N-Doped Carbon Spheres with Varying

Sep 26, 2016 - ... University of Texas at Austin, 1 University Station C0400, Austin, Texas ...... DiLeo , R. A.; Frisco , S.; Ganter , M. J.; Rogers ...
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Facile Synthesis of Ge/N-Doped Carbon Spheres with Varying Nitrogen Content for Lithium Ion Battery Anodes Duck Hyun Youn,† Nicholas A. Patterson,† Hunmin Park,‡ Adam Heller,† and C. Buddie Mullins*,† †

Department of Chemical Engineering and Department of Chemistry, Center for Electrochemistry, University of Texas at Austin, 1 University Station C0400, Austin, Texas 78712-0231, United States ‡ Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, South Korea S Supporting Information *

ABSTRACT: The simple fabrication of composites of germanium nanoparticles dispersed on nitrogen-doped carbon nanospheres (Ge/NC) of varying nitrogen content and their performance in lithium ion battery anodes are reported. A heavily nitrogen-doped carbon gel was formed by condensing m-phenylenediamine with formaldehyde (PF-gel); a less heavily N-doped gel was formed by condensing resorcinol and m-phenylenediamine with formaldehyde (RPF-gel); and an undoped gel was formed by condensing resorcinol with formaldehyde (RF-gel). Pyrolises of the gels with GeCl4 at 750 °C produced nanocrystalline Ge composites with 7.5 atom % N-doped carbon, termed Ge/NC (PF), with 3.9% N-doped carbon, termed Ge/NC (RPF) and undoped carbon, termed Ge/C (RF). The heavily N-doped Ge/NC (PF) anode retained a reversible capacity of 684 mAhg−1 at a specific current of 0.2 Ag−1 after 200 cycles, versus 337 mAhg−1 retained by anode made with Ge/NC (RPF) and 278 mAhg−1 retained by anode made with undoped Ge/C (RF). At a specific current 2.0 Ag−1, the capacity of the Ge/NC (PF) anode was 472 mAhg−1, versus the 210 mAhg−1 of the Ge/ NC (RPF) anode and 83 mAhg−1 of the Ge/C (RF) anode. The enhanced performance of the Ge/NC (PF) anode is attributed to the better electrical conductivity of Ge/NC (PF) and to the higher density of Li+ binding defects in its N-doped carbon. KEYWORDS: germanium, nitrogen-doped carbon, lithium ion battery, anode composite (metal/N-doped carbon etc.) for a LiB anode,5,17,18 because the N-doped carbons are expected to have good electrical conductivity for fast electron transport and a defect structure available for more lithium storage. However, published reports regarding the preparation of Ge/N-doped carbon composites and the effect of the N-doped carbon on the battery performance of the Ge/N-doped carbon electrode are rare. Here we describe the facile syntheses of nanocrystalline Ge/ N-doped carbon composites of controlled nitrogen content and evaluate their performance as LiB anodes. Our undoped carbon is made of the aqueous gel, denoted as RF, is formed by the well-known condensation of resorcinol (R) and formaldehyde (F).19,20 We make similar nitrogen-comprising gels by replacing part or all of the R with m-phenylenediamine (P), i.e., by condensing P and F. When all the R is replaced by P, we form the nitrogen-rich gel. We form a less nitrogen rich gel, RPF, by replacing half of the R with P. When combined with GeCl4 (derived nascent GeO2), and heated to 750 °C, the gels carbothermally reduce the GeO2 to nanocrystalline Ge dispersed on carbon nanospheres. The resulting Ge-carbon composites differ in their nitrogen content: Ge/NC (PF) comprises 7.5 atom % nitrogen; Ge/NC RPF comprises 3.9

