High-rate cycling performance and surface analysis of LiNi_1-xCo_x/2Mn_x/2O₂ (x=2/3, 0.4, 0.2) cathode materials

The electrochemical performance of layered LiNi_1-xCo_x/2Mn_x/2O₂ cathode materials (x = 2/3, 0.4, 0.2; so-called NCM333, NCM622, NCM811) in 1.0 M LiPF₆-dissolved conventional carbonate-based electrolyte during formation at a 0.1 C-rate and consequent cycling at a 1.0 C-rate is measured and considered together with the results of morphology observation, impedance spectroscopy, and surface analysis. X-ray photoelectron spectroscopy (XPS) is carried out on the surface of the cathode materials before and after formation and cycling to investigate the effects of solid electrolyte interphase (SEI) formation on the electrochemical performance. As the Ni content increases, the initial specific capacity increases but the capacity retention ratio decreases. High-rate cycling overrides the SEI formation on the NCM surfaces, but NCM622 suffers great changes in the SEI components with a thick layer resulting in large interfacial resistance. It is also proved that NCM811 shows significant dissolution and accumulation of Ni species on the surface, contributing structural degradation and leading to fast capacity fading.