Chemical Evolution in Silicon–Graphite Composite Anodes

May 14, 2018 - polyacrylate binder (LiPAA), and 2 wt % carbon black (C-NERGY. Super C-45, Imerys). The LiPAA binder was prepared by titration of...
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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Chemical Evolution in Silicon−Graphite Composite Anodes Investigated by Vibrational Spectroscopy Rose E. Ruther,*,† Kevin A. Hays,† Seong Jin An,†,‡ Jianlin Li,†,‡ David L. Wood,†,‡ and Jagjit Nanda*,†,‡ †

Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States Bredesen Center for Interdisciplinary Research and Graduate Education, University of Tennessee, Knoxville, Tennessee 37996, United States

‡

S Supporting Information *

ABSTRACT: Silicon−graphite composites are under development for the next generation of high-capacity lithium-ion anodes, and vibrational spectroscopy is a powerful tool to identify the different mechanisms that contribute to performance loss. With alloy anodes, the underlying causes of cell failure are significantly different in half-cells with lithium metal counter electrodes compared to full cells with standard cathodes. However, most studies which take advantage of vibrational spectroscopy have only examined half-cells. In this work, a combination of FTIR and Raman spectroscopy describes several factors that lead to degradation in full pouch cells with LiNi0.5Mn0.3Co0.2O2 (NMC532) cathodes. The spectroscopic signatures evolve after longer term cycling compared to the initial formation cycles. Several side-reactions that consume lithium ions have clear FTIR signatures, and comparison to a library of reference compounds facilitates identification. Raman microspectroscopy combined with mapping shows that the composite anodes are not homogeneous but segregate into graphite-rich and silicon-rich phases. Lithiation does not proceed uniformly either. A basis analysis of Raman maps identifies electrochemically inactive regions of the anodes. The spectroscopic results presented here emphasize the importance of improving electrode processing and SEI stability to enable practical composite anodes with high silicon loadings. KEYWORDS: Raman, FTIR, silicon, graphite, anode, composite, lithium-ion battery, heterogeneity

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INTRODUCTION Commercial lithium-ion batteries primarily use graphite as the anode material due to its low cost and excellent electrochemical reversibility. Graphite reacts with lithium via an intercalation mechanism, whereby lithium ions insert between the graphene sheets. Lithium intercalation into graphite is limited to a theoretical capacity of 372 mAh/g, corresponding to the formation of LiC6.1 Materials that alloy with lithium at low voltage can offer significantly higher capacity, but this generally comes at the expense of cycle life.2 Alloy reactions typically involve large changes in volume, which can result in particle fracturing and pulverization accompanied by the continuous decomposition of electrolyte. Among the materials that alloy with lithium, silicon is very attractive due to its large theoretical capacity (3579 mAh/g corresponding to the formation of Li15Si4) and low average voltage (0.4 V vs Li/Li+).3,4 Below a critical diameter of 150 nm, silicon nanoparticles have been shown to accommodate large volume expansion (280% for full lithiation) without fracturing.5,6 The problems caused by volume expansion can be further mitigated by forming composites with carbon and/or graphite7−9 or introducing electrode architectures that accommodate volume change.10−15 © XXXX American Chemical Society

Vibrational spectroscopies are versatile techniques to characterize Si anodes. For example, crystalline silicon forms a series of amorphous lithium silicides during lithiation, and crystalline silicon does not re-form even after full delithiation.16,17 The transition from crystalline to amorphous silicon is easily probed by Raman spectroscopy.18−23 Raman mapping can also measure the uniformity of the alloy reaction over the area of the electrode18−22 and show the spatial variation of silicon and carbon in composites.18 Fourier transform infrared spectroscopy (FTIR) provides highly complementary information to Raman spectroscopy and has been used mainly to characterize the formation of the solid electrolyte interphase (SEI),23−29 silicon surface chemistry, and silicon−binder interactions.28−31 While Raman and FTIR spectroscopy have often been used to follow the electrochemistry of silicon anodes in situ, these studies are typically limited to the first few charge−discharge cycles.18,21,22,29 Received: February 5, 2018 Accepted: May 14, 2018

