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Mesoporous cobalt oxide for largely improved lithium storage properties
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Chinese Chemical Letters 23 (2012) 949–952 www.elsevier.com/locate/cclet
Mesoporous cobalt oxide for largely improved lithium storage properties Mai Xia Ma Faculty of Chemistry and Material Science, Langfang Teachers College, Langfang 065000, China Received 21 March 2012 Available online 4 July 2012
Abstract We report the microstructure, application for lithium-ion batteries of mesoporous Co3O4 prepared by modified KIT-6 template method. The sample was characterized by XRD, TEM, HRTEM and nitrogen adsorption. Their electrochemical behaviors as electrode reactants for lithium ion batteries were evaluated by cyclic voltammograms and static charge–discharge. A direct comparison of electrochemical behaviors between mesoporous nanostructure and bulk reflects interesting ‘‘nanostructure effect’’, which is reasonably discussed in terms of how the 3D nanostructures of Co3O4 materials function in tuning their electrochemistry. The results demonstrate that further improvement of electrochemical performance in transition metal-oxide-based anode materials can be realized via the design of multiporous nanostructured materials. # 2012 Mai Xia Ma. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Mesoporous Co3O4; High lithium storage capacity; Nanostructure effect
1. Introduction Transition metal oxide, Co3O4 with normal spinel structure, is a technologically important functional material with applications in gas sensors [1], lithium-ion batteries [2], and electrochromic devices [3]. Nanostructured Co3O4, with a higher surface area and enhanced electrochemical reactivity, is especially attractive for these applications [2,4]. Lithium ion batteries are important electrochemical energy storage and conversion devices. Transition-metal oxides have exhibited high capacity for reversible lithium storage based on the so called ‘‘conversion’’ reaction, which was first reported by Poizot et al. [2]. Among them, cobalt oxide has demonstrated the best electrochemical performance in terms of specific capacity and cyclability [5]. Especially, mesoporous nanostructured Co3O4, with larger particular surface area, larger pore volume, more periodically distributed pores, more peculiar surface properties and more active sites compared with other 3D nanostructure, should bring prominent advantages in approaching higher specific capacity and enhanced high-rate performances. Thus, it is worthwhile to study the electrochemical performances of 3D mesoporous Co3O4. In this letter, ordered mesoporous Co3O4 have been synthesized using modified template method. The as-prepared mesoporous Co3O4 were systematically investigated in detail, which showed excellent performances in lithium-ion batteries.
E-mail address: [email protected]. 1001-8417/$ – see front matter # 2012 Mai Xia Ma. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. http://dx.doi.org/10.1016/j.cclet.2012.06.006
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M.X. Ma / Chinese Chemical Letters 23 (2012) 949–952
2. Experimental In a typical experiment, 1 g of cobalt nitrate hexahydrate (Co(NO3)6H2O, 98%, Alfa Aesar) and 1 g of KIT-6 were first dissolved in 20 mL ethanol at room temperature by intensive magnetic stirring for 2 h to obtain a homogeneous solution. The ethanol was then evaporated off at approximately 60 8C. During the process, the cobalt nitrate was drawn into the pores by capillary action. The dry sample was then thermally decomposed at 300 8C for 1 h and 500 8C for 3 h. The KIT-6 silica template was removed by a 10% solution of HF. The final product was collected by centrifugation and washed with deionized water and absolute ethanol for several times, and then dried at 60 8C to obtain the dark powders.
