Influence of Inversion on Mg Mobility and Electrochemistry in Spinels

Abstract

Magnesium oxide and sulfide spinels have recently attracted interest as cathode and electrolyte materials for energy-dense Mg batteries, but their observed electrochemical performance depends strongly on synthesis conditions. Using first-principles calculations and percolation theory, we explore the extent to which spinel inversion influences Mg 2+ ionic mobility in MgMn 2 O 4 as a prototypical cathode, and MgIn 2 S 4 as a potential solid electrolyte. We find that spinel inversion and the resulting changes of the local cation ordering give rise to both increased and decreased Mg 2+ migration barriers, along specific migration pathways, in the oxide as well as the sulfide. To quantify the impact of spinel inversion on macroscopic Mg 2+ transport, we determine the percolation thresholds in both MgMn 2 O 4 and MgIn 2 S 4 . Furthermore, we analyze the impact of inversion on the electrochemical properties of the MgMn 2 O 4 cathode via changes in the phase behavior, average Mg insertion voltages and extractable capacities, at varying degrees of inversion. Our results confirm that inversion is a major performance limiting factor of Mg spinels and that synthesis techniques or compositions that stabilize the well-ordered spinel structure are crucial for the success of Mg spinels in multivalent batteries.

Ionic Transport in Potential Coating Materials for Mg Batteries

“…Additionally, we only consider local migration pathways that form percolating networks, 36,37 i.e., pathways that are sufficiently connected through the lattice, enabling Mg to diffuse from one end of the lattice to the other along at least one crystallographic direction.…”

Evaluating transition metal oxides within DFT-SCAN and SCAN+U frameworks for solar thermochemical applications

“…To capture the type-II antiferromagnetism of MnO and FeO, 66 we employed a 2 × 2 × 2 supercell of the primitive rocksalt structure. Spinel-Fe3O4 (space group: 𝐹𝑑3 Z 𝑚) exhibits a significant degree of "inversion" 67,68 and electronic conductivity 69 at room temperature. However, Fe3O4 undergoes a Verwey transition at low temperatures (~120 K 70,71 ), which leads to a small opening of the band gap, a slight distortion from the spinel to an orthorhombic structure, and a Fe 2+ -Fe 3+ charge ordering on the octahedral sites.…”

On the Balance of Intercalation and Conversion Reactions in Battery Cathodes

“…The spinel family of compounds, specifically the oxides and sulfides, display significant promise for the development of Mg-cathodes since they host Mg in a less preferred tetrahedral environment and are thermodynamically stable. 8,26,59,85 Alternatively, a metastable charged polymorph can potentially be attained following chemical or electrochemical extraction of a "removable" ion, such as Li, Na or Cu. 5,36 Subsequently, the metastable charged polymorph can reversibly intercalate MV ions and form stable intercalation products.…”