High-entropy strategy for stabilizing oxygen redox in layered oxide cathodes for Na-ion batteries

ABSTRACT: Activating oxygen redox (OR) reactions is an effective strategy for increasing the specific capacity and energy density of Na-ion batteries (NIBs). However, most OR cathodes require a high charge cut-off voltage (≥4.4 V), which severely compromises structural stability and accelerates electrolyte decomposition. In addition, practical cathode materials should possess high sodium content and robust structural stability to ensure high capacity and long cycle life. Herein, we propose a high-entropy O3-type layered oxide, Na 0.9 Li 0.1 Mg 0.1 Ni 0.1 Cu 0.1 Mn 0.4 Ti 0.2 O 2 (HE-NLMNCMTO), as a practical oxygen-redox cathode. Benefiting from the elevated oxygen activity induced by the high-entropy configuration, oxygen redox can be fully activated at a relatively low cut-off voltage of ∼4.2 V. Combined with the intrinsic structural stability of the high-entropy framework, HE-NLMNCMTO delivers a high reversible capacity of 167.1 mA⸱h/g through synergistic cationic and anionic redox reactions and exhibits outstanding cycling stability with 91.2% capacity retention after 400 cycles at 1 C. The remarkable structural stability is confirmed by in-situ XRD, nano-CT and STEM analyses. Moreover, cylindrical cells and anode-free single-layer pouch cells based on HE-NLMNCMTO exhibit excellent electrochemical performance, demonstrating its considerable potential for practical applications. This work provides a viable high-entropy strategy for developing durable and practical oxygen-redox cathodes for NIBs.