Advanced Cathode Design: Ordering LiMn6 Functional Units in Li-Rich Layered Oxides
Li-rich layered oxides (LLOs) xLi2MnO3·(1−x)LiTMO2 (0 < x < 1, TM = Mn, Ni, Co, etc.) are promising high-energy cathode materials for lithium-ion batteries, capable of achieving energy densities up to 1000 Wh kg−1. However, their practical application is hindered by limited cycle life and voltage decay, primarily stemming from structural instability of Li2MnO3 and LiTMO2 domains during cycling, which manifests as transition metal cation migration, oxygen release, and TM dissolution. This review establishes a comprehensive design and modification scheme based on the fundamental structural unit of LLOs: the LiMn6 hexatomic-ring. In this unit, each Li atom in the TM layer is surrounded by six Mn atoms, forming a π-type hybridization between Mn 3d(t2g) and O 2p orbitals, creating an intact π-bond ring that acts as a distributed redox center. Ordering these functional units into specific spatial configurations can enhance capacity, voltage, and structural stability. However, substituting Mn with other elements breaks the symmetry of the π-bond ring, altering redox behavior and reversibility. The spatial arrangement of LiMn6 rings, representing the arrangement of π-bond rings, is critical for redox activity. In Li2MnO3, the typical honeycomb superstructure of LiMn6 rings is discussed. This review highlights the importance of functional unit ordering in material design, drawing parallels from other systems, and proposes a systematic approach to engineer LLOs for improved electrochemical performance.