Abstract
Porous MnO@C core-shell nanowires were prepared via a simple and facile method. The morphologies, the phase purity, the mass contents, and the BET surface area of the composite were characterized by SEM, XRD, TGA, and N2 adsorption test, respectively. When the composite served as an anode for lithium-ion batteries, it showed superior electrochemical performances. The MnO@C composite presented a reversible capacity of 448.1 mAh g−1 after 100 cycles at the current rate of 200 mA g−1.
1. Introduction
With the rapid development of electronic information industry, as well as the existing problem of the depletion of natural resources and environmental deterioration, developing renewable energy and environment-friendly energy was one of the most important fields in the development of new energy storage technology. Meanwhile, much attention has been paid for the new generation of lithium-ion batteries (LIBs) with high energy density and high power density [1]. Traditionally, the anode for LIBs was graphite having a theoretical capacity of 372 mAh g−1. Nowadays, many transitional metal oxides were considered as alternative anode materials for LIBs [2–4]. For example, manganous oxide (MnO) has a theoretical capacity of 755 mAh g−1 [5], which is almost twice as large as that of graphite. To achieve the target of high performance in LIBs, MnO nanoparticles with special nanostructures and special morphologies have been synthesized [5–7].
In recent years, the application of one-dimensional nanometer material in LIBs has become more and more attractive. It was supposed that 1D nanomaterials could efficiently release the stress from the huge volume change during the discharge-charge process [8, 9]. Furthermore, nanowires (NWs) and other 1D nanoscale building blocks can be covered by functional materials to improve their electrochemical performance. Carbonaceous materials are excellent coating materials due to their high electric conductivity and high mechanical strength. For instance, Zhang et al. prepared MnO/C NWs by pyrolyzing MnO2/polypyrrole NWs and used them as anode for LIBs [10].
In this paper, MnOOH nanowires were firstly synthesized via hydrothermal reaction and then coated with a layer of resorcinol-formaldehyde (RF) resin. The final product, MnO@C composite, was obtained by pyrolyzing MnOOH@RF in N2 atmosphere. Then the composites were characterized via scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). It was used as an anode for the lithium-ion batteries, and the electrochemical performances were investigated by cyclic voltammetry and galvanostatic discharge-charge test.
2. Experimental
2.1. Materials
Potassium permanganate (KMnO4) was obtained from Nachang Xinguang Chemical Factory. Resorcinol, formaldehyde solution (38%), ammonia aqueous solution (NH3·H2O, 25%), and ethanol were purchase from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used directly without further purification.
2.2. Preparation of Porous MnOOH Nanowires
MnOOH NWs were synthesized according to a modified procedure described by Zhao et al. [11]. In short, 2.7000 g of KMnO4, 60.00 mL of H2O, and 4.50 mL of ethanol were mixed together. Then the solution was transferred into a 100 mL autoclave and kept at an electronic oven at 150°C for 20 hours. After cooling down, the precipitation was collected by centrifugation, repeatedly washed with ethanol and H2O, and dried at 60°C overnight.
2.3. Preparation of Porous MnO@C Nanowires
MnO@C NWs were prepared as Figure 1 illustrated. Firstly, 0.3160 g of MnOOH was dispersed in a solution containing 0.2000 g of resorcinol, 280 μL of formaldehyde solution, 200 μL of ammonia aqueous solution, 40.00 mL of ethanol, and 40.00 mL of H2O. Then the solution was placed in a Teflon-sealed autoclave and kept at 100°C for 24 hours. The precipitation, MnOOH NWs coated with resorcinol-formaldehyde resin (MnOOH@RF), was collected via centrifugation-rinse cycles. After drying at 60°C overnight, MnOOH@RF NWs were calcined to obtain MnO@C NWs at 600°C for 4 hours in N2 atmosphere. For comparison, Mn2O3 NWs were prepared by directly calcining MnOOH NWs at 600°C for 4 hours in air.
2.4. Characterization
The samples’ morphologies were scanned by scanning electron microscopy (SEM, Hitachi S-3400N). The crystalline phases of the samples were determined by X-ray diffraction (XRD, Bruker D8 Advance). The mass content of MnO in the composite was quantified by thermogravimetric analysis (TGA, Perkin-Elmer Diamond TG/DTA). N2 adsorption-desorption isotherm of MnO@C was measured by BELSORP-mini II instrument.
