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Interconnected MnO2 nanoflakes supported by 3D nanostructured stainless steel plates for lithium ion battery anodes
Electrochimica Acta 121 (2014) 415–420
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Interconnected MnO2 nanoflakes supported by 3D nanostructured stainless steel plates for lithium ion battery anodes Xiuwan Li, Dan Li, Zhiwei Wei, Xiaonan Shang, Deyan He ∗ School of Physical Science and Technology, Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, China
a r t i c l e
i n f o
Article history: Received 2 November 2013 Received in revised form 3 December 2013 Accepted 8 December 2013 Available online 6 January 2014 Keywords: Manganese dioxide Etched 3D stainless steel plate Lithium ion batteries Anode Cyclic voltammetry analysis
a b s t r a c t Interconnected MnO2 nanoflakes supported by 3D nanostructured stainless steel (SS) plates are prepared by a facile hydrothermal synthesis. The resultant architecture is used as binder-free anodes of lithium ion batteries. Cyclic voltammetry analysis is conducted to distinguish the reactions between lithium ions and the active material at various discharge-charge potentials. Galvanostatic battery testing shows that the representative electrode exhibits a reversible capacity up to 1387.1 mA h g−1 at a current rate of 0.2 C after 100 cycles and a capacity higher than 492.9 mA h g−1 at a rate of 5 C. Such an excellent cycling performance, better rate capability and high capacity indicate that the simply etched 3D SS plate is a promising nanostructured current collector. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Owing to the limited capacity of 320–340 mA h g−1 for graphite, the search for anode materials with high capacity and good rate capability has been the most challenge for lithium ion batteries (LIBs). [1–5] Manganese dioxide (MnO2 ) has attracted great interest for its high theoretical capacity of 1230 mA h g−1 , environmentally friendly nature, relatively low cost, and abundant natural reserves. [6–9] It is important to develop more simple techniques for fabricating novel nanostructured MnO2 anodes with high rate capability. One of the effective approaches is to fabricate 3D nanostructured metallic current collectors and then grew the active materials on them, no binders and conductive agents were used. [10–14] Compared with the traditional casting electrodes of powder materials, the enhanced electrochemical performance of the electrodes with an architecture of the active material@3D nanostructured current collector can be attributed to the efficient electron transport channel between the active material and the current collector, the shorter ion and electron transport pathways, and the enough space to accommodate the volume changes of the active material. [15–19] For example, Zhang et al. synthesized highly porous nickel framework through the template method, and prepared MnO2
∗ Corresponding author. Tel.: +86 931 891 2546; fax: +86 931 891 3554. E-mail address: [email protected] (D. He). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.12.064
on the framework by pulsed electrodeposition for the lithiumion cathodes, which showed the ultrafast rate performance. [20] Reddy et al. prepared carbon nanotube arrays as the substrates and fabricated coaxial MnO2 on them as electrodes of LIBs, which showed a capacity of 500 mA h g−1 after 15 cycles. [21] Jiang et al. synthesized 3D metallic nanoframeworks as the core, and MnO2 coatings as the shell to form a 3D interconnected core–shell hybrid electrode by hydrothermal method followed by in-situ reduction, which delivered a specific capacity of 1142 mA h g−1 at a current rate of 369 mA g−1 and a capacity of 269 mA h g−1 at a current rate of 7380 mA g−1 . [22] However, the mentioned preparations of the 3D nanostructured current collectors are complicated, which are difficult to be applied in the fabrication of the next-generation high-power LIBs. In this paper, we report a preparation of 3D nanostructured metallic current collectors via a facile chemical corrosion of stainless steel (SS) plates. The interconnected MnO2 nanoflakes were synthesized on the obtained 3D nanostructured SS plates by hydrothermal method. The resultant architecture was used as binder-free anodes of LIBs. The detailed electrochemical reactions at different potentials were first qualitatively discussed based on the cyclic voltammetry (CV) analysis. It was shown that the electrodes exhibit excellent cycling performance, better rate capability and high areal capacity. The representative electrode delivers a reversible capacity up to 1387.1 mA h g−1 after 100 cycles at a rate of 0.2 C, and a capacity higher than 492.9 mA h g−1 at a rate as high as 5 C.
