Analysis of the material composition and ionic conductivity of bismuth fluoride and barium tin fluoride and how these factors affect their application in fluoride-ion batteries

Abstract

In the coming energy transition, the availability of reliable and affordable energy storage will be of vital importance. Battery storage is a large factor in the energy storage sector, and current battery storage is dominated by lithium-based batteries. However, lately, alternatives to lithium have been given renewed attention, due to the insufficient abundance of lithium in the earth’s crust and the promising theoretical aspects of these other types of batteries. Fluoride-based batteries, for example, have high theoretical capacities and are theorized to be very suitable for application in solid-state battery technology. Fluoride-based batteries have only been produced on a lab-scale relatively recently, with the first reversible solid-state fluoride-ion battery produced in 2011, and the first room temperature reversible fluoride-ion battery produced in 2018, yet interest in this technology has drastically increased over the past few years. The current main issues fluoride- ion batteries are running into are its poor cyclability, its low current densities, and its inability to meet the theoretical capacities. In this report, multiple facets of fluoride-ion batteries have been analyzed in an effort to improve on these characteristics. In particular a focus has been placed on two commonly used materials in fluoride-ion batteries: electrolyte BaSnF4 and cathode material BiF3. The ionic conductivity dependence on pressure was deduced for each material, with BaSnF4 having its highest ionic conductivity at ∼280 MPa and BiF3 having its highest ionic conductivity at ∼680 MPa. To test the effect of oxides in BiF3, which form spontaneously when BiF3 is exposed to air or humidity, various Bi-O-F compounds were synthesized and had their ionic conductivity measured. It was found that each of the compounds that contained oxygen had a drastically lower (factor 1,000-10,000) ionic conductivity than pure BiF3. An attempt was also made to improve the ionic conductivity of BiF3 by doping that material with SnF2. BiF3 doped with SnF2 concentrations of 5-20% were synthesized, and had their ionic conductivity measured. It was found that the ionic conductivity was increased by dopant concentrations of 5% and 10%, with the material with 10% SnF2 having the highest ionic conductivity, while the materials with 15% and 20% SnF2 had a similar ionic conductivity to pure BiF3. Symmetric fluoride-ion batteries were also produced, with BiF3 as an electrode material and BaSnF4 as an electrolyte material. The produced batteries reached charge and discharge capacities with values of at most only 30% of the values reported in literature. The batteries also had poor capacity retention over multiple charge-discharge cycles. Batteries were also produced using BiF3 doped with 5% and 10% SnF2 as an electrode material. These batteries had higher initial capacities but had even poorer capacity retention over subsequent cycles. It was however also found that the critical current density for the batteries had increased as a result of doping with SnF2, allowing higher current densities to be applied to the batteries.