Challenges and strategies on high-manganese Li-rich layered oxide cathodes for ultrahigh-energy-density batteries

Abstract

Benefiting from the synergistic participation of transition metals (TMs) and lattice oxygen in redox reactions, Li-rich layered oxides (LLOs) exhibit a capacity exceeding 250 ​mAh ​g⁻¹, positioning them as promising cathode candidates for next-generation high-energy-density lithium-ion batteries. To further enhance capacity and reduce reliance on environmentally hazardous Co and Ni elements, the development of high-Mn LLOs (HM-LLOs) with ultrahigh capacities surpassing 350 ​mAh ​g⁻¹ ​has emerged as a viable strategy. Elevated Mn content introduces additional Li–O–Li configurations, facilitating greater lattice oxygen involvement in redox reactions, thereby increasing theoretical capacity. However, practical studies reveal that the achievable capacity of HM-LLOs remains significantly lower than theoretical predictions, severely hindering their application. The discrepancy primarily stems from two factors: activation difficulty and irreversible oxygen loss. Despite the higher initial charge capacity, the lattice oxygen utilization efficiency is still limited by incomplete activation. Meanwhile, irreversible oxygen loss leads to low initial coulombic efficiency (ICE). Given these challenges in HM-LLOs, a systematic review is necessary to unravel the origin of these issues and seek valid strategies to promote their application in power batteries. Herein, we elucidate the relationship between high Mn content and theoretical capacity through compositional, structural, and stoichiometric perspectives. Next, we analyze the roles of elemental components in HM-LLOs at the atomic level, followed by an in-depth investigation of unique structural evolution, particularly the formation of large Li₂MnO₃ domains. These factors collectively restrict practical capacity utilization. Low Co content combined with large Li₂MnO₃ domains exacerbate activation issues, while low Ni content and these domains promote irreversible oxygen loss. Building on this mechanistic understanding, we comprehensively categorize various strategies, from precursor synthesis to active material modifications. The mechanisms of precursor synthesis and structural transformations during the sintering process have been detailed. Optimization methods employed during the synthesis process have been thoroughly reviewed. Furthermore, effective modification methods have been elaborated, from the fundamental principles to practical applications. The advantages and disadvantages of these modification methods, as well as potential future optimization directions, have been outlined. Additionally, novel explorations, such as the construction of O2-type structures, innovative activation methods, and the development of sulfur-based host, are discussed. Finally, we propose future directions to bridge the gap between theoretical and practical capacities, including advanced characterization of oxygen redox dynamics and machine learning-guided evaluation of modifications. This review provides critical insights into advancing high-capacity cathode materials, thus accelerating the commercialization of HM-LLOs.