Surface Functionalization of Silicon, HOPG, and Graphite Electrodes

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 24172−24180

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Surface Functionalization of Silicon, HOPG, and Graphite Electrodes: Toward an Artificial Solid Electrolyte Interface Dominique S. Moock,† Sven O. Steinmüller,†,⊥ Isabelle D. Wessely,§ Audrey Llevot,∥,# Benjamin Bitterer,∥ Michael A. R. Meier,∥ Stefan Bräse,‡,§ Helmut Ehrenberg,† and Frieder Scheiba*,†

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Institute for Applied Materials and ‡Institute of Toxicology and Genetics (ITG), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany § Institute for Organic Chemistry (IOC), Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 6, D-76131 Karlsruhe, Germany ∥ Institute for Organic Chemistry (IOC), Karlsruhe Institute of Technology (KIT), Straße am Forum 7, D-76131 Karlsruhe, Germany S Supporting Information *

ABSTRACT: Electrografting of diazonium salts containing a protected alkyne moiety was used for the first functionalization of silicon and highly ordered pyrolytic graphite model surfaces. After deprotection with tetrabutylammonium fluoride, further layers were added by the thiol-yne click chemistry. The composition of each layer was characterized via X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. The same approach was then used to functionalize graphite powder electrodes, which are classically used as negative electrode in lithium-ion batteries. The effect of the coating on the formation of the solid electrolyte layer was investigated electrochemically by cyclovoltammetry and galvanostatic measurements. The modified graphite electrodes showed different reduction peaks in the first cycle, indicating reduced and altered decomposition processes of the components. Most importantly, the electrochemical investigations show a remarkable reduction of irreversible capacity loss of the battery. KEYWORDS: solid electrolyte interface, lithium-ion battery, X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, thiol-yne click chemistry, diazonium salt

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INTRODUCTION In many applications, coatings play an important role in the modification or control of the interface properties of materials. By systematically incorporating defined structures and functional groups into coatings, materials with smart interfaces are obtained. For example, several micrometer thick scratchresistant,1,2 water-repelling, or self-cleaning surface coatings are realized.3,4 The synthesis of nanometer thick, completely defined coatings is of great interest to investigate relationships between composition, structure, and properties. Important techniques for surface functionalizations are the layer-by-layer, grafting-to, and grafting-from methods. The classic layer-by-layer technique deposits oppositely charged polymer chains via electrostatic interactions onto the surfaces.5,6 An important development in the layer-by-layer concept consisted of the substitution of the electrostatic interactions by covalent linkages, thus significantly increasing the stability.7−11 One key advantage of this method is the control of the individual layer sequence that is established orthogonally to the surface. Through the grafting methods, polymer chains are attached perpendicular to the surfaces either via addition of already synthesized (co-) polymers © 2018 American Chemical Society

(grafting-to method) or via direct polymerization on the surface (grafting-from method).12 The disadvantage of the grafting-to method is the low grafting density and that of the grafting-from method is the difficulty to characterize the chain length and sequence. To achieve a better sequence control on surface functionalization, we describe covalent step-by-step approach by successive reactions of defined, functionalized monomers for controlling the layer growth perpendicularly to the surface. In addition, further functional groups for a selective design of coating properties can be incorporated to the monomers. For example, a polarity gradient in the zdirection to the surface could be introduced. A target application of such defined surface coatings, as investigated in this contribution, is the solid electrolyte interface (SEI, usually an in situ formed protective layer) on the negative graphite electrode of the lithium-ion batteries. This protective solid electrolyte interface (SEI) is a lithium ion, but not an electron conductor. The SEI allows the transport of Received: March 26, 2018 Accepted: June 27, 2018 Published: June 27, 2018 24172

DOI: 10.1021/acsami.8b04877 ACS Appl. Mater. Interfaces 2018, 10, 24172−24180

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ACS Applied Materials & Interfaces Scheme 1. Schematic Description of Layer Structure on the Surface

simple, but rigid and well-defined character and easy synthesis. Additional, the trimethylsilyl (TMS)-protection group can be easily removed to obtain the free alkyne moiety, which can be used for further modification with compounds bearing different functional groups to influence polarity. The benefits of the thiol-yne click reaction approach are mild reaction conditions, high yields, and reaction control either catalytically, by radicals,34,35 or by photoinitiation.36 In the latter case, no radical initiators are necessary. This leads to a homogeneous and uniform molecular structure on the surface.34,37 Another possible approach is the use of photomasks to control the location of the functionalization on the substrate. Thiol-yne click chemistry thus should allow a sequence-controlled synthesis of the layers orthogonal to the surface, as described recently.38 In our investigations, model systems consisting of silicon and HOPG were used as substrates before the obtained results were transferred to natural graphite electrodes. Although silicon can be used as anode materials, too, the functionalized silicon wafers in this work were not investigated as electrode materials because of a high volume change during the interand deintercalation of lithium during charge and discharge. However, the use of HOPG and Si model systems provided an excellent reference system for comparison and optimization. Defined layers attached to the anchor groups could be realized and characterized individually by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) to prove the successful sequencecontrolled functionalization. Furthermore, the influence of these layers on the SEI formation, stability, and capacity loss in lithium-ion batteries via cyclovoltammetry and galvanostatic cycling was investigated.

