Improvement of Transport Properties in Li-Conducting Ceramic Oxides

Li-conducting ceramic electrolytes can be used as ion-selective ceramic membranes adding chemical, thermal and electrochemical stability and preventing capacity fading with time. To date there has been low control of the processing conditions of these materials, however Li-ceramic conductors with high alkaline mobilities, have shown to react with moisture (Li-H exchange) and CO₂ even at room temperature1. In recent works we have proven that this corrosion with moisture can promote a huge degradation of the phase purity, microstructure and transport properties and that the control of moisture during processing can improve Li-conductivity in ceramic oxides from 1 to 3 orders of magnitude^1,3.

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: Li_3xLa_2/3-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 24^oC for La₃Zr₂Li_6.55Ga_0.15□_0.3O₁₂which is amongst the highest Li ion conductivities reported for garnet-structured materials to date³.

References

1 a) Boulant, J. F. Bardeau,  A. Jouanneaux, J. Emery, J.-Y. Buzare,  O. Bohnke, Dalton Transactions 39, 3968, 2010 b) G. Larraz, A. Orera, M.L.  Sanjuan, J. Mater. Chem. A., 1, 11419, 2013.

2 F. Aguesse, J. M. Lopez del Amo, V. Roddatis, A. Aguadero, John Kilner Advance Materials: Interfaces, 1300143, 2014

³ C. Bernuy-Lopez, W. Manalastas, A. Aguadero, J. M. Lopez del Amo, F. Aguesse, J. A. Kilner, Chemistry of Materials, 26, 3610, 2014

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