A

DOI: 10.1021/acsami.8b02197 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Half-cells underwent 3 formation cycles at C/20 charge and discharge rates, 94 aging cycles at C/3 rate, and 3 final cycles at C/20 rate. Raman and FTIR Spectroscopy. Electrodes were harvested from discharged cells in an argon-filled glovebox, rinsed with dimethyl carbonate (DMC), and dried under vacuum. Two 5 min washes in DMC were needed to effectively remove residual EC (Figure S1). The Supporting Information has more information on the washing procedure. Anodes were analyzed by Raman and FTIR spectroscopy without air exposure. FTIR spectra were collected in attenuated total reflectance (ATR) mode with a germanium crystal. The FTIR instrument (Bruker Alpha) was housed in an argon-filled glovebox. For analysis by Raman spectroscopy, anodes were sealed under glass in a special cell to prevent air exposure. Raman spectra were acquired with an Alpha 300 confocal Raman microscope (WITec, GmbH), a 20× objective, and a grating with 600 grooves/mm. The laser spot size was approximately 1 μm. Raman spectra were acquired using solid-state lasers with an excitation wavelength of 532 nm for anode analysis and 785 nm for cathode analysis. The laser power was attenuated to 100 μW for anode analysis and 1 mW for cathode analysis to prevent phase transitions from laser-induced heating (Figure S2). More details on the impact of laser power on the Raman spectra can be found in the Supporting Information. Raman maps were acquired over 50 μm × 50 μm areas of the anodes in 1 μm × 1 μm steps with an integration time of 15 s per pixel. Raman maps were analyzed using Witec ProjectPlus software. Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD). SEM images were collected with a Zeiss Merlin VP. X-ray diffraction patterns were collected on a PANalytical X’pert Pro powder diffractometer with Cu Kα radiation (λ ≈ 1.5418 Å).

In this contribution, we apply vibrational spectroscopy to better understand the structural and chemical changes that occur in silicon−graphite (Si−Gr) composite anodes after formation and after long-term electrochemical cycling. This study is unique in that the silicon anodes are harvested from full pouch cells with LiNi0.5Mn0.3Co0.2O2 (NMC532) cathodes. Vibrational spectroscopy has mainly been applied to study the electrochemistry of Si anodes in half-cells with lithium metal counter electrodes. However, different degradation pathways dominate in full cells compared to half-cells.32,33 Half-cells are generally built with a large excess of lithium and electrolyte, which minimizes failure mechanisms related to lithium and electrolyte consumption. Capacity fade in Si half-cells is usually due to particle fracture, particle disconnection, and impedance rise from buildup of SEI products.2 By contrast, the primary failure mechanism in full cells is loss of cyclable lithium due to parasitic side reactions.2,32,33 Here, FTIR analysis of the anode and comparison to model compounds clearly identify some side reactions that consume lithium. The FTIR studies complement Raman microscopy and mapping, which reveals spatial heterogeneity in both chemical composition and electrochemical activity of the anode. Heterogeneity lowers energy density and shortens cycle life,34 and better understanding of nonuniformity will lead directly to improvements in the performance of composite alloy anodes.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION Cycling results from NMC532/Si−Gr pouch cells demonstrate the challenges in incorporating substantial amounts of silicon (15 wt %) into composite anodes (Figure 1). Initially, the full cells with Si−Gr anodes show reasonable cycling performance with a reversible capacity of 125 mAh/gNMC (Figure 1a,b). After 15 cycles the capacity begins to decrease rapidly, and after 100 cycles the cells reach only half of the initial capacity (Figure 1b). The Coulombic efficiency initially increases from 76.4% for the first formation cycle to 99.6% but then steadily decreases to 98.5% after 96 cycles (Figure 1c). The cumulative irreversible capacity losses are also evident in the voltage profiles (Figure 1a). The dc resistance measurements (not shown) indicate that the full cell resistance increases over 40% at 3.6 V between cycles 4 and 97 from 34 to 49 Ω cm2.36 The underlying mechanisms that lead to cell failure must be understood to enable practical cells with high-capacity alloy anodes. The poor performance of the full cell cannot easily be predicted based on the performance of the anode and cathode in half-cells with lithium metal counter electrodes. The anode and cathode half-cells show significantly better capacity retention (Figure S3). After 100 cycles in half-cells, the NMC532 cathode retains 96% of its initial capacity, and the Si− Gr anode retains 71%. The poorer capacity retention in full cells points to loss of cyclable lithium as a major cause of capacity fade.2 Half-cells have a virtually infinite supply of lithium, which eliminates this failure mode. To further illustrate this point, electrodes that underwent 100 cycles in the full cell were harvested from discharged cells and paired with fresh lithium metal counter electrodes (Figure 2). The open circuit voltage (OCV) of the cycled cathode is 3.85 V vs Li/Li+, which shows that the cathode is largely delithiated and could not be fully discharged due to loss of lithium inventory. Both the cycled anode and cathode regain ≥90% of the capacity of the pristine electrodes when the lithium inventory is replenished by

