Highly Stable Metal Mono-Oxide Alloy Nanoparticles and Their

Article pubs.acs.org/JPCC

Highly Stable Metal Mono-Oxide Alloy Nanoparticles and Their Potential as Anode Materials for Li-Ion Battery Gyoung Hwa Jeong,†,∥ Hyoung-Bong Bae,‡,∥ Donghyeuk Choi,‡ Young Hoon Kim,§ Songhun Yoon,*,§ and Sang-Wook Kim*,†,‡ †

Center of Molecular Science and Technology, and ‡Department of Molecular Science and Technology, Ajou University, Suwon 443-749, Korea § Green Chemical Technology Division, Korea Research Institute of Chemical Technology, Daejeon 305-343, Korea S Supporting Information *

ABSTRACT: We report the synthesis of MnxNi1‑xO and MnyCo1‑yO alloy nanoparticles by the thermal decomposition of the metal precursor in a surfactant. The different sized and shaped MnxNi1‑xO and MnyCo1‑yO nanoparticles could be obtained by controlling precursors and surfactants. These alloy nanoparticles are antiferromagnetic and their stability is better than that of pure metal mono-oxides. On the basis of these results, we expect these alloy nanoparticles to have potential applications as electrodes in energy-generating devices such as Li-ion batteries. The higher Ni content (Mn0.19Ni0.81O) electrode exhibited a large reversible capacity (650 mAh g−1), a better initial efficiency (56%), and an improved rate and cycle performance, which was ascribed to higher electrical/electrolyte conductivity or improved surface film property. To our best knowledge, the reversible Li storage in metal oxides like MnO or NiO nanoparticles with about 10 nm diameter material itself has not been reported yet, indicative of the originality of the anode application of our materials. Also, we could expect a higher stability by addition of Mn into theconversion anode and reduction of material cost when compared with the very expensive Sn- or Mo-based oxide materials, electrolyte conductivity, or improved surface film property.

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INTRODUCTION Inorganic nanoparticles with well-defined shapes and sizes have been the focus of much scientific research because of their chemical and physical properties, which are significantly different from those in their bulk state.1 Their synthesis, characterization, and application have been studied widely, both in fundamental and applied research areas. Among the nanoparticles, transition metal oxide nanoparticles have been studied, particularly with regard to their use as catalysts, gas sensors, MR contrast agents, and battery electrodes, because of their magnetic, electronic, and transport properties.2−6 Among them, as promising anode materials in lithium ion batteries (LIBs) based on conversion reaction, various transition metal oxides such as NiO, CoOx, MoOx (x = 2 and 3), and WOx have been intensively investigated.7−10 When they exist as nanostructurized forms such as nanoparticles or mesoporous materials, an improvement of the anode performance has been observed for those materials. Especially, Ni based oxide nanoparticles can be better anode candidates due to their abundance, low toxicity, and low metal price when compared with other metals.7−10 The oxide forms of transition metals such as Co, Ni, and Fe are similar to the oxides of Mn in that they have various oxidation states that exhibit different properties, and much research has been conducted except metal mono-oxide. There have been few reports on the synthesis of rock-salt-structured © 2012 American Chemical Society

metal mono-oxides, in spite of their important applications; this is because their structures are chemically unstable when compared with those of other oxides. Rock-salt-structured metal oxides such as MnO, CoO, and FeO show the trend of existing in the +3 or +4 oxidation states. Lee et al.11 reported on the preparation of MnO nanoparticles by the decomposition of the metal carbonate in organic solvents under solvothermal conditions, but Mn2O3 was obtained as the final product. In a previous study, we also reported the fabrication of Mn-ferrite nanoparticles from MnO nanoparticles by using unstable oxidation state of metal mono-oxide.12 This method could be applied to the fabrication of Co-ferrite nanoparticles from CoO nanoparticles. In addition, Nam et al.13 carried out an experiment on the oxidation state change in rock salt structured CoO with oxidation under air at 240 °C. The CoO nanoparticles synthesized by the thermal decomposition of cobalt acetylacetate in the presence of benzylamine transformed into Co3O4 within 20 min of oxidation. Similarly, the instability of FeO makes it difficult to prepare rock salt structured metal oxides. The metal oxides prepared by the solution phase decomposition of the metal salts can be converted into Fe3O4, γ-Fe2O3, α-Fe2O3, or Fe/Fe3O4 composites.14 Recently, we Received: February 27, 2012 Revised: August 31, 2012 Published: October 3, 2012 23851

