We performed an electrochemical investigation of the relaxation process of the LiMn0.75Fe0.25PO4 single-phase reaction after the termination of lithium extraction/insertion for various C rates and lithium contents using the relaxation analysis method. It was found that the lattice constants changed and the unit cell volume increased during the relaxation process. We propose that formation of the lithium defective domain containing the diffusing lithium with a small unit cell volume, through which lithium prefers to diffuse, is kinetically favored during the lithium extraction/insertion process and that the domain separates into the Li-rich phase and the domain without excess lithium during the relaxation process. (C) 2014 The Electrochemical Society. All rights reserved.