Development of Composite Electrolytes Composed of La0.93Ba0.07F2.93 and a Polymer Electrolyte for Fluoride-Ion Batteries

Introduction Fluoride-ion batteries (FIBs), which based on a fluoride ion shuttle, have high theoretical energy densities surpassing Li-ion batteries. 1 In particular, all-solid-state FIBs with solid electrolytes are nonflammable and provide high safety. Various fluoride-ion conductors have been reported as candidates for the solid electrolyte of FIB. The tysonite-type LaF 3 doped with a few mol% BaF 2 (La 1-x Ba x F 3-x ) has a wide electrochemical window and high ionic conductivity exceeding 10 -5 S cm -1 in single crystal at room temperature. 2 Although La 1-x Ba x F 3-x exhibits high ionic conductivity in single crystals or sintered discs, it is reported that the conductivity of the cold pressed La 1-x Ba x F 3-x is reduced by about two orders of magnitude due to the grain boundary resistance. One way to reduce the grain boundary resistance is to fill the gaps between La 1-x Ba x F 3-x particles with a flexible polymer electrolyte to compensate for the fluoride-ion conduction path. In this study, we prepared composite electrolytes composed of La 1-x Ba x F 3-x and fluoride-ion conducting polymer electrolytes and investigated its conduction mechanism. Experimental The polymer electrolytes were prepared by mixing polyethylene oxide (PEO), tetramethylammonium fluoride (TMAF), and 2,4,6-triphenylboroxin (TPhBX) in a mortar, then hot pressing at 85 °C for 10 min. La 0.93 Ba 0.07 F 2.93 (LBF) was synthesized from LaF 3 and BaF 2 by a solid-state reaction method. Powders of LaF 3 and BaF 2 were mixed in a molar ratio of 93:7, and milled by a ball milling. The mixture was sintered at 900 °C for 6 h in a vacuum-sealed quartz tube. The composite electrolytes were prepared by mixing LBF with PEO, TMAF, and TPhBX in a mortar and hot pressing at 85 °C for 10 min. AC impedance measurements of the polymer electrolyte and the composite electrolyte were performed using gold foil as a current collector. Results & discussions First, we optimized the composition of the polymer electrolytes. Figure 1(a) shows Arrhenius plots of the polymer electrolytes prepared by the molar ratio of the ethylene oxide (EO) of PEO to TMAF at 20:1 and changing the molar ratio of TPhBX ( x ). The conductivity of the polymer electrolytes increased with the addition of TPhBX in the range of x = 0-0.4. This result indicates that TPhBX enhances dissociation of TMAF, resulting in an increase in charge carriers. On the other hand, the conductivity decreased at the composition of x = 0.6. To determine the optimal amount of PEO, the molar ratio of TMAF to TPhBX was at 1:0.4 and the molar ratio of EO ( y ) was changed to prepare the polymer electrolytes. The Arrhenius plots of the obtained polymer electrolytes were shown in Fig. 1(b). The polymer electrolyte with the composition of y = 25 was found to have the highest conductivity. From these results, the optimal composition of the polymer electrolyte was determined to be molar ratio of EO:TMAF:TPhBX = 25:1:0.4. Figure 1(c) shows the conductivity of the composite electrolytes prepared by combining LBF and the optimized polymer electrolyte at 25 ºC for various compositions. The lowest conductivity was obtained for the composite electrolyte with 40 wt% LBF content. Moreover, the composite electrolytes had lower conductivity than the polymer electrolytes and LBF at all compositions. These results were attributed to the high activation energy of the interface resistance between the polymer electrolyte and LBF, which prevents the migration of fluoride ions. Reference [1] M. A. Reddy, and M. Fichtner, J. Mater. Chem ., 21 (2011) 17059. [2] A. Roos, et al., Solid State Ionics , 13 (1984) 191. Acknowledgment This presentation is based on results obtained from a project, JPNP21006, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Figure 1