It has been demonstrated previously that Li may be removed electrochemically from Mn containing oxides beyond the maximum oxidation state of 4+ for Mn in an octahedral oxygen environment. Here we present a comparison of such overcharge behavior in a series of different layered lithium manganese oxides including Li2MnO3, Li-x[Mn1-yLiy]O-2, (y <= 0.2), and Li[NixLi1/3 - 2x/3Mn2/3 -x/3]O-2 (0 <= x <= 0.5). We show that there are two competing mechanisms by which electrochemical extraction of lithium can occur in Mn (4+) systems. In the first Li removal is accompanied by O2- loss (effective removal of Li2O) whilst the second involves oxidation of the non-aqueous electrolyte thus generating H+ ions which exchange for Li+. At 30 degrees C the first mechanism is dominant in all examples studied, whilst at 55 degrees C the proton exchange mechanism becomes more important. At 30 degrees C H+ exchange is more prevalent in Li2MnO3 than in the other two cases. The preference for 0 loss in the Mn/Ni system may be understood in terms of the ease with which MnO2 will lose oxygen. (c) 2005 Elsevier B.V. All rights reserved.