Synthesis, chloride-ion diffusion mechanisms, and anisotropic sintering of 2D layered erbium oxychloride nanoplatelets

Abstract

Increasing interest in halide-ion batteries as a means of long-duration energy storage has led to intense recent interest in mechanisms of anion migration in periodic solids. Lanthanide oxyhalides crystallize in a richly diverse set of structures with well-separated halide-ion slabs and represent potential solid electrolytes of halide batteries. However, oxyhalides of later lanthanides are underexplored with regard to preparatory methods, structural preferences, defect energetics, and anion migration pathways. Here, we report a non-hydrolytic sol–gel condensation route to ligand-passivated single-crystalline ErOCl nanoplatelets, which crystallize in van der Waals layered SmSI and YOF structure types with reduced coordination of the lanthanide centers as compared to the PbFCl structure type preferred by early lanthanide oxyhalides. Pressure-less annealing in an argon ambient environment results in anisotropic sintering of the platelets with strongly preferred grain growth along the ab plane whilst retaining the thin platelet morphology. The preferred grain growth can be traced to strongly anisotropic chloride-ion migration along the ab plane of the bilayered van der Waals solid, which represents the primary mechanism of mass transport across grain boundaries. First-principles nudged elastic band simulations indicate that a low-energy intralayer chloride-ion migration pathway becomes accessible within the chloride-ion slab along the ab plane in the presence of chloride vacancies. The simulations demonstrate the promise of modulation anion conductivity through site-selective modification on the cation and anion lattices. These findings offer valuable insights into an important emerging class of halide electrolytes, illuminate fundamental mechanisms governing anion transport in 2D materials, and provide a blueprint for materials processing to achieve directional ion diffusion pathways.