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MgFe2O4 hollow microboxes derived from metal-organic-frameworks as anode material for sodium-ion batteries
Materials Letters 199 (2017) 101–104
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
MgFe2O4 hollow microboxes derived from metal-organic-frameworks as anode material for sodium-ion batteries Yuan Guo ⇑, Youyu Zhu, Chao Yuan, Chengyang Wang ⇑ Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 23 February 2017 Received in revised form 8 April 2017 Accepted 12 April 2017 Available online 13 April 2017 Keywords: Magnesium ferrite Metal-organic-framework Microstructure Energy storage and conversion Sodium-ion battery
a b s t r a c t Metal-organic-frameworks (MOFs) Prussian blue microcubes are used as the self-sacrificial templates to fabricate hollow MgFe2O4 microboxes via the process of hydrolysis, ion exchange and subsequent calcination. This hollow and porous architecture provides efficient sodium ion diffusion pathway and sufficient space to accommodate the volume expansion during the insertion/extraction of Na+. Obtained MgFe2O4 microboxes exhibit a good capacity of 406 mA h g 1 at a current density of 50 mA g 1 and maintain a reversible capacity of 135 mA h g 1 after 150 cycles when evaluated as an anode material for sodium-ion batteries. With the increase of the amount of Super-P carbon black, the rate performance and conductivity of the MgFe2O4 microboxes electrode can be improved. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Sodium-ion battery (SIB) has been considered as a potential alternative to lithium-ion battery (LIB) due to the abundant and available reserve of sodium resources. However, it remains a huge challenge to develop appropriate electrode materials, which are capable of accommodating the larger Na+ (radius 1.02 Å, compared to Li+ radius 0.59 Å) and enabling insertion/extraction reversibly [1]. The low-cost and high theoretical capacity of iron-based materials have attracted lots of interests to be explored for SIB anode recently [2]. Spinel ferrites AFe2O4 (A = Ni, Co, Mn) have more advantages than iron oxides because the two metal elements can maintain structural stability and serve as the buffer matrix for each other upon cycling [3,4]. Here, we explored MgFe2O4 as SIB anode material, which received few attention before. Challenges of MgFe2O4 are its poor electronic conduction and large volume expansion during discharging/charging, making it difficult to reach the requirement of practical applications. Developing hollow and porous nano/micro-materials has already been proven as a successful strategy in SIB because of their unique structure [5]. This constructed structure is not only beneficial for increasing large contact areas between the electrode and electrolyte,
⇑ Corresponding authors at: Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China. E-mail addresses: [email protected] (Y. Guo), [email protected] (C. Wang). http://dx.doi.org/10.1016/j.matlet.2017.04.069 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
but also conducive to relief inner stress. The metal-organicframeworks (MOFs) assembled by clusters/metal ions coordinated to organic ligands, usually form a uniform size, and possess interior cavities and porous shells after heat treating [6,7]. For example, Prussian blue (PB) containing the (C„N) anions has been proved to have a high energy density, a small volume change during cycling, and stable rate capability in Na half-cells [8,9]. In this work, we report a facile method through MOF Prussian blue (PB) Fe4[Fe(CN)6]3 as a self-sacrificial template to fabricate MgFe2O4 hollow microboxes via the process of hydrolysis, ion exchange and subsequent calcination. When employed as anode for SIBs, the MgFe2O4 microboxes exhibit good cyclability and rate performance due to the unique hollow. To confirm the advantages of Super-P carbon black, we make a comparison through adjusting Super-p ratio in the electrode [10].
