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胡英瑛, 温兆银, 靳俊. 介孔四氧化三锰纳米棒的快速低成本制备与电化学性能. 无机材料学报, 2013, 28(9): 1045-1050
HU Ying-Ying, WEN Zhao-Yin, JIN Jun. Rapid Low-cost Synthesis and Enhanced Electrochemical Properties of Mesoporous Mn3O4 Nanorods . Journal of Inorganic Materials, 2013, 28(9): 1045-1050  
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介孔四氧化三锰纳米棒的快速低成本制备与电化学性能
胡英瑛, 温兆银, 靳俊
中国科学院 上海硅酸盐研究所, 上海200050
修回日期:2013-03-29
摘要

仅以乙醇和四水合醋酸锰为原料, 快速低成本地合成了介孔四氧化三锰纳米棒, 并将其应用于锂离子电池负极材料。通过X 射线衍射、热重分析仪、扫描电子显微镜、透射电子显微镜和比表面积仪等分析手段对四氧化三锰样品进行了表征。实验结果表明: 介孔四氧化三锰纳米棒的平均直径约为150 nm, 孔的尺寸范围为6~20 nm, BET比表面积高达37.3 m2/g。同时, 介孔四氧化三锰纳米棒负极材料在141 mA/g的电流密度下循环100次后可逆充放电容量为676.1和662.4 mAh/g, 而且其在不同的电流密度下继续循环80次后可逆放电容量高达850 mAh/g, 体现出了较高的容量、好的循环稳定性能和倍率性能。

关键词: 氧化锰; 介孔; 纳米棒; 锂离子电池
中图分类号:TB34   文献标志码:A    文章编号:1000-324X(2013)09-1045-06
Rapid Low-cost Synthesis and Enhanced Electrochemical Properties of Mesoporous Mn3O4 Nanorods
HU Ying-Ying, WEN Zhao-Yin, JIN Jun
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China
Corresponding author: WEN Zhao-Yin, professor. E-mail:zywen@mail.sic.ac.cn
Fund:Chinese Science and Technology Ministry (2007CB209700)
Abstract

Mesoporous Mn3O4 nanorods, as anode materials for Li-ion batteries, were fabricated by a novel process using only ethanol and manganese acetate tetrahydrate as the reaction precursors. The as-prepared mesoporous Mn3O4 nanorods were characterized by X-ray diffraction, thermogravimetry-differential scanning calorimetry, scanning electron microscope, transmission electron microscope and Brunauer-Emmett-Teller surface area analyzer. The results indicate that the average diameter of mesoporous Mn3O4 nanorods is about 150 nm, and their pore size is mainly distributed in the range of 6-20 nm with the BET specific surface area as high as 37.3 m2/g. The Mn3O4 nanorods anode displays reversible capacities of 676.1 and 662.4 mAh/g after 100 cycles at a current rate of 141 mA/g, demonstrating a higher capacity and more stable cyclability than Mn3O4 nano-powders. Furthermore, excellent rate capability is realized with the mesoporous Mn3O4 nanorods. A capacity of 850 mAh/g is retained after 80 cycles at various current densities.

Keyword: manganese oxide; mesoporous; nanorod; lithium-ion battery

Recently, transition metal oxides, as electrode materials for rechargeable lithium-ion batteries (LIBs), are very attractive due to their high theoretical capacity, ecological benignity, high safety, etc[ 1]. However, the practical application of transition metal oxides for LIBs suffers from their limited cycling and rate performance caused by the large volume expansion during the Li+ uptake/release process, the difficulties in mass production and high cost[ 2]. Battery research is focusing heavily on integrating conductive and/or buffering networks into the active materials so as to tackle their pulverization and poor electronic conductivities, however, the introduction of low-capacity matrixes leads to decreased capacity of the cells[ 3]. In most cases, it seems that preparation of transition metal oxides with nanometer-scale dimension and morphological specificity has a bright perspective for use in LIBs[ 4, 5].

