Structural Analysis of Tin-Substituted High-Entropy Li-Garnet Electrolytes for Solid-State Batteries

Abstract

Lithium garnets offer promising structural and electrochemical properties and could be used in all solid-state lithium batteries replacing liquid electrolytes. They can operate in a wide electrochemical voltage window and show high ionic conductivities (>10^–4 S cm^–1). The best-studied lithium garnet is Li₇La₃Zr₂O₁₂ (LLZO), which is known to undergo a transition from an ordered, tetragonal form to a disordered cubic modification at elevated temperatures. This is crucial, as the cubic modification offers about 2 orders of magnitude higher ionic conductivities. Applying the high-entropy concept to this material facilitates the stabilization of the cubic structure at ambient conditions. In this work, four different lithium garnet compositions based on Li₆La₃Zr_0.5Nb_0.5Ta_0.5Hf_0.5O₁₂ have been synthesized by mixing Zr⁴⁺, Nb⁵⁺, Ta⁵⁺, and Hf⁴⁺ by Sn⁴⁺, respectively, using two different solid-state approaches. They have been characterized by X-ray diffraction, energy-dispersive X-ray spectroscopy, and impedance spectroscopy to analyze the influence of synthesis parameters and composition on phase purity, elemental distribution, and ionic conductivity. It was found that combining calcination and sintering into one process yields a higher density and ionic conductivity than splitting it into two with intermediate regrinding of the material. Impedance data indicate an increase in ionic conductivity when substituting pentavalent ions for tetravalent ones due to the resulting higher concentration of mobile charge carriers in the structure, compared to Li₆La₃Zr_0.5Nb_0.5Ta_0.5Hf_0.5O₁₂.