Study on the mechanism of the influence of component proportion on the structure and electrochemical performance of Li (Ni_xCo_yMn_1-x-y)O₂ cathode materials

[Background] Li(Ni_xCo_yMn_1-x-y)O₂ (NCM), as a new cathode material of Li-ion battery, has been successfully used in the electric vehicles. It has the advantages of high energy density, good cycling stability and low cost. The crystal structure and electronic structure of Li(Ni_xCo_yMn_1-x-y)O₂ (NCM) cathode are mainly affected by the variable composition of 3d transition metals Ni, Co and Mn. [Purpose] This study aims to investigate the influence of component proportion on the structure and electrochemical performance of Li(Ni_xCo_yMn_1-x-y)O₂ cathode materials. [Methods] First of all, Li(Ni_0.6 Co_0.2 Mn_0.2)O₂ (NCM622), Li(Ni_0.5 Co_0.2 Mn_0.3)O₂ (NCM523) and Li(Ni_0.33 Co_0.33 Mn_0.33)O₂ (NCM333), were selected as three common cathode materials used in industry. Their structures and electrochemical performances were studied by electrochemical methods, scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and X-ray absorption spectroscopy (XAS). Then the corresponding relationship between structure and electrochemistry was established to optimize electrochemical performance of Li-ion batteries, and apply NCM materials to cathode of Li-ion batteries. [Results] Electrochemical measurement results show that NCM materials with higher nickel content have higher capacity density but lower rate capability. In SEM and XRD characterizations, the NCM materials with the three different compositions show similar morphology and crystal structure. XAS at Ni L-edge results reveal that by increasing nickel content in the NCM materials, the oxidation of Ni increases to a higher + 3 state. In the O K-edge XAS, the enhanced intensity of a lower energy peak suggests the increased hybridization of Ni3d-O2p. [Conclusions] Based on the investigations of capacity and rate capability, NCM523 is found to be a well-balanced component. The evolution of Ni and O electron structure is a crucial factor in determining their electrochemical performance. (authors)