Enhanced energy storage and rate performance induced by dense nanocavities inside MnWO₄ nanobars

Abstract

MnWO₄ nanobars with dense nanocavities have been synthesized through a one pot hydrothermal route. Galvanostatic cycling of the MnWO₄ lithium battery anode exhibits an excellent rate performance. The standout electrochemical performance could be attributed to the unique geometry nanocavities and improved accommodation of the transformation strains during cycling.

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Introduction

Over the past few decades, great progress has been made in the field of lithium-ion batteries (LIB). However, the commercially available graphitic anodes of LIB with only a theoretical capacity of 372 mAh g⁻¹ could not meet the growing demand for high energy density and high power density batteries.¹ Therefore, many scientists are constantly looking for some new anode materials with high specific capacity, high energy densities and excellent capacity retention.^2–4 As we all know, the electrochemical properties of electrode material is greatly associated with the particle properties, such as size, morphology, specific surface area and so on. To improve electrochemical performances, nanostructured electrode materials were applied to these batteries due to their small particle size with large surface area leading to shorter Li⁺ insertion/extraction pathways and higher electrode/electrolyte contact area which is beneficial for achieving high rate capability and good cycling performance.⁵ Thus, designing and tailoring the configuration and morphology of materials from the micron to nanometer scale is the major challenging issue to materials scientists.⁶

Transition metal oxides have been considered as promising alternative anode materials owing to their high specific capacity, lower operating voltage, environmental friendliness and they may even have the advantage of being more economical.⁷ However, the large volume changes resulted in poor capacity retention and low energy efficiency during the electrochemical conversion process, which has hindered their applications in LIB.⁸ Hollow nanostructured materials could enhance structural integrity with sufficient interior space for buffering against the local volume variation during the Li⁺ insertion/extraction, which can effectively alleviate pulverization and aggregation of the electrode materials, hence improving the capacity retention.⁹ Therefore, the hollow nanostructures can provide a good opportunity for developing Li-ion storage technologies.

MnWO₄ materials have attracted increasing attention because of their humidity sensing, magnetic properties and shows potential application ranging from medicine, meteorology, to agriculture.^10,11 However, the study into their electric capability has rarely been reported. MnWO₄ belongs to the monoclinic P2/c space group, and it can be described as the Mn and W atoms each being in an approximately octahedral coordination surrounded by six nearest neighbor oxygen atom sites, similar to ternary transition metal oxides. Therefore, MnWO₄ is a potential anode material for LIB, because both Mn and W are electrochemically active metals, just like Li metal. In addition, MnWO₄ exhibits an excellent electrochemical property—ionic conductivity for hydronium ions¹²—which may also be conducive to lithium-ion and electron conduction. MnWO₄ materials have usually been synthesized by recrystallization or chemical reaction in a molten salt media, mechanical grinding in a mill, and high-temperature thermal decomposition processes.¹³ Recently, wet chemical methods have been developed to prepare MnWO₄ nanorods, nanofibers, nanoplates, etc., which made a substantial progress.^14,15

Nanocavities are isolated entities inside an organic or inorganic solid materials and, thus, are very different from nanopores, which connect together and are open to the surface.¹⁶ Moreover, not only do nanocavities provided a framework similar to hollow structures, but also they may supply a useful environment for absorption or reaction. So, internal surfaces of nanocavities are exceptionally useful for enhancing spontaneous emission rate,¹⁷ light emission¹⁸ or optical absorption.¹⁶ Generally, nanocavities were made by neutron irradiation,¹⁹ gas ions,²⁰ thermal decomposition¹⁶ and so on.²¹ However, the materials with nanocavities in the LIB field have not yet attracted much attention.²² Herein, we report an effective hydrothermal route to prepare MnWO₄ nanobars with dense nanocavities. We found that these dense nanocavities significantly enhance the specific capacity and rate performance, thereby providing a new approach to increase the energy storage in the LIB field. The insightful information will benefit the design of the anode materials in LIB.

Results and discussion

The crystal structure of the as-obtained products were identified by X-ray diffraction (XRD) patterns. Fig. 1 shows the XRD pattern of the product synthesized through a typical reaction. All reflection peaks can be indexed as the monoclinc phase of MnWO₄ (JCPDS Card No. 13-0434, space group P2/c). The lattice parameters calculated from the pattern (a = 4.822 Å, b = 5.761 Å and c = 4.986 Å) were in good agreement with the report value (a = 4.829 Å, b = 5.756 Å and c = 4.998 Å). The strong intensity of the sharp diffraction peaks suggests that the MnWO₄ is highly crystallized. No peaks from the impurities could be found in this pattern, which indicates that the samples obtained were pure-phase MnWO₄ materials.

Fig. 1 A typical XRD pattern of MnWO₄ nanobars.

