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Preparation of binder-free three dimensional carbon foam/silicon composite as potential material for lithium ion battery anodes Amit Kumar Roy, Mingjie Zhong, Matthias Georg Schwab, Axel Binder, Shyam Venkataraman, and Željko Tomovi# ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12026 • Publication Date (Web): 24 Feb 2016 Downloaded from http://pubs.acs.org on March 2, 2016

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ACS Applied Materials & Interfaces

Preparation of binder-free three dimensional carbon foam/silicon composite as potential material for lithium ion battery anodes Amit K. Roy, Mingjie Zhong, Matthias Georg Schwab, Axel Binder, Shyam S. Venkataraman*, Željko Tomović* BASF SE, 67056 Ludwigshafen, Germany Abstract We report novel three dimensional nitrogen containing carbon foam/silicon (CFS) composite as potential material for lithium ion battery anodes. Carbon foams were prepared by direct carbonization of low cost, commercially available melamine formaldehyde (MF, Basotect®) foam precursors. The carbon foams thus obtained display a three dimensional interconnected macroporous network structure with good electrical conductivity (0.07 S/cm). Binder free CFS composite used for electrodes were prepared by immersing the as-fabricated carbon foam into silicon nanoparticles dispersed in ethanol followed by solvent evaporation and secondary pyrolysis. In order to substantiate this new approach, preliminary electrochemical testing has been done. First results on CFS electrodes demonstrated initial capacity of 1668 mAh/g with 75% capacity retention after 30 cycles of subsequent charging and discharging. In order to further enhance the electrochemical performance, silicon nanoparticles were additionally coated with a nitrogen containing carbon layer derived from co-deposited poly(acrylonitrile). These carbon coated CFS electrodes demonstrated even higher performance with an initial capacity of 2100 mAh/g with 92% capacity retention after 30 cycles of subsequent charging and discharging. Keywords: carbon foam, carbon-silicon composite, anode material, binder-free anode, lithium-ion battery

* Corresponding authors: BASF SE, 67056 Ludwigshafen, Germany E-mail addresses: [email protected], [email protected]

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1. Introduction Lithium ion batteries (LIBs) are widely used in consumer electronics such as smart phones and notebooks and find increasing use in modern electric vehicles. Graphite, the state of the art anode material, has a theoretical capacity limit of ~ 372 mAh/g and with new applications on the rise, there is an increasing demand for higher capacity anodes.1,2 Silicon has been proposed as the potential candidate to replace graphite due to its high theoretical energy density of ~ 4200 mAh/g.3,4 However, silicon anodes cannot yet be realized for immediate application because they suffer from a rapid capacity fading and poor performance with increasing number of charging and discharging cycles, due to large volume expansion of ~ 300% during the lithiation process, which causes high mechanical stress eventually leading to structural failure of the anode. In order to address the challenge of volume expansion of silicon, several approaches have been reported in the literature, such as the use of nanoparticles instead of micron sized particles5, carbon coating on silicon nanoparticles by pyrolysis6, wrapping of silicon nanoparticles by graphene7, application of silicon nanostructures4,8,9,10 and silicon thin films grown by chemical vapor deposition.11 However, in all of these approaches, standard two dimensional copper current collectors were used to support the active silicon material, thus limiting the form factors which can be realized. In addition to the above, the choice of polymer binder and solvent are crucial for the performance of the final device.12,13 The discharge capacity of the silicon anode can vary depending on the chemical composition of the binder and choice of the solvent during electrode preparation.12 Recent literature indicates the versatility of three dimensional nickel foam as a current collector, specifically for silicon nanoparticles.7,14 Nickel foam provides more space and is able to accommodate volume expansion of silicon during lithiation, and significantly improves the problem of capacity fading in comparison to copper tape current collectors.14 In this work, we apply low cost, lightweight nitrogen containing carbon foam 2


