Probing the Lithium Ion Storage Properties of Positively and

LETTER pubs.acs.org/NanoLett

Probing the Lithium Ion Storage Properties of Positively and Negatively Carved Silicon Sang Hoon Nam,† Ki Seok Kim,† Hee-Sang Shim,‡ Sang Ho Lee,† Gun Young Jung,*,† and Won Bae Kim*,†,‡ †

School of Materials Science and Engineering and ‡Research Institute for Solar and Sustainable Energies (RISE), Gwangju Institute of Science and Technology (GIST), 261 Cheomdan-gwagiro, Buk-gu, Gwangju 500-712, South Korea

bS Supporting Information ABSTRACT: Here, we report Si pillar and well arrays as tailored electrode materials for advanced Li ion storage devices. The well-ordered and periodic morphologies were formed on a Si electrode thin film via laser interference lithography followed by a dry etch process. Two different patterns of negatively or positively carved Si electrodes exhibited highly improved cycle performance as a consequence of the enlarged surface area and the nanoscale pattern effects. The Si well arrays showed the highest energy density, rate capability, and cycling retention among the prepared Si electrodes. This tailored electrode platform demonstrates that these design principles could be applied to future developments in Si electrodes. KEYWORDS: Silicon, laser interference lithography, nanopattern, anode, lithium ion battery

S

ilicon is attracting increasing attention as an alternative anode electrode material for use in Li ion batteries (replacing carbonaceous materials), due to its very high theoretical specific capacity (up to 4200 mAh g
1) and relatively low working potential (∼0.5 V vs Li/Li+).1,2 Although deposited Si film electrodes exhibit a high specific capacity at initial stages, the cycle performance is likely to decay rapidly, with cracking and crumbling occurring with increasing cycle number.3
5 This poor cycle retention, which occurs because of the volume changes (approximately 400%) in Si during Li alloying/dealloying,1 has delayed the development of Si as a next-generation material for Li ion batteries. The most important requirement for successful Si electrode development is the accommodation of the volume expansion/contraction during the cycles. Recently, several research groups have reported interesting Si nanostructures for Li ion batteries, such as nanowires,6
8 nanotubes,9,10 and nanoparticles,11,12 prepared by various bottom-up methods. The nanostructured Si can directly affect the transport characteristics of charges and show induced stress relaxation during lithium insertion/extraction since the interval distance formed between neighboring nanostructures can act as structural buffer space preventing the agglomeration of Si,13 resulting in enhanced capacity and cycle performance. These earlier works provided a base for surface nanopatterning techniques capable of manipulating surface regions through control of both the periodicity and diameter to enter into the picture in the development of Si electrodes for Li ion batteries. Over the last few decades a number of new nanopatterning techniques have been developed, including photolithography,14 nanoimprinting,15 electron beam lithography,16 dip-pen writing,17 and variants thereof; the use of these techniques has allowed a scaling down of the feature size to below 100 nm. The process starts with the design of a r 2011 American Chemical Society

nanopattern in the form of a mask, which is subsequently used for template etching or deposition to create two-dimensional nanopatterns on a substrate. This has also been accomplished using laser interference lithography (LIL), which is a powerful method for fabricating periodic structures over large areas at the nanoscale level. This technique provides an economical, rapid, and reproducible system for manufacturing patterned arrays on flat surfaces.18
21 However, there are few reports on the Li ion storage properties using the nanopattering techniques despite many possible opportunities in Li ion batteries. In this work, we produced carved Si electrodes using the LIL technique under controlled exposure conditions, in combination with a dry etch process. We demonstrated their electrochemical properties and showed that, as a consequence of the structural benefits of tailored Si nanostructures, they are good electrode materials for Li ion storage. The direct fabrication of arrayed or integrated architectures on the substrate provides fast charge transport, thereby allowing every individual nanostructure to participate in the electrochemical reaction.6,22 In the current study, we probed the effects of nanopattern type on the electrochemical properties and Li ion storage performance, using consistently carved Si electrodes. The pattern size, etch depth, and periodicity of the Si electrodes used in this work were controlled to allow a comparative study. Results and Discussion. The LIL technique confers several advantages, including easy processing, large-area patterning, and simple modulation of the density or diameter in periodic dot or hole patterns. In this work, we have utilized LIL with a positive or Received: May 9, 2011 Revised: July 27, 2011 Published: August 22, 2011 3656

