Controlled Prelithiation of Silicon Monoxide for High Performance

Dec 22, 2015 - Although some prelithiation techniques, such as solution processes, engage stabilized Li metal powder (SLMP)(29-31) and prelithiated Si...
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Controlled Prelithiation of Silicon Monoxide for High Performance Lithium-Ion Rechargeable Full Cells Hye Jin Kim,† Sunghun Choi,† Seung Jong Lee,† Myung Won Seo,‡ Jae Goo Lee,‡ Erhan Deniz,*,⊥ Yong Ju Lee,¶ Eun Kyung Kim,¶ and Jang Wook Choi*,† †

Graduate School of Energy, Environment Water, and Sustainability (EEWS) and Center for Nature-Inspired Technology (CNiT) in KAIST Institute NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea ‡ Climate Change Research Division, Korea Institute of Energy Research (KIER), 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea ⊥ Department of Chemistry and Earth Sciences, College of Arts and Sciences, Qatar University, P.O. Box 2713, Doha, Qatar ¶ Battery Research and Development, LG Chem, LTd., Research Park 104-1, Moonji-dong, Yuseong-gu, Daejeon 305-380, Republic of Korea S Supporting Information *

ABSTRACT: Despite the recent considerable progress, the reversibility and cycle life of silicon anodes in lithium-ion batteries are yet to be improved further to meet the commercial standards. The current major industry, instead, adopts silicon monoxide (SiOx, x ≈ 1), as this phase can accommodate the volume change of embedded Si nanodomains via the silicon oxide matrix. However, the poor Coulombic efficiencies (CEs) in the early period of cycling limit the content of SiOx, usually below 10 wt % in a composite electrode with graphite. Here, we introduce a scalable but delicate prelithiation scheme based on electrical shorting with lithium metal foil. The accurate shorting time and voltage monitoring allow a fine-tuning on the degree of prelithiation without lithium plating, to a level that the CEs in the first three cycles reach 94.9%, 95.7%, and 97.2%. The excellent reversibility enables robust full-cell operations in pairing with an emerging nickel-rich layered cathode, Li[Ni0.8Co0.15Al0.05]O2, even at a commercial level of initial areal capacity of 2.4 mAh cm−2, leading to a full cell energy density 1.5-times as high as that of graphite-LiCoO2 counterpart in terms of the active material weight. KEYWORDS: Coulombic efficiency, cycle life, full cell, prelithiation, silicon monoxide

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demonstrated very stable cycling performance.22−26 Nonetheless, most of these phases suffer from inferior performance in a crucial parameter, namely initial Coulombic efficiency (ICE). This shortcoming originates primarily from Li ion trapping in the matrix and SEI layer formation during the first lithiation.27 Hence, in an overall performance viewpoint, while facilitating the long-term cycling performance, the background matrix, in turn, sacrifices the ICE. The compromised ICE would impose a great hurdle in constructing full-cells in practical applications because poor ICE necessitates an excess amount of cathode active material solely for the first cycle, leading to a lessened total energy density. For reference, current commercial LIBs involving graphite anodes usually have ICEs higher than 85% at low C-rates (∼0.1C).28

igh capacity silicon (Si) anodes are expected to play a key role in increasing the energy density of current lithiumion batteries (LIBs) and thereby realizing timely advent of future LIB applications, such as advanced portable IT devices and electric vehicles (EVs), that require greater electricity consumption.1−5 In the case of EV applications, in particular, higher energy density represents increased driving mileage upon each recharge and is therefore critical for their wide propagation in the global markets. In spite of this attractive feature, its huge volume change over repeated charge− discharge cycles impairs the cycle life through pulverization of active particles, film delamination, and unstable solid−electrolyte interphase (SEI) formation, limiting its role as a main active material in practical applications.6−21 As an alternative approach to avoid the known drawbacks, silicon monoxide (SiOx, x ≈ 1) phase has been recently adopted because its SiOy (y near 2) background matrix can buffer the volume expansion of inner Si nanodomains. On the basis of this structural advantage, some SiOx electrodes © XXXX American Chemical Society

Received: September 18, 2015 Revised: December 18, 2015

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Figure 1. (a) Graphical illustration of prelithiation process of c-SiOx electrode and (b) its scalable roll-to-roll process scheme.

