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MnO2/Carbon Nanofibers Material as High Performance Anode for Lithium-Ion Batteries

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20 March 2023

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21 March 2023

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Abstract
High-performance anodes are contributed to nanostructure of transition metal oxides for rechargeable Li-ion batteries (LIBs). In this work, we report the fabrication of high-performance anode materials for lithium-ion batteries, MnO2 nanotube directly grown into fabrics of carbon nanofibers, the MnO2/Carbon nanofibers (CNFs) were investigated by X-ray diffraction, scanning and transmission electron microscopies. When tested as the anode material in LIBs, the MnO2/CNFs exhibit superior performance and excellent long cycling performance with a reversible capacity of 835 mA h g-1 at 0.1 A g-1 after the 133 th cycle, a high initial specific capacity of 1094 mA h g-1 at a current density of 0.1 A g-1. The MnO2/CNFs demonstrates notable specific capacities, specifically, with a coulombic efficiency of 99.5 %, both stability and capacity are conspicuously above literature data. These impressive results indicate that MnO2/CNFs has great potential for high-energy and high-power energy storage applications.
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1. Introduction

Rechargeable lithium-ion batteries are the critical energy storage devices over the last decades, they have been regarded as the first choice for electric automobile, portable electronics because of their high energy density, durable cycle life and eco-friendly [1,2,3]. Electrode materials, especially anodes, they are very important components of LIBs, may play a decisive role in the whole performances [4,5,6]. Up to now, the graphite is still considered as the most common used anode materials in large amounts of LIBs, in fact it is subjected to the low specific capacity (372 mA h g-1), which severely limited the application of LIBs [6,7,8]. Thus, researchers continue to explore advanced anode materials, that it could replace of the graphite [9,10,11,12].
Various transition metal oxides (TMOs) have paid much attention due to their excellent electrochemical properties [13,14,15,16,17,18]. Among of them, manganese oxides are considered as one of the most potential anode materials for high theoretical energy, owing to its low price, high theoretical capacity, variety of crystal structures and environmental friendliness (e.g., MnO2, MnO, Mn2O3, and Mn3O4) [19,20,21]. However, manganese oxide anodes also have many disadvantages, involving volume change during the process of charge/discharge repeatedly and poor electrical conductivity, which result in poor rate capacity and inferior cycling stability [22,23]. To solve the problems and improve the electrochemical performance, many effective strategies have been researched, for instance, materials [24], porous structure, conductive polymers/ metallic materials coating [25,26,27,28].
Sui et al. illustrated semiconducting polypyrrole coated-δ-MnO2 nanosheet arrays on nickel foam (denoted as MnO2@PPy/NF) are prepared via hydrothermal growth of MnO2 followed by the electrodeposition of PPy on the anode in LIBs. The electrode with ∼50 nm thick PPy coating exhibits an outstanding overall electrochemical performance [29]. Lin et al. showed MnO/C cubo-polyhedrons were successfully constructed with carbonizing α-MnO2@ZIF-8 precursors synthesized using sea urchin shaped α-MnO2 microspheres as a template. The as-prepared samples possess an irregular polyhedral cubic structure matrix coated with a thin carbon layer, which has the outstanding long cycling performance with a reversible capacity [30]. Cao et al. demonstrated hollow carbon nanospheres (HCN) loaded with MnO2 (denoted as MnO2@HCN) are investigated as an anode material for lithium-ion batteries. HCN is developed by treatment of 3-aminophenol and formaldehyde resin. MnO2 is loaded on the outer surface of HCN via the reduction of KMnO4 to form porous core-shell structures. SEM, TEM and XRD characterizations indicate that the MnO2@HCN has a spherical morphology with a core consisting of porous carbon nanoparticles and a shell consisting of MnO2 nanoparticles [31].
In this document, MnO2 was firstly synthesized through the hydrothermal, then MnO2 nanotubes (NTs) were obtained by calcination in air. Add to polyacrylonitrile (PAN) particles after MnO2 nanotubes mixed with the dimethylformamide, using heat treatment and electrospinning, and finally the MnO2/Carbon nanofibers (CNFs) was prepared. This material demonstrates the high specific capacity and the retention rate, outstanding performance of long cycling, and the excellent stability.

