MnO₂ nanotubes with a water soluble binder as high performance sodium storage materials

Abstract

Well dispersed MnO₂ nanotubes were synthesized via a hydrothermal method. When tested as anode materials for sodium-ion batteries with sodium polyacrylate (PAANa) as the binder, for the first time, and aluminum as the current collector, the α-MnO₂ nanotubes delivered an initial reversible capacity of 357 mA h g⁻¹ and a capacity retention of 358 mA h g⁻¹ after 40 cycles. Moreover, the α-MnO₂ nanotubes showed a good rate capability. A capacity of 243 mA h g⁻¹ could be obtained at the rate of 400 mA g⁻¹. The sodium storage mechanism of MnO₂ nanotubes and the effects of different binders were also probed through Cyclic Voltammetry (CV), ex situ Scanning Electron Microscope (SEM) and X-ray diffraction (XRD).

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1. Introduction

Owing to the high potential (2.71 V vs. SHE) and abundant resource characters of the sodium element (2.27% in the earth's crust), Sodium Ion Batteries (SIB) have been considered as promising substitutes for lithium ion batteries.¹ Transition-metal based materials such as oxides and sulfides are proposed to be high capacity sodium storage materials.^1–5 The huge volume changes during the charge-discharge process and the poor interface character may cause serious capacity decay in the SIB. One promising method to alleviate the volume expansion is introducing nanostructures to reduce the volume density and facile the ion diffusion such as nanoparticle, nanowire, nanorod, nanosheet, nanotube and hollow sphere morphologies.^6–9 Among them, the nanotube is a particular structure with a thin wall layer and large inner hollow space that can facilitate electrolyte contact and buffer volume expansion. For example, Xia et al. reported that the Fe₃O₄ nanotubes showed a high cycling ability and rate ability.¹⁰ Bi et al. found that TiO₂ nanotubes showed excellent electrochemical performance as binder free anode materials for both lithium and sodium batteries.¹¹ Adopting water soluble binders has recently been proved to be another effective, convenient and environmental-friendly method for improving the electrochemical performance of lithium and sodium batteries.^12–17 Water soluble binders could provide better physical and chemical contact with the electrode surface and assist in building a deformable and stable solid-electrolyte interphase (SEI) and good electrolyte permeability, etc.¹⁸ For example, Dahbi et al. reported that sodium carboxymethyl cellulose (CMC) binder demonstrated a superior reversibility and cyclability with a capacity retention above 97% over 100 cycles, compared to the ordinary poly(vinylidene difluoride) (PVdF) binder on hard carbon sodium battery anode.¹⁹ Bodenes et al. found that CMC could form a thinner and homogeneous passivation layer on the Sb anode for sodium ion batteries that lead to a better cycling ability.²⁰ We also reported that a sodium alginate binder could improve the lithium storage performance of CdO.¹⁷ Komaba et al. compared the effects of different binders with poly(acrylic acid) (PAA), poly(vinylalcohol) (PVA), CMC and conventional PVdF on the lithium storage ability of SiO, and found that PAA had the best electrochemical performance due to its outstanding binding ability and mechanical stability.²¹ Han et al. further found that partial sodiation of the PAA to PAAH_0.2Na_0.8 (PAANa) could result in a better rheological property and porosity. The Si anode showed an excellent capacity retention with 80% neutralized PAH as a binder. However, there is little reported on the use of PAANa binder for SIBs.²²

Manganese dioxide (MnO₂), one of the most attractive transition metal oxides, has attracted considerable attention due to its low cost, environmental friendliness and high theoretical capacity.²³ Some pioneering works have been done to explore and improve the performance of MnO₂ in different shapes and crystalline forms; and it has proved that microstructure and crystalline status strongly affect the electrochemical performance of MnO₂.^24–29 Among these microstructures, the tubular structure is attractive due to the combined advantages of hollow and thin wall thickness, which can facile electrolyte permeation and shorten the ion transport pathway.²⁸ However, to the best of our knowledge, no report on the sodium storage performance of MnO₂ nanotubes exists. Herein, MnO₂ nanotubes were synthesized via a hydrothermal method and tested as the sodium-storage material for the first time. By combining with a water soluble PAAH_0.2Na_0.8 binder, the α-MnO₂ nanotube electrode showed good electrochemical performance.

