Oxygen Electrode Reactions in Dilakylacetamide-Based Electrolytes for Lithium-Air Batteries
Authors: Alsudairi Amell Musaid , Mukerjee Sanjeev , Abraham K.M. 
Published: February 2020

The rechargeability of the Li-air battery remains a persistent challenge despite the fact that significant progress has been made in elucidating the mechanism of oxygen reduction reactions (ORR) in non-aqueous electrolytes1-3. The role of organic solvents on the ORR mechanism through their electron donor property as measured by the Guttmann Donor Number (DN) is well recognized1
,
3. In this paper we report the results of an investigation of the ORR mechanisms in tetrabutyl ammonium ((C4H9)4N+) (TBA+)) and lithium (Li+) salt solutions in dimethylacetamide (DMAc) (DN= 27.8) and diethylacetamide (DEAc) (DN= 32.2)). A prior study of the use of DMAc as a solvent for the Li-air battery has appeared4. Microelectrode voltammetry2 of O2 using an 11 µm diameter carbon microelectrode revealed steady-state limiting current behavior in electrolytes containing TBAPF6, TBACF3SO3 and TBAClO4 in DMAc (Figure1A). This current-voltage profile is indicative of reversibility of the ORR process in these electrolytes as a result of the solubility and efficient mass transport of the initially formed superoxide, O2
-, produced by the one-electron reduction of O2. Cottrell plots derived from chronoamperometry measurements made with the microelectrode were used to determine O2 solubility and diffusion coefficients in the various electrolytes. Typical O2 solubility is 3.11 x 10-06 mol/cm-3 in TBAPF6/DMAc and diffusion coefficient is 5.06 x 10-05 cm2 s-1 in the same electrolyte. Unlike the behavior in TBA salt solutions, current-voltage data for O2 reduction on the microelectrode in Li salt-containing electrolytes revealed irreversible ORR ascribed to the deposition of insoluble products on the electrode surface.

Cyclic voltammetry (CV) data on planar glassy carbon electrode complemented the overall conclusions reached from the microelectrode studies in the TBA and Li salt-based electrolytes in the two amide solvents. The CV of O2 in 0.1M LiPF6/DMAc presented in Figure 1B is typical of the result in Li salt-containing electrolytes. It shows that the overall ORR process is kinetically irreversible while a certain amount of chemical reversibility is revealed by presence of oxidation currents in the anodic scan. Reversal of the cathodic scans at selected potentials in the initial downward sloping region of the voltammogram shows the presence of an anodic peak characteristic of the oxidation of O2- at about 3.3 V that is not seen when the cathodic scan is reversed below the ORR peak potential at about 2.4 V where formation of Li2O2 occurs. Unlike the behavior of O2 in Li salts, reversible ORR was observed in all TBA salt solutions as a result of the stabilization of the dissolved superoxide by the soft acid (TBA+) 3. Mechanistic details of ORR together with the characterization of the O2 reduction products in DMAc and DEAc electrolytes will be presented at the meeting.

References:

(1) Abraham, K. M.: Electrolyte-Directed Reactions of the Oxygen Electrode in Lithium-Air Batteries. Journal of the Electrochemical Society
2015, 162, A3021-A3031.

(2) Gunasekara, I.; Mukerjee, S.; Plichta, E. J.; Hendrickson, M. A.; Abraham, K. M.: Microelectrode Diagnostics of Lithium-Air Batteries. Journal of the Electrochemical Society
2014, 161, A381-A392.

(3) Laoire, C. O.; Mukerjee, S.; Abraham, K. M.; Plichta, E. J.; Hendrickson, M. A.: Influence of Nonaqueous Solvents on the Electrochemistry of Oxygen in the Rechargeable Lithium-Air Battery. Journal of Physical Chemistry C
2010, 114, 9178-9186.

(4) Walker, W.; Giordani, V.; Uddin, J.; Bryantsev, V. S.; Chase, G. V.; Addison, D.: A Rechargeable Li-O-2 Battery Using a Lithium Nitrate/N,N-Dimethylacetamide Electrolyte. Journal of the American Chemical Society
2013, 135, 2076-2079.

Acknoledgement: Partial financial support for this work was provided by the US Army Cerdec thorugh subcontract No: GTS-S-15-015. A scholarship for Amell Alsudairi by the Saudi Arabian Cultural Mission is appreciated.

Figure 1