Nitrogen-Doped TiO2–C Composite Nanofibers with High-Capacity and Long-Cycle Life as Anode Materials for Sodium-Ion Batteries
KeywordsNanofibers Anode materials Sodium-ion batteries Pseudocapacitance Nitrogen-doping
Nitrogen-doped TiO2–C composite nanofibers (TiO2/N–C NFs) are fabricated using green, inexpensive urea as a nitrogen source and pore-forming agent.
X-ray photoelectron spectroscopy results reveal changes in the content of different nitrogen species in detail.
The TiO2/N–C NFs anode exhibits excellent sodium storage performance.
In recent decades, lithium-ion batteries (LIBs) play an important role in daily life (for example, in electric/hybrid vehicles and portable electronic products) owing to their excellent energy densities and long life spans [1, 2, 3, 4, 5]. Nevertheless, the disadvantages of limited lithium resources and high costs limit the commercial application of LIBs in large-scale energy storage. In contrast, sodium-ion batteries (SIBs) are more suitable for low-cost energy storage devices because of the abundance of sodium and affordable price [6, 7, 8, 9]. Nevertheless, it is still challenging to find a suitable host material with a larger space suitable for sodium ion insertion/extraction, which is necessary because the Na+ ion (1.06 Å) is ca. 40% larger than the Li+ ion (0.76 Å) [10, 11]. Therefore, it is vital to investigate suitable electrode materials for SIBs.
There are many reports on anode materials for SIBs, including alloying/dealloying reaction materials (Sn, Sb) [12, 13], conversion reaction materials (FeS2, Fe2O3) [14, 15], and insertion/extraction reaction materials (Na2Ti3O7, TiO2) [16, 17]. In particular, anatase titanium dioxide (TiO2), with a high natural abundance, nontoxicity, a small volume change (less than 4%), and low production cost, has attracted extensive attention as a promising anode material for SIBs . However, TiO2 has inherent defects, such as inferior electrical conductivity (10−12 S cm−1) as well as narrow ionic channels that cannot support rapid transfer of sodium ions [19, 20, 21], resulting in low specific capacity and serious capacity loss at high current densities. In order to improve its sodium storage performance, an important strategy is to increase its conductivity. One typical approach is to decrease the size of TiO2 particles or design novel nanostructures such as nanowires , nanospheres , or nanotubes , which can greatly shorten the sodium ion diffusion distance and promote electronic transport. Another effective method is recombination with carbon or doping of multivalent ions with Fe , S , Nb , or N .
Recently, nitrogen doping has been reported as an effective method to increase both the electronic and ionic conductivities of bulk materials [29, 30]. Nitrogen-doped carbon hollow spheres and carbon nanofibers (NFs) have exhibited excellent electrochemical properties as anode materials for SIBs [31, 32]. Nitrogen doping is effective not only for carbon materials, but also for transition-metal-oxide-based carbon composites. Some nitrogen-doped carbon composite transition metal oxides (such as MnO , Fe2O3 , Co3O4 , and TiO2 ) have been reported and showed satisfactory results. However, at present, the common methods of introducing N atoms are to calcine bulk materials in a poisonous atmosphere of N2/NH3 or to use rare and expensive nitrogen-rich materials, such as 3-hydroxytyramine hydrochloride, diethylenetriamine, polyaniline, and polypyrrole as nitrogen sources.
As a convenient and universal technology for producing polymers or composite material NFs, the electrospinning method has been widely applied in both academic research and industrial applications. Very recently, there have been many studies on the preparation of high-performance electrode materials (such as SnS/C, Na2VPO4F/C, and NiO/C) by electrospinning technology [37, 38, 39]. The obtained one-dimensional NFs with high specific surface areas can provide facile electronic and ionic transport. Further, the porous structure is highly tolerant of stress changes during the reaction in the battery, making it conducive to the realization of a long cycle life [39, 40].
Herein, a simple, economical, and green electrospinning process is proposed to obtain nitrogen-doped TiO2–C composite NFs (denoted as TiO2/N–C NFs). Inexpensive urea is used as the nitrogen source and pore-forming agent. Owing to the advantages of nitrogen doping and the large specific surface area, a TiO2/N–C NF electrode displays outstanding electrochemical properties.
3 Experimental Section
3.1 Synthesis of Materials
The TiO2/N–C NFs were synthesized by electrospinning followed by high-temperature carbonization. The precursor solution for electrospinning was made as follows: First, 5.0 mL of N, N-dimethylformamide (Kermel, 99.5%) and 1.05 g of glacial acetic acid (CH3COOH, Kermel, 99.5%) were mixed; then, 0.1 g of urea [CO(NH2)2, Kermel, 99.5%] and 0.97 g of tetra-n-butyl titanate (C16H36O4Ti, Kermel, 99%) were added with stirring. Next, 0.4 g of polyvinylpyrrolidone (PVP, Mw = 1,300,000, Alfa Aesar) was added to the above mixed solution under stirring for 12 h to acquire a clear precursor solution. The obtained solution was injected into a 10-mL syringe connected to a blunt-tip needle and spun on an electrospinning unit with an applied voltage of 14 kV. The distance between the needle and the collector was set to 14 cm, and the flow velocity was 0.36 mL h−1. The collected NFs were dried at 70 °C for 8 h in a vacuum oven and then precalcined at 200 °C for 2 h. Finally, the TiO2/N–C composite NFs were obtained by calcination at 550 °C for 4 h in an inert atmosphere of Ar, where the ramping rate was set to 4 °C min−1.
