graphite composite anode

Materials Chemistry and Physics 104 (2007) 444–447

Synthesis and electrochemical performance of Si/Cu and Si/Cu/graphite composite anode Pengjian Zuo, Geping Yin ∗ , Xuefeng Hao, Zhanlin Yang, Yulin Ma, Zhenguo Gao Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, China Received 1 March 2006; received in revised form 10 March 2007; accepted 3 April 2007

Abstract The Si/Cu and Si/Cu/graphite composites were synthesized by mechanical ball milling. The phases of composites were analyzed with X-ray diffraction. Their charge–discharge performance of the composite as negative electrodes in Li-ion batteries were tested and the formation process of solid electrolyte interface/interphase (SEI) was discussed by cyclic voltammograms (CV) and electrochemical impedance spectroscopy (EIS). The results show that the materials prepared by ball milling are the composites consisting of Si and Cu3 Si alloy. During cycling the composite electrodes suffer from cracking which cause the large particles pulverize. Therefore, it facilitates the further formation of SEI film on the surface of newly formed particles. The initial reversible capacity and efficiency of composites are improved and the cycle cycleability is enhanced obviously after the ball milling of Si/Cu composite with graphite. The Si/Cu/graphite composite electrode has an initial reversible capacity of 420 mAh g−1 and a charge–discharge efficiency of 65%. © 2007 Elsevier B.V. All rights reserved. Keywords: Li-ion battery; Si/Cu composite; Electrochemical performance

1. Introduction Much research has been carried out in order to meet the requirement of high energy density and long cycling life for the lithium rechargeable batteries recently. Many metals including Sb, Bi, Sn, Cd, Zn, Pb, Ga, Si, Al, etc., which can electrochemically react with lithium under ambient conditions, attract much attention due to their high theoretical specific capacities in comparison with the conventional graphite material [1,2]. Unfortunately, most lithium alloys are easily pulverized by the large volume changes during the electrochemical lithiation/delithiation processes, which results in a less connectivity between the active particles or a separation from the current collector [3–5]. The situation can be improved to some extent by reducing the metal particle size [6–8], distributing them evenly throughout an inactive matrix [9–12] or using other composite structures. ∗

Corresponding authors at: P.O. Box 411#, Laboratory of Electrochemistry, Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, PR China. Tel.: +86 451 86413721; fax: +86 451 86413707. E-mail addresses: [email protected] (P. Zuo), [email protected] (G. Yin). 0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.04.001

Silicon can alloy with lithium to form four different phases Li12 Si7 , Li7 Si3 , Li13 Si4 and Li21 Si5 [13,14], and the theoretical capacity for the reaction of lithium and silicon to Li21 Si5 is about 4000 mAh g−1 which corresponds to 4.2 mol lithium per mol silicon. Si-based alloys such as SiAlMn [15] SiAg [16,17], SiMg [18–20], Si–Mn/C [21] and Si/C composites [22–26] have been reported as anodes for lithium ion batteries. In this study, we succeeded to attain good cycle performance by preparing Si/Cu and Si/Cu/graphite composites through mechanical ball milling. The electrochemical performances of composite powders as anodes in lithium ion batteries are presented. Microstructural analyses are performed and the reaction mechanism of the composites is also discussed. 2. Experimental Si/Cu composites were prepared by ball-milling the mixture of the silicon (−325 mesh, 99.6%, Beijing Dadi Silicon Co.) and copper powders (−325 mesh, 99.6%, Beijing Shuanghuan Chemical) with an atomic ratio of 5:5. In detail, silicon powders and copper powders were mixed at an atomic ratio of 5:5 and then put into stainless vials. The vials were evacuated and filled with pure argon. The ball milling process was performed in a planetary ball milling machine (ND6, Nanjing Tianzun Co.) at a rotation rate of 150 rpm for 60 h. To fabricate the sample of Si/Cu/graphite, the 80:20 (wt%) mixtures of graphite (D50 = 18 ␮m,