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2. Experimental 2.1. Fabrication procedure The interconnected MnO2 nanoflakes were grown on a chemically etched SS plate (Type 304, 100 m thick) by hydrothermal synthesis. SS plate was first sonicated in high-purity acetone for 0.5 h, and then immersed in 1 M HCl solution for 3 h. The temperature of the HCl solution was held at 60 ◦ C. Then 400 mg KMnO4 was dissolved in 30 mL deionized water. The solution as well as the etched SS plate was transferred into a 40 mL of Teflon-lined autoclave, and maintained at 150 ◦ C for 5 h. The mass of the etched SS plate before and after hydrothermal was weighed by a microbalance (Mettler XS105DU) with an accuracy of 0.01 mg, and the active mass of MnO2 was determined by the mass difference of the etched SS plate before and after hydrothermal. The areal density of the active material is about 0.73 mg cm−2 for the representative electrode. 2.2. Structural Characterization The structures of the synthesized materials and the etched SS plates were characterized by X-ray powder diffraction (XRD, Rigaku D/Max-2400 with Cu K␣ radiation, = 0.15418 nm) with 2 range from 10◦ to 90◦ . The morphologies were observed using fieldemission scanning electron microscopy (FE-SEM, Hitachi S-4800) and transmission electron microscopy (TEM, FEI Tecnai G2 F30). 2.3. Electrochemical Characterization The electrochemical measurements were performed using CR2032 coin cells. The cells were assembled in a high-purity argon filled glove box (H2 O < 0.5 ppm, O2 < 0.5 ppm, MBraun, Unilab) using lithium foil as counter electrode and Celgard 2320 as separator membrane. The obtained interconnected MnO2 nanoflakes@3D nanostructured SS plate was used as the working electrode without adding any conductive agent and binder. The electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a 1:1 volume ratio. The galvanostatic dischargecharge cycling and cyclic voltammetry (CV) measurements were carried out at room temperature by using a multichannel battery tester (Neware BTS-610) and an electrochemical workstation (CHI 660 C), respectively. 3. Results and discussion Fig. 1 shows XRD patterns of the as-etched SS plate and the hydrothermally synthesized MnO2 on it. Four distinct diffraction peaks respectively located at 43.9◦ , 65.0◦ , 74.9◦ , and 82.2◦ can be observed for the as-etched SS plate. After MnO2 being hydrothermally synthesized at 150 ◦ C for 5 h, new diffraction peaks appeare at angles of 12.5◦ , 25.2◦ , 37.3◦ , and 51.0◦ , which can be indexed to (001), (002), (-111), and (-113) planes of birnessite MnO2 , respectively (JCPDS 80-1098). No other peaks can be observed except those from the etched SS substrate. Fig. 2(a) shows SEM image of the as-etched SS plate. After corrosion in HCl solution, the surface of the SS plate loses its metallic luster and becomes rough. It can be seen that a 3D network-like structure is formed within a thick layer beneath the surface of the SS plate. Fig. 2(b) shows the morphology of the etched SS plate on which the nanostructured MnO2 being hydrothermally synthesized. Compared with Fig. 2(a), the interspaces in the 3D network-like structure of the etched SS plate have been stuffed by the synthesized MnO2 nanoflowers. The high-magnification SEM image of the interconnected MnO2 nanoflakes is shown in Fig. 2(c). It can be seen that the nanoflakes
Fig. 1. XRD patterns of the as-etched SS plate and the hydrothermally synthesized MnO2 on it.
are interconnected with each other, forming a large number of voids. Such a microstructure will provide a high specific surface area and be beneficial for the electrolyte infiltrating into the electrode material, leading to the reduced diffusion lengths of lithium ions, as reported by the other articles. [8,23–26] The detailed morphological and structural features of the interconnected MnO2 nanoflakes are also examined by TEM. As shown in Fig. 2(d), the interconnected MnO2 nanoflakes grow tightly on the etched SS (the shadow part). The selected-area electron diffraction (SAED) pattern of an individual nanoflake (inset in Fig. 2(d)) reveals that all the diffraction rings can be indexed to those of birnessite MnO2 . As shown in Fig. 2(e), the black pillars suggest the interconnected nature of the nanoflakes, which is consistent with the SEM observation. Fig. 2(f) shows the measured lattice spacings of 0.248 and 0.253 nm, which are in good agreement with (110) and (200) interplanar distances of birnessite MnO2 , respectively. The obtained architecture of the interconnected MnO2 nanoflakes@3D nanostructured SS plate was directly used as a working electrode of CR-2032 coin cell, CV and galvanostatic discharge–charge profiles were recorded to evaluate its lithium storage performance. Fig. 3(a) shows CV profiles for the initial 3 cycles of a representative cell, which were tested in the potential window of 0.02–3.0 V at a scan rate of 0.1 mV s−1 . One cathodic peak can be observed at 0.29 V in the first cycle, which corresponds to the electrochemical reduction of MnO2 and the formation of the solid electrolyte interphase (SEI) layer. For the first anodic scan, two broad overlapping anodic peaks at 1.20 and 1.76 V correspond to the oxidation of Mn to MnO (Mn + Li2 O → MnO + 2Li+ + 2e− ) and the further oxidation to MnO2 (MnO + Li2 O → MnO2 + 2Li+ + 2e− ), respectively. In the subsequent cycles, two cathodic peaks centered at 0.43 and 1.03 V are observed and the anodic peaks shift to 1.25 and 2.00 V. For any potential of the CV test window, provided the current at the potential is greater than zero, there must be an electrochemical reaction of the active material. A complete cathodic or anodic scan curve is therefore composed by the compositions of the different electrochemical reactions. To further make clear the electrochemical reactions, the spectrum unfolding was conducted for the initial three scan curves by assuming that the shape of the current peaks is Lorentzian line. As shown in Fig. 3(b), the first cathodic scan curve can be resolved into four current peaks at 1.47, 0.60, 0.29 and 0 V, respectively. The peak at 1.47 V comes
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Fig. 2. (a) SEM image of the as-etched SS plate; (b) SEM image of the interconnected MnO2 nanoflakes supported by the etched 3D nanostructured SS palte; (c) Highmagnification SEM image of the interconnected MnO2 nanoflakes; (d) Low-magnification TEM image of the MnO2 nanoflakes. The inset is the corresponding SAED pattern; (e) TEM image of the MnO2 nanoflakes; (f) High-magnification TEM image of an individual MnO2 nanoflake.