dx.doi.org/10.1021/jp301899f | J. Phys. Chem. C 2012, 116, 23851−23857

The Journal of Physical Chemistry C

Article

Mn0.40Co0.60O nanocrystals, except that 0.5 mmol of Mn(acac)2, 0.1 mmol of Co(OAc)2, and 2.2 mmol of oleic acid for the Mn0.77Co0.23O and 0.2 mmol of Mn(acac)2, 0.2 mmol of Co(OAc)2, and 1.8 mmol of oleic acid for the Mn0.40Co0.60O nanocrystals were used, respectively. Electrode Fabricaiton and Their Anodes Application. For the preparation of composite anodes, as-prepared prepared nanoparticles were mixed with a conducting agent (Super P) and PVDF (polyvinyliden difluoride) binder with a weight ratio of 8:1:1.16 The mixture was then dispersed in NMP and spread on Cu foil (apparent areas of 1 cm2), followed by pressing and drying at 120 °C for 12 h. The half cell characteristics were analyzed with a coin-type (CR2016) two electrode cell in which lithium foil (Cyprus Co.) was used. The electrolyte was 1.0 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 volume ratio) (Tomiyama Co.). To investigate the anode performance in a lithium ion battery, a galvanostatic charge− discharge test in a voltage range of 2.5 to 0 V vs Li/Li+ was conducted. For the rate performance measurement, the current was varied from 0.1 to 2 C. The cycle performance for 100 cycles was recorded at a 0.5 C rate. All of the electrochemical measurements were made using a WBCS-3000 battery cycler (Wonatech Co.) at ambient temperature in a glovebox filled with argon. For the ac-impedance measurement, a frequency range of 106 Hz−5 mHz was used with an ac amplitude of 10 mV (Ivium potentiostat). Characterizations. TEM images were taken on a FEI Tecnai G2 F30 Super-Twin trasimission electron microscope operating at 300 kV. XRD patterns were obtained using a Rigaku Ultima III diffractometer equipped with a rotating anode and a Cu Ka radiation source (λ = 0.15418 nm). Inductively coupled plasma-optical emission spectra (ICPOES) were measured using the OPTIMA 5300DV, PerkinElmer (U.S.A). STEM-EDX images were obtained using a Titan 80-300TM (FEI) instrument. The superconducting quantum interference device (SQUID) magnetometer was measured MPMS5, Quantum Design.

reported on the synthesis of Mn1‑xFexO nanoparticles by the thermal decomposition of iron acetylacetonate (III) and manganese acetylacetonate (III) precursors.15 The particular stability of Mn1‑xFexO nanoparticles was better than that of MnO and FeO nanoparticles for more than two months, owing to which it could be used as a dual and simultaneous T1 and T2 contrast agent in MRI measurements. Our results established that various types of alloyed transition metal mono-oxide nanoparticles with similar rock-salt structures can be easily synthesized by using simple precursors; moreover, the results show that the synthesized nanoparticles have enhanced stability, owing to which they have several applications. In this manuscript, we describe a facile method to synthesize MnxNi1‑xO and MnyCo1‑yO alloy nanoparticles by the thermal decomposition of the metal precursor in a surfactant. The synthesized nanoparticles were expected to have enhanced stability and different magnetic properties. In order to validate this, we investigated the magnetic properties of the particles, after characterizing them by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). In addition, we tried the first time their potential as anode materials for LIBs.

