Role of dissolution of Mn(III) species in discharge and recharge of chemically-modified MnO2 battery cathode materials

Various experimental techniques, including cyclic-voltammetry, constant current discharge and recharge, with a detecting electrode for determination of soluble Mn(III) intermediate species,were used to study the reaction mechanism involved in the reduction of a chemical modified form of MnO2 that is multiply rechargeable. The results show how dissolution of Mn(III) species plays an important role in the overall path of the reduction reaction and also in the rechargeability of the material. Mainly an heterogeneous reduction mechanism is involved; substantial loss of capacity can arise if diffusion of the soluble Mn(III) species into the bulk electrolyte solution is allowed to take place. In practical cells, this effect must be minimized using cells with 'starved electrolyte'. i.e. with only a small free volume-fraction of KOH electrolyte in MnO2-carbon electrode structure. Soluble Mn(III) species were detected in situ experiments by means of (a) a spectro-electrochemical procedure,(b) a 'collector' electrode system at which dissolved Mn(III) was reoxidized to MnO2, and (c) a rotating ring-disc electrode.