from the initial irreversible formation of SEI layer before the reaction of the active materials, which disappears in the subsequent scans. The peak at 0.60 V may correspond to the reduction of the etched SS substrate (Fe2 O3 + 6Li+ + 4e− → 2Fe + 3Li2 O) owing to the used KMnO4 for hydrothermal synthesis of MnO2 . [27] The peak at 0.29 V corresponds to the reduction of MnO2 to Mn (MnO2 + 4Li+ + 4e− → Mn + 2Li2 O). It is clear that, compared with the peak of MnO2 at 0.29 V, the peak area of Fe2 O3 is much small. The current peak at 0 V may be a result of the formation of polymer/gel-like film (PGF), which is in good agreement with the conclusion that the PGF growth and the electrolyte decomposition are enhanced at low potentials. [27] For the first anodic scan shown in Fig. 3(c), 6 current peaks were obtained by the spectrum unfolding. The peaks at 0 and 0.47 V can be attributed to the overpotential after the first cathodic scan. It is clear that the areas of the two peaks are equal and counteract each other. The peak at 1.20 V comes from the oxidation of Mn to MnO (Mn + Li2 O → MnO + 2Li+ + 2e− ), and the peak at 2.0 V corresponds to the further oxidation of MnO to MnO2 (MnO + Li2 O → MnO2 + 2Li+ + 2e− ). The peak area of the former is larger than that of the latter, which can be attributed to the irreversible formation of Li2 O. The current peak at 1.64 V comes from the oxidation of Fe to Fe2 O3 (2Fe + 3Li2 O → Fe2 O3 + 6Li+ + 4e− ). [24] The above three peaks at the higher potentials are
corresponding to the peaks at 0.29 and 0.60 V shown in Fig. 3(b). The peak at 3.0 V results from the decomposition of PGF, corresponding to the formation of PGF at 0 V. As shown in Fig. 3(d), the second cathodic scan curve can be fitted by 4 current peaks. The peak at 1.1 V should be attributed to the reduction of Fe2 O3 to Fe (Fe2 O3 + 6Li+ + 4e− → 2Fe + 3Li2 O). Unlike the first cathodic scan curve, only one peak can be observed for the reduction of MnO2 to Mn. The current peaks at 0.70 and 0.45 V come from the reduction of MnO2 to MnO and MnO to Mn, respectively, which correspond to the two anodic peaks. More importantly, the area of the peak at 0.70 V is far less than the one at 0.45 V. This phenomenon may be attributed to the loss of oxygen from MnO2 as a result of the formation of the irreversible Li2 O. The current peak at 0 V may result from the formation of PGF, which is consistent with the one at 0 V in the first cathodic scan but its area is larger. The peak fitting of the initial three scan curves illuminates that, in addition to the irreversible SEI layer formed at the first cathodic scan curve, the electrochemical reactions of the interconnected MnO2 nanoflakes electrode during cycling only include the redox reactions of the active materials and the formation and decomposition of PGF on the surface of the active materials. Discharge-charge cycling was performed in a potential window of 0.02–3.0 V versus Li/Li+ for the electrode. As shown in Fig. 4(a), the initial discharge and charge capacities are 1828.5 and
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Fig. 3. Cyclic voltammograms for the electrode of the interconnected MnO2 nanoflakes supported by etched SS plate (a), and the multi-peak fittings for the first cathodic scan curve (b), the first anodic scan curve (c), and the second cathodic scan curve (d).
1285.9 mA h g−1 at a rate of 0.2 C (1 C = 1230 mA g−1 ), respectively. The initial discharge capacity is higher than the theoretical capacity of MnO2 , which should be attributed to the formation of SEI layer during the electrochemical reaction process as shown by Peaks 1 and 4 in Fig. 3(b). The irreversible capacity loss (29.7%) may be mainly attributed to the processes such as electrolyte decomposition and inevitable formation of SEI layer, which are common for the most anode materials. [21,28] Fig. 4(b) shows the discharge-charge voltage profiles of the initial three cycles at a rate of 0.2 C. The electrode exhibits a wide voltage plateau at about 0.43 V during the first discharge. There is a hidden sloping curve before reaching the wide voltage plateau at 0.43 V, which is corresponding to Peaks 3 and 4 shown in Fig. 3(b). The discharge plateau shifts to around 0.54 V in the subsequent discharge process, while the charge plateaus are kept at about 1.21 and 1.97 V. As shown in Fig. 4(a), a reversible capacity of 1297.8 mA h g−1 was achieved in the 2nd cycle, which gradually decreases to about 1184.5 mA h g−1 in the first few cycles. After the 10th discharge-charge cycle, the capacity shows a gradual increasing up to 1387.1 mA h g−1 in the 100th cycle, whereas the coulombic efficiency steadily reaches a value higher than 97.4%. The increasing specific capacity may be ascribed to the reversible growth of the PGF film resulting from kinetically activated electrolyte degradation and the further reaction of the active material, which are common for the anode materials. [29–33] The relative
larger areal capacity suggests that the eched SS plate is a promising nanostructured current collector for enhancing the areal capicity of the eletrodes. To further evaluate electrochemical performance of the electrode, the rate capability is tested and shown in Fig. 4(d). It can be seen that the capacities are perfectly stable at the given current densities. The discharge capacities reach to 1318.4, 1039.0, 893.5, and 695.7 mA h g−1 at rates of 0.2, 0.5, 1, and 2 C, respectively. Even at the rate as high as 5 C, the electrode can deliver a capacity higher than 492.9 mA h g−1 . More importantly, when the current rate is returned to its initial value of 0.2 C after 42 cycles with various C rates, the electrode recovers its initial capacity of 1313.6 mA h g−1 , indicating the excellent rate capability. Fig. 5(a) shows the galvanostatic discharge-charge cycling at 1 C. Before cycling at 1 C, the cell was tested at a rate of 0.2 C for five cycles. The initial discharge and charge capacities are 1819.7 and 1231.0 mA h g−1 , respectively. The electrode can deliver a capacity of 915.4 mA h g−1 in the 7th cycle, and retain a capacity of 813.4 mA h g−1 in the 100th cycle, again indicating that the MnO2 nanoflakes electrode has good cycling stability. The SEM image of the MnO2 nanoflakes electrode after 100 cycles at 1 C is shown in Fig. 5(b). It is obvious that the morphology of the interconnected nanoflakes is almost maintained and no obvious exfoliation of the MnO2 nanoflakes can be found from the nanostructured SS plate.