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with three dimensional network structure as the current collector, which can be successfully used as a host for the silicon nanoparticles in the final electrode. Carbon foams are light weight, electrically conductive and chemically stable, and find applications in many fields. Consequently, carbon foams have been studied for gas storage,15 electromagnetic shielding,16 and were also used as a support in electrochemical capacitor,17 heterogeneous catalysis,18 and electrocatalysis.19 A recent review article20 describes different ways of carbon foam synthesis: 1. Carbonization of template precursors such as polyurethane foam 2. Blowing of precursors, such as polyaromatic hydrocarbons followed by carbonization 3. Exfoliation of graphite and subsequent compression to a foam structure 4. Reduction and freeze-drying of graphene oxide By approaches 1 and 2, either amorphous or graphitic foams can be obtained depending on the carbonization temperature and the nature of the precursor being used. Graphitic carbon foams can be obtained using approaches 3 and 4. Often, carbon foams obtained by carbonization of template precursors such as polyurethane and tannin based foams suffer from poor mechanical stability.21,22,23 On the other hand, carbon foams obtained by carbonization of open cell microporous melamine formaldehyde (MF) foam provide a surprisingly stable structure even after high-temperature treatments, and could be used as a flexible free-standing 3D conductive network.24,25,26 To the best of our knowledge, binder-free nitrogen containing three dimensional carbon foam/silicon (CFS) composite anodes have not been reported so far. The main advantage of the mechanically stable CFS composite electrode is that the open cell three dimensional carbon foam with large voids can behave as a ductile host matrix which can accommodate the volume change of silicon nanoparticles during charging and discharging cycles. Furthermore, the incorporation of nitrogen into amorphous carbon has been reported 3



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to result in better electrochemical performance when compared to pristine non-modified amorphous carbon.27,28 Hence, a nitrogen containing carbon foam scaffold obtained from controlled pyrolysis of commercially available MF foam (Basotect®, BASF) was used for this work. In order to substantiate this new approach we have done preliminary electrochemical testing. First results on CFS electrodes demonstrated initial capacity of 1668 mAh/g with 75% capacity retention after 30 cycles of subsequent charging and discharging. Systematic investigation of the impregnation method, drying conditions and effect of nitrogen containing carbon coating of the silicon particles lead to the fabrication of CFS electrodes exhibiting an initial capacity of 2100 mAh/g with 92% capacity retention after 30 cycles of subsequent charging and discharging. 2. Experimental section 2.1. Preparation of carbon foam. Nitrogen containing carbon foams were prepared by pyrolysis of MF foam (Basotect®, BASF SE, Germany). The pyrolysis equipment consisted of a split tube furnace (HZS 12/600, Carbolite Limited, UK) equipped with a quartz tube. MF foam samples (13.0 x 2.5 x 2.5 cm3) were loaded on a quartz boat and placed inside the tube furnace. Prior to the pyrolysis process, samples were kept at room temperature for 30 minutes under a continuous argon flow of 1000 standard cubic centimeters per minute. The MF foams were pyrolyzed at 1000 °C for 2 hours. A heating rate of 10 °C/minute was applied to reach the top temperature. After pyrolysis, the sample temperature was slowly brought back to 30 °C. The entire heating and cooling process was carried out under a continuous argon flow of 500 standard cubic centimeters per minute (details of the heating and cooling process are shown in the Supporting Information, Figure S1). 2.2. Preparation of carbon foam/silicon (CFS) composites. Four different CFS composites (CFS-1, -2, -3 and -4) were prepared for this work by systematically varying the conditions during the sample preparation. Samples CFS-1, -2 and -3 were prepared by 4


