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Mesocrystal hexagonal Co3O4 nanosheets for high performance lithium and sodium-ion batteries
Materials Letters 209 (2017) 388–391
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Mesocrystal hexagonal Co3O4 nanosheets for high performance lithium and sodium-ion batteries Duqiang Xin a,⇑, Jianfeng Dai b, Jifei Liu b, Qin Wang b, Weixue Li b a b
School of Science, Xijing University, Xi’an 710123, China School of Science, Lanzhou University of Technology, Lanzhou 730050, China
a r t i c l e
i n f o
Article history: Received 6 August 2017 Accepted 10 August 2017 Available online 16 August 2017 Keywords: Crystal structure Diffusion Energy storage and conversion Microstructure Sol-gel preparation
a b s t r a c t Mesocrystal hexagonal Co3O4 nanosheets with highly exposed {1 1 1} crystallographic planes were successfully synthesized by the conversion of hexagonal Co(OH)2 nanosheets. They demonstrate uniform size with length of 400 nm. When applied as anode material for lithium-ion batteries (LIBs) and sodiumion batteries (SIBs), they delivered high specific capacity of 1000 mAh g[email protected] C and 640 mAh g1@2 C, 600 mAh g[email protected] C and 350 mAh g1@2 C, respectively. These excellent electrochemical properties attribute to the ordered ions diffusion pathway originated from the mesocrystal nanostructure. With these outstanding performances, the mesocrystal hexagonal Co3O4 nanosheets enable provide meaningful anode materials for LIBs and SIBs. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Since Sony announced the first version of commercialized lithium-ion batteries (LIBs) in 1990s [1], electrochemical energy storage and conversion devices become crucial in people’s need in daily life [2]. Recently, most commonly used commercial electrode materials are powder materials, such as graphite as anode materials and lithium metal oxides as cathode materials. However, these micrometer powder materials have long diffusion pathways for lithium ion, needless to say the bigger ionic radius of sodium (1.09 Å), even though they exhibit similar electrochemical properties [3]. This may leads to a larger volume expansion for electrode materials, resulting low initially coulomb efficiency, and poor rate performance. Because of the limited specific capacity of graphite powder (～375 mAh g1), transition metal oxides (such as Fe2O3, Co3O4 and NiO etc.) have been considered as candidates of anode materials for L/SIBs due to their high theoretical specific capacity (～1000 mAh g1) [4,5]. However, these oxides are polycrystalline structure with anisotropy crystal structure, and have been demonstrated medium electrochemical performance for LIBs, but poor cycling stability for SIBs, because the ionic radius and mass of sodium is bigger than that of lithium [6]. A significant way to mitigate/resolve the problems is focused on the formation and understanding of ordered nanoparticle superstructure with a vast range of architectures, especially for the mesocrystals [7]. Mesocrystals ⇑ Corresponding author. E-mail address: [email protected] (D. Xin). http://dx.doi.org/10.1016/j.matlet.2017.08.048 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
can be delineated by their high degree of crystallization, porosity, and subunits alignment along a crystallographic register [8]. Based on the high theoretical specific capacity (890 mAh g1) of Co3O4 materials, we report the synthesis of mesocrystal hexagonal Co3O4 nanosheets by hydrothermal method in this letter. Such mesocrystal hexagonal Co3O4 nanosheets could offer numerous highly ordered ionic diffusion pathway, demonstrating superior electrochemical properties for electrochemical batteries. 2. Experimental section 2.1. Synthesis of mesocrystal hexagonal Co3O4 nanosheets First, 1.2 g of Co(NO3)26H2O (99%, Alfa Aesar) were dissolved into a 35 ml of 1:1 mixture of DI water and ethanol solution. Next, 0.5 g of Poly(vinylpyrrolidone) (PVP, K90, Mw = 1,300,000) and 1.0 g of CH3COONa (99%, Alfa Aesar) were added as the surface active agent and the precipitator, respectively. Then, the mixture was sealed in a 50 mL Teflon-lined stainless steel autoclave, heated to and maintained at 120 °C for 10 h. After cooling down to room temperature, the products were washed and collected by centrifugation with DI water and ethanol, and followed by drying at 80 °C for 12 h. 2.2. Materials of physical properties characterization The morphologies and microstructures were examined using a US-800 scanning electron microscope (SEM) and a JEF 2000
D. Xin et al. / Materials Letters 209 (2017) 388–391
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transmission electron microscopy (TEM, equipped with a selectedarea electric diffraction (SAED), and a high-resolution TEM (HRTEM)), respectively.