lithium ions without further reduction of the electrolyte after deposition.13 In a standard lithium-ion battery, the SEI grows as a natural protection film within the first charging cycles due to the decomposition of the electrolyte solvent (usually organic carbonates) and conducting salt because the electrolyte is thermodynamically unstable in the potential range of inter- and deintercalation of lithium into graphite.14−16 This leads to an irreversible loss of capacity.17 However, the SEI is necessary for stable operation of the battery, as it protects the electrolyte from further reduction and is able to remove solvate shells around lithium ions, thus avoiding co-intercalation in the graphite and exfoliation.13 Besides the described irreversible capacity loss, further disadvantages of the natural SEI are its heterogeneous structure, which may lead to inhomogeneous current distributions and degradation of this film during the life-time of the battery, which is one of the major sources for capacity loss and finally battery failure.18 An approach to overcome these challenges is a premodification of the graphite surface before assembling them as electrodes. For this purpose, two possibilities are described in literature. A coating of the surface with polymers19−22 and a functionalization with aryl diazonium salts bearing carboxy23−26 or nitro groups26,27 improved the properties of the graphite electrodes. These electrodes show less reduction reactions in the cyclovoltammogram. Additionally, these layers lead to a decrease in the capacity loss observed during the first charging cycle(s) and to an enhanced cycling stability. Still, the structure of the coating is not fully understood and only partially defined. In this publication, we follow a different approach, which allows to incorporate functional groups within each molecular layer in a defined manner. This enables us to investigate the influence of each individual layer on the lithium transport, electrolyte protection, and desolvation. This will finally also contribute to a better understanding of the SEI layer and how its properties can be designed on a molecular level. First, anchor groups attached to the graphite surface are required for any surface functionalization. Here, diazonium salts are a useful tool to functionalize glassy carbon, highly oriented pyrolytic graphite (HOPG),28 carbon nanotubes,29,30 and graphene31 with a monolayer of functionalized aryls under mild reaction conditions. Furthermore, metal32 and silicon33 surfaces can be modified with such diazonium salts, too. To control the properties along the z-axes, which means perpendicular to the surface, and to allow a precise adjustment and investigation of the properties of the SEI, we used aryl diazonium salts with alkyne moieties for further modification via thiol-yne click chemistry, leading to a defined structured surface (Scheme 1). As aryl diazonium salt 4-(trimethylsilyl)benzene diazonium tetrafluoroborate was selected due to its

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EXPERIMENTAL SECTION

Characterization Methods. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was carried out using a ToF-SIMS 5 instrument (IONTOF GmbH). Surface spectroscopy was performed with Bi3+ as primary ion (ion current about 0.5 pA) for secondary ion generation. Scan rates of measurements were adjusted to keep the ion dose density below 1011 ions/cm2 per polarity, securing that the static SIMS limit of about 1012 ions/cm2 was not reached while performing positive and negative polarity measurements at identical sample positions. High mass resolution measurements were performed on two different positions of each sample to obtain spectral data and surface images of an area of 2000 × 2000 μm2 using 200 pixels per 1000 μm. The spectral data were analyzed and exported with the SurfaceLab software (version 6.7te) of the instrument. The images were obtained using the stage raster scan option in which several smaller image patches, here 400 × 400 μm2, were stacked together for large overview scans. The recorded data were flight time corrected using the advanced ToF correction feature of the SurfaceLab software (version 24173


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Figure 1. Electrografting of 4-(trimethylsilyl)benzene diazonium tetrafluoroborate on HOPG (left) and silicon (right). rate of 20 mV/s. Graphite electrodes were electrografted in the same cell with the same electrolyte, but here the potential was held for 10 min at −0.6 V. To remove the TMS-protecting group, the protected alkyne group was treated with a solution of 0.1 M tetrabutylammonium fluoride in anhydrous tetrahydrofuran (THF) for 30 min and then rinsed with anhydrous THF. The thiol-yne click reaction was performed in pure 2,2′(ethylenedioxy)diethanethiole under UV light (365 nm, 8 W) for 1 h. Afterward, the samples were rinsed with anhydrous acetonitrile. Electrochemical Characterization. Lithium-ion batteries were built in three-electrode cells (EL-CELL electrochemical test equipment) with LP30 (1 M solution of LiPF6 in dimethyl carbonate (DMC)/ethylene carbonate (EC) (1:1, v/v)) as electrolyte and lithium as reference and counter electrode. Galvanostatic cycling was performed between 0.01 and 1.80 V vs Li+/Li with C/20. Cyclovoltammetry was performed with a scan rate of 0.1 mV/s in the potential range of 0.01−1.80 V vs Li+/Li.

