Oxygen Vacancies and Stacking Faults Introduced by Low

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Oxygen Vacancies and Stacking Faults Introduced by Lowtemperature Reduction Improve the Electrochemical Properties of Li2MnO3 Nanobelts as Lithium-ion Battery Cathodes Ya Sun, Hengjiang Cong, Ling Zan, and Youxiang Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12080 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 17, 2017

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ACS Applied Materials & Interfaces

Oxygen Vacancies and Stacking Faults Introduced by Low-temperature Reduction Improve the Electrochemical Properties of Li2MnO3 Nanobelts as Lithium-ion Battery Cathodes Ya Sun†, Hengjiang Cong†*, Ling Zan†, Youxiang Zhang†‡* † College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, P. R. China ‡ Shenzhen Research Institute of Wuhan University, Shenzhen, 518000, P. R. China KEYWORDS: lithium ion batteries, cathode materials, Li-rich manganese-based layered oxides, Li2MnO3, nanobelts

ABSTRACT: Among the Li-rich layered oxides Li2MnO3 has significant theoretical capacity as a cathode material for Li-ion batteries. Pristine Li2MnO3 generally has to be electrochemically activated in the first charge-discharge cycle which causes very low Coulombic efficiency and thus deteriorates its electrochemical properties. In this paper, we show that low-temperature reduction can produce a large amount of structural defects like oxygen vacancies, stacking faults, and orthorhombic LiMnO2 in Li2MnO3. The Rietveld refinement analysis shows that, after a reduction reaction with stearic acid at 340 °C for 8 h, pristine Li2MnO3 changes into a Li2MnO3-

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LiMnO2 (0.71/0.29) composite, and the monoclinic Li2MnO3 changes from Li2.04Mn0.96O3 in the pristine Li2MnO3 (P-Li2MnO3) to Li2.1Mn0.9O2.79 in the reduced Li2MnO3 (R-Li2MnO3), indicating the production of large amount of oxygen vacancies in the R-Li2MnO3. Highresolution transmission electron microscope (HRTEM) images show that a high density of stacking faults is also introduced by the low-temperature reduction. When measured as a cathode material for Li-ion batteries, R-Li2MnO3 shows much better electrochemical properties than PLi2MnO3. For example, when charge–discharged galvanostatically at 20 mA·g−1 in a voltage window of 2.0–4.8 V, R-Li2MnO3 has Coulombic efficiency of 77.1% in the first chargedischarge cycle, with discharge capacities of 213.8 and 200.5 mAh g-1 in the 20th and 30th cycles, respectively. In contrast, under the same charge-discharge conditions, P-Li2MnO3 has Coulombic efficiency of 33.6% in the first charge-discharge cycle, with small discharge capacities of 80.5 and 69.8 mAh g-1 in the 20th and 30th cycles, respectively. These materials characterizations and electrochemical measurements show that low-temperature reduction is one of the effective ways to enhance the performances of Li2MnO3 as a cathode material for Li-ion batteries.

INTRODUCTION Due to their high energy and power densities and long cycle lives, lithium ion batteries (LIBs) have contributed to the commercial success of portable electronics and have also been anticipated to power the electric transportation and realize the full potential of renewable energy sources as part of the electrical distribution grid.1,2 To meet the demand for high energy densities, a family of lithium-rich layered oxides (LLOs) has recently attracted considerable interest as cathode materials for LIBs because of their high capacities and high operating potentials.3 The origin of the high capacity displayed by LLOs are considered to be nested in reversible anionic


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redox processes by forming peroxo-like dimers4-7 and/or formation of localized electron holes on oxygen atoms coordinated by transition metal and lithium ions.8,9 As one of the end members of the LLOs, the manganese-based Li2MnO3 (Li2MO3, M = Mn) has many advantages over the currently-used cathode materials of LIBs. These advantages include significantly higher specific capacity, low cost and nontoxicity of the transition metal Mn. These advantages, especially the ideal specific capacity (459 mAh g-1), make Li2MnO3 an appealing candidate as the next-generation cathode material.10-16 In the intercalationdeintercalation process, Li2MnO3 shows a high voltage plateau at around 4.5 V vs Li+/Li during the first charge process. Since the Mn ions in Li2MnO3 are Mn4+ and cannot expected to be oxidized further, this high voltage plateau is explained as the oxidation of the anionic ions O2- in the lattice.13-16 Although detailed mechanism of lithium extraction from Li2MnO3 is still not fully understood, it is generally accepted that the overall delithiation reaction involves lattice oxygen loss. Localized holes on oxygen (O−) and/or oxygen dimerization (O-O peroxide) and formation of molecular O2 facilitate Mn migration into the vacated lithium layers, causing the corresponding structural rearrangement and phase transformation.17-19 This structural and chemical degradation of Li2MnO3 lead to the capacity and voltage fading, which shows in the form of poor cycling stability. To improve the electrochemical properties and structural stability of Li2MnO3, several different strategies have been proposed.20-26 One strategy is to partially substitute the Li, Mn or O in Li2MnO3 by other elements.20-23 Na-substituted Li2MnO3 (Li2-xNaxMnO3) and fluorinesubstituted Li2MnO3 (Li2MnO3-xFx) were synthesized and evaluated by Dong et al.20,21 The partially substituted samples showed enhanced cycling stabilities, which were ascribed to reduced deintercalation barriers of lithium ions and improved transmission of the electrons in the


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material. Substitution effects of Nb and Mo on the performances of Li2MnO3 were investigated by Chen et al.22,23 The enhanced electrochemical properties were ascribed to extra chargecompensation electrons from the substituted Nb5+ and Mo5+ ions and postponed oxygen release during the delithiation processes. The other proposed strategy to improve the electrochemical properties of Li2MnO3 is embedding spinel phase LiMn2O4 into the layered Li2MnO3.24-26 The spinel-layered integrate structured composites showed both high capacities and good high-rate capabilities. The high capacities of the composite are believed to derive from the Li-rich layerstructured Li2MnO3, and the good high-rate capabilities are attributed to the spinel phase LiMn2O4 which possess three-dimensional Li+ ion diffusion channels. Thermal reduction has also been attempted as a method to improve the capabilities of lithium-rich layered oxides as cathode materials for LIBs.27-28 Abouimrane et al improved discharge capacities and rate capabilities of Li1.12Mn0.55Ni0.145Co0.1O2 through a mild thermal treatment (250 °C for 3 h) under H2 gas.27 When charge-discharged galvanostatically within the voltage window of 2.0–4.9 V (vs. Li/Li+) at a rate of 1.6 C (1 C = 200 mA g-1), the H2-reduced and the pristine material delivered 196 vs. 171 mAh g-1 discharge capacity, respectively. The improvement of the electrochemical properties was attributed to a mixed valence of Mn (Mn4+ and Mn3+) in the thermal-treated material which can promote polaron migration through the cathode. Kubota et al synthesized oxygen-deficient Li2MnO3-x by reducing pristine Li2MnO3 with metal hydrides (CaH2 and LiH).28 When charge-discharged galvanostatically in the range of 2.0-4.6 V (vs Li/Li+) at a rate of 10 mA g-1, the LiH-reduced oxygen-deficient Li2MnO3-x showed extremely large 1st charge and discharge capacities of 388 and 305 mAh g-1, respectively. In this paper, Li2MnO3 nanobelts were synthesized by ion-exchange and oxidation using Na0.44MnO2 nanobelts as precursor, and the electrochemical properties of the pristine Li2MnO3


