; Tsau, C.-H.; Chang, S.-Y. Nanostructured High-Entropy Alloys with Multiple Principal Elements: Novel Alloy Design Concepts and Outcomes. Adv. Eng. Mater. 2004, 6 (5), 299–303. https://doi.org/10.1002/adem.200300567.(4) Wang, Q.; Zhao, C.; Wang, J.; Yao, Z.; Wang, S.; Kumar, S. G. H.; Ganapathy, S.; Eustace, S.; Bai, X.; Li, B.; Wagemaker, M. High Entropy Liquid Electrolytes for Lithium Batteries. Nat. Commun. 2023, 14 (1), 440. https://doi.org/10.1038/s41467-023-36075-1.

Effects of operating temperature on Li-O2 battery with ionic liquid-based binary electrolyte

Awaiting a breakthrough, the Li-oxygen battery (LOB) is considered a promising candidate to meet the high energy demands in the future. Among various critical challenges which hamper its development, the mystery of the electrolyte with optimal properties remains unsolved to this day. In this study, we comprehensively investigated the effects of operating temperature (20C, 40 °C and 60 °C) on the electrochemical performance of LOBs incorporated with room temperature ionic liquid (RTIL) and organic solvent binary electrolyte. We designed and investigated 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2C1im][Tf2N]) RTIL and dimethyl sulfoxide (DMSO) organic solvent at various volume ratios ((4:1), (1:1), (1:4)). Among the binary electrolytes, ([C2C1im][Tf2N]/DMSO (1:4)) delivered the highest discharge capacities of 3.70 Ah g− 1 (20 °C), 4.0 Ah g− 1 (40 °C) and 3.65 Ah g− 1 (60 °C) as compared with pure [C2C1im][Tf2N] and DMSO. Cycling stability tests showed superior stability of the binary electrolyte ([C2C1im][Tf2N]/DMSO (1:4)) irrespective of the operating temperature. From viscosity and ionic conductivity measurements (at 20–60 °C), [C2C1im][Tf2N]/DMSO (1:4) exhibited the highest ionic conductivity and the lowest viscosity compared with other binary electrolytes (even with pure electrolytes) at any given temperature. Cyclic voltammetry (CV) tests revealed the highest reaction rates for [C2C1im][Tf2N]/DMSO (1:4) binary electrolytes than pure electrolytes. The superior performance of [C2C1im][Tf2N]/DMSO (1:4) binary electrolyte was ascribed to enhanced stability against reactive intermediate species during oxygen reduction reaction (ORR), increased ionic conductivity, low viscosity (comparable with organic electrolytes), improved oxygen solubility, and relatively low evaporation rates.

Insights into the structure and Ion transport of pectin-[BMIM][PF6] electrolytes.

We investigate the effect of pectin on the structure and ion transport properties of the room-temperature ionic liquid electrolyte 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) using molecular dynamics simulations. We find that pectin induces intriguing structural changes in the electrolyte that disrupt large ionic aggregates and promote the formation of smaller ionic clusters, which is a promising finding for ionic conductivity. Due to pectin in [BMIM][PF6] electrolytes, the diffusion coefficient of cations and anions is observed to decrease by a factor of four for a loading of 25 wt. % of pectin in [BMIM][PF6] electrolyte. A strong correlation between the ionic diffusivities (D) and ion-pair relaxation timescales (τc) is observed such that D ∼ τc-0.75 for cations and D ∼ τc-0.82 for anions. The relaxation timescale exponents indicate that the ion transport mechanisms in pectin-[BMIM][PF6] electrolytes are slightly distinct from those found in neat [BMIM][PF6] electrolytes (D∼τc-1). Since pectin marginally affects ionic diffusivities at the gain of smaller ionic aggregates and viscosity, our results suggest that pectin-ionic liquid electrolytes offer improved properties for battery applications, including ionic conductivity, mechanical stability, and biodegradability.

