Tailoring of Liquid Electrolytes for Room-Temperature Fluoride Shuttle Batteries

Abstract

Fluoride ion shuttle batteries (FSBs), alternatively referred to as fluoride ion batteries (FIBs), are anion-based rechargeable batteries with fluoride (F-) ions as charge transporting and redox active species, and potentially offer a high electrochemical energy storage capability that overwhelms those of the state-of-the-art lithium ion batteries. The theoretical gravitational capacities for some typical metal/fluoride combinations, e.g., Ag/AgF, Cu/CuF2, Bi/BiF3, and Al/AlF3, amount to 249, 844, 385, and 2980 mAh/g, respectively. Nevertheless, despite some considerable efforts in the past decade or so, FSBs are still far from practical, because of a number of technical limitations to overcome no matter which type of FSB, all-solid type or liquid type, is of major interest. In particular, liquid-electrolyte based FSBs suitable for room-temperature cell operation require high-quality fluoride ion transporting electrolytes ensuring both high enough reactivity of the fluoride ions with the active material of interest and sufficient electrochemical stability of the electrolyte as a whole. Unfortunately, normally none of alkali fluorides are soluble in whatever high boiling battery-oriented organic solvents. One way to solve this problem has been to use organic fluoride salts. Another sophisticated way out has been to utilize a so-called anion acceptor (AA) that makes alkali fluorides substantially soluble even in low polarity solvents such as the glyme series. AA can also play another important role to facilitate the dissolution and re-deposition of the active materials into and from the electrolyte, which are unique pathways available in liquid-electrolyte based FSBs. However, the overall cell performances so far reported in the literature are still limited in capacity and cyclability. In this paper, we introduce lactone-based liquid electrolytes tailored for room-temperature FSBs, where either CsF or KF is dissociated without the help of any special addenda to offer strong electrolytes. Although the maximum fluoride ion concentration achieved by this method is around 0.05 M, it affords the ionic conductivity more than 0.5 mS/cm and the fluoride ions transported between the cathode and anode exhibit high reactivity with various metals to sustain reversible redox reactions to and from their fluoride counterparts. We have also achieved some successful battery operations using practical polymer-composite electrodes. Together with a superior cyclability, the thus tailored liquid fluoride electrolytes offer the opportunity to make a big step forward to establish the room-temperature rechargeable FSBs. Acknowledgement. This work was supported by the Research and Development Initiative for Scientific Innovation of New Generation Battery 2 (RISING2), financially supported by New Energy and Industrial Technology Development Organization (NEDO).