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Mn3O4 submicrospheres as high-performance anode for lithium-ion batteries
Journal of Electroanalytical Chemistry 838 (2019) 1–6
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N-doped carbon encapsulated porous MnO/Mn3O4 submicrospheres as highperformance anode for lithium-ion batteries
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Xiangyang Zhou, Bo Long, Fangyan Cheng, Jingjing Tang, Antao Sun, Juan Yang, Ming Jia, ⁎ Hui Wang School of Metallurgy and Environment, Central South University, Changsha 410083, PR China
ARTICLE INFO
ABSTRACT
Keywords: N-doped carbon shell MnO/Mn3O4 Porous submicrospheres Anodes Lithium-ion batteries
Although Mn-based composites are promising anode materials for advanced lithium-ion batteries (LIBs) owning to their high theoretical capacity (about 2–4 times of commercial graphite), they have some practical limitations such as poor electronic conductivity and large volume expansion, which leading to poor cycle performance. Herein, we developed a versatile strategy to synthesize porous structured MnO/Mn3O4 submicrospheres encapsulated by N-doped carbon shell (MnO/Mn3O4@NC) through an efficient annealing treatment. The as-prepared MnO/Mn3O4@NC composite delivered an excellent electrochemical performance, including a high reversible capacity of 990 mA h g−1 at 0.2 A g−1 over 100 cycles and superior rate capability. The outstanding electrochemical performance of MnO/Mn3O4@NC could be attributed to its porous structure and the well-defined protection by N-doped carbon shell for facilitating the transport of electrons and alleviating the volume expansion.
1. Introduction Rechargeable lithium-ion batteries (LIBs) have drawn tremendous attention for applications in energy storage devices such as portable electronic devices and electric vehicles (EVs), owing to their high energy density, no memory effect and long lifespan [1–3]. With the rapidly development of human society, the commercial graphite is hard to meet the new energy storage requirements. Therefore, it is crucial to explore new electrode materials with improved performance for LIBs [4,5]. Manganese oxides are competitive candidate as electrode materials for enhanced lithium storage due to their high theoretical capacity, low conversion potential, abundant resource and environmental benignity [1,6,7]. However, the manganese oxides are suffered from huge volumetric expansion during the lithiation process and poor electrical conductivity, leading to poor cycle performance. These distinct structural weaknesses of manganese oxides have greatly hindered their practical implementation. Therefore, searching for high-performance anode materials is still necessary for the demand of high energy density LIBs. In the previous researches, numerous efforts have been devoted to solving the above-mentioned issues. One effective strategy is designing nanostructured manganese oxides (such as nanoparticles, [7,8] nanotubes, [9,10] nanowires, [11,12] and so on), which could shorten the
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electron and ion diffusion path and relief the internal strain during cycling process owning to its nanoscale dimensions [5,13]. However, the large surface area of nanomaterials will cause the excessively formation of solid electrolyte interphase (SEI) film, leading to irreversible degradation of capacities and thus increasing the assumption of lithium. Another appealing method is combining bare manganese oxides with the carbonaceous materials (such as graphene, [14] porous carbon, [15,16] carbon nanotube [10] and so on), due to the good electrical conductivity and elastic feature of carbonaceous materials. Extensive researches have shown that coating manganese oxide with a protective and conductive layer is an effective strategy to enhance its electrochemical performance. It is demonstrated that this coating layer can not only alleviate the volume expansion and improve the conductivity, but also prevent the oxides from direct contact with the electrolyte [3,10,17]. In this work, we synthesized porous MnO/Mn3O4@NC submicrospheres through a facial approach. Our approach starts with the preparation of MnCO3 submicrospheres, after being coated by polydopamine (PDA), the porous MnO/Mn3O4@NC submicrospheres were obtained by an annealing process. During the annealing process, the MnCO3 decomposed in to porous MnO/Mn3O4 and coated by a uniform carbon shell.
Corresponding author. E-mail address: [email protected] (H. Wang).
https://doi.org/10.1016/j.jelechem.2019.02.033 Received 26 October 2018; Received in revised form 16 February 2019; Accepted 16 February 2019 Available online 18 February 2019 1572-6657/ © 2019 Elsevier B.V. All rights reserved.
Journal of Electroanalytical Chemistry 838 (2019) 1–6
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Fig. 1. Schematic illustration for the fabrication procedure of the MnO/Mn3O4@NC.
2. Experimental
LiPF6 (1 M) dissolved in a mixture of dimethyl carbonate, diethyl carbonate, and ethylene carbonate (1:1:1 vol%). The cyclic voltammetry (CV) conducted between 0.01 and 3.0 V at a scan rate of 0.2 mV s−1 and electrochemical impedance spectroscope (EIS) with the frequency range from 0.01 to 100 kHz were carried out on a CHI 660D electrochemical workstation.
2.1. Materials synthesis 2.1.1. Synthesis of MnCO3 submicrospheres MnCO3 submicrospheres were prepared by a simple chemical precipitation. In a typical procedure, 1 mmol of MnSO4·H2O and 10 mmol of NH4HCO3 were dissolved in 40 ml distilled water under stirring, respectively. Then, 40 ml of ethanol was added to the MnSO4·H2O solution. After its homogeneous dispersion, the NH4HCO3 solution was slowly poured into the mixture, and the reaction was maintained for 3 h at room temperature. Then solid MnCO3 submicrospheres was obtained by washing with deionized water (DI) and ethanol for several times, and then dried at 60 °C for 12 h.
