Mg2+ intercalation in host materials is more difficult than that of Li+ or Na+, displaying lower ion mobility in solid oxide hosts,3 a tendency toward conversion reactions,4 and a possibly higher desolvation energy penalty.5 In fact, the second material to reversibly intercalate Mg2+, without debilitating decomposition over more than 100 cycles, was only demonstrated last year: the thiospinel Ti2S4.6 Unfortunately, our attempts to insert (or remove) Mg2+ into other 1st row transition metal thiospinels were unsuccessful; however, Mg2+ can be reversibly intercalated from thiospinel Zr2S4, a 2nd row transition metal.
Figure 1 shows the 1st discharge and charge of two coin cells using Mg foil anodes, Mg(CB11H12)2 in tetraglyme electrolyte and either a MgxTi2S4 or MgxZr2S4 electrode. We will present a comparison of the Mg intercalation into the Zr2S4 and Ti2S4 thiospinels, including Mg site occupancy, cell parameter variation, and Mg diffusion coefficients. In doing so, we will explore the crucial parameters that allow facile Mg2+ diffusion for consideration when designing new Mg2+ intercalation materials.
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