From Cation Order to Disorder: Unlocking Ion Transport Pathways in Li-Zn-Zr-Cl Halospinels

Lithium metal chloride halospinels of the general formula Li₂ MCl₄ are a promising class of earth-abundant ion conductors for all-solid-state batteries. However, poor room-temperature ionic conductivity has historically limited their use in practical applications. Here, we substitute Zr⁴⁺ into Li₂ZnCl₄ along the series Li_2-2x/3Zn_1-x Zr_2x/3Cl₄ (x = 0, 0.1, 0.3, 0.4, 0.6, 0.9, and 1.0) to understand how cation disorder and vacancy tuning impact ion transport in "normal" halospinels. Aliovalent Zr⁴⁺ substitution increases ionic conductivity by nearly 5 orders of magnitude, from 1.320(3) × 10^-9 S cm^-1 in Li₂ZnCl₄ to 6.74(1)× 10^-5 S cm^-1 for x = 0.6. Average and local structure characterization through synchrotron X-ray diffraction (SXRD) and neutron pair distribution function (nPDF) analysis reveal that Zr⁴⁺ redistributes the Zn²⁺ and Li⁺ sublattices into previously unoccupied interstitial sites, which form new low-energy hopping pathways that facilitate ion transport. We rationalize the dramatic rearrangement of the cation local structure by considering the coordination preferences of the cations and the potential electrostatic penalties incurred by the higher-valent Zr⁴⁺ cations. This work delivers an atomistic understanding of substitution-induced cation disorder and ion transport properties in a new family of earth-abundant halospinels.