• 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

Open Access Version

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-NaMn1/3Fe1/3Ni1/3O2 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.