Sodium-ion batteries (SIBs) have attracted significant attention because of the abundant resource and low-cost of sodium. Furthermore, flexible and wearable functional electronics have been presented as one of the most important emerging technology. Carbon nanofibers are promising candidates for flexible electrodes due to their high electronic conductivity and high surface area, while it is vital to use non-petroleum-based polymers considering environmental concerns. Developing flexible nanostructured electrodes by using environment friendly polymers with a fast and low-cost technique is critical to develop high performance flexible electronics. Electrochemical properties are influenced by the morphology and average fiber diameters of nanofibers. In this study, poly(vinylpyrrolidone) (PVP) solutions with various concentrations and two different solvent systems (ethanol/water and ethanol/dimethylformamide) were successfully spun into nanofibers by the fast, safe, low-cost, and environment friendly technique of centrifugal spinning. The effect of solvent system and solution concentration was investigated by using scanning electron microscopy images, and the average fiber diameters varied from 436 nm to 3 µm. Moreover, nine different heat treatments were studied, and the effect of time and temperature during stabilization and carbonization on the morphology of carbon nanofibers (CNFs) was investigated. Furthermore, flexible carbon nanofibers were fabricated and used as binder-free anodes in sodium-ion batteries. In order to enhance the electrochemical properties of flexible CNFs, flexible SnO2@CNFs were fabricated by combining centrifugal spinning and heat treatment. The electrochemical performance of the flexible SnO2@carbon nanofiber anodes was evaluated by conducting galvanostatic charge/discharge tests and cycling voltammetry. A high rate of performance was also presented. The high reversible capacity of 400 mA h/g was delivered when flexible centrifugally spun PVP based SnO2@carbon nanofiber electrodes were used in SIBs.
The wide usage of fossil fuels causes vital problems for the environment, and thus renewable energy and energy storage systems including rechargeable batteries have been gaining great interest.1,2 Among rechargeable batteries, lithium-ion batteries (LIBs) have been commonly used in portable electronic devices and electric vehicles due to their high durability, long lifespan, high energy, and high power densities.3,4 However, due to limited resources and the high cost of lithium, sodium-ion batteries (SIBs) have been introduced in recent years. SIBs offer the merits of similar chemical properties with lithium, abundant resources, environmental friendliness, and cheaper production processes.5 However, the ionic radius of sodium (1.02 Å) is much larger than the ionic radius of lithium (0.59 Å), so finding a suitable anode for SIBs is challenging. Carbon-based materials are one of the best choices for SIBs due to their low cost, high capacity, and large abundance.6,7
Carbon nanofibers have been used in many applications such as energy storage devices, sensors, electrode materials in supercapacitors, and rechargeable batteries. Carbon nanofibers (CNFs), one-dimensional sp2-hybridized nanostructures, are appropriate electrode materials due to their low diameters, high electronic conductivity, high strength, and good conductive paths for electrons.8–10 Polyacrylonitrile (PAN) is the most commonly used polymer for carbon nanofiber production due to its high strength, high stability, and good carbon yield.11–17 However, it is a petroleum derived polymer, and it could be dissolved only in aprotic organic solvents such as dimethylformamide (DMF), dimethylacetamide (DMAc), and dimethyl sulfoxide (DMSO), which are highly volatile and hazardous to humans and the environment.18 Considering the environmental and health effects of using petroleum based polymers and organic solvents, finding environment friendly alternatives is important.19
Water-soluble and eco-friendly polymers such as lignin,19,20 polyvinyl alcohol (PVA), and polyvinyl pyrrolidone (PVP)10 are introduced as carbon sources. PVP (C6H9NO)n is a biodegradable and water-soluble polymer that can be used as a carbon precursor due to its environment friendly character (i.e., water solubility and non-toxicity) and structural design. It is composed of the monomer N-vinylpyrrolidone, which consists of linear C–C groups and heteroatoms (i.e., oxygen and nitrogen) in the pyrrolidone group.21