Breaking the passivity wall of metals: Exempli gratia non-aqueous Ti–air battery

A new multivalent metal–air battery employing Titanium (Ti) as active material in EMIm(HF)2.3F room temperature ionic liquid electrolyte is introduced. Ti metal is highly attractive as it is a light metal that can possibly transfer up to 4 electrons. The electrochemical behavior of Ti is known for its passivity in various media. Here, we evaluated the electrochemistry of Ti in EMIm(HF)2.3F with potentiodynamic polarization and galvanostatic discharge experiments. Ti–air battery could successfully be operated under relatively high current densities (up to 0.75 mA cm−2) with an average cell voltage of 1–1.2 V, yielding up to a discharge capacity of 66mAhcm−2. When its full potential is harvested, such a metal–battery holds a unique potential to be the only metal with 4 electrons transfer during its discharge.

(Digital Presentation) Conductivity Measured in Situ from I-E Curve Fitting in Chloroaluminate Electrolytes

Aluminium (Al) batteries are a promising, next-generation technology and current research efforts are aimed at positioning this technology to compete with existing lithium-ion batteries (LIB). The development of non-aqueous electrolyte chemistries for Al battery systems has received renewed attention to address some of the shortcomings associated with LIB. Of particular importance in this development is the liquid electrolyte as its rheology governs the battery chemistry. The goals are to generate a liquid that is compatible with the other (solid) battery components, stable long-term with repeated use, and to optimise the rheology (i.e. target high conductivity and low viscosity). Chloroaluminate room temperature ionic liquid (RTIL) electrolytes made by mixing Lewis acidic aluminium chloride (AlCl3) salt with a (often chloride-containing) Lewis basic salt e.g. 1-ethyl-3-methylimidazolium chloride (EMIM-Cl) has been extensively studied. This tuneable electrolyte provides good thermal stability, good ionic conductivity, and a wide polarizable potential window. While these traits are advantageous these types of Lewis basic salt precursors are generally expensive, difficult to synthesize and in some instances can be toxic. Recently, ionic liquid analogues (ILA) that are made from abundant, inexpensive and often non-toxic materials have begun to be explored. To date the two most common Lewis basic salts examined have been urea and acetamide but their rheological and electrochemical properties need to be improved in order to complete with RTIL-based electrolytes.Our group have recently revealed that amidine-based chloroaluminate ILA electrolytes show promise over urea-, acetamide-, and pyrrolidinium-based electrolytes.[1] Specifically, guanidinium chloride (Guan-Cl) and acetamidinium chloride (Acet-Cl) based salts display reversible electrochemical plating/stripping of Al, good ionic conductivities (e.g. 10 mS cm-1), and moderate viscosities (e.g. 50 cP). Also, in this work we initially proposed a mathematical model to extract the conductivity from these electrolytes by fitting the voltammetric i-E curve (from Al deposition/dissolution) to a linear, modified Butler-Volmer formalism. The characteristic, anodic i-E trace shows a striking linearity often over very large potential ranges (e.g. >2 V), and this response is used to extract ionic conductivity. This represents a novel, electroanalytical method to obtain this important rheological metric.As such, in this contribution we will highlight our latest efforts to expand, benchmark (to common methods for measuring conductivity), and determine the limits of our i-E curve fitting method to measure ionic conductivity.[2] Specifically, we have examined the AlCl3:Acet-Cl electrolyte in depth by looking at the potential scanning rate, potential range probed, and the compositional (mole ratio Lewis acid : Lewis base) effect on the conductivity extracted from our fitting method. We have also studied a range of other common chloroaluminate electrolytes to compare with published literature. All of these values are then benchmarked to conductivity data measured both from a traditional impedance-based method and that from a commercial conductivity probe. Lastly, we also have examined temperature-dependent conductivities from all three of these methods. Overall, we find good agreement between values measured from our in situ i-E curve fitting method to those from more traditional conductivity measurement methods. This electroanalytical work serves to deepen our understanding of the conductivity of chloroaluminate ILAs for Al battery applications.

