Mg-Ion Conduction in Antiperovskite Solid Electrolytes Revealed by 25Mg Ultrahigh Field NMR and First-Principles Calculations

Magnesium-ion batteries hold the potential to outperform the energy density of lithium-ion batteries, given the divalent charge carried by each Mg2+ cation, but remain in an early stage of development. Here, 25Mg solid-state nuclear magnetic resonance (ssNMR) is used to gain insight into the local structure and Mg-ion dynamics of candidate Mg-ion solid electrolytes, the antiperovskites Mg3SbN and Mg3AsN. Using the highest available magnetic field (35.2 T) for high-resolution solid-state NMR, the largest 25Mg quadrupole coupling constants (CQ) yet measured of up to 22 MHz are reported and corroborated by first-principles calculations. Predicted CQ values are shown to correlate with the antiperovskite’s tolerance factor; thus, 25Mg NMR linewidths can report on lattice distortions and phase stability of these antiperovskites. Variable-temperature 25Mg NMR spectra demonstrate changes at elevated temperatures, ascribed to Mg-ion motional effects. 25Mg T1 relaxometry measurements at ultrahigh field reveal a lower activation energy for the more distorted Mg3AsN phase, matching computational predictions of a lower energy barrier for Mg2+ ion migration and suggesting that additional scrutiny of antiperovskites as Mg-ion conductors is warranted. Given the inherent challenges of 25Mg NMR, this work demonstrates the benefits of combining ultrahigh field NMR spectroscopy, advanced pulse sequences, modern signal processing, and first-principles calculations to facilitate NMR of quadrupolar nuclei as a tool to probe the local structure and ion dynamics in beyond-Li battery materials.