There are different ways to produce nanofibers such as chemical vapor deposition (CVD),22 electrospinning,23 and centrifugal spinning.9 Centrifugal spinning is a promising technique considering its low cost, high production rates, high safety, and environment friendly features compared to other carbon nanofiber production techniques. In this technique, surface tension and centrifugal force are the two forces that led to the production of nanofibers in the range of micrometers to nanometers. In centrifugal spinning, high rotation speed is applied to the polymer solution that is inserted into the spinneret. The centrifugal force overcomes the surface tension of the solution when the spinneret reaches the critical speed; hence, liquid jets are ejected from the nozzles and stretched by the centrifugal force. The solidified nanofibers are deposited on the collectors by the evaporation of solvent during the spinning process.24
The overall performance of nanofibers is determined by their morphology and average fiber diameters, which are influenced by the solution and process conditions. Adabi et al.25 synthesized polyacrylonitrile-based carbon nanofibers with different diameters via electrospinning and investigated their electrochemical behavior. The average nanofiber diameter decreased from 143 to 72 nm as the concentration of polymers decreased from 10 to 7 wt. % due to increased polymer chain mobility and decreased polymer chain entanglement.26 Additionally, the current in cyclic voltammetry curves increased as the diameter of CNFs decreased due to increased conductivity. Ghanooni Ahmadabadi et al.27 synthesized Si nanoparticle-carbon nanofiber composites via electrospinning by using PAN as a carbon source with different diameters (230 and 620 nm) and using them as flexible and binder-free electrodes in LIBs. CNFs with smaller average fiber diameters (230 nm) exhibited excellent rate capability and cycle life due to providing a higher surface area for better contact between the electrode and the electrolyte, which increased the diffusion of Li ions.
Until now, there have also been some studies on the spinnability of electrospun nanofibers, and a limited number of studies have investigated the effect of solution and process parameters including concentration and rotational speed on centrifugally spun PVP nanofibers. Xia et al.28 investigated the effect of rotation speed and diameters of spinnerets on the morphology of FeCl3/PVP nanofibers. The results demonstrated that the diameters of nanofibers decreased with increasing rotational speed due to the increasing centrifugal force imposed on the solution. Moreover, decreasing the spinneret diameter led to smaller nanofiber diameters. However, systematic studies that investigate the spinnability of PVP nanofibers in different solvent systems and the effect of heat treatments with different stabilization and carbonization conditions on the morphology of centrifugally spun PVP based carbon nanofibers have not been studied yet. Moreover, herein, flexible centrifugally spun PVP based SnO2@carbon nanofibers were fabricated and used as an anode in SIBs for the first time. Owing to their high electrical conductivity, shortened pathways for ions, and excellent structural stability flexible centrifugally spun PVP based SnO2@carbon nanofiber electrodes delivered a high reversible capacity of around 400 mA h/g.
Polyvinylpyrrolidone (PVP) with an average molecular weight (Mw) of ∼1 300 000, N,N-dimethyl formamide (DMF), SnCI3, and ethanol were purchased from Sigma-Aldrich Gillingham, UK.
B. Preparation of PVP nanofibers
PVP solutions were prepared by dissolving 5, 7, 10, 13, 15, 18, and 20 wt. % PVP in the solvent systems of ethanol/water and ethanol/DMF in ratios of 100/0, 70/30, 50/50, 30/70, and 0/100 under constant stirring for 24 h at room temperature. All solutions are fed into the machine with the aid of a syringe pump at a speed of 60 ml/h, and a rotational speed of 4000 rpm was applied during centrifugal spinning. The spinneret-to-collector distance was set to 10 cm, and the nozzle diameter used in this process was 0.5 mm. The characteristics (viscosity, surface tension, average fiber diameter) of the PVP solutions in solvent systems with different concentrations and the average diameters of nanofibers were presented in Table S1.
C. Preparation of flexible centrifugally spun PVP-based carbon nanofibers
PVP nanofibers were carbonized at diverse stabilization and carbonization temperatures and times, as shown in Table S2. PVP nanofibers were stabilized at 150, 220,, and 280 °C in air and carbonized at 650 and 800 °C in N2. The effect of different stabilization times (0, 2, 8, 24 h) was also investigated (Table S2). Flexible centrifugally spun PVP based SnO2@carbon nanofibers were fabricated by applying stabilization at 50, 150, and 220 °C and then carbonization at 800 °C in N2.
D. Structure characterization
The morphologies and structures of PVP nanofibers and CNFs were characterized by scanning electron microscopy (SEM, Zeiss Sigma 300, Oberkochen, Germany), transmission electron microscope (TEM), x-ray diffraction (PANalytical Empyrean, Malvern, UK) with a step of 0.01 and speed of 4°/min, and Raman spectroscopy (WITech alpha 300R, Ulm, Germany). The diameters of the fibers were calculated by measuring 100 randomly selected nanofibers in SEM images using Revolution software for each sample.
