Herein, we measured the spatial distribution of the Li concentration and its mass transport processes in a porous graphite electrode in-situ. Insertion of Li into graphite changes the color of graphite by altering the visible light absorption; hence, we designed an optical electrochemical analog half-cell to enable direct real-time observations of the color change occurring during lithiation and delithiation. Using the relationship between the electrode color and Li concentration, we determined the distribution of the Li concentration in the electrode during intercalation. Our experimental results show that the distribution of the Li concentration was approximatively linear at the early stage of intercalation. As the Li content in the electrode increased, the distribution of the concentration became nonlinear and exhibited a ladder-like distribution owing to the effects of phase transformations. In addition, the time evolution of the Li concentration distribution curves was unsteady. On the basis of the distribution of the experimental concentration curves at different lithiation times, we determined the normalized overall lithium content within the electrode and its derivative over time (i.e., the Li mass transport rate). These results show that the overall lithium content increased nonlinearly over time. The Li transport rate decreased as the lithium content increased and fluctuated during phase transformation. On the basis of our experimental results, the effects of the Li concentration and the structural phase transformation on the Li transport rate are discussed. (C) 2018 Elsevier Ltd. All rights reserved.