Speaker
Description
The critical role of solid electrolytes in the development and improvement of all-solid-state batteries is apparent, not only regarding lithium but also sodium based secondary cells.[1] A recently reported promising crystalline solid Na-ion conductor candidate is Na11Sn2PS12 with high conductivity in the range of 1.4 - 3.7 mS/cm mediated by intrinsic vacant Na sites.[2,3] Herein, we monitor the induced structural and transport changes upon aliovalent substitution, and thereby stoichiometrically reduced Na vacancy content, in the solid solution series Na11+xSn2P1-xMxS12 with M = Sn, Ge. Rietveld refinements against X-ray synchrotron diffraction at low temperatures show the structural expansion of unit cell, (P1-xMx)S4 tetrahedra and ultimately the Na+ diffusion pathway volumes. AC impedance spectroscopy reveals the associated reduction in activation energy from 0.41 eV in Na11Sn2PS12 down to 0.28 eV in both M = Ge and Sn Na11.625Sn2P0.375M0.625S12 compounds. Further analysing this relation a beneficial effect of increased Na+ density for a reduced activation energy is shown for the compounds investigated.[4] Even though such behavior is typically associated with faster ionic transport, a reduction of room temperature in-grain Na+ conductivity is found, only explained by the overcompensation of the Arrhenius pre-factor σ0 following the Meyer-Neldel rule.
With those results we show that such substitutions are suitable for the design of solid electrolytes in the Na11Sn2PS12 structural family preparing new compounds, shed light on structure- transport relationships and guide future optimization efforts in such materials.
[1] Randau, S. et al. Nat. Energy 2020, 5, 259–270.
[2] Zhang, Z. et al. Energy Environ. Sci. 2018, 11, 87–93.
[3] Duchardt, M. et al. Angew. Chemie Int. Ed. 2018, 57, 1351–1355.
[4] Kraft, M. A. et al. Chem. Mater. 2020, 32, 6566–6576.
| Speaker Country | Germany |
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