Improvement of Transport Properties in Li-Conducting Ceramic Oxides
In this work, we provide a better understanding of the importance of moisture control during the processing of Li-conducting ceramics as it is a limiting factor for their use as solid state electrolytes. For this purpose, we have studied two families of ceramic electrolytes: Li3xLa2/3-xTiO3 (LLTO) and Ga-doped Li7La3Zr2O12. Our approach in the Ga-doped garnet has been to dope in the tetrahedral Li-positions trying to optimize the Li-vacancy content and favour a higher Li-mobility in an octahedral-octahedral path. Impedance spectroscopy combined with 1H and 7Li solid state NMR and neutron power diffraction were used to analyse the cation environments and the effect of moisture on the Li-mobility while electron microscopy was used to evaluate the effect the local structure and microstructure of the dense samples.
We have proven a great effect of moisture on the miscrostructure and transport properties of ceramic oxides. We observed a substantial increase of the densification with densities around 92-95% when sintering under low moisture and CO2 content (< 2ppm). Furthermore, avoiding the corrosion process that leads to the Li-H exchange with the concomitant decrease of Li charge carriers and the formation of secondary phases in the grain surface (Fig. 1), we have increased by 5 times the total conductivity in LLTO2. Moreover, we have achieved a conductivity of 1.3x10-3 S/cm at 24oC for La3Zr2Li6.55Ga0.15□0.3O12which is amongst the highest Li ion conductivities reported for garnet-structured materials to date3.
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2 F. Aguesse, J. M. Lopez del Amo, V. Roddatis, A. Aguadero, John Kilner Advance Materials: Interfaces, 1300143, 2014