graphite composites as anode material for lithium ion batteries

graphite composites as anode material for lithium ion batteries

Electrochemistry Communications 6 (2004) 465–469 www.elsevier.com/locate/elecom Carbon-coated nano-Si dispersed oxides/graphite composites as anode m...

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Electrochemistry Communications 6 (2004) 465–469 www.elsevier.com/locate/elecom

Carbon-coated nano-Si dispersed oxides/graphite composites as anode material for lithium ion batteries Heon-Young Lee, Sung-Man Lee

*

Department of Advanced Materials Science and Engineering, Kangwon National University, Chuncheon, Kangwon-Do 200-701, Republic of Korea Received 13 February 2004; received in revised form 9 March 2004; accepted 9 March 2004 Published online: 9 April 2004

Abstract A carbon-coated nano-Si dispersed oxides/graphite composite has been prepared and investigated as a potential anode material for lithium ion batteries. The nano-Si dispersed oxides were synthesized by mechanochemical reduction of SiO by Al, and carbon has been coated onto the ball milled mixture of nano-Si dispersed oxide/graphite by utilizing pyrolysis of coal–tar pitch at 900 °C in an argon flow. The carbon-coated composites show excellent cycling performance with a reasonable value of the first irreversible capacity. The superior electrochemical characteristics are attributed to the uniform distribution of nano-sized Si phase, the buffering action of graphite and an enhanced connection for electronic and ionic conduction by carbon-coating. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Lithium ion battery; Mechanochemical reduction; Anode materials; Carbon-coating

1. Introduction Recently, there has been considerable interest in finding new electrode materials for a new generation of lithium ion batteries, which might have high energy density compared with the existing system. Silicon is attractive as an alternative anode material due to its very large lithium insertion capacity. However, the large volume change during charge and discharge reactions results in poor cyclability. Many researchers, therefore, have focused on improving the cycle performance of Sibased systems [1–8]. It has been reported that carboncoated silicon, prepared by coating carbon onto the surface of silicon particles using thermal vapor deposition method, shows reversible capacity as high as 1000 mA h/g and improved cycle stability under controlled lithium insertion conditions [4,5]. Our previous study showed that the cyclability of the Si alloys was significantly improved by mechanical mixing with graphite [9]. *

Corresponding author. Tel.: +82-33-250-6266; fax: +82-33-2426256. E-mail address: [email protected] (S.-M. Lee). 1388-2481/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2004.03.005

Based on these studies, it seems to be concluded that the composite structure including active Si phase may be optimized when nano-sized Si elements are dispersed uniformly in a ductile inactive matrix and mixed with graphite powders, followed by carbon-coating. Here, the carbon-coating can be simply done by pyrolysing the carbon precursors to be carbonized through a liquid phase with nano-Si dispersed oxide/graphite mixture [10]. Therefore, preparation of nano-Si/inactive matrix composite particles is a key factor in developing the carbon-coated nano-Si composite material. It is believed that reduction of metal oxides by mechanical alloying method is an ideal method to homogeneously disperse Si fine particles in situ into oxide matrix [11,12]. During ball milling the mixture of SiO (or SiO2 ) and Al powders, the mechanically driven reduction reaction of SiO (or SiO2 ) + Al ! Si + Alx Oy can be induced, which leads to a nano-Si/oxide composite particles. This method was applied to obtain nano-Si dispersed oxide particles. In the present study, we have developed a carboncoated nano-Si dispersed oxide/graphite composite material and investigated its electrochemical performance as anode material for lithium ion batteries.

