Si–AB5 composites as anode materials for lithium ion batteries

Si–AB5 composites as anode materials for lithium ion batteries

Electrochemistry Communications 9 (2007) 713–717 www.elsevier.com/locate/elecom Si–AB5 composites as anode materials for lithium ion batteries X.N. Z...

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Electrochemistry Communications 9 (2007) 713–717 www.elsevier.com/locate/elecom

Si–AB5 composites as anode materials for lithium ion batteries X.N. Zhang, P.X. Huang, G.R. Li, T.Y. Yan, G.L. Pan, X.P. Gao

*

Institute of New Energy Material Chemistry, Department of Materials Chemistry, Nankai University, Tianjin 300071, China Received 5 October 2006; received in revised form 15 October 2006; accepted 26 October 2006 Available online 5 December 2006

Abstract The Si–AB5 (MmNi3.6Co0.7Al0.3Mn0.4 alloy) composites with a high tap density as anode materials for lithium-ion batteries were synthesized by ball-milling. Si nanoparticles are distributed homogeneously on the surface of the AB5 matrix. The electrochemical performance of the Si–AB5 composites as a function of Si content was investigated. It is demonstrated that the Si–AB5 composite delivers a larger reversible capacity and better cycle ability because the inactive AB5 alloy can accommodate the large volume changes of Si nanoparticles distributed on the surface of the Si–AB5 composite during cycling. In particular, the Si–AB5 composite containing 20 wt% Si with the high tap density of 2.8 g/cm3 obtained after ball-milling for 11 h exhibits an initial and maximum reversible (charge) capacity of 370 and 385 mAh/g. The high capacity retention can be achieved after 50 cycles in the potential range from 0.02 to 1.5 V. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Composite; Lithium-ion battery; Anode materials; Silicon; Ball-milling

1. Introduction As anode materials for the lithium-ion battery, silicon possesses a maximum theoretical capacity of 4200 mAh/g (Li4.4Si) [1–4], which is a significant improvement over that of 372 mAh/g of graphite. However, silicon has not yet been applied to commercial lithium-ion batteries due to the mechanical failure of active materials caused by a large volume change during cycling. It is well known that a 300% volume increase is accompanied to the lithium alloying process with Si to form Li4.4Si [5–7]. The dramatic volume variation could pulverize the electrode, and then the capacity fade is remarkably rapid. Considerable effort has been made to overcome this limitation by synthesizing intermetallic silicide alloys with less active or inactive metal elements such as Ca–Si [8], Fe–Si [9–11], Ni–Si [12], Co–Si [13], Mg–Si [14], Cu–Si [15,16], Ag–Si [17] as well as multicomponent Si–M (M = Cr + Ni, Fe, Mn) [18]. Another approach is the use of dual-phase composites consisted of active Si and inactive materials such as Si–SiC [19], Si–

TiC [20,21], Si–TiN [22] and Si-TiB2 [23]. The dispersed inactive materials may accommodate the large volume expansion of the active Si, as a result, the structure of the composites can be retained during the lithium alloying and dealloying processes. Because the limited inner space of lithium ion battery, it is also important to find the composites with the high tap density as anode materials. The higher tap density will lead to higher volumetric capacity in practical cells [24,25]. The aim of this investigation is to utilize the AB5-type hydrogen storage alloy with the high tap density (4.54 g/cm3 for ground powders [26]) and to improve the cycle ability of silicon active materials. In this work, the AB5 alloy was chosen as an inactive material for obtaining the Si–AB5 composites by ball-milling. It is believed that the Si–AB5 composites with Si nanoparticles distributed uniformly on the surface of AB5 particles should show an improved electrochemical performance in view of good electric contact for the Si particles. 2. Experimental

*

Corresponding author. Tel./fax: +86 22 23500876. E-mail address: [email protected] (X.P. Gao).

1388-2481/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.10.056

Si–AB5 composites were synthesized by ball-milling AB5 alloy (MmNi3.6Co0.7Al0.3Mn0.4) and Si powders

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(200 nm). The ball-milling was carried out in a planetary ball mill (Fritsch P-6). The weight fraction of Si powders in Si–AB5 composites was 0%, 10%, 20% and 30%, respectively. The ball-milling was performed with a rotation rate of 250 rpm for 11 h in a charge ratio of 20/1. The powders produced were characterized by X-ray diffractometer (XRD, Rigaku D/max-2500). The morphology and microstructure of ball-milled powders were observed with a scanning electron microscope (SEM, Hitachi S3500N) and a transmission electron microscope (TEM, FEI Tecnai 20). To measure the tap density, 20 g powder was placed in a small glass vial and was tapped on the lap bench for 30 min by tap-density apparatus. The tap density was calculated from the weight and volume of the powders in the measuring cylinder. Electrodes were prepared by 70 wt% active material, 20 wt% acetylene black and 10 wt% polytetrafluoroethylene (PTFE). Lithium metal was used as the counter and reference electrode. The electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate (EC), propylene carbonate (PC), and dimethyl carbonate (DMC). The volume ratio of EC:PC:DMC in the mixture was 6:3:1. The cells were assembled in an atmosphere of high-purity argon in a glove box. Discharge-charge measurements of the cells were carried out at the current density of 50 mA/g with a cut-off potential of 0.02/1.5 V versus Li+/Li using a LANDCT2001A instrument. Cyclic voltammograms (CVs) were obtained using CHI 600A electrochemical workstation at room temperature. 3. Results and discussion XRD patterns of Si powders, AB5 alloy and ball-milled Si–AB5 composites are given in Fig. 1. It is clear that Bragg peaks of Si powders and AB5 alloy become slightly broad after ball-milling for 11 h. No any peaks of silicon can be observed, suggesting the formation of amorphous or nano-