3. Results and discussion Fig. S1 presents typical X-ray diffraction (XRD) patterns of the obtained mesoporous Co3O4 and bulk Co3O4. All the observed diffraction peaks can be assigned to those of cubic Co3O4 (Joint Committee for Powder Diffraction Standards (JCPDS) 43-1003, Space group Fd3m, a = b = c = 0.8084). Fig. S2 shows the N2 adsorption–desorption isotherms and BJH pore size distribution plots of the samples prepared by using KIT-6 as template. The BET surface area of mesoporous Co3O4 was determined to be 114.82 m2/g. Further insight into the morphology and mesoporous Co3O4 was gained using Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) (Fig. 1). From Fig. 1a, it can be seen that this morphology dominates throughout the material. Importantly, the mesoporous Co3O4 material has been maintained. This morphology is excellent replicas of the porous silica templates. From Fig. 1b, it is obvious that the samples are composed of mesoporous nanostructure with mesopores diameter of 5 nm. As shown in Fig. 1b, the HRTEM image was taken from the marked region in Fig. 1a to further reveal the structure. In Fig. 1a, it is apparent that the mesoporous nanostructures are phase-pure Co3O4 showing the planes of (2 2 0) with the spacing of 0.29 nm. The electrochemical performance of the two types samples in lithium ion cells were evaluated in Fig. 2. Obviously, such mesoporous Co3O4 exhibits excellent performances in terms of high specific capacity, good cycling stability and relatively low initial capacity loss. Fig. 2a shows the first and second cycle discharge voltage profiles for the two samples in a potential window of 10 mV–3.0 V. The curves from the two samples are very similar to those reported previously [6–8], indicating a similar electrochemical pathway. Note that the first-discharge capacity values measured here (1250 mAh g 1 for the mesoporous Co3O4 and 1150 mAh g 1 for the bulk Co3O4) are higher than the theoretical capacity. Due to the unavoidable electrolyte decomposition below 0.75 V versus Li metal, the discharge (reduction) capacity of a material is higher than its theoretical discharge capacity [2,9]. Since the second cycle, however, the mesoporous Co3O4 electrode presents much better electrochemical lithium storage performance than the bulk Co3O4 electrode, which may be attributed to the larger electrochemical active surface area of 3D nanostructure.
[(Fig._1)TD$IG]
Fig. 1. (a) TEM images of mesoporous Co3O4. (b) HRTEM image of mesoporous Co3O4. The imaged lattice spacing amounts to 0.29 nm corresponding to the (2 2 0) planes of Co3O4.
[(Fig._2)TD$IG]
M.X. Ma / Chinese Chemical Letters 23 (2012) 949–952
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Fig. 2. (a) Discharge curves of the initial two cycles for the mesoporous Co3O4 and bulk Co3O4 at a current rate of 0.2 C (178 mA/g). (b) Cyclic voltammograms (CVs) of electrodes made by the mesoporous Co3O4 and bulk Co3O4 at a scan rate of 0.1 mV/s. (c) Capacities versus cycle number plots for the mesoporous Co3O4 and bulk Co3O4 at a current rate of 0.2 C (178 mA/g). (d) Stabilized discharge voltage profiles of the mesoporous Co3O4 and bulk Co3O4 cycled at different rates: 0.07 C, 0.2 C, 0.5 C, 1 C and 2 C from left to right.
The CV curves of the mesoporous Co3O4 electrode at a scan rate of 0.1 mV/s are shown in Fig. 2b. In the first scanning cycle, two cathodic peaks were observed at 0.75 V and 1.4 V for the mesoporous Co3O4, corresponding to a multistep electrochemical reduction (lithiation) reaction of Co3O4 to CoO and metallic Co [1]. The reduction peak at 0.75 V is associated with the electrolyte decomposition and the formation of the SEI layer [10,11]. The observed main anodic peak at 2.24 V for Co3O4 ascribed to the oxidation (delithiation) reaction of Co3O4. In the second cycle, the main reduction peak is shifted to 1.2 V for the mesoporous Co3O4. The peak intensity and integral areas of the mesoporous Co3O4 are close to that of the bulk Co3O4 but are obviously decreased for bulk Co3O4. These results indicate that the electrochemical reversibility of the mesoporous Co3O4 is gradually built after the initial cycle and much better than that of the bulk Co3O4. Fig. 2c