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.