Facile Synthesis of Nanoporous γ-MnO2 Structures and Their

Jul 3, 2008 - Cryst. Growth Des. , 2008, 8 (8), pp 2799–2805. DOI: 10.1021/ ... In this work, we report on the preparation of nanoporous γ-MnO2 wit...
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CRYSTAL GROWTH & DESIGN

Facile Synthesis of Nanoporous γ-MnO2 Structures and Their Application in Rechargeable Li-Ion Batteries Jianzhi Zhao, Zhanliang Tao, Jing Liang, and Jun Chen* Institute of New Energy Material Chemistry, Key Laboratory of Energy Material Chemistry (Tianjin) and Engineering Research Center of Energy Storage and ConVersion (Ministry of Education), Chemistry College, Nankai UniVersity, Tianjin 300071, People’s Republic of China

2008 VOL. 8, NO. 8 2799–2805

ReceiVed October 22, 2007; ReVised Manuscript ReceiVed April 9, 2008

ABSTRACT: In this work, we report on the preparation of nanoporous γ-MnO2 with different morphologies and their application in rechargeable Li-ion batteries. Nanoporous γ-MnO2 has been successfully synthesized via a facile route using a hydrothermal treatment and sequential thermal decomposition without employing any template or surfactant. Through simply altering the reactant, nanoporous hollow microspheres and nanocubes can be selectively prepared. The influence caused by different reaction conditions on the structure and morphology of the products has been discussed in detail. It is found that the reactant NH4HCO3 and hydrothermal treatment are necessary for preparing the hollow microspheres. The thermal decomposition of the precursor leads to formation of the nanoporous structure. Both the as-prepared γ-MnO2 hollow microspheres and the nanocubes exhibit promising electrochemical properties as the anode materials of rechargeable Li-ion batteries. In particular, the initial reversible capacity for the hollow microspheres and nanocubes were 1071.1 mAh g-1 and 1041.9 mAh g-1, respectively, showing their potential application in Li-ion batteries.

1. Introduction In recent years, porous inorganic materials have attracted great interest for their wide applications such as in the fields of in ion exchange,1 catalysis processes,2 and lithium-ion batteries3 owing to their ability to interact with ions, atoms and molecules not only at the surfaces, but also throughout the bulk of the material.4 Thus, the change of material structure from solid to porous provides a potential way to improve their properties and application. As an important functional metal oxide, manganese dioxide has attracted great attention due to its special physical and chemical properties. When the basic unit [MnO6] octahedral are linked in different ways, MnO2 can exist in many structural forms, such as R-, β-, γ-, and δ-types, etc.5 Manganese dioxide with different polymorphic forms has wide applications in catalysis and energy storage.6–10 Until now, R-, β-, γ-MnO2 have been prepared in various morphologies, such as wires, rods, belts, and tubes.11–15 Li and co-workers reported the hydrothermal preparation of R-, β-, and γ-MnO2 nanowires/nanorods by a liquid-phase oxidation method.16 Xie’s group synthesized urchinlike R-, γ-MnO2 through a homogeneous catalytic route and direct hydrothermal reaction between MnSO4 and KBrO3.17 Recently, a few reports have been focused on the formation of porous MnO2 structure.18–20 However, the current preparation strategies commonly involve the initial formation of templates to make porous metal oxide materials.19,20 As it is known, the template method is confined by a complex synthesis process and high cost, meaning that a facile and mass production method to prepare porous materials is of great importance. It is noted that nanoporous cadmium oxide has been fabricated by simple thermal decomposition of cadmium carbonate without using a template in the preparation process.21 This novel solid-phase fabrication method shows a good potential to prepare nanoporous metal-oxide structures by employing the corresponding carbonate precursors. In this paper, we report on a facile method to synthesize nanoporous γ-MnO2 architectures via a thermal decomposition * To whom correspondence should be addressed. Fax: (+86) 22-2350-6808; e-mail: [email protected].

of MnCO3 precursor that is synthesized via a hydrothermal route. Through simply altering the reactant of the hydrothermal reaction, nanoporous γ-MnO2 hollow microspheres and nanocubes can be obtained. The influence of different reaction conditions on the product morphology has been discussed in detail. The electrochemical performances of the as-prepared γ-MnO2 as the anode of rechargeable Li-ion batteries have also been investigated. It is found that the as-prepared γ-MnO2 architectures provide more possibility to serve as an ideal host material for the insertion and extraction of lithium ions due to the nanoporous structure.

