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J. Phys. Chem. C 2008, 112, 5160-5164
Transformation from Microcrystalline LiMn1-xCrxO2 to 1D Nanostructured β-Mn1-xCrxO2: Promising Electrode Performance of β-MnO2-Type Nanowires Dae Hoon Park, Hyung-Wook Ha, Sun Hee Lee, Jin-Ho Choy, and Seong-Ju Hwang* Center of Intelligent Nano-Bio Materials (CINBM), DiVision of Nano Sciences and Department of Chemistry, Ewha Womans UniVersity, Seoul 120-750, Korea ReceiVed: NoVember 3, 2007; In Final Form: January 22, 2008
Cr-substituted β-Mn1-xCrxO2 nanowires with nonaggregated morphology have been synthesized by the chemical oxidation of layered LiMn0.9Cr0.1O2 microcrystals under hydrothermal conditions at elevated temperature. According to powder X-ray diffraction analysis, a persulfate treatment at 200 °C gives rise to a unique transformation from a monoclinic-layered structure to a pyrolusite- (β-MnO2-) type structure. Electron microscopic analyses clearly demonstrated that the oxidized derivative consists of well-separated singlecrystalline nanowires with a diameter of ∼30 nm and a length of several micrometers. Mn K-edge and Cr K-edge X-ray absorption spectroscopy and chemical analysis provide straightforward evidence for the substitution of chromium ions for the manganese sites of the pyrolusite-structured manganese oxide nanowires. Of special importance is that the present Cr-substituted β-MnO2-type nanowires show promising electrode performance for Li+ ion batteries, superior to those of pristine LiMn0.9Cr0.1O2, bulk β-MnO2, and other structuretype manganese oxide nanowires including unsubstituted β-MnO2 nanowires. On the basis of the present experimental findings, we are able to conclude that the persulfate treatment under hydrothermal conditions provides a powerful method not only to prepare cation-substituted manganese oxide nanowires but also to improve the electrode performance of microcrystalline metal oxides through nanostructure fabrication.
Introduction Over the past decade, one-dimensional (1D) nanostructured manganese oxides have attracted special attention because of their various potential applications as molecular sieves, catalysts, biosensors, and electrode materials for batteries and supercapacitors.1-5 These nanostructured materials have unique characteristics such as high aspect ratios and large surface-to-bulk ratios. In particular, the narrow diameter (or width) of 1D nanostructures, providing a short Li+ diffusion path, is favorable for their application as electrodes in Li+ ion cells. As a result of intense research activities devoted to this topic, many synthetic methods have been developed for 1D manganate nanostructures such as hydrothermal synthesis, electrophoretic deposition on porous alumina membrane, microwave-assisted synthesis, and heat treatment of MnOOH.6-9 From the viewpoint of crystal structure, a wide variety of manganese oxide phases can be prepared in the form of 1D nanostructures: R-MnO2, β-MnO2, γ-MnO2, δ-MnO2, MnO, and Mn2O3.10-13 Recently, we found that chemical oxidation of microcrystalline manganese oxide leads to the formation of 1D nanostructured homologues with the R-MnO2 and δ-MnO2 structures.14-16 Of special interest is that, using this reaction scheme, we were able to easily control the chemical composition of the nanostructured materials by changing the formula of the solid-state precursor used. It has been well documented that the electrode performance of bulk manganese oxides can be optimized by the partial substitution of Mn by other metal ions,17-19 which spurs interest in the effects of cation substitution on the electrode properties of nanostructured counterparts. In fact, we were successful in demonstrating that the partial replacement of Mn by Cr is also * To whom correspondence should be addressed. Tel.: +82-2-32774370. Fax: +82-2-3277-3419. E-mail:
[email protected].
effective in optimizing the electrochemical functionality of manganate nanowires, as in the case of microcrystalline manganese oxides.16 However, in that study, the nanowires under investigation crystallized with the layered δ-MnO2-type or tunnel R-MnO2-type structure containing considerable amounts of water molecules in the lattice. Such a presence of lattice water molecules has a detrimental effect on the electrochemical properties of the nanowires because the lattice water produces corrosive HF through reaction with the electrolyte. Because pyrolusite- (β-MnO2-) type manganese oxide has no space to accommodate water molecules in the lattice, this nonhydrate phase is considered as a promising candidate for electrode applications. Although there have been several reports on the synthesis and characterization of binary β-MnO2 nanowires,10,20-22 no research groups other than Cheong et al.23 have tried to apply this nanostructured material as electrodes in lithium rechargeable batteries. This is because of the poor electrochemical activity of bulk β-MnO2.24 In fact, Cheong et al. reported that 1D nanostructured β-MnO2 with an interconnected hierarchical superstructure provides only a small discharge capacity in Li+ ion batteries with poor reversibility.23 The electron microscopy data in their article23 seem to suggest that the formation of a hierarchical intergrowth structure would prevent the effective introduction of lithium ions into individual nanowires, leading to the poor electrode performances of their samples. Moreover, at the time of the publication of the present study, we were aware of no reports on the chemical substitution for pyrolusitestructured manganate nanowires to optimize their electrode performance. In this context, we attempted not only to synthesize and electrochemically characterize well-separated β-MnO2 nanowires but also to improve their electrochemical properties through the partial substitution of Mn with Cr.
