Growth of SnO2 Nanoflowers on N-doped Carbon Nanofibers as Anode for Li- and Na-ion Batteries
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KeywordsSnO2 Nanostructures Anode Li-ion battery Na-ion battery
A hybrid structure of SnO2 nanoflowers grown on N-doped carbon nanofibers (NC@SnO2) was successfully constructed.
N-doped carbon nanofiber accelerates the migration of Li+/Na+ ions and guides the growth of the SnO2 nanoflowers.
NC@SnO2 electrode reveals excellent energy storage performance for Li- and Na-ion batteries.
With severe resource constraints and global environmental problems, it is necessary to develop highly efficient energy storage systems to reduce the use of fossil fuels [1, 2, 3, 4, 5]. Nowadays, lithium- and sodium-ion batteries (LIBs and SIBs) have attracted widespread attention all over the world [6, 7, 8]. LIBs have been extensively applied in portable electronic equipment and electric vehicles (EVs) and intelligent power grids because of their outstanding characteristics of high energy density, no memory effect, and small self-discharge [9, 10]. Recently, owing to the lack of lithium resources and the similar chemical property of Na+ to Li+, SIBs have also received increasing attention [11, 12]. As one of the important parts for LIBs or SIBs, the high-performance electrode materials are urgently needed for next-generation battery systems.
As one of the typical transition-metal oxides (TMOs), tin dioxide (SnO2) is widely concerned to be promising electrode materials owing to its non-toxicity, low cost, high theoretical capacity, and outstanding electrochemical performance [13, 14, 15]. Nevertheless, it is similar to the shortcomings of other oxide materials during cycling processes that SnO2 endures the dramatic volume change. This would lead to the capacity decay and poor cycling performance [16, 17, 18]. To improve the electrochemical performance of SnO2, nanostructured SnO2 is employed to reduce the volume variation of SnO2 during the charge/discharge process [19, 20, 21]. However, it is easily agglomerated for nanostructured SnO2 to reduce the specific surface area of the active materials, leading to the attenuation of energy storage. To overcome this problem, a great deal of SnO2/carbon composites has been designed to maintain the structural stability of electrodes and improve the electrical conductivity of composites [22, 23, 24]. In addition, the N-doped carbon composite materials are considered to enhance the electrical conductivity and accelerate the reaction speed of the SnO2 composites, and increase defect sites for the efficient storage of lithium/sodium ions [25, 26, 27].
In this work, we synthesized a hybrid structure of N-doped carbon fibers@SnO2 nanoflowers (NC@SnO2) by electrospinning/hydrothermal methods. When they are used as an anode material in LIBs and SIBs, the as-prepared NC@SnO2 hybrid material displayed excellent electrochemical properties. The high discharge capacity reached 750 mAh g−1 at a current density of 1 A g−1 after 100 cycles in LIBs. Meanwhile, a reversible discharge capacity of 270 mAh g−1 was achieved at a current density of 100 mA g−1 after 100 cycles in SIBs.
3 Experimental Section
3.1 Synthesis of SnO2, N-doped Carbon, and NC@SnO2
All chemical reagents were purchased and used without further treatment. The synthesis of SnO2 nanoflowers was carried out according to the previous literature . The N-doped carbon (NC) nanofibers were synthesized by electrospinning as follows: 0.6 g polyacrylonitrile (PAN, Sigma-Aldrich Co., Ltd. USA) was firstly added into 7 g N, N-dimethylformamide (DMF, Sinopharm Chemical Reagent Co., Ltd., China). Then, the above solution was poured into 10-mL plastic syringe and followed by electrospinning. The NC nanofibers were finally obtained via annealing the precursor at 600 °C in Ar atmosphere. To synthesize NC@SnO2, 4 mmol tin(II) chloride dihydrate (SnCl2·2H2O, Xilong Chemical Co., Ltd., China) and 8 mmol sodium citrate (Na3C6H5O72·H2O, Tianjin Hengxing Chemical Reagent Manufacturing Co., Ltd., China) were firstly dissolved into the mixed solvent of 15 mL ethanol and 15 mL water. After stirring for 30 min, 80 mg NC nanofibers were introduced into the above blend solution. Subsequently, the mixture solution was put into a Teflon-lined stainless steel autoclave at 180 °C for 12 h after continuous ultrasound for 30 min. The precursor samples were taken out the autoclave after the end of the reaction and ultrasonic cleaning with deionized water and ethanol. Finally, the NC@SnO2 samples were obtained with annealing at 500 °C for 3 h in Ar gas.
3.2 Material Characterizations
The X-ray diffraction (XRD) of the samples was conducted with a Shimadzu XRD-6000 instrument, and the morphologies and structural features of the samples were characterized by scanning electron microscopy (SEM, Hitachi S4800) and transmission electron microscopy (TEM; JEOL 2010 with an accelerating voltage of 200 kV). The thermogravimetric analysis (TGA) of the powder sample was surveyed with a WCT-1D instrument (BOIF, China) in air atmosphere from 30 to 800 °C. Brunauer–Emmett–Teller (BET) of the sample was performed with the adsorption of N2 with a nova 2000 e volumetric adsorption analyzer (Kangta, USA), The element composition and chemical bonds of the sample were detected by X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250Xi, USA). Raman spectra of the samples were conducted by utilizing micro-Raman spectrometer (LabRAM HR Evolution, HORIBA).
