Lattice expansion/softening induced superionic conduction in Li2ZrCl6

The lattice expansion/softening can significantly increase ionic conductivity. • endow electrolytes with excellent anti-reduction stability. • The low reduction potential of Li 2 ZrCl 4.8 I 1.2 matches well with ASS Li-S batteries. In this study, I - doping was performed to address the limited ionic conductivity of Li 2 ZrCl 6 arising from the excessively narrow Li + transport channels. Owing to its large ionic radius (2.20 Å) and high polarizability, I - incorporation simultaneously expands and softens the lattice of Li 2 ZrCl 6 by enlarging interplanar spacing and weakening anion-cation interactions, thereby broadening Li + migration channels and reducing Li + migration barriers across all crystallographic directions. Furthermore, the introduction of I - disrupts the symmetry of [ZrX 6 ] (X = Cl, I) octahedra and induces the lattice distortion in Li 2 ZrCl 6 , destroying long-range order of structure with an increase in amorphous content. Nevertheless, excessive lattice distortion and overly large interplanar spacing induced by high iodide content block Li + transport pathways, thereby disrupting migration continuity and impeding Li + migration. Therefore, the ionic conductivity of Li 2 ZrCl 6-x I x (0 < x ≤ 1.5) exhibits a volcano-type dependence on the content of I - . At room temperature, Li 2 ZrCl 4.8 I 1.2 with the lowest migration energy barrier (0.286 eV) shows the highest ionic conductivity of 2.18 mS cm −1 which is more than five times that of Li 2 ZrCl 6 . In addition, the enhanced covalent-bond tendency between Zr 4+ and I - in the structure suppresses the reduction of Li 2 ZrCl 4.8 I 1.2 , decreasing its reduction potential down to 1.67 V (vs. Li/Li⁺). Meanwhile, the Li/ Li 2 ZrCl 4.8 I 1.2 /Li battery achieves stable cycling performance over 1200 h at 0.5 mA cm −2 under overpotential of 0.27 V, demonstrating the excellent anti-reduction stability of the solid-state electrolyte. Download: Download high-res image (292KB) Download: Download full-size image The lattice engineering strategy of introducing iodine ions with a large ionic radius (2.20 Å) and high polarizability simultaneously enhances the ionic conductivity (up to 2.18 mS cm - ¹ at room temperature) and anti-reduction stability of Li 2 ZrCl 6 .