Chemical Evolution in Silicon–Graphite Composite Anodes Investigated by Vibrational Spectroscopy

Abstract

Silicon-graphite composites are under development for the next generation of high-capacity lithium-ion anodes, and vibrational spectroscopy is a powerful tool to identify the different mechanisms that contribute to performance loss. With alloy anodes, the underlying causes of cell failure are significantly different in half-cells with lithium metal counter electrodes compared to full cells with standard cathodes. However, most studies which take advantage of vibrational spectroscopy have only examined half-cells. In this work, a combination of FTIR and Raman spectroscopy describes several factors that lead to degradation in full pouch cells with LiNiMnCoO (NMC532) cathodes. The spectroscopic signatures evolve after longer term cycling compared to the initial formation cycles. Several side-reactions that consume lithium ions have clear FTIR signatures, and comparison to a library of reference compounds facilitates identification. Raman microspectroscopy combined with mapping shows that the composite anodes are not homogeneous but segregate into graphite-rich and silicon-rich phases. Lithiation does not proceed uniformly either. A basis analysis of Raman maps identifies electrochemically inactive regions of the anodes. The spectroscopic results presented here emphasize the importance of improving electrode processing and SEI stability to enable practical composite anodes with high silicon loadings.

Study of the Formation of a Solid Electrolyte Interphase (SEI) on a Silicon Nanowire Anode in Liquid Disiloxane Electrolyte with Nitrile End Groups for Lithium‐Ion Batteries

“…We deduce from the profile of the cycled TmdSx‐CN : LiTFSI that the SEI consists mainly of dissociated nitrile groups such as amine, amide. We propose that the nitrile groups are substituted by carbonyl groups since new vibrations associated with esters appear at 1773 cm −1 ,,, although, the C=O vibration at 1773 cm −1 might be due to soluble imide anion (TFSI − ) present in the disiloxane solvent . After examining the C−H range (Figure c), we further suggest the presence of silicon‐carbon species on the oxidised silicon surface (O x Si−O−CH 3 ) as the typical asymmetric stretch of the methyl group is present at 2990 cm −1 .…”

“…We propose that the nitrile groups are substituted by carbonyl groups since new vibrations associated with esters appear at 1773 cm À1 , [26,37,38] although, the C=O vibration at 1773 cm À1 might be due to soluble imide anion (TFSI À ) present in the disiloxane solvent. [39] After examining the CÀH range (Figure 3c), we further suggest the presence of silicon-carbon [40] species on the oxidised silicon surface (O x SiÀOÀCH 3 ) as the typical asymmetric stretch of the methyl group is present at 2990 cm À1 . [41] The presence of a moderate peak at 1598 cm À1 that corresponds to cyano-vinyl bonds, brings us to infer that the disiloxane bond (SiÀOÀSi) can break under silicon lithiation potentials.…”

“…[38] [44] We did not discern a poly (FEC) peak expected at 1805 cm À1 . [40,45] This might be explained by either a low signal-to-noise ratio as a relative high concentration of FEC (above 15 wt %) is needed to produce a prominent peak, or that the peak is enveloped by the broader and more intense asymmetric peak at about 1830 cm À1 . [46] We assign the peak at 1830 cm À1 to the carbonyl group of the FEC, as well as to RÀCF=O reduction products reported in the presence of lithium.…”

Statistical Raman spectroscopy characterization of carbon additive in low‐C composites: Toward industrial quality control

“…First, besides qualitative characterization of carbon layer in low‐C composites, the sRS provides semiquantitative characterization. Second, one can imply correlational analysis of the obtained Raman spectra parameters …”

“…Unfortunately, this original paper was undeservedly underestimated by the electrochemical society. Few years later, this work was followed by a series of papers utilizing similar statistical Raman microscopy/spectroscopy approach for carbon material structural characterization . In these papers, authors used advanced Raman spectra treatment with correlational analysis of major band parameters—band position, intensity, and full width at half maxima (FWHM).…”