A new Lewis-base ionic liquid comprising a mono-charged diamine structure : a highly stable electrolyte for lithium electrochemistry

A new Lewis-base ionic liquid (IL) based on mono-charged 1,4-diazabicyclo[2.2.2]octane (dabco) was synthesized and its thermal and electrochemical behaviour was characterized. The dabco-based IL with bis(trifluoromethanesulfonyl)amide (TFSA) anion melts at 76 °C when the N-substituted alkyl chain length is 2. The dabco-based IL showed a wide electrochemical window of over 4 V ranging from −3.5 to +1.5 V vs. Fc/Fc⁺ and was able to deposit and strip lithium from a nickel substrate at reasonable efficiency.

Solid state actuators based on polypyrrole and polymer-in-ionic liquid electrolytes

Novel polymer-in-ionic liquid electrolytes (PILEs) have been developed for solid state electrochemical actuators based on polypyrrole. The active polymer electrodes are readily oxidized/reduced without degradation in the PILE. It was found that the actuator cycle life is significantly enhanced in the PILE as is the ‘shelf life’ of the device.

In situ photopolymerization of a gel ionic liquid electrolyte in the presence of iodine and its use in dye sensitized solar cells

We report for the first time an in situ photopolymerization of model co-monomers, 2-hydroxyethyl methacrylate (HEMA) and tetra (ethylene glycol) diacrylate (TEGDA), in an IL electrolyte containing I₂ for DSSCs. TiO₂ nanoparticles were used as the photo-initiator and co-gelator in a charge transfer polymerization reaction. The gel-IL polymer obtained was characterized in terms of the diffusion properties of the electrolyte. Preliminary results from DSSCs assembled using the gel-IL electrolyte showed energy conversion efficiency of 3.9% at 1 sun (AM1.5) and 5.0% at 0.39 sun illumination.

On the role of cyclic unsaturated additives on the behaviour of lithium metal electrodes in ionic liquid electrolytes

Three cyclic vinyl based additives, based respectively on oxygen, sulphur and fluorine, are tested for their ability to improve the cycling of lithium in a hostile ionic liquid medium. Oxygen based vinylene carbonate is found to offer the best protection of the lithium metal whilst allowing very consistent lithium cycling to occur. The vinylene carbonate based system under study is, however, imperfect. Lithium metal is deposited in a dendritic morphology, and vinylene carbonate is rapidly consumed during lithium cycling if it is present in a small quantity. Our results suggest that ionic liquid systems critically relying on a small amount of additive to protect a lithium electrode are not viable for long cycle life secondary batteries. It is suggested that an ionic liquid which itself is lithium metal compatible be used instead.

Phosphonium ionic liquids as lubricants for aluminium-steel

The performance of a series of novel room temperature ionic liquids (ILs) based on the trihexyl(tetradecyl)phosphoniumcation (P66614 +) and a number of novel anions have been studied in pin-on-disk tests using a 100Cr6 steel ball on AA2024 aluminium disks.

The anions coupled to the (P66614 +) cation include diphenyl phosphate (DPP-), dibutyl phosphate (DBP-), bis (2,4,4-trimethyl pentyl) phosphinate (M3PPh-) and bis(2-ethyl hexyl) phosphate (BEH-).

More traditional anions such as bis(trifluoromethanesulfonyl) amide (NTf2 -) and bromide (Br-) were also investigated. Experiments were conducted at various loads to assess the IL film forming abilities.

The results suggest that the structure of the anion is important in forming a surface film that reduces the friction and wear of the aluminium disk. At 30N five of the six ILs tested showed a 30-90% reduction in wear, as determined from wear scar depth measurements, compared to fully formulated diesel oil.

The IL lubricant with a diphenyl phosphate anion achieved the lowest wear coefficient, showing a better performance than a typical fluorine-containing IL anion, NTf2.

To further investigate wear mechanisms and surface interactions the wear scars were analysed using a scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS).

Potentiostatic control of ionic liquid surface film formation on ZE41 magnesium alloy

The generation of potentially corrosion-resistant films on light metal alloys of magnesium have been investigated. Magnesium alloy, ZE41 [Mg−Zn−Rare Earth (RE)-Zr, nominal composition 4 wt % Zn, 1.7 wt % RE (Ce), 0.6 wt % Zr, remaining balance, Mg], was exposed under potentiostatic control to the ionic liquid trihexyl(tetradecyl)phosphonium diphenylphosphate, denoted [P_6,6,6,14][DPP]. During exposure to this IL, a bias potential, shifted from open circuit, was applied to the ZE41 surface. Electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA) were used to monitor the evolution of film formation on the metal surface during exposure. The EIS data indicate that, of the four bias potentials examined, applying a potential of −200 mV versus OCP during the exposure period resulted in surface films of greatest resistance. Both EIS measurements and scanning electron microscopy (SEM) imaging indicate that these surfaces are substantially different to those formed without potential bias. Time of flight-secondary ion mass spectrometry (ToF-SIMS) elemental mapping of the films was utilized to ascertain the distribution of the ionic liquid cationic and anionic species relative to the microstructural surface features of ZE41 and indicated a more uniform distribution compared with the surface following exposure in the absence of a bias potential. Immersion of the treated ZE41 specimens in a chloride contaminated salt solution clearly indicated that the ionic liquid generated surface films offered significant protection against pitting corrosion, although the intermetallics were still insufficiently protected by the IL and hence favored intergranular corrosion processes.

Ionic liquid electrolyte porphyrin dye sensitised solar cells

Ionic liquid electrolytes based on a number of imidazolium, quaternary ammonium and phosphonium cations have been developed for porphyrin dye sensitised solar cells yielding efficiencies of up to 5.2% at 0.68 Sun.

