Li-rich cathodes for rechargeable Li-based batteries: reaction mechanisms and advanced characterization techniques
Due to their high specific capacities beyond 250 mAh g-1, lithium-rich oxides have been considered as promising cathodes for the next generation power batteries, bridging the capacity gap between traditional layered-oxide based lithium-ion batteries and future lithium metal batteries such as lithium sulfur and lithium air batteries. However, the practical application of Li-rich oxides has been hindered by formidable challenges. To address these challenges, the understanding of their electrochemical behaviors becomes critical and is expected to offer effective guidance for both material and cell developments. This review aims to provide fundamental insights into the reaction mechanisms, electrochemical challenges and modification strategies for lithium-rich oxides. We first summarize the research history, the pristine structure, and the classifications of lithium-rich oxides. Then we review the critical reaction mechanisms which are closely related to their electrochemical features and performance, such as the oxidized lattice oxygen, oxygen vacancies, transition-metal migrations, layered to spinel transitions, ‘two-phase mechanism’ and lattice evolutions. These discussions are coupled with state-of-the-art characterization techniques. As a comparison, the anionic redox reactions in layered sodium transition metal oxides are also discussed. Finally, after a brief overview of the correlation among the aforementioned mechanisms, we provide perspectives on the rational design of lithium-rich oxides with high energy density and long cycling stability.