Figure 2. Controlling the degree of prelithiation. (a) Lithiation voltage profiles when different resistances are incorporated in the external circuit. (b) Voltage profile during the external shorting with 100 ohm included in the circuit and OCVs after 10 h of relaxation at different prelithiation points. (c) The first cycle voltage profiles of c-SiOx with different prelithiation durations. (d) Comparison of the specific capacity and ICE after different prelithiation times.

alloying reaction in the first charge. On the basis of this consideration, a sweet spot would be a point where the final potential is below the one that forms the SEI layer and thus circumvents electrolyte decomposition but above the one that progresses the main alloying reaction to a large extent. To this end, we prelithiate the pristine electrode via an electrical short with Li metal foil in the presence of an optimized circuit

In an attempt to catch these two challenging rabbits (cycle life and ICE), the current study has developed delicately controlled prelithiation for SiOx anodes. It should be first noted that a proper degree of lithiation is very critical for stable fullcell operations. While insufficient lithiation leaves Li trapping sites and does not improve the ICE enough, overlithiation removes the possibility of accepting Li ions during the actual B

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Figure 3. Characterization of prelithiated c-SiOx for 30 min. SEM images of (a) pristine c-SiOx and (b) 30 min prelithiated c-SiOx electrodes. XPS spectra of both electrodes obtained at the surface in (c) Si 2p, (d) C 1s, (e) O 1s, and (f) Li 1s branches.

resistance while simultaneously monitoring the voltage between both electrodes. By utilizing a fine-tuning capability in the degree of lithiation, a prelithiation condition that simultaneously maximizes the ICE and cycle life was found and was also engaged for robust full-cell operations by pairing with a commercially available high capacity cathode. Although some prelithiation techniques, such as solution processes, engage stabilized Li metal powder (SLMP)29−31 and prelithiated Si nanoparticles,32,33 to the best of our survey, none of the previous works has covered as delicate control on the degree of lithiation as that in the current study. This investigation also deals with SiOx and so must be directly relevant to the current commercial technology. The carbon-coated SiOx (denoted as c-SiOx) electrode consists of c-SiOx powder, super P (SP), poly(acrylic acid) (PAA) binder, and detailed preparation procedure are described in the Experimental Section. As soon as an external short circuit was constructed between c-SiOx electrode and a piece of lithium metal foil in the presence of electrolyte and separator in between, spontaneous prelithiation was initiated by the potential difference between both electrodes (Figure 1a). A resistor was also included in the external short circuit for the purpose of controlling the speed of prelithiation. Importantly, a quantitative amount (0.2 mL) of the commercial electrolyte (1

M LiPF6 in EC/DEC = 1:1 = v:v with 5 wt % FEC) was introduced in the presence of a separator to mimic the actual battery operation; the lithiation of the c-SiOx electrode is mediated by Li ion flux,34 not direct contact with Li metal. Thus, Li metal plating can be avoided, and the same SEI layer to that generated from electrochemical cycling is formed so the existing electrolyte technology involving additives can still be utilized. In our experiment, to minimize the electrolyte evaporation, the overall prelithiation was processed in a closed coin cell assembly (Figure 1a and Supporting Information Figure S1). The end point of the prelithiation is determined by the cell voltage that is monitored throughout the prelithiation, as details are described in the next paragraph. As portrayed in Figure 1, panel b, the present prelithiation scheme must be scalable in a compatible fashion with the conventional roll-toroll battery manufacturing process. To determine the most appropriate magnitude of the resistor incorporated, various resistances were examined (Figure 2a). If the resistance is higher than 100 ohm, then the prelithiation reaction becomes too slow for practical application. In such slow prelithiation, continuous electrolyte decomposition could also be an issue. On the contrary, if the resistance is lower than 100 ohm, precise voltage monitoring becomes difficult due to large overpotentials. As a result, in our investigation, 100 ohm C

