ARTICLE pubs.acs.org/JPCC
Nitrogen-Doped Graphitic Layers Deposited on Silicon Nanowires for Efficient Lithium-Ion Battery Anodes Yong Jae Cho,† Han Sung Kim,† Hyungsoon Im,† Yoon Myung,† Gyeong Bok Jung,† Chi Woo Lee,† Jeunghee Park,*,† Mi-Hee Park,‡ Jaephil Cho,*,‡ and Hong Seok Kang*,§ †
Department of Chemistry, Korea University, Jochiwon 339-700, Korea Interdisciplinary School of Green Energy, Ulsan National Institute of Science and Technology, Ulsan 689-798, Korea § Department of Nano and Advanced Materials, College of Engineering, Jeonju University, Chonju, Chonbuk 560-709, Korea ‡
bS Supporting Information ABSTRACT: Nitrogen (N)-doped graphitic layers were deposited as shells on pregrown silicon nanowires by chemical vapor deposition. Graphite-like and pyridine-like structures were selectively chosen for 3 and 10% N doping, respectively. Increasing the thickness of the undoped graphitic layers from 20 to 50 nm led to an increase in the charge capacity of the lithium ion battery from 800 to 1040 mA h/g after 45 cycles. Graphitelike 3% N-doping in the 50 nm-thick shell increases the charge capacity by 21% (i.e., to 1260 mA h/g), while pyridine-like 10% N-doping in the 20 nm-thick shell increases it by 36% (i.e., to 1090 mA h/g). This suggests that both pyridine- and graphitelike structures can be effective for lithium intercalation. First principles calculations of the graphene sheets show that the large storage capacity of both N-doping structures comes from the formation of dangling bonds around the pyridine-like local motives upon lithium intercalation.
1. INTRODUCTION Silicon (Si)-based materials have attracted tremendous interest for lithium (Li)-ion battery anodes owing to their extremely high theoretical specific capacity of approximately 4200 mA h/g, which is much higher than that of commercialized graphitic carbon (372 mA h/g).1,2 However, Si suffers from a large volume change of more than 300% during Li alloying and dealloying, which can result in its structural pulverization and electrical disconnection from the current collector, leading to the rapid decline of the cell capacity. Recently, Si-based one-dimensional (1D) nanostructures including nanowires have received much interest, because of their ability to accommodate large strains without pulverization and capacity fading.37 As another simple approach to overcome the volume change problem, the carbon (C) coating of the Si nanostructures has been extensively investigated for many years. In fact, C-coated or -containing Si nanostructures exhibit higher charge capacities than bare Si, because of their good electrical conductivities and stress-buffer nature, which improve the stability of the anode.822 The incorporation of nitrogen (N) atoms into the graphite networks (e.g., carbon nanotubes) is one of the best way to generate n-type conductivity materials, wherein N introduce the donor states near the Fermi level.2330 However, there have been few studies to date on the charge capacities of N-doped C-coated Si nanostructures for Li ion batteries. r 2011 American Chemical Society
A number of experimental and theoretical works showed that the N doping of C materials (including graphene) enhanced the charge capacity of Li ion batteries.3136 It is known that there are two major CN binding structures; one is a direct substitutional graphite-like structure and the other one is a pyridine-like defect structure with lone-pair electrons that requires the rearrangement of the neighboring C atoms.29,30,3739 For polymeric amorphous C, Wu et al. suggested that increasing the N content would increase the charge capacity and reversible capacity, mainly due to the resulting pyridine-like N atoms.31,32 Zhou and coworkers performed first principles calculations on the adsorption of single Li atoms on N-doped CNTs.33,34 They predicted that pyridine-like structures would enhance the capacity due to the large adsorption energy and low energy barrier for Li penetration, while graphite-like N-doping would form an electron-rich structure and hinder Li adsorption. However, more systematic calculations on the adsorption of multiple Li atoms in which the various N doping levels of the CNTs are properly accounted for are lacking. In this respect, a detailed understanding of how two different N-doping structures at various doping levels can modify the charge capacities of Li ion batteries would enable the range of applications of C-containing nanostructures to be extended. Received: February 15, 2011 Revised: March 28, 2011 Published: April 22, 2011 9451
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Herein, Si NWs sheathed with undoped and N-doped graphitic layers were successfully synthesized. We denote these SiC and SiCN coreshell nanocables (NCs) as SiCNx for simplicity. The N doping level was controlled to be either 3 or 10%, leading to selective graphite-like and pyridine-like N structures, respectively. The Si NWs (av diameter = 80 nm) were synthesized by a well-developed metal-assisted chemical etching method and then graphitic layers (thickness = 2050 nm) were subsequently deposited on them via thermal chemical vapor deposition (CVD).4042 The charge capacities of the Si NWs and Si-CNx NCs for Li ion batteries were investigated and showed remarkable enhancements for both the graphite-like and pyridine-like N-doping structures. Furthermore, first principles calculations were performed on the intercalation of various numbers of Li atoms between two graphene sheets, whose interlayer region effectively models that of the 2050 nm-thick graphitic layers with a large diameter (ca. 80180 nm). The results showed that the N doping of graphene increases the capacity of Li intercalation due to the formation of vacancies and dangling bonds around the N doped sites. To the best of our knowledge, this is the first report of Li intercalation in N-doped graphite layers investigated by both experimental and theoretical methods.