3. Results and discussion As illustrated in Fig. 1, the synthesis procedure of MnO/Mn3O4@NC composite involves three steps. Firstly, the MnCO3 submicrospheres were prepared by the precipitation reaction between MnSO4·H2O and NH4HCO3 solution. And then, dopamine serves as a carbon source for the encapsulation of MnCO3. The in-situ polymerization of dopamine enabled a uniform coating of PDA layer on the surface of MnCO3. After a subsequent annealing treatment at 500 °C for 6 h under an inert atmosphere, MnO/Mn3O4@NC submicrospheres were obtained. The phases and crystal structures of the as-prepared products were identified by X-ray diffraction (XRD). The crystal structure of MnCO3 was verified by its XRD pattern (Fig. 2a), in which the diffraction peaks are well indexed to the MnCO3 (JCPDS 24–0734). Fig. 2b displays the XRD patterns of the annealed products. The diffraction peaks of both samples are ascribed to the cubic MnO (JCPDS 07–0230) and the hausmannite Mn3O4 (JCPDS 44–1472) phase. And the intensity of MnO peaks are stronger than that of Mn3O4, suggesting that the content of MnO is higher than Mn3O4 in the final product. No apparent peaks for carbon is detected, which is mainly due to its low amount (about 18.5%) and its amorphous nature [18]. Raman spectrum of the MnO/ Mn3O4@NC submicrospheres is exhibited in Fig. 2c, the peaks at about 224, 260, and 645 cm−1 are corresponding to MnO and Mn3O4 [2]. Two peaks located at 1371 and 1561 cm−1 can be assigned to the disordered carbon (D-band) and graphitic carbon (G-band), respectively [19]. The intensity ratio of the ID/IG is calculated to be about 0.878, indicating a certain degree of graphitization of the carbon shell [20]. The X-ray photoelectron spectroscopy (XPS) is employed to evaluate the chemical composition and the surface electronic states of the MnO/ Mn3O4@NC submicrospheres. As depicted in the top of Fig. 2d, the peaks of C 1 s, Mn 2p, O 1 s, and N 1 s can be clearly observed, suggesting the presence of C, Mn, O, and N element in the MnO/Mn3O4@ NC sample, which is in good agreement with the EDS mapping results (Fig. 3e). The high-resolution XPS spectrum of C 1 s in the bottom left of Fig. 2d can be deconvoluted into four peaks, which can be assigned to graphic carbon (284.19 eV), CeN (285.84 eV), CeO (287.94 eV) and OC=O (291.21 eV), respectively. In addition, the high-resolution spectrum of N 1 s (the bottom right of Fig. 2d) exhibits three peaks at 397.79, 400.24, and 403.07 eV, which could be assigned to pyridinic N, pyrrolic N, and quaternary N, respectively, [19,21] further confirming the successful incorporation of N element into the as-prepared composite. In order to identified the carbon content of the MnO/Mn3O4@NC, the TGA experiments of the MnO/Mn3O4 and MnO/Mn3O4@NC samples were conducted under air flow from room temperature to 800 °C. The total weight loss is the combination effect of the weight gain (from the oxidation of MnO/Mn3O4 to Mn2O3) and the weight loss (from the burning of carbon). From the results of TGA curves displayed in Fig. 2e, the content of carbon in the MnO/Mn3O4@NC sample can confirm to be about 18.5%. The nitrogen adsorption-desorption isotherms are applied
2.1.2. Synthesis of MnO/Mn3O4@NC composites Typically, 100 mg of the as-prepared MnCO3 submicrospheres were dispersed in a Tris-buffer (pH = 8.5) containing 50 ml of DI and 50 ml of ethanol under stirring. After that, 100 mg dopamine hydrochloride was added into the mixture and kept stirring for 24 h. Subsequently, the product (MnCO3@PDA) was collected by centrifugation and washed with DI water and ethanol for several times, and then dried in an oven at 60 °C for 12 h. Finally, it was carbonized in a furnace at 500 °C for 6 h under Ar atmosphere, and then MnO/Mn3O4@NC composites were obtained. For comparison, the MnO/Mn3O4 was synthesized by directly calcination of the MnCO3 submicrospheres. 2.2. Materials characterization X-ray diffraction (XRD) patterns were obtained from a Bruker D8 diffractometer within 2θ range from 10° to 80°, using a Cu Kα radiation source. The morphologies of the samples were collected by a scanning electron microscope (SEM, JSM-6360, Japan) with energy-dispersive spectroscopy (EDS), and transmission electron microscopy (TEM, JEM2100F, Japan). Raman spectrum was performed by a Raman spectrometer with 532 nm laser excitation. XPS experiments were carried out on an X-ray photoelectron spectrometer (K-Alpha 1063). The BrunauerEmmett-Teller (BET) method and the nitrogen adsorption-desorption isotherm were employed to determine the specific surface area and the pore size distribution. Thermogravimetric analysis (TG) analyses were carried out by using TGA (SDQ 600) within the temperature range from room temperature to 800 °C with a heating rate of 10 °C min−1 under air atmosphere. 2.3. Electrochemical measurements The electrochemical behavior was carried out with coin cells (CR2025) assembled in a glovebox filled with Ar atmosphere. Working electrodes were prepared by a typical slurry method using active materials (80 wt%) with acetylene black (10 wt%) and polytetrafluoroethylene (PVDF, 10 wt%) dissolved in N-methyl-2-pyrrodinone (NMP). The formed homogeneous slurry was spread on a copper foil with a thickness of 150 mm. And then the electrodes were dried in a vacuum oven at 120 °C overnight. The active material mass loading in each electrode was controlled to be about 1.0 mg cm−2. Lithium foil was employed as counter and reference electrodes, the polypropylene microporous membrane was used as separator. The electrolyte involves 2
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Fig. 2. XRD patterns of (a) MnCO3, (b) MnO/Mn3O4 and MnO/Mn3O4@NC. Standard XRD cards for MnCO3, MnO and Mn3O4 are inserted as reference. (c) Raman spectra of MnO/Mn3O4@NC. (d) XPS survey spectrum of MnO/Mn3O4@NC (top), the high resolution spectra of C 1 s (bottom left) and N 1 s (bottom right). (e) The TGA curves of MnO/Mn3O4 and MnO/Mn3O4@NC. (f) N2 adsorption-desorption isotherm of MnO/Mn3O4@NC (inset is the pore size distribution curve).
