Letter pubs.acs.org/JPCL
Cite This: J. Phys. Chem. Lett. 2018, 9, 4673−4678
Interactions between Lithium, an Ionic Liquid, and Si(111) Surfaces Studied by X‑ray Photoelectron Spectroscopy Zhen Liu,*,† Guozhu Li,† Andriy Borodin,*,† Xiaoxu Liu,‡ Yao Li,§ and Frank Endres† †
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Institute of Electrochemistry, Clausthal University of Technology, Arnold-Sommerfeld-Strasse 6, 38678 Clausthal-Zellerfeld, Germany ‡ School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, China § Center for Composite Materials and Structure, Harbin Institute of Technology, Harbin, China S Supporting Information *
ABSTRACT: Investigations of the solid-electrolyte interphase formation on a silicon anode are of great interest for future lithium-ion batteries. We have studied the interactions of the ionic liquid 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide ([OMIm]Tf2N) and of lithium with Si(111) surfaces on a molecular level by X-ray photoelectron spectroscopy. The interaction of Li with [OMIm]Tf2N on Si(111) results in the decomposition of both the cation and the anion and the intercalation of lithium. Lithium atoms donate the electrons to the [OMIm]+ cation, forming Li+, and at the same time the alkyl group is detached from the cation. Excessive Li could decompose the imidazolium ring, resulting in CxHy and LiCxHyNz species and interact with the Tf2N− anions, forming LiF, LixO, F3C−O2S−N−Li+, and F3C−O2S−Li+ species. The formation of a stable Si/IL interface and of Si/Li surface alloys was proved to be an effective strategy in stabilizing Li for next-generation Li-ion batteries.
S
Si(111) and with clean Si(111), respectively, by XPS. Crystalline Si(111) substrates have oxide layers on top. Cleaning procedures are thus needed to remove the oxide layer. In general, the sample has to be flashed at 1200 °C, followed by annealing at ∼900 °C in a UHV chamber.15,16 We found that a clean Si(111) surface could be easily obtained by flashing of Cl-terminated Si(111) at 430 °C for 10 s. This study gives an insight into the interfacial properties and reactions of ILs with lithium on a silicon surface, providing information on the formation of SEI layers on semiconductor surfaces in general. Figure 1 presents representative XP spectra of the Hterminated, Cl-terminated, and clean Si(111) surfaces. For a freshly etched H-terminated Si(111) surface, the XP spectrum exhibits peaks for the Si 2p at 99.5 eV and Si 2s at 151.1 eV. In addition, trace amounts of C and O were observed at 285 and 532.5 eV, respectively. Upon chlorination, the XP spectrum shows additional peaks at 201 and 199.5 eV, indicating that this procedure yielded Cl on the surface. A high-resolution XP spectrum in the Si 2p regime shows that the peaks in the Clterminated Si(111) sample shift by 0.2 eV to higher binding energy (BE) (Figure S1) compared with that in the Hterminated Si(111) sample, whose position and intensity were consistent with the formation of a surface Si−Cl bond.17−21 The O 1s signal present on the surfaces was due to adsorbed adventitious oxygen opposed to silicon oxide, as no oxidized
ilicon has been regarded as a potential anode material for lithium-ion batteries due to its high theoretical capacity. However, the silicon anode suffers from huge volumetric changes of ∼400% during lithiation, which results in cracks and breakage and leads to poor conductivity.1−3 A stable solid electrolyte interphase (SEI) layer allows reversible lithiation/ delithiation and should be mechanically stable and flexible to adapt to electrode volume changes. The composition of the SEI layer is dependent on the cathode and anode material, the electrolyte, salt, binder, and additives because it is the decomposition of these components that forms the SEI. An in situ investigation of the composition of the SEI layer is quite challenging because both lithium and silicon are very sensitive to the environment, and the surface can be oxidized even inside of an inert-gas-filled glovebox. Conventional organic solvents are unsuited to investigations under ultrahigh vacuum (UHV) conditions. Ionic liquids (ILs), composed solely of ions, have in contrast extremely low vapor pressure. ILs have been shown as potential electrolytes for Li-ion batteries.4−6 The interfacial properties and the interfacial reaction of ILs at the IL/electrode interface have also been studied.7−9 XPS has been demonstrated to be an effective tool in monitoring the reactions in ILs.10−14 For example, the intercalation and deintercalation of lithium at the IL−graphite(0001) interface,12 the interaction of the IL [BMP][TFSA] with rutile TiO2(110) and coadsorbed lithium,13 and the interaction of lithium with thin films of the IL [OMIm]Tf2N on the copper surface14 were investigated by XPS. In this work, we compared the interaction of lithium and of an IL with each other and with surface-passivated © XXXX American Chemical Society
Received: June 14, 2018 Accepted: August 1, 2018 Published: August 1, 2018 4673
DOI: 10.1021/acs.jpclett.8b01871 J. Phys. Chem. Lett. 2018, 9, 4673−4678
Letter
The Journal of Physical Chemistry Letters
99.5 eV, respectively, indicating that a clean Si(111) surface was obtained. Thin films of the IL [OMIm]Tf2N were first evaporated on H-terminated Si(111). The XP spectra of [OMIm]Tf2N (deposition for 20 min ∼3 monolayers equivalent (MLE)) on a H-terminated Si(111) surface are shown in Figure 2, black curve. In the C 1s region, the peak at the highest BE of 294.2 eV is assigned to the two CF3 groups of the [Tf2N]− anion. The carbon signal at 288 eV is assigned to Chetero , corresponding to carbon atoms bonded to nitrogen in the imidazolium ring, and the peak at 286.1 eV can be identified as arising from Calkyl, corresponding to carbons in the alkyl chain. The N 1s peak at the higher BE of 403.4 eV is due to the nitrogen atoms from the imidazolium cation (Ncation), and the peak at lower BE 400.6 eV is due to the nitrogen atoms from the [Tf2N]− anion (Nanion). Fluorine and the oxygen show peaks at 690.1 eV in the F 1s region and at 533.9 eV in the O 1s region, respectively. These peaks arising from the IL [OMIm]Tf2N are all shifted by ∼1.3 eV to higher BE compared with that on Au(111)7 and on copper.24 These shifts are not due to a surface charging effect because the BE of Si 2p was observed at the same position as reported18,21 but are probably due to the rearrangement of the IL ions at the interface, as revealed by Foelske-Schmitz et al.,25 which leads to a BE shift. We also studied the interaction of adsorbed [OMIm]Tf2N and Li with each other and with the H-terminated Si(111) substrate. XP spectra recorded after stepwise deposition of Li on IL covered H−Si(111) are shown in Figure 2. Upon the
Figure 1. XP survey spectra of H-terminated, Cl-terminated, and clean Si(111) surfaces.
silicon was detected in detailed scans of the Si 2p spectrum (Figure S1), which usually appears at ∼103 eV. A clean Si(111) surface was obtained by annealing the Cl−Si(111) surface under UHV. After annealing, the Cl 2p and O 1s peaks disappear and a C 1s peak (the surface coverage was
