Structural evolution and redox chemistry of robust ternary layered oxide cathode for sodium-ion batteries

Nickel/manganese-based O3-type layered oxides have been widely deemed to be a family of promising cathode materials for sodium-ion batteries (SIBs) owing to the large theoretical specific capacity and low manufacturing cost, but being usually challenged by unsatisfied electrochemical performance. In this study, a novel ternary layered oxide NaNi1/2Mn1/4Ti1/4O2 is reported as a robust high-capacity cathode for SIBs, and its structural evolution and redox chemistry in the potential range of 1.5-4.2 V (vs Na+/Na) are carefully explored by coupling X-ray diffraction technique, X-ray photoelectron spectroscopy, transmission electron microscope, FTIR spectroscopy, and galvanostatic measurement. Experimental facts reveal that the material undergoes multiple structural phase evolutions involving two solid-solution regions and one two-phase region based on the redox chemistry of Ni2+/Ni3+ and Ni3+/Ni4+ couples. It exhibits excellent electrochemical performance with a high reversible capacity of 145.2 mAh g-1 and an impressive capacity retention of 85.1% after 100 cycles due to the unique structure with robust ternary composition and stable coating interface. The coating interface is mainly related with formation of the amorphous carbonate layer at preparation step and the CEI film containing alkoxy group during the first charge. In addition, initial Na extraction from the material suffers from a "potential jump" phenomenon owing to severe electrochemical polarization caused by large interfacial impedance and chargetransfer impedance. This work provides new insights on interfacial change, structural evolution and redox reaction of high-capacity O3-type layered oxide cathode during Na extraction/insertion.