to further confirm the porous feature of the as-prepared MnO/Mn3O4@ NC submicrospheres, and the results are displayed in Fig. 2f. The BET surface area for mesoporous MnO/Mn3O4@NC is determined to be about 25.6 m2 g−1 with a pore volume of 0.06 m3 g−1. The pore size distribution (inset Fig. 2f) exhibits a narrow distribution centered at 2.5 nm, which is in favor of the diffusion of Li+ and the transformation of electrons within the electrode, and the porous structure can also adjust the volume variation during the lithiation/delithiation, thus leading to enhanced lithium storage and excellent cycling stability [7,22,23]. The morphology of the as-synthesized products is investigated by scanning electron microscope (SEM). As shown in Fig. 3a, the MnCO3 product is composed of uniform submicrospheres with rough surface and diameters about 800 nm. After the annealing treatment, it can be observed that the spherical morphology is still maintained (Fig. 3b),
and there are multiple nanopores appeared on the surface of the obtained MnO/Mn3O4 owning to the gas (CO2) evaporation during thermal treatment. After being coated with PDA and the annealing treatment, the surface of the particles became invisible, as exhibited in Fig. 3c, which indicates the effective coating of PDA through our strategy. Transmission electron microscopy (TEM) was used to further confirm the porous structure, as shown in Fig. 3d, the obtained MnO/ Mn3O4@NC composite shows a typical core-shell structure. The elemental mapping (Fig. 3e) revealed that the existence of C, Mn, O and N elements. In addition, the C and N elements are uniformly distributed over the MnO/Mn3O4@NC submicrospheres. The N-doped carbon materials can not only provide more active sites for lithium storage, but also can enhance the electric conductivity and reduce energy barriers for ion transport, therefore, the incorporation of N element is beneficial for improvement of electrochemical performance as anode for LIBs [6].
Fig. 3. SEM images of (a) MnCO3, (b) MnO/Mn3O4, and (c) MnO/Mn3O4@NC. (d) TEM and (e) elemental mapping images of MnO/Mn3O4@NC. 3
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Fig. 4. (a) Cyclic voltammetry curves and (b) Discharge-charge voltage profiles of the 1st, 2nd and 3rd and 10th cycle at a current density of 200 mA g−1 of MnO/ Mn3O4@NC, (c) Cycling performances of MnO/Mn3O4 and MnO/Mn3O4@NC at a current density of 200 mA g−1, (d) Rate performances of MnO/Mn3O4 and MnO/ Mn3O4@NC, (e) Long-term cycling of MnO/Mn3O4@NC at current densities of 500 mA g−1 and 1000 m A g−1.
The electrochemical performance of the MnO/Mn3O4@NC submicrospheres were investigated by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) cycling measurements as an anode for LIBs. Fig. 4a shows the CV curves of MnO/Mn3O4@NC for the initial three cycles at a scan rate of 0.2 mV s−1 within the voltage range from 0.01 V to 3 V. In the first cathodic process, there are three reduction peaks located at 1.43, 0.85, and 0.23 V, which could be ascribed to the reduction of Mn3O4 to MnO (i.e., Mn3O4 + 2Li++2e− → 3MnO
+ Li2O), the decomposition of electrolyte along with the formation of the solid electrolyte interphase (SEI) film on the surface of the electrode, and the reduction of MnO to metallic Mn (i.e., MnO + 2Li++2e− → Mn + Li2O), respectively [22,24]. For the first anodic process, a broad peak appeared at about 1.28 V, which could be assigned to the oxidation of Mn to MnO and the decomposition of Li2O (i.e., Mn + Li2O → MnO + 2Li++2e−) [3]. In the subsequent cycles, the cathodic peak shifts to about 0.33 V, higher than that of the first 4
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Fig. 5. (a) EIS curves of pure MnO/Mn3O4 and MnO/Mn3O4@NC before cycling (inset is the equivalent circuit model). (b) The TEM image of MnO/Mn3O4@NC after 100 cycles at 200 mAh g−1.
