The activation entropy for ionic conduction and critical current density for Li charge transfer in novel garnet-type Li6.5La2.9A0.1Zr1.4Ta0.6O12 (A = Ca, Sr, Ba) solid electrolytes

Abstract

In this paper, we report the activation entropy (ΔS) values for Li-stuffed Li_6.5La_2.9A_0.1Zr_1.4Ta_0.6O₁₂ (A = Ca, Sr, Ba) garnets using a statistical mechanical approach and the role of ceramic processing in Li ion charge transfer between elemental Li and Li-stuffed Li_6.5La_2.9A_0.1Zr_1.4Ta_0.6O₁₂ garnets. The investigated solid electrolytes were prepared by a solid-state reaction and a spark plasma sintering method. The formation of the garnet-type structure was confirmed by powder X-ray diffraction. The homogeneous distribution of the elements in the prepared compositions was investigated using electron probe micro analyzer coupled with a wavelength-dispersive X-ray spectrometer. Li ion conductivity was determined using electrochemical ac impedance spectroscopy. The calculated ΔS value for garnet-type Li_6.5La_2.9A_0.1Zr_1.4Ta_0.6O₁₂ is found to be one order of magnitude higher than that of other solid electrolytes such as Li₄SiO₄ (Li⁺), Na₃ZrMgP₃O₁₂ (Na⁺), AgI (Ag⁺) and Ag-β-alumina (Ag⁺) and comparable to that of the polymer Li ion electrolyte (PEO)₁₀-LiClO₄. SPS-processed Li_6.5La_2.9A_0.1Zr_1.4Ta_0.6O₁₂ samples showed a bulk Li ion conductivity of 5.5 × 10⁻⁴ S cm⁻¹ at 22 °C. Among all the samples investigated in this work, Li_6.5La_2.9Sr_0.1Zr_1.4Ta_0.6O₁₂ showed an impressive Li ion charge transfer resistance (R_CT) of 3.5 Ω cm² at 25 °C without any surface coating of metal oxides, such as Al₂O₃ and ZnO, and exhibited a critical current density of 0.6 mA cm⁻².