Transition Metal Slab Gliding: One Key Process for Activating Anionic Redox Reaction in P2-Type Transition Metal...

Anionic redox chemistry is crucial for determining the capacity and stability of layered oxide cathodes for Na-ion batteries. However, the driving forces of anionic redox remain elusive. Na-O-A (A = alkali metal, alkaline earth metal, or vacancy) configurations have been identified as key to activating anionic redox reactions in transition metal (TM)-deficient oxide materials. However, reversible oxygen redox reactions are also observed in TM-stoichiometric oxide materials, although the Na-O-A configurations cannot be established in these systems. A TM-stoichiometric P2-type Na_2/3Cu_1/3Mn_2/3O₂ cathode and its Ti⁴⁺-substituted analogue (Na_2/3Cu_1/3Mn_1/2Ti_1/6O₂) are herein studied for deciphering the underlying mechanism. The incorporation of Ti⁴⁺ disrupts the ordered arrangement of TM layers, accelerates TM slab gliding, and facilitates TM migration, thus affording new Na-O-vacancy configurations and activating the reversible oxygen redox reaction. Consequently, Na_2/3Cu_1/3Mn_1/2Ti_1/6O₂ exhibits an initial discharge capacity of 153 mAh g^-1 and a capacity retention of 80% after 300 cycles at 2 C rate. This work provides plausible routes for reversible anionic redox reactions and increasing the energy density of SIBs.