Cobalt-Free Nickel-Rich Positive Electrode Materials with a Core–Shell Structure

Abstract

Core–shell or concentration-gradient structures have been reported to improve the structural and chemical stability of Ni-rich electrode materials; however, a core–shell or concentration-gradient structure for cobalt-free systems has not yet been studied. In this work Ni(OH)2 core:Ni0.83M0.17(OH)2 shell precursors (M = Mg, Al, and Mn) were prepared in a continuously stirred tank reactor. Homogeneous Ni0.95M0.05(OH)2 precursors having the same average composition as the core–shell precursors were also prepared. Cross-sectional scanning electron microscopy verified the core–shell structure of all the core–shell precursors. During sintering at about 700 °C to make the lithiated oxides, interdiffusion of the M atoms between core and shell occurred with Mg showing a uniform distribution in the particles while Al and Mn still showed a higher concentration at the particle surfaces. The Mn-containing materials showed significant fractions of Ni atoms on the lithium layer while the Al- and Mg-containing materials showed less than 2% of the Ni in the Li sites. Electrochemical studies of the materials showed that the Al core–shell material had the most superior properties. However, interdiffusion between core and shell is unavoidable rendering it very difficult to prepare exactly the microstructure one desires.