Preparation and Characterization of Porous Silicon and Carbon Composite as an Anode Material for Lithium Rechargeable Batteries

Preparation and Characterization of Porous Silicon and Carbon Composite as an Anode Material for Lithium Rechargeable Batteries

Junsoo Park and Jae-won Lee ^a, *

LG Chem. Ltd., Daejeon 305-380, Korea

a

Department of Energy Engineering, Dankook University, Cheonan-si 330-714, Korea (Received December 10, 2014; Revised January 30, 2015; Accepted February 10, 2015)

···

Abstract The composite of porous silicon (Si) and amorphous carbon (C) is prepared by pyrolysis of a nano-porous Si + pitch mixture. The nano-porous Si is prepared by mechanical milling of magnesium powder with silicon monoxide (SiO) followed by removal of MgO with hydrochloric acid (etching process). The Brunauer-Emmett-Teller (BET) sur- face area of porous Si (64.52 m

g

) is much higher than that before etching Si/MgO (4.28 m

g

) which indicates pores are formed in Si after the etching process. Cycling stability is examined for the nano-porous Si + C composite and the result is compared with the composite of nonporous Si + C. The capacity retention of the former composite is 59.6%

after 50 charge/discharge cycles while the latter shows only 28.0%. The pores of Si formed after the etching process is believed to accommodate large volumetric change of Si during charging and discharging process.

Keywords: Porous, Silicon, Anode, Lithium ion battery, Composite

···

1. Introduction

The limited capacity of graphite which is the most widely used anode material for lithium ion batteries has provoked numerous studied seeking alternative materi- als. Much attention has been given to Si-based anode materials because silicon has the high specific capacity of 4200 mAh g

. However, silicon undergoes a change in volume during charging and discharging, and this results in mechanical instability and poor cycle life performance [1,2]. The mechanical stresses induced by the large volu- metric change should be buffered in order to obtain bet- ter cyclability with silicon-based anodes. Various carbonaceous materials such as graphite [3-7], carbon nanofiber [8-11]

and amorphous carbon [12-15] have been suggested as an electrochemically active component which constitutes a composite with Si for buffering the volumetric change.

Several metallic compounds with high mechanical strength and electronic conductivity have been also proposed to suppress Si volume expansion [16-20].

Porous Si is also considered as one of the candidates to improve the cycling stability. Zhihao et al. reported

microporous silicon replicas with an interconnected net- work of silicon nanocrystals and 3-dimensional frustule mor- phology by simple magnesium reduction method [21]. Chen et al. synthesized Si/MgO composites with reasonable cycling stability by magnesium reduction method [22]. Recently, Ying et al. investigated the electrochemical performances of nano-porous Si prepared by an aluminum redox reaction [23]. The nano-porous Si/graphite/C composite exhibited a reversible capacity of about 700 mAhg

with no capacity loss up to 120

cycle.

In this work, we investigated the electrochemical per- formances of the porous Si + C composite prepared by mechanical milling of SiO with magnesium powder fol- lowed by pyrolysis of pitch. Another composite contain- ing nonporous Si was prepared by same procedure and the properties were compared with those of the porous Si + C composite.

2. Material and Methods

2.1. Synthesis of materials

Si/MgO composite powder was synthesized by mechani-

*Corresponding Author: Jae-won Lee, TEL: +82-41-550-3682, FAX: ,+82-41-559-7914 E-mail: [email protected]

<PM리뷰>

obtained after the milling was put into hydrochloric acid solution while stirring. The etching process lasted for 8 h followed by washing, filtering and drying under vacuum at 80

C for 24 h. By this process, MgO in the composite was dissolved and nano-porous Si (denoted as PSi here- after) was formed.

The obtained PSi was added in a pitch solution in tet- rahydrofuran (THF), and dispersed ultrasonically. The pitch solution containing PSi was dried at 80

C in a con- vection oven for 12 h and finally calcined at 900

C under Ar flow for pyrolysis of the pitch. Nonporous micro- sized Si (B&C, Japan) (denoted as MSi hereafter) under- went the same procedure to prepare MSi + C composite for reference.

