Synthesis and Characterization of Ternary Magnesium-Based Selenide Spinels for the Application as Solid Electrolytes in Magnesium Batteries

Abstract

To meet the growing demand for energy storage systems and the limited lithium availability for today’s lithium-ion batteries, it is essential to develop resource-saving, lithium-free next-generation battery cells. One promising option is the magnesium battery which stands out due to the potential advantages of the magnesium metal anode, such as the high magnesium earth abundance, low costs, and the high theoretical volumetric energy density. In order to ensure a high level of safety and circumvent the drawbacks of passivation/corrosion of the magnesium metal anode in liquid cells, solid-state cells are often the concept of choice. However, the high charge density of the Mg2+-ion leads to strong Coulomb interactions in the solid host-framework, making the development of magnesium-ion-conducting solid electrolytes quite challenging. Among considered classes of magnesium ion conductors, MgB2Se4 spinels (B = Sc, Y, Ln) are predicted to enable high magnesium ion mobility even at room-temperature. To date, a handful of electrochemical studies have been conducted on a single spinel, MgSc2Se4, while unequivocal evidence for magnesium ion transport beyond short-range motion as well as an approach to accurately determine the ionic conductivity in the mixed-conducting spinels are still missing. Apart from that, the nature of the electronic conductivity and the influence of the spinel composition on the magnesium ion conductivity and migration barrier are yet not well understood. This doctoral thesis focuses on a systematic investigation of MgB2Se4 spinels as potential magnesium ion conductors. MgSc2Se4 was used as a pioneer material to study the magnesium ion transport by two independent electrochemical methods, namely reversible magnesium plating/ stripping cycling and electrochemical deposition of magnesium metal. After proving the Mg2+-ion transport, a cell concept with electron blocking interlayer-electrodes was developed to tackle the challenge of determining the partial ionic conductivity of the mixed-conducting spinel from impedance spectroscopy. Thus, a high room-temperature magnesium ion conductivity of MgSc2Se4 and, later on, of three further synthesized spinels (MgY2Se4, MgEr2Se4, MgTm2Se4) was confirmed. Driven by the good accordance of the experimental and computational magnesium ion migration barriers, systematic theoretical periodic density calculations of the static and kinetic energy contribution were performed dependent on the B3+-ion. In this context, general guidelines for achieving small migration barriers in the MgB2Se4 spinel system could be identified, which were later extended by the consideration of the magnesium insertion energy, suggesting an outstanding high magnesium ion mobility in MgYb2Se4. This was finally confirmed by an experimentally high ionic conductivity exceeding 10–4 S cm–1 and a low migration barrier, making the MgYb2Se4 spinel to a strong candidate for a magnesium ion solid electrolyte. Overall, this doctoral thesis is the most comprehensive work on magnesium-ion-conducting selenide spinels so far. Approaches for accurate electrochemical characterization of spinels with mixed ionic-electronic character were developed, which will not only advance the research on MgB2Se4 phases but also pave the way for exploring other magnesium-based mixed conductors. Moreover, the computational studies in this work provide a deeper understanding of the magnesium ion transport in the chalcogenide spinels and could therefore serve as a starting point for further studies on the MgB2Se4 system.