Structure-transport correlations in Na₁₁Sn₂SbSe₁₂and its sulfide solid solutions

We report a new ion conductor - Na11Sn2SbSe12 - as a possible candidate for a solid catholyte in composite cathodes for all-solid state Na-batteries, which exhibits a room temperature ionic conductivity of 0.15 ± 0.03 mS cm-1 and an activation energy of 0.39 ± 0.02 eV. The sulfide solid solutions of Na11Sn2SbSe12, namely, Na11Sn2SbS12-xSex (x = 1 and 6), were also investigated through a combination of Rietveld refinement against powder x-ray diffraction data and electrochemical impedance spectroscopy to reveal the complex structure-property relationships governing ion transport in this class of materials. Meanwhile, broadening of the Na-ion diffusion pathways in Na11Sn2SbSe12 is expected to facilitate Na-ion transport compared to the sulfide-rich member of the solid solution, the opposite holds: increasing the Se fraction in Na11Sn2SbS12-xSex leads to a little change in the activation energy but a reduction in the ionic conductivity. We ascribe this to the lowering of the prefactor, σ0, in the Arrhenius relationship with increasing lattice "softening"as a function of higher Se content.