NANO LETTERS
Ordered Mesoporous Metallic MoO2 Materials with Highly Reversible Lithium Storage Capacity
2009 Vol. 9, No. 12 4215-4220
Yifeng Shi,† Bingkun Guo,‡ Serena A. Corr,§ Qihui Shi,† Yong-Sheng Hu,*,‡ Kevin R. Heier,| Liquan Chen,‡ Ram Seshadri,*,§ and Galen D. Stucky*,†,§ Department of Chemistry and Biochemistry, Materials Department and Materials Research Laboratory, UniVersity of California, Santa Barbara, California 93106, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, and Air Products and Chemicals Inc., Allentown, PennsylVania 18195 Received July 27, 2009; Revised Manuscript Received September 14, 2009
ABSTRACT Highly ordered mesoporous crystalline MoO2 materials with bicontinuous Ia3d mesostructure were synthesized by using phosphomolybdic acid as a precursor and mesoporous silica KIT-6 as a hard template in a 10% H2 atmosphere via nanocasting strategy. The prepared mesoporous MoO2 material shows a typical metallic conductivity with a low resistivity (∼0.01Ω cm at 300 K), which makes it different from all previously reported mesoporous metal oxides materials. Primary test found that mesoporous MoO2 material exhibits a reversible electrochemical lithium storage capacity as high as 750 mA h g-1 at C/20 after 30 cycles, rendering it as a promising anode material for lithium ion batteries.
Molybdenum dioxide (MoO2) with a distorted rutile structure is an unusual and attractive transition metal oxide because of its low, metallic electrical resistivity (8.8 × 10-5 Ω·cm at 300 K in bulk samples), high melting point, and high chemical stability.1 It has promise in applications ranging from catalysis, sensing, electrochromic displays, recording media, electrochemical supercapacitors, and field emission due to its efficient charge transport properties.2 Polycrystalline MoO2 was reported to be useful as an anode material in a Li-ion battery that showed relatively large capacity (400-600 mA h g-1).3 Considering the somewhat high density (6.5 g cm-3) of MoO2, such a high gravimetric capacity is associated with a high volume capacity. In a manner similar to other rutile-type materials such as TiO2 and MnO2, nanosized materials can be differentiated from their bulk counterparts in that bulk materials show poor electroactivity with respect to lithium at room temperature while nanomaterials show both high capacity and excellent cycling performance.4,5 In the past decade, the synthesis of MoO2 nanorods or nanowires, nanospheres, and nanoparticles * To whom correspondence should be addressed. E-mail: (G.D.S.)
[email protected]; (R.S.)
[email protected]; (Y.-S.H.) yshu@ aphy.iphy.ac.cn. † Department of Chemistry and Biochemistry, University of California, Santa Barbara. ‡ Chinese Academy of Science. § Materials Department and Materials Research Laboratory, University of California, Santa Barbara. | Air Products and Chemicals Inc. 10.1021/nl902423a CCC: $40.75 Published on Web 09/23/2009
2009 American Chemical Society
with irregular morphology have been reported using high temperature vapor deposition, electrochemical deposition, carbon nanotubes template synthesis and hydrothermal synthesis.6 However, the smallest dimensions of the MoO2 phase of most of these reported materials is greater than 50 nm, which limits their application in Li-ion battery. A flowerlike MoO2 material consisting of ultrathin nanosheets with a thickness less than 10 nm was reported, which shows a capacity up to 650 mA h g-1, although it is not a pure phase MoO2 material and some Fe2O3 that was present inside the particle center apparently decreased the capacity.3e Recently, ordered mesoporous materials have attracted considerable interest as potential electrode materials in Liion batteries since mesoporous structures can (i) minimize solid-state diffusion lengths for both Li+ and e- due to the nanosized walls (2 nm); and (iii) amplify the charge capacity because of their intrinsic high surface areas.5,7 The lithium storage properties of mesoporous TiO2, Co3O4, Cr2O3, SnO2, MnO2, LiMnO2, LiFePO4, and so forth have been reported and all of them show better performances than the corresponding bulk materials,4,5,8 in accordance with the three advantages listed above. In view of its high bulk conductivity, mesoporous crystalline MoO2 might be expected to be as good as or better than the above studied mesoporous materials. However, until
