Atomic and Electronic Structures of Li0.44MnO2 Nanowires and

Oct 8, 2010 - Recently, Hosono et al. have synthesized high-quality single-crystalline LiMn2O4 nanowires using a phase-conversion technique with ...
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J. Phys. Chem. C 2010, 114, 18358–18365

Atomic and Electronic Structures of Li0.44MnO2 Nanowires and Li2MnO3 Byproducts in the Formation Process of LiMn2O4 Nanowires Jun Kikkawa,*,†,‡ Tomoki Akita,† Eiji Hosono,§ Haoshen Zhou,*,§ and Masanori Kohyama† Research Institute for Ubiquitous Energy DeVices, National Institute of AdVanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan, and Energy Technology Research Institute, AIST, 1-1-1 Umezono, Tsukuba 305-8568, Japan ReceiVed: July 4, 2010; ReVised Manuscript ReceiVed: August 31, 2010

High-quality single crystalline LiMn2O4 nanowires, which can be synthesized by the conversion of Na0.44MnO2 nanowires, are promising electrode materials for high-power lithium ion batteries. Understanding the conversion mechanism is crucial for further improvement of the quality of LiMn2O4 nanowires. In this paper, using advanced techniques of transmission electron microscopy and electron energy-loss spectroscopy, we investigate the atomic and electronic structures of both Li0.44MnO2 nanowires and its byproduct formed after the conversion of Na0.44MnO2 nanowires into Li0.44MnO2 nanowires, as the first half of the process in the conversion of Na0.44MnO2 nanowires into LiMn2O4 nanowires. Results show that Li0.44MnO2 nanowires have a well-defined single-crystalline nature. The byproduct is identified as nanoparticles of Li2MnO3 (space group P3112) different from conventional Li2MnO3 (space group C2/m), formed on the surfaces of Li0.44MnO2 nanowires with the specific crystallographic relationship. The formation mechanism of Li2MnO3 nanoparticles and their role in the conversion of Li0.44MnO2 nanowires into LiMn2O4 nanowires are discussed. Introduction Rechargeable lithium ion batteries, which are widely used in portable electronic equipment, are anticipated to be used as power sources of electric vehicles (EVs) in the near future. Many studies have specifically examined electrode materials with higher energy density, higher power density, longer life, and better safety to achieve higher-performance batteries.1 One of the main subjects is the increase of the power density, which enables rapid charge-discharge cycles with high capacities. Reducing the size of electrode materials to nanometer scale can improve the power density because of nanosize effects: a larger electrode-electrolyte contact area and shorter diffusion lengths for Li ions in crystals.2 Especially, nanowires attract much interest as high power density electrode materials3-7 because nanowires have efficient charge transport along their axes in addition to the nanosize effects. In this context, the development of synthesis techniques for nanowire electrode materials is an important topic. LiMn2O4 is a promising positive electrode material because it is inexpensive, environmentally friendly, and more electrochemically stable than the LiCoO2 used in current portable batteries.8,9 LiMn2O4 has a cubic spinel structure in which the Li ions occupy the tetrahedral A site, and the Mn ions occupy the octahedral B site, forming edge-sharing MnO6 octahedral chains. However, the synthesis of single crystalline LiMn2O4 nanowires has been a challenging mission because the one-dimensional growth of metal oxides with cubic structures is difficult. Recently, Hosono et al. have synthesized high-quality single-crystalline LiMn2O4 nanowires using a phase-conversion technique with Na0.44MnO2 nanowires as a template.10 For this technique, Na0.44MnO2 nanowires with a * To whom correspondence should be addressed. E-mail: kikkawa@ ee.es.osaka-u.ac.jp (J.K.), [email protected]. (H.Z.). † Research Institute for Ubiquitous Energy Devices, AIST. ‡ Present address: Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan. § Energy Technology Research Institute, AIST.