2. Experimental

All the chemicals were purchased from Sinopharm Chemical Reagent Corporation and used without further purification. The α-MnO₂ was produced by a hydrothermal method. In the process of a typical experimental, 5.0 mmol KMnO₄ was first dissolved into 80 ml deionized water to form a well-proportioned solution under magnetic stirring. 20 mmol concentrated hydrochloric acid was added to the solution. The autoclave was sealed and heated at 140 °C for 12 h. The black precipitate was filtered and washed several times. Then, the sample was dried in a vacuum oven at 60 °C for 12 h.^28–30 PAANa binder was prepared by mixing PAA and NaOH with a COOH : NaOH = 1 : 0.8 (molar ratio).²²

Crystallographic phases of the prepared samples were characterized using X-ray diffraction by a Rigaku Dmaxrc diffractometer with Ni-filtered Cu Kα radiation (V = 50 kV, I = 100 mA) at a scanning rate of 4° min⁻¹. The morphology of the as-synthesized samples was investigated by JEOL JEM-2100 high resolution transmission electron microscopy (HRTEM) and SU-70 field emission scanning electron microscopy (FESEM). The electrodes were prepared by the doctor-blade technique using a mixture of the active material, Super P carbon and different binders in the weight ratio of 80 : 10 : 10. Aluminum foil was used as the current collector. Aluminum was chosen as the collector due to its oxidation stability above 3 V (vs. Na/Na⁺) and inertness with sodium metal. The paste was casted onto the aluminum foil and dried in a vacuum oven at 70 °C for 24 h. The loading of the electrode was about 1.5 mg cm⁻². The coin cells were assembled in a glove box filled with argon. The electrochemical measurements were carried out using the two-electrode coin cells (CR2016) with Na sheet as both the reference and counter electrode and glass fiber (Whatman C) as the separator. The electrolyte was 1 M NaPF₆ dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) (1 : 1 v/v). Cyclic voltammetry (CV) was cycled between 0.01 to 4 V at the rate of 0.1 mV s⁻¹. Galvanostatic discharge–charge measurements of the cells were carried out in the voltage range of 0.01 to 4 V at the current density of 50 mA g⁻¹. Ex situ XRD and SEM were tested after certain cycles. The electrodes are taken out and washed by methyl carbonate in the glove box and then dried at 60 °C for 5 h before testing.

3. Results and discussion

Fig. 1 shows the XRD patterns of the as-prepared MnO₂ nanotubes. All the XRD patterns in Fig. 1a can be indexed to a pure α-MnO₂ phase (JCPDS No. 44-0141). No other miscellaneous peaks are detected, which confirms that the products are in a highly pure phase. In general, manganese dioxide exists in several crystal phases such as α, γ, δ and λ forms. These phases can be distinguished because the basic unit MnO₆ octahedra are attached in different ways. α-MnO₂ is a favorite choice for sodium batteries as it has a tunnel structure built from double chains of edge-sharing MnO₆ octahedra, which is beneficial for the insertion/extraction of sodium ions.²⁸ The weak intensity of the peaks indicates the nanosized character of the as-prepared MnO₂ nanotubes. These results are similar with the previous reports.^28–30

Fig. 1 XRD patterns of the as-prepared MnO₂.

Fig. 2 shows the SEM and TEM photographs of the as-prepared MnO₂ nanotubes. The as-synthesized MnO₂ displays as well-dispersed 1-D nanostructured crystals with a particle size of 0.5–1 μm and diameter of 80–100 nm (Fig. 2a). The high-magnification TEM image in Fig. 2b shows that the 1-D nanostructure has a tetragonal open end with an edge length of about 30–50 nm and a wall thickness of several nanometers. The inner diameter is about 25 nm. An HRTEM image of the prepared α-MnO₂ nanotubes is presented in Fig. 2c. It shows clearly lattice fringes along the nanotube, which confirms the single crystal nature of the synthesized α-MnO₂ nanotubes. From Fig. 2d, we can realize that a lattice spacing of 0.49 nm between adjacent planes in the image corresponds to the distance of the (200) planes in the tetragonal α-MnO₂ structure. The image in the upper right corner of Fig. 2d shows that a selected area electron diffraction (SAED) pattern is taken from a single nanotube. The SAED pattern and HRTEM analysis indicate that the nanotube axis is single crystalline. The TEM investigations confirm the high-quality single-crystalline features of the synthesized α-MnO₂ nanotubes.^28–30 The tube-like structure could accommodate large volume changes and facilitate the ion transport, which may improve the electrochemical performance during the conversion process.