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Fig. 5. (a) Cycling performance at a rate of 1 C after the initial 5 cycling at a rate of 0.2 C; (b) SEM image of the interconnected MnO2 nanoflakes@3D nanostructured SS plate after 100 cycles at a rate of 1 C. The inset is its high-magnification SEM image.
and the 3D nanostructured current collector leads to shortened electronic and ionic transport lengths. 4. Conclusions
Fig. 4. (a) Capacity retention of the galvanostatic test at a rate of 0.2 C; (b) The discharge–charge curves for the initial 3 cycles at a rate of 0.2 C; (c) Capacity retention at various C rates.
From the inset in Fig. 5(b), it is clear that the interconnected MnO2 nanoflakes are intact, showing excellent electrochemical stability of the nanostructure in electrolytes. The excellent electrochemical performance of the present anode can be explained as follows. First, the active material of MnO2 nanoflakes firmly adheres to the nanostructured current collector of the etched SS plate, which can provide an efficient electron transport. Second, the interconnected MnO2 nanoflakes with a large specific surface area can offer a larger material/electrolyte contact area and accommodate the volume change caused by the electrochemical reactions. Third, the nanostructrues of the MnO2 material
In summary, we have developed a facile approach to fabricate the electrode consisted of the interconnected MnO2 nanoflakes on the 3D nanostructured SS palte for high-power LIB applications. Such an unique architecture not only favours rapid lithium ion transport but also accommodates the volume changes during the electrochemical reactions. The electrochemical reactions of the interconnected MnO2 nanoflakes electrode during cycling are analysized by fitting the meaured CV curves. In addition to the irreversible SEI film formed at the first cathodic scan, the electrochemical reactions involved in the interconnected MnO2 nanoflakes electrode during cycling include the redox reactions of the active materials and the formation and decomposition of PGF. The representative electrode delivers a reversible capacity up to 1387.1 mA h g−1 after 100 cycles at a rate of 0.2 C, and a capacity higher than 492.9 mA h g−1 at a rate as high as 5 C. Such a superior electrochemical performance indicates that the 3D nanostructured SS plate is a promising current collector for enhancing the areal capicity of the eletrodes. The approach that the anode materials grow directly on the nanostructured current collectors without using binder and conductive agent has been demonstrated to be a favorable route for electrode preparation of LIBs.
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Acknowledgements This project was financially supported by the National Natural Science Foundation of China with grant nos. 11179038 and 10974073, and the Specialized Research Fund for the Doctoral Program of Higher Education with grant no. 20120211130005. X. Li gratefully acknowledges the support of the Fundamental Research Funds for the Central Universities with grant no. lzujbky-2013-232. References [1] M. Winter, G.H. Wrodnigg, J.O. Besenhard, W. Biberacher, P. Novák, J Electrochem Soc 147 (2000) 2427–2431. [2] M. Winter, J.O. Besenhard, M.E. Spahr, P. Novák, Adv Mater 10 (1998) 725–763. [3] S. Ni, T. Li, X. Lv, X. Yang, L. Zhang, Electrochim Acta 91 (2013) 267–274. [4] X.W. Lou, D. Deng, J.Y. Lee, L.A. Archer, J Mater Chem 18 (2008) 4397–4401. [5] L. Wang, H. Gong, C. Wang, D. Wang, K. Tang, Y. Qian, Nanoscale 4 (2012) 6850–6855. [6] M. Kim, Y. Hwang, K. Min, J. Kim, Electrochim Acta 113 (2013) 322–331. [7] J. Fang, Y.F. Yuan, L.K. Wang, H.L. Ni, H.L. Zhu, J.L. Yang, J.S. Gui, Y.B. Chen, S.Y. Guo, Electrochim Acta 112 (2013) 364–370. [8] H. Xia, M. Lai, L. Lu, J Mater Chem 20 (2010) 6896–6902. [9] J. Li, M. Zou, Y. Zhao, Y. Lin, H. Lai, L. Guan, Z. Huang, Electrochim Acta 111 (2013) 165–171. [10] S. Ni, X. Yang, T. Li, J Mater Chem 22 (2012) 2395–2397. [11] Y. Fu, X. Li, X. Sun, X. Wang, D. Liu, D. He, J Mater Chem 22 (2012) 17429–17431. [12] L. Wang, H.W. Xu, P.C. Chen, D.W. Zhang, C.X. Ding, C.H. Chen, J Power Sources 193 (2009) 846–850. [13] Z. Cui, X. Guo, H. Li, Electrochim Acta 89 (2013) 229–238.
