Overcharging manganese oxides: Extracting lithium beyond Mn4+

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, Lix[Mn1 − yLiy]O2, (y ≤ 0.2), and Li[NixLi1/3 − 2x/3Mn2/3 − x/3]O2 (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 °C the first mechanism is dominant in all examples studied, whilst at 55 °C the proton exchange mechanism becomes more important. At 30 °C H+ exchange is more prevalent in Li2MnO3 than in the other two cases. The preference for O loss in the Mn/Ni system may be understood in terms of the ease with which MnO2 will lose oxygen.