Fig. 2 SEM (a), HRTEM (b–d) and SAED (inner graph of Fig. 4d) patterns of the as-prepared MnO₂.

The electrochemical performance of α-MnO₂ nanotubes as SIB anodes was evaluated by static-current charge/discharge cycling. For comparison, we also present the results of α-MnO₂ nanotubes with different binders and the results are revealed in Fig. 3. Fig. 3a and b show the discharge/charge voltage profiles of the 1^st, 2^nd and 40^th cycles for the α-MnO₂ electrodes with PVDF and PAANa binders cycled between 0.01 and 4 V at a current density of 50 mA g⁻¹. From the figure, we can see that both the two electrodes show similar three plateau shapes, indicating the same three phase transformation processes.^29–32 The first discharge capacity of the α-MnO₂ electrodes with PVDF and PAANa binders is almost the same with a capacity of 351 and 360 mA h g⁻¹, respectively, which indicates 1.3 sodium ions intercalate into one MnO₂ unit. However, the capacity retention evolved dramatically differently from the 1^st charge process. In the 1^st charge process, the electrode with PAANa binder showed three charge plateaus with almost 100% coulombic efficiency, while the columbic efficiency of electrode with PVDF binder is only 71%. The improved coulombic efficiency may be ascribed to the PAANa binder, which can form stable bonds with the MnO₂ surface and facilitate the formation of a stable SEI film. Fig. 3c compares the cycling performance of the α-MnO₂ nanotubes as SIB anodes with different binders. All electrodes are cycled between 0.01 and 4 V at a current density of 50 mA g⁻¹ for 40 cycles. From Fig. 3c, we can see that the reversible capacity of α-MnO₂ with PVDF binder decreases from 247 to 74 mA h g⁻¹, which corresponds to 29.96% capacity retention. The best capacity retention of α-MnO₂ nanotubes electrode is the one with PAANa binder. The capacity still remains 358 mA h g⁻¹ (100.003%) after the same cycle. It can be clearly seen that the α-MnO₂ nanotubes electrode with PAANa binder shows an excellent electrochemical stability compared to the PVDF binder. Additionally, the cell with PAANa as the binder shows a much higher initial coulombic efficiency (almost 100% for cell with PAANa and 78.5% for cell with PVDF binder), the high initial coulombic efficiency and good cycle ability may be ascribed to the rich number of carboxylic groups in the PAANa that can form strong bonds with the MnO₂ to maintain the adhesion of electrode and the ionic conductive COONa groups that can help form a stable SEI film on the MnO₂ surface. Similar results have been proved by many research groups.^21,22

Fig. 3 The 1^st, 2^nd and 40^th charge–discharge profiles of MnO₂ with PVDF (a), PAANa (b) binders, cycle ability (c) and rate ability (d) of MnO₂ with different binders.

In addition to the good capacity retention, the α-MnO₂ electrode with PAANa binder also exhibits a good rate capability, as shown in Fig. 3d. With increase of the current density, the α-MnO₂ electrode with PAANa binder can deliver a reversible capacity of 351, 286, 252 and 243 mA h g⁻¹ at the current density of 50, 100, 200 and 400 mA g⁻¹, respectively. A reversible capacity of 298 mA h g⁻¹ can be achieved after the rate backs to 50 mA g⁻¹. The good electrochemical properties may be ascribed to the particular nanotube structure of MnO₂ that promotes sodium ion diffusion and electrolyte permeation, and the stable SEI film promoted by the PAANa binder.¹⁰

