Gradient and De-Clustered Anionic Redox Enabled Undetectable O₂ Formation in 4.5 V Sodium Manganese Oxide Cathodes.

Abstract

Anionic redox chemistry presents a promising approach to enhancing the energy density of oxide cathode materials. However, anionic redox reactions invariably lead to O₂ formation, either as free gaseous O₂ or trapped molecular O₂, 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 O₂ or trapped O₂ 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^-1 at 20 mA g^-1 and exceptional capacity retention of 91.4% after 300 cycles at 300 mA g^-1. In situ X-ray diffraction further suggests that the absence of O₂ 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.