10.1021/jp710579y CCC: $40.75 © 2008 American Chemical Society Published on Web 03/12/2008
Promising Electrode Performance of β-MnO2-Type Nanowires In the present study, we are successful in synthesizing chromium-substituted β-MnO2 nanowires with a well-separated morphology through the chemical oxidation of layered LiMn0.9Cr0.1O2 at elevated temperature under hydrothermal conditions. Because the β-MnO2 phase is more thermodynamically stable than R-MnO2 and δ-MnO2 phases, we carried out the hydrothermal reaction at a temperature of 200 °C, which is higher than the synthesis temperatures of R-MnO2- and δ-MnO2structured nanowires.16 The curling and folding of layered crystallites are well-known to be mainly responsible for the formation of 1D nanostructured manganese oxide in solution.10,14,25 Consequently, we adopted layered LiMn0.9Cr0.1O2 as a precursor for the synthesis of Cr-substituted β-MnO2 nanowires. More interestingly, the precursor LiMn0.9Cr0.1O2 shows promising electrode performance for lithium ion batteries.18 In this regard, the effect of chemical oxidation on the chemical bonding nature and electrode functionality of pristine LiMn0.9Cr0.1O2 can provide valuable insight into the role of the nanostructure in the electrochemical properties of manganese oxides.
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Figure 1. Powder XRD patterns of (a) pristine LiMn0.9Cr0.1O2 and (b) its oxidized derivative. Inset: Structural models for each sample.
Experimental Section Sample Preparation. The pristine LiMn0.9Cr0.1O2 sample was prepared by heating a stoichiometric mixture of Li2CO3, Mn2O3, and Cr2O3 at 960 °C under argon atmosphere for 24 h with intermittent grinding.17 The chemical oxidation of the precursor LiMn0.9Cr0.1O2 powders was carried out by reacting 0.5 g of the pristine material with 40 mL of 0.5 M aqueous (NH4)2S2O8 solution in a 50-mL autoclave with a Teflon liner. The autoclave was heated at 200 °C for 24 h and then air-cooled to room temperature. The resulting black powders were washed thoroughly with distilled water and then dried in air. Sample Characterization. The crystal structures of all of the present materials were studied by powder X-ray diffraction (XRD) using Ni-filtered Cu KR radiation with a graphitediffracted beam monochromator. The chemical composition and thermal behavior of the obtained powders were probed using inductively coupled plasma (ICP) spectrometry and thermogravimetric analysis (TGA), respectively. The error range of the component ratios determined from ICP analysis is (0.01. The crystal morphology and cationic composition of the present samples were examined using field-emission scanning electron microscopy/energy-dispersive spectroscopy (FE-SEM/EDS) with a JEOL JSM-6700F instrument equipped with an energydispersive X-ray spectrometer. Also, high-resolution transmission electron microscopy (HR-TEM) images were collected on a Philips-CM200 microscope with an accelerating voltage 200 kV, together with selected-area electron diffraction (SAED) patterns. X-ray absorption near-edge structure (XANES) experiments were carried out at the Mn K-edge and the Cr K-edge using an extended X-ray absorption fine structure facility installed at the 7C beam line at the Pohang Accelerator Laboratory (PAL) in Pohang, Korea. The XANES spectra were collected at room temperature in transmission mode using gasionization detectors. All of the present spectra were calibrated by measuring the spectrum of Mn or Cr metal foil. The data analysis of the experimental spectra was done by the standard procedure reported previously.26 The electrochemical measurements were performed with a cell of Li/1 M LiPF6 in ethylene carbonate/diethyl carbonate (50:50 v/v)/active material, which was assembled in a drybox. The composite cathode was prepared by thoroughly mixing the active material (70%) with 20% acetylene black and 10% PTFE (polytetrafluoroethylene). All experiments were carried out in galvanostatic mode with a
Figure 2. FE-SEM images of (a) pristine LiMn0.9Cr0.1O2 and (b) its oxidized derivative.