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Interconnected MnO2 nanoflakes supported by 3D nanostructured stainless steel plates for lithium ion battery anodes Xiuwan Li, Dan Li, Zhiwei Wei, Xiaonan Shang, Deyan He ∗ School of Physical Science and Technology, Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, China
a r t i c l e
i n f o
Article history: Received 2 November 2013 Received in revised form 3 December 2013 Accepted 8 December 2013 Available online 6 January 2014 Keywords: Manganese dioxide Etched 3D stainless steel plate Lithium ion batteries Anode Cyclic voltammetry analysis
a b s t r a c t Interconnected MnO2 nanoflakes supported by 3D nanostructured stainless steel (SS) plates are prepared by a facile hydrothermal synthesis. The resultant architecture is used as binder-free anodes of lithium ion batteries. Cyclic voltammetry analysis is conducted to distinguish the reactions between lithium ions and the active material at various discharge-charge potentials. Galvanostatic battery testing shows that the representative electrode exhibits a reversible capacity up to 1387.1 mA h g−1 at a current rate of 0.2 C after 100 cycles and a capacity higher than 492.9 mA h g−1 at a rate of 5 C. Such an excellent cycling performance, better rate capability and high capacity indicate that the simply etched 3D SS plate is a promising nanostructured current collector. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Owing to the limited capacity of 320–340 mA h g−1 for graphite, the search for anode materials with high capacity and good rate capability has been the most challenge for lithium ion batteries (LIBs). [1–5] Manganese dioxide (MnO2 ) has attracted great interest for its high theoretical capacity of 1230 mA h g−1 , environmentally friendly nature, relatively low cost, and abundant natural reserves. [6–9] It is important to develop more simple techniques for fabricating novel nanostructured MnO2 anodes with high rate capability. One of the effective approaches is to fabricate 3D nanostructured metallic current collectors and then grew the active materials on them, no binders and conductive agents were used. [10–14] Compared with the traditional casting electrodes of powder materials, the enhanced electrochemical performance of the electrodes with an architecture of the active material@3D nanostructured current collector can be attributed to the efficient electron transport channel between the active material and the current collector, the shorter ion and electron transport pathways, and the enough space to accommodate the volume changes of the active material. [15–19] For example, Zhang et al. synthesized highly porous nickel framework through the template method, and prepared MnO2
∗ Corresponding author. Tel.: +86 931 891 2546; fax: +86 931 891 3554. E-mail address: [email protected] (D. He). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.12.064
on the framework by pulsed electrodeposition for the lithiumion cathodes, which showed the ultrafast rate performance. [20] Reddy et al. prepared carbon nanotube arrays as the substrates and fabricated coaxial MnO2 on them as electrodes of LIBs, which showed a capacity of 500 mA h g−1 after 15 cycles. [21] Jiang et al. synthesized 3D metallic nanoframeworks as the core, and MnO2 coatings as the shell to form a 3D interconnected core–shell hybrid electrode by hydrothermal method followed by in-situ reduction, which delivered a specific capacity of 1142 mA h g−1 at a current rate of 369 mA g−1 and a capacity of 269 mA h g−1 at a current rate of 7380 mA g−1 . [22] However, the mentioned preparations of the 3D nanostructured current collectors are complicated, which are difficult to be applied in the fabrication of the next-generation high-power LIBs. In this paper, we report a preparation of 3D nanostructured metallic current collectors via a facile chemical corrosion of stainless steel (SS) plates. The interconnected MnO2 nanoflakes were synthesized on the obtained 3D nanostructured SS plates by hydrothermal method. The resultant architecture was used as binder-free anodes of LIBs. The detailed electrochemical reactions at different potentials were first qualitatively discussed based on the cyclic voltammetry (CV) analysis. It was shown that the electrodes exhibit excellent cycling performance, better rate capability and high areal capacity. The representative electrode delivers a reversible capacity up to 1387.1 mA h g−1 after 100 cycles at a rate of 0.2 C, and a capacity higher than 492.9 mA h g−1 at a rate as high as 5 C.