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immersing the carbon foam prepared in previous step into a yellow-colored dispersion of silicon nanoparticles (50 nm, 98% purity, Alfa Aesar) in ethanol (2.5 mg/ml). Sample CFS-4 was prepared by immersing the carbon foam into a dispersion containing both silicon nanoparticles and dissolved PAN (average molecular weight: 150000 g/mol, Sigma Aldrich) in dimethylformamide (DMF). The concentrations were 2.5 mg/ml of silicon nanoparticles and 0.8 mg/ml of PAN, respectively. Samples CFS-2, -3 and -4 were sonicated for 30 minutes to facilitate infiltration of silicon nanoparticles into the carbon foam. After the sonication step, CFS-3 and CFS-4 samples were kept in a vacuum oven at 100 °C, and 200 oC, respectively until complete solvent evaporation. No solvent evaporation was carried out for samples CFS-1 and CFS-2. Secondary pyrolysis of samples CFS-1, -2 and -4 was performed at 1000 °C under argon atmosphere for 2 h. No secondary thermal treatment was applied to CFS-3. The thermal protocol for the secondary pyrolysis step was the same as for the synthesis of pristine carbon foams and is represented in the Supporting Information, Figure S1. 2.3. Characterization of carbon foam and CFS composites. Scanning electron microscopy (SEM) images were recorded using a Phenom ProX microscope (Phenom-World BV, Netherlands). Transmission electron microscopy (TEM) samples were prepared by ultramicrotomy or by grinding of the sample and placement on a carbon film. TEM images were recorded by using electron microscopes (Tecnai G2-F20ST, and Tecnai Osiris, FEI Company, USA) operated at 200 keV under bright-field conditions. Energy dispersive X-ray spectroscopy (EDX) was recorded (Super-X detector, FEI Company, USA and Bruker, Germany) to determine the chemical compositions at distinct spots of the sample. Raman spectra were recorded (NTEGRA, NT-MDT Co., Russia) by using 442 nm excitation wavelength (2.80 eV), a 600/mm grating and a 100X objective. Chemical compositions of the MF foam, and carbon foam were determined by elemental analysis. Gaseous byproducts of the pyrolysis process were characterized by thermo-gravimetric analysis (STA 409, 5



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NETZSCH, Germany), and on-line Fourier transform infrared (FTIR) spectrometry (Tensor 27, Bruker, Germany). The electrical conductivity of the melamine foam was determined by the two point method (according to norm: IEC60093), and for the carbon foam a four point geometry (according to norm: ISO 3915) was used. 2.4. Electrodes fabrication and electrochemical testing. To remove moisture and oxygen, the CFS composites were dried for 3 hours at 80 °C under 1 mbar pressure. The amount of silicon was calculated gravimetrically by subtracting the weight of the CFS composites from the weight of the pristine carbon foam material, and confirmed by elemental analysis. To perform electrochemical testing, standard CR2032 coin cells were assembled inside a glove box (MBraun) under argon atmosphere using CFS composite cylinders (diameter: 10 mm, height: 0.8 mm) as a working electrode and lithium foil as the counter electrode. The weight of the electrodes based on CFS-1, -2, -3 and -4 was 5.4, 6.0, 6.2 and 4.5 mg, respectively. The electrolyte composition was 1.0 M lithium hexafluorophosphate dissolved in a mixture of ethylene carbonate, diethyl carbonate, and fluoroethylene carbonate (3:6:1 ratio). Charge and discharge capacities of the fabricated electrodes were determined by galvanostatic cycling of the half-cells over a potential range between 0.01 V and 1 V using a battery tester (Bio-Logic, France).

3. Results and discussions The carbon foams were obtained by pyrolysis of commercially available MF foam (Basotect®) at 1000 ˚C, as described in Experimental Section. The MF foam has a three dimensional interconnected macroporous network structure with pore diameters between 50 µm to 200 µm. After pyrolysis, a change in color from grey to black was observed, and a significant mass reduction of ~84% and volume shrinkage of ~ 86% were measured. BET 6


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(Brunauer–Emmett–Teller) surface area of the carbon foam was 3 m2/g. Photographic and SEM images of the starting MF foam and the carbon foam after pyrolysis are shown in Figure 1. The SEM image indicates that the carbon foam still contains large voids in the 100 µm to 200 µm range but unlike pristine material numerous smaller pores of < 50 µm are found as a result of the contraction of the material. FTIR characterization of the volatile compounds during the pyrolysis process reveals the successive release of byproducts, such as water, carbon dioxide, carbon monoxide, formaldehyde, methanol, ammonia, hydrogen cyanide, hydrogen isocyanide, etc., which account for the significant mass loss (for details see Supporting Information, Table S1).

