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Nitrogen-doped carbon derived from onion waste as anode material for high performance sodium-ion battery
Solid State Ionics 346 (2020) 115223
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Nitrogen-doped carbon derived from onion waste as anode material for high performance sodium-ion battery Majid Khan1, Nazir Ahmad1, Kewei Lu, Zhihui Sun, Chaohui Wei, Xiangjun Zheng, Ruizhi Yang
T
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College of Energy, Soochow Institute for Energy and Materials Innovations, Soochow University, Suzhou 215006, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Onion Carbon Doping Sodium ion battery Anode
Biomass-derived carbons are attractive and cost-effective anode materials for sodium ion batteries (SIBs), however, they generally suffer from the low capacity and coulombic efficiency. Foreign atom doping has been proved to be an effective approach to tune the chemistry of carbon material and improve their electrochemical performance. In this contribution, nitrogen (N)-doped carbon spheres (NCs) were successfully prepared from onion waste and urea (CH4N2O) by hydrothermal and annealing process, with urea as cheap and additional nitrogen source. The thus-derived NC was used as anode material for SIBs, the optimal sample (NC-800) can harvest a reversible capacity of 152 mAh g−1 at a current density of 0.05 A g−1 after 200 cycles. Impressively, NC-800 can still offer 83 mAh g−1 capacity after 2000 cycles at 1 A g−1, with a capacity retention rate of 69%. The high performance of N-doped carbon (NC-800) can be attributed to the significant increase of defects and the enlargement of interlayer distance caused by nitrogen doping. This work presents a feasible fashion of converting low-value waste into high-value material for energy storage.
1. Introduction The fossil fuel depletion and resultant environmental pollution have forced worldwide attention to the utilization of solar, wind and other clean energies as alternative energy sources. However, their intermittency cannot guarantee long-term and sustainable energy output for practical needs. Such situation has driven the rapid development of energy storage devices via electrochemical process to harvest renewable energies. Among the current electrochemical energy storage (EES) technologies, sodium-ion batteries (SIBs) have drawn enormous attention due to the abundant stock of sodium in nature, and the consequently low cost and high availability [1–4]. Moreover, SIBs are featured by relatively high energy-power densities as the combination of a supercapacitor cathode and battery anode. In turn, the challenges of SIBs lie in the balanced integration of two types of electrodes. Specifically, the insufficient charge transfer in anode, because of the large radius of sodium, can hardly match the fast adsorption/de-sorption process in the cathode. In line with this, enormous effort has been devoted to exploring the material candidates with fast kinetic process and large capacity for high performance SIBs. To date, several types of material have been attempted to be anode material for SIBs, such as metal oxides/sulfides, metals/alloys, and carbonaceous materials [5,6]. Metal-relevant materials own advantages
of high capacity, but poorly afford the volume expansion during repeated sodium diffusions [7]. Contrastingly, carbonaceous materials have come to the stage as an anode material for SIBs, due to the low cost, high intrinsic conductivity, environmental benignity, and good structural stability. Furthermore, carbonaceous material with favorable structure can significantly alleviate the structural collapse, and thus promote the reaction kinetics and cycling stability. Various carbonaceous material, like graphite, hard carbon, carbon black, carbon fibers and graphene, have been applied as the anode material of the SIBs [8]. Unfortunately, the carbonaceous materials suffer from poor Na storage capacity, low initial columbic efficiency and moderate cycling/rate performance [9–13]. To further improve the electrochemical properties of carbon materials, doping with heteroatoms including N [14–17], S [18–20], B [21] and P [22–24] is explored to be an effective strategy. Those dopants can enhance electrical conductivity, increase the active sites, expand interlayer spacing, and hence boost the intercalation/extraction of sodium ions during the electrochemical process [25–27]. Tons of food wastes are produced in modern society, and there are plenty of potential precursors for high-value downstream products. Onion wastes, as one of the common wastes from kitchens, are utilized as the precursor for carbon material in this work. Onion wastes contain N, O atoms binding to the carbon lattice even after carbonization. Although the obtained materials were self-doped with nitrogen, the
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Corresponding author. E-mail address: [email protected] (R. Yang). 1 These two authors contributed equally to this work. https://doi.org/10.1016/j.ssi.2020.115223 Received 24 December 2019; Received in revised form 6 January 2020; Accepted 9 January 2020 0167-2738/ © 2020 Published by Elsevier B.V.
