Reduction Mechanism of Fluoroethylene Carbonate for Stable Solid Electrolyte Interphase Film on Silicon Anode

Silicon (Si) is one of the most promising anode materials for Li‑ion batteries because of its very high specific capacity (4212 mAh/g for Li₂₁Si₅).¹ However, the high specific capacity of silicon is associated with large volume changes (more than 300 percent) when alloyed with lithium. These extreme volume changes can cause severe cracking and disintegration of the electrode and lead to capacity loss. The degradation of the carbonate solvents or the continuous growth of the Solid Electrolyte Interphase (SEI) film during cycling also results in significant irreversible capacity loss because it decreases electrochemical reactivity of electrodes.

Fluoroethylene carbonate (FEC) is a well-known electrolyte additive for silicon anodes which can significantly enhance the stability and flexibility of SEI film and improve the stability and Coulombic efficiency of silicon‑based anodes.^{2, 3} Although FEC has demonstrated significant effect on the electrochemical performance of silicon‑based anodes, the fundamental mechanisms of this effect are still controversial in the literature. One possible reduction mechanism of FEC on silicon-based anodes is that the FEC first converts into vinylene carbonate (VC) through defluorination and then the resultant VC polymerizes.^{2, 4, 5} The main products from this mechanism are LiF and poly(vinylene carbonate). Another possible reduction mechanism is that the FEC is reduced through opening the five-member ring to form a (CH₂CHFOCO₂Li)₂ dimer.⁶ According to these two mechanisms, the FEC reduction products formed in the SEI film should be LiF and poly(vinylene carbonate) or (CH₂CHFOCO₂Li)₂.^{2, 6} In both cases, the lithium-to-fluorine ratio of the reaction products is close to 1. The majority of the fluorine should exist in either LiF or organic products,^4-7but not in both forms. However, our recent analysis found that the ratio of lithium to fluorine is ≥1.5 for the SEI film formed on a silicon-based anode which was cycled in an electrolyte with FEC as additive or pure FEC as solvent (table 1).

Our X-ray Photoelectron Spectroscopy (XPS) study also reveals that fluorine exists in both LiF and organic compounds (Figure 1), which cannot be explained by the previous mechanisms proposed in the literature. Therefore, a new mechanism has been proposed (Figure 2) and verified for the reduction and polymerization of FEC and the formation of a stable SEI layer on anode materials. According to this mechanism, FEC is reduced on the anode surface through the opening of the five-member ring, leading to the formation of lithium poly(vinyl carbonate), LiF, and some dimers.

 Table 1 The percentage of Li, C, O and F in the SEI films in electrolytes (a) 1M LiPF₆ in EC/DMC (1:2 in vol), (b) 1M LiPF₆ in EC/DMC (1:2 in vol) with 10% FEC and (c) 1M LiClO₄in pure FEC

Elements    samples	%Li	%C	%O	%F	Li/F ratio
Electrolyte (a), 2 cycles	27.7	23.3	24.1	24.9	1.11
Electrolyte (a), 35 cycles	22.7	31.3	37.6	8.4	2.70
Electrolyte (b), 2 cycles	22.9	32.5	32.1	12.5	1.83
Electrolyte (b), 100 cycles	21.0	33.8	31.9	13.3	1.58
Electrolyte (c), 2 cycles	21.1	34.7	32.2	12.0	1.76

Figure 1 XPS spectra of F1s for the SEI film formed in LiClO₄/FEC electrolyte

Figure 2Proposed possible reduction reaction of FEC

Acknowledgment

This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, Subcontract No. 18769 under the Batteries for Advanced Transportation Technologies (BATT) program. A portion of the research was performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.

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