Elucidating and Mitigating the Degradation of Cationic–Anionic Redox Processes in Li_1.2Mn_0.4Ti_0.4O₂ Cation-Disordered Cathode Materials

Abstract

Cation-disordered rock-salt oxides with the O^2-/O₂^<i>n</i>- redox reaction, such as Li_1.2Mn_0.4Ti_0.4O₂ (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 O^2-/O₂^<i>n</i>- 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.