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N7.08 - On The Control of Moisture Corrosion Processes for The Optimization of Transport Properties in Fast Li-Conducting Ceramic Electrolytes 
Date/Time:
April 24, 2014   4:00pm - 4:15pm
 
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Fast Li-conducting ceramic electrolytes are foremost components for the development of next generation secondary batteries with increased stability, life and safety. Most efforts have been focused on trying to develop materials with higher conductivity and on the understanding of the bulk ionic conduction mechanisms. However, no special attention has been paid to processing and moisture sensitivity issues (1). In particular, Li3xLa2/3-xTiO3 (LLTO) has been reported to have the highest bulk Li-conductivity in a ceramic material with 10-3 S/cm at 25 C with x = 0.11. However, the high grain boundary resistivity of this system decreases the total conductivity to an order of 10-5 S/cm (2 ). Another kind of materials receiving great attention are Li-stuffed garnet materials with the general formula LixB3C2O12 (x > 3) achieving ionic conductivities higher than 10-4 S/cm at room temperature for the cubic-stabilized phases of different compositions like Ga-doped Li7La3Zr2O12. However sinterability is a major issue for the processing of these materials (3). This study provides for the first time, 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, LLTO and Ga-doped Li7La3Zr2O12 electrolytes were sintered in air, synthetic air and pure oxygen. Impedance spectroscopy combined with 1H an 7Li solid state NMR were used to monitor the Li exchange by protons from moisture and its effect in the Li conductivity while scanning electron microscopy was used to evaluate the effect on the microstructure of the samples. We observed a considerable moisture-dependence on the percentage of densification of the samples and the Li-conductivity with the degree of Li-H exchange. Both sets of materials show a substantial increase of the densification with densities around 95% when sintering in pure O2. Moreover, Li populations with different motilities have been identify and directly correlated to the presence of H in the structures due to Li-H exchange. For instance, the conductivity at the grain boundary (comparing equivalent grain size samples) of LLTO have been increased by more than half a decade to 8.34 10-5 S.cm-1 when preventing protonation of the ceramics using dried synthetic air or pure oxygen. The independency of activation energies on sintering conditions, may indicate that the increase of conductivity is only related to an increase of charge carriers thanks to the hindering of the Li-H exchange and not to a change in the conduction mechanism in these systems. 1-Bohnke, O. et al. Solid State Ionics 188, 2011, 144 2- Inaguma, Y., et al. Solid State Comm 86, 1993, 689 3- Shinawi, H.E et al. Journal of Power Sources 225, 2013, 13
 


 
 
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