P. Zuo et al. / Materials Chemistry and Physics 104 (2007) 444–447 Shenzhen BTR Energy Materials Co. Ltd.) and the acquired Si/Cu composite powders were prepared by ball milling for 20 h. The composite powders were characterized by X-ray diffraction (XRD) which was performed using a D/max␥B diffractometer (Rigaku, Japan) equipped with Cu radiation in the range of 2θ = 20–90◦ . The particle size and microstructure of electrode materials were observed by SEM (Hitachi S-4700). The composite electrodes were prepared by mixing 85 wt% active material, 5 wt% acetylene black and 10 wt% polyvinylidene fluoride (PVDF) dissolved in N-methylpyrrolidinone (NMP) to form a slurry which was then coated on copper foil and dried at 120 ◦ C for 14 h. The electrochemical reactions of Si/Cu and Si/Cu/graphite composites with lithium were investigated by a two-electrode cell using a Li foil as a reference and counter electrode. The cells were assembled in a glove box filled in argon, and the electrolyte was 1 M LiPF6 in a mixture of EC:DEC:EMC = 1:1:1 (v/v/v). A battery test system (BTS, Shenzhen Neware) was used for galvanostatic cycling with a potential range of 1.5–0.01 V at 0.15 mA cm−2 . The CV tests were performed with a scan rate of 0.5 mV s−1 using CHI630a. Electrochemical impedance spectroscopy (EIS) measurements were carried out on a frequency response analyzer (CHI604b) by a three-electrode system using lithium foils as reference and counter electrodes. Before the EIS experiments, the cell was kept for 4 h for equilibrium after desired cycles. The impedance spectra were recorded potentiostatically by applying an ac voltage of 5 mV amplitude over the frequency range from 0.1 MHz to 0.01 Hz.

3. Results and discussion Fig. 1 presents X-ray diffraction patterns of the Si/Cu and Si/Cu/graphite composite powders prepared by mechanical ball milling. Si/Cu composite consists of crystalline Si and Cu3 Si alloy, while Si/Cu/graphite is composite including of Si, Cu3 Si alloy and graphite particles. We infer from the results of XRD and elemental analysis that copper embeds into the crystalline Si lattice to form the Cu3 Si alloy phase especially due to the nanoscale particle size of copper during the ball milling. Fig. 2 shows the images of two composite electrodes. From the images, it can be seen that the particle size of the Si/Cu composite (about 0.5–5 ␮m) is a little larger than that of Si/Cu/graphite composite (∼0.5–2 ␮m), which is partially due to the introduction of graphite powders into the Si/Cu composite and the additional 20 h ball milling during the preparation of Si/Cu/graphite composite. Fig. 3 shows the cyclic voltammograms of the Si/Cu and Si/Cu/graphite composite electrodes. From this figure, it is known that the reductive peak at about 0.5 V during the first

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Fig. 1. XRD patterns of the Si/Cu composite powders prepared by mechanically ball milling.

intercalation process of lithium ions into the composite anode corresponds to the formation of the passivating film (SEI). We can also find that there is another reductive peak at about 0.2–0 V which represents the insertion of lithium into silicon and graphite. From Fig. 3, it can be found that the two anodic peaks at about 0.3 and 0.7 V correspond to the extraction of lithium ions from the graphite and active silicon particles. Electrochemical impedance spectroscopy tests were carried out in order to investigate the electrode resistance changes after the electrodes had performed the desired cycles. Fig. 4 records the Nyquist plots of Si/Cu composite after different cycling numbers. The arc appeared in the high frequency range of the composites is attributed to the formation of SEI film on active material and carbon powder surfaces caused by the decomposition of electrolyte solution [19]. The middle-frequency arc is related to the charge transfer through the electrode–electrolyte interface, and the sloping straight line at low frequency corresponds to the Li-ion diffusion in the bulk materials [25]. The sizes of the arcs in the high frequency range increase during cycling, which can be attributed to the thickening of SEI layer. Generally, the passivating film is stable during the subsequent insertion/extraction process once it forms in the first cycle. However, the drastic volume changes during cycling will

Fig. 2. SEM of two composite electrodes: (a) Si/Cu and (b) Si/Cu/graphite.

