Oxide Spinels with Superior Mg Conductivity

Abstract

Mg batteries with oxide cathodes have the potential to significantly surpass existing Li-ion technologies in terms of sustainability, abundance, and energy density. However, Mg intercalation at the cathode is often severely hampered by the sluggish kinetics of Mg2+ migration within oxides. Here we report a combined theoretical and experimental study addressing routes to identify cathode materials with an improved Mg-ion mobility. Using periodic density functional theory calculations, Mg2+ migration in oxide spinels has been studied, revealing key features that influence the activation energy for Mg2+ migration. Furthermore, the electronic and geometrical properties of the oxide spinels as well as their stability have been analyzed for a series of different transition metals in the spinels. We find that electronegative transition metals enable a high Mg-ion mobility in the oxide spinel frameworks and thus a favorable cathode functionality. Based on the theoretical findings, some promising candidates have been identified, prepared and structurally characterized. Our combined theoretical and experimental findings open up an avenue toward the utilization of functional cathode materials with improved Mg2+ transport properties for Mg-metal batteries.

Ion Mobility in Crystalline Battery Materials

“…The spinel structure possesses a desirable combination of capacity and voltage. [87] Recent studies [101,102] have reported experimental migration barriers of approximately 600 meV for Mg 2+ in Cr-and Mn-spinel oxides, aligning with DFT calculations. It has been observed that structural disorder within the spinel lattice affects the migration of Mg ions, and this disorder can be regulated during synthesis.…”

“…The discovery of these scaling relations has significant implications as it shows the potential to accelerate the process of finding materials with desired ion mobility properties such as oxide spinels. [87] By utilizing this approach, researchers can more efficiently explore and identify materials that exhibit enhanced ion mobility, thus contributing to the development of improved electrochemical devices, including batteries and solid electrolytes.…”

“…[2,87] By utilizing this principle, it was demonstrated that oxide spinels incorporating electronegative transition metal cations of substantial size hold the potential for showcasing elevated Mg 2 + mobility. [87] Sulfide and selenide spinels have garnered attention due to their potential as solid electrolytes in solid-state batteries. 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.…”

“…The octahedra undergo a trigonal distortion, leading to a transformation from O h octahedral symmetry to D 3h octahedral symmetry. [2,87] LIBs have effectively incorporated Mn 2 O 4 spinels into commercial applications as cathodes. Nevertheless, this specific spinel exhibits a significant obstacle to Mg 2 + diffusion, with a barrier of 0.67 eV.…”

Rhombohedral (R) Prussian White as Cathode Material: An Ab‐initio Study

“…Due to the facts that Prussian Blue is a double perovskite type material and that octahedral tilting in Perovskites has been extensively studied, it is known that a phase of this symmetry has to be formed by a cooperative out-of-phase rotation of the FeC 6 and FeN 6 octahedra in all three directions of space (a À a À a À rotation after Glazer). [26][27][28][29][30][31] Such a rotation often causes the trigonal distortion, [32,33] which further splits the d-orbitals by forming one a 1g and two e' g states out of the already favourable t 2g -states. The a 1g state is further lowered in energy in comparison to both the barycenter and the t 2g -states.…”