Intensity

Original AB5

Si powders Ball-milled AB5 Ball-milled (AB5+10%Si) Ball-milled (AB5+20%Si) Ball-milled (AB5+30%Si)

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2θ Fig. 1. XRD patterns of Si powders, AB5 alloy and Si–AB5 composites after ball-milling.

crystalline Si in the Si–AB5 composites. The broad peaks of AB5 alloy indicate the introduction of microstrain and defects, and reduction in grain size in the Si–AB5 composites induced by the ball-milling process. Hence, ball-milling provides an effective route to form composites containing nanocrystalline Si and fine AB5 particles. It is also shown that there is no any undesirable phase formed at the Si and AB5 interfaces of the composites. In addition, compared the XRD patterns of the ball-milled AB5 alloy with those of Si–AB5 composites, it can be concluded that the addition of Si with the increased content into the composites leads to the decrease of the grain size of the AB5 matrix. SEM images of Si powders, AB5 alloy and ball-milled Si–AB5 composites are shown in Fig. 2. In the case of Si powder, fine Si particles with 200–400 nm tend to form large agglomerates. The edges and corners of the AB5 particle surface become unclear after ball-milling. In the ballmilled Si–AB5 composites with 10 wt% Si, many small Si particles coexist separately with larger AB5 particles. When the Si content exceeds 20 wt%, it is apparent that Si particles are well attached tightly to the AB5 particle surface and embedded subsequently in the AB5 matrix by ball-milling. Therefore, the addition of Si with the increased content during ball-milling is beneficial to formation of the composites with the fine Si particles distributed uniformly on the surface of the AB5 matrix. TEM images of the Si nanoparticles collected after the ultrasonic treatment of ball-milled Si–AB5 composite containing 20 wt% Si in ethanol solution are illustrated in Fig. 3. It can be seen that the ball-milled Si collected have irregularly shaped particles up to 80–120 nm in size as shown in TEM image (Fig. 3(a)). Although the most of Si nanoparticles have an amorphous feature after ball-milling, a few of small grains of about 10 nm in size can be observed locally inside the amorphous particle as shown in Fig. 3(b) (HRTEM). The calculated interference fringe spacing of Si from the intensity line profile for a selected area of the Si grain as inserted in Fig. 3(b) is about 0.32 nm, which is consistent with the interplanar distance of (1 1 1) planes of Si in the XRD pattern. The cycle performance of the Si–AB5 composites is shown in Fig. 4. For the ball-milled AB5 alloy, a very low electrochemical capacity could be obtained, indicating the electrochemical inactivity of the ball-milled AB5 alloy. It is can be seen that the initial electrochemical capacity of the composites increases with the addition of the enhanced Si content into AB5 alloy. However, the electrochemical capacity decreases drastically in the second cycle for all the composites and the first cycle efficiency is only about 50%. Compared with rapid decay of the Si–AB5 composite with 30 wt% Si, the electrochemical capacity turns to be stable after the second cycle for the Si–AB5 composite containing 20 wt% Si. The discharge capacity of 420 mAh/g is obtained and the capacity retention of 72% can be retained after 50 cycles. Importantly, the corresponding tap density of the Si–AB5 composite containing

X.N. Zhang et al. / Electrochemistry Communications 9 (2007) 713–717

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Fig. 2. SEM images of Si powders (a), AB5 alloy (b) and Si–AB5 composites after ball-milling (c) AB5 + 10% Si, (d) AB5 + 20% Si and (e) AB5 + 30% Si.