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morphology and element distribution of the optimal NC (NC-800/NC-2) were characterized via scanning electron microscopy (SEM), high-resolution transmission microscopy (HRTEM) and selected area electron diffraction (SAED). The SEM image in Fig. 2a clearly shows the sphere shape of NC-800, indicating the successful synthesis of the spherical structure from onion waste. Both the HR-TEM image in Fig. 2b and the SAED image in Fig. 2c show the lack of the lattice fringes, demonstrating the amorphous carbon structure of NC-800 [28]. As shown in Fig. 2d, the NC-800 is further investigated by element mapping to analyze the element distribution including carbon (C), oxygen (O) and nitrogen (N). It can be seen that nitrogen signals are uniformly and densely spread in the mapping area, implying that homogenous nitrogen doping in NC-800, which is beneficial for the diffusion of Na+ and transport of electrons. The crystallization and defect degree of the as-prepared samples are probed by X-ray diffraction (XRD) and Raman spectroscopy. The XRD patterns of C and NCs at different carbonization temperatures like NC700, NC-800, NC-900, NC-1000 and NC-1100 are shown in Fig. 3a. All the spectra display two broad peaks (002) and (100) located at around 23° and 44°, respectively, which are typical characteristics of amorphous carbon [29]. Noticeably, the (002) peaks of NCs occur at a lower diffraction angle than that of C, revealing the increase of the average layer spacing (d002) in NC spheres. According to Bragg's formula, the d002 distances of C and NC can be obtained and shown in Fig. 3b. Distinctly, all the NCs present larger d002-spacing compared to C, and NC-800 owns d002-spacing greater than the rest of the samples. Specially, the d002 distances of C and NC-800 are 0.358 and 0.384 nm, respectively. Similar findings can be found regarding to the comparisons between C and NC-1–NC-4 as shown in Fig.S1a-b. These results indicate that nitrogen doping enhance the expansion of the d002 interlayer distance, benefiting the intercalation/extraction of the large sodium ions [30]. The Raman spectra of C and NCs at different carbonization temperatures are exhibit in Fig. 3c, showing two peaks at 1331.35 and 1585 cm−1, corresponding to D-band (disorder and defect in carbon materials) and G-band (graphite structure), respectively. From Fig. 3d, the ID/IG ratios of NCs (1.16–1.08) are higher than that of C (1.07), manifesting that NCs have more edge defects and structural deformation than C due to N doping [31]. Moreover, the ID/IG value of the NC-800 is higher than the rest of the NCs annealed at different carbonization temperatures (NC-700, NC-900, NC-1000 and NC-1100). There is also a comparison between C and NC-1–NC-4 in Fig. S1c-d, where NC-2 possesses the greatest ID/IG value among the investigated NCs. The findings suggest relatively low-degree graphitization of NCs, consistent with the HRTEM and SAED results. X-ray photoelectron spectroscopy (XPS) was utilized to analyze the element compositions and chemical state of the as-prepared material to disclose the doping mode of nitrogen dopants. As shown in Fig. S2, there are three peaks at around 284.6, 399.9, and 532 eV in the XPS surveys, corresponding to the C 1 s, N 1 s and O 1 s, respectively [32]. The deconvolution of C 1 s of NC-800 are divided into five separate peaks at 284.6, 285.5, 286.1, 289.7 and 292.5 eV, correlated to CeC, CeN, CeO, C]O and O=C-O groups, respectively [33] as shown in Fig. S3. It has been reported that C]O and CeO groups can participate in the surface redox reaction of sodium ions, thus improving the storage capacity [34]. The NeC peak in the C 1s clearly suggests the successful nitrogen doping in the carbon. The high-resolution N 1s spectrum in the C and NCs at different temperatures and prepared with different amount of urea are shown in (Fig. 4a) and (Fig. S4a), respectively. The four peaks are remarked as pyridinic-N (398.3 eV), pyrrolic N (399.4 eV), graphitic or quaternary-N (400.9 eV) and oxidized-N (402.3 eV), demonstrating that nitrogen atoms are bonded to carbon in different binding states [35,36]. As shown in Fig. 4b and Fig. S4b, the content of the graphitic or quaternary nitrogen in NC-800/NC-2 is higher than the rest of NCs. The high content of the quaternary nitrogen in NC-800/NC-2 is advantageous to enhance charge transfer and consequently rate performance [37]. Furthermore, the content of the
