Effects of surface modification with Ag/C on electrochemical properties of Li[Li0.2Mn0.54Ni0.13Co0.13]O2

Abstract

A lithium-rich solid-solution layered cathode material, Li[Li0.2Mn0.54Ni0.13Co0.13]O2, was synthesized using a fast co-precipitation method, and surface modified with Ag/C via chemical deposition. The electrochemical properties, structures, and morphologies of the prepared samples were investigated using X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), galvanostatic charge-discharge cycling, cyclic voltammetry (CV), electrochemical impedance spectra (EIS), and energy dispersive X-ray spectroscopy (EDS). The XRD results showed that the pristine and Ag/C-coated cathode materials both have hexagonal α-NaFeO2 layered structures with the R3m space group. Microscopic morphological observations and EDS elemental mapping showed that a uniform Ag/C coating layer of thickness 25 nm was deposited on the surfaces of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 particles. The Ag/C-coated Li[Li0.2Mn0.54Ni0.13Co0.13]O2 material gave an excellent electrochemical performance. The initial discharge capacity (0.05C) of the Ag/C-coated sample was 272.4 mAh ∙g-1, with an initial coulombic efficiency of 77.4%, corresponding to 242.6 mAh∙g-1 for the pristine sample, with an initial coulombic efficiency of 67.6%, in the potential range 2.0-4.8 V (vs Li/Li+). After 30 cycles (0.2C), the Ag/C-coated Li[Li0.2Mn0.54Ni0.13Co0.13]O2 retained a capacity of 222.6 mAh∙g-1, which was 14.45% higher than that of Li[Li0.2Mn0.54Ni0.13Co0.13]2. We also found that the Ag/C coating improved the rate capability of the solid-solution material Li[Li0.2Mn0.54Ni0.13Co0.13]O2. The capacity retention (1C) of the Ag/C-coated sample was 81.3%, compared with the capacity at 0.05C. CV and EIS results showed that the Ag/C coating layer suppressed the oxygen release in the initial charge progress and lowered the surface film resistance and electrochemical reaction resistance of the pristine sample.