In Figure , the relative weight fractions of Bi-containing phases, evaluated by Rietveld refinement, are plotted as a function of specific capacity. The evolution of the weight fractions on specific capacity can be described as follows: During the initial discharge process, the disappearance of orthorhombic BiF3 is accompanied by the formation of the intermediate cubic BiF3-x up to a capacity of 120 mAh g -1 , while the content of metallic Bi, though formed to a small extent initially, remains constant. At capacities exceeding 150 mAh g -1 , a tetragonal bismuth oxidefluoride phase (BiOF) is identified, with a weight fraction of ~5 wt%. The formation of BiOF has been observed during electrochemical fluorination process of Bi particles in the presence of Bi2O3 (~29 to 44 wt%) , and it has been reported that oxide presence significantly affects the behavior of fluoride conversion electrodes. In Section 3.1, the elemental analysis reveals that in our case the marginal oxygen content (~1.40 (17) wt%) in BiF3 cathode composites primarily originates from electrolyte during composite preparation and is distributed among particles of different phases, suggesting the correlation between the formation of BiOF and the oxygen impurity. The analysis of lattice parameters of t-BaSnF4 (shown in Figure , discussed in the last paragraph of section 3.2) indicates that, though it has not been reported that t-BaSnF4 is capable of conducting oxygen anions, O 2-ions diffusion along solid-solid interfaces and O-F exchange between electrolyte and BiF3 electrode material are considered possible. This will give the possibility of O 2-agglomeration at interfaces during defluorination.