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

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Abstract
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 Li2–2x/3Zn1–x Zr2x/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.
Funding Information
  • Basic Energy Sciences (DE-AC02-76SF00515)
  • Laboratory Directed Research and Development
  • U.S. Department of Energy (DE-AC36-08GO28308)
  • National Defense Science and Engineering Graduate
  • Department of Energy Office of Critical Minerals and Energy Innovation Transportation Technologies Office

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