Article pubs.acs.org/cm
Synthesis and Lithium Storage Mechanism of Ultrafine MoO2 Nanorods Bingkun Guo,† Xiangpeng Fang,† Bin Li,‡ Yifeng Shi,*,‡ Chuying Ouyang,*,§ Yong-Sheng Hu,*,† Zhaoxiang Wang,† Galen D. Stucky,∥ and Liquan Chen†,⊥ †
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China ‡ College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou, Zhejiang 310036, China § Department of Physics, Jiangxi Normal University, Nanchang 330022, China ∥ Department of Chemistry & Biochemistry, University of California, Santa Barbara, California 93106, United States ⊥ School of Materials Science and Engineering, Chonnam National University, Gwangju, 500-757, Republic of Korea S Supporting Information *
ABSTRACT: Ultrafine MoO2 nanorods with a diameter of ∼5 nm were successfully synthesized by a nanocasting method using mesoporous silica SBA-15 as hard template. This material demonstrates high reversible capacity, excellent cycling performance, and good rate capacity as an anode electrode material for Li ion batteries. The significant enhancement in the electrochemical Li storage performance in ultrafine MoO2 nanorods is attributed to the nanorod structure with small diameter and efficient one-dimensional electron transport pathways. Moreover, density functional theory calculations were performed to elucidate the Li uptake/ removal mechanism in the MoO2 electrodes, which can help us understand the unique cycling behavior of MoO2 material.
KEYWORDS: molybdenum dioxide, nanorods, nanocasting, Li ion batteries
1. INTRODUCTION The term “one-dimensional nanostructure” is generally used to describe structures that have at least one dimension in the size range of 1−100 nm, including nanowires, nanotubes, nanorods, nanobelts, and so on. Since the discovery of carbon nanotubes,1 one-dimensional nanostructures have received increasing interest because of a variety of novel phenomena and their valuable potential applications in various fields.2 Recently, a series of one-dimensional nanostructure materials, such as Si nanowires,3 nanotubes,4 or nanorods,5 SnO2 nanotubes6 or nanowires,7 Ge nanowires,8 SnGe nanowires,9 SnSb nanorods,10 Co3O4 nanobelts,11 TiO2 nanowires12 or nanotubes,13 Li4Ti5O12 nanorods,14 LiFePO4 nanowires15 or nanorods,16 LiMn2O4 nanorods17 or nanowires,18 and so forth, have been widely investigated in the field of Li ion batteries. These materials evidently show significant improvements in their reversible capacities, as well as in their rate and cycling performances, compared with those of the corresponding bulk materials. These improvements are attributed to a short Li solid diffusion pathway in radial direction and the high interfacial contact area with electrolytes of the one-dimensional nanostructures. In addition, the piled porous structure also provides a fast Li diffusion channel, beneficial to the fast charge/discharge cycle of the batteries. © 2011 undefined
Molybdenum dioxide MoO2 has been considered a promising anode electrode material for Li ion batteries because of its high theoretical capacity (840 mA h g−1).19 In addition, the high density (6.5 g cm−3) of MoO2 enables it to store more energy with the same size of the battery compared with that of graphite (2.3 g cm−3) anode-based batteries. Unfortunately, the low storage capacity of the bulk MoO2 material results from the lithiation of the bulk MoO2 electrode that is limited to the addition-type reaction with only one-electron reduction at room temperature. That is, the conversion reaction does not occur during a normal cycling condition due to kinetic barrier in the bulk MoO2 electrode.20 Nanostructuring is an efficient means in improving Li insertion/extraction kinetics because of the short Li-ion solid diffusion pathway in nanomaterials.21 Various MoO2 nanostructures, such as mesoporous MoO2,22 MoO2 nanoparticles,19g and tremella-like MoO2,19d were prepared to improve their performance. Unlike other nanostructures, a one-dimensional nanostructure not only possesses a short Li solid diffusion pathway and a high interfacial contact area with electrolytes due to the reduced Received: August 18, 2011 Revised: December 15, 2011 Published: December 19, 2011 457