2. Experimental section 2.1. Material preparation 2.1.1. Synthesis of MOF Fe4(Fe(CN)6)3 (Prussian Blue, PB) 0.12 g K4Fe(CN)63H2O was added into a 50 mL HCl aqueous solution (0.1 M) containing 4.0 g polyvinyl pyrrolidone (PVP, MW 40,000). After stirring for 45 min, a clear light green solution was obtained. The beaker was sealed and heated for 24 h at 80 °C. The blue deposition was centrifugated and washed with
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Fig. 1. (a) TEM image of PB microcubes, (b) TEM and (c) HRTEM images of porous and hollow MgFe2O4 microboxes; SEM images of (e) PB microcubes, (i–l) MgFe2O4 microboxes and their elemental mappings of Mg, Fe, and O; (d) SAED and (f) XRD patterns of MgFe2O4 microboxes; (g) EDS and (h) SEM patterns of MgFe2O4 microboxes after the 100th charge at 50 mA g 1.
ethanol and deionized water, respectively. It was put in a vacuum oven at 70 °C for 12 h to form the MOF PB microcubes. 2.1.2. Synthesis of MOF MgFe2O4 microboxes Mg(CH3COO)2 and obtained PB with the stoichiometric amount of Mg and Fe mixed were dispersed into 20 mL ethanol under magnetic stirring for 10 min. Then the solution was heated at 70 °C to evaporate the ethanol. The dried powder was calcined in air at 700 °C for 6 h with a slow heating rate at 2 °C min 1 to synthesize MgFe2O4 microboxes. 2.2. Material characterizations Field-emission scanning electron microscopy (FESEM, Hitachi S4800) with an energy dispersive spectrometer (EDS), and transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F) with selected area electron diffraction (SAED) were used to observe the morphologies and structures of the PB and MgFe2O4. X-ray diffraction was carried out on a Bruker AXS D8 Focus X-ray diffractometer using Cu Ka radiation (k = 0.15406 nm). The specific surface area was measured by the Brunauer-Emmett-Teller (BET). 2.3. Electrochemical measurements Electrochemical performance was investigated using CR2430 coin-type cells. Fiberglass and Na metal were used as separator and counter electrode, respectively. The anodes of SIB were fabricated by mixing the active material, Super-P carbon black and polyvinyl difluoride (PVDF) in N-methyl-2-pyrrolidone (NMP) solvent at different proportions (sample A (7:2:1), sample B (8:1:1)), and then coated onto a copper foil. NaClO4 (1 M) in ethylene carbonate/dimethyl carbonate (EC/DMC, 1:1 by volume) with 5 wt%
fluoroethylene carbonate (FEC) added was employed as the electrolyte [11]. The discharge/charge cycling performance was measured by a LAND-CT2001A battery-testing system in the voltage range of 0.005 V–3.0 V. Electrochemical impedance spectroscopy (EIS) was recorded using a CHI660E electrochemical work station in the frequency range from 100 kHz to 10 mHz. 3. Results and discussion TEM image (Fig. 1a) confirms the solid structure of the PB. The hollow structure and porous shells of MOF MgFe2O4 microboxes are presented in Fig. 1b. Fig. 1c shows lattice spacing of 0.171 nm and 0.161 nm for the (422) and (511) crystalline planes of spinel MgFe2O4. Fig. 1f verifies the successful preparation of the MgFe2O4 composite in terms of XRD profile. The diffraction peaks are (2 2 7) space indexed to the pure cubic MgFe2O4 with an Fd3m group (JCPDS 17-0464), which are corresponding to the SAED (Fig. 1d) result. The clear diffraction points can be well indexed as a pure spinel MgFe2O4 phase, indicating a high crystallinity of MgFe2O4. As shown in Fig. 1e, the synthesized MOF PB consists of uniform microcubes with smooth surfaces and a size of 610 nm. After calcination at 700 °C, the synthesized MgFe2O4 particles (Fig. 2b) keep a regular cubic shape but with rough surfaces and a shrunken size of 570 nm. The specific surface area of MgFe2O4 is 39.114 m2 g 1 based on the BET analysis of the nitrogen absorption/desorption isotherms. The elemental distributions of Mg, Fe, and O are homogeneous within the MgFe2O4 particles (Fig. 1i–1l). Cycled anode powder was scraped from the copper foil washed with DMC in the Ar-filled glove box. After dried, the powder was protected by a parafilm from the air [12]. Clearly, some MgFe2O4 particle after 100th charge at 50 mA g 1 still keeps its box structure (Fig. 1h) and has a Na/Mg/Fe/O ratio of 1/1.3/2.5/3.7 (Fig. 1g).
