Enhancement of electrochemical performance of silicon nanowires by homostructured interface used as anode materials for lithium ion batteries

Abstract

A novel and facile approach to fabricating long life silicon nanowires (SiNWs) via interface enhancement by structuring a silicon interface between SiNWs and metallic substrates from a nonequilibrium Si–Au composite based on conventional vapor–liquid–solid mechanism is successfully demonstrated in this paper. The building of a Si transition layer as an interface successfully converts the interface contact of the SiNWs on the substrates from point contact between the SiNWs and the stainless steel substrate into planar contact between the Si film and the stainless steel substrate. Although the interface modification process is very simple, the improved high stability of SiNWs from this way proves that it is a very promising strategy to get long life for high capacity anode materials used in lithium ion batteries. The building of a silicon homo-structural interface between assembled SiNWs and metallic substrates results in ultra long life for SiNWs. The retention capacity of the produced SiNWs is up to 2447 mA h g⁻¹ even after 100 cycles, delivering 80.92% of its second discharge capacity, presenting a very attractive cycling stability.

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Introduction

The fossil energy crisis and environmental issues are facilitating the development of green transportation devices. Rechargeable lithium-ion batteries are the most attractive power sources for environment friendly electric vehicles and hybrid electric vehicles because of their excellent performance over any other conventional batteries. Safety, high energy density and long cyclability are the indispensible factors for lithium ion batteries to be applied successfully in these fields. From a material standpoint, intense worldwide efforts have been devoted to seeking superior electrode materials^1,2 to substitute the current commercialized lithium transition oxides/graphite electrode system, whose specific capacities constrain the overall energy density of lithium ion batteries. Although suffering from poor cyclability triggered by high volume changes,³ which is induced by lithium insertion into and the subsequent extraction from the crystal lattice of silicon, silicon anode material is still attractive due to its high theoretic specific capacity (4200 mA h g⁻¹, ten times the commercialized graphite anode materials) as well as a suitable voltage plateau.

Decreasing the size of the silicon active material has been demonstrated to be feasible to alleviate the structural collapse caused by contrasting volume changes. One-dimensional silicon, e.g. silicon nanowires (SiNWs) or silicon nanotubes, has been proved to possess high mechanical intensity longitudinally to resist against the volume stress caused by lithium insertion and extraction.^4–6 A bottom-up method aided by metal catalysts based on Vapor–Liquid–Solid (VLS) mechanism is a conventional way to synthesize SiNWs, in which metal catalysts generally alloy with silicon to form a eutectic alloy to provide the circumstances for the nucleation of silicon seeds.^7–10 Au is a preferable catalyst for SiNW growth because Si and Au can form a Si–Au eutectic alloy at a relatively low temperature of 368 °C as well as high Si solubility of 12% in this intermediate eutectic.⁶ Although volume expansion in the longitude direction is constrained well to a low extent during lithium insertion and extraction, however, expansion in the radical direction could not be eliminated, which is still up to ca. 300% of its pristine size after lithium insertion.⁵ An interesting phenomenon should be noted that is very hard to obtain a long life for SiNWs despite many references demonstrating the structural stability of SiNWs during electrochemical lithium insertion and extraction, although there was almost no fracture found in SiNWs.^6,9–11

Some insightful research was conducted to elucidate the reason for the unpromising cycling life in our group. In our previous study, we found the disconnection occurred essentially between the substrates and SiNWs when we opened the batteries after deep depth cycling. Metals with stable property in electrolyte are preferred as the current collector in lithium ion batteries, so stainless steel and nickel foil are always chosen as the substrate for SiNW growth, considering their good thermal resistance and stable chemical performance in organic electrolytes for lithium ion batteries, so we deduced that the interface between the metallic substrate and the SiNWs is very weak, which should be induced by the crystal lattice mismatch between deposited SiNWs and their substrates, therefore interface de-bonding behavior would likely take place when serious expansion–shrinkage in volume occurs during the correspondingly electrochemical lithium insertion–extraction.