structure on the surface of hexagonal Co3O4 nanosheets, and the related HRTEM (Fig. 1(h)) indicates that the interplanar distance of 0.275 nm is consistent with the (0 1 0) crystallographic plane of Co3O4 material. More importantly, the related SAED pattern
2.3. Electrochemistry analysis
(Fig. 1(i)) shows a diffraction dot matrix, including the (1 1 0),
Working electrodes were fabricated by mixing the active materials, Super-P, and PVDF in N-methyl-2-pyrrolidone (NMP) solvent at a ratio weight of 80:10:10. The slurry was uniform plastered on a copper foil current collector, and subsequently drying under vacuum at 80 °C for 12 h. The loading level is 2 mg cm2. 2025 type coin cells were assembled to examine the electrochemical properties, and lithium and sodium metal as the reference electrodes, respectively. The electrolyte of LIB is a mixture (1:1 by volume) of ethylene carbonate (EC) and diethyl carbonate (DC) containing 1 M LiPF6. The electrolyte of SIB is a mixture (1:1 by volume) of ethylene carbonate (EC) and dimethyl carbonates (DMC) containing 1 M NaClO4. Cell assembly was carried out in an argon-filled glove box with moisture and oxygen concentrations below 1.0 ppm. The cyclic voltammetry (CV) was measured by a Wuhan Corrtest electrochemical workstation at a scanning rate of 0.1 mV/s. The galvanostatic charge/discharge was performed on a Wuhan LAND-CT2001A battery testing system. 3. Results and discussion The morphologies of the as-prepared hydrothermal specimen in the SEM image (Fig. 1(a)) reveals the successfully synthesis of the uniform and tiny hexagonal Co(OH)2 nanosheets, and the corresponding highly magnified SEM image (Fig. 1(b)) indicates they predominantly 400 nm in length with smooth surface. Microstructure observation in TEM image (Fig. 1(c)) shows many dark and write-coloured dots, indicating a nanoporous structure of the specimen. The related SAED pattern in Fig. 1(d) shows a diffraction dot matrix, revealing a mesocrystal structure of the hexagonal Co(OH)2 nanosheets exposed {2 2 0} crystallographic planes. After this Co (OH)2 specimen was annealed at 450 °C for 2 h, the morphology of the obtained Co3O4 specimen is showed in Fig. 1(e), it maintained the hexagonal structure with a large-scale production. The highly magnified SEM image (Fig. 1(f)) shows the hexagonal Co3O4 nanosheets reveals a roughness surface, originated from the crystallization of mesocrystal Co(OH)2 nanosheets. Microstructure observation (TEM image in Fig. 1(g)) reveals many nanoporous
(0 1 0), (1 0 0), (1 1 0), (0 1 0), (1 0 0) crystallographic planes, which are ascribe to Co3O4 {1 1 0} crystallographic planes. These results indicate that the successfully synthesis of mesocrystal hexagonal Co3O4 nanosheets. The electrochemical properties of the mesocrystal hexagonal Co3O4 nanosheets were investigated in this section. Fig. 2(a) shows the CV testing of the hexagonal Co3O4 nanosheets vs. Li+/Li. A sharply reduction peak located at 1 V during the first negative scan indicates Co3O4 reduced to Co, and the corresponding oxidation peak located at 2 V, which is ascribe Co oxide to Co3O4. Fig. 2 (b) shows the CV testing of hexagonal Co3O4 nanosheets vs. Na+/ Na. A weak reduction peak located at 0.36 V and its intensity decreased at the secondary scans, indicating an irreversible decomposition of the electrolyte, as well as the formation of solidelectrolyte interface. A big reduction peak at 0.96 V ascribe to the conversion reaction of Co3O4 reduced to Co, as well as the formation of