now the successful synthesis of an ordered mesoporous MoO2 material has not been reported. Ordered mesoporous metal oxides can be synthesized by a soft template method via the cooperative assembly of inorganic species with organic surfactants.9 However, this approach depends on careful kinetic control of the competing reaction processes that take place during the assembly and is temperature limited in this regard, and also by the synthesis/calcination temperature that can be used. Mesoporous metal oxides can also be synthesized by nanocasting using mesoporous silica or carbon materials as hard templates.10 The nanocast mesostructured metal oxides show higher crystallinity because the hard template can provide a stable matrix during heat treatment over a larger temperature range. Since the first report of ordered mesoporous crystalline Cr2O3,10d more than 10 nanocast metal oxides have been reported in the past six years, including iron, cobalt, nickel, tungsten, cerium, indium, aluminum, titanium, copper, manganese, and magnesium oxides10c However, none of these mesoporous oxides are conducting materials, so that their applications in electrochemistry are limited. Most of these syntheses were carried out in air atmosphere using higher valence inorganic molecular precursors. In order to get lower valence mesostructured metal oxides, a second postreducing treatment has been adopted in previous reports. For example, mesoporous Fe3O4 can be obtained by heating mesoporous Fe2O3 in H2 flow and mesoporous CoO can be synthesized by reducing mesoporous Co3O4 by glycerol.11 In some special cases, the volume change is quite small and only in those cases does the mesostructure survive the postreducing process. For example, for Fe3O4 and CoO the volume shrinkage ratios from Fe2O3 and Co3O4 are only about 2 and 12%, respectively. However, in most cases, the reducing processes cause large volume shrinkages so that the mesostructures collapse after treatment. For MoO2, an estimate of the volume shrinkage from MoO3 to MoO2 calculated using the bulk densities is about 36%, which means that mesoporous MoO2 cannot be synthesized from mesoporous MoO3 even if it is possible initially to make mesoporous MoO3, which, to the best of our knowledge has not been previously reported. We report here a facile and large-scale nanocasting synthesis method for directly producing highly ordered mesoporous crystalline MoO2 in 10% H2 atmosphere using phosphomolybdic acid (PMA), H3PMo12O40, as a precursor. Mesoporous silica KIT-6 material was adopted as a hard template and filled with the PMA precursor by employing the solvent evaporation induced capillary condensation method.12 The prepared PMA@KIT-6 composite was then heated to 500 °C at a rate of 2 °C min-1 in a mixed gas flow of 10% H2 and 90% Ar. During this treatment, the precursor PMA decomposed and was reduced in situ from Mo6+ to Mo4+ and transformed to crystalline MoO2 inside the mesopore of the KIT-6 template. Finally, the silica template was removed using 4% HF as etchant, and a highly ordered mesoporous crystalline MoO2 was collected as the product. The obtained mesoporous MoO2 4216
Figure 1. (a) Small-angle XRD patterns of mesoporous silica template KIT-6 and its replica mesoporous MoO2 materials. (b) Wide-angle XRD patterns of replicated materials prepared at different temperatures under a reducing atmosphere. The asterisk indicates the appearance in the XRD pattern of metallic Mo.
material shows a positive temperature coefficient of resistance and a low resistivity (∼0.01 Ω cm at 300 K, measured as a pressed pellet), indicating that it shows a metallic conductivity as expected. As far as we know, it is the first ordered mesoporous metal oxide material with metallic conductivity, which makes it different from all previously reported mesoporous metal oxides materials. Our results show that mesoporous MoO2 material exhibits an reversible electrochemical lithium capacity as high as 750 and 670 mA h g-1 at C/20 (41.9 mA g-1) and C/10 (83.8 mA g-1) after 30 and 20 cycles, respectively, which are much higher than the capacity of commentarially used carbon materials. These primary test results suggest that the mesoporous MoO2 material reported here is a promising anode material for lithium ion batteries. Mesoporous silica template KIT-6 was prepared according to the literature report.13 Small-angle X-ray diffraction (XRD) pattern of the produced KIT-6 (Figure 1a) shows seven wellresolved diffraction peaks, which can be indexed to 211, 220, 321, 400, 420, 332, and 431 Bragg reflections of the bicontinuous cubic gyroidal mesostructure (space group of Ia3d), indicating a highly ordered mesostructure. The cell parameter of KIT-6 template is 23.5 nm, calculated based on the d211-spacing value (9.60 nm) from the XRD pattern. The nitrogen sorption isotherms of KIT-6 template (Supporting Information Figure S1) show typical type IV isotherms with an obvious capillary condensation at p/p0 from 0.7 to 0.8, indicating uniform mesoporosity. Calculations based on the isotherms show that the KIT-6 template has a surface area of 902 m2 g-1, a mean pore size of 8.6 nm, and a pore volume of 1.2 cm3 g-1, which are all in agreement with literature data.13 The small-angle XRD pattern of the mesoporous MoO2 replica material (Figure 1a), prepared at 500 °C, shows one intense peak at 2θ ) 1.0° with a weak shoulder, corresponding to the 211 and 220 diffraction peaks, and a broad step at 2 θ ) 1.7°, corresponding to the 420 and 332 diffraction peaks. These results indicate that the structural regularity of Nano Lett., Vol. 9, No. 12, 2009
Figure 3. High resolution SEM images of mesoporous MoO2 material.
