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Highly ordered three-dimensional TiO2@C nanotube arrays as freestanding electrode for sodium-ion battery
Materials Letters 207 (2017) 149–152
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Highly ordered three-dimensional TiO2@C nanotube arrays as freestanding electrode for sodium-ion battery Jun Yang a, Ziliang Chen b, Hao Wang b, Feng Liang a, Rongsheng Chen a,⇑, Renbing Wu a,b,⇑ a b
Institute of Advanced Materials and Nanotechnology, The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China Departmental of Materials Science, Fudan University, Shanghai 200433, China
a r t i c l e
i n f o
Article history: Received 13 May 2017 Received in revised form 5 July 2017 Accepted 14 July 2017 Available online 14 July 2017 Keywords: Carbon-coated TiO2 nanotube array Freestanding Sodium-ion battery
a b s t r a c t Highly ordered three-dimensional carbon-coated TiO2 nanotube arrays (TiO2@C NTAs) directly grown on Ti current collector have been fabricated using a combination of anodization and hydrothermal method. The grown TiO2@C NTAs were vertically aligned on the Ti foil with a diameter of around 100 nm and length of 10 lm. Benefiting from their structure features, the as-fabricated TiO2@C NTAs anode exhibits excellent sodium storage in terms of high reversible capacity (232 mA h g 1 over 500 cycles at current density of 200 mA g 1) and superior rate capability (68 mA h g 1 at current density of 6.4 A g 1). Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction As the main component of sodium-ion batteries (SIBs), anodes play a critical role in improving the electrochemical performance [1–3]. Within the family of various anodes, TiO2 is certainly of special interest by the virtue of its natural abundance, low cost, environmental friendliness and attractive theoretic specific capacity of 335 mA h g 1 [4]. Unfortunately, it still has some deficiencies, like a poor electric conductivity and low sodium ion diffusivity, which usually lead to unsatisfactory electrochemical performance and thus severely limits its widely application in SIBs. A common and effective strategy to increase the electric conductivity is to construct TiO2-based hybrid materials by incorporating conductive carbon additives [5–10]. On the other hand, designing onedimensional (1D) nanostructure, which not only can provide high specific surface area but also facilitate sodium ion transport and diffusion into the TiO2 host lattice, has already demonstrated a successful strategy to further improve the electrochemical performance [11–14]. Specifically, highly ordered TiO2 nanotube arrays features intrinsic merits for sodium storage for increasing the contact of TiO2 with an electrolyte and the ability to release inner stress. Herein, aiming at the integration of both hybridization and 1D nanostructure engineering for the improved electrochemical per⇑ Corresponding authors at: Institute of Advanced Materials and Nanotechnology, the State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China. E-mail addresses: [email protected] (R. Chen), [email protected] (R. Wu). http://dx.doi.org/10.1016/j.matlet.2017.07.066 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
formance, we report on the preparation of three-dimensional (3D) highly ordered carbon-coated TiO2 nanotube arrays (TiO2@C NTAs) on Ti foil substrate, as illustrated in Fig. 1. The growth of nanotube arrays on Ti current collector used as binder-free anode in SIBs can simultaneously offer sufficient electric conductivity, low ion diffusion resistance and good self-integrity, thus resulting in excellent sodium storage in terms of high reversible capacity and superior rate capability. 2. Experiment The highly ordered TiO2 nanotube arrays (TiO2 NTAs) was fabricated by electrochemical anodization of a Ti foil (10 10 0.89 mm3, Alfa Aesar) with a direct current power supply (IT6834, ITECH, China). The electrolyte was ethylene glycol containing 0.5 wt% NH4F, 5 vol% methanol and 5 vol% distilled water. Anodization was conducted at 60 V for 30 min with the Ti foil served as the anode and a graphite foil served as the cathode. The anodized Ti foil was rinsed in distilled water and then annealed in air at 450 °C for 3 h. The annealed Ti foil was immersed in a 60 mL glucose solution (0.05 mol/L) within a sealed steel autoclave and was heated to 200 °C for 10 h. The obtained foil was rinsed in distilled water and further annealed in nitrogen flow at 550 °C for 3 h to obtain the finally product, which was designated as TiO2@C NTAs. The TiO2@C NTAs were directly employed as electrodes without adding any conductive additive and polymeric binder. The average mass loading of the grown NTAs on Ti foil is about 0.8 mg cm 2 and the thickness of electrode used in the half cells are 0.9 mm. Pure Na foils and a glass fiber (Whatman) were used
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Fig. 1. Schematic illustration of the formation of highly ordered 3D TiO2@C nanotube arrays electrode configuration.