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EXPERIMENTAL SECTION Materials. Manganese(II) acetylacetonate (Mn(acac)2, 98%), nickel(II) acetate tetrahydrate (Ni(OAc)2, 98%), cobalt(II) acetate tetrahydrate (Co(OAc)2, >99.9%), and 1octadecene (ODE, 90%) were purchased from Sigma-Aldrich. Oleic acid (OA, 99%) was obtained by TCI-GR. Synthesis of MnxNi1−xO Alloy Nanoparticles. The MnxNi1‑xO, MnyCo1‑yO, and other nanoparticles were synthesized in a one-step process. In a standard synthesis, the Mn0.69Ni0.31O alloy nanoparticle was successfully synthesized by using Mn(acac)2 (0.3 mmol), Ni(OAc)2 (0.1 mmol), oleic acid (1.2 mmol), and 1-octadecene (10 mL) in a three neck flask. This mixture was heated to 110 °C with vigorous stirring under a vacuum for 3 h and then further heated to 320 °C at a rate of 2.1 °C/min and maintained at this temperature for 40 min under N2 gas. When the reaction temperature nearly reached 320 °C, the solution changed color to dark-brown. After the reaction was finished, the solution was cooled to room temperature, and then nanoparticle was precipitated by using acetone/isopropyl alcohol (the volume ratio 3:1) with a centrifugugation. The same procedure was used to prepare Mn0.43Ni0.57O and Mn0.19Ni0.81O nanocrystals, except that 0.2 mmol of Mn(acac)2 and 0.2 mmol of Ni(OAc)2 for Mn0.43Ni0.57O; 0.1 mmol of Mn(acac)2 and 0.3 mmol of Ni(OAc)2 for Mn0.19Ni0.81O; and 1.8 mmol of OA, 0.5 mmol of Mn(acac)2, and 0.1 mmol of Ni(OAc)2 for Mn0.80Ni0.20O (0.5/ 0.1 mmol) were used. Synthesis of MnyCo1−yO Alloy Nanoparticles. In a standard synthesis, Mn0.71Co0.29O alloy nanoparticles were successfully synthesized by using Mn(acac)2 (0.3 mmol), Co(OAc)2 tetrahydrate (0.1 mmol), oleic acid (1.8 mmol), and 1-octadecene (10 mL) in a three neck flask. This mixture was heated to 110 °C with vigorous stirring under vacuum for 3 h, and then further heated to 320 °C at a rate of 2.1 °C/min and maintained at this temperature for 50 min under nitrogen gas. When the reaction temperature nearly reached 320 °C, the solution changed color to dark blue. After the reaction was finished, the solution was cooled to room temperature. Also, this nanoparticle was precipitated by using acetone/isopropyl alcohol (the volume ratio 3:1) with a centrifugation. The same procedure was used to prepare Mn 0.77 Co 0.23 O and

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RESULTS AND DISCUSSION Thermal decomposition of various precursors in hot surfactant solution is one of the most common methods for fabricating highly crystalline and size-controlled transition metal oxide crystals.2−4 Various precursors such as oleates, acetates, acetylacetonates, and carbonyls have been used in this method. For example, Sun et al.2 synthesized uniform Mn3O4, CoO, and CuO nanoparticles by using metal formate precursors in a coordinating solvent. Monodisperse and size-controlled MnO and Mn3O4 nanocrystals were prepared from acetate and acetylacetonate, respectively.17,18 Chen et al.19 synthesized dotlike NiO and flower-like CoO nanoparticles using Ni-oleate and Co-oleate as the precursors. From the results of their experiments, it was found that in the synthesis of metal oxides, the metal precursors, including the metal, surfactants, and solvents, as well as the synthetic conditions, including temperature, affect the size and shape of the formed crystals. In our experiments, MnxNi1‑xO and MnyCo1‑yO alloy nanoparticles were synthesized by the thermal decomposition of the acetylacetonates (acac) and acetates (OAc) of Mn, Ni, and Co as the precursors, in the presence of oleic acid as the surfactant and 1-octadecene as the solvent. The particles were successfully synthesized by the “heating up” method in a high boiling-point solvent, the boiling point of 1-octadecene being 320 °C.20 During the reaction, the metal precursors combined with oleic 23852



