As is common for studies of electrode materials for fluoride ion batteries, temperature stability of the BiF3 composite was examined by heating the composite to the cell's operating temperature in argon atmosphere for ~50 h and ~75 h (which corresponds to a typical cell operation time). It is indicated that the composite exhibits good thermal stability (see Figure 5 and Table S 4 for a comparison of diffraction patterns as well as structure information before and after heating). The observed changes are minor and fall within the margin of error as determined by Rietveld analysis.