Image courtesy of Brett L. Lucht, University of Rhode Island, email@example.com
Schematic figure of solid-electrolyte interphase (SEI) layer formed during the first charging cycle illustrating a interphase layer comprised predominantly of a mixture of lithiumethylene dicarbonate (LEDC) and lithium fluoride (LiF).
Two in-situ experimental techniques were used to unambiguously identify the structure and chemistry of the solid electrolyte interphase (SEI) in a model lithium (Li) ion battery system. This approach allowed differences in the SEI thickness and its uniformity as well as the chemical composition of different phases to be determined, crucial information for the control and understanding of this important microstructural feature for Li-ion batteries.
SEI formation is one of the most important and fundamental reactions for Li-ion batteries and is critical to reversible cycling performance. A better understanding of the structure and function of the SEI would likely lead to improved Li-ion battery cycle life and safety.
Interest in lithium ion (Li-ion) batteries for consumer electronic devices has been steadily increasing over the last two decades. However, recent interest in their use in electric vehicles has spawned a new wave of research and development as performance limitations still exist with regard to their cycle life, safety, and cost. A critical component of Li-ion batteries is the solid electrolyte interphase or SEI, a complex layer that forms from the decomposition products from the battery’s electrolyte, the substance in batteries that acts as a medium to conduct lithium ions between electrodes. The role of the SEI has been studied for decades, but the exact details of the SEI composition, formation mechanism, and mechanism of function are still not fully understood because the layer is thin (10−100 nm) and very sensitive to water and oxygen. In addition, the components of the SEI have structural similarity to the components of the electrolyte, making spectroscopic discrimination difficult. To overcome these difficulties, researchers from the University of Rhode Island have combined a novel microscopy technique with multi-nuclear magnetic resonance (NMR) to analyze the structure and composition of the SEI. The first is a transmission electron microscopy (TEM) energy dispersive X-ray spectroscopy (EDX) method with the TEM grid integrated into the electrode. This affords simultaneous, in-situ structural and chemical information. The second method utilizes NMR spectroscopy to make the unambiguous identification of chemical compounds. To demonstrate the utility of combining the two techniques, the structure and composition of the SEI was investigated in several model systems in which the electrochemical cells were cycled with LiPF6, a common electrolyte, and one or two additional solvents or their blends. This approach allowed assessment of the SEI thickness, its uniformity, and the chemical composition of different phases. While many results suggest that upon additional cycling and aging, the structure, composition, and thickness of the anode SEI change, these experiments show the initial composition could be relatively simpler than earlier believed. The information contained in this study provides computational investigators an experimental basis for modeling the SEI structure and its fundamental electrochemical properties.
University of Rhode Island
Department of Energy Office of Science, Basic Energy Sciences, EPSCoR Implementation award
M. Nie, D. Chalasani, D. P. Abraham, Y. Chen, A. Bose and B. L. Lucht, "Lithium ion battery graphite solid electrolyte interphase (SEI) revealed by microscopy and spectroscopy." J. Phys. Chem. C. 117, 1257-1267 (2013).
University, DOE Laboratory