(Invited) Novel Fluoride Batteries, Fluoride Shuttle Batteries, and Their Positive Electrodes

Abstract

Progress of battery technology has opened new applications, which requires development of batteries of high performances, especially high energy density. There are two directions to improve the energy densities, i.e., enhancing cell voltage and increasing capacity of active materials. High cell voltage meets, however, the difficulty of stability of electrolyte.Batteries based on rocking-chair type reactions are limited in their capacity due to requirement of host lattice. On the other hand, conversion type active materials including traditional dissolution-deposition type ones can give higher capacity by using multi electron reactions. Anion mediating system may be able to be operated various metal electrodes using the common charge carrier. Among anion mediating systems, fluoride mediating systems, fluoride shuttle batteries (FSBs), give high capacity because metal fluoride is generally low formal weight.The authors’ group has focused on developing FSBs based on reactions of metal fluoride and metal, MFm/M. Metal fluoride are usually insulator, while some metal fluorides show ionic conductivity for F-. In this talk BiF3 and CuF2 are focused as positive active material. Electrolyte is a key material for activation of MFm/M electrodes. Solid electrolyte, liquid electrolyte including IL, and polymer electrolyte have been examined.Addition of anion acceptors to organic electrolyte enhances charge and discharge performances of BiF3/Bi electrode. The selection of adding anion acceptors is critical for each MFm/M electrode. The addition of anion acceptor, fluorobis(2,4,6-trimethylphenyl)borane, in tetragryme-CsF electrolyte increases remarkably the capacity and cyclability of BiF3/Bi electrode1). Copper fluoride is known as a typical insulator and when Cu electrode is oxidized to CuF2 only the skin-layer of Cu metal, ca. 6 nm, is oxidized to CuF2 and is passivated using lanthanum fluoride solid electrolyte operated at 140 oC. The addition of BaF2 to CuF2 powder activates CuF2/Cu electrode2). Konishi, T. Minato, T. Abe, Z. Ogumi, J. Electrochem. Soc., 2017 161, A3702.Morita, T. Abe, Z. Ogumi, et al., in preparation