Fig. 2. The as-synthesized TiO2@C NTAs: (a) cross-section and (b) top view FESEM images, (c) XRD patterns, (d) Raman spectra, (e) TEM and (f) HRTEM images.
J. Yang et al. / Materials Letters 207 (2017) 149–152
as the counter electrodes and separator, respectively. The electrolyte was 1 M sodium perchlorate (NaClO4) in a mixture of ethylene carbonate and dimethyl carbonate. Galvanostatic chargedischarge cycling tests were performed with a Land CT2001A battery testing system at room temperature. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted with an Autolab PGSTAT 302N electrochemical workstation. The morphology and structure of the TiO2@C NTAs were characterized by field-emission scanning electron microscopy (FESEM, FEI Nova 450 Nano), Transmission Electron Microscope (TEM, JEOL JEM-2100), Raman spectroscopy (LabRAM HR800), and X-ray diffraction (XRD, Philips X’Pert Pro diffractometer with copper Ka radiation).
3. Results and discussion Fig. 2a and b show the FESEM images of the cross section and top view of TiO2@C NTAs, respectively. It can be observed that the grown TiO2@C NTAs were vertically aligned on the Ti foil with a diameter of around 100 nm and their length is up to 10 lm. The powder XRD pattern shown in Fig. 2c indicated that the electrochemically grown TiO2 nanotubes transferred from amorphous state to crystalline anatase phase (JCPDS No. 21-1272) after annealing at 550 °C. Raman spectra revealed that the existence of carbonaceous materials after annealing due to the appearance of a D-band at 1339 cm 1 and G-band at 1595 cm 1, corresponding to the induced disorder mode and graphitic CAC stretching mode
151
of sp2 hybridized carbon, respectively (Fig. 2d) [8]. As shown in Fig. 2e, the tubular morphology of the as-synthesized products has an outer and inner diameter of about 120 and 80 nm, respectively. The lattice fringes with a d-spacing value of 0.35 nm in the high-resolution TEM (HRTEM) image are assigned to the (1 0 1) planes of anatase TiO2 (Fig. 2f). In addition, the amorphous layers with the disordered carbon phase decorated on the surface of the TiO2 are also clearly observed. Fig. 3a shows the CV curves of the 3D TiO2@C NT arrays, from which as the irreversible peak at around 0.7 V during the first cathodic scan can be attributed to the formation of the solid electrolyte interface (SEI). During the initial anodic sweep, a broad peak located in the range of 0.5–1.2 V can be observed, corresponding to the Ti3+/Ti4+ redox couple. The galvanostatic charge/discharge profiles of the TiO2@C NTAs electrode exhibited sloped charge/discharge profiles without obvious plateaus (Fig. 3b), indicating a homogenous Na insertion into the TiO2 lattice [15]. The initial discharge and charge capacities are 758 and 382 mA h g 1, corresponding to a Coulombic efficiency of 50.3% at the first cycle. The irreversible capacity loss and low initial Coulombic efficiency could be mainly due to the irreversible electrolyte decomposition, the formation of SEI layer and the irreversible trapping of Na+ in the carbon matrix [16]. Fig. 3c demonstrates the cycling performance together with the Coulombic efficiency of the TiO2@C NTAs electrode at a constant current density of 200 mA g 1. There was a substantial capacity loss during the first 100 cycles, which may result from the continuous formation of SEI and the electrolyte decomposition. At the end of 500 charge-discharge cycles, a
Fig. 3. (a) CV curves scanned at the rate of 0.1 mV s 1 and (b) galvanostatic charge–discharge voltage profiles measured at the current density of 0.2 A g 1 for the TiO2@C NTAs electrode; (c) cycling performances of TiO2@C NTAs and TiO2 NTAs, and Coulombic efficiency of TiO2@C NTA; (d) rate capabilities of TiO2@C NTAs at various current densities from 0.2 to 6.4 A g 1.