The morphology of the as-prepared MnWO₄ is characterized by transmission electron microscopy (TEM) (Fig. 2). Overall TEM observation show that the nanobars have similar shape and size distributions in the range of 20–30 nm (Fig. 2a). Furthermore, these nanobars are single crystals and with dense nanocavities inside. The typical size of the nanocavities is a statistical distribution in the range of 5–8 nm, and with a sharp polyhedral shape, as shown in the high-magnification image viewed along the principle directions (Fig. 2b). The high-resolution TEM (HRTEM) image recorded from the middle part of a randomly selected nanobar clearly shows that the spacing between any two adjacent lattice fringes is 2.95 Å, corresponding to that of the (111) planes of the monoclinic MnWO₄ phase. The corresponding energy-dispersive X-ray spectroscopy (EDS) showed that the element here is mainly W, Mn and O (Fig. S1, see ESI for details†). The electron diffraction pattern (Fig. 2c) taken from the whole nanobar was indexed as the [010] zone axis of MnWO₄, indicating that the nanobar is still a single crystal phase. Fig. 2d shows a typical MnWO₄ crystal structure which belongs to the monoclinic P2/c space group, and consists of edge-sharing [MnO₆] and [WO₆] octahedra that form zigzag chains along the c-axis; tungsten atoms and manganese atoms are arranged in alternating sheets parallel to (100). Interestingly, the nanocavities inside the MnWO₄ nanobars are rarely present near the edge of the nanobars, which is similar to the TiO₂ nanorods from the thermal decomposition of H₂Ti₃O₇.¹⁶ The process of formation of the dense nanocavities in the MnWO₄ nanobars is analogous to the mechanism of inside-out Ostwald ripening,²³ that is, selective dissolution of the interior and re-deposition on the shell caused by the supersaturation and vacancy. Hematite spindles,²⁴ SnO₂ hollow spheres,²⁵ Cu₂O nanocubes,²⁶etc., are observed to undergo similar selective dissolution of the interiors under hydrothermal or solvothermal conditions with the exterior protected by adsorbed inorganic salt anions.

Fig. 2 TEM image of MnWO₄ nanobars with dense nanocavities (a), HRTEM image taken on randomly selected nanobars (b), a typical selected area electron diffraction pattern (c), schematic structure of MnWO₄ crystal (d).

To gain insight into electrochemical reaction mechanism, the cyclic voltammograms (CV) of the MnWO₄ are measured and illustrated in Fig. 3. The CV curves of MnWO₄ electrode carried out at a scan rate of 0.1 mV s⁻¹ in the voltage range of 0.01–3.0 V versus Li/Li⁺. The first discharge cycle differs from the other charge/discharge cycles because of the irreversible destruction of the MnWO₄ structure. In the first cathodic scan, two obvious reduction peaks are observed at around 1.17 and 0.63 V, possibly corresponding to the reduction of Mn²⁺, W⁶⁺, and the formation of Mn, W (Fig. S2, see ESI for details†) and amorphous Li₂O, respectively. Moreover, stepwise redox processes are readily observed in the charge profile. The subsequent charge cycles showed anodic peaks located in 1.1–1.3 V, which could be attributed to the formation of MnO²⁷ and WO₃.^28,29 The following cycle showed a pair of redox peaks at around 1.56 and 0.34 V in the discharge cycle and at around 1.20 and 2.0 V in the charge cycle. The oxidation peak at 2.0 V that disappeared in the subsequent cycles could be attributed to an irreversible process. Furthermore, a positive shift and increased intensity of the pairs of redox peaks in Fig. 3 could be attributed to the conversion reaction of WO₃ with Li, which is similar to the ZnWO₄ anode materials.³⁰ Therefore, we expected the following electrochemical reaction mechanism would be responsible for the reaction of MnWO₄ with Li.

MnWO₄ + 8Li⁺ + 8e⁻ → Mn + W + 4Li₂O (1)

Mn + W + 4Li₂O ↔ MnO + WO₃ + 8Li⁺ + 8e⁻ (2)

Fig. 3 Cyclic voltammetry of the MnWO₄ nanobars with dense nanocavities at a scan rate of 0.1 mV s⁻¹ in the voltage range of 0.01–3.0 V vs. Li⁺/Li.

