Operando X-Ray Absorption Spectroscopy of Solid Electrolyte Interphase Formation on Silicon Anodes

Abstract

Lithium-ion batteries (LIBs) are key to the transition from fossil fuels towards increased use of renewable energy sources. However, more widespread deployment requires improvements in energy density, cost and cycle-lifetime. Various cathode and anode materials are under consideration for next-generation LIBs, and the interfacial stability of these materials in contact with the electrolyte is a critical consideration. Interface-sensitive operando characterization techniques are thus urgently needed to reveal the reactions occurring in working batteries.1,2 The solid electrolyte interphase (SEI) that forms on Li-ion battery anodes is critical to their long-term performance, however observing SEI formation processes at the buried electrode-electrolyte interface is a significant challenge. Here we show that operando soft X-ray absorption spectroscopy in total electron yield mode can resolve the chemical evolution of the SEI during electrochemical formation in a Li-ion cell, with nm-scale interface sensitivity. O, F, and Si K-edge spectra, acquired as a function of potential, reveal when key reactions occur on high-capacity amorphous Si anodes cycled with and without fluoroethylene carbonate (FEC).3 Cross-referencing to cycling data, complementary bulk sensitive fluoresecent yield (FY) XAS measurements, and density functional theory (DFT) calculated spectra enables identification of the electrolyte and SEI species, and the dominant mechanisms of SEI formation. Without FEC present, LiF formation is detected at 0.6 V vs. Li/Li+ prior to significant lithiation of the a-Si, whilst at lower potentials the SEI grows in thickness with an increased contribution from organic components containing -C(=O)O- species. The observed sequential formation of inorganic and organic components is implicated in layering of the SEI. With FEC as an additive we see the onset of SEI formation at much higher potentials (1.0 V vs. Li/Li+), and attribute the improved cycle life seen with this additive to the rapid healing of SEI defects formed during delithiation. Operando TEY-XAS offers new insights into the formation mechanisms of electrode-electrolyte interphases and their stability for a wide variety of electrode materials and electrolyte formulations. References Wu et al. Phys. Chem. Chem. Phys. 2015, 17, 30229.Weatherup et al. Top. Catal. 2018, 61, 2085.Swallow et al. Nature Commun. 2022, 13, 6070.