doping degree was unsatisfied, thus the urea was adopted to further increase the nitrogen content. The doped carbon spheres with rich nitrogen (NC) were simply synthesized through a two-step approach involving hydrothermal and annealing processes. Different temperatures were investigated, and it was found that the annealing temperature of 800 °C was the optimal. As the anode of sodium ion battery, the thusderived NC-800 could deliver high reversible specific capacity of 226.7 mAh g−1 at 0.05C, and retained 77.9 mAh g−1 of reversible capacity at 2.5C. When the current density dropped back to 0.05C, the SIB can harvest a reversible capacity of 217.1 mAh g−1, demonstrating satisfying electrochemical performance. Moreover, the SIBs with NC800 demonstrated good cycling stability with 73.2% of capacity retention after 200 cycles at 0.05C. This contribution offers a cost-effective approach to transform the daily wastes into high performance material for SIBs. 2. Experimental 2.1. Preparation of nitrogen-doped carbon The onion wastes collected from kitchen were firstly washed with ultrapure water thoroughly. Then the onion wastes were soaked in antibiotics solution to wash away impurities followed by drying at 130 °C for 24 h. After being grounded into fine powder, the onion wastes were ready for use. NC spheres were synthesized from the above onion powder and urea (CH4N2O) by a two-step reaction process. Typically, 3 g of onion powder and 0.12 g of urea (CH4N2O) were dissolved into 60 mL of deionize water under vigorous stirring overnight, until a uniform solution was formed. Then the obtained mixture was hydrothermally treated for 12 h in a high-pressure autoclave at 180 °C. Subsequently, the material was then collected through the vacuum filtration with distilled water for several times, till the pH value of the filtrate reached about 7, and next dried at 80 °C for 12 h. Thusthrived powder was heated up to 800 °C upon the heating rate of 3 °C min−1 and kept at 800 °C for 3 h in the nitrogen atmosphere. The product was denoted as NC (NC-800/NC-2). For comparisons, NC was carbonized at different temperatures from 700 to 1100 °C, marked as NC-700, NC (NC-800/NC-2), NC-900, NC-1000 and NC-1100, respectively. The mass ratios between onion powder and urea were ranged from (3:0.08), (3:0.12), (3:0.24) to (3:0.36) under the same temperature of 800 °C, thus-obtained materials were marked as NC-1, NC (NC2/NC-800), NC-3 and NC-4, respectively. Moreover, the carbon without additional N-doping carbonized at 800 °C was also prepared to compare with other samples doped with nitrogen. 2.2. Cell assembly The NC, acetylene black, and polyvinyl fluoride (PVDF) were well mixed under a mass ratio of 7:2:1, and then uniformly coated on a copper foil. After the coated foil were dried at 70 °C for 12 h, it was cut into 13 mm diameter discs. The mass loading of active material in all electrodes were between 1.5 and 2.5 mg cm−2. The prepared electrode was used as the working electrode and sodium metal as the reference/ counter electrode, while glass fiber (GF/D, Whatman) as separator. The coin cells (CR2032) were assembled in an argon-filled glove box. The electrolyte used was 1.0 M NaClO4 in diethyl carbonate/ethylene carbonate (v/v, 1:1) with 5% fluoroethylene carbonate added. The LandCT2001A battery test system was used for the cell test at different current densities in the voltage range of 0.01 to 3 V. The cyclic voltammetry and electrochemical impedance spectroscopy (EIS) of the cells were tested with the CHI 770. 3. Results and discussion Fig. 1 schematically illustrate the preparation process of N-doped carbon which involves hydrothermal and carbonization process. The 2
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Fig. 1. Schematic illustration of the synthesis of N-doped carbon sphere.
