Effect of Fluoroethylene Carbonate Electrolytes on the Nanostructure of the Solid Electrolyte Interphase and Performance of Lithium Metal Anodes

Abstract

The mechanism for the performance enhancement of lithium metal electrodes by fluoroethylene carbonate (FEC) is revealed. Electrolytes containing FEC, 1.2 M LiPF6 in ethylene carbonate (EC):ethyl methyl carbonate (EMC) (3:7, vol) with 10% FEC (mass %) and 1.2 M LiPF6 in FEC, improve the electrochemical performance of both Li∥Li and Cu∥LiFePO4 cells compared to the baseline electrolyte, 1.2 M LiPF6 in EC:EMC (3:7, vol). Ex situ surface analysis of lithium metal electrodes after the initial plating demonstrates that the solid electrolyte interphase (SEI) generated from FEC containing electrolytes is similar to the SEI generated from the baseline electrolyte, yet the corresponding Coulombic efficiencies are markedly different. Electron microscopy investigations reveal the presence of a unique SEI containing nanostructured LiF particles for the lithium electrode plated from the 1.2 M LiPF6 in FEC electrolyte. The presence of the nanostructured LiF particles correlate with the improved cycling performance, suggesting that the morphology of the SEI is as important as the composition of the SEI.

Study of the Formation of a Solid Electrolyte Interphase (SEI) on a Silicon Nanowire Anode in Liquid Disiloxane Electrolyte with Nitrile End Groups for Lithium‐Ion Batteries

“…We suggest that the addition of FEC resulted in mainly fluorinated‐rich SEI with LiF, and trapped LiTFSI . This is because the C 1s XPS spectrum is dominated by carbon‐fluorine bonds at 291.8 eV (LiTFSI, C‐F x ) and at 287.1 eV assigned to C−CF x .…”

“…It was suggested that LiF nanoparticles precipitate on the electrode‘s surface under a certain diameter an ultra‐microelectrode‐like surface termination may form. In turn, this termination leads to a uniform diffusion field giving rise to more uniform stripping and plating of lithium on copper current collectors . In addition, we suggest that the presence of fluorine throughout the SEI may indicate that some of the FEC reduction products encapsulate residual lithium anion salt (TFSI).…”

“…[48] We also detect Li 2 O with its characteristic peaks of O 1s at 528.6 eV, and Li 1s at 54.45 eV. [48] The SiÀO bond has a very broad Si 2p peak at 100.95 eV. Another, at 103 eV has an intensity on the same scale as the signal-to-noise ratio.…”

“…[38][44] We did not discern a poly(FEC) peak expected at 1805 cm −1 ,. This might be explained by either a low signal‐to‐noise ratio as a relative high concentration of FEC (above 15 wt %) is needed to produce a prominent peak, or that the peak is enveloped by the broader and more intense asymmetric peak at about 1830 cm −1 . We assign the peak at 1830 cm −1 to the carbonyl group of the FEC, as well as to R−CF=O reduction products reported in the presence of lithium .…”

“…On the outer layer of the SEI, the layer facing the electrolyte solution (defined as t sputter = 0 minutes), we note the presence of Li 2 CO 3 as evident by peaks of C 1s at 290.1 eV, O 1s at 531.65 eV, and Li 1s at 55.1 eV. [48] We also detect Li 2 O with its characteristic peaks of O 1s at 528.6 eV, and Li 1s at 54.45 eV. [48] The SiÀO bond has a very broad Si 2p peak at 100.95 eV.…”