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Figure 4. Cyclic voltammograms of (a) the pristine c-SiOx and (b) 30 min prelithiated c-SiOx cycled between 3.0 and 0.01 V at 0.5 mV/s for 10 consecutive cycles. (c) Galvanostatic discharge−charge profiles of the pristine and 30 min prelithiated c-SiOx in the first five cycles at 100 mA g−1. (d) Cycling performance of both half-cells at 0.07C (1C = 1500 mA g−1) for five precycles and 1C for the remaining cycles. The areal loading of cSiOx is 2.2 mg cm−2 for all the data in this figure.

was selected for a fine control of prelithiation. As the external shorting time between c-SiOx and Li metal was increased, the voltage continuously went down (Figure 2b), reflecting the progressive reaction with c-SiOx to form Li−Si alloy. Since the voltage changes dynamically along a single plateau, open circuit voltage (OCV) after 10 h relaxation was monitored at every 10 min. At 40 min of the shorting, the OCV reached 0.31 V, which is slightly lower than the voltage (∼0.34 V) corresponding to the Li−Si alloy formation (Li0→1.71Si).35 Hence, our prelithiation was conducted in the time range up to 40 min. In this given time constraint, galvanostatic charging/discharging scan (0.01−1.5 V versus Li/Li+, 100 mA g−1) was used to identify the optimal shorting time for the prelithiation (Figure 2c). Compared to the 0 min case, the 10 and 20 min prelithiated samples show peculiarly higher lithiation capacities in spite of the prelithiation. This phenomenon is attributed to the fact that the prelithiation accelerates the activation of Si domains that would otherwise take place during the beginning of cycles.36 However, after 20 min, the lithiation capacity decreases due to the lower OCV as well as the more progressed SEI formation that prevents further electrolyte decomposition. In contrast, the delithiation capacity remains at nearly constant values around 1350 mAh g−1 regardless of the shorting time except for 0 min. The 0 min sample exhibits the smallest delithiation capacity (1157.4 mAh g−1) because the prelithiated samples have a benefit of the aforementioned activation of Si domains during the prelithiation. As a result, the ICE increases from 73.6% at 0 min to 107.9% at 40 min (Figure 2d), implying that while insufficient shorting time is not as effective in improving ICE due to the insufficient irreversible phase formation, excessive shorting time eliminates the chance of inserting Li ions into the SiOx host during the actual first charge and increases the

possibility of Li dendrite growth owing to overlithiation. At the shorting time of 30 min, the OCV becomes 0.38 V, and the lithiation and delithiation capacities are 1443.6 mAh g−1 and 1369.3 mAh g−1, respectively, leading to an optimal ICE of 94.9%. On the basis of these results, in-depth characterization and electrochemical testing hereafter were focused on the 30 min prelithiated sample. The morphology and elemental composition of c-SiOx were characterized using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). According to the SEM analyses, the pristine c-SiOx particles have smooth surfaces, and their average particle size is around 5 μm (Figure 3a). After 30 min prelithiation, however, the surfaces of c-SiOx become rougher owing to the coverage of SEI layers (Figure 3b). Moreover, after the prelithiation, the electrode does not pulverize because the rate and depth of the lithiation can be accurately controlled by adjusting the circuit resistance and contact duration, respectively. Figure 3, panels c−f display the XPS spectra in the Si 2p, C 1s, O 1s, and Li 1s branches, respectively. In the Si 2p peaks, the pristine c-SiOx exhibits peaks at 99.8 and 103.8 eV corresponding to elemental Si and surface oxide, respectively.37 After the 30 min prelithiation, only one peak at 102.0 eV was detected, which is attributed to formation of lithium silicates (LixSiOy).37,38 Lithium silicates are well-known irreversible products in the first cycle of SiOx, and this observation verifies that the present prelithiation largely mimicks the first cycle in electrochemical cycling and is an effective approach to boost the ICE. In the C 1s spectrum of the pristine c-SiOx, the two neighboring peaks at 284.0 and 285.0 eV are assigned to the surface carbon coating layer and hydrocarbon surface D

