Superior Lithium ElectroactiveMesoporous Si@Carbon Core-ShellNanowires for Lithium Battery AnodeMaterialHyesun Kim and Jaephil Cho*

Department of Applied Chemistry, Hanyang UniVersity, Ansan, 426-791 Korea

Received June 27, 2008; Revised Manuscript Received September 10, 2008

ABSTRACT

Mesoporous Si@carbon core-shell nanowires with a diameter of ∼6.5 nm were prepared for a lithium battery anode material using a SBA-15template. As-synthesized nanowires demonstrated excellent first charge capacity of 3163 mA h/g with a Coulombic efficiency of 86% at a rateof 0.2 C (600 mA/g) between 1.5 and 0 V in coin-type half-cells. Moreover, the capacity retention after 80 cycles was 87% and the rate capabilityat 2 C (6000 mA/g) was 78% the capacity at 0.2 C.

Si metal has been known to reach a highest lithium capacityof ∼4200 mA h/g corresponding to Li4.4Si;1 however a largevolume change (>300%) during lithium alloying and deal-loying can result in pulverization of the particles andelectrical disconnection from the current collector.2 Theelectrical disconnection leads to a rapid capacity fade of thecell. Many reports have focused on reducing such volumechange via composites with carbon and Si nanoparticles.2,9

These methods showed some improvement of the capacityretention because the carbon acts as an electron conductorbetween the pulverized particles. However, in order toachieve relatively good capacity retention, the carbon contentin the composite should be greater than 50 wt %, and thecapacity retention after 50 cycles less than 1500 mA h/g.

Alternatively, reports on one-dimensional (1D) siliconnanowires for anode materials are very rare. Si nanowiresprepared by the laser ablation system described by Lieberet al.10 had a wire diameter of ∼10-30 nm,11 and the firstdischarge and charge capacities of the nanowires were ∼1300and ∼900 mA h/g, respectively, even though the reason forthe low specific capacities was not reported. Si nanowireswith a diameter <50 nm prepared by vapor-liquid-solid(VLS) template-free growth was reported by Cui et al.12 Theyreported a reversible capacity of ∼2000 mA h/g at a rate of0.2 C between 2 and 0 V, and no capacity fade up to 10cycles. Semiconducting nanowire growth by the VSL,solution-liquid-solid (SLS), and template-free synthesishave been reported.13,21 However, these methods did notproduce well-aligned nanowires with ordered separationdistance. It is very important to array the Si nanowires with

uniform interwire distance (same pore size) so that theordered pores can act as a buffer layer for the uniformvolume changes. One such example is mesoporous SnO2 andmesoporous tin phosphates prepared by soft templates,21

which showed excellent capacity retention compared to thenanosized counterpart, thus demonstrating the role of me-sopores.

Recently, Ryoo’s group reported preparation of orderedmesoporous carbon or metal oxides using highly orderedmesoporous silica templates.22 After an annealing process,the parent silica materials were selectively removed. Theseframework compositions should be stable under conditionsused to dissolve the mold, that is, stability in relativelyconcentrated NaOH or HF for silica.23 This method is highlyreproducible and can use silica templates. To prepare thenanowires, a hexagonal SBA-15 silica template with p6 mmsymmetry was used, which contains two-dimensional, paral-lel cylindrical pores arranged with a hexagonal symmetry.

In this study, mesoporous silicon-carbon nanowiresprepared by a SBA-15 hard template are reported. Eventhough metallic semiconductors cannot be used in thistemplate due to reaction at higher temperature, butylterminators in the Si-C4H9 precursor are converted to carbonshell layers later in the annealing process. Hence, the carbonlayer blocks a direct reaction between the Si core and theSiO2 template. These mesoprous carbon@shell nanowiresshow not only excellent lithium reactivity with a reversiblecapacity of 3163 mA h/g at a rate of 0.2C but also excellentrate capability at 2 C rate.

