Preparation of Porous MnO@C Core-Shell Nanowires as Anodes for Lithium-Ion Batteries

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 N 2 adsorption test, respectively. When the composite served as an anode for lithium-ion batteries, it showed superior electrochemical performances. TheMnO@C composite presented a reversible capacity of 448.1mAh g after 100 cycles at the current rate of 200mAg.


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][3][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][6][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 MnO 2 /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 N 2 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.

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 KMnO 4 , 60.00 mL of H 2 O, 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 H 2 O, and dried at 60 ∘ C overnight.

Preparation of Porous MnO@C Nanowires. MnO@C
NWs were prepared as Figure 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 KMnO 4 and ethanol was MnOOH, whose XRD pattern was consistent with the JCPDS card number 41-1379 [12].  (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.  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 Mn 2 O 3 [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 N 2 adsorptiondesorption 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 m 2 g −1 and 0.188 cm 3 g −1 , respectively. They were much larger than those of MnOOH (2.67 m 2 g −1 and 0.0197 cm 3 g −1 , resp.). The larger Relative pressure (P/P 0 ) 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 CO 2 and H 2 O and generated MnO@C NWs with the mesoporous structure.

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 Mn 2+ to Mn 0 , 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 dischargecharge 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 C MnO@C = m MnO × C MnO + m carbon × C carbon (2) in which m MnO and m carbon were the mass content of MnO and carbon and the m MnO and m carbon 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 dischargecharge 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 Mn 2 O 3 was also investigated as an anode for LIBs. Although the theoretical capacity of Mn 2 O 3 (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 Mn 2 O 3 . 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.

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.