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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 stu...

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... in the occupied tetrahedral and octahedral sites, as shown for some ternary spinels by the blue symbols in Fig. 2b. According to our calculations, for the MgAl 2 S 4 system characterized by a ratio of about k 64 = 1.08, the octahedral and the tetrahedral site become energetically degenerate with regard to the Mg occupation, as illustrated in Fig. 6a. This can be explained by a competition between bond length and coordination as a function of the ratio k 64 . The octahedral site has the higher coordination than the tetrahedral site, but obviously in the ideal structure the elongation of the Mg-X bond length by 1.15 with respect to the tetrahedral sites makes the octahedral site ...
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... at this value the larger bond length is compensated by the higher coordination of the octahedral site. For even smaller values of k 64 , as for example in MgMn 2 S 4 with k 64 = 1.05, the octahedral site is energetically more favorable whereas for larger values of k 64 as in MgSc 2 S 4 with k 64 = 1.10, the tetrahedral site becomes preferred (see Fig. ...
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... of all note that this ratio of 1.5 is much larger than the value of k 64 = 1.08 at which there is an equilbrium between tetrahedral site and octahedral site in MgAl 2 S 4 . In addition, the fact whether a spinel exhibits a tetrahedral or an octahedral site preference does not only depend on the ratio k 64 , but also on the anion parameter u. In Fig. 6b, we again show the ratio k 64 as a function of the anion parameter u, but now we also include some additional data points for the low Mg-concentration limit. In addition, we have inserted a dividing line given by k div 64 = 4.78(1 − 2u). In spinels above this line, the migrating Mg ions prefer the tetrahedral site whereas in those ...

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... 29 However, we could recently show that the stability of ions in chalcogenide spinels can only be understood if deviations from a purely ionic interaction are taken into account. 48 It is essential to realize that the considered binary materials span the whole range of interaction characteristics between metallic and ionic bonding. Such bonding characteristics can in fact been classified in so-called Van Arkel-Ketelaar triangles 49 in which compounds are placed according to the mean electronegativity χ mean (x-axis) and the electronegativity difference ∆χ (y-axis) of the constituting elements. ...
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Ion mobility is a critical performance parameter in electrochemical energy storage and conversion, but also in other electrochemical devices. Based on first-principles electronic structure calculations, we have derived a descriptor for the ion mobility in battery electrodes and solid electrolytes. This descriptor is entirely composed of observables that are easily accessible: ionic radii, oxidation states and the Pauling electronegativities of the involved species. Within a particular class of materials, the migration barriers are connected to this descriptor through linear scaling relations upon the variation of either the cation chemistry of the charge carriers or the anion chemistry of the host lattice. The validity of these scaling relations indicates that a purely ionic view falls short of capturing all factors influencing ion mobility in solids.The identification of these scaling relations has the potential to significantly accelerate the discovery of materials with desired mobility properties.