Investigation of the structural and electrochemical performance of Li1.2Ni0.2Mn0.6O2 with Cr doping

Cr-doped layered oxides Li[Li0.2Ni0.2 - x Mn0.6 - x Cr-2x ]O-2 (x = 0, 0.02, 0.04, 0.06) were synthesized by co-precipitation and high-temperature solid-state reaction. The materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TRTEM), X-ray photoelectron spectroscopy (XPS), and electrochemical impedance spectroscopy (EIS). XRD patterns and HRTEM results indicate that the pristine and Cr-doped Li1.2Ni0.2Mn0.6O2 show the layered phase. The Li1.2Ni0.16Mn0.56Cr0.08O2 shows the best electrochemical properties. The first discharge specific capacity of Li1.2Ni0.16Mn0.56Cr0.08O2 is 249.6 mA h g(-1) at 0.1 C, while that of Li1.2Ni0.2Mn0.6O2 is 230.4 mA h g(-1). The capacity retaining ratio of Li1.2Ni0.16Mn0.56Cr0.08O2 is 97.9% compared with 93.9% for Li1.2Ni0.2Mn0.6O2 after 80 cycles at 0.2 C. The discharge capacity of Li1.2Ni0.16Mn0.56Cr0.08O2 is 126.2 mA h g(-1) at 5.0 C, while that of the pristine Li1.2Ni0.2Mn0.6O2 is about 94.5 mA h g(-1). XPS results show that the content of Mn3+ in the Li1.2Ni0.2Mn0.6O2 can be restrained after Cr doping during the cycling, which results in restraining formation of spinel-like structure and better midpoint voltages. The lithium-ion diffusion coefficient and electronic conductivity of Li1.2Ni0.2Mn0.6O2 are enhanced after Cr doping, which is responsible for the improved rate performance of Li1.2Ni0.16Mn0.56Cr0.08O2.