A relatively shallow depth by limiting the lithium inserting voltage to a higher cutoff range is a feasible way to alleviate the disconnection in the interface.^6,10 Surface modification via depositing a carbon layer around the SiNWs or microstructure modification via architecting core–shell microstructure, in which the inert core or inert coating film acts as the scaffold of the active center to support a stable mechanical structure and fine electrical connection, has been proved feasible to improve the cycling performance of SiNWs.^11–16 Another promising way is to increase the contact area between substrates and the active silicon centre, which has been demonstrated to be efficient in silicon films prepared on metal substrates.^2,11,16–18 These surface modifications and fabrication of a carbon matrix composite were the conventional ways to modify these high capacity materials, and many reports also demonstrated the feasibility of these methods. However, the interface issues are almost ignored, and have never been noted and mentioned in most of the previous reports.

Our exploration found that fabricating a suitable interface could be the most effective way to realize the life stability of SiNWs. In this paper, a novel and facile approach to fabricating long life silicon nanowires (SiNWs) via interface enhancement by a homostructured interface between SiNWs and metallic substrates from a nonequilibrium Si–Au composite catalyst is successfully demonstrated. Although the interface modification process is very simple, the improved high stability of SiNWs by this way proves that it is a very promising strategy to get long life anode materials with high capacity for lithium ion batteries. Our exploration found that fabricating a suitable interface could be the most effective way to realize the life stability of SiNWs. The building of a silicon homo-structural interface between assembled SiNWs and metallic substrates results in ultra long life for SiNWs, whose capacity is up to 2447 mA h g⁻¹ even after 100 cycles, retaining 80.92% of its second discharge capacity.

Experimental section

Stainless steel was cleaned thoroughly by ultrasonic vibration in acetone, alcohol and 1 wt% H₂SO₄ solvent in order for 10 min, respectively, followed by washing in distilled water untill the pH value was about 7. The preparation process for SiNWs is shown in Fig. 1. Silicon and Au layers were evaporated onto the surface of stainless steel in order by E-beam with a thickness of 20 angstroms and 100 angstroms, respectively (seen in Fig. 1). The stainless steel substrate deposited with Si–Au bi-layer film was placed in a tubular furnace and then the furnace was heated up to 600 °C with a continuous flow of pure H₂ for three hours. Then the substrate was cooled to room temperature in the furnace with H₂ flow as the protective gas. The surface color of the stainless steel became light brown after heat-treatment in comparison with the golden color before heat-treatment, and then the stainless steel was cut into circle discs with a diameter of 15 mm to be used as the substrate for SiNW growth. SiNW was prepared at 550 °C for 20 min with 10% SiH₄ as the Si source gas balanced by hydrogen in the tubular furnace.

Fig. 1 Schematic of the preparation process for SiNWs.

X-ray photoelectron spectroscopy (XPS) measurements were conducted with a K-Alpha 1063 system (Thermo Fishier Scientific) with a monochromatic aluminium Kα source and a source power of 200 W. Scanning electronic microscope (SEM) images were obtained with a Hitachi 4700 FESEM system (Hitachi, Japan).

The electrochemical performance of the SiNWs was tested in 2025 coin cells, in which the deposited SiNWs were used as the working electrode and Li foil acted as the counter-electrode. 1 M LiPF6/EC + DMC (1 : 1 in volume) solution was applied as the electrolyte. The electrochemical behavior of the cells was investigated at constant current charging–discharging measurement within the stable voltage window of 0.02–1.5 V. The cyclic voltammogram for the half cell of SiNWs was performed between 0.02 V and 2.0 V at a scan rate of 0.05 mV s⁻¹. Electrochemical impedance measurement was performed in the frequency range of 1 Hz–0.1 MHz with a CHI660C (Chenhua Co., China).