depicts the cycling performance of the two samples up to 100 cycles. It is apparent that the mesoporous Co3O4 shows the higher reversible capacity of 900 mAh g 1 even after 100 cycles than that of the bulk Co3O4 with 600 mAh g 1. One possible reason for the observation of a capacity for the mesoporous Co3O4, which is higher than the theoretical value (890 mAh g 1) is that there exist many irreversible reactions, such as decomposition of electrolyte [7] during the charge–discharge process. A reversible capacity of as high as 900 mAh g 1 can still be retained after 100 cycles, which should be considered advantageous compared to other reported Co-based anodes tested under similar conditions [12–14]. This is probably because that its highly rough surface and porosity can provide more active sites during charging–discharging processes, and 3D mesoporous nanostructure can provide longterm structural stability against the volume change during charging–discharging processes. Fig. 2d stabilized
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M.X. Ma / Chinese Chemical Letters 23 (2012) 949–952
discharge voltage profiles of the mesoporous Co3O4 cycled at different rates: 0.07 C, 0.2 C, 0.5 C, 1 C and 2 C from left to right. It shows that the mesoporous Co3O4 exhibits a good rate capability as the anode material. Even at a high current rate of 2 C, the Co3O4 nanowires can still deliver a capacity of 750 mAh g 1. According to the results presented above, the electrodes of mesoporous Co3O4 displayed superior electrochemical performance. The most likely interpretation is as follows. First, as is known, chemical and physical phenomena are strongly affected when materials become nanometer-sized. With decreasing particle size, an increasing proportion of the total number of atoms lies on the surface, making the structural strains more and more active for lithium electrochemical reaction. Second, the highly mesoporous nanostructure with higher surface-to-volume ratio, makes the electrochemical reaction with lithium more active. Third, the cycling performances of the mesoporous nanostructure are better than that of the bulk electrodes, indicating that orderly arrangements of 3D nanostructures exhibit better capacity retention. This may lighten the stress caused by volume change during the numerous charge– discharge cycles and suppress the degradation of the electrode. 4. Conclusions In summary, the mesoporous nanostructured Co3O4 have been prepared with KIT-6 silica as templates. Their electrochemical lithium-storage behaviors evaluated by CVs and discharge–charge cycling tests reveal explicit ‘‘nanostructure effect’’. The mesoporous Co3O4 display high discharge capacity and superior cycling reversibility. Observations from this research are expected to extend to other transition metal oxides being extensively investigated, and provide valuable guidance for producing practically reliable electrodes based on a series of convertible metal oxides in the future. Acknowledgment This work was supported by the Specialized Research Fund of Langfang Teachers College for the scientific research. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.cclet.2012.06.006. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
W.Y. Li, L.N. Xu, J. Chen, Adv. Funct. Mater. 15 (2005) 851. P. Poizot, S. Laruelle, S. Grugeon, et al. Nature 407 (2000) 496. T. Maruyama, S. Arai, J. Electrochem. Soc. 143 (1996) 1383. C.L. Shao, H.Y. Guan, S.B. Wen, et al. Chin. Chem. Lett. 15 (2004) 492. K.T. Nam, D.W. Kim, P.J. Yoo, et al. Science 312 (2006) 885. X.W. Lou, D. Deng, J.Y. Lee, et al. J. Mater. Chem. 18 (2008) 4397. X.W. Lou, D. Deng, J.Y. Lee, et al. Adv. Mater. 20 (2008) 258. G. Binotto, D. Larcher, A.S. Prakash, et al. Chem. Mater. 19 (2007) 3032. P. Poizot, S. Laruelle, S. Grugeon, et al. J. Electrochem. Soc. 149 (2002) A1212. W.L. Yao, J. Yang, J.L. Wang, et al. J. Electrochem. Soc. 155 (2008) A903. J. Yao, X.P. Shen, B. Wang, et al. Electrochem. Commun. 11 (2009) 1849. Y. Wang, H. Xia, L. Lu, et al. ACS Nano 4 (2010) 1425. D. Barreca, M. Cruz-Yusta, A. Gasparotto, et al. J. Phys. Chem. C 114 (2010) 10054. M.K. Shaju, F. Jiao, A. Debart, et al. Phys. Chem. Chem. Phys. 9 (2007) 1837.