Electrochemical performance of the sample was also examined at a current rate of 100 mA g⁻¹, at room temperature (25 °C), in the potential window range from 0.01 V to 3 V with Li foil as a counter electrode. As shown in Fig. 4a, in the initial discharge process, the voltage drops rapidly down to 0.7 V, then exhibits a long plateau at 0.68 V, followed by a sloping profile until the lower cutoff limit of 0.01 V for the MnWO₄, possibly attributed to the reduction of MnWO₄ to Mn⁰, W⁰ and the formation of amorphous Li₂O. In the first discharge cycle, the voltage plateau corresponded well with the cathodic peak of the CV profile. The first-charge curve, up to 3.0 V shown in Fig. 4a, is smooth for MnWO₄ up to about 2.0 V with a slight indication of a plateau at 1.5–2.0 V. Above 2 V, the voltage rises rapidly, indicating little capacity contribution above this voltage. The initial discharge and charge specific capacities are up to about 1230 mAh g⁻¹ and 810 mAh g⁻¹, respectively. The extra capacity over the theoretical specific capacity (708 mAh g⁻¹) might arise from the insertion of lithium ions into acetylene black and interfacial storage. Subsequent discharge–charge cycles at 100 mA g⁻¹ show poor capacity retention, which could be attributed to the irreversible formation of a stable SEI layer on the electrode surface as well as establishment of an intimate electric contact of the “composite” with the current collector.³¹ The capacities slightly increased after 60 cycles, which is a similar phenomenon to ZnWO₄.³⁰ Hereafter, the electrode displays an excellent cycling performance and the coulombic efficiency is higher than 98%. The electrode still maintains a capacity of 600 mAh g⁻¹ after 160 cycles at a current density of 100 mA g⁻¹, which is better than ZnWO₄ and molybdate salt electrode materials with the same structure.^30,32,33 The excellent performance of the MnWO₄ nanobars is derived from its dense inner nanocavities structure, endowing them with a hollow texture, providing shorter electron transport pathways, large surface-to-volume ratio, more efficient lithium-ion storage sites, and effectively relieving the volume change capability when lithium ions insert into/out the electrode. This strategy is widely adapted in sulfur–porous carbon composites for Li/S batteries,^34,35 and even lithium/iodine batteries.³⁶

Fig. 4 (a) Discharge/charge curves of the electrode made from the as-prepared MnWO₄ nanobars with dense nanocavities, (b) Plot of capacity versus cycle number for the MnWO₄ nanobar electrode in the voltage range 0.01–3.0 V at a current rate of 100 mA g⁻¹.

Besides the high specific capacity and good cycle performance, the rate capabilities are also a very important evaluation method for LIB. The rate capabilities of the prepared MnWO₄ nanobars are also researched at different current densities (Fig. 5). It was investigated using five cycles at 100, 200, 500, 1000, and 2000 mA g⁻¹, and the average discharge capacities were 680, 540, 380, 300 and 220 mAh g⁻¹, respectively. Additionally, after reaching a huge discharge–charge current density, the specific capacity was still be able to return to more than 570 mAh g⁻¹ when the current density was reduced to 100 mA g⁻¹. The initial specific capacity could almost be recovered, which implies that the structure of the electrode is stable even during the high current density. The enhanced cycle performance and rate capability could be attributed to the unique nanocavity geometry and improved accommodation of the transformation strains during cycling. The above results indicate that the as-prepared MnWO₄ nanobars with dense nanocavities are promising anode candidates to replace commercial graphite as the LIB anode.

Fig. 5 Rate capabilities of the MnWO₄ nanobars at different current rates.

Conclusions

The MnWO₄ nanobars with dense nanocavities were successfully synthesized through an effective hydrothermal route. The formation mechanism of the nanocavity structures inside the nanobars is analogous to the process of inside-out Ostwald ripening. The prepared MnWO₄ nanobars with dense nanocavities show excellent rate performance and high specific capacity in LIB. The outstanding electrochemical performance could be ascribed to the unique nanocavity geometry and improved accommodation of the transformation strains during cycling, which allow for efficient electrode–electrolyte contact and short electron transport pathways, at the same time offering sufficient lithium-ion storage sites during the Li⁺ insertion/extraction.

Acknowledgements

This work was supported by the 973 Project of China (No. 2011CB935901), the National Nature Science Foundations of China (No. 11179043, Grant No. 91022033), and China Postdoctoral Science Foundation (2012M511927).

