Surface Treatments for Controlling Solid Electrolyte Interphase

Article pubs.acs.org/JPCC

Surface Treatments for Controlling Solid Electrolyte Interphase Formation on Sn/Graphene Composite Anodes for HighPerformance Li-Ion Batteries Gul Zeb,† Peter Gaskell,‡ Kaiwen Hu,† Young Nam Kim,§ Xingcheng Xiao,§ Thomas Szkopek,‡ and Marta Cerruti*,† †

Mining & Materials Engineering, McGill University, 3610 University Street, Montreal, QC H3A 0C5, Canada Electrical & Computer Engineering, McGill University, 3480 University Street, Montreal, QC H3A 2A7, Canada § General Motors R&D Center, 30500 Mound Road, Warren, Michigan 48090, United States ‡

S Supporting Information *

ABSTRACT: Sn is a candidate anode material for high energy density Li-ion batteries, owing to its high specific capacity, low cost, and high electronic conductivity, but its practical applications are hindered by mechanical degradation induced by the large volume change during cycling. Graphene can be used as a buffer material for Sn volume expansion while also improving mechanical strength and electronic conductivity of the composite structure. We report here the synthesis of a composite of Sn nanoparticles and graphene through surface functionalization of graphene using diazonium grafting and subsequent Sn nanoparticle deposition. We further applied two types of surface treatments on the anode surface to improve the nucleation of the solid electrolyte interphase, which is formed due to the reduction of the electrolyte solution. These treatments include refunctionalizing the anode surface with graphene oxide sheets or sulfophenyl groups, which provide ample sites on the anode surface for the nucleation of the solid electrolyte interphase. These treatments result in the formation of a stable layer of solid electrolyte interphase, as evidenced from lower and stable charge transfer resistance at the anode interface during cycling. The anodes treated with layers of graphene oxide and sulfophenyl groups delivered reversible capacities which were 39% and 85% higher than the untreated anode. We related the enhanced electrochemical performance of the treated anodes to the formation of a stable solid electrolyte interphase layer.

1. INTRODUCTION Rechargeable Li-ion batteries based on graphite anodes are widely used in portable electronics.1−3 Graphite anodes remain dimensionally stable during lithium intercalation, as the interlayer distance between graphene layers increases moderately (10.3%), and exhibit a theoretical storage capacity of 372 mAh g−1 thanks to the formation of the intercalation compound LiC6.4−6 Current research focuses on engineering higher energy density anode materials to enable the use of Liion batteries in electric vehicles.3,7 Sn is a promising alternative to graphite owing to its higher theoretical capacity of 994 mAh g−1, due to the formation of a Li4.4Sn alloy.8−10 However, bulk Sn anodes are afflicted by quick fading of reversible capacity due to mechanical degradation during cycling. 11 The mechanical degradation of Sn-based anode particles is caused by the huge volume changes (as much as 259%) occurring during lithiation and delithiation;12 this results in pulverization and/or loss of electronic interparticle contact. Three important strategies have been developed to address this problem: (i) use of Sn nanostructures,13,14 (ii) use of compounds, intermetallics, and oxides,10,12,15−17 and (iii) use of Sn−C composites, where C acts as buffer for Sn expansion as well as provides a conducting network.18−23 © 2017 American Chemical Society

More recently, composites of Sn nanoparticles (NPs) and graphene have drawn much attention.24−32 Graphene is a suitable matrix for Sn dispersion, which offers ballistic electronic conduction,33 high surface to volume ratio resulting in smaller lithium diffusion length and improving rate capability,34 wrinkled structure which can accommodate Sn volume expansion,35,36 and mechanical strength to improve the overall stability of the composite anode.37 Some of the techniques for synthesizing Sn/graphene composites include thermal reduction of SnO2 in the presence of graphene,24,25 mechanical ball milling of Sn particles and graphitic carbon,32 thermal evaporation and subsequent deposition of Sn on graphene,29 and chemical reduction of Sn2+ or Sn4+ on graphene in solution.38 The latter strategy is performed at room temperature and produces samples with good electrical contact between Sn and graphene. In this case, the oxygencontaining groups present on graphene oxide act as nucleation sites for Sn deposition, and simultaneous reduction of graphene Received: March 23, 2017 Revised: June 17, 2017 Published: July 10, 2017 16682

