Nano-FTIR Spectroscopy and Imaging of Electrochemical Interfaces in Li-Ion Batteries

Electrochemical interfaces are central to the function and performance of electrochemical energy storage devices. Thus, the development of new methods to characterize these interfaces, in conjunction with electrochemical performance, is essential for bridging the existing knowledge gaps and accelerating the development of energy storage technologies. These analytical hurdles related to sensitivity, specificity, selectivity and environmental control related to deployment of X-ray, electron, neutron, optical, NMR, and scanning probe methods stimulate the development of new experimental approaches to characterize electrochemical interfaces to overcome some subset of these challenges for a variety of specific materials and interface architectures. Of particular need is the ability to characterize surfaces or interfaces in a non-destructive way with adequate resolution to discern individual structural and chemical building blocks.<br/>Optical spectroscopy techniques such as Raman and Fourier transform infrared spectroscopy (FTIR) have been regarded as a gold standard for nondestructive chemical and structural fingerprinting of electrode materials and electrochemical interfaces. This is due to the relatively low energy of visible and IR light, and the techniquess sensitivity to changing electric dipole moments and/or polarizability, such as those in molecular and crystal lattice vibrations. Moreover, the vast majority of electrode and electrolyte materials are vibrationally active and possess a unique spectrum signature, thus optical spectroscopies are ubiquitous in both academia and industry. However, because of the relatively long wavelengths of VUS/IR light and related diffraction limit, the spatial resolution for optical techniques has been limited to <i>ca.</i> 1 - 1,000 micrometers. Thus, their utilization during the so-called “nano-revolution” during the last <i>ca.</i> 35 years has played an insignificant role in the characterization of nanostructures and associated nanoscale phenomena due to its inadequate spatial resolution.<br/>However, over the last decade, with the coalescence of scattering-type, scanning near-field optical microscopy (s-SNOM), high power broadband IR sources, IR interferometry, and lock-in amplification techniques, an emerging class of infrared near-field nanoimaging and nanospectroscopy (nano-FTIR methodologies has been realized to study electrochemical energy storage materials and interfaces, non-destructively, with nanoscale resolution, and in some cases, while within their native environment. To this end, sub-diffraction-limit low-energy optical probes that exploit near-field interactions, such as pseudoheterodyne imaging, photothermal AFM-IR, and nanoscale Fourier transform infrared spectroscopy, are powerful emerging techniques for electrochemical science and technology.<br/>Moving toward the characterization of electrochemically controlled surfaces and interfaces in echargeable battery materials and systems is critical for catalyzing advanced energy storage technologies. Most ecent efforts progressing to this end based on infrared near-field probes will be outlined. The working mechanisms and implementations of the scattering- and photothermal- types, and highlighted works in which these tools were employed to characterize energy storage materials, surface chemistry and structure, and interfaces and interphases will be discussed together with the key detection and processing steps involved in producing scattering-type near-field nanoscale Fourier transform infrared spectra. <i>In situ </i>and <i>operando </i> approaches by the integration of bulk electrochemistry and custom nanofabrication, with near-field IR nanoimaging and/or spectroscopy of the Si/electrode interface will also be described.