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2. Experimental 2.1. Fabrication procedure The interconnected MnO2 nanoflakes were grown on a chemically etched SS plate (Type 304, 100 m thick) by hydrothermal synthesis. SS plate was first sonicated in high-purity acetone for 0.5 h, and then immersed in 1 M HCl solution for 3 h. The temperature of the HCl solution was held at 60 ◦ C. Then 400 mg KMnO4 was dissolved in 30 mL deionized water. The solution as well as the etched SS plate was transferred into a 40 mL of Teflon-lined autoclave, and maintained at 150 ◦ C for 5 h. The mass of the etched SS plate before and after hydrothermal was weighed by a microbalance (Mettler XS105DU) with an accuracy of 0.01 mg, and the active mass of MnO2 was determined by the mass difference of the etched SS plate before and after hydrothermal. The areal density of the active material is about 0.73 mg cm−2 for the representative electrode. 2.2. Structural Characterization The structures of the synthesized materials and the etched SS plates were characterized by X-ray powder diffraction (XRD, Rigaku D/Max-2400 with Cu K␣ radiation, = 0.15418 nm) with 2 range from 10◦ to 90◦ . The morphologies were observed using fieldemission scanning electron microscopy (FE-SEM, Hitachi S-4800) and transmission electron microscopy (TEM, FEI Tecnai G2 F30). 2.3. Electrochemical Characterization The electrochemical measurements were performed using CR2032 coin cells. The cells were assembled in a high-purity argon filled glove box (H2 O < 0.5 ppm, O2 < 0.5 ppm, MBraun, Unilab) using lithium foil as counter electrode and Celgard 2320 as separator membrane. The obtained interconnected MnO2 nanoflakes@3D nanostructured SS plate was used as the working electrode without adding any conductive agent and binder. The electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a 1:1 volume ratio. The galvanostatic dischargecharge cycling and cyclic voltammetry (CV) measurements were carried out at room temperature by using a multichannel battery tester (Neware BTS-610) and an electrochemical workstation (CHI 660 C), respectively. 3. Results and discussion Fig. 1 shows XRD patterns of the as-etched SS plate and the hydrothermally synthesized MnO2 on it. Four distinct diffraction peaks respectively located at 43.9◦ , 65.0◦ , 74.9◦ , and 82.2◦ can be observed for the as-etched SS plate. After MnO2 being hydrothermally synthesized at 150 ◦ C for 5 h, new diffraction peaks appeare at angles of 12.5◦ , 25.2◦ , 37.3◦ , and 51.0◦ , which can be indexed to (001), (002), (-111), and (-113) planes of birnessite MnO2 , respectively (JCPDS 80-1098). No other peaks can be observed except those from the etched SS substrate. Fig. 2(a) shows SEM image of the as-etched SS plate. After corrosion in HCl solution, the surface of the SS plate loses its metallic luster and becomes rough. It can be seen that a 3D network-like structure is formed within a thick layer beneath the surface of the SS plate. Fig. 2(b) shows the morphology of the etched SS plate on which the nanostructured MnO2 being hydrothermally synthesized. Compared with Fig. 2(a), the interspaces in the 3D network-like structure of the etched SS plate have been stuffed by the synthesized MnO2 nanoflowers. The high-magnification SEM image of the interconnected MnO2 nanoflakes is shown in Fig. 2(c). It can be seen that the nanoflakes
Fig. 1. XRD patterns of the as-etched SS plate and the hydrothermally synthesized MnO2 on it.
are interconnected with each other, forming a large number of voids. Such a microstructure will provide a high specific surface area and be beneficial for the electrolyte infiltrating into the electrode material, leading to the reduced diffusion lengths of lithium ions, as reported by the other articles. [8,23–26] The detailed morphological and structural features of the interconnected MnO2 nanoflakes are also examined by TEM. As shown in Fig. 2(d), the interconnected MnO2 nanoflakes grow tightly on the etched SS (the shadow part). The selected-area electron diffraction (SAED) pattern of an individual nanoflake (inset in Fig. 2(d)) reveals that all the diffraction rings can be indexed to those of birnessite MnO2 . As shown in Fig. 2(e), the black pillars suggest the interconnected nature of the nanoflakes, which is consistent with the SEM observation. Fig. 2(f) shows the measured lattice spacings of 0.248 and 0.253 nm, which are in good agreement with (110) and (200) interplanar distances of birnessite MnO2 , respectively. The obtained architecture of the interconnected MnO2 nanoflakes@3D nanostructured SS plate was directly used as a working electrode of CR-2032 coin cell, CV and galvanostatic discharge–charge profiles were recorded to evaluate its lithium storage performance. Fig. 3(a) shows CV profiles for the initial 3 cycles of a representative cell, which were tested in the potential window of 0.02–3.0 V at a scan rate of 0.1 mV s−1 . One cathodic peak can be observed at 0.29 V in the first cycle, which corresponds to the electrochemical reduction of MnO2 and the formation of the solid electrolyte interphase (SEI) layer. For the first anodic scan, two broad overlapping anodic peaks at 1.20 and 1.76 V correspond to the oxidation of Mn to MnO (Mn + Li2 O → MnO + 2Li+ + 2e− ) and the further oxidation to MnO2 (MnO + Li2 O → MnO2 + 2Li+ + 2e− ), respectively. In the subsequent cycles, two cathodic peaks centered at 0.43 and 1.03 V are observed and the anodic peaks shift to 1.25 and 2.00 V. For any potential of the CV test window, provided the current at the potential is greater than zero, there must be an electrochemical reaction of the active material. A complete cathodic or anodic scan curve is therefore composed by the compositions of the different electrochemical reactions. To further make clear the electrochemical reactions, the spectrum unfolding was conducted for the initial three scan curves by assuming that the shape of the current peaks is Lorentzian line. As shown in Fig. 3(b), the first cathodic scan curve can be resolved into four current peaks at 1.47, 0.60, 0.29 and 0 V, respectively. The peak at 1.47 V comes
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Fig. 2. (a) SEM image of the as-etched SS plate; (b) SEM image of the interconnected MnO2 nanoflakes supported by the etched 3D nanostructured SS palte; (c) Highmagnification SEM image of the interconnected MnO2 nanoflakes; (d) Low-magnification TEM image of the MnO2 nanoflakes. The inset is the corresponding SAED pattern; (e) TEM image of the MnO2 nanoflakes; (f) High-magnification TEM image of an individual MnO2 nanoflake.