2. EXPERIMENTAL SECTION The Si NWs were fabricated by metal-assisted chemical etching. The (100) silicon wafers (2 2 cm2) were washed with water and acetone and then immersed in an oxidant solution containing H2SO4 (97%) and H2O2 (35%) with a volume ratio of 3:1 for 10 min at room temperature to remove any organics and silicon oxide. They were etched with a 5% aqueous solution of HF for 3 min at room temperature to produce a fresh H-terminated Si surface. The Si wafers were immediately placed into a Ag coating solution containing 4.8 M HF and 0.005 M AgNO3 and slowly stirred for 5 min under atmosphere ambient. After the uniform coating of the Ag nanoparticles (Ag NPs), the Si wafers were washed with distilled water to remove any excess Ag ions and then immersed in an etchant solution composed of 4.8 M HF and 0.20.4 M H2O2. After 530 min of etching in the dark at room temperature, the Si wafers were washed repeatedly with water and immersed in dilute HNO3 (1:1 v/v) to dissolve the Ag NP catalyst. The deeply blackened Si wafers were washed with 5% HF again to remove the oxide layer and cleaned with distilled water. The Si NWs were placed inside a CVD reactor tube and methane (CH4) and hydrogen (H2) were introduced at flow rates of 100 and 300 sccm, respectively, for 530 min, when the temperature reached 1100 °C. The thicknesses of the graphitic outerlayers were controlled by adjusting the deposition time. For the N-doped outerlayers, NH3 (flow rate = 20100 sccm) was mixed with CH4/H2 at the same temperature. The products were analyzed by scanning electron microscopy (SEM, Hitachi S-4700), field-emission transmission electron microscopy (TEM, Jeol JEM 2100F and FEI TECNAI G2 200 kV), high-voltage TEM (HVEM, Jeol JEM ARM 1300S, 1.25 MV), energy-dispersive X-ray fluorescence spectroscopy (EDX), and electron energy-loss spectroscopy (EELS, GATAN GIF2000). The Raman spectra were measured using the 514.5 nm line of an argon ion laser. X-ray photoelectron spectroscopy (XPS) was performed using the 8A1 beamline of the PLS.
Figure 1. (a) SEM micrograph of the vertically aligned SiC NC array synthesized on the Si substrates. The inset corresponds to the SiC NCs separated from the substrates. HRTEM images showing SiC NCs with (b) 50 and (c) 20 nm-thick shells. The lattice-resolved TEM images and FFT ED patterns (insets) confirm the (d) crystalline graphitic layer shells and (e) the single-crystalline Si NW core with the [100] growth direction. (f) EELS spectrum showing the 3 and 10% N doping in the graphitic layers.