Fig. 2. (a) SEM image of NC-800. (b) HRTEM image of NC-800. (c) SAED of NC-800. (d) STEM image and the mapping area of NC-800 and the corresponding elemental mapping of O, C and N.
can be divided into two sections: the first section is a smooth plateau below 0.1 V corresponding to the Na intercalation among the carbon layers, and the second section is a slant above 0.1 V matching to Na+ binding with defects [40,41]. The NC-800/NC-2 exhibits higher capacities in both smooth plateau and slant section as compared to the rest of the samples, attributed to the increase of layer spacing and defects caused by N-doping in carbon spheres. The rate performances are also investigated to estimate the kinetic properties of C and NC samples, as illustrated in Fig. 5c. The C provides the rate capability with a reversible capacity of 59.4, 52.3, 44, 34.6, 26.9, 20.1 and 67.5 mAh g−1 at 0.05, 0.1, 0.2, 0.5, 1, 2.5 and 0.05C, respectively. Contrastingly, NC800/NC-2 presents better rate capability with a reversible capacity of 226.7, 167, 144.2, 121.2, 102.4, 77.9 and 217.1 mAh g−1 at 0.05, 0.1, 0.2, 0.5, 1, 2.5 and 0.05C, respectively. When the current density switches back to 0.05C, the capacity can recover to 95.7% of the initial capacity. The marked performance of NC-800/NC-2 can be attributed to the increased conductivity, layer spacing and active sites induced by nitrogen doping, enabling the shortened channel of ion diffusion, enlarged pool ion buffer and consequently additional capacity [42]. The cycling performance of C and NCs at different carbonization temperatures from 700 to 1100 °C at a current density of 0.05 A g−1 for
pyrrolic N in NC-800 is comparatively higher than other NCs. The pyrrolic N tends to locate at the edge plane and defect position in the carbon skeleton, increasing the conductivity and layer distance to enhance the diffusion of sodium ion [38]. Overall, NC-800 is considered to be a promising anode candidate for SIB, owing to the considerable amorphous structure and heteroatom doping, which improves the conductivity and active sites of the material [39]. The characteristics of sodium ion storage of C and NC samples were studied by Galvano charge-discharge (GCD) technique. The GCD profiles of C and NC with different carbonization temperatures and different ratios of urea were carried out in the voltage ranging from 0.01 to 3 V at a constant current density of 0.05 A g−1, for the first cycle as shown in Fig. 5a and Fig. 5b, respectively. The NC-800 exhibits the highest reversible capacity of 225.7 mAh g−1, while the capacity of C, NC-700, NC-900, NC-1000 and NC-1100 are 129.5, 183.8, 190.4, 145.9 and 141.5 mAh g−1, respectively, as illustrated in Fig. 5a. The reversible capacities rise with the carbonization temperature from 700 to 800 °C, while descend after 800 °C. In Fig. 5b, C, NC-1, NC-2, NC-3 and NC-4 achieve 129.5, 172.8, 225.7, 190.5 and 177.8 mAh g−1. Clearly, due to the less defects induced by nitrogen source, C shows the lowest capacities as compared to NC. The GCD profile of all NC samples and C 3
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Fig. 3. (a) XRD patterns of C and NC treated at different temperatures. (b) The bar chart of interlayer spacing of NC and C obtained from the XRD patterns in (a). (c) Raman spectra of C and NC treated at different temperatures. (d) The bar chart of ID/IG obtained from the Raman spectra in (c) .