Figure 2. TEM images of the mesoporous MoO2 material with different magnifications and a typical SAED pattern.
mesoporous MoO2 is only slightly lower than that of its template KIT-6. Compared to the XRD pattern of KIT-6, the diffraction peaks of mesoporous MoO2 are slightly shift to higher angles, indicating a small shrinkage of the mesostructure after replication. The cell parameter of mesoporous MoO2 is calculated to be 23.3 nm from the d211value and the volume shrinkage of the replica is about 2.5%. Wide-angle XRD patterns (Figure 1b) show that the materials crystallize as the pseudotetragonal rutile MoO2 phase (JCPDS: 02-0422) with thermal treatment to 500-700 °C. Treating the PMA@silica composite below this temperature leads to an amorphous phase product, and increasing the temperature above 800 °C leads to the formation of metallic molybdenum in the final products as shown in Figure 1b (indicated by an asterisk). As demonstrated in their wideangle XRD patterns, samples prepared at different temperature show a similar crystallinity, which can be attributed to the confinement effect from the KIT-6 template’s 8.6 nm uniform mesopore. Transmission electron microscopy (TEM) investigations also reveal that the mesostructure regularity is well preserved after the nanocasting replication, as shown in Figure 2. The TEM images suggest that the MoO2 product has ordered mesostructure throughout the whole particle domain. No large nonporous impurity particles were observed, indicating that almost all the PMA was successfully filled inside the mesopore and then transformed in situ to the MoO2 material during the preparation. High-resolution TEM (HRTEM) image (Figure 2c) shows that the obtained mesoporous MoO2 is well crystallized, in agreement with the wide-angle XRD results. Selected area electron diffraction (SAED) patterns (Figure 2d) display individual spots associated with concentric rings, confirming that the prepared mesoporous MoO2 materials contain polycrystalline MoO2. Nano Lett., Vol. 9, No. 12, 2009
Energy-dispersive X-ray spectrum (see Supporting Information Figure S2) analysis revealed only molybdenum and oxygen. No silica and phosphorus is observed, indicating that the silica template and atomic phosphorus that is part of the PTA precursor was removed. The representative thermogravimetric analysis (TGA) of the ordered mesoporous MoO2 as measured in air exhibits two significant weight change steps (Supporting Information Figure S3). First step is a weight increase from about 200 to 350 °C, corresponding to the oxidation of MoO2 to MoO3. After 350 °C, the weight increases to 670 °C as a result of the complete oxidation to MoO3. The second large weight decrease above 670 °C can be ascribed to the sublimation of MoO3. The total weight increase recorded between 100 and 670 °C is 11.6 wt %, which is similar to the theoretical value of 12.5 wt %, confirming that the initial product is almost pure MoO2. MoO2 has a high electrical conductivity, 8.8 × 10-5 Ω.cm in bulk samples at room temperature, so that it can be directly spread on SEM sample holders for SEM observation without requiring any further coating by gold or carbon. This unique property makes the mesostructure easily imaged by SEM (Figure 3). It was found that a highly ordered mesostructure can be clearly observed for each product particle and that the surface of the particle is quite clean, indicating that almost all of the PMA was filled into the mesopores of the KIT-6 and transformed in situ to MoO2, which is in agreement with the results from the TEM observation. SEM images obtained at low magnification (Supporting Information Figure S5) show that mesoporous MoO2 material has a particle size distribution and morphology similar to that of the KIT-6 template (Supporting Information Figure S4); however, macropores (>100 nm) are observed in the mesoporous MoO2 particles as shown in Figure 3a, which means that the 5-200 µm particles actually are composed of smaller particles with sizes in the range of several hundred nanometers. These results suggests that the mesoporous MoO2 particles are not entirely composed of integrated frameworks, which can be explained by the volume shrinkage on going from the PMA precursor to the MoO2 product. Calculations based on molecular weights and densities suggest that 1.00 cm3 PMA transforms theoretically to only 0.52 cm3 MoO2. It means that about half (52%) of the original template pore volume is occupied by MoO2 after one impregnation-reduction cycle and the unoccupied domain is responsible for the macropores. N2 sorption isotherms of mesoporous MoO2 (Supporting Information Figure S6) show typical type-IV curves with several capillary condensation steps at the range of p/p0 of 4217