Chinese Chemical Letters 23 (2012) 949–952 www.elsevier.com/locate/cclet
Mesoporous cobalt oxide for largely improved lithium storage properties Mai Xia Ma Faculty of Chemistry and Material Science, Langfang Teachers College, Langfang 065000, China Received 21 March 2012 Available online 4 July 2012
Abstract We report the microstructure, application for lithium-ion batteries of mesoporous Co3O4 prepared by modified KIT-6 template method. The sample was characterized by XRD, TEM, HRTEM and nitrogen adsorption. Their electrochemical behaviors as electrode reactants for lithium ion batteries were evaluated by cyclic voltammograms and static charge–discharge. A direct comparison of electrochemical behaviors between mesoporous nanostructure and bulk reflects interesting ‘‘nanostructure effect’’, which is reasonably discussed in terms of how the 3D nanostructures of Co3O4 materials function in tuning their electrochemistry. The results demonstrate that further improvement of electrochemical performance in transition metal-oxide-based anode materials can be realized via the design of multiporous nanostructured materials. # 2012 Mai Xia Ma. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Mesoporous Co3O4; High lithium storage capacity; Nanostructure effect
1. Introduction Transition metal oxide, Co3O4 with normal spinel structure, is a technologically important functional material with applications in gas sensors [1], lithium-ion batteries [2], and electrochromic devices [3]. Nanostructured Co3O4, with a higher surface area and enhanced electrochemical reactivity, is especially attractive for these applications [2,4]. Lithium ion batteries are important electrochemical energy storage and conversion devices. Transition-metal oxides have exhibited high capacity for reversible lithium storage based on the so called ‘‘conversion’’ reaction, which was first reported by Poizot et al. [2]. Among them, cobalt oxide has demonstrated the best electrochemical performance in terms of specific capacity and cyclability [5]. Especially, mesoporous nanostructured Co3O4, with larger particular surface area, larger pore volume, more periodically distributed pores, more peculiar surface properties and more active sites compared with other 3D nanostructure, should bring prominent advantages in approaching higher specific capacity and enhanced high-rate performances. Thus, it is worthwhile to study the electrochemical performances of 3D mesoporous Co3O4. In this letter, ordered mesoporous Co3O4 have been synthesized using modified template method. The as-prepared mesoporous Co3O4 were systematically investigated in detail, which showed excellent performances in lithium-ion batteries.
E-mail address: [email protected]. 1001-8417/$ – see front matter # 2012 Mai Xia Ma. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. http://dx.doi.org/10.1016/j.cclet.2012.06.006
950
M.X. Ma / Chinese Chemical Letters 23 (2012) 949–952
2. Experimental In a typical experiment, 1 g of cobalt nitrate hexahydrate (Co(NO3)6H2O, 98%, Alfa Aesar) and 1 g of KIT-6 were first dissolved in 20 mL ethanol at room temperature by intensive magnetic stirring for 2 h to obtain a homogeneous solution. The ethanol was then evaporated off at approximately 60 8C. During the process, the cobalt nitrate was drawn into the pores by capillary action. The dry sample was then thermally decomposed at 300 8C for 1 h and 500 8C for 3 h. The KIT-6 silica template was removed by a 10% solution of HF. The final product was collected by centrifugation and washed with deionized water and absolute ethanol for several times, and then dried at 60 8C to obtain the dark powders.
