Near-Field IR Nanoscale Imaging of the Solid Electrolyte Interphase on a HOPG Electrode

Abstract

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...

Solid Electrolyte Interphase Instability in Operating Lithium-Ion Batteries Unraveled by Enhanced-Raman Spectroscopy

“…16−18 More recently, tip-enhanced Raman spectroscopy (TERS) using silver or gold scanning nanoprobes demonstrated the ex situ mapping of the SEI composition distribution with nanoscale resolution on a non-SERS active electrode material (silicon anode 19 ) in a similar way as nanoinfrared mapping was achieved on tin anodes a few years before. 20 TERS, despite recent significant advances since its demonstration under operando conditions, 21,22 has not been yet implemented in a LIB electrolyte because of the difficulty to control the atmosphere and prevent the electrolyte evaporation in open cells. Alternatively, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) 23 using metal@SiO 2 core−shell nanoparticles as plasmonic signal amplifiers deposited on pristine electrode materials has been carried out to study chemical changes at various electrode/electrolyte interfaces upon cycling in a LIB electrolyte, including silicon 24 and carboncoated Zn 0.9 Fe 0.1 O anodes 25 and LiNi x Mn y Co 1−x−y O 2 (NMC) cathodes.…”

“…To circumvent this limitation, Raman signal enhancement techniques using the plasmon resonance properties of metal nanostructures were successfully implemented to extract compositions of surface films developed on various electrode materials upon contact with organic carbonate electrolytes. Surface-enhanced Raman spectroscopy (SERS) has been carried out operando on SERS-active silver and gold nanostructured electrodes starting from 2000. − More recently, tip-enhanced Raman spectroscopy (TERS) using silver or gold scanning nanoprobes demonstrated the ex situ mapping of the SEI composition distribution with nanoscale resolution on a non-SERS active electrode material (silicon anode) in a similar way as nanoinfrared mapping was achieved on tin anodes a few years before . TERS, despite recent significant advances since its demonstration under operando conditions, , has not been yet implemented in a LIB electrolyte because of the difficulty to control the atmosphere and prevent the electrolyte evaporation in open cells.…”

Functional Scanning Force Microscopy for Energy Nanodevices

“…The images and the chemical contrast allowed it to be identified that there existed a particulate top layer rich in Li 2 CO 3 and a flat base layer rich in EtOLi (Figure c,e). Interestingly, fine granular features were observed in chemical images (Figure d,f) but not in topographical images (Figure a,b), disclosing the advantage of high resolution IR‐aNSOM technique over standard AFM and conventional FTIR …”

“…Fine granular feature is marked with a small square. a–f) Reproduced with permission . Copyright 2015, Electrochemical Society.…”

Capturing electrochemical transformations by tip-enhanced Raman spectroscopy

“…In this context, the emerging Tip Enhanced Raman Spectroscopy (TERS), which associates near-field microscopies and vibrational spectroscopy, has attracted lately a considerable attention [28] because of its extraordinary sensitivity and acute spatial resolution without the need for ultra-high vacuum (UHV) and also because of the ease to work in aqueous and non-aqueous solvent contrary to NanoInfrared spectroscopies [29][30][31][32], opening the way to in situ, in vivo and in operando (EC-TERS) measurements.…”