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

a 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 nonvolatile 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 LiFePO 4 in such electrolytes and discharge capacities above 130 mAh.g À1 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-LiFePO 4 battery configuration and demonstrate over 150 mAh.g À1 discharge capacity without any significant capacity fading at 80 C. Broader context Lithium battery technologies have applications in large-scale energy storage such as grid storage and electric vehicles. Safety, energy density, and cost are very crucial factors in the choice of large scale devices. Currently, mainstream lithium-ion battery electrolytes are based on volatile and ammable organic carbonates which introduce signicant safety issues. Ionic liquid (IL) electrolytes, which are inherently non-volatile and non-ammable, offer a safer alternative. Additionally, ILs offer the promise of enabling a rechargeable lithium metal electrode to deliver signicant increases in energy. Amongst the various possible cation and anion combinations, ILs using the dicyanamide (DCA) anion, which contain no uorine reducing cost, offer exceptionally low viscosities and high conductivities -highly desirable characteristics for Li electrolyte solvents. We have investigated the ionic liquid N-methyl-N-butylpyrrolidinium dicyanamide to form lithium battery electrolytes which enable good capacity retention for a lithium metal-LiFePO 4 battery with discharge capacities above 130 mAh.g À1 at 50 C. We also describe solid state electrolytes, based on plastic crystals with the DCA anion, in a lithium metal-LiFePO 4 battery and demonstrate over 150 mAh.g À1 discharge capacity without any signicant capacity fading at 80 C

The \u27filler-effect\u27 in organic ionic plastic crystals : Enhanced conductivity by the addition of nano-sized TiO2

The addition of nano-sized ceramic particles to the plastic crystal ethyl-methyl pyrrolidinium bis(trifluoromethane sulfonyl)amide (P12TFSA) has been investigated by means of DSC and conductivity. The thermal behaviour of the plastic crystal as a function of filler content suggests that the filler particles decrease the onset temperature of the melting slightly at high loadings, however they do not decrease the crystallinity of the material. Furthermore, the IV &rarr; III transition decreases in intensity, indicating that the addition of filler increases the possibility for the crystal to remain in metastable rotator phases also at lower temperatures. The conductivity shows a more than one order of magnitude increase with the addition of filler, with a filler concentration dependence that levels out above&nbsp;~ 10 wt.% TiO2.<br /

Nanoparticle enhanced conductivity in organic ionic plastic crystals : space charge versus strain induced defect mechanism

High conductivity in solid-state electrolytes is a critical requirement for many advanced energy and other electrochemical applications. Plastic crystalline materials have shown promise in this regard, and the inclusion of nanosized inorganic particles in both amorphous and crystalline materials has indicated order of magnitude enhancements in ion transport induced by space charge or other defect enhancement. In this paper we present conductivity enhancements in the plastic crystal N,N&lsquo;-ethylmethylpyrrolidinium bis(trifluoromethanesulfonyl)amide ([C2mpyr][NTf2]) induced by nanosized SiO2 particles. The addition of the nanoparticles dramatically increases plasticity and ion mobility. Positron annihilation lifetime spectroscopy (PALS) measurements indicate an increase in mean defect size and defect concentration as a result of nanoparticle inclusion. The scaling of the conductivity with size suggests that a &ldquo;trivial space charge&rdquo; effect is operable, although a strain induced enhancement of defects (in particular extended defects) is also likely given the observed increase in plasticity.<br /