E. Performance evaluation
The electrochemical performance of flexible centrifugally spun PVP based SnO2@carbon nanofiber anodes was evaluated by using coin-type (CR2032) half cells. A Whatman glass microfiber filter membrane was used as a separator, and sodium metal plates were used as the counter electrodes. The electrolyte was composed of 1.0M NaClO4 in ethylene carbonate (EC) and propylene carbonate (PC) with a 1:1 volume. Galvanostatic charge/discharge measurements were performed to investigate the cycling stability of the material. A constant current was applied to the working electrode between two potential values of 0–2.5 V. Cyclic voltammetry (CV) curves were investigated in the range of 0–2.5 V at a scan rate of 0.1 mV s−1. The rate capabilities for all electrodes were evaluated at high current densities ranging from 0.1 to 2 A/g at room temperature.
III. RESULTS AND DISCUSSION
A. Effect of the concentration and solvent systems on the morphology and average fiber diameters of centrifugally spun PVP fibers
Figures 1 and 2 display SEM images of centrifugally spun PVP nanofibers obtained by using different concentrations ranging from 5 to 20 wt. % in ethanol/distilled water (DW) and ethanol/DMF solvent systems, respectively. Figures S1 and S2 also show SEM images of PVP nanofibers in ethanol/DW and ethanol/DMF solvent systems at higher magnifications. The spinnability for different concentrations and solvent systems can be seen in Fig. 1 for ethanol/DW with the ratios of 100/0, 70/30, 50/50, 30/70, and 0/100 and concentrations ranging from 5 to 20 wt. %. As seen in Fig. 1, PVP nanofibers with a concentration of 5 wt. % were prepared when only ethanol was used as the solvent. When the concentration was increased to 7 wt. %, PVP nanofibers with average fiber diameters of 1467 and 677 nm were produced by using the solvent system of ethanol/DW with ratios of 100/0 and 70/30, respectively. Adding DW resulted in a significant decrease in average fiber diameters. Ethanol is a solvent with high volatility compared to DW, and adding a low volatile solvent (DW) to the polymer solution resulted in the formation of thinner fibers owing to longer stretching.29,30 It has been assumed that solvents with higher volatility (ethanol) result in higher viscosity during the formation of fiber in centrifugal spinning, which leads to the formation of fibers with high average diameters.31 Similar result was also reported by Yang et al.,32 who showed that mixing DMF (low volatility) with ethanol (high volatility) led to the production of nanofiber with low diameters. Solvent type also has an influence on the interrelation between the process parameters and the average fiber diameters. Nasouri et al.33 investigated the effect of solvent type (DMF and ethanol) on the diameter of PVP nanofibers. When DMF-contained solution was used, the average diameter of nanofibers increased with increasing applied voltage due to the high boiling point of DMF; however, decreasing the applied voltage increased the average diameter of nanofibers for PVP/ethanol solution due to the lower boiling point of ethanol, which led to evaporation quickly.
For the same solvent system of 100/0 in ethanol/DW, the average diameter increased with increasing concentrations. Increasing concentration led to higher viscosity and larger fiber diameters because of the higher stress relaxation time, which restricted the evaporation of solvents during the formation of fibers and the higher amount of polymers coming from nozzles. When the concentration was 10 wt. %, fiber formation was seen for 100/0, 70/30, and 50/50 ratios in ethanol/DW. The average fiber diameter decreased from 2468 to 727 nm as the water content increased. When the concentration was 15%, no fiber formation was seen in the 100/0 system due to the high volatility of ethanol.34,35 Hemamalini and Giri Dev36 also reported that the average diameter of starch polymer nanofibers increased with increasing the content of ethanol due to the rapid evaporation of ethanol. In 70/30, 50/50, and 30/70 ethanol/DW, fibers with average fiber diameters of 3744, 1632, and 640 nm were obtained. When 18 wt. % PVP was used, fiber formation was seen for the systems 50/50, 30/70, and 0/100, and the average fiber diameters decreased from 2423 to 436 nm as water content increased. As the concentration increased to 20 wt. %, the average fiber diameters were 952 and 614 nm, respectively, for 30/70 and 0/100 in ethanol/DW systems.