tunnel structure are first converted to Li0.44MnO2 nanowires using a Na/Li ion-exchange method. They are then converted to LiMn2O4 nanowires with the spinel structure by sintering. The structural difference between Li0.44MnO2 and LiMn2O4 is confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The tunnel-spinel transformation technique produces LiMn2O4 nanowires with much higher aspect ratios than those of LiMn2O4 nanorods, which are synthesized from β-MnO2 nanorods.11 The high-quality LiMn2O4 nanowires actually exhibit a higher-rate charge-discharge cycling performance than commercial LiMn2O4 powders of several hundred nanometers to several micrometers in size.10 The phase conversion of Li0.44MnO2 nanowires into LiMn2O4 nanowires with high crystallinity is a key issue in the present technique as an innovation for battery technology; understanding the mechanism underlying this conversion is crucial for further improvement of the quality of LiMn2O4 nanowires. XRD and selected-area electron diffraction revealed the coexistence of both Li0.44MnO2 nanowires and its byproduct, considered to be Li2MnO3, after the ion-exchange process.10 Results of that study suggest that the byproduct phase plays a key role for conversion of Li0.44MnO2 nanowires into high-quality LiMn2O4 nanowires. To clarify the phase conversion mechanism, it is necessary to clarify the details of morphology, the interior structures, and the electronic structures of both the Li0.44MnO2 nanowire and the byproduct. In this study, we investigate atomic and electronic structures of both Li0.44MnO2 nanowires and the byproduct resulting from the conversion process of Na0.44MnO2 nanowires into Li0.44MnO2 nanowires, using high-resolution TEM (HRTEM), nanobeam electron diffraction (NBED), and scanning transmission electron microscopy-electron energy-loss spectroscopy (STEM-EELS). Investigating Li0.44MnO2 nanowires is necessary because there is no detailed experimental study on their atomic and electronic structures, although Li0.44MnO2-based materials are also applicable to high-rate lithium ion batteries.12-14 We discuss both

10.1021/jp1061732  2010 American Chemical Society Published on Web 10/08/2010

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the formation mechanism of the byproduct and their role in the subsequent conversion of Li0.44MnO2 nanowires into LiMn2O4 nanowires. Experimental Methods Sample Synthesis. A commercial Mn3O4 powder (0.1 g) was dispersed into a NaOH aqueous solution of 5 mol dm-3 (40 mL), which was placed in a Teflon-lined autoclave with capacity of 45 mL. The autoclave was heated at 205 °C for 4 days to synthesize Na0.44MnO2 nanowires. The Na0.44MnO2 nanowires were washed repeatedly in deionized water. The washed Na0.44MnO2 nanowires were dried at room temperature under vacuum conditions. The Na/Li ion exchange and the addition of more lithium were performed in molten salts of LiNO3 (88 mol %) and LiCl (12 mol %) at 450 °C for 1-2 h. Then this sample was washed repeatedly in deionized water and dried at room temperature under vacuum conditions. The sample was dispersed on a copper mesh grid with carbon films for electron microscopy analyses. Electron Microscopy and XRD Analyses. A transmission electron microscope (JEM3000F; JEOL) with a Gatan imaging filter was used for HRTEM, NBED, and STEM-EELS analyses at an accelerating voltage of 297 kV. The NBED patterns were obtained with parallel electron probes of 30-50 nm diameter using a small condenser aperture of 20 µm diameter.16,17 For the STEM-EELS measurements, an electron probe (ca. 1 nm) was scanned across the objects, acquiring an EELS spectrum at each probe position. Respective settings of the energy dispersion and acquisition time of each EELS spectrum were 0.1 eV pix-1 and 0.5 s [Figures 5b and S3b (Supporting Information)], 0.1 eV pix-1 and 2.0 s (Figure 7) and 0.3 eV pix-1 and 2.0 s [Figures 6a and S3c (Supporting Information)]. Convergence semiangles were set at 10 mrad. Collection semiangles were set at 5 mrad [Figures 5b and S3b (Supporting Information)] and 12 mrad [Figures 6a, 7, and S3c (Supporting Information)]. The energy resolution was about 2.7 eV [Figures 6a and S3c (Supporting Information)] and 1.6 eV [Figures 5a, 7, and S3b (Supporting Information)]. EELS spectra from a reference material, monoclinic Li2MnO3 with the space group of C2/m prepared as in ref 18, were obtained under similar experimental conditions. DigitalMicrograph software was used for EELS data processing. For quantitative elemental analysis [Figures 6b and S3d (Supporting Information)], the modified hydrogenic model for the white line and the Hartree-Slater model were used, respectively, to evaluate the partial differential cross sections of Mn-L2,3 and O-K edges. CrystalKit software was used for electron diffraction simulations. XRD profiles were measured with Cu KR radiation by an XRD diffractometer (D8 Advance; Bruker AXS) whose goniometer is equipped with an optical encoder and verified with a standard alumina: the position of the detector is reproduced and certified within 0.01°. Crystal models (Figures 2, 9, 10e, and 11) were drawn with Visualization of Crystal Structures (VICS) software. Results and Discussion

