Elucidating the SEI Formation and Composition As a Function of Binder on Si Anodes Using in Situ Neutron Reflectometry

Abstract

This talk will highlight the use of in situ neutron reflectometry (NR) to explore the effect polymeric binders used in composite slurries have on the composition and formation of the solid electrolyte interface (SEI) on amorphous silicon (aSi) anodes. Due to the high-volume expansion, upwards of 300%, a polymeric binder is needed for added mechanical strength; however, little is known about binder interactions with Si and its effect on SEI formation. Two different binder systems, polyacrylic acid (PAA), the most commonly used binder to date, and PEFM, an electronically conductive binder, were explored using NR at various potential steps. Thin film architectures were used to provide a model electrode surface to better probe interfacial reactions. In this case vapor deposited aSi films were spin coated with low weight percent polymer binder solution to create an aSi/binder interface. NR measurements indicate changes in both the polymer layer and resulting SEI as determined by a change in material scattering length density (SLD), a measure of the composition of a layer, during the initial lithiation stage. Ex situ techniques, in this case x-ray photoelectron spectroscopy, were used in conjunction with NR measurements to confirm models of the data. Preliminary results indicate the formation of a predominantly inorganic SEI layer. Neutrons interact with the nucleus of an atom, unlike x-rays, and as such scattering cross-section is not dependent on Z number. This allows for probing of lighter elements, especially Li, inherent to batteries. Because of their wave-like nature, neutrons follow Snell’s Law in which the beam is reflected and refracted at interfaces separating layers with different indices of refraction providing information on the thickness, roughness, and/or diffuseness of a layer and most importantly the composition described by the SLD. NR is an ideal tool to study interfaces, including buried such as those found in materials for energy storage. Acknowledgement: This research was supported by the Vehicle Technologies Office, Hybrid Electric Systems Program, David Howell (Manager), Battery R&D, Brian Cunningham and Peter Faguy (Technology Managers), at the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy. Neutron reflectometry measurements were carried out on the Liquids Reflectometer at the Spallation Neutron Source which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.