The correlation of the liquidus curves and valence electron structures of a ternary lithium halide molten–salt electrolyte for liquid metal batteries

Abstract

Liquid metal batteries have received considerable attention owing to their excellent properties. However, an electrolyte with low melting temperature is required to decrease operating temperature for the safety of liquid metal batteries and for saving energy. For revealing the mechanism of low liquefaction temperature, an empirical electron theory of solid molecules was used to study the thermal properties of pure lithium halides and their ternary-phase systems systematically. The theoretical bond lengths, melting points, liquefaction temperatures and mixed energies of pure lithium halides and their ternary phases match the experimental values well. The mechanism of liquefaction temperature for ternary lithium halides depends on their valence electron structures. The liquefaction temperature can be stabilized on a liquidus line or curve through the modulation of the constant number of covalent electrons (n_c) and lattice electrons (n_l). The liquefaction temperatures on various liquidus lines and curves are positively related to the linear density of valence electron pairs on the strong Li–X bond, bonding factor, and number of valence electrons in the s orbital but are negatively related to the number of valence electrons in the p orbital. With an increase in the linear density of the valence electron pair number and bonding factor, bond strength is enhanced, which increases the resistance of the strong Li–X bond against the break force induced by thermal phonon vibrations, and more thermal phonons with high vibrating energy are required for breaking the strongest Li–X bond at a higher temperature; therefore, the liquefaction temperature increases.