Nano-FTIR Spectroscopy of the Solid Electrolyte Interphase Layer on a Thin-Film Silicon Li-ion Anode

Si anodes for Li-ion batteries are notorious for their large volume expansion during lithiation and the corresponding detrimental effects on cycle life, however, the calendar life is the main roadblock for widespread adoption. During the calendar life aging, the origin of the impedance increase and capacity fade is attributed to the instability of the solid electrolyte interphase (SEI).¹ In this work, nano-FTIR spectroscopy and XPS depth profiling is used to characterize the structure and composition of the SEI on Si thin film electrodes that underwent an accelerated calendar aging protocol.<br/>Nano-FTIR spectroscopy is a novel scanning probe technique in which a metallically coated atomic force microscope (AFM) tip is illuminated with a broadband infrared light source.² By operating the AFM in constant tapping mode, the background from the scattered light can be separated from the near-field response with lock in amplification. The result is FTIR spectra with high spatial resolution (ca. 20 nm) that match well with standard far-field and ATR-FTIR references.^2–5 Nano-FTIR is an ideal method to study the SEI because of its high spatial resolution, ultrahigh sensitivity, and specificity for identifying inorganic and organic SEI compounds.<br/>We first evaluate the structure and composition of the SEI after different washing procedures with dimethyl carbonate (DMC). We find that brief washing (&lt; 5 seconds) results in large chemical and topographic changes to the surface of the Si electrodes in comparison to non-washed electrodes. While the non-washed electrode is homogenous in terms of topography, IR imaging and nano-FTIR spectra, the washed electrode displays heterogeneity on multiple length scales demonstrating how brief washing can disrupt the delicate SEI layer and potentially result in misleading conclusions. This is further supported by XPS depth profiling experiments showing that the SEI layer thickness also decreases with increased washing time.<br/>After establishing the undesirable effects of washing on the SEI layer, we focus on characterizing the initial SEI formation (non-washed) to compare it to the SEI after the accelerated calendar aging protocol which invovles a 12 hour hold at 0.05 V. From the characterization on the initial SEI formation, we identify Li ethylene dicarbonate (LiEDC), polyethylene oxide (PEO) and LiF as the major components of the SEI layer. After the accelerated aging protocol, we find that the amount of PF₆^- reduction products is much higher than the initial SEI suggesting that there is accumulation of salt related decomposition products during the aging process. Based on this result, we propose that PF₆^- mass transport into the SEI layer is a major source of parasitic reactions and could be prevented by increasing the fraction of the inorganic scaffold (e.g. LiF) in the SEI or improving the Li⁺transference number of the polymer like SEI species.<br/><br/><b>Acknolwedgements</b><br/>This research was supported by the US Department of Energy (DOE)’s Vehicle Technologies Office under the Silicon Consortium Project directed by Brian Cunningham and managed by Anthony Burrell.<br/><br/><b>References</b><br/>(1) McBrayer, J. D. Calendar Aging of Silicon-Containing Batteries. <i>Nat Energy</i> <b>2021</b>, <i>6</i>, 866–872.<br/>(2) Huth, F.; Govyadinov, A.; Amarie, S.; Nuansing, W.; Keilmann, F.; Hillenbrand, R. Nano-FTIR Absorption Spectroscopy of Molecular Fingerprints at 20 Nm Spatial Resolution. <i>Nano Lett</i> <b>2012</b>, <i>12</i>, 3973–3978.<br/>(3) Mester, L.; Govyadinov, A. A.; Chen, S.; Goikoetxea, M.; Hillenbrand, R. Subsurface Chemical Nanoidentification by Nano-FTIR Spectroscopy. <i>Nat Commun</i> <b>2020</b>, <i>11</i>.<br/>(4) He, X.; Larson, J. M.; Bechtel, H. A.; Kostecki, R. In Situ Infrared Nanospectroscopy of the Local Processes at the Li/Polymer Electrolyte Interface. <i>Nat Commun</i> <b>2022</b>, <i>13</i>.<br/>(5) Lu, Y. H.; Larson, J. M.; Baskin, A.; Zhao, X.; Ashby, P. D.; Prendergast, D.; Bechtel, H. A.; Kostecki, R.; Salmeron, M. Infrared Nanospectroscopy at the Graphene-Electrolyte Interface. <i>Nano Lett</i> <b>2019</b>, <i>19</i>, 5388–5393.