Tritium, which is used as a fuel in a thermonuclear fusion reactor, is produced in the fusion reactor blanket by the reaction of neutrons with lithium whose 6Li-isotope-ratio is 90%. Other than the mercury amalgam method, which has a large environmental impact, no practical lithium isotope enrichment technology has been established. Electrodialysis using the lithium ion conductive electrolyte La0.57Li0.29TiO3 (LLTO) is promising, as reported to achieve high isotope enrichment [1]. In this study, we investigate the effect of applied voltage in electrodialysis on the isotope separation factor. An anode (primary solution side) and a cathode (secondary solution side) were formed on both front and back surfaces of LLTO. One reference electrode was also formed on each surface of the LLTO. DC voltage of various magnitudes was applied between the anode and the cathode. The electrochemical impedance between the reference electrode on the primary solution side and the anode during the application of this voltage was measured by AC impedance spectroscopy. At the same time, the electrochemical impedance between each of the secondary solution side reference electrode and the cathode was also measured. The amount of transferred lithium and the isotope ratio of lithium were measured by an inductively coupled plasma mass spectrometer (ICP-MS). The lithium isotope separation factor increased linearly with decreasing applied voltage. Here we assume the following: ① The existence probability of such atoms with respect to the minimum value of the energy required for lithium in the crystal lattice to move to the adjacent site has a normal distribution, ② In 6Li and 7Li, the standard deviation of the normal distribution is the same and the energy average the values are different. Then, in a narrow energy region near the activation energy of 6Li or 7Li transfer, it is expected that the ratio of 6Li in the energy transfer lithium decreases linearly with the increase in energy. Our experimental results seem to follow this prediction. [1] S. Honda, K. Shin-mura and K. Sasaki, J. Ceram. Soc. Japan, 126, [5], 331-335 (2018).
Demand for lithium-ion batteries (LIB) is rapidly increasing with the electrification of automobiles and the spread of new energy management systems. It is desired to recover lithium resources, whose supply and demand are tight, from spent LIB and seawater. Lithium for LIB must have a high purity of 99.9% or higher. The electrodialysis method using a lithium ion electrolyte has a high potential for recovering such high-purity lithium. Kunugi et al. [1] reported that only lithium ions could be recovered from LiCl-NaCl-KCl mixed aqueous solution by electrodialysis using La0.55Li0.35TiO3 as an electrolyte. Hoshino [2] demonstrated the possibility of using lithium ion conductive glass ceramics (LICGCTM). However, in either method, the lithium recovery rate is too low for practical use. We have devised an electrodialysis system with two power sources and three electrodes, which is different from a general electrodialysis cell consisting of two electrodes and one power source, and experimentally confirmed a large lithium recovery rate. In this study, we investigate the effect of electrode overvoltage in the secondary solution on the lithium recovery rate by this new electrodialysis method. An anode (primary solution side) and a cathode (secondary solution side) were formed on both sides of the La0.57Li0.29TiO3 (LLTO) by firing a Pt paste. As a third electrode, a Ni mesh electrode was placed in the secondary solution at a position apart from the cathode. A constant DC voltage was applied between the anode and the cathode. A constant DC voltage of another magnitude was also applied between the cathode and the Ni mesh electrode. The electric resistance of the solution between the cathode and the third electrode was changed by changing the lithium concentration in the secondary side solution or the distance between the cathode and the third electrode. The change in the lithium recovery rate due to this change in electrical resistance was investigated. The recovery rate was calculated using the lithium concentration in the secondary side solution measured by inductively coupled plasma optical emission spectroscopy. The electrical resistance of the solution between the cathode and the third electrode decreased due to the increase of the lithium-ion concentration in the secondary side solution, and the overpotential for the electrochemical reaction on the cathode and the third electrode increased. As a result, the lithium recovery rate was increased. [1] S. Kunugi, Y. Inaguma and M. Itoh, Solid State Ionics, 122, 35-39 (1999). [2] T. Hoshino, Desalination, 359, 59-63 (2015).
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