Heteroatom-Substituted P2-NaNiMgMnO Cathode with {010} Exposing Facets Boost Anionic Activity and High-Rate Performance for Na-Ion Batteries.

As an attractive cathode candidate for sodium-ion batteries, P2-type NaNiMnO is famous for its high stability in humid air, attractive capacity, and high operating voltage. However, the low Na transport kinetics, oxygen-redox reactions, and irreversible structural evolution at high-voltage areas hinder its practical application. Herein, a comprehensive study of a microbar P2-type NiNiMgMnO material with {010} facets is presented, which exhibits high reversibility of structural evolution and anionic redox activity, leading to outstanding rate capability and cyclability. The notable rate performance (53 mA h g at 20 C, 2.0-4.3 V) contributed to the high exposure of {010} facets via controlling the growth orientation of the precursor, which is certified by density functional theory calculation and lattice structural analysis. Mg substitution strengthens the reversibility of anionic oxygen redox and structural evolution in high-voltage areas that was confirmed by the in situ X-ray diffraction and ex situ X-ray photoelectron spectroscopy tests, leading to outstanding cyclic reversibility (68.9% after 1000 cycles at 5 C) and slowing down the voltage fading. This work provides new insights into constructing electrochemically active planes combined with heteroatom substitution to improve the Na transport kinetics and structural stability of layered oxide cathodes for sodium storage.