Cycle Performance of Si Film Negative Electrodes with Lithium Fluoride Coating As an Artificial SEI
Authors: Sonoda Mayu , Haruta Masakazu 
Published: December 2024

Introduction

There is a growing need for high-capacity lithium-ion batteries as a power source for electric vehicles and a backup power source for renewable energy. Si negative electrodes, which have about ten times the capacity of conventional graphite electrodes, are expected to be used practically to increase the capacity of lithium-ion batteries. However, there are still issues that need to be addressed before Si electrodes can be used practically. One of them is the decomposition of electrolytes on the negative electrode surface. In conventional lithium-ion batteries, a small amount of additive such as FEC is added to the electrolyte to suppress electrolyte decomposition. However, suppression of the electrolyte decomposition is still difficult for Si electrodes. On the other hand, it has been found that lithium fluoride (LiF) in the SEI film formed from the decomposition products of the electrolyte improves charge and discharge properties of Si electrode.1-3 In this study, we aimed to suppress electrolyte decomposition and improve cycle life by forming a LiF layer as an artificial SEI on the Si electrode using a thin-film technique.

Experimental

Amorphous Si thin films, 100 nm in thickness, were prepared on copper foil by RF magnetron sputtering. Subsequently, LiF layers were deposited on the Si surfaces by sputtering with varying thicknesses. The thickness of the LiF layers was measured by a stylus surface profiler. The LiF-coated Si films were transferred to an Ar-filled glovebox without exposure to air and punched into disks 13 mm in diameter. A coin-type half-cell (CR2032) was assembled with the LiF-coated Si film as a working electrode and a Li disk of 14 mm in diameter as a counter electrode. A solution of 1 M LiPF6 dissolved in a mixture of ethylene carbonate and diethyl carbonate was used as an electrolyte. Charge and discharge characteristics were measured with a C/3 rate in a thermostatic chamber at 30°C. After cycling, the coin cells were disassembled in the glovebox, the Si electrodes were rinsed with dimethyl carbonate, and the Si electrode surfaces were observed with a scanning electron microscope (SEM).

Result and Discussion

Figure 1 shows variations in discharge capacity of the Si film electrodes with and without LiF coating. The discharge capacity of the Si electrode without LiF rapidly dropped after 10 cycles, but LiF coating significantly improved cyclability. Capacity retention at the 100th cycle, compared to that at the 5th cycle, increased from 4% without LiF to 52% with LiF coating. LiF coating also improved initial Coulombic efficiency from 89.9% to 92.5%, suggesting that LiF coating suppresses electrolyte decomposition. Si electrode surfaces after 30 cycles were observed by SEM as shown in Fig. 2. Exfoliation of the Si film without LiF was observed in many areas after 30 cycles, but it was suppressed by LiF coating. LiF coating is effective in improving the cycle performance of Si negative electrodes as an artificial SEI.

Acknowledgments

This work was supported by GteX Program Japan Grant Number JPMJGX23S3 and JSPS KAKENHI Grant Number 2201967.

References

[1] M. Haruta et al., Electrochimica Acta 267 (2018) 94.
[2] M. Haruta et al., J. Electrochem. Soc. 165 (2018) A1874.
[3] M. Haruta et al., Nanoscale 19 (2018) 17257.

Figure 1