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

We report the experimental discovery of Li₇Si₂S₇I_0.89Cl_0.11, a triclinic (P1̄) analogue of the recently discovered monoclinic (P2₁/n) Li₇Si₂S₇I (LSSI), which retains the high computationally predicted Li⁺ ion conductivity of LSSI. Li₇Si₂S₇I_0.89Cl_0.11, 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. Li₇Si₂S₇I_0.89Cl_0.11 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, Li₇Si₂S₇I_0.89Cl_0.11 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 S^2-/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.