A systematic study of ²⁵Mg NMR in paramagnetic transition metal oxides: applications to Mg-ion battery materials

Rechargeable battery systems based on Mg-ion chemistries are generating significant interest as potential alternatives to Li-ion batteries. Despite the wealth of local structural information that could potentially be gained from Nuclear Magnetic Resonance (NMR) experiments of Mg-ion battery materials, systematic ²⁵Mg solid-state NMR studies have been scarce due to the low natural abundance, low gyromagnetic ratio, and significant quadrupole moment of ²⁵Mg (I = 5/2). This work reports a combined experimental ²⁵Mg NMR and first principles density functional theory (DFT) study of paramagnetic Mg transition metal oxide systems Mg₆MnO₈ and MgCr₂O₄ that serve as model systems for Mg-ion battery cathode materials. Magnetic parameters, hyperfine shifts and quadrupolar parameters were calculated ab initio using hybrid DFT and compared to the experimental values obtained from NMR and magnetic measurements. We show that the rotor assisted population transfer (RAPT) pulse sequence can be used to enhance the signal-to-noise ratio in paramagnetic ²⁵Mg spectra without distortions in the spinning sideband manifold. In addition, the value of the predicted quadrupolar coupling constant of Mg₆MnO₈ was confirmed using the RAPT pulse sequence. We further apply the same methodology to study the NMR spectra of spinel compounds MgV₂O₄ and MgMn₂O₄, candidate cathode materials for Mg-ion batteries.