The limiting component of lithium-ion batteries continues to be the cathode component. Since the layered materials, such as LiCoO2, have observed capacities of roughly half of their theoretical capacities, advances have been made in attempts to improve their stability and thus capacity. One such attempt is adding Li2MnO3, thus creating Li2MnO3-LiMO2 materials, where M is typically a transition metal ion or combination of transition metals. These Li2MnO3 stabilized materials have been shown to be promising with >200mAh/g but still suffer from performance issues. The Li2MnO3 component is observed to lose oxygen during the first charge cycle and thus creates Li2-XMnO3-δ. These oxygen vacancies are related to some of the cathode performance issues. However, the amount of oxygen released and the role of the oxygen vacancies are still not very well understood. Therefore, this work takes an atomic level computational approach using density functional theory calculations to explore the impact of oxygen vacancies and the correlated effects on voltage, capacity, lithium diffusion, chemical strain, dopants and electrolyte decomposition. Despite the extensive computational work in the literature on lithium transition metal oxide cathode materials, little work has been devoted to the correlated effects of two vacancy types in these materials. Therefore, this work offers novel approaches to model both vacancy types and their impacts on each other. First, it was found that the oxygen vacancies can decrease the formation energy of lithium vacancies. Less hopping of lithium atoms is observed and the energy barrier for lithium hopping is increased when oxygen vacancies are present. The calculated diffusion coefficient decreases by ~5 order of magnitude from the perfect crystal structure. This suggests oxygen vacancies cause an increased capacity but at the expense of decreased rate capability of these materials. The chemical strain associated with both non-dilute lithium vacancies and dilute vacancies were analyzed with an anisotropic model. It was found that the oxygen vacancies and lithium vacancies are highly correlated causing the associated chemical expansion to not be a linear sum of the individual vacancy types. The predicted chemical strain due to a low energy VLi-VO-VLi¬ dumbbell structure can be correlated with the in situ experimentally measured stress. To investigate if the amount of oxygen vacancies can be controlled, the effects of Si and Al dopants were also studied. The silicon was shown to decrease the oxygen vacancy formation energy in neighboring octahedral to the silicon, thus suggested to activate the manganese and increase the capacity of the materials, consistent with experimental observations. Lastly, the impact of surface oxygen vacancies on adsorption and decomposition of an electrolyte component, ethylene carbonate (EC), on the Li¬2MnO3 surface was investigated. A two proton removal reaction from EC to Li2MnO3 (131) was discovered, suggesting some beneficial effect on the perfect Li2MnO3 surface. However, an EC appears to be repelled near a surface oxygen vacancy. The released oxygen can react with the EC molecule and trigger different decomposition reactions. Overall, the oxygen vacancies generated in the lithium-rich layered cathode materials are shown to have a very highly correlated impact on lithium, dopant and electrolyte-surface interactions which therefore can significantly impact battery performance and life.
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