Electrodes. The Cell Analysis, Modeling, and Prototyping (CAMP) Facility at Argonne National Laboratory (ANL) provided the silicon−graphite composite anodes (A-A006A) and LiNi0.5Mn0.3Co0.2O2 cathodes (A-C013A).35 The anode contained 73 wt % graphite (Hitachi MAGE), 15 wt % silicon (50−70 nm, Nanostructured and Amorphous Materials, Inc.), 10 wt % lithium polyacrylate binder (LiPAA), and 2 wt % carbon black (C-NERGY Super C-45, Imerys). The LiPAA binder was prepared by titration of an aqueous poly(acrylic acid) (PAA) solution with LiOH to pH 7. The cathode contained 90 wt % LiNi0.5Mn0.3Co0.2O2 (NCM-04ST, Toda), 5 wt % carbon black (C-NERGY Super C-45, Imerys), and 5 wt % polyvinylidene difluoride binder (PVDF, Solef 5130, Solvay). The anode and cathode had reversible capacities of 1.92 and 1.48 mAh/ cm2, respectively, at 1C rate, yielding an N/P ratio of 1.3. Cell Fabrication. Pouch cells were assembled in the DOE Battery Manufacturing R&D Facility (BMF) dry room (dew point ≤−55 °C) at Oak Ridge National Laboratory as described in earlier publications.36,37 Briefly, each pouch cell had one single-side coated anode and one single-side coated cathode divided by a porous polymer separator (Celgard 2325). The electrolyte was 90 wt % Gen 2 (Tomiyama) and 10 wt % fluoroethylene carbonate (FEC, Solvay, 30 ppm of H2O by Karl Fischer titration). Gen 2, the baseline electrolyte formulation adopted by ANL, is 1.2 M LiPF6 in ethylene carbonate (EC):ethyl methyl carbonate (EMC) (3:7 by weight). The electrolyte volume factor (defined as the volume of electrolyte divided by the total pore volume in the anode, cathode, and separator) was 3.5−4.5.36 Electrochemical Cycling. Pouch cells were cycled between 3.0 and 4.1 V for 1, 2, and 100 cycles. Cells that underwent 1 or 2 cycles were charged and discharged at C/20 rate, defined by the cathode capacity of 150 mAh/g. The cell that cycled 100 times underwent three formation cycles at C/20 charge and discharge rates, an initial dc resistance test (1 cycle), 92 aging cycles at C/3 rate, a final dc resistance test (1 cycle), and 3 final cycles at C/20 rate.35 During the aging cycles, the voltage was held at 4.1 V after the constant current charge until the current dropped to C/20. For comparison, some electrodes were also cycled in coin cells (CR2032, Hohsen) with lithium metal counter electrodes (half-cells) and the same electrolyte. Cathode half-cells were cycled between 3.0 and 4.2 V, and anode halfcells were cycled between 0.005 and 1.5 V. The cycling protocol for the half-cells was similar to the full cells without the dc resistance test. B

DOI: 10.1021/acsami.8b02197 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. Voltage profiles of pristine electrodes (black traces) and electrodes harvested from full pouch cells after 100 cycles (red traces). The electrodes were cycled at C/20 rate in coin cells with lithium metal counter electrodes and fresh electrolyte. (a) NMC532 cathode cycled between 3.0 and 4.2 V vs Li/Li+. (b) Si−Gr anode cycled between 0.005 and 1.5 V vs Li/Li+.

