(Invited) Spectroscopic and Microscopic Characterizations of Silicon-Electrolyte Interfacial Chemistry and Silicon-Based Electrode Aging Behaviors

Abstract

Due to inherent properties of the silicon (Si)-electrolyte interphase (SEI)—complexity, high reactivity and continuous evolution—it remains a poorly understood topic in advanced Si-based Li-ion battery (LiB) research,1,2 and its detailed and real-time analysis is a great challenge. Vibrational spectroscopy, such as Raman and Fourier-transform infrared spectroscopy (FTIR), is one of the most important analytical tools for understanding and quantifying the interfacial chemical reactions. These techniques are used extensively by the battery community in their ex situ form, but developing in situ variants of these techniques and combing in situ analyses from both techniques can provide new insight and help to elucidate the mechanism of interfacial failure in battery systems. We have optimized in situ vibrational spectroscopy methods that show reproducible and stable performance over multiple cycles in terms of both electrochemistry and spectroscopy, which enables us to monitor the surface chemistry and evolution of the SEI. This is essential to gaining a better understanding of the electrochemical performance of Si-based LiBs.3-5 Further, the reactions leading to SEI growth and evolution in Si-based LiBs may be more comprehensively probed by monitoring changes in the gas and liquid phases. To this end, we are developing novel in situ gas chromatography (GC) techniques, which may be applied to both coin-type cells and larger-format pouch cells. Using custom cell designs, we track the evolution of electrolyte and degradation species in the gaseous and liquid6 phases at various stages of battery cycling. We achieve both qualitative identification and quantification through coupled mass-spectrometry (MS) and flame ionization detection (FID). This GC-MS-FID analysis complements our in situ vibrational spectroscopy studies, providing a true multi-modal and multi-phase approach to SEI analysis. Taken together, these studies can provide an in-depth understanding of the underlying chemistry and physics and mechanical explanation of various reactions/interactions within the SEI, thus enabling the rational design of electrolytes and electrodes to stabilize the SEI.Additionally, we have developed advanced techniques to assess the structural and electronic properties of Si-based composite electrodes. Comprehensive characterization of pristine and cycled composite electrode architectures is complicated by several inherent properties. During battery operation, Si-based composite electrodes experience localized lithiation and reaction rates due to heterogeneous conditions and distributions of electrode components. Further, (electro)chemical and structural properties continuously evolve during cycling. Finally, the nanoscale diameter of particles falls beneath the lateral and depth resolution of most laboratory-based instruments. To address these challenges, we have utilized scanning spreading resistance microscopy (SSRM) to image the three-dimensional nanostructure of composite anodes via contrast in the electronic properties of the distinct components.7 Specifically, Si-based composite anode components, such as SiOx nanoparticles, graphite, carbon black, and lithium polyacrylate binder, are all readily distinguished by their intrinsic electronic properties, with measured electronic resistivity closely matching known material properties. In combination with scanning electron microscopy/energy dispersive X-ray (SEM-EDX) and electron energy loss spectroscopy (EELS), we expect to compare the electrical, chemical and structural changes of Si-based composite electrodes before and after cycling. This can provide a fundamental understanding of localized degradation mechanisms, which informs our understanding of SEI formation and evolution. This technique may also be applied to assess heterogeneous aging within the electrode, and is more broadly applicable to other battery systems. In particular, it may be used to better understand particle dispersion, localized lithiation, electrolyte-electrode interphase formation/evolution, and degradation processes in composite electrodes for the development of next-generation batteries.