Fabrication and Electrochemical Performance of Interconnected

Apr 9, 2013 - The interconnected nanowire structures assisted in retaining a high Coulombic efficiency upon cycling by limiting the loss of active mat...
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Fabrication and Electrochemical Performance of Interconnected Silicon Nanowires Synthesized from AlCu Catalyst Zhongsheng Wen, Johanna Stark, Rajarshi Saha, Jack Parker, and Paul A. Kohl* School of Chemical and Biomolecular Engineering, 311 Ferst Drive NW, Georgia Institute of Technology, Atlanta, Georgia 30332-0100, United States ABSTRACT: Highly interconnected silicon nanowires (SiNW) were prepared by a vapor−liquid−solid process using a multilayered AlCu film as the catalyst. The resulting SiNWs had a branched and interconnected structure where the AlCu catalyst was incorporated into the SiNWs. The electrochemical lithiation of the SiNWs was investigated through both deep and shallow cycling. The first cycle Coulombic efficiency for lithiation/delithiation was 93.6% which increased to 99.96% upon cycling. The interconnected nanowire structures assisted in retaining a high Coulombic efficiency upon cycling by limiting the loss of active material during nanowire fracture and providing shorter/multiple electrical paths to the substrate. The performance for repeated shallow discharges, as encountered in a microbattery configuration for wireless sensors, was evaluated.

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INTRODUCTION High power and energy density are important attributes for portable power sources. The power source for wireless devices, such as sensors, requires a small footprint and the ability to deliver high current pulses for short periods of time followed by periodic recharging from a wired source, or trickle charged from an energy harvester, such as a photovoltaic cell. Thus, it is highly desirable to have a lithium ion anode which has high surface area so that high currents can be obtained, and can undergo a very large number of shallow discharge cycles as the sensor performs its periodic function. Silicon is a promising anode material for compact lithium ion batteries due to its high specific capacity and low lithium insertion voltage.1−3 However, silicon undergoes a large volume change on deep cycling when lithium ions are inserted and extracting from the silicon lattice. The volumetric changes can result in silicon fracture and isolation of the individual silicon structures.4−10 The volumetric changes can also disrupt the solid electrolyte interphase (SEI) layer resulting in consumption of the electrolyte. Silicon anodes have been modified in several ways including (i) formation of a silicon composite, (ii) silicide formation, and (iii) creation of nanostructured silicon, including silicon nanoparticles, nanofilms, nanotubes, and nanowires. Silicon composites can be made with a high conductivity matrix in order to increase the reversibility of the lithiation reaction and mitigate the effect of stress created by volume expansion.10−15 However, the improvement in the cycling efficiency comes at the expense of reduced capacity due to the decreased fraction of silicon. The structural change due to lithium ion insertion can be controlled by forming a silicidebased anode which also lowers the volumetric expansion; however, the irreversible decomposition of silicides during lithiation results in poor cycling life. © 2013 American Chemical Society

Silicon nanofilms have shown promising results compared to micrometer size silicon composite particles.1,4−6,16 Silicon nanofilms composed of two-dimensional nanostructures are better able to accommodate large volume changes and efficient charge transport compared to bulk silicon. Films composed of silicon nanostructures also exhibit better electrochemical performance, especially in terms of cyclability and reversible capacity retention.1,4−7,10 Research has also shown anode improvements due to the size reduction from the micrometer to nanometer scale and from the change from 3D structures to 1D structures due to the high specific surface area. The improvement in mechanical properties has been observed with respect to volume expansion, and anisotropic microstructure of the SiNW.1,4−6 SiNW can be grown by the vapor−liquid−solid (VLS) mechanism using a metal catalyst. Au is widely used as the catalyst for SiNW growth due to its high silicon solubility, low eutectic temperature with Si, and chemical stability. Au-free catalysts for SiNW growth are of interest due the cost of materials; however, they have proven more challenging to find. Numerous metals including Ni, Cu, Al, Ag, and Ti were investigated as the catalyst for SiNW growth.17−23 Al is a promising candidate because Si−Al has a similar phase diagram as that of Si−Au, although the eutectic temperature is slightly higher. Previous reports claimed that SiNWs could be fabricated at relatively lower temperature with an Al catalyst via a vapor−solid−solid mechanism rather than the VLS mechanism.24 However, ultrahigh vacuum is required when using an Al catalyst for SiNW growth due to the reactivity of Received: September 28, 2012 Revised: April 4, 2013 Published: April 9, 2013 8604

