Abstract
A Si thick film with a thickness of c.a. 2 µm was deposited on a copper substrate by electron cyclotron resonance - chemical vapor deposition (ECR-CVD) under a mixture of silane (SiH4) and argon (Ar) gases. The surface modification of the silicon anode was carried out by the metal-assisted chemical etching method. Observation by SEM showed that the silicon on the copper substrate had a columnar structure as the result of the etching process. The electrochemical performance of the anode prepared using the modified Si thick film showed a specific capacity of more than 2000 mAh with 84% of the discharge capacity remaining ever after 25 cycles of charge and discharge, as compared with 35% of the discharge capacity remaining for the pristine Si thick film.
Keywords: lithium-ion battery anode, modified silicon anode, columnar structure, metal-assisted chemical etching
Introduction
Silicon is of special interest as an anode material for Li-ion batteries, since it has a large theoretical speciï¬c capacity of c.a. 4000 mAh/g [1, 2], which corresponds to the fully lithiated state of Li21Si5 alloy and is more than 10-fold greater than that of graphite [3]. However, the application of bulk silicon anodes faces some major problems, which are derived from the well-known 400% volume change of Li-Si alloy during repeated lithium insertion and extraction, which generates great mechanical stresses inside the silicon lattice, leading to cracking, pulverization, capacity fading and a high irreversible capacity [2]. Much research has been focused on silicon nanowires (SNWs) as a solution to the problem of the large volume change of Li-Si alloy [4, 5]. Various methods of preparing SNWs have been developed [6-9]. However, despite the success of these works, their growth mechanisms have some drawbacks. It is only possible to grow SNWs on a limited area of a substrate. Moreover, they generally need a high temperature or a high vacuum, templates and complex equipment.
Nowadays, the metal-assisted chemical etching (MACE) method is well known as a good candidate for the preparation of high quality SNWs with the advantages of low cost and simplicity [10-17]. This technique has been widely used to manufacture silicon wire for many different applications. However, the number of studies in which this method was used for the preparation of nanosilicon as the anode in Li-ion batteries is quite limited. Several authors used this method to prepare SNWs from a silicon wafer for use as the anode material for Li-ion batteries. However, various difficulties originating from the poor contact between the SNWs and collector limited the performance of the anode [18, 19, 20].
In this work, for the first time, the metal-assisted chemical etching method was applied to a silicon thick film to render it more stress-dissipative structure as the anode for Li-ion batteries. Silicon with a nanorod-like structure was formed on a copper substrate by the modification of the silicon thick film in HF/H2O2 solution with a silver catalyst. The effect of the modification process on the electrochemical performance of the anode was studied by the cycling test performed on the pristine and modified Si thick films used as the anodes in Li-ion batteries.
Experimental
Si thick ï¬lms were deposited on copper foil from a SiH4/Ar gas mixture using a plasma-enhanced CVD system. The plasma was generated by an electron cyclotron resonance (ECR) operating at 2.45 GHz with a microwave power of 700 W. The working pressure was 1.5x10-2 torr. The flow rates of Ar and SiH4 were 30 sccm and 20 sccm, respectively. The substrate temperature was kept stable at 250 oC.
The modification of the silicon thick film was performed at room temperature using the metal-assisted chemical etching method. First, Ag particles were deposited on the surface of the Si films by dipping them in a solution of 0.01M AgNO3 and 1M HF. Then, the silver-covered thick films were etched by a solution containing 0.1M H2O2 and 0.5M HF. After the etching process, the Si thick films were rinsed with deionized water and dried for 2 hours in a vacuum oven at 80 oC.
Half cells, which were fabricated in a dry room, were assembled in a polyethylene bag using the Si thick films as the working electrodes, polyethylene as the separator and lithium metal foils as the counter electrodes. The anode area was 4cm2 (2cm x 2cm). The liquid electrolyte was 1M LiPF6 in ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate (1:1:1 volume ratio). The morphology of the samples was observed using scanning electron microscopy (SEM) (Hitachi S-5500). The electrochemical characteristics of the batteries were measured by a battery test system (Maccor Series 4000) at room temperature. Galvanostatic discharge-charge cycling was carried out in the potential range of 2-0V (versus Li/Li+) at a current density of 250µAcm-2.
Results and discussion
Figure 1(a) shows the surface SEM-image of the as-deposited Si thick film on the copper foil. The surface of the film was quite rough with grain sizes ranging from 300 to 800 nm. The SEM observation showed that the thickness of the thick film was about 2.1 μm. The distribution and morphology of the silver nano-particles on the surface of Si thick film are shown in figure 1(b). At a deposition time of 15 seconds, the diameters of the silver particles were about 30-120 nm. After the deposition of the silver particles, the Si thick film was etched in a solution of HF and H2O2. Experimental observation showed that after 22 minutes of etching, when the silver particles encountered the Cu foil, the Cu foil was oxidized immediately by the etching solution, resulting in the detachment of the Si film from the Cu substrate. Therefore, to keep the bond between the Si thick film and Cu foil, the etching time was fixed at 20 minutes. As shown in figures 1(c) and 1(d), The etching process resulted in the formation of the Si columns on the Cu substrate with diameters in the range of 30-200 nm.
