MoSe₂ Complex with N and B Dual-Doped 3D Carbon Nanofibers for Sodium Batteries

Abstract

The sodium battery is one of the best energy storage technologies due to its abundant resource reserves and excellent energy storage ability. As a two-dimensional layered transition metal, molybdenum selenide (MoSe₂) has large interlayer spacing and a high theoretical capacity (470 mAh∙g⁻¹). Its structure is suitable for the negative electrode of sodium-ion batteries, with a large ionic radius and slow ion diffusion kinetics. However, it is difficult for the rate capability and cycling performance of MoSe₂ to meet practical needs due to a weak intrinsic electron transport ability and volume expansion during sodium absorption. The hydrothermal synthesis method was used to synthesize the MoSe₂ complex based on boron and nitrogen dual-doped 3D carbon fibers obtained from bacterial cellulose membranes (MoSe₂/N&B-BCM) for sodium batteries. Additionally, electrochemical analysis and experimental characterization were performed. In summary, the experimental analysis shows that MoSe₂/N&B-BCM has excellent conductivity, structural integrity, cyclability (328 mAh∙g⁻¹ after 100 cycles at a 0.5 c constant rate), and rate stability.

1. Introduction

With the massive consumption of fossil resources, the importance of new energy sources is becoming more and more prominent. In recent years, high specific energy, no memory effect, safety, long cycle life, and high efficiency have become part of the features of energy storage used in different devices, such as cell phones, laptops, and electric vehicles [1,2,3]. As one of the most widely used new energy sources, lithium-ion batteries are widely used in human daily life [4,5]. However, the very limited reserves of lithium on earth and their uneven distribution have limited the development of lithium-ion batteries in China. Therefore, the search for new low-cost and resource-rich high-performance secondary battery systems to replace lithium-ion batteries has become the key to sustainable energy development. Sodium-based energy storage systems have shown high promise due to their abundant sodium sources, low cost, and wide availability. Therefore, sodium-ion batteries have become one of the most promising directions for the future development of energy storage batteries [6,7,8].

However, the main obstacle to the development of sodium-ion batteries is the large radius of Na⁺, as this results in slow reaction kinetics. This limits the practical applications of sodium batteries [9] due to the large radius of Na⁺ (0.102 nm) (compared with 0.076 nm for Li⁺) and a large diffusion potential barrier. Therefore, novel sodium storage materials are urgently required, and the development of high-capacity, long-cycle-life anode materials is particularly critical. In recent years, carbon materials, alloy materials, and metal–oxygen/sulfur/selenide have been widely used (Figure 1), of which carbon materials have a stable cycling performance but low capacity [10]; alloys have a high theoretical capacity but huge volume expansion [11]; and metal oxides have a high theoretical capacity but low electrical conductivity [12]. The ideal anode material for sodium-ion batteries needs to meet the requirements of excellent conductivity, small volume expansion, and long cycle life.

MoSe₂ has a sandwich structure bonded between in-plane atoms and coupled with weak vertical van der Waals forces. It is beneficial for the intercalation/deintercalation of Na ions. It has been widely studied as an anode material for sodium-ion batteries due to its high theoretical capacity (470 m Ah g⁻¹) and large layer spacing (0.646 nm). MoSe₂ has many attractive features, including low cost, high quantity, environmental friendliness, and stable rate performance. Therefore, it is considered to be the most promising anode material for sodium-ion batteries. Despite so many advantages, there is still considerable room for improving the performance of MoSe₂ anode materials before practical applications, which are mainly limited by the limited-contact active sites of sodium ions and long ion diffusion paths [13]. Moreover, repeated Na⁺ embedding and de-embedding during long-term cycling causes significant mechanical stress and volume changes in MoSe₂, which may cause MoSe₂ accumulation and aggregation, leading to poor cycling stability [14]. To solve these problems, various strategies have been developed to improve the electrical conductivity and structural stability of MoSe₂, such as the optimization of particle size or morphology and the fabrication of MoSe₂/carbon hybrids [15,16,17,18,19,20].