DOI: 10.1021/acs.jpcc.7b02784 J. Phys. Chem. C 2017, 121, 16682−16692

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The Journal of Physical Chemistry C oxide and Sn2+ is carried out using NaBH4 reducing agent to form a Sn/graphene composite.38 The volume change during lithiation and delithiation affects not only mechanical stability of Sn as the active material but also the electrochemical stability of the solid electrolyte interphase (SEI) layer. SEI layers are formed by the irreversible reduction of electrolyte on the anode surface. A thin, continuous, conductive, and robust SEI layer is desirable for low resistance for Li-ion diffusion and stable and high capacity.39−41 However, because of the volume change in the active materials during lithium insertion and extraction, the SEI layer continuously cracks and reforms, thus becoming thick and nonuniform and causing electrical isolation of the active mass, high interparticle contact resistance, and capacity fading. Researchers have attempted to stabilize SEI formation on graphite by noncovalent polymer coating,42 surface fluorination,43 deposition of Ag NP,44 and mild oxidation to bond the SEI to the surface carboxylic groups.4,45 Adding fluoroethylenecarbonate to the electrolyte has been shown to help the formation of a thin SEI and lower capacity fading in graphene-, Si-, and Sn-based nanoanodes.41,46,47 The encapsulation of Sn-based nanoparticles with a passivating Co shell is another strategy to prevent the direct contact of Sn with the electrolyte and maintain the structural and interfacial integrity of the Sn-based core.48 Diazonium chemistry was proposed as a method to covalently bind thin organic layers on graphite, which both controlled SEI nucleation and prevented graphite exfoliation.40,49 The same method was later applied to Si-NPbased anodes and yielded higher reversible capacity and better cyclability compared to anodes made with bare Si NPs.50 These results were attributed to a robust and flexible SEI film formed on the surfaces of the modified Si NP. Here we report two types of surface treatments of a Sn/ graphene anode to form a stable SEI (Scheme 1). We

Sn/graphene anode by slurry casting (c) and treated its surface by either depositing a graphene oxide layer via electrophoresis (d) or grafting a sulfophenyl functional layer via diazonium chemistry (d′). Our results show that indeed SEI formation was stabilized through these surface treatments. In both cases, we achieved much higher reversible capacities than untreated Sn/ graphene composite anodes. The diazonium-based chemical route is simpler than the electrophoretic route and results in better electrochemical performance of the anode.

2. EXPERIMENTAL SECTION 2.1. Functionalization of Graphene. Graphene nanoplatelets (GNPs) with an average flake size of ∼10 μm and thickness of ∼3 nm were obtained from Graphene Supermarket (USA). GNP was functionalized through −C6H4−NH2 groups via diazonium chemistry. The procedure involved preparing a solution of para-phenylenediamine (0.54 g, Sigma) and sodium nitrite (0.35 g, Sigma) in 0.5 M hydrochloric acid (100 mL) to achieve 0.1 M aminophenyl diazonium cation solution. GNP (100 mg) was then dispersed in the solution under magnetic stirring for 2 h. During the process GNP reduces the diazonium cations to radicals which graft on the surface of GNP. The functionalized GNP was filtered and washed with water for at least 3 times and denoted as A-GNP. 2.2. Deposition of Sn Nanoparticles on Graphene. SnCl2 (300 mg, ≥99.99%, Sigma) was added to a mixture of ascorbic acid (300 mg, Enzo) and citric acid (300 mg, 99.5%, Sigma) in 100 mL of deionized water solution. A-GNP (100 mg) was then added to the solution under stirring. The pH of the solution was adjusted to 4.2 using NaOH (1 M). After 1 h, NaBH4 solution (100 mg in 30 mL of deionized water, Sigma) was introduced dropwise into the solution over the course of 2 h, to obtain metallic Sn over A-GNP. A composite of Sn and AGNP was obtained after the filtration over 0.8 μm cellulose acetate filter paper (Whatman). The composite was further taken through three cycles of resuspension in deionized water and filtrations, dried in air, and designated as Sn-A-GNP. 2.3. Anode Preparation. The working anodes made of SnA-GNP, A-GNP, and GNP were synthesized using a slurry casting method with polyvinylidene fluoride as binder (20% by weight), N-methyl-2-pyrrolidone as solvent, and Cu foil (25 μm, MTI Corporation) as current collector. The casted anodes were dried overnight at 60 °C in Ar atmosphere. 2.4. Anode Surface Treatment via Deposition of Graphene Oxide. The electrophoretic deposition of GO on Sn-A-GNP was carried out in an aqueous GO suspension (∼2 g/L, 0.5−5 μm flake size, Graphene Supermarket USA). A potential of 2 V was applied between Sn-A-GNP anode and stainless steel foil (50 μm, McMaster Carr USA) cathode, held 1 mm apart, for a duration of 10 min. Later, the treated electrode was washed with deionized water and designated as GO−[Sn-A-GNP]. 2.5. Anode Surface Treatment via Sulfophenyl Treatment. The grafting of sulfophenyl groups on Sn-A-GNP electrode was performed by immersing the electrodes in a solution of 0.5 M 4-sulfophenyl diazonium cations (produced in situ via reaction of NaNO2 and 4-aminobenzenesulfonic acid in HCl) for 10 min. The electrode was later washed with deionized water and designated as S−[Sn-A-GNP]. 2.6. Material Characterization. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific K-Alpha monochromatic spectrometer equipped with an Al Kα X-ray source (1486.6 eV) and an ultrahigh vacuum chamber