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To further investigate the alloy structures of the prepared particles, the MnxNi1‑xO nanoparticles were characterized using powder XRD. Figure 2a shows the XRD patterns of the

acid, thereby forming metal oleates when the temperature increased up to 320 °C. It is known that carboxylic acids with long alkyl chains, such as oleic acid, are frequently used in the synthesis of transition metal oxide nanoparticles, because acid groups can easily form metal complexes,21 following which they undergo decomposition. It is believed that, at times, the transitions between these two steps are instantaneous. After the decomposition of the compounds, the metal oxide nuclei were formed and grown into nanoparticles for aging at the reaction temperature (320 °C). The compositions of these alloy nanoparticles were varied by varying the precursor ratios of Mn to Ni and Mn to Co. MnxNi1‑xO Alloy Nanoparticles. The MnxNi1‑xO alloy nanoparticles were prepared by heating the Mn and Ni precursor mixtures; Mn to Ni mole ratios of 5/1, 3/1, 1/1, and 1/3 resulted in the formation of Mn0.80Ni0.20O, Mn0.69Ni0.31O, Mn0.43Ni0.57O, and Mn0.19Ni0.81O, respectively, which were measured using inductively coupled plasma-optical emission spectroscopy (ICP-OES). The results showed that the reactivity of the Ni precursor was better than that of the Mn precursor (Supporting Information, S1). Figure 1 shows the

Figure 2. (a) XRD patterns of MnxNi1‑xO alloy nanoparticles: Black line, Mn0.80Ni0.20O; red line, Mn0.69Ni0.31O; blue line, Mn0.43Ni0.57O; and green line, Mn0.19Ni0.81. The top reference line is NiO and the bottom reference line is MnO. (b) XRD pattern changes of Mn0.69Ni0.31O at 120 °C for testing heat stability.

MnxNi1‑xO nanoparticles; each pattern exhibits five diffraction peaks that can be indexed to diffraction from the (111), (200), (220), (311), and (222) planes of the cubic rock-salt structure at all compositions.22 The diffraction peaks of the alloy particles represented a shift from pure MnO to pure NiO as the Mn content decreased, and a good linear relationship was found to exist between the peak position and the mole fraction of MnO. The linear shifts in the peak due to a variation in composition can be regarded as evidence that the alloy particles conform to Vegard’s law23 (Supporting Information, S3). Additionally, Nirich alloy nanoparticles such as the Mn0.19Ni0.81O nanoparticles showed a relatively high peak intensity ratio of the (111) plane to the (200) plane, which implies that NiO has a higher (200) plane surface energy when compared to MnO. In the cubic rock-salt structure, the three low-energy surfaces are the {100}, {110}, and {111} surfaces, with surface energy ratio of 1/1.4/ 1.73.24 However, it has been shown that MnO nanoparticles tend to form the octahedron with the {111} surface dominantly.25 Therefore, by increasing the amount of Ni contents, this can be attributed to the formation of the pod structures so as to minimize the {111} surface of high surface energy. This above-mentioned hypothesis can be proved by TEM (Figure 1c,d). The alloy nanoparticles have particular stability, whereas pure rock salt MO has a metastable property. The XRD patterns of the Mn0.69Ni0.31O nanoparticles observed for 24 h at 120 °C are shown in Figure 2b. The sample after 24 h has the same peak pattern and a similar intensity compared with those of original particles, whereas pure MnO nanoparticles transformed into Mn3O4 (Supporting Information, S4a). Also, we could observe the XRD pattern after six months, which indicates that their structure is not changed and quite stable (Supporting Information, S4b). The reason is not convincing, however, because it is assumed that particular stability resulted from alloyed cubic structure prevent them from transforming to the more stable state, tetragonal Mn3O4. That is, pure metal mono-oxide easily transforms to a more stable structure, for example, Mn3O4, Co3O4, and Fe3O4; however, the alloy structures, which stabilized each other by Hume−Rothery rules, maintain their structure for a long time, otherwise their phases are segregated after oxidation due to the different crystal structures of Mn3O4, Co3O4, and Fe3O4.