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reversible capacity of 232 mA h g 1 can be retained, which is much better than those of TiO2@C NTAs electrode (78 mA h g 1) and most reported TiO2-based nanostructured electrodes (Table S1, Supplementary data). To understand Na-storage properties, the electrochemical impedance spectra (EIS) of TiO2@C NTAs electrode after different cycles were investigated. The SEI and charge-transfer resistance for the 500th cycle was smaller than that in the 5th cycle, indicating the activating process and the improved kinetics of the reaction upon cycling (Fig. S1, Supplementary data). FESEM image of the electrodes collected after 500 cycles indicated that the characteristic of hollow tube structure was basically retained (Fig. S2, Supplementary data). The rate capability of TiO2@C NTAs electrode was also investigated (Fig. 3d) and they could deliver average capacities of 451, 216, 135, 90 and 68 mA h g 1 at current densities of 0.2, 0.8, 1.6, 3.2, 6.4 A g 1, respectively. The excellent rate performance may be ascribed to the unique nanoarchitecture design of the 3D TiO2@C NT arrays, where the fast electron transport and high ion accessibility can be simultaneously realized. Specifically, i) the vertically alignment of the nanotubes not only offers favourable Naion diffusion path along the longitude direction to reach the whole surface of the TiO2 host but also enable full exposure of active materials to the electrolyte and thus enhancement of ion diffusion; ii) the carbon coating provides an improved electronic conductivity thus reducing the charge transfer resistance.
4. Conclusions Highly ordered TiO2@C NTAs on Ti foil substrate have been fabricated by electrochemical anodization and subsequent hydrothermal treatment. Such TiO2@C NTAs offer many advantages including ease in scale-up production, no additives, high ion accessibility and fast electron transport and excellent electrode integrity during fast charge/discharge process. Therefore, the TiO2@C NTAs electrode exhibits superior sodium storage properties in terms of high reversible capacity and robust rate capability.
Acknowledgements This work is supported by the Natural Science Foundation of China (51672049), the Start-up Grant from Fudan University, China (JJH2021103), the Fund of the State Key Laboratory of Refractories and Metallurgy (G201709) and Thousand Young Talents Program of China. 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.matlet.2017.07. 066. References [1] H.S. Hou, X.Q. Qiu, W.F. Wei, Y. Zhang, X.B. Ji, Adv. Energy Mater. 7 (2017) 1602898. [2] G.Q. Zou, X.N. Jia, Z.D. Huang, S.M. Li, H.X. Liao, et al., Electrochim Acta 196 (2016) 413–421. [3] L.J. Fu, K. Tang, K.P. Song, P.A. van Aken, Y. Yu, et al., Nanoscale 6 (2014) 1384– 1389. [4] S.H. Guo, J. Yi, Y. Sun, H.S. Zhou, Energy Environ. Sci. 9 (2016) 2978–3006. [5] G.Q. Zou, J. Chen, Y. Zhang, C. Wang, Z.D. Huang, et al., J. Power Sources 325 (2016) 25–34. [6] G.Q. Zou, H.S. Hou, Y. Zhang, Z.D. Huang, X.Q. Qiu, et al., J. Electrochem. Soc. 163 (2016) A3117–A3125. [7] M.N. Tahir, B. Oschmann, D. Buchholz, X.W. Dou, I. Lieberwirth, et al., Adv. Energy Mater. 6 (2016) 1501489. [8] Y. Zhang, Y.C. Yang, H.S. Hou, X.M. Yang, J. Chen, et al., J. Mater. Chem. A 3 (2015) 18944–18952. [9] J. Hwang, S.T. Myung, J.H. Lee, A. Abouimrane, I. Belharouak, et al., Nano Energy 16 (2015) 218–226. [10] D. Bresser, B. Oschmann, M.N. Tahir, F. Mueller, I. Lieberwirch, et al., J. Electrochem. Soc. 162 (2015) A3013–A3020. [11] Y. Wu, X.W. Liu, Z.Z. Yang, L. Gu, Y. Yu, Small 12 (2016) 3522–3529. [12] J.F. Ni, S.D. Fu, C. Wu, J. Maier, Y. Yu, et al., Adv. Mater. 28 (2016) 2259–2265. [13] Z.H. Bi, M.P. Paranthaman, P.A. Menchhofer, R.R. Dehoff, C.A. Bridges, et al., J. Power Sources 222 (2013) 461–466. [14] H. Xiong, M.D. Slater, M. Balasubramanian, C.S. Johnson, T. Rajh, J. Phys. Chem. Lett. 2 (2011) 2560–2565. [15] Y. Xu, M. Zhou, L.Y. Wen, C.L. Wang, H.P. Zhao, et al., Chem. Mater. 27 (2015) 4274–4280. [16] J.H. Kim, W. Choi, H. Jung, S.H. Oh, K.Y. Chung, et al., J. Alloy Compd. 690 (2017) 393–396.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Highly ordered three-dimensional TiO2@C nanotube arrays as freestanding electrode for sodium-ion battery Jun Yang a, Ziliang Chen b, Hao Wang b, Feng Liang a, Rongsheng Chen a,⇑, Renbing Wu a,b,⇑ a b
Institute of Advanced Materials and Nanotechnology, The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China Departmental of Materials Science, Fudan University, Shanghai 200433, China