2. Experimental

2.1. Preparation of MnO2

The transition metal Oxide MnO2 was prepared by hydrothermal method. Firstly, 0.2 g KMnO4 was mixed with 25 mL H2O and stirred for 10 minutes. Then, 1.25 mL HNO3 was added and fully stirred and transferred to a 50 mL Teflon lined stainless steel reactor for hydrothermal reaction at 120 ℃ for 12 h, natural cooling, centrifugal washing at 10000 rates, and then dry. MnO2 powder was prepared.

2.2. Preparation of MnO2 NTs

MnO2 was calcined to produce MnO2 NTs. MnO2 powder was placed in a tubular furnace and placed in 400℃ air for 4 hours for heat treatment to obtain MnO2 NTs.

2.3. Preparation of MnO2 Nanofibers (NFs)

The polymer containing 5% MnO2 NFs and 20% MnO2 NFs were prepared by electrospinning. Weighing out the required of MnO2 NTs, respectively , and N,N dimethylformamide (Tianjin Kaitong Chemical Reagent Co., LTD.) mixed, put in the ultrasonic cleaning machine fully dissolved, then add the appropriate amount of polyacrylonitrile (PAN), continue to put in the ultrasonic cleaning machine until all dissolved, mixed into the required slurry into the electrostatic spinning machine for spinning. After re-coating by electrospinning, containing the polymer of 5% MnO2 NFs and 20% MnO2 NFs were prepared.

2.4. Preparation of MnO2 /CNFs

MnO2 NFs polymer was calcined to produce MnO2/CNFs. 5% MnO2 NFs polymer and 20% MnO2 NFs polymer were placed in tubular furnace respectively and carbonized in Ar atmosphere at 600℃ for 4 hours to obtain 5% MnO2/CNFs and 20% MnO2/CNFs (Figure 1).

2.5. Materials Characterization

The composition of MnO2 NTs, 5% MnO2/CNFs and 20% MnO2/CNFs were characterized by X-ray diffractometry (XRD, D8Advance, Bruker AXS, Germany), scanning electron microscope (SEM, JSM-6360LV, JEOL, Japan) and transmission electron microscope (TEM, Fei Tecnai G2 F20S-Twin ) were used to further observe the morphology of the tissue. The batteries were assembled in the glove box. 5 % MnO2/CNFs and 20 % MnO2/CNFs were selected as anode materials, and dry at room temperature for 24 h.

3. Results and Discussion

3.1. Morphology and Structure

The morphology and structure images of MnO2 NTs, 5 % MnO2 /CNFs and 20 % MnO2 /CNFs were obtained. Figure 2 characterizes the morphology of MnO2 NTs by scanning electron microscopy (SEM) at different magnifications. Figure 2a displays the tubular structure of MnO2 NTs with the longest of about 3.3 μm, the morphologies of MnO2 NTs have better uniformity, have almost no impurity particles. As shown in Figure 3b, hollow tubes are gained with an average diameter of 100 nm (±100). Figure 3a,b show that the 5 % MnO2/CNFs, and 3c-d show the 20 % MnO2/CNFs, both of them are the uniform nanofiber with a diameter of approximately 300–500 nm, and uniformly distributed nanoparticles are in the high component, it reveals that the uniform MnO2/CNFs can be prepared [32,33].
The crystal structures and phase information of MnO2 NTs, 5 % MnO2/CNFs and 20 % MnO2/CNFs were characterized by X-ray powder diffraction (XRD), the results are demonstrated in Figure 4, the diffraction peaks of MnO2 NTs, 5 % MnO2/CNFs and 20 % MnO2/CNFs are in line with the standard card of MnO2 (JCPDS NO. 44-0141), the XRD patterns of MnO2 NTs, 5 % MnO2/CNFs and 20 % MnO2/CNFs show no noticeable change. The typical diffraction peaks correspond to the (110), (200), (220), (310), (211), (301), (411), (431), (521), (002), (541), (312) and (332) crystal planes of manganese dioxide, respectively. This indicated that manganese dioxide was integrate into the composite material, which present the successful preparation. The peak of 20 % MnO2/CNFs is strong and narrow, confirmed that the good crystallinity and high purity [34,35].
In order to obtain further investigation into the structure of 20 % MnO2/CNFs samples, TEM characterization is carried out in Figure 5. The prepared 20 % MnO2/CNFs showed a nanotube structure with the diameter of 250 nm which is filled with the MnO2 particles in Figure 5b. In Figure 5c, the high resolution TEM image of the MnO2/CNFs exhibits that the lattice spacings are 0.182 nm and 0.161 nm, which are consistent with the spacing of the (411) and (431) planes of MnO2, obviously, demonstrating the composition of the nanofibers to be MnO2 again [35,36,37].

