Silicon Few-Layer Graphene Nanocomposite as High-Capacity and

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Silicon-Few Layer Graphene Nanocomposite as HighCapacity and High-Rate Anode in Lithium-Ion Batteries Stefano Palumbo, Laura Silvestri, Alberto Ansaldo, Rosaria Brescia, Francesco Bonaccorso, and Vittorio Pellegrini ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01927 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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ACS Applied Energy Materials 1

Silicon-Few Layer Graphene Nanocomposite as High-Capacity and High-Rate Anode in Lithium-Ion Batteries Stefano Palumbo1,2,3*, Laura Silvestri1, Alberto Ansaldo1, Rosaria Brescia4, Francesco Bonaccorso1,5*, Vittorio Pellegrini1,5 1

Istituto Italiano di Tecnologia, IIT, Graphene Labs, Via Morego 30, Genova, Italy

Politecnico di Torino, Dipartimento di Elettronica e Telecomunicazioni, Corso Duca degli Abruzzi 24, Torino, Italy 2

Istituto Nazionale di Ricerca Metrologica, INRIM, Metrology for Quality of life Dept., Strada delle Cacce 91, Torino, Italy 3

4

Electron Microscopy Facility, Istituto Italiano di Tecnologia, IIT Via Morego 30, Genova, Italy

5

Bedimensional Spa, Via Albisola 121, Genova, Italy

KEYWORDS Silicon, Graphene, nano-structured composite, large scale production, high capacity anode, lithiumion batteries. ABSTRACT: A silicon-graphene hetero-structure provides optimal electrochemical performance as anode nanomaterial both in half and full cells with a commercial NMC111 (LiNi1/3Mn1/3Co1/3O2) cathode. The anode consists of carbon-coated polycrystalline silicon nanoparticles in between a parallel oriented few-layers graphene flakes (FLG). Electrochemical tests in lithium cells display high capacity values (≈2300 mAh/g) with a Coulombic efficiency (CE) reaching 99% at current density of 350 mA/g and 1000 mAh/g at current density values up to 3.5 A/g (CE=99%). The laminated graphene-based structure yields a protective coating to the silicon nanoparticles still enabling exposure to lithium ions. The method of production of the laminated silicon-graphene nanocomposite is scalable and low-cost, offering a practical route to the introduction of high silicon content anodes in lithium-ion batteries.

INTRODUCTION The continuous development of lithium-ion batteries (LIBs) technology (1-2-3-4) over the years led to large improvements in terms of specific capacity, energy density and cycle life stability, enabling the introduction of LIBs into the market of hybrid and electric vehicles (5). For example, LIBs increased their efficiency by 5–10% every year for the past 25 years (6). In fact, while in 1990s the highest energy density value for LIBs was ≈90 Wh kg−1 (7), currently their value approaches 260 Wh kg−1 (5). Nevertheless, development of LIBs able to satisfy the growing demand in terms of energy densities and fast charging times poses new challenges, which cannot be overcome by exploiting conventional electrode materials, due to their intrinsic theoretical limitations (6,8-9). Conventional LIBs consist of graphite-based anodes and lithiated transition metal oxide cathodes, such as LiCoO2 (2). These cells typically supply ≈120 Whkg-1 (2). In these electrochemical systems, the redox process is based on intercalation/de-intercalation of lithium ions between the two layered materials (10), a

mechanism that displays fundamental limitations preventing further improvements. In fact, considering the anode for example, graphite is able to exchange only 1 Li+ every 6C atoms, achieving a theoretical specific capacity of 372 mAhg-1 (11-12). Alternative processes such as alloying (13) or conversion (14) are exploited to overcome these limitations, leading to higher capacity values, which consequently increase the energy density of the currents LIBs (10,15). Silicon, currently, represents one of the most promising alloying anode material to replace graphite-based anodes (13,16–18). Silicon can theoretically react in a lithium cell through an alloying reaction to form Li4.4Si, achieving a specific capacity of 4200 mAhg-1 (19). However, several issues affect silicon, as well as other alloying materials, hindering its implementation in commercial batteries (20). In particular, the main issue of silicon is associated to the significant volume changes (>300%) during the lithiation/de-lithiation processes (21). The volume change produces cracks and pulverization of the electrode leading to the loss of electrical contact and the consequent poor cycling performance (22). The continuous volume expansion/contraction of