Ionic liquids and reactions at the electrochemical interface

Ionic liquids (ILs) represent a fascinating, and yet to be fully understood, medium for a variety of chemical, physical and biological processes. Electrochemical processes form an important subset of these that are particularly of interest, since ILs tend to be good electrochemical solvents and exhibit other properties which make them very useful as electrolytes in electrochemical devices. It is important therefore to understand the extent to which electrochemical reactions and processes behave in a relatively “normal”, for example aqueous solution, fashion as opposed to exhibiting phenomena more uniquely the product of their organic ionic nature. This perspective examines a range of electrochemical reactions in ionic liquids, in many cases in the context of real world applications, to highlight the phenomena as far as they are understood and where data gaps exist. The important areas of lithium and conducting polymer electrochemistry are discussed in detail.

Lithium-functionalised silica nanoparticles for enhanced ionic conductivity in an organic ionic plastic crystal

Addition of silica nanoparticles functionalised with lithium propane sulfonate to the organic ionic plastic crystal N-ethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide ([C₂mpyr][NTf₂]) results in a significant increase in ionic conductivity. Analysis of these nanocomposites by impedance spectroscopy, NMR, positron annihilation lifetime spectroscopy (PALS) and Raman spectroscopy suggests that this is the result of higher matrix mobility due to an increase in defect size and concentration. The effect of these functionalised nanoparticles is compared to that previously observed for unfunctionalised nanoparticles in the lithium-doped and pure plastic crystal.

Organic ionic plastic crystals : recent advances

Investigations into the synthesis and utilisation of organic ionic plastic crystals have made significant progress in recent years, driven by a continued need for high conductivity solid state electrolytes for a range of electrochemical devices. There are a number of different aspects to research in this area; fundamental studies, utilising a wide range of analytical techniques, of both pure and doped plastic crystals, and the development of plastic crystal-based materials as electrolytes in, for example, lithium ion batteries. Progress in these areas is highlighted and the development of new organic ionic plastic crystals, including a new class of proton conductors, is discussed.

The effect of potential bias on the formation of ionic liquid generated surface films on Mg alloys

An electrochemical approach to the formation of a protective surface film on Mg alloys immersed in the ionic liquid (IL), trihexyl(tetradecyl)phosphonium–bis 2,4,4-trimethylpentylphosphinate, was investigated in this work. Initially, cyclic voltammetry was used with the Mg alloy being cycled from OCP to more anodic potentials. EIS data indicate that, under these circumstances, an optimum level of protection was achieved at intermediate potentials (e.g., 0 or 0.25 V versus Ag/AgCl). In the second part of this paper, a small constant bias was applied to the Mg alloy immersed in the IL for extended periods using a novel cell design. This electrochemical cell allowed us to monitor in situ surface film formation on the metal surface as well as the subsequent corrosion behaviour of the metal in a corrosive medium. This apparatus was used to investigate the evolution of the surface film on an AZ31 magnesium alloy under a potential bias (between ±100 mV versus open circuit) applied for over 24 h, and the film evolution was monitored using electrochemical impedance spectroscopy (EIS). A film resistance was determined from the EIS data and it was shown that this increased substantially during the first few hours (independent of the bias potential used) with a subsequent decrease upon longer exposure of the surface to the IL. Preliminary characterization of the film formed on the Mg alloy surface using ToF-SIMS indicates that a multilayer surface exists with a phosphorous rich outer layer and a native oxide/hydroxide film underlying this. The corrosion performance of a treated AZ31 specimen when exposed to 0.1 M NaCl aqueous solution showed considerable improvement, consistent with electrochemical data.

The electrochemistry of lithium in ionic liquid/organic diluent mixtures

The electrochemistry of lithium is investigated in a number of electrolytes that consist of a lithium salt dissolved in a combined ionic liquid-organic diluent medium. We find that ethylene carbonate and vinylene carbonate improve electrochemical behaviour, while toluene and tetrahydrofuran are less promising.We also present insights into the electrode passivation caused by these diluents in an ionic liquid electrolyte during lithium cycling. We observe that during lithium cycling those electrolytes with carbonate based diluents are the most able to utilise their previously reported improved lithium ion diffusivities. Conversely, tetrahydrofuran, the most promising diluent of those studied in terms of its known ability to increase lithium ion diffusivity is found not to be as advantageous as a diluent. It appears that the poor electrochemical interfacial properties of the tetrahydrofuran electrolyte prevented the realisation of the benefits of the high solution lithium ion diffusivity.

The influence of different nanoparticles on a range of organic ionic plastic crystals

The addition of nanoparticles to an organic ionic plastic crystal can result in orders of magnitude increases in ionic conductivity, which makes these materials of interest as solid state electrolytes. However, this effect is not universal and depends on both the nature of the organic ionic plastic crystal and on the type of nanoparticle used. The effect of addition of TiO₂, Al₂O₃ and SiO₂ nanoparticles to a range of ionic materials with varying plasticity and rotator phase behaviour has been studied by thermal analysis and conductivity and the effect on the different materials is compared.

Transition in wear performance for ionic liquid lubricants under increasing load

The effect of ionic liquid (IL) lubrication for aluminium/steel systems is highly dependant on the applied load and the IL structure. This study illustrates that a change in anion of an IL lubricant results in different physicochemical properties that will alter its performance at a given load. As the load is increased there is a shift in lubricant performance and mechanism of the IL. Up to a load of 30 N the lowest wear coefficient was achieved by a phosphonium diphenylphosphate IL, whilst above 30 N a phosphonium bis(trifluoromethanesulfonyl)amide IL was able to form a more tenacious tribolayer that resulted in the lowest wear.