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Synthesis of tungsten disulfide (WS2) nanoflakes for lithium ion battery application
Electrochemistry Communications 9 (2007) 119–122 www.elsevier.com/locate/elecom
Synthesis of tungsten disulfide (WS2) nanoflakes for lithium ion battery application Chuanqi Feng b
a,b
, Lunfeng Huang a, Zaiping Guo
b,c,*
, Huakun Liu
b,c
a Department of Chemistry and Chemical Engineering, Hubei University, Wuhan 430062, China Institute for Superconducting and Electronic Materials, University of Wollongong, Northfield Avenue, Wollongong, NSW 2522, Australia c ARC Centre of Excellence for Electromaterials Science, University of Wollongong, NSW 2522, Australia
Received 7 August 2006; accepted 23 August 2006 Available online 28 September 2006
Abstract A novel method (a rheological phase reaction) was used to synthesize WS2 nanoflakes by adding oxalic acid as a reducing reagent. High resolution electron microscopy observations revealed that the as-prepared WS2 nanoflakes had started to curve and that WS2 nanotubes were partly formed. The lithium intercalation/deintercalation behavior of as-prepared WS2 electrode was also investigated. It was found that the WS2 nanoflake electrode exhibited higher specific capacity with very good cycling stability compared to WS2 nanotube or nanoparticle electrodes. The reasons for the improved electrochemical performance of the nanoflake electrodes are also discussed. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Tungsten disulfide; Nanoflakes; Lithium ion battery; Rheological phase reaction; Anode material
1. Introduction Graphite or coke-based carbon materials are commonly used as anodes in most commercial products on the market today. However, the reversible capacity of such anodes is limited to 372 mA h/g [1,2]. In order to increase the specific energy of lithium ion batteries, new anode materials with higher capacity are urgently needed. Several new materials, such as tin oxide-based materials [3], transition metal oxides [4], transition metal sulfides [5,6], and MnV2O6 [7] have aroused a great deal of attention. These materials have higher lithium storage capacity and have been identified as possible candidates for the next generation of anode materials. WS2, MoS2, and many other transition metal sulfides were found to have a similar structure to carbon and could *
Corresponding author. Address: Institute for Superconducting and Electronic Materials, University of Wollongong, Northfield Avenue, Wollongong, NSW 2522, Australia. Tel.: +61 2 4221 5727; fax: +61 2 4221 5731. E-mail address: [email protected] (Z. Guo). 1388-2481/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.08.048
form inorganic fullerene-like structures. A key feature of these sulfides is the existence of van der Waals forces across the gaps between the S–M–S sheets, which provides the space for guest reactants in intercalation reactions. During the intercalation of lithium ions, a complete charge transfer occurs, which involves not only the reduction of M4+ to M3+ but also the diffusion of Li+ into the Van der Waals gaps [8]. Thus, these sulfides could be developed as an intercalation host to form a promising electrode material in high energy density batteries [5,6,9]. A few attempts have been made to investigate the electrochemical activity of MS2 (M = Mo, W, etc.) electrodes. For example, Julien [10] reported on the electrochemical behavior of crystalline WS2 powders and found that the lithium insertion capacity was only 0.6 mol Li+ per mole of crystalline WS2, while Dominko et al. [5] found that 1.7–3.0 mol of Li could be inserted into one mole of MoS2xIy nanotube electrode, depending on the quality of the MoS2xIy nanotubes. More recently, work from our group [6] demonstrated that WS2 nanotube electrode could deliver a reversible capacity of above 500 mA h/g, corresponding to 4.7 mol lithium per mole of WS2 nanotubes. We attributed such a high
C. Feng et al. / Electrochemistry Communications 9 (2007) 119–122
2. Experimental
(003)
(100)
(002)
capacity to lithium intercalation into intratubular and intertubular sites of WS2 nanotubes as well as diffusion into the WS2 structure to form LixWS2 intercalation compounds. Since lithium ions can intercalate into small holes/channels, it may be imagined that WS2 nanotubes with plenty of defects can deliver even higher lithium intercalation capacity. In this communication, a novel synthesis method (rheological phase reaction) [11,12] is reported to synthesize WS2 nanoflakes (partly formed WS2 nanotubes). The samples prepared can reversibly store lithium with a capacity of 790 mA h/g in a voltage range of 0.01–3.0 V vs Li/Li+, corresponding to 7.4 mol lithium per mole of WS2, which is the highest capacity reported on WS2 electrodes so far.