Scheme 1. Schematic Representation of the Synthesis and Surface Treatments of Sn/Graphene Anodes

synthesized a Sn/graphene composite by functionalizing graphene with aminophenyl functional groups via diazonium chemistry (a) and subsequently deposited Sn nanoparticles on the functionalized graphene through electroless deposition (b). We previously showed that this strategy leads to the deposition of Sn nanoparticles that are in intimate contact with highly ordered pyrolytic graphite (HOPG).51 We then fabricated the 16683


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The Journal of Physical Chemistry C (10−9 Torr). The spectral energies were calibrated by setting the binding energy of the C1s component corresponding to C− CC bonds to 284.4 eV. Thermogravimetric analysis (TGA) was carried out on a Q500 (TA Instruments) at a heating rate of 20 °C/min in air. Electron micrographs were recorded on a JEOL JSM7600F scanning electron microscope (SEM) operated at 5 kV and a Philips CM200 transmission electron microscope (TEM) operated at 200 kV. Powder X-ray diffraction (XRD) was carried out on a Bruker D8 Discover system with Co Kα (λ = 1.78896 Å) radiation. 2.7. Coin Cell Preparation and Testing. CR 2032 coin cells were fabricated in an Ar-filled glovebox with the working electrode and the Li metal as the reference and counter electrode. A microporous trilayered polypropylene (PP) and polyethylene (PE) polymer membrane (Celgard, USA) were used as the separator and 1 M LiPF6 in a mixed solution of ethylene carbonate and dimethyl carbonate (1:1 volume ratio, Novolyte, USA) as the electrolyte. Galvanostatic charging and discharging were carried out between 0.01 and 2 V for 100 cycles with a current density of 18.65 mA g−1 for GNP and AGNP and 25.8 mA g−1 for Sn-A-GNP, GO−[Sn-A-GNP], and S−[Sn-A-GNP], corresponding to the C/20 rate based on the weight of the active materials. The electrochemical impedance spectroscopy (EIS) measurements were carried out using a BioLogic VMP3 Potentiostat analyzer. The frequency range was from 1 MHz to 0.02 Hz. All impedance measurements were carried out in the fully delithiated state.

on the experimental conditions (iv), giving rise to a multilayer structure.52−54 We confirmed the functionalization of GNPs through XPS and TGA. Figure 1a shows XPS survey scans of GNP and functionalized GNP (A-GNP). See SI-1 for the complete elemental analysis. While on GNP no elements other than carbon and a small quantity of oxygen (∼1%) are found, the spectrum measured on A-GNP also reveals the presence of nitrogen (∼2%). This confirms the successful functionalization of A-GNP, which introduces N in the form of amino and azo groups (Scheme 2). These groups are evidenced on the highresolution N1s spectrum of this sample (Figure 1b), which can be fitted with three components. The component centered at ∼399 eV is assigned to the amino groups,51 and the component at ∼400.0 eV is usually attributed to an azo bridge (−NN−), indicating the growth of multilayers.55,56 The component at 401.6 eV is probably related to some ammonium salt present on the surface, as confirmed by the presence of some Cl (∼1%) detected on the survey scan.56 In addition to ammonium chloride, the formation of ammonium bicarbonate, through the reaction of amino groups, atmospheric carbon dioxide, and water, is also possible.57,58 The high-resolution C1s spectra of GNP and A-GNP are shown in Figure 1c. The broadening of the C1s peak upon functionalization can be related to the introduction of C−N bonds (at ∼285.3 eV) in GNP and is consistent with the literature.59 TGA curves of GNP and AGNP are shown in Figure 1d. The TGA of GNP shows that burning of graphene takes place between ∼500 and 700 °C in the presence of oxygen. The TGA curve of A-GNP shows an additional weight loss of up to 4% between ∼125 and 400 °C in comparison with GNP, which has to be related to the presence of functional groups on GNPs.60 The SEM image of GNPs (Figure 1e) reveals the typical wrinkled structure of the GNP. The TEM image of GNP edges (Figure 1f) indicates that the thickness of GNP is ∼2−3 nm (7−10 graphene layers). The SEM of A-GNP (Figure 1g) is very similar to that of the GNP. The functional layer on A-GNP is too thin to be measured. This is different from the thick aminophenyl layer observed on the optical micrographs of highly ordered pyrolitic graphite (HOPG) functionalized in a similar way.51 This difference is due to the comparatively very high loading of GNP (substrate) in the diazonium solution. The thin functional layer produced on the GNP is actually a desirable feature for anode applications since it is likely to generate lower resistance to electron flow. Figures 2a and b show the galvanostatic charge/discharge and correspondingly calculated Coulombic efficiency (CE) profiles for GNP and A-GNP. The first specific discharge and specific charge capacities for GNP are 2186 and 350 mAh g−1 with a CE of only 16.0%. Such a low CE for the first cycle is often observed in nanomaterials which have high surface area, due to various side reactions of the electrode and electrolytes and irreversible entrapment of Li in the SEI layer.61 The capacities and CE profiles stabilize around cycle # 40, and a reversible capacity of 225 mAh g−1 is obtained after 100 cycles with CE of 94%. The differential capacity plots for GNP (Figure 2c) show multiple cathodic peaks centered between ∼1.0 and ∼0.4 V for the first discharge half-cycle, which are associated with SEI formation.62 The insertion of Li between the GNP sheets takes place at potentials below ∼0.2 V versus Li and is observed as an envelope on the dQ/dV plot. This plot confirms that the irreversible charge consumption due to SEI formation is responsible for the very low CE observed during