3. Results and discussion Fig. S1 presents typical X-ray diffraction (XRD) patterns of the obtained mesoporous Co3O4 and bulk Co3O4. All the observed diffraction peaks can be assigned to those of cubic Co3O4 (Joint Committee for Powder Diffraction Standards (JCPDS) 43-1003, Space group Fd3m, a = b = c = 0.8084). Fig. S2 shows the N2 adsorption–desorption isotherms and BJH pore size distribution plots of the samples prepared by using KIT-6 as template. The BET surface area of mesoporous Co3O4 was determined to be 114.82 m2/g. Further insight into the morphology and mesoporous Co3O4 was gained using Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) (Fig. 1). From Fig. 1a, it can be seen that this morphology dominates throughout the material. Importantly, the mesoporous Co3O4 material has been maintained. This morphology is excellent replicas of the porous silica templates. From Fig. 1b, it is obvious that the samples are composed of mesoporous nanostructure with mesopores diameter of 5 nm. As shown in Fig. 1b, the HRTEM image was taken from the marked region in Fig. 1a to further reveal the structure. In Fig. 1a, it is apparent that the mesoporous nanostructures are phase-pure Co3O4 showing the planes of (2 2 0) with the spacing of 0.29 nm. The electrochemical performance of the two types samples in lithium ion cells were evaluated in Fig. 2. Obviously, such mesoporous Co3O4 exhibits excellent performances in terms of high specific capacity, good cycling stability and relatively low initial capacity loss. Fig. 2a shows the first and second cycle discharge voltage profiles for the two samples in a potential window of 10 mV–3.0 V. The curves from the two samples are very similar to those reported previously [6–8], indicating a similar electrochemical pathway. Note that the first-discharge capacity values measured here (1250 mAh g 1 for the mesoporous Co3O4 and 1150 mAh g 1 for the bulk Co3O4) are higher than the theoretical capacity. Due to the unavoidable electrolyte decomposition below 0.75 V versus Li metal, the discharge (reduction) capacity of a material is higher than its theoretical discharge capacity [2,9]. Since the second cycle, however, the mesoporous Co3O4 electrode presents much better electrochemical lithium storage performance than the bulk Co3O4 electrode, which may be attributed to the larger electrochemical active surface area of 3D nanostructure.
[(Fig._1)TD$IG]
Fig. 1. (a) TEM images of mesoporous Co3O4. (b) HRTEM image of mesoporous Co3O4. The imaged lattice spacing amounts to 0.29 nm corresponding to the (2 2 0) planes of Co3O4.
[(Fig._2)TD$IG]
M.X. Ma / Chinese Chemical Letters 23 (2012) 949–952
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Fig. 2. (a) Discharge curves of the initial two cycles for the mesoporous Co3O4 and bulk Co3O4 at a current rate of 0.2 C (178 mA/g). (b) Cyclic voltammograms (CVs) of electrodes made by the mesoporous Co3O4 and bulk Co3O4 at a scan rate of 0.1 mV/s. (c) Capacities versus cycle number plots for the mesoporous Co3O4 and bulk Co3O4 at a current rate of 0.2 C (178 mA/g). (d) Stabilized discharge voltage profiles of the mesoporous Co3O4 and bulk Co3O4 cycled at different rates: 0.07 C, 0.2 C, 0.5 C, 1 C and 2 C from left to right.
The CV curves of the mesoporous Co3O4 electrode at a scan rate of 0.1 mV/s are shown in Fig. 2b. In the first scanning cycle, two cathodic peaks were observed at 0.75 V and 1.4 V for the mesoporous Co3O4, corresponding to a multistep electrochemical reduction (lithiation) reaction of Co3O4 to CoO and metallic Co [1]. The reduction peak at 0.75 V is associated with the electrolyte decomposition and the formation of the SEI layer [10,11]. The observed main anodic peak at 2.24 V for Co3O4 ascribed to the oxidation (delithiation) reaction of Co3O4. In the second cycle, the main reduction peak is shifted to 1.2 V for the mesoporous Co3O4. The peak intensity and integral areas of the mesoporous Co3O4 are close to that of the bulk Co3O4 but are obviously decreased for bulk Co3O4. These results indicate that the electrochemical reversibility of the mesoporous Co3O4 is gradually built after the initial cycle and much better than that of