The mechanism of the etching process is explained quite clearly in the literature [11-14, 18]. In this work, in spite of its rough surface, the silver particles covered the surface of the Si thick film almost completely. In the figures 1(c) and 1(d), it can be seen that the etching direction was almost perpendicular to the general surface of the Si thick film. This seems to originate from the etching of the network of Ag particles connected together, as observed by H Fang et al. [18].
Figure 1. SEM images showed the top view of the as-deposited Si thick film on the Cu plate (a) and the Si thick film covering by Ag particles (b); the top view (c) and the cross-section (d) of the Si thick film etched in HF/H2O2 solution for 20 minutes.
For the investigation of their electrochemical characteristics, the pristine and modified Si thick films were used as the anodes in half cells. Figure 2a shows a comparison of the cycling behavior and coulombic efï¬ciency of the pristine and modified Si thick films used as electrodes. In the initial cycles, the specific capacity of the modified Si thick film was lower than that of the pristine thick film. It should noted that at the same current density during charge and discharge, the Coulomb rate of the modified Si thick film was higher than that of the pristine Si thick film, because of the weight loss of the modified Si thick film during the etching process. On the other hand, because of the specific capacity of silver was lower than that of silicon [24], the contribution of silver particles resulted in the decrease of the specific capacity of the modified Si thick film. During the first charge and discharge, the specific capacity of the modified Si thick film respectively were 3057 mAh and 2911 mAh, indicating a coulombic efï¬ciency of 95%. After the first cycle, in parallel with the slight decrease of the specific capacity, the coulombic efï¬ciency increased and remained higher than 97% after the first 25 cycles. Both the charge and discharge capacities of the modified Si thick film remained quite stable. After 25 cycles, the discharge capacity still remained at 84% of the value observed in the 1st cycle and 86% of that observed in the 2nd cycle. These results are considerably better than the previously reported results [19-21]. In the case of the pristine Si thick film, after 10 cycles, the coulombic efï¬ciency began to become unstable and decreased to 93% at the 23rd cycle. Beside that, the fading of the capacity took place quite fast and resulted in a decrease of the discharge capacity of c.a. 65% after 25 cycles.
Figure 2. (a) comparison of cycling behavior and coulombic efï¬ciency of pristine and modified Si thick films used as electrodes; (b) profile of voltage vs. capacity for pristine and modified Si thick films used as electrodes at 1st, 2nd, 5th, 15th and 25th cycles; (c) the differential capacity curves at the 2nd cycle for the pristine and modified Si thick films used as electrodes.
Figure 2(b) shows the profile of the voltage versus capacity for the pristine and modified Si thick films used as electrodes. In the proï¬le of the ï¬rst discharge curve, the polarization during the ï¬rst lithiation of the modified Si thick ï¬lm was higher than that of the pristine Si thick ï¬lm. This can be attributed to the increase of the surface area of the modified Si thick film. Moreover, in the fist discharge curve, a "potential overshoot" at around 0.03-0.05V appeared for both the pristine and modified Si thick films. This phenomenon results from the existence of a nucleation barrier in the ï¬rst process of Li-Si phase formation, as explained by R. A. Huggins in [22].
A comparison of the differential capacity curves of the pristine and modified Si thick films used as electrodes at the 2nd cycle is shown in figure 2(c). In the discharge process of the pristine Si thin film used as the anode, the appearance of four peaks at ~0.34V, ~0.24V, ~0.08V, and ~0.04V indicates that four differential Si-Li alloys existed during the lithiation process, as observed by C.J. Wen et. al. [23]. The shift of the peak at ~0.04V and the increase of the relative intensity of the peak at ~0.08V may result from the contribution of silver particles on the modified Si thick film [24]. Moreover, the change in the relative intensity of the peak at ~0.34V may result from the mechanical stress in the pristine Si thick film at the initial stage of the lithiation process.
The differential capacity curves of the modified Si thick films used as electrodes at the differential cycles also showed the stability of the peaks with cycling. This result indicates the stability of the electrochemical reactions in the electrode and the excellent capacity retention of the composite with cycling, as a result of the modification technique.
Conclusions
Silicon with a columnar structure can be prepared on a copper substrate by the metal-assisted chemical etching process and employed as the anode for lithium secondary batteries without any further treatment. The cycling test showed that the modified Si thin anode showed better performance than the pristine Si thick film. This excellent electrochemical performance is attributed to the columnar structure of the silicon anode, which provides free vacancies and absorbs the stress caused by the lithiation process. It can be concluded that the optimal void structure of the silicon anode prepared herein enabled it to exhibit better cycling performance when used as the Si anode for lithium-ion batteries.