1. INTRODUCTION Lithium ion batteries (LiBs) are essential power sources in portable electronics and electric vehicles.1 Novel electrode materials are being explored to meet the demand for higher energy density and longer cycle life.2 Because the high theoretical capacity of anodes made with the Group IV elements silicon, germanium, and tin versus the capacity of the widely used graphite, these anode materials are being intensely studied.3−7 Germanium has a theoretical capacity of 1600 mAhg−1 when cycling between Ge and Li4.4Ge, well above the 372 mAhg−1 theoretical capacity of graphite. Furthermore, the diffusivity of Li+ in Ge is 400 times faster than in Si, and the electrical conductivity of Ge is 100 times higher than that of Si.8 However, the 370% volume change upon lithiation of Ge pulverizes the Ge. Pulverization leads to loss of electrical continuity and results in rapid fading of the capacity during cycling. Various efforts have been devoted to overcome this problem including the use of nanostructured Ge and Ge/carbon composites. Stress-relaxation in nanostructured Ge is faster than in bulk Ge, resulting in improved cycling stability.9,10 Additionally, the carbonaceous materials in the Ge/carbon composites, such as porous carbons, carbon nanotubes, and graphenes can act as a cushion to buffer the volume change and reduce aggregation of Ge.11−16 Thus, combining a nanostructured Ge with a carbon matrix might be an effective way to enhance the performance of LiBs. Recently, N-doped carbons have been employed as a support material to fabricate a © 2016 American Chemical Society

Received: August 8, 2016 Accepted: September 26, 2016 Published: September 26, 2016 27788

DOI: 10.1021/acsami.6b09857 ACS Appl. Mater. Interfaces 2016, 8, 27788−27794

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of the preparation and structure of the Ge/NC (PF) composite.

Figure 2. a) XRD patterns of the composites. Vertical lines indicate reference pattern of Ge (JCPSD 00-004-0545). SEM images of b) Ge/NC (PF), c) Ge/NC (RPF), and d) Ge/C (RF). scanning electron microscope was used for the SEM, and the energy dispersive spectroscopic (EDS) imaging; TEM images were obtained with a JEOL, JEM-2100F high resolution transmission electron microscope. Time-of-flight secondary ion mass spectrometry (TOFSIMS) depth profiles were collected using an ION-TOF GmbH, TOF.SIMS 5. X-ray photoelectron spectra (XPS) were obtained with a Kranos, Axis Ultra DLD. A Witec, Alpha 300 was used for the Raman spectra. Thermogravimetric analysis (TGA) was carried out with air flowing over the samples heated at 10 °C/min with a Mettler-Toledo TGA/DSC1. Specific surface areas were obtained from N2-sorption isotherms measured at 77 K (Quantachrome, Nova 2200e). Electrical conductivities were measured with a four-point probe (Keithley 2400). 2.4. Electrochemical Measurements. The Ge/NC (PF), Ge/ NC (RPF), and Ge/C (RF) composites were dispersed in deionized water with 90 kDa CMC (carboxymethyl cellulose) binder and Super PLi carbon at an 8:1:1 weight ratio. The slurries were cast onto a copper foil using a notch bar and dried at 80 °C for 12 h in a vacuum oven. The mass loadings of all electrodes was 0.5−0.7 mg cm−2. Coin cells were assembled with a lithium foil counter and reference electrode, a polypropylene membrane (Celgard 2400) separator, and 1 M LiPF6 in fluoroethylene carbonate/diethyl carbonate (FEC/DEC, 1:1 v/v) electrolyte in an argon atmosphere glovebox. The cells were galvanostatically charged/discharged with a BT 2143 Arbin battery tester in a potential range of 0.01−1.5 V. Electrochemical impedance spectra (EIS) were measured in the 105 to 10−1 Hz range with a modulation amplitude of 5 mV, using a potentiostat (CHI 680D, CH

atom % nitrogen; and Ge/C (RF) has no nitrogen. As will be seen, the three composites provide quite different LiB anodes.