Figure 1: Digital photos and SEM images of MF foam (a, and c) and carbon foam (b, and d) obtained by pyrolysis of MF foam at 1000 °C (2 h). Elemental analysis shows that the pyrolyzed MF foam contains carbon, nitrogen, oxygen, and a small amount of sodium (for details see Supporting Information, Table S2). The obtained three dimensional porous carbon foam is surprisingly flexible and can be easily cut into a 7



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stable cylindrical shaped electrode suitable for coin cell testing (see Supporting Information, Figure S2). The densities of the pristine MF foam and the carbon foam were ~ 9 mg/cm3 and ~ 6 mg/cm3, respectively. The electrical conductivity of the starting MF foam was found to be 2.3 x 10-11 S/cm. This value was increased to 0.07 S/cm after pyrolysis, thus meeting the requirements to be used as a free-standing current collector for LIBs. The preparation of four different CFS based composites is described in detail in Experimental Section and summarized in Table 1. During the preparation of the CFS-1 sample, agglomerates of silicon nanoparticles were visible on the outer surface of the carbon foam indicating poor diffusion of the particles (Figure 2a and 2b). Therefore, an ultra-sonication step was introduced to break down these agglomerates and allow for easier diffusion of the particles into the foam matrix during the preparation of CFS-2. As compared to CFS-1, significantly more silicon nanoparticles were now observed inside the foam, indicating overall better distribution of silicon nanoparticles for sample CFS-2 (Figure 2c and 2d).

Sample

Impregnation

CFS-1

Nano-Si in ethanol Nano-Si in ethanol Nano-Si in ethanol Nano-Si + PAN in DMF

CFS-2 CFS-3 CFS-4

Sonication time during impregnatio n 0

-

yes

29.6

Initial discharge capacity (mAhg-1) 544

30 min

-

yes

32.2

1668

75.4

30 min

2 h at 100 °C 3 h at 200 °C

-

36.7

2258

24.9

yes

11.1

2103

92.5

30 min

Drying proces s

Secondary pyrolysis (2 h at 1000 °C)

Amount of Silicon (wt.%)

Capacity retention after 30 cycles (%) 90.3

Table 1: CFS composites prepared from carbon foam and silicon nanoparticles at various process parameters.

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Figure 2: SEM images of the composite CFS-1 obtained without ultra-sonication: (a) surface, (b) cross-section view, and of the composite CFS-2 obtained with ultra-sonication: (c) surface, (d) cross-section view.

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TEM and HR-TEM (Figure 3a-d) of samples CFS-2 and CFS-4 indicated that the silicon nanoparticles are crystalline in nature and contain a thin amorphous surface layer. EDX

analysis revealed that the thin amorphous layer of the CFS-2 sample contained predominantly oxygen and silicon leading to the conclusion on the presence of native silicon oxide originating from the starting material. The sample CFS-4 was coated additionally with PAN prior to the secondary pyrolysis step (compare Table 1). As a result, the thin layer in CFS-4 sample contained carbon, nitrogen, oxygen and silicon indicating the presence of nitrogen incorporated carbon coating on top of the native silicon oxide.

