The diffusion coefficient of Li in solid Si was determined by using a bipolar cell system in which a sheet of Si sample was sandwiched as a bipolar electrode between 2 facing bipolar cell compartments. Counter and reference electrodes were mounted in both of the cell compartments. In one of the cell compartments (cell A), an electrolyte containing Li+ was placed, while the facing compartments (cell B) were filled with an electrolyte containing Na+ instead of Li+, and a needle-like W/WOx electrode, which could sense the Li+ concentration, was mounted near to 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 cathodic polarization in cell-A, Li+ was incorporated into the bipolar electrode, migrating from the cell-A side to the cell-B side, and transferred to the electrolyte in cell-B. As a result, the signal of the Li+ sensing electrode began to increase after a delay which corresponded to the time required for Li to migrate in the bipolar electrode from the cell-A side to the cell-B side. From the value of the time delay, the diffusion coefficient could be calculated by the use of a diffusion equation in which the thickness of the bipolar electrode was involved. The obtained value of the diffusion coefficient of the vacuum-deposited film was 10−9cm2/s; regardless of the polarization potential. In contrast, a Si single crystal sample obtained by thinning a Si single crystal wafer gave about 2 x 10−11cm2/s. The larger value obtained with vacuum-deposited film was attributed to the loose structure of the deposited film.

Measurement of the Diffusion Rate of Li in Silicon by the Use of Bipolar Cells. K.Yoshimura, J.Suzuki, K.Sekine, T.Takamura: Journal of Power Sources, 2007, 174[2], 653-7