On the Lithiation Mechanism of Amorphous Silicon Electrodes in Li-Ion Batteries

Amorphous silicon is a high-capacity negative electrode material for use in advanced lithium-ion batteries. We investigated the mechanism of Li incorporation into and removal from this material during electrochemical lithiation and delithiation using a combination of in operando neutron reflectometry and ex situ secondary ion mass spectrometry. The results indicate that a heterogeneous lithiation mechanism is present for the first cycle and also for subsequent cycles during lithiation and delithiation, where a highly lithiated phase penetrates the silicon electrode. During the first lithiation half-cycle, a two-step process takes place, which is not present for delithiation and higher cycles. In the first step, a Li-poor phase penetrates the silicon electrode leading to about 10% of maximum capacity. Afterward, during the second step, a Li-rich phase moves into the electrode leading to complete lithiation in a slower process. The different phases are separated by a relatively sharp interface of only several nanometers. The Li-poor phase extended over the entire electrode is still present after delithiation in the form of irreversibly trapped Li.