the bulk Co3O4. Fig. 2c depicts the cycling performance of the two samples up to 100 cycles. It is apparent that the mesoporous Co3O4 shows the higher reversible capacity of 900 mAh g 1 even after 100 cycles than that of the bulk Co3O4 with 600 mAh g 1. One possible reason for the observation of a capacity for the mesoporous Co3O4, which is higher than the theoretical value (890 mAh g 1) is that there exist many irreversible reactions, such as decomposition of electrolyte [7] during the charge–discharge process. A reversible capacity of as high as 900 mAh g 1 can still be retained after 100 cycles, which should be considered advantageous compared to other reported Co-based anodes tested under similar conditions [12–14]. This is probably because that its highly rough surface and porosity can provide more active sites during charging–discharging processes, and 3D mesoporous nanostructure can provide longterm structural stability against the volume change during charging–discharging processes. Fig. 2d stabilized
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discharge voltage profiles of the mesoporous Co3O4 cycled at different rates: 0.07 C, 0.2 C, 0.5 C, 1 C and 2 C from left to right. It shows that the mesoporous Co3O4 exhibits a good rate capability as the anode material. Even at a high current rate of 2 C, the Co3O4 nanowires can still deliver a capacity of 750 mAh g 1. According to the results presented above, the electrodes of mesoporous Co3O4 displayed superior electrochemical performance. The most likely interpretation is as follows. First, as is known, chemical and physical phenomena are strongly affected when materials become nanometer-sized. With decreasing particle size, an increasing proportion of the total number of atoms lies on the surface, making the structural strains more and more active for lithium electrochemical reaction. Second, the highly mesoporous nanostructure with higher surface-to-volume ratio, makes the electrochemical reaction with lithium more active. Third, the cycling performances of the mesoporous nanostructure are better than that of the bulk electrodes, indicating that orderly arrangements of 3D nanostructures exhibit better capacity retention. This may lighten the stress caused by volume change during the numerous charge– discharge cycles and suppress the degradation of the electrode. 4. Conclusions In summary, the mesoporous nanostructured Co3O4 have been prepared with KIT-6 silica as templates. Their electrochemical lithium-storage behaviors evaluated by CVs and discharge–charge cycling tests reveal explicit ‘‘nanostructure effect’’. The mesoporous Co3O4 display high discharge capacity and superior cycling reversibility. Observations from this research are expected to extend to other transition metal oxides being extensively investigated, and provide valuable guidance for producing practically reliable electrodes based on a series of convertible metal oxides in the future. Acknowledgment This work was supported by the Specialized Research Fund of Langfang Teachers College for the scientific research. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.cclet.2012.06.006. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
W.Y. Li, L.N. Xu, J. Chen, Adv. Funct. Mater. 15 (2005) 851. P. Poizot, S. Laruelle, S. Grugeon, et al. Nature 407 (2000) 496. T. Maruyama, S. Arai, J. Electrochem. Soc. 143 (1996) 1383. C.L. Shao, H.Y. Guan, S.B. Wen, et al. Chin. Chem. Lett. 15 (2004) 492. K.T. Nam, D.W. Kim, P.J. Yoo, et al. Science 312 (2006) 885. X.W. Lou, D. Deng, J.Y. Lee, et al. J. Mater. Chem. 18 (2008) 4397. X.W. Lou, D. Deng, J.Y. Lee, et al. Adv. Mater. 20 (2008) 258. G. Binotto, D. Larcher, A.S. Prakash, et al. Chem. Mater. 19 (2007) 3032. P. Poizot, S. Laruelle, S. Grugeon, et al. J. Electrochem. Soc. 149 (2002) A1212. W.L. Yao, J. Yang, J.L. Wang, et al. J. Electrochem. Soc. 155 (2008) A903. J. Yao, X.P. Shen, B. Wang, et al. Electrochem. Commun. 11 (2009) 1849. Y. Wang, H. Xia, L. Lu, et al. ACS Nano 4 (2010) 1425. D. Barreca, M. Cruz-Yusta, A. Gasparotto, et al. J. Phys. Chem. C 114 (2010) 10054. M.K. Shaju, F. Jiao, A. Debart, et al. Phys. Chem. Chem. Phys. 9 (2007) 1837.