Graphite Composite as Anode for Li-ion Batteries

Electrochimica Acta 142 (2014) 11–17

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Si-SiOx -Cristobalite/Graphite Composite as Anode for Li-ion Batteries Yurong Ren a , Mingqi Li b,∗ a b

School of Materials Science and Engineering, Changzhou University, Changzhou 213164, China College of Chemistry and Chemical Engineering, China West Normal University, Nanchong 637009, China

a r t i c l e

i n f o

Article history: Received 6 May 2014 Received in revised form 2 July 2014 Accepted 18 July 2014 Available online 12 August 2014 Keywords: Li-ion battery Gentle synthesis Multicomponent silicon-based composite Anode Electrochemical performance

a b s t r a c t A gentle route is developed to fabricate multicomponent nanosilicon-based composite by using SiO, NaOH and graphite as starting materials. The composite is composed of nanosilicon, nonstoichiometric SiOx , cristobalite and graphite (Labeled as Si-SiOx -cristobalite/graphite). Used as anode material in Liion batteries, the Si-SiOx -cristobalite/graphite exhibits high lithium storage capacity with good cycling stability and rate capability. At a current density of 100 mA g−1 , the composite shows a stable reversible capacity of about 990 mAh g−1 (Calculated on the total mass of the composite) and the capacity retention is nearly 100% after 150 cycles. Even at 1600 mA g−1 , the stable discharge capacity of the composite is also over than the theoretic capacity of graphite. In the composite, cristobalite is inert to lithium, while the other components are involved in lithiation process during the first discharge. The excellent electrochemical performance of the Si-SiOx -cristobalite/graphite electrode is attributed to the uniform distribution of active components in inert media and the conductivity enhanced by graphite. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction Due to a high theoretical capacity, satisfactory operation potential and abundant resources, silicon is considered as one of the most attractive candidate anodes for high-energy Li-ion batteries. However, a large volume variation of silicon (∼300%) during lithium insertion/extraction process, which causes the pulverization of silicon particle itself and the peeling-off of electrode material from the current collector, severely influences its cycle performance[1]. To date, many attempts have been made to resolve the problems associated with the volume change of silicon. For example, the fabrication of silicon nanostructures, such as nanoparticles, nanotubes [2] and nanowires [3,4], the design of porous silicon [5–7], the encapsulation of nanosilicons in conductive coating layers [8–11] or the dispersion of nanosilicons in buffer matrix, have extensively been explored. Compared with pure nanosilicon, since nanosilicon-based composites, in which nanosilicon is dispersed or encapsulated in matrix with high conductivity and good elasticity, not only have a relatively small total volume expansion/contraction, but also can avoid the electrochemical sintering of silicon nanoparticles and relieve the side reactions between silicon and electrolyte during cycling, they are considered as the most promising silicon-based anode materials. However, in practice, it is

∗ Corresponding author. Tel.: +86 817 2321461. E-mail address: [email protected] (M. Li). http://dx.doi.org/10.1016/j.electacta.2014.07.101 0013-4686/© 2014 Elsevier Ltd. All rights reserved.

a large challenge how to realize the uniform distribution of silicon in dispersion media when nanostructured silicon is chosen as starting materials [11–16]. Theoretically, the size of silicon should be as small as possible to mitigate the inner strain caused by the volume change. Nevertheless, a less size means a higher specific surface energy, which makes it more difficult to uniformly disperse silicon in matrixes, especially in the case of a large-scale production [17]. Since the undispersed nanosilicon aggregations are liable to bring about the local structural destruction of the electrode, the advantages of nanosilicons is compromised. Additionally, nanosilicon is usually prepared by vapor deposition or sputtering technique, which not only has a high cost, but also is hard to scale up, severely affecting the wide application of its corresponding composites [18]. As an alternative to nanosilicon, using SiO as the precursor of nanosilicon-based composite materials can circumvent the above problems. Many studies [19–22] have demonstrated that SiO can be decomposed into nanosilicon and SiOx or amorphous SiO2 at high temperature, and the formed silicon is surrounded by SiOx . During the initial discharge, SiOx reacts with lithium to form elemental silicon, Li2 O and a series of lithium silicate, in which the latter two have good buffer to the volume effect of silicon [19,23–25]. However, the preparation route still needs further improvement because the thermal disproportionation temperature of SiO exceeds 900 ◦ C with a long annealing time, in which the produced nanosilicons are easily sintered to form big particles. In this work, a gentle route is developed to prepare multi-component nanosilicon composite using SiO, NaOH and graphite as starting materials. In comparison

