Rate-Determining Process at Electrode/Electrolyte Interfaces for All-Solid-State Fluoride-Ion Batteries

Room-temperature reversible F-ion batteries based on sulfone electrolytes with a mild anion acceptor additive

Rechargeable fluoride ion batteries (FIBs) as an emerging anion shuttle system are attracting much attention due to their potential advantages in terms of energy density, cost and safety. A liquid electrolyte system enables the FIB operation at low or room temperature due to its higher ionic conductivity than that of a solid F-ion electrolyte. However, the insolubility of fluoride salts in aprotic solvents limits the development of liquid F-ion electrolytes. Although the boron-based anion acceptors (AAs) can facilitate the dissolution of F-ion salts, they are prone to lead to a tough desolvation process for F- due to strong Lewis acidity and therefore an inferior electrochemical performance. Here, a new non-boron AA (6-thioguanine) with moderate Lewis acidity is proposed to dissolve F- in the sulfone solvent. The ionic conductivity of the corresponding electrolytes reaches a level of mS cm-1 at room temperature. A model FIB coin cell is successfully operated with high conversion reaction reversibility based on the coupled defluorination/fluorination mechanism of electrodes, enabling a low overpotential of 0.36 V and a reversible capacity of 126 mA h g-1 after 40 cycles.

Challenges for fluoride superionic conductors: fundamentals, design, and applications

Electronics, which harnesses the properties of electrons, has made remarkable progress since its inception and is a cornerstone of modern society. Ionics, which exploits the properties of ions, has also had a profound impact, as demonstrated by the award of the Nobel Prize in Chemistry in 2019 for achievements related to lithium-ion batteries (LIBs). Ionic conduction in solids is the flow of carrier ions through a solid owing to an electrical or chemical bias. Some ionic materials have been studied intensively because their ionic conductivities are higher than those of liquids, even though they are solids. Among various conductive species, fluoride ions are the most promising charge carriers for fluoride-ion batteries (FIBs) as post LIBs. Increasing fluoride-ion conductivity toward the superionic conductive region at room temperature would be a breakthrough for the room-temperature operation of all-solid-state FIBs. This review focuses on fluoride-ion conductors, from the general concept of ions to the characteristics of fluoride ions. Fluoride-ion conductors are classified according to material type and form, and our current understanding, identification of problems, and future directions are discussed from experimental and theoretical physics perspectives.