Enhancing the Chemical Stability of Fluoride-Ion Batteries

Abstract

Fluoride-ion batteries (FIBs) are a promising post-lithium-ion technology that has the potential to achieve energy densities of >600 Wh/kg at the stack level and use Earth-abundant materials.1 These characteristics make them ideally suited to meet the requirements of the green energy transition. The progress of FIBs, however, has been hindered by the lack of a suitable liquid electrolyte as well as a reliable and practical electrochemical testing configuration. The chemical compatibility of solvent, salts as well as other inactive components in the presence of the highly reactive F-ion is often overlooked, resulting in the generation of HF, HF2 - or other by-products. For example, polyvinylidene fluoride (PVDF) is still the standard binder used in FIBs, despite its well-known chemical instability in the presence of F-ions.2 Moreover, most liquid FIBs are tested using flooded cells, where the large excess of electrolyte used and, the absence of a separator often underplay more fundamental issues such as electrolyte degradation and capacity fading.In this work, we report a F-ion conducting liquid electrolyte with excellent chemical stability and good transport properties. We accomplish this, by combining an organic fluoride salt with an imidazolium-based ionic liquid, both free from β-hydrogens. We also adapt electrode dry-processing methods, widely used in lithium-ion batteries, to produce chemically stable Pb-PbF2 counter electrodes. Our results demonstrate that chemical stability and realistic testing conditions can be achieved in FIBs. Xiao, A.W., Galatolo, G., Pasta, M.; The Case for Fluoride-ion Batteries. Joule 2021, 5, 2823–2844Marshall, J.E.; On the Solubility and Stability of Polyvinylidene Fluoride. Polymers 2021, 13, 1354