Although the determination of site occupancies is often a major goal in Rietveld refinement studies, the accurate refinement of site occupancies is exceptionally challenging due to many correlations and systematic errors that have a hidden impact on the final refined occupancy parameters. Through the comparison of results independently obtained from neutron and synchrotron powder diffraction, improved approaches capable of detecting occupancy defects with an exceptional sensitivity of 0.1% (absolute) in the class of layered NMC (Li[Ni x Mn y Co z ]O 2) Li-ion battery cathode materials have been developed. A new method of visualizing the diffraction parameter space associated with crystallographic site scattering power through the use of f * diagrams is described, and this method is broadly applicable to ternary compounds. The f * diagrams allow the global minimum fit to be easily identified and also permit a robust determination of the number and types of occupancy defects within a structure. Through a comparison of neutron and X-ray diffraction results, a systematic error in the synchrotron results was identified using f * diagrams for a series of NMC compounds. Using neutron diffraction data as a reference, this error was shown to specifically result from problems associated with the neutral oxygen X-ray atomic form factor and could be eliminated by using the ionic O 2− form factor for this anion while retaining the neutral form factors for cationic species. The f * diagram method offers a new opportunity to experimentally assess the quality of atomic form factors through powder diffraction studies on chemically related multi-component compounds.
While it is accepted that paired Ni Li and Li Ni antisite defects are present in the important family of NMC cathode materials with the general formula Li(Ni x Mn y Co z )O 2 , their formation mechanism and influence on properties are not well understood due to the difficulty of accurately quantifying defects. In this work, novel high-precision powder diffraction methods have been used to elucidate the dependence of defect concentration on NMC composition. Formation energies for paired antisite defects (calculated under the assumption of equal state degeneracy) are observed to vary from about 320 to 160 meV, contradicting the constant defect formation energy that would be expected based on the previously proposed atomistic defect formation mechanism (size similarity of Ni 2+ and Li + cations). The present data support an alternative mechanism in which the equilibrium defect concentration is determined by the average size of transition-metal sites and thus suggest a new route by which chemical substitutions can be used to tune defect concentrations to optimal levels.
Higher energy densities in rechargeable batteries can be achieved using thicker cathode films, though it is a challenging endeavor since the electrochemical performance of thick electrodes is substantially worse than that of the conventional thin electrodes due to a variety of transport limitations, which are thus far poorly understood. Operando synchrotron studies have been, for the first time, applied to thick film samples to determine the depth-dependent state of charge (SOC) distribution inside 170 micron thick Li(Ni0.8Mn0.1Co0.1)O2 cathode films using an unconventional radial diffraction experiment geometry, allowing the SOC to be probed with both high spatial resolution (20 microns) and high temporal resolution (hundreds of time steps) in a single experiment. The resulting data allow the evolution of vertical inhomogeneity within these thick cathode films to be determined during cycling and they reveal a number of unexpected phenomena, such as the continuation of charging at some heights within the cathode during the discharge cycle of the cell. The new availability of comprehensive depth-dependent SOC data will drive the parameterization and advancement of whole-cell models, leading to an improved understanding of large-scale transport phenomena and enhanced capabilities for the rational design of thick electrodes with improved performance.
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