Analytical Study of SEI Formation Process on Lithium Metal
Authors: Sonoki Hidetoshi , Kuwata Hiroko , Matsui Masaki , Imanishi Nobuyuki 
Published: February 2020

Introduction

Lithium metal is one of the most promising negative electrode materials for rechargeable battery because of its high theoretical capacity (>3800 mAh g-1) and highly negative equilibrium potential (-3.045 V vs. SHE). However the dendritic lithium deposited during charging process causes several undesirable phenomena, hence rechargeable lithium batteries have not been successfully commercialized. Even though a lot of studies have been conducted to suppress the dendritic growth of lithium metal [1], the formation of the dendritic lithium still remains as the biggest problem in the lithium negative electrode.
The dendritic growth occurs due to an inhomogeneous solid electrolyte interphase (SEI) film formation which causes locally focused current. In the present study, we investigated the SEI formation process in various electrolyte solutions using the electrochemical quartz crystal microbalance (EQCM) technique.

Experimental

1 mol dm-3 LiPF6/EC:DEC (1:1 vol.%), LiTFSI/EC:DEC (1:1 vol.%) and LiTFSI/diglyme electrolyte solutions were used for the EQCM measurements. The measurements were carried out using a three-electrode EQCM cell. A quartz crystal resonator with nickel electrode was employed as the working electrode. Lithium foil was used as the counter and the reference electrode.
Cyclic voltammetry was measured for 5 cycles to investigate the behavior of the SEI formation process. All the measurements were performed in a dried and argon-filled glove box at room temperature. The frequency change was converted into the mass change (Δm) by Sauerbrey equation [2].

Results and Discussion

Fig. 1 (a), (b) and (c) shows cyclic voltammograms (blue) and mass change-potential curves (red) of LiPF6/EC:DEC (1:1 vol.%), LiTFSI/EC:DEC (1:1 vol.%) and LiTFSI/diglyme electrolyte solutions respectively. The graphs corresponding to the 1st cycle are shown in the darkest color and the color of the graphs gets lighter as the cycle number increases. Reduction current and an increase in Δm were observed at the same time. In all cases, the irreversible mass change at 1st cycle was greater than that at any other cycle. It clearly shows that most of the SEI film is formed at 1st cycle and almost all mass changes after 2nd cycle are based upon adsorption and desorption on the working electrode.
Δm started to increase at 2.1 V in LiPF6/EC:DEC (a). Furthermore, it showed the greatest irreversible mass change in these three electrolyte solutions. On the other hand, Δm started to increase at 1.4 V in LiTFSI/EC:DEC (b). It is supposed that LiPF6 is reduced relatively easily than LiTFSI.
The irreversible mass change in LiTFSI/diglyme (c) was greater than LiTFSI/EC:DEC (b). However, ether-based electrolyte solutions generally have high reductive stability. It indicates that the anion can be reduced and greater amount of the SEI film is formed because diglyme chelates the Li+ ion resulting in producing the naked anion. According to the above results, the anion of a lithium salt, which has high reductive stability, will be suitable for the suppression of the SEI formation.

Reference

[1] K. Kanamura, S. Shiraishi, Z. Takehara, J. Electrochem. Soc., 143, 2187-2197 (1996).
[2] G. Sauerbrey, Zeitschrift für Physik, 155, 206-222 (1959).

Figure 1