The structure, composition, and electronic state of electrochemically lithiated Si(111) were investigated by using soft X‐ray emission spectroscopy (SXES) combined with scanning electron microscopy (SEM) and by using X‐ray diffraction (XRD) with synchrotron radiation (SR). When the SXES results were compared with the calculated density of state (DOS), we found that three kinds of electrochemically lithiated Si (EC‐LiSi) phases were formed on the Si(111) substrate, that is, a single‐crystalline Li15Si4 (sc‐Li15Si4) alloy phase, an amorphous phase of Li15Si4 and/or Li13Si4 (a‐Li15Si4 and/or a‐Li13Si4), and a mixed phase of a‐Li15Si4 and/or a‐Li13Si4 (52 %) and crystalline Si (c‐Si) (48 %),. The shape of the sc‐Li15Si4 phase was a micrometer‐sized, three‐fold‐symmetric triangular pyramid, which reflects the atomic arrangement of the Si(111) surface, and the XRD patterns, obtained by using SR, revealed that the crystal orientation of sc‐Li15Si4 is in the same direction as the Si(111) substrate. Based on the band center energy shifts, we confirmed that electrons are partially transferred from Li to Si atoms in the LixSi alloy.

The electrochemical quartz crystal microbalance technique was employed to study the initial stage of the electrodeposition and dissolution of lithium utilizing three kinds of electrolyte solutions such as LiPF, LiTFSI, or LiFSI in tetraglyme. The native-SEI (solid-electrolyte interphase) formed by a potential prescan before lithium deposition/dissolution in all three solutions. Simultaneous additional SEI (add-SEI) deposition and its dissolution with lithium deposition and dissolution, respectively, were observed in LiPF and LiTFSI. Conversely, the add-SEI dissolution with lithium deposition and its deposition with lithium dissolution were observed in LiFSI. Additional potential precycling resulted in the accumulation of a "pre-SEI" layer over the native-SEI layer in all of the solutions. With the pre-SEI, only lithium deposition/dissolution were significantly observed in LiTFSI and LiFSI. On the basis of the potential dependences of the mass and resistance changes, the anion-dependent effects of such a pre-SEI layer presence/absence on the lithium deposition/dissolution processes were discussed.

Introduction. It is well-known that LiSi alloy forms by electrochemical lithiation of the Si substrate [1]. Since the high performance of lithiation amount of Si is theoretically predicted [2-4], LiSi alloy formation/deformation, i.e., lithiation/delithiation of Si, is very attractive interest not only in the Li ion based battery research field but also fundamental electrochemistry area [5]. However, there is no report to investigate in detail the lithiation/delithiation process into Si substrate. X-ray photoelectron spectroscopy (XPS) method was often used for electronic state analysis of LiSi alloy [5]. However, information from XP spectrum is limited only about inner shell electron and then, electronic state of LiSi alloy has not been clarified yet. On the other hand, soft X-ray emission spectroscopy (SXES) provides us the information about valence band electron and then, we can obtain the electronic states of lithiated/delithiated Si in details. In this study, we electrochemically prepared the samples with different lithiation/delithiation amounts and investigated the geometric and electronic structures during electrochemical lithiation/delithiation process of the Si(111) and Si(100) substrate by scanning electron microscopy (SEM), SXES, and surface x-ray diffraction (SXRD) using synchrotron radiation as an x-ray source. Experimentals. After the surfaces of n-Si(111) and n-Si(100) substrates (phosphorous-doped, 1 – 10 W cm) were hydrogen terminated [6], Lithiated samples were prepared in the glove box under Ar atmosphere as below. The potential of the Si electrode was negatively scanned from open circuit potential (OCP) (ca. 2.4 V vs. Li/Li+) to 0.01 V with a scan rate of 1 mV s-1 in the electrolyte solution (1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (EC : DMC = 1 : 1, v/v%). At 0.01 V, the potential was kept for several periods. After that, the potential was back to OCP, namely the Si electrode was disconnected. Delithiated samples were prepared by the potential scan from 0.01 V to 2.4 V after keeping the potential of 0.01 V for 60 min. After washing, the sample was transferred to the SEM chamber or the SXRD cell under Ar atmosphere without any exposure in air. Then, SEM, SXES, and SXRD measurements were carried out. Results and Discussion. Time dependences of current density and electrode potential during the lithiation/delithiation of the Si(111) and Si(100) electrodes were measured. At both electrodes, similar behaviors were observed. When the electrode potential was negatively scanned from OCP, small cathodic current, which is 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 when the potential scan was stopped at 0.01 V. After keeping the potential of 0.01 V for several periods, when the potential was positively scanned from 0.01 V to 2.4 V, large anodic current due to delithiation started to flow around 0.05 V, reached a maximum around 0.5 V, and then decreased to zero, indicating that all lithium was desorbed. Surface SEM image of the lithiated samples showed that the shape of the surface layer was a triangular and rectangular pyramids on Si(111) and Si(100), respectively. The longer the lithiation period, the larger the pyramid. Cross sectional SEM image shows that both lithiated Si(111) and Si(100) consist of four kinds of layers. Based on the results of SXES and SXRD, these layers can be assigned in the order from the surface as follows, single crystalline Li15Si4 alloy phase, amorphous Li15Si4 and/or Li13Si4 mixed alloy phase, mixed phase of amorphous Li15/13Si4 alloy and crystalline Si, and crystalline Si phase containing Li atoms [7]. After delithiation, the first and second layers, i.e., single crystalline Li15Si4 alloy phase and amorphous Li15Si4 and/or Li13Si4 mixed alloy phase, were peeling off and these layers became the amorphous Si phase. The third and fourth layers were back to the crystalline Si phase. References. [1] V. A. Sethuraman, et al., J. Electrochem. Soc., 160 (2013) A394 and references there in. [2] B. A. Boukamp, et al., J. Electrochem. Soc., 128 (1981) 725. [3] C. K. Chan, et al., Nature Nanotechnol., 3 (2008) 31. [4] C. K. Chan, et al., J. Power Sources, 189 (2009) 34. [5] S.-O. Kim and A. J. Manthiram, J. Mater. Chem. A, 4 (2015) 2399 and references there in. [6] S. Nihonyanagi, et al., J. Am. Chem. Soc., 126 (2004) 7034. [7] N. Aoki, et al., ChemElectroChem, in press (2016).

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The structure, chemical composition, and electronic state of the Li x Si y alloy, which formed on the Si(111) surface by electrochemical lithiation of Si, were investigated in detail by soft x-ray emission spectroscopy (SXES) combined with scanning electron microscopy (SEM) and by surface x-ray diffraction (SXRD) using synchrotron radiation (SR) as an Xray source. Electrochemically lithiated Si(111) (EC-Li x Si y ) consists of three layers; from top layer to bottom, a singlecrystalline of Li 15 Si 4 (sc-Li 15 Si 4 ) alloy, which was epitaxially grown on Si(111), an amorphous Li 15/13 Si 4 (a-Li 15/13 Si 4 ) alloy, and a mixed phase of a-Li 15/13 Si 4 alloy and crystalline Si (c-Si).