Figure 1a shows the schematic diagram of the preparationprocedure for the mesoporous Si@carbon core-shell nanow-* Corresponding author, jpcho@hanyang.ac.kr.

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10.1021/nl801853x CCC: $40.75 2008 American Chemical SocietyPublished on Web 10/25/2008

ires using the SBA-15 template. Butyl-capped Si precursorwas impregnated into the template and annealed at 900 °Cunder vacuum. The template was removed using HF. Figure1b displays the transmission electron microscopy (TEM)image of carbon@Si nanorods with a diameter of 4 nm anda length of 20 nm obtained after the first impregnation,annealing at 900 °C, and HF etching. The (111) lattice fringeand selected area diffraction pattern (SADP) (inset of c)confirm the formation of the diamond cubic Si phase.However, after the fourth impregnation, fully grown nanow-ires are observed (Figure 1d). The light lines in Figure 1dare considered projections of the mesopore channels, whereasthe dark lines are the Si walls. In addition, a very thinamorphous carbon layer is observed (Figure 1e) and CHSanalysis confirmed that the carbon content was 6 wt %. Theordering of the carbon layer was examined by surface-enhanced Raman spectral analysis (Figure 1f). The mode at1582 cm-1, referred to as the G mode, was assigned to the“in-plane” displacement of carbons strongly coupled inthe hexagonal sheets.24 When disorder was introduced intothe graphite structure, the bands broadened. Further, the bandnear 1357 cm-1 is typically called the “disorder-induced”or D mode, and the integrated intensity ratio ID/IG isindicative of the degree of carbonization.24 A smallerintensity ratio indicates a higher degree of carbonization. Thevalue was 0.09 for ordered synthetic graphite, but that valueof the Si nanowires was 1.45, indicating the formation ofdisordered carbon layer.

Figure 2a exhibits a low-angle X-ray diffraction (XRD)pattern of the synthesized nanowires after removing thetemplates. Two resolved peaks indexed as (100) and (110)

confirmed a well-ordered hexagonal mesoporous structurewith a space group of p6 mm. The intense (100) peakcorresponds to a d-spacing of 8.8 ( 0.1 nm, and the high-angle XRD pattern of the annealed sample clearly showedthe presence of Si nanocrystals (inset of Figure 1a). Thecrystal size of the sample, calculated using the Scherrerformula, was estimated to be approximately 6.5 nm.

The nitrogen-adsorption isotherm of the annealed sample,with a Brunauer-Emmett-Teller (BET) surface area of 74m2/g, is shown in Figure 2b. The nitrogen adsorption-desorp-tion isotherms of both templates were type IV with a sharpcapillary condensation step with high relative pressures andH1 hysteresis loops, indicative of large channel-like poresin a narrow size distribution. The average pore size in theannealed sample was approximately 2.3 nm (Barrett-Joyner-Halenda (BJH) analysis), which is consistent with the XRDand TEM data (Figures 1 and 2, respectively). The pore wallthickness was estimated from the difference in the d-spacings,and the pore size was determined from the desorption branchof the isotherm using the BJH method. Accordingly, thecalculated pore-wall size of the nanowires was 6.5 nm,consistent with the TEM result.

Figure 3a shows voltage profiles of the mesoporouscarbon@Si nanowires after 1, 30, 60, and 80 cycles, andthe first discharge and charge capacities were 3664 and 3163mA h/g, respectively, with an initial Coulombic efficiencyof 86%. An irreversible capacity ratio of 14% may be dueto the side reaction with the electrolytes. In the side reaction,solvent molecules and salt anions are reduced on the activesurface, forming insoluble Li salts that precipitate to form apassivating film surface.25 In addition, the relatively largesurface area of the nanowires leads to the intensified surfacereactions and, hence, to their high irreversible capacity.Another source that may contribute to the irreversiblecapacity is the formation of SiOx, and the decompositionplateau of SiOx to Si and Li2O should appear near 0.8 V.26

However, the cycling curve during the first cycle does notshow decomposition, indicating the formation of the SiOx

Figure 1. (a) Schematic view of the preparation of Si@carboncore-shell nanowires. (b) TEM image of the Si@carbon core-shellnanorods obtained from first impregnation. (c) Expanded TEMimage of (b) (inset is SADP of (c)). (d) TEM image of theSi@carbon core-shell nanowires obtained from fourth impregna-tion. (e) Expanded TEM image of (d) and (f) Raman spectrum ofSi@carbon core-shell nanowires.

