Towards Understanding Li-O2 Electrochemistry on Precious Metal Electrocatalyst Surfaces in Non-Aqueous Electrolytes

With expanding expectations for vehicle electrification, energy storage systems with high specific energy densities are attracting increased interest.  In particular, Lithium-Oxygen (Li-O₂) batteries are considered one of the best candidates for post lithium ion batteries due to their high specific energy density (2600Wh/kg-electrode mass).  However, Li-O₂batteries have several major issues including electrolyte decomposition[1] and carbon support corrosion[2] which inhibits cell reversibility.  The reaction products and mechanism of product formation were investigated on various precious metal electrocatalyst surfaces using cyclic voltammetry (CV), rotating ring-disk electrodes (RRDE), in-situ electrochemical scanning probe microscopy (EC-SPM), and in-situ Raman spectroscopy.

Gold, palladium, and platinum were investigated as Li-O₂ oxygen reduction electrocatalysts under various operating conditions.  Oxygenated 0.1M LiTFSI/DMSO and 0.1M LiClO₄/DMSO were employed to investigate the influence of electrolyte salt on product formation.  In order to obtain electrochemical characteristics of the various electrocatalyst surfaces, CV and RRDE were conducted using Li metal as the counter electrode and a Ag/Ag⁺reference electrode.  For EC-SPM and in-situ Raman spectroscopy experiments, a two electrode configuration was utilized where Li metal served as both the counter and reference electrode.  Additionally, constant current and voltage control modes were used case by case.

Through our experiments, we have confirmed that the discharge product morphology and reversibility of the Li-O₂ electrochemical reaction were influenced by the solvent, the electrolyte salt species, electrocatalyst metal, and electrochemical operating condition (current density and discharge cut-off voltage). The fact that discharge conditions can induce different Li₂O₂ product morphologies has been reported [3].  Once large Li₂O₂ particles form, decomposition becomes more difficult and larger charging overpotentials are observed.  Additionally, under certain experimental conditions, a transition from Li₂O₂ formation via disproportionation to a 2^nd electron transfer mechanism is observed, which also influences decomposition overpotentials.  Finally, Li salts containing the perchlorate anion produce a uniform film-like morphology as opposed to a particle morphology produced by the TFSI anion containing electrolyte.  Thus, a complex interaction between the solvent, electrolyte salt anion, LiO₂ intermediate species, and operating condition appears to influence Li₂O₂formation mechanism and morphology and thus apparent reversibility.  Further details of this investigation will be presented.

[1]  S.A. Freunberger, J.Am.Chem.Soc., 2011, 133, 8040–8047.

[2]  B.D. McCloskey, J. Phys. Chem. Lett. 2012, 3, 997-1001.

[3]  B.D. Adams, Energy Environ. Sci., 2013,6, 1772-1778.