Measuring T1 Relaxation in Paramagnetic Solids with Solid-State NMR: a Case Study on the Milling Induced Phase Transition in Li6CoO4.

Solid-state NMR has been a vital tool for the study of structural evolution of cathodes in lithium-ion and sodium-ion batteries. However, the differentiation of relaxation parameters for certain sites is difficult owing to limited spectral resolution associated with strong anisotropic hyperfine interaction. Here we propose a novel IR-pjMATPASS method that can measure T1 relaxation with site-specific resolution for paramagnetic solids. We apply this method to the characterization of ball-milling induced order-disorder phase transition in Li6CoO4 as a case study. The quasi-quantitate 7Li NMR enables the synthetic optimization of high energy ball-milling conditions to harvest a disordered cubic phase through site-specific 7Li T1 measurements. The example study shown here provides a quantitative strategy for NMR studies of paramagnetic solids.