Mono vs Divalent Cation Diffusion in Thiospinel Ti2S4

a) Mg2+ diffusion (DMg) is slower than Li diffusion in Ti2S4, and DMg drops off significantly faster than DLi as x increases, limiting MgxTi2S4 discharge, kinetically. Mg2+ tetrahedral site occupation is partially to blame for this. b) he slope of these Arrhenius plots yield the diffusion energy barriers (EDiff) for Mg2+ at x = 0.35. Similar data from two cells, A and B, show that the experiment is reproducible.

Scientific Achievement

We showed why the positive electrode material, MgxTi2S4, does not work well at room temperature, unlike Li2xTi2S4, which works very well. Furthermore, we confirmed that theoretical diffusion calculations predict accurate results at low x values.

Significance and Impact

Since diffusion rates can be accurately predicted, the search for new Mg positive electrode materials would be more efficient if diffusion coefficients were calculated for candidate materials before investing time in making and testing them. A diffusion barrier of less than about 500 meV will be required for a useful, dense Mg positive electrode material.

Research Details

  • Diffusion coefficients were measured using a novel Galvanostatic Alternating Pulse method & pressed pellets.
  • The decrease in DMg above about x = 0.6 could be due to either (or both) an increase in Mg2+ – Mg2+ interactions within the lattice or to Mg tetrahedral site occupation.

Work performed at the University of Waterloo (JCESR collaborator) and Argonne National Laboratory (JCESR managing partner) by P. Bonnick, X. Sun, K. Lau, C. Liao, and L.F. Nazar, J. Phys. Chem. Lett., 2017, 8, 2253-2257

DOI: 10.1021/acs.jpclett.7b00618

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