6.7) of the instrument. Brighter colors indicate higher intensity values. The color scales of specific fragments have always the same values. The sample was prepared under argon atmosphere in the glovebox of the XPS spectrometer. The transfer of the samples into the interlock of the ToF-SIMS was done in a special transfer box from IONTOF GmbH. This ensured that the samples had no contact with the atmosphere at any time. X-ray photoelectron spectroscopy (XPS) was performed with a KAlpha XPS instrument (Thermo Fisher Scientific) with an attached glovebox. Four random points on each sample were analyzed using a microfocussed, monochromated Al Kα X-ray source (400 μm spot size). The spectra were fitted with several Voigt profiles (binding energy uncertainty of ±0.2 eV). Plasmon loss features of the silicon substrate were additionally fitted to gain the real signal background and therefore correct signals of elements (e.g., Cl 2p spectra). The analyzer transmission function, Scofield sensitivity factors,39 and effective attenuation lengths (EALs) for photoelectrons were applied for quantification. EALs were calculated using the standard TPP-2M formalism.40 All the spectra were referenced to the C 1s peak (C−C, C−H) at 285.0 eV binding energy controlled by the means of the well-known photoelectron peaks of metallic Cu, Ag, and Au. All the samples were prepared under argon atmosphere in the glovebox attached to the XPS. Electrochemical experiments were carried out using a VMP3 potentiostat (BioLogic science instruments) in a climate chamber at 25 °C for battery testing and a VSP potentiostat (BioLogic science instruments) for the electrografting of aryl diazonium salts. The sample preparation was performed under argon atmosphere in a glovebox. Materials. Silicon wafers with [100] orientation were purchased from Si-Mat Silicon Materials. Highly oriented pyrolytic graphite (HOPG) was purchased from SPI in SPI-2 grade. Graphite electrodes were produced of 90% natural graphite Mechano-Cap 1P1 (H. C. Carbon GmbH) and 10% poly(vinylidene fluoride) (PVDF). A slurry from 2.0 g of this mixture and 7.0 mL N-methyl-2-pyrrolidone was prepared. This slurry was coated onto a copper foil with a layer thickness of 250 μm (wet). Surface Modifications. The following surface modifications were performed in an argon-filled glovebox with oxygen and water less than 0.1 ppm. Before use, the silicon wafer was dipped in a 40% hydrogen fluoride solution for 7 min to remove the native oxygen layer. Electrografting of the aryl diazonium salt was performed in an inhouse built cell with platinum mesh as the counter electrode and Ag/ AgNO3 in 0.01 M tetrabutylammonium perchlorate in acetonitrile as the reference electrode. The electrografting of the HOPG and silicon surface was performed in a solution of 0.01 M 4-(trimethylsilyl)benzene diazonium tetrafluoroborate dissolved in 0.1 M tetrabutylammonium tetrafluoroborate in anhydrous acetonitrile. The potential was shifted between −1.0 and +0.2 V vs Ag/AgNO3 with a scanning

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RESULTS AND DISCUSSION In a first step, HOPG and silicon wafers were used as model surfaces and 4-(trimethylsilyl)benzene diazonium tetrafluoroborate as the anchor group. In both cases, the diazonium salt was attached via electrograftication as proven by cyclovoltammetry (Figure 1).41,42 Both diagrams show a reduction current in the first cycle, which decreases in the following cycles. The disappearance of the reduction current is due to the saturation of the reaction sites on the surface and the steric shielding effect of the TMS group that prevents multilayer grafting. The observed reduction current is consistent with the grafting process of diazonium salts described in literature, which is initiated by a single electron transfer from the surface to the diazonium salt while nitrogen is degassed and an aryl radical is formed. The latter then reacts with the surface to form a covalent bond.28 A great challenge in the transfer of the grafting process on HOPG to natural graphite is the higher number of edge sites in natural graphite compared to the high oriented structure of the HOPG, which is a basal-plane-oriented graphite in which the functionalization mainly occurs on the basal-plane. The less oriented structure of natural graphite results in more edge sites. To achieve a regular coating, a grafting on these edge sites is necessary. Therefore, a graphite model electrode with mainly edge site orientation (ALS Co., Ltd.) was electrografted, too. The cyclovoltammogram in Figure 2 shows that the grafting on HOPG occurs in the same potential range as on the HOPG 24174


