A comprehensive understanding on the anionic redox chemistry of high-voltage cathode materials for high-energy-density lithium-ion batteries†

The electrification of transportation is an important contributor to reducing global carbon dioxide emissions. However, this progress is constrained by anxiety regarding the driving range of vehicles, which is well recognized to originate from the low specific energy of the employed state-of-the-art energy storage devices. Therefore, further promoting the specific energy of lithium-ion batteries (LIBs) is an inevitable need, where the development of cathode materials with high energy densities, i.e. high specific capacity and/or high working voltage, is essential. Accordingly, numerous research efforts are ongoing worldwide, where several materials stand out, including LiCoO₂ (LCO), Ni-rich oxides and Li-rich cathodes, mainly because of their potential to deliver high capacities when operating at high voltages. However, the elevated operating voltage turns out to be a double-sided sword for these materials as achieving high specific capacity is always accompanied by the oxygen redox process, which shows unsatisfactory reversibility and has a significant impact on their structure stability and electrochemical performance. Consequently, understanding the failure mechanism of anionic redox chemistry and finding solutions to this issue are crucial for realizing the practical application of these high-voltage materials. Although many studies have been reported on the anionic redox chemistry of different materials, the corresponding reviews have predominantly focused on Li-rich cathode materials. Hence, the reviews on high-voltage LCO and Ni-rich oxides remain incomplete, and a unified understanding of their behavior at high voltages has not been established yet. This lack of comprehensive understanding has hindered the further development and application of high-voltage cathode materials. Thus, this review highlights the similarities and differences in the anionic redox chemistry of LCO, Li-rich and Ni-rich high-voltage cathode materials, emphasizing on a unified mechanistic picture and the related challenges and countermeasures. We aim to provide an outlook for future guidelines in material exploration with anionic redox chemistry, thus unlocking the full potential of high-voltage LIBs for diverse applications.