Characterizing Oxygen Local Environments in Paramagnetic Battery Materials via ¹⁷O NMR and DFT Calculations

Experimental techniques that probe the local environment around O in paramagnetic Li-ion cathode materials are essential in order to understand the complex phase transformations and O redox processes that can occur during electrochemical delithiation. While Li NMR is a well-established technique for studying the local environment of Li ions in paramagnetic battery materials, the use of ¹⁷O NMR in the same materials has not yet been reported. In this work, we present a combined ¹⁷O NMR and hybrid density functional theory study of the local O environments in Li₂MnO₃, a model compound for layered Li-ion batteries. After a simple ¹⁷O enrichment procedure, we observed five resonances with large ¹⁷O shifts ascribed to the Fermi contact interaction with directly bonded Mn⁴⁺ ions. The five peaks were separated into two groups with shifts at 1600 to 1950 ppm and 2100 to 2450 ppm, which, with the aid of first-principles calculations, were assigned to the ¹⁷O shifts of environments similar to the 4i and 8j sites in pristine Li₂MnO₃, respectively. The multiple O environments in each region were ascribed to the presence of stacking faults within the Li₂MnO₃ structure. From the ratio of the intensities of the different ¹⁷O environments, the percentage of stacking faults was found to be ca. 10%. The methodology for studying ¹⁷O shifts in paramagnetic solids described in this work will be useful for studying the local environments of O in a range of technologically interesting transition metal oxides.