Figure 1. TEM images of MnxNi1‑xO alloy nanoparticles . (a) Mn0.80Ni0.20O, (b) Mn0.69Ni0.31O, (c) Mn0.43Ni0.57O, and (d) Mn0.19Ni0.81O. The scale bar size is 10 nm. In the inset, the scale bar size is 5 nm.

TEM images of the MnxNi1‑xO alloy nanoparticles. As shown in Figure 1a,b, the prepared Mn0.80Ni0.20O and Mn0.69Ni0.31O nanocrystals have nearly spherical shapes, and the Mn0.69Ni0.31O nanocrystals are smaller than the Mn0.80Ni0.20O nanocrystals. The average particle sizes of the Mn0.80Ni0.20O and Mn0.69Ni0.31O nanoparticles are 9.94 and 9.26 nm, respectively (Supporting Information, S2). In addition, the prepared Mn0.43Ni0.57O and Mn0.19Ni0.81O nanoparticles have tri- or tetra-pod structures with irregular branches (Figure 1c,d). From Figure 1, it can be confirmed that the molar ratio between the Mn and Ni precursor influenced the size and shape of the synthesized particles. The TEM images and ICP-OES data show that the composition of the MnxNi1‑xO alloy nanoparticles does not correspond to the initial stoichiometry of the metal precursor and that the monodispersity and regularity of the synthesized particles become worse as the amount of the Ni precursor is increased. 23853



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MnyCo1‑yO Alloy Nanoparticles. The MnyCo1‑yO alloy nanoparticles were prepared by heating the Mn and Co precursor mixtures; Mn to Co mole ratios of 5/1, 3/1, and 1/1 resulted in the formation of Mn0.77Co0.23O, Mn0.72Co0.28O, and Mn0.41Co0.59O, respectively, which were measured using ICPOES (Supporting Information, S1). The data were obtained reproducibly in repeated reactions, which showed that there are no strong stoichiometric effects of the molar ratio between the Mn and Co precursors. As shown in the TEM images in Figure 3, the shape trend of the MnyCo1‑yO nanoparticles was different from that of the MnxNi1‑xO nanoparticles. Figure 4. XRD patterns of MnxCo1‑xO alloy nanoparticles. Blue line, Mn0.77Co0.23O; red line, Mn0.72Co0.28O; and black line, Mn0.41Co0.59O. The top reference line is CoO and bottom reference line is MnO.

Figure 3. TEM images of MnyCo1‑yO alloy nanoparticles. (a) Mn0.77Co0.23O, (b) Mn0.72Co0.28O, and (c) Mn0.41Co0.59O, the scale bar size is 10 nm. (d) HR-TEM image of Mn0.72Co0.28O alloy nanoparticles. The scale bar size is 5 nm.

When compared with the MnxNi1‑xO nanoparticles, the MnyCo1‑yO nanoparticles had nearly spherical structures with different sizes, depending on the composition. The average sizes of the Mn0.77Co0.23O, Mn0.72Co0.28O, and Mn0.41Co0.59O alloy nanoparticles were 14.41, 11.67, and 10.72 nm, respectively (Supporting Information, S2). As the amount of the Mn precursor decreased, the sizes of the MnyCo1‑yO nanoparticles also decreased; however, there was no change in their shapes (Figure 3a−c). We further analyzed the Mn0.77Co0.23O nanoparticles by using HR-TEM. Figure 3d showed that the prepared particles have a good crystalline structure with well-defined lattice fringes. The powder XRD data exhibited five distinct diffraction patterns that can be indexed to diffraction from the (111), (200), (220), (311), and (222) planes of the cubic rock-salt structure at all compositions.26 The data of MnyCo1−yO nanoparticles is similar to that of MnxNi1‑xO nanoparticles in that the linear shifts in the peak due to a variation in composition occurred18 (Figure 4). We increased the amount of oleic acid to 3.6 mmol to observe the effect of surfactants. As a result, as shown in Figure 5a,b, the MnyCo1‑yO alloy nanoparticles, which have tetrapodshapes with long and short branches, were obtained. From the images of the nanoscale element mapping, it is confirmed that Mn and Co atoms were distributed evenly in the structure, which implied the MnyCo1‑yO nanoparticles still retain their alloy structures in spite of different shapes (Figure 5c,d). In