Figure 3: TEM and HRTEM images of the composites (a, b) CFS-2, and (c, d) CFS-4. 10


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Raman spectra of the silicon nanoparticles, carbon foam and CFS-2 composites were collected as shown in Figure 4. A peak related to crystalline silicon (1st order silicon peak) at 519 cm-1 is visible in the spectra of both the silicon particle and the composite material.29 Two prominent peaks are visible between 1000 cm-1 to 2000 cm-1 in the spectra of the carbon foam

G

Intensity (a.u.)

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CFS-2 composite C-foam Silicon particles Si1st order

500

D

Si2nd order

1000

1500

2000

Raman Shift (1/cm) and CFS-2, respectively. The first one at ~1393 cm-1 is conventionally termed as the D peak indicating disordered carbon, and the second one at ~1594 cm-1 is known as the G peak, indicating a certain amount of graphitic content.25 From the presence of these peaks it may be concluded that the carbon foam is predominantly amorphous in nature, and that the crystalline silicon nanoparticles are successfully incorporated into the foam structure.

Figure 4: Raman spectra of silicon nanoparticles, carbon foam, and CFS-2 composite.

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The amount of silicon in the CFS composites was determined by gravimetric measurement to be 29.6 wt.%, 32.2 wt.%, and 36.7 wt.%, in the CFS composites -1, -2 and -3, respectively. Half-cell testing with the composite anode resulted in initial discharge capacities of 544 mAh/g (CFS-1), 1668 mAh/g (CFS-2) and 2258 mAh/g (CFS-3). In comparison, the bare carbon foam without silicon nanoparticles delivered a discharge capacity of only 220 mAh/g in a control experiment (Figure 5). Therefore, for all capacity calculations only the

Discharge capacity (mAh/g)

amount of silicon has been used.

(a)

2500

Bare foam CFS-1 CFS-2 CFS-3

2000 1500 1000 500 0

0

10 20 Number of Cycles

30

(b)

100 Coulombic efficiency (%)

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Bare foam CFS-1 CFS-2 CFS-3

80

60

0

10 20 Number of Cycles 12


30

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Figure 5: Cycling performance of the bare carbon foam and CFS-1, CFS-2 and CFS-3 composites (a) discharge capacity, and (b) Coulombic efficiency. For the first cycle the discharge rate was C/20, afterward it was C/10 for all other cycles. The higher initial discharge capacity of CFS-2 when compared to CFS-1 can be considered as the result of an overall better distribution of the silicon nanoparticles throughout the carbon scaffold due to the ultra-sonication step. The improved initial discharge capacity of CFS-3 when compared to the other two samples may be rationalized by the higher silicon content over CFS-2 (4.5 wt.% more). After 30 cycles, the capacities were 491 mAh/g (CFS-1), 1257 mAh/g (CFS-2) and 562 mAh/g (CFS-3), which correspond to 90.3%, 75.4% and 24.9% retention of their initial capacities, respectively. The rapid capacity fading in the case of CFS3 (prepared without the secondary pyrolysis step) can be attributed to the loss in electrical contact between silicon nanoparticles and carbon foam. These results highlight the importance of an additional thermal treatment step for our novel concept. For the experiments, the initial Coulombic efficiencies were 58.1% (CFS-1), 68.6% (CFS-2) and 61.1% (CFS-3), respectively. Low initial Coulombic efficiency during the first charge and discharge process is due to the formation of Li2O and solid electrolyte (SEI) interface.30 The Coulombic efficiency increases for CFS-1 to 97.8 %, CFS-2 to 98.3 % and CFS-3 to 96.6 %, after 5 charge and discharge cycles. Even though the cycling performance of CFS-2 was superior to that of CFS3, the discharge capacity slowly faded with increasing number of cycles. In a final experiment, effect on electrochemical performance for a CFS composite with additional nitrogen containing carbon coating was also analyzed (sample CFS-4). The amount of silicon in CFS-4 was determined to be 11.1 wt.%. Figure 6 shows the discharge capacity and Coulombic efficiency for samples CFS-2 and CFS-4. The initial discharge capacity of the CFS-4 was 2103 mAh/g. Although the silicon content of sample CFS-4 was 21.1% lower in comparison to the CFS-2, the initial discharge capacity of CFS-4 was found to be 26.1% 13



higher than CFS-2. After 30 cycles, the capacity for CFS-4 was still 1946 mAh/g, which correspond to 92.5% capacity retention. Overall, CFS-4 showed higher discharge capacity and

Discharge capacity (mAh/g)

less capacity fading relative to CFS-2.

