Mechanism of Magnesium Transport in Spinel Chalcogenides

Abstract

In the area of sustainable energy storage, batteries based on multivalent ions such as magnesium have been attracting considerable attention due to their potential for high energy densities. Furthermore, they are typically also more abundant than, e.g., lithium. However, as a challenge their low ion mobility in electrode materials remains.This study addresses the ionic conductivity in spinel host materials which represent a promising class of cathode and solid-electrolyte materials in Mg-ion batteries. Based on periodic density functional theory calculations, we identify the critical parameters which determine the mobility and insertion of ions. We will in particular highlight the critical role that trigonal distortions of the spinel structure play for the ion mobility. In detail, we will show that it is the competition between coordination and bond length that governs the Mg site preference in ternary spinel compounds upon trigonal distortions. This can only be understood by also taking covalent interactions into account. Furthermore, our calculations suggest that anionic redox plays a much more important role in sulfide and selenide spinels than in oxide spinels. Based on our theoretical study, we rationalize the impact of the metal distribution in the host material and the ion concentration on the diffusion process. Furthermore, cathode-related challenges for practical devices will be addressed. Our findings shed light on the fundamentional mechanisms underlying ionic conductivity in solid hosts and thus may contribute to improve ion transport in battery electrodes.

Ion Mobility in Crystalline Battery Materials

“…Among these compounds, MgSc 2 Se 4 has emerged as a promising contender with a remarkably low migration barrier of E a = 0.37 eV. [ 2,31–33 ] Nevertheless, its suitability as a solid electrolyte is impeded by a significant partial electronic conductivity. Moreover, studies have indicated that the mobility of magnesium in chalcogenide spinels that include lanthanoids escalates proportionally with the lanthanoid's size.…”

“…Ternary materials such as inorganic oxides, [ 26,27 ] hydrides, [ 28–30 ] and chalcogenides [ 2,25,31–33 ] display commendable bulk ionic conductivities. Among these, chalcogenide spinel lattices have been recognized as ionic conductors with high ion mobility, [ 31,33 ] a crucial property for solid electrolytes.…”

“…Conversely, closed‐shell systems like Sc and Y exhibit relatively low electron conductivity, qualifying them for use as solid electrolytes. [ 2,25,31 ]…”

“…This concept was derived after they had previously shown, using Bader charge analysis, that the stability of ions in chalcogenide spinels can only be understood if deviations from a purely ionic interaction are taken into account. [ 2 ]…”

“…In recent years, significant progress has been made in our conceptual understanding of ion mobility in crystalline electrode materials, especially concerning the role of non‐ionic interaction between the migrating ion and the host lattice. [ 2–5 ] In this review, we will discuss the materials' properties that correlate with ionic conductivity, in particular with the height of the activation barrier, using the descriptor concept. Both experimental and theoretical studies will be addressed in order to establish strategies to identify solid electrolytes and electrodes with improved mobility.…”