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The working electrodes were prepared by mixing the MoO2 nanorod samples, carbon black, and poly(vinylidene difluoride) at a weight ratio of 80:10:10. The slurry was pasted on pure Cu foil, dried under an infrared lamp for some time, and then placed in a vacuum oven at 100 °C for 12 h. The mass of the active materials is between 2.5 and 3.5 mg cm−2. A polypropylene film (Celgard 2300) was used as a separator. Pure Li foil (Aldrich) was used as a counter electrode. The electrolyte consisted of a solution of 1 M LiPF6 in ethylene carbonate/dimethyl carbonate (1:1 volume ratio) obtained from the Ferro Corporation. The cells were assembled in an argon-filled glovebox. Galvanostatic discharge−charge experiments were conducted at current rates of C/20 in the voltage range of 0.01−3.0 V on a Land BT2000 battery test system (Wuhan, China). Cyclic voltammogram measurements were performed on a CHI660 electrochemical workstation in the voltage range of 0.01−3.0 V at a scan rate of 0.05 mV s−1. Computations are performed with the PWSCF code using a spindependent density functional theory plus Hubbard U method and ultrasoft pseudopotentials. The valence electrons wave functions and the augmented electron density are expanded in plane-wave sets with cutoff energies of 30 and 140 Ry, respectively.
physical distance but also has efficient one-dimensional electronic transport pathways. Therefore, MoO2 nanowires or nanorods with ultrafine diameters (100 nm). In addition, although Dahn and McKinnon19b have studied the Li uptake/removal mechanism in MoO2 between 3.0 and 1.0 V, the conversion reaction process for MoO2 nanostructures below 1.0 V is still not clearly understood so far. In most published reports, the capacities of different nanostructured MoO2 always evidently increased during the first 1−20 cycles, which is different from most other materials and it has not been clearly understood yet. In the present study, the nanocasting method was used to synthesize a MoO2 nanorod sample that has a diameter of ∼5 nm, considerably smaller than previously reported samples, using mesoporous silica SBA-15 as hard template. The resulting sample shows a high reversible Li storage capacity of 830 mA h g−1 (99% of the theoretical capacity) and an excellent cycling performance as an anode for Li ion batteries. Moreover, the Li uptake/removal mechanism of MoO2 nanorods between 3.0 and 0.01 V was carefully investigated using ex situ X-ray diffraction (ex situ XRD). The obtained results suggest that the MoO2 nanorods undergo the conversion reaction. In addition, density functional theory (DFT) calculations were performed to understand the Li uptake/removal mechanism in the MoO2 electrodes.
3. RESULTS AND DISCUSSION The pore structure of a typical SBA-15 material is generally described as 2D hexagonally packed cylindrical pores.25,26 These primary mesopores are isolated and separated from each other by the silica wall in an ideal structural model. However, in a typical real SBA-15 material, a large amount of randomly distributed secondary micro-/mesopores are present in the silica wall that connect all the primary mesopores together to form a continuous pore space.27 This “un-ideal” structure makes SBA-15 suitable for use as a hard template to fabricate self-supported nanowires array replicas, including carbons, metal oxides, metal sulfides, nitrides, fluorides, etc.28 In the present work, a low hydrothermal treatment temperature (80 °C) and a high calcination temperature (900 °C) were adopted in the synthesis of SBA-15 to diminish the secondary pores. Therefore, isolated MoO2 nanorods can be fabricated using this special SBA-15 as a hard template. As shown in Figure 1a, three distinct diffraction peaks were recorded in the small-angle XRD pattern of the prepared SBA-