Figure 1. Electrochemical performance of Si-Gr/NMC full cells. (a) Voltage profiles during cycles 1, 2, and 100. (b) Capacity fade. (c) Coulombic efficiency.

the lithium metal counter electrode (Figure 2). XRD and Raman analysis of the NMC532 cathode did not detect any phase changes after cycling (Figure S4). Clearly, most of the capacity fade in the full cells is due to loss of active lithium and not intrinsic electrode failure such as particle fracture, particle isolation, or other structural changes. The electrochemical reversibility of silicon anodes depends critically on the choice of binder,8,20,31,38,39 electrolyte,19,24,40 additives,25,27,35,39,41−44 and the resultant SEI.26,32 The polymer binders and SEI components have vibrational bands that are IRactive, and monitoring changes in these signatures can provide insights into the chemical reactions that consume active lithium and lead to capacity fade. The dominant IR modes in the pristine Si−Gr anode arise from the oxide layer on silicon, the LiPAA binder, and the carbon black additive (Figure 3). Silicon oxide shows a broad absorption band from 1000 to 1250 cm−1. LiPAA has two strong modes at 1544 and 1406 cm−1 that are attributed to the asymmetric and symmetric stretches of the carboxylate (COO−) groups.45,46 The LiPAA binder used in this study has approximately 80% of the acidic protons from carboxylic acid (COOH) groups substituted by Li+.47 The broad shoulder near 1700 cm−1 in the spectrum of LiPAA is assigned to the carbonyl stretch of the remaining 20% of the COOH groups.45,46 Interactions of the binder with the silicon surface may improve the electrochemical reversibility of the anode.8,9,31,48 However, due to the relatively large fraction of binder in the anodes studied here, most of the LiPAA cannot bind directly to a solid surface. (More details and estimations of

Figure 3. FTIR spectra of the pristine Si−Gr electrode, LiPAA binder (dry), C-45 carbon black, and NanoAmor Si.

surface coverage can be found in the Supporting Information.) FTIR is sensitive to changes in the chemistry of the silicon oxide and LiPAA but not their surface interactions. Both the silicon oxide and LiPAA binder are reduced after one electrochemical cycle, contributing to irreversible lithium loss. The reactions can be followed through the changes in the FTIR spectra (Figure 4). The broad SiO2 band from 1000 to 1250 cm−1 largely disappears consistent with the electrochemical reduction of the oxide.49−51 SiO2 may be reduced to Si0 and lithium silicates (LixSiO2y) according to the following reactions: SiO2 + 4Li → Si + 2Li 2O

(1)

ySiO2 + x Li → LixSiO2y + (y − 1)Si

(2)

The formation of lithium silicates is electrochemically irreversible49−51 and contributes to the large first-cycle capacity loss (Figure 1a). The FTIR spectrum of the anode shows clear evidence for the formation of lithium silicates after one C

DOI: 10.1021/acsami.8b02197 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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about 0.5 mL of gas would be generated in the single-layer pouch cells used in this study. This volume of gas is still significant, though, since it is larger than the total pore volume in the anode, cathode, and separator (0.2 mL). The LiPAA used in this study has a capacity around 83 mAh/g assuming all the protons are reducible. Therefore, reaction 3 could contribute about 9 mAh/gSi+Gr to the irreversible capacity of the anode. This is about 10% of the measured first-cycle irreversible capacity loss for Si−Gr anodes (Figure 2b). Other side reactions that consume active lithium become apparent after long-term electrochemical cycling (Figure 4). After 100 cycles, the most obvious change is the relative increase in intensity of the mode near 1400 cm−1. This is due to the formation of lithium carbonate, which has a broad band that overlaps with the carboxylate stretches from the LiPAA binder. Li2CO3 is commonly found in the SEI formed in EC-based electrolytes, although it is debated whether Li2CO3 is an inherent component of the SEI or an artifact of water contamination.55,56 For example, lithium alkyl carbonates, which are typical SEI components observed on Si anodes,57 will react with trace water to form Li2CO3.58,59 While the Si anodes were never exposed to ambient air during sample preparation or analysis, anodes made with LiPAA binder are processed from water and tend to have higher moisture content.60 It is likely that Li2CO3 observed by FTIR is due to residual water in the electrodes and not an artifact of ex situ analysis. The buildup of Li2CO3 over 100 cycles seen here with FTIR is consistent with the imperfect passivation of the silicon anode,61 loss of lithium inventory, and decrease in Coulombic efficiency during cycling (Figure 1c).62 After 100 cycles, significantly more EC remains after washing, providing further evidence for the thickening of the SEI (Figure 4). The SEI layer on Si has been shown to be porous and trap unreacted electrolyte.63 Residual EC, characterized by ν(CO) near 1800 cm−1, is almost completely removed from the relatively thin SEI formed after 1 or 2 cycles (Figure 4). Residual EC cannot be removed from the thicker SEI that forms after 100 cycles. Even very long soaking times (2 h in DMC) were not adequate to remove the remaining EC (Figure S5). The SEI that forms on the Si−Gr anode after 100 cycles in a full cell is substantially different from the SEI that forms after 100 cycles in a half-cell with a lithium metal counter electrode (Figure S6). Si−Gr anodes cycled in half-cells show significant amounts of lithium alkoxides and lithium oxalate that do not appear when the electrodes are cycled in full cells. The differences in SEI chemistry could be due to differences in water content, cycling voltage window, or crossover of chemical species from the counter electrode. The lithium metal counter electrode used in half-cells is a strong reducing agent that could scavenge water and other reactive species. In contrast, the NMC532 cathode used in full cells is a strong oxidizing agent. Products of electrolyte oxidation could cross over to the anode and incorporate into the SEI.64,65 While the specific causes for differences in SEI chemistry are unknown, the results nonetheless emphasize that measurements with half-cells may not fully represent the chemical and electrochemical pathways that dictate full cell performance. The FTIR results describe several side reactions that lead to poor Coulombic efficiency in full cells with Si−Gr anodes including continuous SEI growth, irreversible reduction of SiO2, and deprotonation of the LiPAA binder. While the unstable SEI on silicon is likely the largest factor leading to cell failure,