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TMS groups is also confirmed by ToF-SIMS imaging of the surface showing a significant contribution of fragments assigned to the TMS-protecting group like SiC3H9+ or SiCH3+ ions (Figure 5 and ToF-SIMS images Figure S1). Additionally, the binding energies related to the PVDF binder of the electrode are the same before and after the grafting process. Therefore, reactions with or an influence of PVDF on electrografting seems unlikely. In a second step, the TMS-protecting group was removed with tetrabutylammonium fluoride to prepare the anchor groups for further functionalization via thiol-yne click chemistry using the reaction conditions established in previous works for deprotecting TMS groups on the solid phase.43,44 This step was confirmed by the disappearance of the Si−C peak in the XPS (Figure 6) and the SiC3H9+ ion intensity for all three substrates in ToF-SIMS (Figure 5 and ToF-SIMS images Figure S1). The C 1s spectra are shown in the SI (Figure S3) and confirm these results, too. The obtained free alkynes were used as reactive sites for the next layer. A further thiol-yne reaction was performed with 2,2′-(ethylenedioxy)diethanethiol under UV light. This commercially available dithiol allows the introduction of polar ether functionalities. By introducing the polar groups, a positive effect on the lithium-ion transport through the grafted layers was expected. In previous works by our group, the functionalization of silicon surfaces via thiol-ene reaction was carried out with radical initiators under UV light.38 However, this can lead to impurities because the initiator remains in the layers and must be washed out. Thiol-yne reactions without a photoinitiator are described in literature.34 Therefore, an initiator-free approach was followed in this work. The formation of the thioether was verified by the presence of fragments assigned to the dithiol (C2S−, S−, CH3O−) in the ToF-SIMS measurements of the Si-wafer, the HOPG sample, and the graphite electrode (Figure 5, and ToF-SIMS images Figure S2). The XPS S 2p spectrum shows a new peak at 163.7 eV assigned to the S−C bond for both carbon substrates (Figure 6). For the Si-wafer, the most intense S 2p peak cannot be used for verification because a plasmon loss feature of silicon interferes with the S 2p peak. Therefore, the S 2s spectrum, which shows a new peak at 228.0 eV that can be attributed to the S−C bond of the thioether group (Figure 6), was measured instead.

Figure 2. Electrografting of 4-(trimethylsilyl)benzene diazonium tetrafluoroborate on edge side electrode with a diameter of 3 mm.

basal planes. This confirms that the anchor groups are also successfully attached on the graphitic edge sites. In contrast to the Si- and HOPG model electrodes, the grafting of the graphite electrode was performed at a constant potential of −0.6 V vs Ag/AgNO3 over a period of 10 min as shown in Figure 3. The constant potential grafting was necessary because the copper current collector of the electrode becomes strongly oxidized at potentials exceeding −0.5 V vs Ag/AgNO3 (Figure 3, left). Therefore, the potential was held at −0.6 V to avoid oxidation of the copper current collector while still being in the potential range of the grafting reaction (Figure 3). The reduction of the diazonium salt and its reaction with the surface are responsible for the decrease in current over time. The electrochemical data indicate a successful attachment of the aryl diazonium salt on HOPG, silicon, and natural graphite electrode. To confirm these results, XPS and ToF-SIMS measurements were additionally performed. The XPS Si 2p spectra of both substrates clearly depict the appearance of a new peak at 100.9 eV assigned to the Si−C bond of the trimethylsilyl (TMS)-protecting group. The Si wafer still shows a peak of Si−Si metal bond at 99.4 eV (Figure 4), but additionally a peak characteristic for Si−C appears. The C 1s spectra are shown in the Supporting Information (SI) (Figure S3) and confirm these results. The presence of the

Figure 3. Electrografting of 4-(trimethylsilyl)benzene diazonium tetrafluoroborate onto a real graphite electrode. Left: cyclovoltammogram of the electrode in the range of −1.0−0.4 V vs Ag/AgNO3; right: current vs time plot for a constant potential at −0.6 V. 24175


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Figure 4. Si 2p XPS spectra of graphite, HOPG, and silicon. Left: untreated surfaces; middle: electrografted diazonium salt surfaces; right: deprotected alkyne surfaces. The Si 2p spectral lines are split into two-component peaks p1/2 and p3/2 arising from spin−orbit coupling.

Figure 5. ToF-SIMS spectra of silicon, HOPG, and graphite electrode; untreated surface (black), electrografted diazonium salt surfaces (red), deprotected alkyne surfaces (blue), and thioether (green).

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Figure 7. Untreated graphite electrode (1) and three functionalized graphite electrode samples (2−4) used for electrochemical investigations. Figure 6. XPS S 2p spectra (bottom) and XPS S 2s (top) spectra. Left side shows the deprotected surfaces of HOPG (middle) and graphite electrode (bottom), as well as silicon (top). Right side shows the surfaces after the thiol click reaction on HOPG (middle), graphite (bottom), and silicon (top). The S 2p spectral lines are split into twocomponent peaks p1/2 and p3/2 arising from spin−orbit coupling.

