Controversy on necessity of cobalt in nickel-rich cathode materials for lithium-ion batteries

[Display omitted] •Controversy on necessity of Co in Ni-rich cathode materials was analyzed in detail.•Co plays double-edged roles, not just for enhancing electrochemical performance.•Efforts and new working mechanisms for restraining degradation and OER caused by Co.•Developing Co-free cathodes is an inevitable trend for Li+-rechargeable batteries. Since the layered oxide LiCoO2 as the cathode material for commercial Li ion batteries, especially, nickel-rich layered oxide cathode materials are consolidating their status as the cathode material of choice and enabling a significant success of the passenger electric vehicle industry. Generally, cobalt in cathodes has been considered necessary in enhancing electrochemical performance. However, they are still facing critical challenges in further commercialization. For instance, cobalt caused more severe structural degradation and capacity degradation at high potential. Additionally, it triggered O2 and heat release, which eventually cause the interfacial instability and thermal instability of the cathode materials. Prior studies also confirmed that cobalt plays double-edged roles in cathodes, and questioned its necessity. Meanwhile, not only is it facing a roadblock caused by high-cost restrictions, but more importantly, 50% of world mine production originates from copper-cobalt ore in the Democratic Republic of the Congo (DRC), where geopolitical instability and harsh working conditions could halt cobalt exports. Therefore, many studies have explored the possibility of cobalt-free materials. This review shed new lights on understanding the role of cobalt and reveals the perspectives of technical challenges in current state by the practical aspect for cobalt-free cathode materials, thereby helping to advance the future development of next-generation low-cost and long-calendar-life batteries.