Critical Role of Framework Flexibility and Disorder in Driving High Ionic Conductivity in LiNbOCl4

Understanding Li-ion transport is key for the rational design of superionic solid electrolytes with exceptional ionic conductivities. LiNbOCl₄ is reported to be one of the most highly conducting materials in the recently realized new class of soft oxyhalide solid electrolytes, exhibiting an ionic conductivity of ∼11 mS·cm^-1. Here, we apply X-ray/neutron diffraction and pair distribution function analysis─coupled with density functional theory/ab initio molecular dynamics (AIMD)─to determine a structural model that provides a rationale for the high conductivity that we observe experimentally in this nanocrystalline solid. We show that it arises from unusually high framework flexibility at room temperature. This is due to isolated 1-D [NbOCl₄]^- anionic chains that exhibit energetically favorable orientational disorder that is─in turn─correlated to multiple, disordered, and equi-energetic Li⁺ sites in the lattice. As the Li ions sample the 3-D energy landscape with a fast predicted diffusion coefficient of 5.1 × 10^-7 cm²/s at room temperature (σ_i^calc = 17.4 mS·cm^-1), the inorganic polymer chains can reorient or vice versa. The activation energy barrier for Li migration through the frustrated energy landscape is especially reduced by the elastic nature of the NbO₂Cl₄ octahedra evident from very widely dispersed Cl-Nb-Cl bond angles in AIMD simulations at 300 K. The phonon spectra are predominantly influenced by Cl vibrations in the low energy range, and there is a strong overlap between the framework (Cl, Nb) and Li partial density of states in the region between 1.2 and 4.0 THz. The framework flexibility is also reflected in a relatively low bulk modulus of 22.7 GPa. Our findings pave the way for the investigation of future "flex-ion" inorganic solids and open up a new direction for the design of high-conductivity, soft solid electrolytes for all-solid-state batteries.