The reaction mechanism of a high capacity lithium-and manganese-rich metal oxide, 0.4Li 2 MnO 3-0.6LiMn 0.5 Ni 0.5 O 2 , has been investigated at the atomic level. High-resolution synchrotron X-ray powder diffraction (HRPD) and X-ray absorption spectroscopy (XAS) were used, respectively, to evaluate the electrochemical charge and discharge reactions in terms of local and bulk structural changes, and variations in the oxidation states of the transition metal ions. Ni Kedge XAS data indicate the participation of nickel in reversible redox reactions, whereas Mn K-edge absorption spectra show that the manganese ions do not participate in the electrochemical reactions. Rietveld refinements of the oxygen occupancy during charge and discharge provide evidence of reversible oxygen release and re-accommodation by the host structure; this unique oxygen participation is likely the main reason for the anomalously high capacity of these electrodes. The HRPD data also show that during the early cycles, characteristic peaks of the Li 2 MnO 3 component disappear when charged to 4.7 V, but reappear on discharge to 2.5 V, consistent with a reversible lithium and oxygen extraction process. The results provide new insights into the charge compensation mechanisms that occur when high capacity, lithium-and manganese-rich electrode materials are electrochemically cycleda topic that is currently being hotly debated in the literature.

For the over-lithiated-oxides (OLOs), a composite of layered Li 2 MnO 3 and LiMO 2 (M = Mn, Co, Ni), the Li 2 MnO 3 part is not stable after the 1st charge-discharge cycle and partly transforms into layered LiMnO 2 , which in practice indicates that the phase used is actually a mixture of both Li 2 MnO 3 and LiMnO 2 . In the present work, the influences of 10 cationic (Mg, Ti, V, Nb, Fe, Ru, Co, Ni, Cu, Al) and 2 anionic (N, F) dopants on the phase stability, redox potential, ionic and electronic conductivity of both Li 2 MnO 3 and LiMnO 2 are investigated in detail using density functional theory. The calculations show that all the cationic dopants and F can be thermodynamically stable into the layered structures. The redox potential of both oxides is quite sensitive to some of the dopants, like V, Nb, Ru, due to the appearance of gap states introduced by those dopants. The Jahn-Teller effect has a strong influence on the Li vacancy diffusion behavior in both LiMnO 2 and its doped phases. Li vacancy diffusion behavior in Li 2 MnO 3 , including both interlayer and intralayer pathways, is relatively more complex and some dopants like Mg, Ti, Nb, Ru can decrease the barriers of the diffusion paths. The calculations also show the evidences of hole polaron formation in LiMnO 2 and electron polaron formation in Li 2 MnO 3 which should be the reason why these phases have low electronic conductivities. Based on these findings, possible ways to improve the electronic conductivity through the doping process are discussed.

In this work, we present a density-functional theory (DFT) investigation of the phase stability, electrochemical stability and phase transformation mechanisms of the layered and over-lithiated Mn oxides. This study includes the thermodynamic stability of Li and oxygen vacancies, to examine the electrochemical activation mechanisms of these cathode materials. The DFT calculations provide phase diagrams of the Li-Mn-O system in both physical and chemical potential spaces, including the crystals containing vacancies as independent phases. The results show the ranges of electrochemical activity for both layered LiMnO2 and over-lithiated Li2MnO3. By using a thermodynamic model analysis, we found that the required temperature for oxygen evolution and Li vacancy formation is too high to be compatible with any practical synthesis temperature. Using solid-state transition calculations, we have identified the key steps in the phase transition mechanism of the layered LiMnO2 into the spinel phase. The calculated effects of pH on the Li-Mn-O phase stability elucidated the mechanism of Mn(2+) formation from the spinel phase under acidic conditions.