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Fig. 2. Electrochemical properties of MgFe2O4 microboxes electrodes at different active material ratios A (7:2:1) and B (8:1:1). (a) Discharge/charge curves and (b) cyclic performance of sample A at a current density of 50 mA g 1; (c) Rate performance and (d) Nyquist plots of samples A and B.
Therefore, MgFe2O4 microboxes could resist the deformation during cycling. Fig. 2a shows the representative discharge-charge curves of the sample A (7:2:1) at 50 mA g 1. Two potential plateaus (0.75 and 0.55 V) of the electrode are delivered in the initial discharge process, which results from the formation of the solid electrolyte interface (SEI) film and the reduction of Fe3+ to Fe0, respectively [13]. The initial discharge/charge capacities are 406/207 mA h g 1 with a coulombic efficiency of 51%. The capacity loss is mainly caused by the irreversible intercalation of Na+ into the crystal lattice and the formation of SEI film. Moreover, the addition of FEC in the electrolyte can strengthen the electrode architecture, enhance the structural stability of SEI film, and suppress the side reactions between electrode and electrolyte [14]. Fig. 2b presents the cyclic performance of sample A at 50 mA g 1. After 150 cycles, the electrode still maintains a stable capacity of 135 mA h g 1 with the capacity retention rate of 98.6%, suggesting favourable cycle stability. Rate performance of samples A (7:2:1) and B (8:1:1) is displayed in Fig. 2c. The average discharge capacities of sample A are 255, 172, 150, 112, and 85 mA h g 1 at current densities of 0.05, 0.1, 0.2, 0.5, and 1 A g 1 respectively, which are much high than sample B electrodes. A reversible capacity of 168 mA h g 1 can be retained when the current density decreases from 1.0 to 0.05 A g 1, demonstrating structure stability and favourable reversibility. The improved electrochemical performance due to the hollow structure, which can tolerate the volume expansion, effectively buffer the mechanical strain, and suppress the particle aggregation [15]. Fig. 2d presents the Nyquist plots of samples A
and B electrode before discharge and charge. Ri is referred to the electrolyte resistance. A semicircle in the small high-frequency region represents the SEI layer resistance (Rf) and the other semicircle in the medium frequency region denotes charge transfer resistance (Rct). A sloping line in the low-frequency region referred as the impedance ascribed to the Na+ diffusion (Zw) [13,16]. It can be seen that both sample A and sample B are diffraction resistance predominated process. Zw of sample A is greatly decreased compared to that of sample B. Therefore, the electrochemical performance of sample A is greater than the case of sample B with a faster kinetic process. Hence, the Super-P carbon black can enhance the electrochemical performance and the electrical conductivity of the MgFe2O4 electrode. 4. Conclusions In summary, novel MgFe2O4 hollow microboxes with a uniform size of 570 nm are synthesized through template-engaged reaction. The hollow and porous architecture can not only facilitate Na+ diffusion, but also provide extra space to tolerate the volume expansion during the insertion/extraction of Na+. With the increase of the amount of Super-P carbon black, the rate performance and conductivity of the MgFe2O4 microboxes electrode were improved. Acknowledgment This research is supported by the National Nature Science Foundation of China (51172160).
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References [1] N. Yabuuchi, K. Kubota, M. Dahbi, S. Komaba, Chem. Rev. 114 (2014) 11636– 11682. [2] Z.L. Jian, B. Zhao, P. Liu, F.J. Li, M.B. Zheng, M.W. Chen, et al., Chem. Commun. 50 (2014) 1215–1217. [3] F.M. Wu, X.W. Wang, M. Li, H. Xu, Ceram. Int. 42 (2016) 16666–16670. [4] J.-M. Feng, X.-H. Zhong, G.-Z. Wang, L. Dong, X.-F. Li, D.-J. Li, J. Mater. Sci. 52 (2017) 3124–3132. [5] X. Yang, Z.A. Zhang, Y. Fu, Q. Li, Nanoscale 7 (2015) 10198–10203. [6] H. Yu, H.S. Fan, B.L. Yadian, H.T. Tan, W.L. Liu, H.H. Hng, et al., ACS Appl. Mater. Interfaces 7 (2015) 26751–26757. [7] L. Zhang, H.B. Wu, S. Madhavi, H.H. Hng, X.W. (David) Lou, J. Am. Chem. Soc. 134 (2012) 17388–17391.