For comparison, the pristine TiO2–C NFs and the other two types of TiO2/N–C NFs with different N contents were prepared using similar methods by adjusting the amount of urea to 0, 0.05, and 0.2 g, respectively.
3.2 Structural Characterization
The as-prepared materials were examined by X-ray diffraction (XRD) in a Rigaku D/Max-2500 powder diffractometer with Cu Kα radiation (λ = 1.5418 Å). The morphologies of the synthesized samples were observed using scanning electron microscopy (SEM, JEOL, SM-71480) and transmission electron microscopy (TEM, JEOL, JEM-100CX). The chemical composition of the as-prepared materials was analyzed using X-ray photoelectron spectroscopy (XPS, ThermoFisher, K-Alpha+). N2 adsorption–desorption isotherms were obtained using TriStar II 3020 (Micromeritics, USA) at liquid nitrogen temperature (77.3 K). The specific surface area (SBET) was calculated by the conventional Brunauer–Emmett–Teller (BET) method. Thermogravimetry was performed using a TGA Q50 (TA Instruments) analyzer. Raman spectra were obtained using a Raman spectrometer (Renishaw, Model 1000) at an excitation wavelength of 514 nm.
3.3 Electrochemical Measurements
Polyvinylidene fluoride binder (10 wt%), 20 wt% carbon black, and 70 wt% active material (TiO2/N–C NFs or TiO2–C NFs) were dissolved in an appropriate amount of N-methyl-2-pyrrolidinone. The obtained slurry was evenly coated on copper foil and placed in a vacuum oven at 110 °C for 12 h. Circular pieces 1 cm in diameter were punched from the dried copper foil and used as working electrodes; their mass load was 1.2 ± 0.2 mg cm−2. In an argon-filled glove box, CR2025-type coin cells were assembled; metallic sodium was used as the counter electrode and separated from the work electrode by a glass fiber (Whatman, GF/C). The electrolyte was a solution of 1 mol L−1 NaClO4 dissolved in propylene carbonate/ethylene carbonate (1:1 by volume). The coin cells were cycled in galvanostatic discharge–charge measurements using a battery testing system (Neware, China) at room temperature at voltage intervals of 0.01 and 2.5 V. Both cyclic voltammetry (CV) tests and electrochemical impedance spectroscopy (EIS) experiments were conducted on a CHI660E electrochemistry workstation (Chenhua, Shanghai).
4 Results and Discussion
The larger specific surface area ensures full infiltration of the active material and electrolyte, thereby shortening the transport path to accelerate the rapid transfer of Na+/e− . The TiO2 content of the composites was determined by TGA. As shown in Fig. S1, the weight losses of the composites are ~ 28.9% and 26.4% in air, which implies a TiO2 content of 71.1 wt% in the TiO2/N–C NFs and 73.6 wt% in the TiO2–C NFs.
Figure 3c shows the rate performance of the two samples. The TiO2/N–C NF electrode could release reversible capacities of 265.8, 236.8, 202.4, 187.2, 175.6, 153.7, 136.4, and 132.1 mAh g−1 at current densities of 0.05, 0.1, 0.2, 0.5, 1, 2, 3, and 4 A g−1, respectively. Even at 5 A g−1, a reversible capacity of 124.5 mAh g−1 could be achieved. The discharge capacity could be maintained at 236.2 mAh g−1 when the current density recovered to 0.05 A g−1, which represents an excellent rate capability. In contrast to that of the TiO2/N–C NFs, the capacity of the TiO2–C NFs decreased significantly as the current density increased and dropped to 43.1 mAh g−1 at 5 A g−1. The outstanding rate properties may be due mainly to the improved conductivity resulting from the incorporation of N atoms. Figure 3d shows the corresponding discharge–charge curves. The discharge capacity of the TiO2/N–C NFs gradually decreases with an increase in current density. Nonetheless, the TiO2/N–C NF anode reveals less polarization than the TiO2–C NF anode (Fig. S2b), which further demonstrates the excellent rate capability. An ultra-long-term high-rate cycling performance test was performed to further verify the electrochemical performance of the TiO2/N–C NF anode. As shown in Fig. 3e, the specific capacity of the TiO2/N–C NF anode remains at 118.1 mAh g−1 after 2000 cycles at 5 A g−1 and exhibits almost no capacity decay. In order to explore the effect of adding urea to the precursor solution on the TiO2/N–C NF anode, the electrochemical properties of samples with different amounts of urea (0.05, 0.1, and 0.2 g) are shown in Fig. S3. The composite with 0.1 g of added urea obviously exhibits the best cycle stability and rate performance.
