From Additive to Cosolvent: How Fluoroethylene Carbonate Concentrations Influence Solid–Electrolyte Interphase Properties and Electrochemical Performance of Si/Gr Anodes

Abstract

Silicon-containing anodes perform best when the solid–electrolyte interphase (SEI) accommodates the high volume changes of silicon particles, as this reduces side reactions and extends the cell lifetime. With this work, we investigate the influence of different fluoroethylene carbonate (FEC) electrolyte concentrations on the SEI composition and thickness and correlate these SEI properties to the electrochemical performance. Three electrolytes (i.e., 2FEC:98LP30, 20FEC:80DMC, and 50FEC:50DMC) are cycled with 9% Si/Gr anodes, and their SEIs are characterized postmortem using photoelectron spectroscopy (XPS). We propose a fitting model for the XPS results in which FEC decomposition yields −C–O, DO (1,3-dioxolan-2-one), −CO2Li, Li2CO3, and LiF. −C–O, DO, and −CO2Li are most probably incorporated in a cross-linked polymeric network. Due to its distinct chemical environments, detecting DO can be unambiguously linked to the presence of FEC decomposition products in the SEI. The presence of DO-type species in the C 1s spectra is correlated to the electrochemical performance: A higher retention in silicon activity was observed for the 20 and 50 vol % FEC-containing electrolytes, where FEC decomposition products (i.e., DO) were present even after 100 cycles. By contrast, when cycling in the 2FEC:98LP30 electrolyte, the silicon activity cannot be retained, and FEC decomposition products are barely detected after 100 cycles. We suggest that the presence of the −C–O-, DO-, and −CO2Li-containing polymeric network positively influences the SEI during silicon volume changes. Additionally, we show that the interaction of FEC and LiPF6 plays an important role in the formation of SiO x F y species.

Domino Reactions Enabling Sulfur-Mediated Gradient Interphases for High-Energy Lithium Batteries

“…47 This is the motivation for using FEC as the electrolyte additive for Si-based anodes. 48,49 The content of LiF is higher for the SE-based cell than for the RE-based one [e.g., 70.2% (SE) vs 54.6% (RE)], which signifies that elemental sulfur may accelerate the conversion of F-containing species into LiF.…”

“…Generally, the reductive decompositions of PF 6 – anions are more reluctant than FEC, owing to the higher chemical stability of the former species . This is the motivation for using FEC as the electrolyte additive for Si-based anodes. , The content of LiF is higher for the SE-based cell than for the RE-based one [e.g., 70.2% (SE) vs 54.6% (RE)], which signifies that elemental sulfur may accelerate the conversion of F-containing species into LiF.…”

Interphase formation with carboxylic acids as slurry additives for Si electrodes in Li-ion batteries. Part 2: a photoelectron spectroscopy study

“…One of the reaction pathways of FEC leads to polycarbonates (-[CO 3 -R] n -), e.g. poly-VC [55,56], which is indicated by the small signal at the highest binding energy (BE) at around 290.6 eV (dark yellow peak). The signal intensity remains about constant within a margin of 2% across different probing depths, indicating their comparatively even distribution throughout the SEI.…”

“…The F1s spectra recorded at the excitation energies of 1000 eV, 2360 eV and 7080 eV are shown in figure 9. The most dominant peak across all excitation energies is the characteristic LiF signal at 685 eV, independent of the electrode preparation, and originates from both the degradation of PF 6 -salt to LiF and alkyl fluorophosphates, and from FEC degradation processes [55,56,65]. It is therefore difficult to associate differences in the F1s spectra of the two electrode formulations directly to electrolyte salt degradation, as LiF is formed through several reactions.…”

Simultaneous Realization of Multilayer Interphases on a Ni-Rich NCM Cathode and a SiOx Anode by the Combination of Vinylene Carbonate with Lithium Difluoro(oxalato)borate

“…The interface of a Si-based anode is continuously exposed to the electrolyte, and various studies have attempted to develop effective strategies to ensure stability at the interface by using electrolyte additives suitable for Si-based anode materials. Note that the reductive potential of the carbonate-based conventional electrolyte is higher than the alloy reactions between Li + and Si-based anode materials, thereby leading to unavoidable side reactions of the electrolyte. , This issue can be overcome by using fluoroethylene carbonate (FEC), which can form a robust solid-electrolyte interphase (SEI) layer on Si-based anodes through electrochemical reduction. , When FEC undergoes electrochemical reductive decomposition, lithium fluoride (LiF)-based SEI layers are released at the Si interface. The LiF-based SEI layers possess high mechanical stiffness, which can efficiently control the pulverization of Si-based anode materials. , Moreover, LiF can effectively prevent the tunneling of electrons at the electrode interfaces based on its low electronic conductivity, thereby providing suitable electronic characteristics to the SEI layers .…”