Abstract
The Mg battery cathode material, thiospinel MgxZr2S4 (0 ≤ x ≤ 1), exhibits negligible volume change (ca. 0.05%) during electrochemical cycling, providing valuable insight into the limiting factors in divalent cation intercalation. Rietveld refinement of XRD data for MgxZr2S4 electrodes at various states of charge, coupled with EDX analysis, demonstrates that Mg2+ can be inserted into Zr2S4 at 60 °C up to x = 0.7 at a C/10 rate (up to x = 0.9 at very slow rates) and cycled with a high Coulombic efficiency of 99.75%. HAADF-STEM studies provide clear visual evidence of Mg-ion occupation in the lattice, whereas XAS studies show that Zr4+ was reduced upon Mg2+ intercalation. Operando and synchrotron XRD studies reveal the creation of two phases during the latter stages of discharge (x > 0.5) as the lattice fills and Mg2+ ions begin occupying tetrahedral (8a) sites in addition to octahedral (16c) interstitial sites. Compared to the isostructural Ti2S4 thiospinel, Zr2S4 presents a slightly larger cell volume and hence an almost ideal zero-strain lattice on Mg2+ insertion. Nonetheless, its 4-fold lower electronic conductivity results in a diffusion coefficient for Mg2+ ions (DMg; 1 × 10-10 to 1 × 10-9 cm2/s) that is more than a factor of 10 lower than in Ti2S4. This shows that delocalization of the electron charge carriers in the framework is a very important factor in governing multivalent ion diffusivity in the thiospinel framework and, by extension, in other materials.
| Original language | English |
|---|---|
| Pages (from-to) | 4683-4693 |
| Number of pages | 11 |
| Journal | Chemistry of Materials |
| Volume | 30 |
| Issue number | 14 |
| DOIs | |
| State | Published - Jul 24 2018 |
| Externally published | Yes |
Funding
*E-mail: [email protected]. ORCID Patrick Bonnick: 0000-0002-4448-0072 Chang-Wook Lee: 0000-0002-9636-310X Linda F. Nazar: 0000-0002-3314-8197 Present Addresses §P.B. is currently at Toyota Research Institute of North America, 1555 Woodridge Ave., Ann Arbor, Michigan, 48105, USA ||X.S. is currently at Department of Chemistry, Northeastern University, No. 11, Lane 3, WenHua Road, HePing District, Shenyang, Liaoning, 110819, China Author Contributions †P.B. and L.B. contributed equally to this work. P.B. and L.F.N. conceived the experiments and study; P.B. and L.B. carried out the materials synthesis, electrochemical studies, and materials characterization/analysis with the help of X.S. S.H.V. carried out the TEM studies and analyzed the data; C.-W.L. and M.B. performed the XAS study and analysis and contributed discussion to the text; P.B., L.B., and L.F.N. wrote the manuscript. Funding This work was supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the US Department of Energy (DOE), Office of Science, Basic Energy Sciences. NSERC is acknowledged by L.F.N. for a Canada Research Chair, and support through the NSERC Discovery Grant program. X-ray absorption spectroscopy measurements were performed at the Argonne National laboratories at the APS. Sector 20 operations are supported by the DOE Office of Science. Synchrotron XRD measurements were performed using beamline 08B1−1 at the Canadian Light Source, which is supported by the Canada Foundation for Innovation, Natural Sciences and Engineering Research Council of Canada, the University of Saskatchewan, the Government of Saskatchewan, Western Economic Diversification Canada, the National Research Council Canada, and the Canadian Institutes of Health Research. Notes The authors declare no competing financial interest. We are grateful to Dr. Joel Reid at the Canadian Light Source for performing the synchrotron XRD experiments. Electron Microscopy work was carried out at the Canadian Centre for Electron Microscopy (CCEM), a facility supported by NSERC and McMaster University. P.B. also thanks Dr. Jung-Soo Kang for his help with XPS.