Monoclinic phase LiFeSO4F has a triplite like structure in which Li+/Fe2+ is fully mixing. Not 100% Li can be extracted from the lattice easily even at a rate of C/20, and the valence of Fe changes between ca +2 and +2.5 observed by XANES for a Li/LiFeSO4F cell cycled between 2.2 and 4.6 V. Two-phase reaction mechanism is verified by GITT due to the appearance of a flat plateau, and large polarization appears after more than 50% Li extracted from triplite-LiFeSO4F. A core–shell model has been mentioned to explain its extreme polarization. In the fully mixing structure, there is no intact long-range pathway for Li+, so leading to sluggish kinetics effect and “inert” Li+ in the lattice.