Y. Ren, M. Li / Electrochimica Acta 142 (2014) 11–17

with the method mentioned above, the formation temperature for nanosilicon decreases to about 800 ◦ C and the thermostatic period is shortened to only about 10 min. Subsequently, simple mechanical ball milling is used to further reduce the particle size of the as-prepared Si-SiOx -cristobalite and mix it with graphite powder. Our experimental results show that the resultant Si-SiOx cristobalite/graphite exhibits a stable reversible capacity of about 990 mAh g−1 with excellent cycling stability and rate capability. 2. Experimental

O1s

Intensity / A.U.

12

(a)

Na1s OKLL

2.1. Synthesis of Si-SiOx -cristobalite

Si2s Si2p

Si-SiOx -cristobalite was synthesized by a modified method [20]. Typically, 6.0 g SiO (45 ␮m) was mixed with 0.4 g NaOH dissolved in 10 mL ethanol. After drying, the mixture was transferred to a tube furnace. The furnace was heated to 800 ◦ C at a rate of 5 ◦ C min−1 under an argon stream, and then remained at this temperature for 10 min. Subsequently, it was cooled down to room temperature. The as-prepared sample was washed with aqueous ammonia (pH = 9.5) to prevent the hydrolysis of sodium silicate formed during annealing, and then dried at 100 ◦ C in vacuum for 3 h.

NaKLL

1200

1000

800

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200

0

Binding Energy / eV CSP

Si

2.2. Synthesis of Si-SiOx-cristobalite/graphite

Si

(b)

3+

Si

Intensity / A. U.

2.8 g the as-prepared Si-SiOx - cristobalite and 1.2 g graphite (17 ␮m) were batched into 100 mL steel vial with a ratio of 1:10 as ball to powder. The ball milling was conducted in a planetary ball milling machine (QM-3SP2, Nanjing, China) for 4 h at a speed of 550 rpm under the protection of argon.

600

C1s

2+

4+

Si

1+

Si

108

106

104

102

0

100

98

Binding Energy / eV Fig. 2. XPS spectra of Si-SiOx -cristobalite (a) and fitting components of Si2p peak (b).

2.3. Material characterization and electrochemical measurement

Fig. 1. XRD patterns of SiO, Si-SiOx -cristobalite and Si-SiOx -cristobalite/graphite (a) and HRTEM image of Si-SiOx -cristobalite (b).

The crystal structures of samples were measured on an X-ray diffractometer (X’Pert MPD X) at 2500 V using Cu Ka radiation. The XRD measurements of the working electrode at different charge-discharge states were performed under the protection of argon. XPS analysis was performed by a multi-technique ultra-high vacuum Imaging XPS Microprobe system (Thermo VG Scientific ESCALab 250). The morphology and structure of samples were characterized by SEM (JEOL 5900LV) and TEM (JEOL JEM-2100F). The N2 adsorption and desorption isotherms were collected at 77 K with a Micrometrics ASAP2010 Gas Adsorption Analyzer. The electrochemical test was performed using 2032 coin-type cells. The tested electrode was made of 80wt% the as-prepared material as active material, 10wt% Super P as conductive agent, and 10wt% sodium alginate as binder. The loading mass of the active material on the electrodes was about 3-3.5 mg cm−2 (excluding binder and conductive carbon). The current collector was copper foil. The counter electrode and reference electrode were lithium foil. The electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate in 1:1:1 (V/V/V) ratio with 10wt% fluorinated ethylene carbonate (FEC) and 2wt% vinylene carbonate (VC) as the electrolyte additives. The separator