[8] Y. You, H.-R. Yao, S. Xin, Y.-X. Yin, T.-T. Zuo, C.-P. Yang, et al., Adv. Mater. 28 (2016) 7243–7248. [9] Y. Jiang, S. Yu, B. Wang, Y. Li, W. Sun, Y. Lu, et al., Adv. Funct. Mater. 26 (2016) 5315–5321. [10] Y.H. Yin, B. Zhang, X.T. Zhang, J.J. Xu, S.T. Yang, J. Sol-Gel. Sci. Technol. 66 (2013) 540–543. [11] J.W. Zhou, J. Qin, L.C. Guo, N.Q. Zhao, C.S. Shi, E.-Z. Liu, et al., J. Mater. Chem. A 4 (2016) 17370–17380. [12] K. Zhang, Z. Hu, X. Liu, Z.L. Tao, J. Chen, Adv. Mater. 27 (2015) 3305–3309. [13] X.J. Zhang, T.Q. Chen, D. Yan, W. Qin, B.W. Hu, Z. Sun, L.K. Pan, Electrochim. Acta 180 (2015) 616–621. [14] Y.C. Liu, N. Zhang, C.M. Yu, L.F. Jiao, J. Chen, Nano Lett. 16 (2016) 3321–3328. [15] H.-G. Wang, D.P. Liu, Y.H. Li, Q. Duan, Mater. Lett. 172 (2016) 64–67. [16] Z.Q. Shi, C.B. Chong, J. Wang, C.Y. Wang, X.W. Yu, Mater. Lett. 159 (2015) 341– 344.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
MgFe2O4 hollow microboxes derived from metal-organic-frameworks as anode material for sodium-ion batteries Yuan Guo ⇑, Youyu Zhu, Chao Yuan, Chengyang Wang ⇑ Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 23 February 2017 Received in revised form 8 April 2017 Accepted 12 April 2017 Available online 13 April 2017 Keywords: Magnesium ferrite Metal-organic-framework Microstructure Energy storage and conversion Sodium-ion battery
a b s t r a c t Metal-organic-frameworks (MOFs) Prussian blue microcubes are used as the self-sacrificial templates to fabricate hollow MgFe2O4 microboxes via the process of hydrolysis, ion exchange and subsequent calcination. This hollow and porous architecture provides efficient sodium ion diffusion pathway and sufficient space to accommodate the volume expansion during the insertion/extraction of Na+. Obtained MgFe2O4 microboxes exhibit a good capacity of 406 mA h g 1 at a current density of 50 mA g 1 and maintain a reversible capacity of 135 mA h g 1 after 150 cycles when evaluated as an anode material for sodium-ion batteries. With the increase of the amount of Super-P carbon black, the rate performance and conductivity of the MgFe2O4 microboxes electrode can be improved. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Sodium-ion battery (SIB) has been considered as a potential alternative to lithium-ion battery (LIB) due to the abundant and available reserve of sodium resources. However, it remains a huge challenge to develop appropriate electrode materials, which are capable of accommodating the larger Na+ (radius 1.02 Å, compared to Li+ radius 0.59 Å) and enabling insertion/extraction reversibly [1]. The low-cost and high theoretical capacity of iron-based materials have attracted lots of interests to be explored for SIB anode recently [2]. Spinel ferrites AFe2O4 (A = Ni, Co, Mn) have more advantages than iron oxides because the two metal elements can maintain structural stability and serve as the buffer matrix for each other upon cycling [3,4]. Here, we explored MgFe2O4 as SIB anode material, which received few attention before. Challenges of MgFe2O4 are its poor electronic conduction and large volume expansion during discharging/charging, making it difficult to reach the requirement of practical applications. Developing hollow and porous nano/micro-materials has already been proven as a successful strategy in SIB because of their unique structure [5]. This constructed structure is not only beneficial for increasing large contact areas between the electrode and electrolyte,