Manganese oxides have been considered as potential electrode materials in LIBs on the basis of their high theoretical lithium-storage capacities: 615 mAh/g for MnO2, 679 mAh/g for Mn2O3, and 703 mAh/g for Mn3O4[ 6]. Nowadays, it is crucial to fabricate high-quality nanostructured manganese oxides (especially Mn3O4) quickly and efficiently through low-cost and green methods. There were several means available for the synthesis of nanostructured Mn3O4, such as organic solvent-assisted thermal decomposition[ 7], polymeric-precursor[ 8], microwave-assisted[ 9], surfactant-mediated[ 10], and solvothermal[ 11, 12] routes. Almost all the methods involved the emission of organic pollutants at the higher reaction temperature with auxiliary energy supply or with long heat treatment time. Up to now, there are few researches related to the application of Mn3O4 as anode materials in LIBs[ 13], partly because of their limited capacity and rate performance even with Co doping. Lately, Mn3O4 nanoparticles on reduced graphene oxide sheets were developed with a high capacity of 730 mAh/g and good rate and cycling performance[ 14], showing their excellent potential for LIB application. Spongelike nanosized Mn3O4 exhibited a high initial capacity of 869 mAh/g and reversible capacity of around 700 mAh/g after 40 charge and discharge cycles at the rate of 30 mA/g and significantly enhanced first coulombic efficiency. However, these improved results were obtained at low current rates, and no work showing the reversible capacity affer 100 cycles was presented[ 15].

In this study, we report a rapid low-cost and high-efficiency method for the fabrication of mesoporous Mn3O4 nanorods with enhanced electrochemical performance as the anode materials for LIBs. The Mn3O4 nanorod based anode exhibits a higher capacity and more stable cyclability compared to Mn3O4 nano-powders and other labs reports[ 13, 14, 15].

1 Experimental Sections
1.1 Sample preparation and characterization

All chemicals are of analytical-grade without further purification, and were supplied by Sinopharm Chemical Reagent Co. Ltd (China). In a typical synthesis, 0.1 mol/L manganese acetate tetrahydrate (Mn(CH3COO)2·4H2O was dissolved in 30 mL of anhydrous alcohol at room temperature. The solution was vigorously stirred to give a transparent mixture, followed by formation of white precipitates with stirring constantly. After several hours, the amount of precipitates no longer increased. A white powder was collected by centrifugation, then washed twice with alcohol, and finally dried at 50℃ for 6 h. The precipitates were calcined in air at 450℃ for 2 h to obtain nanostructured Mn3O4. The reference sample was obtained by annealing commercial Mn(CH3COO)2·4H2O in air at 450℃ for 2 h. The products were subjected to powder X-ray diffraction (XRD) (Rigaku Ultima IV, Cu Kα radiation, λ=0.15418 nm), scanning electron microscope (SEM) (Hitachi S-3400N, 15.0 kV), and transmission electron microscope (TEM and HRTEM) (JEOL-JEM 2010 type, 100 kV). Thermogravimetry-differential scanning calorimetry (TG-DSC, NETZSCH 409PC) was carried out with well ground samples in flowing air at a heating rate of 5 ℃/min. Nitrogen sorption isotherms were measured on a Micrometitics Tristar 3000 system.

1.2 Electrochemical reaction of Li with nanostructured Mn3O4

The working electrodes were prepared by casting a slurry consisting of 75wt% active material, 15wt% carbon black, and 10wt% polyvinylidene fluoride (PVDF) onto a copper foil. CR2025 type coin cells were assembled in an argon-filled glove box with H2O and O2 contents lower than 1×10-6 by using the as-prepared working electrodes as the anode, a lithium foil as the counter and reference electrodes, Celgard 2400 as the separator, and 1 mol/L solution of LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by volume) as the electrolyte. The galvanostatic charge-discharge cycling was performed at room temperature on a LAND CT2001A battery test system in a voltage range of 0-3.0 V vs Li/Li+.

2 Results and discussion

The X-ray diffraction patterns of the calcined products of commercial Mn(CH3COO)2·4H2O and the as-obtained precipitates are shown in Fig. 1. All the peaks of the products are indexed to single phase tetragonal Mn3O4 (JCPDS 24-0734; a=0.576 nm and c=0.946 nm). No impurity phases are detected in the samples.