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nanobelts (P-Li2MnO3) and low-temperature reduced Li2MnO3 nanobelts (R-Li2MnO3) were compared as cathode materials for LIBs. X-ray diffraction (XRD), Rietveld refinement analysis, selected-area electron diffraction (SAED), high-resolution transmission electron microscope (HRTEM) and Raman spectroscope characterizations showed that a large amount of structural defects like oxygen vacancies, stacking faults, and orthorhombic LiMnO2 were produced in the R-Li2MnO3. Electrochemical measurements showed that the R-Li2MnO3 can provide much higher charge/discharge capacities and better high-rate capabilities than the P-Li2MnO3 when used as cathode materials for LIBs. Especially, R-Li2MnO3 did not show the characteristic long voltage plateau at around 4.5 V (vs Li/Li+) in the first charge process. EXPERIMENTAL SECTION Synthesis of Pristine Li2MnO3 Nanobelts. The pristine Li2MnO3 nanobelts (P-Li2MnO3) were synthesized by ion-exchange and oxidation using Na0.44MnO2 nanobelts as precursor, while the Na0.44MnO2 nanobelts were prepared via a molten-salt method.29 To prepare Na0.44MnO2 nanobelts, 1 mmol of MnCO3 and 0.23 mmol of Na2CO3 were mixed with 5 g of NaCl, which were ground to be homogeneous in a mortar. The mixed powder was then placed in an alumina crucible and annealed at 850 °C for 5 h. After cooling to room temperature, the resulting product was washed several times with distilled water and dried at 100 °C. To synthesize the pristine Li2MnO3 nanobelts P-Li2MnO3, Na0.44MnO2 nanobelts were annealed at 500 °C for 1 h in air using a mixture of LiNO3 and LiCl (88 :12 in mole ratio) as the molten salt. Synthesis of the Reduced Li2MnO3 Nanobelts. The chemically reduced Li2MnO3 nanobelts (RLi2MnO3) were reduced by stearic acid at a low temperature using a simple rheological phase method. A typical procedure is as follows. P-Li2MnO3, stearic acid and an appropriate amount of ethanol were mixed (P-Li2MnO3: stearic acid = 1: 0.04 in mol ratio) and grounded for several


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minutes to get a homogeneous solid-liquid rheological body, which looks like a kind of mushy slurry. The rheological body was then calcined at 340 °C for 8 h under Ar atmosphere. The product was cooled to room temperature and washed with distilled water one time during the calcination. Characterization. The crystalline phases of the materials were determined by powder X-ray powder diffraction (XRD, Bruker D8 ADVANCE, and Rigaku SMARTLAB, with Cu Kα radiation). Morphologies and microstructures of the materials were investigated by scanning electron microscope (SEM, SIRION, FEI) and transmission electron microscope (TEM, JEM 2010-FEF, and JEM-2100F, JEOL Ltd.). The Raman spectra were obtained by using a RM-1000 Renishaw confocal Raman Micro-spectroscope with 514.5 nm laser radiation at a laser power of 0.04 mV in the range of 200~800 cm-1. The X-ray photoelectron spectra (XPS) were obtained using an ESCALAB 250 X-ray photoelectron spectroscope (Thermo Fisher Scientific, USA) with a focused monochromatic Al Kα source (1486.6 eV). The areas of the samples to be analyzed were 500 µm × 500 µm. The binding energies were calibrated by the C1s line at 285.0 eV. The Li/Mn ratio in the R-Li2MnO3 was analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES) using IRIS Intrepid II XSP (Thermo Fisher Scientific USA). The concentrations of Li and Mn were 0.089 and 0.43 mg L-1, respectively, and the ratio of Li/Mn was then calculated as 1.63. The carbon content in the R-Li2MnO3 was determined by VARIOEL III elemental analyzer (Elementar Analysen System GmbH, Germany) and showed to be 4.11%. Evaluation of Electrochemical Performances. The electrochemical measurements were performed using CR2016 coin-type cell with lithium metal disks as the counter electrodes. The working electrodes were made by pressing mixtures of Li2MnO3, acetylene black and


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polytetrafluoroethylene (PTFE) binder, with a weight ratio of 75:20:5, on stainless steel which were used as the current collectors. The weight of active materials in the cathode electrode varied between 3.0 and 4.0 mg cm-2 for each cell. The electrolytes of the cells were composed by LiPF6 (1 mol L-1) in the solvent of ethylene carbonate and dimethyl carbonate (1:1 v/v). The separators in the cells were Celgard 2300 microporous films. The cells were assembled in glovebox filled with high purity Ar gas. The electrochemical tests were performed galvanostatically at different current densities with voltage window of 2.0-4.8 V on Neware battery test system (Shenzhen, China) at room temperature. All the charge and discharge specific capacities were calculated on the net mass of the Li2MnO3 excluding the carbon content. Electrochemical impedance spectroscopies (EIS) were obtained using a CHI760C electrochemistry workstation. The AC amplitude was 5 mV, with the applied frequency ranging from 100 kHz to 0.01 Hz. RESULTS AND DISCUSSION Figure 1 shows the XRD pattern, SEM, TEM and high-resolution TEM images of the Na0.44MnO2 precursor prepared by molten-salt method. All the diffraction peaks in the pattern (Fig. 1a) can be well indexed to orthorhombic Na4Mn9O18 (JCPDS 27-0750), demonstrating the high purity of the precursor.29 From the SEM images of the precursor (Fig. 1b), we can see that the Na0.44MnO2 has one-dimensional (1D) nanobelts morphology with widths ranging from 100 to 200 nm and lengths up to tens of micrometers. The TEM images (Fig. 1c) confirm the nanobelts morphology of the Na0.44MnO2 precursor. In the high-resolution TEM image (Fig. 1d), the crystal fringes of the nanobelts can be clearly seen. From the image, distances between adjacent crystalline planes can be measured to be 0.45 nm which corresponds to the (200) planes of the orthorhombic Na0.44MnO2. The selected area electron diffraction (SEAD) pattern viewed along [010] zone axis (shown in Supporting Information) indicates that the nanobelts are single


