Furthermore, starting at a capacity of 150 mAh g -1 weak reflections at angles of 27.72° and 33.73° appear. These weak reflections can be assigned to tetragonal BiOF (space group P4/nmm ), and they persist with only slightly reductions in intensity even as the cell potential decreases to -775 mV. Throughout the defluorination, the reflections attributed to metallic Bi (and thus the corresponding weight fraction) increase continuously, with their strongest increase per discharge capacity at a specific capacity of around 120 mAh g -1 ; a numerical derivative of the weight fraction vs. capacity gives a slope around 0.64 wt% g (mAh) -1 at 120 mAh g 1 , which is far larger compared to what would be expected for the direct defluorination of o-BiF3 to form fully crystalline metallic Bi (100 wt%/302 mAh g -1 = 0.33 wt-% g (mAh) -1 ). Thus, the observed behavior of the reflections from different phases can only be explained if o-BiF3 does not transform directly to metallic Bi on the defluorination process, but that the defluorination is happening via a most likely defect rich cubic intermediate phase with composition BiF3-x, which is chemically different to the cubic phase initially present in the BiF3 composite (where the high symmetry is induced from the combination of oxygen incorporation and fluoride vacancies). This is contrary to a straight reconstructive transformation model between the most stable modification of BiF3 (Pnma) and metallic Bi (R-3m) .