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Electrochemical properties of Sn-Co electrode with various kinds of binder materials for sodium ion batteries
SOSI-13873; No of Pages 5 Solid State Ionics xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Electrochemical properties of Sn-Co electrode with various kinds of binder materials for sodium ion batteries Yuhki Yui a,b, Masahiko Hayashi a, Katsuya Hayashi c, Jiro Nakamura a,b a b c
NTT Device Technology Labs., NTT Corporation, Atsugi, Kanagawa, Japan Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Tokyo, Japan NTT Network Technology Labs., NTT Corporation, Atsugi, Kanagawa, Japan
a r t i c l e
i n f o
Article history: Received 17 July 2015 Received in revised form 24 November 2015 Accepted 2 January 2016 Available online xxxx Keywords: Sodium ion batteries Anode Binder Sn-Co In situ light microscopy SAICAS
a b s t r a c t Discharge–charge properties of Sn-Co anode materials with various kinds of binders [polyvinylidene difluoride (PVdF), polyacrylic acid (PAA), sodium polyacrylate (PAANa), sodium carboxymethyl cellulose (CMC), and polyimide (PI)] for sodium ion batteries were investigated to determine the correlation between cycle performance and the properties of electrode with binder. Sn-Co electrodes with PAA or CMC binders exhibited better cycle properties (discharge capacities of more than 400 mAh/g up to 20 cycles) than with PVdF, PAANa, or PI. These cycle properties with PAA or CMC were due to smaller changes in the electrode volume that occurred during cycling, as revealed by in situ light microscopy. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Lithium-ion batteries (LIBs) are used for power storage in mobile devices, electrical products, and electric vehicles, and the demand for them is likely to increase. However, lithium is expensive because it is not an abundant metal [1]. On the other hand, sodium is abundant and cheap [1], and interest in sodium-ion batteries (SIBs) has been growing. The materials that have been studied for use as SIB anodes include hard carbon [2–5], tin [6–8], tin-based materials [9–10], antimony-based materials [11–12], germanium [13], and oxide materials [14–21]. However, there are few materials that satisfy the requirements for both large capacity and good cyclability. In a previous study, we focused on Sn-Co, and found that it shows better cycling performance than tin electrodes [9]. In addition, to improve the cyclability of alloy-based anodes for LIBs in recent years, various kinds of binders have been studied, such as polyacrylic acid (PAA) [22–23], sodium polyacrylate (PAANa) [24], sodium carboxymethyl cellulose (CMC) [22–23], and polyimide (PI) [25]. In this study, to further improve the cyclability, we investigated the electrochemical properties of anodes with PAA, PAANa, CMC, and PI as binder materials. Furthermore, an investigation of the change in volume and adhesion strength of electrodes with binders revealed the relationship between them and cycle performance. 2. Experimental Sn-Co powder (element ratio: Sn:Co = 9:1) was obtained from Mitsubishi Materials Corporation as reported in our previous study [9].
PVdF (molecular weight (Mw) = 1,000,000, Kureha Corp.), PAA (Mw = 1,250,000, Sigma-Aldrich Co. Ltd.), PAANa (Mw = 15,000, Sigma-Aldrich Co. Ltd.), CMC (Mw = 250,000, Sigma-Aldrich Co. Ltd.), and PI (I.S.T. Corp.) were used as binders. The working electrodes (WEs) were prepared by mixing 80 wt% of Sn-Co powder, 10 wt% of Ketjen Black EC600JD (Lion Co.) and 10 wt% of the binder in N-methylpyrrolidone (PVdF, PI) or distilled water (PAA, PAANa, CMC) and then coating the mixture on a Cu sheet and drying it at 90 °C. The electrochemical performance was evaluated with a 2032 coin-type cell using a WE (0.02 mm thick and 14 mm in diameter), an electrolyte solution (1 mol/l NaPF6/EC:DEC 1:1 in volume, Tomiyama Pure Chemicals Industries Ltd.), a polypropylene separator (Celgard, 19 mm in diameter), and Na metal sheets (0.6 mm thick and 15 mm in diameter) as a counterelectrode (CE). All cells were assembled in an Ar-filled glove box. Cycling tests were performed using an automatic galvanostatic discharge–charge system (Hokuto HJ1001SD8) at a constant current density of 25 mA/g between 0.01 and 1.5 V at 25 °C. After the cell voltage had reached 0.01 V, the cell was kept at a constant voltage of 0.01 V for 10 h. To analyze the surface of the WEs after the fifth cycle with a scanning electron microscope (SEM, JEOL Ltd., JSM890) with accelerating voltage of 7 kV, the cells were opened after the fifth cycle in the Ar-filled glovebox and the WEs were washed with dimethyl carbonate and dried. Changes in WE volume during the first cycle were directly observed by using in situ light microscopy (Lasertec Corp., ECCS B310). The rate for expansion and contraction of WEs was estimated in the line analysis mode. The cross-sectional surface of the semicircular cell of WE/
http://dx.doi.org/10.1016/j.ssi.2016.01.007 0167-2738/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: Y. Yui, et al., Electrochemical properties of Sn-Co electrode with various kinds of binder materials for sodium ion batteries, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.01.007
Y. Yui et al. / Solid State Ionics xxx (2016) xxx–xxx
separator/CE (Na) was monitored via an observation window made of sapphire by using light microscopy in the same manner as reported previously [9]. Discharge–charge tests were performed in the same condition as with the coin-type cell mentioned above, and the results were similar to the case of coin-type cell. An analysis of adhesion strength between WE composites incorporating various kinds of binders and a Cu sheet was performed using a surface and interfacial cutting analysis system (SAICAS, Daipla Wintes Co., Ltd.) as shown in Fig. 1(a) [26–27]. The WE composite side was attached to a glass plate with epoxy. A diamond blade (width: 1 mm) cuts downward at a 20° shear angle through the Cu sheet. During the test, the blade moved in the horizontal direction at 1 μm/s. When the blade reached the WE composite, it moved horizontally, peeling off the copper. The adhesion strength is the average cutting load during the peeling off of the copper as shown in Fig. 1(b). 3. Results and discussion
2 PAA CMC PI PAANa PVdF
1.5
Voltage (V)
2
1 Voltage range: 0.01 – 1.5 V Current density: 25 mA/g
0.5
0
-0.5
Fig. 2 shows the first discharge–charge curves of Na/Sn-Co cells with various kinds of binders in the 0.01 to 1.5 V range. The first discharge capacities of the Sn-Co electrode with PVdF, PAA, PAANa, CMC, and PI were 569, 523, 598, 486, and 537 mAh/g, respectively. All cells using the various kinds of binders showed two distinct plateaus (at about 0.01–0.25 and 0.45–0.65 V for the charge process), and the type of binder did not affect plateau voltage. This suggests that the binders were not involved in the discharge–charge reaction. Fig. 3 shows the cycle properties of the cells. The WEs with PAA and CMC showed better cycle properties than those with PI, PAANa, and PVdF, and the discharge capacity of the former two reached more than 400 mAh/g after 20 cycles. The difference in the cycle properties may be due to a difference in the change in WE volume during cycling. If there is a difference in WE volume, the WE surface after the cycle would be changed. Accordingly, we observed the WE surface before and after cycle tests using SEM. Fig. 4 shows SEM images of the surface of pristine (a)–(e) and fifth-cycled (a′)–(e′) WEs. Before cycling, the surface morphologies were almost the same as for WEs with the various kinds of binders, and there were no cracks and the Sn-Co cube particles were 0.7– 2.0 μm in size. After the fifth cycle, the WE surface with PAA and
(a) Start of cut
Start of peeling
0
100
200 300 400 Capacity (mAh/g)
500
600
Fig. 2. First discharge–charge curves of Na/Sn-Co cells with PAA, CMC, PI, PAANa, and PVdF as binders in the 0.01 to 1.5 V range at a current density of 25 mAh/g.
CMC did not have any cracks and the remaining Sn-Co particles maintained their original size and cube shape as shown in Fig. 4(a, a′) and (b, b′), respectively. On the other hand, the WEs with PI, PAANa, and PVdF clearly changed. The Sn-Co particles on the surface of the WEs with PI and PAANa remained swollen to more than double their original size as shown in Fig. 4(c, c′) and (d, d′), respectively. This indicates poor electrical contact between the particles and current collector (Cu sheet). As reported in ref. [8], large volume changes during alloying of Sn-Co with Na can result in loss of electrical contact between the cracked and isolated Sn-Co particles and current collector. The surface of the WE with PVdF had a large crack, and Sn-Co cube particles collapsed and fractured as shown in Fig. 4(e, e′). Such cracks and collapsed particles were observed in a wide area. These results suggest that the WEs with PVdF and Sn-Co particles considerably expanded from their original size. Then, result of tensile stress was generated in WE, and cracks occurred. Therefore, the electrical contact between particles and current collector becomes poor and Sn-Co particles deactivate. As a result, the capacity also decreases.
Cu sheet WE composite Glass
(b)
Cutting load
Blade cuts Into Cu
Blade moves horizontally Adhesion strength
Discharge capacity (mAh/g)
Blade
600 500
CMC PAA
400 300 PI
200 100
PVdF PAANa
0 Cutting time Fig. 1. (a) Schematic diagram of measurement process of surface and interfacial cutting analysis system (SAICAS). (b) Cutting time versus cutting load of electrode.
0
5
10 15 Cycle number
20
Fig. 3. Cycle properties of Na/Sn-Co cells with PAA, CMC, PI, PAANa, and PVdF as binders.
Please cite this article as: Y. Yui, et al., Electrochemical properties of Sn-Co electrode with various kinds of binder materials for sodium ion batteries, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.01.007
Y. Yui et al. / Solid State Ionics xxx (2016) xxx–xxx
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Fig. 4. SEM images of WEs with (a and a′) PAA, (b and b′) CMC, (c and c′) PI, (d and d′) PAANa, and (e and e′) PVdF as binders: (a)–(c) as prepared; (a′)–(e′), after fifth cycle.
The expansion and contraction behaviors of Sn-Co electrodes should have a clear correlation with cycle performance. We therefore evaluated the change in WE volume during the first cycle by in situ light microscopy. The WE thickness was measured by line analysis to obtain a quantitative expansion rate in the same manner as reported previously [9]. Fig. 5 shows the expansion rate versus the state of charge (SOC) during cycling. Here, the thickness of the as-prepared WE is defined as expansion rate = 100%. SOC values of 100% and 0% mean full charge and full discharge, respectively. The WE with PVdF expanded 227% after discharge, and did not shrink at all after charge. This result would explain the large surface crack in the SEM image. The WEs with PAANa and PI expanded a little, 146% and 128%, respectively, even though they did not shrink after charge. This indicates that the WE might repeatedly expand every cycle. Clarifying this point requires further investigation. In addition, this result would explain why the Sn-Co particles remained swollen in the SEM images. The WEs with PAA and CMC expanded a little, 130% and 137%, respectively, and shrank to almost their original size. To ensure reproducibility, these measurements were conducted several times and confirmed to be highly reproducible. Consequently, with a correlation between cycle performance and WE expansion and contraction, the
WEs with PAA and CMC showed good cycle performance. It is conceivable that the rates of expansion and shrinkage of electrodes have a strong correlation with the adhesion strength of binders. This is because of the binding of the Sn-Co particles and the conductive material and the Cu sheet by the binder. For this reason, we investigated the adhesion strength between WE composites incorporating the various kinds of binders and Cu sheet using the SAICAS. Fig. 6 shows cutting time versus cutting load of a WE with PAA, obtained by using the SAICAS. Fig. 6(a) is an overall view of the SAICAS result, and Fig. 6(b) shows the details of the peeling mode. At 255 s, the blade reached the WE composite, and the cutting load dropped to 0 kN/m as shown in Fig. 6(a). Then, the Cu sheet was peeling from the WE composite. The cutting load during the peel-off of the copper shown in Fig. 6(b) corresponds to the adhesion strength, and the adhesion strength between WE composites incorporating the PAA and Cu sheet was 0.69–0.89 kN/m. For comparison, the adhesion strength was calculated as the average values for 400 to 500 s as shown in Fig. 6, and the values are shown in Table 1. Average adhesion strength of WEs with CMC, PI, PVdF, and PAANa were also obtained in the same manner, and the results are shown in Table 1 with the results obtained
Please cite this article as: Y. Yui, et al., Electrochemical properties of Sn-Co electrode with various kinds of binder materials for sodium ion batteries, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.01.007
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Y. Yui et al. / Solid State Ionics xxx (2016) xxx–xxx
Discharge
Expansion rate (%)
250
Voltage range: 0.01 – 1.5 V 200
Table 1 Average adhesion strength and the rate for expansion and contraction of WEs with PAA, CMC, PI, PAANa, and PVdF as binders.
Charge
Adhesion strength (kN/m)
PAA CMC PI PAANa PVdF
Current density: 25 mA/g
150
PAA CMC PI PVdF PAANa
0.72 0.62 0.58 0.60 0.65
Expansion rate (%) After discharge
After charge
130 137 128 227 146
111 125 125 198 143
100 100
100
100 SOC (%)
Fig. 5. Expansion rate of WEs with PAA, CMC, PI, PAANa, and PVdF as binders versus SOC during first discharge and first charge by line analysis of the image of the cross-sectional surface of WEs obtained by in situ light microscopy.
in Fig. 5. The order of adhesion strength for the binders is PAA (0.72 kN/ m) N PAANa (0.65 kN/m) N CMC (0.62 kN/m) N PVdF (0.60 kN/m) N PI (0.58 kN/m). To check the reproducibility of the measurements, we carried out the measurements twice and confirmed that the error was 2% or less. The good adhesion strength of PAA, PAANa, and CMC is due to hydrogen bonds with the Cu sheet [22]. In addition, the order of increasing expansion after the first cycle was PAA (111%) b CMC (125%) = PI (125%) b PAANa (143%) b PVdF (198%). This suggests that the better adhesion strength between the Sn-Co particles, conducting materials, and current collector leads to electrode shrinkage during the charging
Cutting load (kN/m)
(a) 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0
Cutting mode
Cutting load (kN/m)
4. Conclusions We investigated the electrochemical properties of a Sn-Co electrode as anode material with various kinds of binders for SIBs, which were analyzed by using SEM, in situ light microscopy, and the SAICAS. The cycle properties of Sn-Co electrodes with PVdF, PAANa, and PI were poor; in contrast, the WEs with PAA and CMC showed good cycle stability (over 400 mAh/g up to 20 cycles). This is attributed to the shrinkage of the WEs with PAA (expansion rate after first cycle: 111%) and CMC (125%) during cycling because of the good adhesion strength of PAA (0.72 kN/m) and CMC (0.62 kN/m). Acknowledgements
Peeling mode
We are grateful to Mitsubishi Material Corp. for supplying the Sn-Co powder.
Adhesion strength References
0
(b)
process. Despite the good adhesion strength of PAANa, it is not clear why the electrode did not shrink. The likely reason is that the molecular weight of PAANa is smaller than that of the other binders. Clarifying the reason requires further investigation. As described above, there is a significant correlation between the properties of WEs incorporating binders and their electrochemical properties. A good adhesion strength of electrodes with binders leads to little expansion and considerable shrinkage during cycling, which leads to good cycle performance. Hence, good adhesion is a key factor in improving capacity retention.
100
1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
200 300 400 Cutting time (s)
500
600
Adhesion strength
250
300
350 400 450 500 Cutting time (s)
550
Fig. 6. Cutting time versus cutting load of WE with PAA by using SAICAS.
600
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Please cite this article as: Y. Yui, et al., Electrochemical properties of Sn-Co electrode with various kinds of binder materials for sodium ion batteries, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.01.007
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Please cite this article as: Y. Yui, et al., Electrochemical properties of Sn-Co electrode with various kinds of binder materials for sodium ion batteries, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.01.007
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Electrochemical properties of Sn-Co electrode with various kinds of binder materials for sodium ion batteries Yuhki Yui a,b, Masahiko Hayashi a, Katsuya Hayashi c, Jiro Nakamura a,b a b c
NTT Device Technology Labs., NTT Corporation, Atsugi, Kanagawa, Japan Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Tokyo, Japan NTT Network Technology Labs., NTT Corporation, Atsugi, Kanagawa, Japan
a r t i c l e
i n f o
Article history: Received 17 July 2015 Received in revised form 24 November 2015 Accepted 2 January 2016 Available online xxxx Keywords: Sodium ion batteries Anode Binder Sn-Co In situ light microscopy SAICAS
a b s t r a c t Discharge–charge properties of Sn-Co anode materials with various kinds of binders [polyvinylidene difluoride (PVdF), polyacrylic acid (PAA), sodium polyacrylate (PAANa), sodium carboxymethyl cellulose (CMC), and polyimide (PI)] for sodium ion batteries were investigated to determine the correlation between cycle performance and the properties of electrode with binder. Sn-Co electrodes with PAA or CMC binders exhibited better cycle properties (discharge capacities of more than 400 mAh/g up to 20 cycles) than with PVdF, PAANa, or PI. These cycle properties with PAA or CMC were due to smaller changes in the electrode volume that occurred during cycling, as revealed by in situ light microscopy. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Lithium-ion batteries (LIBs) are used for power storage in mobile devices, electrical products, and electric vehicles, and the demand for them is likely to increase. However, lithium is expensive because it is not an abundant metal [1]. On the other hand, sodium is abundant and cheap [1], and interest in sodium-ion batteries (SIBs) has been growing. The materials that have been studied for use as SIB anodes include hard carbon [2–5], tin [6–8], tin-based materials [9–10], antimony-based materials [11–12], germanium [13], and oxide materials [14–21]. However, there are few materials that satisfy the requirements for both large capacity and good cyclability. In a previous study, we focused on Sn-Co, and found that it shows better cycling performance than tin electrodes [9]. In addition, to improve the cyclability of alloy-based anodes for LIBs in recent years, various kinds of binders have been studied, such as polyacrylic acid (PAA) [22–23], sodium polyacrylate (PAANa) [24], sodium carboxymethyl cellulose (CMC) [22–23], and polyimide (PI) [25]. In this study, to further improve the cyclability, we investigated the electrochemical properties of anodes with PAA, PAANa, CMC, and PI as binder materials. Furthermore, an investigation of the change in volume and adhesion strength of electrodes with binders revealed the relationship between them and cycle performance. 2. Experimental Sn-Co powder (element ratio: Sn:Co = 9:1) was obtained from Mitsubishi Materials Corporation as reported in our previous study [9].
PVdF (molecular weight (Mw) = 1,000,000, Kureha Corp.), PAA (Mw = 1,250,000, Sigma-Aldrich Co. Ltd.), PAANa (Mw = 15,000, Sigma-Aldrich Co. Ltd.), CMC (Mw = 250,000, Sigma-Aldrich Co. Ltd.), and PI (I.S.T. Corp.) were used as binders. The working electrodes (WEs) were prepared by mixing 80 wt% of Sn-Co powder, 10 wt% of Ketjen Black EC600JD (Lion Co.) and 10 wt% of the binder in N-methylpyrrolidone (PVdF, PI) or distilled water (PAA, PAANa, CMC) and then coating the mixture on a Cu sheet and drying it at 90 °C. The electrochemical performance was evaluated with a 2032 coin-type cell using a WE (0.02 mm thick and 14 mm in diameter), an electrolyte solution (1 mol/l NaPF6/EC:DEC 1:1 in volume, Tomiyama Pure Chemicals Industries Ltd.), a polypropylene separator (Celgard, 19 mm in diameter), and Na metal sheets (0.6 mm thick and 15 mm in diameter) as a counterelectrode (CE). All cells were assembled in an Ar-filled glove box. Cycling tests were performed using an automatic galvanostatic discharge–charge system (Hokuto HJ1001SD8) at a constant current density of 25 mA/g between 0.01 and 1.5 V at 25 °C. After the cell voltage had reached 0.01 V, the cell was kept at a constant voltage of 0.01 V for 10 h. To analyze the surface of the WEs after the fifth cycle with a scanning electron microscope (SEM, JEOL Ltd., JSM890) with accelerating voltage of 7 kV, the cells were opened after the fifth cycle in the Ar-filled glovebox and the WEs were washed with dimethyl carbonate and dried. Changes in WE volume during the first cycle were directly observed by using in situ light microscopy (Lasertec Corp., ECCS B310). The rate for expansion and contraction of WEs was estimated in the line analysis mode. The cross-sectional surface of the semicircular cell of WE/
http://dx.doi.org/10.1016/j.ssi.2016.01.007 0167-2738/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: Y. Yui, et al., Electrochemical properties of Sn-Co electrode with various kinds of binder materials for sodium ion batteries, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.01.007
Y. Yui et al. / Solid State Ionics xxx (2016) xxx–xxx
separator/CE (Na) was monitored via an observation window made of sapphire by using light microscopy in the same manner as reported previously [9]. Discharge–charge tests were performed in the same condition as with the coin-type cell mentioned above, and the results were similar to the case of coin-type cell. An analysis of adhesion strength between WE composites incorporating various kinds of binders and a Cu sheet was performed using a surface and interfacial cutting analysis system (SAICAS, Daipla Wintes Co., Ltd.) as shown in Fig. 1(a) [26–27]. The WE composite side was attached to a glass plate with epoxy. A diamond blade (width: 1 mm) cuts downward at a 20° shear angle through the Cu sheet. During the test, the blade moved in the horizontal direction at 1 μm/s. When the blade reached the WE composite, it moved horizontally, peeling off the copper. The adhesion strength is the average cutting load during the peeling off of the copper as shown in Fig. 1(b). 3. Results and discussion
2 PAA CMC PI PAANa PVdF
1.5
Voltage (V)
2
1 Voltage range: 0.01 – 1.5 V Current density: 25 mA/g
0.5
0
-0.5
Fig. 2 shows the first discharge–charge curves of Na/Sn-Co cells with various kinds of binders in the 0.01 to 1.5 V range. The first discharge capacities of the Sn-Co electrode with PVdF, PAA, PAANa, CMC, and PI were 569, 523, 598, 486, and 537 mAh/g, respectively. All cells using the various kinds of binders showed two distinct plateaus (at about 0.01–0.25 and 0.45–0.65 V for the charge process), and the type of binder did not affect plateau voltage. This suggests that the binders were not involved in the discharge–charge reaction. Fig. 3 shows the cycle properties of the cells. The WEs with PAA and CMC showed better cycle properties than those with PI, PAANa, and PVdF, and the discharge capacity of the former two reached more than 400 mAh/g after 20 cycles. The difference in the cycle properties may be due to a difference in the change in WE volume during cycling. If there is a difference in WE volume, the WE surface after the cycle would be changed. Accordingly, we observed the WE surface before and after cycle tests using SEM. Fig. 4 shows SEM images of the surface of pristine (a)–(e) and fifth-cycled (a′)–(e′) WEs. Before cycling, the surface morphologies were almost the same as for WEs with the various kinds of binders, and there were no cracks and the Sn-Co cube particles were 0.7– 2.0 μm in size. After the fifth cycle, the WE surface with PAA and
(a) Start of cut
Start of peeling
0
100
200 300 400 Capacity (mAh/g)
500
600
Fig. 2. First discharge–charge curves of Na/Sn-Co cells with PAA, CMC, PI, PAANa, and PVdF as binders in the 0.01 to 1.5 V range at a current density of 25 mAh/g.
CMC did not have any cracks and the remaining Sn-Co particles maintained their original size and cube shape as shown in Fig. 4(a, a′) and (b, b′), respectively. On the other hand, the WEs with PI, PAANa, and PVdF clearly changed. The Sn-Co particles on the surface of the WEs with PI and PAANa remained swollen to more than double their original size as shown in Fig. 4(c, c′) and (d, d′), respectively. This indicates poor electrical contact between the particles and current collector (Cu sheet). As reported in ref. [8], large volume changes during alloying of Sn-Co with Na can result in loss of electrical contact between the cracked and isolated Sn-Co particles and current collector. The surface of the WE with PVdF had a large crack, and Sn-Co cube particles collapsed and fractured as shown in Fig. 4(e, e′). Such cracks and collapsed particles were observed in a wide area. These results suggest that the WEs with PVdF and Sn-Co particles considerably expanded from their original size. Then, result of tensile stress was generated in WE, and cracks occurred. Therefore, the electrical contact between particles and current collector becomes poor and Sn-Co particles deactivate. As a result, the capacity also decreases.
Cu sheet WE composite Glass
(b)
Cutting load
Blade cuts Into Cu
Blade moves horizontally Adhesion strength
Discharge capacity (mAh/g)
Blade
600 500
CMC PAA
400 300 PI
200 100
PVdF PAANa
0 Cutting time Fig. 1. (a) Schematic diagram of measurement process of surface and interfacial cutting analysis system (SAICAS). (b) Cutting time versus cutting load of electrode.
0
5
10 15 Cycle number
20
Fig. 3. Cycle properties of Na/Sn-Co cells with PAA, CMC, PI, PAANa, and PVdF as binders.
Please cite this article as: Y. Yui, et al., Electrochemical properties of Sn-Co electrode with various kinds of binder materials for sodium ion batteries, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.01.007
Y. Yui et al. / Solid State Ionics xxx (2016) xxx–xxx
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Fig. 4. SEM images of WEs with (a and a′) PAA, (b and b′) CMC, (c and c′) PI, (d and d′) PAANa, and (e and e′) PVdF as binders: (a)–(c) as prepared; (a′)–(e′), after fifth cycle.
The expansion and contraction behaviors of Sn-Co electrodes should have a clear correlation with cycle performance. We therefore evaluated the change in WE volume during the first cycle by in situ light microscopy. The WE thickness was measured by line analysis to obtain a quantitative expansion rate in the same manner as reported previously [9]. Fig. 5 shows the expansion rate versus the state of charge (SOC) during cycling. Here, the thickness of the as-prepared WE is defined as expansion rate = 100%. SOC values of 100% and 0% mean full charge and full discharge, respectively. The WE with PVdF expanded 227% after discharge, and did not shrink at all after charge. This result would explain the large surface crack in the SEM image. The WEs with PAANa and PI expanded a little, 146% and 128%, respectively, even though they did not shrink after charge. This indicates that the WE might repeatedly expand every cycle. Clarifying this point requires further investigation. In addition, this result would explain why the Sn-Co particles remained swollen in the SEM images. The WEs with PAA and CMC expanded a little, 130% and 137%, respectively, and shrank to almost their original size. To ensure reproducibility, these measurements were conducted several times and confirmed to be highly reproducible. Consequently, with a correlation between cycle performance and WE expansion and contraction, the
WEs with PAA and CMC showed good cycle performance. It is conceivable that the rates of expansion and shrinkage of electrodes have a strong correlation with the adhesion strength of binders. This is because of the binding of the Sn-Co particles and the conductive material and the Cu sheet by the binder. For this reason, we investigated the adhesion strength between WE composites incorporating the various kinds of binders and Cu sheet using the SAICAS. Fig. 6 shows cutting time versus cutting load of a WE with PAA, obtained by using the SAICAS. Fig. 6(a) is an overall view of the SAICAS result, and Fig. 6(b) shows the details of the peeling mode. At 255 s, the blade reached the WE composite, and the cutting load dropped to 0 kN/m as shown in Fig. 6(a). Then, the Cu sheet was peeling from the WE composite. The cutting load during the peel-off of the copper shown in Fig. 6(b) corresponds to the adhesion strength, and the adhesion strength between WE composites incorporating the PAA and Cu sheet was 0.69–0.89 kN/m. For comparison, the adhesion strength was calculated as the average values for 400 to 500 s as shown in Fig. 6, and the values are shown in Table 1. Average adhesion strength of WEs with CMC, PI, PVdF, and PAANa were also obtained in the same manner, and the results are shown in Table 1 with the results obtained
Please cite this article as: Y. Yui, et al., Electrochemical properties of Sn-Co electrode with various kinds of binder materials for sodium ion batteries, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.01.007
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Y. Yui et al. / Solid State Ionics xxx (2016) xxx–xxx
Discharge
Expansion rate (%)
250
Voltage range: 0.01 – 1.5 V 200
Table 1 Average adhesion strength and the rate for expansion and contraction of WEs with PAA, CMC, PI, PAANa, and PVdF as binders.
Charge
Adhesion strength (kN/m)
PAA CMC PI PAANa PVdF
Current density: 25 mA/g
150
PAA CMC PI PVdF PAANa
0.72 0.62 0.58 0.60 0.65
Expansion rate (%) After discharge
After charge
130 137 128 227 146
111 125 125 198 143
100 100
100
100 SOC (%)
Fig. 5. Expansion rate of WEs with PAA, CMC, PI, PAANa, and PVdF as binders versus SOC during first discharge and first charge by line analysis of the image of the cross-sectional surface of WEs obtained by in situ light microscopy.
in Fig. 5. The order of adhesion strength for the binders is PAA (0.72 kN/ m) N PAANa (0.65 kN/m) N CMC (0.62 kN/m) N PVdF (0.60 kN/m) N PI (0.58 kN/m). To check the reproducibility of the measurements, we carried out the measurements twice and confirmed that the error was 2% or less. The good adhesion strength of PAA, PAANa, and CMC is due to hydrogen bonds with the Cu sheet [22]. In addition, the order of increasing expansion after the first cycle was PAA (111%) b CMC (125%) = PI (125%) b PAANa (143%) b PVdF (198%). This suggests that the better adhesion strength between the Sn-Co particles, conducting materials, and current collector leads to electrode shrinkage during the charging
Cutting load (kN/m)
(a) 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0
Cutting mode
Cutting load (kN/m)
4. Conclusions We investigated the electrochemical properties of a Sn-Co electrode as anode material with various kinds of binders for SIBs, which were analyzed by using SEM, in situ light microscopy, and the SAICAS. The cycle properties of Sn-Co electrodes with PVdF, PAANa, and PI were poor; in contrast, the WEs with PAA and CMC showed good cycle stability (over 400 mAh/g up to 20 cycles). This is attributed to the shrinkage of the WEs with PAA (expansion rate after first cycle: 111%) and CMC (125%) during cycling because of the good adhesion strength of PAA (0.72 kN/m) and CMC (0.62 kN/m). Acknowledgements
Peeling mode
We are grateful to Mitsubishi Material Corp. for supplying the Sn-Co powder.
Adhesion strength References
0
(b)
process. Despite the good adhesion strength of PAANa, it is not clear why the electrode did not shrink. The likely reason is that the molecular weight of PAANa is smaller than that of the other binders. Clarifying the reason requires further investigation. As described above, there is a significant correlation between the properties of WEs incorporating binders and their electrochemical properties. A good adhesion strength of electrodes with binders leads to little expansion and considerable shrinkage during cycling, which leads to good cycle performance. Hence, good adhesion is a key factor in improving capacity retention.
100
1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
200 300 400 Cutting time (s)
500
600
Adhesion strength
250
300
350 400 450 500 Cutting time (s)
550
Fig. 6. Cutting time versus cutting load of WE with PAA by using SAICAS.
600
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Please cite this article as: Y. Yui, et al., Electrochemical properties of Sn-Co electrode with various kinds of binder materials for sodium ion batteries, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.01.007