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paid attention to CEs in the beginning period of cycling, as SiOx intrinsically suffers from poor initial CEs. In the first five precycles at 0.07C, the pristine one shows CEs of 73.6%, 94.7%, 96.6%, 97.5%, and 98.0%, whereas the prelithiated one exhibits 94.9%, 95.7%, 97.2%, 97.9%, and 98.3%. Notably, the prelithiation not only improves the ICE, but also the CEs in the following cycles. According to time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis (Supporting Information Figure S3), the SEI layer derived from prelithiation is thicker and has more inorganic species such as LiF and Li2O. These distinct SEI properties are attributed to varying current density during the lithiation of the prelithiated electrode in comparison with the normal electrode operated at a constant current density. Despite the different CEs in the early period of cycling, the CEs of both samples saturated to similar values at the end of cycling, reflecting that SEI properties become similar.45 The superior CEs of the prelithiated sample verify that the well-controlled prelithiation overcomes the very shortcoming of SiOx in a commercial level of the areal capacity. To examine the validity of such impact of the prelithiated cSiOx, full-cells upon integration with Li[Ni0.8Co0.15Al0.05]O2 (NCA) (half-cell data in Supporting Information Figure S4) were tested in the voltage range of 2.5−4.2 V at a current density of 10 mA g−1 (Figure 5). Notably, NCA is one of the emerging high capacity cathode materials46,47 and would raise the energy density markedly when paired with stable Si anodes. In the first cycle (Figure 5a), the 30 min prelithiated one shows a higher first discharge capacity of 165.09 mAh gNCA−1 compared to that (106.33 mAh gNCA−1) of the pristine sample because the prelithiation compensates the irreversible capacity loss by the preformation of SEI layer and irreversible amorphous matrix elements (Li2O, LixSiOy, etc.). As a result, the ICE drastically increases from 58.85% to 85.34%. As depicted in Figure 5, panel b, at 1C (1.53 mA cm−2 and 2.18 mA cm−2, respectively), the higher capacity of the prelithiated full cell was preserved over entire 100 cycles. After 100 cycles, the capacity of the prelithiated c-SiOx/NCA full cell drops from 2.36 mAh cm−2 to 1.44 mAh cm−2, while the pristine counterpart starts at 1.52 mAh cm−2 and ends at 1.17 mAh cm−2. The average CE of the prelithiated one during the cycle number of 1−100 (excluding five precycles) is 99.0%, which is higher than that of the pristine one (98.8%) for the same cycling period. These CE values are expected to improve further once commercial electrode coating and pressing processes are employed under an accurate n/p ratio control. It would be instructive to see how much the energy density (ED) can be enhanced once this prelithiation technique allows one to use c-SiOx exclusively as an anode material. For fair comparison across different battery systems, only active material was taken into account in the weight consideration; other cell components, including separator, current collector, pouch/case, etc., were not considered. From ED = {(Ccathode × Canode)/(Ccathode + Canode)}Vnominal, the conventional graphite/ LiCoO2 cell (Ccathode = 120 mAh g−1, Canode = 370 mAh g−1 and 3.7 V) yields ED = 335.3 Wh kg−1. On the basis of the same metric, the capacity (Ccathode = 165.1 mAh g−1 and Canode = 977.9 mAh g−1) and voltage (3.6 V) of our prelithiated c-SiOx/ NCA cell yield ED = 508.5 Wh kg−1, which is 1.5-times that of the graphite/LiCoO2 system. This gravimetric ED can be translated to a volumetric ED of 1082.6 Wh L−1 by consideration of the densities of NCA and c-SiOx in the electrodes: 2.8 g cm−3 and 0.88 g cm−3, respectively. As shown in Figure 5, panel c, other emerging battery systems are

