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

Abstract

special environment for its characterization appear as steps which in most cases might strongly modify the active material. [ 1 ] Benefi ting from characterization techniques able to analyze the electrodes in situ and in operando therefore appears of uttermost importance for obtaining information of direct relevance regarding the electrode processes. We show in the present work that such pieces of information can be obtained by using in situ Fourier-transform infrared (FTIR) spectroscopy in a suitable geometry, which avoids the traditional limitations associated with this technique and allows for quantitative information to be extracted. [ 2 ] This approach therefore brings significant complementary information to other useful techniques operating in the electrochemical environment, which focus either on electrode materials, such as nuclear magnetic resonance (NMR) [ 3,4 ] or X-ray absorption, diffraction or imaging, [ 5,6 ] or on electrode surfaces, such as scanning probe microscopies. [ 7,8 ] Using infrared spectroscopy at the electrochemical interface has already been attempted in the context of the study of battery electrodes. [9][10][11] The geometry adopted for these studies was however severely limiting the sensitivity, and imposing stringent limitations for the electrochemical conditions (especially in terms of mass-transport limitations), which have direct impact on the obtained results. [ 12 ] Here, we use an alternative approach based on the multiple-internal-refl ection geometry, which allows for benefi ting from an enhanced sensitivity and working in electrochemical conditions similar to those of standard batteries for studying thin-fi lm electrodes. As shown hereafter, such a capability provides useful information of direct relevance for both interfacial and bulk electrode processes.The present study has been devoted to silicon electrodes, studied in a half-cell confi guration against a lithium-metal electrode. Silicon represents an impressive gain in energy density for negative electrodes in Li-ion batteries. One strategy to overcome the poor capacity retention associated with the massive volume expansion of silicon during lithiation is the use of nanosized geometries, for instance, particles, [13][14][15][16] wires, [17][18][19] and tubes. [ 20,21 ] The use of a thin-fi lm geometry also prevents Multiple-internal-refl ection infrared spectroscopy allows for the study of thin-fi lm 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-refl ection 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 ...

Lithiation Mechanism of Methylated Amorphous Silicon Unveiled by Operando ATR‐FTIR Spectroscopy

“…[19] This twophase picture has been evidenced by microscopic investigations on silicon nanowires. The "hysteresis" behavior during the first cycle has been explained as follows: the gradual increase of the IR absorbance during the first lithiation is consistent with the formation of a heavily lithiated phase progressively invading the silicon layer at a constant (current driven) speed.…”

“…[19], the large IR background growing at high energy (i.e., high wavenumbers) is not a Drude-like absorption like in metals and it is not consistent with a light scattering process. [19], the large IR background growing at high energy (i.e., high wavenumbers) is not a Drude-like absorption like in metals and it is not consistent with a light scattering process.…”

“…At low wavenumbers (≈1000-2000 cm −1 ) and as discussed elsewhere, they exhibit several peaks ascribed to the solid electrolyte interphase (SEI) layer and electrolyte absorption. [19], the baseline of the IR spectra gradually increases during the first lithiation, whereas it sharply decreases during the subsequent delithiation. [19], the baseline of the IR spectra gradually increases during the first lithiation, whereas it sharply decreases during the subsequent delithiation.…”

“…[19] Operando measurements were carried out with a homemade program for synchronization of the IR measurements with the GCPL measurement. Galvanostatic cycling with potential limitation (GCPL) was performed with a home-made galvanostat.…”

“…One way to increase the energy density is to use Li alloys. [19] We use here this approach in which the active material is directly deposited as a homogeneous thin film on the IR window, which makes the observation of changes inside the active material possible while controlling the lithium incorporation. [3] Among these materials, silicon has the highest theoretical specific capacity (3580 mA h g −1 for Li 3.75 Si).…”