(110) (112)
120
*
a
b
10
20
30
40
50
60
70
80
2theta (degree)
The WS2 material was synthesized by using analytically pure (NH4)6W7O24 Æ 4H2O, sulfocarbamide (CS(NH2)2), and oxalic acid (H2C2O4 Æ 2H2O) as starting materials. The W/S/H2C2O4 molar ratio was 1:2:1. These powders were mixed and thoroughly ground in an agate mortar, then a few drops of water were added to form a rheological state mixture. The mixture was then put into a 50 ml sealed teflon lined autoclave and maintained at 200 °C for 24 h to form a dark gray solid. After that, the solid mixture was packed into alumina crucibles and calcined in a tube furnace at 600 °C for 2 h under a flow of argon. Powder X-ray diffraction (Rigaku D/max-ra) using Cu ˚ ) with a graphite monochromaKa radiation (k = 1.5406 A tor was employed to identify the crystalline phase of the synthesized materials. The morphology of the resulting compound was observed using a transmission electron microscope (TEM). The electrochemical characterizations were performed using coin cells. The anode was prepared by dispersing 70 wt% as-prepared WS2 powder and 20 wt% carbon black in 10 wt% PTFE solution. The WS2 and carbon black powders were first added to a solution of PTFE in isopropanol to form a homogeneous slurry. The slurry was then spread onto a copper foil. The coated electrodes were dried at 125 °C for 24 h in vacuum and then pressed to enhance the contact between the active materials and the conductive carbon. Coin test cells were assembled in an argon filled glove box, where the counter electrode was Li metal and the electrolyte was 1 mol L1 LiPF6 dissolved in a 50/ 50 vol% mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). These cells were galvanostatically charged and discharged in the voltage range of 0.01– 3.0 V at room temperature to measure the electrochemical response. The current density used was 47.5 mA h/g. 3. Results and discussion 3.1. Reaction mechanism and structure characterization X-ray diffraction (XRD) was performed on the intermediate product (before 600 °C calcination) and the final product
Fig. 1. XRD patterns of as-prepared WS2 sample (the peak marked * corresponds to WO2): (a) after calcination at 600 °C and (b) before calcination.
WS2 (Fig. 1). The results show that WS2 did not form even after hydrothermal treatment in an autoclave at 200 °C for 24 h (Fig. 1b). The hexagonal phase of WS2 formed after sintering the intermediate product at 600 °C for 2 h under a flow of argon, with XRD revealing a small impurity peak corresponding to WO2 (Fig. 1a). The strong and clear peaks indicate the high crystallinity of the WS2 sample. Fig. 2a shows a typical TEM image of the as-prepared WS2 samples. It is obvious that the products are cluster of nanoflakes with irregular shapes. High resolution TEM image (Fig. 2b) of the samples reveals that the thin WS2 nanoflakes had started to curve and that WS2 nanotubes were partly formed. The growth process of the asprepared WS2 is to some extent similar to the mechanism proposed by Ye et al. [13], which involves curving followed by seaming of molecular layers, for the tube-formation process in materials with layered structures. This indicates that WS2 nanotubes could be formed under proper conditions. However, as proposed by Wilson and Yoffe [14], tubular WS2 is difficult to synthesize under moderate conditions, because the plate-like crystals usually grow in the 2Hb polytype, which has a stable form up to 1000 °C. Therefore, a higher sintering temperature (above 1000 °C) may be needed for WS2 nanotube formation using a similar rheological phase reaction method. Based on the literature [9,15,16] and the experimental conditions we used, the formation of WS2 may undergo a complex process which contains three steps: (a) the hydrolysis of CS(NH2)2; (b) the reduction of W(VI) and the formation of WO2 (evidenced by the WO2 impurity peak in the XRD pattern (Fig. 1a); and (c) the formation of WS2. The oxalic acid plays key roles as the reducing reagent, as well as a pH adjustment agent during the reaction process, while the sulfocarbamide works as a sulfurization reagent. The reaction process for the synthesis of WS2 could be expressed as follows:
C. Feng et al. / Electrochemistry Communications 9 (2007) 119–122
121
Fig. 2. Transmission electron micrographs of WS2 sample.