Maccor multichannel galvanostat/potentiostat in the voltage range of 1.0-4.4 V at a constant current density of 20 mA/g. Results and Discussion Powder XRD, FE-SEM/EDS, and TEM/SAED Analyses. The evolution of the crystal structure of layered lithium manganese oxide upon chemical oxidation was examined by powder XRD analysis. Figure 1 shows powder XRD patterns of layered LiMn0.9Cr0.1O2 and its oxidized derivative. A reaction with 0.5 M (NH4)2S2O8 solution at 200 °C results in a phase transformation from the monoclinic-layered structure of LiMn0.9Cr0.1O2 to the pyrolusite (β-MnO2) structure with tetragonal symmetry, as illustrated in the inset of Figure 1. The observation of sharp and intense XRD peaks provides clear evidence of the high crystallinity of the obtained β-MnO2 phase. According to a least-squares fitting analysis, the lattice parameters were determined to be a ) 5.423 Å, b ) 2.828 Å, c ) 5.387 Å, and β ) 115.68° for pristine LiMn0.9Cr0.1O2 and a ) 4.400 Å and c ) 2.870 Å for β-Mn1-xCrxO2 nanowires. We probed the effect of chemical oxidation on the crystallite morphology of the pristine material using FE-SEM. As shown in Figure 2, the precursor LiMn0.9Cr0.1O2 consists of semispherical particles with a diameter of ∼5 µm, whereas the oxidation reaction results in the formation of nanowires with diameters of about 30 nm and lengths of several micrometers. Such a morphological transformation from microcrystals to nanowires was further confirmed by HR-TEM analyses, as shown in Figure 3. The dimension of the nanowires is in good agreement with the estimated value from the FE-SEM results. The nanowires show a characteristic SAED pattern that matches well with the pyrolusite (β-MnO2) structure, attesting to its single-crystalline nature. Recently, we found that a persulfate treatment for pristine layered LiMn0.9Cr0.1O2 gives rise not only to an increase of the Mn oxidation state but also to the partial disintercalation of interlayer lithium ions and the simultaneous intercalation of water molecules.16 As a consequence, a layered
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Figure 3. (a) High-resolution TEM image/SAED pattern and (b) EDS diagram for the β-Mn1-xCrxO2 nanowires.
δ-MnO2-structured phase with expanded interlayer spacing is formed.16 Because the interlayer interaction is much weaker for this hydrated phase than for the pristine compound, each manganese chromium oxide layer can be easily transformed into nanowires through a rolling and curling process. In this regard, we believe that the synthesis of β-MnO2 nanowires is initiated by the formation of an intermediate layered δ-MnO2 structure and the subsequent curling and folding of this intermediate phase.10 Of special interest is that, in comparison to R-MnO2or δ-MnO2-structured manganese oxide nanowires,14,16 the β-MnO2 nanowires appear to be more uniform in size and morphology. Although the mechanism for this morphological difference among the manganese oxide nanowires is not clear at present, the better uniformity of the β-MnO2 nanowires could be related to its higher preparation temperature. Under such synthesis conditions, the disproportionation of Mn3+ ions into insoluble Mn4+ and soluble Mn2+ ions is promoted, resulting in a considerable dissolution of manganese ions from the solidstate product. Such enhanced Mn dissolution and recrystallization of manganese oxide would be responsible for the observed uniform size and shape of the β-MnO2 nanowires. In fact, the higher Mn oxidation state in the β-MnO2 nanowires than in the other types of manganese oxide nanowires was experimentally evidenced by the Mn K-edge XANES results, as discussed in the corresponding section below. Elemental and Thermal Analyses. We tried to verify the incorporation of chromium ions into the manganese oxide lattice using EDS and ICP analysis. As illustrated in Figure 3, the EDS diagram clearly demonstrated the coexistence of Mn and Cr ions in the manganate nanowires. The ICP analysis showed that the present β-MnO2-structured manganate nanowires contain Cr ions with a content of 0.03 per formula unit. It was also found that no lithium ions remain in the nanowires, implying the oxidation of Mn and Cr ions to the tetravalent state upon persulfate treatment. From the viewpoint of crystal structure, the complete removal of lithium from the layered lithium manganate lattice leads to the destabilization of the layered lattice through an increasing electrostatic repulsion between negatively charged oxide ligands of MnO2 layers directed toward the interlayer space. To relieve the instability of the disintercalated structure and the electrostatic repulsion between oxide ligands, a fraction of manganese ions migrate into the interlayer space of the layered lattice, resulting in a phase transformation into the pyrolusite-type structure (inset of Figure 1). We investigated the thermal behavior of the β-Mn1-xCrxO2 nanowires using TGA. In Figure 4, the present nanowires display only a negligible weight loss in the temperature range of 100250 °C, strongly suggesting the absence of water molecules in this material. This result is in good agreement with our expectation based on the nonporous pyrolusite structure of the β-MnO2 phase. A distinct weight decrease corresponding to oxygen loss occurs at around 550-660 °C, resulting in the formation of a (Mn,Cr)2O3 phase. The observed amount of
Figure 4. TGA curve of the β-Mn1-xCrxO2 nanowires.