Near-edge X-ray absorption fine structure (NEXAFS) measurements were performed at the U7 beamline of the PLS. For the electrochemical tests, the electrodes for the battery test cells were made of the active material (Si NWs and Si-CNx NCs), super P carbon black, and polyvinylidene fluoride (PVDF) binder at a weight ratio of 8:1:1. The slurry prepared by thoroughly mixing a N-methyl-2-pyrrolidone (NMP) solution of PVDF, carbon black, and the cathode material was coated onto Cu foil with a thickness of 50 μm. The coated electrode was dried at 150 °C for 20 min and then roll-pressed. The coin-type half cells (2016 R-type) prepared in a helium-filled glovebox contained an electrode, a Li metal anode, a microporous polyethylene separator, and an electrolyte solution of 1 M LiPF6 in ethylene carbonate/dimethyl carbonate (EC/DMC) (1:1 vol %). Each anode with an area of 1 cm2 contained 12 mg of the Si NWs (or SiCNx NCs). Total energy calculations were carried out using the Vienna ab initio simulation package (VASP).43,44 The electronion interactions were described by the projected augmented wave (PAW) method, which was basically a frozen-core all-electron calculation.45 The exchange-correlation effect was treated by the generalized gradient approximation presented by Perdew, Burke, and Ernzerhof (PBE).46 A supercell geometry was adopted with large supercells that guaranteed that the interatomic distances between neighboring cells along the direction perpendicular to the graphene plane were greater than 11.00 Å. The lattice parameter of the 4 4 graphene sheet along the two directions of the hexagonal lattice was 9.864 Å, and k-point sampling was done using 4 4 points in the first Brillouin zone. The cutoff energy was set high enough (= 400 eV) to ensure accurate results, 9452
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The Journal of Physical Chemistry C and the conjugate gradient method was employed to optimize the geometry until the HellmannFeynman force exerted on an atom was less than 0.03 eV/Å.
3. RESULTS AND DISCUSSION Figure 1a shows the SEM image of the vertically aligned SiC coreshell NC arrays grown on the Si substrates. The inset shows the SEM image of the SiC NCs separated from the substrates. They average 7 μm in length and 180 nm in diameter. The high-resolution TEM (HRTEM) image shows the homogeneously coated graphitic layers on the Si NW core (av diameter = 80 nm) with a thickness of 50 nm (Figure 1b). Reducing the deposition time or CH4 flow rate produced thinner graphitic layers. Figure 1c shows the HRTEM image of the SiC NCs having a shell thickness of 20 nm. Figure 1d,e shows the latticeresolved images of C shell and Si core for a selected 20 nm-thick SiC NC, respectively. The C shells consisted of crystalline graphitic layers, whose (002) planes are separated by a distance of 3.4 Å. In the fast Fourier-transformed electron diffraction (FFT ED) pattern, the spots aligned along the NW axis can be indexed as the (002) basal planes of the graphite, confirming the existence of ordered graphitic layers (inset). The (200) fringes of Si, perpendicular to the wire axis, are separated by approximately 2.7 Å, which is close to that of face-centered-cubic Si (JCPDS 800018; a = 5.392 Å). The FFT ED pattern, at the [011] zone axis,
Figure 2. Fine-scanned XPS N 1s peaks of (a) 3% N-doped (SCN1) and (b) 10% N-doped (SCN2) C-shelled Si-CNx NCs. The data points (open circles) are fitted by two Voigt functions (filled with slashed lines); PN1 (red) and PN2 (blue) for the pyridine-like and graphite-like structures, respectively. The black line represents the sum of the resolved bands.
