After discharging to a certain capacity (120 mAh g -1 in our case), the reduced Bi/F composition and the O 2-agglomeration promote the formation of thermodynamically more stable bismuth oxidefluoride. The mechanism underlying BiOF formation will be discussed in detail later, along with additional in-situ analysis. However, this phase does not disappear at higher discharge states under the formation of more metallic Bi, which could be assigned to the strong O-Bi bond, relatively poor O 2-conductivity of composite or the accessibility loss from electron/F - conducting particles due to the volume change. Though a direct reduction from BiF3 to Bi metal cannot be entirely ruled out from ex-situ results, the phase transformation from o-BiF3 to c-BiF3 prior to the significant increase in Bi content is clearly revealed. The correlation between the two structures of BiF3 will be further discussed along with in-situ results.