2. EXPERIMENTAL SECTION Material Synthesis. SBA-15 hard template was prepared according to the following established procedure:25 Triblock copolymer P123 (20.0 g) was dissolved in 750 mL of 1.6 M HCl water while stirring at 38 °C to obtain a clear solution. Tetraethyl orthosilicate (42.0 g) was then added during rigorous stirring, and the mixture was kept at constant temperature for another 24 h. Then the mixture was transferred into a thick-walled polypropylene bottle and heated up to 80 °C for 24 h. The white precipitate was recovered by filtration and dried at 50 °C after being washed with deionized water and acetone. The products were then calcined at 900 °C for 2 h with a heating rate of 2 °C min−1 in air. SBA-15 hard template (5.0 g) was mixed with H3PMo12O40 (5.0 g) precursor in 30 mL of ethanol while stirring in an open crucible. After the ethanol solvent was evaporated at 45 °C, the obtained yellow powders were loaded on a quartz boat and heated to 600 °C in a tube furnace at a rate of 2 °C min−1 under a constant (200 mL min−1) Ardiluted H2 gas flow and was maintained at this temperature for 5 h. The obtained MoO2/SBA-15 composite was treated with 200 mL of 2 M NaOH aqueous solution to remove the silica template. Synthesis of bulk MoO2: 5 g of H3PMo12O40 without any mesoporous silica template was directly reduced to bulk MoO2 by Ar-diluted H2 gas flow at 600 °C for 5 h. Characterization. Powder X-ray diffraction (XRD) patterns were collected on a Scintag PADX diffractometer using Cu Kα radiation (45 kV, 35 mA). The electrode for ex situ XRD measurement was rinsed with dimethyl carbonate (DMC) and dried in a vacuum chamber. During the measurement, the electrodes were covered by a thin Myler film in the glovebox. TEM images were taken using an FEI Tecnai T20 Sphera electron microscope operating at 200 keV. SEM images were acquired using an FEI XL30 Sirion FEG digital scanning electron microscope. Electrochemical experiments were performed using homemade two-electrode cells.
Figure 1. (a) Small-angle XRD pattern and (b) nitrogen sorption isothermals and the corresponding pore size distribution curve of the 900 °C-calcined SBA-15 template.
15 template material, indicating a highly ordered mesostructure. The unit cell parameter estimated from the XRD pattern is 8.9 nm, which is significantly smaller than the typical SBA-15 materials (∼11 nm). This result can be explained by the lowtemperature (80 °C) hydrothermal treatment and hightemperature calcination (900 °C). The corresponding hydrothermal and calcination temperatures in the synthesis of a typical SBA-15 material are 100−130 and 550 °C, respectively. Nitrogen sorption isotherms analyses revealed that the prepared SBA-15 has a smaller pore size (∼5 nm) because of 458
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the structure shrinkage caused by high-temperature calcination, but the pore size distribution is still quite narrow, as shown in Figure 1b. Accordingly, the pore volume and specific surface area are reduced to 0.25 cm3 g−1 and 357 m2 g−1, respectively. All these results confirmed that the prepared SBA-15 hard template still possesses highly ordered mesostructure with a uniform pore size despite the 900 °C high-temperature calcination. During the nanocasting process, H3PMo12O40 was used as a precursor to fill the mesopore space of the SBA-15 template by solvent evaporation-induced capillary condensation as described previously in another study.29 H3PMo12O40 was then reduced in situ to crystalline MoO2 by 10% H2 + 90% Ar gas flow at 600 °C for 5 h. MoO2 nanorods were obtained as final products after removing the silica template by using a NaOH aqueous solution as an etching agent. The wide-angle XRD pattern of the synthesized sample shows intensive X-ray diffraction peaks (Figure 2), which
Figure 3. SEM images (a, b), TEM (c) image, and SAED pattern (d) of the MoO2 nanorods.