Ultra-stable ionic-liquid-based electrochromism enabled by metal-organic frameworks

Room temperature ionic liquid (RTIL) electrolytes are excellent candidates for highly stable electrochromic (EC) devices because of their wide electrochemical windows and good environmental stability. However, the small crystal spacings and molecular gaps of conventional EC materials limit the ion-diffusion rate of RTILs’ large cations. The multiple insertions/extractions of large cations can destroy the structure of the conventional EC materials, leading to structural collapse and poor EC performance. Here, we demonstrate that metal-organic frameworks (MOFs) with large intrinsic pores in crystals are promising materials for stable and high-performance RTIL-based EC systems. The MOFs’ large intrinsic hexagonal channels provide suitable ion-transport channels for RTILs, leading to ideal electrochemical stability. We further demonstrate a patterned large-area MOF/RTIL-based EC device, suggesting the promise in future digital displays.

The Current Progress of the Titanium Preparation by Electrolysis in the Room-Temperature Ionic Liquid Electrolytes

Titanium is a beneficial metallic material due to its excellent properties. However, the large-scale application of titanium is inhibited by the high production cost of the Kroll process. To address this challenge, researchers have proposed many new strategies based on electrochemical technology over the past decades. Those electrochemical methods show potential practical value to replace the Kroll process. Nevertheless, many of them are conducted in high-temperature melts, limiting the rapid development of those methods. Accordingly, room-temperature electrolysis in ionic liquid electrolytes was employed in titanium production. At present, there is no systematic and in-depth summary on room-temperature titanium electrolysis, although many pathways in room-temperature melts have been reported. In this review, we briefly outline the development of the titanium electrolysis methods firstly and summarize the room-temperature titanium electrolysis in ionic liquid electrolytes. Furthermore, we have discussed the fundamental mechanisms and key challenges occurring in room-temperature titanium electrolysis. Finally, we proposed the opportunities and research direction on room-temperature titanium electrolysis. We hope this review will be a valuable roadmap for room-temperature titanium electrolysis.

Electrolyte Speciation and Electrochemical Performance of Idinium Based Ionic Liquid Analogues for Aluminium Battery Applications

The development of non-aqueous electrolyte chemistries for aluminium (Al) battery systems has recently received renewed attention to address some of the challenges associated with lithium-ion batteries. Of particular importance in this development is the electrolyte, which governs the battery chemistry. Chloroaluminate room temperature ionic liquid (RTIL) electrolytes made by mixing Lewis acidic aluminium chloride (AlCl3) salt with a (often chloride-containing) Lewis basic salt e.g. 1-ethyl-3-methylimidazolium chloride (EMIM-Cl), 1-butyl-3-methylimidazolium chloride (BMIM-Cl), and 1-butyl-1-methylpyrrolidinium chloride (BMP-Cl) have been studied. These tuneable electrolytes provide good thermal stability, good ionic conductivity, and a wide polarisable potential window. While these traits are advantageous the Lewis basic salt precursors are often very costly, challenging to synthesise and in some instances can be toxic. Recently, ionic liquid analogues (ILA) that are made from abundant, inexpensive and often non-toxic materials have begun to be explored. To date the two most common Lewis basic salts examined have been urea and acetamide (both contain oxygen functionality) but their rheological properties need to be improved to complete with the RTIL-based electrolytes.Our group has recently examined a range of different chloroaluminate ILA electrolytes, and we have identified a class of Lewis base salt precursors that show promise over urea, acetamide and BMP-Cl based electrolytes. Specifically, –idinium chloride salts with nitrogen functionality (i.e. guanidinium chloride [Guan-Cl], acetamidinium chloride [Acet-Cl], and formamidinium chloride [Form-Cl]) display reversible electrochemical plating/stripping of Al, good ionic conductivities (10 – 14 mS cm-1), and viscosities (50 – 80 cP). The choice of Lewis base salt can have a profound impact on not only the electrochemical performance but also the chemical and rheological properties of the electrolyte.As such, this contribution will highlight our efforts in studying the chemical functionality of the Lewis base by changing from oxygen-, nitrogen- and sulphur-based ligands. We will also examine the compositional differences and determine the liquidus range by systematically varying the molar ratios of the Lewis acid : Lewis base. We will report how this affects the chemical speciation of the electrolyte, the ionic conductivity, and the electrolyte viscosity. This will help us identify an optimal electrolyte composition for testing in a coin cell configuration. Temperature-dependent electrochemical performance of these electrolytes will be evaluated using cyclic voltammetry (CV), electrochemical quartz crystal microbalance (EQCM), and electrochemical impedance spectroscopy (EIS). The chemical speciation of Al in the different electrolytes will be analysed using 27Al NMR and Raman spectroscopy. This will help identify if Al2Cl7 -, AlCl4 - or other ionic species are responsible for their behaviour. Overall, this work will provide a deeper, fundamental understanding of these novel electrolytes and their application in aluminium-based batteries.

