Multimodal evaluation of the lithium ion battery solid electrolyte interphase: quantifying elementary chemistry via in operando neutron reflectivity and electrochemical quartz crystal microbalance

Abstract

The solid electrolyte interphase (SEI) is an interfacial layer that forms in lithium-ion batteries due to instability of the electrolyte at low voltages. While the SEI in theory passivates the electrolyte from further degradation, in reality continued SEI growth shortens battery lifetime and leads to myriad problems, including capacity fade, reduced charge/discharge rates, and contributions to thermal runaway and catastrophic battery fires. Despite years of study, the SEI remains a central challenge to lithium-ion battery durability, largely due to poor understanding of the basic chemistry responsible for its formation and evolution. Neutron reflectometry (NR) is a promising technique that is able to characterize the SEI layer under a range of electrochemical and thermal conditions. Neutrons can penetrate many sample environments with little attenuation, are sensitive to isotopes of elements such as Li and H, and impart little ionizing or thermal energy. Furthermore, NR provides sub-Ångström sensitivity for the layer thickness, making NR perhaps the perfect in operando probe for enhancing the understanding of SEI chemistry. Although initial NR studies of the SEI show great sensitivity to the chemistry and thickness of the SEI, additional sensitivity is required to provide true quantitative insight into phenomena such as the layered ordering of the SEI, its evolution as a function of electric potential, and its thermal degradation at elevated temperatures. This study extends neutron reflectometry experiments published in 2012 on the SEI by improving the ability of the technique to quantify the chemistry and structure of the SEI. These improvements include fabrication and optimization of non-intercalating anodes for enhanced NR contrast and the use of complementary electrochemical measurements and mass uptake measurements via quartz crystal microbalance with dissipation (QCM-D). In this study, an SEI is grown on a non-intercalating, thin-film W anode via CV, and its properties are measured in operando by parallel NR and QCM-D measurements to create a depth profile of the SEI chemical composition. A dual-layer SEI structure is observed, which has been proposed in literature based on other techniques but has never previously been observed in operando via NR. The NR data and QCM-D data are then combined to create a proposed chemical composition as well as physical structure, which shows an inner layer (adjacent to the electrode) that is composed mainly of Li2O (25%), LiF (20%), LiOH (27%), Li2CO3 (15.5%), and electrolyte (12.5%), and an outer layer (adjacent to the electrolyte) of LEDC (51%), LEC (6%), and electrolyte (43%). This study not only opens the way for highly sensitive NR measurements, it allows for a detailed quantitative understanding of the SEI composition and formation chemistry, which has previously not been well explored.