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

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.