Operando Analysis of Electrochemical Reactions in All-Solid-State Battery Using XPS

We develop an operando X-ray photoelectron spectroscopy apparatus equipped with a bias application sample holder and an air-tight sample transfer system and apply it to the electrochemical lithiation/delithiation reactions of an amorphous Si thin film electrode sputter-deposited on a Li6.6La3Zr1.6Ta0.4O12 solid electrolyte sheet. In the initial stage of the first lithiation, Si 2p peaks corresponding to bulk Si and suboxide drastically shifts to a lower binding energy due to the formation of Li-silicide and Li-silicate, respectively. The Li-silicide peak further shifts monotonically with increasing the capacity density (Li content in Li-silicide) while the Li-silicate peak remains unchanged. In the successive delithiation after charging up to ca. 2200 mAh g−1, the position of Li-silicide peak monotonically recovers to a higher binding energy. In the successive delithiation after charging up to ca. 3400 mAh g−1 where the crystalline Li15Si4 phase forms, however, a drastic shift of Li-silicide peak occurs in the capacity range of ca. 1500–1800 mAh g−1 where the crystalline Li15Si4 phase transforms to the amorphous phase.