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Figure 1. Schematic diagrams illustrating the fabrication of positively or negatively carved Si arrays. The scheme and images for the laser interference setup and pitch size differentiation with increasing incident angle (θ), and the developed polymer templates were additionally described in Figure S1 in the Supporting Information. Figure 3. X-ray spectra of the sputtered Si thin film: (a) X-ray diffraction and (b) X-ray photoelectron spectroscopy of Si 2p obtained from the bare Si film. The inset figure indicates the depth profile using Ar ion etch.

Figure 2. Scanning electron microscopy plane- and cross-view images of SPA (a, b), SWA (c, d), and STF (e, f). The inset figures present highly magnified images of the plane-view of the prepared Si electrodes.

negative photoresist mask to prepare nanoscale patterned periodic Si pillar arrays (SPA) or Si well arrays (SWA), as illustrated in Figure 1. The prepared Si thin film (STF) was spin coated with a diluted positive- or negative-tone photoresist. The 4-fold symmetry array pattern was modulated by irradiating a standing wave onto the sample, which was rotated by α (α = 0° and 90°). A double exposure was performed to fabricate a square lattice array. Two beams, one directly from a 325 nm He
Cd laser and the other reflected from a Lloyd mirror, converged coherently and caused constructive and destructive interference on the

photoresist surface. The periodic pattern array was given by Λ = λ/(2 sin θ), where Λ, λ, and θ indicate the pitch, wavelength of the UV laser, and incident angle, respectively. After the development process, two-dimensional periodic patterns of photoresist dots or holes were obtained (see Figure S1 in the Supporting Information). The development process was then performed with acetone. The positively or negatively carved Si electrodes were then constructed by reactive ion etching (RIE). Scanning electron microscopy (SEM) images of the SPA (Figure 2a,b) and SWA (Figure 2c,d) revealed that the carved Si nanostructures showed clear, well-defined periodic nanopatterns of SPA or SWA, consistent with the schematics in Figure 1. The period, radius, and etch depth of the carved shapes were fixed to ca. 400, 200, and 150 nm, respectively. The developed pattern of 1.29  109 dots was generated uniformly over an area of 1.77 cm2. For comparison, an STF without a pattern was also prepared (Figure 2e,f). The structural and chemical properties of the prepared Si electrodes were investigated using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Figure 3a shows the XRD patterns for the sputtered Si on the stainless steel substrate. Except the characteristic peaks observed at 2θ = 43.8°, 44.7°, and 50.8° from the substrate itself, the sputtered Si indicated no crystalline peaks due to its amorphous nature. Also, transmission electron microscopy (TEM) image and selective area electron diffraction (SAED) pattern indicated that the phase of deposited Si in this work is amorphous (see Figure S2, Supporting Information). It is reported that many nanostructures and thin films of Si remain amorphous with cyclings.6
12 It is reported that the amorphous feature can show a higher electrochemical performance than crystalline Si.23
25 Figure 3b shows the XPS spectra of the sputtered Si, which gave two main peaks, corresponding to elemental Si (at 99.3 eV) and SiOx (at 103.3 eV). The SiOx is believed to be oxidized Si species present on the 3657

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Figure 4. The galvanostatic process of the prepared Si electrodes: the voltage profiles for the first, second, and tenth cycle at a constant current density of 0.04C rate: (a) SPA, (b) SWA, and (c) STF, respectively. (d) Corresponding calculated dQ/dV plots for tenth cycle.

