The lithium-ion battery has mitigated the reliance on fossil fuels and alleviated the increasing pressure on environment as a new form of energy storage. Compared with other conventional cathodes, lithium-rich layered oxide is considered to be one of the most potential candidates for the next generation cathode materials due to its low cost and higher reversible capacity, which is a promising application especially in the fields of electric vehicles and large-scale energy storage grids. In this review, we start from the point of the structures of lithium-rich layered oxides then mainly focus on the differences of lattice configuration between a solid solution structure and a "composite" structure. Various characterization methods which are used to identify these differences are introduced as well. The origins of the 4.5 V plateau and several representative mechanisms proposed in recent years are summarized. From different perspective, these mechanisms explain the origin of the abnormal capacity of lithium-rich layered oxides when they are charged-discharged at the first cycle. According to the current research, the validity of these mechanisms is elaborated. Meanwhile, some major problems which hinder the further development of lithium-rich layered oxides, including irreversible capacity loss at first cycle, cycle performance, rate capability, are introduced. At the same time, the effects generated by the surface spinel phase and several typical modification methods are elaborated. Finally, the future development and the prospects of lithium-rich layered oxides as cathode materials for lithium-ion batteries are also proposed.

Contents
1 Introduction
2 Structures
2.1 Solid solution structure
2.2 “Composite” structure
3 Origins of the 4.5 V plateau
3.1 “Oxygen loss” mechanism
3.2 “Reversible oxygen redox” mechanism
3.3 “Proton exchange” mechanism
3.4 Other mechanisms
4 Capacity loss mechanisms and modifications
4.1 Reasons and modifications of irreversible capacity loss during first cycle
4.2 Reasons and modifications of voltage decay upon cycles
4.3 Reasons and modifications of poor C-rate capability
5 Conclusion