Advances and Challenges in Li-Excess Cathode Additives for Next-Generation Li Rechargeable Batteries

The development of high-energy-density rechargeable batteries requires the integration of Li-excess cathode additives (LECAs) to mitigate irreversible Li+ loss. However, the practical implementation of these additives remains challenging without a clear understanding of their reaction mechanisms. LECAs can be broadly classified into transition-metal-free (TM-free), overlithiated, and hyperlithiated compounds, each exhibiting distinct Li+ release behaviors and electrochemical properties. Despite their potential advantages, the absence of comprehensive mechanistic insights hampers their optimization and commercial application. This review systematically examines the reaction mechanisms of LECAs through advanced in situ and ex situ analytical techniques, providing a detailed understanding of their Li+ compensation processes, structural evolution, and electrochemical performance. Furthermore, we evaluate structural modification strategies aimed at enhancing their stability, redox kinetics, and compatibility with electrolyte systems, focusing on TM and anion cosubstitution, particle size control, and surface engineering. We also identify key technical challenges, such as oxygen evolution, interfacial degradation, and Li+ utilization inefficiencies, highlighting critical obstacles to their practical implementation. Finally, we outline future research directions, emphasizing the importance of precise material design, interface stabilization, and scalable fabrication techniques to address these challenges. This review establishes a strategic framework for the rational design and optimization of LECAs, providing insights into their potential for next-generation Li rechargeable batteries.