Making Magnesium Mobile: Cathodes for Multivalent Battery Systems


While Li-ion batteries are currently the preferred energy storage technology, multivalent alternatives such as Mg should be considered. Magnesium metal has a high volumetric capacity and has been shown to cycle with no dendrite formation. However, the highly charged Mg2+ ion cannot easily diffuse into the oxide cathodes favored in the lithium ion community. Selecting structures with favorable Mg mobility, which requires understanding Mg2+ diffusion in solid state structures, is essential in designing Mg ion cathodes. MgCr2S4 was chosen as a synthetic target among the thiospinels owing to its high predicted insertion voltage and high predicted magnesium mobility. The cubic material was synthesized for the first time using a solid state method, however the reaction of Cr2S3 and MgS was slow, consistent with the small formation energy (-0.02 eV/f.u.). It was shown that conventional electrochemical methods were insufficient to remove magnesium from the structure, which was attributed to the high instability of the charged Cr2S4 phase. MoO2.8F0.2 was synthesized by a hydrothermal method. To better understand the differences seen in the magnesium intercalation chemistry of MoO2.8F¬0.2 and MoO3, the lithium intercalation chemistry was studied. The diffusivity of the intercalating species measured by the galvanostatic intermittent titration technique showed that the extent of magnesium intercalation in both the oxide and the fluoride doped system correlated to processes with chemical diffusion coefficients greater than 5·10-13 cm2/sec upon lithium intercalation. Studying the diffusivity of Li intercalation processes provides a facile route to probe materials for Mg intercalation capacity. Vanadium Oxyfluoride (VOF3) was identified as a target material to study the effects of ordered layered oxide fluorides on magnesium intercalation. NH4VOF3 was studied as a potential synthetic precursor in a relatively safe hydrothermal route to VOF3. NH4VOF3 has two distinct polymorphs, both which crystalize in the space group Pbam and have similar unit cell volumes (371.3 Å3 vs 375.4 Å3 at 100K). Synthetic control between the two polymorphs can be achieved by varying the reaction time. To develop a functional magnesium ion battery, it is necessary to actively design battery cathodes with the mobility of highly charged cations in mind. By choosing structure types with high ionic mobility, and by selectively doping materials to bypass sluggish electrochemical properties a suitable magnesium cathode can be synthesized. Such a material would be of great commercial and scientific interest.

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