Gradient and De-Clustered Anionic Redox Enabled Undetectable O Formation in 4.5 V Sodium Manganese Oxide Cathodes.

Anionic redox chemistry presents a promising approach to enhancing the energy density of oxide cathode materials. However, anionic redox reactions invariably lead to O formation, either as free gaseous O or trapped molecular O, which destabilizes the material's structure. Here, this critical challenge is addressed by constructing a crystal structure with both gradient redox activity and de-clustered redox-active oxygen. This design strategy is directly validated by operando differential electrochemical mass spectrometry and ex situ 50 K electron paramagnetic resonance, revealing no release of O or trapped O in the 4.5 V P2-type sodium manganese-based layered oxide. Notably, the material exhibits a highly reversible capacity of 247 mA h g at 20 mA g and exceptional capacity retention of 91.4% after 300 cycles at 300 mA g. In situ X-ray diffraction further suggests that the absence of O formation suppresses the typical P2-O2 phase transition, resulting in a minimal lattice volume change of only 0.5%. Ex situ neutron diffraction studies and theoretical calculations further elucidate that the locally ordered lattice is well-preserved, attributable to reduced cationic migrations during cycling.