In the past few decades, MoSe₂ complexes with carbon (MoSe₂/C, MoSe₂/BCM, and MoSe₂@N,P-C) have produced good electronic results. Therefore, a MoSe₂ complex with carbon is a good choice for energy storage devices. Liu et al. synthesized sheet-like MoSe₂/C composites [21]. After 50 cycles, the reversible specific capacity is 576.7 mAh∙g⁻¹ at a current density of 100 mA∙g⁻¹. Zhang et al. successfully synthesized 3D core–sheath-structured carbon nanofiber@nitrogen-doped porous carbon for sodium-ion batteries (SIBs) [22]. It shows an outstanding electrochemical performance, including a high reversible specific capacity (240 mAh∙g⁻¹ at 100 mA∙g⁻¹ over 100 cycles), an excellent rate performance (146.5 mAh∙g⁻¹ at 1000 mA∙g⁻¹), and superior cycling stability (148.8 mAh∙g⁻¹ at 500 mA∙g⁻¹ over 400 cycles). Dual nitrogen and boron elements on a bacterial cellulose membrane (BCM) significantly improve both the cathode and anode performance of sodium-ion batteries, thus alleviating the mismatch between the two electrodes [23].

Herein, to improve the cycle performance of MoSe₂, we design and synthesize a novel MoSe₂ complex with boron and nitrogen dual-doped 3D carbon nanofibers (MoSe₂/N&B-BCM) for sodium batteries by using the hydrothermal synthesis method. MoSe₂/N&B-BCM was compared with MoSe₂/BCM, N&B-BCM, and BCM to identify the better electronic performance. After optimization, MoSe₂/N&B-BCM exhibits excellent cyclability (328 mAh∙g⁻¹ after 100 cycles at a 0.5 A∙g⁻¹ current density) and good rate performance. These outstanding electrochemical performances should be associated with the novel 3D architecture, in which the N and B elements are mainly homogeneously dispersed on the fibers of the BCM. MoSe₂ complexed with N- and B-doped BCM produced stable composites due to the dual doping with N and B elements; its electronic point actives were also improved, resulting in an excellent rate and stability.

2. Experimental Section

2.1. Reagents and Instruments

Se, 99.9% by mass; hydrazine hydrate, 98.5% by mass; ammonium borate tetrahydrate, 98.3% by mass; sodium molybdate dihydrate, 99.5% by mass; and polyvinylidene fluoride (PVDF), battery grade, were obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). NaClO4, 1.0 mol/L; ethylene carbonate (EC); and diethyl carbonate (DC) were obtained from China National Chemical Engineering Co., Ltd. (Beijing, China). Anhydrous ethanol, 99.7% mass fraction, was obtained from Yantai Far East Fine Chemical Co., Ltd. (Yantai, China) and bacterial cellulose membrane from Guilin Qihong Technology Co., Ltd. (Guilin, China).

Glove box: SG1200/750TS, Michelona (China) Co., Ltd (Shanghai, China). Ultrasonic cleaner: KQ5200DE, Kunshan Ultrasonic Instruments Co., Ltd. (Suzhou, China). Magnetic mixer: DF-101S, Gongyi Yuhua Instrument Co., Ltd. (Zhengzhou, China). Blast drying oven: GZX-9076 MBE, Shanghai Boxun Industrial Co., Ltd. (Shanghai, China). Tube furnace: MFLGKD405-12, Shanghai Muffle Furnace Technology Instruments Co., Ltd. (Shanghai, China). Electrochemical workstation: CHI660E, Shanghai Chenhua Instruments Co., Ltd. (Shanghai, China). Battery testing system: CT2001A, Wuhan Blue Electric Electronic Co., Ltd. (Wuhan, China). Three mouth flasks, beakers, vacuum pumps, pipettes, etc.: Beijing Xinville Glass Instruments Co., Ltd. (Beijing, China). Electronic balance: FA2004N, Shanghai Jinghai Instrument Co., Ltd. (Shanghai, China). Tablet press: 50T press, Jiaxing Tongli Hardware Machinery Co., Ltd. (Jiaxing, China).

X-ray diffractometer (XRD): D8 Advance, Bruker, Germany. Confocal laser micro spectrometer (Raman): InviaQontor, Renishaw, London, UK. Transmission electron microscope (TEM): JEM-2100F, JEOL, Tokio, Japan. Scanning electron microscope (SEM): LYRA 3 XMH, TESCAN, Brno, Czech Republic. Energy spectrometer (EDS): X-MaxN 80T IE250, Oxford, UK. X-ray Photoelectron Spectrometer (XPS): AXIS Supra, Kratos Analytical Ltd., Manchester, UK. LAND battery programmable tester: CT2001A, Wuhan Blue Electric Electronics Co., Wuhan, China.

