Beyond Inducing Anionic Redox: Controllable Migration Sequence of Li Ions in Transition Metal Layers Toward Highly Stable Li-Rich Cathodes

Abstract

The energy density of layered oxides of Li-ion batteries can be enhanced by inducing oxygen redox through replacing transition metal (TM) ions with Li ions in the TM layer. Undesirably, the cathodes always suffer from unfavorable structural degradation, which is closely associated with irreversible TM migration and slab gliding, resulting in continuous capacity and voltage decay. Herein, attention is paid to the Li ions in the TM layer (Li_TM) and find their extra effects beyond inducing oxygen redox, which has been rarely mentioned. With the aid of ⁷Li solid-state NMR and density functional theory (DFT) calculations, the controllable migration of Li_TM is verified. The mystery is uncovered that the preferential migration of Li_TM plays an imperative role in preventing the structural transformation by postponing the slab gliding of the layered structure. Integrated with the inhibited TM migration, the structural robustness and reversibility of Li₂RuO₃ can be drastically improved after Zr-substitution, providing a solid foundation for achieving ultra-stable electrochemical performance even after thousands of cycles (2500 cycles). The discovery highlights the significance of Li_TM with respect to the structural robustness and provides a potential route toward high-energy-density Li-ion batteries.