Superionic lithium mobility in low symmetry Li7Si2S7I polymorph accessed via Si2S7 dimer reorientation.

We report the experimental discovery of Li7Si2S7I0.89Cl0.11, a triclinic (P1̄) analogue of the recently discovered monoclinic (P21/n) Li7Si2S7I (LSSI), which retains the high computationally predicted Li+ ion conductivity of LSSI. Li7Si2S7I0.89Cl0.11, which is effectively a polymorph of LSSI, maintains the same ordered anion packing based on the packing of spheres in the NiZr intermetallic, however demonstrates a distinct ordering of the Si4+ framework-forming cations. While both structures feature Si2S7 dimers within hexagonal close-packed (hcp) anion motifs, their different arrangement in the triclinic material results in the alternating stacking of silicon-free and silicon-rich layers. Li7Si2S7I0.89Cl0.11 has an additional Li+ position compared to LSSI, sixteen in total, which maintains the large number of redundant low energy pathways favourable for superionic conduction. Thus, Li7Si2S7I0.89Cl0.11 has a predicted ionic conductivity of 0.019(7) S cm-1 derived from molecular dynamics simulation of the experimentally measured structure and a theoretical activation energy for bulk Li+ ion transport of 0.16(4) eV, within error of those of monoclinic LSSI. These results demonstrate the structural resilience of the ordered S2-/I- anion net to changes in cation positions and crystal system, further exemplifying the ability of the net to afford diverse low barrier Li+ ion transport pathways and thus generate a predicted superionic conductivity. Polymorphism is generally thought to have a profound impact on ion transport, however the computational results here suggest that there are privileged anion frameworks with an intrinsic robustness to changes in cation distribution where superionic transport can persist.