Structural Study of Electrochemically Lithiated Si
Authors: Omachi Asami , Aoki Nana , Uosaki Kohei , Kondo Toshihiro 
Published: February 2020

Introduction.

Recently, silicon (Si) has paid much attention as an alternative anode material of conventional lithium-ion battery (LIB) and next-generation lithium-air battery (LAB), because of its high lithium capacity (theoretical capacity: 3580 mAh g-1 as Li15Si4), which is about ten times larger than that of conventional carbon material (320 mAh g-1)　[1]. Many researchers have studied performance of silicon anode, but there have been only a few reports about the electrochemical LiSi alloy formation [2-3] and not clarified their structure, composition, and electronic state in detail. In this study, therefore, the structure, composition and electronic state of the LiSi alloy, which was electrochemically prepared on the Si(111) surface, were investigated by soft x-ray emission spectroscopy (SXES) combined with scanning electron microscopy (SEM) and surface x-ray diffraction (SXRD) using synchrotron radiation (SR) as an X-ray source [4].

Experimentals.

For the preparation of the LiSi alloy sample, the potential of the Si(111) electrode was scanned from 2.4 V (vs. Li/Li+), where is closed to an open circuit potential (OCP), to 0.01 V in an ethylene carbonate (EC) and dimethyl carbonate (DMC) mixed solution (1:1 vol%) containing 1 M LiPF6 with a scan rate of 1 mV s-1 and then hold at 0.01 V for 1 h in the pure argon gas filled glove box. Just after 1 h, the sample electrode was disconnected and washed with DMC. Using transfer vessel under pure argon atmosphere, the SEM images and SXRD patterns of the prepared LiSi alloy sample were measured under ambient pressure environment and pure argon atmosphere, respectively.

Results and Discussion.

When the electrode potential was negatively scanned from 2.4 V, a small cathodic peak due to the formation of solid electrolyte interphase (SEI) was observed around 1.50 V. Around 0.05 V, large cathodic current due to lithiation flowed and it continued to flow after the potential scan was stopped at 0.01 V. When the potential was kept at 0.01 V, cathodic current became larger, had a maximum, gradually decreased, and then became constant at -0.01 mA cm-2.
From the cross-sectional SEM image, three kinds of the LiSi alloy phases were found. The surface was shaped as a µm-sized three-fold symmetric triangular pyramid, which reflects the atomic arrangement of the Si(111) substrate. Based on the site-selective SXE spectra of each layer, we can assigned the first, second, and third layer phases to crystalline Li15Si4, amorphous Li15/13Si4, and crystalline Si containing Li atoms, respectively. Because there were no other peaks except for the Li15Si4(444) reflection in the out-of-plane SXRD pattern, only the Li15Si4(02-2) and (04-4) reflections were observed in the in-plane pattern along the [01-1] scan of the Si(111) substrate, and only the Li15Si4(11-2) and (22-4) reflections were observed in the in-plane pattern along the [11-2] scan of Si(111), we can conclude that the first layer of the electrochemically lithiated Si(111) is a single-crystalline phase of Li15Si4 (sc-Li15Si4), which was epitaxially grown on Si(111).

References.

[1] V. A. Sethuraman, et al., J. Electrochem. Soc., 160 (2013) A394 and references there in.
[2] M. Gu, et al., ACS Nano., 7 (2013) 6303.
[3] M. N. Obrovac, et al., Electrochem. Solid-State Lett.,
7 (2004) A93.
[4] N. Aoki, et al., ChemElectroChem., in press (2016).