Figure 1. (a) Bright-field TEM image of Li0.44MnO2 nanowires. (b) HRTEM image of a single Li0.44MnO2 nanowire viewed along the [530]ort direction showing that the nanowire is extended in the [001]ort direction. (c) HRTEM image and (d) NBED pattern of a single Li0.44MnO2 nanowire viewed along the [111j]ort direction. (e) Simulated ED pattern of Li0.44MnO2 with the crystal structure (Figure 2).

Structural Analysis of Li0.44MnO2 Nanowires. A brightfield TEM image of Li0.44MnO2 nanowires after the Na/Li ion-exchange in Na0.44MnO2 nanowires is shown in Figure 1a. The single Li0.44MnO2 nanowires have diameters of around 20-100 nm (Figure S1, Supporting Information), and bundled Li0.44MnO2 nanowires have diameters of several hundred nanometers (Figure S2, Supporting Information). These Li0.44MnO2 nanowires have lengths of several hundred micrometers. Figure 1b shows an HRTEM image of a single

Li0.44MnO2 nanowire viewed along the [530]ort direction, where the subscript “ort” denotes an orthorhombic cell (Figure 2). Lattice fringes of the (002)ort plane are clearly observed in thin areas near the edge of the Li0.44MnO2 nanowire, although lattice fringes of the (001)ort plane are predominantly observed inside the nanowire (Figure 1b). The Li0.44MnO2 nanowires are extended in the [001]ort direction (Figure 1b). Figure 1c also shows an HRTEM image of a single Li0.44MnO2 nanowire

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Figure 2. Schematic drawing of the crystal structure of Li0.44MnO2 (space group Pbam), where the orthorhombic unit cell is shown as the solid line.

viewed along the [111j]ort direction, where the angle between the electron incident direction and the nanowire axis, the [001]ort direction, is set to be about 125°. Figure 1d shows a NBED pattern of the Li0.44MnO2 nanowire obtained from a region containing the HRTEM observation area (Figure 1c), which certainly corresponds to the simulated [111j]ort electron diffraction (ED) pattern of Li0.44MnO2 with the space group Pbam.12-14 The crystal structure of Li0.44MnO2 is illustrated in Figure 2. The Li0.44MnO2 comprises sheets of edge-sharing MnO6 octahedra and columns of edge-sharing MnO5 square pyramids, both of which have M-O frameworks lying parallel to the nanowire axis forming one-dimensional tunnels of two kinds: narrow tunnels and S-shaped wide tunnels.12,13 The Li ions are located in these tunnels (Figure 2). The charge neutrality for Li0.44MnO2 suggests that the presence of valence fluctuation of Mn ions and the ratio of Mn3+ to Mn4+ ions should be 0.44-0.56. Figure 3a shows the XRD pattern from the sample after the Na/Li ion exchange. The (140)ort lattice spacing of the Li0.44MnO2 nanowires is estimated as 0.5013 nm by Gaussian fitting of the 140ort peak (Figure 3b), the value of which corresponds to the (140)ort lattice spacing of 0.5012 nm for bulk Li0.44MnO2 with the lattice parameter of aort ) 0.890 79 nm, bort ) 2.425 32 nm, cort ) 0.282 78 nm.14 Because the (140)ort lattice spacing varies sensitively depending on Li content in the tunnels,12 we can say with certainty that the nanowires have a Li0.44MnO2 composition. We were also able to confirm the excellent homogeneity of the chemical composition along the Li0.44MnO2 nanowires axis by EELS (Figure S3, Supporting Information). The EELS analysis demonstrates the ideal Mn/O ratio of Li0.44MnO2. The combination of HRTEM, NBED, XRD, and EELS analyses showed clearly that Li0.44MnO2 nanowires have a welldefined single-crystalline nature, and no precipitation with different crystal structures exists in Li0.44MnO2 nanowires. However, nanometer-sized particles that adhere to the faces of Li0.44MnO2 nanowires are observed irregularly, as shown in the annular dark-field (ADF) STEM images (Figure 4). EELS Analysis of the Byproduct Particles. To investigate the chemical composition of nanoparticles on Li0.44MnO2 nanowires, a series of EELS spectra were acquired successively by scanning an electron probe across both a single Li0.44MnO2 nanowire and a byproduct nanoparticle, as shown in Figure 5a: the measurement positions are indicated by i-ix. The relative thickness19 t/λ at the positions of the nanoparticle and nanowire