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silicon during cycling also negatively affects the electrode-electrolyte interface (23). In fact, the exposure of fresh electrode surface (due to expansion) determines a continuous electrolyte reduction during the subsequent cycles, resulting in an unstable SEI (solid electrolyte interface) layer formation (23). To overcome the aforementioned issues several strategies have been adopted mainly focussing on the design of advanced electrode structures (24), including, in particular, the exploitation of silicon nanoparticles (25) and their encapsulation into a carbon matrix (26). Several experimental works (25,27-28) have proven the existence of a relationship between silicon particles size and the electrochemical performance, demonstrating that the use of not-agglomerated nanosized silicon can suppress particles cracking upon lithiation. However, besides controlling the size of silicon particles, also their morphology (i.e., nanospheres (29–31), nanowires (32) or nanotubes (33)) is known to affect the electrochemical properties. In addition to the nanostructuring approach (29– 33), the realization of a composite of silicon nanoparticles with a conductive carbon material represents another established way to mitigate the volumetric changes, thus improving the performance of the electrode in terms of both cycle life and rate, ensuring the electrical contact of the electrode (34– 36). This approach proved to be effective only when a carbon nanostructure is able to enclose the silicon nanoparticles (34–36). In fact, only minor improvements are observed by simply mixing silicon nanoparticles with an amorphous carbon or by carbon coating methods (17, 37). Contrariwise, more complex strategies, such as the encapsulation of silicon into carbon nanostructure with empty spaces (38) as well as their infiltration into a nanoporous matrix, allow silicon nanoparticles to freely expand and contract, with a positive effect on cyclability (18). Among the explored conductive carbon structures (34–36), graphene (39–41) has recently emerged as a promising silicon partnering material owing to its excellent mechanical properties (42) and electrical conductivity (43). Several approaches have been proposed during the last years to successfully integrate graphene and silicon nanoparticles to obtain anodes with long cycle life (44–56). So far, the most common graphene-based material used in combination with silicon is reduced graphene oxide (RGO) (57–61). In fact, the presence of oxygencontaining functional group makes the RGO flakes hydrophilic, thus enabling the possibility to use a large variety of solvents, e.g., water (41,62). This means that through simple synthetic strategies, graphene can be functionalized or modified with electroactive species through covalent or noncovalent bond (63–65). Another strategy relies on the use of

single- or few-layers graphene (SLG and FLG, respectively), which, from one hand accommodate the electroactive material and from the other hand provides a much more effective pathway for electrons conductivity (66-67). Further examples to improve the performance of silicon-based electrodes make use of nitrogen-doped graphene (68). In fact, it was experimentally demonstrated (69-72) that the doping of graphene flakes with nitrogen atoms modifies the graphene electronic structure (73) and improves the lithium diffusion kinetics and transfer (74), resulting in a beneficial effect on its electrochemical performance (75–77). Different methods have been explored to produce silicon-graphene composites (26,66-67,78–83). Although the simple physical mixing of silicon nanoparticles and graphene flakes could represent an easy way to have a low cost and scalable method for the industrial production of electrodes (80-81), it has not always a validation in the cell electrochemical performance (82). Alternative methodologies enabling the in-situ growth of graphene in order to obtain a controlled morphology of the silicongraphene composite (26,66-67,83-84) as well as the synthesis of silicon nanoparticles onto graphene surface (78-79,85), demonstrated to be more effective to optimize the performance in lithium cell, but with the downside to be hardly compatible with large scale industrial production (86–88). Here, we present a laminated silicon-FLG composite to be used as anodic material in lithium cell that combines a facile and low-cost method for its preparation with high and stable electrochemical performances. Pristine materials include commercial silicon nanoparticles and FLG flakes produced by a wet jet mill (WJM) process, which enables the scaling-up of the production of high quality few-layers graphene (89). Electrode preparation exploits a three-step process. Firstly, by using ultra-sonication, FLG flakes and silicon nanoparticles are dispersed in a common solvent. Then, the obtained slurry is directly deposited on the copper current collector. Finally, the electrode is subjected to a thermal treatment in a reducing atmosphere (90). We show that the as-prepared electrodes are formed by FLG flakes with a preferential orientation parallel to the electrode surface, that entrap polycrystalline carbon-coated silicon nanoparticles with overall silicon content as high as 57%. The resulting laminated silicon-FLG electrode shows high specific capacity values (>2000 mAhg-1), being able to sustain current densities up to 7 A/g, with a Coulombic Efficiency (CE) of 99%. Furthermore, we also demonstrate the use of silicon-FLG electrode in a full lithium-ion cell using NMC111 (LiNi1/3Mn1/3Co1/3O2) as positive electrode. Electrochemical tests at C/2 shows that the cell can exchange 1800 mAhg-1


