Probing the Structural Transition Kinetics and Charge Compensation of the P2-NaAlNiMnO Cathode for Sodium Ion Batteries.

Although the layered P2-type NaNiMnO materials show high discharge voltage and specific capacity, they suffer from severe structural instabilities and surface reaction upon Na exchange for sodium-ion batteries (SIBs). Therefore, it is quite necessary to reveal the underlying structural evolution mechanism and diffusion kinetics to improve the structural/electrochemical stability for application in SIBs. Here, we synthesize a P2-type NaAlNiMnO material by a small dose of Al replacing the Mn, aiming at enhancing the structural stability without sacrificing the average discharge voltage and theoretical capacity. The etching X-ray photoelectron spectroscopy and energy-dispersive X-ray mapping/line scan results indicate that the Al doping induces dual effects of the AlO surface coating and the bulk lattice doping, which efficiently suppress the accumulation of structural irreversible changes from P2 to O2, the volume changes, and surface reactions at high voltage. Obvious improvements are further found on the diffusion kinetics of Na ions as well as the decrease of overall voltage polarization. Interestingly, the dual effects of Al doping lead to the significant increase of capacity retention after 50 cycles and improvement of rate capability compared with the undoped counterpart between 2.0 and 4.5 V. Hence, this work sheds new light on stabilizing the P2-Na-Ni-Mn-O materials, which provides a rewarding avenue to develop better SIBs.