Article Cite This: J. Phys. Chem. C 2017, 121, 23333-23346
pubs.acs.org/JPCC
Auger Electrons as Probes for Composite Micro- and Nanostructured Materials: Application to Solid Electrolyte Interphases in Graphite and Silicon-Graphite Electrodes Kaushik Kalaga,† Ilya A. Shkrob,*,† Richard T. Haasch,‡ Cameron Peebles,† Javier Bareño,† and Daniel P. Abraham*,† †
Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States Frederick Seitz Material Research Laboratory, University of Illinois at UrbanaChampaign, Urbana, Illinois 61801, United States
‡
S Supporting Information *
ABSTRACT: In this study, Auger electron spectroscopy (AES) combined with ion sputtering depth profiling, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) have been used in a complementary fashion to examine chemical and microstructural changes in graphite (Gr) and silicon/graphite (Si/Gr) blends contained in the negative electrodes of lithium-ion cells. We demonstrate how AES depth profiling can be used to characterize morphology of the solid electrolyte interphase (SEI) deposits in such heterogeneous media, complementing well-established methods, such as XPS and SEM. In this way we demonstrate that the SEI does not consist of uniformly thick layers on the graphite and silicon; the thickness of the SEI layers in cycle life aged electrodes follows an exponential distribution with a mean of ca. 13 nm for the graphite and ca. 20−25 nm for the silicon nanoparticles (with a crystalline core of 50−70 nm in diameter). A “sticky-sphere” model, in which Si nanoparticles are covered with a layer of polymer binder (that is replaced by the SEI during cycling) of variable thickness, is introduced to account for the features observed.
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spectroscopy (XPS) has been used extensively8,10−21 to characterize the chemical composition and evolution of SEI deposits on Si particles. The results for Si/Gr blends are qualitatively similar, suggesting that silicon is the main contributor to SEI formation and the capacity loss in these Si/Gr blends. Many important details of SEI formation in Si and Si/Gr electrodes have been recognized. The pristine Si particles are covered with a thin silica native layer. Carboxyl groups in the polymer binder covalently bond to siloxyl groups on the surface, holding the particles together.22 Typically, the electrolyte contains organic carbonate solvents and LiPF6 salt. Hydrolysis/solvolysis of the hexafluorophosphate accelerates SEI formation in Si-containing electrodes, as the released F− replaces O2− in (the partially lithiated) SiO2 in the outer shell of the particle and also directly reacts with the Si cores.14,23 An inner (mineral) SEI layer of LiF, LixSiOy, and LiSixOyFz is formed around the core; these deposits block electrolyte access to the reactive surface, preventing its continuous reduction. Further out, the solvent reduction is incomplete, and poorly
INTRODUCTION Silicon has gained considerable attention as a promising alternative to graphite to address the emerging demand for higher energy density in lithium ion battery (LIB) cells. With the ability to store x = 3−4 Li atoms per silicon, the LixSi alloy offers a theoretical gravimetric capacity of ∼3600 mAh/g.1−3 However, amorphization of Si, considerable volume expansion and contraction during cycling, formation of unstable solid electrolyte interphase (SEI) layers, and dissolution of the active material into the electrolyte have all been shown to reduce cycle life of Si-bearing LIB cells; see, for example, refs 4−10. As an alternative to Si nanocomposites, electrodes containing blends of Si nanoparticles with graphitic carbon (Gr) are being developed.4−6 These Si/Gr blends exploit the dual advantages of high graphite conductivity and the high specific capacity of silicon, while limiting undesired electrode volume changes on cycling. We have characterized the performance of such Si/Gr electrodes as part of the U.S. DOE’s Applied Battery Research for Transportation program. As the Si and Si-Gr electrodes are cycled, scanning electron microscopy (SEM) images indicate the formation of thick SEI deposits on the surface and in the pores of the electrode matrix. Eventually these deposits isolate electrochemically active particles, and lithium cannot be deposited or extracted from these particles. X-ray photoelectron © 2017 American Chemical Society
Received: August 18, 2017 Revised: October 2, 2017 Published: October 5, 2017 23333
DOI: 10.1021/acs.jpcc.7b08279 J. Phys. Chem. C 2017, 121, 23333−23346
Article
The Journal of Physical Chemistry C
used to gain new and useful structural information. In this study, we attempt to close this knowledge gap. This is an important concern, as the familiar XPS depth profiling (e.g., ref 36) generally yields limited insight, as the effects of textural heterogeneity prevail over the features of interest. We show that this is not the case for AES. The supporting tables and figures have been placed in the Supporting Information (SI). When referenced in the text, these materials have the designator “S”, as in Figure S1.