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(referred to silicon mass) reaching a CE of 99%. These results surpass previous attempts to develop commercially-suitable solutions of high silicon content composites for Li-ion cells.

MATERIAL AND METHODS Materials Silicon nanoparticles ( 97%, Sigma Aldrich) by a mechanical stirrer (Eurostar digital Ika-Werke) (89). A hydraulic mechanism and a piston supplied a pressure between 180-250 MPa to push the as-prepared mixture into a set of 5 different perforated and interconnected disks, called processor. Two jet streams were generated at the second disk, which have two holes with diameters of 1 mm. Then, the jet streams collided between the second and the third disk, which consists of a nozzle (i.e., a half-cylinder channel) of a diameter of 0.3 mm. The shear force generated by the solvent when the sample passes through such nozzle promotes the graphite exfoliation (89). The sample was then separated in two jet streams, recombining in the last disk before leaving the processor. Immediately after the processor, the sample was cooled down by means of a chiller. The processed sample was then collected. The wet-jet milling process was repeated passing the sample through the 0.15 mm nozzle. Finally, a third exfoliation step was carried out by adjusting the diameter of the nozzle to 0.10 mm. The dispersion in NMP was then subjected to rotovapor and freeze drying to completely remove the solvent. Electrode Preparation Silicon-FLG electrodes were prepared by dispersing equal masses of silicon nanoparticles, FLG flakes and polyacrylic acid (PAA) in ethanol by ultra-sonication for 90 minutes. The mass loading of the active material was 30 mg/ml. 50 μl of the obtained dispersion (corresponding to a mass of 1.5 mg of active material) was subsequently deposited, by drop casting, onto a copper disk (15 mm diameter) and then dried in air for 5 minutes. Finally, the electrodes were annealed in a tubular furnace under a controlled atmosphere (vacuum + 5 sccm of H2) for 30 minutes at 750 °C with a heating rate of 15° C / min up to 700 °C slowing down to 5 °C/min to reach 750 °C. To preserve the samples from an undesired bending of the copper disk, a steel foil was super-imposed during the process. Physical-Chemical Characterization

The as-prepared silicon-FLG electrode was characterized via a set of techniques. Scanning electron microscopy (SEM) images of the electrodes were acquired by a JEOL JSM-6490LA instrument. High-resolution transmission electron microscopy (HRTEM) and energy-filtered TEM (EFTEM) analyses were carried out on an image-corrected JEOL JEM2200FS TEM (Schottky emitter source), operated at 200 kV, equipped with an in-column image filter (Ώtype) and a Bruker XFlash 5060 energy-dispersive Xray spectrometer (EDS). EFTEM maps reported here were obtained using the three-window method at the C K and O K ionization edges. To prepare the grid for TEM analysis, the electrodes were first sonicated in ethanol and then 100 μL of each dispersion were drop cast onto a holey amorphous carbon-coated copper grid. Raman measurements were carried out for siliconFLG electrodes before and after the annealing process. The Raman spectra were acquired by using a Renishaw inVia micro-Raman spectrometer with the 514.5nm line of an Ar+ laser. The incident laser was focused by a 50× optical microscope objective with a numerical aperture of 0.95 and then the scattered light was detected in a back-scattering geometry dispersed by a 2400 l/mm holografic grating. All the spectra were collected by acquiring 3 accumulations of 30s each. Acquired spectra were analysed by OriginLab OriginPro 2016 using Voight curves in order to deconvolute the main Lorentzian components. X-ray diffraction (XRD) patterns were recorded on a PANalytical Empyrean X-ray diffractometer equipped with a 1.8kW CuKα ceramic X-ray tube, PIXcel3D 2×2 area detector and operating at 45 kV and 40 mA. The diffraction patterns were collected in air at room temperature using Parallel-Beam (PB) geometry and symmetric reflection mode. X-ray diffraction data analysis was carried out using HighScore 4.7 software (91) from PANalytical. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos Axis UltraDLD spectrometer, using a monochromatic Al Kα source operated at 15 kV and 20 mA. Highresolution narrow scans were acquired on Si 2p core levels at constant pass energy of 10 eV in steps of 0.1 eV. The photoelectrons were detected at a take-off angle of Φ= 0° with respect to the surface normal. The data were processed with Casa XPS software, version 2.3.17. Silicon spectra fitting has been performed by considering Voigt profiles; for elemental silicon, an intensity ratio of 2:1 between the two components of the doublet and a doublet separation of 0.6 eV have been assumed (92). To evaluate the silicon content in the composite, a thermogravimetric analysis (TGA) was performed using a TGA Q500-TA Instrument. As analytic gas, air with a flux of 50 ml/min was implied. The method used during the measurements provides an initial stabilization of the sample at 30 °C for 5 minutes,