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crystalline. Figure 2 shows the XRD pattern, SEM, TEM and high-resolution TEM images of the PLi2MnO3 synthesized by ion-exchange and oxidation using the Na0.44MnO2 nanobelts as precursor. The XRD pattern (Fig. 2a) of P-Li2MnO3 is the typical X-ray diffraction pattern of layer-structured monoclinic phase Li2MnO3 (JCPDS 86-1634) with a C2/m space group.30 For monoclinic Li2MnO3, the feature of the XRD pattern is the superstructure peaks between 20° and 25° arising from the LiMn6 cation ordering in the [Li1/3Mn2/3] transition metal layers. The SEM image (Fig. 2b) of P-Li2MnO3 shows that the P-Li2MnO3 preserves the 1D nanobelts morphology of the Na0.44MnO2 precursor through the ion-exchange and oxidation process. Fig. 2c is a typical TEM image of P-Li2MnO3. It seems that although the length and width of PLi2MnO3 can keep unchanged as those of the Na0.44MnO2 precursor, the surfaces of P-Li2MnO3 are much rougher than the precursor. Fig. 2d is a typical high-resolution TEM image of PLi2MnO3. From the image, an interplanar spacing of 0.47 nm can be clearly measured which corresponds to the plane distance of (001) for monoclinic Li2MnO3. Figure 3 shows the XRD pattern, SEM, TEM images and the selected-area electron diffraction (SAED) pattern of the R-Li2MnO3 produced by low-temperature reduction of the PLi2MnO3. Compared with the XRD pattern of P-Li2MnO3, the XRD pattern of R-Li2MnO3 (Fig. 3a) shows two features. One feature is that the diffraction peaks of monoclinic Li2MnO3 preserve, but with decreased intensities. Another feature is that several new peaks appear in the pattern (labeled with stars). These new peaks can be indexed to the orthorhombic LiMnO2 (PDF NO. 860356).31 The diffraction peaks at 15.5°, 39.2°, 61.3° and 66.5° correspond respectively to the (010), (200), (221) and (002) planes of orthorhombic LiMnO2. Typical SEM and TEM images of R-Li2MnO3 are shown in Fig. 3b and Fig. 3c, respectively. It shows that the R-Li2MnO3


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preserves the nanobelts morphology of the P-Li2MnO3 after the chemical reduction reaction at 340 °C for hours. The selected-area electron diffraction (SAED) pattern corresponding to Fig. 3c is shown in Fig. 3d. To show the diffraction spots more clearly, these spots are shown in two different colors (blue and red, respectively) in Fig. 3e. As we can see, the diffraction spots should be indexed to two different crystal structures. While the blue diffraction spots can be indexed to monoclinic phase Li2MnO3, the red spots can be indexed to orthorhombic phase LiMnO2 as shown in Fig. 3f. The zone axes applied are [100] and [001] for the Li2MnO3 and LiMnO2, respectively. To confirm that two different crystal structures co-exist in the R-Li2MnO3, we characterized the material with high-resolution TEM images. Figure 4a is an example of such high-resolution TEM images. In Fig. 4a, area A and area B have different crystal structures. The image of area A is enlarged in Fig. 4b with the corresponding electron diffraction pattern of Fig. 4b, obtained by fast Fourier transformation (FFT), is inset at the top of the image. The spots of the electron diffraction pattern can be indexed to layer-structured monoclinic phase Li2MnO3, with [-538] as the zone axes applied. The interplanar distance is measured to be 0.19 nm, which corresponds to the (131) lattice planes of monoclinic Li2MnO3. Image of area B is enlarged in Fig. 4c, with its electron diffraction pattern obtained by FFT shown at the top of its inset. The electron diffraction spots can be indexed to orthorhombic LiMnO2, with [0-12] as the zone axes applied. The interplanar distance in Fig. 4c is measured to be 0.22 nm, which corresponds to the (200) lattice planes of orthorhombic LiMnO2. The structures of P-Li2MnO3 and R-Li2MnO3 were determined by X-ray powder diffraction and Rietveld analysis. The structural refinement was based on the layered Li[Li1/3Mn2/3]O2 and LiMnO2 structures.32,33 The structure model of Li2MnO3 with space group C2/m used for the


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refinement was as follows: Li1 and Mn1 in 2b (0, 0.5, 0); Li2 in 2c (0, 0, 0.5); Li3 in 4h (0, 0. 686(2), 0.5); Li4 and Mn2 in 4g (0, 0.1763(4), 0.0); O1 in 4i (0.2043(19), 0.0, 0.1950(15)); and O2 in 8j (0.2497(18), 0.3145(6), 0.2274(8)). The structure model of LiMnO2 with space group Pmnm used for the refinement was as follows: Li1 and Mn1 in 2a (0.25, 0.647(5), 0.25); Li2 and Mn2 in 2a (0.25, 0.100(4), 0.25); O1 in 2b (0.75, 0.12(2), 0.25); and O2 in 2b (0.75, 0.63(2), 0.25). Figures 5 and 6 show the Rietveld analysis results for P-Li2MnO3 and R-Li2MnO3, and Tables 1, 2 and 3 summarize the R factors, lattice parameters and final structure parameters determined by the Rietveld refinement. The diffraction peaks of Li2MnO3 are indexed by space group of monoclinic C2/m, and the diffraction peaks of LiMnO2 are indexed by space group of orthorhombic Pmnm. For P-Li2MnO3, the profile fittings provide a Rwp factor of 1.72%. For RLi2MnO3, the fittings determine its compound composition to be 71.45% monoclinic Li2MnO3 plus 28.55% orthorhombic LiMnO2 with a Rwp factor of 1.50%. The cell parameters of Li2MnO3 in P-Li2MnO3 are: a = 4.9340(6), b = 8.5265(11), c = 5.0172(6) Å, β = 108.896(6)o while in RLi2MnO3: a = 4.9314(8), b = 8.5207(17), c = 5.0211(10) Å, β = 109.094(11)°. Remarkably, while the occupancy parameter at the oxygen 4i site is 1.0 for P-Li2MnO3, it is only 0.785 for RLi2MnO3. This large drop of oxygen occupancy parameter indicates that a large amount of oxygen vacancies exist at the 4i site for the monoclinic phase in R-Li2MnO3. With the introduction of high density of oxygen vacancies, the volume of the unit cells shrinks from 199.70(4) Å for P-Li2MnO3 to 199.38(7) Å for R-Li2MnO3, and the molecular formula changes from Li2.04Mn0.96O3 to Li2.1Mn0.9O2.79 for the monoclinic phase Li2MnO3 in the materials. Interestingly, the occupancy parameters at the oxygen 2b sites for the O1 and O2 in the orthorhombic phase LiMnO2 remain as 1.0, demonstrating that while there is high density of oxygen vacancies in the monoclinic phase Li2MnO3, no oxygen vacancies exist in the


