Constructing a Composite Structure by a Gradient Mg Doping Strategy for High-Performance Sodium-Ion Batteries.

Layered P2-NaMnNiO has been considered an attractive cathode material for sodium-ion batteries (SIBs). Nevertheless, it is still burdened with hazardous phase transformation of P2-O2 under high voltage and harmful reactions at the interface of the electrode and electrolyte. These result in unfavorable structural degradation and rapid capacity decay. Herein, a gradient Mg doping approach is proposed to trigger a structural transformation. During the annealing process, the bulk-diffused Mg and surface residual Mg induce the formation of the P2/P3@MgO structure. Consequently, this method combines the merits of the composite phases, bulk doping, and surface modification. In consequence, Na diffusion kinetics and electrochemical performances are remarkably enhanced. The cells using P2/P3@MgO show 69.7% capacity retention at 0.2 C within a voltage range of 1.5-4.5 V for 100 cycles, compared with the 42.6% for P2-NaMnNiO. This work offers new insights into further developments of advanced layered oxide cathodes for SIBs.