3.2. Electrochemical Performance

The lithium electrochemical performance of MnO2 NTs, 5 % MnO2/CNFs and 20 % MnO2/CNFs were evaluated using coin cells. The CV curves of MnO2 NTs, 5 % MnO2/ CNFs and 20 % MnO2/ CNFs for the first several cycles at a scan rate of 0.1 mV s-1 in the potential range 0.01-3.0 V, respectively, in Figure 6a–c. Compared with MnO2 NTs and 5 % MnO2/CNFs in Figure 6a,b, in the first cathodic cycle creates a broad peak at 0.45 V in Figure 6c, which vanishes in the following cathodic cycles, it illustrates that the reaction is irreversible and the energy difference prompts the formation of the solid electrolyte interface (SEI) on the electrode surface [30,31]. In addition to this, a peak decreases sharply below 0.35 V is because of the reduction of Mn4+ to Mn0 (MnO2 + 4Li+ + 4e— → Mn + 2Li2O) [30,38]. In the first anodic cycles, the strong peak at 1.32 V is corresponding to the dissolution of Li+ from Li2O and the oxication of Mn0 to Mn2+ (Mn + 2Li2O → MnO2 + 4Li+ + 4e—) [30,31,38]. In the other anodic cycles, the reaction is reversible because of the peaks have no obvious changes and almost in the same place, the excellent electrochemical stability was revealed.
The discharge/charge graph of 5 % MnO2/ CNFs and 20 % MnO2/ CNFs at a current density of 0.1Ag-1 are displayed in Figure 7a,b. The curves of lap 1, lap 20 and lap 50 were regarded as the samples, there is an obvious discharge voltage plateaus at about 0.76V (Figure 7a) and 0.57V (Figure 7b), the specific capacity of the first discharge are 1023 mA h g-1 and 1094 mA h g-1, respectively, the following cycles, the specific capacity are around 531-558 mA h g-1 and 658-704 mA h g-1, an SEI film was formed due to the lost capacity, which is irreversible, it is the same as cyclic voltammetry curves (Figure 7a,b), the mechanism of alloying lithiation is proved. Meanwhile, the discharge/charge capacity of the 20% MnO2/CNFs remain higher than the 5% MnO2/CNFs, it is indicated that not only the high performance, but also the excellent battery reversibility [39,40].
The cycling performance of 5 % MnO2/CNFs for 160 cycles and 20 % MnO2/CNFs for 200 cycles at a current density of 0.1 A g-1 are shown in Figure 8a,b. The initial discharge/charge specific capacities of 20 % MnO2/CNFs are 1094 mA h g-1 and 687 mA h g-1, higher than 5 % MnO2/CNFs. By the cycles continues, the specific capacities 5 % MnO2/CNFs maintains at around 610 mA h g-1 from the 117 th cycle to the 160 th cycle, while 20 % MnO2/CNFs remains at around 835 mA h g-1 from the133 th cycle to the 200 th cycle. During the whole process, coulombic efficiency of them hold at around 98.9 % and 99.5 %, respectively, 20 % MnO2/CNFs owns the higher coulombic efficiency obviously, these confirm that the more percentage of MnO2/CNFs, the better reversibility and structural stability [41,42,43].
To compare the discharge/charge capacity, the rate performance of MnO2 NTs, 5 % MnO2/CNFs and 20 % MnO2/CNFs are tested at various current rates ranging from 0.1 to 0.5 A g-1 (Figure 9a–c). MnO2 NTs does not contain the MnO2/CNFs in it, so the structure is damaged easily and the cycle is unstable (Figure 9a). On the contrary, because of the carbon nanofibers, 5 % MnO2/CNFs and 20 % MnO2/CNFs have the good structural stability (Figure 9b,c), but 20 % MnO2/CNFs show the superior capacity, the corresponding invertible capacities are 796, 611, 552, 486 mA h g-1 at 0.1, 0.2, 0.3, 0.5 A g-1. When test again at 0.1, 0.2, 0.3 A g-1, the reversible capacity of 835, 620,565 mA h g-1 are reached. In addition, the MnO2/CNFs shows better rate capability compared to the MnO2 NTs nanoparticle electrode at various rates from 0.1 to 0.5 A g-1 (Figure 9).