2.2. Analysis

The crystal structure of the obtained powder was exam- ined by powder X-ray diffraction (XRD, Rigaku D/Max- 2500/PC, Japan) using a Cu K α radiation source. The particle size was analyzed using a particle size analyzer (PSA, Partica LA-950V2, Horiba, Japan) and the parti- cle morphology was observed by field emission scan- ning microscopy (FE-SEM, S-800, Hitachi, Japan).

2.3. Electrochemical performance test

A coin-cell (CR2016) was used to determine the elec- trochemical performance of the obtained samples. A neg- ative electrode was made by coating a slurry of the active material, Ketjen black and a polyvinylidene fluoride (PVdF) binder (Kurea, Japan) at a weight ratio of 80:10:10 onto a copper foil current collector. The electrolyte used was a 1.0 M LiPF

solution in ethylene carbonate/ethyl-methyl carbonate (EC/EMC) (1/1 vol%) (Cheil Ind., Korea). The galvanostatic charge and discharge cycle tests of the cell were carried out at room temperature between 0.01 V and 1.5 V. The cell was charged (lithiation) with con- stant current-constant voltage (CC-CV) mode at 0.1C

3. Results and Discussion

Fig. 1 presents the XRD pattern of the Si/MgO mix- ture obtained after mechanical alloying and PSi obtained after the etching process. The strong peaks in Fig. 1(a) correspond to Si and MgO, and no peaks of metallic magnesium are identified. In addition, some weak peaks are detected, which are assignable to Mg

SiO

. These materials should be the by-products of reaction (1).

SiO + Mg → Si + MgO (1)

In Fig . 1(b), diffraction peaks for MgO and Mg

SiO

are not observed which indicates these compounds were almost removed in the etching process. Considering the intensity and sharpness of the peak in this figure, Si parti- cles with high crystallinity was obtained in this process.

The SEM images of PSi is given in Fig. 2. Sub-microme- ter sized particles are aggregated to form secondary parti- cles. Looking into the surface of the particles with higher magnification, nanometer sized particles are observed to be aggregated and the mesopores are supposed to exist among the nanoparticles.

Fig. 1 X-ray diffraction (XRD) pattern of (a) Si/MgO and (b)

porous Si (PSi) obtained after the etching process.

Specific surface area (SSA) measured by Brunauer- Emmett-Teller (BET) method are listed in Table 1. The SSA for PSi is 64.52 m

g

, which is much larger than 4.28 m

g

for the Si/MgO composite. The BJH pore size distribution for PSi is shown in Fig. 3. Most pores have diameter ranging from 9 to 15 nm showing 9.31 nm of d

value. These pores are believed to be formed from removal of MgO nanoparticles embedded in the Si/MgO composites.

Assuming no reaction between the pitch and Si, the sil-

icon content in the PSi/C composite is calculated to be 18.2 wt%.

Fig. 4 presents the charge and discharge profile of composites (PSi/C and MSi/C) at the first cycle. PSi/C composite shows the first charging capacity of 1012 mAh g

and discharging capacity of 607 mAh g

(Coulombic efficiency = 60.0%) while MSi/C composite exhibits 1300 mAh g

and 879 mAh g

(Coulombic efficiency = 67.6%) for the first charge and discharge respectively. The first charge and discharge capacity of PSi/C composite is less than that of MSi/C, which may be due to hidden impuri- ties – possibly amorphous oxidized product of SiO

gen- erated from the milling process – unidentified from XRD analysis. PSi/C composite shows lower Coulombic effi- ciency than MSi/C composite. In general, electrode mate- rial with large surface area has low Coulombic efficiency because of active side reaction consuming lithium ion. It should be noted that the charge capacity represented at the interval of CC-mode is larger for PSi/C than MSi/C composite, which indicates the resistance in lithiation process is lower for PSi/C than MSi/C composite. The shorter lithium diffusion length of PSi/C composite together with enhanced electronic conductivity due to carbon in the pores of silicon can be the reasons for the lower resistance in lithiation. Cycling stability of the composites is shown in Fig. 5. The reversible capacities of PSi/C and MSi/C composite retained after 50 cycles are 378 mAh g

(59.6% capacity retention) and 255 mAh g

(28.0% capacity retention). These cycling stabil- ity curves distinctly show that the capacity fading of Fig. 2. Scanning electron microscope (SEM) images of PSi.