Figure 5. Mn K-edge XANES spectra for pristine LiMn0.9Cr0.1O2 (solid lines), β-Mn1-xCrxO2 nanowires (dashed lines), δ-HyMn1-xCrxO2 nanowires (dot-dashed lines), in comparison with those for the references Mn2O3 (dotted lines) and MnO2 (crosses). The inset presents an expanded view of preedge region for 6538-6545 eV.
weight loss in this temperature range (∼9.0%) is in good agreement with the theoretical value (∼9.2%) corresponding to the reduction process of 2(Mn0.97Cr0.03)O2 f (Mn0.97Cr0.03)2O3 + 1/2O2. Mn K-Edge and Cr K-Edge XANES Analyses. The local crystal structure and electronic configuration of metal ions in the β-Mn1-xCrxO2 nanowires were investigated using XANES analysis at the Mn and Cr K-edges. Figure 5 shows the Mn K-edge XANES spectra of pristine LiMn0.9Cr0.1O2 and β-Mn1-xCrxO2 nanowires, in comparison to those of δ-HyMn1-xCrxO2 nanowires, Mn+32O3, and Mn4+O2.27 Whereas the pristine LiMn0.9Cr0.1O2 compound shows an edge position similar to that of the reference Mn+32O3, the edge energy of the β-Mn1-xCrxO2 nanowires is nearly identical to that of the reference Mn4+O2. This clearly demonstrates the increase of the Mn oxidation state upon persulfate treatment, as suggested from the ICP results. As can be seen clearly from the inset of Figure 5, the β-Mn1-xCrxO2 nanowires exhibit weak preedge peaks P and P′, corresponding to the 1s f 3d transitions.28-30 Because these transitions are not allowed for centrosymmetric octahedral symmetry, the observed weak intensity of these features indicates the stabilization of the Mn ions in the octahedral site.31 Taking into account the fact that the intensity of the higherenergy peak P′ is proportional to the average oxidation state of the manganese ion,30 the similar intensities of this feature for the β-Mn1-xCrxO2 nanowires and the reference Mn4+O2 can be regarded as further evidence for the tetravalent oxidation state of manganese ions in these materials. In the main-edge region,
Promising Electrode Performance of β-MnO2-Type Nanowires
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Figure 6. Cr K-edge XANES spectra for pristine LiMn0.9Cr0.1O2 (solid lines), β-Mn1-xCrxO2 nanowires (dashed lines), the δ-HyMn1-xCrxO2 nanowires (dot-dashed lines), in comparison with those for the references Cr2O3 (dotted lines) and CrO3 (crosses).