Na2O. There are two oxidation peaks emerged at 0.43 V and 0.82 V during the reversed positive scan, which are corresponding to the conversion reaction of the Co oxidized to Co3O4. The electrochemical cycling properties of mesocrystal hexagonal Co3O4 nanosheets vs. Li+/Li and Na+/Na cycling at 0.1 C (100 mA g1), and the corresponding charge-discharged curves are showed in Fig. 2(d) and (e), respectively. The initially discharge specific capacity of 1030 mAh g1 for LIB, which is higher than 642 mAh g1 for SIB. After 50 cycles, they also maintain their initially specific capacity, indicate reversible cycle performances of these batteries. The rate performance of mesocrystal hexagonal Co3O4 nanosheets vs. Li+/Li and Na+/Na (Fig. 2(f)) deliver reversible specific capacities of 1000, 898, 800, 740, 660, 1000 mAh g1 and 634, 600, 520, 450, 250, 450 mAh g1 at the current densities of 0.1, 0.2, 0.5, 1, 2, 0.1 C, respectively. More importantly, their correspond specific capacity fully recovered when the current density resumed at different rates. In this regard, such a unique electrochemical performance is primarily attributes to the mesocrystal structure could offer many highly ordered ion diffusion pathways, resulting in a better electric and ionic transfer kinetics. In order to investigate the effect of mesocrystal improves the electrochemistry properties, their ionic diffusion coefficients of Li
Fig. 1. (a)-(b) SEM images, (c) TEM image, (d) SAED pattern of hexagonal Co(OH)2 nanosheets, (e)-(f) SEM image, (g) TEM image, (h) HRTEM image, and (i) SAED image of mesocrystal hexagonal Co3O4 nanosheets.
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D. Xin et al. / Materials Letters 209 (2017) 388–391
Fig. 2. CV curves of mesocrystal hexagonal Co3O4 nanosheets vs. (a) Li+/Li and (b) Na+/Na. (c) Cycling performance and (d)-(e) the corresponding charge-discharge curves cycling at 1st, 2nd and 50th. (f) The rate performance at different current density.
Fig. 3. (a) EIS of Co3O4 electrodes vs. Li and Na at full discharge state. (b) The relationship between Zreal and x1=2 at low frequency. RS represents the contact and solution resistance, RSEI represents the surface film resistance, RCT represents the charge transfer resistance, Cdl and Cint represent the constant phase elements, and W (Warburg resistance).
(DLi+) and Na (DNa+) were further calculated. The Nyquist plots of the electrode and a simple equivalent circuit was built to simulate the EIS in Fig.3(a), and the relationship between Zreal and x1=2 at low frequency as showed in Fig.3(b). The calculation according to the following equation:
DLiþ ¼
R2 T 2 2A2 n2 TF 4 C 2Li
r2
where, R is the gas constant, T is the absolute temperature. A is the contract area between the electrode and electrolyte. The theoretical BET surface values of mesocrystal hexagonal nanosheets can be calculated according to their geometric outlines, which is 206 m2/ g. n is the number of transferred electrons, F the Faradaic constant (96,486 C/mol), C is the concentration of lithium and sodium ions, r is the Warburg factor which is related to Zreal. By plotting Zreal vs. x1=2 plots at low frequency, r can then be explored from the slope of the fitting line (Fig. 3(b)). Based on these parameters, the Li (DLi+) and Na (DNa+) of mesocrystal hexagonal Co3O4 nanosheets are 5.71019 cm2 s1 and 2.11019 cm2 s1. These results clearly
suggest that the superior electrochemistry energy storage properties is attribute to the highly ordered ionic diffusion pathway.