Figure 2. (a) Low- and high-angle (inset) XRD pattern of theSi@carbon core-shell nanowires. (b) Nitrogen adsorption-desorptionisotherm of the Si@carbon core-shell nanowires. (c) Pore sizedistribution of the Si@carbon core-shell nanowires.

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layer did not occur. Alternatively, XPS shows a very weakpeak at ∼100 eV, which is assigned to the formation of aSiOx layer27 and is believed to be negligible (see SupportingInformation, Figure S1).

After the first cycle, the Coulombic efficiency remainsrelatively stable throughout the cycles, indicating that theformed surface film remained intact. The capacity after 80cycles was 2738 mA h/g, which corresponds to 87% capacityretention (Figure 3b). In the case of carbon-coated Sinanoparticles (Si:C ) 44:56 wt %), a first charge capacityof 1857 mA h/g with a irreversible capacity ratio of 29%has been reported, and the charge capacity after 20 cycleswas 1489 mA h/g between 0.02 and 1.2 V at a cycling rateof 100 mA/g, with a capacity retention of 80%.4 It has beenreported that in order to reduce the volume change andimprove the cycle life performance, the discharge voltagewas limited above 0.02 V because the most contribution fromthe capacity decay is reported to be from an inhomogeneousvolume change between amorphous LixSi and crystallineLi15Si4 below 0.02 V.28 However, the results herein demon-strate that controlling the wire diameter and pore size aremore important than controlling the discharge cutoff voltage.Alternatively, nestlike silicon nanoparticles showed com-pletely different voltage curves than Si nanowires, withprofiles slowly declining to 0.2 V.29 The nanoparticles hada first charge capacity of ∼4000 mA h/g but showed rapidcapacity fade after 50 cycles to 1000 mA h/g. These resultsindicate that particle pulverization occurred (electrical dis-connect from the current collector) after cycling. On thecontrary, due to the nature of the nanowire/carbon with thebranched morphology and carbon cell, direct contact amongthe nanowires can be minimized and less aggregation isexpected.30 Moreover, the ordered mesopores provide thebuffer zone for volume changes between the nanowires.

Figure 3c shows differential curves during the 1st, 2nd,and 30th cycles, and only one broad peak from 0.18 and 0V was observed during the first discharge (lithium alloy),which was due to the phase transition of amorphous LixSito crystalline Li15Si4.28,31 Upon the second discharge, threepeaks were observed at 0.23, 0.08, and 0.05 V as opposedto the first discharge. The first two peaks were broader thanthe latter (0.05 V) and therefore amorphous-like phasetransition occurred. The peak at 0.23 and 0.08 V may bedue to the phase transition between amorphous LixSi becausethis region is the only dominant region for amorphous LixSiphases.31 The peak at 0.04 V has been assigned to the phasetransition of amorphous LixSi from crystalline Li15Si4.However, the different differential curves during the first andremaining discharge cycles are not clear. During charges,the peaks at 0.3 and 0.48 V are from phase transitionsbetween amorphous LixSi phases.31 However, such broadpeaks are also observed in the very small crystallites withsize of <20 nm,32 and therefore we cannot rule out thepossibility for the presence of nanosized LixSi crystallites.

