A Metastable Oxygen Redox Cathode for Lithium‐Ion Batteries

Simultaneously harnessing cation and anion redox activities in the cathode is crucial for the development of high energy‐density lithium‐ion batteries. However, achieving long‐term stability for both mechanisms remains a significant challenge due to pronounced anisotropic volume changes at low lithium content, unfavorable cation migration, and oxygen loss. Here, we demonstrate exceptionally stable cation and anion redox behavior in a metastable, cobalt‐free layered oxide, Li0.693[Li0.153Ni0.190Mn0.657]O2 (LLNMO). After 50 cycles at 50 mA/g (~0.2 C), the cathode retains 97.4% of its initial capacity (222.4 mAh/g) with negligible voltage decay. This remarkable stability is attributed to its metastable rhombohedral symmetry (R‐3m) with unique local structures. The face‐sharing connectivity between lithium layers and alternating transition metal (TM) layers effectively suppresses TM migration‐induced voltage decay during anion redox. Additionally, the structure balances interlayer cation/cation and anion/anion repulsions, resulting in minimal expansion and contraction during de‐/lithiation (< 2.3% along the c‐axis) and excellent structural reversibility. These findings highlight that layered oxides with a metastable framework are promising cathode candidates for next‐generation ultra‐high‐energy lithium‐ion batteries.