4. Conclusion Mesocrystal hexagonal Co3O4 nanosheets were synthesized by hydrothermal method. They delivered a reversible specific capacity of 1000 mAh g1 and 600 mAh g1 at a current density of 0.1 C when applied as anode materials for LIB and SIB, respectively. These findings indicate that the mesocrystal hexagonal Co3O4 nanosheets could offer highly ordered ionic diffusion pathway, which enabling effectively commercialization of energy storage applications.
Acknowledgment This work is supported by Xijing University Research Foundation (XJ150203).
D. Xin et al. / Materials Letters 209 (2017) 388–391
References [1] B. Fu, X. Zhou, Y.P. Wang, Co3O4 carbon nanofiber mats as negative electrodes for sodium-ion batteries, Mater. Lett. 170 (2016) 21–24. [2] N. Nitta, F. Wu, J.T. Lee, G. Yushin, Li-ion battery materials: present and future, Mater. Today 18 (5) (2015) 252–264. [3] M.D. Slater, D. Kim, E. Lee, C.S. Johnson, Sodium-ion batteries, Adv. Fun. Mater. 23 (8) (2013) 947–958. [4] T. Tachikawa, T. Majima, Metal oxide mesocrystals with tailored structures and properties for energy conversion and storage applications, NPG Asia Mater. 6 (5) (2014) e100.
391
[5] Y. Ding, P. Zhang, Z. Long, Y. Jiang, J. Huang, W. Yan, G. Liu, Synthesis and electrochemical properties of Co3O4 nanofibers as anode materials for lithiumion batteries, Mater. Lett. 62 (19) (2008) 3410–3412. [6] F. Klein, B. Jache, A. Bhide, P. Adelhelm, Conversion reactions for sodium-ion batteries, Phys. Chem. Chem. Phys. 15 (38) (2013) 15876–15887. [7] E. Uchaker, G. Cao, Mesocrystals as electrode materials for lithium-ion batteries, Nano Today 9 (4) (2014) 499–524. [8] M.-G. Ma, H. Cölfen, Mesocrystals-applications and potential, Curr. Opin. Colloid Inter. Sci. 19 (2) (2014) 56–65.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Mesocrystal hexagonal Co3O4 nanosheets for high performance lithium and sodium-ion batteries Duqiang Xin a,⇑, Jianfeng Dai b, Jifei Liu b, Qin Wang b, Weixue Li b a b
School of Science, Xijing University, Xi’an 710123, China School of Science, Lanzhou University of Technology, Lanzhou 730050, China
a r t i c l e
i n f o
Article history: Received 6 August 2017 Accepted 10 August 2017 Available online 16 August 2017 Keywords: Crystal structure Diffusion Energy storage and conversion Microstructure Sol-gel preparation
a b s t r a c t Mesocrystal hexagonal Co3O4 nanosheets with highly exposed {1 1 1} crystallographic planes were successfully synthesized by the conversion of hexagonal Co(OH)2 nanosheets. They demonstrate uniform size with length of 400 nm. When applied as anode material for lithium-ion batteries (LIBs) and sodiumion batteries (SIBs), they delivered high specific capacity of 1000 mAh g[email protected] C and 640 mAh g1@2 C, 600 mAh g[email protected] C and 350 mAh g1@2 C, respectively. These excellent electrochemical properties attribute to the ordered ions diffusion pathway originated from the mesocrystal nanostructure. With these outstanding performances, the mesocrystal hexagonal Co3O4 nanosheets enable provide meaningful anode materials for LIBs and SIBs. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Since Sony announced the first version of commercialized lithium-ion batteries (LIBs) in 1990s [1], electrochemical energy storage and conversion devices become crucial in people’s need in daily life [2]. Recently, most commonly used commercial electrode materials are powder materials, such as graphite as anode materials and lithium metal oxides as cathode materials. However, these micrometer powder materials have long diffusion pathways for lithium ion, needless to say the bigger ionic radius of sodium (1.09 Å), even though they exhibit similar electrochemical properties [3]. This may leads to a larger volume expansion for electrode materials, resulting low initially