The Effect of Lithium Content and Core to Shell Ratio on Structure and Electrochemical Performance of Core-Shell Li(1+x)[Ni0.6Mn0.4](1−x)O2 Li(1+y)[Ni0.2Mn0.8](1−y)O2 Positive Electrode Materials

Abstract

Core-shell positive electrode materials with a core:shell mass ratio of 2:1 and 4:1 were synthesized in a two-step reaction. Powder X-ray diffraction, SEM and spatial EDS measurements were used to characterize the core and shell phases in the precursors and lithiated products. It was determined using EDS that the precursor and lithiated products are both core-shell and the two phases can be easily resolved with laboratory grade XRD equipment. Two phase Rietveld refinement was completed on the core-shell lithiated products. The results of these refinements in conjunction with contour plots of the lattice parameters within the Li-Ni-Mn oxide layered single phase region were used to position the core and shell of each sample on the Li-Ni-Mn-O phase diagram as a function of the amount of Li2CO3 used in synthesis. The shell phase retained an approximately fixed amount of Li while the Li content of the core phase increased as the overall Li content of the core-shell sample increased. Both the core and shell were electrochemically active. A specific capacity of 220 mAh/g was achieved in a core shell material between 2.5-4.6 V vs. Li/Li+.