As already discussed, the surface-grafted materials (2−4) show higher overpotentials as untreated graphite electrode (1). This is also visible in differential capacity analysis (dq/dV against V, Figure 9) of the galvanostatic measurements. The peak maxima for the charging processes of all three modified graphite electrodes (2−4) in the dq/dV diagram are shifted to more negative/positive potentials for reduction/oxidation, respectively, compared to the untreated graphite electrode (1). The peaks corresponding to the intercalation stages of lithium into graphite are observed at 0.20, 0.12, and 0.08 V for the untreated graphite electrode (1). Differences are detectable for the functionalized electrodes. Here, the maxima can be found at 0.20, 0.11, and 0.07 V for the electrode after electrografting (2) at 0.19, 0.10, and 0.07 V for the deprotected electrode (3) and at 0.18, 0.10, and 0.07 V for the graphite electrode with the thioether (4) (Figure 9). The deintercalation process shows a similar electrode behavior with peak maxima shifting to higher potentials. In summary, the functionalized surfaces show larger overpotentials, suggesting increased kinetic limitations for the lithium intercalation and deintercalation reactions. Compared to the untreated graphite electrode (1), the electrode after electrografting (2) shows a smaller capacity at the beginning and a stronger decrease within the first 10 cycles (Figure 10). As the deprotected electrode (3) shows no capacity loss within the first 10 cycles, a reaction or interaction with the trimethylsilyl group seems to be a possible explanation for the different behavior. The same behavior with a nearly constant capacity around 330 mAh/g is observed for the thioether functionalized electrode (4). The untreated graphite electrode (1) has a slightly larger initial capacity of 358 mAh/ g, but decreases more rapidly than the deprotected (3) and thioether (4) electrodes (Figure 10), evidencing a stabilizing effect of functionalization. The ether groups of the poly-

After confirming the successful functionalization of the graphite samples (Figure 7), they were used as electrodes in lithium-ion batteries. As electrolyte, a mixture of dimethyl carbonate (DMC) and ethylene carbonate (EC) without any additive was used. Cyclovoltammetry was performed for an untreated graphite electrode as a reference (1) and the functionalized graphite samples (2−4) discussed above. As presented in Figure 8, the untreated graphite electrode (1) shows three oxidation peaks and three reduction peaks related to the inter- and deintercalation of lithium within graphite.45 Compared to this, the peaks of the surface-modified electrodes (2−4) are significantly broadened in the potential range between 0.2 and 0.0 V vs Li+/Li, indicating a slowed down lithium intercalation. For the deintercalation process, the oxidation peaks are slightly shifted to higher potentials and broadened as well. This effect can be attributed to a higher overpotential of the coated materials toward lithium (de)intercalation. For a closer look at the formation of the SEI, the potential range between 0.5 and 2.5 V vs Li+/Li where electrolyte decomposition may occur was further investigated (Figure 7). The reduction peak at 0.6 V of the untreated graphite electrode (1) is generally attributed to the reduction of ethylene carbonate leading to the SEI formation.46,47 This peak clearly decreases for all three functionalized electrodes (2−4). Especially for the samples modified with the thioether (4), it disappears almost completely. In exchange, a reduction peak appears at 1.9 V for the graphite electrode after deprotection of the TMS group (3) and at 1.7 V for the thioether (4)-treated surface, respectively. 24177


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Figure 8. First cycle of the cyclovoltammogram of untreated graphite (1) and modified graphite electrodes (2−4) (left). Zoom in of the potential range between 0.5 and 2.7 V (right).

Figure 9. dq/dV plot of the first galvanostatic cycle of reference (1), electrografted (2), deprotected (3), and thioether (4) graphite electrodes.

Figure 10. Capacity of the untreated graphite (1), electrografted (2), deprotected (3), and thioether (4) graphite electrodes.

(ethylene glycol) moieties were introduced as described before to increase the polarity of the surface layers and, therefore, to enhance the diffusion of the lithium ions in the film. As can be seen, the difference between the surface with alkyne units (3) and the thioether-grafted layer (4) is very small, both in respect to intercalation kinetics (Figure 9) and cycling stability (Figure 10). A possible explanation could be that the electrografted layer is quite dense and rigid, so that the thioether layer does not significantly improve the lithium ion mobility through the electrografted layer. In addition, alkyne or alkene units may remain even after reaction with the thioether because complete reaction is impeded sterically. This could mean that the presence of alkyne or alkene units is beneficial to the formation of a stable SEI and hence for the observed improved cycling stability. This may also explain the poorer performance of the unprotected electrode sample because the alkyne group is sterically not accessible due to the TMS groups. All four electrode samples show an increase in the Coulombic efficiency after the first cycle (Figure 11). For

the untreated graphite (1) and the electrografted electrode (2), the maximum Coulombic efficiency is reached in the second cycle and decreases again afterward, whereas the efficiency loss for the untreated graphite electrode (1) is lower than that for the electrode with the first layer (2). For the two other samples, the Coulombic efficiency stays constant (Figure 11). The smaller Coulombic efficiency in the first cycle is caused by the reduction in the electrolyte and the SEI formation. In an ideal battery, the SEI would be stable after the first cycle and no further side reactions or electrolyte decomposition occurs. This would be confirmed with a Coulombic efficiency of 100% after the first cycle. The detected decrease in the untreated graphite (1) and the electrografted electrode (2) can be related to an unstable SEI. In contrast, the almost constant Coulombic efficiency of 99.5% for the deprotected electrode (3) and the thioether functionalized graphite (4) illustrates that the SEI formed on the coated electrodes shows an improved SEI stability. 24178



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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b04877.