0.45-0.95, suggesting a distribution in the mesopore structure. Accordingly, bimodal pore size distributions are derived from the adsorption branch of the isothermal based on the BJH model included in the Supporting Information. The first peak at the mean value of 3.2 nm is attributed to the void space generated after the removal of silica wall. The mesopores at about 18 nm may relate to the special structure of KIT-6 type mesoporous materials. The KIT-6 template has a minimal surface, bicontinuous pore structure with cubic Ia3d symmetry consisting of an enantiomeric pair of threedimensional (3D) mesoporous channel systems.13,14 These two pore systems are separated by the silica wall corresponding to the gyroidal infinite periodic minimal surface (G surface). During the nanocasting synthesis, in some cases the guest materials only occupy one channel systems in some regions, which means that only half of the mesostructure was replicated to the reactant materials.10c,12a,14b,15 The absence of the complete enantiomeric pair is reflected in the large pore size of the replicated material. Figure 3b shows such a half replicated domain, which can be frequently observed in our SEM observation. Calculations based on the isotherms show that mesoporous MoO2 product has a specific BET surface area of 66 m2 g-1 and a pore volume of 0.29 cm3 g-1. The heteropoly acid PMA was adopted as the molybdenum precursor in this nanocasting preparation because of its high solubility in ethanol and high molybdenum content. These two properties ensure that PMA easily enters the mesopores of the template and leads to a high yield of product. PMA (4.2 g) can be filled into a 1 g mesoporous silica template (3.5 g PMA per 1 cm3 pore volume), leading to more than 3 g of final product with almost no PMA found outside the template, as demonstrated by TEM and SEM results. This optimized precursor/template ratio is close to the literature report for using phosphotungtic acid or PMA as precursors in the nanocasting synthesis of WO3 or MoS2.16 The synthesis can be easily scaled up 10 times and 30 g products can be produced in one batch in our lab. As mentioned above, PMA has a high volume yield up to 52%, which is about 5-10 times larger than those of metal nitrates precursors.16b This property enables that not only can the mesostructure of the template be replicated by MoO2, but also that its macroscale morphology can be readily copied, as seen from SEM images (Supporting Information Figure S4 and S5). As a more evident example, when spherelike mesoporous silica is employed as a template, mesoporous MoO2 spheres can be produced (Supporting Information Figure S7). This kind of macroscale morphology copy for nanocasting synthesized mesoporous metal oxide materials has rarely been reported. Mesoporous MoO2 that was synthesized at 500 °C was loaded onto an interdigital fingers patterned gold electrodes for resistance measurements, as illustrated in the schematic inset of Figure 4, by dropping its water suspension on to the substrate and drying in vacuum at room temperature. Temperature-dependent 2-probe DC electrical resistance measurement results (Figure 4) clearly show a positive temperature coefficient of resistance, characteristic of a metallic sample, indicating our sample shows a metallic 4218
Figure 4. Two-probe resistance as a function of temperature of mesoporous MoO2 powders precipitated from water dispersion on to the interdigital fingers electrode as displayed in the inset.
Figure 5. (a) Cyclic voltammograms of mesoporous MoO2 at a scan rate of 0.1 mV s-1 in the voltage range of 1-3 V. (b) Cyclic voltammograms of mesoporous MoO2 at a scan rate of 0.2 mV s-1 in the voltage range of 0.01-3 V. (c) Galvanostatic discharge (Li uptake, voltage decreases)/charge (Li remove, voltage increases) curves of mesoporous MoO2 electrodes cycled at a rate of C/20. (d) Cycling performance of the mesoporous MoO2 electrode at current rates of C/20 and C/10.
