The oxygen redox reaction in lithium-rich layered oxide battery cathode materials generates extra capacity at high cell voltages (i.e., >4.5 V). However, the irreversible oxygen release causes transition metal (TM) dissolution, migration and cell voltage decay. To circumvent these issues, we introduce a strategy for tuning the Coulombic interactions in a model Li-rich positive electrode active material, i.e., Li_1.2Mn_0.6Ni_0.2O₂. In particular, we tune the Coulombic repulsive interactions to obtain an adaptable crystal structure that enables the reversible distortion of TMO₆ octahedron and mitigates TM dissolution and migration. Moreover, this strategy hinders the irreversible release of oxygen and other parasitic reactions (e.g., electrolyte decomposition) commonly occurring at high voltages. When tested in non-aqueous coin cell configuration, the modified Li-rich cathode material, combined with a Li metal anode, enables a stable cell discharge capacity of about 240 mAh g⁻¹ for 120 cycles at 50 mA g⁻¹ and a slower voltage decay compared to the unmodified Li_1.2Mn_0.6Ni_0.2O₂.

Tuning the anionic redox chemistry (O²⁻ → O₂ ⁿ⁻) activity and reversibility by crystal and/or electronic modulation is essential for Li-rich oxide cathode materials. Herein, we report a facile strategy to improve the activity and reversibility of both anionic and cationic redox by integrating oxygen vacancies and the spinel phase. The initial specific capacity (216.1 mA h g⁻¹vs. 316.3 mA h g⁻¹), coulombic efficiency (80% vs. 94.8%), long-term cycling stability (1000 cycles at 5C) and voltage decay have all been greatly improved due to the largely suppressed irreversible oxygen release. The underlying modulation mechanism has been unraveled. Firstly, the introduction of oxygen vacancies decreases the covalency of TM–O and the density of states of the O 2p band, which mitigates the irreversible oxygen release during oxygen redox. Secondly, the spinel phase integration induced by oxygen vacancies not only improves the Li-ion conductivity and the rate capability due to its 3D Li⁺ channel and the expanded Li layer but also enhances the structural stability. Thirdly, the first-principles calculations indicate that the increase of delocalized electrons around the transition metal also intensifies the MnO₆ octahedral distortion and the inactive Mn-ions are partially activated during the first cycle and participate in the charge compensation. This study sheds some new light on designing high-performance Li-rich layered oxide cathode materials by regulating the anionic and cationic redox with the incorporation of oxygen vacancies and the spinel phase.

This review systematically discusses the structure, reaction mechanisms, and challenges as well as countermeasures for high-voltage Li-rich, LiCoO₂, and Ni-rich cathode materials, attributing their anion redox activity to O 2p non-bonding states.