Si is a promising negative-electrode material for next-generation lithium-ion batteries due to its high gravimetric and volumetric capacities. A solid electrolyte interphase (SEI) forms on the Si negative electrode owing to reductive decomposition of an electrolyte solution in the initial charging process. Although the SEI layer strongly affects the charge/discharge performance, details of its formation process have not been clarified yet. Therefore, understanding of the SEI formation process is important to improve the cycle performance of Si negative electrodes.

In recent years, glyme-based concentrated electrolyte solutions have attracted attentions because of their striking features; dissolution of polysulfide anions is suppressed in Li-S batteries, and dendrite formation is suppressed in Li metal batteries [1, 2]. In this study, the SEI formation process on Si-thin-film model electrodes was observed by in situ atomic force microscopy (AFM) coupled with cyclic voltammetry (CV) in lithium bis(fluorosulfonyl)imide (LiFSI)/tetraglyme (G4) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)/G4 electrolyte solutions.

Si thin films (100 nm) were deposited on mirror polished Cu substrates by RF magnetron sputtering. In situ AFM coupled with CV was performed using a liquid immersion sample stage designed for electrochemical measurements, consisting of the Si-film working electrode, and Li-wire counter and reference electrodes, at a sweep rate of 0.5 mV s^-1 between 1.9 and 0.02 V (vs. Li⁺/Li). The electrolyte solution used was 4.5 M LiFSI or LiTFSI dissolved in G4. AFM scan was conducted in the contact mode at room temperature, and typical scan area was 5 × 5 mm². In situ AFM observation was conducted in an Ar-filled glovebox with a dew point of about -60°C.

Small cathodic currents were observed at potentials below about 1.8 V during the initial potential sweep in both LiFSI/G4 and LiTFSI/G4. The cathodic currents suddenly increased at around 1 V due to the reductive decomposition of the electrolyte solutions. The cathodic current in LiFSI/G4 was larger than that in LiTFSI/G4, indicating that the LiFSI/G4 is more vulnerable to reduction. In fact, in situ AFM observation revealed that some precipitates appeared on the Si surface at around 1 V in LiFSI/G4, which suggests the formation of SEI. In contrast, no morphological change was seen in LiTFSI/G4 during the initial charging process. These results suggest that the decomposition products of LiTFSI/G4 can hardly form a stable SEI on the Si electrode in the initial cycle. The surface cracked after the 2nd potential cycle in both the electrolyte solutions. The crack progressed with repeated potential cycling, which was more significant in LiTFSI/G4. The redox currents due to the alloying/dealloying reaction of Si with Li decreased more rapidly with potential cycling in LiTFSI/G4. Based on the above results, the formation of a stable SEI in the initial cycling in LiFSI/G4 largely contributes to the suppression of crack formation and the superior performance of charge/discharge cycles.

The effect of a fluoroethylene carbonate (FEC)-derived SEI on morphological changes of the Si film electrode will also be presented in our poster [3].

Reference

[1] H. Wang et al., ChemElctroChem., 2, 1144-1151 (2015)

[2] K. Yoshida et al., J. Am. Chem, Soc., 133, 13121-13129 (2011).

[3] M. Haruta et al., J. Electrochem. Soc., 165, A1874-A1879 (2018)