from the initial irreversible formation of SEI layer before the reaction of the active materials, which disappears in the subsequent scans. The peak at 0.60 V may correspond to the reduction of the etched SS substrate (Fe2 O3 + 6Li+ + 4e− → 2Fe + 3Li2 O) owing to the used KMnO4 for hydrothermal synthesis of MnO2 . [27] The peak at 0.29 V corresponds to the reduction of MnO2 to Mn (MnO2 + 4Li+ + 4e− → Mn + 2Li2 O). It is clear that, compared with the peak of MnO2 at 0.29 V, the peak area of Fe2 O3 is much small. The current peak at 0 V may be a result of the formation of polymer/gel-like film (PGF), which is in good agreement with the conclusion that the PGF growth and the electrolyte decomposition are enhanced at low potentials. [27] For the first anodic scan shown in Fig. 3(c), 6 current peaks were obtained by the spectrum unfolding. The peaks at 0 and 0.47 V can be attributed to the overpotential after the first cathodic scan. It is clear that the areas of the two peaks are equal and counteract each other. The peak at 1.20 V comes from the oxidation of Mn to MnO (Mn + Li2 O → MnO + 2Li+ + 2e− ), and the peak at 2.0 V corresponds to the further oxidation of MnO to MnO2 (MnO + Li2 O → MnO2 + 2Li+ + 2e− ). The peak area of the former is larger than that of the latter, which can be attributed to the irreversible formation of Li2 O. The current peak at 1.64 V comes from the oxidation of Fe to Fe2 O3 (2Fe + 3Li2 O → Fe2 O3 + 6Li+ + 4e− ). [24] The above three peaks at the higher potentials are
corresponding to the peaks at 0.29 and 0.60 V shown in Fig. 3(b). The peak at 3.0 V results from the decomposition of PGF, corresponding to the formation of PGF at 0 V. As shown in Fig. 3(d), the second cathodic scan curve can be fitted by 4 current peaks. The peak at 1.1 V should be attributed to the reduction of Fe2 O3 to Fe (Fe2 O3 + 6Li+ + 4e− → 2Fe + 3Li2 O). Unlike the first cathodic scan curve, only one peak can be observed for the reduction of MnO2 to Mn. The current peaks at 0.70 and 0.45 V come from the reduction of MnO2 to MnO and MnO to Mn, respectively, which correspond to the two anodic peaks. More importantly, the area of the peak at 0.70 V is far less than the one at 0.45 V. This phenomenon may be attributed to the loss of oxygen from MnO2 as a result of the formation of the irreversible Li2 O. The current peak at 0 V may result from the formation of PGF, which is consistent with the one at 0 V in the first cathodic scan but its area is larger. The peak fitting of the initial three scan curves illuminates that, in addition to the irreversible SEI layer formed at the first cathodic scan curve, the electrochemical reactions of the interconnected MnO2 nanoflakes electrode during cycling only include the redox reactions of the active materials and the formation and decomposition of PGF on the surface of the active materials. Discharge-charge cycling was performed in a potential window of 0.02–3.0 V versus Li/Li+ for the electrode. As shown in Fig. 4(a), the initial discharge and charge capacities are 1828.5 and
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Fig. 3. Cyclic voltammograms for the electrode of the interconnected MnO2 nanoflakes supported by etched SS plate (a), and the multi-peak fittings for the first cathodic scan curve (b), the first anodic scan curve (c), and the second cathodic scan curve (d).
1285.9 mA h g−1 at a rate of 0.2 C (1 C = 1230 mA g−1 ), respectively. The initial discharge capacity is higher than the theoretical capacity of MnO2 , which should be attributed to the formation of SEI layer during the electrochemical reaction process as shown by Peaks 1 and 4 in Fig. 3(b). The irreversible capacity loss (29.7%) may be mainly attributed to the processes such as electrolyte decomposition and inevitable formation of SEI layer, which are common for the most anode materials. [21,28] Fig. 4(b) shows the discharge-charge voltage profiles of the initial three cycles at a rate of 0.2 C. The electrode exhibits a wide voltage plateau at about 0.43 V during the first discharge. There is a hidden sloping curve before reaching the wide voltage plateau at 0.43 V, which is corresponding to Peaks 3 and 4 shown in Fig. 3(b). The discharge plateau shifts to around 0.54 V in the subsequent discharge process, while the charge plateaus are kept at about 1.21 and 1.97 V. As shown in Fig. 4(a), a reversible capacity of 1297.8 mA h g−1 was achieved in the 2nd cycle, which gradually decreases to about 1184.5 mA h g−1 in the first few cycles. After the 10th discharge-charge cycle, the capacity shows a gradual increasing up to 1387.1 mA h g−1 in the 100th cycle, whereas the coulombic efficiency steadily reaches a value higher than 97.4%. The increasing specific capacity may be ascribed to the reversible growth of the PGF film resulting from kinetically activated electrolyte degradation and the further reaction of the active material, which are common for the anode materials. [29–33] The relative
larger areal capacity suggests that the eched SS plate is a promising nanostructured current collector for enhancing the areal capicity of the eletrodes. To further evaluate electrochemical performance of the electrode, the rate capability is tested and shown in Fig. 4(d). It can be seen that the capacities are perfectly stable at the given current densities. The discharge capacities reach to 1318.4, 1039.0, 893.5, and 695.7 mA h g−1 at rates of 0.2, 0.5, 1, and 2 C, respectively. Even at the rate as high as 5 C, the electrode can deliver a capacity higher than 492.9 mA h g−1 . More importantly, when the current rate is returned to its initial value of 0.2 C after 42 cycles with various C rates, the electrode recovers its initial capacity of 1313.6 mA h g−1 , indicating the excellent rate capability. Fig. 5(a) shows the galvanostatic discharge-charge cycling at 1 C. Before cycling at 1 C, the cell was tested at a rate of 0.2 C for five cycles. The initial discharge and charge capacities are 1819.7 and 1231.0 mA h g−1 , respectively. The electrode can deliver a capacity of 915.4 mA h g−1 in the 7th cycle, and retain a capacity of 813.4 mA h g−1 in the 100th cycle, again indicating that the MnO2 nanoflakes electrode has good cycling stability. The SEM image of the MnO2 nanoflakes electrode after 100 cycles at 1 C is shown in Fig. 5(b). It is obvious that the morphology of the interconnected nanoflakes is almost maintained and no obvious exfoliation of the MnO2 nanoflakes can be found from the nanostructured SS plate.
