graphene with enhanced initial coulombic efficiency and rate performance for lithium ion batteries

Journal of Alloys and Compounds 584 (2014) 76–80

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Tin–indium/graphene with enhanced initial coulombic efficiency and rate performance for lithium ion batteries Hongxun Yang ⇑, Ling Li School of Biology & Chemical Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China

a r t i c l e

i n f o

Article history: Received 11 July 2013 Received in revised form 16 August 2013 Accepted 4 September 2013 Available online 14 September 2013 Keywords: Tin–indium alloy Graphene nanosheet Anode Lithium ion batteries

a b s t r a c t Tin is an attractive anode material replacing the current commercial graphite for the next generation lithium ion batteries because of its high theoretical storage capacity and energy density. However, poor capacity retention caused by large volume changes during cycling, and low rate capability frustrate its practical application. In this study, a new ternary composite based on tin–indium alloy (Sn–In) and graphene nanosheet (GNS) was prepared via a facile solvothermal synthesis followed by thermal treatment in hydrogen and argon at 550 °C. Characterizations show that the tin–indium nanoparticles with about 100 nm in size were wrapped between the graphene nanosheets. As an anode for lithium ion batteries, the Sn–In/GNS composite exhibits a remarkably improved electrochemical performance in terms of lithium storage capacity (865.6 mAh g1 at 100 mA g1 rate), initial coulombic efficiency (78.6%), cycling stability (83.9% capacity retention after 50 cycles), and rate capability (493.2 mAh g1 at 600 mA g1 rate after 25 cycles) compared to Sn/GNS and Sn–In electrode. This improvement is attributed to the introduction of lithium activity metal, indium, which reduces the charge transfer resistance of electrode, and the graphene nanosheet which accommodates the volume change of tin–indium nanoparticles during cycling and improves electrical conductivity of material. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Tin-based alloys, as one of the most promising materials to replace graphite for the next generation lithium ion batteries, have been extensively studied as anode materials because of their high theoretical storage capacities on both the gravimetric and volumetric bases [1–4]. However, the poor cycle performance arising from large volume expansion/contraction during Li+ insertion/ extraction leading to the pulverization of the electrode and the electrical disconnection from current collectors impedes its practical application. Many researchers have made great efforts in order to resolve those existing problems. One of the successful strategies is to fabricate core–shell nanostructure, such as metals and alloys with carbon shells [5,6]. This structure can offer a more expansion space, enhanced electronic transport framework, a good isolator layer between the high-activity metal sites and electrolytes, hence improved cycling stability [5,6]. The other approach is the incorporation of lithium inactive element, such as, Sn–Cu [7–10], Sn–Co [11–15], Sn–Ni [16,17], Sn–Fe [18], and Sn–Al [19,20] into tin active material to form a Sn-based alloy electrode. These hybrid systems provide distinct advantages, especially in mechanics, such as buffering the volume expansion of Sn phase during cycling, and

⇑ Corresponding author. Tel./fax: +86 511 84401181. E-mail address: [email protected] (H. Yang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.09.033

higher electrical conductivity. These are beneficial for achieving enhanced cycle performance compared to the single metal systems, but at the expense of the battery capacity. The introduction of lithium active metal material into tin to form alloy electrode has been reported rarely [21,22]. Indium metal is reactive towards lithium and has higher electrical conductivity compared to tin [23– 26]. The reported heterostructured SnO2–In2O3 nanowires showed improved reversible capacity compared to single SnO2 nanowires because of indium lithium activity after first cycle [27]. However, the thermal evaporation synthesis process of SnO2–In2O3 nanowires usually has some serious drawbacks, such as, high cost (requirement for (1 0 0) Si substrate and gold coating) and very low productivity. Therefore, it is very significant to synthesize Sn–In nanoparticles composites via a low cost and mass production method for extensive application in high performance lithium ion batteries. Until now, there is no report about tin–indium alloy anode in the application of lithium ion batteries. On the other hand, it was found that the introduction of a conductor buffer such as carbon [28], carbon nanotube [29,30], carbon fibres [31], or graphene nanosheet [32–34], could lead to an improvement in electrochemical performance. Especially, graphene nanosheet (GNS) with a two-dimensional structure of carbon atoms packed into planar honeycomb lattice, has been proposed as a potential electrode material for energy storage devices because of its superior electrical conductivity, high surface area,