3. RESULTS AND DISCUSSION 3.1. Functionalization of Graphene. We have demonstrated that the functionalization of graphite via chemical grafting of diazonium ions allows for controlled nucleation of Sn nanoparticles from solution.51 In this paper we extended this approach to graphene to synthesize Sn/graphene composites. We used the chemical grafting technique to functionalize GNP with aminophenylene-like organic coating (Scheme 2). Briefly, the reaction of para-phenylenediamine and NaNO2 in an aqueous acidic solution yields aminophenyl diazonium salt (i). When the GNP is introduced in this solution, diazonium cations are reduced to phenyl and azophenyl radicals (ii), which are spontaneously grafted on the surface of the GNP (iii). Further growth of the phenyl layer may take place depending Scheme 2. Mechanism of Grafting of the Aminophenyl Layer on Graphene

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Figure 1. XPS survey spectra (a), high-resolution spectra in N1s (b), and C1s (c) regions and TGA curves (d) of GNP and A-GNP. SEM (e) and TEM (f) images of GNP and SEM image of A-GNP (g).

capacity of 279 mAh g−1 after 100 cycles and CE of 100%. Such a big improvement compared to the results obtained on GNP highlights that a functionalization step using simple aqueous processing on GNP can significantly improve its anode performance. To understand the mechanism behind the improved performance of A-GNP, we analyzed the samples post-mortem, after 100 cycles. The SEM images of GNP and A-GNP anodes after cycling are shown in Figure 3a and b, respectively. The SEM images of GNP and A-GNP anodes before cycling (SI-4) show very similar morphology to those in Figure 1e and g, respectively. The SEI on A-GNP after cycling appears to be more continuous and consistent than on GNP (Figure 3a,b); this may be responsible for the better anode performance of AGNP. In fact, the cracks observed on GNP indicate that the SEI on this sample is mechanically instable leading to an overall thicker layer that ends up electrically isolating the active material.63,64 The EIS plots obtained for GNP and A-GNP anodes before cycling in the fully charged (delithiated) state are shown in Figure 3c. The elements of the equivalent circuit of adsorption model (inset of Figure 3c) consist of the bulk electrolyte resistance (Re), the charge transfer resistance (Rct), and the double-layer capacitance at the electrode/electrolyte interface (CPEdl) and a Warburg impedance (W) due to the Liion diffusion. The semicircle is associated with the charge transfer impedance on the interface of the electrode/electrolyte interface, and the inclined line corresponds to the Li-ion diffusion process within the anode. The charge transfer resistance (Rct) is evaluated as the diameter of the semicircle.

