Oxygen Reaction at Carbonaceous Materials with Different Structure in Electrolytes Based on Lithium Perchlorate and Aprotic Solvents

Abstract

The necessity of the studying of carbonaceous materials differing in their surface area and structure is called for by the fact that these materials are used until now in the designing of positive electrodes for lithium-oxygen current sources. Under the model conditions, the effect of some factors on the effectiveness of oxygen reduction reaction at the positive electrode is studied. Among them are: properties of the dimethylsulfoxide- and acetonitrile-based electrolytes, the carbonaceous material (ХС 72, Super P, and carbon nanotubes) structure and its relevant transport processes depending on the electrode active layer mass (thickness) and the polarization current density, which determines the oxygen reaction effectiveness at the carbonaceous material. The electrochemically active surface area is shown to increase with the specific surface area, which is determined by the carbonaceous material porous structure, its mass at the electrode, the solvent properties, and the reaction rate. The active layer thickness and current density must be chosen for each carbonaceous material individually, depending upon its structure. At that, the active layer entire surface must be electrochemically accessible; it must make possible the lithium peroxide formation and subsequent decomposition. In the dimethylsulfoxide-based electrolyte (high donor number), the oxygen reduction reaction is highly reversible; the lithium peroxide formation here occurs via disproportionation in the solution bulk and results in the formation of Li2O2 particles with disordered (in all probability, toroidal) structure. This facilitates the back reaction (Li2O2 anodic decomposition), in good agreement with literature data [1]. In acetonitrile-based electrolyte (low donor number), the oxygen reduction reaction occurs in adsorbed state, producing LiО2 that disproportionates at the electrode surface forming a lithium peroxide insulating film whose oxidation needs high overvoltage. On the strength of all the parameters, carbon nanotubes are most effective in the oxygen reduction reaction in the dimethylsulfoxide-based electrolyte, because the carbon nanotubes have large volume of mesopores for the reactant transport, high electrochemically active surface area for the Li2O2 accumulation, and thus provide high characteristics per electrode.