Fig. 1 XRD patterns of the calcined products from (a) commercial manganese acetate tetrahydrate and (b) the as-obtained precipitates

The thermal behavior of the commercial manganese acetate tetrahydrate is displayed in Fig. 2(a). There are two endothermic peaks blow 110℃ with a net weight loss of 25%, indicating the release of 3.5 molecules of water[ 16]. The sharp exothermic peak at about 315.2℃ in the DSC curve is as cribed to the decomposition of Mn(CH3COO)2·0.5H2O to Mn3O4. The total weight loss in the process is 65%, that is close to the theoretical value of 62%. Nevertheless, the TG curve of the as-obtained precipitates in Fig. 2(b) only shows 2.5% weight loss at low temperature (<100℃), corresponding to the removal of adsorbed water rather than the crystallization water. For the decomposition of anhydrous Mn(CH3COO)2 to Mn3O4, the theoretical weight loss is 55.8%, similar to the result observed in the measurement (52%). It is therefore suggested that the formation of the precipitates is a process of dehydration and recrystallization.

Fig. 2 TG-DSC curves of (a) the commercial manganese acetate tetrahydrate and (b) the as-obtained precipitates in air

The morphology of the as-obtained precipitates is displayed in Fig. 3(a) and (b). Plate-shaped structure consisting of some rods with average diameter and length of 250 nm and 6 μm was observed. As shown in the Fig. 3(c) and (d), the Mn3O4 converted from the precipitates displays a crosslinked nanorod shape with an average diameter of 150 nm, which preserves the morphology of the precursors.

Fig. 3 SEM images of the as-precipitated manganese acetate (a, b) and Mn3O4 nanorods (c, d)

The structure of the products is further confirmed by transmission electron microscope (TEM) in Fig. 4. Figure 4(a) and (b) show typical morphology of Mn3O4 nanorods. Clearly, a large number of nanopores are widely distributed in the Mn3O4 nanorods, the yield of which would rely on the pyrolysis of acetate and the release of gases (CO2, H2O) during the calcination process. Figure 4(c) shows a typical high-resolution TEM (HRTEM) image of the mesoporous Mn3O4. Interplanar distances of 0.25 and 0.5 nm can correspond to the (211) plane of tetragonal Mn3O4 particles.

Fig. 4 TEM images of mesoporous Mn3O4 nanorods (a, b) and HRTEM analysis of Mn3O4 nanorods (c)

SEM image of the calcined products of commercial manganese acetate tetrahydrate in air is shown in Fig. 5. Serious agglomeration of the nano Mn3O4 particles is observed.

Fig. 5 SEM image of the calcined products from commercial manganese acetate tetrahydrate in air

The pore size distribution and specific surface area of mesoporous Mn3O4 nanorods were further investigated by the BET measurement in Fig. 6. The nitrogen adsorption-desorption isotherm displays a typical H1 hysteresis loop[ 17], indicating the existence of mesopores. The pore distribution curve (inset in Fig. 6) indicates that the pores are mainly distributed in the range of 6-20 nm. The BET specific surface area of mesoporous Mn3O4 is 37.3 m2/g, which is much higher than the commerical Mn3O4 crystals (1 m2/g), even 4 times as high as that of the synthetical Mn3O4 nano-octahedra (9 m2/g)[ 18].

Fig. 6 Nitrogen adsorption/desorption isotherms and pore size distribution curve (inset) of the mesoporous Mn3O4 nanorods

Figure 7(a) shows the charge/discharge voltage profiles of the Mn3O4 nanorod based anode for the 1st and 2nd cycles at a constant current rate of 0.2 C (1 C = 703 mA/g). The first and second discharge capacity are 1094.4 and 618.6 mAh/g, respectively. The coulombic efficiency for the first cycle is 54.2%, but it rises to 87.7% for the second cycle. The first efficiency is among the highest values reported up to now[ 16, 18]. The relatively low first efficiency may ascribed to the well-known formation of a surface-electrolyte interphase (SEI) film and the unavoidable irreversible reaction involving the decomposition of Mn3O4 into MnO[ 6, 19]:Mn3O4+ 2Li++ 2e-  3MnO + Li2O (1)