Figure 5. HAADF-STEM image and the corresponding EDS mapping of Mn0.72Co0.28O alloy nanoparticles. (a) Low-magnatification TEM image, (b and c) high-magnatification TEM images, (d) Mn mapping image, (e) Co mapping image, and (f) overlap image of Mn and Co mapping. The scale bar sizes are (a) 200 and (b) 20 nm.

addition, when the mixed surfactants with a 1:1 ratio of oleic acid and oleyamine were used, the size of MnyCo1‑yO alloy nanoparticles was very large (50−100 nm) and the shapes were similar to a sea urchin with small spines (Supporting Information, S5). Magnetic Properties of MnxNi1‑xO and MnyCo1‑yO Alloy Nanocrystals. The magnetic properties of the MnxNi1‑xO and MnyCo1‑yO alloy nanocrystals were studied using a superconducting quantum interference device 23854



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was close to the theoretical maximum capacity of 735 mAh g−1 that is based on the following reaction:

(SQUID). MnO nanoparticles were synthesized using a similar method as that used for the synthesis of alloy nanoparticles in order to compare their magnetic properties. TEM image and XRD pattern of the prepared MnO nanoparticles are shown in the Supporting Information, S6. Figure 6 shows the magnet-

MnxNi1 − xO + 2Li+ + 2ye → MnxNi1 − xO1 − y + y Li 2O (1)

When considering the increase of the reversible capacity with larger Ni content, the Mn metal itself was supposed to be less active for Li storage and thus NiO could provide better charge storage sites, which has not been reported by other groups. Because homogeneous Mn1‑xNixO nanoparticles were successfully prepared in our approach, the different Li storage activity of Ni and Mn could be clarified. For the purpose of better investigation of Li conversion reaction, the differential capacity was plotted against voltage in Figure 7b. Clearly, two charge storage peaks were observed in the 1 − x = 0.57 and 0.81 cases, which are characteristic in the conversion reaction described by eq 1. Here, y is variable because the reversible capacity was changed according to composition x. In order to the investigate synergistic effect of the Ni−Mn mono oxide binary, we prepared NiO and MnO separately. When applied into anode materials, the NiO anode exhibited two clear discharging peaks at 1.3 and 2.3 V vs Li/Li+, which is similar to those of Mn1−xNixO electrodes (Supporting Information, S7). In the case of the MnO anode, however, the discharge voltage range was broadly located from 0.5 to 1.5 V, indicating that Li+ in the Mn1−xNixO anode was discharged in a similar pattern to NiO. In the case of cycle performance of NiO and MnO (Figure S8(b)), poor cycle property of NiO anode was observed when compared with that of the Mn1−xNixO anode. Hence, one can expect that the existence of a more inert phase of MnO can be helpful to improve the cycle performance of the Mn1−xNixO anode, indicative of a synergistic effect of complete nanosized alloy. Because the Mn0.19Ni0.81O electrode showed a very low reversible capacity, the anode performance of the other two electrodes was analyzed in depth. For a rate capability analysis of the two anodes, the current rate was varied from 0.1 to 2 C, and the results are displayed in Figure 8a. Owing to the

Figure 6. Hysteresis loops of the synthesized nanoparticles.

ization curve as a function of the external magnetic field at room temperature (300 K) and at a low temperature (5 K). It shows that the induced magnetization (M) of the Mn0.69Ni0.31O and Mn0.72Co0.28O alloy nanoparticles was similarly small compared with that of the MnO nanoparticles and their hysteresis disappeared at high temperature (300 K). It means that MnO, Mn0.69Ni0.31O, and Mn0.41Co0.59O particles exhibited similar antiferromagnetic behavior at room temperature. These phenomena revealed that the properties of the rock-saltstructured alloy metal oxide nanoparticles are similar to that of pure rock-salt metal oxide nanoparticles. Application as the Anode Material in LIBs. Figure 7 shows the galvanostatic charge−discharge patterns of the

Figure 7. Galvanostatic charge−discharge patterns of the Mn0.19Ni0.81O anodes at current rates of 0.1 C from 2.5 to 0 V vs Li/Li+.