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then a heat rump of 5 °C/min up to a final temperature of 800 °C maintained for one hour. For electrochemical characterization, coin cells 2032 were assembled facing the as-prepared silicongraphene electrode with a lithium foil. A solution consisted of LP30 (1M LiPF6 in DMC:EC) and 10% v/v FEC (fluoro-ethylene carbonate) embedded in a Whatman borosilicate separator was used as electrolyte. The assembled lithium cells were tested in a Biologic VMP3 potentiostat through galvanostatic cycling and potentiodynamic cycling with galvanostatic acceleration (PCGA) measurements. Galvanostatic cycling was carried out using the potential range of 0.1-1.0 V at different current density from 0.35A/g to 7A/g. Electrochemical Impedance

spectra were acquired upon cycling by applying a potential signal of 5 mV amplitude in the frequency range 100kHz-100mHz. The PCGA measurements were performed setting a potential step of 5mV and a cut-off of C/20 between 0.1 V and 1.0 V. Before any electrochemical test, a formation cycle was performed between 0.01-2.0 V and applying a current density of 0.35A/g, corresponding to a C rate of ≈C/10. Finally, a complete lithium ion cell was assembled using silicon-FLG electrode as anode, a commercial NMC111, with a nominal capacity of 2mAh/cm2 electrode as cathode and LP30+10% v/v of FEC as electrolyte. The electrochemical behaviour of the asassembled cell was tested by galvanostatic cycling in the potential range of

Figure 1. (a-c) Cross section and top view SEM micrographs of the silicon-FLG electrode before and (d-f) after annealing. 2.8-4.0 V, using a C-rate of C/2. In order to provide a stable electrode-electrolyte interphase, a formation procedure was carried out before cycling the Li-ion cell and consisting of: (i) a first charge to 4.0 V at C/10, followed by a potentiostatic step with a current cut-off corresponding to C/20; (ii) first discharge at 2.8 V at C/10 followed by a rest time of 3 hours; then (iii) two charge-discharge cycles between 4.0-2.8 V at C/2, with a potentiostatic step at 4.0 V with a current cutoff corresponding to C/20. For all the electrochemical tests, specific capacity is referred to the mass of silicon.

RESULTS AND DISCUSSION Morphological and structural characterization of the silicon-FLG electrodes The structural and morphological properties of the FLG produced by WJM are reported in Ref. (89). Here we report the morphological characterization of the as-prepared silicon-FLG electrodes carried out by SEM. Figure 1 reports the comparison of the

electrodes before (a-c) and after the annealing process (d-f). Figures 1a-b show the cross section of the electrode once deposited onto the copper foil. We can clearly distinguish the two components, with the isotropic silicon nanoparticles in between the FLG flakes. Observing the top view of the electrode in Figure 1c, we can see that also on the surface of the electrode the presence of silicon nanoparticles and the FLG flakes. After the annealing process, the electrode morphology is significantly changed. The electrode cross section in Figure 1d shows that the graphene flakes are oriented parallel one to each other, the dimension of the silicon aggregates appear reduced and they are homogeneously distributed between the FLG flakes. As evident in Figure 1e, FLG flakes entrap the silicon nanoparticles creating a well-organized and ordered structure. Differences between electrodes before and after annealing are also evident in the surface of the electrode (Figure 1f), where FLG flakes cover a higher surface compared to the electrode without annealing.