2.5. Electrochemical Measurements
The electrochemical performance of MnO@C NWs was investigated using button batteries (CR2032). The batteries were composed of lithium foil (the reference and counter electrode) and the MnO@C working electrode, which were separated by a microporous polypropylene separator soaked in an electrolyte of 1.0 mol L−1 LiPF6 dissolved in ethylene carbonate/dimethyl carbonate (1 : 1 by volume). The MnO@C electrode was prepared by pasting slurry on copper foil. The slurry was mixed with polyvinylidene fluoride, carbon black, and the MnO@C composite (1 : 1 : 8 by mass) in N-methyl-2-pyrrolidinone. The loading mass of the active materials on copper foil was about 0.47 mg cm−2. The batteries were galvanostatically tested on a multichannel batteries testing system (Land CT2001A). Cyclic voltammetry (CV) was recorded on an electrochemical workstation (CHI760E).
3. Results and Discussion
3.1. Characteristics of MnO@C
The phase purity of the samples was characterized by XRD. As shown in Figure 2, the hydrothermal product of the solution containing KMnO4 and ethanol was MnOOH, whose XRD pattern was consistent with the JCPDS card number 41-1379 [12]. The direct pyrolysis product of MnOOH NWs in air was Mn2O3, which was confirmed by the series of XRD peaks in the figure (JCPDS card number 41-1442). The pyrolysis product of MnOOH@RF NWs had different XRD patterns. The peaks at 2θ = 35.1, 40.7, 58.9, 70.3, and 73.9° could be well indexed to the (111), (200), (220), (311), and (222) planes of MnO (JCPDS number 07-0230) [13], which indicated MnOOH had turned to MnO, and the carbon generated from RF resin provided a reduction atmosphere in the pyrolysis.
The morphologies of the samples were displayed in Figure 3. Both sample MnOOH and sample MnO@C presented nanowires characteristic. As shown in Figure 3(a), most of sample MnOOH was nanowires with the diameters in range of 0.2~0.7 μm and the lengths over 28μm. And the surface of MnOOH NWs was smooth. MnO@C NWs were derived from MnOOH NWs (Figure 3(b)), their diameters were distinctively larger than those of MnOOH NWs, and their surface was rougher than that of MnOOH NWs. It suggested that carbon had covered MnO NWs.
(a)
(b)
The mass content of components could be evaluated by TGA at air atmosphere (Figure 4). The weight of MnO@C NWs slightly decreased until 205.8°C, which should be ascribed to the evaporation of water adsorbed in the pore of the sample. It fluctuated from 205.8 to 629.2°C, in which the oxidation of MnOOH and RF resin resulted in the weight increase and decrease, respectively. The weight turned to steady when the temperature was beyond 629.2°C, indicating that MnOOH@RF had thoroughly oxidized to Mn2O3 [14]. According to the weight of 98.5% at 205.8°C and the weight of 74.3% at 800.0°C shown in the figure, the mass contents of MnO and carbon calculated were 67.7% and 32.3%, respectively.
The textures of samples were measured by N2 adsorption-desorption isotherm. The MnO@C sample presented a typical IV isotherm with H3 hysteresis loop (Figure 5) [15], which implied the mesoporous structure in it. The BET surface area and the total pore volume of MnO@C were 249 m2 g−1 and 0.188 cm3 g−1, respectively. They were much larger than those of MnOOH (2.67 m2 g−1 and 0.0197 cm3 g−1, resp.). The larger BET surface area and the larger total pore volume should be attributed to RF resin covered on MnOOH NWs. During the pyrolysis, MnOOH@RF NWs released CO2 and H2O and generated MnO@C NWs with the mesoporous structure.
3.2. Electrochemical Performance of the MnO@C Anode
Sample MnO@C anode was investigated in an assembled coin-type half cell by cyclic voltammogram and galvanostatic discharge-charge test. The electrochemical mechanism of MnO@C during the discharge-charge process was estimated by cyclic voltammetry in the voltage range of 0.01~3.00 V at a scan rate of 0.2 mV s−1. As shown in Figure 6, in the first negative scan, the peak around 0.60 V should be ascribed to irreversible decomposition of the electrolyte and the formation of solid electrolyte interface (SEI) film, which disappeared in the following cycles. The reduction peak between 0.12 and 0.51 V and the oxidation peak at 1.28 V could be expressed as the following equation [16, 17]:
Since the second cycle, the CV curves tended to overlap, indicating a stable and reversible electrochemical behavior of MnO@C anode.