0.1 A g−1, a recoverable capacity of 1000 mA h g−1 can be obtained, implying the excellent structural integrity of the MnO/Mn3O4@NC submicrospheres. It is clearly seen that the rate performance of MnO/ Mn3O4@NC submicrospheres is superior to that of the MnO/Mn3O4 electrode. The long-term cycling performance of the MnO/Mn3O4@NC submicrospheres electrode at current densities of 0.5 A g−1 and 1 A g−1 were measured. As depicted in Fig. 4e, the MnO/Mn3O4@NC submicrospheres electrode delivers a high reversible capacity of 1068 mA h g−1 over 200 cycles at 0.5 A g−1. Even at a high density of 1 A g−1, it still exhibits a reversible capacity of 953 mA h g−1 after 350 cycles, demonstrating superior rate capability and excellent cyclic stability. This could be benefit from the well combination of the high conductivity provided by N-doped carbon shell and the good buffer volume expansion ability offered by its porous structure. The electrochemical impedance spectroscopy (EIS) measurements were conducted to better understand the electrochemical properties of the two electrodes, as shown in Fig. 5a. All the Nyquist plots are exhibited a depressed semicircle at high frequency and an inclined line at low frequency, which are represented the charge transfer impedance (Rct) on the electrode/electrolyte interface and Li+ diffusion impedance (Warburg impedance, Ws) of the electrode, respectively [3]. The EIS data are fitted based on an equivalent circuit model, the fitting results are shown in Table 1. It can be clearly seen that the semicircle diameter for MnO/Mn3O4@NC is much smaller than that of MnO/Mn3O4, suggesting that N-doped carbon coating can effectively promote the charge transfer, thus giving a significant improvement for the cycling performance of the MnO/Mn3O4@NC electrode [23]. To further investigate the structural integrity of MnO/Mn3O4@NC electrode after cycling, we disassembled the cycled cells and observed its morphology by TEM. From the TEM image (Fig. 5b) of the cells after 100 cycles at 0.2 A g−1, we could see that the MnO/Mn3O4@NC can still maintains the spherical shape, suggesting that the carbon shell can effectively accommodate the volume variation during cycling, leading to the improved electrochemical performance.
Table 1 The fitting results of pure MnO/Mn3O4 and MnO/Mn3O4@NC electrodes. Electrode MnO/Mn3O4 MnO/Mn3O4@NC
Rs (Ω)
Rct (Ω)
2.2 1.8
887.5 212.5
cycle, which can be attributed to the improved reaction kinetics after the initial activation process [25,26]. Fig. 4b displays GCD profiles of the 1st, 2nd, 3rd, and 10th for MnO/Mn3O4@NC at a current density of 200 mA g−1. It can be seen the first discharge profile exhibits a voltage plateau at 1.5–0.3 V corresponds to the reduction of Mn3O4 to MnO, and then it shifts to a higher voltage in the subsequent cycles [20]. Another plateau at about 0.3 V is related to the reduction of MnO to Mn, which is in consistent with the CV results. In addition, the initial discharge and charge capacities of MnO/Mn3O4@NC are 1456.8 and 914.3 mA h g−1, respectively, giving an initial coulombic efficiency of 62.8%, which is much higher than that of the MnO/Mn3O4 (about 37.7%). The initial irreversible capacity loss is mainly attributed to the decomposition of the electrolyte and the generation of SEI film during cycling process [23,27,28]. To study the effect of the N-doped carbon coating on the electrochemical performance, the cyclic and rate performance of MnO/Mn3O4 and MnO/Mn3O4@NC anodes are evaluated. Fig. 4c exhibits the cycling performance of the pristine MnO/Mn3O4 and MnO/Mn3O4@NC submicrospheres at a current density of 200 mA g−1. It shows that the MnO/Mn3O4@NC delivers a reversible capacity of 990 mA h g−1 over 100 cycles with superior cyclic stability, which is higher than that of pristine MnO/Mn3O4 (350 mA h g−1 after 100 cycles). It can be clearly observed that the MnO/Mn3O4 and MnO/Mn3O4@NC anodes suffer from a capacity decline during the first several cycles, which is mainly ascribed to the SEI film generated on the surface of the electrodes. After 20 cycles, it is obviously observed that the capacities of the MnO/ Mn3O4@NC composite gradually increased with cycling, which could be ascribed to the activation reaction of the porous structure. The porous structure might be beneficial for more electrolyte get access to the inner structure of the electrode, thus contribute the capacity during cycling process [28]. Meanwhile, the elevated capacity can be due to the formation of higher valence manganese oxides in the final product and the enhanced electrochemical kinetics [27]. Fig. 4d displays the rate performance of the MnO/Mn3O4@NC submicrospheres anode at different current densities ranging from 0.1 to 1 A g−1. The reversible capacities were measured to be 850, 800, 660, and 520 mA h g−1 at current densities of 0.1, 0.2, 0.5, and 1 A g−1, respectively. Remarkably, after the current density is recovered to
4. Conclusions In summary, we demonstrated a facial and efficient strategy to fabricate porous MnO/Mn3O4@NC submicrospheres. The submicrospheres were successfully synthesized via an annealing treatment of spherical MnCO3 precursor under inert atmosphere. The resulting porous structure is formed through the decomposition of MnCO3. When evaluated as anode for LIBs, the as-prepared MnO/Mn3O4@NC electrode could deliver a reversible capacity of 990 mA h g−1 at 0.2 A g−1 over 100 cycles, and exhibit excellent rate capability and good cycling performance, which could be attributed to the porous structure and the 5
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N-doped carbon coating. The porous MnO/Mn3O4@NC offers enough spaces to alleviate the volume expansion during repeated lithiation/ delithiation processes and the uniform carbon coating can not only enhance the electronic conductivity of the composites, but also provide a protective layer for MnO/Mn3O4@NC from pulverization, thus leading to excellent electrochemical performance. The method used in this work provide a facial, low cost and easy controllable way to prepare porous MnO/Mn3O4@NC submicrospheres, which could be extended to other materials toward high-performance anode for LIBs.