Figure 4. FTIR spectra of the pristine Si−Gr anode and the Si−Gr anodes after 1 formation cycle, after 2 formation cycles, and after 100 cycles. FTIR spectra of Li2SiO3, Li4SiO4, Li2O, NanoAmor silicon, Li2CO3, LiPAA, and EC are also shown for comparison.

formation cycle (Figure 4). Broad overlapping bands between 650 and 1100 cm−1 are a good match for both Li2SiO3 and Li4SiO4. Studies with photoelectron spectroscopy49 and NMR51 suggest that Li4SiO4 is the most likely silicate that forms, but the FTIR spectra shown here cannot rule out the formation of other silicates including the possibility of amorphous phases.52 The reduction of some of the SiO2 to metallic Si and Li2O is also possible, but this reaction would not create unique signatures in FTIR. Li2O has a broad absorbance below 750 cm−1 that overlaps with silicate bands, and Si metal is transparent in the IR. To estimate the impact of the oxide layer on the irreversible capacity, we make two assumptions: (1) the silicon particles are 60 nm spheres with a 3 nm oxide shell based on unpublished transmission electron microscopy (TEM) collected by ANL, and (2) all of the oxide is converted to Li4SiO4 and Si metal. Under these conditions, the oxide layer would contribute about 25 mAh/gSi+Gr to the irreversible capacity or about 30% of the first cycle irreversible loss measured in half-cells (Figure 2b). While this value is only approximate, it gives insights into the impact that even very thin oxide layers can have on Coulombic efficiency. A second notable change in the FTIR spectrum of the Si−Gr anode after cycling is loss of the COOH carbonyl stretch near 1700 cm−1 from the LiPAA binder. The acidic protons are reduced, generating hydrogen gas: 1 −COOH + Li+ + e− → −COOLi + H 2 (3) 2 For comparison, the protons in acetic acid are reduced at −1.7 V vs SCE (approximately 1.5 V vs Li/Li+).53 The electrochemical conversion of poly(acrylic acid) (PAA) to LiPAA has also been proposed by Lucht and co-workers based on similar results from FTIR spectroscopy.54 The pouch cells showed no signs of swelling from gas generation. However, even if all the acidic protons in LiPAA were reduced to hydrogen gas, only D

DOI: 10.1021/acsami.8b02197 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. (a) SEM image of the pristine Si−Gr composite anode. (b) Optical image of the pristine Si−Gr composite anode. The area analyzed by Raman mapping is shown in the black square. (c) Raman map of the pristine Si−Gr composite anode analyzed using cluster analysis. (d) Average Raman spectra corresponding to the two clusters mapped in panel c. (e) Expanded view of the Raman spectra shown in panel d to highlight bands with lower intensity.

