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, Li_0.693[Li_0.153Ni_0.190Mn_0.657]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 O6-type structure (R-3m symmetry) with unique local geometry. 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.