storage behavior in the ultrafine MoO2 nanorods. Figure 4a illustrates the CV curves of the ultrafine MoO2 nanorod electrode at a scan rate of 0.05 mV s−1 in the voltage range of 0.01−3.0 V. A pronounced reduction (Li insertion) peak was observed at 1.16 V and two obvious oxidation (Li extraction) peaks were observed at 1.56 and 1.86 V during the first cycle. The reduction peak at 1.16 V can be ascribed to a phase transition from the orthorhombic to the monoclinic phase upon Li insertion as suggested by Dahn and McKinnon.19b The two oxidation peaks at 1.56 and 1.86 V can be attributed 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. These two phase transitions are highly reversible in the subsequent cycles as indicated by the two pronounced reduction/oxidation pairs (1.43/1.86 V and 1.20/1.56 V) even after 23 cycles. This reversibility demonstrates the advantages of our ultrafine MoO2 nanorod structural design. In addition, for the 21st, 22nd, and 23rd cycles, a stronger reduction peak below 1.0 V was clearly observed, which suggests a more complete conversion reaction from MoO2 to Mo and Li2O after the initial activated process. Figure 4b shows the discharge (Li uptake)/charge (Li removal) curves of the ultrafine MoO2 nanorod electrode at a current rate of C/20 (1C refers to 4 Li uptake into MoO2 per formula unit in 1 h, that is, 838 mA g−1). For comparison, the bulk MoO2 electrode was evaluated under the same condition and its results are also shown in Figure 4b. In the first cycle, two short discharge plateaus at approximately 1.54 and 1.35 V and two charge plateaus at 1.43 and 1.70 V were observed and were also reversible in the subsequent cycles, which agree well with the CV results. The bulk MoO2 electrode exhibits a similar behavior. However, when the discharge voltage is below 1.0 V, the difference between both samples is pronounced. The capacity contribution below 1.0 V for the bulk electrode is much lower than that of the ultrafine MoO2 nanorod electrode. The reversible capacity of the bulk MoO2 electrode is just about 210 mA h g−1 in the voltage range of 3.0−0.01 V and most of the capacity comes from the reaction above 1.0 V. These results
Figure 2. Wide-angle XRD patterens of the MoO2 nanorods and bulk MoO2.
clearly indicates the crystalline nature of the sample. All recorded peaks can be ascribed to those of MoO2 with monoclinic symmetry (JCPDS: 78-1069), and no peaks of MoO3 or other molybdenum oxides are observed, indicating that the synthesized sample is phase-pure MoO2. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the MoO2 nanorods are shown in Figure 3. These images portray markedly that most of the products are isolated nanorods. The relatively uniform diameters of the nanorods, at approximately 5 nm, agree well with the pore diameter of the mesoporous silica template, confirming the template replication synthesis. Such a small diameter provides a short and solid-state transfer path for Li diffusion during insertion and extraction. On the other hand, the length of the nanorods is widely distributed from 100 to 800 nm, because of the nonconfined pore space in this direction in the SBA-15 template and the volume shrinkage during the conversion of H3PMo12O40 into MoO2. The crystalline nature of the sample can also be clearly demonstrated by its selected-area electron diffraction (SAED) pattern (Figure 3d), as indicated by its wide-angle XRD result. Cyclic voltammetry (CV) and galvanostatic discharge− charge experiments were conducted to investigate the Li 459
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Figure 4. (a) Cyclic voltammograms of the ultrafine MoO2 nanorods at a scan rate of 0.05 mV s−1; (b) the galvanostatic discharge−charge curves of the ultrafine MoO2 nanorod and bulk MoO2 electrodes at a current rate of C/20 in the voltage range of 0.01−3.0 V; (c) the discharge−charge curves of the ultrafine MoO2 nanorods; (d) the cycling performance of the ultrafine MoO2 nanorods and (e) rate performance of the ultrafine MoO2 nanorods.
