Progress on Modification Strategies of Layered Lithium-Rich Cathode Materials for High Energy Lithium-Ion Batteries

High-performance rechargeable lithium-ion batteries have been widely used in portable electronic devices, electric vehicles and other fields of electrochemical energy storage. However, in order to achieve a wider range of commercial applications, the energy density of lithium-ion batteries needs to be further improved. Layered lithium-rich oxide materials with a high reversible specific capacity of over 250 mAh center dot g-1 are regarded as commercially promising cathodes for next-generation high-energy lithium-ion batteries. The high capacity of layered lithium-rich materials can be attributed to its unique oxygen redox chemistry, which can achieve additional charge storage thus increasing its capacity. However, many challenges must be addressed, including high-voltage oxygen release, structural changes from layered to rock-salt phase and structural degradation owing to the migration of transition metal ions, before it can be applied practically. These existing challenges result in low initial Coulombic efficiency, voltage/capacity decay, and insufficient cycle life. In view of the above issues, the modification of layered lithium-rich materials is an effective method. This review systematically introduces the composition and structure of lithium-rich materials, and then analyzes the electrochemical mechanism and internal causes which affect the electrochemical performance of lithium-rich materials. Furthermore, recent material modification strategies are discussed with regards to the current challenges. In addition, current methods and developmental trends of modification strategies such as bulk doping, surface coating, defect design, ion exchange and microstructure regulation are summarized in detail. According to the different charge properties, the doping modification can be divided into cationic doping, anion doping and anion-cation co-doping. Among them, cationic doping can be further categorized into transition metal layer doping substitution and lithium layer doping substitution, depending on the doping site. Two tables for the doping and ion exchange modifications were tabulated, and the representative scientific research was summarized. Recent research conducted on hotspot high-entropy materials were also mentioned. Finally, design ideas for high-capacity, long-cycle layered lithium-rich materials and high specific energy lithium-ion batteries were prospected. This comprehensive review is expected to promote further lithium-rich oxide materials research.