Thin Tantalum Film Electrodeposition from an Ionic Liquid—Influence of Substrate Nature, Electrolyte Temperature and Electrochemical Parameters on Deposits’ Quality

This paper discusses the feasibility of tantalum electrodeposition carried out under potential control from 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [BMPyr][TFSI] room temperature ionic liquid electrolyte containing tantalum pentafluoride (TaF5). The influence of electrode nature, electrolyte temperature and electrolysis duration on the morphology, the adherence, the thickness and the crystalline properties of tantalum deposits is reported. The obtained tantalum deposits are analyzed by SEM, XRD, Raman spectroscopy and XPS surface analytical techniques. Pulsed potential electrodeposition is investigated in order to enhance deposit properties. Post electrodeposition thermal treatments are also reported.

Engineering the Interlayer Spacing by Pre‐Intercalation for High Performance Supercapacitor MXene Electrodes in Room Temperature Ionic Liquid

AbstractMXenes exhibit excellent capacitance at high scan rates in sulfuric acid aqueous electrolytes, but the narrow potential window of aqueous electrolytes limits the energy density. Organic electrolytes and room‐temperature ionic liquids (RTILs) can provide higher potential windows, leading to higher energy density. The large cation size of RTIL hinders its intercalation in‐between the layers of MXene limiting the specific capacitance in comparison to aqueous electrolytes. In this work, different chain lengths alkylammonium (AA) cations are intercalated into Ti3C2Tx, producing variation of MXene interlayer spacings (d‐spacing). AA‐cation‐intercalated Ti3C2Tx (AA‐Ti3C2), exhibits higher specific capacitances, and cycling stabilities than pristine Ti3C2Tx in 1 m 1‐ethly‐3‐methylimidazolium bis‐(trifluoromethylsulfonyl)‐imide (EMIMTFSI) in acetonitrile and neat EMIMTFSI RTIL electrolytes. Pre‐intercalated MXene with an interlayer spacing of ≈2.2 nm, can deliver a large specific capacitance of 257 F g−1 (1428 mF cm−2 and 492 F cm−3) in neat EMIMTFSI electrolyte leading to high energy density. Quasi elastic neutron scattering and electrochemical impedance spectroscopy are used to study the dynamics of confined RTIL in pre‐intercalated MXene. Molecular dynamics simulations suggest significant differences in the structures of RTIL ions and AA cations inside the Ti3C2Tx interlayer, providing insights into the differences in the observed electrochemical behavior.

Amperometric gas sensors based on screen printed electrodes with porous ceramic substrates

In the present study, an amperometric gas sensor based on room temperature ionic liquid (RTIL) electrolyte and screen-printed electrodes (SPE) was fabricated. In order to compose the sensor device, a porous alumina ceramic was used as gas diffusion barrier and substrate of SPE, simultaneously. While RTIL electrolytes were also solidified with casting method to form a solid film and attached onto porous ceramics equipped with printed electrodes. It was found that with this design, the amperometric sensor could become smaller and easier to intergrade. Additionally, the porous ceramics as the diffusion barrier as substrate enable gas molecules diffuse into the three-phase (gas/electrode/electrolyte) interface directly without passing through RTIL membranes. For demonstration, an amperometric nitrogen dioxide (NO2) sensor was composed with our proposed structure and the sensing performance were characterized in terms of sensitivity, selectivity and stability. It was indicated that indeed our NO2 sensor show a very rapid response speed, only 20 s and 10 s for responding and recovering, respectively. This is 4 times shorter than the traditional sensor structure based on RTIL. In addition, the sensor has a good sensitivity (33 nA / ppm) and limit of detection (LOD) around 0.02 ppm with a signal noise ratio (SNR) around 3. It was also detected that the sensor has a wide operating temperature range (-20 to 40 °C), suggesting our sensor devices could work in extreme environments.