Figure 3. (a) XRD profile of the sample containing Li0.44MnO2 nanowires and byproduct after Na/Li ion exchange in Na0.44MnO2 nanowires. Indexes show representative peaks from Li0.44MnO2 nanowires. (b) Enlarged 140 peak in part a, where a Gaussian fitting line with the maxima at 17.69 is illustrated.

Figure 4. (a and b) ADF-STEM images of nanoparticles on the faces of Li0.44MnO2 nanowires.

(Figure 5a) are estimated as around 0.24, which is sufficiently thin for analyzing each EELS spectrum. Figure 5b shows raw EELS spectra including Mn-M2,3 and Li-K edges at positions i-ix (Figure 5a), where EELS spectra of the reference material, Li2MnO3 (space group C2/m),18 are also indicated. The MnM2,3 and Li-K edges originate in electron transitions from Mn3p states to Mn-4d states and from Li-1s states to Li-2p states, respectively. In Figure 5b, it is clear that spectrum shapes of both Mn-M2,3 and Li-K edges at positions v-ix on the nanoparticle closely resemble those of the reference Li2MnO3. Consequently, the local electronic structures around Mn-3p, 4d and Li-1s, 2p states are similar for both the nanoparticle and the reference Li2MnO3. Furthermore, the Li content relative to the Mn contentsthe Li/Mn ratio of the nanoparticlesis considered to be about 2, as is the Li/Mn ratio of Li2MnO3. Although spectrum shapes of Mn-M2,3 edges at positions i-iii on the Li0.44MnO2 nanowire also resemble those of the reference Li2MnO3, the Li-K edges at positions i-iii on the Li0.44MnO2 are weaker and broader than those from the reference Li2MnO3 because of the smaller content of Li in Li0.44MnO2 nanowires than that in Li2MnO3. The EELS spectrum at the position iv reflects Mn-M2,3, and Li-K edges originating from both the Li0.44MnO2 nanowire and the nanoparticle, which overlap along the brighter area in Figure 5a. Figure 6a shows a series of EELS spectra within an energyloss region including both O-K and Mn-L2,3 edges at positions

Formation Process of LiMn2O4 Nanowires

Figure 5. (a) ADF-STEM image of a single Li0.44MnO2 nanowire and a single nanoparticle. (b) A series of STEM-EELS spectra including Mn-M2,3 and Li-K edges acquired at each probe position from i to ix are shown as the circles in part a with EELS spectra of the reference Li2MnO3.