H. Yang, L. Li / Journal of Alloys and Compounds 584 (2014) 76–80

structural flexibility, and chemical tolerance [35–39]. GNS as buffering matrix not only accommodate the large volume change of Sn during the cycling process, but also enhance the electron transport properties of the electrode due to their excellent electronic and mechanical properties [32–34]. Recently, the reported SnO2– In2O3/GNS composite improved the rate and cycle performances, but the initial coulombic efficiency is relatively lower due to the irreversible reaction between SnO2–In2O3 and lithium ion [40]. This inspired us to design and prepare Sn–In/GNS as anode for lithium ion batteries with enhanced initial coulombic efficiency. To our knowledge, Sn–In/GNS composite has not been reported, yet. Herein, we intend to combine the advantages of indium and graphene nanosheet to synthesize a new ternary composite of tin–indium (Sn–In)/graphene nanosheet (GNS) through a facile solvothermal synthesis following annealing method. Indium, as lithium activity metal, not only delivers higher capacity, but also has higher electrical conductivity compared to tin which reduces the charge transfer resistance of electrode. GNS in the Sn–In/GNS could accommodate the volume change of tin–indium nanoparticles during cycling and improve electrical conductivity of material. It could be expected that Sn–In/GNS electrode for lithium ion batteries could achieve better capacity retention and enhanced capability. 2. Experimental section 2.1. Synthesis of graphene oxide In a typical synthesis method, graphene oxide (GO) was synthesized by a modified Hummers method [40–42]. Briefly, 80 mL concentrated H2SO4 was added into a flask containing 1 g graphite powder and 0.5 g NaNO3 with an ice bath. Under vigorous stirring, 3 g KMnO4 was added gradually at below 20 °C. After removing the ice bath, the mixture was stirred at 35 °C for 2 days. Successively, 50 mL of H2O was slowly added to the pasty mixture. Addition of water into the concentrated H2SO4 medium will release a large amount of heat; therefore, water should be added while keeping the mixture in an ice bath to keep the temperature below 35 °C. After dilution with 350 mL of H2O, 5 mL of 30% H2O2 was added to the mixture, and the diluted solution color changed to brilliant yellow along with bubbling. After continuously stirring for 2 h, the mixture was centrifuged and washed with 10% HCl aqueous solution, distilled water, and ethanol to remove other ions. Finally, the resulting solid was dried by vacuum at 35 °C for 2 days. 2.2. Synthesis of Sn–In/GNS composites 50 mg GO was dispersed in 60 mL ethanol under ultra-sonication for two hours as Part A. A mixture of 0.285 g SnCl22H2O and 0.096 g InCl3 was dissolved in 10 mL ethanol solution as Part B. Part A and Part B were then mixed under stirring to form a uniform suspension. Then, the suspension was transferred to 100 mL autoclave at 150 °C for 10 h. The solvothermal product was centrifuged and washed using ethanol for three times, and dried under vacuum at 80 °C for overnight. To get Sn–In/ graphene composite, the obtained SnO2–In2O3/graphene powder was reduced at 550 °C for 6 h within a mixture Ar (50%)/H2 (50%). For comparison, Sn–In nanoparticles were also synthesized at similar conditions without the presence of graphene oxide. Sn/graphene composites were synthesized at similar conditions without the presence of In. 2.3. Characterizations and measurements The Sn–In/GNS composites were characterized by a field emission scanning electron microscope (FE-SEM, JEOL JSM-7600F), field emission transmission electron microscope (FE-TEM, JEOL JEM-2100F), X-ray diffraction analyser (XRD, Rigaku D/MAX RINT-2000), and Raman Spectra (Raman Microprobe, Renishaw

