In this study, we investigated the relaxation processes of LiMnPO4-based material after the termination of electrochemical insertion or extraction of lithium by using X-ray diffraction (XRD) and the Rietveld method. It was found that the relative mole fraction of Li-rich phase just after lithium insertion increased as the lithium insertion rate increased and the amount of Li-rich phase increased and that of Li-lean phase decreased with the relaxation time. It is considered that the Li-rich phase was preferable for lithium diffusion. Further, it is considered that Li-lean phase with excess lithium was formed and it changed to Li-rich phase with a low rate due to a high energy barrier at the lithium insertion process, and that Li-lean phase with excess lithium changed to Li-rich phase at the relaxation process. For lithium extraction, Li-rich phase increased and Li-lean phase decreased after lithium extraction. It is considered that the volume reduction transition from Li-rich phase to Li-lean phase even including excess lithium increased the lithium diffusion path outside the crystal and the transition was promoted for faster lithium diffusion at the lithium extraction process, and that Li-lean phase including excess lithium changed to Li-rich phase at the relaxation process. (C) 2013 Elsevier B.V. All rights reserved.