i-ix in Figure 5a. Both background signals and plural scatterings are removed using conventional methods,19 and the spectrum intensity is normalized by the peak R at 530.9 eV (Figure 6a). The intensities of peaks β and γ at positions v-ix are larger than those at i-iii, although the peak intensities of the Mn-L2,3 edges at v-ix are lower than those at i-iii. The Mn/(Mn + O) ratio was estimated roughly from the EELS spectrum and is shown in Figure 6b: the error bars show errors of 10%, due to uncertainties in the calculation of the cross sections.19 The Mn/ (Mn + O) ratios at positions v-ix on the nanoparticle are smaller than those at i-iii on the Li0.44MnO2 nanowire. The experimental Mn/(Mn + O) ratios of both Li0.44MnO2 nanowires and nanoparticles range around values of theoretical ratios: 0.33 for Li0.44MnO2 and 0.25 for Li2MnO3, respectively, within the acceptable error range. As shown in the inset in Figure 6a, the Mn-L2,3 edges from the Li0.44MnO2 nanowire arise at a slightly lower energy-loss position than those from the nanoparticle, which indicates that the Mn valence state of the nanoparticle is higher than that of the Li0.44MnO2 nanowire, i.e., +3.56 on average.20 Figure 7 shows EELS O-K edges acquired at positions i-ix in Figure 5a and from the reference Li2MnO3. The EELS O-K edges of both the Li0.44MnO2 nanowire and the nanoparticle have three main peaks indicated as R, β, and γ (Figure 7). It should be first remarked that the EELS O-K edges at positions v-ix on the nanoparticle have shapes similar to those from the reference Li2MnO3. From the EELS analyses of Li-K, Mn-M2,3, Mn-L2,3, and O-K edges, we therefore conclude that nanoparticles on Li0.44MnO2 nanowires have both a chemical composition and an electronic structure that closely resemble those of

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Figure 6. (a) A series of STEM-EELS spectra acquired from probe positions i-ix are shown by the circles in Figure 5a. The spectrum intensity is normalized by the peak R at the O-K edge. The inset shows a magnification of the Mn-L2,3 edges, indicating the blue-shift of the Mn-L2,3 edges at i-iii from those at v-ix. (b) Calculated Mn/(Mn + O) ratios at positions i-ix. Error bars show 10% uncertainties for EELS quantitative analysis. Dotted lines show the Mn/(Mn + O) values calculated from the chemical formula of Li0.44MnO2 and Li2MnO3.

Figure 7. EELS O-K edges acquired at positions i-ix in Figure 5a and from the reference Li2MnO3.

the reference Li2MnO3 within the energy resolution; the details of the crystal structure are analyzed in the next section. Further on, we shall refer to the structural difference of EELS O-K edges in the case of nanoparticles and Li0.44MnO2 nanowires. In Figure 7, the threshold energy, indicated by “th”, and the shape of the peak R are common for the EELS O-K edges at positions i-iii (Li0.44MnO2 nanowire) and v-ix (nanoparticle), indicating that the binding energy of O-1s orbitals in both Li-Mn oxides has the same level. The maxima of the peaks β and γ at positions v-ix are respectively located at 542.1-542.9 eV and 566.4 eV, which are, respectively, lower energy-loss positions than those at i-iii: about 544.0-544.5 and 570.1 eV. The peak β can be attributed mainly to electron transitions from

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Figure 8. (a and b) NBED patterns of nanoparticles on Li0.44MnO2 nanowires. Simulated ED patterns of Li2MnO3 (C2/m-type) in parts c and d and Li2MnO3 (P3112-type) in parts e and f with electron incident directions respectively corresponding to parts a and b.

O-1s states to O-2p orbitals hybridized with Mn-4sp and presumably Li-2sp orbitals.21,22 Consequently, the result indicates that the energy gap separating O-1s states and the hybridized O-2p states in Li0.44MnO2 nanowires is larger than that of the nanoparticle or the reference Li2MnO3. The peak γ is a part of the extended energy-loss fine structure (EXELFS), which corresponds to the extended X-ray-absorption fine structure (EXAFS).19 Oscillations in EXELFS arise due to interference effects resulting from the multiple scattering of the excited electron toward the neighboring atoms and thus sensitive to local structures. We consider that the peak γ can be ascribed to resonant scattering between the excited oxygen atom and its nearest-neighbor oxygen atoms, because oxygen ions can be strong backscatters and give rise to potential cage or barriers for electron scattering, as has been discussed in previous studies on transition metal oxides.21 Kurata et al. reported that higher energy-loss values of EXELFS peaks above the threshold energy reflect smaller first-neighbor O-O distances in transition-metal oxides with rock-salt or rutile structures according to the EELS analysis.21 The rocksalt and rutile structures are consisting of sheets of edge-sharing MnO6 octahedra, which are main building blocks for tunnel Li0.44MnO2 nanowires. In Figure 7, the energyloss values of the peak γ maxima above the threshold energy defined as ∆Eγ are estimated as 38.8-39.6 eV for the EELS spectra at v-ix and from the reference Li2MnO3, whereas the ∆Eγ value for the EELS spectra at i-iii was estimated as 41.9-42.5 eV. The first-neighbor O-O distance in Li0.44MnO2 is 0.245-0.285 nm,22 whereas the O-O distance in Li2MnO3 (C2/m) is 0.274-0.276 nm.18 The presence of shorter O-O distances in Li0.44MnO2 can generate larger ∆Eγ values than those of the Li2MnO3 (C2/m), although it is not easy to fully interpret of the peak γ at present, because of the complex structure of Li0.44MnO2 with the edge-sharing MnO5 square pyramids (Figure 2). Structural Analysis of Li2MnO3 Nanoparticles. Figure 8a,b shows typical NBED patterns obtained from nanoparticles on