2.2. Materials Syntheses

2.2.1. Preparation of BCM

First, the bacterial cellulose membrane (BCM) was cut into species (40 × 60 mm) and washed several times with deionized water, and the prepared BCM was immersed in 100 mL of DI water for 24 h, slowly stirred at room temperature, and then freeze-dried for 24 h. The freeze-dried product was placed in a tube furnace and sintered in nitrogen at 600 °C for 3 h. After carbonization, the product was thoroughly washed in hot deionized water and dried overnight at 80 °C.

2.2.2. Preparation of MoSe₂/BCM

First, the BCM was stirred to break it up. An amount of 158 mg of selenium powder was dissolved in 5 mL of hydrazine, as solution A, and then 242 mg of sodium molybdate was dissolved in a mixture of ethanol–water (ethanol:water, 1:1). The loading volume of the reaction solution (a combination of ethanol and water) was 50%, as solution B. Moreover, solution A was stirred with the BCM and then slowly added to solution B. The resulting mixed liquid was sealed in a reaction vessel and reacted at 180 °C for 10 h. The resulting membrane was washed with deionized water and ethanol, and then freeze-dried for 24 h. The freeze-dried product was placed in a tube furnace and sintered in nitrogen at 350 °C for 1 h and then heated to 800 °C for another 1 h. After carbonization, the product was thoroughly washed in hot deionized water and dried overnight at 80 °C.

2.2.3. Preparation of N&B-BCM

First, after cutting the BCM into species (40 × 60 mm) and washing several times with deionized water, the prepared BCM was immersed in 100 mL of a 0.1 M NH₄HB₄O₇ aqueous solution for 24 h, slowly stirred at room temperature, and then freeze-dried for 24 h. Then, the freeze-dried product was placed in a tube furnace and sintered in nitrogen at 600 °C for 3 h. After carbonization, the product was thoroughly washed in hot deionized water and dried overnight at 80 °C.

2.2.4. Preparation of MoSe₂/N&B-BCM

First, after cutting N&B-BCM into species (40 × 60 mm) and washing several times with deionized water, the prepared BCM was immersed in 100 mL of a 0.1 M NH₄HB₄O₇ aqueous solution for 24 h, slowly stirred at room temperature, and then freeze-dried for 24 h. Then, the freeze-dried product was placed in a tube furnace and sintered in nitrogen at 600 °C for 3 h. After carbonization, the product was thoroughly washed in hot deionized water and dehydrated overnight at 80 °C.

Then, the N&B-BCM was stirred to break it. An amount of 158 mg of selenium powder was dissolved in 5 mL of hydrazine, as solution A, and then 242 mg of sodium molybdate was dissolved in a mixture of ethanol–water (ethanol:water, 1:1). The loading volume of the reaction solution (a combination of ethanol and water) was 50%, as solution B. Moreover, solution A was stirred with N&B-BCM and then slowly added to solution B. The resulting mixed liquid was sealed in a reaction vessel and reacted at 180 °C for 10 h. The resulting membrane was washed with deionized water and ethanol, and then freeze-dried for 24 h. The freeze-dried product was placed in a tube furnace and sintered in nitrogen at 350 °C for 1 h and then heated to 800 °C for another 1 h. After carbonization, the product was thoroughly washed in hot deionized water and dried overnight at 80 °C.

2.3. Electrochemical Measurements

Preparation of electrodes:

(1)

The slurry preparation: The active material (the composite material detailed above), conductive material (acetylene black), and binder (the solute was PVDF, the solvent was NMP, and the mass fraction was 8 wt%) were weighed. The active material and the conductive material were ground in an agate mortar for 40 min, and then the mixed powder was poured into a 5 mL glass bottle with a cover. The binder was added and then stirred at a constant speed for 3 h.

(2)

The electrode preparation: A 10 × 15 cm piece of copper foil was wiped with an alcohol cotton ball to remove surface dust; the evenly stirred slurry was coated onto the surface of the copper foil; the foil was kept in a vacuum drying oven for 12 h, and the temperature was set at 80 °C; the dried copper foil was further impacted with a punch to obtain an electrode sheet with a diameter of 14 mm; the obtained electrode sheet was placed in a tablet press with a 10 MPa pressure and was pressed down and kept as such for 3 s; and the quality of the electrode sheet was weighed. A card machine was used to make the active material copper foil into several Ø = 14 mm copper foils, and they were weighed, and the average mass of the Ø = 14 mm copper foils was calculated. Combining the average mass of the electrode sheet and copper foil of Ø = 14 mm with the slurry ratio, the mass of the active material on the surface of the electrode sheet was calculated. The weight of the electrode was 2 mg/cm².