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Figure 2. a) Superposition of the normalized Raman spectra for the Si-FLG anode pre-annealing (red) and post annealing (blue). HRTEM images and corresponding fast Fourier transforms (FFTs) for regions in the (b) pristine and (c) annealed Si-FLG electrode on top of an amorphous carbon film. The FFTs indicate a [001]-oriented graphite (ICSD 76767) pattern, due to the FLG flakes, and randomly oriented cubic silicon (ICSD 51688) nanometer-sized domains. The aim of the annealing process is twofold: (i) achieve a higher conductivity of the composite by the formation of a carbon coating over the Si-NP and in between, i.e., a better electrical contact between the Si-NPs and the FLG flakes and (ii) increase the anode specific capacitance by removing the binder. To validate this hypothesis, Raman spectra in the 1100 – 1900 cm-1 range were acquired. The spectra were then normalized to the intensity of the carbon G peak and the peaks were deconvoluted in their Lorentzian components. In figure 2a the typical Raman spectra of pristine and annealed samples are reported. On the pristine sample spectrum, the typical components of graphene spectrum in the range acquired (D, G and D’ (93)) can be clearly identified. The peaks are well defined and separated and the I(G)/I(D) ratio is 0.064. On the contrary, after the annealing process, the intensity of the D peak was strongly increased, and the D and G peaks are partially overlapped. By the deconvolution of the Raman spectrum, 5 different components can be identified. Of them, 3 can be ascribed to the graphene flakes, i.e., D, G, and D’ peaks, while the other two reveal the typical features of the amorphous carbon. An explanation of these features can be linked to

the carbon-content present in the binder that turned into a carbon coating after the annealing process. This hypothesis was then further explored by means of HR-TEM and EFTEM imaging. A deeper analysis on the Raman spectra is reported in Fig S3. The fast Fourier transform (FFT) of the HRTEM image, shown in Figures 2b-c, clearly show the [001] orientation of the FLG flakes, while the silicon spots are arranged in rings, corresponding to polycrystalline silicon. Further details of the electrodes have been highlighted by EFTEM elemental mapping and XPS measurements. Figure 3 shows the EFTEM analysis for the silicongraphene electrode pre- (3a-c) and post-annealing (3d-f). By comparing the C maps in Figures 3b and 3e it is possible to note an inhomogeneous 3 nm-thick carbon coverage all over the silicon nanoparticles, in the case of the annealed sample. On the contrary, in the pristine sample there is a low amount of carbon. Furthermore, a clear surface oxidation is observed in the annealed sample (figure 3f), less prominent in the pristine sample (figure 3c). From the EDS quantification, the annealed sample is clearly much richer in carbon and slightly richer in oxygen compared to the pristine sample (Figure S2).



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Figure 3. (a,d) Zero-loss filtered Bright Field TEM (BF-TEM) image and (b-c, e-f) corresponding EFTEM elemental maps for carbon (red) and oxygen (blue) on regions of (a-c) pristine and (d-f) annealed Si-FLG electrode, partly suspended on a hole in the holey carbon support film. The electrode surface has been further analysed by XPS measurements. Figure 4 reports the spectra obtained for the annealed silicon-FLG electrode in comparison to the not annealed one and the pristine silicon sample. Both spectra are characterized by the presence of a doublet of narrow peaks, with the most intense one centred at (99.4±0.2) eV, typical of elemental silicon (92), accompanied by a broad peak centred at (103.4±0.2) eV, corresponding to SiO2 (92). The relative intensity of the oxide peak increased with the annealing process, passing from 19.6% to 28.3% of the whole silicon content, as obtained by the fitting procedure (Figure S1). Besides, there is not a significant difference between the pristine silicon and the silicon-FLG electrode not subjected to the annealing process.