⇑ Corresponding authors at: Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China. E-mail addresses: [email protected] (Y. Guo), [email protected] (C. Wang). http://dx.doi.org/10.1016/j.matlet.2017.04.069 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
but also conducive to relief inner stress. The metal-organicframeworks (MOFs) assembled by clusters/metal ions coordinated to organic ligands, usually form a uniform size, and possess interior cavities and porous shells after heat treating [6,7]. For example, Prussian blue (PB) containing the (C„N) anions has been proved to have a high energy density, a small volume change during cycling, and stable rate capability in Na half-cells [8,9]. In this work, we report a facile method through MOF Prussian blue (PB) Fe4[Fe(CN)6]3 as a self-sacrificial template to fabricate MgFe2O4 hollow microboxes via the process of hydrolysis, ion exchange and subsequent calcination. When employed as anode for SIBs, the MgFe2O4 microboxes exhibit good cyclability and rate performance due to the unique hollow. To confirm the advantages of Super-P carbon black, we make a comparison through adjusting Super-p ratio in the electrode [10].
2. Experimental section 2.1. Material preparation 2.1.1. Synthesis of MOF Fe4(Fe(CN)6)3 (Prussian Blue, PB) 0.12 g K4Fe(CN)63H2O was added into a 50 mL HCl aqueous solution (0.1 M) containing 4.0 g polyvinyl pyrrolidone (PVP, MW 40,000). After stirring for 45 min, a clear light green solution was obtained. The beaker was sealed and heated for 24 h at 80 °C. The blue deposition was centrifugated and washed with
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Fig. 1. (a) TEM image of PB microcubes, (b) TEM and (c) HRTEM images of porous and hollow MgFe2O4 microboxes; SEM images of (e) PB microcubes, (i–l) MgFe2O4 microboxes and their elemental mappings of Mg, Fe, and O; (d) SAED and (f) XRD patterns of MgFe2O4 microboxes; (g) EDS and (h) SEM patterns of MgFe2O4 microboxes after the 100th charge at 50 mA g 1.
ethanol and deionized water, respectively. It was put in a vacuum oven at 70 °C for 12 h to form the MOF PB microcubes. 2.1.2. Synthesis of MOF MgFe2O4 microboxes Mg(CH3COO)2 and obtained PB with the stoichiometric amount of Mg and Fe mixed were dispersed into 20 mL ethanol under magnetic stirring for 10 min. Then the solution was heated at 70 °C to evaporate the ethanol. The dried powder was calcined in air at 700 °C for 6 h with a slow heating rate at 2 °C min 1 to synthesize MgFe2O4 microboxes. 2.2. Material characterizations Field-emission scanning electron microscopy (FESEM, Hitachi S4800) with an energy dispersive spectrometer (EDS), and transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F) with selected area electron diffraction (SAED) were used to observe the morphologies and structures of the PB and MgFe2O4. X-ray diffraction was carried out on a Bruker AXS D8 Focus X-ray diffractometer using Cu Ka radiation (k = 0.15406 nm). The specific surface area was measured by the Brunauer-Emmett-Teller (BET). 2.3. Electrochemical measurements Electrochemical performance was investigated using CR2430 coin-type cells. Fiberglass and Na metal were used as separator and counter electrode, respectively. The anodes of SIB were fabricated by mixing the active material, Super-P carbon black and polyvinyl difluoride (PVDF) in N-methyl-2-pyrrolidone (NMP) solvent at different proportions (sample A (7:2:1), sample B (8:1:1)), and then coated onto a copper foil. NaClO4 (1 M) in ethylene carbonate/dimethyl carbonate (EC/DMC, 1:1 by volume) with 5 wt%