MnO +2Li+ +2e- ↔ Mn + Li2O (2)

Fig. 7 Voltage-capacity profiles of (a) the galvanostatic charge-discharge cycles, cycling behaviour (b) and rate performance (c) of the Mn3O4 nanorods based electrode; (d) Capacity vs cycle number of the nano Mn3O4 aggregates

The first discharge curve displays a sloping plateau at 0.4 V reflecting the Li+ charge reaction with Mn3O4, and the lithium extraction reaction occurs at 1.2 V, which is consistent with the previous reports[ 12]. Figure 7(b,d) illustrate the cycling behaviour at 0.2 C (141 mA/g) of Mn3O4 nanorods (r-Mn3O4) and nanoparticle aggregates (a-Mn3O4) obtained from commercial manganese acetate tetrahydrate, respectively. After 47 cycles, the discharge capacity of a-Mn3O4 is 244 mAh/g, retaining only 19.3% of the initial capacity. Mn3O4 nano aggregates with a large surface area can yield a high capacity for the initial 10 cycles, but the particles tend to pulverize during cycling, resulting in a lower capacity. Significant improvement in the cycling performance was achieved with the r-Mn3O4 which could deliver reversible capacities of 676.1 and 662.4 mAh/g even after 100 cycles, obviously higher than that of a-Mn3O4 and the previous reports[ 13, 14, 15]. Moreover, after a normal initial degrade of the capacity within nearly 25 cycles, the capacity of the r-Mn3O4 increases. According to Liu’s study, the reasons contributing to the capacity decrease upon the first 25 cycles are probably low lithium-diffusion processes and high polarization[ 20], while the capacity increases after 25 cycles could be explained that the particle size of the mesoporous structure decreases during the electrochemical milling, and a larger surface area can yield a higher capacity, which is in agreement with Hassan and Chen’s results[ 21, 22]. Hence, the good cycling performance of the r-Mn3O4 is proposed to be related to its nano and mesoporous structure.

It should be emphasized that the outstanding rate performance of the r-Mn3O4 is observed, as illustrated in Fig. 7(c). Even after the sample experienced long cycling (100 cycles), the reversible capacity remains 680 mAh/g at 0.2 C.

With the rapid increase in the charge-discharge current density, the capacity decreases slowly. Even at 2 C, which corresponds to a time of 30 min to fully discharge the total capacity, the measured discharge capacity is still as high as 340 mAh/g, which is 48.3% of its theoretical capacity. A capacity of 850 mAh/g is retained when the current density directly returns to the initial value (0.2 C) after 80 cycles, indicating its excellent rate performance and cyclability. It was reported that at the nanometer scale, both the reversible formation of Li2O and the Li-bearing solid-electrolyte interface catalyzed by metal nanoparticles can occur[ 23]. For a mesoporous structure, these processes would consume a long time, which might be the reason for the increasing capacity.

3 Conclusions

Only with ethanol and manganese acetate tetrahydrate as the reactive reagents, we developed a simple rapid low-cost method for the synthesis of mesoporous Mn3O4 nanorods with uniform nanopores and high surface area. The as-prepared Mn3O4 exhibited a crosslinked nanorods with average diameter of 150 nm, in which nanopores with diameter of 6-20 nm were widely distributed. Their electrochemical properties were measured as the anode materials for Li-ion batteries. The Mn3O4 nanorod delivered reversible capacities of 676.1 and 662.4 mAh/g even after 100 cycles, showing much higher capacity and more stable cyclability than the agglomerated nano Mn3O4 powder directly from commercial manganese acetate tetrahydrate. Moreover, a capacity of 850 mAh/g was retained after 80 cycles of charge and discharge at various current densities. This work proves the mesoporous Mn3O4 nanorods as potential anode materials for high-performance lithium-ion batteries.

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