The coexistence state of two phases (Li4Ti5O12 and lithiated Li7Ti5O12) in spinel lithium titanium oxide (LTO; Li4Ti5O12) is examined using scanning transmission electron microscopy with electron energy-loss spectroscopy (STEM-EELS). A single crystalline LTO specimen is prepared from a TiO2 wafer and is then partially lithiated. STEM-EELS spectrum imaging of the lithiated specimen reveals that the two phases exist separately inside the specimen with phase boundaries, and no apparent misfit strains or misorientations are detected. The phase interface appears to be preferentially formed parallel to the (100) plane owing to preferential Li diffusion along the < 011 > and equivalent directions in the LTO crystal. The observed two-phase morphology can be explained by the fact that relative to solid solutions, the Li7Ti5O12 phase can accommodate inserted Li atoms with negligible lattice distortions or volume changes in the common Ti-O bond framework. (C) 2014 Elsevier B.V. All rights reserved.