Patrick C. Howlett

Energy applications of ionic liquids

Ionic liquids offer a unique suite of properties that make them important candidates for a number of energy related applications. Cation–anion combinations that exhibit low volatility coupled with high electrochemical and thermal stability, as well as ionic conductivity, create the possibility of designing ideal electrolytes for batteries, super-capacitors, actuators, dye sensitised solar cells and thermo-electrochemical cells. In the field of water splitting to produce hydrogen they have been used to synthesize some of the best performing water oxidation catalysts and some members of the protic ionic liquid family co-catalyse an unusual, very high energy efficiency water oxidation process. As fuel cell electrolytes, the high proton conductivity of some of the protic ionic liquid family offers the potential of fuel cells operating in the optimum temperature region above 100 °C. Beyond electrochemical applications, the low vapour pressure of these liquids, along with their ability to offer tuneable functionality, also makes them ideal as CO2 absorbents for post-combustion CO2 capture. Similarly, the tuneable phase properties of the many members of this large family of salts are also allowing the creation of phase-change thermal energy storage materials having melting points tuned to the application. This perspective article provides an overview of these developing energy related applications of ionic liquids and offers some thoughts on the emerging challenges and opportunities.

High Lithium Metal Cycling Efficiency in a Room-Temperature Ionic Liquid

A room-temperature ionic liquid (RTIL) solvent, N-methyl, N-alkyl pyrrolidinium bis(trifluoromethanesulfonyl)amide (P 1 X (Tf) 2 N), has been investigated for use in a lithium metal battery. An average cycling efficiency of>99% is achieved at 1.0 mA cm - 2 (I d e p = I d i s s ) on platinum. At deposition rates up to 1.75 mA cm - 2 optical micrographs indicate that the deposit is uniform and nondendritic. Above 1.75 mA cm - 2 , the deposit becomes dendritic and efficiency decays. High cycling efficiency (>99%) can also be obtained on copper, but at relatively low current density (0.1 mA cm - 2 ). The deposition/cycling history also influences the cycling behavior of the deposit.

Electrochemistry at Negative Potentials in Bis(trifluoromethanesulfonyl)amide Ionic Liquids

The bis(trifluoromethanesulfonyl)amide (TFSA) anion is widely studied as an ionic liquid (IL) forming anion which imparts many useful properties, notably electrochemical stability. Here we present electrochemical and spectroscopic evidence indicating that reductive decomposition of the bis(trifluoromethanesulfonyl)amide (TFSA) anion begins at ~ −2.0 V vs. Fc+/Fc, well before the reported cathodic limit for many of these ILs. These processes are shown to be dependent upon the electrode substrate and are influenced by the water content of the IL. Supporting ab initio calculations are presented which suggest a possible mechanism for the anion decomposition. The products appear to passivate the electrode surface and the implications of this behaviour are discussed.

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.

Lithium electrochemistry and cycling behaviour of ionic liquids using cyano based anions

Lithium based battery technologies are increasingly being considered for large-scale energy storage applications such as grid storage associated with wind and solar power installations. Safety and cost are very significant factors in these large scale devices. Ionic liquid (IL) electrolytes that are inherently non-volatile and non-flammable offer a safer alternative to mainstream lithium battery electrolytes, which are typically based on volatile and flammable organic carbonates. Hence, in recent years there have been many investigations of ionic liquid electrolytes in lithium batteries with some highly promising results to date, however in most cases cost of the anion remains a significant impediment to widespread application. Amongst the various possible combinations the dicyanamide (DCA) anion based ionic liquids offer exceptionally low viscosities and high conductivities – highly desirable characteristics for Li electrolyte solvents. DCA ILs can be manufactured relatively inexpensively because DCA is already a commodity anion, containing only carbon and nitrogen, which is produced in large amounts for the pharmaceutical industry. In this study we use the non-fluorinated ionic liquid N-methyl-N-butylpyrrolidinium dicyanamide to form non-volatile lithium battery electrolytes. We demonstrate good capacity retention for lithium metal and LiFePO4 in such electrolytes and discharge capacities above 130 mAh.g−1 at 50 °C. We show that it is important to control moisture contents in this electrolyte system in order to reduce capacity fade and rationalise this observation using cyclic voltammetry and lithium symmetrical cell cycling. Having approximately 200 ppm of moisture content produces the optimum cycling ability. We also describe plastic crystal solid state electrolytes based on the DCA anion in the lithium metal–LiFePO4 battery configuration and demonstrate over 150 mAh.g−1 discharge capacity without any significant capacity fading at 80 °C.