MnxNi1‑xO anodes at current rates of 0.1 C from 2.5 to 0 V vs Li/Li+. For all electrodes, a characteristic plateau near 0.5 V followed by a sloped decrease in voltage was observed during the first charging process, which is relevant to the conversion reaction of NiO.1 During the discharging process, a distinct voltage plateau was hard to observe, which is discussed in detail from the differential capacity plot. Obviously, the initial charge (Cch) and discharge capacities (Cdis) of the MnxNi1‑xO anodes became larger according to the increase of Ni content (1 − x). For 1 − x = 0.81 electrode, Cch and Cdis were as high as 1160 and 650 mAh g−1, respectively. Its initial efficiency (IE) was 56%. This also indicates that 3.18 Li/MnxNi1‑x was charged and 1.77 Li/MnxNi1‑x was reversibly extracted. The observed Cdis

Figure 8. (a) Rate capability analysis of two anodes. (b) Cycle performance of two electrodes. Blue line, Mn0.19Ni0.81O; black line, Mn0.43Ni0.57O.

intrinsically sluggish feature of the conversion reaction, a large capacity decrease was inevitable. As shown in the Supporting Information, S8, the initial voltage drop during voltage change at 0 and 2.5 V remained unchanged, but a distinct decrease of available capacity was observed, which revealed again the kinetic nature of the conversion reaction.7−10,16,27 Generally, the nanosized Mn grains create a large contacting surface between Mn and Li2O that makes the reverse reaction kinetically favorable. The conversion reaction depends on the 23855



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MnOx phase structure, amount of oxygen, x, in MnOx, and particle size. However, the anode materials with MnOx particles include MnO particles have the problem which is the poor cycling performance as well as their stability.28,29 In addition, although the conversion reaction shows a reasonable performance in charge and discharge processes, MnO shows a smaller performance than those measured on the other transition metal oxides.30 Therefore, in our case, as the content of Ni is increased, a better cycling performance of prepared electrode (Mn0.19Ni0.81O) is observed . Interestingly, however, a better rate capability was observed in an electrode of higher Ni content (1 − x = 0.81) as listed in Table 1, which is probably

As seen, the EIS spectra were composed of one or two semicircles in the high frequency region followed by a straight line with decreasing frequency, which is characteristic in the anode for an LIB.16,35 At the highest frequency, the intercept resistance in the Zreal axis (Rb) corresponded to a bulk electrolyte resistance bewteen working and counter electrodes.16 The resistance from the distributed semicircle (Rs) can encompass overall electrical/electrolyte conduction in the electrode or ionic transport within the surface film.16,36 Polymeric species by electrolyte decomposition can deposit on a metal oxide surface, which indicated that Rs can also contain the resistance of the Li+ conduction within this polymeric film. As seen in Figure 8, it was hard to separate the charge transfer reaction at the interface (Rct) and the Warburg diffusion. In the initial state, the EIS speactra was similar for bothe electrodes. After 100 cycles, however, a large increase of Rs was observed in the 1 − x = 0.5 electrode, while Rs for Mn0.19Ni0.81O electrode remained invariant, which reflected that no increase of electrical/electrolyte conducvity or surface film of the latter electrode appeared in the large Ni content electrode.

Table 1. Electroanalysis of Materials as Ni Content Cch,a Cdis,b mAh g−1 mAh g−1 Mn0.69Ni0.31O Mn0.43Ni0.57O Mn0.19Ni0.81O

561 1150 1160

88 550 650

IE (%)

rate (%) 2C/0.1 C

cycle (%)/100 cycle (0.5 C)

16 47 56

19 34

71 88

a

Initial charging capacity (Cch) at 0.1 C rate. bInitial discharging capacity (Cas) at 0.1 C rate.