demonstrated that with the combined approach of ultra-sonication and annealing, we obtained a siliconFLG electrode with a well-organized structure. The annealing step leads to the formation of a carbon coating onto the silicon surface, which could be the main cause of the partial oxidation of the silicon (less than 10%). Thermo-gravimetric analysis revealed silicon content of 57% (Figure S4). In summary, the as-prepared electrodes consist of a laminated structure in which small silicon aggregates are entrapped between FLG flakes. Electrochemical characterization Potentiodynamic cycling with galvanostatic acceleration (PCGA) measurements were performed to have an insight on the redox processes related to the as-prepared silicon-FLG electrodes. Figure 5 reports the differential potential profile obtained during the formation cycles (blue line) and the subsequent cycles (red line).

Figure 4. XPS spectrum comparison of the silicon particles and of the silicon-FLG electrode postannealing. The increase of oxygen content might be due to annealing conditions and by the presence of oxygen atoms in the PAA. Structural and morphological analyses have

Figure 5. PCGA response of the annealed silicongraphene electrode in lithium cell. Voltage range used for the formation cycle is 0.01-2V, while for the other cycles it is in the 0.1-1V range.


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In agreement with previous work (94), formation cycle shows a completely different feature when compared to the other cycles. During the discharge, it is possible to observe the occurrence of multiple processes occurring below 0.13V, associated to the contemporary intercalation of lithium into FLG layers and the complete lithiation of Si. As already reported (94-95), the lithiation of the crystalline silicon phase initially form amorphous LixSi alloys, according to: Si + xLi+ + xe− → LixSi (0 ≤ x ≤ 4.4) followed by the formation of Li15Si4 below 0.06V. In the recharge process, two small peaks, associated with the redox process of FLG, are observable at 0.1 and 0.15V, followed by two large peaks at 0.3 and 0.45V (labelled C and D, respectively) corresponding to the multistep de-lithiation process of amorphous Li-Si alloys (96). In the second cycle, limiting the potential cut-offs to 0.1-1V, we also limited the contribution of FLG flakes into redox process, while the two-anodic process centred at 0.25 and 0.1V, according to a-Si +xLi+ → aLixSi; a-LixSi +yLi+ → a-Li(y+x)Si (96), and two cathodic peaks at 0.3V and 0.45V, related to silicon delithiation process, are still observable. Figure 6a shows the cycling performance of the annealed silicon-FLG electrode, obtained in galvanostatic mode with a current density of 0.35 A/g. When discharged at 0.01V, the silicon-FLG electrode achieves a specific capacity >3500 mAh/g, almost reaching the theoretical one. The recharge to 2V shows a CE of almost 89%. In the other cycles, limiting the potential range between 0.1-1V, the tested electrode discharges a capacity of ~2300 mAh/g with an initial Coulombic efficiency (ICE) of 95% that reach 99% after 12 cycles. The specific capacity achieved at the 2nd cycle remains rather stable during cycling (with a fading per cycle of 0.13%), delivering 1800 mAh/g after 90 cycles, which correspond to ~78% of the initial value. Figure 6b shows the potential profiles of some representative cycles. We can observe that the charge-discharge curves exhibit similar shape in every cycle, confirming the reversibility of the process. Furthermore, we can distinguish the typical sloping curve associated to the alloying/de-alloying of silicon (93–96). In fact, in agreement with the PCGA analysis, discharge curves are characterized by two slopes at 0.20 V and 0.1 V, while recharge processes show an initial plateau at 0.15 V, and two slopes around 0.3 V and 0.4 V. Finally, the formation of SEI has been evaluated acquiring impedance spectra at different cycles (figure S5). The sequence of Nyquist plots in figure S5a describes a rather regular evolution of a non-blocking interphase to a blocking one, compatible with a reactive interphase which is progressively passivated from electrolyte decomposition products. Upon cycling, the resistance of the SEI rises from 13 Ω to 38 Ω after 100 cycles (figure S5b). For comparison, in figure S6 is reported the

behaviour of the same electrode when it is not subjected to the annealing step. This electrode shows a lower specific capacity, not being able to sustain prolonged cycling. In fact, in a few cycles, it is possible to observe a loss of specific capacity of ~40%. Moreover, after less than 50 cycles the material starts to degrade, with irreversible damage after only 100 cycles.

