Understanding the Formation and Composition of the Solid-Electrolyte Interphase at Silicon SurfacesSummary: Silicon materials have been proposed for use as electrodes in the next generation of lithium ion batteries. One reason is because silicon materials can incorporate a large amount of lithium (i.e. silicon has a high gravimetric energy density), theoretically 4.4 Li atoms for every Si atom. For comparison, conventional materials like graphite can incorporate 1 Li for every 6 C atoms. However, when silicon electrodes are lithiated, they form a solid-electrolyte interphase (SEI) at their surface. The SEI is formed from solvent and electrolytic salt that is electrochemicall reduced to oligomers and inorganic crystals on the silicon surfaces. The SEI acts as a barrier between the electrolyte solution and the electrode. Consequently, SEI plays a major role in how lithium ions move into the silicon (ionic transport and alloying kinetics) and how the electrons interact with the lithium (electron transport at the surface).
The SEI is compositionally very heterogeneous and results from multiple competing reactions that resist simple analysis and characterization. Furthermore, the solvents that react to form the SEI, and the SEI itself, are very sensitive to oxygen and water content, making anoxic and in situ techniques a necessity. The Webb group, in collaboration with the groups of Dr. Brian Korgel and Dr. Keith Stevenson, are developing new analytical and synthetic methods to understand the formation and composition of SEI on silicon. This collaboration aims to further the understanding of SEI formation reaction mechanisms and their dependence on the structure of silicon surfaces and electrochemical potential.
Strategy: The Webb group is developing in situ and anoxic techniques, including Fourier tranform infrared spectroscopy (FTIR), spectroscopic ellipsoetry, and X-ray photoelectron spectroscopy (XPS) to monitor surface chemistry of silicon materials at SEI. Advanced electrochemical technques and XPS metrology to limit oxygen exposure during sample prepration have lead to high-resolution measurements of SEI species composition. The development of semiconductor industry-standard polished materials allows us to explore well-defined single crystal surfaces and integrate the materials into spectroscopic measurements. For example, careful control studes and alighment makes it possible to use crystalline silicon both as an electrode and as a waveguide in attenuated total-internal reflection (ATR) surface analysis. In this way, surface chemistry can be correlated with the appearance of chemical functionalities. The high reflectivity of a polished wafer further allows for detailed ellipsometric studeis of SEI thickness and dielectric properties.
By comparing spectroscopic results for various electrochemical treatments to each other, control studies and values reported in the literature, we are determining which mechanism dominates at each surface and how this is altered by applied potential and surface chemical preparation.
People: Kjell Schroder
Publications: Schroder, K. W.; Dylla, A. G.; Bishop, L. D. C.; Kamilar, E. R.; Saunders, J.; Webb, L. J.;* Stevenson, K. S.* "The Effects of Solute-Solvent Hydrogen Bonding on Non-Aqueous Electrolyte Structure." J. Phys. Chem. Lett. 2015, 6, 2888-2891. pdf
Schroder, K.; Dylla, A.; Harris, S.; Webb, L. J.;* Stevenson, K. S.* "The Role of Surface Oxide in the Formation of Solid Electrolyte Interphases at Silicon Electrodes for Lithium-ion Batteries." ACS Appl. Mater. Interface. 2014, 6, 21510-21524. pdf
Schroder, K.; Celio, H.; Webb, L. J.;* Stevenson, K. S.* "Examining Solid Electrolyte Interphase Formation on Crystalline Silicon Electrodes: Influence of Electrochemical Preparation and Ambient Exposure Conditions. J. Phys. Chem. C 2012, 116, 19737-19747. pdf