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CONCLUSIONS MnxNi1‑xO and MnyCo1‑yO alloy nanoparticles were obtained using a facile synthetic method, which consists of thermal decomposition of the metal precursor, nuclei formation, and growth in oleic acid as a surfactant and octadecine as a solvent. The different sized and shaped MnxNi1‑xO and MnyCo1‑yO nanoparticles could be obtained by controlling precursors and surfactants. Furthermore, we showed that the these alloy nanoparticles are antiferromagnetic and that their stability is better than that of metal mono-oxides. On the basis of these results, we expect these alloy nanoparticles to have potential applications as electrodes in energy-generating devices such as Li-ion batteries. The higher Ni content (Mn0.19Ni0.81O) electrode exhibited a large reversible capacity (650 mAh g−1), a better initial efficiency (56%), and an improved rate and cycle performance, which was ascribed to higher electrical/electrolyte conductivity or improved surface film property. To the best of our knowledge, the reversible Li storage in metal oxides like MnO or NiO nanoparticles with about 10 nm diameter material itself has not been reported yet, indicative of the originality of the anode application of our materials. Also, we could expect a higher stability by addition of Mn into conversion anode and reduction of material cost when compared with the very expensive Sn- or Mo-based oxide materials.

attributed to higher electrical conductivity and better activity of Li storage. In Figure 8b, cycle performance of two electrodes was displayed at 0.5 C rate until 100 cycles. For both electrodes, an initial capacity decrease and an additional gradual capacity increase were observed that are characteristic behaviors of conversion reaction for nanosized metal oxide. As reported by Yang et al., this is attributed to losing crystallinity or transforming to an amorphous-like structure during cycling, which improves the Li diffusion kinetics and accommodate higher Li storage.31 In addition, the electrode of higher Ni content (1 − x = 0.81) exhibited a better cycle retention after 100 cycles (88%) than the 1 − x = 0.57 case, indicative of an improved capacity recovery. In the Introduction, we mentioned other oxide anode materials in Li-ion battery. In most papers, anode materials combined with support materials like as mesoporous materials which have a large surface area. On the other hand, microsized particles are used as anode materials for Li-ion battery.32−34 Although the capacity fade of NiO and MnO2 was severe in the previous paper,35,36 our capacity retention is high. In order to investigate the resistance change after cycles, electrochemical impedance spectroscopy (EIS) was carried out for the two electrodes in the 5 × 10−3 to 105 Hz frequency region at the begining and after 100 cycles. Figure 9 displays the EIS spectra as Nyquist plots.

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ASSOCIATED CONTENT

S Supporting Information *

ICP data, TEM images, XRD patterns, and electrochemical data for the synthesized nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +82 31 2192522. Fax: +82 31 2191592. E-mail: [email protected]; [email protected]. Author Contributions ∥

These authors contributed to this work equally.

Notes

Figure 9. EIS spectra as Nyquist plots: (a) initial and (b) after 100 cycles. Blue line, Mn0.19Ni0.81O; black line, Mn0.43Ni0.57O.

The authors declare no competing financial interest. 23856



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(34) Wu, Z. S.; Ren, W.; Wen, L.; Gao, L.; Zhao, J.; Chen, Z.; Zhou, G.; Li, F.; Cheng, H. M. ACS Nano 2010, 4, 3187. (35) Subramanian, V.; Zhu, H.; Wei, B. Pure Appl. Chem. 2008, 80, 2327. (36) Yuan, L.; Guo, Z. P.; Konstantinov, K.; Munroe, P.; Liu, H. K. Electrochem. Solid-State Lett. 2006, 9, A524. (37) Bard, L. R. F. A. J., Ed.; Electrochemical Methods: Fundamentals and Applications; John Wiley & Sons: New York, 1988.

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) Grant No. 2010-0008823 and Priority Research Centers Program (2009-0093826), Republic of Korea.

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