shown to be mainly derived from a low voltage range, especially that below 0.9 V. This phenomenon indicates that the MoO2 nanorod electrode could undergo a conversion reaction below 1.0 V.20 In addition, the reversible capacity of the ultrafine MoO2 nanorod electrode gradually increases with cycling (Figure 4c). Actually, this activated process for nanostructured MoO2 materials has also been reported by other groups but without any clear explanation.19d,e Figure 4d displays the cycling performance of the ultrafine MoO2 nanorod electrode. This electrode exhibits a high reversible capacity that does not decline over 29 cycles, contrary to that of the bulk MoO2 electrode, which has a low
indicate that the bulk MoO2 is lithiated by an addition-type reaction with only one electron reduction, and that the conversion reaction does not occur.20 In contrast, the ultrafine MoO2 nanorod electrode exhibits a first discharge capacity of 775 mA h g−1 (Li3.7MoO2, including the contribution of the formation of solid electrolyte interphase (SEI) 30) and a first charge capacity of 521 mA h g −1 (Li2.5MoO2) in the voltage range of 3.0−0.01 V (Figure 4b), which is considerably higher than the capacity of bulk MoO2. Comparing the discharge/charge curves of the ultrafine MoO2 nanorods and that of the bulk MoO2 electrodes (Figure 4b), the extra capacity of the ultrafine MoO2 nanorod electrode is 460
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capacity that decays rapidly with cycling (Figure S3, Supporting Information). The capacity of MoO2 nanorods even reaches 830 mA h g−1 after 29 cycles, which is very close to the theoretical capacity of MoO2 (840 mA h g−1). For transition metal oxides, if the conversion reaction occurs, the materials will generally suffer from poor cycling stability resulting from the significant volume variation and the aggregation of the pulverized and/or nanosized particles during repeated charge/ discharge processes.31,32 However, the capacity of the MoO2 nanorod electrode does not decay with cycling, which should be ascribed to the ultrafine nanorod structure of MoO2. The diameter of the MoO2 nanorods is only approximately 5 nm, enabling its accommodation of a large amount of strain without pulverization during cycling. As the charge/discharge rate is increased from C/20 to C/5, C/2, and 1C, the capacity of the MoO2 nanorod electrode decreases from 740 to 560, 400, and 260 mA h g−1 (Figure 4e), respectively. This rate capability is much higher than that of the mesoporous MoO2.22 In that case, the capacity of mesoporous MoO2, the capacity decreased from 700 to 500, 180, and 80 mA h g−1, when the chare/discharge rate is increased from C/20 to C/5, C/2, and 1C. The better rate performance of the nanorod material in this research can be attributable to the ultrasmall diameter and efficient onedimensional electronic transport pathways. Ex situ XRD was performed to study the Li uptake/removal mechanism in the MoO2 nanorod electrode. Figure 5b displays the ex situ XRD patterns at various discharge/charge states. When the MoO2 electrode was discharged to 1.32 V in the first cycle, a slight downshift of the (110) peak was observed ((2) curve), suggesting that MoO2 underwent a single-phase reaction. However, when electrode was discharged to 1.0 V, several new peaks appeared ((3) curve), indicating that the electrode underwent a possible two-phase reaction. After further discharged to 0.01 V, only the Li0.98MoO2 phase can be detectable ((4) curve). In the subsequent charge, no new peak was initially observed when the MoO2 electrode was charged to 1.30 V ((5) curve). However, when charged to 1.62 V, a peak at 24.9° at the same position as in the curve of that discharged to 1.0 V appeared ((6) curve). Finally, when charged to 3.0 V, the MoO2 recovered ((7) curve). On the basis of the above discussions including electrochemical data, it can be concluded that the MoO2 nanorods could partially undergo a conversion reaction and then decompose into Mo and Li2O during the first discharge process. However, the characteristic peaks of Mo and Li2O were not detected by XRD ((4) curve), which could be attributed to the nanoparticle size and/or the amorphous natures of Mo and Li2O.20,33 The Li0.98MoO2 phase observed at 0.01 V suggests that a part of MoO 2 undergoing an addition-type reaction does not decompose into Li2O and Mo in the first cycle, which could be the reason for the first discharge capacity of MoO2 nanorods being only 775 mA h g−1 (Li3.7MoO2), which is between 205 mA h g−1 (Li0.98MoO2) and 840 mA h g−1 (Li4MoO2). During the first charge, the MoO2 electrode remained at the Li0.98MoO2 phase, and no MoO2 phase was detected when the electrode was charged to 1.30 V. This result indicates that Li0.98MoO2, not MoO2, could be first formed from Li2O and Mo composite, which will be further discussed later. The structural information after 15 cycles is also collected to determine whether the MoO2 nanorod electrode is stable with cycling. The reaction process after 15 cycles is the same as that in the first cycle ((8) and (9) curves in Figure 5b). However, the characteristic peak intensity of the Li0.98MoO2 evidently
Figure 5. (a) The first galvanostatic discharge/charge voltage profiles of the MoO2 nanorods and (b) ex situ XRD patterns of (1) the starting MoO2 nanorods electrode, (2) discharged to 1.32 V, (3) discharged to 1.00 V, (4) discharged to 0.01 V, (5) charged to 1.30 V, (6) charged to 1.62 V, (7) charged to 3.00 V, (8) discharged to 0.01 V after 15 cycles, and (9) subsequently charged to 3.00 V.