Examination of Reducing Agents Used for Aluminum Electroless Deposition

1. Introduction Because of its properties such as low electrical resistance, simple to structure in dry etch processes, aluminum is widely used for wiring material in the field of semiconductors. However, it is necessary to use large instruments such as sputtering and glow plasma methods to deposit aluminum on the silicon substrate. If the technique of electroless plating of aluminum is established, it will be able to obtain the thin film on the silicon substrate easily without electricity. We reported an electroless plating of aluminum on the glass by using a room-temperature ionic liquid composed of aluminum chloride (AlCl3) and 1-ethyl-3-methylimidazolium chloride (EMIC) with lithium hydride (LiH) or di-isobutyl aluminum hydride (DIBAH) as reducing agents1). In this research, we tried electroless plating of aluminum on silicon substrate coated with tungsten (W) or with titanium nitride (TiN) which has not been reported so far. And the possibility of shortening the time of electroless plating was investigated by dissolving various new reducing agents in the plating solution.2. ExperimentalTo perform the electroless aluminum plating experiment, the silicon substrate coated with W or with TiN were degreased, pickled, and catalyzed. Thereafter, reducing agents (DIBAH or various new reducing agents) were added to a plating solution of 66.7 mol% AlCl3 + 33.3 mol% EMIC ionic liquid heated to 70 ℃, and the substrate was immersed in the solution to perform electroless aluminum plating. The linear sweep voltammetry measurements were employed, a three-electrode cell consisting of a tungsten working electrode, an aluminum counter electrode, and an aluminum reference electrode by the scan rate was 5 mV/s. An aluminum wire (99.99%, φ:2mm) immersed in a 66.7 mol% AlCl3 – 33.3 mol% EMIC melt in a Pyrex tube separated by a G4 glass frit was used as the reference electrode. The electrochemical behavior of reducing agents in 50mol% AlCl₃ + 50mol%EMIC ionic liquid was investigated. All experimental operations were performed inside a glove box under an inert gas atmosphere.3. Results and DiscussionA plating film was provided on silicon substrate coated with W and with TiN from the 66.7 mol% AlCl₃ + 33.3 mol% EMIC ionic liquid with the DIBAH reducing agent. The characteristics of the plated films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM). The observed X-ray diffraction peaks of the plated film agreed with those of aluminum in the literatures. However, it took more than 6 hours to get few ㎛ thickness of aluminum plating. We tried to find other reducing agent to shorten the time of electroless plating. The self-potential of reducing agents in the 50mol% AlCl₃ + 50mol% EMIC ionic liquid were measured. Some reducing agents showed less-novel potential, indicating that they can be used as a reducing agent as well as DIBAH for electroless Al plating. It was confirmed that electroless plating was performed by using those reducing agents. Especially, reducing agent A shortens the plating time to about 1-2 minutes to get few ㎛ thickness. Also, it was found that when the concentration of the reducing agent E was high, the reactivity became too strong, and the liquid decomposition occurred rapidly, so that plating could not be performed. From these results, we could say that the reducing agent A was very useful, but it was important to control the reactivity by adjusting the concentration.The electroless plating mechanism was studied by the polarization curves of a tungsten electrode in room-temperature ionic liquids. It was concluded that the electroless plating reaction consisted of anodic oxidation reaction of reducing agent and cathodic reduction of Al2Cl7-.4. ConclusionElectroless Al plating is possible for W-coated silicon substrate and TiN-coated silicon substrate from 66.7 mol% AlCl₃ + 33.3mol% EMIC ionic liquid with DIBAH reducing agent. The results of self-potential measurement showed that reducing agents with less-novel potential could be used as reducing agents for Al electroless plating. The results of electroless plating experiments suggested that the use of reducing agent A could shorten the plating time significantly.5. References1) Setsuko Koura, Souta Ooshima, Daiki Kudo, Electroless deposition of Aluminum from a Room Temperature Ionic Liquid Electrolyte, 2016 / 230th ECS Meeting Electrochemical Society (USA and Japan) (2016)

