Surface Surgery of the Nickel-Rich Cathode Material LiNi0.815Co0.15Al0.035O2: Toward a Complete and Ordered Surface Layered Structure and Better Electrochemical Properties

A complete and ordered layered structure on the surface of LiNi0.815Co0.15Al0.035O2(NCA) has been achieved via a facile surface-oxidation method with Na2S2O8. The field-emission transmission electron microscopy images clearly show that preoxidation of the hydroxide precursor can eliminate the crystal defects and convert Ni(OH)2into layered ß-NiOOH, which leads to a highly ordered crystalline NCA, with its (006) planes perpendicular to the surface in the sintering process. X-ray photoelectron spectroscopy and Raman shift results demonstrate that the contents of Ni2+and Co2+ions are reduced with preoxidization on the surface of the hydroxide precursor. The level of Li+/Ni2+disordering in the modified NCA determined by the peak intensity ratio I(003)/I(104)in X-ray diffraction patterns decreases. Thanks to the complete and ordered layered structure on the surface of secondary particles, lithium ions can easily intercalate/extract in the discharging–charging process, leading to greatly improved electrochemical properties.