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†
In this paper, we report the activation entropy (ΔS) values for Li-stuffed Li6.5La2.9A0.1Zr1.4Ta0.6O12 (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 Li6.5La2.9A0.1Zr1.4Ta0.6O12 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 Li6.5La2.9A0.1Zr1.4Ta0.6O12 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 Li6.5La2.9A0.1Zr1.4Ta0.6O12 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, Li6.5La2.9Sr0.1Zr1.4Ta0.6O12 showed an impressive Li ion charge transfer resistance (RCT) 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.