coulomb efficiency, and poor rate performance. Because of the limited specific capacity of graphite powder (～375 mAh g1), transition metal oxides (such as Fe2O3, Co3O4 and NiO etc.) have been considered as candidates of anode materials for L/SIBs due to their high theoretical specific capacity (～1000 mAh g1) [4,5]. However, these oxides are polycrystalline structure with anisotropy crystal structure, and have been demonstrated medium electrochemical performance for LIBs, but poor cycling stability for SIBs, because the ionic radius and mass of sodium is bigger than that of lithium [6]. A significant way to mitigate/resolve the problems is focused on the formation and understanding of ordered nanoparticle superstructure with a vast range of architectures, especially for the mesocrystals [7]. Mesocrystals ⇑ Corresponding author. E-mail address: [email protected] (D. Xin). http://dx.doi.org/10.1016/j.matlet.2017.08.048 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
can be delineated by their high degree of crystallization, porosity, and subunits alignment along a crystallographic register [8]. Based on the high theoretical specific capacity (890 mAh g1) of Co3O4 materials, we report the synthesis of mesocrystal hexagonal Co3O4 nanosheets by hydrothermal method in this letter. Such mesocrystal hexagonal Co3O4 nanosheets could offer numerous highly ordered ionic diffusion pathway, demonstrating superior electrochemical properties for electrochemical batteries. 2. Experimental section 2.1. Synthesis of mesocrystal hexagonal Co3O4 nanosheets First, 1.2 g of Co(NO3)26H2O (99%, Alfa Aesar) were dissolved into a 35 ml of 1:1 mixture of DI water and ethanol solution. Next, 0.5 g of Poly(vinylpyrrolidone) (PVP, K90, Mw = 1,300,000) and 1.0 g of CH3COONa (99%, Alfa Aesar) were added as the surface active agent and the precipitator, respectively. Then, the mixture was sealed in a 50 mL Teflon-lined stainless steel autoclave, heated to and maintained at 120 °C for 10 h. After cooling down to room temperature, the products were washed and collected by centrifugation with DI water and ethanol, and followed by drying at 80 °C for 12 h. 2.2. Materials of physical properties characterization The morphologies and microstructures were examined using a US-800 scanning electron microscope (SEM) and a JEF 2000
D. Xin et al. / Materials Letters 209 (2017) 388–391
389
transmission electron microscopy (TEM, equipped with a selectedarea electric diffraction (SAED), and a high-resolution TEM (HRTEM)), respectively.
structure on the surface of hexagonal Co3O4 nanosheets, and the related HRTEM (Fig. 1(h)) indicates that the interplanar distance of 0.275 nm is consistent with the (0 1 0) crystallographic plane of Co3O4 material. More importantly, the related SAED pattern
2.3. Electrochemistry analysis
(Fig. 1(i)) shows a diffraction dot matrix, including the (1 1 0),
Working electrodes were fabricated by mixing the active materials, Super-P, and PVDF in N-methyl-2-pyrrolidone (NMP) solvent at a ratio weight of 80:10:10. The slurry was uniform plastered on a copper foil current collector, and subsequently drying under vacuum at 80 °C for 12 h. The loading level is 2 mg cm2. 2025 type coin cells were assembled to examine the electrochemical properties, and lithium and sodium metal as the reference electrodes, respectively. The electrolyte of LIB is a mixture (1:1 by volume) of ethylene carbonate (EC) and diethyl carbonate (DC) containing 1 M LiPF6. The electrolyte of SIB is a mixture (1:1 by volume) of ethylene carbonate (EC) and dimethyl carbonates (DMC) containing 1 M NaClO4. Cell assembly was carried out in an argon-filled glove box with moisture and oxygen concentrations below 1.0 ppm. The cyclic voltammetry (CV) was measured by a Wuhan Corrtest electrochemical workstation at a scanning rate of 0.1 mV/s. The galvanostatic charge/discharge was performed on a Wuhan LAND-CT2001A battery testing system. 