Figure 2. Cycling performance (a) and CE profiles (b) of GNP and AGNP anodes. The dQ/dV plots for the 1st, 10th, and 20th cycle for GNP (c) and A-GNP (d).

the first cycle of GNP. On A-GNP, the cathodic peak at ∼0.9 V is probably related to the reduction of the multilayers present as functional groups on this sample. The first cycle discharge and charge capacities for A-GNP are 2294 and 471 mAh g−1 with a CE of 20.5%, i.e., 4.5% more than for GNP, indicating an improvement due to surface functionalization. The A-GNP anode capacities stabilize around cycle # 40, with a reversible 16685


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similar Rct before cycling. The EIS plots obtained for GNP and A-GNP anodes after the 30th cycle in the fully charged (delithiated) state, and the equivalent circuit according to the surface layer model, are shown in Figure 3d. The circuit elements CPESEI and RSEI are the capacitance and resistance of the SEI film. In this case the semicircle in the high-frequency region is due to the formation of SEI film and the contact resistance, and the semicircle in the medium frequency is associated with the charge-transfer impedance on the interface of the electrode/electrolyte interface. The values of Rct and RSEI for GNPs after the 30th cycle were quantified at 474 and 351 Ω, respectively (Table 1). The values of Rct and RSEI for A-GNP were quantified at 355 and 246 Ω, respectively. Rct and RSEI decrease more after 30 cycles for the A-GNP anode than for the unfunctionalized GNP anode, suggesting the formation of a more compact SEI and easier charge transfer on the A-GNP anode. A few other researchers have reported the functionalization of graphite anodes using diazonium chemistry. Verma and Novák reported cyclability results of in situ grafted paracarboxylic benzene on graphite in aqueous solution, with and without the reduction of −COOH groups in the second step.40 According to their report, the capacity of the functionalized graphite did not increase compared to pristine graphite. However, Pan et al. observed an increase in anode capacity due to grafting of nitrobenzene groups on graphite.49 The authors also reported that the presence of nitrophenyl groups allowed for the formation of a compact and uniform SEI. Our results are thus more similar to those of Pan et al. since we show both an improvement in the capacity of the GNP-based anodes after diazonium functionalization and the formation of a more stable SEI. 3.2. Sn Deposition on the Functionalized Graphene. In order to further improve the capacity of A-GNP, Sn NPs were electrolessly deposited on A-GNP, taking advantage of the aminophenyl groups on A-GNP which can provide nucleation sites for Sn NP deposition.51 The resulting composite of Sn and

Figure 3. SEM images of GNP (a) and A-GNP (b) anodes after cycling. EIS Nyquist plots before cycling (c) and after the 30th cycle (d) for GNP and A-GNP anodes (inset: equivalent circuit models for EIS plot fitting).

The values of Rct for GNP and A-GNP were found to be 572 Ω and 559 Ω, respectively (Table 1), and both anodes exhibit Table 1. Impedance Parameters Calculated from the Equivalent Circuit Models for GNP and A-GNP Anodes before cycling GNP A-GNP

after 30 cycles

Rct (Ω)

RSEI (Ω)

Rct (Ω)

572 559

351 246

474 355

Figure 4. SEM (a), TEM (b), XRD pattern (c), XPS survey spectrum (d), and XPS depth profile in the Sn3d region (e) of Sn-A-GNP. 16686


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Figure 5. Cycling performance (a) and the corresponding Coulombic efficiency profile (b) at a rate of C/20, the dQ/dV plots for the 1st, 10th, and 20th cycle (c), and EIS plots and the calculated values of Rct and RSEI before cycling and after the 1st, 10th, and 30th cycles (d,e) of the Sn-A-GNP anode.

reversibly consumed for the lithiation of Sn (below ∼0.65 V versus Li), as the peak at ∼1.0 V is negligible. The EIS plots and the calculated values of the circuit elements Rct and RSEI obtained for the Sn-A-GNP anode before cycling and after the 1st, 10th, and 30th cycle in the fully charged (delithiated) state are shown in Figure 5d and e, respectively. The values of Rct are calculated to be 322 Ω (before cycling), 357 Ω (after 1st cycle), 107 Ω (after 10th cycle), and 189 Ω (after 30th cycle). The values of RSEI are 235 Ω (after 1st cycle), 84 Ω (after 10th cycle), and 158 Ω (after 30th cycle). Thus, the Rct and RSEI show large variability, and there is no clear trend from cycle to cycle. This is likely to be a result of unstable SEI formation. A stable SEI is required for enhanced electrochemical cycling performance; therefore, in the following we attempt to improve the stability of SEI formed on Sn-A-GNP anodes through anode surface treatments. 3.3. Anode Surface Treatment via Deposition of Graphene Oxide. We tested two different anode surface treatments to provide nucleation sites for SEI formation. In the first treatment, we deposited graphene oxide (GO) on Sn-AGNP through electrophoresis. The method of GO deposition though electrophoresis under high electric field is also known for in situ partial reduction of the deposited GO film.65 In the present work, the functional groups present on the partially reduced GO are anticipated to provide nucleation sites for SEI formation and thereby control the morphology of SEI. The partial reduction of GO is also important for the electronic conductivity of the anode. We used XPS to confirm the deposition of GO on Sn-A-GNP (Figure 6 and SI Table A2). The XPS spectra were recorded on the slurry-casted Sn-A-GNP anode before and after the deposition of GO. The attenuation of Sn/C atomic ratio by a factor of ∼1.7 upon surface treatment evidences the deposition of the functional layers on the Sn NP. The O/C atomic ratio for GO was found to be ∼0.5 (SI-3). The O/C atomic ratio for the [Sn-A-GNP] anode increased after GO deposition, from 0.15 to 0.34, by a factor of ∼2. This increase in O/C suggests the deposition of GO on the [Sn-AGNP] anode.