electrode homogeneity is also critical to maximize capacity and cycle life in lithium-ion batteries.34 The uniformity of the composition and electrochemical reactivity of the Si−Gr anodes is explored using Raman microspectroscopy. Phase segregation is evident in the SEM images of the anode (Figure 5a).36 Some of the graphite (larger, micrometer-sized particles) is relatively bare and not uniformly coated with silicon and carbon black (smaller, submicrometer particles). Raman microspectroscopy and mapping further illustrate the chemical heterogeneity (Figure 5b−e). A cluster analysis divides the Raman map of the pristine anode into two groups. The first cluster (shown in blue in Figure 5c−e) is dominated by the strong first-order optical phonon (1TO) mode of crystalline silicon at 520 cm−1 (Figure 5d).66 The first cluster also shows broad D and G bands near 1360 and 1590 cm−1 that are characteristic of carbon black67 and a C−H stretching mode at 2930 cm−1 from the LiPAA binder (Figure 5e).68 The second cluster (shown in red in Figure 5c−e) shows much weaker features from crystalline silicon and LiPAA. Also, the G band is much sharper and the D band is less intense for the second cluster, indicating that the carbon signal comes mainly from graphite (Figure 5e). The Raman maps confirm that the electrode segregates into two phases. One phase (cluster 1) is primarily silicon, carbon black, and LiPAA, while the second phase (cluster 2) is primarily graphite. These graphite-rich regions also appear brighter in the optical image of the pristine electrode (Figure 5b). The pristine anode is not a uniform composite. Spatial variations in the electrochemical reactivity can also be mapped by following changes in the silicon vibrational modes after cycling (Figure 6). The sharp band at 520 cm−1 largely disappears after one cycle, consistent with the conversion of crystalline silicon (c-Si) to amorphous silicon (a-Si) (Figure 6a).2,16 The a-Si is characterized by much weaker, broad bands near 480 cm−1 (Figure 6b).69 Most, but not all, of the silicon becomes amorphous after one electrochemical cycle. Raman maps collected before and after cycling follow the uniformity of

Figure 6. (a) Average of 2500 spectra from Raman maps (50 μm × 50 μm scan area) of a pristine Si−Gr anode and the anode after one formation cycle. (b) Expanded view of the Raman spectra shown in panel a to highlight bands with lower intensity.

the electrochemical reactions. The Raman spectra of the cycled Si−Gr anodes show contributions from four major components: c-Si, a-Si, graphite, and carbon black (Figures 6 and 7). To map the relative contribution of each of these components across the anodes, we assume that the Raman spectra of the composite are a linear combination of the Raman spectra for these components. Spectra for each component of the composite were acquired, normalized to the same unit intensity, and used as basis vectors (Figure 7). The Raman spectrum for a-Si is taken from a thin film deposited by RF sputtering.70 This approach to deconvolute the Raman spectra is not quantitative but provides insights into the relative amount of each component at each location across the map. E

DOI: 10.1021/acsami.8b02197 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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higher impedance. Regions with low impedance experience higher current densities, which leads to more rapid cell aging.34 Carbon coverage is an important criterion to consider when evaluating the local state of charge of electrodes.71 The electrochemically inactive silicon could be in regions with insufficient conductive additives. Nanoscale silicon particles are often highly agglomerated. While significant effort was made to break up these agglomerates using high energy ball milling with carbon black, some silicon agglomerates may remain and form electrochemically inactive regions. To test the hypothesis that the uncycled regions had lower concentrations of carbon additives, the spectra collected after one cycle were binned into two groups and averaged (Figure 9). One group consisted of areas with the highest concentration of c-Si (shown in red), while the other group consisted of the remaining electrode area (shown in blue) (Figure 9a). The average carbon signal from the two groups shows the same intensity, independent of the amount of c-Si (Figure 9b). This suggests that the conductive carbon black is uniformly mixed with the Si, at least on the length scale probed by the Raman microscope (approximately 1 μm). The lack of electrochemical activity could be due to submicrometer silicon agglomerates. The large volume expansion and SEI growth during lithiation of the anode could also lead to electronic isolation of some silicon particles.2,20 Further optimization is needed to improve the electrode uniformity, minimize the amount of inactive silicon, and maximize capacity utilization.

Figure 7. Raman spectra of the four primary components of cycled Si−Gr anodes: c-Si, a-Si, graphite, and carbon black.

