Observing Real-Time SEI Formation on Silicon(-containing) Anodes By Operando ATR-FTIR Spectroscopy
Authors: Weiling Matthias , Lechtenfeld Christian , Pfeiffer Felix , Frankenstein Lars , Diddens Diddo , Wang Jian-Fen , Nowak Sascha , Baghernejad Masoud 
Published: December 2024

Increasing the silicon content of anodes is a promising approach to increase the energy density of lithium-ion batteries, due to its notably higher gravimetric capacity compared to state-of-the-art graphite. However, severe volume expansion during lithiation and volume contraction during delithiation cause particle pulverization. This leads to rapid capacity fading, short cycle-life, and ongoing electrolyte consumption, which is further accompanied by continuous electrolyte consumption for solid-electrolyte interphase (SEI) (re-)formation. In order to overcome the previous mentioned challenges, film forming electrolyte additives can be utilized to tune the interphase properties. In this study the effect of a sulfur-containing additive, 2-Sulfobenzoic anhydride (2-SBA) as an SEI-forming electrolyte additive was investigated through complementary theoretical, experimental, and operando techniques.[1]

With linear sweep voltammetry, supported by density functional theory calculations, indicate the reduction reaction of 2-SBA on the anode, which may lead to SEI formation. The electrochemical performance of high-voltage NMC811||AG+20%SiOx (4.5 – 2.8 V) multilayer pouch cells was investigated by means of galvanostatic charge/discharge cycling. These exhibit a lifetime of 65 cycles (until 80% state-of-health) with the baseline electrolyte (1 M LiPF6 in EC/EMC) without additives, whereas cells containing the optimizing electrolyte formulation with 2-SBA exhibit a cycle life of 105 cycles, corresponding to an increase of +60%. Furthermore, with 2-SBA the capacity retention of the cells is kept on a constant level of >99.9%, whereas a decreasing capacity retention is observed without 2-SBA. Further improvements of the cell cycle-life are achieved by the combination of 2-SBA with lithium difluorophosphate (LiDFP), a common high-voltage electrolyte additive, increasing the cycle-life to 165 cycles (+146%). Interestingly, LiDFP alone is not able to increase the cycle-life significantly (90 cycles, +38%), but also leads to lower and non-constant capacity retention. This indicates a beneficial purpose-selective approach, which is found to be superior to cells with fluoroethylene carbonate (63 cycles, -3%) or vinylene carbonate (116 cycles, +78%) as state-of-the-art electrolyte additives. Thus, the analysis indicates a beneficial effect of 2-SBA on SEI formation on silicon-containing anodes, which is further analyzed by operando attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy.

Operando ATR-FTIR spectroscopy is a powerful technique to study the composition of interphases, as the interfering signals of i.e. bulk electrolyte species are minimized.[2] The IR permeability of crystalline silicon enables probing the SEI with the internal reflection mode. Furthermore, depth-dependent spectra from 0.1 to about 2 μm can be obtained by variation of the incident angle of the IR beam, enabling depth-profiling of the SEI from the electrode towards bulk electrolyte.

Based on the obtained results we proposed an interphase model, where the 2-SBA additive is reduced, ring-opened, and accumulated on the silicon surface. By combination with an ion chromatography technique, different 2-SBA reaction products after reduction are identified, including products from electrolyte solvent degradation. Transferring this interphase model to composite electrodes containing graphite and SiOx particles, the identified organic decomposition products of 2-SBA can be considered beneficial to improve the cycle-life and performance of Si-containing anodes in high-voltage lithium-ion batteries.

[1] M. Weiling, C. Lechtenfeld, F. Pfeiffer, L. Frankenstein, D. Diddens, J.-F. Wang, S. Nowak, M. Baghernejad, Mechanistic Understanding of Additive Reductive Degradation and SEI Formation in High-Voltage NMC811||SiOx-Containing Cells via Operando ATR-FTIR Spectroscopy. Adv. Energy Mater. 2024, 14, 2303568. https://doi.org/10.1002/aenm.202303568

[2] M. Weiling, F. Pfeiffer, M. Baghernejad, Vibrational Spectroscopy Insight into the Electrode|electrolyte Interface/Interphase in Lithium Batteries. Adv. Energy Mater. 2022, 12, 2202504. https://doi.org/10.1002/aenm.202202504