ChBE Seminar Series: Joseph A. Dura
Tuesday, March 3, 2015
11:00 a.m.-12:15 p.m.
Room 2108, Chemical and Nuclear Engineering Building
Professor Ganesh Sriram
Energy Storage Materials and Interfaces Characterized by In-Operando Neutron Reflectometry
Joseph A. Dura
NIST Center for Neutron Research
National Institute of Standards and Technology
The interfacial reactions that drive energy storage in batteries produce chemical and structural changes in the electrodes often creating new material layers on the surface. A full understanding of these interfacial processes requires detailed knowledge of the interfaces throughout their evolution. Neutron reflectometry, NR, is ideally suited for in-operando characterization of interface structures. It provides a depth profile of the scattering length density, SLD, (which is determined from the composition) with sub-angstrom precision. It is highly sensitive to light elements and in particular to Lithium and Hydrogen which have scattering lengths that vary considerably for different isotopes. Furthermore, since neutrons are weakly interacting they do not perturb the materials of interest, and robust in-operando NR sample environments can be designed to preserve the highly reactive and delicate battery structures during measurements at the applied potential of interest.
In one example that illustrates the capabilities of NR, we determine the SLD depth profile of the Solid Electrolyte Interface, SEI as a function of potential. The SEI is a layer that forms on electrodes in Li-ion batteries from the decomposition products of the electrolyte. An ideal SEI passivates the surface and prevents continued electrolyte decomposition, however in actuality, sustained growth of the SEI is one leading cause of capacity fade. Our in-operando studies reveal how the SEI evolves within a cycle.
The second example sheds light on a mechanism which may be used to improve the durability of Si high capacity anodes. This material is currently limited in practice by fracturation which occurs over many charge/discharge cycles and is caused by its high volume expansion. The lithiation and thickness of an amorphous Si anode was determined from in-operando NR at different states of charge for the first and sixth cycles. From this information we can infer that the evolution of the porosity is responsible for the non-linear volume expansion observed in the literature. The porosity helps to accommodate the volume expansion during lithiation in this study, and is reestablished upon delithiation. Scaling this accommodation approach to larger degrees of lithiation might promote stable Si anodes and enable higher capacity batteries.