Li0.44MnO2 nanowires. Simulated ED patterns of well-known monoclinic Li2MnO3 with a space group of C2/m are shown in Figure 8c,d, where electron incident directions correspond to those in parts a and b of Figure 8, respectively. Although the NBED pattern (Figure 8a) corresponds to the simulated [101j]mon ED pattern (Figure 8c), the extra reflections in Figure 8b cannot be explained by C2/m-type Li2MnO3 (Figure 8d). The experimental NBED patterns are explainable by hexagonal Li2MnO3 with a space group of P3112,23-25 as shown in the simulated ED patterns in Figure 8e,f. Both C2/m-type and P3112-type Li2MnO3 (also written as Li{Li1/3Mn2/3}O2) comprise alternative stacking of Li and {Li1/3Mn2/3} layers separated by close-packed oxygen arrays, as shown in Figure 9a. The basic difference between C2/m-type and P3112-type Li2MnO3 can be seen in the stacking sequence of the {Li1/3Mn2/3} layer along the equivalent directions, i.e., [103]mon and [001]hex directions.23-25 This stacking difference is explainable with a single {Li1/3Mn2/3} layer (Figure 9b). In C2/m-type Li2MnO3, the position of the Li atom at the A site in the first {Li1/3Mn2/3} layer shifts to the D site in the second {Li1/3Mn2/3} layer with the transfer vector of 1/3(bhex + chex). This shifts to the E site in the third {Li1/3Mn2/3} layer and shifts to the F site in the fourth {Li1/3Mn2/3} layer (Figure 9b). In P3112-type Li2MnO3, the position of the Li atom at the A site in the first {Li1/3Mn2/3} layer shifts to the B site in the second {Li1/3Mn2/3} layer with the transfer vector of 1/3(-ahex - bhex + chex), shifts to the C site in the third {Li1/3Mn2/3} layer with the transfer vector of 1/3(ahex + chex), and then goes back to the A site in the fourth {Li1/3Mn2/3} layer with 1/3(bhex + chex) (Figure 9b). It must be noted that discrimination between P3112-type and C2/m-type in Li2MnO3 nanoparticles was achieved for the first time using the NBED technique in this study. Layered rock-salt structures with P3112 symmetry have been observed in other lithium transition-metal oxides such as LiNi0.5Mn0.5O2 and LiNi1/3Mn1/3Co1/3O2.23-26 Returning to the EELS analysis, the EELS spectra show that both C2/m-type and P3112-type Li2MnO3 crystals have similar

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Figure 9. Schematic drawings of (a) the crystal structure of Li2MnO3 (P3112-type) and (b) the single Li1/3Mn2/3 layer in Li2MnO3 (C2/mand P3112-type).