2. EXPERIMENTAL SECTION 2.1. Syntheses of RF, RPF, and PF Gels. The RF-gel was prepared by a condensing resorcinol and formaldehyde without a base catalyst in an aqueous solution. Two g of resorcinol was dispersed in 50 mL of deionized water and 2.75 mL of formaldehyde (37 wt % solution in water) was added dropwise under stirring. After 30 min, the solution was transferred to a Teflon-lined stainless steel autoclave and hydrothermally treated at 120 °C for 24 h. The resulting gel was collected by centrifugation, washed with deionized water and dried in an oven at 80 °C. The RPF and PF-gels were similarly prepared, except that either 2 g of a 1:1 mixture of resorcinol and mphenylendiamine or 2 g of m-phenylenediamine were used. 2.2. Synthesis of Ge/NC (PF), Ge/NC (RPF), and Ge/C (RF). One g of PF-gel was dispersed in 100 mL of ethanol and 8 mL deionized water. A solution of 2 mL of GeCl4 (3.688 g) in 8 mL of ethanol was added to the PF-gel dispersion, and the mixture was stirred for 3 h. The precipitated GeO2/PF-gel was collected by centrifuging, washed, dried, and heated to 750 °C for 3 h under flowing argon gas to produce the Ge/NC (PF). The other composites were made similarly by substituting the PF-gel with RPF-gel or with RF-gel. 2.3. Characterization Equipment. XRD patterns were obtained with a Rigaku, R-axis Spider diffractometer. A Quanta FEG 650 27789

DOI: 10.1021/acsami.6b09857 ACS Appl. Mater. Interfaces 2016, 8, 27788−27794

Research Article

ACS Applied Materials & Interfaces Instruments). The EIS spectra were fitted to circuits by Z-view software.

3. RESULTS AND DISCUSSION 3.1. Physicochemical Characteristics. Figure 1 shows schematically the preparation and structure of the Ge/NC (PF) composite. SEM images and digital photographs of the gels are shown in Figure S1a−c of the Supporting Information. After the PF-gel is mixed with GeCl4 and the GeCl4 is hydrolyzed to GeO2, the product GeO2/PF-gel (SEM image shown in Figure S1d) was heated to 750 °C, where the GeO 2 was carbothermally reduced to Ge while the PF-gel was carbonized, such that Ge nanoparticles dispersed on N-doped carbon spheres formed. The method allowed adjustment of the nitrogen doping in the range between 0 and 7.5 atom % by exclusion of PF, i.e., full or partial substitution of P by R. Figure 2a shows the XRD patterns of the composites annealed at 750 °C. The XRD patterns of the Ge/NC (PF) composite were consistent with the cubic Ge reference patterns (vertical lines, JCPDS 00-004-0545) and had no impurity peaks. The Ge/NC (RPF) and the Ge/C (RF) composites showed generally identical metallic Ge patterns with those of the Ge/NC (PF). The XRD patterns of the Ge/NC (PF) composite annealed at various temperatures are shown in Figure S2a; when annealed at 650 °C, GeO2 was still present; it was, however, reduced to Ge at 700 °C. Unlike the XRD patterns of the Ge/NC (PF) the pattern of Ge/C (RF) showed incomplete GeO2 reduction to Ge at 700 °C (Figure S2b); complete reduction required heating to above 750 °C. These results indicate that the N-doped carbon contributed to the reduction of the GeO2 species. Nitrogen atoms possess sufficient unpaired electrons which can effectively assist in the reduction of GeO2. Similar observations are reported in previous published reports, where the N-doped carbon or its precursor showed a clear effect of reducing the loaded metal species compared to the undoped carbons.5,21,22 For their complete reduction to Ge, the composites of this study were annealed at 750 °C. The SEM images of the Ge/NC (PF) composite (Figure 2b) shows that it consists of 300−600 nm diameter spheres. Elemental mapping images of the Ge/NC (PF) spheres show that they are N-doped carbon and contain Ge (Figure S3a). The line scan images of the Ge/NC (PF) sphere along the scan line (red arrow) revealed that the Ge species are distributed within the N-doped carbon sphere (Figure S3b). The morphology of the Ge/NC (RPF) (Figure 2c) is similar to that of Ge/NC (PF), except that the diameters of the spheres are slightly larger, 400−700 nm; elemental mapping (Figure S3c) shows that the Ge is dispersed in the N-doped carbon spheres. The carbon spheres of the nitrogen-free Ge/C (RF) composite (Figures 2d and S3d) are larger, of 2−5 μm, and also contain dispersed Ge. As reported earlier,20 the size of the carbon spheres shrinks when the initial solution pH is higher; the initial solution pH is lower for resorcinol than it is for mphenylenediamine. Figure 3 shows the TEM images of the three composites. A porous N-doped carbon structure for the Ge/NC (PF) is observed in Figure 3a. The inset of Figure 3a shows that Ge nanoparticles (Ge NPs) of 10 nm size are distributed in the Ndoped carbon. The TEM-EDS mapping images of the Ge/NC (PF) is shown in Figure 3b. Like the SEM-EDS results in Figure S3a, the mapping image of carbon is consistent with that of nitrogen and germanium, revealing that the Ge metals are