CSðNH2 Þ2 þ 2H2 O ! 2NH3 þ CO2 þ 2H2 S
ð1Þ
ðNH4 Þ6 W7 O24 þ 7H2 C2 O4 ! 6NH3 þ 7WO2 þ 14CO2 þ 10H2 O WO2 þ 2H2 S ! WS2 þ 2H2 O
ð2Þ ð3Þ
The overall reactions could be expressed as (4) ðNH4 Þ6 W7 O24 þ 7H2 C2 O4 þ 14CSðNH2 Þ2 þ 4H2 O ! 7WS2 þ 28CO2 þ 34NH3
ð4Þ
During these reactions, the increased reaction entropy facilitates the formation of the expected product (WS2). Besides this, the H+ that comes from the oxalic acid also plays a catalytic role during the reaction process. This novel method provides a simple and easily applicable route to synthesize WS2 at moderate temperature. It can also be used to synthesize other transition metal sulfides. 3.2. Electrochemical properties The electrochemical properties of the as-prepared WS2 were measured via coin cell testing. Fig. 3 shows the charge/discharge profiles of as-prepared WS2 electrode. In the first cycle, the WS2 electrode delivered a lithium 3.0
1st charge
20th
10th
+
Voltage (Vvs Li / Li)
2.5 2.0 1.5 1.0 0.5
1st discharge
2nd 20th
0.0 0
200
10th 400
600
800
1000
1200
1400
1600
1800
Specific capacity (mAh/g)
Fig. 3. Typical charge and discharge curves of an as-prepared WS2 electrode. Current density: 47.5 mA/g.
insertion capacity of about 1690 mA h/g, which is much higher than the reported data on WS2 nanotubes or nanopowder [6,10]. The as-prepared WS2 electrode retained a reversible capacity of 680 mA h/g after 20 cycles. Three lithium insertion plateaus at 1.4 V, 1.1 V and 0.8 V as well as a slope starting from 0.5 V down to the cutoff voltage of 0.01 V were observed in the first discharge curve. During the first charge, the charge voltage increased gradually at first, then a short plateau at about 2.3 V vs Li/Li+ was observed. From the second cycle, the discharge plateau is located at about 2.0 V, while the discharge plateaus at lower potential observed in the first cycle have disappeared. The charge/discharge behavior of the as-prepared WS2 electrode is similar to that of WS2 nanotube electrodes, which is described in other reports [5,6], except for the two discharge plateaus at 1.4 V and 1.1 V. Based on our previous research [6], we suggest that, in the first lithiation process, lithium ions intercalate into WS2 nanoclusters, defect sites, and intratubal sites (the hollow core) in WS2 nanoflakes (partly formed nanotubes). Once these sites are saturated with Li+, lithium ions will then diffuse into the WS2 structure to form LixWS2 compounds. During the charging process, lithium ions first deintercalate from the nanoclusters, defect sites, and intratubal sites in partly formed WS2 nanotubes, which contributes to the slope in the charging curves, then the lithium ions deintercalate from LixWS2 structures at higher potentials (observed as the plateau on the charging curves). It should be noted that the irreversible capacity in the first cycle is very high (900 mA h/g), which may caused by (1) the decomposition of electrolyte on the surface of the WS2 to form a passivation layer on the electrode and (2) a fraction of Li+ ions that were trapped in the nanoclusters or defect sites/intratubal sites in partly formed WS2 nanotubes induced irreversible capacity. From the second cycle, only one plateau (2.3 V for the charge curve and 2.0 V for the discharge curve) connected with a slope can be observed for both charge and discharge curves, indicating that the
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ions intercalate into nanoflake clusters; (2) lithium ions intercalate into defect sites in nanoflakes (partly formed nanotubes); (3) lithium ions intercalate into intratubal sites (the hollow core) through the open end; and (4) lithium ions intercalate into the WS2 structure and form LixWS2. All these four possibilities contribute to the high lithium insertion capacity of WS2 nanoflakes electrodes. Nanoflakes, i.e. partly formed nanotube materials, may indicate a new possible direction to further improve the electrochemical performance of electrochemically active materials.
