An Azo-Spiro Mixed Ionic Liquid Electrolyte for Lithium Metal–LiFePO[sub 4] Batteries

Using a binary ionic liquid (IL) electrolyte composed of N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide and 2-oxo-3,9-dioxa-6-azonia-spiro[5.5]undecane bis(trifluoromethanesulfonyl)imide at an optimized ratio, we investigated battery cycling behavior at temperatures of 50°C and above. We achieved an extended cycle life using the binary electrolyte system as the problem of cell short-circuiting (stemming from lithium dendrite growth) was significantly reduced. We provide evidence that 2-oxo-3,9-dioxa-6-azonia-spiro[5.5]undecane bis(trifluoromethanesulfonyl)imide assists in short-circuit prevention through the suppression of lithium dendrites. For batteries employing a lithium metal anode and a LiFeP0 4 cathode, we report capacities in excess of 120 mAh g -1 , and by utilizing a pulse charging technique we were able to charge a cell at a current density (0.1 mA cm -2 ), which we were not able to achieve using a normal charging regime. We also present lithium cycling data for 2-oxo-3,9-dioxa-6-azonia-spiro[5.5]undecane bis(trifluoromethanesulfonyl)imide in the absence of another IL. We show that poor capacity and a necessarily high operating temperature make this system undesirable although we observed no short-circuiting. We conclude that the structure of the components of the IL and the nature of the charging regime employed can cause a significant reduction in dendrite-induced short-circuiting.