Figure 6. Electrochemical performance of the annealed silicon-FLG electrode in lithium cell: a) cycling performance; b) potential profiles obtained at 0.35 A/g; and c) rate capability measured at different current density values. For all the measurements, before cycling a formation cycle (indicated as cycle 0) has been performed between 0.01-2V at 0.35A/g. In summary, the electrode after annealing demonstrated enhanced electrochemical properties compared to the reference electrode. On the one hand, the annealing process determines the creation of a well-organized structure. The graphene flakes entrapping silicon particles can buffer the volumetric expansion/contraction of silicon during cycling, beneficial for the electrochemical performance. On



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the other hand, annealing causes also the pyrolysis of the not conductive PAA, creating a carbonaceous network that improve the conductivity and physically sustain the silicon-graphene structure. To evaluate the rate capability, the annealed silicon-FLG electrode was further cycled increasing the current density from 0.35A/g (corresponding to C/10) to 7 A/g (≈ 2C). As reported in Figure 6c, the annealed silicon-FLG electrode exhibits excellent rate performance. However, we noticed that the exchanged capacity decreases from 2300 mAh/g at 0.35A/g to 750 mAh/g at 7 A/g (2C). In addition, when the current density returns to the initial value of 0.35A/g, the specific capacity raises again to ~2000 mAh/g, which is the same value obtained during first cycles. Since the annealed silicon-FLG electrode demonstrated cyclability at high current densities, it was decided to perform a prolonged discharge/recharge test of the anode at 3.5 A/g. The obtained results are shown in Figure S7a. During the first 50 discharge-recharge cycles, it is possible to observe an initial drop of the specific capacity (from 1500 mAh/g to less than 1000 mAh/g), corresponding to a loss of ~30% of the initial value. Then, in the subsequent cycles the specific capacity reaches a stable value of ~1000 mAh/g. After 250 charge/discharge cycles the specific capacity reached a final value of 750 mAh/g, with capacity retention of 75%. Discharge/charge profiles in Figure S7b show the signature of the electrochemical lithiation/delithiation of silicon, as evident from the shapes of the curves, showing behaviour similar to the electrode cycled at lower currents. Ex-situ Morphological Analysis In order to study the changes occurred in the electrode material upon cycling, an ex-situ morphological and structural analysis has been performed by the use of SEM and HR-TEM. For the analysis, the electrodes were firstly cycled under galvanostatic condition in lithium cell using a current density of 0.35A/g. Afterward, the tests were stopped

at different cycle numbers (i.e., 1, 10, 50 and 100) and the electrode recovered and washed with anhydrous DMC to eliminate the residual lithium salt on the electrode surface. The micrographs obtained for the electrode after 100 cycles are reported in Figure 7.

Figure 7.a) HRTEM image and (inset) corresponding fast Fourier transform (FFTs), (b) zero-loss filtered BFTEM image and corresponding elemental maps of (c) C and (d) O for Si nanoparticles from the post-mortem electrode (after 100 cycles), suspended on holes in the carbon support film. After the 1st forming cycle, the composite does not show any appreciable changes neither in the structure nor in the morphology. The latter is similar to the one of the post-annealing electrode (Figure 1a). The graphene flakes and the silicon nanoparticles are clearly visible (Figure S8a,e). According to EFTEM mapping (Figure S9a), silicon nanoparticles are still covered by a carbon coating (Figure S9b) of ~3 nm thick together with an oxide-reach shell (Figure S9c). HRTEM images analysis (Figure S10a,e) shows that silicon nanoparticles are still polycrystalline, as already observed just after the annealing process.

Figure 8. a) Specific Capacity vs cycle number plot and b) cell voltage profiles in a silicon-FLG/NMC111 cell.


