Identifying the Pivotal Role of the Conjugation π Bond for Stabilizing the Anionic Redox in Lithium-Rich Layered Oxide Cathodes

Lithium-rich layered oxide cathode materials have attracted significant attention due to their high specific energy. However, upon long cycling within the high-voltage window, the irreversible redox of O and potential transition metal (TM) ion migration could trigger structural distortion toward the spinel/rock-salt structure, thus leading to the large voltage hysteresis and fast capacity fading. Therefore, the comprehensive understanding of the charge compensation and Li + storage mechanism in the different structures, i.e., layered, spinel, and rock-salt, is the paradigm toward practical application of Li-rich cathode materials. Herein, Li 1.2 Ni 0.2 Mn 0.6– x Ti x O 2 is chosen as the prototype for the systematic and comparison study of the structure–property interrelationship for the O3-layered structure, spinel, and ordered rock-salt (OR) structures. Electrochemical studies show that the three compounds exhibit large specific capacities, whereas the resulting O3–Li 1.2 Ni 0.2 Mn 0.5 Ti 0.1 O 2 displays supreme capacity retention with the smallest voltage plateau drop. Among them, O3 exhibits a capacity retention of 95.4% after 100 cycles significantly higher than the spinel (36.9%) and ordered rock-salt (38.5%) structures. In situ X-ray diffraction and X-ray absorption spectroscopy spectrum unambiguously disclose O3-type layered oxides render more stable structure evolution with more reversible O redox behavior. A theoretical study using Wannier orbital calculations as the implement further reveals that the O3 structure could form a conjugated π-type bonding network between TM and O, stabilizing oxygen. This work provides valuable insights into O redox chemistry in different types of layered oxide cathodes, which inspires the fundamental understanding of the electrochemical degradation in the Li-rich compounds during cycling, paving the way of practical application for Li-rich cathode materials.