Lithium Redox Behavior at Liquid Lithium-Lead Alloy Electrodes in Molten LiCl–KCl
Authors: Ito Ryo , Kitagawa Yuzuha , Mukai Keisuke , Yagi Juro 
Published: December 2024

Introduction

Fusion energy is vital in a carbon-free society and stable energy supply. The advanced liquid blanket concepts allow high-efficiency (~50%) power generation by applying gas-turbine cycles. In particular, liquid Li17Pb83 (lithium-lead eutectic alloy) coolant enables higher-temperature (850~1000 ℃) heat applications, such as effective hydrogen and synthetic fuel production, and process heat for industrial uses. For realizing the Li17Pb83 blanket systems, molten salt electrochemistry technologies can be applied to lithium isotope enrichment, high-purity Li17Pb83 production, and online lithium replenishment for the lithium burn-up through neutron nuclear reaction: 6Li(n,t)4He, under the operation of a fusion reactor. These application developments need to obtain the chemical activity of lithium (a
Li) and to clarify lithium redox behavior at liquid LixPb100-x electrodes; 0 < x ≤ 17 at.%. Previous studies investigated the a
Li in liquid LixPb100-x
[1,2], and lithium redox potential (E
Li) was estimated according to the following Nernst equation: E
Li
=E
Li
0+(RT/F)ln(a
Li+
/a
Li in alloy). Thus, this study aims to clarify the lithium oxidation/reduction behavior at the LixPb100-x electrodes.

Experimental

The experimental apparatus was installed in a glove box under a purified argon gas atmosphere. The working electrodes were a coil-like Ni wire (φ: 0.5 mm, A: 1.0 cm2), liquid Pb (A: 0.28 cm2), and liquid LixPb100-x (x: 4.6 or 14.8, A: 0.28 cm2). The liquid Pb and LixPb100-x were stored in J-shaped glass tubes (made of borosilicate glass, φ: 8 mm, t: 1 mm). The molar ratio of the LixPb100-x was quantified by ICP-MS (Inductively Coupled Plasma-Mass Spectrometry). The quasi-reference electrode was a coil-like Mo wire (φ: 0.5 mm), and the redox potential was calibrated with Li+/Li standard potential. The counter electrode was a glassy carbon plate (t: 1 mm). The electrolytic bath was LiCl-KCl eutectic, and the bath temperature was controlled within ±2 ℃. Cyclic voltammetry and amperometry were performed at 500 ℃ to investigate the unsteady-state and steady-state polarization behaviors, respectively. In polarization measurements, the open circuit potential and chrono-amperometry were applied to adjust the lithium concentration of the LixPb100-x electrodes.

Results and Discussion

Figure 1 shows cyclic voltammograms obtained at Ni, Pb, Li4.6Pb95.4, and Li14.8Pb85.2 electrodes at 500 ℃. Li+-ion was reduced and formed into lithium metal at 0.0 V on the surface of the Ni electrode. Nickel is significantly difficult to alloy with lithium[3], thus deposited lithium can be regarded as a pure substance: a
Li = 1.0. Li+-ion was reduced and immediately dissolved in the liquid Pb, Li4.6Pb95.4, and Li14.8Pb85.2 electrodes. The relationship of E
Li was summarized as follows: Ni < Li14.8Pb85.2 < Li4.6Pb95.4 < Pb. According to the Nernst equation, these results suggest that the relationship of a
Li is estimated as follows: 0.0 < Pb < Li4.6Pb95.4 < Li14.8Pb85.2 < Ni = 1.0. The increasing tendency of a
Li is in good agreement with the previous studies[1,2].

Figure 2 shows steady-state polarization curves obtained at Li4.6Pb95.4 and Li14.8Pb85.2 electrodes at 500 ℃. In the case that the absolute value of overpotential (η) was large enough, current density (i) was an exponential of η in both electrodes. The polarization curves were fitted well with the following Tafel equations: η » 0, i ≃ i
a = i0exp(αa
Fη/RT), η « 0, i ≃ i
c = i0exp(αc
Fη/RT). These results suggest that a charge transfer is the rate-limiting step of lithium redox behavior at the LixPb100-x electrodes. The lithium redox behaviors at different temperatures will be presented at the meeting.

References

[1] Hubberstey, P. & Sample, T. Pb-17Li-water interactions: a thermodynamic and experimental characterization. J. Nucl. Mater. 199, 149–158 (1993)

[2] Hubberstey, P. & Sample, T. Thermodynamics of the interactions between liquid breeders and ceramic coating materials. J. Nucl. Mater. 248, 140–146 (1997)

[3] Nohira, T. & Ito, Y. Electrochemical behavior of hydride ion in a LiCl-KCl eutectic melt. J. Electrochem. Soc. 149, E159-E165

Figure 1