Significant Suppression of Silicon Negative Electrode for Lithium-Ion Battery in Ionic-Liquid Electrolyte
Authors: Domi Yasuhiro , Usui Hiroyuki , Yamaguchi Kazuki , Yodoya Shuhei , Sakaguchi Hiroki 
Published: February 2020

Elemental Silicon (Si) has a high theoretical capacity of 3580 mA h g-1 and has attracted attention as an active material of negative electrode for high energy density lithium-ion batteries. However, it shows poor cycle stability, which is mainly due to a massive volume change in Si during lithiation and delithiation. Here, we report that combination an ionic liquid electrolyte with a charge capacity limit of 1000 mA h g-1 significantly suppresses Si volume expansion, improving the cycle life. On the other hand, the Si layer expands largely in an organic electrolyte even with the charge capacity limit and even in an ionic-liquid electrolyte without the limit. We demonstrated that the homogeneously distributed Si lithiation−delithiation, phase-transition control from the Si to Li-rich Li-Si alloy phases, and formation of a surface film with structural and/or mechanical stability contribute to suppressing Si volume expansion.1

A Si working electrode was fabricated by gas-deposition method, which is without any binder and conductive agent. A 2032-type coin cell was constructed, comprising a Si electrode, a glass fiber filter as the separator, and a Li metal sheet as the counter electrode. An ionic liquid electrolyte used was 1 mol dm-3 (M) lithium bis(fluorosulfonyl)amide (LiFSA) dissolved in N-methyl-N-propylpyrrodinum bis(fluorosulfonyl)amide (Py13-FSA). For comparison, 1 M lithium bis(trifluoromethanesulfonyl)amide (LiFSA) dissolved in propylene carbonate (PC) was employed as a conventional organic electrolyte. Galvanostatic charge-discharge testing was performed with a charge capacity limit of 1000 mA h g-1 unless stated otherwise. The current density was set at 0.36 A g-1 (0.1 C) during the first cycle and 1.44 A g-1 (0.4 C) during subsequent cycles. An electrode cross section was observed by field emission scanning electron microscopy (FE-SEM). The cross section was fabricated using a cross-section polisher or focused ion beam. The electrode was not exposed to the atmosphere until it was introduced into the chamber of the FE-SEM using a transfer vessel.
A reversible capacity of the Si electrode decayed at around the100th cycle in the organic electrolyte (1 M LiTFSA/PC). Conversely, in the ionic-liquid electrolyte (1 M LiFSA/Py13-FSA), the Si electrode exhibited a better cycle life with a reversible capacity of 1000 mA h g-1 after the 600th cycle. To determine the difference between the cycle lives of Si electrode in organic and ionic-liquid electrolytes, we investigated the thickness of the lithiated Si active material layers by FE-SEM.
The thickness of the nonlithiated Si layer before charge-discharge testing was 1.6 ± 0.3 mm.2 In the organic electrolyte, the Si layer expnads with the cycle number, developing several cracks and becoming porous after the 50th and 100th cycles. The thickness reached ca. 25 mm after the 100th cycle when the capacity faded. The amorphous Li1.0Si (a-Li1.0Si) phase may mainly form with a charge capacity limit of 1000 mA h g-1 because it has a theoretical capacity of 950 mA h g-1. The expansion rate of the Si layer was about 1460%, whereas the calculated rate of increase in thickness from Si to a-Li1.0Si phases is 17%. In the ionic-liquid electrolyte, the Si electrode retained a thickness of 2.7 mm over 300 cycles, with an expansion rate of ca. 69%. Unexpectedly, the Si layer did not expand significantly after repeated cycling. There are no reports on the significant suppression of Si volume expansion after such long-term cycling. However, the expansion rate of 69% is still above the calculated rate of 17% for the a-Li1.0Si phase. After the 600th cycle and just before capacity fading, the layer thickness increased to 13.3 mm with some void and crack formation inside the layer. We discuss differences in the performances of the Si-alone electrode in the organic and ionic-liquid electrolytes based on the Li storage distribution, phase transition, and surface film formation.

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[1] Y. Domi, H. Usui, K. Yamaguchi, S. Yodoya, H. Sakaguchi, ACS Appl. Mater. Interfaces,.2019, in press.
[2] K. Yamaguchi, Y. Domi, H. Usui, H. Sakaguchi, ChemElectroChem, 2017, 4, 3257-3263.