Influence of inversion on Mg mobility and electrochemistry in spinels

TL;DR

This study investigates how spinel inversion affects magnesium ion mobility and electrochemical performance in Mg-based spinel materials, revealing that inversion can both hinder and enhance Mg transport and is a key factor limiting battery performance.

Contribution

The paper uses first principles calculations and percolation theory to quantify the impact of spinel inversion on Mg mobility and electrochemical properties in Mg spinels, highlighting its role as a performance limiter.

Findings

Inversion alters Mg$^{2+}$ migration barriers along specific pathways.

Percolation thresholds are affected by inversion, influencing Mg transport.

Inversion impacts phase behavior, voltages, and capacities of MgMn$_2$O$_4$.

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.