2500

(a)

2000 1500 1000 CFS-2 CFS-4

500 0 0

Coulombic efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10 20 Number of Cycles

30

(b)

100

CFS-2 CFS-4

80

60

0

10 20 Number of Cycles 14 ACS Paragon Plus Environment

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Figure 6: Cycling performance of CFS-2 and CFS-4 composites: (a) discharge capacity, and (b) Coulombic efficiency. For the first cycle the discharge rate was C/20, afterward it was C/10 for all the other cycles. It has been reported that a carbon barrier layer on silicon anode might be able to accommodate the stress, and facilitate the lithium ion transportation.6,31 Moreover, the recent literature indicates that the nitrogen containing carbon coatings on silicon could enhance the capacity compared to a pristine carbon coating on silicon.32,33 Cho et al. applied pure and nitrogen doped carbon coatings on silicon nanowires via chemical vapor deposition, and thereby demonstrated that silicon nanowires with a nitrogen doped carbon layer exhibit higher charge capacity relative to one without nitrogen.32 Tao et al. also investigated nitrogen containing carbon coatings on silicon nanoparticles33 and reported that with increasing nitrogen content, the discharge capacity was notably enhanced. The initial results that we have presented here suggest that the combined effect of both carbon coating and the presence of nitrogen in the coating might be the origin of the better performance of the sample CFS-4 in comparison to the other composites.

4. Conclusion MF-derived carbon foams were prepared by direct carbonization of low cost, commercially available Basotect® foam precursor. CFS composites were prepared from the carbon foams thus obtained and directly applied as free-standing anodes for LIBs, without the use of additional metal current collector or polymer binder. The carbon foam support exhibits a dual role: Acting as a current collector as well as serving as a ductile host matrix for silicon nanoparticles to accommodate volume expansion and contraction. It was demonstrated that a 15



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secondary pyrolysis step is required to firmly attach the silicon nanoparticles to the struts of the underlying carbon foam structure thus ensuring good electrical contact throughout the electrode. Ultra-sonication leads to a more uniform infiltration of silicon nanoparticles deep into the pores of the carbon foam skeleton. In addition, a PAN-derived nitrogen containing carbon coating on CFS composites can further improve the contact between silicon particles as reflected in a significant enhancement of the discharge capacity and cycle stability of the anode. Whereas the gravimetric charge storage capacity of the presented composites makes them attractive candidates for further exploration, the volumetric storage capacity still needs to be enhanced as outlined in a recent article.34 This could be achieved by designing smallerpore polymer precursors in the future as compared to the currently available commercial raw material. In summary, we have demonstrated a simple and economical approach for the preparation of novel three dimensional CFS composite which might be useful for lithium ion battery anodes. This process holds promise for scale-up, owing to the commercial availability of the silicon nanoparticles and carbon foam precursors used for this study.

Supporting Information. Heating profile used for the pyrolysis, gaseous byproducts detected via FTIR during carbonization of foam, elemental analysis of MF and carbon foam. Notes. The authors declare no competing financial interest. Funding sources. This work was sponsored by BASF SE, Ludwigshafen, Germany.

Acknowledgements

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We would like to thank to Dr. Philipp Müller and Mr. Ulrich Flörchinger for SEM, TEM and EDXS analysis, Dr. Uwe Häussel for elemental analysis, Mr. Lutz Höring for FTIR characterizations, and Dr. Ingolf Hennig for electrical conductivity measurements.

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