To further explore the electrochemical behavior along with Na-ion uptake/extraction, CV was applied for the evaluation of the anode, as shown in Fig. 4a. During the first cathodic process, a wide peak at 2 V and a sharp peak at 0.4 V are clearly observed, indicating the Na insertion into MnO₂ and the reduction of MnO₂.³¹ Correspondingly, in the voltage profiles (Fig. 3b), three plateaus in the range of 0.01 to 4 V are also observed. During the responding anodic process, three clear reversible peaks at 0.5, 3 and 3.9 V correspond to the reversible de-sodiation process, which is in good accordance with the three plateaus in the voltage profiles. The CV curves overlap well in the following cycles suggesting the reaction is reversible. Several excellent publications have focused on the sodium mechanism of MnO₂ as cathodes with the discharge cut-off voltage above 1 V. The sodium storage mechanism is ascribed to an intercalation mechanism: MnO₂ + Na ↔ Na_xMnO₂.^7,31 While as the anode for SIBs, the sodium storage mechanism of MnO₂ is proved to be a conversion type and the final valence of Mn is in an average 2⁺ and 3⁺ when discharged to 0 V.²⁷ To clarify the sodium mechanism of the MnO₂ nanotubes, we probed the ex situ XRD patterns of MnO₂–PAANa composite electrodes under different sodiation statuses from 0.01 to 4 V. Fig. 4b shows the XRD patterns of the fresh, discharged to 0.01 V and charged to 4 V MnO₂–PAANa electrodes. From the figure, we can see that when discharged to 0.01 V, all the XRD patterns of MnO₂ disappeared and only the XRD patterns of Al remains, which indicates that the crystalline MnO₂ is converted to an amorphous form. When charged to 4 V, the MnO₂ is still in an amorphous pattern. The results indicate that when discharged to 0.01 V, the MnO₂ undergoes an irreversible conversion progress, which is similar with the conversion reactions of metal oxides in lithium batteries. When discharged, the reduction of MnO₂ forms amorphous MnO_x surrounded by the Na₂O matrix.³² The redox sodiation/de-sodiation progresses are carried out in the composites. Our experiments further confirmed that the Al can be an anode current collector for SIBs.^33–36

Fig. 4 CV of MnO₂–PAANa half cells (a), XRD patterns of the fresh (1), discharged to 0.01 V (2) and charged to 4 V MnO₂–PAANa electrodes (3) (b).

In order to gain a better understanding of why PAANa binder exhibits such a superior electrochemical performance compared to PVDF binder, ex situ SEM was also performed using half cells with different binders, as shown in Fig. 5. The SEM comparison between electrodes with different binders after 40 cycles was investigated and presented in Fig. 5a and b. From the figures, we can see that the electrode with PVDF binder shows a coarse and discontinuous character, which may be according to the large volume expansion during the repeated charge/discharge progress. In comparison, the electrode with PAANa binder shows a compact and smooth morphology. The integral character of the electrode can maintain a good conducting network between the current collector and active materials and accomodate the volume expansion during the sodium storage progress. The better buffering ability of PAANa binder may be due to the following reasons:^37–43 (1) comparing to the neutralized PVDF binder, the negative charges on the polymer chains could suppress aggregation of the polymer chains through electrostatic repulsion and results in a more homogeneous coating on the MnO₂ surface. (2) The carboxylic groups could form additional bonds with MnO₂, which increases the binding ability of binder and contributes the stable SEI film formation. (3) It has been proved that 80% neutralized PAANa could provide a porous structure inside the electrode composite because of its unique rheological properties during the drying process; the self-formed porous structure is beneficial for buffering the volume expansion. (4) It is reported that the stiffness of PVDF binder was significantly decreased by contact with diethyl carbonate (DEC) while PAANa does not change. The combination of the above advantages leads to the excellent electrochemical performance of the MnO₂ electrode with PAANa as the binder.

Fig. 5 Ex situ SEM images of MnO₂ with PVDF binder (a) and PAANa binder (b) after 40 cycles.

4. Conclusions

In summary, hydrothermally grown α-MnO₂ nanotubes with PAANa binder were tested as sodium storage materials. The α-MnO₂–PAANa hybrids exhibit enhanced reversible capacity, good cycle performance and high rate performance. The improved electrochemical performance of the composites electrode is due to the unique MnO₂ microstructure and the binder, which could buffer the volume change, facilitate the ion transport and form a stable SEI film. Our finding also confirmed that aluminum could be used as the anode current collector for sodium ion batteries.

