First, the formation of another intermediate phase as a result of fluoride ions extraction was observed. This phase is metastable and cannot be observed at lower temperature (room temperature for ex-situ XRD); its nature was derived from the changes of the XRD patterns described in the following: Beyond a capacity of 75 mAh g -1 two shoulders at 2-theta of 29.2° and 31°-32° appear (marked with dashed black rectangles in Figure ). This position is around the (200) reflection of c-BiF3, which decreases in intensity once the additional reflections appear (below 75 mAh g -1 , it was only observed to shift its position from 30.58° to 30.70° upon defluorination). These additional reflections increase in intensity until a capacity of 125 mAh g -1 , after which these signals decrease in intensity and they completely disappear at ~250 mAh g -1 . Once these new additions decrease and disappear, the formation of t-BiOF with its indicator reflections at ~27.7° and ~33.4° are observed.