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orthorhombic phase LiMnO2 in the composite. To study the microstructures of the R-Li2MnO3, high-resolution TEM images were taken and are shown in Figure 7. As can be seen in Fig. 7a, a high density of stacking faults can be found in this high-resolution TEM image of the low-temperature reduced Li2MnO3. To see the stacking faults more clearly, areas A, B and C in Fig. 7a are enlarged to be Fig. 7b, 7c and 7d, respectively, and in these enlarged images the stacking faults can be easily found and are labeled with rectangles. High density of stacking faults leads to quite different interplanar distances for the same lattice planes. For example, the interplanar distances of the (-111) lattice planes are measured to be 0.359, 0.363 and 0.366 nm, respectively, in Fig. 7b, Fig. 7c and 7d. These highresolution TEM images show that low-temperature reduction can cause high density of disordered arrangements and stacking faults in the material. Raman spectroscopies of the P-Li2MnO3 and R-Li2MnO3 were collected to probe the nearneighbor environments of oxygen coordination around the lithium and manganese cations in the two samples and are shown in Figure S2 in the Supporting Information. In the Raman spectrum of P-Li2MnO3 (Fig. S2a), there are nine well-resolved peaks, located at 250, 312, 325, 372, 415, 438, 498, 568 and 617 cm-1, respectively, in the 200 cm-1 to 800 cm-1 wave number window. These nine peaks were explained and assigned to the Li-O stretching vibrations of LiO6 octahedrons and Mn-O stretching vibration of MnO6 octahedrons in monoclinic Li2MnO3, respectively, by Julien and Massot et al.34 In comparison with the spectrum of P-Li2MnO3, the Raman spectrum of R-Li2MnO3 has poor resolution, but with a characteristic broad and overlapping peak in 550-700 cm-1. This characteristic broad peak of the Raman spectrum is associated with the strongest Mn-O bonds of the R-Li2MnO3. To resolve this broad and overlapping peak, the Raman spectrum of R-Li2MnO3 has been fitted and shown in Fig. S2b.


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This characteristic broad and overlapping peak in 550-700 cm-1 can be fitted into four peaks, located at 580, 605, 632 and 658 cm-1, respectively. These fitted peaks have almost the same frequencies as the peaks measured in the Raman spectrum of orthorhombic LiMnO2 in the Pmmn ଵଷ (‫ܦ‬ଶ௛ ) space group.35

The electrochemical properties of the low-temperature reduced Li2MnO3 nanobelts (RLi2MnO3) as cathode material for LIBs were tested and compared with the as-synthesized pristine Li2MnO3 nanobelts (P-Li2MnO3). Figure 8 shows the electrochemical properties of the two samples when charge-discharged galvanostatically at 0.1 C (1 C = 200 mA g-1) in the voltage range of 2.0-4.8 V at room temperature. Figure 8a shows the voltage-capacity curves in the first charge-discharge cycle for P-Li2MnO3 and R-Li2MnO3. As expected, the charge profile of P-Li2MnO3 displays the characteristic long voltage plateau at 4.7 V (vs. Li+/Li), which has been interpreted as an index of structural change due to the irreversible oxygen removal from the Li2MnO3.36 In the following discharge process, only a smooth sloping profile appears. In this first charge-discharge cycle, the charge capacity is 182.0 mAh g-1 and the discharge capacity is 61.2 mAh g-1, resulting in low Columbic efficiency of 33.6%. This poor performance of PLi2MnO3 is not surprising. The low Columbic efficiency is the result of irreversible oxygen removal and structure reconstruction. The R-Li2MnO3 shows totally different charge-discharge profile from the P-Li2MnO3. The charge profile of R-Li2MnO3 shows two features. First, two voltage plateaus, locating at 3.4 V and 4.1 V, respectively, appear in this first-cycle charge profile. The other feature is that no apparent voltage plateau appears at around 4.7 V. In the discharge profile, two voltage plateaus, locating at 3.9 V and 3.0 V, respectively, appear. In this first charge-discharge cycle of R-Li2MnO3, the charge capacity is 225.7 mAh g-1 and the discharge capacity is 174.1 mAh g-1, resulting in Columbic efficiency of 77.1%. Figure 8b shows


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the differential capacity (dQ/dV) plots in the first charge-discharge cycle for P-Li2MnO3 and RLi2MnO3. Peaks in dQ/dV plots (Fig. 8b) correspond to voltage plateaus in the charge-discharge profiles (Fig. 8a). The area under a peak is a measure of the capacity at that voltage. In the differential capacity plot of P-Li2MnO3, a sharp peak at 4.7 V appears in the charge process, with no apparent peak in the discharge process. In the differential capacity plots of R-Li2MnO3, two peaks locating at 3.4 V and 4.1 V, respectively, appear in the charge process, and two peaks locating at 3.0 V and 3.9 V, respectively, arise in the discharge process. Different from PLi2MnO3, the oxidizing peak at ~4.7 V in the dQ/dV plot for the R-Li2MnO3 is pretty weak. This oxidizing peak corresponding to the voltage plateau at ~4.7 V in Fig. 8a becomes weaker and weaker, and totally disappears in the 20th charge-discharge cycle. Figure 8c shows the chargedischarge profiles of the R-Li2MnO3 in the 1st, 2nd, 5th, 10th, 20th and 30th charge-discharge cycles. Figure 8d shows the cyclability of R-Li2MnO3 in the initial 30 charge-discharge cycles, with the cyclability of P-Li2MnO3 showed for comparison. For R-Li2MnO3, discharge capacity of 213.8 mAh g-1 is reached in the 20th cycle. After slow decreasing, the discharge capacity declines to 200.5 mAh g-1 in the 30th cycle, which is 93.8% of the capacity in the 20th cycle and 115.2% of the capacity in the 1st cycle. In contrast, the discharge capacities of P-Li2MnO3 are 80.5 and 69.8 mAh g-1, respectively, in the 20th and 30th charge-discharge cycles. The charge profile of R-Li2MnO3 in Fig. 8a shows two features. A remarkable feature is that, although Li2MnO3 is still the major constituent of R-Li2MnO3, its charge profile in the first cycle does not show the characteristic long voltage plateau at around 4.7 V. The disappearance of this voltage plateau should be the result of structure defects, including the stacking faults and oxygen vacancies, inside the crystallines of R-Li2MnO3,28,37. For stoichiometric Li2MnO3, the characteristic voltage plateau in the 1st charge process was generally considered to be an oxygen