electronic structures in spite of the different stacking sequence of the {Li1/2Mn2/3} layers, which is consistent with statements in a recent report of a first principles study.27 The crystallographic orientation relationship between P3112type Li2MnO3 nanoparticles and Li0.44MnO2 nanowires was also investigated by NBED. Figure 10b shows a NBED pattern obtained from the region including both a Li2MnO3 nanoparticle and a Li0.44MnO2 nanowire as indicated by the larger circle b in Figure 10a. Figure 10c,d show NBED patterns obtained from the smaller circle c within the Li2MnO3 nanoparticle and the dotted circle d within the Li0.44MnO2 nanowire, respectively. Crystal structures of P3112-type Li2MnO3 and Li0.44MnO2 viewed from the electron incident direction are drawn in Figure 10e. A set of NBED patterns (Figure 10b,c) indicates that the reciprocal lattice vectors, g030 in Figure 10c and g002 in Figure 10d, are parallel and have similar vector magnitudes: the NBED spots of 030hex (Figure 10c) and 002ort (Figure 10d) are very close to each other and overlap, as the arrows in Figure 10b indicate. Looking into the NBED spots in Figure 10b, we found that 030hex and 03j0hex spots are located slightly closer to the center as compared to 002ort and 002jort spots. Thus, the lattice spacing of the (002)ort plane is slightly smaller than that of the (030)hex plane. In the real space, the [120]hex axis of Li2MnO3 nanoparticles is parallel to the Li0.44MnO2 nanowire axis, [001]oth (Figure 10e). The triangular shapes of the Li2MnO3 nanoparticles in the ADF-STEM and TEM images (Figures 4 and 10a) can be attributed to the lower energy of the (112)hex and (12j2)hex close-packed surfaces (Figure 10e). In any case, a specific relationship of crystallographic orientation exists between Li2MnO3 nanoparticles and Li0.44MnO2 nanowires, which suggests the formation of a specific chemical bonding at the Li2MnO3/Li0.44MnO2 interface, although the interface structure is unclear at present.

Figure 10. (a) TEM image of Li2MnO3 nanoparticles on a Li0.44MnO2 nanowire. NBED patterns obtained from areas indicated by the larger circle (b), smaller circle (c), and dotted circle (d), respectively, in part a. The arrows in part b show overlapping of the 030hex spot in part c with the 020ort spot in part d. (e) Schematic drawing of the crystallographic orientation relationship between the Li2MnO3 (P3112-type) nanoparticle and Li0.44MnO2 nanowire.

Formation Mechanism of Li2MnO3 Nanoparticles and Their Role in Phase Conversion. In this section, we shall briefly discuss the formation mechanism of Li2MnO3 nanoparticles and their role in the conversion of Li0.44MnO2 nanowires into LiMn2O4 nanowires. First, experiments show that it is likely that Li2MnO3 nanoparticles on a Li0.44MnO2 nanowire are formed by partial oxidization of the nanowire rather than by complete oxidization of other nanowires in a molten salt of LiNO3 and LiCl at 450 °C during the Na/Li ion-exchange process. This oxidization process is expressed as follows:10

2Li0.44MnO2 + 3.12LiNO3 f 2Li2MnO3 + 3.12NO + 2.12O2v (1) Because Li2MnO3 nanoparticles are not precipitated inside the nanowires, the oxidation occurs on all the surfaces of the Li0.44MnO2 nanowires, resulting in the formation of chemical species of Li, Mn, and O at the nanowire surfaces or in the solution. These chemical species diffuse along the nanowire surfaces or in the solution. The nuclei of Li2MnO3 are formed at certain positions on the nanowire surfaces because of the fluctuation of local composition, temperature, and pH. The nuclei have a specific crystallographic orientation with respect to the Li0.44MnO2 nanowires, i.e., [120]hex (Li2MnO3)//[001]ort (Li0.44MnO2), forming energetically favorable Li2MnO3/ Li0.44MnO2 interfaces with the lower density of dangling bonds. In addition, the nuclei grow with the stable (112)hex

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Figure 11. Schematic drawings of the crystal models of Li0.44MnO2 and LiMn2O4 viewed along each nanowire axis. The rectangle in the model represents the projected unit cell. The dotted enclosure in the Li0.44MnO2 represents a sheet of MnO6 octahedra, which possibly becomes a part of the framework in the LiMn2O4.

