Potential Dependences of Discharge Characteristics of Li-O₂ Batteries with Various Electrolytes

Practical application of non-aqueous Li-O2 batteries is strongly expected, as its theoretical energy density is much larger than Li ion batteries. However, due to non-conductive Li2O2 formed on the cathode during the discharge, the discharge capacity is limited to be much smaller than the theoretical value. Two different pathways, the surface and solution pathway, have been proposed for the discharge process forming Li2O2, the latter being considered preferred for the sustained discharge. Recent works revealed that the formation pathway of Li2O2 is determined by the adsorption/desorption equilibrium of LiO2, a reaction intermediate of the discharge reaction. Considering that the operating potential of the cathode is one of the factors which determines the adsorption/desorption equilibrium of LiO2, it is effective to conduct the discharge tests under potential-controlled modes for deeper understanding of the electrochemistry of the discharge reaction. In the present work, we examined the discharge profiles of non-aqueous Li-O2 batteries under potential-controlled conditions. We put a particular interest on revealing the generality and specificity of the potential dependences of the discharge characteristics among Li-O2 batteries carrying different solvents, including dimethyl sulfoxide (DMSO), acetonitrile (MeCN), and tetraglyme (G4). For all the systems investigated qualitatively similar potential dependences were observed. Namely, the higher (or the lower) the potential was, the smaller (or larger) the current value was and the larger (or smaller) the discharge capacity was, suggesting that the solution and surface pathway preferentially proceeds in higher and lower potentials, respectively, irrespective of the solvents. On the other hand, it was also found that the sharpness of the potential dependence strongly depends on the kind of solvents used. For example, the potential dependence of the discharge capacity in MeCN was much sharper than that in DMSO. These results indicate that it is important to tune the working potential of the cathode so that the discharge capacity can be larger, especially in a solvent which has the strong potential dependency. Figure 1