13


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extraction process (Li2MnO3 → MnO2 + 2Li+ + O + e-) that electrochemically activated this Mn4+-containing material. However, in the R-Li2MnO3, XRD Rietveld analysis showed that this layered rock-salt oxide is oxygen-deficient, with a chemical formula of Li2.1Mn0.9O2.79. This implies that large amount of oxygen vacancies are produced by the low-temperature reduction reaction and that large amount of oxygen in the material has been extracted before the material is charged as the cathode of LIB. Also, high-resolution TEM images showed that the oxygendeficient Li2.1Mn0.9O2.79 in the R-Li2MnO3 contains other structure disorders, such as stacking faults. It might also be possible that these disordered structures can charge-discharge without being electrochemically activated in the 1st charge process. The other feature of the charge profile of R-Li2MnO3 is that, compared with the P-Li2MnO3, two new voltage plateaus locating at 3.4 V and 4.1 V appears in the first charge profile. The voltage plateau at 3.4 V should correspond to the Li-ion deinsertion from the orthorhombic LiMnO2 in the material. The voltage plateau at 4.1 V is not apparent but can be visible. Since no spinel phase LiMn2O4 was detected inside R-Li2MnO3, this voltage plateau could be due to the structure transformations of LiMnO2 into spinel-like structure, upon Li+ removal, in the first charge process.38 In Fig. 8c, the voltage plateau locating at 3.4 V in the first charge falls to 3.2 V in the second charge profile, which is a characteristic for orthorhombic LiMnO2 as a cathode material for LIB.31 After slow and persistent decline, this voltage plateau moves to and stables at ~3.1 V in the 30th chargedischarge cycle. Figure 9 shows the performances of R-Li2MnO3 as cathode material for LIBs at high rates. Figure 9a is the typical voltage-capacity curves at 0.5 C, 1 C, 2 C, and 5 C, respectively. Similar to the curves at low rates, these voltage-capacity curves do not show voltage plateaus at around 4.7 V, but show voltage plateaus at around 3.0 V and 4.1 V, respectively. Figure 9b is the dQ/dV


14

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plots corresponding to the charge-discharge profiles in Fig. 9a. Figure 9c is the cyclability of RLi2MnO3 when charge-discharged continuously from the rate of 0.1 C to the rate of 5 C, with each rate charging-discharging galvanostatically for 10 cycles. The cyclability of P-Li2MnO3 is also showed in the figure for comparison. Obviously, R-Li2MnO3 has much better high-rate performances than P-Li2MnO3. Figure 9d is the cyclability of R-Li2MnO3 when chargedischarged galvanostatically at the rates of 0.5 C, 1 C, 2 C and 5 C, respectively. After long time galvanostatical charging-discharging (50 cycles for the rates of 0.5 C; 100 cycles for the rates of 1 C, 2 C, and 5 C), the discharge capacities stay at 131.6, 119.2, 105.8 and 81.7 mAh g-1, respectively. Electrochemical impedance spectroscopy (EIS) measurements were further performed to identify the kinetic properties of the P-Li2MnO3 and R-Li2MnO3. Figure 10a displays Nyquist plots of the two materials, with the equivalent circuit of Nyquist plot inserted into the figure. EIS spectra are generally composed of a semicircle at the high to medium frequency region and an inclined line in the low frequency range. The intercepts along the real axis (Zre) at high frequency zone represent the ohmic resistance of the electrolyte (Re). The diameters of the semicircles on the Zre axis in the middle frequency range represent the charge-transfer resistance (Rct) through the solid-electrolyte interfaces.39 As shown in Fig. 10a, the charge-transfer resistance (Rct) value of R-Li2MnO3 is 226.5 Ω, which is smaller than that of P-Li2MnO3 (271.0 Ω). Figure 10b shows the relationship between Zre and the reciprocal square root of the frequency (ω-1/2) in the low frequency region, with the slope of the fitting lines as the Warburg coefficient σ. Using the resulting σ, the apparent Li+ diffusion coefficient (DLi+) can be evaluated.39 Based on this relationship between DLi+ and σ, the diffusion coefficients of lithium ions for the R-Li2MnO3 and P-Li2MnO3 were calculated as 1.32 × 10-15 and 6.10 × 10-16 cm2 s-1, respectively. Thus, the


15


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apparent Li+ diffusion coefficient of R-Li2MnO3 is about two times faster than that of P-Li2MnO3. The above data show that low-temperature reduction process can not only enhance the electronic conductivity between electrode and electrolyte, but also efficiently improve the lithium ion diffusions in the material. To see how well the morphology and the crystallinity of R-Li2MnO3 can maintain, a Li cell, after being charge-discharged 100 times galvanostatically at the rate of 1 C, was disassembled and its cathode material was characterized. The XRD, TEM and high-resolution TEM images are showed as Figure S3 in the Supporting Information. The XRD pattern (Fig. S3a) reveals that RLi2MnO3 still has good crystallinity after 100 charge-discharge cycles. The weak intensities of the diffraction peaks are due to the little amount of electrode materials (~3 mg) in one Li cell. Figure S3b shows that the nanobelt morphology of R-Li2MnO3 is essentially preserved after 100 charge-discharge cycles. Clear lattice fringes in the high-resolution TEM image (Fig. S3c) proves that R-Li2MnO3 is still highly crystalline, which is consistent with the XRD result. This essential preservation of both morphology and crystallinity highly demonstrates that R-Li2MnO3 have good structural and cycling stabilities. CONCLUSIONS In summary, pristine Li2MnO3 nanobelts and low-temperature reduced Li2MnO3 nanobelts were synthesized, characterized and measured as the cathode materials for Li-ion batteries. X-ray diffraction, Rietveld refinement analysis, selected-area electron diffraction, high-resolution transmission electron microscope and Raman spectroscope characterizations showed that a large amount of structural defects like oxygen vacancies, stacking faults, and orthorhombic LiMnO2 were produced in the low-temperature reduced Li2MnO3 nanobelts. The reduced Li2MnO3 showed much better performances than the pristine Li2MnO3. The much-better performances of


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the reduced Li2MnO3 should be ascribed to the LiMnO2 and the structure defects inside. The LiMnO2 produced in the nanobelts is electrochemically active, thus offering extra capacities, and has better electronic conductivity than the pristine Li2MnO3 phase, which results in less electrode polarization. The reduced Li2MnO3 did not show the characteristic long voltage plateau at around 4.5 V in the first charge process, which should be associated with the oxygen vacancies and stacking faults produced by the reduction reaction in the material. The high capacities and highrate capabilities of these low-temperature reduced Li2MnO3 may shed light on developing this Li and Mn-rich metal oxide materials for high-power lithium ion batteries.

Figure 1. The XRD pattern (a), SEM (b), TEM (c) and high-resolution TEM (d) images of the Na0.44MnO2 precursor. Figure 2. The XRD pattern (a), SEM (b), TEM (c) and high-resolution TEM (d) images of the PLi2MnO3. Figure 3. The XRD pattern (a), SEM (b), TEM (c) and selected area electron diffraction pattern (d, e, f) of the R-Li2MnO3. Figure 4. A high-resolution TEM image of the R-Li2MnO3 (a), with areas A and B in the image are enlarged and shown in (b) and (c), respectively. The corresponding electron diffraction patterns of the images b and c are inset at the images’ top right. Figure 5. Observed (blue), calculated (dotted solid red line) and difference plots (solid green line) for the X-ray Rietveld analysis of the P-Li2MnO3.