[12] [13]
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Acknowledgements
[16]
This work was supported by the National Natural Science Foundation of China (Grant No: 51871247, 51774343 and 51802354).
[17]
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N-doped carbon encapsulated porous MnO/Mn3O4 submicrospheres as highperformance anode for lithium-ion batteries
T
Xiangyang Zhou, Bo Long, Fangyan Cheng, Jingjing Tang, Antao Sun, Juan Yang, Ming Jia, ⁎ Hui Wang School of Metallurgy and Environment, Central South University, Changsha 410083, PR China
ARTICLE INFO
ABSTRACT
Keywords: N-doped carbon shell MnO/Mn3O4 Porous submicrospheres Anodes Lithium-ion batteries
Although Mn-based composites are promising anode materials for advanced lithium-ion batteries (LIBs) owning to their high theoretical capacity (about 2–4 times of commercial graphite), they have some practical limitations such as poor electronic conductivity and large volume expansion, which leading to poor cycle performance. Herein, we developed a versatile strategy to synthesize porous structured MnO/Mn3O4 submicrospheres encapsulated by N-doped carbon shell (MnO/Mn3O4@NC) through an efficient annealing treatment. The as-prepared MnO/Mn3O4@NC composite delivered an excellent electrochemical performance, including a high reversible capacity of 990 mA h g−1 at 0.2 A g−1 over 100 cycles and superior rate capability. The outstanding electrochemical performance of MnO/Mn3O4@NC could be attributed to its porous structure and the well-defined protection by N-doped carbon shell for facilitating the transport of electrons and alleviating the volume expansion.
1. Introduction Rechargeable lithium-ion batteries (LIBs) have drawn tremendous attention for applications in energy storage devices such as portable electronic devices and electric vehicles (EVs), owing to their high energy density, no memory effect and long lifespan [1–3]. With the rapidly development of human society, the commercial graphite is hard to meet the new energy storage requirements. Therefore, it is crucial to explore new electrode materials with improved performance for LIBs [4,5]. Manganese oxides are competitive candidate as electrode materials for enhanced lithium storage due to their high theoretical capacity, low conversion potential, abundant resource and environmental benignity [1,6,7]. However, the manganese oxides are suffered from huge volumetric expansion during the lithiation process and poor electrical conductivity, leading to poor cycle performance. These distinct structural weaknesses of manganese oxides have greatly hindered their practical implementation. Therefore, searching for high-performance anode materials is still necessary for the demand of high energy density LIBs. In the previous researches, numerous efforts have been devoted to solving the above-mentioned issues. One effective strategy is designing nanostructured manganese oxides (such as nanoparticles, [7,8] nanotubes, [9,10] nanowires, [11,12] and so on), which could shorten the
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electron and ion diffusion path and relief the internal strain during cycling process owning to its nanoscale dimensions [5,13]. However, the large surface area of nanomaterials will cause the excessively formation of solid electrolyte interphase (SEI) film, leading to irreversible degradation of capacities and thus increasing the assumption of lithium. Another appealing method is combining bare manganese oxides with the carbonaceous materials (such as graphene, [14] porous carbon, [15,16] carbon nanotube [10] and so on), due to the good electrical conductivity and elastic feature of carbonaceous materials. Extensive researches have shown that coating manganese oxide with a protective and conductive layer is an effective strategy to enhance its electrochemical performance. It is demonstrated that this coating layer can not only alleviate the volume expansion and improve the conductivity, but also prevent the oxides from direct contact with the electrolyte [3,10,17]. In this work, we synthesized porous MnO/Mn3O4@NC submicrospheres through a facial approach. Our approach starts with the preparation of MnCO3 submicrospheres, after being coated by polydopamine (PDA), the porous MnO/Mn3O4@NC submicrospheres were obtained by an annealing process. During the annealing process, the MnCO3 decomposed in to porous MnO/Mn3O4 and coated by a uniform carbon shell.
Corresponding author. E-mail address: [email protected] (H. Wang).
https://doi.org/10.1016/j.jelechem.2019.02.033 Received 26 October 2018; Received in revised form 16 February 2019; Accepted 16 February 2019 Available online 18 February 2019 1572-6657/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic illustration for the fabrication procedure of the MnO/Mn3O4@NC.
2. Experimental
LiPF6 (1 M) dissolved in a mixture of dimethyl carbonate, diethyl carbonate, and ethylene carbonate (1:1:1 vol%). The cyclic voltammetry (CV) conducted between 0.01 and 3.0 V at a scan rate of 0.2 mV s−1 and electrochemical impedance spectroscope (EIS) with the frequency range from 0.01 to 100 kHz were carried out on a CHI 660D electrochemical workstation.
2.1. Materials synthesis 2.1.1. Synthesis of MnCO3 submicrospheres MnCO3 submicrospheres were prepared by a simple chemical precipitation. In a typical procedure, 1 mmol of MnSO4·H2O and 10 mmol of NH4HCO3 were dissolved in 40 ml distilled water under stirring, respectively. Then, 40 ml of ethanol was added to the MnSO4·H2O solution. After its homogeneous dispersion, the NH4HCO3 solution was slowly poured into the mixture, and the reaction was maintained for 3 h at room temperature. Then solid MnCO3 submicrospheres was obtained by washing with deionized water (DI) and ethanol for several times, and then dried at 60 °C for 12 h.