Silicon anode design for Li ion batteries: Synergic effects of Ag nanoparticles and ionic liquid electrolytes

The infinite volume expansion and subsequent stability issues of Si and other alloy-based anodes have relentlessly hindered their application in Li-ion batteries. Moreover, the highly inflammable conventional organic electrolytes have always raised serious safety concerns for Li secondary batteries. In the present study, a novel electrode design based on porous Si nanostructures is demonstrated for application in stable Li-ion batteries to enhance the performance. Silver nanoparticles are embedded within the silicon aerogel-based structures in a viable approach and used in combination with room temperature ionic liquid (RTIL) electrolytes. The mechanical, as well as electrochemical properties of Si anode, were found to be significantly improved with high rate performance as well as cyclic stability. A discharge capacity as high as 2712 mA h g−1 was obtained, maintains a value of 1899 mA h g−1 at the end of 100 cycles at 100 mA g−1.

SnSe nano-particles as advanced positive electrode materials for rechargeable aluminum-ion batteries

With the rapid consumption of fossil energy, the development of renewable energy storage technology has received much attention. Rechargeable aluminum-ion batteries (RAIBs) are expected to constitute large-scale energy storage systems because of the aluminum resources’richness and low cost. Therefore it is necessary to study the new type of RAIBs positive electrode material, but there is not a systematic design principle now. It is noted that SnSe, as a novel positive electrode material of aluminum-ion battery based on aluminium chloride/1-ethyl-3-methylimidazolium chloride (AlCl3/[EMIm]Cl) room temperature ionic liquid electrolyte for the first time, exhibits well-defined discharge voltage plateaus near 1.6 V and a high first cycle specific discharge capacity of 582 mAh g−1 (coulomb efficiency of 83.27%) at a current density of 0.3 A g−1. After 100 cycles, the specific discharge capacity can be maintained at 107 mAh g−1 (coulomb efficiency of 99.55%). X-ray photoelectron spectroscopy (XPS) and constant current Charge/Discharge results show that the mechanism of the cell is the reaction between AlCl4- and SnSe, and both Sn2+ and Se2– participate in the oxidation–reduction reaction. Just from the high capacity point of view, the novel Al-ion battery based on the AlCl4- reaction mechanism can be a multi-ion reaction.

Chain length matters: Structural transition and capacitance of room temperature ionic liquids in nanoporous electrodes

Room-temperature ionic liquids (RTILs) have shown great potential in promoting the widespread application of energy storage devices. The relationship between the electrical double layer (EDL) structure and capacitive performance of different RTILs is vital to the rational design of novel RTIL electrolytes. Herein, we have investigated the microscopic structures of three types of RTILs containing cations with varying tail lengths in nanopores, their charging behaviors, and their EDL capacitances using classical density functional theory (CDFT). The results show that the interfacial structure changes from a monolayer to a bilayer as the neutral chain length of the cations increases, which is in agreement with previous experimental results (Smith et al., 2013). Moreover, the RTILs containing cations with longer chains can significantly enhance the capacitance in nearly neutral pores, while they dampen capacitance under negative electrical potential and show little disadvantage when positive potential is applied.