Figure 3. TEM images of a) Ge/NC (PF), c) Ge/NC (RPF), and d) Ge/C (RF). (b) Elemental mapping images of Ge, C, and N in Ge/ NC (PF).

dispersed in the N-doped carbon matrix. Figure S4a shows other mapping images for a single sphere of the Ge/NC (PF) composite, which further demonstrate the uniform distribution of the Ge species in the carbon sphere. The TOF-SIMS depth profiles of the Ge/NC (PF) are shown in Figure S4b. The 70 Ge− signal increases with increasing sputtering time (sputtering rate: 0.042 nm/s), indicating that the Ge species exist in the N-doped carbon sphere. The Ge/NC (RPF) and the Ge/C (RF) images resemble those of Ge/NC (PF) in Figure 2c,d. It is commonly observed that the Ge metals are dispersed in the porous carbon matrix in the Ge/NC (RPF) and the Ge/C (RF) composites. X-ray photoelectron spectroscopic (XPS) survey scans of Ge/NC (PF) and Ge/NC (RPF) show Ge, carbon, nitrogen, and oxygen (Figure 4a) and, as expected, the survey scan of Ge/C (RF) is similar except for the absence of the N 1s peak. The scans show that Ge/NC (PF) contains 7.5 atom % nitrogen and that Ge/NC (RFP) contains 3.9 atom % nitrogen. At high resolution the N 1s spectrum of Ge/NC (PF) in Figure 4b has three deconvoluted peaks at 401.3, 400.1, and 398.3 eV, corresponding respectively to quternary, pyrrolic and pyridinic nitrogen.23 The deconvoluted N 1s peaks of Ge/NC (RPF) are similar (Figure 4c). In the Ge 3d spectrum of the Ge/NC (PF) composite (Figure S5a), the three dominant peaks at 29.8, 30.9, and 32.5 eV are assigned to Ge, GeO, and GeO2, respectively.24 The spectrum of Ge/NC (RPF) in Figure S5b is similar to spectrum of the Ge/NC (PF), but in Ge/C (RF) an intense GeO2 peak is observd (Figure S5c), also suggesting, like the XRD results of Figure S2, that N-doping assists the reduction of GeO2. Figure 4d shows Raman spectra of the three composites. The D peak at 1350 cm−1 corresponds to the defect-induced breathing mode of sp2 rings, while the G peak at 1590 cm−1 originates in first-order scattering of the E2g mode the of sp2 domains.25 The numbers represent the intensity ratios of D and G peaks (ID/IG ratios). In general, the ID/IG ratio is used to 27790

DOI: 10.1021/acsami.6b09857 ACS Appl. Mater. Interfaces 2016, 8, 27788−27794

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Figure 4. a) Survey XPS spectra of the composites. b) Deconvoluted N 1s XPS spectrum of Ge/NC (PF). c) Deconvoluted N 1s XPS spectrum of Ge/NC (RPF). d) Raman spectra of the three composites.