77

Instruments, England). Elemental analyses (C) were carried on an Elementar Vario EL III analyzer. Sn and In were determined by a Jobin Yvon Ultima2 ICP atomic emission Spectrometer. The working electrodes were prepared by mixing 80 wt% active material, 10 wt% Super P, and 10 wt% polyvinylidene fluoride binder dissolved in Nmethyl-2-pyrrolidinone. The resulting slurry was pasted onto copper foil and dried in a vacuum oven at 90 °C for overnight, and the electrodes were then pressed under a pressure of approximately 180 kg cm2. Coin-type half cells (2032 R type), the as-prepared composites as working electrode, pure lithium metal as counter electrode and 1 M LiPF6 in ethylene carbonate/diethyl carbonate (EC/DEC, 1:1 vol%) as the electrolyte, were fabricated to evaluate the electrochemical properties using a battery cycle tester (TOSCAT 3000, Toyo System, Tokyo, Japan). Cyclic voltammetry (CV) measurements were performed on a Sn–In/GNS working electrode using an electrochemical workstation (Parstat 2273) between 2 and 0.01 vs (Li/ Li+)/V at a sweep rate of 0.2 mV s1. Electrochemical impedance spectroscopy (EIS) tests were also measured on an electrochemical workstation (Parstat 2273) operating in the frequency range of 0.1 Hz to 106 Hz with ac amplitude of 10 mV.

3. Results and discussion 3.1. Characterizations of Sn–In/GNS composite Fig. 1 illustrates the synthesis of Sn–In nanoparticles on graphene nanosheet via solution-phase reaction method followed by thermal treatment in hydrogen and argon. Fig. 2 shows the scanning electron microscope (SEM) and transmission electron microscope (TEM) images of Sn–In/GNS composite. As can be seen in Fig. 2a and b, Sn–In/GNS composite composed of graphene nanosheets and nanoparticles. These nanoparticles are wrapped between graphene nanosheets. TEM images of the Sn–In/GNS composite are exhibited in Fig. 2c and d. The mean particle size of Sn–In nanoparticles was found to be 100 nm according to TEM images. TEM characterization further confirms the intimate contact between the graphene nanosheets and Sn–In nanoparticles. It is believed that, on the one hand, the presence of graphene nanosheets is favorable to improve the dispersion and growth of Sn–In nanoparticles. On the other hand, the assembly of nanoparticles on the graphene surface to some extent prevents the stacking of graphene nanosheets due to van der Waals interactions, leading to a large available surface area for energy storage. Fig. 3a shows the typical X-ray diffraction pattern of the as-prepared Sn–In/GNS composite. From its XRD patterns, the as-prepared composite consists of b and c phases, including Sn and In composition. The weight ratio between Sn and In was found to be 75:25 via the ICP atomic emission Spectrometer, corresponding to the Sn weight content of 67.5% and In weight content of 22.5%. This result is in accordance with the binary In–Sn equilibrium phase diagram ([43], Fig. S1). The carbon content of 10% is determined by the element analyses. Raman spectra for Sn–In/GNS and graphene oxide were also investigated. It is well known that the intensity ratio of the D to G band (ID/IG) reflects the graphitization degree of carbonaceous materials and the defect density [39]. The ID/IG for Sn–In/GNS (1.72) is much larger than that for graphene oxide (1.04), showing the transformation of graphene sheets from graphene oxide sheets after the solvothermal treatment. Based on above data, SEM and TEM images of the as-prepared material, it can be concluded that the Sn–In/GNS composite has been successfully synthesized.

Fig. 1. Synthesis process of Sn–In/GNS composite.

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H. Yang, L. Li / Journal of Alloys and Compounds 584 (2014) 76–80

Fig. 2. (a) Low magnification SEM image of Sn–In/GNS composite. (b) High magnification SEM image. (c) TEM image of Sn–In/GNS composite in low magnification. (d) High magnification TEM image of Sn–In/GNS composite.

3.2. Electrochemical performances of Sn–In/GNS composite We subsequently examined the electrochemical properties of the Sn–In/GNS composite as an anode material for lithium ion

2.0

γ ( 100)

Potential vs (Li/Li+)/V

Intensity (a.u.)

(a)

batteries. Fig. 4a shows the representative charge/discharge curves of Sn–In/GNS electrodes for the 1st and 2nd cycle in the range of 0.01–2.0 vs (Li/Li+)/V at a rate of 100 mA g1. As can be seen, the Sn–In/GNS electrode materials delivered a high initial charge

γ(101) γ( 001) β(101) β(211) γ(102) β(220)

γ(110) γ(002)

β(110)

(a)

1.5 1.0 0.5

1st 2nd

0.0

20

30

40

50

60

70

0

80 6

D

4

Current (mA)

Intensity (a.u.)