Results and discussion

As illustrated in Fig. 1, Au and Si layers were deposited on cleaned stainless steel substrate in order, and then the substrate was heat-treated to trigger the inter-diffusion of the Si and Au layers so as to get a Si-rich Au composite catalyst layer. The Si-rich Au composite plays a very important role in the formation of the silicon interface layer. Different from the Au-catalyzed VLS process, where SiNWs grow directly on the substrates, in our Si-rich Au composite catalyzed process, a very thin silicon film could be formed on the stainless steel substrate preferably earlier than the growth of SiNWs, according to the Si–Au binary phase diagram. Therefore, the SiNWs were subsequently grown on the pre-formed silicon film during the assembling process. Although this process did not increase the essential contacting area of the SiNWs on the substrate, the silicon film between the SiNWs and the substrate acts as transition layer between them, which not only increases the intrinsic contact area on the substrate, but also alleviates the lattice mismatch between SiNWs and their substrates. It is rational to derive that the mechanical stability between stainless steel substrates and SiNWs is increased correspondingly because the contact mode was converted from between SiNWs and stainless steel substrate to between Si film and stainless steel substrate, and the lattice mismatch was avoided technically. In addition, the diameter of the SiNWs could also be controlled well by the nonequilibrium Si–Au alloying process based on the nonquilibrium heating–cooling process, in which Si–Au alloy and Au particles were formed and precipitated with controllable size.

The saturated atomic percent of silicon in the Si–Au eutectic is 18.6% at 363 °C, according to the triple phase point on the Si–Au binary phase diagram. In our case, one layer of Si and one layer of Au were pre-deposited on the stainless steel substrate before heat treatment. According to our experimental data, the calculated mol ratio of silicon/Au is about 15% in the predeposited layer on the presumption of free of holes in the compact layers. Therefore, it is possible to get a Si-rich Au composite layer consisting of Au primary crystals and a Si–Au eutectic alloy via nonequilibrium heating–cooling process. Fig. 2 shows the binding energy changes of substrates coated with Si and Au layers before and after heat treatment. A chemical shift occurred after heat treatment, and the binding energy at 84.88 eV represents the existence of a Si–Au eutectic alloy after heat treatment.

Fig. 2 XPS spectra of Au 4f on the surface of the stainless steel substrate coated with Si–Au layers before and after heat treatment.

It is necessary to elucidate why we chose the heated substrates coated with Si–Au layers at the start of SiNW growth rather than Si–Au coated substrates without any further heat treatment. In our previous study, we found the diameter distribution of produced SiNWs could not be controlled well when using Au-coated stainless steel substrates directly because the size of the formed Si–Au eutectic droplets was uncontrollable because of the merging behavior of these Si-saturated Au droplets and the different deposition rate of silicon at different locations on the substrates. So controlling the distribution range of the final diameter of produced SiNWs in a facile way is challenging. Generally, the diameter of the prepared SiNWs is determined definitely by the initial particle size of the catalysts. One optional approach to control the diameter of SiNWs is to make the catalyst layer by E-beam nanolithography with pre-designed nanopatterns consistent with the final diameter of the SiNWs. But E-beam nanolithography is a long, very expensive and delicate process, and too many steps are involved to obtain defined Au nanopatterns.

In our typical process in this paper, Si–Au layer coated stainless steel substrates were heated to 600 °C and kept for 3 hours to make Si and Au atoms diffuse thoroughly so as to form a Si–Au eutectic composite, and then the substrates were cooled in an oven under H₂ atmosphere at a high cooling rate. In this nonequilibrium process, the formed Si–Au eutectic would keep its highly Si–Au mixed state, and highly dispersed primary Si–Au alloy particles can thus remain in a Si-rich state. This nonequilibrium Si-rich Au composite acted as the initial catalyst for the subsequent SiNW growth. Scattered dots were observed on the surface of Si–Au coated substrates after heat-treatment (as seen in Fig. 3a). During the process of SiNW growth, actually the nonequilibrium catalyst layer experienced the process of re-precipitation and re-crystallization. The composition of the Si-rich Au composite would change with the decomposition of the Si gas source, which saturated the Si–Au eutectic all the time, so Si in the Si–Au alloy would be precipitated preferably before the growth of SiNWs. This preferable precipitation process could be demonstrated by an interesting phenomenon where the surface of the substrates was blue in color when the deposited SiNWs were removed by moderate scratching, and XPS results also proved that the blue surface is Si film. That is, a very thin transition layer of silicon was actually formed between the prepared SiNWs and the stainless steel substrates during SiNW depositing. The existence of this silicon transition layer could definitely alleviate the lattice mismatch between SiNWs and metallic substrates. It is understandable that better interface affinity between subsequently produced SiNWs and pre-deposited Si thin film could be gained (scheme in Fig. 3d). Fig. 3b and c exhibit the morphological changes of SiNWs with different growth times. Before SiNW growth, there are some fine particles dispersed on the surface of the stainless steel. After 20 minutes of SiNW growth (seen in Fig. 3c), SiNWs could be obviously detected on the substrate with uniform diameter between 100 and 120 nm, demonstrating the feasibility of this predeposition-heat-treatment-VLS combined method to control the diameter distribution of the final SiNWs. In the following part, SiNWs prepared by this process were designated as Si–Au catalyzed SiNWs.