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steel disks were mounted in 2025 coin cells and used as the working electrode with a Li foil reference and counter electrode for electrochemical tests. The electrolyte was 1 M LiPF6 in an equal volume mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The SiNW electrodes were lithiated and delithiated at constant current where the voltage was maintained at 0.02−2.0 V versus Li reference. The cyclic voltammograms for the SiNW half cells were obtained at a scan rate of 0.05 mV/s. Full cell lithium ion batteries were assembled using the SiNW as the negative electrode (anode) and LiCoO2 as the positive electrode (cathode). The full cells underwent charge/discharge cycles within the voltage range of 2.8 to 3.9 V.

aluminum with oxygen. The formation of aluminum oxide could prevent silane decomposition at the Al catalyst preventing the nucleation of SiNWs. Cu is an interesting catalyst for SiNW growth; however, copper silicide can be formed initially according to the phase diagram. The formation of copper silicide was also shown to be necessary for the formation of the primary seed for SiNW growth.18 The deposition temperature should be above 800 °C for copper silicide to be formed for the VLS process. In this paper, we report the use of an Al−Cu alloy as the VLS catalyst. The Al−Cu eutectic is formed at 548 °C which is lower than the melting point of Al or Cu. In addition, Si, Cu, and Al form a ternary Si−Al−Cu eutectic at 525 °C, as shown in Figure 1. In this study, we show an Au-free ternary catalyst

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RESULTS AND DISCUSSION Growth of the SiNWs. In this study, the SiNWs were deposited by thermally pyrolyzing SiH4 inside a tube furnace. The total reactor pressure was 15 Torr. The flow rate of silane was 5 sccm, and the flow rate of hydrogen was 500 sccm. The catalyst layer was deposited onto the stainless steel by alternately evaporating Cu and Al, as shown in Figure 2a.

Figure 1. Phase diagram for Al−Cu−Si.25

for SiNW VLS growth at a relatively lower temperature. The SiNWs grown from the Al−Cu catalyst are different from those grown from a Au catalyst. The Au catalyst produces relatively isolated nanowires whereas the SiNWs grown from the Al−Cu catalyst are highly interconnected and branched. The electrochemical performance of the Al−Cu SiNW has been characterized as the anode in a lithium ion battery.

Figure 2. (a) Schematic of Al−Cu multilayer catalyst on substrate. (b) SEM image of the stainless steel substrate with multilayered Al−Cu film (2 μm marker bar in lower right). (c) SEM image of SiNW deposited on the stainless steel substrate (20 μm marker bar in lower right).

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EXPERIMENTAL SECTION Stainless steel substrates were sequentially cleaned in acetone and isopropyl alcohol for 10 min in an ultrasonicate cleaner, followed by a 20% HCl solution clean to remove the surface oxide. The substrate was then rinsed in flowing distilled water. The multilayered Cu−Al film was evaporated onto the cleaned stainless steel in a layer-by-layer sequence using an electronbeam evaporator at 1.8 × 10−6 Torr. After the Al−Cu catalyst layer was deposited, the stainless steel substrates were cut into 15 mm diameter circular disks with a diameter diamond saw. Each disc was weighed before and after growth to determine the weight of the SiNWs on the substrate. Unless stated otherwise, the SiNWs were grown at 570 °C and 15 Torr pressure. The gas composition was 1% SiH4 and the balance H2. The average weight of the SiNWs on the stainless steel disk was about 1.02 mg. Scanning electron microscope (SEM) and transmission electron microscope (TEM) images were obtained to characterize the morphology and structure of the SiNW. The stainless

The thickness of the individual Al and Cu layers was 1 and 0.3 nm, respectively. The total film thickness of the Al/Cu film was 6.5 nm. The morphology of the Al/Cu multilayer structure on stainless steel is shown in Figure 2b. The top layer was copper in an attempt to limit the oxidation of the aluminum by residual oxygen in the furnace. Thus, ultrahigh vacuum conditions may not be necessary in this multilayered metal structure. A uniform distribution of SiNWs was formed after 50 min of deposition, as shown in Figure 2c. This demonstrates that the multilayered Cu/Al film acted as an effective catalyst for SiNW growth. It is worth noting that most of the previous VLS SiNW growth studies with an Au-free catalyst were carried out at a relatively high temperature (>600 °C), high pressure (>100 Torr), and high silane partial pressure (>5%), compared to the conditions here.16−19 The more extreme conditions can be attributed to the difficulty of nucleating the silicon with a catalyst which has low silicon solubility. The experiments performed here show 8605

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Figure 3. Morphology of the silicon deposition layer with different catalyst: (a) Cu catalyst (5 μm marker bar in lower right); (b) Al catalyst (50 μm marker bar in lower right).