3. Results and discussion As illustrated in Fig. 1, the synthesis procedure of MnO/Mn3O4@NC composite involves three steps. Firstly, the MnCO3 submicrospheres were prepared by the precipitation reaction between MnSO4·H2O and NH4HCO3 solution. And then, dopamine serves as a carbon source for the encapsulation of MnCO3. The in-situ polymerization of dopamine enabled a uniform coating of PDA layer on the surface of MnCO3. After a subsequent annealing treatment at 500 °C for 6 h under an inert atmosphere, MnO/Mn3O4@NC submicrospheres were obtained. The phases and crystal structures of the as-prepared products were identified by X-ray diffraction (XRD). The crystal structure of MnCO3 was verified by its XRD pattern (Fig. 2a), in which the diffraction peaks are well indexed to the MnCO3 (JCPDS 24–0734). Fig. 2b displays the XRD patterns of the annealed products. The diffraction peaks of both samples are ascribed to the cubic MnO (JCPDS 07–0230) and the hausmannite Mn3O4 (JCPDS 44–1472) phase. And the intensity of MnO peaks are stronger than that of Mn3O4, suggesting that the content of MnO is higher than Mn3O4 in the final product. No apparent peaks for carbon is detected, which is mainly due to its low amount (about 18.5%) and its amorphous nature [18]. Raman spectrum of the MnO/ Mn3O4@NC submicrospheres is exhibited in Fig. 2c, the peaks at about 224, 260, and 645 cm−1 are corresponding to MnO and Mn3O4 [2]. Two peaks located at 1371 and 1561 cm−1 can be assigned to the disordered carbon (D-band) and graphitic carbon (G-band), respectively [19]. The intensity ratio of the ID/IG is calculated to be about 0.878, indicating a certain degree of graphitization of the carbon shell [20]. The X-ray photoelectron spectroscopy (XPS) is employed to evaluate the chemical composition and the surface electronic states of the MnO/ Mn3O4@NC submicrospheres. As depicted in the top of Fig. 2d, the peaks of C 1 s, Mn 2p, O 1 s, and N 1 s can be clearly observed, suggesting the presence of C, Mn, O, and N element in the MnO/Mn3O4@ NC sample, which is in good agreement with the EDS mapping results (Fig. 3e). The high-resolution XPS spectrum of C 1 s in the bottom left of Fig. 2d can be deconvoluted into four peaks, which can be assigned to graphic carbon (284.19 eV), CeN (285.84 eV), CeO (287.94 eV) and OC=O (291.21 eV), respectively. In addition, the high-resolution spectrum of N 1 s (the bottom right of Fig. 2d) exhibits three peaks at 397.79, 400.24, and 403.07 eV, which could be assigned to pyridinic N, pyrrolic N, and quaternary N, respectively, [19,21] further confirming the successful incorporation of N element into the as-prepared composite. In order to identified the carbon content of the MnO/Mn3O4@NC, the TGA experiments of the MnO/Mn3O4 and MnO/Mn3O4@NC samples were conducted under air flow from room temperature to 800 °C. The total weight loss is the combination effect of the weight gain (from the oxidation of MnO/Mn3O4 to Mn2O3) and the weight loss (from the burning of carbon). From the results of TGA curves displayed in Fig. 2e, the content of carbon in the MnO/Mn3O4@NC sample can confirm to be about 18.5%. The nitrogen adsorption-desorption isotherms are applied
2.1.2. Synthesis of MnO/Mn3O4@NC composites Typically, 100 mg of the as-prepared MnCO3 submicrospheres were dispersed in a Tris-buffer (pH = 8.5) containing 50 ml of DI and 50 ml of ethanol under stirring. After that, 100 mg dopamine hydrochloride was added into the mixture and kept stirring for 24 h. Subsequently, the product (MnCO3@PDA) was collected by centrifugation and washed with DI water and ethanol for several times, and then dried in an oven at 60 °C for 12 h. Finally, it was carbonized in a furnace at 500 °C for 6 h under Ar atmosphere, and then MnO/Mn3O4@NC composites were obtained. For comparison, the MnO/Mn3O4 was synthesized by directly calcination of the MnCO3 submicrospheres. 2.2. Materials characterization X-ray diffraction (XRD) patterns were obtained from a Bruker D8 diffractometer within 2θ range from 10° to 80°, using a Cu Kα radiation source. The morphologies of the samples were collected by a scanning electron microscope (SEM, JSM-6360, Japan) with energy-dispersive spectroscopy (EDS), and transmission electron microscopy (TEM, JEM2100F, Japan). Raman spectrum was performed by a Raman spectrometer with 532 nm laser excitation. XPS experiments were carried out on an X-ray photoelectron spectrometer (K-Alpha 1063). The BrunauerEmmett-Teller (BET) method and the nitrogen adsorption-desorption isotherm were employed to determine the specific surface area and the pore size distribution. Thermogravimetric analysis (TG) analyses were carried out by using TGA (SDQ 600) within the temperature range from room temperature to 800 °C with a heating rate of 10 °C min−1 under air atmosphere. 2.3. Electrochemical measurements The electrochemical behavior was carried out with coin cells (CR2025) assembled in a glovebox filled with Ar atmosphere. Working electrodes were prepared by a typical slurry method using active materials (80 wt%) with acetylene black (10 wt%) and polytetrafluoroethylene (PVDF, 10 wt%) dissolved in N-methyl-2-pyrrodinone (NMP). The formed homogeneous slurry was spread on a copper foil with a thickness of 150 mm. And then the electrodes were dried in a vacuum oven at 120 °C overnight. The active material mass loading in each electrode was controlled to be about 1.0 mg cm−2. Lithium foil was employed as counter and reference electrodes, the polypropylene microporous membrane was used as separator. The electrolyte involves 2
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Fig. 2. XRD patterns of (a) MnCO3, (b) MnO/Mn3O4 and MnO/Mn3O4@NC. Standard XRD cards for MnCO3, MnO and Mn3O4 are inserted as reference. (c) Raman spectra of MnO/Mn3O4@NC. (d) XPS survey spectrum of MnO/Mn3O4@NC (top), the high resolution spectra of C 1 s (bottom left) and N 1 s (bottom right). (e) The TGA curves of MnO/Mn3O4 and MnO/Mn3O4@NC. (f) N2 adsorption-desorption isotherm of MnO/Mn3O4@NC (inset is the pore size distribution curve).
