Ionic liquids (ILs) form a novel class of electrolytes with unique properties that make them attractive candidates for electrochemical devices. In the present study a range of electrolytes were prepared based on the IL N-methyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl) amide ([C(3)mpyr][NTf(2)]) and LiNTf(2) salt. The traditional organic solvent diluents vinylene carbonate (VC), ethylene carbonate (EC), tetrahydrofuran (THF) and toluene were used as additives at two concentrations, 10 and 20 mol%, leading to a ratio of about 0.6 and 1.3 diluent molecules to lithium ions, respectively. Most promisingly, the lithium ions see the greatest effect in the presence of all the diluents, except toluene, producing a lithium self-diffusion coefficient of almost a factor of 2.5 times greater for THF at 20 mol%. Raman spectroscopy subtly indicates that THF may be effectively breaking up a small portion of the lithium ion-anion interaction. While comparing the measured molar conductivity to that calculated from the self-diffusion coefficients of the constituents indicates that the diluents cause an increase in the overall ion clustering. This study importantly highlights that selective ion transport enhancement is achievable in these materials.
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 conductivitieshighly 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 conductivitieshighly 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 ca...
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