Understanding the second electron discharge plateau in MnO2-based alkaline cells

We present a study of the MnOOH --> Mn(OH)(2) reduction process leading to the second electron plateau occasionally occurring during the electrochemical discharge of MnO2-based alkaline cells. The origin of the second plateau is nested in a dissolution-reduction-precipitation mechanism, and its onset potential is governed by Mn+3/Mn+2 solution species through the Nernst law. Also, the appearance of this second plateau in MnO2-based alkaline cells is a strong function of the nature of the counter electrode, and is, for instance, rarely observed when Zn slurries are used as the counter electrode. Direct experimental evidence, by means of in situ XRD, SEM, and EDX analyses, is given for the formation of the spinel ZnMn2O4 during the first electron reduction of MnO2. An ion exchange reaction is proposed to account for the formation of this phase. Based on these findings, we show that the likelihood of raising the second potential through chemical approaches, thereby using the second electron capacity in MnO2-based alkaline cells, is low. (C) 2001 The Electrochemical Society.