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  Yu, Y.; Zhang, J.; Gao, R.; Wong, D.; An, K.; Zheng, L.; Zhang, N.; Schulz, C.; Liu, X.: Triggering Reversible Anion Redox Chemistry in O3-Type Cathode through Tuning Na/Mn Anti-Site Defects. Energy & Environmental Science 16 (2023), p. 584-597

10.1039/D2EE03874A
Open Access Version

Abstract:
Oxygen anion redox (OAR) plays a crucial role on the capacity and stability of oxide cathodes in sodium-ion batteries but the intrinsic mechanism is poorly understood. How to trigger and stabilize OAR is challenging, particularly for O3-type transition metal (TM) oxide cathodes. Herein, we clarify Na/Mn anti-site defects mainly trigger OAR in O3-NaMn_1/3Fe_1/3Ni_1/3O₂ cathode, and OAR activity and reversibility can be enhanced through tuning Na/Mn anti-site defects with Ho doping. Ho³⁺ replacing Fe³⁺ site promotes more Na/Mn anti-site defects, enabling more O lone-pair electrons to participate in charge compensation. Meanwhile, Ho³⁺ enlarges O-O bond and O-TM-O angle, which maintain the single-electron oxygen hole configuration of (O-)-TM-(O-) and inhibit O-O shortening caused by electron loss, avoiding forming (O2)²- dimer. Furthermore, Ho³⁺ induces the splitting of TM 3d orbital energy band above Fermi level and generates low energy orbitals of Mn eg^* and Ni eg^*, which promotes the transition of O lone-pair electrons and Ni eg^* orbital electrons, and simultaneously activates redox activity of anions and cations. After regulation, the capacity rises from 146.8 to 184.9 mAh g^-1 and the capacity retention increases from 40.3 to 90.0%. This study reveals OAR mechanism in O3-type cathode and present insights on how to trigger and stabilize OAR.