Enhancing the Reversibility of Lattice Oxygen Redox Through Modulated Transition Metal–Oxygen Covalency for Layered Battery Electrodes

• Published in: Advanced materials (Weinheim) Vol. 34; no. 20; pp. e2201152 - n/a
• Main Authors: Cheng, Chen, Chen, Chi, Chu, Shiyong, Hu, Haolv, Yan, Tianran, Xia, Xiao, Feng, Xuefei, Guo, Jinghua, Sun, Dan, Wu, Jinpeng, Guo, Shaohua, Zhang, Liang
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 01-05-2022; Wiley
• Subjects: Covalence; Electrochemical analysis; Electrodes; Electronegativity; ENERGY STORAGE; Inelastic scattering; lattice oxygen redox; layered electrodes; metal-oxygen covalency; Na-ion batteries; Oxidation; Oxygen; Rechargeable batteries; RIXS; structural stability; Transition metals; metal-oxygen covalency; lattice oxygen redox; structural stability; RIXS; layered electrodes; Na-ion batteries
• ISSN: 0935-9648; 1521-4095
• DOI: 10.1002/adma.202201152

Utilizing reversible lattice oxygen redox (OR) in battery electrodes is an essential strategy to overcome the capacity limitation set by conventional transition metal redox. However, lattice OR reactions are often accompanied with irreversible oxygen oxidation, leading to local structural transformations and voltage/capacity fading. Herein, it is proposed that the reversibility of lattice OR can be remarkably improved through modulating transition metal–oxygen covalency for layered electrode of Na‐ion batteries. By developing a novel layered P2‐Na0.6Mg0.15Mn0.7Cu0.15O2 electrode, it is demonstrated that the highly electronegative Cu dopants can improve the lattice OR reversibility to 95% compared to 73% for Cu‐free counterpart, as directly quantified through high‐efficiency mapping of resonant inelastic X‐ray scattering. Crucially, the large energetic overlap between Cu 3d and O 2p states dictates the rigidity of oxygen framework, which effectively mitigates the structural distortion of local oxygen environment upon (de)sodiation and leads to the enhanced lattice OR reversibility. The electrode also exhibits a completely solid‐solution reaction with an ultralow volume change of only 0.45% and a reversible metal migration upon cycling, which together ensure the improved electrochemical performance. These results emphasize the critical role of transition metal–oxygen covalency for enhancing the reversibility of lattice OR toward high‐capacity electrodes employing OR chemistry.
Instead of activating the oxygen oxidation activity by construction of overwhelming nonbonding oxygen states, it is more crucial to enhance the reversibility of lattice oxygen redox reactions of layered battery electrodes through modulating transition metal (TM) O covalency, which concurrently suppresses the phase transition and irreversible TM migration, thus leading to the improved structural stability and electrochemical performance.