(3)

The battery assembly: The entire battery assembly process was carried out in a vacuum glove box filled with Ar. All the batteries used in this paper were CR2025 button cells. From bottom to top, they have a positive electrode shell, an electrode sheet (the side coated with the active material facing up), a separator, a sodium metal sheet, nickel foam, and a negative electrode shell. Two~three drops of electrolyte were added. After assembly was complete, a paper towel was used to wipe off the excess electrolyte that spilled out of the battery to prevent the battery from short-circuiting.

3. Results and Discussion

The phase structure of the samples was investigated using X-ray powder diffraction (XRD). Figure 2 shows the XRD patterns of MoSe₂/N&B-BCM, MoSe₂/BCM, N&B-BCM, and BCM. Compared with pure BCM, N&B-BCM, MoSe₂/N&B-BCM, and MoSe₂/BCM show 3 peaks at around 30°, 38°, and 55°, respectively. These correspond to the (100), (103), and (110) planes and suggest that N and B dual-doped BCM carbonates materials. Except for the carbon peaks, MoSe₂ (JCPDF 29-0914) is easily located from the 13.60°, 32.10°, 38.32°, 47.29°, and 56.98° diffraction peaks. It is shown that MoSe₂ grew well on the BCM and N&B-BCM. There are 2 peaks corresponding to the (100) and (110) peaks of MoSe2. A total of 2 obvious peaks appear at 2θ = 13.7°, 37.9°, which correspond to the (002) and (103) planes, suggesting that the (002) and (103) interlayer spacing of the freshly prepared MoSe₂ was dramatically changed. After the N-B doping treatment, the diffraction peaks become much sharper with increased intensity, indicating improved crystallinity. Besides the 2 (100) and (110) diffraction peaks, 2 additional peaks at 13.7 and 37.9° are observed, which can be correlated with the (002) and (103) diffraction peaks of MoSe₂ (JCPDF 29-0914).

To analyze the chemical composition on the surface and the valence of states in the sheet-like MoSe₂/N&B-BCM composites, X-ray photoelectron spectroscopy (XPS) measurements were carried out in the region of MoSe₂/N&B-BCM. The survey spectrum of the composites (Figure 3a) further shows the C1s spectrum, which could be deconvoluted into one peak. The sharp peak at around 284.6 eV corresponds to sp2-C with C=C bonds. As shown in Figure 3b, the N1s spectrum can be deconvolved into 2 peaks, which can be assigned to pyridinic N at 395.8 eV and quaternary N at 402.1 eV. The high-resolution Mo3d spectrum of the composites exhibits 2 peaks at 229 eV and 232.1 eV (Figure 3c), corresponding to the Mo3d5/2 and 3d3/2 spin-orbit peaks of MoSe2, confirming the existence of the Mo IV state. The 3d height of the Se element is split into 2 well-defined peaks at 54.5 eV and 55.4 eV (Figure 3d), corresponding to the Se3d5/2 and Se3d3/2 characterized peaks, further illustrating the products are MoSe₂/N&B-BCM [24,25,26,27].

The morphologies of all the samples were investigated using scanning electron microscopy (SEM). When BCM was doped with N and B (Figure 4c), it exhibited an interconnected fiber morphology with a diameter of 1 um. When was complexed with MoSe₂ (Figure 4d), it showed a nanoflower structure, confirming that MoSe₂ had already grown on the carbon fibers. Figure 4e,f shows the MoSe₂/N&B-BCM system, which exhibits a stable structure like a rice spike. The elemental mapping images of MoSe₂/N&B-BCM in Figure 4g display that the Mo, Se, C, N, and B elements are evenly distributed (Figure 4h–l). In the synthesis of MoSe₂/N&B-BCM, hydrazine hydrate and ethylenediamine (EDA) were used, so the N and B elements were present in the composite. The EDS spectra further confirm the existence of Mo, Se, C, N, and B.