Figure 4. SEM image of SiNW from different temperature: (a) 570 °C; (b) 600 °C.

synthesis of highly uniform SiNW with the Al/Cu catalyst, as shown in the images to follow. The low eutectic temperature of the AlCu alloy likely provides the liquid-like conditions for the catalyst where a greater concentration of silicon atoms can be achieved in the catalyst compared to Cu alone. The greater concentration of silicon atoms in the catalyst facilitates the nucleation of SiNW in the VLS process. The SiNWs had an unexpected microstructure when the multilayered Al/Cu catalyst was used. The SiNW structures produced from the Al/Cu catalyst was examined by TEM and SEM imaging. A highly interconnected SiNW network was formed rather than the forest of isolated SiNWs observed with the other catalysts. The diameter of the SiNWs prepared at 570 °C was mostly between 100 and 130 nm, as shown in Figure 4a. When the growth temperature was increased to 600 °C, the diameter of the SiNW increased to 200 to 220 nm, as shown in Figure 4b. We note that the only two previous references have mentioned interconnected SiNW, and those references have reported improved electrochemical performance.26,27 Nguyen et al. have claimed that the formation of the interconnected SiNWs in their study was due to a thermal gradient which resulted in isotropic Si growth with kinking and entanglement.26 In contrast to the Au or AuSi eutectic used in the above-mentioned references, the interconnected SiNWs grown

that SiNW growth with a multilayered Al/Cu catalyst occurs at a relatively low temperature, pressure, and silane partial pressure (only 1% required here). To examine the interaction between Al and Cu in the multilayered structure for the SiNW nucleation, stainless steel substrates with only Cu or Al as the catalyst were used for SiNW growth under the same conditions as described above for the Al/Cu catalyst. The morphology after 50 min growth time is shown in Figure 3. Figure 3a shows the silicon layer produced with the Cu catalyst, and Figure 3b shows the silicon layer produced with the Al catalyst. The deposited Si film was not homogeneous and was composed of a mixture of silicon particles, silicon nanowires, and kinked nonuniform microwires. The results are similar to the results from other studies.18 Insufficient silicon atom diffusion occurred in these experiments which limits the nucleation of SiNWs. The nucleation of SiNWs is lower with the Al catalyst, and only a few SiNWs were found spread sparsely across the surface, compared to the Cu catalyst sample. Most of the area was covered with silicon particles, which exhibited a much different morphology from that with a Cu catalyst. This phenomenon was likely caused by the negative effects of the residual oxygen on the Al catalyst. The oxidized aluminum makes the nucleation of SiNWs difficult. This experiment also demonstrates that Al is an important component in the 8606

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Figure 5. (a) TEM images of SiNW with branches. (b) The SAED diffraction of the SiNW.

here with the multilayered Al/Cu catalyst did not seem to have thermal gradients as the cause of the interconnected network because little kinking was observed. The TEM and electron diffraction results demonstrate that the SiNWs were essentially amorphous, as shown in Figure 5a,b. The diffraction rings correspond to an amorphous structure in the deposited SiNWs. Figure 5 also shows the alignment of two interconnected silicon nanowires. When two wires touch each other during growth, we propose that they can merge because the Al−Cu catalyst resides on the SiNW surface, as will be shown in the following results. The multilayered catalyst plays a key role in SiNW growth. Since the eutectic is formed at the interface between the Al and Cu layer, it is likely that this is the location where the SiNW seed is initially formed. The silicon growth is accompanied by Al−Cu interdiffusion and the continuous formation of a new eutectic phase. The last layer of the multilayered catalyst film on the substrate was copper. The diffusion of Al−Cu was shown by XPS analysis, Figure 6. The catalyst coated substrate was heat treated for 2 h at 550 °C in hydrogen gas. The XPS scans in the top part of Figure 6 show that the XPS signal for copper remains constant after different ion etching times. However, after heat treatment (bottom of Figure 6), the copper signal is small before ion etching due to alloying. The XPS depth profile showed that the Cu on the top of the multilayered stack almost disappeared after the heat-treatment, which verified that interdiffusion between the Cu and Al layers took place at high temperature. The composition of the SiNWs deposited with a multilayered Al/Cu catalyst was analyzed by secondary ion mass spectrometry (SIMS). The ion image maps are shown in Figure 7. Analysis of the 300 nm diameter SiNWs showed the presence of masses corresponding to Si, SiH, and a trace of H on the surface of the sample. Trace amounts of the AlCu catalyst were abserved over the entire SiNW surface, although no pure metal head was found on each nanowire. The distribution of the catalyst over the sidewalls of the SiNWs is different from the traditional VLS catalyst location because the catalyst is usually restricted to the head of the nanowire rather than distributed over the sidewalls. To verify the difference in these results compared to a traditional catalyst location, a sample with Au catalyst was examined. In the case of Au, the catalyst was only found on the tip of the SiNW and not

