Composition dependence of average and local structure of xLi(Li1/3Mn2/3)O2–(1 − x)Li(Mn1/3Ni1/3Co1/3)O2 active cathode material for Li ion batteries

Abstract

This study focused on the composition dependence of the crystal structure and cathode performance of xLi(Li1/3Mn2/3)O2–(1 − x)Li(Mn1/3Ni1/3Co1/3)O2 (0 ≤ x ≤ 1) prepared by co-precipitation. From charge-discharge tests, it was found that there was large differences in the battery performance between the composition of x = 0.2 and x ≥ 0.3. For the samples, the average crystal structures were determined by the Rietveld refinement of neutron diffraction patterns. The results indicated that for x = 0.2, Li existed at both 2b and 4g sites, whereas for x ≥ 0.3, Li was localized at 2b sites and Mn occupied the 4g sites. In order to examine the local crystal structure, pair distribution function (PDF) and extended X-ray absorption fine structure (EXAFS) analyses were carried out. From the PDF analysis, a significant difference was seen in the short-range peaks associated with the transition metal layer for x = 0.2, 0.4 and 0.6. The EXAFS results indicated a large difference in the local structure around Mn for x = 0.2 and x ≥ 0.3. This is thought to be due to ordering of LiMn6 and LiMn5Ni, which is likely to affect cathode performance.