In Situ AFM Observation of Amorphous Si Thin-Film Electrode in Ethylene Carbonate-Based Electrolyte Solutions with Vinylene Carbonate

Abstract

Si has drawn much attention as a promising next-generation negative electrode material for lithium ion batteries (LIBs) because Si has a high theoretical capacity of ca. 4200mAh g-1, which is approximately 11 times higher than that of graphite. However, several technical issues, such as large volumetric changes of Si upon alloying/dealloying with Li and a large irreversible capacity particularly in the initial cycles, remain to be solved for practical use. Hence, the morphological changes need to be controlled to improve the charge/discharge performance. In this study, an amorphous Si thin-film was used as a model electrode and its morphological changes, which originate from the formation of solid electrolyte interphase (SEI) and volumetric changes upon alloying/dealloying with Li, was observed by in situ atomic force microscopy (AFM). The effects of electrolyte additives on morphological changes of Si were discussed. Amorphous Si thin-films were deposited on 500-μm-thick Cu sheet by electron beam evaporation. The thickness of Si was ca. 100 nm. The electrochemical properties were investigated by cyclic voltammetry (CV) (SP-300, Biologic) at a sweep rate of 0.5 mV s-1 between 3.0 and 0.02 V, using a laboratory-made three-electrode cell. The effective electrode surface area was limited to 1.77 cm2 by an O-ring. Both reference and counter electrodes were Li wire. All potentials are referred to as V vs. Li+/Li. The electrolyte solution used was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and ethylmethylcarbonate (EMC) (1:1, by volume) with and without 10 wt.% vinylene carbonate (VC). Electrochemical AFM observations coupled with CV were conducted using a 5500 AFM/SPM Microscope (Agilent) in a contact mode with pyramidal silicon nitride tips (spring constant: 0.02 N m-1, Olympus). The scan area was set to 10 × 10 μm2. AFM observation was carried out at a room temperature of about 25 oC in an Ar-filled glovebox with a dew point of less than −60 oC. Figure. 1 shows AFM images of amorphous Si thin-film electrodes during the potential scan from 0.48 to 0.02 V in 1 M LiPF6 / EC+EMC (a) without and (b) with 10 wt.% VC additives. In Fig.1a, a very large blister appeared on the Si surface at potentials below 0.35 V. Li1.7 1Si is known to form at around 0.35 V [1], and hence the formation of the blisters should be closely related to alloying reaction of Li-Si. The AFM observation was interrupted below 0.20 V due to the significant morphological changes. The blisters appeared below 0.35 V even when VC was used, while the sizes was relatively small and they were uniformly distributed. We observed the fact that a very thin surface film was formed evenly on a Si electrode at higher potentials. The alloying/dealloying reactions with Li seem to proceed in a relatively uniform way on the whole surface of a Si thin-film electrode through the VC-derived effective surface film. This assumption was proved by ex situ Raman spectroscopy. Figure 2 shows Raman spectra of an amorphous Si thin-film electrode after charging (lithiation) in VC-containing electrolyte solution, together with that of an as-prepared film as a reference. The line that peaks at around 480 cm-1, which is identified as an amorphous Si, decreased upon charging particularly at a blister (B in Fig.2), as compared with a smooth region (A in Fig.2). These results indicate that the blister consisted of Li-Si alloy; Si significantly expands upon alloying with Li and its mechanical strength lowers with an increase in Li content [2], and hence the blister should form. Such non-uniform morphological changes of Si upon lithiation can be suppressed by using VC additives, which results in high cycleability of Si electrode.