A-GNP was designated as Sn-A-GNP. SEM images (Figure 4a) show that Sn nanoparticles of 30−50 nm size uniformly cover the surface on GNPs. These particles are composed of ∼4 nm primary particles, as evidenced by TEM (Figure 4b). XRD (Figure 4c) reveals that the structure of Sn NP is tetragonal; some SnO2 is observed too, due to the formation of a native oxide layer on the particles. The XPS survey confirms the presence of Sn (∼3%) and an increase in the amount of O (∼12%) on the surface of the samples (Figure 4d and SI-1 for the elemental analysis). Sn3d high-resolution scans performed at different depths (Figure 4e) show that a layer of oxidized Sn is present on the surface (Sn3d5/2 at 487.3 eV), covering metallic Sn (Sn3d5/2 at 485.3 eV), which increases in intensity at increasing depth. These results confirm the metallic nature of the Sn particles and the presence of a thin native oxide layer surrounding them. The weight of Sn in Sn-A-GNP is estimated at 40% through TGA (SI-2). Figure 5a,b shows the galvanostatic charge/discharge and the CE profile for the composite Sn-A-GNP anode. The first cycle discharge and charge capacities for the composite are 1901 and 641 mAh g−1, respectively. The first cycle CE of 33.8% is significantly higher than those measured for GNP and A-GNP. The anode assumes a stable capacity profile at around cycle # 30 and delivers a CE of around 100% onward. The reversible capacity achieved after 100 cycles is 427 mAh g−1, which is 89% higher than that for bare GNP. The average capacity fading rate over the last 50 cycles is only 0.14 mAh g−1/cycle, which indicates a good cyclability of the composite anode. The dQ/ dV plot for the composite Sn-A-GNP (Figure 5c) shows that Sn lithiation takes place through a series of phase changes between 0.7 and 0.2 V versus Li; at least two delithiation peaks (at 0.46 V, 0.60 V) can be distinctly identified. The peaks at 0.9−1.0 V versus Li appearing on the first lithiation half-cycle can be related to the reduction of multilayer functional groups (Figure 2d) and partially to the reduction of SnOx (native oxides) to form metallic Sn. A portion of this peak may also be assigned to SEI formation. Therefore, it is difficult to ascertain the consumption of charge for each reaction. It is however worth noting that after the first cycle most of the charge is 16687


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Sn and result in increased contribution of Sn toward Li storage. While the morphological analysis of the anodes after cycling does not show significant differences between GO−[Sn-AGNP] and Sn-A-GNP anodes (SI-6), EIS analysis shows that the values of RSEI and Rct for the surface-treated anode (Figure 7d,e) are smaller than those measured on Sn-A-GNP and, most importantly, decrease, albeit slightly, throughout the cycles. The values of Rct are calculated to be 116 Ω (before cycling), 84 Ω (after first cycle), 81 Ω (after 10th cycle), and 55 Ω (after 30th cycle). The values of RSEI are 85 Ω (after first cycle), 73 Ω (after 10th cycle), and 63 Ω (after 30th cycle). We speculate that anode treatment results in more stable SEI formation, as shown by the lower and consistent RSEI and Rct; this in turn led to higher Li storage capability of Sn. The GO treatment of Sn-A-GNP has been performed in open air, room temperature, and aqueous acidic solution; however, it requires an electrochemical setup for electrophoretic deposition. Next we explore a simpler route for anode treatment based on diazonium chemistry. 3.4. Anode Surface Treatment via Sulfophenyl Treatment. The second anode functionalization method involved exploiting again the spontaneous grafting of in situ generated diazonium radicals. We grafted sulfophenyl radicals on the SnA-GNP anode using diazonium chemistry. Essentially the mechanism of grafting of the sulfophenyl layer on Sn-A-GNP is similar to that for aminophenyl grafting on GNP (Scheme 1). We detected an S2p peak on the XPS spectrum on S−[Sn-AGNP], which was not present on the Sn-A-GNP anode, confirming the grafting of sulfophenyl groups on the Sn-AGNP anode (Figure 8). Furthermore, the decrease in Sn/C atomic ratio by a factor of ∼1.3 on the S−[Sn-A-GNP] versus the [Sn-A-GNP] anode suggests the success in grafting functional layers on Sn NP (Figure 8 and SI Table A2). After confirming the functionalization of the [Sn-A-GNP] anode with sulfophenyl groups, we carried out galvanostatic charge/discharge tests (Figure 9a). We observed a striking

Figure 6. High-resolution XPS spectra in C1s Sn3d and O1s regions for the GO−[Sn-A-GNP] anode in comparison with the untreated [Sn-AGNP] anode.