a r t i c l e
i n f o
Article history: Received 13 May 2017 Received in revised form 5 July 2017 Accepted 14 July 2017 Available online 14 July 2017 Keywords: Carbon-coated TiO2 nanotube array Freestanding Sodium-ion battery
a b s t r a c t Highly ordered three-dimensional carbon-coated TiO2 nanotube arrays (TiO2@C NTAs) directly grown on Ti current collector have been fabricated using a combination of anodization and hydrothermal method. The grown TiO2@C NTAs were vertically aligned on the Ti foil with a diameter of around 100 nm and length of 10 lm. Benefiting from their structure features, the as-fabricated TiO2@C NTAs anode exhibits excellent sodium storage in terms of high reversible capacity (232 mA h g 1 over 500 cycles at current density of 200 mA g 1) and superior rate capability (68 mA h g 1 at current density of 6.4 A g 1). Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction As the main component of sodium-ion batteries (SIBs), anodes play a critical role in improving the electrochemical performance [1–3]. Within the family of various anodes, TiO2 is certainly of special interest by the virtue of its natural abundance, low cost, environmental friendliness and attractive theoretic specific capacity of 335 mA h g 1 [4]. Unfortunately, it still has some deficiencies, like a poor electric conductivity and low sodium ion diffusivity, which usually lead to unsatisfactory electrochemical performance and thus severely limits its widely application in SIBs. A common and effective strategy to increase the electric conductivity is to construct TiO2-based hybrid materials by incorporating conductive carbon additives [5–10]. On the other hand, designing onedimensional (1D) nanostructure, which not only can provide high specific surface area but also facilitate sodium ion transport and diffusion into the TiO2 host lattice, has already demonstrated a successful strategy to further improve the electrochemical performance [11–14]. Specifically, highly ordered TiO2 nanotube arrays features intrinsic merits for sodium storage for increasing the contact of TiO2 with an electrolyte and the ability to release inner stress. Herein, aiming at the integration of both hybridization and 1D nanostructure engineering for the improved electrochemical per⇑ Corresponding authors at: Institute of Advanced Materials and Nanotechnology, the State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China. E-mail addresses: [email protected] (R. Chen), [email protected] (R. Wu). http://dx.doi.org/10.1016/j.matlet.2017.07.066 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
formance, we report on the preparation of three-dimensional (3D) highly ordered carbon-coated TiO2 nanotube arrays (TiO2@C NTAs) on Ti foil substrate, as illustrated in Fig. 1. The growth of nanotube arrays on Ti current collector used as binder-free anode in SIBs can simultaneously offer sufficient electric conductivity, low ion diffusion resistance and good self-integrity, thus resulting in excellent sodium storage in terms of high reversible capacity and superior rate capability. 2. Experiment The highly ordered TiO2 nanotube arrays (TiO2 NTAs) was fabricated by electrochemical anodization of a Ti foil (10 10 0.89 mm3, Alfa Aesar) with a direct current power supply (IT6834, ITECH, China). The electrolyte was ethylene glycol containing 0.5 wt% NH4F, 5 vol% methanol and 5 vol% distilled water. Anodization was conducted at 60 V for 30 min with the Ti foil served as the anode and a graphite foil served as the cathode. The anodized Ti foil was rinsed in distilled water and then annealed in air at 450 °C for 3 h. The annealed Ti foil was immersed in a 60 mL glucose solution (0.05 mol/L) within a sealed steel autoclave and was heated to 200 °C for 10 h. The obtained foil was rinsed in distilled water and further annealed in nitrogen flow at 550 °C for 3 h to obtain the finally product, which was designated as TiO2@C NTAs. The TiO2@C NTAs were directly employed as electrodes without adding any conductive additive and polymeric binder. The average mass loading of the grown NTAs on Ti foil is about 0.8 mg cm 2 and the thickness of electrode used in the half cells are 0.9 mm. Pure Na foils and a glass fiber (Whatman) were used
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Fig. 1. Schematic illustration of the formation of highly ordered 3D TiO2@C nanotube arrays electrode configuration.