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Fig. 3. Cyclic voltammograms (CVs) of two composite electrodes: (a) Si/Cu and (b) Si/Cu/graphite.

Fig. 4. (a) Impedance spectra of Si/Cu composite anode at different charge states in a three-electrode lithium cell. (b) Zoom part in high frequency region after (1) 1 cycle; (2) 2 cycles; (3) 6 cycles; (4) 10 cycles; (5) 20 cycles; (6) 30 cycles.

result in cracking and crumbling of the active materials when using metal-based materials as anodes. Consequently, the fresh surface of active materials caused by this change will lead to additional formation of the SEI film during the subsequent insertion of lithium ions into the alloy anodes. It is the ununiformity of SEI film formation that leads to the increasing of the layer thickness. Fig. 5 shows the scanning electron micrograph of the Si/Cu composite electrode after 40 cycles. It can be seen that the dispersed particles coalesce into a block structure after cycling. The

morphology variation, which was electrochemical sintering, is generally considered as the main source of irreversible capacity loss during cycling [27]. The charge and discharge curves of the Si/Cu composite electrodes for the initial three cycles are shown in Fig. 6. It is obvious that the Si/Cu composite electrode has a charge capacity of 380 mAh g−1 and a large irreversible capacity loss (∼44%). Fig. 7 shows the curves of coulombic efficiency and cyclic capacity versus cycle number for Si/Cu/graphite composite electrode. For comparison, the electrochemical performance of the graphite anode is also presented in Fig. 7. It can be found that the electrochemical performance is enhanced obvi-

Fig. 5. SEM images of Si/Cu composite electrode after 40 cycles.

Fig. 6. Charge and discharge curves of Si/Cu composite electrode.

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Acknowledgements This work was partially supported by The Natural Science Foundation of China (No. 20673032) and The Key Scientific & Technological Programme of Heilongjiang Province of China (No. GB06A309). We gratefully acknowledge useful discussions with Dr. Qinmin Pan. References

Fig. 7. Coulombic efficiency and cyclic capacity vs. cycle number for Si/Cu/graphite composite electrode.

ously after mixing the Si/Cu composite powders and graphite by mechanical ball milling. The Si/Cu/graphite composite electrodes possess an initial reversible capacity of 420 mAh g−1 and maintain a capacity of 282 mAh g−1 after 40 cycles which is higher than that for the graphite electrode. The coulomb efficiency of composite electrodes becomes constant to be over 98% after the fifth cycle even though the initial efficiency is not high (∼65%), which is available for the composites as anodes for rechargeable lithium ion batteries during cycling. 4. Conclusion In this study, Si/Cu and Si/Cu/graphite composites were prepared by mechanically ball milling and the electrochemical performance was investigated as anode materials for lithium ion batteries. The new alloy phase of the Cu3 Si was formed after 60 h ball milling. The composite particles agglomerate into block and then the electrode encounter a crack as anode materials caused by their extreme changes with the insertion/extraction of lithium ions, which results in the decreasing of cyclic performance of composite electrodes. Moreover, the blend of graphite into the Si/Cu composite has enhanced the cycle performance of composite materials further. The initial discharge capacity of the Si/Cu/graphite composite anode was 420 mAh g−1 with a reversible capacity after 40 cycles being 282 mAh g−1 . It is believed that appropriate particle size is beneficial for the formation of SEI film during cycling, which will improve the electrochemical performances of the alloy negative electrode. Further work should be conducted to optimize the microstructure of the Si/Cu/graphite to decrease the initial irreversible capacity loss and improve the cycling performance of the composite electrodes.

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