Experimental and computational studies of electrochemical anion intercalation into graphite from target-oriented designed borate-based ionic liquid electrolytes

Here, we report on the fundamental experimental and computational analyses of target-oriented designed ionic liquid (IL) electrolytes composed of small and (electro)chemically stable borate-based anions with respect to their anion intercalation/de-intercalation behavior in graphite positive electrodes for dual-ion batteries (DIBs). Due to their relatively small size, borate-based anions (e.g., BF4ˉ) and electrolytes are of high interest for DIB cells in order to achieve a high specific capacity, which can, however, be impeded by electrolyte solvation effects. In order to exclude solvent effects, we develop and synthesize novel room-temperature IL electrolytes (RTILs), i.e., Pyr1101BF4/LiBF4 and Pyr1101CF3BF3/LiCF3BF3, which are characterized with respect to stability and anion intercalation behavior. These studies are combined with computational studies to gain fundamental insights into the electronic structures of the BF4ˉ and CF3BF3ˉ acceptor-type graphite intercalation compounds (GICs), staging stoichiometries, theoretical capacities and anion transport properties.

Capacitive Charging Mechanisms at the Molybdenum Disulfide/Ionic Liquid Interface

Molybdenum disulfide (MoS2) is a 2D material of the transition metal dichalcogenide family which has recently gained significant attention for use in supercapacitors. MoS2 can exist as several polymorphs such as the 2H and 1T which are semiconducting and semi-metallic, respectively. The 2H phase, found in nature, is the most stable form of MoS2. The 1T phase is metastable and can be obtained by chemical methods such as via lithium intercalation. It shows a much higher conductivity (100 S/m). Furthermore, restacked single layers of 1T-MoS2 can be intercalated by smaller ions such as Na+, Li+, H+ and K+, in aqueous electrolyte, allowing the electrolyte to access their interlayer spacing and therefore providing impressively high volumetric capacitance of between 400 – 700 F/cm3. Hence, molybdenum disulfide is a promising material for supercapacitor application.While these high values are achieved in aqueous electrolyte, few studies have examined their capacitive behavior in room temperature ionic liquid (IL) electrolytes which exhibit much higher electrochemical stability windows > 3-4 V. The potential of coupling high capacitance materials with high voltage operation has the potential to lead to high energy densities due to a supercapacitor’s square dependence of the energy density on cell voltage. However, it is currently unknown whether these electrolytes can intercalate between restacked 1T-MoS2. Furthermore, the intrinsic capacitance of the 1T-MoS2/IL electrolyte remains largely unexplored along with the impact that defects and oxidation may have. To investigate the charging behavior in IL electrolytes, we examined the electrochemical behaviour of monolayer MoS2 deposited on highly oriented pyrolytic graphite (HOPG), using cyclic voltammetry and electrochemical impedance spectroscopy. The monolayer MoS2 is deposited using a Langmuir Blodgett (LB) deposition method. Multilayers are deposited by sequential layer-by-layer deposition. The negligible roughness and surface coverage of the monolayers are confirmed and estimated by atomic force and scanning electron microscopies. Highly purified 1-ethyl, 3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM TFSI) is used as the electrolyte and the monolayers are electrochemically probed in custom-made O-ring sealed electrochemical cell. Using these analyses, we probe the frequency and potential-dependence of the intrinsic capacitance of the MoS2/IL interface and provide design criteria for building improved IL-based supercapacitors.

Ionophore-Assisted Electrochemistry of Neutral Molecules: Oxidation of Hydrogen in an Ionic Liquid Electrolyte.

The electrochemical properties of gas molecules are of great interest for both fundamental and applied research. In this study, we introduce a novel concept to systematically alter the electrochemical behavior and, in particular, the redox potential of neutral gas molecules. The concept is based on the use of an ion-binding agent, or "ionophore", to bind and stabilize the ionic electrochemical reaction product. We demonstrate that the ionophore-assisted electrochemical oxidation of hydrogen in a room-temperature ionic liquid electrolyte is shifted by almost 1 V toward more negative potentials in comparison to an ionophore-free electrolyte. The altered electrochemical response in the presence of the ionophore not only yields insights into the reaction mechanism but also can be used to determine the diffusion coefficient of the ionophore species. This ionophore-modulated electrochemistry of neutral gas molecules opens up new avenues for the development of highly selective electrochemical sensors.