2. Experimental Section 2.1. Synthesis. All chemicals were of analytical grade and used as purchased without further purification. Distilled water was used throughout. To prepare nanoporous γ-MnO2 hollow microspheres, 4.5 mmol of NH4HCO3 was added into 15 mL of 0.2 M MnSO4 aqueous solution. After 5 min of stirring, the obtained mixture was transferred into a Teflon-lined stainless steel autoclave, sealed, and heated at 160 °C for 4 h. A typical heating rate was 5.3 °C/min. The white solid product MnCO3 was centrifuged, and sequentially washed with water and ethanol repeatedly to remove the residual reactant. Then, the precursor MnCO3 was dried in air at 60 °C. Finally, nanoporous γ-MnO2 hollow microspheres were obtained by thermal treatment of the asprepared MnCO3 precursor at 400 °C for 4 h. To prepare nanoporous γ-MnO2 nanocubes, all the procedures and parameters were similar to the above experiment except using the reactant NaCO3 rather NH4HCO3. In order to investigate the influence of different conditions on the product morphology, a series of parallel experiments were carried out by altering the reaction parameters such as reaction concentration and reaction time. 2.2. Characterization. The products were characterized with various analytical techniques. The powder X-ray diffraction (XRD) pattern was measured on a Rigaku D/max 2500 X-ray diffractometer (Cu KR radiation). Field-emission SEM (FE-SEM) images were taken using JEOL JSM-6700F microscope operated at the accelerating voltages of 10 kV. Transmission electron microscope (TEM) characterization was performed with a Philips Tecnai F20 microscope with the accelerating voltage of 200 kV. 2.3. Electrochemical Measurements. The electrochemical measurements were carried out using CR 2032 coin cells. Test electrodes were prepared by mixing 85 wt % the as-prepared MnO2 samples, 10

10.1021/cg701044b CCC: $40.75  2008 American Chemical Society Published on Web 07/03/2008

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Figure 1. The XRD patterns of (a) the as-prepared MnCO3 hollow sphere precursor and the resulting products through the thermal treatment at different temperatures (b) 300 °C, (c) 400 °C, and (d) 500 °C.

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Figure 2. The XRD patterns of the as-prepared (a) MnCO3 nanocubic precursor and (b) γ-MnO2 nanocubes.

wt % Vulcan XC-72 carbon, and 5 wt % polyvinylidene fluoride (PVDF) with N-methylpyrrolidone (NMP) as the solvent and coated onto a copper foil substrate. The electrodes were dried at 110 °C in a vacuum furnace for 12 h before assembling, and Li foil was used as the counter electrode. The CR 2032 coin cells were assembled in a glovebox under an argon atmosphere. The electrolyte was a solution of 1 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (50/50 vol. %). For cycling experiments, the cells were charged and discharged between 0.02 and 3.3 V (versus Li/Li+) at a constant current density of 100 mA g-1. The electrochemical measurement was carried out using the Land battery measurement system (Wuhan, P. R. China) at room temperature.

3. Results and Discussion 3.1. XRD Results of the Samples. Figure 1 shows the XRD patterns of the as-prepared MnCO3 hollow sphere precursor (Figure 1a) and the resulting products (Figure 1b-d) through the thermal treatment at different temperatures for 4 h. All of the reflections of the XRD pattern in Figure 1a can be readily indexed to pure hexagonal phase of MnCO3, according to reported data (JCPDS Card No. 86-0173). As shown in Figure 1b, the diffraction peaks emerged at 2θ ) 37.3°, 42.2°, and 56.5° are ascribed to the (020), (121), and (221) planes of γ-MnO2, respectively. The peaks in the XRD pattern marked with stars can be attributed to the residue MnCO3, demonstrating that the MnCO3 precursor cannot be decomposed completely at 300 °C in 4 h. The XRD pattern in Figure 1c shows that the as-obtained product is pure phase hexagonal γ-MnO2.22 After thermal treatment of MnCO3 precursor at 500 °C for 4 h, only R-Mn2O3 has been obtained. All the peaks in Figure 1d can be indexed to the cubic structure of R-Mn2O3, corresponding to that of JCPDS Card No. 78-390. The broadening peaks in the XRD pattern are mainly caused by the poor crystallinity and small average grain size of the as-obtained products. The XRD results indicate that the thermal treatment temperature has a significant influence on the phase composition of product. The optimal temperature of obtaining γ-MnO2 is 400 °C in this experiment. Therefore, the thermal decomposition of nanocubic carbonate precursor was carried out at 400 °C. The corresponding XRD patterns of the as-prepared nanocubic products are given in Figure 2, which are similar to that showed in Figure 1, confirming that γ-MnO2 have been successfully prepared from the pure phase hexagonal MnCO3.