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2. Experimental

3. Results and discussion Preliminary experiments showed that the addition of Li2 O2 improved the electrochemical properties such as the cycle life of the resulting electrode. Hence, we included Li2 O2 in starting mixture for the mechanochemical reduction reaction, although the exact function of Li2 O2 is not clear at present. Fig. 1 shows X-ray diffraction (XRD) patterns of the SiO–Al–Li2 O2 powder mixture milled for different times. Before milling, pattern consists of a weak broad signal in the range of 15–30° associated with the amorphous SiO signal, and of sharp peaks for Al and Li2 O2 . After 6 h of milling, the Si peaks are observed while the Li2 O2 diffraction peaks disappear and the relative intensity of the Al peaks is reduced. The intensity of Si

15h milling

Intensity (Arb.Units)

The nano-Si dispersed oxide particles were prepared by mechanical ball milling the mixture of SiO (or SiO2 ), Al and Li2 O2 powders using a SPEX-8000 high-energy mechanical mill. Commercial elemental powders of SiO (Aldrich – 325 mesh), Al (99%) and Li2 O2 (99%) were used as starting materials. The starting materials were weighted (SiO:Al:Li2 O2 ¼ 1:1:0.2 in molar ratio), mixed and loaded into a stainless-steel vial containing stainlesssteel balls. All the process were conducted inside the glove box filled with argon. Carbon-coated Si dispersed oxide/graphite composite material was prepared as follows. The Si dispersed oxide particles obtained by mechanical reduction process were mixed (1:1 by weight) with graphite and then the mixture was milled with coal–tar pitch followed by heating at 900 °C for 1 h under argon atmosphere. The carboncoated product was ground using variable speed rotormill (Fritch, pulverisette 14) to control the size and shape of the composite material. The sample was characterized by X-ray diffraction (XRD) with Cu-Ka radiation to identify the phases formed. Morphology of the sample was observed by a scanning electron microscope (SEM). Electrodes were prepared by pasting a slurry containing 85 wt% active materials, 5 wt% carbon black and 10 wt% polyvinylidene fluoride (PVDF) as a binder, dissolved in N-methyl-2-pyrrolidone (NMP) onto a copper mesh. The electrodes were then dried overnight at 120 °C under vacuum and then pressed. The electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by volume, provided by Cheil Industries Inc., South Korea). Half-cells were assembled in an argon-filled glovebox and galvanostatically charged and discharged in the voltage range 0.0–1.2 V vs. Li/Liþ using a current density of 0.2 mA/cm2 .

Si Al Al2O3 Li2O3

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Diffraction Angle (2θ) Fig. 1. X-ray diffraction patterns of SiO:Al:Li2 O2 ¼ 1:1:0.2 composites milled for 0, 6 and 15 h.

diffraction peaks increased with increasing milling time up to 15 h and the peak corresponding to Al2 O3 was also observed. The crystallite size of Si appears to be about 16 nm, as estimated from the diffraction peaks of Si as per [13]. At this point, it could be argued that the nano-sized crystalline Si particles are formed with oxides through the reduction of SiO by Al using a ball-milling method although the stoichiometry of the resultant oxides is not identified at present. Hereafter this material will be denoted as nano-Si dispersed oxide. On the one hand, for the sample milled up to 15 h, any XRD peaks related to Al have not been observed. The starting powder mixture includes a little excessive amount of Al compared to the calculated for complete reduction of SiO, in which the remained Al would react with Li2 O2 . In fact, on milling more than 15 h, the intensities of the Al2 O3 peaks slightly increased without any increase of Si peak intensity. It is therefore assumed that the effect of residual SiO and Al on the electrochemical properties would be negligible. We prepared a composite material by milling the mixture of nano-Si dispersed oxide and graphite at the weight ratio of 1:1. Fig. 2 shows the discharge capacity as a function of cycle number for the composite electrodes obtained by milling for different times. The capacity retention is very good for at least 40 cycles with a

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Fig. 2. Discharge capacity vs. cycle number of nano-Si dispersed oxide/ graphite composites with milling time.

reversible capacity over 600 mA h/g. However, the initial capacity loss is considerable, which increases with milling time for the preparation of the composites. This irreversible capacity occurs mainly near 0.8 V as shown in Fig. 3, which is caused by side reactions with electrolyte at the surfaces of the graphite [14,15]. It has been found that carbon-coating onto graphite particles is very effective in reducing the first irreversible capacity associated with the surfaces of graphite particles [10,16–18]. Fig. 4 shows the XRD pattern of carbon-coated composite powder which was prepared by heating the mixture of composite material and coal–tar pitch (70 wt%) at 900 °C in an argon flow. For comparison, the XRD pattern of the composite powders before carboncoating is also given. It is worthwhile to note that the broad signal of Si peak remained even after heat treatment for carbon-coating, while diffraction peaks for

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Fig. 3. Discharge and charge curves of nano-Si dispersed oxide/ graphite composites with milling time.

