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 LiNi_0.815Co_0.15Al_0.035O₂ (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)₂ into layered β-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.