In the ethanol/DMF solvent system, the average diameter of nanofibers increased with increasing concentrations as well. When the ethanol concentration was 5 wt. %, the average fiber diameter was 1214 nm, while the average fiber diameter increased to 1467 nm when a 7 wt. % ethanol solution was used. When the concentration was 10 wt. %, fiber formed only in the system of 70/30 ethanol/DMF with an average fiber diameter of 1461 nm. As the concentration increased to 13 wt. %, fiber formation was seen for the systems of 70/30 and 50/50, and the average diameter of fibers decreased from 1525 to 850 nm as the DMF content increased. Cay et al.37 and Hsu and Shivkumar38 also reported that the average diameters of fibers decreased with the addition of DMF due to its less volatile properties. With increasing the concentration to 15 wt. %, the fibers formed in the systems of 70/30, 50/50, and 30/70, with the nanofiber diameters decreasing from 1753 to 634 nm. The average diameters of fibers were 3016, 1423, 1316, and 688 nm for the 70/30, 50/50, 30/70, and 0/100 systems when the concentration was 18 wt. %. Moreover, when concentration increased to 20 wt. %, fiber formation was observed in 50/50, 30/70, and 0/100 systems with average diameters of 1593, 1545, and 1125 nm, respectively. In a 0/100 ethanol/DMF system, the fibers formed when concentration increased to 18 and 20 wt. %, respectively. In centrifugal spinning techniques, the average fiber diameters are influenced by diverse parameters, such as solution properties like concentration, surface tension, viscosity, temperature, solvent type, and operational conditions such as nozzle diameter, rotation speed, flow rate, and spinneret to collector distance.24 Viscosity is one of the most critical factors that affects the formation of fibers directly. If the viscosity of the solution is too high, the nanofibers can not be produced due to their inability to stretch the liquid jet. By contrast, if the viscosity is too low, droplets are produced instead of fibers.39 The concentration and rotational speed also have significant effects on the fiber formation and average fiber diameters.40 As seen in Table S1, the average fiber diameters increased as the concentration and viscosity of polymers increased for both solvents. At a constant rotational speed (4000 rpm), increasing concentration led to increased viscosities and larger average fiber diameters. In the 100/0 ethanol/DW solvent system, the viscosities were 435, 435, 555, and 1525 (Pa s) when PVP concentrations were 5, 7, 10, and 13 wt. %, respectively. In the 70/30 ethanol/DW solvent system, the viscosities were 465, 895, 2125, and 2234 (Pa s) when 7, 10, 13, and 15 wt. % PVP solutions were used. In the 50/50 ethanol/DW solvent system, the viscosities increased to 1105, 2935, 3453, and 4206 (Pa s), respectively, as the concentrations were 10, 13, 15, and 18 wt. %. The viscosities were 2872, 4606, and 5500 (Pa s) when the concentrations were 15, 18, and 20 wt. % in the system of 30/70 ethanol/DW. The viscosities increased from 2410 to 3330 (Pa s) when the concentration increased from 18 to 20 wt. % in the 0/100 system. Deitzel et al.41 also reported that the concentration and average diameters of nanofibers increased with increasing viscosity. In the 70/30 ethanol/DMF solvent system, the viscosities of PVP solutions with concentrations of 10, 13, 15, and 18 wt. % were 385, 412, 508, and 764 (Pa s). In the 50/50 system, the viscosities were 671, 675, 819, and 1058 (Pa s) for the PVP solutions with concentrations of 13, 15, 18, and 20 wt. %, respectively. As the concentrations of PVP solutions increased from 15, 18 to 20 wt. %, the viscosities also increased from 875, 976 to 1237 (Pa s), respectively, in the 30/70 ethanol/DMF solvent system. The same results were observed for the 0/100 ethanol/DMF solvent system: as the concentration increased from 18 to 20 wt. %, the viscosities increased from 1395 to 1507 (Pa s), respectively.
B. Effect of the carbonization conditions on the morphology and average fiber diameter
After centrifugal spinning, PVP nanofibers were converted to carbon nanofibers (CNFs) by applying heat treatment including stabilization and carbonization. In order to investigate the effect of heat treatment on the morphology of PVP based carbon nanofibers, we examined nine different heat treatment processes with diverse stabilization and carbonization times and temperatures. The results of the heat treatment processes are demonstrated in Table S2. The effects of stabilization temperatures of 150 and 280 °C, carbonization temperatures of 650 and 800 °C, and stabilization times varying from 0–26 h were studied.