Next, galvanostatic charge/discharge tests were carried out to observe the effect of GO deposition treatments on the anode performance (Figure 7a). We observe an improvement in anode capacity after treatment through GO. The GO−[Sn-AGNP] anode delivers a reversible capacity of 603 mAh g−1 after 60 cycles, which amounts to a relative increase in reversible capacity of 39%, versus Sn-A-GNP (434 mAh g−1 after 60 cycles). Still, the first cycle CE of the GO-treated anode (36.4%) is close to that of the untreated anode (33.8%), and the overall CE profile is similar to the profile measured on the untreated anode (Figure 7b). We further analyze the normalized dQ/dV versus V plots for GO−[Sn-A-GNP]. The peaks related to Sn lithiation are observed between ∼0.7 and ∼0.2 V versus Li. The peaks related to delithiation of Sn (∼0.45, ∼0.60 versus Li) are more evident on the 20th cycle plot. The peak intensities in the Sn lithiation and delithiation region are higher for the GO−[Sn-A-GNP] anode than for the Sn-A-GNP anode (Figure 5), indicating the contribution of Sn toward enhancement in capacities (SI-5). We speculate that surface treatments result in more consistent SEI formation on

Figure 7. Cycling performance (a) and the corresponding Coulombic efficiency profile (b) at a rate of C/20, the dQ/dV plots for the 1st, 10th, and 20th cycle (c), EIS plots and the calculated values of Rct and RSEI before cycling and after the the 1st, 10th, and 30th cycle (d,e) of the GO−[Sn-AGNP] anode. 16688


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(after 10th cycle), and 78 Ω (after 30th cycle). Surface treatment through sulfophenyl groups results in a smaller and decreasing value of RSEI and Rct through electrochemical cycling, and the anode delivers much higher reversible capacity than the untreated anode. The 1st (formation), 30th, and 60th cycle-specific discharge and charge capacities and CE for the anodes have been tabulated in Table 2. We see improvements in reversible capacity and CE after aminophenyl functionalization of GNP. This functionalization allowed controlled deposition of Sn NPs, and the resulting composite Sn-A-GNP delivered a reversible capacity of 434 mAh g−1 after 60 cycles. The GO-treated Sn-AGNP anode delivered a reversible capacity of 603 mAh g−1 after 60 cycles, which amounts to an increase of 39%, versus Sn-AGNP. The sulfophenyl-treated Sn-A-GNP delivered a reversible capacity of 802 mAh g−1 after 60 cycles, with an increase in reversible capacity of 85% versus Sn-A-GNP. This improvement in capacity is related to stable SEI formation. The larger reversible capacity of S−[Sn-A-GNP] than GO−[Sn-A-GNP] is likely due to the larger fraction of electrochemical active element (Sn) present in the former sample (SI-7). In fact, the sulphophenyl functionalization leads to the grafting of only an additional layer of molecules,66 while the GO deposition likely generated at least a few layers of GO coatings on the anode surface.

Figure 8. High-resolution XPS spectra in S2p, Sn3d, and O1s regions for the S−[Sn-A-GNP] anode in comparison with the untreated [Sn-AGNP] anode.

enhancement in anode capacity after sulfophenyl treatment, and the anode delivered a reversible capacity of 802 mAh g−1 after 60 cycles, with an increase in reversible capacity of 85% versus Sn-A-GNP. The CE profile is similar to that for GO− [Sn-A-GNP] and Sn-A-GNP anodes (Figure 9b). The lower gravimetric capacity of GO−[Sn-A-GNP] versus S−[Sn-AGNP] may partially be related to the comparatively high loading of GO layers on GO−[Sn-A-GNP] which are electrochemically less active toward Li storage. dQ/dV versus V plots reveal that the peak intensities in the Sn lithiation and delithiation region are stronger for the S−[Sn-A-GNP] anode than for the GO−[Sn-A-GNP] and Sn-A-GNP anodes (Figures 5c and 7c), indicating higher contribution of Sn toward enhancement in anode capacity. Although, again, the postmortem analysis of SEI morphology is not conclusive (SI-6), the EIS plots show the consistency of the SEI layer throughout electrochemical cycles (Figure 7d,e). The values of Rct are calculated to be 166 Ω (before cycling), 116 Ω (after the first cycle), 112 Ω (after the 10th cycle), and 92 Ω (after the 30th cycle). The values of RSEI are 85 Ω (after first cycle), 81 Ω

