High magnesium mobility in ternary spinel chalcogenides

Abstract

Magnesium batteries appear a viable alternative to overcome the safety and energy density limitations faced by current lithium-ion technology. The development of a competitive magnesium battery is plagued by the existing notion of poor magnesium mobility in solids. Here we demonstrate by using ab initio calculations, nuclear magnetic resonance, and impedance spectroscopy measurements that substantial magnesium ion mobility can indeed be achieved in close-packed frameworks (~ 0.01-0.1 mS cm^-1 at 298 K), specifically in the magnesium scandium selenide spinel. Our theoretical predictions also indicate that high magnesium ion mobility is possible in other chalcogenide spinels, opening the door for the realization of other magnesium solid ionic conductors and the eventual development of an all-solid-state magnesium battery.

Fig. 1

First-principles Mg and Zn migration barriers in sulfides, selenides, and tellurides AX₂Z₄ spinels (with A = Mg or Zn). a tet–oct–tet migration path in the AX₂Z₄ framework, with energy of the tet, oct, and transition sites indicated by E _tet, E _oct, E _a, respectively. E _a corresponds to the migration energy. b Effect of the anion size on the shared (triangular) face between tet and oct sites. c and d computed Mg and Zn migration barriers (orange bars in meV) in AX₂Z₄ spinel and volume per anion (blue bars), respectively, with X = Sc, Y, and In, and Z = S, Se and Te. e Mg probability density in MgSc₂Se₄ at 900 K obtained from ab initio molecular dynamic simulations (AIMDs). f Mg diffusivities as extrapolated from AIMD in MgSc₂Se₄ (orange) and MgY₂Se₄ (blue), with dashed lines and error bars indicating Arrhenius fits and SD, respectively

Fig. 2

X-ray diffractions and electrochemical impedance characterizations of MgSc₂Se₄. a Rietveld refinement of the synchrotron XRD pattern for MgSc₂Se₄. The square root of the intensity is plotted on the y-axis. The observed and calculated curves are shown in blue and red in the top panel, and the difference curve is shown in dark gray in the bottom panel. Reflections corresponding to MgSc₂Se₄ (blue), Mg (black), MgSe (red), Sc₂O₃ (magenta), and Sc₂Se₃ (dark yellow) are shown with tick marks of the respective colors. b Impedance spectrum of the Ta/MgSc₂Se₄/Ta cell, and the circuit utilized in the fitting of the impedance data (Supplementary Fig. 10). The observation is shown in blue circles and the fit is displayed in red. The equivalent circuit utilizes two Jamnik–Maier elements, which are tentatively attributed to contributions from bulk and grain boundary, respectively. Supplementary Fig. 12 shows the impedance behavior at low Z _Re

Fig. 3

Characterization of Mg transport in MgSc₂Se₄ via ²⁵Mg solid-state NMR. a Stack plot of ²⁵Mg magic angle spinning (MAS) variable temperature NMR of MgSc₂Se₄ collected at 11.7 T with a spinning speed of 20 kHz. *Spinning sidebands. b ²⁵Mg static variable temperature spin lattice relaxation data collected at 7.02 T plotted as function of temperature and Arrhenius fit (blue line). Dashed dark blue line illustrates the deviation of experimental data from the fit. The light blue dashed parabolic curve mimics the expected inverse SLR maxima vs. recorded data. The inset of panel a shows the enlargement of the ²⁵Mg MAS NMR signal at ~ 53 p.p.m.