Acknowledgements

This work was supported by Key Research Plan of Shandong Province (2015GGE27286). The National Natural Science Foundation of China (No. 21371108), the Sate Key Program of National Natural Science of China (No. 51532005) Shandong Provincial Natural Science Foundation for Distinguished Young Scholar (JQ201304), the Project of the Taishan Scholar (No. ts201511004), Independent Innovation Foundation of Shandong University.

References

1. N. Yabuuchi, K. Kubota, M. Dahbi and S. Komaba, Chem. Rev., 2014, 114, 11636–11682.
2. Y. Z. Jiang, M. Hu, D. Zhang, T. Z. Yuan, W. P. Sun, B. Xu and M. Yan, Nano Energy, 2014, 5, 60–66.
3. S. Hariharan, K. Saravanan and P. Balaya, Electrochem. Commun., 2013, 31, 5–9.
4. Y. J. Zhu, L. M. Suo, T. Gao, X. L. Fan, F. D. Han and C. S. Wang, Electrochem. Commun., 2015, 54, 18–22.
5. L. Wu, H. Y. Lu, L. F. Xiao, J. F. Qian, X. P. Ai, H. X. Yang and Y. L. Cao, J. Mater. Chem. A, 2014, 2, 16424–16428.
6. M. X. Tang, A. B. Yuan and J. Q. Xu, Electrochim. Acta, 2015, 166, 244–252.
7. J. Liu, R. Younesi, T. Gustafsson, K. Edstrom and J. F. Zhu, Nano Energy, 2014, 10, 19–27.
8. F. Zhang, Y. L. An, W. Zhai, X. P. Gao, J. K. Feng, L. J. Ci and S. L. Xiong, Mater. Res. Bull., 2015, 70, 573–578.
9. Y. F. Yuan, L. Ma, K. He, W. T. Yao, A. Nie, X. X. Bi, K. Amine, T. P. Wu, J. Lu and R. Shahbazian-Yassr, Nano Energy, 2016, 19, 382–390.
10. H. Xia, Y. H. Wan, G. L. Yuan, Y. S. Fu and X. Wang, J. Power Sources, 2013, 241, 486–493.
11. Z. H. Bi, M. P. Paranthaman, P. A. Menchhofer, R. R. Dehoff, C. A. Bridges, M. F. Chi, B. K. Guo, X. G. Sun and S. Dai, J. Power Sources, 2013, 222, 461–466.
12. J. Li, H. M. Dahn, L. J. Krause, L. Dinh-Ba and J. R. Dahn, J. Electrochem. Soc., 2008, 155, 812–816.
13. J. X. Song, M. J. Zhou, R. Yi, T. Xu, M. L. Gordin, D. H. Tang, Z. X. Yu, M. Regula and D. H. Wang, Adv. Funct. Mater., 2014, 24, 5904–5910.
14. R. Y. Zhang, X. Yang, D. Zhang, H. L. Qiu, Q. Fu, H. Na, Z. D. Guo, F. Du, G. Chen and Y. J. Wei, J. Power Sources, 2015, 285, 227–234.
15. X. Zheng, Z. C. Zhang and K. Amine, Electrochem. Commun., 2013, 34, 86–89.
16. I. Kovalenko, B. Zdyrko, A. Magasinski, B. Hertzberg, Z. Milicev, R. Burtovyy, I. Luzinov and G. Yushin, Science, 2011, 334, 75–79.
17. J. K. Feng, S. L. Xiong, Y. T. Qian and L. W. Yin, Electrochim. Acta, 2014, 129, 107–112.
18. I. Kovalenko, B. Zdyrko, A. Magasinski, B. Hertzberg, Z. Milicev, R. Burtovyy, I. Luzinov and G. Yushin, Science, 2011, 334, 75–79.
19. M. Dahbi, T. Nakano, N. Yabuuchi, T. Ishikawa, K. Kubota, M. Fukunishi, S. Shibahara, J. Y. Son, Y. T. Cui, H. Oji and S. Komaba, Electrochem. Commun., 2014, 44, 66–69.