Fig. 3 SEM images of the substrate before and after SiNW deposition with different growth times. (a) Morphology of the surface of Si–Au layer coated stainless steel after heat-treatment. (b and c) Image of Si–Au catalyzed SiNWs produced with a deposition time of 10 min and 20 min, respectively. (d) Schematic illustration of the interface between the Si–Au catalyzed SiNWs and the stainless steel substrate.

SiNWs catalyzed by pure Au with the same preparation process were also produced in comparison with Si–Au catalyzed SiNWs. SiNWs produced from Au catalyst were designated as Au-catalyzed SiNWs. The impedance comparison of SiNW electrodes from different catalysts is shown in Fig. 4. The total impedance of the cell composed of Si–Au catalyzed SiNWs and Li foil is about 134.8 ohms with the specific area impedance of 76.3 ohm cm⁻², much higher than the impedance of the Au catalyzed SiNWs at 61.9 ohms. It is worth noting that the impedance actually consists of the impedance of the solid–liquid interfaces between the SiNWs and the electrolyte, the SiNWs and the Si transition film, the Si transition film and the substrate. The morphology of Au catalyzed SiNWs is similar to that of Si-Au catalyzed SiNWs (Fig. 5). Although it is very hard to figure out the impedance of the silicon transition interface, however, the mass load of these two samples was controlled almost the same, and the difference between them is mainly from the catalyst precursors, so it could be understandable that the increment in electrochemical resistance is caused by the precipitated silicon transition layer between substrates and SiNWs during SiNW growth, which could definitely increase the overall resistance of the electrode due to the semiconductivity property of silicon. The circuit mode should be researched further to understand the intrinsic electrochemical behavior of the Si–Au catalyzed SiNWs.

Fig. 4 Electrochemical impedance spectra of silicon nanowires prepared with different catalysts.

Fig. 5 The morphology of Au catalyzed SiNWs.

Although with larger resistance, the electrochemical performance of the Si–Au catalyzed SiNWs from the alloyed catalyst shows higher reversibility, compared with that of the Au catalyzed SiNWs. The coloumbic efficiency for the first cycle for the SiNW electrode from the Si–Au catalyst is up to 91.91% (seen in Fig. 6a) with a discharge capacity of 3457.6 mA h g⁻¹ and a charge capacity of 3177.8 mA h g⁻¹, demonstrating the high reversibility for lithium insertion–extraction. It is interesting to note that there is no obvious behaviour of the formation of a solid electrolyte interface (SEI) film taking place during lithium ion insertion for the first cycle, similar to previous reports.^4,5,10,11 This is probably another reason for high reversibility in the first cycle. The inset profile is the cyclic voltammogram profile of the as-prepared SiNW electrode for the first cycle. The behavior of the Si–Au catalyzed SiNW electrode is similar to that previously reported.^4,5,13,14 There is no SEI formation peak observed on the cathodic curves, especially in the first cycle, consistent with the results of the galvanostatic charging–discharging test. The cathodic peaks appearing at 0.18 V and 0.02 V represent the formation of lithium silicide with different mol ratios of Li/Si, mostly Li₁₂Si₇ and Li₂₂Si₅.^3,13 Two corresponding anodic peaks appeared at 0.34 V and 0.51 V, respectively.

