Due to the lack of diffusion data of Li in solid silicon at room temperature, we decided to determine the diffusion coefficient of Li in solid silicon by the use of a bipolar cell system where a sheet of silicon sample is sandwiched as the bipolar electrode between two facing bipolar cell compartments. In both of the cell compartments counter and reference electrodes were mounted. In one of the cell compartments (Cell A), an electrolyte containing Li+ was filled, while the facing compartments (Cell B) was filled with an electrolyte containing Na+ instead of Li+, and a needle-like W/WOx electrode, which can sense the Li+ concentration, was mounted near the surface of the bipolar electrode. At first the potential of the bipolar electrode in Cell A was set at the natural potential, and the potential in Cell B was polarized at a sufficiently positive potential, then the potential in Cell A was set at a definite cathodic potential and the signal-time curve of the W/WOx electrode in Cell B was recorded. During the cathodic polarization in Cell A Li+ was incorporated in the bipolar electrode, migrating from the Cell A side to the Cell B side, and transferred to the electrolyte in Cell B compartment. As a result, the signal of the Li+ sensing electrode began to increase after a time delay which corresponds to the time for Li to migrate in the bipolar electrode from the Cell A side to the Cell B side. From the value of time delay we could calculate the diffusion coefficient by the use of a diffusion equation where a parameter of the thickness of the bipolar electrode is involved. The obtained value of the diffusion coefficient of the vacuum-deposited film was 10(-9) cm(2) s(-1) independent of the polarization potential. In contrast, the silicon single crystal sample obtained by thinning the silicon single crystal wafer gave about 2 x 10(-11) cm(2) s(-1). The larger value obtained with the vacuum-deposited film is attributed to the loose structure of the deposited film. (C) 2007 Elsevier B.V. All rights reserved.