Table 1. Electrical Properties of the Ge/NC (PF), Ge/NC (RPF), and Ge/C (RF) Electrodes catalysts Ge/NC (PF) Ge/NC (RPF) Ge/C (RF)

sheet resistancea (kΩ□−1) 3.57 (±0.78) × 10 6.25 (±1.03) × 10 2.37 (±0.44) × 102

conductivitya (Sm1−) −1

2.80 × 10 1.60 × 10−1 4.22 × 10−2

Rctb (Ω)

capacitanceb (μF)

54.8 89.2 120.9

2.70 × 102 1.69 × 102 6.50 × 10

From a 4-point probe method using an electrode composed of Ge/NC (80 wt %) and CMC (20 wt %) on a Cu foil. bExtracted from fitting EIS spectra to an equivalent circuit.

a

3.2. Electrochemical Properties. The Ge/NC (PF), Ge/ NC (RPF), and Ge/C (RF) electrodes were galvanostatically charged and discharged in the 0.01−1.5 V window at a current density of 0.2 Ag−1. The 1st, 2nd, 10th, 50th, and 100th voltage profiles of the electrodes are shown in Figure S8. The calculated specific capacity values are based on the total mass of the composites. In the voltage profile of the Ge/NC (PF) electrode (Figure S8a), the plateaus at ∼0.7 V and below 0.3 V during the first discharge scan correspond to the decomposition of the electrolyte and the formation of LiGe alloys. In the following charge scan, the plateau at ∼0.5 V is ascribed to the dealloying reaction.4,31,32 The Ge/NC (PF) electrode delivered an initial discharge and charge capacity of 1303 and 763 mAhg−1, respectively, with an initial Coulombic efficiency of 59%. The initial capacity loss is attributed to the formation of a solid electrolyte interphase (SEI) and to irreversible lithium insertion into the nanocomposites.12,28 From the 2nd cycle to 100th cycle, the voltage profiles of the Ge/NC (PF) electrode were largely unchanged, with a capacity of 732 mAhg−1 at the 100th cycle, implying good cycling stability. The Coulombic efficiency of >99% for up to 200 cycles after the 10th cycle (see Figure S9) also suggests improved reversibility. Figure S8b shows the cyclic voltammetry (CV) curves of the Ge/NC (PF) electrode at a scan rate of 0.1 mVs−1. The CV curves are generally correlated with the above voltage profiles in Figure

estimate the degree of disorder in the graphitized structure.25−27 The 1.03 ID/IG ratio of Ge/NC (PF) is slightly higher than the 1.01 ratio of Ge/NC (RPF) and is considerably higher than the 0.95 ratio of Ge/C (RF), showing that nitrogen-doping results in defects. The electrical conductivities of the Ge/NC (PF), Ge/NC (RPF), and Ge/C (RF) electrodes made with CMC coated on Cu foil are listed in Table 1. The electrical conductivity of the Ge/NC (PF) electrode is 0.28 S m−1, about twice the 0.16 S m−1 conductivity of Ge/NC (RPF) electrode, and about 7 times that of the 0.042 S m−1 conductivity of Ge/C (RF) electrode. Evidently, nitrogen-doping improves the electrical conductivity of the carbon, consistent with previous reports.28,29 Considering that high electrical conductivity is a basic requirement for a good electrode material, enhanced battery performance is expected when electrodes are made with more conductive carbon. The Ge/NC (PF) and Ge/NC (RPF) composites exhibit type IV mesopore-suggesting isotherms (Figure S6a). The BET surface areas are 61 m2/g for Ge/NC (PF), 30 m2/g for Ge/NC (RPF). The Ge/C (RF) shows a microporous structure with a BET surface area of 198 m2/g. Thermogravimetric analyses (TGA) in Figure S7 show respective Ge contents of 41.6, 38.5, and 44.7 wt % for Ge/ NC (PF), Ge/NC (RPF), and Ge/C (RF) by assuming the residual product is GeO2.30 27791

