A New Strategy to Build a High-Performance P′2-Type Cathode Material through Titanium Doping for Sodium-Ion Batteries

Herein, Ti4+ in P ' 2-Na-0.67[(Mn0.78Fe0.22)(0.9)Ti-0.1]O-2 is proposed as a new strategy for optimization of Mn-based cathode materials for sodium-ion batteries, which enables a single phase reaction during de-/sodiation. The approach is to utilize the stronger Ti-O bond in the transition metal layers that can suppress the movements of Mn-O and Fe-O by sharing the oxygen with Ti by the sequence of Mn-O-Ti-O-Fe. It delivers a discharge capacity of approximate to 180 mAh g(-1) over 200 cycles (86% retention), with S-shaped smooth charge-discharge curves associated with a small volume change during cycling. The single phase reaction with a small volume change is further confirmed by operando synchrotron X-ray diffraction. The low activation barrier energy of approximate to 541 meV for Na+ diffusion is predicted using first-principles calculations. As a result, Na-0.67[(Mn0.78Fe0.22)(0.9)Ti-0.1]O-2 can deliver a high reversible capacity of approximate to 153 mAh g(-1) even at 5C (1.3 A g(-1)), which corresponds to approximate to 85% of the capacity at 0.1C (26 mA g(-1)). The nature of the sodium storage mechanism governing the ultrahigh electrode performance in a full cell with a hard carbon anode is elucidated, revealing the excellent cyclability and good retention (approximate to 80%) for 500 cycles (111 mAh g(-1)) at 5C (1.3 A g(-1)).