Spectroscopic Insight into Li‐Ion Batteries during Operation: An Alternative Infrared Approach

Multiple‐internal‐reflection infrared spectroscopy allows for the study of thin‐film amorphous silicon electrodes in situ and in operando, in conditions typical of those used in Li‐ion batteries. It brings an enhanced sensitivity, and the attenuated‐total‐reflection geometry allows for the extraction of quantitative information. When electrodes are cycled in representative electrolytes, the simultaneously recorded infrared spectra give an insight into the solid/electrolyte interphase (SEI) composition. They also unravel the dynamic behavior of this SEI layer by quantitatively assessing its thickness, which increases during silicon lithiation and partially decreases during delithiation. Li‐ion solvation effects in the vicinity of the electrode indicate that lithium incorporation in the solid phase is the rate‐determining step of the electrochemical processes during lithiation. The lithiation of the active material also results in the irreversible consumption of a large quantity of hydrogen in the pristine material. Finally, the evolution of the electronic absorption of the electrode material suggests that lithium diffusion is much easier after the first lithiation than in the pristine material. Therefore, in situ Fourier‐transform infrared spectroscopy performed in a well‐suited configuration efficiently extracts original and quantitative pieces of information on the surface and bulk phenomena affecting Li‐ion electrodes during their operation in realistic conditions.