surface layer, which can contribute to an irreversible capacity resulting from the decomposition of SiOx to Si and xLi2O.26 After Ar ion sputtering, the XPS spectrum of the fresh surface was obtained (inset Figure 3b). The etched Si indicated a single strong peak at 99.3 eV, without any SiOx peak. The demonstrated Si (pillar arrays, well arrays, and thin film) electrodes were inserted into a coin-type half-cell, and the Li
Si alloying/dealloying properties were investigated with discharge/ charge processes. As shown in the panels of Figure 4, a potential window from 0.01 to 1.5 V (vs Li/Li+) was studied at a constant C rate of 0.04C (ca. 168 mA g
1). The discharge capacity mainly occurred below 0.3 V, and the charge capacity appeared from 0.3 to 0.8 V. As shown in panels a
c of Figures 4, the voltage profiles of the prepared Si electrodes indicated a plateau in the first discharge and a smoother and sloped shape after the second discharge. The irreversible voltage profile at the initial cycles seems to be associated with the formation of a solid electrolyte interface (SEI) layer or native oxide reduction which is related to phase transformation.27 It was also found that the voltage profiles at the initial cycles could be affected by the surface morphology of the electrodes. In the first discharge, the potentials of the SPA, SWA, and STF rapidly dropped to the voltages of 0.10, 0.43, and 0.22 V, respectively. The potential plateau formed around 0.2 V in cases of the SPA and STF and then gradually decreased to 0.01 V. The STF showed the steeper decreases of potential from the second discharge curves than those of the carved Si patterns in the range 0.2
0.6 V. The physical constraint involved in the silicon electrodes can influence the capacity and energy dissipation due to plastic deformation.28,29 These voltage fluctuations may be related to the electrochemical resistance of Si against electrochemical reactions. Consequently, the control of active Si via the pattern type in this manner could have led to the different potential profiles and cell performances. The reversible 20th cycle showed a distinct difference between the carved Si electrodes and the STF electrodes. Whereas the

STF electrode potential rapidly dropped to about 0.4 V, the carved Si electrodes showed sluggish potential drops in the discharge process, implying that the carved Si electrodes have smaller internal resistance compared with the STF. From the calculated differential capacity (dQ/dV) in Figure 4d, the plot of dQ/dV vs V shows redox couples at the 10th cycle in terms of the positions and relative intensity of the peaks. The obtained peaks were attributed to the potential dependence of Li
Si alloys of different compositions. Cathodic peaks below 0.3 V, as well as anodic peaks over 0.25 V, were observed; these were related to the alloying/dealloying in the Si electrode. In the alloying process, two broad cathodic peaks at around 0.23 and 0.10 V appear in the case of SWA and they are associated with the formation of amorphous LixSi phase via a single-phase transition. The appearance of the two peaks might be caused from two different sites with different energies in the structures.30 In the dealloying processes, lithium is extracted from the amorphous LixSi, and the amorphous silicon is then formed. The constructed Si structures will be re-formed when the potential reaches 0.8 V, which is consistent with the changes in the Si electrodes after cycling, as will be discussed below. A decrease in the anodic-tocathodic peak-to-peak separation was observed in the carved Si electrodes, as compared with the STF. The peak separations in the SWA sample appeared to be ca. 0.15 V, whereas the SPA and STF were found to have a peak separation of ca. 0.30 V. In previous research,31,32 such peak separations occurred at low overpotentials, and the peak positions were usually reported for doped Si and metal-incorporated Si. We can therefore postulate that the carved Si electrodes had better conductivity (see Table S1, Supporting Information) along with the small potential differences, which implies that the constructed Si electrode had a small internal resistance with a lower overpotential needed for the cycling of the carved Si electrode. The SWA sample showed the largest peak magnitudes, due to the effective integration of active Si in the alloying/dealloying process associated with the 3658

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Figure 5. (a) Capacity vs cycle number of the prepared Si electrodes and (b) Coulombic efficiency vs cycle number for a half-cell cycled between 1.5 and 0.01 V. (c) The relative capacity at the different C rates (0.02C to 0.2C), shown in percentage form.

