Influence of Ni/Mn distributions on the structure and electrochemical properties of Ni-rich cathode materials.

To reveal the influence of element distribution on the structure and electrochemical performances of Ni-rich layered cathode materials, LiNi0.68Co0.13Mn0.19O2 (NCM) with four types of Ni/Mn distributions (homogeneous, core-shell, multi-shell and concentration-gradient structures) is designed and synthesized with a combination of co-precipitation and high-temperature solid-state method. Ni/Mn distributions of the as-prepared NCM cathode materials are investigated with focused ion beam (FIB) and energy disperse X-ray spectrum (EDS) line scanning on the cross-section of single particles, which illustrate that NCM materials with the desired Ni/Mn distributions are successfully prepared. For the three spherical heterogeneous NCM materials, the center is the Ni-rich component while the surface is the Mn-rich component. Ni/Mn distributions between the center and surface components are in different forms. Studies imply that the heterogeneous samples exhibit smaller cation disordering, lower charge transfer resistance, higher Li+ diffusion coefficient and higher structural stability than the homogeneous one. Therefore, the heterogeneous samples, especially the multi-shell and concentration-gradient ones, display improved cycling and thermal stability compared to the homogeneous one. These results manifest that multi-shell and concentration-gradient structures are effective strategies to modify the layered NCM cathode materials for Li-ion batteries.