Oxide Speciation and Oxy-Fluoro Complexes in Molten Fluoride Salts
Authors: Williams Haley , Ratso Sander , Scarlat Raluca O. 
Published: December 2024

The presence and speciation of oxides are relevant to various engineering applications of molten fluoride salts. Here, our studies are directed toward two primary applications: 1) as coolants, fuel salts, or breeding blankets in advanced fission and fusion reactors, and 2) as the reaction medium for the novel direct synthesis of Li-ion battery cathodes from Li-ores.

In an advanced nuclear reactor, oxide might be introduced to the molten fluoride coolant/blanket via ingress of oxygen or moisture from air. Oxygen and moisture act as oxidants, reacting with the metal fluoride to produce metal oxides or hydroxides, along with corrosive HF. By these reactions, the presence of oxide is an indicator of leaks as well as of the progression of corrosion. Quantification of oxides by square wave voltammetry (SWV) has been proposed as a technique for electrochemical oxide sensors [1]. The intensity of the peak current for the oxidation of oxide to oxygen gas is proportional to the quantity of oxide present, so comparison to a calibration curve can indicate how much oxide is present in the melt. True engineering systems, however, would likely contain multiple solutes including corrosion products, fission products, and activation products. Multi-component chemistries are further complicated by differences in solvent-solute interactions; different fluoride solvents have varying degrees of polymerization, so they incorporate solutes into their structure differently. Practically, this means that oxides may exist in different complexes depending on the solvent and other solutes, therefore oxide quantification via SWV is complicated by solvent and solute chemistry.

The synthesis of Li-ion battery cathodes via reaction in molten fluoride salts is currently being studied within a DOE Office of Science BES project as a lower-carbon emissions alternative to traditional cathode manufacturing. This concept involves the dissolution of spodumene (LiAlSi2O6) in molten fluorides and the electrochemical deposition of disordered transition metal (M) oxyfluoride anion rocksalt (Li1+xTM1-xO2-zFz) for use as a battery cathode. The anion disorder in the rocksalt is relevant due to its relation to cathode performance. Energy storage capacity from the transition metal redox states increases as F is substituted for O. Cathode synthesis, therefore, requires tuning of the chemical potentials and transport properties in melts containing fluoride and oxide anions. In this context, a deeper understanding of the structure and speciation of oxide in molten fluorides is critical to formulating the reaction pathway and required composition and configuration of the rocksalt product.

Given these applications, we investigate the speciation of oxide and its existence within oxo-fluoro complexes in molten fluoride salts using electrochemistry. We present results of electroanalytical studies of oxides in molten 2LiF-BeF2 and 46.5LiF-11.5NaF-42KF, using cyclic voltammetry and SWV. We make connections between solute speciation and solvent structure, allowing for mutually enhanced understanding of solubility and fluoroacidity. Broadly, electroanalytical studies provide a deeper understanding of oxide’s speciation in molten fluoride systems which, in turn, informs electrochemical sensor development and electrochemical synthesis processes. By investigating the fundamental science of oxide and oxide complexes, we hope to advance technologies which enable the transition to clean energy.

[1] L. Massot, L. Cassayre, P. Chamelot, and P. Taxil, “On the use of electrochemical techniques to monitor free oxide content in molten fluoride media,” J. Electroanal. Chem., vol. 606, no. 1, pp. 17–23, Aug. 2007, doi: 10.1016/j.jelechem.2007.04.005.