Stoichiometry matters: correlation between antisite defects, microstructure and magnetic behavior in the cathode material Li1−zNi1+zO2

Abstract

As contemporary battery applications such as electric vehicles demand higher energy densities, layered LiNiO2 (LNO) could contribute as the end-member of the LiNi1−x−yCoxMnyO2 (NCM) family with the highest extractable specific capacity in a practical voltage window. Achieving high capacities requires among other things a defect free crystal structure, which is not easily achieved due to the natural occurrence of Ni excess on the Li site (NiLi) and/or antisite defects where Ni and Li switch crystallographic sites. Here, we present a study of the evolution of point defects in a series of LNO samples varying from underlithiated to fully lithiated stoichiometry in layered Li1−zNi1+zO2 with −0.05 ≤ z ≤ 0.35. Using the high angular resolution of synchrotron X-ray diffraction complemented with the different elements contrast provided by neutron diffraction, we are able to identify two defect regimes. In the first regime, at the underlithiated end, both NiLi as well as Li on the Ni site (LiNi) defects are present. Inhibited crystal growth during synthesis is found to coincide with the presence of these LiNi defects for z ≥ 0.15. Upon decreasing z values and the vanishing of LiNi, the primary particle size distribution as well as average refined crystallite size increases. Investigation of the local structure by nuclear magnetic resonance reveals the presence of a Li environment not detected by diffraction methods at low z, the Li-rich end of the sample series. Finally, magnetometry data suggest the onset of the ferrimagnetic-to-antiferromagnetic transition in LNO correlates with the elimination of LiNi defects in the structure. The present study thus not only highlights the correlation between defect chemistry and physical properties, but also shows the relationship to crystal growth, a field highly relevant for industrial battery cathode materials engineering.