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confirms the existence of single-crystalline Si with the [100] direction along the wire axis (inset). The 50 and 20 nm-thick C shelled samples are referred to as SC1 and SC2, respectively. N-doped graphitic layers with N contents of either 3 or 10% were deposited on the Si NW arrays. Their SEM and TEM images are quite similar to those of the SiC NC samples. The N content was controlled by adjusting the flow rate of NH3. Under the experimental conditions used herein, the pyridine-like N structures become more favorable as the N content increases, especially in the case of the 20 nm-thick shell.47 We selectively synthesized a 3% N-doped 50 nm-thick shell containing only graphite-like structures and 10% N-doped 20 nm-thick shell having mostly pyridine-like structures. These 3% and 10% N-doped samples are respectively referred to as SCN1 and SCN2. The electron energy-loss spectroscopy (EELS) spectra of these samples show their N contents in the graphitic layers (Figure 1f). The average N doping percentages of SCN1 and SCN2 were obtained from the XPS spectra measured using an energy of 630 eV. Figure 2 shows the fine-scanned N 1s spectra. SCN1 shows an unresolved band centered at 401.1 eV. The band of SCN2 was deconvoluted into two bands: PN1 at 398.2 eV and PN2 at 401.2 eV. These bands were respectively assigned to the pyridine-like and graphite-like CN bonding structures, which is consistent with other works detailing N-doped CNTs.29,30,3739 At an N content of 3% (SCN1), only graphite-like structures (PN2 band) are allowed. At an N content of 10% (SCN2), as much as 80% of the structures are pyridine-like ones (PN1 band). The 3 and 10% N doping results primarily in the graphite- and pyridine-like structures, respectively, which enables their individual properties to be studied. Figure 3a shows the first cycle voltage profiles of these samples. The first discharge and charge capacities of the Si NWs were 3290 and 1410 mA h/g, respectively, with an initial Coulombic efficiency of 33%. The deposition of a thicker graphitic layer gives rise to a more significant increase of the first charge capacity and Coulombic efficiency; the samples with the 50 (SC1) and 20 nm-thick (SC2) layers showed first charge capacities of 2100 and 1390 mA h/g, along with initial Coulombic efficiencies of 89 and 95%, respectively. The 3% N doping in the 50 nm shell (SCN1) led to a slight increase of the first charge capacity to 2140 mA h/g. In contrast, 3% N doping in SCN2 (with graphitee-like structures) led to the more significant enhancement of the first charge capacity to 1950 mA h/g,
Figure 3. (a) First chargedischarge voltage profiles of half cells using Si NWs and Si-CNx NCs. (b) Charge capacities and Coulombic efficiencies vs cycle number for a half cell cycled with a rate of 600 mA/g tested between 0.01 and 1 V. 9453
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Table 1. Performance Parameters of Li Ion Battery Based on Si NWs and Si-CNx CoreShell NCs charge capacity (mA h/g) sample name shell thickness N content (%)
first cycle
45 cycles
Si NW
0
1410
195
SC1
50 nm
0
2100
1040
SC2 SCN1
20 nm 50 nm
0 3
1390 2140
800 1260
SCN2
20 nm
10
1950
1090
corresponding to a 40% increase, relative to SC2. The initial Coulombic efficiencies of SCN1 and SCN2 reached 92 and 85%, respectively. Figure 3b shows the charge capacity and Coulombic efficiency as a function of the cycle number up to 45 cycles at a rate of 600 mA/g). The capacity of the Si NWs decays rapidly to 195 mA h/g after 45 cycles. SC1 and SC2 exhibited better cycling properties with stable reversible capacities of 1040 and 800 mA h/g, respectively, after 45 cycles at the same rate. SCN1 demonstrated a reversible capacity of 1260 mA h/g, which is 21% (220 mA h/g) higher than that of SC1. SCN2 had a reversible capacity of 1090 mA h/g, which is 36% (290 mA h/g) higher than that of SC2. The Coulombic efficiencies of all of the CNx shell samples remained at >99% after the initial cycle, showing nearly complete reversibility. The charge capacities are summarized in Table 1. Cui and co-workers reported max 3124 mAh/g (after 10 cycles), using the Si NWs directly grown on the stainless steel substrates, which is much higher than our data (195 mA h/g after 45 cycles).3 Our low value would ascribe to the deteriorated contact with current collector, following the significant volume change. Furthermore, as shown in the TEM images of Figure S2 (Supporting Information), most of Si NWs crack into amorphous or crystalline phase nanoparticles (NPs), which can reduce the charge capacity. Wang and Han reported the charge capacity of 242 mAh/g (after 20 cycles) for the Si NPs (mixed with carbon black), which is similar to our value.7 For the C-coated Si NWs, Zhu and co-workers reported 1326 mAh/g (after 40 cycles), which is comparable to our value (1260 mAh/g after 45 cycles).11 The Yang group reported 2150 mAh/g (after 30 cycles), using the Si NWs directly grown on the stainless steel substrates.18 This higher value would be due to the better contact with current collector. A number of works showed that the C layers reduce the capacity degradation of Si NWs or NPs by the protection against the many possible side reactions with the electrolyte.822 The present crystalline graphitic layers consistently operate as efficient buffer and protection layers for the Si NWs. Figure S2 (Supporting Information) shows the TEM images and ED patterns of the SC1 and SCN1 after 45 cycles. In contrast to the Si NW samples, the NCs degraded not much. Their Si NW core partially break into the LixSi alloy phase NPs (size e5 nm) embedded in the crystalline graphitic layers, which is similar to the result reported in previous works on C-shelled Si NWs where the NPs were dispersed in an amorphous C matrix.8,12 The improved charge capacities and cycle lifetimes of the thicker C shell indicate that the graphitic layers act as good electrical conductors and serve as additional sites for Li insertion/extraction. The 30% increase in the charge capacity (800 versus 1040 mA h/g) observed upon increasing the shell
Figure 4. Atomic structures of two 4 4 Gr sheets with two H4-type doping patterns when (a) no Li atoms are intercalated and (b,c) 16 Li atoms (purple-colored spheres) are intercalated. Two different views are shown for the latter case. Five C atoms with dangling bonds, C1C5, are represented by star marks in the superscripts. The atoms in the upper Gr sheet are represented by the ball-and-stick model, while those in the lower sheet are represented by the stick model.
