Abstract: Current bottlenecks in cobalt supply have negatively impacted commercial lithium-ionbattery application and inspired the development of Co-free cathodes with high energy density. Li-Mn-rich layered oxides (LMRs) are promising to be next generation cathodes for lithium-ion batteries due to their high specific capacity (>250 mAh/g) and low cost, but the voltage decay and capacity fading during cycling are the main challenge for their commercialization. Herein, Co-containing and Co-freeLMRs were synthesized for investigating the function of Co on the electrochemical performance. It is found that Co can promote the oxidation of O2- and contribute more capacity above 4.5V, which leads to oxygen release and irreversible structural transformations. Moreover, this report will review the research progress of LMRs in the understanding of precursor process route selection, modification effect and mechanism of bulk doping, surface coating, liquid phase post-treatment, and the design of new special structures.