contamination, respectively. The two peaks at 286.5 and 289.0 eV corresponding to C−O and O−CO bonds originate from the PAA binder. After the prelithiation, the peak at 284.0 eV indicative of the surface carbon disappears. Instead, a new peak at 290.0 eV newly appears, which is ascribed to the formation of SEI layer components such as lithium carbonate (Li2CO3) or lithium alkyl carbonates.39,40 Consistent with the Si 2p spectra, the O 1s spectra support the formation of LixSiOy after the prelithiation (Figure 3e); the peak at 533.0 eV, indicative of silicon oxide (SiO2) in the pristine form, is replaced by the peak at 531.4 eV, a signature of LixSiOy. In the case of Li 1s spectra (Figure 3f), although the peak at 55.8 eV after the prelithiation is not clearly interpreted, it is anticipated that the peak is merged from subpeaks at 56.0 and 55.5 eV, assigned to lithium fluoride (LiF) and Li2CO3, respectively.41 Importantly, all of the XPS results match well with those of Si and SiOx electrodes in their first lithiation and therefore support the scenario that our prelithiation replicates well the actual battery operation even via the electrical shorting process. The inclusion of the electrolyte in the prelithiation step is critical in this sense. To evaluate the electrochemical performance of the prelithiated c-SiOx, cyclic voltammetry (CV) and galvanostatic lithiation/delithiation tests were first carried out under half-cell configuration. The CV tests were conducted for both the pristine and 30 min prelithiated c-SiOx cells for 10 consecutive cycles between 3.0 and 0.01 V at a scan rate of 0.5 mV s−1 (Figure 4a,b). The pristine c-SiOx cell exhibited a broad peak at ∼1.6 V in the first cathodic scan due to the formation of SEI layer. The cathodic peak below 0.2 V and the anodic peaks at 0.4 and 0.56 V, respectively, correspond to Li alloying and dealloying process with Si nanodomains that are generated during the first lithiation.42 Interestingly, these alloying and dealloying peaks grow with cycling, implying that an increasing portion of the Si nanodomains are accessible to Li ions. On the contrary, the lithiation profile of the 30 min prelithiated c-SiOx lacks any signal indicative of SEI formation because of the low OCV (0.40 V) (Figure 4b), pointing to the fact that the SEI layer formed during the prelithiation prevents further electrolyte decomposition. Furthermore, the alloying and dealloying peaks of this prelithiated sample show clearly greater intensities compared to those of the pristine one, reconfirming that the prelithiation contributes to activation of Si nanodomains to some extent. The more pronounced activation of Si nanodomains is indeed reflected in the galvanostatic voltage profiles that exhibit larger capacities for the prelithiated c-SiOx electrode (Figure 4c). Figure 4, panel d displays the capacity retentions and CEs of the pristine and 30 min prelithiated c-SiOx during cycling. Cycling performances of prelithiated c-SiOx with different prelithiation times are shown in Supporting Information Figure S2. Notably, to examine the commercial viability, the loading of c-SiOx was chosen in a way that the initial areal capacity was 2.56 mAh cm−2 for all of the measurements shown in this paper, which belongs to the range for most mobile IT applications. After 100 cycles consisting of five precycles at 0.07C (1C = 1500 mA g−1) followed by faster cycles at 1C, both the pristine and prelithiated electrodes exhibited similar capacities of 904.6 and 905.5 mAh g−1, respectively, implying that the prelithiation does not influence the inherent cycling performance of c-SiOx. The capacity retentions (76.4% and 74.3%, respectively) after 100 cycles are relatively lower than those reported in the literature43,44 because of the significantly higher areal capacities in the current electrodes. We have also E

