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

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 (C_Q) yet measured of up to 22 MHz are reported and corroborated by first-principles calculations. Predicted C_Q 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.