Cycleability of SiOx Nano-Flake Anode in Glyme-Based Localized High Concentration Electrolytes Using Fluorinated Diluents
Authors: Nakanishi Mai , Inaba Minoru , Doi Takayuki 
Published: December 2024

Introduction

Further improvement of energy density is required for lithium-ion batteries for wide-spread use of EVs. Silicon-based materials are attracting attention as high-capacity active materials for the anode of lithium-ion secondary batteries, but their capacity is significantly degraded due to pulverization associated with large volume changes. For this purpose, we have demonstrated that the use of amorphous Si nano-flake powder (LeafPowder® Si, Si-LP, 100 nm thickness) exhibits good cycleability[1]. However in conventional EC-based electrolytes, even the Si-LP anode has a problem that the electrode layer expands significantly after repeated cycling (approximately 12 times after 200 cycles). Possible reasons include not only the expansion of the active material itself due to the lithium alloying/dealloying reactions, but also the accumulation of decomposition products of the electrolyte inside the electrode layer. Therefore, we considered using highly concentrated electrolyte (HCE) systems suitable for lithium metal anode, which changes the shape significantly like Si anodes[2]. Furthermore, we investigated localized HCE (LHCE) using a fluorinated diluent to reduce the high viscosity of the HCEs.

Experimental

Partially oxidized silicon nano-flake powder (SiO-LP) (OIKE & Co., Ltd., thickness: 100-130 nm, average particle size: approximately 5.6 μm, O/Si = 0.68) was used as the anode active material. Ketjen black (KB, EC-600JD, Lion Corporation), and carboxymethylcellulose was used as the conducting agent the binder, respectively. They were mixed at a weight ratio of 83.3:5.6:11.1 to obtain a slurry. The slurry was coated on the copper-foil current collector and dried under vacuum at 80°C for 12 hours to obtain a SiO-LP electrode.
Bis(fluorosulfonyl)imide (LiFSI) electrolyte was dissolved in diglyme (G2, Kishida Chemical) to the saturated concentration to obtain a HCE. A fluorinated solvent, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE), was added as the diluent at a 1:1 volume ratio to obtain a LHCE.
A two-electrode coin-type half-cell was used for charge and discharge tests. The cell was assembled with the SiO-LP working electrode, a lithium metal counter electrode, a Celgard®3501 separator, and 60 µl of the electrolyte w. Charge and discharge tests were performed at 0.5 C between 0.02 and 1.5V vs. Li/Li+ in a constant current (CC)-constant voltage (CV) mode with a battery test system (TOSCAT-3100, Toyo System). The CV mode was terminated at 1/10 of the current value. The cell temperature was 30°C.

Results and Discussion

Figure 1 shows the Raman spectra of the S-N stretching mode of the FSI anions in LiFSI/G2 electrolyte solutions. Peaks at 720, 732, 744 and 760 cm-1 are assigned to free anion (or solvent-separated ion-pairs, SSIPs), CIPs of Li-FSI, and AGGs, respectively. Addition of HFE as a diluent increased the ratio of CIPs or/and AGGs, although the viscosity was reduced to 1/15. The addition of HFE maintained the local structure of the HCE.
Figure 2 shows the variations of discharge capacity of the SiOx anode in LiFSI/G2 (HCE), LiFSI/G2/HFE (LHCE), and conventional 1M LiPF6/EC+DEC (1:1 by vol.) electrolyte. When the conventional electrolyte was used, the SiO-LP anode significantly deteriorated on cycling, and the capacity decreased 28% at the 500th cycle, On the other hand, the glyme-based HCE and LHCE gave the maximum discharge capacity after about 100 cycles, and the discharge capacity gradually decreased thereafter with a much lower degradation rate. The capacity retention after 500 cycles was improved to 96.9% and 88.4.
Figure 3 shows the electrode expansion rate of the SiOx anode after 100 cycles in the three types of electrolytes. While the electrode expanded to 370% in the conventional EC-based electrolyte after 100 cycles, the expansion was suppressed to 164% and 162% in the LiFSI/G2 and LiFSI/G2/HFE electrolytes, respectively.
Elemental analysis of the electrode surface after cycling revealed that fluorine was detected uniformly on the electrode surface. The improvement of cycleability and the suppression of electrolyte decomposition inside the electrode are due to the presence of a large amount of LiF in SEI. It is thought that lithium fluoride improved the robustness of the SEI and suppressed crack formation in the electrode.
Furthermore, since the expansion rate was reduced slightly when HFE was added as the diluent, there is a possibility that the electrode expansion can be further reduced by selecting a more effective fluorinated diluent. Such new electrolyte systems will be reported on our poster.

Reference

[1] M. Inaba et al., Electrochemistry, 85(10), 623–629 (2017).
[2] C. Zhu et al., ACS Energy Lett., 7, 1338-1347 (2022).

Figure 1