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13

2.00

(a)

E/V

1.75 1.50

1st

1.25

2nd

1.00 0.75 0.50 0.25 0.00 0

200

400

600

800

1000

Specific Capacity / mAh g

1200

1400

1600

-1

1.5

(b)

1-5

1.0 0.5

1st

I / mA

0.0 -0.5 -1.0 -1.5 -2.0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

curves

(b)

1.4

E/V Fig. 3. SEM images of Si-SiOx -cristobalite (a) and Si-SiOx -cristobalite/graphite (b). Fig. 5. Charge-discharge cristobalite/graphite.

was Celgard 2400 membrane. The charge–discharge measurements were carried out at different current densities with cutoff potentials of 1.5/0.0 V vs Li+ /Li. All current density and specific capacity were calculated on the total mass of the prepared composite. CV and EIS were performed using an M283 electrochemical workstation (EG&G Corporation). CV was conducted at a scanning

(a)

and

CV

of

Si-SiOx -

rate of 0.2 V s−1 over a potential window of 0.0-1.5 V. For the impedance spectra measurement, the utilized type of cell was two-electrode cell in which Li foil was adopted as the reference and counter electrode. After the cell reached a desired cycle number, open-circuit state was kept for 1 h to make the electrode reach an equilibrium status. Then, the electrochemical impedance measurements were carried out by applying an ac voltage of 10 mV over the frequency range from 100 kHz to 100 mHz. 3. Results and discussion

3

BJH Desorption dV/dw (cm /g)

0.0003

profiles

0.0002

0.0001

0.0000 0

20

40

60

80

100

120

140

160

180

Pore Width / nm Fig. 4. Barrete Joynere Halenda (BJH) analysis of desorption isotherm of the Si-SiOx cristobalite/graphite.

In order to synthesize nanosilicon-based composite, commercially available SiO powder was mixed NaOH and then annealed at 800 ◦ C for 10 min. Subsequently, the reaction products were washed by aqueous ammonia and dried in vacuum. From Fig. 1 (a), the raw SiO is amorphous, which is in agreement with the previous reports [26,27]. After annealing, the pronounced peaks for crystalline Si (PDF# 27-1402) and cristobalite (PDF# 39-1425) appear in the XRD pattern of the as-prepared composite. Based on Scherrer formula, the average crystallite size of Si and cristobalite is about 8 nm and 14 nm, respectively. The interplane spacing of (1 1 1), (2 2 0) and (3 1 1) lattice plane family for the formed nanosilicon is respectively about 0.31, 0.19 and 0.16 nm, which are consistent with values usually obtained for bulk silicon [5]. Many small crystalline nanoparticles can be observed in HRTEM image of the sample and they are surrounded by amorphous matrix,

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♦ Si

Graphite

• Cristobatile

♣ Li 2 O

♣

•

Charged to 1.50 V ♣

Intensity / A.U.