3.3 Electrochemical Measurements
The working electrodes of LIBs and SIBs were fabricated by using 80 wt% of active materials (NC@SnO2, SnO2, and NC), 10 wt% of acetylene black, and 10 wt% of carboxymethylcellulose sodium (CMC). The mixture was uniformly distributed in the deionized water and ethanol and coated on the copper foil which dried at 60 °C in a vacuum drying oven for a day. CR2025-type coin half-batteries of as-prepared electrodes were assembled in the glove box with water and oxygen content of less than 0.5 ppm. The microporous polypropylene (Celgard 2400) and glass microfiber filter membranes (Whatman, Grade GF/A) were utilized as a separator of LIBs and SIBs, respectively. And corresponding metal plates were used as the counter electrodes of batteries. The electrolyte of LIBs was composed of 1.0 M of LiPF6 solution which mixed ethylene carbonate (EC) and dimethyl carbonate (DMC) with 1:1 in volume, and the electrolyte of SIBs was constituted by 1.0 M of NaClO4 solution which mixed EC with DMC (1:1 in volume), accompanied with 5% fluoroethylene carbonate (FEC) of additive agent. The electrochemical property and cyclic voltammetry measurement of LIBs and SIBs were performed with Neware Battery Testing System and CHI 660C Electrochemical Workstation, respectively.
4 Results and Discussion
The charge/discharge profiles of NC@SnO2 at the 1st, 2nd, 3rd, and 5th cycle were displayed at in Fig. 5b. The voltage platforms of charge–discharge can be observed to be consistent with the oxidation–reduction peaks of above CV curves. The initial discharge–charge capacities of NC@SnO2 are 1463.6 and 1009.8 mAh g−1, respectively. And the low initial coulombic efficiency of 67.0% may be associated with the formation of SEI film and the irreversible reactions of SnO2 material in the first cycle [32, 43]. The cycling performance of NC@SnO2, SnO2, and NC is shown in Fig. 5c. The discharge capacity of NC@SnO2 is about 750 mAh g−1 at 1 A g−1 after 100 cycles, while the discharge capacities of SnO2 and NC only remain 480 and 220 mAh g−1, respectively. In Fig. 5d, one can see that the average capacities of NC@SnO2 are about 1100, 850, 763, 684, 615, 568, and 905 mAh g−1 at different current densities of 0.2, 0.5, 1, 2, 4, 6, and 0.2 A g−1, respectively. However, the average capacities of SnO2 are only about 966, 842, 765, 685, 525, 370, and 770 mAh g−1 at 0.2, 0.5, 1, 2, 4, 6, and 0.2 A g−1, respectively. And the NC electrode exhibits the capacities less than 550 mAh g−1 at various current densities.
Figure 6b displays the discharge/charge capacities of 555.7/212.5 mAh g−1 in the first charge/discharge cycle, respectively, with a coulombic efficiency of 38.2%. The low coulombic efficiency can be attributed to the formation of SEI film, and the irreversible reaction of SnO2 with sodium ion to form NaxSn alloys in the first discharge process [46, 47]. In this work, the SnO2 and NC electrodes are used as a reference. In Fig. 6c, one can see that the discharge capacity of NC@SnO2 is about 270 mAh g−1, compared with 55 and 220 mAh g−1 of SnO2 and NC at 100 mA g−1 after 100 cycles. The rate performances for the three electrodes were also studied as shown in Fig. 6d. When the current densities were set at 0.05, 0.1, 0.2, 0.4, 0.8, 1, and 0.1 A g−1, the NC@SnO2 electrode exhibits the discharge capacities of about 295, 300, 280, 247, 202, 193, and 300 mAh g−1, respectively. These results are better than those of SnO2 and NC electrodes.
In summary, we have successfully prepared a hybrid structure of NC@SnO2 by electrospinning/hydrothermal methods. The NC nanofibers of the hybrid NC@SnO2 can prevent the agglomeration of SnO2 nanoflowers and effectively accelerate the transition of Li+/Na+ ion to promote the rate capability. Moreover, the structure can make more surface of the nanoflower exposed and buffer the volume expansion of SnO2 to enhance discharge capacity and cycling performance during cycling process. In addition, the hybrid NC@SnO2 could deliver a discharge capacity of 750 mAh g−1 after 100 cycles at 1 A g−1 for Li-ion battery and 270 mAh g−1 after 100 cycles at 100 mA g−1 for Na-ion battery.
This work was supported by the National Natural Science Foundation of China (Grant No. 51302079) and the National Natural Science Foundation of Hunan Province (Grant No. 2017JJ1008).
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