phase transition. Thereby, the carved Si is a better anode material with low discharge/charge voltage hysteresis then that of STF. Panels a and b of Figure 5 compare the cycle performance of Si electrodes in terms of their discharge/charge capacity and Coulombic efficiency, which is summarized in panels a
c of Figure 4. In Figure 5a, the carved Si electrodes appear to show substantially higher capacities, with improved capacity retention compared with that of bare STF. Small capacity fading of below 0.4% per cycle (after the fifth cycle) was observed in the carved Si electrode, whereas the STF showed 1.3% per cycle. This means that the carved Si electrodes had greater facile stress relaxation than the bare STF, since the capacity retention is related to the film stress during cycling.33 Considering that the Si nanopatterns were etched to 50% of the total Si film thickness, the change and improvement of the electrochemical properties for the carved Si nanostructures should be averaged by the remaining film layer part because the unetched Si layer also contributes to the electrochemical performance. A plot of Coulombic efficiency as a function of cycle number is provided in Figure 5b. The Coulombic efficiency is the ratio of the number of charges entering the electrode to the number of charges extracted from the electrode. For SWA, the Coulombic efficiency at the first cycle was ca. 77.5%, which was similar to those reported for onedimensional nanostructured Si,6
10,13 implying an improved reversibility in electrochemical reaction compared with that of the bulk Si. SWA showed the largest capacity and reversibility

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among the Si electrodes, indicating a more effective integration of active Si compared with SPA or STF. Within five cycles, the carved Si electrode reached the highest Coulombic efficiency of more than 99%, indicating that the carved Si was highly reversible than the STF. In addition, the improved capacity retention of the carved Si was also correlated to the enhanced Coulombic efficiency, which could contribute to the long-term cycling stability.34 Figure 5c shows results obtained from experiments for the performance of the carved Si electrode at higher C rates, varying from 0.02C (ca. 84 mA g
1) to 0.2C (ca. 840 mA g
1) rates between 1.5 and 0.01 V in the coin-type half-cells. (See also Figure S3 in the Supporting Information for the discharge capacity profiles with cycle numbers.) Comparing the discharge capacity at 0.2C rate, the carved Si electrodes preserved 80% of the capacity at 0.02C rate. In contrast, the capacity of the STF sample rapidly decreased to 53% retention by the increase in the input current density to 0.2C rate. We believe that the remarkable rate capability of the carved Si electrode resulted from the wellconstructed architecture. A well-constructed architecture can ensure enlarged active sites for electrochemical reactions with open space among neighboring nanostructures for Li ion diffusion of the electrolyte.35 This feature is particularly helpful for highpower applications when the battery is discharged or charged at high current. The Li diffusion coefficient in the carved Si electrode was measured by cyclic voltammetry,36,37 with various scan rates from 0.05 to 0.5 mV s
1. As shown in panels a
c of Figure 6 (which show current vs potential profiles with respect to various scan rates, shown from low scan rates to high scan rates), the ratedetermining steps of the electrochemical reaction may have changed from surface reactions to semi-infinite solid-state diffusion of Li ions. For semi-infinite diffusion, the peak current is proportional to the square root of the scan rate (υ1/2). We found that the peak current at different scan rates was in proportion to square root of the scan rate (Figure 6d), indicating that in this work the reaction kinetics were controlled by the semi-infinite diffusion of Li. Peak currents for both the cathodic and anodic peaks increased with increasing potential scan rate. The Li diffusion coefficient can be derived by the following peak current equation38 Ip ¼ ð2:69  105 Þn3=2 AD1=2 C0 υ1=2 where n is the number of electrons transferred (1 for Li+), Ip is the peak current (A), A is the apparent surface area of the electrode (cm2), D is the diffusion coefficient of the Li ions (cm2 s
1), C0 is bulk concentration of Li ions, and υ is the scan rate (V s
1). The surface areas of SPA and SWA were calculated to be ca. 3.48 cm2, while that of STF was ca. 1.77 cm2. For the relation of Ip and υ1/2 shown in Figure 6d, SPA and SWA had steeper slopes than STF. SWA appeared to have a diffusion coefficient of ca. 5.9  10
9 cm2 s
1, which was approximately four times higher than that for the STF sample (ca. 1.6  10
9 cm2 s
1). The diffusion coefficient for SPA (ca. 1.3  10
9 cm2 s
1) was similar to that of STF. The electrode performance of SPA showed insignificant improvement over that of STF, despite the enlarged active sites. Therefore, the improved SPA performance may have been related to the reduced diffusion length of Li ions and the decrease in resistance.39 From the nanopattern carving, the effective thickness of the silicon layer could be changed as compared with that of uncarved STF. Considering the results of Figures 4 and 6, the SPA appeared to show an analogous behavior to the film 3659

