An Azo-Spiro Mixed Ionic Liquid Electrolyte for Lithium Metal– LiFePO4 Batteries

Abstract

Using a binary ionic liquid (IL) electrolyte composed of -methyl--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 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 cathode, we report capacities in excess of , and by utilizing a pulse charging technique we were able to charge a cell at a current density , 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.