Fig. 6 The electrochemical cycling performance of SiNWs: (a) the charging–discharging profiles of Si–Au catalyzed SiNWs; (b) the comparison of the electrochemical cyclability of SiNW electrodes produced from different catalysts.

The cycling performance of the Si–Au eutectic catalyzed SiNWs was also investigated, and the comparative cyclability of Si–Au catalyzed SiNWs and Au catalyzed SiNWs is shown in Fig. 6b. Very excellent cycling performance of Si–Au catalyzed SiNWs was gained. The cycling performance and reversibility of the Si–Au catalyzed SiNWs presents much better than that of the Au-catalyzed SiNWs. The retention specific capacity of the Si–Au catalyzed SiNWs is up to 80.92% of its second discharge capacity after 100 cycles, whereas that of the Au catalyzed SiNWs is only 2539.6 mA h g⁻¹, 74.2% after 50 cycles although the deposition process is controlled the same. The above part already shows that there is a very thin transition layer of silicon film existing between the produced SiNWs and the stainless steel substrate. Therefore, it could be deduced that although there is some increment in impedance because of the existence of the silicon transition layer, however, this thin silicon transition layer makes the lattice between the SiNWs and the substrate match better, which is the main reason for the dramatically increased cycling stability.

As shown in the Raman profiles of the Si–Au catalyzed SiNWs before cycling in Fig. 7a, there is a strong peak at Raman shift of 507.6 cm⁻¹ rather than at the characteristic Raman shift of 521 cm⁻¹ for the bulk cubic crystalline silicon, presenting a wurtzite nanocrystalline structure in the as-prepared SiNWs, which shifts to 505.08 cm⁻¹ after cycling, presenting the crystal changing tendency toward amorphous structure. The tendency in the structure changes of Au-catalyzed SiNWs is almost the same as that of the Si–Au catalyzed SiNWs before and after cycling. However, the characteristic Raman shift for Au catalyzed SiNWs at 518.13 cm⁻¹ presents more crystallization, which is much closer to the shift of bulk crystalline silicon (Fig. 7b). After lithium insertion–extraction, the Au catalyzed SiNWs change into amorphous, resulting in the Raman shift at 505.62 cm⁻¹, but the relative intensity became very weak, due to the serious peeling off of SiNWs from the substrate. The peeling off phenomenon was not detected on the Si–Au catalyzed SiNWs electrode. This is probably due to the stability of the silicon interface between the SiNWs and the substrate. In addition, the irreversibility in structure would trap more lithium ions in the defects occurring in the course of electrochemical lithium insertion–extraction, so the difference in the start of crystallization of SiNWs from different catalysts shows different cycling capability. Fig. 7c presents the morphology of the Si–Au catalyzed SiNWs after cycling. The surface of the SiNWs becomes very coarse for the formation of a SEI film, but there are no obvious defects embedded on the nanowires (Fig. 7c).

Fig. 7 (a) Raman shift of Si–Au catalyzed SiNWs before and after cycling; (b) Raman shift of Au catalyzed SiNWs before and after cycling; (c) SEM image of Si–Au catalyzed SiNWs after cycling.

Conclusions

In conclusion, the present work demonstrates a new and facile approach based on interface enhancement to fabricate long life SiNWs with a simply modified Si-rich Au composite as the catalyst. The utilization of this Si-rich Au catalyst can induce the formation of a Si transition layer between the as-deposited SiNWs and the stainless steel substrates, which not only improves the lattice match between them, but also increases the contact area of the Si active center with the substrates by converting the interface contact from point contact between SiNWs and stainless steel substrate to planar contact between Si film and stainless steel substrate. The electrochemical performance could thus be enhanced dramatically. Structuring a homo-structural interface offers a promising strategy by interface modification to alleviate the irreversibility of silicon or other high capacity anode materials.

Acknowledgements

This research is financially supported by the Natural Science Foundation of China (Grant no. 21476035), the Fundamental Research Funds for the Central Universities (Grant no. 3132014077 and Grant no. 3132014323), the Research Funds of Education Department of Liaoning Province (Grant no. L2013204), and the Natural Science Foundation of Liaoning Province (Grant no. 2014025016).

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