Abstract:
Li(Ni_xMnyCo_z)O₂ (NMCs) layered oxides are very promising cathodes for Li-ion batteries due to their high theoretical capacities and high working voltages. However, a variety of key defects, such as Li/Ni anti-sites, compositional off-stoichiometry, local distortion and short-range ordering, exist in NMC compounds and likely influence their electrochemical performance. Identifying and further quantifying these intrinsic defects are important for understanding their structure/properties relationships and providing a guidance to synthesize the best NMC compounds. In this work, comparative study across various NMC compositions with best synchrotron and neutron Bragg diffraction data was carried out to develop methods for precise quantification of long-range occupational defects. A new structure factor f* diagram was introduced to map different types of defects efficiently. With this method, we are able to figure out the issues with atomic scattering factors in the Rietveld refinement against X-ray powder diffraction data.  Furthermore, the pair distribution function method was applied to determine the composition dependent local structures in various NMC samples. A simple method is developed to quantify the degree of cation ordering.