Operando Magnetism on Oxygen Redox Process in Li-Rich Cathodes

Oxide ions in lithium-rich layered oxides can store charge at high voltage and offer a viable route toward the higher energy density batteries. However, the underlying oxygen redox mechanism in such materials still remains elusive at present. In this work, a precise in situ magnetism measurement is employed to monitor real-time magnetization variation associated with unpaired electrons in Li_1.2Mn_0.6Ni_0.2O₂ cathode material, enabling the investigation on magnetic/electronic structure evolution in electrochemical cycling. The magnetization gradually decreases except for a weak upturn above 4.6 V during the initial charging process. According to the comprehensive analyses of various in/ex situ characterizations and density functional theory (DFT) calculations, the magnetization rebound can be attributed to the interaction evolution of lattice oxygen from π-type delocalized Mn─O coupling to σ-type O─O dimerization bonding. Moreover, the magnetization amplitude attenuation after long-term cycles provides important evidence for the irreversible structure transition and capacity fading. The oxygen redox mechanism concluded by in situ magnetism characterization can be generalized to other electrode materials with an anionic redox process and provide pivotal guidance for designing advanced high-performance cathode materials.