Lithium Sensors Based on Ionic Conducting Ceramics
Authors: Abella Jordi , Colominas Sergi , Nel-lo Marc , Ferre Oscar 
Published: May 2020

Tritium is one of the required elements in future fusion reactors. This hydrogen isotope has to be continuously produced in order to supply the fusion reactor. Tritium can be obtained from the fussion reaction of lithium 6. Because of that, future fusion reactors are designed to have a tritium breeding module surrounding the reactor. One of the breeding blanket that has been proposed is based on lead-lithium eutectic melt [1]. Nevertheless, lithium monitoring in the breeding blanket is of great importance for the performance of the reactor. A deviation from the eutectic composition can cause a substantial change on the tritium production.

Therefore, in order to control the lithium dosage needed and keep the adequate composition in the melt, on-line lithium measurements will be required. For this purpose, lithium probes must be designed to withstand the high temperatures of the breeding modules (~400 ºC) and the chemical reactivity of lithium. Finally, the constructed sensors need to be tested at different lithium concentrations in order to determine its viability and range of application.

Sensors based on solid state electrolytes have several advantages: stable compounds which can withstand the harsh chemical environment of the melts, its ionic conductivity tends to increase frequently with the temperature [2] and the output signal (cell potential or electrochemical current) is easy to measure. Lithium conducting electrolytes for molten metals are under development at the Electrochemical Methods Laboratory at Institut Quimic de Sarria (IQS), Barcelona. Its qualification and performance are being tested. Li-probes for molten metals will be based on the use of ceramic type lithium conducting solid-state electrolytes.

In the present work, lithium conducting solid electrolytes Li6BaLa2Ta2O12 [3] and Li6La3Ta1.5Y0.5O12 [4] were synthesized. The crystallographic phases were characterized with XRD, its surface microstructure with SEM and its lithium content was determined. The resulting ceramics were shaped in a disc form. The potentiometric probe consisted in the union of the sintered ceramic disc with an alumina tube using a glass sealant. The design of the sensor involved a two electrode system that permitted to measure the potential difference between a working and a reference electrode. The reference electrode was stablished as a lithium alloy with a fixed and known composition which was placed inside the alumina tube. The working electrode was the alloy where the assembly was submerged. A molybdenum wire was used to allow the electron flow from the electrodes to the measuring system.

The sensors response was measured potentiometrically in two different lithium alloys Sn/Li and Pb/Li. The experiments that were performed for each lithium alloy consisted on the measurement of the potential difference between the working and the reference electrode. In each experiment, the lithium concentration was increased in the working electrode while the lithium content in the reference electrode was kept constant. The lithium concentration in the melts (working electrode) covered a range from 3at% up to 30at%. The measurements were performed at three different temperatures: 400 ºC, 500 ºC and 600 ºC. Correlation curves between the measured potential and the lithium composition were evaluated. The slopes of the obtained correlations were in good agreement with the slopes of the Nernst equation.

References

[1] F. De Schutter, J. Dekeyser, J. Luyten, H. Tas, Electrochemical sensors for oxygen and lithium detection in Pb17Li, Fusion Eng. Des. 14 (1991) 241–248. https://doi.org/10.1016/0920-3796(91)90008-E.

[2] V. Thangadurai, S. Narayanan, D. Pinzaru, Garnet-type solid-state fast Li ion conductors for Li batteries: critical review, Chem. Soc. Rev. 43 (2014) 4714. https://doi.org/10.1039/c4cs00020j.

[3] M. Nel-lo, S. Colominas, J. Abellà, Lithium conducting ceramics for future electrochemical sensors in molten metals, Fusion Eng. Des. (2019). https://doi.org/10.1016/j.fusengdes.2019.02.043.

[4] A.K. Baral, S. Narayanan, F. Ramezanipour, V. Thangadurai, Evaluation of fundamental transport properties of Li-excess garnet-type Li5+2xLa3Ta2-xYxO12 (x = 0.25, 0.5 and 0.75) electrolytes using AC impedance and dielectric spectroscopy, Phys. Chem. Chem. Phys. 16 (2014) 11356–11365. https://doi.org/10.1039/c4cp00418c.