Interfacial Studies of the SEI Layer on a Tin Electrode

Development of new innovative experimental approaches and enabling methodologies to understand the function and mechanism of operation of materials, electrodes and electrochemical interfaces in electrical energy storage systems is critical for clean and/or renewable energy technologies. A better understanding of the underlying principles that govern these phenomena is inextricably linked with successful implementation of high energy density materials.

Several analytical techniques have been implemented for the physico-chemical characterization of the materials, interfaces and interphases. In many cases limitations in studying the real system are imposed by ex situ methods that require excitation or detection of electrons or ions in vacuum environment and/or they suffer from inadequate sensitivity, selectivity and specificity in the in situ environment. The advent of  near-field optical methods during the past decades has led to the development of new advanced techniques for chemical analysis. This presentation provides a brief overview of novel in and ex situexperimental approaches aimed at probing battery materials and electrodes in electrical storage systems at an atom, molecular or nanoparticulate level.

A strong understanding of the solid electrolyte interphase (SEI) (1,2) layer in lithium-ion batteries is necessary for further development of the technology, leading the path to application of high energy density intermetallic anodes However, the identification of chemical compounds in this surface layer, as well as their spatial distribution, has been limited by diffraction to the 5-10 μm wavelength scale available to traditional Fourier Transform Infrared spectroscopy (FTIR). This study demonstrates subwavelength imaging of the SEI using infrared (IR) near-field scanning optical microscopy (NSOM). NSOM (3) is a technique based on light confinement at the apex of an atomic force microscope (AFM) probe. The tight optical focusing combined with the placement of the probe within small fractions of a wavelength allow resolution well below the classical diffraction limit, while simultaneously obtaining topographic information about the sample surface and the present SEI. Additionally, this microscope is highly surface-sensitive, allowing selective optical imaging of the interfacial layers. The NSOM used here is configured as part of a pseudo-heterodyne interferometer, which allows high signal-to-noise ratio as well as collection of the optical phase.

The observed significant difference of Sn reactivity toward the electrolyte as a function of Sn surface crystalline orientation suggests radically different reaction paths, reduction products, and properties of the surface film. These results were supported by reactivity of polycrystalline tin that was falling in between the two single crystal surfaces. Our most recent measurements however shed new light on these results. We have identified that the native oxide layer present on tin exposed to air, and appreciable oxygen solubility in Sn have a major contribution to the electrochemical response of the Sn electrode. The presented experimental methodologies exploit the micro and nano-manipulation techniques and Sn single crystal model electrodes to provide sufficient sample definition suitable for advanced X-ray, far- and near-field Raman, FTIR and electrochemical characterization techniques. Examples of detailed in situmolecular characterization of electrode surface and bulk processes at the nano-level scale  limit will be discussed. These include elemental and molecular depth profiling at nanometer-scale depth resolution of a solid electrolyte interface (SEI) layer formed on Sn polycrystalline and monocrystral model electrodes in an organic carbonate-based electrolyte.

Acknowledgement

Part of this work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy, under contract no. DE-AC02-05CH11231. It has also been supported in part by the Northeastern Center for Chemical Energy Storage, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award number DESC0001294. This work was also supported in part by the Chemical Science Division, Office of Basic Energy Sciences, Office of Nuclear Nonproliferation, and the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

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