thickness may result from the increase of the C wt % (ca. 42 and 70% for SC2 and SC1, respectively). The charge capacity of these graphitic layers appears to be larger than the theoretical value of graphite. Nevertheless, nanosize graphitic layers would be expected to exhibit a larger charge capacity; for instance, 540 mA h/g for graphene.48 We suggest that the large diameter graphene-like graphitic layers, sheathing around the Si NW, maintain the rigidity and stability toward Li intercalation. The increased charge capacity of the N-doped graphitic layers is consistent with that of the N-doped carbons, suggesting that the N atoms can accommodate the Li intercalation by producing active binding sites.3134 The first charge capacity of SCN1 is comparable to that of SC1 but exceeds it after 34 cycles, which indicates the higher stability of the graphite-like structure toward Li intercalation. In contrast, the first charge capacity of SCN2 with the pyridine-like structure is much higher than that of SC2 but undergoes more degradation compared to SCN1. Considering the different N contents of the two samples, the stable graphite-like structures can be as effective as the pyridine-like structures for the enhancement of the charge capacity, which is different from the previous theoretical prediction.34 In order to understand the present experimental results, herein we performed a systematic investigation of the Li-intercalation energy on graphene (Gr) sheets at various N doping levels using first-principles calculations, providing a quantitative comparison of the charge capacity. A detailed, first-principles investigation of the relation between the N doping level and underlying atomic structure was previously performed for CNTs and Gr.45 The Gr sheets are equivalent to the 80180 nm diameter graphitic layers sheathing the Si NWs. It was found that the pyridine-like local structures are characterized by divacancies around the doped sites and are more stable than the graphite-like structures at the higher N-content (10%). The graphite-like structures are stable at an N content of 3%, which is consistent with the present results. On the basis of these results, the energy changes associated with the intercalations (= ΔEinter1) of 9, 12, and 16 Li atoms between two parallel 4 4 Gr sheets were calculated. These numbers were chosen to obtain systems with much higher symmetries than if other amounts of Li atoms were chosen, thereby vastly reducing the number of possible isomers. In order to model SCN1 with graphite-like N atoms, 3 (= 4.7%) of the total of 64 C atoms were substituted by N atoms at random positions (“SYSTEM C”). For the model of SCN2 (“SYSTEM H”) with pyridine-like motives, two H4-type divacancies with 8 C atoms (= 12.9%) were substituted by N atoms, in such a way that 9454
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Table 2. Comparison of Li-Intercalation Energies (= ΔEinter1) Per Li Atom between Two 4 4 Graphene Sheets with No Defects, Three Graphite-Like N Atoms, and Two H4-Type Divacancies Forming Pyridine-like Motives system N (0%)a number of Li atoms
N-doping pattern
system C (4.7%)
system H (12.9%)
three
two H4-type divacancies
graphite-like N atoms
leading to pyridine-like motives
b
Figure 5. Atomic structures of SYSTEM C with a monovacancy around a graphite-like N nitrogen atom when (a) 12 Li atoms and (b,c) 16 Li atoms (purple-colored spheres) are intercalated. Two different views are shown for the latter case. The two C atoms with dangling bonds, C1 and C2, are represented by star marks in the superscripts. The atoms in the upper graphene sheet are represented by the ball-and-stick model, while those in the lower sheet are represented by the stick model.