1800
Specific capacity (mAh/g)
1600 1400
discharge charge
1200 1000 800 600 400 200
Acknowledgement
0 0
5
10
15
20
Cycle number
Fig. 4. Typical cycling performance of an as-prepared WS2 electrode. Current density: 47.5 mA/g.
intercalation reaction dominates the electrochemical process after the first cycle. With the continuing trend toward lower operating voltages in electronic devices, WS2 electrode, although it has a relatively high discharge potential, still can be accepted as an anode material for most electronic devices. The cycling behavior within 20 cycles is shown in Fig. 4. As mentioned above, we can observe the large irreversible capacity in the first cycle, but the coulombic efficiency in the subsequent cycles is high (90%), and the average fade in specific capacity was just 0.69% per cycle. Considering the high reversible capacity and good cycling stability of the as-prepared WS2 nanoflake electrodes, WS2 nanoflakes could be a promising alternative anode material for lithium ion batteries. 4. Conclusions In this study, a novel method (the rheological reaction method) was used to synthesize WS2 nanoflakes (partly formed nanotubes). The synthesized WS2 shows stable cyclability over a wide voltage range. (The reversible capacity remains 680 mA h/g after 20 cycles, which is 86.2% of the initial capacity.) There are four possibilities for lithium intercalation in the WS2 nanoflake electrodes: (1) lithium
This work was supported by the Australian Research Council (ARC) through Linkage Project ‘‘Nano anode materials for lithium-ion batteries’’ with industry partner Sons of Gwalia Limited. References [1] T. Tran, J. Feikert, X. Song, et al., J. Electrochem. Soc. 142 (1995) 3297. [2] B.A. Johnson, R.E. White, J. Power Sources 70 (1998) 48. [3] I.A. Courtney, J.R. Dahn, J. Electrochem. Soc. 144 (1997) 2045. [4] F. Badway, I. Plitz, S. Grugeon, et al., Electrochem. Solid State 5 (2002) 115. [5] R. Dominko, D. Arcon, A. Mrzel, A. Zorko, P. Cevc, P. Venturini, M. Gaberscek, M. Remskar, D. Mihailovic, Adv. Mater. 14 (2002) 1531. [6] G.X. Wang, S. Bewlay, Jane Yao, H.K. Liu, S.X. Dou, Electrochem. Solid State 7 (2004) A321. [7] T. Morishita, K. Nomura, T. Inamasu, et al., Solid State Ionics 176 (2005) 2235. [8] B.G. Sibernagel, Solid State Commun. 17 (1975) 361. [9] X.L. Li, Y.D. Li, J. Phys. Chem. B 108 (2004) 13893. [10] C.M. Julien, Mater. Sci. Eng. R 40 (2003) 47. [11] J.T. Sun, W. Xie, L.J. Yuan, K.L. Zhang, Mater. Sci. Eng. B 64 (1999) 157. [12] C.Q. Feng, H. Tang, K.L. Zhang, J.T. Sun, Mater. Chem. Phys. 80 (2003) 573. [13] C.H. Ye, G.W. Meng, Z. Jiang, Y.H. Zhang, G.Z. Wang, L.D. Zhang, J. Am. Chem. Soc. 124 (2002) 15180. [14] J.A. Wilson, A.D. Yoffe, Adv. Phys. 18 (1969) 193. [15] X.L. Li, J.P. Ge, Y.D. Li, Chem. Eur. J. 10 (2004) 6163. [16] Y.Q. Zhu, T. Sekine, K.S. Brigatti, et al., J. Am. Chem. Soc. 125 (2003) 329.
