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_6.6La₃Zr_1.6Ta_0.4O₁₂ are dynamically analyzed by operando X-ray photoelectron spectroscopy. In the initial lithiation, the Si 2p peak corresponding to bulk Si⁰ significantly shifts to a lower binding energy due to the formation of LiSi and then monotonically with increasing capacity, i.e., the Li content x in LiSi. 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 LiSi 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 Li₁₅Si₄ 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 LiSi, and positions of XPS peaks.