each Gr sheet included one divacancy, as shown in Figure 4a. “SYSTEM N” denotes the system with no N defects (as the model of SC1 and SC2). Figures S3∼S5 (Supporting Information) show the optimized structures of the most stable configurations (SYSTEMS N, H, and C) for the intercalation of 9, 12, and 16 Li atoms obtained from the various initial configurations considered, respectively. Table 2 compares the ΔEinter1 per Li atom of these three model systems. Only the intercalation energies of the most stable configurations are shown for each N-doping pattern and Liintercalation level. For SYSTSEM N, the small value (0.19 eV) of ΔEinter1(12f16 Li) indicates that the intercalation of an additional 4 Li atoms into the 12 Li atom system is not practically possible in the interlayer regions, and that at most 12 Li (= 18.8%) atoms can be intercalated between a 64-C domain of two adjacent supershells. This is because the present model assumes that the hexagons in the two Gr sheets are perfectly matching. In a real system, however, the Li atoms will intercalate in the multilayers, whose adjacent layers have different diameters and chiralities, thereby increasing the charge capacity. Next analyzed is SYSTEM H, in order to describe SCN2. Because of the broken symmetry of the hexagonal network caused by the introduction of divacancies, at least twice as many configurations were considered than for SYSTEM C. In the systems that were investigated, the Li atoms will be stored more tightly at all of the Li contents considered. Specifically, the large value (1.02 eV) of ΔEinter1(12f16) shows that the additional intercalation of 4 Li atoms in the 12 Li-intercalated system resulting in 16 Li-intercalation is quite plausible. Therefore, the intercalation level will be higher than that of SYSTEMS N and C, being at least 16 (= 25.0%) Li atoms, which explains why the highest charge capacity was obtained for SCN2. Figure 4b,c shows that the high intercalation energy can be ascribed to the formation of dangling bonds around the divacancies by the rearrangement of the C and N atoms. Upon the intercalation of 16 Li atoms, the C1C2 and C2C3 bonds are broken, the C3N2 pairs are rearranged in such a way that N2 becomes bonded to N1, and the C2 and N3 atoms move to the lower Gr
sheet. As a result, a total of five C atoms, C1∼C5 (marked by superscript *), and two N atoms, N2 and N3, form dangling bonds, which stabilize the Li atoms concentrated around the divacancies. The extra Li atoms are concentrated around the dangling bonds, which is manifested by the significantly shorter LiN and LiC distances (∼1.5 Å) than those (>2.0 Å) in the other configurations. Finally, SYSTEM C was analyzed. This system had at least twice as many configurations to study than SYSTEM N. At most, 12 Li atoms can be intercalated, considering that the value (0.07 eV) of ΔEinter1(12f16 Li) is even smaller than that of SYSTEM N. This apparently disagrees with the observation that the graphite-like N doping (SCN1) increases the charge capacity by 21%. In order to rationalize this high charge capacity, a structural rearrangement is proposed. The monovacancy formation energy (ΔEvac) was calculated, which is defined as the process of Gr f Gr with one vacancy þ C(Gr). If the vacancy is adjacent to an N atom, such that a pyridine-like structure and dangling bonds are formed, the formation of vacancies is more favorable than that for SYSTEM N, especially when a certain amount of Li atoms have already been intercalated; the ΔEvac values are 3.93 and 1.47 eV when the number of intercalated Li atoms is 0 and 12, respectively.49 In fact, a separate calculation of ΔEinter1(12f16 Li) (= 0.33 eV) in the presence of such a vacancy shows that SYSTEM C can intercalate ∼16 Li atoms. Figure 5a corresponds to SYSTEM C with 12 intercalated Li atoms with a pyridine-like monovacancy around one N atom (N1). The C1C2 bond (marked by superscript *) is weakened, leading to the formation of a five-member C ring. The CC distance of 1.69 Å is shorter than the value of 1.91 Å obtained when no Li atoms are intercalated. In turn, this weak C1C2 bond is stabilized by the interactions with the adjacent Li atoms; LiR makes a close approach to these two C atoms with a distance of 2.18 Å, which is shorter than that (= 2.37 Å) usually adopted when no vacancy is involved. In the case of the intercalation of 16 Li atoms, the additional four Li atoms induce the breaking of this C1C2 bond and the formation of dangling bonds that stabilize the intercalation of the Li atoms concentrated around the monovacancy site, and vice versa (Figure 5b,c). All of the Li atoms are rearranged so as to be located on top of the center of the hexagons of the lower Gr sheet, thereby reducing the LiLi repulsion. Note that this is different from the case where 12 Li atoms are intercalated where some of the Li atoms are offcentered. The displacement of two C atoms toward the lower Gr sheet stabilizes the dangling bonds. As a result, the C1Li1 and C1Li2 distances are shortened from 2.16 to 1.98 Å. A similar