Fig. 2. The as-synthesized TiO2@C NTAs: (a) cross-section and (b) top view FESEM images, (c) XRD patterns, (d) Raman spectra, (e) TEM and (f) HRTEM images.
J. Yang et al. / Materials Letters 207 (2017) 149–152
as the counter electrodes and separator, respectively. The electrolyte was 1 M sodium perchlorate (NaClO4) in a mixture of ethylene carbonate and dimethyl carbonate. Galvanostatic chargedischarge cycling tests were performed with a Land CT2001A battery testing system at room temperature. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted with an Autolab PGSTAT 302N electrochemical workstation. The morphology and structure of the TiO2@C NTAs were characterized by field-emission scanning electron microscopy (FESEM, FEI Nova 450 Nano), Transmission Electron Microscope (TEM, JEOL JEM-2100), Raman spectroscopy (LabRAM HR800), and X-ray diffraction (XRD, Philips X’Pert Pro diffractometer with copper Ka radiation).
3. Results and discussion Fig. 2a and b show the FESEM images of the cross section and top view of TiO2@C NTAs, respectively. It can be observed that the grown TiO2@C NTAs were vertically aligned on the Ti foil with a diameter of around 100 nm and their length is up to 10 lm. The powder XRD pattern shown in Fig. 2c indicated that the electrochemically grown TiO2 nanotubes transferred from amorphous state to crystalline anatase phase (JCPDS No. 21-1272) after annealing at 550 °C. Raman spectra revealed that the existence of carbonaceous materials after annealing due to the appearance of a D-band at 1339 cm 1 and G-band at 1595 cm 1, corresponding to the induced disorder mode and graphitic CAC stretching mode
151
of sp2 hybridized carbon, respectively (Fig. 2d) [8]. As shown in Fig. 2e, the tubular morphology of the as-synthesized products has an outer and inner diameter of about 120 and 80 nm, respectively. The lattice fringes with a d-spacing value of 0.35 nm in the high-resolution TEM (HRTEM) image are assigned to the (1 0 1) planes of anatase TiO2 (Fig. 2f). In addition, the amorphous layers with the disordered carbon phase decorated on the surface of the TiO2 are also clearly observed. Fig. 3a shows the CV curves of the 3D TiO2@C NT arrays, from which as the irreversible peak at around 0.7 V during the first cathodic scan can be attributed to the formation of the solid electrolyte interface (SEI). During the initial anodic sweep, a broad peak located in the range of 0.5–1.2 V can be observed, corresponding to the Ti3+/Ti4+ redox couple. The galvanostatic charge/discharge profiles of the TiO2@C NTAs electrode exhibited sloped charge/discharge profiles without obvious plateaus (Fig. 3b), indicating a homogenous Na insertion into the TiO2 lattice [15]. The initial discharge and charge capacities are 758 and 382 mA h g 1, corresponding to a Coulombic efficiency of 50.3% at the first cycle. The irreversible capacity loss and low initial Coulombic efficiency could be mainly due to the irreversible electrolyte decomposition, the formation of SEI layer and the irreversible trapping of Na+ in the carbon matrix [16]. Fig. 3c demonstrates the cycling performance together with the Coulombic efficiency of the TiO2@C NTAs electrode at a constant current density of 200 mA g 1. There was a substantial capacity loss during the first 100 cycles, which may result from the continuous formation of SEI and the electrolyte decomposition. At the end of 500 charge-discharge cycles, a
Fig. 3. (a) CV curves scanned at the rate of 0.1 mV s 1 and (b) galvanostatic charge–discharge voltage profiles measured at the current density of 0.2 A g 1 for the TiO2@C NTAs electrode; (c) cycling performances of TiO2@C NTAs and TiO2 NTAs, and Coulombic efficiency of TiO2@C NTA; (d) rate capabilities of TiO2@C NTAs at various current densities from 0.2 to 6.4 A g 1.
