A systematic first-principles calculation based on density functional theory is carried out to discuss the redox mechanism of Li2MnO3. The lattices of structural models having C2/m- and C2/c-type stacking sequences can be regarded as hexagonal, while their symmetry is monoclinic. Different stacking sequences of [Mn2/3Li1/3] layers do not cause differences in the energy or crystallographic structure, suggesting a disordered stacking sequence. A calculation for Li2−x MnO3 assuming topotactic lithium removal indicates that lithium removal can occur at a potential of about 4.6V with a wide potential plateau. The electronic structure of Li2−x MnO3 shows that the manganese ions remain in the charge state of Mn4+ and the charge of the removed lithium ions is compensated by the oxidation of oxygen.