Trace Ti/Mg Co-Doped O3-type Layered Oxide Cathodes with Enhanced Kinetics and Stability for Sodium-Ion Batteries

O3-type layered oxide cathodes are identified as a promising cathode material for sodium-ion batteries (SIBs) due to their high specific capacity and moderate operating voltage, yet it usually suffers from sluggish Na-ion diffusion and structure instability. Herein, we demonstrate the synthesis of a trace Ti/Mg co-doped NaNi1/ 3Fe1/3Mn1/3O2 (TiMg-NFM) cathode, in which Ti-ion is distributed in transition metal layers and Mg-ion is doped into sodium layers. The co-doping can enlarge the Na-layer spacing inside the layered structure to reduce Na-ion diffusion barrier. Meanwhile, the structural stability is effectively reinforced via the stronger Ti-O bond and the pillar effect of Mg-ions, further reducing surface side reaction with electrolyte. These merits endow a high reversible capacity of the TiMg-NFM cathode with 144.5 mAh g-1 at 0.1C, and much higher capacity retention of 111.4 mAh g-1 at 5C versus the pristine NFM (62.1 mAh g-1). More impressively, 80.1% of initial capacity can be maintained after 300 cycles at 2C. This work presents a feasible strategy to enhance the reaction kinetics and structure stability for developing advanced O3-type layered oxide cathodes.