Figure 3. FESEM images of (a, b) MnCO3 hollow microspheres, (c, d, e) MnO2 hollow microspheres, and TEM images of (f) MnO2 hollow microspheres, (g) the square area in (f), (h) a broken MnO2 hollow sphere.

3.2. FESEM and TEM Results of the Hollow Sphere Samples. Figure 3 shows the FESEM and TEM images of the as-prepared MnCO3 hollow microspheres and nanoporous MnO2 hollow microspheres. Figure 3a presents a panoramic FESEM image of the MnCO3 precursor, which is composed of uniform

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Figure 4. SEM images of MnCO3 precursor samples prepared under different experimental conditions: (a) without hydrothermal treatment, and with hydrothermal treatment of (b) 0.5 h, (c) 12 h, (d) 0.033 M MnSO4. The molar ratio of MnSO4/NH4HCO3 is 1:1.5.

microspheres with diameters of 2-3 µm. As shown in the magnified image of Figure 3b, the cavities on the shells can be clearly seen, confirming the hollow structure of the as-prepared MnCO3 microsphere. Moreover, the surface of the hollow sphere is not smooth because the sphere shell is made up with irregular MnCO3 nanoparticles. Figure 3c shows the morphology of γ-MnO2 sample synthesized through the thermal decomposition of MnCO3 hollow sphere precursor, indicating that the asobtained MnO2 maintains the frame structure of the precursor. The cavity and rough surface can also be seen from the picture. The shell of the MnO2 hollow sphere is formed by sheet particles with sizes of 100-200 nm according to the magnified images shown in Figures 3d,e. A lot of lacunas among the particles on the microsphere shell, as marked with white arrows in Figure 3d, can be observed, which are attributed to the irregular growth of the precursor particles and the particle shrinking during the thermal treatment. TEM has been employed to further investigate the structure of the as-prepared MnO2 hollow microspheres. Figure 3f displays the TEM image of a representative MnO2 hollow microsphere with a diameter of about 2.5 µm and a 500 nm thick shell. The different brightnesses in the TEM image can also illuminate the hollow structure of the sample, and the diameter of the cavity is about 1 µm. Figure 3g presents the magnified TEM image of the stochastically selected area on the shell of the hollow sphere shown in Figure 3f. It can be seen that the shell is not solid but full of pores with sizes of 5-30 nm among the MnO2 particles. However, some hollow spheres with very thin walls can also be observed in the obtained product, as shown in Figure 3h. 3.3. Formation Process of the Nanoporous Hollow Spheres. For illustrating the growing mechanism of the asprepared MnCO3 hollow microspheres, a series of parallel experiments were carried out. Figure 4 shows the SEM images of MnCO3 precursors obtained in different experimental conditions. Figure 4a displays the morphology of the sample synthesized via the direct mix of reactants without hydrothermal treatment. The product exists in spheres with a diameter of about 1 µm. However, no hollow structure can be observed from the