Synthesis of tungsten disulfide (WS2) nanoflakes for lithium ion battery application Chuanqi Feng b
a,b
, Lunfeng Huang a, Zaiping Guo
b,c,*
, Huakun Liu
b,c
a Department of Chemistry and Chemical Engineering, Hubei University, Wuhan 430062, China Institute for Superconducting and Electronic Materials, University of Wollongong, Northfield Avenue, Wollongong, NSW 2522, Australia c ARC Centre of Excellence for Electromaterials Science, University of Wollongong, NSW 2522, Australia
Received 7 August 2006; accepted 23 August 2006 Available online 28 September 2006
Abstract A novel method (a rheological phase reaction) was used to synthesize WS2 nanoflakes by adding oxalic acid as a reducing reagent. High resolution electron microscopy observations revealed that the as-prepared WS2 nanoflakes had started to curve and that WS2 nanotubes were partly formed. The lithium intercalation/deintercalation behavior of as-prepared WS2 electrode was also investigated. It was found that the WS2 nanoflake electrode exhibited higher specific capacity with very good cycling stability compared to WS2 nanotube or nanoparticle electrodes. The reasons for the improved electrochemical performance of the nanoflake electrodes are also discussed. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Tungsten disulfide; Nanoflakes; Lithium ion battery; Rheological phase reaction; Anode material
1. Introduction Graphite or coke-based carbon materials are commonly used as anodes in most commercial products on the market today. However, the reversible capacity of such anodes is limited to 372 mA h/g [1,2]. In order to increase the specific energy of lithium ion batteries, new anode materials with higher capacity are urgently needed. Several new materials, such as tin oxide-based materials [3], transition metal oxides [4], transition metal sulfides [5,6], and MnV2O6 [7] have aroused a great deal of attention. These materials have higher lithium storage capacity and have been identified as possible candidates for the next generation of anode materials. WS2, MoS2, and many other transition metal sulfides were found to have a similar structure to carbon and could *
Corresponding author. Address: Institute for Superconducting and Electronic Materials, University of Wollongong, Northfield Avenue, Wollongong, NSW 2522, Australia. Tel.: +61 2 4221 5727; fax: +61 2 4221 5731. E-mail address: [email protected] (Z. Guo). 1388-2481/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.08.048
form inorganic fullerene-like structures. A key feature of these sulfides is the existence of van der Waals forces across the gaps between the S–M–S sheets, which provides the space for guest reactants in intercalation reactions. During the intercalation of lithium ions, a complete charge transfer occurs, which involves not only the reduction of M4+ to M3+ but also the diffusion of Li+ into the Van der Waals gaps [8]. Thus, these sulfides could be developed as an intercalation host to form a promising electrode material in high energy density batteries [5,6,9]. A few attempts have been made to investigate the electrochemical activity of MS2 (M = Mo, W, etc.) electrodes. For example, Julien [10] reported on the electrochemical behavior of crystalline WS2 powders and found that the lithium insertion capacity was only 0.6 mol Li+ per mole of crystalline WS2, while Dominko et al. [5] found that 1.7–3.0 mol of Li could be inserted into one mole of MoS2xIy nanotube electrode, depending on the quality of the MoS2xIy nanotubes. More recently, work from our group [6] demonstrated that WS2 nanotube electrode could deliver a reversible capacity of above 500 mA h/g, corresponding to 4.7 mol lithium per mole of WS2 nanotubes. We attributed such a high
C. Feng et al. / Electrochemistry Communications 9 (2007) 119–122
2. Experimental
(003)
(100)
(002)
capacity to lithium intercalation into intratubular and intertubular sites of WS2 nanotubes as well as diffusion into the WS2 structure to form LixWS2 intercalation compounds. Since lithium ions can intercalate into small holes/channels, it may be imagined that WS2 nanotubes with plenty of defects can deliver even higher lithium intercalation capacity. In this communication, a novel synthesis method (rheological phase reaction) [11,12] is reported to synthesize WS2 nanoflakes (partly formed WS2 nanotubes). The samples prepared can reversibly store lithium with a capacity of 790 mA h/g in a voltage range of 0.01–3.0 V vs Li/Li+, corresponding to 7.4 mol lithium per mole of WS2, which is the highest capacity reported on WS2 electrodes so far.
