Synchronously Restraining the Phase Transition and Structural Defect through a Unique Dopant Strategy for Manganese-Based Layered Cathodes for Sodium-Ion Batteries

The P2-type sodium manganese-based layered oxide cathodes suffer from an unsatisfactory phase transition and structural defects due to the instability of the bulk and interfacial structure. In this work, we proposed a manganese-based layered oxide cathode, P2-Na0.67Ni0.25Mn0.75O2@Fe2O3@Ta2O5 (Na2575-Fe-Ta), to increase the bulk and interfacial stability synchronously during cycling. Partially substituting Fe ions into the TMO2 layer in the bulk lattice structure mitigates the unfavorable phase transition and suppresses the variation of the lattice parameters during charge and discharge, retarding structural degradation. Moreover, the in situ formed NaTaO3 layer via doping Ta2O5 not only reduces the irreversible release of lattice oxygen but also mitigates electrolyte consumption and parasitic reactions on the electrode-electrolyte interface, which is ascribed to the generation of structural defects after repeated Na+ ion insertion/extraction. Consequently, the well-designed sample delivers 214.9 mA h/g under 0.1 C and exhibits 64.6% capacity retention after 200 cycles under 0.5 C, much better than those of the pristine, 19.5 mA h/g and 9.7%. Herein, we demonstrated that the synergistic improvement of bulk and interfacial stability by doping multiple transition metal ions in a one-step method is promising for the application of Na0.67Ni0.25Mn0.75O2 for sodium-ion batteries.