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

High Resolution Image Download MS PowerPoint Slide Lithium metal chloride halospinels of the general formula Li 2 M Cl 4 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 Zr 4+ into Li 2 ZnCl 4 along the series Li 2–2 x /3 Zn 1– x Zr 2 x /3 Cl 4 ( 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 Zr 4+ substitution increases ionic conductivity by nearly 5 orders of magnitude, from 1.320(3) × 10 –9 S cm –1 in Li 2 ZnCl 4 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 Zr 4+ redistributes the Zn 2+ 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 Zr 4+ cations. This work delivers an atomistic understanding of substitution-induced cation disorder and ion transport properties in a new family of earth-abundant halospinels.