conductivity as expected. Using this technique, it is difficult to obtain meaningful sample geometries so that no attempt was made to extract the resistivity from the resistance. The room temperature (300 K) resistivity was instead measured to be about 0.01Ω cm on a pressed bar of the mesoporous MoO2 sample by the four-probe method. As a consequence of the bars not being sintered, we note that the magnitudes of the resistivities reported here only provide an upper bound, and the true resistivity values of the material are likely to be much lower. Nevertheless, the values of the resistivities are quite low, confirming that it is a metallic sample. Lithium storage properties of the 500 °C-synthesized mesoporous MoO2 sample were investigated by performing cyclic voltammetry (CV) and galvanostatic discharge-charge experiments in different voltage ranges and at different current rates. Figures 5a and b show the CV curves of mesoporous MoO2 electrode cycled in 1 M LiPF6 EC/DMC electrolyte solution at scan rates of 0.1 and 0.2 mV s-1, respectively. In the first cycle at a voltage range of 1-3 V, a pronounced reduction (Li insertion) peak was observed at 1.15 V and two oxidation (Li extraction) peaks were observed at 1.50 and 1.78 V. The reduction peak at 1.15 V could be Nano Lett., Vol. 9, No. 12, 2009
ascribed to a phase transition from the orthorhombic to the monoclinic phase due to the Li insertion as suggested by Dahn and McKinnon.3b The two oxidation peaks at 1.50 and 1.78 V could be assigned to the phase transitions from the monoclinic to the orthorhombic phase and from the orthorhombic to the monoclinic phases in the Li extraction process, respectively.3b In contrast to the previous reports obtained using a bulk MoO2 electrode,3a,b after the first cycle, these two phase transitions are highly reversible in the reduction and oxidation processes as indicated by the pronounced two reduction/oxidation pairs (1.48/1.75 V and 1.21/1.47 V). Furthermore, when the voltage range was extended to 0.01-3 V after 3 cycles, they are also highly reversible (Figure 5b). This reversibility could be the result of the unique MoO2 mesostructure, which has a uniform nanosized framework structure. Figure 5c displays the discharge (Li uptake)/charge (Li remove) curves of the mesoporous MoO2 electrode at a current rate of C/20 (1C refers to 4 Li uptake into MoO2 in 1 h.). As in the first cycle, two short discharge plateaus at about 1.52 and 1.25 V and two charge plateaus at 1.41 and 1.71 V were observed and are also reversible in the following few cycles, which are in good agreement with CV results. The mesoporous MoO2 electrode shows a first discharge capacity of 960 mA h g-1 (Li4.6MoO2) and a charge capacity of 630 mA h g-1 (Li3MoO2) in the voltage range of 0.01-3.0 V. Moreover, the surprising point is that the reversible capacity gradually increases upon cycling and reaches 750 mA h g-1 after 30 cycles (Figure 5c,d), which corresponds to 3.58 mol Li being reversibly uptaken/removed into/from the mesoporous MoO2 electrode. In the case of the current rate of C/10, an even pronounced similar phenomenon has been observed (Figure 5d and Supporting Information Figure S8a). The first charge capacity is about 450 mA h g-1 and gradually increases to 670 mA h g-1 after 20 cycles. This activated process for nanostructured MoO2 materials has also been reported by other groups3e and could be due to the material partially losing its crystallinity or transforming to an amorphous-like structure during cycling, thus improving the Li diffusion kinetics so that more Li can be removed/ uptaken from/into the material. There is also a possibility that the increased capacity might be due to the reversible formation/decomposition of solid electrolyte interphase (SEI) or interfacial Li storage as proposed by Maier,17,18 since the extra capacity mainly comes from the low voltage range. Ex-XRD patterns of fully discharged and charged MoO2 samples are shown in Supporting Information Figure S9. It can be seen that no Mo metal/Li2O are formed but some new unknown peaks appear at the end of first discharge, probably suggesting that reaction mechanism is not a traditional conversion reaction as observed for other metal oxides, for example, iron/cobalt oxides.21 Upon recharging to 3 V, MoO2 is recovered, indicating it is a reversible process. The Li storage behavior for the mesoporous MoO2 electrode is very different from that of the bulk MoO2 electrode where the capacity decays rapidly with cycling once a cell is discharged below 1.0 V.3a,b,d The high reversible capacity of 750 mA h g-1 at C/20 is two times higher than Nano Lett., Vol. 9, No. 12, 2009
that of commercial graphite (320-350 mA h g-1) and also higher than the previous reports for MoO2 materials (200-650 mA h g-1).3c,e However, as the charge/discharge rate is increased from C/20 to C/5, C/2, and 1C, the capacity of the mesoporous MoO2 electrode decreased from 750 to 500, 200, and 90 mA h g-1 (Supporting Information Figure S8b), respectively, even though the electrode material still has metallic electronic conductivity. This could be due to the sluggish Li diffusion kinetics in the electrode. The significant improvement of the electrochemical Li storage performance is attributed to the unique mesoporous structure of MoO2, which has a variety of favorable properties. First, the hierarchical mesoporosity makes facile the liquid electrolyte diffusion into the bulk of the electrode material and hence provides fast conductive ion transport channels for the conductive ions (e.g., solvated Li+ ions).19 These mesostructured channels are also expected to buffer well against the local volume change during the Li uptake/ removal reactions and thus to enhance the structural stability. TEM observations showed that the ordered mesostructure is preserved quite well after discharge and recharge processes, as illustrated in Supporting Information Figure S10. Second, the MoO2 wall with thickness of