17


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Figure 6. Observed (blue), calculated (dotted solid red line) and difference plots (solid green line) for the X-ray Rietveld analysis of the R-Li2MnO3. Figure 7. A high-resolution TEM image of the R-Li2MnO3 (a), with areas A, B and C in the image are enlarged and shown in (b), (c) and (d), respectively. In these enlarged high-resolution TEM images (b, c, d) stacking faults in the R-Li2MnO3 can be easily seen. Figure 8. The electrochemical properties of the P-Li2MnO3 and R-Li2MnO3 at 0.1 C in 2.0-4.8 V voltage range. The voltage-capacity curves (a) and the corresponding differential capacity plots (b) of the P-Li2MnO3 and the R-Li2MnO3 in the first charge-discharge cycle; the voltagecapacity curves of the R-Li2MnO3 in the initial 30 charge-discharge cycles (c); and cyclabilities of the P-Li2MnO3 and R-Li2MnO3 (d). Figure 9. The electrochemical properties of the P-Li2MnO3 and R-Li2MnO3 at high rates. The typical voltage-capacity curves (a) and the corresponding dQ/dV plots (b) of the R-Li2MnO3 at the rates of 0.5 C, 1 C, 2 C, and 5 C, respectively; the cyclability of R-Li2MnO3 from the rate of 0.1 C to the rate of 5 C, with the cyclability of the P-Li2MnO3 at the same rates shown for comparison (c); the cyclability of the R-Li2MnO3 when charge-discharged galvanostatically for long time at the rates of 0.5 C, 1 C, 2 C and 5 C, respectively, (d). Figure 10. The Nyquist plots and equivalent circuit (a) and relationship between Zre and ω-1/2 in the low frequency region (b) of the P-Li2MnO3 and R-Li2MnO3. SUPPORTING INFORMATION


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TEM image of a Na0.44MnO2 nanobelt and the selected area electron diffraction pattern; Raman spectrums of the P-Li2MnO3 and R-Li2MnO3; XRD, TEM and high-resolution TEM images of a cell’s cathode material after the cell was charge-discharged at the rate of 1 C for 100 times AUTHOR INFORMATION Corresponding Authors *Email: [email protected]. (H.-J. Cong) *Email: [email protected]. Tel: +86-27-65783395; Fax: +86-27-68754067 (Y.-X. Zhang)

ACKNOWLEDGMENT This study was supported by the National Natural Science Foundation of China (grant No. 21271145), the Natural Science Foundation of Hubei Province (grant NO. 2015CFB537, 2016CFB382) and the Science and Technology Innovation Committee of Shenzhen Municipality (contract NO. JCYJ20170306171321438).

REFERENCES (1) Armand, M.; Tarascon, J.-M., Building Better Batteries Nature 2008, 451, 652-657. (2) Tarascon, J. M.; Armand, M. Issues and Challenges Facing Rechargeable Lithium Batteries Nature 2001, 414, 359-367. (3) Delmas, C. Operating Through Oxygen Nat. Chem. 2016, 8, 641-643.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 62

(4) Sathiya, M.; Rousse, G.; Ramesha, K.; Laisa, C. P.; Vezin, H.; Sougrati, M. T.; Doublet, M-L.; Foix, D.; Gonbeau, D.; Walker, W.; Prakash, A. S.; Ben Hassine, M.; Dupont, L.; and Tarascon, J-M. Reversible Anionic Redox Chemistry in High-capacity Layered-oxide Electrodes Nat. Mater. 2013, 12, 827-835. (5) Sathiya, M.; Abakumov, A. M.; Foix, D.; Rousse, G.; Ramesha, K.; Saubanère, M.; Doublet, M. L.; Vezin, H.; Laisa, C. P.; Prakash, A. S.; Gonbeau, D.; VanTendeloo G.; Tarascon, J-M. Origin of Voltage Decay in High-capacity Layered oxide Electrodes Nat. Mater. 2015, 14, 230-238. (6) Seo, D.-H.; Lee, J.; Urban, A.; Malik, R.; Kang, S. Y.; Ceder, G. The Structural and Chemical Origin of the Oxygen Redox Activity in Layered and Cation-disordered Li-excess Cathode Materials Nat. Chem. 2016, 8, 692-697. (7) McCalla, E.; Abakumov, A. M.; Saubanère, M.; Foix, D.; Berg, E. J.; Rousse, G.; Doublet, M.-L.; Gonbeau, D.; Novák, P.; Van Tendeloo, G.; Dominko, R.; Tarascon, J.-M. Visualization of O-O Peroxo-like Dimers in High-capacity Layered Oxides for Li-Ion Batteries Science 2015, 350, 1516-1521. (8) Luo, K.; Roberts, M. R.; Hao, R.; Guerrini, N.; Pickup, D. M.; Liu, Y.-S.; Edström, K.; Guo, J.; Chadwick, A. V.; Duda L. C.; Bruce, P. G. Charge-compensation in 3d-Transition-metal Oxide Intercalation Cathodes Through the Generation of Localized Electron Holes on Oxygen Nat. Chem. 2016, 8, 684-691. (9) Saubanere, M.; McCalla, E.; Tarascon J.-M. and Doublet, M.-L. The Intriguing Question of Anionic Redox in High-energy Density Cathodes for Li-Ion Batteries Energy Environ. Sci. 2016, 9, 984-991.


20

Page 21 of 62

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(10) Wang, R.; He, X. Q.; He, L. H.; Wang, F. W.; Xiao, R. J.; Gu, L.; Li, H.; Chen, L. Q. Atomic Structure of Li2MnO3 after Partial Delithiation and Re-Lithiation Adv. Energy Mater. 2013, 3, 1358-1367. (11) Xiao, L.; Xiao, J.; Yu, X. Q.; Yan, P. F.; Zheng, J. M.; Engelhard, M.; Bhattacharya, P.; Wang, C. M.; Yang, X.-Q.; Zhang, J.-G. Effects of Structural Defects on the Electrochemical Activation of Li2MnO3 Nano Energy 2015, 16, 143-151. (12) Shi, J.-L.; Zhang, J.-N.; He, M.; Zhang, X.-D.; Yin, Y.-X.; Li, H.; Guo, Y.-G.; Gu, L.; Wan, L.-J. Mitigating Voltage Decay of Li-Rich Cathode Material via Increasing Ni Content for Lithium-Ion Batteries ACS Appl. Mater. Interfaces 2016, 8, 20138−20146. (13) Lu, Z. H.; MacNeil, D. D. and Dahn, J. R. Layered Cathode Materials Li[NixLi(1/32x/3)Mn(2/3-x/3)]O2

for Lithium-Ion Batteries Electrochem. Solid-State Lett. 2001, 4, A191-A194.