3. Results and discussion The morphologies of the as-prepared hydrothermal specimen in the SEM image (Fig. 1(a)) reveals the successfully synthesis of the uniform and tiny hexagonal Co(OH)2 nanosheets, and the corresponding highly magnified SEM image (Fig. 1(b)) indicates they predominantly 400 nm in length with smooth surface. Microstructure observation in TEM image (Fig. 1(c)) shows many dark and write-coloured dots, indicating a nanoporous structure of the specimen. The related SAED pattern in Fig. 1(d) shows a diffraction dot matrix, revealing a mesocrystal structure of the hexagonal Co(OH)2 nanosheets exposed {2 2 0} crystallographic planes. After this Co (OH)2 specimen was annealed at 450 °C for 2 h, the morphology of the obtained Co3O4 specimen is showed in Fig. 1(e), it maintained the hexagonal structure with a large-scale production. The highly magnified SEM image (Fig. 1(f)) shows the hexagonal Co3O4 nanosheets reveals a roughness surface, originated from the crystallization of mesocrystal Co(OH)2 nanosheets. Microstructure observation (TEM image in Fig. 1(g)) reveals many nanoporous
(0 1 0), (1 0 0), (1 1 0), (0 1 0), (1 0 0) crystallographic planes, which are ascribe to Co3O4 {1 1 0} crystallographic planes. These results indicate that the successfully synthesis of mesocrystal hexagonal Co3O4 nanosheets. The electrochemical properties of the mesocrystal hexagonal Co3O4 nanosheets were investigated in this section. Fig. 2(a) shows the CV testing of the hexagonal Co3O4 nanosheets vs. Li+/Li. A sharply reduction peak located at 1 V during the first negative scan indicates Co3O4 reduced to Co, and the corresponding oxidation peak located at 2 V, which is ascribe Co oxide to Co3O4. Fig. 2 (b) shows the CV testing of hexagonal Co3O4 nanosheets vs. Na+/ Na. A weak reduction peak located at 0.36 V and its intensity decreased at the secondary scans, indicating an irreversible decomposition of the electrolyte, as well as the formation of solidelectrolyte interface. A big reduction peak at 0.96 V ascribe to the conversion reaction of Co3O4 reduced to Co, as well as the formation of Na2O. There are two oxidation peaks emerged at 0.43 V and 0.82 V during the reversed positive scan, which are corresponding to the conversion reaction of the Co oxidized to Co3O4. The electrochemical cycling properties of mesocrystal hexagonal Co3O4 nanosheets vs. Li+/Li and Na+/Na cycling at 0.1 C (100 mA g1), and the corresponding charge-discharged curves are showed in Fig. 2(d) and (e), respectively. The initially discharge specific capacity of 1030 mAh g1 for LIB, which is higher than 642 mAh g1 for SIB. After 50 cycles, they also maintain their initially specific capacity, indicate reversible cycle performances of these batteries. The rate performance of mesocrystal hexagonal Co3O4 nanosheets vs. Li+/Li and Na+/Na (Fig. 2(f)) deliver reversible specific capacities of 1000, 898, 800, 740, 660, 1000 mAh g1 and 634, 600, 520, 450, 250, 450 mAh g1 at the current densities of 0.1, 0.2, 0.5, 1, 2, 0.1 C, respectively. More importantly, their correspond specific capacity fully recovered when the current density resumed at different rates. In this regard, such a unique electrochemical performance is primarily attributes to the mesocrystal structure could offer many highly ordered ion diffusion pathways, resulting in a better electric and ionic transfer kinetics. In order to investigate the effect of mesocrystal improves the electrochemistry properties, their ionic diffusion coefficients of Li
Fig. 1. (a)-(b) SEM images, (c) TEM image, (d) SAED pattern of hexagonal Co(OH)2 nanosheets, (e)-(f) SEM image, (g) TEM image, (h) HRTEM image, and (i) SAED image of mesocrystal hexagonal Co3O4 nanosheets.