Sn-In/GNS Graphene oxide G

400

600

800

1000

Capacity (mAh g-1)

2θ

(b)

200

(b)

1st

2

2nd

0 -2 -4 -6

1000

1200

1400

1600

1800

2000

0.0

0.5

1.0

1.5

2.0

Potential vs (Li/Li+)/V

Wavenumbers /cm-1 Fig. 3. (a) Powder XRD pattern of Sn–In/GNS composite and (b) Raman spectra for Sn–In/GNS and graphene oxide samples.

Fig. 4. (a) The 1st and 2nd discharge/charge voltage profiles of Sn–In/GNS anode in the range of 0.01–2.5 vs (Li/Li+)/V at a rate of 100 mA g1 and (b) cyclic voltammogram profiles for Sn–In/GNS composite at a sweep rate of 0.2 mV s1.

H. Yang, L. Li / Journal of Alloys and Compounds 584 (2014) 76–80

capacity of 860.51 mAh g1 with a coulombic efficiency of 73% at the first cycle (Fig. S3). In the first cycle, the two obvious voltage plateaus are around 0.68 V and 0.27 V, similar to previous reports [25,26,40]. These plateaus are still clear after the first cycle, indicating the reversible capacity of the first cycle. To further investigate the mechanism of the electrochemical reaction, cyclic voltammogram profiles for the Sn–In/GNS electrode from the first to the 2nd cycle are carefully studied between 0.01 and 2.0 vs (Li/ Li+) V measured at a scan rate of 0.2 mV s1 (Fig. 4b). In the first cycle, the two redox peaks at 0.55/0.75 and 0.35/0.55 V correspond to the reversible reaction (1) and (2), respectively [26,40]: þ

In þ 4:33Li þ 4:33e Li4:33 In

ð1Þ

þ

Sn þ 4:4Li þ 4:4e Li4:4 Sn

ð2Þ

To understand the role of indium and GNS in the composite Sn–In/GNS, we compared the capacity retentions of the Sn/GNS, Sn–In, and Sn–In/GNS as anode materials at the discharge/charge rate of 100 mA g1, as shown in Fig. 5a. With the increase of cycle numbers, the differences between Sn/GNS and Sn–In/GNS composite became pronounced. At 50 cycles, the composite Sn/GNS delivered a charge capacity of 616.1 mAh g1 with capacity retention of 71.6%, while the composite Sn–In/GNS still keep a charge capacity of 726.2 mAh g1 with capacity retention of 83.9% compared to the initial charge capacity. The enhancement lithium storage capacity of Sn–In/GNS is related to the improvement of the electrochemical properties of Sn coming from the synergistic effect between Sn and In, resulting in the decrease of the irreversible capacity of Sn. As above-mentioned for CV studies, during the first discharge process, indium can firstly react with Li+ ions at higher potential (0.57 V) before tin (0.26 V), following by the formation of metallic indium nanograins. Tin can be surrounded by the pre-formed indium

100 80

800

60 600

400

200

40

Sn-In/GNS Sn/GNS Sn-In

20

(a) 0

10

20

30

40

Coulombic Efficiency (%)

Charge capacity (mAh g-)

1000

0

50

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nanograins, leading to the suppression of volume expansion and the prevention of the Sn products from aggregation in the following oxidation of Sn. Thus, the capacity fading can be relieved [42]. For the Sn–In anode, there was a rapid fading of charge capacity due to the severe pulverization. After 30 cycles the Sn–In electrode almost lost active, while the composite Sn–In/GNS still keep higher charge capacity of 740 mA g1. According to previous reports, this improvement may be attributed to the GNS as a buffering matrix which could accommodate the volume change during insertion and extraction and prevent aggregation of metal oxide nanoparticles, and the synergistic effect between Sn–In and graphene nanosheets. For the purpose of further comparison, we also evaluated the rate capabilities of lithium ion batteries made from the Sn/GNS, Sn–In, and Sn–In/GNS as anode materials under the similar electrochemical test conditions (Fig. 5b). At lower charge/discharge rate of 100 mA g1 or 200 mA g1, the charge capacity of Sn–In/GNS is slightly larger than that of Sn/GNS, and much higher than that of the Sn–In showing the excellent properties of graphene nanosheets. With the increase of the current rate, the differences among the Sn–In/GNS, Sn/GNS, and Sn–In became much obvious. At the current rate of 800 mA g1, the Sn–In/GNS contains a specific capacity of 493.2 mA g1, which is higher than that of the Sn/ GNS binary composite (313.8 mA g1), the Sn–In (77.4 mA g1) and the graphite material (372 mA g1). Such enhancement rate capability and reversibility could be attributed to the introduction of In and graphene nanosheets, and the synergistic effect not only between Sn–In and GNS but also between Sn and In. In order to further probe the enhanced rate capability, EIS measurements were carried out using half cells consisting of Sn–In/ GNS, Sn/GNS, or Sn–In nanoparticles as the working electrode after two discharge/charge cycles. As shown in Fig. 6, they are composed of a depressed semicircle in the high frequency region and a sloping line in the low frequency region. The impedance spectra are fitted using the equivalent circuit model (see the inset in Fig. 6), and the fitted impedance parameters are listed in Table 1. The equivalent circuit model includes Rs, a constant phase element (CPE) associated with the interfacial resistance, and the semi-circle which is correlated with the Li+ charge transfer resistance at the interface Rct. The linear portion is designated to Warburg impedance (W1), which is attributed to the diffusion of Li+ into the bulk of the electrode materials. Rs includes the ionic resistance of the electrolyte, the intrinsic resistance of the active material, and the contact resistance at the interface active material/current collector. As shown in Table 1, Rs is much smaller than Rct, indicating the cell impedance is mainly attributed to charge-transfer resistance. It is