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reversible capacity of 232 mA h g 1 can be retained, which is much better than those of TiO2@C NTAs electrode (78 mA h g 1) and most reported TiO2-based nanostructured electrodes (Table S1, Supplementary data). To understand Na-storage properties, the electrochemical impedance spectra (EIS) of TiO2@C NTAs electrode after different cycles were investigated. The SEI and charge-transfer resistance for the 500th cycle was smaller than that in the 5th cycle, indicating the activating process and the improved kinetics of the reaction upon cycling (Fig. S1, Supplementary data). FESEM image of the electrodes collected after 500 cycles indicated that the characteristic of hollow tube structure was basically retained (Fig. S2, Supplementary data). The rate capability of TiO2@C NTAs electrode was also investigated (Fig. 3d) and they could deliver average capacities of 451, 216, 135, 90 and 68 mA h g 1 at current densities of 0.2, 0.8, 1.6, 3.2, 6.4 A g 1, respectively. The excellent rate performance may be ascribed to the unique nanoarchitecture design of the 3D TiO2@C NT arrays, where the fast electron transport and high ion accessibility can be simultaneously realized. Specifically, i) the vertically alignment of the nanotubes not only offers favourable Naion diffusion path along the longitude direction to reach the whole surface of the TiO2 host but also enable full exposure of active materials to the electrolyte and thus enhancement of ion diffusion; ii) the carbon coating provides an improved electronic conductivity thus reducing the charge transfer resistance.
4. Conclusions Highly ordered TiO2@C NTAs on Ti foil substrate have been fabricated by electrochemical anodization and subsequent hydrothermal treatment. Such TiO2@C NTAs offer many advantages including ease in scale-up production, no additives, high ion accessibility and fast electron transport and excellent electrode integrity during fast charge/discharge process. Therefore, the TiO2@C NTAs electrode exhibits superior sodium storage properties in terms of high reversible capacity and robust rate capability.
Acknowledgements This work is supported by the Natural Science Foundation of China (51672049), the Start-up Grant from Fudan University, China (JJH2021103), the Fund of the State Key Laboratory of Refractories and Metallurgy (G201709) and Thousand Young Talents Program of China. 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.matlet.2017.07. 066. References [1] H.S. Hou, X.Q. Qiu, W.F. Wei, Y. Zhang, X.B. Ji, Adv. Energy Mater. 7 (2017) 1602898. [2] G.Q. Zou, X.N. Jia, Z.D. Huang, S.M. Li, H.X. Liao, et al., Electrochim Acta 196 (2016) 413–421. [3] L.J. Fu, K. Tang, K.P. Song, P.A. van Aken, Y. Yu, et al., Nanoscale 6 (2014) 1384– 1389. [4] S.H. Guo, J. Yi, Y. Sun, H.S. Zhou, Energy Environ. Sci. 9 (2016) 2978–3006. [5] G.Q. Zou, J. Chen, Y. Zhang, C. Wang, Z.D. Huang, et al., J. Power Sources 325 (2016) 25–34. [6] G.Q. Zou, H.S. Hou, Y. Zhang, Z.D. Huang, X.Q. Qiu, et al., J. Electrochem. Soc. 163 (2016) A3117–A3125. [7] M.N. Tahir, B. Oschmann, D. Buchholz, X.W. Dou, I. Lieberwirth, et al., Adv. Energy Mater. 6 (2016) 1501489. [8] Y. Zhang, Y.C. Yang, H.S. Hou, X.M. Yang, J. Chen, et al., J. Mater. Chem. A 3 (2015) 18944–18952. [9] J. Hwang, S.T. Myung, J.H. Lee, A. Abouimrane, I. Belharouak, et al., Nano Energy 16 (2015) 218–226. [10] D. Bresser, B. Oschmann, M.N. Tahir, F. Mueller, I. Lieberwirch, et al., J. Electrochem. Soc. 162 (2015) A3013–A3020. [11] Y. Wu, X.W. Liu, Z.Z. Yang, L. Gu, Y. Yu, Small 12 (2016) 3522–3529. [12] J.F. Ni, S.D. Fu, C. Wu, J. Maier, Y. Yu, et al., Adv. Mater. 28 (2016) 2259–2265. [13] Z.H. Bi, M.P. Paranthaman, P.A. Menchhofer, R.R. Dehoff, C.A. Bridges, et al., J. Power Sources 222 (2013) 461–466. [14] H. Xiong, M.D. Slater, M. Balasubramanian, C.S. Johnson, T. Rajh, J. Phys. Chem. Lett. 2 (2011) 2560–2565. [15] Y. Xu, M. Zhou, L.Y. Wen, C.L. Wang, H.P. Zhao, et al., Chem. Mater. 27 (2015) 4274–4280. [16] J.H. Kim, W. Choi, H. Jung, S.H. Oh, K.Y. Chung, et al., J. Alloy Compd. 690 (2017) 393–396.