•

Discharged to 0.00 V •

♦

♣

Discharged to 0.25 V •

♦

Discharged to 0.40 V •

♦

Before cycling 10

15

20

25

30

35

40

2 Theta Fig. 6. XRD patterns of Si-SiOx -cristobalite/graphite at different charge-discharge states.

further confirming the formation of nanosilicon and cristobalite (Fig. 1 (b)). Park proposed that the chemical reaction occurring on the surface between SiO and NaOH might be as follows: 3SiO + 2NaOH = Na2 SiO3 + H2 ↑ + Si + SiO2 , and the amorphous SiO2 were liable to transform into cristobalite at the presence of NaOH [20]. XPS was also used to investigate the composition of the as-prepared composite. In order to eliminate the surface contamination, the sample was etched by Ar+ for 20 min. From Fig. 2 (a), in addition of Si and O, a weak Na peak is also observed, which can come from the uncleaned salt in the inner of material. Fitting analysis of the XPS for the sample shows that the ratio of Si to O is about 1: 0.89. The Si 2p peak can be split five peaks corresponding to various chemical states of Si0 , Si+ , Si2+ , Si3+ and Si4+ , the atomic concentrations of which are about 12.4%, 12.5%, 28.8%, 33.3%, and 13.0%, respectively (Fig. 2 (b)). The as-prepared Si-SiOx -cristobalite was further mixed with commercial graphite by high-energy mechanical milling to improve its conductivity. The prepared composite is labeled as SiSiOx -cristobalite/graphite. Fig. 3 is the SEM images of the Si-SiOx and Si-SiOx -cristobalite/graphite. The particle size of the Si-SiOx cristobalite is far less than 45 ␮m of the raw SiO and has a wide distribution. It was found that the annealed SiO became crisp and the neighbor particles were bound by the side product sodium silicate. During washing, with the dissolution of sodium silicate, the annealed SiO aggregates were scattered to small powder particles under stirring. After mixed with graphite by high-energy ball milling, the Si-SiOx - cristobalite is reduced in particle size. Combination with the XPS results, it can be calculated that the SiSiOx -cristobalite/graphite is composed of about 8.7wt% Si, 52.2wt%

Fig. 7. TEM images of Si-SiOx -cristobalite/graphite composite before cycling (a) and after 30 cycles ((b), (c) and (d) with different magnification times).

Y. Ren, M. Li / Electrochimica Acta 142 (2014) 11–17

15

100

2000 Si-SiOx-cristobatile/graphite

1800 1600

Si-SiOx-cristobatile

1400

pristine SiO/graphite

1200

graphite

90

(a) 80

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800 50

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Coulombic Efficiency / %

Discharge Specific Capacity / mAh g

-1

2200

400 40 200 30 150

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Cycle Number 2000

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-1

Specific Capacity / mAh g

(b)

Discharge capacity

90 80

Coloumbic efficiency

1400

70 1200

100 mA g 1000

100 mA g

-1 -1

-1

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50

-1

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Coulombic Efficiency / %

1600

0 100

Cycle Number Fig. 8. Cycle performance profile (a) and rate capability (b) of Si-SiOx -cristobalite/graphite.

SiOx , 9.1wt% cristobalite and 30wt% graphite. From the Barrete Joynere Halenda (BJH) analysis of desorption isotherm of the SiSiOx -cristobalite/graphite in Fig. 4, the composite possesses a wide size distribution of pore, containing mesopores and macropores, which is favorable to accommodate the expansion of silicon upon cycling. The charge-discharge profiles of Si-SiOx -cristobalite/graphite in the first two cycles are shown in Fig. 5 (a). The composite presents an initial discharge capacity of 1570 mAh g−1 and an initial charge capacity of 993 mAh g−1 , respectively. In the second cycle, the discharge capacity is 1024 mAh g−1 and the charge-discharge is about 983 mAh g−1 . The Si-SiOx -cristobalite/graphite has a slightly higher discharge potential plateau than graphite (close to 0 V), therefore the composite is safer as anode in a full cell. High reversible capacity and low lithium extraction voltage platform suggest that the composite is promising to become an anode material for highenergy lithium-ion batteries. Fig. 5 (b) shows the CV curves of the Si-SiOx -cristobalite/graphite in the initial five cycles. In the

first cycle, the two peaks at 1.18 V and 0.70 V correspond to the reductive decomposition of electrolyte and the formation of SEI film. The former comes from the additive FEC [28], while the latter arises from other solvents such as VC and EC. To understand these peaks located at 0.65-0 V, the XRD patterns of the working electrode at different charge-discharge states were recorded (Fig. 6). Before the electrode is discharged to 0.6 V, no change is observed in the XRD pattern (It is not given), indicating lithiation of the electrode material does not happen. When the potential of the electrode reaches 0.4 V, a broad and weak peak appears at 15-20o . As the discharge voltage descends to 0.25 V, a sharp peak assigned to crystalline Li2 O appears at about 30.7o . These phenomena should be associated with a series of reaction of SiOx with lithium. During the investigation of SiOx electrochemical mechanism, Sohn [29] found that SiOx was reduced to amorphous silicon by lithium and Li2 Si2 O5 and Li6 Si2 O7 phases were formed when the potential of the SiOx electrode reached 0.4 V during the first discharge. Li4 SiO4 phase was yielded at 0.15 V. Among these lithium silicates, Li2 Si2 O5