PVP nanofibers must be stabilized sufficiently to keep the nanofibrous structure intact and hindered the thermoplastic polymer from melting. During the stabilization process, PVP nanofibers become more stable due to cyclization and crosslinking reactions.42 The oxygen that exists during stabilization processes chemisorbs into the carbon surface and produces carbon-oxygen groups, which lead to a change in the color of nanofibers from white to yellow due to increased carbon content.43 The stabilization process includes crosslinking, oxidation, and dehydrogenation; in contrast, carbonization only has dehydrogenation under nitrogen or argon atmospheres. Carbon groups (C–C) are produced by breaking the linkages of polymer chains (C–N bonds) with increasing temperature during carbonization.44–46
As seen from Table S2 and Fig. 3, during the C7 heat treatment process, the PVP nanofibers were stabilized at 150 °C for 24 h and at 280 °C for 6 h, and then carbonized at 800 °C, which produced a carbon yield of 7.14%. In C1 heat treatment, nanofibers were stabilized at 280 °C followed by carbonization at 800 °C, and the carbon yield increased to 13.5%. When the carbonization temperature decreased to 650 °C, the amount of carbon yield increased to 18.62% during the C5 process. With no stabilization treatment (C8) and only carbonization at 650 °C, the carbon yield was 17.38%. With only stabilization at 280 °C and carbonization at 650 °C (C6), the carbon yield was 18.21%. In the C3 process, 2 h of stabilization at 150 °C and carbonization at 650 °C were applied, which led to a carbon yield of 15.43%. Moreover, when the stabilization time at 150 °C was increased to 24 h (C4), the carbon yield was 15.86%. When two-step stabilization is applied at 150 and 280 °C followed by carbonization at 650 °C (C2), the carbon yield decreases to 13.22%, which shows that stabilization at 280 °C is appropriate to achieve a higher carbon yield. The SEM images of C1 to C8 heat treatment processes demonstrated that the fibrous morphology of nanofibers was preserved; however, some of the carbon nanofibers appear to be broken. During the final process (C9), nanofibers stabilized at 50, 150, and 220 °C followed by carbonization at 50, 220, 250, and 800 °C leading to the production of the highest carbon yield of 23.10%, which proved that sufficient stabilization conditions improved the carbon yield. A suitable flexible electrode material should have both excellent mechanical strength and high electrochemical performance. In our recent work,10 we investigated the effects of stabilization and carbonization conditions on the morphology of centrifugally spun PVA/PVP-blend nanofibers. Results demonstrated that the use of appropriate multistage stabilization temperatures (50, 150, and 220 °C) followed by carbonization at 50, 220, 250, and 800 °C preserved the fibrous structure of carbon nanofibers with a high carbon yield. As seen from the photographs and SEM images of PVP based carbon nanofibers in different carbonization conditions (Table S2 and Fig. 3), only the C9 process led to flexible carbon nanofibers with fibrous structures. Although the fibers shrank after the carbonization process, they remained flexible (Fig. 4) due to enough stabilization treatment, which allows PVP based CNFs to be used as binder-free anodes in SIBs.
C. Flexible centrifugally spun PVP based SnO2@carbon nanofibers
SnO2 has been presented as a promising anode material due to its abundant sources and high capacity, but its low conductivity and large volume change lead to poor cycling performance. It has been known that nanostructured SnO2/carbon electrodes improve the electrochemical properties of batteries, owing to benefits including a peculiar current path, shortened diffusion lengths for ions, and a large contact area between the electrodes and electrolyte. Moreover, binder free flexible carbon nanofiber electrodes not only increase energy density but also provide high structural stability.47–49
Here, flexible PVP based carbon nanofibers were produced via centrifugal spinning and heat treatment. In order to further improve the electrochemical properties of flexible electrodes, a certain amount of SnCI3 was added to the PVP solution, and then the SnCI3/PVP solution was centrifugally spun to prepare SnCI3/PVP nanofibers. After that, heat treatment was applied to fabricate flexible PVP based SnO2/CNFs. Figure 4 presents a schematic illustration and photographs of flexible centrifugally spun PVP based SiO2@CNFs. The video was also displayed in the supplementary material to show the flexibility of the resultant electrodes.