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D. Xin et al. / Materials Letters 209 (2017) 388–391
Fig. 2. CV curves of mesocrystal hexagonal Co3O4 nanosheets vs. (a) Li+/Li and (b) Na+/Na. (c) Cycling performance and (d)-(e) the corresponding charge-discharge curves cycling at 1st, 2nd and 50th. (f) The rate performance at different current density.
Fig. 3. (a) EIS of Co3O4 electrodes vs. Li and Na at full discharge state. (b) The relationship between Zreal and x1=2 at low frequency. RS represents the contact and solution resistance, RSEI represents the surface film resistance, RCT represents the charge transfer resistance, Cdl and Cint represent the constant phase elements, and W (Warburg resistance).
(DLi+) and Na (DNa+) were further calculated. The Nyquist plots of the electrode and a simple equivalent circuit was built to simulate the EIS in Fig.3(a), and the relationship between Zreal and x1=2 at low frequency as showed in Fig.3(b). The calculation according to the following equation:
DLiþ ¼
R2 T 2 2A2 n2 TF 4 C 2Li
r2
where, R is the gas constant, T is the absolute temperature. A is the contract area between the electrode and electrolyte. The theoretical BET surface values of mesocrystal hexagonal nanosheets can be calculated according to their geometric outlines, which is 206 m2/ g. n is the number of transferred electrons, F the Faradaic constant (96,486 C/mol), C is the concentration of lithium and sodium ions, r is the Warburg factor which is related to Zreal. By plotting Zreal vs. x1=2 plots at low frequency, r can then be explored from the slope of the fitting line (Fig. 3(b)). Based on these parameters, the Li (DLi+) and Na (DNa+) of mesocrystal hexagonal Co3O4 nanosheets are 5.71019 cm2 s1 and 2.11019 cm2 s1. These results clearly
suggest that the superior electrochemistry energy storage properties is attribute to the highly ordered ionic diffusion pathway.
4. Conclusion Mesocrystal hexagonal Co3O4 nanosheets were synthesized by hydrothermal method. They delivered a reversible specific capacity of 1000 mAh g1 and 600 mAh g1 at a current density of 0.1 C when applied as anode materials for LIB and SIB, respectively. These findings indicate that the mesocrystal hexagonal Co3O4 nanosheets could offer highly ordered ionic diffusion pathway, which enabling effectively commercialization of energy storage applications.
Acknowledgment This work is supported by Xijing University Research Foundation (XJ150203).
D. Xin et al. / Materials Letters 209 (2017) 388–391
References [1] B. Fu, X. Zhou, Y.P. Wang, Co3O4 carbon nanofiber mats as negative electrodes for sodium-ion batteries, Mater. Lett. 170 (2016) 21–24. [2] N. Nitta, F. Wu, J.T. Lee, G. Yushin, Li-ion battery materials: present and future, Mater. Today 18 (5) (2015) 252–264. [3] M.D. Slater, D. Kim, E. Lee, C.S. Johnson, Sodium-ion batteries, Adv. Fun. Mater. 23 (8) (2013) 947–958. [4] T. Tachikawa, T. Majima, Metal oxide mesocrystals with tailored structures and properties for energy conversion and storage applications, NPG Asia Mater. 6 (5) (2014) e100.
391
[5] Y. Ding, P. Zhang, Z. Long, Y. Jiang, J. Huang, W. Yan, G. Liu, Synthesis and electrochemical properties of Co3O4 nanofibers as anode materials for lithiumion batteries, Mater. Lett. 62 (19) (2008) 3410–3412. [6] F. Klein, B. Jache, A. Bhide, P. Adelhelm, Conversion reactions for sodium-ion batteries, Phys. Chem. Chem. Phys. 15 (38) (2013) 15876–15887. [7] E. Uchaker, G. Cao, Mesocrystals as electrode materials for lithium-ion batteries, Nano Today 9 (4) (2014) 499–524. [8] M.-G. Ma, H. Cölfen, Mesocrystals-applications and potential, Curr. Opin. Colloid Inter. Sci. 19 (2) (2014) 56–65.