Thick electrodes upon biomass-derivative carbon current collectors: High-areal capacity positive electrodes for aluminum-ion batteries

Rechargeable aluminum-ion batteries (AIBs) are considered as low-cost safe energy storage devices beyond lithium-ion batteries, due to the high volumetric capacity and rich abundance of aluminum. However, the highly corrosive room temperature ionic liquid electrolyte causes serious problems in the traditional metal current collectors, and electrochemical inert metals with higher density have been commonly used, which leads to dramatic decreased material utilization in the positive electrode. For addressing such issues, here a biomass-derivative carbon (BDC) current collector is demonstrated to construct millimeter-scale thick positive electrodes based on two types of graphite active materials (with different planar sizes) and two electrode configurations. The results suggest the strategy of using single-side casting process with graphite of smaller planar size would be more promising to promote the overall electrochemical properties and energy storage performance. The as-assembled thick electrode (with thickness up to ∼2 mm) using the same type of graphite is able to deliver much enlarged areal loading (36 mg cm−2), showing greater areal capacity for energy storage. The strategy and mechanism in this work offers a considerable approach for improving the current collectors and promoting the graphite proportion in AIBs.

Reversible Electrochemical Mirror Devices Using Space Compliant Ionic Liquid Electrolytes

Reversible electrochemical mirror devices function through reversible redox reactions that alternate between deposition of a highly reflective thin metallic film (hence the term mirror) and complete oxidation of the metallic film during the erasure cycle. These devices generally utilize transparent, conductive substrates such as those based on indium tin oxide type films applied to glass or plastic transparent substrates, though other substrate materials could be used in the build of these devices. REM devices may be built to either facilitate reflection and transmission or reflection and absorption of radiation sources depending on the nature of the counter electrode used. REM devices may be used for various applications including auto-dimming mirrors for the automotive industry, electrochromic windows for the aviation industry, smart glass for the architectural industry and mirrors or thermal emitters for deployment on orbital platforms. Conventional REM devices utilize electrolytes based on organic solvents such as gamma-butylrolactone and dimethyl sulfoxide. For space based applications, these electrolytes are unsuitable due to their vapor pressures and potential for evaporation if the cell seal is compromised. Room temperature ionic liquid electrolytes (RTIL) are an attractive alternative to these conventional systems due to their negligible vapor pressure in addition to their excellent chemical and thermal stability and their large electrochemical windows. RTIL base electrolytes may be tailored to deliver specific properties based on their cation and anion species, making the electrolytes tunable for specific applications. RTILs with anionic species such as chloride, bromide and iodide have been evaluated as potential alternatives to the conventional organic electrolytes for REM devices. These systems have shown promise in terms of mirror formation and long cycle life; however, they exhibit high sensitivity to water, including atmospheric moisture, which can limit their usefulness in REM devices. The present work focuses on deposition and stripping of silver thin films from RTIL with relative low moisture sensitivity. Simple, two electrode cells were built using transparent electrodes with electrically conductive films (i.e. indium tin oxide, platinum and silver) and air and moisture stable RTIL electrolytes for electrochemical characterization and plating/stripping cycling experiments. Highly reflective and reproducible silver mirror formation using air and moisture stable RTIL based electrolytes has been demonstrated. In the current work, experiments are being conducted to explore how the cell configuration and cell components influence deposition and oxidation processes as well as device cycling lifetime. Use of pulsed deposition and erasure steps have also been explored in simple REM systems containing RTIL electrolytes. Pulse and pulse reverse processes have the ability to improve the deposition process of reversible silver systems as compared to operation under constant voltage or constant current electric fields, namely the ability to enhance mass transport properties of the system as well as driving nucleation of the silver thin films over grain growth, which is anticipated to help in the stripping of the film from the substrate. Operation under pulse/pulse reverse electric fields is expected to achieve highly reflective metallic mirror films while maximizing the cycling lifetimes and preventing operational degradation of the electrolyte and cell components. This work outlines how cell configuration and device operating conditions influence deposition and erasure reactions as well as device cycle lifetime. Acknowledgement: Funding for this work is gratefully acknowledged from Air Force STTR Contract Number FA9453-17-C-0490. Platinum coated ITO electrodes were prepared by Dr. John Bryan Plumley.