Journal of The Electrochemical Society, 162 (12) A2281-A2288 (2015) A2281 0013-4651/2015/162(12)/A2281/8/$33.00 © The Electrochemical Society The Mechanism of SEI Formation on Single Crystal Si(100), Si(110) and Si(111) Electrodes U. S. Vogl, a,b S. F. Lux, a,b, * P. Das, b A. Weber, b, * T. Placke, a, * R. Kostecki, b, *, z and M. Winter a,c, **, z a Westf¨ alische Wilhelms-Universit¨ at M ¨ unster, MEET Battery Research Center, Institute of Physical Chemistry, 48149 M ¨ unster, Germany b Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley 94611, California, USA c Helmholtz Institute M ¨ unster (HI MS), 48149 M ¨ unster, Germany Subsequent to our previous studies on the SEI formation mechanism on the single crystal silicon (100) surface, here we report on complementary studies of the SEI formation on Si surfaces with the crystal orientations (111) and (110). The differences in electrochemical behavior of the different crystal orientations are discussed - especially with regard to the effect of the SEI forming electrolyte additives fluoroethylene carbonate (FEC) and vinylene carbonate (VC) added to ethylene carbonate (EC)/diethyl carbonate (DEC) based electrolytes. Fourier transform infrared spectroscopy (FTIR) of the SEI during early stages of SEI formation and physico-chemical investigations (wetting behavior) indicate a strong dependence of the chemical composition of the SEI on the surface orientation and the electrolyte composition during the early stages of lithiation of Si. However, at a higher lithiation degree less difference in the chemical composition of the SEI can be observed. These findings are in agreement with those made for the SEI formation on the Si(100) surface. © 2015 The Electrochemical Society. [DOI: 10.1149/2.0361512jes] All rights reserved. Manuscript submitted May 11, 2015; revised manuscript received July 31, 2015. Published September 1, 2015. Graphite is the current state-of-the-art anode material in lithium- ion batteries. 1–4 However, silicon is thought to replace graphite due to its higher specific capacity of 3,500 mAh/g 5 and because of its high natural abundance. 6 However, silicon is encumbered with serious drawbacks hindering its application, such as large volumetric changes of ±300% during the lithiation/de-lithiation processes, poor intrinsic electronic conductivity of silicon, and a high interfacial instability of Si electrodes in organic electrolytes. 7–14 The consequences of these huge volume changes are cracking and disintegration of the silicon electrode surface resulting in a loss of electronic contact and rapid fading of reversible capacity. 7,15,16 The performance of silicon anodes in the cell depends on the characteristics of the solid electrolyte interphase (SEI) – regularly consisting of decomposition products of the electrolyte salt, which is typically LiPF 6 , 17 and the organic carbonate solvents, which are usu- ally a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and/or diethyl carbonate (DEC), 18,19 - which is formed within the first cycles during the charge reaction by electrolyte decomposition. The SEI is essential for a good performance of any anode, including car- bonaceous and intermetallic lithium ion battery anodes. 20–25 On silicon electrodes, the electrolyte decomposition of LiPF 6 -based electrolytes starts at a potential of 1.8 V vs. Li/Li + . 26 Between 1.8 V vs. Li/Li + and 0.6 V vs. Li/Li + , the decomposition products of the electrolyte start the formation of films on the silicon surface. At 0.4 V vs. Li/Li + , the lithiation of silicon starts and goes along with the formation of new SEI compounds and new Si compounds, e.g. Si-Li, Si-F, F-Si-Li. 26 Electrolyte additives like vinylene carbonate (VC), and fluoroethy- lene carbonate (FEC) are known to affect the composition and phys- ical properties of the SEI layer on the silicon surface resulting in an improvement of the SEI formation process, the electrode cycling sta- bility, and consequently, battery lifetime and Coulombic efficiency of silicon anodes. 27–30 In our previous work, 31 the SEI on a model silicon surface with the single crystal orientation (100) was investigated. Using this model system with a silicon wafer of a defined surface orientation excludes influencing factors coming from the composite electrode composi- tion, e.g. of the binder, the current collector, or the conductive agents. Our studies of the SEI formation and composition in dependence of different charging (= lithiation) cut-off potentials and various elec- trolyte compositions, i.e., the addition of VC or FEC to the standard ∗ Electrochemical Society Active Member. ∗∗ Electrochemical Society Fellow. z E-mail: r_kostecki@lbl.gov; martin.winter@uni-muenster.de electrolyte 1M LiPF 6 EC:DEC [3:7] led to various novel findings. We could manifest different morphologies and chemical compositions of the SEI after addition of FEC and VC at a potential of 500 mV vs. Li/Li + , i.e., when the silicon is only partially lithiated, by means of SEM, EDX, IR and XPS. Remarkably, the SEI formed in different electrolytes, i.e., with and without the FEC and VC additive, shows the same composition and morphology on the crystal orientation 100 when the silicon becomes fully lithiated at a potential of 10 mV vs. Li/Li + . This could be explained by the huge impact of the volume ex- pansion and the transition of crystalline into amorphous silicon which leads to cracking of the pristine SEI and subsequent repair by new SEI products 32–34 at an operation state where the electrolyte additives have been to a large part consumed and the base electrolyte compo- nents, e.g. EC and DEC solvents and the electrolyte salt, are the major contributor in the SEI formation process. In this continuing study, we additionally focus on different crystal orientations of the Si anode, i.e., in addition to the Si(100) surface, we will also regard the Si(110) and Si(111) surfaces. Figure 1 displays the unit cells of these different surfaces as well as the view along the different directions of the diamond cubic lattice in which the black marked silicon atoms belong to a particular plane. The surface orientations strongly differ in their atomic density and surface energy. For instance, the Si(111) plane exhibits the highest atomic density and the lowest surface energy among them, whereas the Si(100) plane has the lowest atomic density and the highest surface energy. Table I lists the atomic density, spacing and surface energy (unrelaxed status) of the three different surface orientations. 35,36 From Figure 1, it can be observed that the crystalline silicon structure exhibits relatively large interstitial spaces between the atoms along the (110) direction, which are larger than those along the (100) or (111) directions. Lee et al. reported that lithium ions enter crystalline silicon preferentially through the (110) channels during the initial stages of lithiation, thus causing a volume expansion along this direction. 37 In this study we will focus on the differences in the SEI formation mechanism at the different silicon surfaces. Experimental Silicon wafers were obtained from Silicon Quest International, San Jose, CA, in the crystal orientations (100), (110) and (111). All silicon wafers are single-side polished, 500–550 μm thick, n-type doped with phosphorus and have a resistivity range of 5–10 ohm · cm. After thorough cleaning and dicing into smaller pieces of 1 × 1 cm 2 , ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.240.225.32 Downloaded on 2016-02-06 to IP