soluble organic compounds, including semicarbonates and oligo- and polymeric materials, form a semisolid, porous layer (the outer SEI) covering the hard inner layer.14,20,23−25 Unlike graphite (that shows ∼10% expansion on lithiation), the significant volume expansion and contraction of LixSi alloys (∼280%) generates cracks in the SEI, allowing fresh electrolyte access the reactive core; so the SEI continues to form, and it becomes detached during the contraction of the particles. Eventually, these active particles become isolated by the detached SEI deposits and/or the electrolyte dries out, and the cell ceases to cycle. While that much has been established, key details of the chemical and microstructural evolution are not known. Even such fundamental characteristics as the SEI thickness are generally not known for the composite electrodes, as only thin films on flat surfaces can be examined using ellipsometry,26 neutron reflectometry,27 tapping mode atomic force microscopy,28 and other appropriate techniques. As was observed in ref 28 quite different SEI thickness estimates were obtained using different methodologies: ∼50 nm with transmission electron microscopy, 2−90 nm by X-ray photoelectron spectroscopy, up to 250 nm using secondary ion mass spectrometry, 10−500 nm by atomic force microscopy, and 5−8 nm by neutron reflectometry.27 Various methods have been pursued to provide more structural insight including, to name a few, focused ion beam (FIB) cross-sectioning,11 FIB/ SEM tomography,20,29 synchrotron X-ray computer tomography,30 electron-dispersive X-ray spectroscopy (EDX), scanning transmission electron microscopy−electron energy loss spectrometry,11 and noninvasive XPS.14 Despite this rapid progress in methods development, an understanding of the interplay between structural and chemical aspects of the SEI remains elusive. To address these concerns, we turned to a technique that so far has been infrequently used in the studies of electrochemical systems: Auger electron spectroscopy, AES.31 The only example of using this technique for lithium-ion cells was the recent study by Radvanyi et al.32,33 who demonstrated the formation of Li3Si alloy during Si lithiation. This insight was possible because AES can have enhanced chemical state selectivity as compared to XPS.31,34 However, this feature is not the only advantage that AES presents over XPS. With AES, tightly focused electron beams, rather than X-rays, can be used to excite microstructures at the surface; the spectrum quantifies the yield of the emitted Auger electrons vs their kinetic energy. Thus, AES can be easily combined with SEM, and tightly focused electron beams can be used to study micro- and nanostructures on the surface. Using modern instrumentation, electron beams with focal spots of ∼10 nm at the surface can be used to probe individual particles (see ref 35). In comparison, for common X-ray sources this spatial resolution is 1−10 μm, whereas for the synchrotron Xrays it is typically ∼100 nm. Furthermore, like XPS, AES can be combined with ion sputtering: the ion beam removes the material, and AES (which is inherently a surface technique due to the short travel distance of Auger electrons from the material) is used to probe the exposed surfaces. While these are obvious advantages, we also realized that there is a general lack of understanding as to what features can be observed and in what way these features can be interpreted when AES is applied to heterogeneous, porous, and textured media. To gain full advantage of the improved spatial resolution offered by AES it is necessary to learn how this method can be
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METHODS Materials and Aging Conditions. Table S1 in the SI lists details of the materials used in our experiments. The NCM523 electrode contained Li1.03(Ni0.5Co0.2Mn0.3)0.97O2 as active material, C45 carbon particles for improved electron conduction, and polyvinylidene fluoride as the binder. The Si/Gr electrodes contained 73 wt % graphite, C45 carbons (2 wt %), and lithiated poly(acrylic acid) (LiPAA) binder; the same LiPAA binder was used for Gr electrodes. The porosity of the composite electrodes was 42.4% for Si/Gr electrodes and 34.1% for Gr electrodes. The electrolyte consisted of 1.2 M lithium hexafluorophosphate (LiPF6) in a liquid mixture of ethylene carbonate (EC):ethyl methyl carbonate (EMC) (3:7 w/w) and 10 wt % fluoroethylene carbonate as an SEIenhancing additive. For electrochemical testing, 2032-type coin cells were assembled in argon (20 μm (Table S1 in SI), so this sputtering depth corresponds to