3.3. Analysis of Electrode Structure after Circulation

In order to further prove the relationship between the fiber structure and the cycling stability, the electrode was disassembled after the cycling, the structure and morphology are shown in Figure 10. It is very clearly to see that 20 % MnO2/CNFs still maintain the fiber structure, and the fiber structure is not broken, the porous fiber can release the expanded volume of the alloy effectively during the lithium process, thus maintaining the stability of the structure and improving the stability and efficiency of the cycle.

4. Conclusions

This study provides a simple method to prepare MnO2/CNFs material as high performance anode, MnO2/CNFs were prepared by electrospinning, the electrochemical properties of MnO2 NTs, 5 % MnO2/CNFs and 20 % MnO2/CNFs are evaluated. Comparison with them, 20 % MnO2/CNFs anode can provide the initial reversible capacity of 1094 mA h g-1 at 0.1 mA g-1 and remains 835 mA h g-1 after 133 cycles. The results show that 20 % MnO2/CNFs had better the good structural stability, high specific capacity, excellent reversibility, the long cycling performance, and superior ionic conductivity, which provided ideas and prospect for designing a high performance anode for LIBs.

Acknowledgments

The authors acknowledge financial support from Fundamental Research Funds for the Central Universities of Heilongjiang Province of China (2022-KYYWF-0603), Research Fund for the Doctoral Program of Higher Education of China (JMSUBZ 2020-02) and Fundamental Research Funds for the Central Universities of Heilongjiang Province of China (2021-KYYWF-0554).

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. Schematic of the synthesis procedure of the MnO2/Carbon nanofibers.
Figure 1. Schematic of the synthesis procedure of the MnO2/Carbon nanofibers.
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Figure 2. SEM images of MnO2 NTs (a,b).
Figure 2. SEM images of MnO2 NTs (a,b).
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Figure 3. SEM images of the MnO2/CNFs at different components: 5 % MnO2/CNFs (a,b); 20 % MnO2/CNFs (c,d).
Figure 3. SEM images of the MnO2/CNFs at different components: 5 % MnO2/CNFs (a,b); 20 % MnO2/CNFs (c,d).
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Figure 4. XRD patterns of MnO2 NTs, 5 % MnO2/ CNFs and 20 % MnO2/ CNFs.
Figure 4. XRD patterns of MnO2 NTs, 5 % MnO2/ CNFs and 20 % MnO2/ CNFs.
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Figure 5. TEM images of 20% MnO2 /CNFs (a,b) and HRTEM image of 20% MnO2 /CNFs (c).
Figure 5. TEM images of 20% MnO2 /CNFs (a,b) and HRTEM image of 20% MnO2 /CNFs (c).
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Figure 6. Cyclic Voltammetry curves of MnO2 NTs (a), 5 % MnO2/ CNFs (b) and 20 % MnO2/ CNFs (c) at a scan rate of 0.1 mV s-1 in the potential range 0.01-3.0 V.
Figure 6. Cyclic Voltammetry curves of MnO2 NTs (a), 5 % MnO2/ CNFs (b) and 20 % MnO2/ CNFs (c) at a scan rate of 0.1 mV s-1 in the potential range 0.01-3.0 V.
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Figure 7. Discharge/charge curves in selected cycles for 5 % MnO2/ CNFs (a) and 20 % MnO2/ CNFs (b).
Figure 7. Discharge/charge curves in selected cycles for 5 % MnO2/ CNFs (a) and 20 % MnO2/ CNFs (b).
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Figure 8. Cycling performance of 5 % MnO2/CNFs for 160 cycles (a) and 20 % MnO2/CNFs for 200 cycles (b) at a current density of 0.1 A g-1.
Figure 8. Cycling performance of 5 % MnO2/CNFs for 160 cycles (a) and 20 % MnO2/CNFs for 200 cycles (b) at a current density of 0.1 A g-1.
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Figure 9. Rate performance of MnO2 NTs (a), 5% MnO2/ CNFs (b) and 20% MnO2/ CNFs (c) at various current rates ranging from 0.1 to 0.5Ag-1.
Figure 9. Rate performance of MnO2 NTs (a), 5% MnO2/ CNFs (b) and 20% MnO2/ CNFs (c) at various current rates ranging from 0.1 to 0.5Ag-1.
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Figure 10. SEM images of the 20% MnO2/CNFs after 200 cycles.
Figure 10. SEM images of the 20% MnO2/CNFs after 200 cycles.
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