(110) (112)
120
*
a
b
10
20
30
40
50
60
70
80
2theta (degree)
The WS2 material was synthesized by using analytically pure (NH4)6W7O24 Æ 4H2O, sulfocarbamide (CS(NH2)2), and oxalic acid (H2C2O4 Æ 2H2O) as starting materials. The W/S/H2C2O4 molar ratio was 1:2:1. These powders were mixed and thoroughly ground in an agate mortar, then a few drops of water were added to form a rheological state mixture. The mixture was then put into a 50 ml sealed teflon lined autoclave and maintained at 200 °C for 24 h to form a dark gray solid. After that, the solid mixture was packed into alumina crucibles and calcined in a tube furnace at 600 °C for 2 h under a flow of argon. Powder X-ray diffraction (Rigaku D/max-ra) using Cu ˚ ) with a graphite monochromaKa radiation (k = 1.5406 A tor was employed to identify the crystalline phase of the synthesized materials. The morphology of the resulting compound was observed using a transmission electron microscope (TEM). The electrochemical characterizations were performed using coin cells. The anode was prepared by dispersing 70 wt% as-prepared WS2 powder and 20 wt% carbon black in 10 wt% PTFE solution. The WS2 and carbon black powders were first added to a solution of PTFE in isopropanol to form a homogeneous slurry. The slurry was then spread onto a copper foil. The coated electrodes were dried at 125 °C for 24 h in vacuum and then pressed to enhance the contact between the active materials and the conductive carbon. Coin test cells were assembled in an argon filled glove box, where the counter electrode was Li metal and the electrolyte was 1 mol L1 LiPF6 dissolved in a 50/ 50 vol% mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). These cells were galvanostatically charged and discharged in the voltage range of 0.01– 3.0 V at room temperature to measure the electrochemical response. The current density used was 47.5 mA h/g. 3. Results and discussion 3.1. Reaction mechanism and structure characterization X-ray diffraction (XRD) was performed on the intermediate product (before 600 °C calcination) and the final product
Fig. 1. XRD patterns of as-prepared WS2 sample (the peak marked * corresponds to WO2): (a) after calcination at 600 °C and (b) before calcination.
WS2 (Fig. 1). The results show that WS2 did not form even after hydrothermal treatment in an autoclave at 200 °C for 24 h (Fig. 1b). The hexagonal phase of WS2 formed after sintering the intermediate product at 600 °C for 2 h under a flow of argon, with XRD revealing a small impurity peak corresponding to WO2 (Fig. 1a). The strong and clear peaks indicate the high crystallinity of the WS2 sample. Fig. 2a shows a typical TEM image of the as-prepared WS2 samples. It is obvious that the products are cluster of nanoflakes with irregular shapes. High resolution TEM image (Fig. 2b) of the samples reveals that the thin WS2 nanoflakes had started to curve and that WS2 nanotubes were partly formed. The growth process of the asprepared WS2 is to some extent similar to the mechanism proposed by Ye et al. [13], which involves curving followed by seaming of molecular layers, for the tube-formation process in materials with layered structures. This indicates that WS2 nanotubes could be formed under proper conditions. However, as proposed by Wilson and Yoffe [14], tubular WS2 is difficult to synthesize under moderate conditions, because the plate-like crystals usually grow in the 2Hb polytype, which has a stable form up to 1000 °C. Therefore, a higher sintering temperature (above 1000 °C) may be needed for WS2 nanotube formation using a similar rheological phase reaction method. Based on the literature [9,15,16] and the experimental conditions we used, the formation of WS2 may undergo a complex process which contains three steps: (a) the hydrolysis of CS(NH2)2; (b) the reduction of W(VI) and the formation of WO2 (evidenced by the WO2 impurity peak in the XRD pattern (Fig. 1a); and (c) the formation of WS2. The oxalic acid plays key roles as the reducing reagent, as well as a pH adjustment agent during the reaction process, while the sulfocarbamide works as a sulfurization reagent. The reaction process for the synthesis of WS2 could be expressed as follows:
C. Feng et al. / Electrochemistry Communications 9 (2007) 119–122
121
Fig. 2. Transmission electron micrographs of WS2 sample.
CSðNH2 Þ2 þ 2H2 O ! 2NH3 þ CO2 þ 2H2 S
ð1Þ
ðNH4 Þ6 W7 O24 þ 7H2 C2 O4 ! 6NH3 þ 7WO2 þ 14CO2 þ 10H2 O WO2 þ 2H2 S ! WS2 þ 2H2 O
ð2Þ ð3Þ
The overall reactions could be expressed as (4) ðNH4 Þ6 W7 O24 þ 7H2 C2 O4 þ 14CSðNH2 Þ2 þ 4H2 O ! 7WS2 þ 28CO2 þ 34NH3
ð4Þ
During these reactions, the increased reaction entropy facilitates the formation of the expected product (WS2). Besides this, the H+ that comes from the oxalic acid also plays a catalytic role during the reaction process. This novel method provides a simple and easily applicable route to synthesize WS2 at moderate temperature. It can also be used to synthesize other transition metal sulfides. 3.2. Electrochemical properties The electrochemical properties of the as-prepared WS2 were measured via coin cell testing. Fig. 3 shows the charge/discharge profiles of as-prepared WS2 electrode. In the first cycle, the WS2 electrode delivered a lithium 3.0
1st charge
20th
10th
+
Voltage (Vvs Li / Li)
2.5 2.0 1.5 1.0 0.5
1st discharge
2nd 20th
0.0 0
200
10th 400
600
800
1000
1200
1400
1600
1800
Specific capacity (mAh/g)
Fig. 3. Typical charge and discharge curves of an as-prepared WS2 electrode. Current density: 47.5 mA/g.
