Electrochemical Lithiation and Delithiation in Amorphous Si Thin Film Electrodes Studied by Operando X-ray Photoelectron Spectroscopy.

The electrochemical lithiation/delithiation in amorphous Si thin film electrodes deposited on a L 6.6 La 3 Zr 1.6 Ta 0.4 O 12 are dynamically analyzed by operando X-ray photoelectron spectroscopy. In the initial lithiation, the Si 2p peak corresponding to bulk Si 0 significantly shifts to a lower binding energy due to the formation of Li x Si and then monotonically with increasing capacity, i.e., the Li content x in Li x Si. When the lithiation stops at capacity of 2200 mAh g -1 ( x = ∼2.3), the peak recovers monotonically to a higher binding energy throughout the successive delithiation. When Li is inserted into Li x Si up to 3400 mAh g -1 ( x = 3.5), however, the peak drastically shifts in the capacity range of 1520-1920 mAh g -1 ( x = 1.6-2.0) in the successive delithiation. This shift is attributed to the phase transition of crystalline Li 15 Si 4 formed in the preceding lithiation to the amorphous phase. The mechanism of initial lithiation/delithiation at each step is summarized on the basis of the state of charge, Li content x in Li x Si, and positions of XPS peaks.