Diffusion Measurements of Mg in High Capacity Thiospinel Mg_xTi₂S₄

The demand for smaller batteries capable of storing the same amount of energy as conventional Li-ion technology has led to the pursuit of several new technologies including rechargeable Mg batteries. Mg metal is attractive as a negative electrode material because it has a higher volumetric capacity density (3833 mAh/mL) than Li metal (2062 mAh/mL), is the 8 th most abundant element in the earth’s crust, is safe to handle in ambient atmosphere, and can be electrodeposited (charged) without the formation of dendrites. 1 The seminal work by Aurbach et al. in 2000 2 established an electrolyte and a positive electrode material, the Chevrel phase (Mo 6 S 8 ), that was paired with Mg metal to form the first rechargeable Mg battery. Mg 2+ intercalation in host materials is more difficult than that of Li + or Na + , displaying lower ion mobility in solid oxide hosts 3 and a probable higher desolvation energy penalty. 4 No further positive electrode materials with both notable capacity and cycle life have been demonstrated since the Chevrel phase, until now. In this presentation, we will demonstrate that the thiospinel Ti 2 S 4 reversibly intercalates Mg 2+ with a 2 nd cycle capacity of about 165 mAh/g, which drops to only 140 mAh/g after 40 cycles at C/10 as shown in Figure 1. Of crucial scientific importance is that Ti 2 S 4 provides a second example of a material that supports facile Mg 2+ diffusion, which could help elucidate why Mg 2+ intercalation is so difficult in other potential cathode materials. In exploring Mg 2+ diffusion, the first step is to measure the chemical diffusion coefficient, D. The galvanostatic intermittent titration technique (GITT) 5 is a versatile method of carrying this out for an intercalant like Mg 2+ if a reliable cell can be constructed that has a long enough diffusion length to produce a linear potential vs time response reflecting Fick’s laws of diffusion. Figure 2 shows a typical GITT experiment on thiospinel Mg x Ti 2 S 4 , which displays the required potential vs time response. This talk will elaborate on the results of the diffusion measurements we have performed and compare those results to theory. References J. Muldoon, C. B. Bucur, and T. Gregory. Chem. Rev. 114 , 11683-11720 (2014). D. Aurbach, Z. Lu, A. Schechter, Y. Gofer, H. Gizbar, R. Turgeman, Y. Cohen, M. Moshkovich and E. Levi. Nature 407 , 724-727 (2000). E. Levi, Y. Gofer, and D. Aurbach. Chem. Mater. 22 , 860-868 (2010). L. F. Wan, B. R. Perdue, C. A. Apblett and D. Prendergast. Chem. Mater. 27 , 5932-5940 (2015). W. Weppner and R. A. Huggins. Solid-State Science and Technology 124 , 1569-1578 (1977). Figure 1

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