Fig. 4. X-ray diffraction patterns of (a) nano-Si dispersed oxide/ graphite composites (5 min milling), (b) carbon-coated nano-Si dispersed oxide/graphite composites (70 wt% coal–tar pitch, 900 °C for 1 h under argon atmosphere).

Al2 O3 became visible. This indicates that the carboncoated composite material includes nano-sized Si. Here the Si phase is truly nano-crystalline (20 nm) as determined from peak broadening analysis [13]. Fig. 5 shows SEM micrographs of the nano-Si dispersed oxide, the nano-Si dispersed oxide/graphite composite and the carbon-coated composite. In the case of nano-Si dispersed oxide/graphite composite, oxide particles were uniformly distributed in the graphite. Notice that the carbon-coated composite was obtained by manual grinding followed by milling using a speed rotor-mill at 12000 rpm, which leads to the formation of spherical, agglomerated particles. The first cycle discharge–charge curves of the carbon-coated composite electrode is given in Fig. 6. As expected, the irreversible capacity is significantly reduced by carbon-coating treatment. It can be also seen that the plateau at around 0.45 V in the charge curve is more visible after the carboncoating treatment. This plateau is due to the Li extraction from the Lix Si alloy [5]. It can be considered that the enhanced plateau is attributed to the increase of Si crystal size after carbon-coating treatment, as measured from XRD data. The cycling performance is also further improved compared with that for bare composite of Fig. 2, as shown in Fig. 7. These results demonstrate an evident effect of carbon-coating treatment on the improvement of the electrochemical performance of nano-Si dispersed oxide/graphite composite. The coal–tar pitch carboncoating effectively depresses the irreversible side reactions by covering the active sites for electrolyte decomposition. Moreover, the resultant powder shows a spherically aggregated shape with reduced specific surface area. As a result of carbon-coating treatment, it is also supposed that the core active material of nano-Si

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Fig. 5. Scanning electron micrographs of (a) nano-Si dispersed oxide, (b) nano-Si dispersed oxide/graphite composite, (c) carbon-coated nano-Si dispersed oxide/graphite composite obtained by manual grinding after carbon-coating, (d) carbon-coated nano-Si dispersed oxide/graphite composite, manually ground and then milled by the speed rotor-mill.

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Fig. 6. Discharge and charge curve of carbon-coated nano-Si dispersed oxide/graphite composite.

based oxide is more closely connected with graphite particles, and the electrical conductivity of the composite material is enhanced. In situ formed inactive oxides and the graphite act as a buffering matrix for Li–Si alloying of nano-Si active material during cycling. Therefore, the excellent electrochemical performance is attributed to the combined effects of the uniform distribution of nano-Si phase in inactive oxides, the graphite mixing and the carbon coating.

Fig. 7. Capacity vs. cycle number of carbon-coated nano-Si dispersed oxide/graphite composite.

4. Conclusions A nano-Si dispersed oxide is synthesized by mechanochemical reduction induced by ball milling of the powder mixture of SiO and Al. The composites, obtained by ball milling the nano-Si dispersed oxide/ graphite mixture, show a stable capacity retention during cycling, but ball milling of graphite leads to initial irreversible capacity loss. It appeared that carbon-coat-

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ing treatment on the composites provides improved cycling ability of the composites with reduced irreversible capacity. The superior properties are attributed to the combined effects of the uniform distribution of nano-Si phase in inactive oxides, the graphite mixing and the carbon-coating.

Acknowledgements This research was supported by University IT Research Center Project.

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