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Figure 6. Cyclic voltammetry of (a) SPA, (b) SWA, and (c) STF at various scan rates from 0.05 to 0.5 mV s
1. (d) Relationship between the peak current density and the square root of the scan rate.

structure that having a decreased film thickness,23,39,40 while the SWA seemed to reveal the different features from the others with the nanostructure effects,6
13,41 in light of electrochemical characteristics such as irreversible capacity, internal resistance, overpotential, and lithium ion diffusion coefficient. To seek a correlation between the tailored Si electrode configuration and the electrochemical results, we compared the morphology of the samples before and after the cycles. SEM images for SPA and SWA samples after 30 cycles are shown in panels a and b of Figure 7. The carved Si electrodes seemed to remain relatively stable, with a few cracks and delamination appearing in the surface region, whereas the STF showed serious pulverization from the substrate. In the STF case, repetitive aggregation, pulverization, and large volume expansions/contractions may have led to the formation of a large cracked surface. The results showed that the capacity retention of the STF was unstable and subject to prolonged degradation. Accordingly, part of the active Si became detached from the current collector, resulting in a rapid fading of the capacity. In contrast, the carved Si electrodes can play an effective role in facile stress relaxation, thereby alleviating the cracking and crumbling of the Si electrode. This capability may have been responsible for the better reversible capacity performance in the Li ion battery. The carved Si electrodes showed remarkable electrochemical properties compared with the STF. The carving and nanopatterning of the Si electrodes reduced the internal resistance by increasing the number of active sites for Li alloying/dealloying. Simultaneously, the extended surface area and generated interior nanocavities provided extra contact regions for the electrolyte, resulting in a facilitated Li ion transport. Furthermore, the carved Si electrodes exhibited improved capacity retention upon repeated cycling. The enlarged active sites in these Si electrodes may have contributed to the facile stress relaxation, with smaller volume changes during cycling. This approach with the precisely patterned electrodes of SPA and SWA using the LIL method can

allow a comparative model study through the systematically carved Si nanopatterns. It is also important to probe the electrochemical properties of nanostructured Si electrodes for the Li ion battery with respect to the morphological evolution. The promise of the carved architectures for more advanced shapes in the Si electrodes can provide more general design guidelines for the advanced structures of the Li ion battery electrodes. In summary, we have fabricated positively and negatively carved Si nanopattern electrodes using laser interference lithography and have investigated them as anode materials for rechargeable Li ion batteries. To ensure a systematic study, the fabricated Si nanopattern arrays were controlled in terms of the pattern size, etch depth, and periodicity. The electrochemical results described in this work suggest that nanopatterning may play a significant role in next-generation electrode materials. The improved electrochemical features of the carved Si electrodes could be attributed to the low internal resistance, facilitated charge transport, and facile stress relaxation resulting from the engraving of the Si into periodic nanopatterns. Consequently, the carved Si electrodes prepared using LIL exhibited a superior Li storage capacity, a high rate capability, and long cycling properties, indicating their potential as anode materials in high energydensity Li ion batteries. Experimental Section. Preparation of Si Thin Film (STF). A Si thin film with ca. 300 nm thickness was deposited on a substrate with size of 1.77 cm2 using a radio frequency magnetron sputtering system at a base pressure of less than 5  10
6 Torr and a working pressure of 10 mTorr. An inert Ar (99.999%) gas at 40 sccm was carried to the chamber at room temperature. The thickness and mass of as-deposited Si thin film was carefully measured using a surface profiler (KLA Tencor, Alpha-step IQ) and microbalance (Sartorius, M3P). The measured mass and thickness was ca. 0.175 mg and ca. 300 nm, respectively. Fabrication of Si Carved Structures (SPA and SWA). The Si thin film for pattern fabrication was first treated by coating a 3660

