Solvent-Coupled Catalysis of the Oxygen Electrode Reactions in Lithium-Air Batteries

The electrocatalysis of oxygen reduction reactions (ORR) in non-aqueous electrolytes is coupled to the ability of the solvents to modulate the Lewis acidity of Li+. This is accomplished through chemical interactions of Li+ with the solvent to form acid-base complexes of the general formula, Li(solvent)n+, which determine the relative stability of the ORR intermediates and the final products formed. In high Donor Number solvents such as dimethyl sulfoxide (DMSO), the ORR proceeds via an outer Helmholtz plane (OHP) reaction pathway, conforming to a homogeneous catalysis of the reaction, irrespective of the presence of a catalyst in the cathode. In low Donor Number solvents exemplified by tetraethylene glycol dimethyl ether (TEGDME) and CH3CN, catalysts such as cobalt phthalocyanine (CoPC), Pt and Au promote heterogeneous electrocatalysis at the inner Helmholtz plane (IHP) of the electrical double layer on the electrode. The catalysis in this case involve the adsorption of O2 as well as the ORR intermediates on the catalyst surface leading to lower activation energy of the reactions and increases in the discharge voltages of Li-air cells compared to uncatalyzed cells. The heterogeneous catalysis at the IHP may promote the full electrochemical reduction of O2 to O2−. © 2014 The Electrochemical Society. [DOI: 10.1149/2.0981410jes] All rights reserved.