Figure 6. XPS curves of multilayered Cu−Al before (top figure) and after (bottom figure) heat-treatment. The scan identified as “Before Etching” was taken before ion etching in both figures. The other scans show the copper signal after different ion etching times.

distributed over the sidewalls, like the AlCu catalyst. The existence of trace amounts of AlCu catalyst on the sidewalls of the nanowires could create low melting point regions on the surface of SiNW. The presence of the AlCu catalyst on the sidewalls could be responsible for the unusual, interconnected growth structure of the SiNWs observed here. As the SiNWs are growing they can come into contact with other nanowires. 8607

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Figure 8. Cyclic voltammogram profile for as-prepared SiNW electrode.

curve became reproducible, similar to other reports. Previous studies found that Li12Si7 and Li21Si5 were the main lithium products formed with lithium ion insertion. Similarly, two cathodic peaks were observed after the first cycle. The first peak at about 0.22 V was related to the formation of Li12Si7. The peak at 0.02 V was associated with the presence of Li21Si5 or Li22Si5. The two main anodic peaks were consistent with the cathodic ones.1,28 A small peak was observed at about 0.43 V after the first cycle. The CV scans for the multilayer AlCu film demonstrated that the peaks were not due to the metal catalyst layer alone. Thus, this peak may be due to the presence of Al or Cu alloyed Si. The analysis of this peak may be the subject of a future study. It is interesting to note that there was no progressive activation step for the SiNW sample where the current density increase with scan number, as observed in previous studies.7,8,10 For crystalline silicon structures, a transformation from crystalline to amorphous occurred during lithiation. In the case of the AlCu catalyzed samples, the SiNWs were predominantly amorphous at the start, so that the transformation from crystalline to amorphous was not significant. The transformation from crystalline to amorphous for the Si anode can produce a large number of defects and dangling bonds that can irreversibly trap many lithium ions. This is responsible for irreversibility cycling effects, especially during the first cycle. Thus, the growth of amorphous SiNWs could be structurally better than traditional crystalline SiNWs. Figure 9a shows the charge−discharge profile of a SiNW anode with lithium foil as the counter electrode at 0.1 C rate within the voltage cutoff windows of 0.02−2.0 V versus Li metal reference. The SiNWs have a charge capacity (lithium insertion) of up to 3025 mA h g−1 and discharge capacity of 2836 mA h g−1 for the first cycle, yielding a Coulombic efficiency of 93.7% on the first cycle. Figure 10a shows the voltage cycling for a full cell (coin cell) consisting of a LiCoO2 cathode and SiNW anode at the rate of 0.2 C with a cutoff cell voltage between 2.8 and 3.9 V. The Coulombic efficiency for the first cycle was 85%. The Coulombic efficiency for subsequent cycles was 99.999 37%, which is comparable to commercial LiCoO2/graphite electrodes. It is worth noting that this Coulombic efficiency is higher than most reports related to silicon anodes. This higher efficiency is likely attributed to the amorphous state and interconnected structure of the SiNWs grown from the mutilayer Cu/Al catalyst. Electrical isolation of the SiNWs can occur more easily with noninterconnected

Figure 7. SIMS image of SiNWs. The upper left figure shows the SiH distribution. The upper right shows the silicon distribution. The lower left figure shows the AlCu distribution. The lower right figure shows the total ion distribution.

