Lithiation of the crystalline silicon as analyzed using soft X-ray emission spectroscopy and windowless energy dispersive X-ray spectroscopy

There is no clear consensus on the exact compounds that are generated during the room temperature lithiation of Si, which may include amorphous LixSi or a crystalline Li15Si4 alloy. Here, the lithiation behavior of a single-crystal Si(1 1 1) anode was studied using scanning electron microscopy (SEM), soft X-ray emission spectroscopy (SEXS), and windowless energy dispersive X-ray spectroscopy (EDS). A Li-Si alloy generated electrochemically via potentiostatic lithiation was found to have a layered structure that contained the 1st layer of crystalline Li15Si4 (c-Li15Si4) alloy pyramids, the 2nd layer of amorphous Li13Si4 (a-Li13Si4) alloy, and the 3rd layer of Li-diffused LixSi alloy. A maximum cathodic current appeared during this process, and both the c-Li15Si4 and Li-diffused LixSi alloys were observed to grow during the initial lithiation and to maintain almost constant thicknesses prior to reaching the maximum current, while the 2nd layer began to grow after the maximum current. Comparing the experimental and theoretical currents showed that the lithiation of crystalline Si is diffusion-controlled. The low Li diffusion coefficients in the bulk LixSi alloy and bulk Si evidently result in a low current density during Si lithiation.