Figure 5a indicates that MoSe₂/N&B-BCM exhibits an outstanding electrochemical performance as an anode. MoSe₂/N&B-BCM has the highest capacity of 328 mAh∙g⁻¹ at 0.5 A∙g⁻¹. According to Figure 5b, MoSe₂/N&B-BCM has excellent cyclability when compared with BCM, N&B-BCM, and MoSe₂/BCM as an anode, and MoSe₂/N&B-BCM exhibits the highest capability at 328 mAh∙g⁻¹ at 0.5 A∙g⁻¹. MoSe₂/N&B-BCM had a stable rate performance at 328 mAh∙g⁻¹, 260 mAh∙g⁻¹, 202 mAh∙g⁻¹, 153 mAh∙g⁻¹, 121 mAh∙g⁻¹, 205 mAh∙g⁻¹, 246 mAh∙g⁻¹, and 309 mAh∙g⁻¹, at a 0.1 A∙g⁻¹, 0.2 A∙g⁻¹, 0.5 A∙g⁻¹, 1 A∙g⁻¹, 2 A∙g⁻¹, 0.5 A∙g⁻¹, 0.2 A∙g⁻¹, and 0.1 A∙g⁻¹ current density (Figure 5c). Figure 5d shows the charge–discharge curve of the MoSe₂/N&B-BCM electrode at a magnification of 0.1 A∙g⁻¹. The composite anode has a satisfactory initial discharge and charge capacity of 480 mAh∙g⁻¹ and 328 mAh∙g⁻¹, respectively, with a coulombic efficiency of 68.3%. The SEI film provides such an irreversible capacity. In the following two cycles, both curves coincide very well, implying stable SEI film formation and high electrochemical reversibility [28].

The initial 4 cycles of the current voltage curves of MoSe₂/N&B-BCM in the range 0.01 V–3 V (vs. Na⁺/Na) at a scan rate of 0.1 mV∙s⁻¹ are presented in Figure 5e, which are different from those of BCM, N&B-BCM, and MoSe₂/BCM. An irreversible cathodic peak was observed for the MoSe₂/N&B-BCM electrode at 1.08 V in the first cycle but disappeared in the subsequent processes, showing a typical behavior of a solid electrolyte interface (SEI) [29]. In sharp contrast, a pair of redox peaks at 1.85 and 1.10 V were observed in all CV curves of MoSe₂/N&B-BCM, which are attributed to the redox reactions between Mo and Na⁺ [30]. Moreover, the CV curves of the second, third, and fourth cycles were nearly identical, suggesting the excellent cycling stability of the MoSe₂/N&B-BCM electrode.

As shown in Figure 5f and

Table 1. The impedance of BCM, N&B-BCM, MoSe2/BCM, and MoSe2/N&B-BCM with sodium. (Rct: charge migration resistance, Rs: internal resistance).

Resistance Value	BCM	N&B-BCM	MoSe2/BCM	MoSe2/N&B-BCM
Rs	28.5 Ω	20.82 Ω	28.46 Ω	11.96 Ω
Rct	1719 Ω	604 Ω	453 Ω	182.4 Ω

, we tested the BCM, N&B-BCM, MoSe₂/BCM, and MoSe₂/N&B-BCM sodium batteries’ EIS in order to understand the superior reaction dynamics of the MoSe₂/N&B-BCM electrode. The semicircle in the medium-frequency region in the Nyquist plots typically indicates the electrochemical reaction impedance. In contrast, the sloping line in the low-frequency region corresponds to Na⁺ diffusion in the electrodes [31].

Obviously, the diameter of the semicircle related to MoSe₂/N&B-BCM is smaller than that of BCM, N&B-BCM, and MoSe₂/BCM in a new cell, implying augmented charge transfer efficiency. Meanwhile, the steeper slope of the MoSe₂/N&B-BCM electrode also suggests improved ion diffusion, which accords well with the above charge–discharge analysis.

4. Conclusions

Compared with the capacity of BCM (98 mAh∙g⁻¹), N&B-BCM (101 mAh∙g⁻¹), and MoSe₂/BCM (232 mAh∙g⁻¹), the effects of dual-doped N and B elements with 3D carbon nanofibers on sodium storage properties were systematically studied. MoSe₂/N&B-BCM exhibited excellent cyclability in the sodium test (328 mAh∙g⁻¹ after 100 cycles at 0.5 A∙g⁻¹) and rate performance. These outstanding electrochemical performances should be associated with the novel 3D architecture, in which the N and B elements are mainly homogeneously dispersed on the fibers of the BCM. Due to the dual-doped N and B features, MoSe₂ complexed with N- and B-doped BCM produces stable composites. Its electronic point actives had already been enhanced, which resulted in stable cyclability and excellent rate performance.