the intense and sharp resonance peak B corresponding to the dipole-allowed 1s f 4p transition can be observed at ∼6558 and ∼6563 eV for pristine LiMn0.9Cr0.1O2 and δ-HyMn1-xCrxO2 nanowires, respectively. This feature becomes broader and weaker for the present β-Mn1-xCrxO2 nanowires. It is wellknown that the sharpness and intensity of this peak are proportional to the relative proportion of edge-sharing to cornersharing MnO6 octahedra.28-30 In this regard, the observation of a sharp and intense feature B for LiMn0.9Cr0.1O2 and δ-HyMn1-xCrxO2 is consistent with their layered crystal structure composed of a network of edge-shared MnO6 octahedra. In contrast, the broadening of this peak upon chemical oxidation confirms the structural modification from the layered structure to the pyrolusite-type structure consisting of corner- and edge-sharing of MnO6 octahedra. Also, Cr K-edge XANES analysis was carried out for the pristine LiMn0.9Cr0.1O2 compound, the β-Mn1-xCrxO2 nanowires, and some references. As shown in Figure 6, the chemical oxidation process for the pristine compound leads to a blue shift in the edge position, and hence, the edge energy of the β-Mn1-xCrxO2 nanowires is higher than that of Cr3+2O3 but lower than that of Cr6+O3, strongly suggesting the tetravalent oxidation state of chromium ions in this material. In the preedge region, the pristine LiMn0.9Cr0.1O2 and the nanowires show a small peak P corresponding to the 1s f 3d transition, indicating the octahedral local symmetry of chromium ions. In contrast, the reference CrO3 with tetrahedral Cr6+ ions displays a very intense preedge peak P, because this transition becomes partially allowed in the tetrahedral geometry through d-p orbital mixing. The energy of the preedge peak P is slightly higher for the β-Mn1-xCrxO2 nanowires than for pristine LiMn0.9Cr0.1O2, confirming the increase of Cr oxidation state upon persulfate treatment. As for the Mn K-edge region, the main-edge peak B corresponding to the dipole-allowed 1s f 4p transition shows a broad feature for the β-Mn1-xCrxO2 nanowires, which is fairly distinguishable from the sharp spectral features of pristine LiMn0.9Cr0.1O2 and the δ-HyMn1-xCrxO2 nanowires. The observed similar characteristics of the main-edge feature B for the Mn and Cr K-edge XANES regions provide strong evidence for the fact that both Mn and Cr ions indeed exist in the same crystal sites of the pyrolusite structure. Electrochemical Measurement. We tested the electrode performance of the β-Mn1-xCrxO2 nanowires in comparison with that of pristine LiMn0.9Cr0.1O2, in order to probe the influences of nanowire formation and phase transformation on the electrochemical properties of the bulk metal oxide. As plotted in Figure 7, the present β-Mn1-xCrxO2 nanowires provide a larger discharge capacity (345 mAh/g for the second cycle, 329 mAh/g
Figure 7. Potential profiles of (a) pristine LiMn0.9Cr0.1O2 and (b) β-Mn1-xCrxO2 nanowires for the 2nd (solid lines), 10th (dashed lines), and 30th cycles (dot-dashed lines).
Figure 8. Dependence of discharge capacity on number of cycles for pristine LiMn0.9Cr0.1O2 (triangles), Cr-substituted β-Mn1-xCrxO2 nanowires (circles), unsubstituted β-MnO2 nanowires (squares), and bulk β-MnO2 microcrystal (diamonds). The error range of the discharge capacity is (2 mAh/g.
for the 30th cycle) with a wider 3 V plateau than the pristine LiMn0.9Cr0.1O2 microcrystal. More importantly, as the cycle proceeds, the discharge capacity of the nanowires in the higherpotential region of 2-4.4 V becomes larger (169 mAh/g for the second cycle, 223 mAh/g for the 10th cycle, 213 mAh/g for the 30th cycle). This behavior is contrasted with the decreasing capacity of the pristine layered material in this potential region (Figure 7a), verifying the usefulness of nanostructure formation in improving the electrode activity of manganese oxide. For comparison, we measured the electrode performance of the unsubstituted binary β-MnO2 nanowires synthesized by a previously reported hydrothermal procedure.32 As can be seen from Figure 8, the Cr-substituted β-Mn1-xCrxO2 nanowires display less prominent capacity loss during the initial cycles and a larger discharge capacity than their unsubstituted counterparts. In this regard, the positive effect of Cr substitution appears obvious, although the improvement of electrode performance upon Cr substitution is not as large because of the low content of Cr substituent.18 In fact, a similar improvement upon Cr substitution has been reported for δ-MnO2- and R-MnO2-structured manganese oxide nanowires,16 reflecting the universal usefulness of Cr substitution. In light of the positive effect of Cr substitution, we tried to synthesize manganese oxide
5164 J. Phys. Chem. C, Vol. 112, No. 13, 2008 nanowires with higher Cr contents by adopting layered Li1.2Mn0.4Cr0.4O2 as a precursor. However, a persulfate treatment for this precursor under hydrothermal conditions failed to produce nanowire-type manganese oxide. This result seems to suggest that a higher concentration of manganese ion in the precursor is necessary for the formation of nanowire-type compounds. On the other hand, the present experimental findings underscore that, in terms of a large discharge capacity and the flatness of operating potential, the present pyrolusite- (β-MnO2-) structured nanowires are much more promising as an electrode material than other structure-type manganese oxide nanowires previously reported;6,33 for instance, δ-MnO2-type manganate nanowires show a much smaller discharge capacity of ∼220 mAh/g for the 10th cycle under the same conditions. In contrast to the present results, Cheng et al. reported that the 1D nanostructure of β-MnO2 with an interconnected superstructure exhibits poor electrode performance for lithium ion cells, i.e., a small discharge capacity of
