Using Rotating Ring Disc Electrode Voltammetry to Quantify the Superoxide Radical Stability of Aprotic Li-Air Battery Electrolytes

Despite the promising high specific energy density of lithium-air batteries, their commercialization remains hindered by numerous issues, including the poor stability of the electrolyte due to its reaction with the superoxide radical (O-2(center dot-)) produced upon discharge at the battery's cathode. In this work, we have used rotating ring disc electrode (RRDE) voltammetry to study this reaction and to quantify the stability of the electrolyte against O-2(center dot-) by its pseudo-first-order reaction constant, k. Our results confirm the recently reported reactivity of propylene carbonate (PC, which was used in many of the initial works on Li-air batteries), while unveiling the enhanced stability of 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Pyr(14)TFSI), with a k value at least 3 orders of magnitude lower than that estimated in PC. Moreover, our RRDE-transient measurements indicate that the diffusion of O-2(center dot-) in this ionic liquid is approximate to 70 times slower than that in PC, which could partially explain the poor discharge capacities observed in Li-air battery tests using Pyr(14)TFSI.