DOI: 10.1021/acsami.6b09857 ACS Appl. Mater. Interfaces 2016, 8, 27788−27794

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ACS Applied Materials & Interfaces S8a. During the first cathodic scan, the broad reduction peak at 0.7 V corresponds to the formation of the SEI film. The large peak below 0.5 V is assigned to the reduction of Ge. From the second cycle, reduction peaks observed at 0.48 and 0.01 V can be attributed to the formation of LixGe alloys. In the anodic scan, oxidation peaks at 0.41 and 0.58 V are ascribed to the dealloying reaction of LixGe.16 The Ge/NC (RPF) and the Ge/ C (RF) electrodes show Li insertion/extraction behaviors similar to the Ge/NC (PF) electrode, with initial Coulombic efficiencies of 63% and 45%, respectively. However, their cycling stabilities are worse than the cycling stability of the Ge/ NC (PF) electrode: the capacity of the Ge/NC (RPF) electrode decreases to 400 mAhg−1 at the 100th cycle (Figure S8c) and the capacity of the Ge/C (RF) electrode decays to 302 mAhg−1 at the 100th cycle (Figure S8d). As seen in Figure 5a, the Ge/NC (PF) electrode, which exhibits a much better cycling stability than the Ge/NC (RPF)

(PF) electrode is far better than that of the other electrodes. Even at 2.0 Ag−1 the capacity of the Ge/NC (PF) electrode is still as high as 472 mAhg−1, much above the capacity of the graphite anode. In contrast, the Ge/C (RF) electrode has a capacity of only 83 mAhg−1 and the Ge/NC (RPF) electrode only has a capacity of 210 mAhg−1 at 2.0 Ag−1 specific current. Furthermore, when the specific current is returned to 0.2 Ag−1 from 5.0 Ag−1, the Ge/NC (PF) electrode recovers its initial capacity, indicating its good reversibility. The voltage profiles of the electrodes corresponding to Figure 5b are presented in Figure S10. It is observed that as the current density is increased, the increase in polarization of the Ge/NC (PF) electrode is much lower compared to the other electrodes. Figure 5c shows the Nyquist plots derived from the electrochemical impedance spectra (EIS) of the three electrodes after their rate capability tests (60 cycles). The data fitted to the equivalent circuit of the inset of Figure 5c provide the parameters listed in the Table 1. The diameter of the semicircle in the high frequency region for the Ge/NC (PF) electrode is smaller than the diameter of the Ge/NC (PRF) or the Ge/C (RF) electrode. The charge transfer resistances (Rct) of Ge/NC (PF), the Ge/NC (RPF), and the Ge/C (RF) electrodes are 54.8, 89.2, and 120.9 Ω, suggesting that the electron transfer is faster in the Ge/NC (PF) electrode enabling its better cycling stability and rate capability compared to the other electrodes.33,34 The morphology of the Ge/NC (PF) electrode after 200 cycles (at fully charged state) were investigated by SEM. In the SEM image of the fresh Ge/NC (PF) electrode (Figure S11a), the Ge/NC (PF) spheres are surrounded by Super PLi carbon. After 200 cycles, the Ge/NC (PF) spheres still remain their spherical morphology with the formation of SEI layers on the spheres in Figure S11b. And the corresponding elemental mapping images (Figure S11c) show that the Ge species are distributed in the carbon spheres after 200 cycles. In the electrochemical measurements, the Ge/NC (PF) electrode with 7.5 atom % nitrogen showed better cycling performance and rate capability compared to the Ge/NC (RPF) electrode with 3.9 atom % nitrogen and the Ge/C (RF) electrode without nitrogen. It is concluded that nitrogendoping in the carbon contributed to the improved battery performance. First, by inducing the defects suggested by the Raman spectra (Figure 4d), nitrogen-doping apparently provides additional lithium sorbing sites, increasing the Li+ concentration and thereby the Li+ permeability of the composite. Indeed, as shown in Figure S12, the PF carbon electrode itself (the PF-gel annealed at 750 °C without the Ge precursor) shows a higher lithium storage capacity than the RPF carbon or the RF carbon: At the 200th cycle, the reversible capacity of the PF carbon electrode is 270 mAhg−1, while the reversible capacity is 225 mAhg−1 for the RPF carbon electrode and 179 mAhg−1 for the RF carbon electrode. Thus, the Ge/ NC (PF) electrode, possessing more defects in the carbon sphere might be able to store more lithium due to the effects of nitrogen-doping. Second, the nitrogen-doping appears to improve the electrical conductivity compared to the undoped carbon. As shown in the electrical conductivity measurements in Table 1, the Ge/NC electrode showed higher electrical conductivity than the other electrodes. Better electrical conductivity could facilitate charge transfer in the electrode, which is reflected as a low charge transfer resistance value for the Ge/NC (PF) electrode in the Nyquist plots (Figure 5c). Thus, the nitrogen-doping could create additional defect sites