SEM images of flexible centrifugally spun PVP based CNFs and SnO2@CNFs are presented in Figs. 5(a) and 5(c), respectively. A nanofibrous structure with a rough surface was seen, and the average fiber diameters were around 280 and 420 nm for flexible CNFs and SnO2@CNFs, respectively.
TEM images of PVP-based carbon nanofibers and SnO2@CNFs are displayed in Figs. 5(b) and 5(d), respectively. They have a long thin interconnected fibrous structure with uniform diameters, which leads to excellent mechanical properties and stability during the electrochemical process. Additionally, SnO2@CNFs demonstrated that the tin nanoparticles were partially embedded into the carbon nanofiber with a rougher surface, which resulted from the thermal decomposition of PVP during the heat treatment process. The rough surface provides more surface area between SnO2 and the electrolyte, which is favorable for sodium ion diffusion.50–52
The structure of flexible centrifugally spun PVP based CNFs and SnO2@CNFs was investigated by XRD measurement [Figs. 5(e) and 5(g)]. The XRD pattern of flexible centrifugally spun PVP based CNFs shows two broad diffraction peaks at around ∼22° and ∼45°, which are attributed to (002) and (101) planes of carbon. The peak at ∼22° is attributed to the graphite structure, while the peak at ∼45° is related to the crystallinity and turbostratic carbon structure, which demonstrated the amorphous nature of PVP based carbon nanofibers.53,54 XRD pattern was also used to quantitatively measure the interchain spacing (d-spacing) of carbon nanofibers using the Bragg equation λ = 2d sin θ, where λ is the wavelength of the incident x-ray source from Cu (λ = 1.5406 Å), and θ is the diffraction angle for the peak position. The d(002) value of flexible centrifugally spun PVP based CNFs was calculated to be around 0.355 nm, which was larger than that of graphite (0.34 nm).55 Upon the addition of the SnO2, the diffraction peaks for SnO2 appear in the pattern of flexible centrifugally spun PVP based SnO2@CNFs, as shown in Fig. 5(g). The peaks are indexed to polycrystalline SnO2 with tetragonal rutile structure (JCPDS card No. 41-1445).47 The peaks with 2θ values of 22.6°, 33.9°, and 51.8° are attributed to the crystal planes of (110), (101), and (211) of SnO2.56,57
Raman spectra of flexible centrifugally spun PVP based CNFs and SnO2@CNFs are shown in Figs. 5(f) and 5(h), respectively. The two main peaks are situated at 1350 and 1580 cm−1, which are attributed to the D band and the G band, respectively. The G band is assigned to graphitic carbon, while the D band is attributed to amorphous carbon.58 Additionally, the relative intensity of the D band to the G band (ID/IG) represents the degree of disorder in the graphite structure. The Raman spectra of flexible centrifugally spun PVP based CNFs and SnO2@CNFs show that the ratio of ID/IG is around 0.9, which demonstrated a disordered structure that was compatible with the XRD spectra.
TEM energy dispersive x ray (EDX) mapping images of flexible centrifugally spun PVP based SnO2@CNFs are also presented in Fig. 6 to prove the homogeneous distribution of SnO2.
Figures 7(a) and 7(b) show the cyclic voltammetry (CV) curves of the initial three cycles of the flexible centrifugally spun PVP based CNFs and SnO2@CNFs electrodes from 0.0 to 2.5 V vs Na/Na+ at a scanning rate of 0.1 mV s−1, respectively. For the flexible PVP based CNFs, three cathodic peaks were observed at ∼0, 0.54, and 1.17 V during the first scan, which demonstrated the insertion of sodium ions into the carbon matrix,59 the decomposition of sodium electrolyte and subsequent solid electrolyte interphase (SEI) formation,50 and the reaction of sodium ions on the carbon nanofiber surface,60 respectively. In the anodic scan of PVP-CNFs, a small peak was observed at about 0.3 V in all three cycles, which was ascribed to the extraction of sodium ions from carbon.60 In the first cathodic scan of SnO2@CNFs, a broad peak at ∼0.6 V can be ascribed to the irreversible reaction of SnO2 with Na+ to form Sn and Na2O with the formation of the solid electrolyte interface (SEI) layer.61–64 The anodic peaks at ∼0.6, 1.1, and 1.9 demonstrated the reversible dealloying of NaxSn. The CV curves in the second and third cycles were similar, which indicates that the electrochemical reactions of SnO2@CNFs are highly reversible.65,66
The first discharge charge curves are presented in Figs. 7(c) and 7(d). The first discharge and charge capacities are 694/471, 478/458, 477/457 mA h/g for flexible centrifugally spun PVP based SnO2@CNFs whereas those are 177/92, 93/87, 92/86 mA h/g for flexible centrifugally spun PVP based CNFs. Teng et al.67 prepared SnO2 carbon anode and the first discharge capacity was around 160 mA h/g. The high initial capacity of flexible centrifugally spun PVP based SnO2@CNFs could be ascribed to high electronic conductivity and short ion diffusion path of flexible CNFs.