In order to determine the mechanism of the outstanding cycling stability of the TiO2/N–C NF electrode, a Na half-cell tested at a current density of 1 A g−1 for 1000 cycles was disassembled, and the morphology and microstructure of the TiO2/N–C NF electrode after cycling were observed by TEM, as shown in Fig. S4a, b. The morphology of the TiO2/N–C NFs remained essentially integrated. Further, as shown in the energy-dispersive X-ray spectroscopy (EDS) elemental mapping images in Fig. S4b, C, N, Ti, and O were still uniformly distributed in the NFs after a long cycling duration, indicating the mechanical stability of the fibers. Furthermore, the presence of Na in the EDS element mapping images also illustrates the process of sodium insertion/extraction during the cycle. An HRTEM image of the TiO2/N–C NFs after 1000 cycles further reveals that the crystal structure of the TiO2/N–C NFs remained integrated. From the above discussion, the preservation of the morphology and crystal structure of the TiO2/N–C NF electrode after cycling further explains the excellent electrochemical properties.
Nitrogen content of samples to which different amounts of urea were added
Nitrogen content (wt%)
N–TiO2/C NFs (0.05 g urea)
N–TiO2/C NFs (0.1 g urea)
N–TiO2/C NFs (0.2 g urea)
As the pseudocapacitive energy storage occurs on the surface or near the surface of the electrode, the ion diffusion is a type of diffusion on the surface or in the liquid phase, which has a much faster velocity than that in the solid phase. The characteristics of rapid ion diffusion greatly increase the ability of electrode materials to charge and discharge rapidly at high current densities [63, 66, 67, 68]. As shown in Fig. 7e, the contribution of the pseudocapacitance to the overall capacity increases with an increase in scan rate. Specifically, the contribution of the capacitive effect to the total charge stored at 0.1 mV s−1 is 24.5% and increases to 80.2% at 10 mV s−1 (Fig. 7f). However, the TiO2–C NFs show a low pseudocapacitance contribution of 16.2% at 0.1 mV s−1, and the value is only 67.4% even at 10 mV s−1 (Fig. S3). The large contribution of the pseudocapacitive contribution to the overall capacity may be correlated with the large specific surface area and the participation of nitrogen, which cause the TiO2/N–C NFs to exhibit excellent electrochemical performance as an anode in SIBs at ultra-high current density.
Comparison of EIS parameters of TiO2/N–C NFs and TiO2/C NFs
I0 (mA cm−2)
DNa+ (cm2 s−1)
5.8 × 10−13
2.0 × 10−13
The Nyquist plots of the TiO2/N–C NFs with different amounts of urea are used to explain the causes of these different electrochemical properties (Fig. S7). The simulated results are shown in Table S2. It can be seen that the Rs values of TiO2/N–C NFs with different amounts of urea are similar. However, the Rct value of TiO2/N–C NFs with 0.1 g of added urea is 85.5 Ω, which is smaller than that of the TiO2/N–C NFs with 0.05 g of added urea and much smaller than that of TiO2/N–C NFs with 0.2 g of added urea. Hence, it can be deduced that TiO2/N–C NFs with 0.1 g of added urea exhibit the smallest electrochemical resistance, indicating the best electron conductivity and electrochemical activity.
In summary, nitrogen-doped TiO2–C composite NFs were fabricated by a facile and green electrospinning method. Inexpensive urea was used as a nitrogen source and pore-forming agent. The as-prepared TiO2/N–C NFs exhibited a large specific surface area (213.04 m2 g−1) and a suitable nitrogen content (5.37 wt%). These characteristics not only contribute to increasing the contact area with the electrolyte and thus shortening the ion/electron diffusion distance, but also essentially enhance the electronic conductivity. As anodes in SIBs, the TiO2/N–C NFs exhibit a high reversible capacity (265.8 mAh g−1 at 0.05 A g−1), an outstanding rate performance (202.4 and 153.7 mAh g−1 at 0.2 and 2 A g−1, respectively), and an ultra-long cycling durability (118.1 mAh g−1 at 5 A g−1 after 2000 cycles). This work will open the way to the use of TiO2/N–C NFs as one of the most promising anode materials for low-cost SIBs.
This work was supported financially by the National Natural Science Foundation of China (Grant No. 51672234), Hunan 2011 Collaborative Innovation Center of Chemical Engineering and Technology with Environmental Benignity and Effective Resource Utilization, Program for Innovative Research Cultivation Team in University of Ministry of Education of China (1337304), and the 111 Project (B12015).
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