PNNL

Abstract

LiNi1-x-yMnxCoyO2 (NMC) is an important class of high energy density cathode materials. The possibility of changing both x and y in the chemical formula provides numerous materials with diverse electrochemical and structural properties. It is highly desirable to have guidance on correlating NMC structural and electrochemical properties with their chemical composition for material designing and screening. Here, using synchrotron-based X-ray diffraction, X-ray absorption spectroscopy, electrochemical characterization, and literature survey, the content difference between Mn and Co (denoted as x-y in NMC) is identified as an effective indicator to estimate Li/transition metal (Li/TM) cation mixing ratio and 1st cycle coulombic efficiency. In addition, a linear relationship between oxygen position “z” and the size difference between Li+ and TM cation (normalized by the c axis length) is found and such linearity can be used to accurately predict the oxygen position in NMC materials by considering the average TM cation size and c axis length. It is also concluded that the shortest O-O distance in the bulk of the NMC materials could not be shorter than 2.5 Å even at highly charged state. Therefore, oxygen release is not likely to take place from the bulk if the structure maintains the R3 ¯m symmetry.