Table 1. The specific surface area of the Si/MgO composite before and after the etching

Sample Surface area (m

g

) Pore size (nm)

Before etching 4.28 21.21

After etching 64.52 9.31

Fig. 3. BJH pore size distribution of PSi.

Fig. 4. Charge-discharge profile of the 1

cycle of the PSi/C

and MSi/C composite.

racks appear to be developed from the initial in case of MSi/C electrode because there is no buffer to absorb the volumetric change during cycling.

The microstructural changes before and after the cycling tests are seen in the SEM images of PSi/C and MSi/C (Fig. 6). We can see that the composite PSi/C

Fig. 5. Cycling stability of the PSi/C and MSi/C composite.

(current density: 0.1C charge/0.1C discharge)

Fig. 6. The morphological change of the composite electrodes:

(a) PSi and (b) MSi after 50 cycles.

Fig. 7. Cyclic voltammogram of (a) PSi/C and (b) MSi/C

composite.

electrode keeps the homogeneous surface morphology and shows less cracks than MSi/C electrode even after 50 cycles. The structure of PSi/C composite is more endur- able than MSi/C composite against volumetric change of Si during charge and discharge and this structural stabil- ity might give less cracks. The cracks at the surface of MSi/C electrode are expected to break electronic conduc- tive path and cause severe capacity fading.

The cyclic voltammograms in Fig. 7 show electro- chemical behavior of the composites during charging and discharging in the potential range from 0.0 to 3.0V. No peaks around 0.6 to 1.5 V which represent electrolyte decomposition did not appear for both the composite. In the case of PSi/C, a cathodic current is observed at about 0.3 V at the first and second scanning, which corre- sponds to electrochemically driven amorphization, while

no peaks are found with MSi/C composite at this region.

The 50

scanning shows no obvious cathodic or anodic current peaks at certain voltage but weak current all over the range for both the composite.

Electrochemical impedance spectroscopy (EIS) data measured at frequency from 10 mHz to 100 kHz are shown in Fig. 8. The size of the semicircles is similar for the two composites at initial stage (after 1

cycle) but MSi/C composite shows larger increase in the size after 51

cycle. It means that increase in polarization resis- tance of MSi/C electrode is larger than that of PSi/C composite, which may be attributed to less degradation of the PSi/C electrode due to cycling stability. Resis- tance in lithium diffusion can be estimated from the EIS curve of the cycled electrodes at low frequency range.

PSi/C electrode exhibits lower resistance than MSi/C electrode, which is attributed to shorter lithium diffu- sion length of the porous structure of PSi/C composite and crack formation in MSi/C electrode during the cycling.

4. Conclusions

The effects of porous structure on the cycling stability of Si as an anode material were investigated by prepar- ing a composite of porous Si + carbon. The nano-porous Si was prepared by a simple magnesium reduction pro- cess and the etching process gave mesoporous Si with mean pore diameter of 9.31 nm. The BET surface area of porous Si (64.52 m

g

) was extremely higher than that of before etching Si/MgO (4.28 m

g

). The electrode prepared with the porous Si (PSi/C composite) exhibited improved cycling stability, compared with micro-sized Si (MSi/C). The PSi/C composite retained 59.6% of the first cycle after 50 cycles while MSi/C showed only 28.0% of the capacity retention. It is believed that the pores of Si formed after removal of MgO and Mg

SiO

can accom- modate the large volumetric change of Si during charge/

discharge process.

Acknowledgements

This study was supported by a grant from Chungcheong leading industry promotion project of the Korean Minis- try of Knowledge Economy.

Fig. 8. Electrochemical impedance spectra (EIS) of (a) PSi/C

and (b) MSi/C composite.

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