product. The SEM images of samples obtained via hydrothermal treatment for 0.5 and 12 h are shown in Figure 4, panels b and c, respectively. After 0.5 h hydrothermal treatment, some hollow spheres are already formed, and meanwhile the product has a large size distribution. When the dwell time of the hydrothermal treatment is prolonged to 12 h, only a few of orbicular microspheres can be found from the product, suggesting that the long time hydrothermal treatment destroys the hollow structure. The SEM image of those broken hollow spheres shown in Figure 4c indicates that the shell of the as-prepared sample is becoming thicker to some extent with the increasing reaction time. By decreasing the concentration of the reactants with constant molar ratio of MnSO4/NH4HCO3 ) 1:1.5, the microcubic sample was obtained (Figure 4d). Most of the microcubes present a stack-like surface, meaning the incomplete growth of the sample due to the low concentration of the reactant as well as the short dwell time. As indicated by the white arrows in Figure 4d, some microcubes also have flat surfaces and sharp edges/corners. The influence of temperature has also been examined. The heating rate and temperature can affect the morphology of the obtained spheres according to the experimental results. The typical heating rate is 5.3 °C/min and the keeping temperature is 160 °C. The product mainly exists in the form of irregular particles under a slower heating rate of 2.7 °C/min, while hollow spheres with a large size distribution can be obtained by promoting the heating rate. When the keeping temperature decreases to 140 °C, the product mainly consists of numerous irregular particles as well as some hollow spheres. The obtained hollow spheres have a large size distribution and many spheres are broken as the heating temperature increases to 180 °C. Therefore, the moderate heating rate and temperature are necessary to obtain relatively uniform and intact spheres. On the basis of the above results, the formation process of the MnCO3 hollow microsphere can be proposed. Hydrothermal treatment is the necessary condition in the formation of hollow structure, and the dwell time plays an important role in obtaining the uniform and orbicular structure. When the reactants are

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Scheme 1. Formation Process for Nanoporous γ-MnO2 Hollow Microspheres

mixed, the generated MnCO3 small particles spontaneously aggregate to microspheres. However, the as-formed microsphere structure is not stable and can be separated into small particles during the hydrothermal process. The exceeding reactant NH4HCO3 decomposes and releases NH3 and CO2 under the hydrothermal treatment to form microbubbles that could serve as an aggregation center similar to the reported case.23 Consequently, driven by the minimization of interfacial energy, the dispersed MnCO3 small particles aggregate around the gas-liquid interface between the microbubbles and water, forming MnCO3 hollow microspheres. Further irregular growth of MnCO3 particles leads to a thicker shell and rough surface. The diversity existing in the bulk of the as-prepared microsphere and the shell thickness, which have been detected in the FESEM and TEM images, is mainly caused by the difference in the bubble size and the growing time. After the thermal decomposition of the MnCO3 precursor, γ-MnO2 hollow microspheres with nanoporous structure can be obtained along with the release of CO2 gas. The formation process of the nanoporous γ-MnO2 hollow microspheres is illustrated in Scheme 1.

3.4. FESEM and TEM Results of the Nanocubic Samples. When the reactant Na2CO3 was used instead of NH4HCO3, the MnCO3 nanocubic precursor was synthesized through hydrothermal treatment. After thermal decomposition of the precursor, γ-MnO2 nanocubes with a nanoporous structure were successfully obtained in succession. Figure 5 shows the SEM images of the as-prepared MnCO3 and MnO2 samples. From the different magnification images of the MnCO3 sample, numerous nanocubes with a flat surface can be seen (Figure 5a,b). Correspondingly, some cubes with larger bulk have also been found to exist in the product. Figure 5c presents the panoramic image of the obtained MnO2 sample, which consists of predominant nanocubes with a favorable dispersed state as well as some cubes with larger bulk. There is no obvious change in the overall morphology compared with the MnCO3 precursor. However, quantities of nanopores can be clearly seen in the larger cubes from the magnified image shown in Figure 5d. The image of some broken cubes reveals the inner nanoporous structure throughout the whole cubes.

Figure 5. SEM images of (a, b) as-prepared MnCO3 nanocubic precursor, (c, d) nanoporous γ-MnO2 nanocubes.

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Figure 6. TEM images of the as-prepared MnO2 nanocubic samples at different magnifications.

Figure 7. The SEM images of γ-MnO2 samples with different magnifications. The reactants of the hydrothermal reaction were Na2CO3 and MnSO4, and the dwell time was 0.5 h.

Figure 6 shows the TEM images of the MnO2 nanocubic sample. Figure 6a presents a panoramic image, which is composed of many nanocubes and one cube with larger bulk. From the magnified image of the larger cube shown in Figure 6b, the porous structure can be clearly observed. Though the ultrasonic process before the TEM characterization may induce some damages to the morphology of the cube, the obvious corners/edges can be found. The image in Figure 6c indicates that the predominant nanocubes also have the porous structure. As we can see in Figure 6d, the nanocube is filled by the pores with a size of 5-10 nm so that the structure can be described as a nanocage. When the dwell time of the hydrothermal treatment is decreased to 0.5 h, the as-prepared MnO2 sample is composed of uniform spheres with diameters of 600-900 nm (Figure 7a). The magnified image shown in Figure 7b indicates that the sphere is a buildup of hundreds of nanoparticles, indicating that