became very weak, indicating that a larger portion of MoO2 undergoes a conversion reaction and decomposes into Li2O and Mo upon cycling. Transition metal oxides, such as CoO, FeO, MnOx, and so on, can be directly formed from the discharged product of metals and Li2O during the charging process.30a,33,34 However, ultrafine MoO2 nanorods apparently possess different behavior during the charging process. The obtained results indicate that only a part of intermediate Li0.98MoO2 undergoes a conversion reaction in the first discharge cycle, and the remaining Li0.98MoO2 gradually decomposes with cycling. If the Mo reacts directly with Li2O to form MoO2, the current densities of the two oxidation peaks (1.56 and 1.86 V) in the CV curves (Figure 4a) should gradually decrease because a larger portion of MoO2 decomposes into Mo and Li2O, which may directly revert to MoO2 when charged to 3.0 V. However, the current densities of these oxidation peaks in the CV curves were stable and did not decrease even after 23 cycles. This phenomenon may be attributed to the initial formation of Li0.98MoO2, not MoO2, from the Li2O and Mo nanocomposite during the charging process, as shown by the ex situ XRD patterns, in contrast to the previous perception on other transition metal oxides. Thus, the reactions of the MoO2 nanorods could be summarized as follows: MoO2 ↔ Li 0.98MoO2 ↔ Mo + Li2O 461
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empty eg orbital in the spin-up channel. These DFT calculation results confirm that the discharged state LiMoO2 has much worse kinetic properties than the MoO2 compound. From these discussions, we could determine the reason for the high reversible capacity and activated process of the ultrafine MoO2 nanorods. The ultrafine MoO2 nanorod materials could partially decompose into amorphous Mo and Li2O in the first discharge cycle at room temperature and demonstrate higher reversible capacity compared with that of bulk MoO2. However, only a number of the ultrafine MoO2 nanorods undergo a conversion reaction because of the poor kinetics of Li0.98MoO2 in the first few cycles. This could have resulted in the lower reversible capacity (521 mA h g−1) compared with the theoretical capacity (840 mA h g−1) for a conversion reaction in the first cycle. The conversion reaction causes MoO2 to partially lose its crystallinity or to transform into an amorphous-like structure, as shown by the ex situ XRD patterns, enhancing the ion diffusion kinetics and/or weakening Mo−O band strength. Therefore, a larger portion of MoO2 undergoes a conversion reaction, and the reversible capacity of the MoO2 nanorod electrode gradually increases with cycling. The significant enhancement in the electrochemical Li storage performance of the ultrafine MoO2 nanorods compared with that of the bulk MoO2 sample could be attributed to the unique nanorod structure, which exhibits a variety of favorable properties. First, the approximately 5 nm diameter of MoO2 nanorods (Figure 3c) significantly reduces the solid-state transport length for Li diffusion. Second, the small diameter of the MoO2 nanorods can reduce volume variation during repeated charge/discharge processes and consequently improve cycling stability. Third, these nanorods have a very large surface area, which allows Li to easily transfer from a liquid electrolyte to the solid electrode. Finally, the MoO2 nanorods demonstrate high electronic conductivity and efficient one-dimensional electronic transport pathways, different from other nanostructure materials.
