context: The need to increase the energy density of Li-ion batteries leads to continual development of electrode material technology, including engineering of the physicochemical properties of cathode materials. Among them, Li-rich layered oxides are attracting considerable research attention as a candidate for high performance positive electrodes due to their high specific charge (capacity), exceeding 200 mA h g−1. This group of compounds, proposed firstly by Thackeray and Johnson, include pristine Li2MnO3 or Li2MnO3 with partial substitution of Mn with other transition metals (TMs) and is often described as: xLi2MnO3·(1 − x)LiMO2 (or alternatively Li[LiyMn1−y−zMz]O2) (M – transition metal). Although, initially Thackeray et al. reported that Li2MnO3 (theoretical capacity 468 mA h g−1) is electrochemically inactive because Mn4+ cannot be oxidized to higher states, further studies showed that by means of appropriate activation processes Li2MnO3 can deliver higher discharge capacities than LiMnO2. Further studies by Kobobuchi et al. suggested that upon delithiation the local electronic state of lattice oxygen in Li2MnO3 may change, affecting the material performance. XAS studies of the O K-edge and Mn L-edge showed that in the Li-rich NMC sample charged up to 4.7 V vs. Li+/Li, oxygen ions also contribute to the reduction of manganese via electron transition from O-1s to the hybridized bands composed of O-2p and Mn-3dx2−y2. As a result, the change of the local electronic state surrounding Mn4+ ions triggers their migration within TMs and/or into the Li layers, which may be reflected in phase transformation of the initial monoclinic (C2/m) layered structure into either a rhombohedral (Rm) and/or a spinel structure. As confirmed by Amalraj et al. phase transition of Li2MnO3 from layered to spinel-like structures occurs even at potentials as low as 4.3 V, while at 4.5 V Li2MnO3 by partial loss of Li2O forms Li2−xMnO3−x/2 with enhanced stability associated with the existence of a composite structure built up with Li2MnO3 domains in a LiMnO2 matrix.