fluoroethylene carbonate (FEC) added was employed as the electrolyte [11]. The discharge/charge cycling performance was measured by a LAND-CT2001A battery-testing system in the voltage range of 0.005 V–3.0 V. Electrochemical impedance spectroscopy (EIS) was recorded using a CHI660E electrochemical work station in the frequency range from 100 kHz to 10 mHz. 3. Results and discussion TEM image (Fig. 1a) confirms the solid structure of the PB. The hollow structure and porous shells of MOF MgFe2O4 microboxes are presented in Fig. 1b. Fig. 1c shows lattice spacing of 0.171 nm and 0.161 nm for the (422) and (511) crystalline planes of spinel MgFe2O4. Fig. 1f verifies the successful preparation of the MgFe2O4 composite in terms of XRD profile. The diffraction peaks are (2 2 7) space indexed to the pure cubic MgFe2O4 with an Fd3m group (JCPDS 17-0464), which are corresponding to the SAED (Fig. 1d) result. The clear diffraction points can be well indexed as a pure spinel MgFe2O4 phase, indicating a high crystallinity of MgFe2O4. As shown in Fig. 1e, the synthesized MOF PB consists of uniform microcubes with smooth surfaces and a size of 610 nm. After calcination at 700 °C, the synthesized MgFe2O4 particles (Fig. 2b) keep a regular cubic shape but with rough surfaces and a shrunken size of 570 nm. The specific surface area of MgFe2O4 is 39.114 m2 g 1 based on the BET analysis of the nitrogen absorption/desorption isotherms. The elemental distributions of Mg, Fe, and O are homogeneous within the MgFe2O4 particles (Fig. 1i–1l). Cycled anode powder was scraped from the copper foil washed with DMC in the Ar-filled glove box. After dried, the powder was protected by a parafilm from the air [12]. Clearly, some MgFe2O4 particle after 100th charge at 50 mA g 1 still keeps its box structure (Fig. 1h) and has a Na/Mg/Fe/O ratio of 1/1.3/2.5/3.7 (Fig. 1g).
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Fig. 2. Electrochemical properties of MgFe2O4 microboxes electrodes at different active material ratios A (7:2:1) and B (8:1:1). (a) Discharge/charge curves and (b) cyclic performance of sample A at a current density of 50 mA g 1; (c) Rate performance and (d) Nyquist plots of samples A and B.
Therefore, MgFe2O4 microboxes could resist the deformation during cycling. Fig. 2a shows the representative discharge-charge curves of the sample A (7:2:1) at 50 mA g 1. Two potential plateaus (0.75 and 0.55 V) of the electrode are delivered in the initial discharge process, which results from the formation of the solid electrolyte interface (SEI) film and the reduction of Fe3+ to Fe0, respectively [13]. The initial discharge/charge capacities are 406/207 mA h g 1 with a coulombic efficiency of 51%. The capacity loss is mainly caused by the irreversible intercalation of Na+ into the crystal lattice and the formation of SEI film. Moreover, the addition of FEC in the electrolyte can strengthen the electrode architecture, enhance the structural stability of SEI film, and suppress the side reactions between electrode and electrolyte [14]. Fig. 2b presents the cyclic performance of sample A at 50 mA g 1. After 150 cycles, the electrode still maintains a stable capacity of 135 mA h g 1 with the capacity retention rate of 98.6%, suggesting favourable cycle stability. Rate performance of samples A (7:2:1) and B (8:1:1) is displayed in Fig. 2c. The average discharge capacities of sample A are 255, 172, 150, 112, and 85 mA h g 1 at