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line transferred samples were rotated 90° and another exposure was performed to generate a 4-fold symmetry array pattern. The development process was performed using a developer (MIF500, Clariant), for a time period of 35 s at room temperature. This process removed the undeveloped regions, and the fabricated resists were baked at 110 °C for 1 min on a hot plate. A dryetching process with a CHF3 gas mixture (100 W power at 10 mTorr) was performed for 65 s to transfer the positively or negatively developed photoresist nanopatterns on the STF. Finally, the masses of the SPA and SWA were characterized, giving 0.09 and 0.15 mg, respectively, of active material. Microstructural and Electrochemical Characterization. The morphologies of the prepared electrodes were examined by SEM (Hitachi S-4800). After cycling, delithiated electrodes were removed from the configured cell in an Ar filled glovebox, since the reacted electrodes were sensitive to air and moisture. The electrodes were washed with acetonitrile to remove the remaining electrolyte and dried at room temperature before the SEM characterization. The microstructures were characterized by XRD (Rigaku Ru-200B), TEM (JEOL S7600), and XPS (VG Multilab2000). The composition of the deposited Si was characterized by XPS with a monochromatic Al Kα X-ray source (E = 1486.6 eV). Data processing was performed using Avantage 4.54 software. The background was corrected using the Shirley method, and the binding energy of the C 1s peak from the support at 284.5 eV was taken as an internal standard. The electrochemical tests were performed, using a twoelectrode system fabricated with the prepared materials for the working electrode and metallic lithium for the counter electrode in an Ar-circulating glovebox. The mirror-like polished stainless steel disk was used as the substrate for all the electrochemical tests and characterizations in this work except the samples for the cross-section SEM images that employed crystalline Si substrates. A 1 M LiPF6 solution in a 1:1 volume mixture of ethylene carbonate and diethyl carbonate was used as the electrolyte. The galvanostatic discharge/charge process at various C rates from 0.02C to 0.2C was conducted with a potential window of 1.5
0.01 V (vs Li/Li+) using a battery cycler (WonA tech, WBCS3000). Cyclic voltammetry was performed over a potential range of 1.50
0.01 V at various scan rates of 0.05, 0.1, 0.2, and 0.5 mV s
1, using a Solartron 1470E multistat system.

’ ASSOCIATED CONTENT Figure 7. The morphological changes of Si electrodes after the 30th cycle in the galvanostatic process at 0.04C rate. SEM images of (a) SPA, (b) SWA, and (c) STF after the finished discharge/charge process.

20 nm thick adhesion promoter layer of hexamethyldisilazane (HMDS, Fluka), followed by annealing at 90 °C for 2 min (these steps are not included in Figure 1). An He
Cd laser (λUV = 325 nm, intensity = 0.75 mW cm
2) was used for the LIL process. The positive (AZ6612, Clariant) or negative (AZ6600 series, Clariant) tone photoresists were mixed with a thinner (AZ1512, Clariant), using a 1:2 volume ratio. The photoresists were spin-coated on the HMDS layer-coated Si thin film. In this method, two coherent He
Cd laser beams were used to produce a periodic interference pattern on a photoresist-coated Si thin film, as shown in Figure 1. In the first exposure, some parts of the photoresist coated Si thin film were exposed, and nanoscale parallel lines were formed. After the first exposure, the parallel

bS

Supporting Information. Figures showing schematic diagram of laser interference setup and developed polymer template, TEM image and SAED pattern for the Si thin film, and discharge capacity vs cycle number for the prepared Si electrodes and table of sheet resistance and the initial voltage drop of the prepared Si electrodes. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (W. B. Kim); [email protected] (G. Y. Jung).

’ ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government 3661

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Nano Letters (MEST) (No. 20110016600, Midcareer Researcher Program) and by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. R15-2008-006-03002-0). We also appreciate the financial support by the GIST Specialized Research International Cooperation (GSR_IC/GIST) Project, the Program for Integrated Molecular Systems (PIMS/GIST), and the Core Technology Development Program from the Research Institute of Solar and Sustainable Energies (RISE/GIST).

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