Y. Ren, M. Li / Electrochimica Acta 142 (2014) 11–17

120

(a) 100

before cycling 2nd cycle 100th cycle

80

Zim / Ω

phase was reversible, while the Li4 SiO4 and Li6 Si2 O7 phases were irreversible. When the discharge voltage reaches 0 V, the diffraction peaks assigned to both Si and graphite disappear but no new peak appears, indicating that the lithiation reactions of Si and graphite happen between 0-0.25 V and the formed alloy Lix Si is amorphous or low crystalline. When the cell is charged to 1.5 V, the diffraction peak assigned to Li2 O is still kept but the peak of crystalline silicon is not restored, suggesting that Li2 O is inert and crystalline silicon is transformed into amorphous silicon after the first cycle. Here, it should be mentioned that some literatures reported that the formed Li2 O during the lithiation of silicon oxide was amorphous rather than crystalline. In our studies, we found that the diffraction peak ascribed to Li2 O would disappear quickly if the cycled electrode was exposed to air, which might result from the chemical reaction between Li2 O and H2 O in air. Additionally, it can be seen that the diffraction peak of cristobalite has no change during lithium insertion/extraction process, meaning it does not take part in lithiation process. In CV curve for the first cycle, the two anodic peaks at 0.20 and 0.6 V mainly correspond to lithium extraction from Lix C and Lix Si, respectively [30]. Since the capacity contribution of Li2 Si2 O5 phase is relatively little, no obvious peak is observed. In order to further confirm the above reaction mechanism, TEM was used to examine the Si-SiOx - cristobalite/graphite before cycling and after 30 cycles (Fig. 7)). It can be clearly seen that Si-SiOx -cristobalite particles is mixed with graphite sheets and the morphology of the Si-SiOx -cristobalite/graphite does not show obvious difference before and after cycling. However, a significant change happens in the inner composition and structure of the SiSiOx -cristobalite. After cycling, most of the observed crystalline fingerprints before cycling disappear and large amount of quantum dots with a uniform distribution appear in the material, which are well in accordance with Yuan’ observation [31]. The decrease of crystalline fingerprints is because silicon transforms from crystalline to amorphous after cycling. These quantum dots correspond to lithium silicate and are expected to have a good buffer to the volume change of silicon altogether cristobalite and Li2 O. The cycle performance profiles of the samples at a current density of 100 mA g−1 with cutoff potentials of 1.5/0.0 V are shown in Fig. 8 (a). The Si-SiOx -cristobalite/graphite exhibits a stable reversible capacity of about 990 mAh g−1 and the capacity has been maintained in the tested 150 cycles. Although the coulombic efficiency for the first cycle is only 63.2%, it rapidly increases to nearly 100% after several cycles. In contrast, although the unannealedSiO/graphite delivers a discharge capacity of about 2100 mAh g−1 , its capacity cannot keep stable in subsequent cycles and is lower than the Si-SiOx -cristobalite/graphite’s after 40 cycles. The previous study has shown that although SiO can be reduced to elemental silicon by lithium during the first discharge, the particle size of some silicon is too big, which is disadvantageous over the structural stability of the electrode. Herein, the SiO/graphite shows a higher reversible capacity than the Si-SiOx -cristobalite/graphite in the initial 40 cycles is because cristobalite in the latter is inactive to lithium. However, single heat treatment of SiO is not enough for getting a good electrochemical performance. From Fig. 8 (a), the SiSiOx -cristobalite only presents a first discharge capacity of about 1026 mAh g−1 and its discharge capacity rapidly fades to 200 mAh g−1 after 10 cycles with unstable coulombic efficiency. Because of poor conductivity, electronic contact is easily lost with the expansion and contraction of material. After being milled with graphite, the electrochemical performance of the Si-SiOx -cristobalite is significantly enhanced due to the reduction of Si-SiOx -cristobalite particle size and the improvement of its conductivity. Fig. 8 (b) is rate capability of the Si-SiOx -cristobalite/graphite at current densities ranging from 100 to 1600 mA g−1 . A remarkable rate capability is achieved. Even at a current density of 1600 mA g−1 , the Si-SiOx -cristobalite/graphite still delivers a stable discharge