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Since the 10th cycle, silicon undergoes a major structural change, turning from polycrystalline to amorphous, as pointed out by the HRTEM analysis (Figure S10b,f). This can explain the formation of a small valley in the cycling performance of Figure 6a. This amorphous structure is preserved also for higher cycles (Figure 7a). From SEM imaging, (Figure S8b,f in SI), a gradual evolution of the Si nanoparticles into a flat, continuous layer is observed. After 50 cycles, SEM images (Figure S8c,g) show how the cluster of silicon has almost covered all the electrode’s interface. It is still possible to point the most superficial individual graphene flakes. Moreover, the morphology of the silicon nanoparticles becomes more and more porous with the cycling and can no longer be neglected after 50 cycles (Figure S9d,e). In the same way, the oxidation of the silicon nanoparticles seems to increase with the cycling, with the first appreciable evidence only after 50 cycles (Figure S9f). For higher cycling, the aggregation and deaggregation of silicon nanoparticles leads to a new structure, as shown in the top view by Figure S8d. The amorphous silicon nanoparticles now are aggregate in individual cluster, and it is again possible to observe graphene flakes on the surface. Contrary, the graphene flakes are not significantly affected by the cycling (Figure S10d,h), an expected result which confirm that the silicon nanoparticles are the active material. Development of lithium-ion battery After having demonstrated the electrochemical activity of the silicon-graphene electrode in a lithium cell, such electrode was tested in a complete lithiumion cell. To this end a commercial NMC111 with a nominal capacity of 2mAh/cm2 was chosen as the positive electrode, while as electrolyte the same mixture of LP30 and 10% v/v of FEC used in half-cell experiments, was adopted. Anode and cathode masses were balanced considering the anode reversible capacity (2300 mAh/g) and the reversible capacity of the cathode (137 mAh/g as reported in Figure S8). After the formation procedure (Figure S9), the cell achieved a specific capacity of ~1800 mAh/g (a value referred to the silicon mass) with a CE >99 %, see Figure 8. After 20 cycles, the cell’s specific capacity is still higher than 1600 mAh/g. Unfortunately, the specific capacity decreases over prolonged cycling and after 200 cycles only 33% of the initial capacity is retained (Figure S9).

CONCLUSION We designed and developed a nanostructured silicon-few-layer graphene (FLG) electrode to be exploited as anode for lithium ion battery. The

nanostructured silicon-FLG electrode comprises horizontally oriented FLG flakes entrapping the silicon nanoparticles. The realization of such nanostructured anode materials is rather simple, being directly fabricated onto the copper current collector, exploiting the use of ultra-sonication followed by an annealing process in reducing atmosphere. The latter process allows the pyrolysis of poliacrylic acid, which increases the electrical conductivity of the silicon nanoparticles and the bond with the graphene flakes. The as-prepared nanostructured electrodes have shown excellent electrochemical performances, achieving high specific capacity in lithium cell (>2000 mAh/g), being able to sustain high currents (over 750 mAh/g at 7 A/g) maintaining high reversibility (Coulombic efficiency -CE- ≈ 99%). These results are in line with those already reported in literature (table S1). Finally, we demonstrated the use of silicon-FLG electrode in a complete lithium-ion battery. Our full cell delivered a first cycle specific capacity of 1800 mAh/g and a good stability (CE 99%). We believe that such laminated silicon-FLG graphene nanocomposite represents a potential hybrid morphology for high silicon loading Li-ion cells displaying high-rate capabilities and excellent stability. The developed nanostructured silicon-FLG anode open the way for further optimization of the electrode by, for example, exploring different morphologies of silicon nanoparticles in combination with FLG.

ASSOCIATED CONTENT Supporting Information. Additional XPS spectra and galvanostatic cycling measurements as well as EDS maps, TGA curves and tables with the state-of the art are available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

[email protected];

Notes

The authors declare no competing financial interest.

Funding Sources We acknowledge the European Union’s Horizon 2020 research and innovation program under grant agreement no. 785219, GrapheneCore2.

ACKNOWLEDGMENT We thank Sergio Marras and Mirko Prato from the Materials Characterization Facility at the Fondazione Istituto Italiano di Tecnologia for help with X-Ray Diffraction measurements and XPS analysis. We thank Simone Lauciello for SEM and TEM



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characterization, Luca Gagliani and Manuel Crugliano for their contribution to the production of few-layer graphene flakes, Giammarino Pugliese for thermal analysis, and Christoph Stangl of Varta Micro Innovation gmbH for useful discussion.

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