Li-Rich Layered Oxides and Their Practical Challenges: Recent Progress and Perspectives

Lithium-rich layered oxides (LLOs), also known as Li1+xM1−xO2 or xLi2MnO3-(1–x)LiMO2 (M = Ni, Co, Mn), have been regarded as some of the highest capacity lithium cathodes and have attracted increasing attention from battery researchers and engineers in recent years. This is because LLOs possess maximum possible capacities of ~ 280 to 310 mAh g−1 with a high working potential of ~ 3.7 V (vs. Li+/Li0) and an astounding energy density of ~ 900 Wh kg−1. Despite these promising properties, these technologically important cathodes have not yet been successfully commercialized due to low initial Coulombic efficiency, poor rate capabilities and gradual capacity/voltage fade during electrochemical cycling as well as further complications from continuous structural changes during cycling. Here, researchers have concluded that these issues mainly originate from the electrochemical activation of Li2MnO3 components, which, although it provides anomalously high capacity performances, also causes associated complex anionic redox activities of O and irreversible structural and phase transformations during charging at potentials greater than 4.5 V (vs. Li+/Li0). To provide perspectives, this review will summarize various attempts made towards addressing these issues and present the connections between electrochemical properties and structural change. In addition, this review will discuss redox chemistries and mechanistic behaviours during cycling and will provide future research directions to guide the commercialization of LLOs.