60

40

20

0 0

20

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Zre / Ω

(b)

28 24

Before cycling After the 2nd cycle After the 100th cycle

20

Zim / Ω

16

16 12 8 4 0 0

5

10

15

20

25

30

35

40

45

50

Zre / Ω Fig. 9. EIS of Si-SiOx -cristobalite and Si-SiOx -cristobalite/graphite electrodes before and after cycling.

capacity of about 380 mAh g−1 . When current density is tuned back to 100 mA g−1 , the discharge capacity is completely restored, indicating the structure of the Si-SiOx -cristobalite/graphite electrode is very stable. Fig. 9 presents the EIS of the Si-SiOx -cristobalite and SiSiOx -cristobalite/graphite electrodes before and after cycling. The resistance of the former is much bigger than that of the latter. Before cycling, the diameter of the depressed semicircle for the Si-SiOx cristobalite electrode in high-medium frequency range is about 80 , while that for the Si-SiOx -cristobalite/graphite electrode in high-medium frequency range is only about 35 . After cycling, two electrodes also show significant difference. From the 2nd to 100th cycle, there is just a slight increase in high-frequency intercept of the Si-SiOx -cristobalite/graphite electrode, which corresponds to solution resistance and electronic contact resistance [32], while the high-frequency intercept of the Si-SiOx -cristobalite electrode increases to about 24  from 10  in the second cycle. In addition, the diameter of the depressed semicircle for the Si-SiOx -cristobalite electrode in high-medium frequency range practically keeps constant and no obvious change is observed in the slope of line in low frequency region during cycling. These results suggest that the mechanical milling and the addition of graphite significantly improve the interface contact between material particles and collector, and the structural stability of the electrode. Fig. 10 is the SEM images of the Si-SiOx -cristobalite/graphite electrode before cycling

Y. Ren, M. Li / Electrochimica Acta 142 (2014) 11–17

17

4. Conclusions In conclusion, we have developed a gentle and scalable process to fabricate a new Li-ion battery negative material using SiO and graphite as starting materials. A high reversible capacity and excellent cycle stability can be achieved even under the conditions of a high mass loading per unit area and without capacity limitation. The improved electrochemical performance should be attributed to the high structural stability of the electrode and the conductivity enhanced by graphite. The composite is promising to become an alternative anode material for high-energy lithium-ion batteries if its initial coulombic efficiency is improved. Acknowledgment This research was a project supported by Natural Science Foundation of China (51374175) and (51342010). References

Fig. 10. SEM images of Si-SiOx -cristobalite/graphite electrode before cycling and after 100 cycles.

and after 100 cycles. After 100 cycles, a layer of SEI film is formed on the surface of the electrode materials, but no obvious structural crack or pulverization is observed, further confirming the structural stability of the Si-SiOx -cristobalite/graphite electrode. High structural stability of the electrode should be ascribed to the special composition and structure of the composite material. Firstly, silicon with several nm is uniformly inserted into SiOx matrix (Fig. 1 b and Fig. 7), in which lithium silicate and lithium oxide are generated during the first lithiation process and have been confirmed to have a good suppression function on the volume effect of silicon [23,24]. Secondly, the pores in electrode material provide room to some degree for the volume expansion of silicon [25]. Thirdly, the addition of ductile graphite not only improves the conductivity of the electrode, but also has a good buffer to the volume change of active material.

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