Light anions swim upstream
A molten-salt secondary battery.
Rudolf Simon, CC BY-SA 3.0
When table salt dissolves in water, the dissociated ions behave as an electrolyte: An applied electric field pushes sodium ions toward the negative electrode and chloride ions toward the positive one, thereby producing a net current. Logic says that adding more ions should generate a proportional increase in the current, and that’s true—to a point. At sufficiently high ion concentrations, interactions between the charged particles reduce ion mobilities, which results in lower-than-expected conductivities.
Compared with their solvated counterparts, ions in molten salts are densely packed. That closeness makes them particularly susceptible to anomalous behavior caused by interactions with each other. A 2018 experimental study found that in such systems, ions can even travel the wrong way in an electric field—a particularly undesirable quality for a material that’s used to facilitate charge transport in batteries. Now Marie-Madeleine Walz and David van der Spoel at Uppsala University in Sweden have used molecular-dynamics simulations to visualize ion transport in molten salts. Their results help explain the microscopic origins of backward-traveling ions.
The researchers simulated lithium halide salts with three different anions: fluoride, chloride, and iodide. In each individual melt, the cation traveled along the direction of an applied electric field and the anion traveled against it. Although transient bonds formed between the two species, they weren’t strong enough to reverse the direction of travel.
When the salts were mixed (LiF–LiCl–LiIeut), however, the lifetimes of the Li+–Cl− and Li+–F− bonds increased—from 1.7 to 2.4 ps and from 1.5 to 4.3 ps, respectively, at 1200 K. Those longer lifetimes indicate an increased interaction strength, which enabled the Li+ cations to drag the Cl− and F− anions in the direction of the electric field. The Li+–I− bond lifetime decreased from 2.2 to 2.0 ps with mixing, so the heavier I− ions still traveled toward the positive electrode. Each ion’s contribution to the overall conductivity was consistent with Sundheim’s golden rule, which is based on the law of conservation of momentum.
The simulations from Walz and van der Spoel highlight the subtle connections between electric mobility, number density, and composition in molten salts. Accounting for those relationships can help optimize an electrolyte’s conductivity for battery applications. (M.-M. Walz, D. van der Spoel, Commun. Chem. 4, 9, 2021 .)