The contact points could create a lower melting point region compared to the free surface. This region could then cause the two nanowires to merge together at that point. Thus, the existence of the AlCu residue on the surface of SiNW appears to play a key role in forming the interconnected nanowire structure observed here. In addition to facilitating the formation of interconnected SiNWs, the AlCu catalyst on the surface could also cause the widening of the nanowires by catalyzing Si deposition on the sidewalls, rather than the exclusive deposition at the nanowire tip, as usually observed in VLS growth. The lateral growth of the SiNWs was verified by observing the diameter of the nanowires with growth time, under constant growth rate conditions. The diameter of the SiNWs was less than 40 nm after 10 min of growth. The diameter increased to more than 100 nm after 50 min growth time. Some branching of the SiNWs and tip-merging was also observed in these samples. In summary, the formation of the interconnected SiNW microstructure results from the multilayered catalyst film, which provided a location for SiNW seed formation so as to increase the contact ratio between the neighboring SiNWs. Second, trace amounts of Cu−Al catalyst on the surface of the SiNWs created a lower melting point region to make the merging of SiNWs and Si deposition around much easier. Electrochemical Performance of the SiNW. Silicon has a high theoretical capacity for lithiation, up to 4200 mAh/g. The Si−Li phases, Li3Si7, LiSi2, Li12Si7, and Li4.4Si, can exist at high temperature. Thus, lithiation can occur in multiple steps corresponding to the different lithium silicide phases during electrochemical lithiation at room temperature. Figure 8 shows the cyclic voltammetric (CV) curves for lithiation of the SiNW. The cathodic process corresponds to lithium insertion, and the anodic process corresponds to delithiation. There was an obvious difference between the first and the successive CV cycles. After the first and second cycles, the shape of the CV 8608

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Figure 9. (a) Charge−discharging profiles of SiNW electrode. (b) The cyclability of SiNW electrode.

Figure 10. (a) Charging−discharging profiles of the full coin cell consists of LiCoO2 /SiNWs. (b) The cyclability of the full coin cell with shallow charge−discharge process.

SiNWs. Fracture in a single nanowire would electrically isolate that portion of the electrode.8 In addition, the amorphous SiNWs improve the reversibility compared to crystalline SiNWs which must undergo a crystal transformation from crystallineto-amorphous. In addition, the interconnected SiNW structures make the SiNWs more robust to mechanical failure. Nucleation of the SiNWs grown on a silicon wafer proceeds more efficiently in a Si-rich environment due to the lattice matching between the nanowire and the substrate. However, the electrochemical performance of the silicon substrate during lithiation is unacceptable for battery applications due to the volumetric expansion of the substrate and resulting mechanical failure. When the SiNWs were grown on a metal substrate, the lattice mismatch between the SiNWs and the substrate may be a weak point. It is possible that, during lithiation and delithiation, the interface between the SiNW and the metal substrate weakens and causes electrical contact problems. It is possible that the AlCu catalyzed SiNWs have less of a lattice mismatch problem because they are mostly amorphous SiNW. The Coulombic efficiency after the first cycle increased to 99.96% with cycling, as shown in Figure 9b. This demonstrates that the interconnected amorphous structure could be the source of the improved Coulombic efficiency. The coin cell was cycled through many shallow discharge cycles to mimic the performance of a battery in a wireless

sensor application. In these applications, the battery can remain at open circuit or under trickle charge for relatively long periods of time followed by short, high current discharges during sensing or transmission. In the experiment, the coin cell was first charged to 4.0 V at 0.1C rate. The cell was then discharged at 10 mA cm−2 for 0.1 s followed by charging at 4.0 V for 0.9 s repeatedly. The discharge voltage was recorded after 8 100 000 cycles, as shown in Figure 10b. The inset in Figure 10b shows the cycling profile for the last 30 000 cycles. Thus, the high surface area of the SiNWs is particularly useful in performing a large number of shallow discharge cycles without significant loss in capacity.

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CONCLUSIONS Multilayer Al/Cu films were successfully used as the catalyst for SiNW growth by the VLS method at a relatively low temperature. The prepared SiNW have a highly interconnected microstructure and excellent electrochemical performance even a high loading of about 1.02 mg per substrate. Trace amounts of AlCu were found on the SiNW sidewalls, which was beneficial in creating the interconnected SiNW structure. The initial Coulombic efficiency was 93.7% with a charge and discharge capacity of 3025 and 2836 mA h g−1, respectively. 8609

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The Coulombic efficiency of subsequent cycles increased to 99.96%.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 404-894-2893. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the intellectual contributions of Michael Filler and the financial support the Interconnect Focus Center, one of six SRC focus centers.

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REFERENCES

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