Redox Evolution of Li-Rich Layered Cathode Materials

Li-rich layered oxides utilizing reversible oxygen redox are promising cathodes for high-energy-density lithium-ion batteries. However, they exhibit different electrochemical profiles before and after oxygen redox activation. Therefore, advanced characterization techniques have been developed to explore the fundamental understanding underlying their unusual phenomenon, such as the redox evolution of these materials. In this review, we present the general redox evolution of Li-rich layered cathodes upon activation of reversible oxygen redox. Various synchrotron X-ray spectroscopy methods which can identify charge compensation by cations and anions are summarized. The case-by-case redox evolution processes of Li-rich 3d/4d/5d transition metal O3 type layered cathodes are discussed. We highlight that not only the type of transition metals but also the composition of transition metals strongly affects redox behavior. We propose further studies on the fundamental understanding of cationic and anionic redox mixing and the effect of transition metals on redox behavior to excite the full energy potential of Li-rich layered cathodes.