20. L. Bodenes, A. Darwiche, L. Monconduit and H. Martinez, J. Power Sources, 2015, 273, 14–24.
21. S. Komaba, K. Shimomura, N. Yabuuchi, T. Ozeki, H. Yui and K. Konno, J. Phys. Chem. C, 2011, 115, 13487–13495.
22. Z. J. Han, N. Yabuuchi, K. Shimomura, M. Murase, H. Yuib and S. Komab, Energy Environ. Sci., 2012, 5, 9014–9020.
23. Y. Q. Zhang, H. C. Xuan, Y. K. Xu, B. J. Guo, H. Li, L. T. Kang, P. D. Han, D. H. Wang and Y. W. Du, Electrochim. Acta, 2016, 206, 278–290.
24. G. Y. Zhao, D. Zhang, L. Zhang and K. N. Sun, Electrochim. Acta, 2016, 202, 8–13.
25. Z. F. Ma and T. B. Zhao, Electrochim. Acta, 2016, 201, 165–171.
26. S. W. Lee, C. W. Lee, S. B. Yoon, M. S. Kim, J. H. Jeong, K. W. Nam, K. C. Roh and K. B. Kim, J. Power Sources, 2016, 312, 207–215.
27. Y. T. Weng, T. Y. Huang, C. H. Lim, P. S. Shao, S. Hy, C. Y. Kuo, J. H. Cheng, B. J. Hwang, J. F. Lee and N. L. Wu, Nanoscale, 2015, 7, 20075–20081.
28. W. Xiao, H. Xia, J. Y. H. Fuh and L. Lu, J. Power Sources, 2009, 193, 935–938.
29. V. Subramanian, H. W. Zhu, R. Vajtai, P. M. Ajayan and B. Q. Wei, J. Phys. Chem. B, 2005, 109, 20207–20214.
30. J. Luo, H. T. Zhu, H. M. Fan, J. K. Liang, H. L. Shi, G. H. Rao, J. B. Li, Z. M. Du and Z. X. Shen, J. Phys. Chem. C, 2008, 112, 12594–12598.
31. D. W. Su, H. J. Ahn and G. X. Wang, NPG Asia Mater., 2013, 5, e70.
32. M. V. Reddy, G. V. S. Rao and B. V. R. Chowdari, Chem. Rev., 2013, 113, 5364–5457.
33. H. Xia, M. O. Lai and L. Lu, J. Mater. Chem., 2010, 20, 6896–6902.
34. S. L. Chou, J. Z. Wang, M. Choucair, H. K. Liu, J. A. Stride and S. X. Dou, Electrochem. Commun., 2010, 12, 303–306.
35. J. X. Song, Z. X. Yu, M. L. Gordin, X. L. Li, H. S. Peng and D. H. Wang, ACS Nano, 2016, 9, 11933–11941.
36. J. X. Song, M. J. Zhou, R. Yi, T. Xu, M. L. Gordin, D. H. Tang, Z. X. Yu, M. Regula and D. H. Wang, Adv. Funct. Mater., 2014, 24, 5904–5910.
37. S. B. Ni, X. H. Lv, T. Li, X. L. Yang and L. L. Zhang, J. Mater. Chem. A, 2013, 1, 1544–1547.
38. D. Su, H. J. Ahn and G. X. Wang, J. Mater. Chem. A, 2013, 15, 4845–4850.
39. J. Wang, M. J. Zhou, G. Q. Tan, S. Chen, F. Wu, J. Lu and K. Amine, Nanoscale, 2015, 7, 8023–8034.
40. G. Q. Tan, F. Wu, Y. F. Yuan, R. J. Chen, T. Zhao, Y. Yao, J. Qian, J. R. Liu, Y. S. Ye, R. Shahbazian-Yassar, J. Lu and K. Amine, Nat. Commun., 2016, 7, 1038.
41. Q. Wang, D. A. Zhang, Q. Wang, J. Sun, L. L. Xing and X. Y. Xue, Electrochim. Acta, 2014, 146, 411–418.
42. L. L. Xing, B. He, Y. X. Nie, P. Deng, C. X. Cui and X. Y. Xue, Mater. Lett., 2013, 105, 169–172.
43. S. L. Chou, Y. D. Pan, J. Z. Wang, H. K. Liu and S. X. Dou, Phys. Chem. Chem. Phys., 2014, 16, 20347–20359.

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