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Synthetic procedure of diazonium salt; additional XPS spectra and ToF-SIMS images (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Dominique S. Moock: 0000-0002-2692-6181 Michael A. R. Meier: 0000-0002-4448-5279 Stefan Bräse: 0000-0003-4845-3191 Present Addresses ⊥

Since 03/2018: VDE Renewables GmbH, Siemensstrasse 30, D-63755 Alzenau, Germany (S.O.S.). # Since 09/2017: Bordeaux INP, Univ. Bordeaux, CNRS, LCPO, UMR 5629, F-33600 Pessac, France (A.L.).

Figure 11. Coulombic efficiency for 10 cycles for the untreated graphite (1), electrografted (2), deprotected (3), and thioether (4) graphite electrode.

Author Contributions

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The manuscript was written through contributions of all the authors. All the authors have given approval to the final version of the manuscript.

CONCLUSIONS We showed the successful functionalization of Si- and HOPG model electrodes by anchoring functionalized aryl diazonium salts via electrografting. After deprotection, an additional functionalization step with thiol-yne click chemistry was confirmed by XPS and ToF-SIMS. After the achievement of synthesis control of these model systems, the process was transferred to a real graphite electrode for lithium-ion batteries. The function of the attached layers as an artificial SEI in a lithium-ion battery was demonstrated. We investigated three samples on the graphite electrode after electrografting (2), after the deprotection (3), and after thiol-yne click reaction (4) and compared their performance to an untreated graphite electrode (1). The electrografted electrode (2) shows a trend similar to the untreated graphite electrode (1), but with an increased irreversible capacity loss. In contrast, the electrodes after deprotection of the TMS group (3) and after functionalization with the thioether (4) display less capacity fading than the untreated graphite electrode. This proves that the artificial protection layer enhances the stability of the SEI formed during the first cycle. However, the dq/dV diagrams show higher overpotentials for the surface functionalized samples (2−4) compared to the untreated graphite electrode (1). Because the molecular structure of the grafted layers has not yet been optimized for lithium-ion transport, this overpotential may be caused by low lithium-ion mobility in the grafted layers, which leads to a slower (de)intercalation of lithium into graphite. Nevertheless, we could demonstrate that the effect of each attached molecular layer on the electrochemical properties and the SEI formation is different and can be investigated. Hence, we successfully established a toolkit for further optimization of the artificial SEI, e.g., by adding polar functional groups to enhance lithium transport or by adding cross-linking functionality to avoid solvent migration through the film by size exclusion. This may finally lead to tailored SEI design with improved properties compared to the natural SEI film while avoiding the capacity loss caused by its formation.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the SFB 1176 funded by the German Research Foundation (DFG) in the context of project B2 for funding. This work was partially carried out with the support of the Karlsruhe Nano Micro Facility (KNMF, www.knmf.kit. edu), a Helmholtz research infrastructure at Karlsruhe Institute of Technology (KIT, www.kit.edu).

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REFERENCES

(1) Bauer, F.; Flyunt, R.; Czihal, K.; Buchmeiser, M. R.; Langguth, H.; Mehnert, R. Nano/Micro Particle Hybrid Composites for Scratch and Abrasion Resistant Polyacrylate Coatings. Macromol. Mater. Eng. 2006, 291, 493−498. (2) Mathiazhagan, A.; Joseph, R. Nanotechnology-A New Prospective in Organic Coating -Review. Int. J. Chem. Eng. Appl. 2011, 2, 225−237. (3) Lee, H. J.; Michielsen, S. Lotus Effect: Superhydrophobicity. J. Text. Inst. 2006, 97, 455−462. (4) Marmur, A. The Lotus Effect: Superhydrophobicity and Metastability. Langmuir 2004, 20, 3517−3519. (5) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Ultrathin Polymer Coatings by Complexation of Polyelectrolytes at Interfaces: Suitable Materials, Structure and Properties. Macromol. Rapid Commun. 2000, 21, 319−348. (6) Richardson, J. J.; Cui, J.; Björnmalm, M.; Braunger, J. A.; Ejima, H.; Caruso, F. Innovation in Layer-by-Layer Assembly. Chem. Rev. 2016, 116, 14828−14867. (7) Feng, X.; Cumurcu, A.; Sui, X.; Song, J.; Hempenius, M.; Vancso, G. J. Covalent Layer-by-Layer Assembly of Redox-Active Polymer Multilayers. Langmuir 2013, 29, 7257−7265. (8) Bechler, S. L.; Lynn, D. M. Reactive Polymer Multilayers Fabricated by Covalent Layer-by-Layer Assembly: 1,4-Conjugate Addition-Based Approaches to the Design of Functional Biointerfaces. Biomacromolecules 2012, 13, 1523−1532. (9) Broderick, A. H.; Manna, U.; Lynn, D. M. Covalent Layer-byLayer Assembly of Water-Permeable and Water-Impermeable