The signal of c-Si dominates the Raman map of the pristine anode, except for a few isolated regions where the graphite signal is most prominent (Figure 8). This is consistent with the cluster analysis of the pristine anode (Figure 5). After the first formation cycle, the signal due to a-Si grows at the expense of cSi, as expected. The relative amount of c-Si decreases with increasing cycle number, but some c-Si is still present even after 100 cycles. This indicates that some silicon remains electronically or ionically isolated from the rest of the electrode and does not lithiate. This is true even for Si−Gr anodes cycled in half-cells with lithium metal counter electrodes (Figure S7). Even in cells with abundant lithium, the alloy reaction does not proceed uniformly. XRD also verified the presence of crystalline Si in the electrodes (Figure S8). While inadequate passivation of the anode is the main reason for capacity fade (Figure 1), a uniform composite structure is desirable to maximize cell performance. Even dispersion of the silicon within the graphite matrix will accommodate volume expansion more effectively than large silicon aggregates. Inhomogeneities in ionic and/or electronic conductivity also lead to underutilization of regions of the electrode with locally

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CONCLUSIONS Raman and FTIR spectroscopy provide highly complementary information to characterize Si−Gr anodes and understand factors that impact performance in full cells. Fundamental studies on full cells are important because interactions between the anode and cathode change the cell chemistry and failure

Figure 8. Raman maps of Si−Gr anodes taken before cycling (pristine), after 1 formation cycle, after 2 formation cycles, and after the full cycling protocol (100 cycles). Raman maps were generated by deconvoluting each spectrum into the sum of the four components shown in Figure 7. F

DOI: 10.1021/acsami.8b02197 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 9. (a) Raman map of the Si−Gr anode after one cycle. The Raman spectra were binned into regions with a large fraction of c-Si (red) and a small fraction of c-Si (blue). (b) Average Raman spectra from the anode mapped after one cycle comparing regions rich in c-Si to regions rich in a-Si.

Department of Energy (DOE) under Contract DE-AC0500OR22725. The work was sponsored by the Office of Energy Efficiency and Renewable Energy (EERE) Vehicle Technologies Office (VTO). We thank Bryant Polzin, Stephen Trask, and Alison Dunlop for providing the electrodes used in this study. The electrodes were produced at the DOE CAMP (Cell Analysis, Modeling and Prototyping) Facility at Argonne National Laboratory. The CAMP Facility is fully supported by VTO within the core funding of the Advanced Battery Research for Transportation Program. XRD and SEM analyses were conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. We thank Dr. Jong Kahk Keum for assistance with XRD. We thank Dr. Gabriel Veith and Jaclyn Coyle for providing the lithium silicates as reference material. We thank Dr. Ethan Self for helpful discussions during the preparation of this manuscript.

mechanisms. FTIR analysis shows that the chemical composition of the SEI is distinctly different after long-term cycling in full cells compared to half-cells with lithium metal counter electrodes. The primary cause of capacity fade in full cells with Si−Gr anodes is loss of cyclable lithium. By comparing vibrational spectra of the anodes to a library of known reference compounds, FTIR identifies several side-reactions that consume active lithium and lower the Coulombic efficiency. These include the formation of LixSiO2y and Li2CO3 and the reduction of LiPAA binder. Electrode uniformity is another area for improvement. Raman microspectroscopy combined with cluster and basis analysis evaluates heterogeneity in both electrode composition and electrochemical reactivity. The graphite does not uniformly mix with silicon in the composite anode; rather, significant phase segregation occurs between graphite and silicon. Crystalline silicon remains in isolated, micrometer-sized regions of the anode after cycling, which indicates that some silicon is electrochemically inactive and does not participate in the alloy reaction with lithium. Complete analysis of the vibrational modes enables the identification and isolation of several factors that lead to poor electrochemical performance and will advance the development of high-energy batteries with alloy anodes.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b02197. Additional FTIR and Raman spectra, half-cell cycling performance, and XRD characterization (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*(R.E.R.) E-mail [email protected]; Tel (865) 946-1578. *(J.N.) E-mail [email protected]; Tel (865) 241-8361. ORCID

Rose E. Ruther: 0000-0002-1391-902X Kevin A. Hays: 0000-0002-2085-8202 Seong Jin An: 0000-0001-7981-4418 Jianlin Li: 0000-0002-8710-9847 David L. Wood: 0000-0002-2471-4214 Jagjit Nanda: 0000-0002-6875-0057 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was conducted at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. G

DOI: 10.1021/acsami.8b02197 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.8b02197 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX