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Mildly expanded graphite for anode materials of lithium ion battery synthesized with perchloric acid
Electrochimica Acta 116 (2014) 170–174
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Mildly expanded graphite for anode materials of lithium ion battery synthesized with perchloric acid Yuxiao Lin, Zheng-Hong Huang ∗ , Xiaoliang Yu, Wanci Shen, Yongping Zheng, Feiyu Kang Key Laboratory of Advanced Materials of Ministry of Education, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 29 September 2013 Received in revised form 9 November 2013 Accepted 9 November 2013 Available online 20 November 2013
a b s t r a c t Mildly expanded graphite (MEG) was synthesized by using perchloric acid as both intercalating agent and oxidizing agent. Its performance as anode material for lithium ion battery was investigated. SEM, XRD, TEM, nitrogen adsorption and TGA/DSC were used to characterize the sample. Charge/discharge tests show that the MEG exhibits a rate capacity as high as 397 mAh/g at 0.2 C and 250 mAh/g at 1.6 C. © 2013 Elsevier Ltd. All rights reserved.
Keywords: Mildly expanded graphite Porous structure Lithium ion battery High capacity
1. Introduction Portable electronic devices, such as mobile phone and notebook computer, are becoming more and more popular, and even electrical vehicles have already been put into applications. They have more demands for lithium ion battery in all respects, such as the capacity, the performance under high charge/discharge rate, the stability under cycling, safety, cost, etc. Natural flake graphite has been playing a dominating role in the field of the anode material in lithium ion battery due to its excellent electrochemical performance and low cost. However, there are still two issues limiting its further development: poor cycle stability resulting from the volume change during the intercalation and deintercalation of lithium ions, and relatively low theory capacity (372 mAh/g) decided by its microstructure [1–3]. Up to now, all kinds of modification methods including mild oxidation [4–7], metals or metal oxides deposition [8], coating [9,10], expansion [11–14] and edge exfoliation [15], have been applied to improve the performance of natural graphite as anode material in lithium ion battery, among which mild expansion was an effective method put forward by our group [14]. By the process of intercalating reaction and deintercalating heat treatment, the cycling performance of the obtained mildly expanded graphite is greatly improved. However, the temperature of the heat treatment is as high as 360 ◦ C and the time is as long as 28 hours because of the
∗ Corresponding author. Tel.: +86 10 62773752; fax: +86 10 62773752. E-mail address: [email protected] (Z.-H. Huang). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.11.057
high boiling point of sulfuric acid, resulting in some trouble in its industrial manufacture. What’s more, the sulfur remaining in the mildly expanded graphite is also pollutive. In the present work, we synthesized mildly expanded graphite using perchloric acid as both intercalating and oxidizing agent. Because of its stronger oxidizing effect compared with sulfuric acid, perchloric acid can be intercalated to the graphite without other strong oxidizing agent. In addition, due to its relatively lower boiling point, the deintercalating process can be performed under lower temperature in less time. Moreover, the capacity of the obtained mildly expanded graphite is improved, indicating that the two limitations and other problems relating to the mild expansion were fundamentally solved.
2. Experimental Raw materials were used as follows: spherical natural flake graphite (NFG) from Jixi, China with carbon content of 99 wt % and average particle size of 22.4 m, commercially available perchloric acid (72 wt %). Some previous researches about expanded graphite [16–18] were referred to determine parameters chosen in the synthesis, but the treatment afterwards was quite different. In a typical synthesis, NFG was added into the solution of perchloric acid in a three-neck flask, and stirred at 120 ◦ C. The mass ratio of graphite and perchloric acid was 1:4. After the reaction for 0.5 h, the mixture became glutinous slurry indicating that the reaction was completed. The obtained HClO4 –GIC was washed and centrifugally separated repeatedly until the pH value became 4 and then dried at
Y. Lin et al. / Electrochimica Acta 116 (2014) 170–174
171
Fig. 1. SEM micrographs of NFG (a, b), washed HClO4 –GIC (c, d) and MEG (e, f).
100 ◦ C. Finally, the mildly expanded graphite (MEG) was obtained by heating the HClO4 –GIC up to 200 ◦ C with the ramping rate of 1.5 ◦ C/min and the holding time of 2 h. The working electrode was prepared by casting the slurry of mildly expanded graphite (MEG, 85 wt %), polyvinylidene fluoride (PVDF, 10 wt %) and carbon black (5 wt %) dissolved in N-methylpyrrolidone (NMP) on a copper foil. The foil was dried at 80 ◦ C for 8 h and then dried in vacuum at 120 ◦ C for 12 h. Lithium foil was used as the counter electrode and 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) (1:1:1, v/v) served as the electrolyte. The working and the counter electrodes were separated with Celgard 2400 separator. The cell was galvanostatically cycled between 0 and 1.50 V vs. Li/Li+ . 3. Results and discussion Fig. 1 shows the morphologies of NFG, washed HClO4 –GIC and MEG. The particle of NFG is spherical and its surface is quite smooth. However, after the reaction, many folds and cracks appeared and still existed even after heat treatment, making the particles not only rough but also irregular. The specific surface area increases from 4 m2 /g to 36 m2 /g according to the result of nitrogen adsorption test, regardless of the contribution of closed pores which cannot be detected in this test [19]. The mean particle size and expansion
volume also increase from 22.4 m and 0.97 ml/g to 42.7 m and 3.89 ml/g, respectively. Obviously, the surface of MEG is rougher. This change will influence its performance as anode material of lithium ion battery in two opposite ways. The first cycle columbic efficiency will be greatly decreased because there will be more surfaces covered by the solid state-electrolyte interface film. On the contrary, its capacity will be improved because more defects will be introduced to the particle, which will provide more space for lithium ions during charging and discharging. TEM is used to further investigate the defects of the assynthesized MEG. The TEM micrographs of NFG and MEG are shown in Fig. 2. It is easy to observe the nanopores with diameter about 3–5 nm in MEG. There is a great opportunity that during the process of mild expansion, the percloric acid was decomposed and finally deintercalated from the HClO4 –GIC. The deintercalation process is also verified by the result of TGA/DSC test, which is shown in Fig. 3. As we can see, there is a remarkable exothermic peak around 175 ◦ C in the heating curve of the HClO4 –GIC, indicating the deintercalation of perchloric acid. In addition, the test of Energy Dispersive Spectrometer (EDS) shows that the weight percentages of chlorine in the HClO4 –GIC unwashed, HClO4 –GIC washed completely and mildly expanded graphite are 3.31%, 0.34% and 0.17%. This indicates that perchloric acid is suitable in this work due to its low boiling point, strong oxidizing property and high tendency to deintercalation. Comparing to other works using sulfur acid and other
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Intensity (a.u.)
GIC
NFG
MEG 20
30
40
50 2θ (°)
60
70
80
Fig. 4. XRD patterns of NFG, HClO4 –GIC and MEG.
Fig. 2. TEM micrographs of NFG (a) and MEG (b).
intercalating agent, the temperature and time required in heat treatment are largely reduced. The content of chlorine remained in MEG is also much less than that of chlorine or sulfur in other experiments [20,21]. XRD patterns shown in Fig. 4 provide some information for the change inside the crystal of graphite and the data calculated from the XRD patterns are listed in Table 1. It is obvious that curves of both NFG and MEG have a sharp peak of (002) at around 26.5◦ and a small peak of (004) at 54.0◦ , while the (002) peak of HClO4 –GIC has two branches, the major one of which shifted toward the left side when the other stays almost at the same position. It is obvious that
2 Exothemic
175.78
100
DSC (mW/mg)
TGA 90 0
mass (%)
95
1
curves of both NFG and MEG have a sharp peak of (002) at around 26.5◦ and a small peak of (004) at 54.0◦ , while the peak of HClO4 –GIC has two branches, the major one of which shifted toward the left side when the other stays almost at the same position. Besides these two branches, it is also worth noting that there are other three small peaks at 29.6◦ , 51.8◦ and 56.6◦ . All these peaks are characteristics of many stages HClO4 –GIC [20–22]. Furthermore, our calculation confirms the formation of stage 4 or 5 HClO4 –GIC. It can be explained that after the reaction with perchloric acid, the majority of NFG particles are intercalated, contributing to the 4 peaks at 25.3◦ ,29.6◦ , 51.8◦ and 56.6◦ , while tiny part of NFG particles are not intercalated, contributing to the relatively week peak of (002) and (004) staying almost at original position. After washing and mild expansion, the perchloric acid are decomposed and deintercalated from NFG, making the sample resume to a state quite similar to the original state. This suggests that the treatment of mild expansion preserved the original layer structure of the NFG. However, according to the data calculated from the XRD patterns, there is a decrease of 13.6 nm in the micro crystallite size and a certain increase of 0.0002 nm in the interlayer space. Larger interlayer distance means less resistance for the intercalation and deintercalation of the lithium ions which is beneficial to its cycle performance. And the decrease in the microcrystallite size can be explained that some graphite crystals are cracked. Then there will be many defects appeared on each cracked crystals, which also provide extra space for the lithium ions during intercalation and deintercalation. From all these data above, we can probably deduce that perchloric acid is successfully intercalated into the natural flake graphite during the reaction, and largely removed during the following process of washing and mild expansion. Even though the rough surface with large specific surface area may have a harmful influence on the first columbic efficiency, many defects introduced during the process of intercalation and deintercalation, such as nanopores and cracks between crystal will contribute to the enhancement of capacity by providing extra space for lithium ion during charging and discharging.
85
DSC
80
-1 100
200
300
Temperature Fig. 3. The TGA/DSC curve of HClO4 –GIC.
400
Table 1 data calculated from the XRD patterns. Sample
2(◦ )
Micro crystallite size (nm)
d002 (nm)
NFG GIC MEG
26.566 25.350 26.546
58.9 45.3
0.3356 0.3511 0.3358
Y. Lin et al. / Electrochimica Acta 116 (2014) 170–174
450
(a) 1.5
MEG
1.0
NFG
MEG
Capacity (mAh/g)
400
Voltage (V)
173
0.2C
0.2C
0.4C 0.8C
350 NFG 300
1.6C 250
0.5
200 150 0
0.0 0
100
200
300
400
20
40
60
Cycle number
Specific Capacity (mAh/g) Fig. 6. Electrochemical performance of NFG and MEG.
(b) 1.5
1.0 Voltage (V)
NFG
MEG
0.5
0.0 0
100
200
300
400
Specific Capacity (mAh/g) Fig. 5. Charge/discharge curves of NFG and MEG at 1st cycle (a) and 30th cycle (b).
Figs. 5 and 6 show the charge/discharge curves and electrochemical performance of natural flake graphite and mildly expanded graphite. As we can see, the discharge capacity of NFG and MEG at the 1st cycle is 334.5 and 366.5 mAh/g, while the charge capacities of NFG and MEG at the 1st cycle is 362.5 and 429.4 mAh/g, respectively. Therefore the columbic efficiency of the first cycle is 91.3% and 84.4%. It seems that the treatment does have contribution to the specific capacity. However, most of the improvement lies in the irreversible part, making the columbic efficiency much lower than that of NFG. Fig. 5a shows that the major difference between the two curves lies in the beginning part of the discharge curve, indicating the formation of SEI layer, which is the main reason for the irreversible capacity. As the cycling process goes on, the capacities of both samples have rebounded. At the 30th cycle, the capacity of NFG and MEG recovered to 360 and 395 mAh/g, respectively. Moreover, when the charge/discharge current density increased afterward, MEG shows much better performance than NFG. Typically, when the charge/discharge density is increased to 1.6 C, the capacity of MEG still remains at 260 mAh/g, almost twice as much as NFG. When the current density resumes to 0.2 C, the capacity of MEG maintains at 397 mAh/g, which is still higher than that of NFG. In addition, when measuring the depth of the electrode foil
before and after charging, we observed a 12.5% increase for the MEG sample, which is lower than the 16% increase for the NFG sample. Therefore the volume change during cycling is buffered in the MEG. It is quite worthy of note that the specific capacity at current density of 0.2 C is much higher than the theoretical capacity of graphite (372 mAh/g). As shown in Fig. 2, there are many defects such as cracks on the surface and nanopores inside the particles in MEG. Also, it is confirmed that the expansion volume of MEG increases nearly 3 times. These two characteristics are well in agreement with the cavity mechanism proposed by Akihiro Mabuchi in which lithium ions can both be intercalated into interlayers and inserted into cavities [23]. What’s more, the cavity mechanism are also confirmed in our previous work [14]. Besides, decreased micro crystallite size also indicates that it could not only shorten the distance of lithium ion channels in graphite, but also provide more space to store lithium ions [24]. To sum up, all these defects will contribute to the enhancement in capacity because these defects will provide extra space for lithium ions storage during the process of charging and discharging [4,25]. However, the specific surface area increased due to the cracks. Thus, the columbic efficiency will be greatly affected because more surfaces will be covered by the solid state-electrolyte interface film. In the cycling performance, both the defects and slightly increased interlayer space play roles. It is well known that the lithium ions will encounter some resistance during the process of charging and discharging, making the volume of graphite change greatly and thus causing severe damage to the electrode [3]. In contrast, the electrode prepared by MEG will provide less resistance and thus suffer less damage from the volume change during cycling especially at high charge/discharge current density [15], which is in agreement with the measure of the electrode depth. It means that the larger part of capacity will be maintained when cycled at high charge/discharge current density. In comparison with previous method to synthesize mildly expanded graphite using sulfuric acid as intercalative agent and hydrogen peroxide as oxidizing agent, there are lots of advantages for this method. First, the oxidizing ability of perchloric acid is strong enough to open the edges of graphite, so perchloric acid can serve both as oxidizing agent and intercalating agent, avoiding usage of other strong oxidizing agent. Moreover, as the boiling point of perchloric acid is much lower, both the temperature and time required to deintercalate greatly decreased. It takes 28 hours under 360 ◦ C in previous method, while 4 hours and 200 ◦ C are required in this method. Also, only 0.17% weight percent of chlorine remained in MEG is quite acceptable in comparable to the content of chlorine
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or sulfur in other experiments [26,27]. Last, the capacity as high as 396 mAh/g of as prepared MEG is also higher than 378 mAh/g of MEG obtained by the previous method. 4. Conclusion MEG with many defects inside and slightly increased interlayer space was synthesized using perchloric acid as both intercalating and oxidizing agent. The as-prepared MEG exhibits a rate capacity as high as 395 mAh/g at 0.2 C and 250 mAh/g at 1.6 C. The defects help to improve both the capacity and cycling performance by providing extra space to store lithium ions and buffering the volume change during charging and discharging. And the slightly increased interlayer space also helps to reduce the influence of volume change by weakening the resistance encountered by lithium ions. What’s more, there are many advantages for the method using only perchloric acid to prepare MEG, including lower temperature and less time of deintercalation, less maintaining pollutive element and higher capacity. Acknowledgement The authors thank the financial support from the National Nature Science Foundation of China under grant No. 51232005 and the National Key Technology R & D Program of China under Grant No. 2008BAE60B08. References [1] L.J. Fu, H. Liu, C. Li, Y.P. Wu, E. Rahm, R. Holze, H.Q. Wu, Solid State Sciences 8 (2006) 113–128. [2] M. Endo, C. Kim, K. Nishimura, T. Fujino, K. Miyashita, Carbon 38 (2000) 183–197.
[3] M. Wissler, Journal of Power Sources 156 (2006) 142–150. [4] C. Menachem, Y. Wang, J. Flowers, E. Peled, S.G. Greenbaum, Journal of Power Sources 76 (1998) 180–185. [5] Y.P. Wu, R. Holze, Journal of Solid State Electrochemistry 8 (2003) 73–78. [6] Y. Wu, C. Jiang, C. Wan, E. Tsuchida, Electrochemistry Communications 2 (2000) 272–275. [7] J. Shim, K.A. Striebel, Journal of Power Sources 164 (2007) 862–867. [8] W.-J. Zhang, Journal of Power Sources 196 (2011) 13–24. [9] Y.-S. Wu, Y.-H. Wang, Y.-H. Lee, Journal of Alloys and Compounds 426 (2006) 218–222. [10] L. Zou, F. Kang, X. Li, Y.-P. Zheng, W. Shen, J. Zhang, Journal of Physics and Chemistry of Solids 69 (2008) 1265–1271. [11] A. Bhattacharya, A. Hazra, S. Chatterjee, P. Sen, S. Laha, I. Basumallick, Journal of Power Sources 136 (2004) 208–210. [12] F. Kenji, K. Kazuhiko, I. Kennichi, Y. Masaki, Journal of Power Sources 69 (1997) 165–168. [13] S. Yang, H. Song, X. Chen, Electrochemistry Communications 8 (2006) 137–142. [14] L. Zou, F. Kang, Y.-P. Zheng, W. Shen, Electrochimica Acta 54 (2009) 3930–3934. [15] J.-S. Park, M.-H. Lee, I.-Y. Jeon, H.-S. Park, J.-B. Baek, H.-K. Song, ACS Nano 6 (2012) 10770–10775. [16] Z. Ying, X. Lin, Y. Qi, J. Luo, Materials Research Bulletin 43 (2008) 2677–2686. [17] X.H. Wei, L. Liu, J.X. Zhang, J.L. Shi, Q.G. Guo, Materials Letters 63 (2009) 1618–1620. [18] X.H. Wei, L. Liu, J.X. Zhang, J.L. Shi, Q.G. Guo, Materials Letters 64 (2010) 1007–1009. [19] K. Mergia, K.L. Stefanopoulos, N. Ordás, C. García-Rosales, Microporous and Mesoporous Materials 134 (2010) 141–149. [20] Jihui-Li, Huifang-Da, Qian-Liu, Shufen-Liu, Materials Letters 60 (2006) 3927–3930. [21] M.S. Zhou, L.M. Liu, C.J. Li, Y. Tao, New Carbon Materials 25 (2010) 389–394. [22] X.H. Wei, L. Liu, J.X. Zhang, J.L. Shi, Q.G. Guo, New Carbon Materials 22 (2007) 342–348. [23] A. Mabuchi, K. Tokumitsu, H. Fujimoto, T. Kasuh, J Electrochem Soc 142 (1995) 1041–1046. [24] E. Peled, C. Menachem, D. BarTow, A. Melman, J Electrochem Soc 143 (1996) L4–L7. [25] W. Mao, J. Wang, Z. Xu, Z. Niu, J. Zhang, Electrochemistry Communications 8 (2006) 1326–1330. [26] J.M. Skowronski, Journal of Physics and Chemistry of Solids 57 (1996) 695–701. [27] X.L. Chen, K.M. Song, J.H. Li, J.P. Liu, Carbon 34 (1996) 1599–1600.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Mildly expanded graphite for anode materials of lithium ion battery synthesized with perchloric acid Yuxiao Lin, Zheng-Hong Huang ∗ , Xiaoliang Yu, Wanci Shen, Yongping Zheng, Feiyu Kang Key Laboratory of Advanced Materials of Ministry of Education, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 29 September 2013 Received in revised form 9 November 2013 Accepted 9 November 2013 Available online 20 November 2013
a b s t r a c t Mildly expanded graphite (MEG) was synthesized by using perchloric acid as both intercalating agent and oxidizing agent. Its performance as anode material for lithium ion battery was investigated. SEM, XRD, TEM, nitrogen adsorption and TGA/DSC were used to characterize the sample. Charge/discharge tests show that the MEG exhibits a rate capacity as high as 397 mAh/g at 0.2 C and 250 mAh/g at 1.6 C. © 2013 Elsevier Ltd. All rights reserved.
Keywords: Mildly expanded graphite Porous structure Lithium ion battery High capacity
1. Introduction Portable electronic devices, such as mobile phone and notebook computer, are becoming more and more popular, and even electrical vehicles have already been put into applications. They have more demands for lithium ion battery in all respects, such as the capacity, the performance under high charge/discharge rate, the stability under cycling, safety, cost, etc. Natural flake graphite has been playing a dominating role in the field of the anode material in lithium ion battery due to its excellent electrochemical performance and low cost. However, there are still two issues limiting its further development: poor cycle stability resulting from the volume change during the intercalation and deintercalation of lithium ions, and relatively low theory capacity (372 mAh/g) decided by its microstructure [1–3]. Up to now, all kinds of modification methods including mild oxidation [4–7], metals or metal oxides deposition [8], coating [9,10], expansion [11–14] and edge exfoliation [15], have been applied to improve the performance of natural graphite as anode material in lithium ion battery, among which mild expansion was an effective method put forward by our group [14]. By the process of intercalating reaction and deintercalating heat treatment, the cycling performance of the obtained mildly expanded graphite is greatly improved. However, the temperature of the heat treatment is as high as 360 ◦ C and the time is as long as 28 hours because of the
∗ Corresponding author. Tel.: +86 10 62773752; fax: +86 10 62773752. E-mail address: [email protected] (Z.-H. Huang). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.11.057
high boiling point of sulfuric acid, resulting in some trouble in its industrial manufacture. What’s more, the sulfur remaining in the mildly expanded graphite is also pollutive. In the present work, we synthesized mildly expanded graphite using perchloric acid as both intercalating and oxidizing agent. Because of its stronger oxidizing effect compared with sulfuric acid, perchloric acid can be intercalated to the graphite without other strong oxidizing agent. In addition, due to its relatively lower boiling point, the deintercalating process can be performed under lower temperature in less time. Moreover, the capacity of the obtained mildly expanded graphite is improved, indicating that the two limitations and other problems relating to the mild expansion were fundamentally solved.
2. Experimental Raw materials were used as follows: spherical natural flake graphite (NFG) from Jixi, China with carbon content of 99 wt % and average particle size of 22.4 m, commercially available perchloric acid (72 wt %). Some previous researches about expanded graphite [16–18] were referred to determine parameters chosen in the synthesis, but the treatment afterwards was quite different. In a typical synthesis, NFG was added into the solution of perchloric acid in a three-neck flask, and stirred at 120 ◦ C. The mass ratio of graphite and perchloric acid was 1:4. After the reaction for 0.5 h, the mixture became glutinous slurry indicating that the reaction was completed. The obtained HClO4 –GIC was washed and centrifugally separated repeatedly until the pH value became 4 and then dried at
Y. Lin et al. / Electrochimica Acta 116 (2014) 170–174
171
Fig. 1. SEM micrographs of NFG (a, b), washed HClO4 –GIC (c, d) and MEG (e, f).
100 ◦ C. Finally, the mildly expanded graphite (MEG) was obtained by heating the HClO4 –GIC up to 200 ◦ C with the ramping rate of 1.5 ◦ C/min and the holding time of 2 h. The working electrode was prepared by casting the slurry of mildly expanded graphite (MEG, 85 wt %), polyvinylidene fluoride (PVDF, 10 wt %) and carbon black (5 wt %) dissolved in N-methylpyrrolidone (NMP) on a copper foil. The foil was dried at 80 ◦ C for 8 h and then dried in vacuum at 120 ◦ C for 12 h. Lithium foil was used as the counter electrode and 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) (1:1:1, v/v) served as the electrolyte. The working and the counter electrodes were separated with Celgard 2400 separator. The cell was galvanostatically cycled between 0 and 1.50 V vs. Li/Li+ . 3. Results and discussion Fig. 1 shows the morphologies of NFG, washed HClO4 –GIC and MEG. The particle of NFG is spherical and its surface is quite smooth. However, after the reaction, many folds and cracks appeared and still existed even after heat treatment, making the particles not only rough but also irregular. The specific surface area increases from 4 m2 /g to 36 m2 /g according to the result of nitrogen adsorption test, regardless of the contribution of closed pores which cannot be detected in this test [19]. The mean particle size and expansion
volume also increase from 22.4 m and 0.97 ml/g to 42.7 m and 3.89 ml/g, respectively. Obviously, the surface of MEG is rougher. This change will influence its performance as anode material of lithium ion battery in two opposite ways. The first cycle columbic efficiency will be greatly decreased because there will be more surfaces covered by the solid state-electrolyte interface film. On the contrary, its capacity will be improved because more defects will be introduced to the particle, which will provide more space for lithium ions during charging and discharging. TEM is used to further investigate the defects of the assynthesized MEG. The TEM micrographs of NFG and MEG are shown in Fig. 2. It is easy to observe the nanopores with diameter about 3–5 nm in MEG. There is a great opportunity that during the process of mild expansion, the percloric acid was decomposed and finally deintercalated from the HClO4 –GIC. The deintercalation process is also verified by the result of TGA/DSC test, which is shown in Fig. 3. As we can see, there is a remarkable exothermic peak around 175 ◦ C in the heating curve of the HClO4 –GIC, indicating the deintercalation of perchloric acid. In addition, the test of Energy Dispersive Spectrometer (EDS) shows that the weight percentages of chlorine in the HClO4 –GIC unwashed, HClO4 –GIC washed completely and mildly expanded graphite are 3.31%, 0.34% and 0.17%. This indicates that perchloric acid is suitable in this work due to its low boiling point, strong oxidizing property and high tendency to deintercalation. Comparing to other works using sulfur acid and other
172
Y. Lin et al. / Electrochimica Acta 116 (2014) 170–174
Intensity (a.u.)
GIC
NFG
MEG 20
30
40
50 2θ (°)
60
70
80
Fig. 4. XRD patterns of NFG, HClO4 –GIC and MEG.
Fig. 2. TEM micrographs of NFG (a) and MEG (b).
intercalating agent, the temperature and time required in heat treatment are largely reduced. The content of chlorine remained in MEG is also much less than that of chlorine or sulfur in other experiments [20,21]. XRD patterns shown in Fig. 4 provide some information for the change inside the crystal of graphite and the data calculated from the XRD patterns are listed in Table 1. It is obvious that curves of both NFG and MEG have a sharp peak of (002) at around 26.5◦ and a small peak of (004) at 54.0◦ , while the (002) peak of HClO4 –GIC has two branches, the major one of which shifted toward the left side when the other stays almost at the same position. It is obvious that
2 Exothemic
175.78
100
DSC (mW/mg)
TGA 90 0
mass (%)
95
1
curves of both NFG and MEG have a sharp peak of (002) at around 26.5◦ and a small peak of (004) at 54.0◦ , while the peak of HClO4 –GIC has two branches, the major one of which shifted toward the left side when the other stays almost at the same position. Besides these two branches, it is also worth noting that there are other three small peaks at 29.6◦ , 51.8◦ and 56.6◦ . All these peaks are characteristics of many stages HClO4 –GIC [20–22]. Furthermore, our calculation confirms the formation of stage 4 or 5 HClO4 –GIC. It can be explained that after the reaction with perchloric acid, the majority of NFG particles are intercalated, contributing to the 4 peaks at 25.3◦ ,29.6◦ , 51.8◦ and 56.6◦ , while tiny part of NFG particles are not intercalated, contributing to the relatively week peak of (002) and (004) staying almost at original position. After washing and mild expansion, the perchloric acid are decomposed and deintercalated from NFG, making the sample resume to a state quite similar to the original state. This suggests that the treatment of mild expansion preserved the original layer structure of the NFG. However, according to the data calculated from the XRD patterns, there is a decrease of 13.6 nm in the micro crystallite size and a certain increase of 0.0002 nm in the interlayer space. Larger interlayer distance means less resistance for the intercalation and deintercalation of the lithium ions which is beneficial to its cycle performance. And the decrease in the microcrystallite size can be explained that some graphite crystals are cracked. Then there will be many defects appeared on each cracked crystals, which also provide extra space for the lithium ions during intercalation and deintercalation. From all these data above, we can probably deduce that perchloric acid is successfully intercalated into the natural flake graphite during the reaction, and largely removed during the following process of washing and mild expansion. Even though the rough surface with large specific surface area may have a harmful influence on the first columbic efficiency, many defects introduced during the process of intercalation and deintercalation, such as nanopores and cracks between crystal will contribute to the enhancement of capacity by providing extra space for lithium ion during charging and discharging.
85
DSC
80
-1 100
200
300
Temperature Fig. 3. The TGA/DSC curve of HClO4 –GIC.
400
Table 1 data calculated from the XRD patterns. Sample
2(◦ )
Micro crystallite size (nm)
d002 (nm)
NFG GIC MEG
26.566 25.350 26.546
58.9 45.3
0.3356 0.3511 0.3358
Y. Lin et al. / Electrochimica Acta 116 (2014) 170–174
450
(a) 1.5
MEG
1.0
NFG
MEG
Capacity (mAh/g)
400
Voltage (V)
173
0.2C
0.2C
0.4C 0.8C
350 NFG 300
1.6C 250
0.5
200 150 0
0.0 0
100
200
300
400
20
40
60
Cycle number
Specific Capacity (mAh/g) Fig. 6. Electrochemical performance of NFG and MEG.
(b) 1.5
1.0 Voltage (V)
NFG
MEG
0.5
0.0 0
100
200
300
400
Specific Capacity (mAh/g) Fig. 5. Charge/discharge curves of NFG and MEG at 1st cycle (a) and 30th cycle (b).
Figs. 5 and 6 show the charge/discharge curves and electrochemical performance of natural flake graphite and mildly expanded graphite. As we can see, the discharge capacity of NFG and MEG at the 1st cycle is 334.5 and 366.5 mAh/g, while the charge capacities of NFG and MEG at the 1st cycle is 362.5 and 429.4 mAh/g, respectively. Therefore the columbic efficiency of the first cycle is 91.3% and 84.4%. It seems that the treatment does have contribution to the specific capacity. However, most of the improvement lies in the irreversible part, making the columbic efficiency much lower than that of NFG. Fig. 5a shows that the major difference between the two curves lies in the beginning part of the discharge curve, indicating the formation of SEI layer, which is the main reason for the irreversible capacity. As the cycling process goes on, the capacities of both samples have rebounded. At the 30th cycle, the capacity of NFG and MEG recovered to 360 and 395 mAh/g, respectively. Moreover, when the charge/discharge current density increased afterward, MEG shows much better performance than NFG. Typically, when the charge/discharge density is increased to 1.6 C, the capacity of MEG still remains at 260 mAh/g, almost twice as much as NFG. When the current density resumes to 0.2 C, the capacity of MEG maintains at 397 mAh/g, which is still higher than that of NFG. In addition, when measuring the depth of the electrode foil
before and after charging, we observed a 12.5% increase for the MEG sample, which is lower than the 16% increase for the NFG sample. Therefore the volume change during cycling is buffered in the MEG. It is quite worthy of note that the specific capacity at current density of 0.2 C is much higher than the theoretical capacity of graphite (372 mAh/g). As shown in Fig. 2, there are many defects such as cracks on the surface and nanopores inside the particles in MEG. Also, it is confirmed that the expansion volume of MEG increases nearly 3 times. These two characteristics are well in agreement with the cavity mechanism proposed by Akihiro Mabuchi in which lithium ions can both be intercalated into interlayers and inserted into cavities [23]. What’s more, the cavity mechanism are also confirmed in our previous work [14]. Besides, decreased micro crystallite size also indicates that it could not only shorten the distance of lithium ion channels in graphite, but also provide more space to store lithium ions [24]. To sum up, all these defects will contribute to the enhancement in capacity because these defects will provide extra space for lithium ions storage during the process of charging and discharging [4,25]. However, the specific surface area increased due to the cracks. Thus, the columbic efficiency will be greatly affected because more surfaces will be covered by the solid state-electrolyte interface film. In the cycling performance, both the defects and slightly increased interlayer space play roles. It is well known that the lithium ions will encounter some resistance during the process of charging and discharging, making the volume of graphite change greatly and thus causing severe damage to the electrode [3]. In contrast, the electrode prepared by MEG will provide less resistance and thus suffer less damage from the volume change during cycling especially at high charge/discharge current density [15], which is in agreement with the measure of the electrode depth. It means that the larger part of capacity will be maintained when cycled at high charge/discharge current density. In comparison with previous method to synthesize mildly expanded graphite using sulfuric acid as intercalative agent and hydrogen peroxide as oxidizing agent, there are lots of advantages for this method. First, the oxidizing ability of perchloric acid is strong enough to open the edges of graphite, so perchloric acid can serve both as oxidizing agent and intercalating agent, avoiding usage of other strong oxidizing agent. Moreover, as the boiling point of perchloric acid is much lower, both the temperature and time required to deintercalate greatly decreased. It takes 28 hours under 360 ◦ C in previous method, while 4 hours and 200 ◦ C are required in this method. Also, only 0.17% weight percent of chlorine remained in MEG is quite acceptable in comparable to the content of chlorine
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or sulfur in other experiments [26,27]. Last, the capacity as high as 396 mAh/g of as prepared MEG is also higher than 378 mAh/g of MEG obtained by the previous method. 4. Conclusion MEG with many defects inside and slightly increased interlayer space was synthesized using perchloric acid as both intercalating and oxidizing agent. The as-prepared MEG exhibits a rate capacity as high as 395 mAh/g at 0.2 C and 250 mAh/g at 1.6 C. The defects help to improve both the capacity and cycling performance by providing extra space to store lithium ions and buffering the volume change during charging and discharging. And the slightly increased interlayer space also helps to reduce the influence of volume change by weakening the resistance encountered by lithium ions. What’s more, there are many advantages for the method using only perchloric acid to prepare MEG, including lower temperature and less time of deintercalation, less maintaining pollutive element and higher capacity. Acknowledgement The authors thank the financial support from the National Nature Science Foundation of China under grant No. 51232005 and the National Key Technology R & D Program of China under Grant No. 2008BAE60B08. References [1] L.J. Fu, H. Liu, C. Li, Y.P. Wu, E. Rahm, R. Holze, H.Q. Wu, Solid State Sciences 8 (2006) 113–128. [2] M. Endo, C. Kim, K. Nishimura, T. Fujino, K. Miyashita, Carbon 38 (2000) 183–197.
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