Divergent Electrochemical Behaviors of O2 and O3 Phases in Li‐Rich Layered Oxides

ABSTRACT Lithium‐ion batteries (LIBs) have dominated the landscape of electrochemical energy storage for decades. The growing market demands for higher energy densities are pushing against the limitations of conventional cathode materials. Li‐rich layered oxides (LLOs) present a promising path forward by harnessing both cationic and anionic redox reactions, thereby unlocking higher energy densities. Their crystal structures primarily adopt two stacking configurations, O2 and O3 types, which exhibit different electrochemical behaviors. The O3‐type LLOs typically deliver higher initial capacity but suffer from severe voltage decay and oxygen release. In contrast, the O2‐type variants often exhibit superior structural resilience and voltage retention, albeit at the expense of capacity. This performance dichotomy originates from their distinct anionic redox reactivity and transition metal migration pathways. The mechanistic origins of this divergence remain a critical yet unresolved question in the field. In this review, we describe the fundamental electrochemical performance of these two cathode types, focusing on the origin of their distinct behaviors. In addition, we further compare their respective advantages and limitations, highlight current research progress and offer perspectives for future investigation.