The long-term cycling performance of the prepared samples at 0.1 A g−1 is shown in Figs. 8(a) and 8(b). Flexible PVP based CNFs show a capacity of ∼90 mA h/g in 200 cycles. Flexible PVP based SnO2@CNFs electrode retained the reversible capacity of ∼400 mA h/g in 200 cycles. The Columbic efficiency of flexible PVP based SnO2@CNFs electrodes is also presented. The electrodes show good Columbic efficiency of almost 99% after the first cycle. SnO2 decorated 3D nanostructured flexible carbon nanofibers could absorb the volume expansion of SnO2 nanoparticles, which resulted in an enhancement in cycling performance.68 Cheng and Tian47 anchored SnO2 on carbon nanoribbons, and good cycling stability was explained by the high structural stability of nanostructured carbon and well distributed SnO2. Wang et al.69 fabricated SnO2 coated carbon cloth anodes, and the reversible capacity was around 200 mA h/g in 300 cycles. The superior capacity and very good cycling stability of flexible PVP based SnO2@CNFs electrodes could be ascribed to the high conductivity, large number of active sites, and large surface area of nanostructured flexible CNFs.
The C rate performance of a flexible PVP based SnO2@CNFs electrode is presented in Fig. 8(c). The reversible capacities of 437, 393, 353, and 300 mA h/g were observed at the current densities of 0.1, 0.2, 0.5, 1, and 2 A/g, respectively. Yang et al.70 also prepared SnO2/CNF electrodes, and high C rate performance with good cycling stability was observed. The result was attributed to the excellent buffer effect of CNFs. In summary, flexible centrifugally spun PVP based SnO2@CNFs were prepared for the first time and used as anodes in SIBs. Excellent reversible capacity and C rate performance were achieved owing to the high conductivity, robust structure, large number of active sites, and short ion diffusion paths of flexible CNFs.
In summary, PVP nanofibers have been successfully prepared via a fast, safe, low cost, and environmental friendly centrifugal spinning technique compared to other fiber spinning techniques by using different solvent systems including ethanol/DW and ethanol/DMF with varying ratios of 0/100, 30/70, 50/50, 70/30, and 100/0. Nine different heat treatment procedures with varying stabilization times and temperatures were studied to investigate the effect of heat treatment on the morphology of PVP based carbon nanofibers. Flexible centrifugally spun PVP based SnO2@carbon nanofibers preserved their fibrous structure after heat treatment and delivered a reversible capacity of around 400 mA h g−1 in 200 cycles with good Columbic efficiency of almost 99%. Therefore, it is demonstrated that non-petroleum based PVP can be used in a fast, safe, and environment friendly nanofiber spinning technique, centrifugal spinning, to fabricate flexible carbon nanofibers, and flexible centrifugally spun PVP based SnO2@carbon nanofibers are promising anode candidates for flexible SIBs.
SEM images and the video of TDCNF electrodes were provided as supplementary material.
This research was supported by ITU BAP (Grant No. MGA-2021-43328), the national research foundation of Korea (Grant No. NRF-2021R1F1A1061200), and the Korea Institute of Industrial Technology (Grant No. PEO23050).
Conflict of Interest
The authors have no conflicts to disclose.
Meltem Yanilmaz: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Supervision (equal); Writing – original draft (equal); Writing – review & editing (equal). Elham Abdolrazzaghian: Investigation (equal); Writing – original draft (equal). Lei Chen: Supervision (equal); Writing – review & editing (equal). Bülin Atıcı: Investigation (equal). Juran Kim: Supervision (equal); Writing – review & editing (equal).
The data that support the findings of this study are available from the corresponding author upon reasonable request.