Bare and Ag-doped MnO2 powders as electrocatalysts for Li/Air cathode batteries

According to its electrochemical and thermodynamic properties, manganese and its oxides are good candidates as materials for positive or negative electrodes of primary and secondary batteries, because it is stable in both aqueous and organic electrolytes and is reasonable in cost. Manganese dioxide can be used for many purposes: cathode for both aqueous and nonaqueous batteries, in corrosion protection of stainless steels, catalyst for chemical oxidation of various compounds in combination with other transition metal elements, and electrocatalyst for oxygen reduction in alkaline solutions. Since the first announcement in 1996 by Abraham, Li/Air batteries have suffered of the problem to remove, during the charge cycle, the “combustion” products, Li2O2 and Li2O, of the discharge cycle from the cathode material, which represents nowadays the weak link of this new powerful accumulator. Unfortunately, Li2O2 and Li2O can clog the cathode pores thus reducing the performance and lifetime of the batteries, as their complete removal during the charge cycle, used to restore the anode material, is hard to accomplish without an appropriate catalyst. It is necessary to add an electrocatalyst to the cathode material, which can be involved in the charge cycle helping in the removal of the Li oxides. Up to now electrolytic MnO2 has been used in this purpose, but the performance of the cathode material added with this oxide is still not enough to guarantee long life cycles to the Li/Air batteries. In this context we present a new composite material based on ordered mesoporous carbon cathode containing MnO2 electrocatalytic nanopowders prepared via hydrothermal method trough the oxidation of Mn2+ by ammonium peroxodisulphate. The synthesis was carried out for 24 h at different temperatures (60, 90, and 120 °C) also in the presence of silver dopant. The obtained black powders were finally dried at 60 °C for 24 h. The results are discussed in term of cyclic voltammetry of microcavity filled with the cathode material, in the potential window typical for the Li/Air batteries, in order to elucidate the electrochemical reaction and the electrocatalytic behaviour of the powder, and in term of laboratory Li/Air battery life cycles.