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.

References:

1. D. Aurbach, Z. Lu, A. Schechter, Y. Gofer, H. Gizbar, R. Turgeman, Y. Cohen, M. Moshkovich and E. Levi, Nature 407, 724 (2000).

2. X. Sun, P. Bonnick, V. Duffort, M. Liu, Z. Rong, K. A. Persson, G. Ceder and L. F. Nazar, Energy Environ. Sci. 9, 2273 (2016).

3. X. Sun, P. Bonnick and L. F. Nazar, ACS Energy Lett. 1, 297 (2016).

4. C. Ling and F. Mizuno, Chem. Mater. 25, 3062 (2013).