To verify this hypothesis, an elemental analysis of the BiF3 powder was conducted, revealing that the purchased BiF3 has an oxygen content of 0.78 (37) wt%. This indicates that the cubic BiF3 is primarily a defect BiF3-δ accompanied by small amounts of oxygen impurities. Furthermore, apart from t-BaSnF4 and BiF3 with two modifications identified, no other additional phases were observed in the as-prepared cathode composite. In addition, we also performed an elemental analysis of the as-prepared BiF3 cathode composite (see Table 2) as well as the individual other ingredients (t-BaSnF4, CNF as well as the pristine materials for electrolyte preparation). The analysis revealed an overall oxygen content in the cathode composite of 1.40 (17) wt%, which mainly originates from the solid electrolyte (1.08(4) wt% of oxygen content measured). The increased oxygen content in the cathode composite likely results from the electrolyte and carbon nanofibers during the milling process, suggesting that additional oxygen species (O 2-) are present in the phases in contact with the CAM BiF3.