Li(Ni,Mn,Co)O2 (NMC) battery cathode materials, and other formulations related to LixMO2, have at least three atoms that occupy the M site. X-ray diffraction (XRD) is the tool most often used to determine crystallographic site occupancies, but it is mathematically impossible to use a single XRD measurement to quantify the Ni, Mn, and Co occupancies of the M site. A single XRD measurement cannot determine the occupancy of any crystallographic site occupied by more than 2 atoms.
Anomalous X-ray diffraction addresses this problem by collecting multiple diffraction patterns using different wavelengths of X radiation. When X-rays above and below the absorption edge of an atom are used for successive measurements, then the scattering factor of the X-rays from that atom varies and provides contrast to resolve the site occupancies in complicated structures. Anomalous XRD measurements are conventionally performed at synchrotron beamlines, where the X-ray wavelength can be precisely tuned.
Anomalous XRD measurements using a laboratory diffractometer typically require tedious exchange of X-ray tubes, which may change too many experimental parameters to allow precise analysis of the multiple diffraction patterns. However, the latest energy sensitive detectors, such as the Malvern Panalytical 1Der, allow measurements using the K-alpha and K-beta radiation produced by a single X-ray tube. These different energies can be selected without changing any optics or removing/remounting the sample, reducing the chance of discrepancies between measurements. Collecting data with these two radiations can provide the scattering factor contrast necessary for precise site occupancy refinement. This study will present the analysis of NMC 1-1-1 and NMC 8-1-1 using combined analysis of K-alpha and K-beta radiation from Cu and Co X-ray tubes. The benefits and limitations of the laboratory solution is also presented.