to further confirm the porous feature of the as-prepared MnO/Mn3O4@ NC submicrospheres, and the results are displayed in Fig. 2f. The BET surface area for mesoporous MnO/Mn3O4@NC is determined to be about 25.6 m2 g−1 with a pore volume of 0.06 m3 g−1. The pore size distribution (inset Fig. 2f) exhibits a narrow distribution centered at 2.5 nm, which is in favor of the diffusion of Li+ and the transformation of electrons within the electrode, and the porous structure can also adjust the volume variation during the lithiation/delithiation, thus leading to enhanced lithium storage and excellent cycling stability [7,22,23]. The morphology of the as-synthesized products is investigated by scanning electron microscope (SEM). As shown in Fig. 3a, the MnCO3 product is composed of uniform submicrospheres with rough surface and diameters about 800 nm. After the annealing treatment, it can be observed that the spherical morphology is still maintained (Fig. 3b),
and there are multiple nanopores appeared on the surface of the obtained MnO/Mn3O4 owning to the gas (CO2) evaporation during thermal treatment. After being coated with PDA and the annealing treatment, the surface of the particles became invisible, as exhibited in Fig. 3c, which indicates the effective coating of PDA through our strategy. Transmission electron microscopy (TEM) was used to further confirm the porous structure, as shown in Fig. 3d, the obtained MnO/ Mn3O4@NC composite shows a typical core-shell structure. The elemental mapping (Fig. 3e) revealed that the existence of C, Mn, O and N elements. In addition, the C and N elements are uniformly distributed over the MnO/Mn3O4@NC submicrospheres. The N-doped carbon materials can not only provide more active sites for lithium storage, but also can enhance the electric conductivity and reduce energy barriers for ion transport, therefore, the incorporation of N element is beneficial for improvement of electrochemical performance as anode for LIBs [6].
Fig. 3. SEM images of (a) MnCO3, (b) MnO/Mn3O4, and (c) MnO/Mn3O4@NC. (d) TEM and (e) elemental mapping images of MnO/Mn3O4@NC. 3
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Fig. 4. (a) Cyclic voltammetry curves and (b) Discharge-charge voltage profiles of the 1st, 2nd and 3rd and 10th cycle at a current density of 200 mA g−1 of MnO/ Mn3O4@NC, (c) Cycling performances of MnO/Mn3O4 and MnO/Mn3O4@NC at a current density of 200 mA g−1, (d) Rate performances of MnO/Mn3O4 and MnO/ Mn3O4@NC, (e) Long-term cycling of MnO/Mn3O4@NC at current densities of 500 mA g−1 and 1000 m A g−1.
The electrochemical performance of the MnO/Mn3O4@NC submicrospheres were investigated by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) cycling measurements as an anode for LIBs. Fig. 4a shows the CV curves of MnO/Mn3O4@NC for the initial three cycles at a scan rate of 0.2 mV s−1 within the voltage range from 0.01 V to 3 V. In the first cathodic process, there are three reduction peaks located at 1.43, 0.85, and 0.23 V, which could be ascribed to the reduction of Mn3O4 to MnO (i.e., Mn3O4 + 2Li++2e− → 3MnO
+ Li2O), the decomposition of electrolyte along with the formation of the solid electrolyte interphase (SEI) film on the surface of the electrode, and the reduction of MnO to metallic Mn (i.e., MnO + 2Li++2e− → Mn + Li2O), respectively [22,24]. For the first anodic process, a broad peak appeared at about 1.28 V, which could be assigned to the oxidation of Mn to MnO and the decomposition of Li2O (i.e., Mn + Li2O → MnO + 2Li++2e−) [3]. In the subsequent cycles, the cathodic peak shifts to about 0.33 V, higher than that of the first 4
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Fig. 5. (a) EIS curves of pure MnO/Mn3O4 and MnO/Mn3O4@NC before cycling (inset is the equivalent circuit model). (b) The TEM image of MnO/Mn3O4@NC after 100 cycles at 200 mAh g−1.