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ACS Applied Materials & Interfaces Polymer Multilayers on Highly Water-Soluble and Water-Sensitive Substrates. Chem. Mater. 2012, 24, 1786−1795. (10) Yang, W. J.; Pranantyo, D.; Neoh, K.-G.; Kang, E.-T.; Teo, S. L.-M.; Rittschof, D. Layer-by-Layer Click Deposition of Functional Polymer Coatings for Combating Marine Biofouling. Biomacromolecules 2012, 13, 2769−2780. (11) Gu, C.; Huang, X.; Shi, M.; Zhang, M.; Wang, X.; Zheng, Z. Bio-Inspired Layer-by-Layer Assembled Hybrid Films for Improvement of Mechanical Properties. Polym. Compos. 2013, 34, 1261−1268. (12) Bhat, R. R.; Tomlinson, M. R.; Wu, T.; Genzer, J. SurfaceGrafted Polymer Gradients: Formation, Characterization, and Applications. Surface-Initiated Polymerization II; Springer-Verlag: Berlin/Heidelberg, 2006; Vol. 198, pp 51−124. (13) Winter, M. The Solid Electrolyte Interphase − The Most Important and the Least Understood Solid Electrolyte in Rechargeable Li Batteries. Z. Phys. Chem. 2009, 223, 1395−1406. (14) Goodenough, J. B.; Kim, Y. Challenges for Rechargeable Li Batteries. Chem. Mater. 2010, 22, 587−603. (15) Verma, P.; Maire, P.; Novák, P. A Review of the Features and Analyses of the Solid Electrolyte Interphase in Li-Ion Batteries. Electrochim. Acta 2010, 55, 6332−6341. (16) Soto, F. A.; Ma, Y.; Martinez de la Hoz, J. M.; Seminario, J. M.; Balbuena, P. B. Formation and Growth Mechanisms of SolidElectrolyte Interphase Layers in Rechargeable Batteries. Chem. Mater. 2015, 27, 7990−8000. (17) An, S. J.; Li, J.; Daniel, C.; Mohanty, D.; Nagpure, S.; Wood, D. L. The State of Understanding of the Lithium-Ion-Battery Graphite Solid Electrolyte Interphase (SEI) and Its Relationship to Formation Cycling. Carbon 2016, 105, 52−76. (18) Xu, K. Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries. Chem. Rev. 2004, 104, 4303−4418. (19) Guo, K. Poly(Acrylonitrile) Encapsulated Graphite as Anode Materials for Lithium Ion Batteries. J. Power Sources 2002, 111, 350− 356. (20) Li, F.-S.; Wu, Y.-S.; Chou, J.; Winter, M.; Wu, N.-L. A Mechanically Robust and Highly Ion-Conductive Polymer-Blend Coating for High-Power and Long-Life Lithium-Ion Battery Anodes. Adv. Mater. 2015, 27, 130−137. (21) Veeraraghavan, B.; Paul, J.; Haran, B.; Popov, B. Study of Polypyrrole Graphite Composite as Anode Material for Secondary Lithium-Ion Batteries. J. Power Sources 2002, 109, 377−387. (22) Wu, N.-L.; Weng, Y.-T.; Li, F.-S.; Yang, N.-H.; Kuo, C.-L.; Li, D.-S. Polymeric Artificial Solid/Electrolyte Interphases for Li-Ion Batteries. Prog. Nat. Sci.: Mater. Int. 2015, 25, 563−571. (23) Pan, Q.; Wang, H.; Jiang, Y. Covalent Modification of Natural Graphite with Lithium Benzoate Multilayers via Diazonium Chemistry and Their Application in Lithium Ion Batteries. Electrochem. Commun. 2007, 9, 754−760. (24) Verma, P.; Novák, P. Formation of Artificial Solid Electrolyte Interphase by Grafting for Improving Li-Ion Intercalation and Preventing Exfoliation of Graphite. Carbon 2012, 50, 2599−2614. (25) Pan, Q.; Jiang, Y. Effect of Covalently Bonded Polysiloxane Multilayers on the Electrochemical Behavior of Graphite Electrode in Lithium Ion Batteries. J. Power Sources 2008, 178, 379−386. (26) Assresahegn, B. D.; Brousse, T.; Bélanger, D. Advances on the Use of Diazonium Chemistry for Functionalization of Materials Used in Energy Storage Systems. Carbon 2015, 92, 362−381. (27) Pan, Q.; Wang, H.; Jiang, Y. Natural Graphite Modified with Nitrophenyl Multilayers as Anode Materials for Lithium Ion Batteries. J. Mater. Chem. 2007, 17, 329−334. (28) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Savéant, J.-M. Covalent Modification of Carbon Surfaces by Aryl Radicals Generated from the Electrochemical Reduction of Diazonium Salts. J. Am. Chem. Soc. 1997, 119, 201−207. (29) Strano, M. S.; Dyke, C. A.; Usrey, M. L.; Barone, P. W.; Allen, M. J.; Shan, H.; Kittrell, C.; Hauge, R. H.; Tour, J. M.; Smalley, R. E. Electronic Structure Control of Single-Walled Carbon Nanotube Functionalization. Science 2003, 301, 1519−1522.

