Triggering reversible anion redox chemistry in O3-type cathodes by tuning Na/Mn anti-site defects

Oxygen anion redox (OAR) plays a crucial role in 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 that Na/Mn anti-site defects mainly trigger OAR in the O3-NaMn_1/3Fe_1/3Ni_1/3O₂ cathode, and OAR activity and reversibility can be enhanced by tuning Na/Mn anti-site defects with Ho doping. Replacing the Fe³⁺ site by Ho³⁺ promotes more Na/Mn anti-site defects, enabling more O lone-pair electrons to participate in charge compensation. Meanwhile, Ho³⁺ enlarges the O-O bond and ∠O-TM-O angle, and maintains the single-electron oxygen hole configuration of (O⁻)-TM-(O⁻) and inhibits O-O shortening caused by electron loss, avoiding forming an (O₂)²⁻ dimer. Furthermore, Ho³⁺ induces the splitting of the TM 3d orbital energy band above the Fermi level and generates low energy orbitals of Mn e_g* and Ni e_g*, which promotes the transition of O lone-pair electrons and Ni e_g* orbital electrons, and simultaneously activates the redox activity of anions and cations. After regulation, the capacity increases from 146.8 to 184.9 mA h g⁻¹ and the capacity retention increases from 40.3 to 90.0%. This study reveals the OAR mechanism in the O3-type cathode and presents insights into how to trigger and stabilize OAR.