Lithium diffusion in a small Li4/3Ti5/3O4 (LTO) particle was investigated from kinetic viewpoints of two-phase transition process based on a core-shell model by means of galvanostatic and potentiostatic measurements of thin LTO composite electrodes. High-rate galvanostatic charge (insertion) - discharge (extraction) properties of the thin LTO composite electrode showed that the insertion into the LTO particle was significantly slower than the extraction. An apparent chemical diffusion coefficient (D-app) of lithium in the LTO particle during the insertion and extraction was evaluated from the results of potential step chronoamperometry (PSCA) with a spherical finite diffusion model. The phase-boundary movements between the two phases in the cathodic and the anodic potential steps for a long-time region were controlled by lithium diffusion through Li7/3Ti5/3O4 rock-salt (LTO-rock-salt) and Li4/3Ti5/3O4 spinel (LTO-spinel) shell, respectively. D-app in the LTO-rock-salt and the LTO-spinel phase were estimated to be approximately 1 x 10(-12) cm(2)/s and 1.6 x 10(-11) cm(2)/s, respectively. The slower insertion was mainly due to a D-app value one order of magnitude smaller in the LTO-rock-salt than that in the LTO-spinel phase. Electrochemical kinetic properties of the LTO particle with the core-shell structure were interpreted by lithium diffusion through the LTO-rock-salt shell during the insertion and the LTO-spinel shell with the low electron conductivity during the extraction. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3574037]