current densities of 0.05, 0.1, 0.2, 0.5, and 1 A g 1 respectively, which are much high than sample B electrodes. A reversible capacity of 168 mA h g 1 can be retained when the current density decreases from 1.0 to 0.05 A g 1, demonstrating structure stability and favourable reversibility. The improved electrochemical performance due to the hollow structure, which can tolerate the volume expansion, effectively buffer the mechanical strain, and suppress the particle aggregation [15]. Fig. 2d presents the Nyquist plots of samples A
and B electrode before discharge and charge. Ri is referred to the electrolyte resistance. A semicircle in the small high-frequency region represents the SEI layer resistance (Rf) and the other semicircle in the medium frequency region denotes charge transfer resistance (Rct). A sloping line in the low-frequency region referred as the impedance ascribed to the Na+ diffusion (Zw) [13,16]. It can be seen that both sample A and sample B are diffraction resistance predominated process. Zw of sample A is greatly decreased compared to that of sample B. Therefore, the electrochemical performance of sample A is greater than the case of sample B with a faster kinetic process. Hence, the Super-P carbon black can enhance the electrochemical performance and the electrical conductivity of the MgFe2O4 electrode. 4. Conclusions In summary, novel MgFe2O4 hollow microboxes with a uniform size of 570 nm are synthesized through template-engaged reaction. The hollow and porous architecture can not only facilitate Na+ diffusion, but also provide extra space to tolerate the volume expansion during the insertion/extraction of Na+. With the increase of the amount of Super-P carbon black, the rate performance and conductivity of the MgFe2O4 microboxes electrode were improved. Acknowledgment This research is supported by the National Nature Science Foundation of China (51172160).
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References [1] N. Yabuuchi, K. Kubota, M. Dahbi, S. Komaba, Chem. Rev. 114 (2014) 11636– 11682. [2] Z.L. Jian, B. Zhao, P. Liu, F.J. Li, M.B. Zheng, M.W. Chen, et al., Chem. Commun. 50 (2014) 1215–1217. [3] F.M. Wu, X.W. Wang, M. Li, H. Xu, Ceram. Int. 42 (2016) 16666–16670. [4] J.-M. Feng, X.-H. Zhong, G.-Z. Wang, L. Dong, X.-F. Li, D.-J. Li, J. Mater. Sci. 52 (2017) 3124–3132. [5] X. Yang, Z.A. Zhang, Y. Fu, Q. Li, Nanoscale 7 (2015) 10198–10203. [6] H. Yu, H.S. Fan, B.L. Yadian, H.T. Tan, W.L. Liu, H.H. Hng, et al., ACS Appl. Mater. Interfaces 7 (2015) 26751–26757. [7] L. Zhang, H.B. Wu, S. Madhavi, H.H. Hng, X.W. (David) Lou, J. Am. Chem. Soc. 134 (2012) 17388–17391.
[8] Y. You, H.-R. Yao, S. Xin, Y.-X. Yin, T.-T. Zuo, C.-P. Yang, et al., Adv. Mater. 28 (2016) 7243–7248. [9] Y. Jiang, S. Yu, B. Wang, Y. Li, W. Sun, Y. Lu, et al., Adv. Funct. Mater. 26 (2016) 5315–5321. [10] Y.H. Yin, B. Zhang, X.T. Zhang, J.J. Xu, S.T. Yang, J. Sol-Gel. Sci. Technol. 66 (2013) 540–543. [11] J.W. Zhou, J. Qin, L.C. Guo, N.Q. Zhao, C.S. Shi, E.-Z. Liu, et al., J. Mater. Chem. A 4 (2016) 17370–17380. [12] K. Zhang, Z. Hu, X. Liu, Z.L. Tao, J. Chen, Adv. Mater. 27 (2015) 3305–3309. [13] X.J. Zhang, T.Q. Chen, D. Yan, W. Qin, B.W. Hu, Z. Sun, L.K. Pan, Electrochim. Acta 180 (2015) 616–621. [14] Y.C. Liu, N. Zhang, C.M. Yu, L.F. Jiao, J. Chen, Nano Lett. 16 (2016) 3321–3328. [15] H.-G. Wang, D.P. Liu, Y.H. Li, Q. Duan, Mater. Lett. 172 (2016) 64–67. [16] Z.Q. Shi, C.B. Chong, J. Wang, C.Y. Wang, X.W. Yu, Mater. Lett. 159 (2015) 341– 344.