Figure 5. a) Cycle number dependence of the specific capacity retained at a specific current of 0.2 Ag−1. b) Dependence of the specific capacity on the specific current. C) Nyquist plots of the electrodes.

or the Ge/C (RF) electrode, retains after the 200th cycle at a specific current of 0.2 Ag−1, a capacity of 684 mAhg−1 versus 337 mAhg−1 for the Ge/NC (RPF) and 278 mAhg−1 for the Ge/C (RF) electrode. Figure 5b shows the retained capacity of the electrodes when they were cycled at specific currents between 0.2 and 5.0 Ag−1. The rate capability of the Ge/NC 27792

DOI: 10.1021/acsami.6b09857 ACS Appl. Mater. Interfaces 2016, 8, 27788−27794

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(8) Wu, S.; Han, C.; Iocozzia, J.; Lu, M.; Ge, R.; Xu, R.; Lin, Z. Germanium-Based Nanomaterials for Rechargeable Batteries. Angew. Chem., Int. Ed. 2016, 55, 7898−7922. (9) Seo, M.-H.; Park, M.; Lee, K. T.; Kim, K.; Kim, J.; Cho, J. High performance Ge nanowire anode sheathed with carbon for lithium rechargeable batteries. Energy Environ. Sci. 2011, 4, 425−428. (10) Chockla, A. M.; Klavetter, K. C.; Mullins, C. B.; Korgel, B. A. Solution-Grown Germanium Nanowire Anodes for Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2012, 4, 4658−4664. (11) Ngo, D. T.; Kalubarme, R. S.; Le, H. T. T.; Fisher, J. G.; Park, C.-N.; Kim, I.-D.; Park, C.-J. Carbon-Interconnected Ge Nanocrystals as an Anode with Ultra-Long-Term Cyclability for Lithium Ion Batteries. Adv. Funct. Mater. 2014, 24, 5291−5298. (12) Cui, G.; Gu, L.; Zhi, L.; Kaskhedikar, N.; van Aken, P. A.; Müllen, K.; Maier, J. A Germanium−Carbon Nanocomposite Material for Lithium Batteries. Adv. Mater. 2008, 20, 3079−3083. (13) DiLeo, R. A.; Frisco, S.; Ganter, M. J.; Rogers, R. E.; Raffaelle, R. P.; Landi, B. J. Hybrid Germanium Nanoparticle−Single-Wall Carbon Nanotube Free-Standing Anodes for Lithium Ion Batteries. J. Phys. Chem. C 2011, 115, 22609−22614. (14) Xue, D.-J.; Xin, S.; Yan, Y.; Jiang, K.-C.; Yin, Y.-X.; Guo, Y.-G.; Wan, L.-J. Improving the Electrode Performance of Ge through Ge@C Core−Shell Nanoparticles and Graphene Networks. J. Am. Chem. Soc. 2012, 134, 2512−2515. (15) Zhong, C.; Wang, J.-Z.; Gao, X.-W.; Wexler, D.; Liu, H.-K. In situ one-step synthesis of a 3D nanostructured germanium-graphene composite and its application in lithium-ion batteries. J. Mater. Chem. A 2013, 1, 10798−10804. (16) Xu, Y.; Zhu, X.; Zhou, X.; Liu, X.; Liu, Y.; Dai, Z.; Bao, J. Ge Nanoparticles Encapsulated in Nitrogen-Doped Reduced Graphene Oxide as an Advanced Anode Material for Lithium-Ion Batteries. J. Phys. Chem. C 2014, 118, 28502−28508. (17) Xie, X.; Su, D.; Zhang, J.; Chen, S.; Mondal, A. K.; Wang, G. A comparative investigation on the effects of nitrogen-doping into graphene on enhancing the electrochemical performance of SnO2/ graphene for sodium-ion batteries. Nanoscale 2015, 7, 3164−3172. (18) Chang, K.; Geng, D.; Li, X.; Yang, J.; Tang, Y.; Cai, M.; Li, R.; Sun, X. Ultrathin MoS2/Nitrogen-Doped Graphene Nanosheets with Highly Reversible Lithium Storage. Adv. Energy Mater. 2013, 3, 839− 844. (19) Al-Muhtaseb, S. A.; Ritter, J. A. Preparation and Properties of Resorcinol−Formaldehyde Organic and Carbon Gels. Adv. Mater. 2003, 15, 101−114. (20) ElKhatat, A. M.; Al-Muhtaseb, S. A. Advances in Tailoring Resorcinol-Formaldehyde Organic and Carbon Gels. Adv. Mater. 2011, 23, 2887−2903. (21) Chew, L. M.; Kangvansura, P.; Ruland, H.; Schulte, H. J.; Somsen, C.; Xia, W.; Eggeler, G.; Worayingyong, A.; Muhler, M. Effect of nitrogen-doping on the reducibility, activity and selectivity of carbon nanotube-supported iron catalysts applied in CO2 hydrogenation. Appl. Catal., A 2014, 482, 163−170. (22) Park, H.; Youn, D. H.; Kim, J. Y.; Kim, W. Y.; Choi, Y. H.; Lee, Y. H.; Choi, S. H.; Lee, J. S. Selective Formation of Hägg Iron Carbide with g-C3N4 as a Sacrificial Support for Highly Active Fischer− Tropsch Synthesis. ChemCatChem 2015, 7, 3488−3494. (23) Matter, P. H.; Zhang, L.; Ozkan, U. S. The role of nanostructure in nitrogen-containing carbon catalysts for the oxygen reduction reaction. J. Catal. 2006, 239, 83−96. (24) Meng, X.; Zhao, J.; Li, H.; Endres, F.; Li, Y. Enhanced photoluminescence of ordered macroporous germanium electrochemically prepared from ionic liquids. Opt. Express 2012, 20, 9421−9430. (25) Choi, E.-Y.; Han, T. H.; Hong, J.; Kim, J. E.; Lee, S. H.; Kim, H. W.; Kim, S. O. Noncovalent functionalization of graphene with endfunctional polymers. J. Mater. Chem. 2010, 20, 1907−1912. (26) Xiao, Y.; Cao, M.; Ren, L.; Hu, C. Hierarchically porous germanium-modified carbon materials with enhanced lithium storage performance. Nanoscale 2012, 4, 7469−7474. (27) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Synthesis

