Enhanced Electrochemical Performance of Na0.67Fe0.5Mn0.5O2 Cathode with SnO2 Modification

P2-type layered oxide Na0.67Fe0.5Mn0.5O2 is recognized as a very promising cathode material for sodium-ion batteries due to the merits of high capacity, high voltage, low cost, and easy preparation. However, its unsatisfactory cycle and rate performances remain huge obstacles for practical applications. Here, we report a strategy of SnO2 modification on P2-type Na0.67Fe0.5Mn0.5O2 to improve the cycle and rate performance. Scanning electron microscope(SEM) and transmission electron microscope(TEM) images indicate that an insular thin layer SnO2 is coated on the surface of Na0.67Fe0.5Mn0.5O2 after medication. The coating layer of SnO2 can protect Na0.67Fe0.5Mn0.5O2 from corrosion by electrolyte and the cycle performance is well enhanced. After 100 cycles at 1 C rate(1 C=200 mA/g), the capacity of SnO2 modified Na0.67Fe0.5Mn0.5O2 retains 83 mA·h/g(64% to the initial capacity), while the capacity for the pristine Na0.67Fe0.5Mn0.5O2 is only 38 mA·h/g(33.5% to the initial capacity). X-Ray photoelectron spectroscopy reveals that the ratio of Mn4+ increases after SnO2 modification, leading to less oxygen vacancy and expanded lattice. As a result, the capacity of Na0.67Fe0.5Mn0.5O2 increases from 178 mA·h/g to 197 mA·h/g after SnO2 modification. Furthermore, the rate performance of Na0.67Fe0.5Mn0.5O2 is enhanced with SnO2 coating, due to high electronic conductivity of SnO2 and expanded lattice after SnO2 coating. The capacity of SnO2 modified Na0.67Fe0.5Mn0.5O2 at 5 C increases from 21 mA·h/g(pristine Na0.67Fe0.5Mn0.5O2) to 35 mA·h/g.