Ionic Liquids as Antiwear Additives in Base Oils: Influence of Structure on Miscibility and Antiwear Performance for Steel on Aluminum

The use of ionic liquids as additives to base oil for the lubrication of steel on aluminum was investigated. The miscibility and wear performance of various phosphonium, imidazolium, and pyrrolidinium ionic liquids in a range of polar and nonpolar base oils was determined. The structure and ion pairing of the ionic liquids was found to be important in determining their miscibility in the base oils. In wear tests, some of the miscible base oil/IL blends reduced the aluminum wear depth when compared to that found with the base oil alone. The nonpolar base oil/IL blends were able to withstand higher wear-test loads than the polar base oil/IL blends. At 10 N, as little as 0.01 mol/kg of IL, or 0.7-0.9 wt %, in the nonpolar base oils was enough to drastically reduce the wear depth on the aluminum. XPS analysis of the wear surfaces suggested that the adsorbing of the IL to the surface, where it can form low-shear layers and also react to form tribofilms, is important in reducing friction and wear. The largest reductions in wear at the highest load tested were found for a mineral oil/P6,6,6,14 (i)(C8)2PO2 blend.

New Insights into the Fundamental Chemical Nature of Ionic Liquid Film Formation on Magnesium Alloy Surfaces

Ionic liquids (ILs) based on trihexyltetradecylphosphonium coupled with either diphenylphosphate or bis(trifluoromethanesulfonyl)amide have been shown to react with magnesium alloy surfaces, leading to the formation a surface film that can improve the corrosion resistance of the alloy. The morphology and microstructure of the magnesium surface seems critical in determining the nature of the interphase, with grain boundary phases and intermetallics within the grain, rich in zirconium and zinc, showing almost no interaction with the IL and thereby resulting in a heterogeneous surface film. This has been explained, on the basis of solid-state NMR evidence, as being due to the extremely low reactivity of the native oxide films on the intermetallics (ZrO2 and ZnO) with the IL as compared with the magnesium-rich matrix where a magnesium hydroxide and/or carbonate inorganic surface is likely. Solid-state NMR characterization of the ZE41 alloy surface treated with the IL based on (Tf)2N(-) indicates that this anion reacts to form a metal fluoride rich surface in addition to an organic component. The diphenylphosphate anion also seems to undergo an additional chemical process on the metal surface, indicating that film formation on the metal is not a simple chemical interaction between the components of the IL and the substrate but may involve electrochemical processes.

An Ionic Liquid Surface Treatment for Corrosion Protection of Magnesium Alloy AZ31

We describe here the development of protective surface films on a Mg alloy in ionic liquids (ILs) based on the bis(trifluoromethanesulfonyl)amide (TFSA) anion. The film is shown to provide improved corrosion resistance to the alloy against humid environments as well as in the presence of chloride. Film formation is the result of the extreme reactivity of the metal in the IL, which provides a unique, concentrated medium of the desired reactive species, in this case the TFSA anion. The duration of the treatment is shown to influence the thickness and morphology of the resulting film and hence its protectiveness.

A comparison of phosphorus and fluorine containing IL lubricants for steel on aluminium

Ionic liquids have been shown to be highly effective lubricants for a steel on aluminium system. This work shows that the chemistry of the anion and cation are critical in achieving maximum wear protection. The performance of the ILs containing a diphenylphosphate (DPP) anion all showed low wear, as did some of the tris(pentafluoroethyl)trifluorophosphate (FAP) and bis(trifluoromethanesulfonyl)amide (NTf(2)) anion containing ILs. However, in the case of the FAP and NTf(2) based systems, a cation dependence was observed, with relatively poor wear resistance obtained in the case of an imidazolium FAP and two pyrrolidinium NTf(2) salts, probably due to tribocorrosion caused by the fluorine reaction with the aluminium substrate. The systems exhibiting poor performance generally had a lower viscosity, which also impacts on their tribological properties. Those ILs that exhibited low wear were shown to have formed protective tribofilms on the aluminium alloy surface.

An organic ionic plastic crystal electrolyte for rate capability and stability of ambient temperature lithium batteries

Reliable, safe and high performance solid electrolytes are a critical step in the advancement of high energy density secondary batteries. In the present work we demonstrate a novel solid electrolyte based on the organic ionic plastic crystal (OIPC) triisobutyl(methyl)phosphonium bis(fluorosulfonyl)imide (P1444FSI). With the addition of 4 mol% LiFSI, the OIPC shows a high conductivity of 0.26 mS cm−1 at 22 °C. The ion transport mechanisms have been rationalized by compiling thermal phase behaviour and crystal structure information obtained by variable temperature synchrotron X-ray diffraction. With a large electrochemical window (ca. 6 V) and importantly, the formation of a stable and highly conductive solid electrolyte interphase (SEI), we were able to cycle lithium cells (Li|LiFePO4) at 30 °C and 20 °C at rates of up to 1 C with good capacity retention. At the 0.1 C rate, about 160 mA h g−1 discharge capacity was achieved at 20 °C, which is the highest for OIPC based cells to date. It is anticipated that these small phosphonium cation and [FSI] anion based OIPCs will show increasing significance in the field of solid electrolytes.