(14) Lu, Z. H. and Dahn, J. R. Understanding the Anomalous Capacity of Li/Li[NixLi(1/32x/3)Mn(2/3-x/3)]O2

Cells Using In Situ X-Ray Diffraction and Electrochemical Studies J.

Electrochem. Soc. 2002, 149, A815-A822. (15) Robertson A. D. and Bruce, P. G. Mechanism of Electrochemical Activity in Li2MnO3 Chem. Mater. 2003, 15, 1984-1992. (16) Tran, N.; Croguennec, L.; Menetrier, M.; Weill, F.; Biensan, Ph.; Jordy, C. and Delmas, C. Mechanisms Associated with the “Plateau” Observed at High Voltage for the Overlithiated Li1.12(Ni0.425Mn0.425Co0.15)0.88O2 System Chem. Mater. 2008, 20, 4815-4825.


21


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(17) Phillips, P. J.; Bareño, J.; Li, Y.; Abraham, D. P. and Klie, R. F. On the Localized Nature of the Structural Transformations of Li2MnO3 Following Electrochemical Cycling Adv. Energy Mater. 2015, 5, 1501252. (18) Oishi, M.; Yamanaka, K.; Watanabe, I.; Shimoda, K.; Matsunaga, T.; Arai, H.; Ukyo, Y.; Uchimoto, Y.; Ogumi Z. and Ohta, T. Direct Observation of Reversible Oxygen Anion Redox Reaction in Li-Rich Manganese Oxide, Li2MnO3, Studied by Soft X-Ray Absorption Spectroscopy J. Mater. Chem. A 2016, 4, 9293-9302. (19) Chen H. and Saiful Islam, M. Lithium Extraction Mechanism in Li-Rich Li2MnO3 Involving Oxygen Hole Formation and Dimerization Chem. Mater. 2016, 28, 6656-6663. (20) Dong, X.; Xu, Y. L.; Xiong, L. L.; Sun, X. F.; Zhang, Z. W. Sodium Substitution for Partial Lithium to Significantly Enhance the Cycling Stability of Li2MnO3 Cathode Material J. Power Sources 2013, 243, 78-87. (21) Dong, X.; Xu, Y. L.; Yan, S.; Mao, S. C.; Xiong L. L. and Sun, X. F. Towards Low-cost, High Energy Density Li2MnO3 Cathode Materials J. Mater. Chem. A 2015, 3, 670-679. (22) Ma, J.; Zhou, Y. N.; Gao, Y. R.; Kong, Q. Y.; Wang, Z. X.; Yang, X. Q. and Chen L. Q. Molybdenum Substitution for Improving the Charge Compensation and Activity of Li2MnO3 Chem. Eur. J. 2014, 20, 8723-8730. (23) Gao, Y. R.; Wang, X. F.; Ma, J.; Wang, Z. X. and Chen, L. Q. Selecting Substituent Elements for Li-Rich Mn-Based Cathode Materials by Density Functional Theory (DFT) Calculations Chem. Mater. 2015, 27, 3456-3461.


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Page 23 of 62

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(24) Wu, F.; Li, N.; Su, Y. F.; Zhang, L. J.; Bao, L. Y.; Wang, J.; Chen, L.; Zheng, Y.; Dai, L. Q.; Peng, J. Y. and Chen, S. Ultrathin Spinel Membrane-Encapsulated Layered Lithium-Rich Cathode Material for Advanced Li-Ion Batteries Nano Lett. 2014, 14, 3550-3555. (25) Liu, H.; Du, C. Y.; Yin, G. P.; Song, B.; Zuo, P. J.; Cheng, X. Q.; Ma Y. L.; and Gao, Y. Z. An Li-Rich Oxide Cathode Material with Mosaic Spinel Grain and a Surface Coating for High Performance Li-Ion Batteries J. Mater. Chem. A 2014, 2, 15640-15646. (26) He, H. B.; Cong, H. J.; Sun, Y.; Zan, L. and Zhang, Y. X. Spinel-Layered Integrate Structured Nanorods with Both High Capacity and Superior High-Rate Capability as Cathode Material for Lithium-Ion Batteries Nano Res. 2017, 10, 556-569. (27) Abouimrane, A.; Compton, O. C.; Deng, H. X.; Belharouak, I.; Dikin, D. A.; Nguyen, S. T. and Amine, K. Improved Rate Capability in a High-Capacity Layered Cathode Material via Thermal Reduction Electrochem. Solid-State Lett. 2011, 14, A126-A129. (28) Kubota, K.; Kaneko, T.; Hirayama, M.; Yonemura, M.; Imanari, Y.; Nakane, K.; Kanno, R. Direct Synthesis of Oxygen-Deficient Li2MnO3-x for High Capacity Lithium Battery Electrodes J. Power Sources 2012, 216, 249-255. (29) Zhang, X. K.; Tang, S. L.; Du, Y. W. Controlled Synthesis of Single-Crystalline Li0.44MnO2 and Li2MnO3 Nanoribbons Mater. Res. Bull. 2012, 47, 1636-1640. (30) Thackeray, M. M.; Kang, S. H.; Johnson, C. S.; Vaughey, J. T.; Benedek, R.; Hackney, S. A. Li2MnO3-Stabilized LiMO2 (M = Mn, Ni, Co) Electrodes for Lithium-Ion Batteries J. Mater. Chem. 2007, 17, 3112-3125.


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(31) Reimers, J. N.; Fuller, E. W.; Rossen, E. and Dahn, J. R. Synthesis and Electrochemical Studies of LiMnO2 Prepared at Low Temperatures J. Electrochem. Soc. 1993, 140, 3396-3401. (32) Matsunaga, T.; Komatsu, H.; Shimoda, K.; Minato, T.; Yonemura, M.; Kamiyama, T.; Kobayashi, S.; Kato, T.; Hirayama, T.; Ikuhara, Y.; Arai, H.; Ukyo, Y.; Uchimoto Y. and Ogumi, Z. Dependence of Structural Defects in Li2MnO3 on Synthesis Temperature Chem. Mater. 2016, 28, 4143-4150. (33) Croguennec, L.; Deniard, P.; Brec R. and Lecerf, A. Preparation, Physical and Structural Characterization of LiMnO2 Samples with Variable Cationic Disorder J. Mater. Chem. 1995, 11, 1919-1925. (34) Julien, C. M.; Massot, M. Lattice Vibrations of Materials for Lithium Rechargeable Batteries III. Lithium Manganese Oxides Mater. Sci. Eng., B 2003, 100, 69-78. (35) Zhao, L. Z.; Chen, Y. W.; Wang, G. R. Raman Spectra Study of Orthorhombic Li2MnO3 Solid State Ionics 2010, 181, 1399-1402. (36) Armstrong, A. R.; Holzapfel, M.; Novák, P.; Johnson, C. S.; Kang, S. H.; Thackeray, M. M.; Bruce, P. G. Demonstrating Oxygen Loss and Associated Structural Reorganization in the Lithium Battery Cathode Li[Ni0.2Li0.2Mn0.6]O2 J. Am. Chem. Soc. 2006, 128, 8694-8698. (37) Lim, J.; Moon, J.; Gim, J.; Kim, S.; Kim, K.; Song, J.; Kang, J.; Im W. B. and Kim, J. Fully Activated Li2MnO3 Nanoparticles by Oxidation Reaction J. Mater. Chem. 2012, 22, 11772-11777. (38) Pistoia, G.; Antonini, A.; Zane, D. Synthesis of LiMnO2 and Its Characterization as a Cathode for Rechargeable Li Cells Solid State Ionics 1995, 78, 115-122.