Figure 7 showed the voltage profile of MnO@C for the first three cycles between 0.01 and 3.00 V at a current rate of 200 mA g−1. It could be seen that a plateau appeared between 0.6 and 0.7 V in the initial discharge curve, indicating the formation of SEI film. Another long and flat discharge plateau appearing at ~0.2 V was related to the reduction reaction of Mn2+ to Mn0, which shifted to ~0.5 V since the second cycle [10, 18]. After that, a plateau from 1.2 to 1.3 V was revealed on the charge curve, indicating the oxidation of Mn to MnO. Since the second cycle, the galvanostatic discharge-charge curves turned to almost identical, which was in good agreement with the CV curves of MnO@C.
The galvanostatic discharge-charge test of MnO@C for 100 cycles was shown in Figure 8. The sample MnO@C should have a theoretical capacity of 631.3 mAh g−1 according to the equation in which and were the mass content of MnO and carbon and the and were the theoretical capacity of MnO and graphite, respectively. However, at the first cycle, the MnO@C anode delivered a discharge capacity of 1115.8 mAh g−1 which was significantly larger than the theoretical capacity. The larger discharge capacity at the first cycle should result from the irreversible decomposition of the electrolyte and the formation of SEI film as CV suggested. Correspondingly, the fact that the value of discharge capacity was greatly larger than that of the charge capacity led to a low Coulombic efficiency (CE) of only 59.9% at the first cycle. At the second cycle, the MnO@C anode delivered a discharge capacity of 639.4 mAh g−1, which was close to its theoretical capacity. Since the following cycles, the gradual decrement of discharge capacity came along with the gradual increment of CE. The discharge capacity kept steady around 410 mAh g−1 in the range from the 40th cycle to the 80th cycle. After that, it began to slowly increase. When cycling up to 100 cycles, the MnO@C anode recovered to a discharge capacity of 448.1 mAh g−1. The discharge capacity of MnO@C was larger than that of graphite, which should be attributed to larger theoretical capacity of MnO and less aggregation of MnO nanoparticles in the composite during the discharge-charge cycle [19].
The rate capability of MnO@C was presented in Figure 9. MnO@C anode delivered reversible capacities of 428.1, 322.7, 273.4, 234.6, and 209.1 mAh g−1 at the current rates of 200, 400, 600, 800, and 1000 mA g−1, respectively. When the current rate returned to 200 mA g−1, its reversible capacity could recover back to 420.8 mAh g−1, which manifested that the MnO@C anode had a fine rate performance and good reversibility.
In a control experiment, sample Mn2O3 was also investigated as an anode for LIBs. Although the theoretical capacity of Mn2O3 (1018 mAh g−1) is larger than that of MnO (755 mAh g−1), both Figures 8 and 9 showed that the electrochemical performance of sample MnO@C was more superior to that of sample Mn2O3. The superior electrochemical performance of MnO@C should be attributed to the following reasons: The morphology of sample MnO@C was 1D nanowire, which was helpful to release the stress resulting from the huge volume change during the discharge-charge cycles. The carbon layer on the composite could avoid the aggregation of MnO nanoparticles in the composite during the discharge-charge cycles. And the carbon layer could also improve the electric conductivity of the whole composite. The composite had mesoporous structure, which could shorten the diffusion length of ion.
4. Conclusions
In summary, MnO@C NWs were prepared by a facile, simple, and low cost method. When they acted as anodes for LIBs, they showed a reversible capacity of 448.1 mAh g−1 at a current rate of 200 mA g−1 after being cycled for 100 times. Even at a current rate of 1000 mA g−1, its reversible capacity was still as large as 209.1 mAh g−1. The superior electrochemical performance of MnO@C NWs should be ascribed to the mesoporous structure in them and the conductive carbon protect layer on MnO.
Competing Interests
The authors declare that there are no competing interests regarding the publication of this paper.
Acknowledgments
This work was financially supported by National Natural Science Foundation of China (21465014 and 21465015), Science and Technology Support Program of Jiangxi Province (20123BBE50104 and 20133BBE50008), Young Scientist Foundation of Jiangxi Province (20122BCB23011), Natural Science Foundation of Jiangxi Province (20142BAB203101), the Ministry of Education by the Specialized Research Fund for the Doctoral Program of Higher Education (20133604110002), and the Ground Plan of Science and Technology Projects of Jiangxi Educational Committee (KJLD14023).