Figure 3d shows rate capabilities of the meseoporousSi@C nanowires at 0.1, 0.5, 1, and 2 C rates between 1.5and 0 V in coin-type half-cells. The charge capacities at theserates were 2954, 2757, and 2462 mA h/g, respectively, andthe rate capability at 2 C, compared to 0.2 C, was 78%.Under a 3 C rate cycling, the first charge capacity was 2000mA h/g and capacity retention after 20 cycles was 95% (seeSupporting Information Figure S2). There is only one studyon the rate capability of Si in which Si nanowire directlygrown on a metallic current collector showed 3124 mA h/gat a rate of 0.05 C. However a rapid capacity decrease to∼2500 and ∼ 2000 mA h/g at a rate of 0.2 and 1 C rates,respectively, were observed.12 Practically, mesoporous ma-terials have ordered internal pores that can be flooded with

Figure 3. (a) Voltage profiles of the Si@carbon core-shell nanowire electrode after 1, 30, 60, and 80 cycles at a rate of 0.2 C between 1.5and 0 V in coin-type half-cells. (b) Plot of charge capacity and Coulombic efficiency of the cell (a) vs cycle number. (c) Differential curvesof the Si@carbon core-shell nanowire electrode after 1, 2, and 30 cycles. (d) Voltage profiles of the rate capabilities of the Si@carboncore-shell nanowire electrode at rates of 0.2, 0.5, 1, and 2 C between 1.5 and 0 V in coin-type half-cells (same charge and discharge rateswere used).

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electrolytes, ensuring a high surface area in contact with theelectrode and hence a high flux of lithium across theinterface.33,34 Additionally, unlike the porosity that existsbetween particles in an electrode, the size of which is randomand highly distributed, the uniformity of pore size andregularity in the arrangement of the pores (ordered porosity)ensures an even distribution of electrolyte in contact withthe electrode surface. The thin walls, of uniform dimensionsthroughout, ensure short diffusion paths for lithium ions onalloy/dealloy and electrons and hence equal high rates oftransport throughout the material.

Figure 4 shows the TEM images of the nanowires after80 cycles (after charging to 1.5 V). Compared to the imageof the pristine sample, some mesoporous Si nanowires weredemolished. In addition, morphology of the pristine nanow-ires tuned into the nanowires consisted of aggregatednanoparticles (textured side walls). A similar phenomenonwas observed in Si nanowires after 10 cycles.12 Furthermore,the nanowire diameter of the pristine sample increases to∼12 nm after 80 cycles despite the large volume change. Innanomaterials, the energy barriers for alloy formations aresmaller than those for the bulk materials because a largefraction of the Si atoms is in high energy states on the highcurvatured surfaces.11 In this regard, the large volume changedue to nucleation of the new phase (LixSi) can be readilyaccommodated. The cycled nanowires consisted of amor-phous and crystalline phases, indicating that pristine crystal-line structure was mostly collapsed. Moreover, the aggregatednanoparticles were mostly amorphous phase. We believe sucha presence of the crystalline Si phase even after extensivecycling has not been reported in previous studies. On theother hand, the XRD pattern of the cycled electrode showsonly an amorphous phase (Supporting Information FigureS3). The formation of the aggregated nanoparticles may bedue to the nonuniform volume changes among nanowires.

In conclusion, a new synthesis method for mesoporousSi@C using a SBA-15 template was demonstrated. As-synthesized nanowires had a diameter of 6.5 nm and well-ordered mesopores of 2.3 nm. Due to this alignment,excellent capacity retention was maintained after 60 cycles,and the initial capacity was 3163 mA h/g and the capacityretention after 80 cycles was 87%.

Acknowledgment. This work was supported by the ITR&D program of MKE/IITA (Core Lithium Secondary

Battery Anode Materials for Next Generation Mobile PowerModule, 2008-F-019-01).

Supporting Information Available: Experimental detailsand XPS, rate capability at 3 C, and ex situ XRD pattern ofthe Si@carbon core-shell nanowires. This material isavailable free of charge via the Internet at http://pubs.acs.org.

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NL801853X

Figure 4. (a) TEM image of the Si@carbon core-shell nanowireelectrode after 80 cycles (after cycling, the cell was charged to1.5V). (b) Expanded image of (a). (c and d) SADPs of the (b). (e)Expanded image of (a).

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