Elucidation of the Reaction Behavior of Silicon Negative Electrodes in a Bis(fluorosulfonyl)amide‐Based Ionic Liquid Electrolyte

Abstract

Excellent cycling performance of an electrode composed of silicon alone was achieved in a bis(fluorosulfonyl)amide (FSA)‐based electrolyte, with a high discharge capacity of 950 mA h g−1 observed even at the 500th cycle. To elucidate the reaction behavior of the Si electrode in an FSA‐based ionic liquid electrolyte, we investigated the change in the cross‐sectional morphology of the Si‐active material layer, the distribution of Li in the layer, and the crystallinity of Si on the electrode surface. By cross‐sectional scanning electron microscopy, we confirmed that the electrode thickness increased with the cycle number. The increase in thickness was less noticeable in the FSA‐based electrolyte than in an organic electrolyte. An elemental analysis of the electrode material revealed that a film derived from the electrolyte was formed not only on the surface but also inside of the electrode. Soft X‐ray emission spectroscopy demonstrated that the distribution of Li in the FSA‐based electrolyte was more uniform for the cross‐section of the cycled electrode compared to that in an organic electrolyte. The results of Raman spectroscopy indicated that domains of amorphous Si were homogeneously distributed on the electrode surface in the FSA‐based electrolyte. The uniform distribution of the lithiation−delithiation reaction should help to suppress disintegration of the active material layer.

Lithiation and Delithiation Reactions of Binary Silicide Electrodes in an Ionic Liquid Electrolyte as Novel Anodes for Lithium‐Ion Batteries

“…In contrast, a surface film derived from ionic liquid electrolytes is homogeneous and thin. The lithiation of Si uniformly occurs in the ionic liquid electrolytes, which leads to the spread of the stress over the entire electrode surface . No accumulation of strain suppresses the severe disintegration of the Si electrode.…”

“…This result demonstrates that the decomposition of the FSA anion (F 2 NO 4 S 2 ) occurs on the FeSi 2 electrode, whereas the Py13 cation (C 8 H 18 N) hardly decomposes. We previously reported that PC mainly decomposed on the cross‐section of the Si‐alone electrode, whereas no decomposition of LiTFSA occurred . Therefore, the properties (component, thickness, and so on) of the surface films formed on different electrodes should differ even in the same electrolyte.…”

“…Figure provides the Raman spectra of all synthesized silicides. Peaks assigned to c ‐Si and a ‐Si appear at approximately 520 and 490 cm −1 , respectively ,. Each silicide also gives no Raman peaks which indicates that neither c ‐Si nor a ‐Si is included in the synthesized powders.…”

“…A surface film is widely accepted to form through the reductive decomposition of the electrolyte. We have previously reported that the thickness of the film formed on the Si‐alone electrode is inhomogeneous in PC‐based organic electrolytes ,. Li + should be preferentially stored in the electrode through not the thicker parts but the thinner parts of the film, because the thicker parts inhibit the lithiation reaction of Si.…”

“…Ionic liquids have excellent physicochemical properties as electrolyte solvents, such as negligible vapor pressure, non‐flammability, high conductivity, and wide electrochemical windows . We have reported that a Si‐anode exhibits a better electrochemical performance in some ionic liquid electrolytes compared to that in conventional organic electrolytes . In addition, we have applied the ionic liquid electrolytes to the composite electrodes.…”