Cycle number 1000 Sn-In/GNS Sn/GNS Sn-In

Charge capacity (mAh g-)

-1

100 mA g

-1

800

200 mA g

-1

300 mA g

-1

400 mA g

600

-1

600 mA g

400 200 0

(b) 0

5

10

15

20

25

Cycle number Fig. 5. (a) Cycle performances for Sn–In/GNS, Sn–In, and Sn/GNS composites at a rate of 100 mA g1 and (b) rate capabilities of Sn–In/GNS, Sn–In, and Sn/GNS as anodes for lithium ion batteries at different rates.

Fig. 6. Electrochemical impedance spectroscopy (EIS) spectra of Sn–In/GNS, Sn/ GNS, and Sn–In as anodes for lithium ion batteries after 2nd cycle discharge/charge. Inset shows the equivalent circuit.

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H. Yang, L. Li / Journal of Alloys and Compounds 584 (2014) 76–80

Table 1 Rs and Rct values of Sn–In/GNS, Sn/GNS, and Sn–In after 2nd cycle.

References

Samples

Rs (X)

Rct (X)

Sn–In/GNS Sn/GNS Sn–In

2.59 3.92 5.91

27.53 51.22 103.2

obvious that the Rct of Sn–In/GNS (27.53 X) is much smaller in comparison with the Sn/GNS (51.22 X) and Sn–In (103.21 X) composite. This is attributed to the higher electrical conductivity of metallic indium than that of metallic tin. The Sn–In/GNS alloy maintains lower charge transfer resistance than that of the binary Sn/GNS composite during cycling, thus leading to enhancement rate capability. Moreover, GNS in the presence of Sn–In/GNS composite acts as a conductive-buffer-spacer which improves the electron transport property of the electrode and stabilizes the electronic and ionic conductivity, therefore leading to fast lithium ion diffusion and lower charge transfer resistance compared to Sn–In electrode. This result is also in agreement with the superior rate capability of Sn–In/GNS composite since charge transfer process is the rate-determining step for conversion reactions [40].

4. Conclusions In summary, a ternary composite of Sn–In/GNS has been designed and synthesized as an anode material for high performance lithium ion batteries. The obtained Sn–In nanoparticles with about 100 nm in size were anchored on the surface of electrically conductive graphene nanosheets. The ternary Sn–In/GNS composite exhibits enhanced initial coulombic efficiency, excellent capacity retention and improvement rate capability. This improvement could be attributed to the introduction of indium and GNS, and the synergistic effect among the GNS, tin and indium. This work highlights the advantages of combination with different metal nanoparticles anchoring on GNS for the maximum utilization of electrochemically activity, and GNS for energy storage applications in high performance lithium ion batteries. This research also supplies a facile method to produce a large scale of Sn–In/GNS nanomaterials as an advanced anode for lithium ion batteries.

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Acknowledgements

[36] [37]

This work was financially supported by High-level Scientific Research Founds for the oversea talent of JUST (No. 635211301).

[38] [39] [40]

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.jallcom.2013.09.033.

[41] [42] [43]

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