for the more insertion of lithium and increase the electrical conductivity for fast electron transfer, resulting in the enhanced cycling stability and rate performance of the Ge/NC (PF) electrode.

4. CONCLUSIONS The Ge/NC composites with various nitrogen content (7.5, 3.9, and 0%) were successfully prepared by using PF-, RPF-, and RF-gels as N-doped carbon or undoped carbon sources. The Ge/NC (PF) electrode showed enhanced cycling stability and rate capability compared to the Ge/NC (RPF) and the Ge/C (RF) electrodes. The Ge/NC (PF) electrode retained a reversible capacity of 684 mAhg−1 at 0.2 Ag−1 after 200 cycles, while the Ge/NC (RPF) and the Ge/C delivered only 337 mAhg−1 and 278 mAhg−1, respectively. The enhanced performance of the Ge/NC (PF) electrode might be attributed to the increased electrical conductivity (for easier charge transfer) and induced defect sites (for more lithium insertion) by the nitrogen-doping.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b09857. XRD, SEM, XPS, BET, and TGA results for the composites (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.B.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was generously supported by the Welch Foundation through grants F-1131 (A.H.) and F-1436 (C.B.M.). D.H.Y. thanks Dr. Andrei Dolocan for experimental assistance and for helpful conversations.

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DOI: 10.1021/acsami.6b09857 ACS Appl. Mater. Interfaces 2016, 8, 27788−27794