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Fig. 5. (a) Cycling performance at a rate of 1 C after the initial 5 cycling at a rate of 0.2 C; (b) SEM image of the interconnected MnO2 nanoflakes@3D nanostructured SS plate after 100 cycles at a rate of 1 C. The inset is its high-magnification SEM image.
and the 3D nanostructured current collector leads to shortened electronic and ionic transport lengths. 4. Conclusions
Fig. 4. (a) Capacity retention of the galvanostatic test at a rate of 0.2 C; (b) The discharge–charge curves for the initial 3 cycles at a rate of 0.2 C; (c) Capacity retention at various C rates.
From the inset in Fig. 5(b), it is clear that the interconnected MnO2 nanoflakes are intact, showing excellent electrochemical stability of the nanostructure in electrolytes. The excellent electrochemical performance of the present anode can be explained as follows. First, the active material of MnO2 nanoflakes firmly adheres to the nanostructured current collector of the etched SS plate, which can provide an efficient electron transport. Second, the interconnected MnO2 nanoflakes with a large specific surface area can offer a larger material/electrolyte contact area and accommodate the volume change caused by the electrochemical reactions. Third, the nanostructrues of the MnO2 material
In summary, we have developed a facile approach to fabricate the electrode consisted of the interconnected MnO2 nanoflakes on the 3D nanostructured SS palte for high-power LIB applications. Such an unique architecture not only favours rapid lithium ion transport but also accommodates the volume changes during the electrochemical reactions. The electrochemical reactions of the interconnected MnO2 nanoflakes electrode during cycling are analysized by fitting the meaured CV curves. In addition to the irreversible SEI film formed at the first cathodic scan, the electrochemical reactions involved in the interconnected MnO2 nanoflakes electrode during cycling include the redox reactions of the active materials and the formation and decomposition of PGF. The representative electrode delivers a reversible capacity up to 1387.1 mA h g−1 after 100 cycles at a rate of 0.2 C, and a capacity higher than 492.9 mA h g−1 at a rate as high as 5 C. Such a superior electrochemical performance indicates that the 3D nanostructured SS plate is a promising current collector for enhancing the areal capicity of the eletrodes. The approach that the anode materials grow directly on the nanostructured current collectors without using binder and conductive agent has been demonstrated to be a favorable route for electrode preparation of LIBs.
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Acknowledgements This project was financially supported by the National Natural Science Foundation of China with grant nos. 11179038 and 10974073, and the Specialized Research Fund for the Doctoral Program of Higher Education with grant no. 20120211130005. X. Li gratefully acknowledges the support of the Fundamental Research Funds for the Central Universities with grant no. lzujbky-2013-232. References [1] M. Winter, G.H. Wrodnigg, J.O. Besenhard, W. Biberacher, P. Novák, J Electrochem Soc 147 (2000) 2427–2431. [2] M. Winter, J.O. Besenhard, M.E. Spahr, P. Novák, Adv Mater 10 (1998) 725–763. [3] S. Ni, T. Li, X. Lv, X. Yang, L. Zhang, Electrochim Acta 91 (2013) 267–274. [4] X.W. Lou, D. Deng, J.Y. Lee, L.A. Archer, J Mater Chem 18 (2008) 4397–4401. [5] L. Wang, H. Gong, C. Wang, D. Wang, K. Tang, Y. Qian, Nanoscale 4 (2012) 6850–6855. [6] M. Kim, Y. Hwang, K. Min, J. Kim, Electrochim Acta 113 (2013) 322–331. [7] J. Fang, Y.F. Yuan, L.K. Wang, H.L. Ni, H.L. Zhu, J.L. Yang, J.S. Gui, Y.B. Chen, S.Y. Guo, Electrochim Acta 112 (2013) 364–370. [8] H. Xia, M. Lai, L. Lu, J Mater Chem 20 (2010) 6896–6902. [9] J. Li, M. Zou, Y. Zhao, Y. Lin, H. Lai, L. Guan, Z. Huang, Electrochim Acta 111 (2013) 165–171. [10] S. Ni, X. Yang, T. Li, J Mater Chem 22 (2012) 2395–2397. [11] Y. Fu, X. Li, X. Sun, X. Wang, D. Liu, D. He, J Mater Chem 22 (2012) 17429–17431. [12] L. Wang, H.W. Xu, P.C. Chen, D.W. Zhang, C.X. Ding, C.H. Chen, J Power Sources 193 (2009) 846–850. [13] Z. Cui, X. Guo, H. Li, Electrochim Acta 89 (2013) 229–238.
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