and (12j 2)hex facets and finally their surface energy decreases, resulting in the triangular shapes from the projected images (Figures 4 and 10a). The phase conversion of Li0.44MnO2 nanowires into LiMn2O4 nanowires is achieved by sintering at 800 °C for 1 h.10 Tunnel Li0.44MnO2 nanowires should be directly transformed into spinel LiMn2O4 nanowires, because one-dimensional growth of LiMn2O4 with a cubic structure does not occur even if the LiMn2O4 nuclei are formed, apart from the Li0.44MnO2 nanowires or through the structural transformation of the byproduct Li2MnO3 nanoparticles. Partial tunnel-spinel transformation at 500 °C was previously reported by Doeff et al., although the transformation mechanism was not presented in detail.28 In our method, the spinel LiMn2O4 nanowires are extended in the [011]cub direction.10 Cross-sectional crystal models of both Li0.44MnO2 and LiMn2O4 nanowires are drawn in Figure 11. The tunnel-spinel transformation requires the MnO6 octahedral to rotate on the nanowire axis, and the transformation mechanism is complicated in comparison to other ramsdellitespine1,29 orthorhombic-spine1,30 and rutile-spinel31 transformations in manganese oxide materials. Aligned MnO6 sheets as indicated by an arrow in Figure 11 can be parts of the framework of the spinel structure. The mechanism of tunnel-spinel structural transformation is scientifically interesting and will be certainly studied in the near future. In the previous study, it was confirmed experimentally that sintering of a single phase of Li0.44MnO2 causes the formation of Mn3O4 precipitates in the LiMn2O4 phase:10

25Li0.44MnO2 f 11LiMn2O4 + Mn3O4 + O2v

(2)

A pure phase of single-crystalline LiMn2O4 nanowires is formed only by sintering of Li0.44MnO2 nanowires with Li2MnO3 nanoparticles. Consequently, Li2MnO3 nanoparticles can react with constituents of Mn3O4, resulting in the disappearance of both Li2MnO3 byproduct and Mn3O4 precipitates and the formation of the spinel LiMn2O4 phase:10

Li2MnO3 nanoparticles formed on Li0.44MnO2 nanowires during the Na/Li ion exchange process play a crucial role in the synthesis of high-quality single-crystalline LiMn2O4 nanowires. The ideal ratio of Li2MnO3 nanoparticles to Li0.44MnO2 nanowires is expected to be 1/25, as expressed in the total reaction10

25Li0.44MnO2 + Li2MnO3 f 13LiMn2O4 + 1/2O2

(4) The control of the amount of Li2MnO3 nanoparticles is important in a practical sense for additional improvement of the quality of LiMn2O4 nanowires. Conclusion Using HRTEM, NBED, and STEM-EELS, we investigated the atomic and electronic structures of the mixture of Li0.44MnO2 nanowires and byproduct nanoparticles formed after the conversion of Na0.44MnO2 nanowires into Li0.44MnO2 nanowires. Results showed that the byproduct are P3112-type Li2MnO3 nanoparticles. They adhere to the faces of single-crystalline Li0.44MnO2 nanowires with the specific relationship of crystallographic orientations between the Li2MnO3 nanoparticles and Li0.44MnO2 nanowires. The STEM-EELS spectra demonstrated that the P3112-type Li2MnO3 has an electronic structure closely resembling that of C2/m-type Li2MnO3. It is considered that the P3112-type Li2MnO3 nanoparticles are formed by partial oxidation of each Li0.44MnO2 nanowire and that they react with constituents of Mn3O4 that appear in the conversion process of Li0.44MnO2 nanowires to LiMn2O4 nanowires. The P3112-type Li2MnO3 nanoparticles play a crucial role in synthesizing highquality single-crystalline LiMn2O4 nanowires for Li ion batteries with high power density.

(3)

Acknowledgment. J.K. thanks K. Okazaki, S. Tanaka, and K. Tanaka (AIST) for fruitful discussions and M. Tabuchi (AIST) for preparation of Li2MnO3 (space group C2/m) as the reference material.

This reaction can occur at the nanowire surfaces in the conversion process from Li0.44MnO2 into LiMn2O4. Therefore,

Supporting Information Available: Additional figures showing experimental results. This material is available free of charge via the Internet at http://pubs.acs.org.

Li2MnO3 + Mn3O4 + 1/2O2 f 2LiMn2O4

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