4. CONCLUSIONS The spontaneous grafting of an in situ generated diazonium radical is a simple and fast process performed in open air, aqueous solutions, at room temperature, and without the assistance of any electrochemical setup, which make it viable for industrial applications. In this work we used this chemistry to first synthesize Sn/graphene anodes and later used it again as a postfabrication surface treatment of the anode; we compared this method to another postfabrication surface treatment involving electrophoretic deposition of GO on the anode.

Figure 9. Cycling performance (a) and the corresponding Coulombic efficiency profile (b) at a rate of C/20, the dQ/dV plots for the 1st, 10th, and 20th cycle (c), and EIS plots and the calculated values of Rct and RSEI before cycling and after the 1st, 10th, and 30th cycle (d,e) of the S−[Sn-AGNP] anode. 16689


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Table 2. 1st (Formation), 30th, and 60th Cycle-Specific Discharge and Charge Capacities and Coulombic Efficiencies for the Anodes cycle # 1

anode

specific discharge capacity (mAh g−1)

specific charge capacity (mAh g−1)

GNP A-GNP Sn-A-GNP GO−[Sn-A-GNP] S−[Sn-A-GNP]

2186 2294 1901 2713 3727

350 471 642 988 1289

cycle # 30 Coulombic efficiency (%)

specific discharge capacity (mAh g−1)


16.0 20.5 33.8 36.4 34.6

240 304 444 625 859

216 290 441 634 854


Coulombic efficiency (%)

89.7 95.4 99.5 101.4 99.4

228 287 432 593 803

212 281 434 603 802

93.1 98.0 100.4 101.8 99.8

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank General Motor Canada, Centre de Caractérisation Microscopique des Matériaux (CM)2, Centre for SelfAssembled Chemical Structures (CSACS), Canada Research Chairs Program, Natural Sciences and Engineering Research Council, Fonds de recherche du Québec − Nature et technologies (FRQNT) for supporting this work, and Dr. X. T. Le for useful discussion.

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REFERENCES

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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/acs.jpcc.7b02784. Surface elemental analysis of GNP, A-GNP, and Sn-AGNP before mixing with binder and slurry casting and [Sn-A-GNP], GO−[Sn-A-GNP], and S−[Sn-A-GNP] anodes after mixing with binder and slurry casting, as obtained by XPS surveys (SI-1); TGA of Sn-A-GNP (SI2); XPS of GO (SI-3); SEM images of GNP and A-GNP anodes before cycling (SI-4); comparison of differential capacity plots for the composite anodes (SI-5); SEM images of [Sn-A-GNP], GO−[Sn-A-GNP], and S−[SnA-GNP] anodes after cycling (SI-6); strategies for optimizing anode coatings (SI-7) (PDF)

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Coulombic efficiency (%)

Notes

EIS data showed that both postfabrication treatments resulted in small and stable resistance of the SEI layer. This result suggests controlled nucleation of SEI and represents a great improvement compared to the unstable SEIs formed on untreated anodes. dQ/dV plots reveal that the enhancement in capacity after both surface treatments can be related to a much higher contribution of Sn than what was achieved in the untreated anodes. However, the diazonium-based treatment resulted in higher reversible capacity than the GO-treated anode, likely due to the larger fraction of electrochemically active mass than in the GO-treated anode. The sulfophenyl treatment is carried out in aqueous solution, at room temperature in the open, without the requirement of any electrochemical setup, which makes the whole process facile and viable to be carried out at larger scale. Future work should aim at improving the first-cycle Coulombic efficiency of the surface-treated anodes and optimizing the loading of the GO layers. Chemical reduction of the oxygen-rich sulfophenyl groups prior to anode assembly may provide a possible solution to the initial capacity loss (SI7).40 The rate capability tests of the surface-treated anodes and the influence of higher cycle rate on SEI formation also need further investigation.

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cycle # 60 specific discharge capacity (mAh g−1)

AUTHOR INFORMATION

Corresponding Author

*Fax: +1 (514) 398 6878. Tel.: +1 (514) 398 5496. E-mail: [email protected]. ORCID

Gul Zeb: 0000-0003-1647-6380 Kaiwen Hu: 0000-0002-4715-2998 Young Nam Kim: 0000-0001-7101-9399 16690


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Surface Treatments for Controlling Solid Electrolyte Interphase

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