The fundamental concepts and some successful examples of FSB systems that had been developed in the RISING battery project were addressed by one of the present co-authors (Z. Ogumi) in IBM2017 and IBA2018. We here focus on the recent advance in liquid-type FSBs that came out through the continued RISING2 battery project. It should be noted that none of alkali fluorides are soluble in common buttery-oriented organic solvents no matter how large static dielectric constants they have. Used as alternative electrolytes have thus typically been organic fluoride salts (such as alkylammonium or substituted alkylammonium fluorides) in combination with preferably ionic liquids. Another, more-or-less sophisticated, way out is to utilize so-called anion receptors that make even alkali fluorides substantially soluble in common organic solvents.

We here show nonaqueous plain liquid fluoride electrolytes, which, according to our comprehensive and careful analyses, are apparently free from any addenda but somehow solvated Cs⁺ (or K⁺) cations and F^- anions at a molar concentration of ~50 mM to give total ionic conductivity of typically ~0.8 mS/cm. The simple electrolytes exhibit versatile performance to make a variety of combinations of electrodes (ranging from Au/AuF₃ to at least Zn/ZnF₂) work reversibly in terms of the expected conversion reaction between the respective metal and its fluoride counterpart.

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).