References

1. M. S. Whittingham, Dalton Trans., 2008, 5424.
2. H. Ma, F. Cheng, J. Y. Chen, J. Z. Zhao, C. S. Li, Z. L. Tao and J. Liang, Adv. Mater., 2007, 19, 4067.
3. A. Manthiram and D. Applestone, RSC Adv., 2012, 2, 5411.
4. J. Ma and A. Manthiram, RSC Adv., 2012, 2, 3187.
5. A. Magasinski, P. Dixon, B. Hertzberg, A. Kvit, J. Ayala and G. Yushin, Nat. Mater., 2010, 9, 353.
6. Y.-G. Guo, J.-S. Hu and L.-J. Wan, Adv. Mater., 2008, 20, 2878.
7. J. Liu, Y. Zhou, F. Liu, C. Liu, J. Wang, Y. Pan and D. Xue, RSC Adv., 2012, 2, 2262.
8. J. Cabana, L. Monconduit, D. Larcher and M. R. Palacín, Adv. Mater., 2010, 22, E170.
9. X. W. Lou, D. Deng, J. Y. Lee and L. A. Archer, Chem. Mater., 2008, 20, 6562.
10. W. Qu, W. Wlodarski and J.-U. Meyer, Sens. Actuators, B, 2000, 64, 76.
11. Y.-X. Zhou, Q. Zhang, J.-Y. Gong and S.-H. Yu, J. Phys. Chem. C, 2008, 112, 13383.
12. L. Zhang, C. Lu, Y. Wang and Y. Cheng, Mater. Chem. Phys., 2007, 103, 433.
13. M. Jang, T. J. R. Weakley and K. M. Doxsee, Chem. Mater., 2001, 13, 519.
14. S. H. Yu, B. Liu, M. S. Mo, J. H. Huang, X. M. Liu and Y. T. Qian, Adv. Funct. Mater., 2003, 13, 639.
15. S.-J. Chen, X.-T. Chen, Z. Xue, J.-H. Zhou, J. Li, J.-M. Hong and X.-Z. You, J. Mater. Chem., 2003, 13, 1132.
16. W. Q. Han, L. J. Wu, R. F. Klie and Y. M. Zhu, Adv. Mater., 2007, 19, 2525.
17. E. J. A. Kroekenstoel, E. Verhagen, R. J. Walters, L. Kuipers and A. Polman, Appl. Phys. Lett., 2009, 95, 263106.
18. M. Makarova, V. Sih, J. Warga, R. Li, L. D. Negro and J. Vuckovic, Appl. Phys. Lett., 2008, 92, 161107.
19. J. H. Evans, Nature, 1971, 229, 403.
20. H. Seel and R. Brendel, Thin Solid Films, 2004, 451–452, 608.
21. W.-Q. Han and A. Zettl, Appl. Phys. Lett., 2004, 84, 2644.
22. Q. Li, J. Zhang, B. Liu, M. Li, R. Liu, X. Li, H. Ma, S. Yu, L. Wang, Y. Zou, Z. Li, B. Zou, T. Cui and G. Zou, Inorg. Chem., 2008, 47, 9870.
23. Y. Xiong, B. Wiley, J. Chen, Z.-Y. Li, Y. Yin and Y. Xia, Angew. Chem., Int. Ed., 2005, 44, 7913.
24. C.-J. Jia, L.-D. Sun, Z.-G. Yan, L.-P. You, F. Luo, X.-D. Han, Y.-C. Pang, Z. Zhang and C.-H. Yan, Angew. Chem., Int. Ed., 2005, 44, 4328.
25. X. W. Lou, Y. Wang, C. Yuan, J. Y. Lee and L. A. Archer, Adv. Mater., 2006, 18, 2325.
26. J. J. Teo, Y. Chang and H. C. Zeng, Langmuir, 2006, 22, 7369.
27. Y. Deng, S. Tang, Q. Zhang, Z. Shi, L. Zhang, S. Zhan and G. Chen, J. Mater. Chem., 2011, 21, 11987.
28. N. Sharma, G. V. S. Rao and B. V. R. Chowdari, Electrochim. Acta, 2005, 50, 5305.
29. W.-J. Li and Z.-W. Fu, Appl. Surf. Sci., 2010, 256, 2447.
30. H.-W. Shim, I.-S. Cho, K. S. Hong, A.-H. Lim and D.-W. Kim, J. Phys. Chem. C, 2011, 115, 16228.
31. G. Binotto, D. Larcher, A. S. Prakash, R. H. Urbina, M. S. Hegde and J. M. Tarascon, Chem. Mater., 2007, 19, 3032.
32. N. Sharma, K. M. Shaju, G. V. S. Rao, B. V. R. Chowdari, Z. L. Dong and T. J. White, Chem. Mater., 2003, 16, 504.
33. W. Xiao, J. S. Chen, C. M. Li, R. Xu and X. W. Lou, Chem. Mater., 2009, 22, 746.
34. X.-P. Gao and H.-X. Yang, Energy Environ. Sci., 2010, 3, 174.
35. B. Zhang, X. Qin, G. R. Li and X. P. Gao, Energy Environ. Sci., 2010, 3, 1531.
36. Y. L. Wang, Q. L. Sun, Q. Q. Zhao, J. S. Cao and S. H. Ye, Energy Environ. Sci., 2011, 4, 3947.

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details. See DOI: 10.1039/c2ra20813j

‡ En Zhang contributed equally with Zheng Xing, and they are co-first authors for this paper.

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