Investigation of Oxide Cathode Materials for Rechargeable Mg Batteries
Authors: Sun Xiaoqi , Blanc Lauren , Bonnick Patrick , Nolis Gene M , Cabana Jordi , Nazar Linda F 
Published: February 2020

Rechargeable Mg batteries are considered as promising candidates for energy storage. The Mg metal anode with high volumetric capacity (3833 mAh mL-1) and low redox potential (-2.37 V vs. S.H.E.) provides high energy density for the system, while a low price, good safety and long term utilization is maintained by the inexpensiveness, air stability and limited-dendritic growth of Mg. However, the sluggish solid diffusion of the divalent Mg ion presents limited candidates for cathode materials. One strategy to facilitate Mg2+ mobility is to utilize polarizable anions in the lattice so as to weaken the ionic interaction, as noted with the first Mg cathode (the Chevrel phase Mo6S8) as well as with more recent spinel and layered phases of titanium sulfide.1-3 Although the sulfide materials exhibit reasonable Mg2+ mobility, they limit the voltage and have relatively high mass, thus lowering the energy density of the cathode. Switching to oxides is the solution towards the search for high energy density cathodes; however, a structure that allows facile Mg2+de/intercalation is obviously necessary.
In this presentation, we will discuss the prospect of oxide materials for Mg2+ de/intercalation. One such cathode candidate is the “calcium ferrite” (CF) –type structured oxide in which a low Mg2+ migration barrier has been predicted by first principles calculations.4 A sodium vanadium-titanium oxide compound with the CF structure was obtained by solid state synthesis. Na+ ions reside in the spacious tunnels of the CF structure in this material, and need to be either removed or replaced by Mg2+ in order to allow Mg2+ de/intercalation. Desodiation (Fig. 1a) was carried out with both electrochemical and chemical methods, resulting in ~ 55% and 85% Na+ extraction, respectively. The chemically desodiated material exhibits ~ 85 mAh g-1 discharge capacity in a Mg full cell (Fig. 1b), showing a promising Mg2+ intercalation capability in the structure, although only half of this is reversible in these preliminary studies. XAS studies suggest that vanadium is the redox centre. We also examined Mg2+ ion exchange for the CF material, finding that ~ half of the Na+ ions were replaced with Mg2+ (Fig. 1a). Interestingly, we observe a phase transformation reaction accompanying major desodiation or ion exchange. This demonstrates a trend towards the rearrangement of the polyhedral framework in the structure when the large ion tunnel is emptied or occupied by the smaller Mg2+ ion, the origins of which will be discussed in this talk. Nevertheless, the promising electrochemistry of the desodiated material upon Mg2+ de/intercalation, as well as the ability to partially exchange Na+ with Mg2+, suggests moderately good Mg2+ diffusion in the CF structure, and sheds light on the possibility of iso-structural compounds as cathode materials for Mg batteries that are stable or metastable upon Mg2+de/intercalation. Other promising cathode materials for Mg batteries, not strictly limited to oxides, will also be the topic of the presentation.
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Figure 1