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chemical cycling of typical Si anodes. This sophisticated technique allows the ICE to reach as high as 94.9% without sacrificing the structural stability of SiOx at all throughout cycling, thus enabling us to catch the two challenging rabbits (cycle life and ICE) in Si anode operations simultaneously. The robust full cell performance in pairing with a high Ni layered cathode was observed even with a commercial level of the initial areal capacity (2.4 mAh cm−2). The given procedure is compatible with the existing roll-to-roll manufacturing line and so must be immediately applicable to the current state of the art LIB anodes containing certain contents of SiOx. Experimental Section. Materials and Electrode Preparation. Slurry was prepared using c-SiOx (KSC1064, Shin-Etsu Chemical Co., Ltd.), PAA (Aldrich, Mv ≈ 3000000), and Super P (TIMCAL) in a mass ratio of 8:1:1 in N-methyl-2pyrrolidone (NMP). The slurry was then cast onto a copper foil and dried under vacuum at 70 °C for 12 h. For c-SiOx prelithiation, 2032 coin type assembly was used. The c-SiOx electrode was first placed on the bottom of a coin cell assembly, and 0.2 mL of electrolyte (1.0 M lithium hexafluorophosphate (LiPF6) in 1:1 w/w ethyl carbonate (EC)/diethyl carbonate (DEC), 5 wt % fluoroethylene carbonate (FEC)) was dropped on the c-SiOx electrode. After separator (Celgard 2400) and Li metal foil were sequentially located on top of the c-SiOx electrode, a spacer and a wave were placed to fill the void space of the coin cell. The assembly was closed with a coin cell cap and then a resistor was connected to the ends of both electrodes to construct a short circuit. During the external shorting, the voltage was monitored using a multimeter (HIOKI 3244) for a predetermined duration in an Ar-filled glovebox. Cathode electrodes were fabricated by preparing a slurry containing NCA (Li[Ni0.8Co0.15Al0.05]O2, Ecopro), polyvinylidene fluoride binder (PVDF, Aldrich, MW ∼ 534,000), and Super P (TIMCAL) in a mass ratio of 94:3:3 in NMP. The slurry was then cast onto aluminum foil and dried under vacuum. The NCA and c-SiOx full-cell was prepared in a way that the anode has 10% excess capacity. The mass loading of active material was 2.2 mg cm−2 for the pristine and prelithiated c-SiOx anodes and 14 mg cm−2 for the NCA cathode. Materials Characterization. The morphologies of the pristine and prelithiated c-SiOx were examined by fieldemission scanning electron microscopy (HITACHI, S-4800). Surface components were investigated using XPS (Thermo VG Scientific, K-alpha) and TOF-SIMS (ION-TOF GmbH, TOFSIMS5). Electrochemical Measurements. Electrodes were punched into circular discs for fabrication of 2032 coin type cells. The coin cells were assembled in an Ar-filled glovebox using Celgard 2400 separator. CV measurements were carried out using an electrochemical cycler (VMP3, BioLogic). Galvanostatic cycling was performed using a battery tester (PEBC05−0.01, PNE solution). The half-cell tests of c-SiOx were conducted in the potential range of 0.01−1.5 V versus Li/Li+ at 0.07C (100 mA g−1) under CC mode for both charge and discharge in the first five cycles and then continued at 1C thereafter. The full-cells were cycled in the potential range of 2.5−4.2 V at 0.05 C (10 mA g−1) under CC/CV mode for charge and CC mode for discharge in the first five cycles, and the C-rate was then raised to 1C thereafter. In the CC/CV mode, after the top cutoff potential was reached, the voltage was on hold until the current density became 0.01C (2 mA g−1).

Figure 5. Electrochemical characterization of c-SiO x / Li[Ni0.8Co0.15Al0.05]O2 full-cells made of the pristine and 30 min prelithiated c-SiOx. (a) Voltage profiles of the first cycles. (b) Cycling performance with regard to the areal capacity. (c) Comparison of the gravimetric energy densities for various full-cells. LMO, LCO, LFP, NMC, and NCA indicate LiMn 2 O 4 , LiCoO 2 , LiFePO 4 , Li[Co1/3Ni1/3Mn1/3]O2, and Li[Ni0.8Co0.15Al0.05]O2, respectively.

compared together.48 Remarkably, the experimental ED of the current prelithiated c-SiOx/NCA cell is higher than the theoretical values of other available graphite-based LIBs and is also 155% as high as that of the pristine c-SiOx/NCA cell. This result reveals that prelithiation solely can have a considerable effect on the EDs of the LIBs with any established cathodes. Details on the calculation are given in the Supporting Information. In conclusion, we have developed a prelithiation technique of c-SiOx where the degree of prelithiation can be delicately controlled via voltage monitoring. Various analyses verify that the generated SEI layer during the prelithiation has very similar characteristics to those of the SEI layers created by electroF

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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/acs.nanolett.5b03776. Details of energy density calculation, prelithiation setting, additional characterization, and electrochemical data of prelithiated c-SiOx and NCA electrodes (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.W.C. acknowledges the financial support by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (NRF-2012-R1A2A1A01011970 and NRF-2014R1A4A1003712). This work was also made possible by NPRP Grant No. NPRP 7-301-2-126 from the Qatar National Research Fund (a member of Qatar Foundation). This work was also supported partly by the framework of Research and Development Program of the Korea Institute of Energy Research (KIER) (B5-2442-05).

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DOI: 10.1021/acs.nanolett.5b03776 Nano Lett. XXXX, XXX, XXX−XXX