20 wt% Si is measured to be as high as 2.8 g/cm3, which is much higher than that of commercialized anode materials. The higher tap density of the composite is useful for improving the volumetric energy density of lithium ion battery. The cycle stability of the electrode is directly correlated to the composite structure. The improvement of the cycle stability of the Si–AB5 composite containing 20 wt% Si could be attributed to a high dispersion and a fixation of active Si embedded in the AB5 matrix, which can alleviate the volume variation of the composite during cycling and reduce the mechanical stress within the electrode. In addition, the lack of pulverization cracks within the active/inactive composite structure also supports the reasoning for the reversible cycling [20]. The rapid capacity fading of the Si– AB5 composite with 10 wt% Si corresponds to the Si agglomerates separately between larger AB5 particles,

which could not be embedded in the AB5 matrix as shown in SEM observation (Fig. 2(c)). Usually, the active materials with a lower content are difficult to construct composites with a uniform distribution of active and inactive materials during ball-milling process. The similar phenomenon was also found in LaMg12–Ni composites with a lower Ni content during ball-milling process [27]. When Si content of 30 wt% is added into the composite, it seems that there is insufficient surface area of AB5 particles to depress the agglomerate of Si nanoparticles, and then to buffer the volume expansion of the Si domain during lithium alloying and dealloying processes. Therefore, it is assumed that the rapid capacity fading of the Si–AB5 composite with 30 wt% Si may be caused by the pulverization of Si agglomerates in the composites to some extent. As a result of the pulverization, the electric contact between active particles or active/inactive interfaces turns bad grad-

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Fig. 3. TEM image (a) and HRTEM image (b) of Si nanoparticles collected after the ultrasonic treatment of the Si–AB5 composite containing 20 wt% Si. The intensity line profile is inserted in Fig. 3(b).

ually, leading to a poor utility of Si particles. This effect seriously compromises the cycle stability of the composite with high Si content due to the loss of electronic particle to particle contact and increase in an electrical resistivity [28]. Therefore, the cycle performance of the Si–AB5 composite strongly depends on the ratios of AB5 alloy to Si powders. The discharge and charge profile of the Si–AB5 composite containing 20 wt% Si is indicated in Fig. 5. It is found that the potential curve of the first discharge is obviously different from the second one. The Si–AB5 composite electrode delivers the discharge capacity of about 800 mAh/g in the first cycle, and the discharge and charge capacities of about 420 and 370 mAh/g in the second cycle, respectively. The maximum reversible (charge) capacity of the Si–AB5 composite electrode is 385 mAh/g after 9 cycles. The irreversible capacity is mainly used to form a solid electrolyte interface (SEI) film on the surface of the electrode. A discharge potential plateau at 0.6–0.8 V can be observed in the first discharge, corresponding to the formation of SEI film [17,29]. It has been reported that when lithium ions are inserted into amorphous Si, sloping potential plateaus are observed during the second and subsequent charges [30,31]. For the Si–AB5 composite, no distinctive discharge plateaus are changed further from the second cycle, indicating the characteristic of amorphous Si. This observation is in good agreement with the above XRD and TEM analysis of the composite. Cyclic voltammograms (CVs) of the Si–AB5 composite containing 20 wt% Si for the first five cycles are shown in Fig. 6. One cathodic peak at 0.6 V in the first cycle, which is related to the discharge potential plateau in Fig. 5, disappears in the following cycles. This irreversible reaction is associated with SEI formation to a great extent [32–34]. The ball-milling enlarge the surface area and lattice defects of Si–AB5 composite greatly, which capture more charges for the SEI formation, leading to the appearance of the

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solid: discharge open: charge

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Cycle number (n) Fig. 4. Cycle performance of ball-milled AB5 alloy and Si–AB5 composites.

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Capacity (mAh/g) Fig. 5. Charge and discharge curves of the ball-milled Si–AB5 composite containing 20 wt% Si.

X.N. Zhang et al. / Electrochemistry Communications 9 (2007) 713–717

0.5

5th 4th 3th 2nd

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References

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Potential (V, vs. Li+/Li) Fig. 6. Cyclic voltammograms of the ball-milled Si–AB5 composite containing 20 wt% Si.

large irreversible cathodic peak. The SEI film formed will be stable under subsequent lithium alloying and dealloying processes [35]. It is evidenced by the disappearance of the cathodic peak of 0.6 V from the second cycle. Two anodic peaks, located at 0.34 and 0.52 V, respectively, gradually evolve from the first cycle, and become more distinct after the following cycles. The observed two pairs of redox peaks, indexed as b/b 0 and b/b00 , are related to the alloying/dealloying process of LixSi alloys with different compositions. The observation in cyclic voltammograms is similar to that of thin-film Si electrodes and Si–C electrodes [6,16,35]. 4. Conclusions The Si–AB5 composites as anode materials, comprised of an electrochemically active Si and an inactive AB5 alloy, were synthesized by ball-milling. The inactive AB5 alloy can accommodate the large volume changes of Si nanoparticles distributed on the surface of Si–AB5 composites during cycling. The optimization of suitable ratio of AB5 alloy to Si powders can improve the cycle performance of the Si– AB5 composite. The Si–AB5 composite containing 20 wt% Si provides the discharge capacity of 420 mAh/g and a good capacity retention (72%) after 50 cycles, which suggests that the Si–AB5 composite with the high tap density is a prospective anode material for lithium ion battery with a high volumetric energy density in future. Acknowledgement This work is supported by the 973 Program (2002CB211800), the NCET (040219) and the CPSF (20060390665), China.

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