insertion capacity of about 1690 mA h/g, which is much higher than the reported data on WS2 nanotubes or nanopowder [6,10]. The as-prepared WS2 electrode retained a reversible capacity of 680 mA h/g after 20 cycles. Three lithium insertion plateaus at 1.4 V, 1.1 V and 0.8 V as well as a slope starting from 0.5 V down to the cutoff voltage of 0.01 V were observed in the first discharge curve. During the first charge, the charge voltage increased gradually at first, then a short plateau at about 2.3 V vs Li/Li+ was observed. From the second cycle, the discharge plateau is located at about 2.0 V, while the discharge plateaus at lower potential observed in the first cycle have disappeared. The charge/discharge behavior of the as-prepared WS2 electrode is similar to that of WS2 nanotube electrodes, which is described in other reports [5,6], except for the two discharge plateaus at 1.4 V and 1.1 V. Based on our previous research [6], we suggest that, in the first lithiation process, lithium ions intercalate into WS2 nanoclusters, defect sites, and intratubal sites (the hollow core) in WS2 nanoflakes (partly formed nanotubes). Once these sites are saturated with Li+, lithium ions will then diffuse into the WS2 structure to form LixWS2 compounds. During the charging process, lithium ions first deintercalate from the nanoclusters, defect sites, and intratubal sites in partly formed WS2 nanotubes, which contributes to the slope in the charging curves, then the lithium ions deintercalate from LixWS2 structures at higher potentials (observed as the plateau on the charging curves). It should be noted that the irreversible capacity in the first cycle is very high (900 mA h/g), which may caused by (1) the decomposition of electrolyte on the surface of the WS2 to form a passivation layer on the electrode and (2) a fraction of Li+ ions that were trapped in the nanoclusters or defect sites/intratubal sites in partly formed WS2 nanotubes induced irreversible capacity. From the second cycle, only one plateau (2.3 V for the charge curve and 2.0 V for the discharge curve) connected with a slope can be observed for both charge and discharge curves, indicating that the
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ions intercalate into nanoflake clusters; (2) lithium ions intercalate into defect sites in nanoflakes (partly formed nanotubes); (3) lithium ions intercalate into intratubal sites (the hollow core) through the open end; and (4) lithium ions intercalate into the WS2 structure and form LixWS2. All these four possibilities contribute to the high lithium insertion capacity of WS2 nanoflakes electrodes. Nanoflakes, i.e. partly formed nanotube materials, may indicate a new possible direction to further improve the electrochemical performance of electrochemically active materials.
1800
Specific capacity (mAh/g)
1600 1400
discharge charge
1200 1000 800 600 400 200
Acknowledgement
0 0
5
10
15
20
Cycle number
Fig. 4. Typical cycling performance of an as-prepared WS2 electrode. Current density: 47.5 mA/g.
intercalation reaction dominates the electrochemical process after the first cycle. With the continuing trend toward lower operating voltages in electronic devices, WS2 electrode, although it has a relatively high discharge potential, still can be accepted as an anode material for most electronic devices. The cycling behavior within 20 cycles is shown in Fig. 4. As mentioned above, we can observe the large irreversible capacity in the first cycle, but the coulombic efficiency in the subsequent cycles is high (90%), and the average fade in specific capacity was just 0.69% per cycle. Considering the high reversible capacity and good cycling stability of the as-prepared WS2 nanoflake electrodes, WS2 nanoflakes could be a promising alternative anode material for lithium ion batteries. 4. Conclusions In this study, a novel method (the rheological reaction method) was used to synthesize WS2 nanoflakes (partly formed nanotubes). The synthesized WS2 shows stable cyclability over a wide voltage range. (The reversible capacity remains 680 mA h/g after 20 cycles, which is 86.2% of the initial capacity.) There are four possibilities for lithium intercalation in the WS2 nanoflake electrodes: (1) lithium
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