0.1 A g−1, a recoverable capacity of 1000 mA h g−1 can be obtained, implying the excellent structural integrity of the MnO/Mn3O4@NC submicrospheres. It is clearly seen that the rate performance of MnO/ Mn3O4@NC submicrospheres is superior to that of the MnO/Mn3O4 electrode. The long-term cycling performance of the MnO/Mn3O4@NC submicrospheres electrode at current densities of 0.5 A g−1 and 1 A g−1 were measured. As depicted in Fig. 4e, the MnO/Mn3O4@NC submicrospheres electrode delivers a high reversible capacity of 1068 mA h g−1 over 200 cycles at 0.5 A g−1. Even at a high density of 1 A g−1, it still exhibits a reversible capacity of 953 mA h g−1 after 350 cycles, demonstrating superior rate capability and excellent cyclic stability. This could be benefit from the well combination of the high conductivity provided by N-doped carbon shell and the good buffer volume expansion ability offered by its porous structure. The electrochemical impedance spectroscopy (EIS) measurements were conducted to better understand the electrochemical properties of the two electrodes, as shown in Fig. 5a. All the Nyquist plots are exhibited a depressed semicircle at high frequency and an inclined line at low frequency, which are represented the charge transfer impedance (Rct) on the electrode/electrolyte interface and Li+ diffusion impedance (Warburg impedance, Ws) of the electrode, respectively [3]. The EIS data are fitted based on an equivalent circuit model, the fitting results are shown in Table 1. It can be clearly seen that the semicircle diameter for MnO/Mn3O4@NC is much smaller than that of MnO/Mn3O4, suggesting that N-doped carbon coating can effectively promote the charge transfer, thus giving a significant improvement for the cycling performance of the MnO/Mn3O4@NC electrode [23]. To further investigate the structural integrity of MnO/Mn3O4@NC electrode after cycling, we disassembled the cycled cells and observed its morphology by TEM. From the TEM image (Fig. 5b) of the cells after 100 cycles at 0.2 A g−1, we could see that the MnO/Mn3O4@NC can still maintains the spherical shape, suggesting that the carbon shell can effectively accommodate the volume variation during cycling, leading to the improved electrochemical performance.
Table 1 The fitting results of pure MnO/Mn3O4 and MnO/Mn3O4@NC electrodes. Electrode MnO/Mn3O4 MnO/Mn3O4@NC
Rs (Ω)
Rct (Ω)
2.2 1.8
887.5 212.5
cycle, which can be attributed to the improved reaction kinetics after the initial activation process [25,26]. Fig. 4b displays GCD profiles of the 1st, 2nd, 3rd, and 10th for MnO/Mn3O4@NC at a current density of 200 mA g−1. It can be seen the first discharge profile exhibits a voltage plateau at 1.5–0.3 V corresponds to the reduction of Mn3O4 to MnO, and then it shifts to a higher voltage in the subsequent cycles [20]. Another plateau at about 0.3 V is related to the reduction of MnO to Mn, which is in consistent with the CV results. In addition, the initial discharge and charge capacities of MnO/Mn3O4@NC are 1456.8 and 914.3 mA h g−1, respectively, giving an initial coulombic efficiency of 62.8%, which is much higher than that of the MnO/Mn3O4 (about 37.7%). The initial irreversible capacity loss is mainly attributed to the decomposition of the electrolyte and the generation of SEI film during cycling process [23,27,28]. To study the effect of the N-doped carbon coating on the electrochemical performance, the cyclic and rate performance of MnO/Mn3O4 and MnO/Mn3O4@NC anodes are evaluated. Fig. 4c exhibits the cycling performance of the pristine MnO/Mn3O4 and MnO/Mn3O4@NC submicrospheres at a current density of 200 mA g−1. It shows that the MnO/Mn3O4@NC delivers a reversible capacity of 990 mA h g−1 over 100 cycles with superior cyclic stability, which is higher than that of pristine MnO/Mn3O4 (350 mA h g−1 after 100 cycles). It can be clearly observed that the MnO/Mn3O4 and MnO/Mn3O4@NC anodes suffer from a capacity decline during the first several cycles, which is mainly ascribed to the SEI film generated on the surface of the electrodes. After 20 cycles, it is obviously observed that the capacities of the MnO/ Mn3O4@NC composite gradually increased with cycling, which could be ascribed to the activation reaction of the porous structure. The porous structure might be beneficial for more electrolyte get access to the inner structure of the electrode, thus contribute the capacity during cycling process [28]. Meanwhile, the elevated capacity can be due to the formation of higher valence manganese oxides in the final product and the enhanced electrochemical kinetics [27]. Fig. 4d displays the rate performance of the MnO/Mn3O4@NC submicrospheres anode at different current densities ranging from 0.1 to 1 A g−1. The reversible capacities were measured to be 850, 800, 660, and 520 mA h g−1 at current densities of 0.1, 0.2, 0.5, and 1 A g−1, respectively. Remarkably, after the current density is recovered to
4. Conclusions In summary, we demonstrated a facial and efficient strategy to fabricate porous MnO/Mn3O4@NC submicrospheres. The submicrospheres were successfully synthesized via an annealing treatment of spherical MnCO3 precursor under inert atmosphere. The resulting porous structure is formed through the decomposition of MnCO3. When evaluated as anode for LIBs, the as-prepared MnO/Mn3O4@NC electrode could deliver a reversible capacity of 990 mA h g−1 at 0.2 A g−1 over 100 cycles, and exhibit excellent rate capability and good cycling performance, which could be attributed to the porous structure and the 5
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N-doped carbon coating. The porous MnO/Mn3O4@NC offers enough spaces to alleviate the volume expansion during repeated lithiation/ delithiation processes and the uniform carbon coating can not only enhance the electronic conductivity of the composites, but also provide a protective layer for MnO/Mn3O4@NC from pulverization, thus leading to excellent electrochemical performance. The method used in this work provide a facial, low cost and easy controllable way to prepare porous MnO/Mn3O4@NC submicrospheres, which could be extended to other materials toward high-performance anode for LIBs.
[12] [13]
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Acknowledgements
[16]
This work was supported by the National Natural Science Foundation of China (Grant No: 51871247, 51774343 and 51802354).
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