There are several phase transitions between MoO2 and Li0.98MoO2, as suggested by Dahn and McKinnon.19b On the basis of this discussion and on other reports,19b,20 it can be seen that both bulk MoO2 and ultrafine MoO2 nanorods are found to be easily lithiated to form Li0.98MoO2 at above 1.0 V. However, the bulk MoO2 electrode cannot be further lithiated when discharged to 0.01 V at room temperature. These data signify that the primary difficulty hindering the bulk MoO2 materials from undergoing a conversion reaction at room temperature is the discharged product of Li0.98MoO2. The decomposition of Li0.98MoO2 above 0.01 V is feasible at room temperature from a thermodynamic viewpoint. Therefore, the poor kinetics in Li0.98MoO2 hinders the conversion reaction above 0.01 V in bulk MoO2 materials at room temperature. For conversion reaction in metal oxides, the kinetic barrier mainly comes from heterogeneous charge transfer and Li+ and/or O2− diffusion in solid state.20,35 However, as Li0.98MoO2, poor electronic conductivity could be another barrier. To confirm these findings, we calculated the electronic structure of the MoO2 and its discharged product LiMoO2 using the density functional theory (DFT). The results are given in Figure 6. Very clearly, the MoO2 compound exhibits
4. CONCLUSIONS Ultrafine MoO2 nanorods with a diameter of ∼5 nm have been successfully synthesized through a nanocasting method using mesoporous silica SBA-15 as a hard template. The assynthesized nanorods exhibit a high reversible capacity of 830 mA h g−1 between 3.0 and 0.01 V after 29 cycles, very close to the theoretical capacity (840 mA h g−1) of MoO2, via a conversion reaction. These ultrafine MoO2 nanorods also exhibit excellent cycling performance and good rate capability. The significant enhancement in the electrochemical Li storage performance of MoO2 nanorods is attributable to the small diameter and efficient one-dimensional electronic transport pathways of the nanorod structure. Ex situ XRD results indicate that only a number of MoO2 nanorods undergo a conversion reaction, and the remaining MoO2 only undergo an additiontype reaction in the first few cycles because of poor kinetics. The partial conversion reaction enhances the ion diffusion kinetics and/or weakening Mo−O band strength of MoO2. Therefore, a larger portion of MoO2 undergoes a conversion reaction, and the capacity of the MoO2 nanorod electrode gradually increases with cycling. In addition, the DFT results confirm that the discharged intermediate LiMoO2 has much worse kinetic properties than that of the MoO2 itself. This condition could be a reason why the bulk MoO2 material is limited to the addition-type reaction with only one-electron transfer at room temperature.
Figure 6. Atomic structures and density of states of monoclinic (a) LiMoO2 and (b) MoO2. The Fermi level is chosen as 0 eV. The large gray and red spheres and small purple spheres are Mo, O, and Li atoms, respectively.
metallic electronic structure, indicating a very good electronic conductive property. However, an energy gap of approximately 0.6 eV is observed when the MoO2 compound is lithiated to LiMoO2, which could substantially reduce the electronic conductivity of this compound. The Mo4+ is in the electronic state of Mo-3d2(↑)3d0(↓), where the t2g orbital of the spin-up channel is partially situated in an octahedral crystal field, resulting in the observed metallic electronic structure. In contrast, Mo3+ ions in the lithiated state exhibit an electronic configuration of Mo-3d3(↑)3d0(↓). Therefore, an energy gap is open between the fully occupied t2g orbital and completely 462
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ASSOCIATED CONTENT
S Supporting Information *
SEM image, XRD pattern, and cycling performance of the bulk MoO2. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected];
[email protected].
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ACKNOWLEDGMENTS This work was supported by funding from “863” Project (2009AA033101), “973” Projects (2009CB220104, 2010CB833102), NSFC (50972164), the 100 Talent Project of the Chinese Academy of Sciences, Zhejiang Provincial NSFC (Y4110369), Key Project of Chinese Ministry of Education (211066), United States National Science Foundation (Grant DMR08-05148), and the WCU (World Class University) program through the Korea Science and Engineering Foundation funded by the Ministry of Education, Science and Technology of Korea (R32-2009-000-20074-0). The use of MRSEC facilities (through No. DMR 0520415) is also acknowledged.
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dx.doi.org/10.1021/cm202459r | Chem. Mater. 2012, 24, 457−463
