The SEI layer on graphitic carbon electrodes is well known to protect effectively the electrode from further electrolyte reduction during long-term charge-discharge cycling process. Many different techniques have been applied to characterize the chemical and structural composition of this complex surface film. The standard vibrational optical spectroscopies, which offer molecular-level information are subject to the diffraction limit, which restricts their ability to probe at the nanoscale level of the SEI building blocks. This work exploits infrared apertureless near-field microscopy that operates below the diffraction limit to characterize the SEI layer on a model HOPG electrode. Variations in surface topography and chemical contrast are discussed in the context of SEI composition and function. The solid electrolyte interphase (SEI) layer is integral to the functionality of Li-ion battery cells.1,2 It consists of products of cathodic reduction of the electrolyte at the negative electrode, which in stable battery systems is a self-limiting process.3 The SEI layer's stability determines Li-ion battery longevity, electrochemical performance and safety. By contrast, in unstable systems besides poor passivating properties and/or gradual dissolution, the SEI can be broken up by volumetric changes of the electrode active material during lithiation/delithiation. These detrimental phenomena cause the SEI to (re)grow continuously and/or reform during battery cycling, consuming the electrolyte, raising electrode impedance, shifting lithium inventory in the cell and reducing cell discharge capacity.Understanding what chemical components perform the key structural and chemical roles in functional SEI layers is prerequisite to the ability of stabilizing Li-ion electrodes that operate outside of the window of stability of the organic electrolyte. Numerous attempts have been made to characterize the structure and chemical composition of the SEI, involving a vast array of techniques, including but not limited to Fourier transform infrared spectroscopy (FTIR), 4 atomic force microscopy (AFM), 5,6 scanning electron microscopy (SEM), 7 X-ray techniques such as X-ray photoelectron spectroscopy (XPS) 8 and X-ray absorption spectroscopy (XAS), 9 Raman spectroscopy and microscopy, 10,11 and fluorescence spectroscopy. 12 Vibrational spectroscopies such as FTIR and Raman are particularly useful for the characterization of structures and molecular bonds to reconstruct the identity of chemical compounds in the layer.However, the spatial resolution of standard optical spectroscopic and imaging techniques is limited by diffraction to light beam sizes on the order of the wavelength of the source i.e., ∼10 μm for IR and ∼500 nm in the visible range. This often implies a lack of surface sensitivity, low spatial resolution in imaging, and spectral convolution in spectroscopy. Therefore a typical FTIR spectrum of an SEI layer consists of a multitude of overlapping peaks, which are difficult to deconvolute into independent spectra of constituent...
Beta-glucans are polysaccharides of D-glucose monomers linked by (1-3) betaglycosidic bonds, are found to have a potential immunogenicity risk in biotherapeutic products, and are labeled as process contaminants. A common source of beta-glucans is from the cellulose found in traditional depth filter media. Typically, beta-glucan impurities that leach into the product from the primary clarification depth filters can be removed by the subsequent bind-and-elute affinity chromatography capture step. Beta-glucans can also be removed by a bind-and-elute cation exchange chromatography step, which is useful for removing beta-glucans introduced by a post-Protein A depth filtration step. However, the increasing prevalence of flowthrough polishing chromatography poses a challenge for beta-glucan removal due to the lack of any bind-and-elute chromatography steps after the post-Protein A depth filter. In this work, a depth filter flush strategy was developed to control beta-glucan leaching into the product pool. Different loading conditions for the depth filtration and subsequent chromatography steps were evaluated to determine the robustness of the optimized flush strategy. Carry through runs demonstrated greater than twofold reduction in beta-glucan levels using the optimized wash as compared to standard filter flush conditions.
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