the dwell time plays an important role in the formation of nanocubes with a favorable dispersed state. A properly prolonged dwell time is beneficial to obtain the dispersed product. In addition, there is no hollow structure observed in the product, confirming that the reactant NH4HCO3 is a key factor in the formation of hollow structure. 3.5. Application of the As-Prepared Nanoporous Samples in Lithium Ion Batteries. The transition-metal oxides (NiO, CuO, CoO, etc.), proposed by Poizot et al.,24 can be used as new anode materials for lithium-ion batteries with higher reversible capacities than that of the traditional graphite. Recent research on hollow and porous structure has shown that this structural character is an ideal host for reversible Li+ intercalation with a high stability.20,25 Motivated by these interests, the preliminary electrochemical properties of the as-prepared γ-MnO2 nanoporous samples as anode materials for lithiumion batteries have been investigated. Figure 8a presents the

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samples have larger reactive areas, shorter diffusion length of Li-ions, and more sufficient contact with conductive carbon during the process of preparing the electrode. The cycling behavior of the as-prepared γ-MnO2 samples is shown in Figure 8b. The obvious capacity decay in the first five cycles may be caused by the complicated side-reactions and irreversible structure transformation. Then, only tiny capacity fading existed in the sequential cycle for both samples, exhibiting a favorable cycle performance. The discharge capacity of the nanocubes is a little higher than that of the microspheres for each cycle. The discharge capacities after 20 cycles for the nanocubes and microspheres are 656.5 and 602.1 mAh g-1, respectively, which are still much higher than the theoretical capacity of commercial graphite (372 mAh g-1). With respect to the facile preparation and high yield of the product, the present nanoporous γ-MnO2 structures show a potential application in lithium ion batteries.

4. Conclusion In summary, γ-MnO2 hollow microspheres and porous nanocubes were successfully prepared. The hollow microspheres of γ-MnO2 were obtained via the straightforward thermal decomposition of the MnCO3 precursors. Meanwhile, the γ-MnO2 porous nanocubes were synthesized by simply using NaCO3 instead of NH4HCO3. The electrochemical measurement of lithium-ion batteries showed that the as-prepared γ-MnO2 nanocubes and microspheres exhibited high initial capacities and favorable cycle performance. After 20 cycles, the discharge capacities of the nanocubes and microspheres are 656.5 and 602.1 mAh g-1, showing potential anode materials for rechargeable lithium ion batteries. Furthermore, the as-prepared γ-MnO2 with porous and hollow structures may also find possible application in the areas of sensors, catalysis, and capacitors.

Figure 8. (a) First discharge-charge curves of γ-MnO2 hollow microspheres (red lines) and nanocubes (blue lines) between 0.02 and 3.3 V vs Li/Li+ at a constant current density of 100 mA g-1. (b) Cycling behavior for electrodes of hollow microspheres (red squares) and nanocubes (blue squares).

voltage-capacity curves of the as-prepared γ-MnO2 nanoporous hollow microspheres and nanocubes in the first discharge-charge cycle. During the discharge process, an obvious wide plateau can be found at about 0.4 V for both samples. For the γ-MnO2 nanocubes, a high discharge capacity of 1992.6 mAh g-1 was obtained in the first discharge process, larger than that of nanowires (1760 mAh g-1).26 In comparison, a lower discharge capacity of 1289.0 mAh g-1 was achieved for the hollow microspheres. During the charge process, the capacity of each electrode was lower than that of the corresponding discharge process. The charge capacity of the nanocubes was 1041.9 mAh g-1 and the ratio of the charge retention is 52.3%, while the charge capacity of the hollow microspheres was 1071.1 mAh g-1 and the ratio of the charge retention is 83.1%. The large irreversible capacity in the first cycle may be attributed to the formation of solid-electrolyte interphase (SEI) film onto the surface of the electrode materials.26 This difference in the electrochemical results shows that the morphology of γ-MnO2 has a remarkable effect on the electrochemical performance. The smaller particle size and an efficient filling of micro- and nanocubes are more favorable for improving the electrochemical properties than that of the microspheres because the cubic

Acknowledgment. This work was supported by the National Key-BasicResearchProgram(2005CB623607),NSFC(20703026), and Tianjin Basic & High-Tech Programs (07ZCGHHZ00700 and 08JCZDJC21300).

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