Elucidating and Mitigating the Degradation of Cationic–Anionic Redox Processes in Li1.2Mn0.4Ti0.4O2 Cation-Disordered Cathode Materials

Cation-disordered rock-salt oxides with the O2–/O2n– redox reaction, such as Li1.2Mn0.4Ti0.4O2 (LMTO), are critical Li-rich cathode materials for designing high-energy-density batteries. Understanding the cationic–anionic redox accompanying the structural evolution process is really imperative to further improve the performance. In this work, the cationic–anionic redox and capacity degradation mechanism of carbon-coated LMTO during (dis)charge processes are elucidated by combining in situ X-ray diffraction, X-ray absorption near-edge spectroscopy, differential electrochemical mass spectrometry, transmission electron microscopy, and electrochemical analyses. It is concluded that the redox reaction of Mn2+/Mn4+ is quite stable, while the severe degradation is mainly caused by the O2–/O2n– redox process. Moreover, we clearly clarify how the cationic–anionic interplay governs sluggish kinetics, large polarization, and capacity fading in LMTO, and reveal for the first time that a certain amount of carbon coating is capable of suppressing the irreversible lattice oxygen loss and results in an encouraging cycling performance. In summary, we elucidate the degradation of cationic–anionic redox processes in cation-disordered cathode materials and propose strategies for adjusting the electronic/ionic conductivity of the electrodes to modulate the oxygen redox reactions, setting a new direction for the design of better cation-disordered oxides.