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(39) Bard, A. J.; Faulkner, J. R. Electrochemical methods, second edn. Wiley 2001, p. 231.


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For Table of Contents Only


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Table 1 X-ray Rietveld refinement results for P-Li2MnO3 Atom

Site

g

x

y

z

B/Å2

Li(1)

2b

0.572(5)

0

0.5

0

0.5

Mn(1)

2b

=1-g(Li(1))

0

0.5

0

0.5

Li(2)

2c

1

0

0

0.5

1.0

Li(3)

4h

1

0

0.686(2)

0.5

1.0

Li(4)

4g

0.258(5)

0

0.1763(4)

0

0.5

Mn(2)

4g

=1-g(Li(4))

0

=y(Li(4))

0

0.5

O(1)

4i

1

0.2043(19)

0

0.1950(15)

0.8

O(2)

8j

1

0.2497(18)

0.3145(6)

0.2274(8)

0.8

Unit cell: monoclinic C2/m (12): a = 4.9340 (6) Å, b = 8.5265 (11) Å, c = 5.0172 (6) Å, β = 108.896 (6)°, Rwp = 1.72, Rp = 1.13.


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Table 2 X-ray Rietveld refinement results for R-Li2MnO3 Atom

Site

g

x

y

z

B/Å2

Li(1)

2b

0.492(10)

0

0.5

0

0.5

Mn(1)

2b

=1-g(Li(1))

0

0.5

0

0.5

Li(2)

2c

1

0

0

0.5

1.0

Li(3)

4h

1

0

0.701(3)

0.5

1.0

Li(4)

4g

0.354(11)

0

0.1828(5)

0

0.5

Mn(2)

4g

=1-g(Li(4))

0

=y(Li(4))

0

0.5

O(1)

4i

0.785(11)

0.107(3)

0

0.153(2)

0.8

O(2)

8j

1

0.228(2)

0.3240(8)

0.2204(11)

0.8

Unit cell: monoclinic C2/m (12): a = 4.9314 (8) Å, b = 8.5207 (17) Å, c = 5.0211 (10) Å, β = 109.094 (11)°, ω= 0.72, Rwp = 1.50, Rp = 1.06.


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Table 3 X-ray Rietveld refinement results for LiMnO2 Atom

Site

g

x

y

z

B/Å2

Li(1)

2a

0.468(13)

0.25

0.647(5)

0.25

0.5

Mn(1)

2a

=1-g(Li(1))

0.25

=y(Li(1))

0.25

0.5

Li(2)

2a

0.440(14)

0.25

0

0.25

1.0

Mn(2)

2a

=1-g(Li(2))

0.25

0.701(3)

0.25

1.0

O(1)

2b

1

0.75

0.12(2)

0.25

0.8

O(2)

2b

1

0.75

0.63(2)

0.25

0.8

Unit cell: orthorhombic Pmnm (59): a = 4.5905 (9) Å, b = 5.725 (2) Å, c = 2.8235 (11), ω= 0.28, Å, , Rwp = 1.50, Rp = 1.06.


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Figure 1. The XRD pattern (a), SEM (b), TEM (c) and high-resolution TEM (d) images of the Na0.44MnO2 precursor. 128x93mm (150 x 150 DPI)


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Figure 2. The XRD pattern (a), SEM (b), TEM (c) and high-resolution TEM (d) images of the P-Li2MnO3. 129x107mm (150 x 150 DPI)


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Figure 3. The XRD pattern (a), SEM (b), TEM (c) and selected area electron diffraction pattern (d, e, f) of the R-Li2MnO3. 122x94mm (150 x 150 DPI)


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Figure 4. The high-resolution TEM images of the R-Li2MnO3. 214x190mm (150 x 150 DPI)


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Figure 5. Observed (blue), calculated (dotted solid red line) and difference plots (solid green line) for the Xray Rietveld analysis of the P-Li2MnO3. 181x169mm (150 x 150 DPI)



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Figure 6. Observed (blue), calculated (dotted solid red line) and difference plots (solid green line) for the Xray Rietveld analysis of the R-Li2MnO3. 181x166mm (150 x 150 DPI)


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Figure 7. High density of stacking faults in the high-resolution TEM images of the R-Li2MnO3. 192x190mm (122 x 122 DPI)



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Figure 8. The electrochemical properties of the P-Li2MnO3 and R-Li2MnO3 at 0.1 C in 2.0-4.8 V voltage range. The voltage-capacity curves (a) and the corresponding differential capacity plots (b) of the PLi2MnO3 and the R-Li2MnO3 in the first charge-discharge cycle; the voltage-capacity curves of the RLi2MnO3 in the initial 30 charge-discharge cycles (c); and cyclabilities of the P-Li2MnO3 and R-Li2MnO3 (d). 133x102mm (150 x 150 DPI)



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Figure 9. The electrochemical properties of the P-Li2MnO3 and R-Li2MnO3 at high rates. The typical voltagecapacity curves (a) and the corresponding dQ/dV plots (b) of the R-Li2MnO3 at the rates of 0.5 C, 1 C, 2 C, and 5 C, respectively; the cyclability of R-Li2MnO3 from the rate of 0.1 C to the rate of 5 C, with the cyclability of the P-Li2MnO3 at the same rates shown for comparison (c); the cyclability of the R-Li2MnO3 when charge-discharged galvanostatically for long time at the rates of 0.5 C, 1 C, 2 C and 5 C, respectively, (d). 189x142mm (150 x 150 DPI)



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Figure 10. The Nyquist plots and equivalent circuit (a) and relationship between Zre and ω-1/2 in the low frequency region (b) of the P-Li2MnO3 and R-Li2MnO3. 151x121mm (150 x 150 DPI)



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Figure 10. The Nyquist plots and equivalent circuit (a) and relationship between Zre and ω-1/2 in the low frequency region (b) of the P-Li2MnO3 and R-Li2MnO3. 163x121mm (150 x 150 DPI)


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Oxygen Vacancies and Stacking Faults Introduced by Low

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