--
9b
1.32 1.27
12c
0.49 0.55
2.50 1.25
16d
0.19 0.07 (0.33)e
1.02
a
The number inside parentheses denotes the N atomic percentage. The intercalation energy is defined for the process graphene þ 9 Li f (9 Li-intercalated)-graphene. c The intercalation energy is defined for the process (9 Li-intercalated)-graphene þ 3 Li f (12 Li-intercalated)graphene. d The intercalation energy is defined for the process (12 Liintercalated)-graphene þ 4 Li f (16 Li-intercalated)-graphene. e The intercalation energy is defined for the process (12 Li-intercalated)graphene with one vacancy þ4 Li f (16 Li-intercalated)-graphene with one vacancy, where the vacancy is generated in such a way that a pyridine-like local structure is formed.
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The Journal of Physical Chemistry C phenomenon is observed for the C1Li2 and C1Li3 distances. We want to point out that more than one monovacancy may be introduced in SYSTEM C, which can further enhance the Liintercalation. In summary, we were able to explain the enhanced charge capacity of the N-doped graphitic layers using models in which the dangling bonds around the divacancy or monovacancy sites (formed by the rearrangement of the atoms) stabilize the intercalated Li atoms.
4. CONCLUSIONS Vertically aligned Si NW arrays were synthesized by an Ag metal-assisted chemical-etching method and then the graphitic layers were deposited as shells on the Si NWs via the subsequent thermal CVD of CH4/H2/NH3. Two undoped samples with shell thicknesses of 50 nm (SC1) and 20 nm (SC2) were prepared, along with their N-doped counterparts having N contents of 3% (SCN1) and 10% (SCN2), respectively. Only graphite-like structures exist at 3% N doping, while pyridine-like structures are dominant at 10% N doping. For Li ion batteries, 3% N doping with a graphite-like structure increases the charge capacity by 21% from 1040 mA h/g to 1260 mA h/g after 45 cycles. The pyridine-like N-doping (10%) increases the charge capacity from 800 mA h/g to 1090 mA h/g, that is, by 36%. The improved charge capacities of the N-doped graphitic layers suggest that the graphitic layers serve as efficient Li intercalation sites in addition to successfully protecting the Si NW core. The graphite-like structures exhibit higher stability and are as effective as the pyridine-like structures for Li storage. First principles calculations showed that the higher Li storage capacity of the pyridine-like local structures is due to the formation of dangling bonds as a result of the rearrangement of the C and N atoms around the divacancy sites, which stabilizes the intercalated Li atoms. The graphite-like local structures easily generate monovacancies around the N atoms when a certain amount of Li atoms have already been intercalated and enhance the Li storage to a significant degree. The present work demonstrated the role of the structural factors of the graphitic layers that might be crucial in further enhancing the performance of Li ion batteries in the future. ’ ASSOCIATED CONTENT
bS
Supporting Information. XPS, TEM, and configurations for intercalation. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail: (J.P.)
[email protected]; (J.C.)
[email protected]; (H.S.K.)
[email protected].
’ ACKNOWLEDGMENT This study was supported by the NRF (20100000181; 2009-82528; 2010-0029164), IITA (2008-C1090-0804-0013), and WCU program (R31-10035). This work was also supported by the Basic Science Research Program through the NRF of Korea funded by the Ministry of Education, Science and Technology (2010-0007815). The HVEM (Daejeon) measurements were performed at the KBSI. The experiments at the PLS were partially supported by MOST and POSTECH. Computations
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were performed using a supercomputer of the Korea Institute of Science and Technology Information under the Contract No. KSC-2008-S03-0008.
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