(30) Bahr, J. L.; Yang, J.; Kosynkin, D. V.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. Functionalization of Carbon Nanotubes by Electrochemical Reduction of Aryl Diazonium Salts: A Bucky Paper Electrode. J. Am. Chem. Soc. 2001, 123, 6536−6542. (31) Niyogi, S.; Bekyarova, E.; Itkis, M. E.; Zhang, H.; Shepperd, K.; Hicks, J.; Sprinkle, M.; Berger, C.; Lau, C. N.; DeHeer, W. A.; Conrad, E. H.; Haddon, R. C. Spectroscopy of Covalently Functionalized Graphene. Nano Lett. 2010, 10, 4061−4066. (32) Adenier, A.; Bernard, M.-C.; Chehimi, M. M.; Cabet-Deliry, E.; Desbat, B.; Fagebaume, O.; Pinson, J.; Podvorica, F. Covalent Modification of Iron Surfaces by Electrochemical Reduction of Aryldiazonium Salts. J. Am. Chem. Soc. 2001, 123, 4541−4549. (33) de Villeneuve, C. H.; Pinson, J.; Bernard, M. C.; Allongue, P. Electrochemical Formation of Close-Packed Phenyl Layers on Si(111). J. Phys. Chem. B 1997, 101, 2415−2420. (34) Lowe, A. B. Thiol-Yne ‘Click’/Coupling Chemistry and Recent Applications in Polymer and Materials Synthesis and Modification. Polymer 2014, 55, 5517−5549. (35) Lowe, A. B.; Hoyle, C. E.; Bowman, C. N. Thiol-Yne Click Chemistry: A Powerful and Versatile Methodology for Materials Synthesis. J. Mater. Chem. 2010, 20, 4745. (36) Escorihuela, J.; Marcelis, A. T. M.; Zuilhof, H. Metal-Free Click Chemistry Reactions on Surfaces. Adv. Mater. Interfaces 2015, 2, No. 1500135. (37) Lowe, A. B. Thiol−ene “Click” Reactions and Recent Applications in Polymer and Materials Synthesis: A First Update. Polym. Chem. 2014, 5, 4820−4870. (38) Llevot, A.; Steinmüller, S. O.; Bitterer, B.; Ridder, B.; Berson, J.; Walheim, S.; Schimmel, T.; Bräse, S.; Scheiba, F.; Meier, M. A. R. Sequence-Controlled Molecular Layers on Surfaces by Thiol−ene Chemistry: Synthesis and Multitechnique Characterization. Polym. Chem. 2017, 8, 5824−5828. (39) Scofield, J. H. Hartree-Slater Subshell Photoionization CrossSections at 1254 and 1487 EV. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129−137. (40) Tanuma, S.; Powell, C. J.; Penn, D. R. Calculations of Electron Inelastic Mean Free Paths. V. Data for 14 Organic Compounds over the 50−2000 EV Range. Surf. Interface Anal. 1994, 21, 165−176. (41) Pinson, J.; Podvorica, F. Attachment of Organic Layers to Conductive or Semiconductive Surfaces by Reduction of Diazonium Salts. Chem. Soc. Rev. 2005, 34, 429. (42) Delamar, M.; Hitmi, R.; Pinson, J.; Saveant, J. M. Covalent Modification of Carbon Surfaces by Grafting of Functionalized Aryl Radicals Produced from Electrochemical Reduction of Diazonium Salts. J. Am. Chem. Soc. 1992, 114, 5883−5884. (43) Wezeman, T.; Zhong, S.; Nieger, M.; Bräse, S. Synthesis of Highly Functionalized 4-Aminoquinolines. Angew. Chem., Int. Ed. 2016, 55, 3823−3827. (44) Protasova, I.; Bulat, B.; Jung, N.; Bräse, S. Synthesis of Diaziridines and Diazirines via Resin-Bound Sulfonyl Oximes. Org. Lett. 2017, 19, 34−37. (45) Winter, M.; Besenhard, J. O.; Spahr, M. E.; Novák, P. Insertion Electrode Materials for Rechargeable Lithium Batteries. Adv. Mater. 1998, 10, 725−763. (46) Imhof, R. In Situ Investigation of the Electrochemical Reduction of Carbonate Electrolyte Solutions at Graphite Electrodes. J. Electrochem. Soc. 1998, 145, 1081. (47) Wang, Y.; Nakamura, S.; Ue, M.; Balbuena, P. B. Theoretical Studies to Understand Surface Chemistry on Carbon Anodes for Lithium-Ion Batteries: Reduction Mechanisms of Ethylene Carbonate. J. Am. Chem. Soc. 2001, 123, 11708−11718.

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Surface Functionalization of Silicon, HOPG, and Graphite Electrodes

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