Using in situ Neutron Reflectometry to Study Solid-­Electrolyte Interphase Formation in aSi and Sn Anode Materials

J. Kim, M. Doucet, G. Veith, J. Browning
Oak Ridge National Laboratory,
United States

Keywords: interfacial reactions, solid-electrolyte interphase, neutron reflectometry


Interfacial reactions between liquid electrolytes and electrode surfaces in energy storage devices are responsible for their long-term behavior and may mediate certain safety concerns regarding unwanted oxidation/reduction of the electrolyte. These reactions occur when an aprotic electrolyte is reduced, or oxidized, on the surface of an electrode at a given potential forming the so­‐called solid-­electrolyte interphase (SEI). These redox reactions form a passivating layer on the electrode surface that has been shown to be a mixture of inorganic and organic/polymeric species. A properly formed SEI prevents additional redox reactions from occurring and enables long term cycling. A poor SEI layer leads to safety issues, such as fires and gassing, as well as lifetime and power limitations due to consumption of electrolyte and the resistance of the SEI to both ionic and electronic transport. Understanding these reactions in situ is difficult since they occur at the liquid-­solid interface of optically absorbing materials that hinder the use of traditional spectroscopic techniques. Furthermore, since some interfaces involve liquids it is necessary to use an analytical technique that can “see” through structural materials required to contain the liquid. Neutron reflectometry (NR) is a neutron scattering technique highly sensitive to morphological and compositional changes occurring across surfaces and interfaces, including buried interfaces and those occurring at the boundary between a liquid and a solid. Neutrons, by virtue of their nature, are deeply penetrating and therefore ideally suited as a probe to study materials in complicated environments, such as electrochemical cells. NR can be used to study thin film morphology and composition over lengths scales extending 1 nm to hundreds-­of-nanometers. We will present results of the application of NR to the study of SEI formation on the high-capacity anode materials of aSi and Sn as a function of charge, electrolyte and electrolyte additives.