Fig. 4. (a)The deconvolution of N 1s for the NC treated at different temperatures. (b) The content of different N species in the C and NC treated at different temperatures. 4
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Fig. 5. GCD profiles of C and N-doped carbon (NC) at different reaction temperatures (a) and different ratio of onion to urea (b). (c) Rate capability of C and NC-800/ NC-2. (d) Cycling performance of C and NC-700~NC-1100 at 0.05 A g−1.
lines in low-frequency area related to the Warburg impedance [49–51]. The Rct of NC-800 after 1st and 10th cycle are 111.5 and 87.19 Ω, respectively. Interestingly, the Rct decreases slightly after 10 cycles, implying the activation and stability of the electrode was boosted at the initial stage of the cycling. The Nyquist plot of C and NC-800 prior to test was also performed, as shown in Fig. S5. The first value of X axis represents the equivalent circuit resistance (ESR), including the resistivity of the material, electrolyte and contact resistance. The NC-800 shows smaller ESR than that of C as shown in Fig. S5, implying that the conductivity of NC is improved by nitrogen doping, as the ESR difference is caused by the resistance diversity of the electrode material. The Rct of NC is also lower than that of C, probably caused by enriched nitrogen dopants, leading to the increase of conductivity and layer spacing in NC spheres. These result in the enhancement of fast Na+ diffusion and charge transfer at the interface between the electrolyte and electrode. Furthermore, ultra-long cycling performance of NC-800 and C at a high current density of 1 A g−1 is displayed in Fig. 6c. The initial specific capacity of NC-800 is 120.1 mAh g−1, while that of C is 23.9 mAh g−1 at 1 A g−1. Remarkably, NC-800 can still provide a reversible capacity of 83 mAh g−1 with a capacity retention percentage as high as 69% after 2000 cycles. The high capacity and stable cycling performance of NC-800 can be attributed to the following factors: 1) the improved conductivity, enlarged d002 layer distance and increased defects in carbon due to N doping, enhancing the diffusion of sodium ion; 2) stable structure, alleviating the structural deformation during cycling.
200 cycles are shown in Fig. 5d. NC-800/NC-2 harvests a reversible capacity of 152 mAh g−1 with the capacity retention of 72.2% after 200 cycles, superior to other samples with respect to cycle stability. The rest of NC samples and C such as NC-700, NC-900, NC-1000, NC-1100 and C deliver a reversible capacity of 138.3, 149.7, 98.2, 108.4 and 51.4 mAh g−1, respectively. The stable cycling performance of NC-800/ NC-2 can be attributed to the nitrogen doping, which facilitates the transport of sodium ion and alleviating the structural deformation during cycling. The cyclic voltammograms (CVs) of NC-800/NC-2 in the first five cycles under the scan rate of 0.1 mV s−1 and voltage ranging from 0.01 to 3.0 V (vs Na+/Na) are shown in Fig. 6a. Of particular note, there are three cathode peaks located at 1, 0, 0.4, and one anodic peak at 0.2 V in the first cycle. The first peak at 1.0 V can be ascribed to the reaction of sodium ions with functional groups in carbon surface [43]. The peak at 1.0 V recur in the following CVs, demonstrating the constant contribution from functional groups in carbon materials to the ultimate capacity in each cycle [44]. The peaks occur at 0.4 V can be attributed to the decomposition of electrolyte and the formation of solid electrolyte interphase (SEI) [45]. After the first charge/discharge process, those irreversible reduction peaks disappear, and the subsequent CVs almost overlap. The cathode peak near 0 V probably resulted from the insertion of sodium ion in carbonaceous materials [46]. Regarding the anodic process, there is a clear and sharp peak observed at 0.2 V, corresponding to the extraction of sodium ion [47,48]. The EIS measurement of the half-cell was conducted after the 1st and 10th cycle to investigate the ohmic resistance and Na+ diffusion in NC anodes. The Nyquist plots obtained at a frequency range of 0.01–100 kHz are shown Fig. 6b, consisting of semicircles at high-frequency region representing charge transfer resistance (Rct), and oblique
4. Conclusion In conclusion, NC spheres with amorphous and disordered structure 5
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Fig. 6. (a) CVs of NC-800 at the scan rate of 0.1 mV s−1 from 0 to 3 V vs. Na+/Na. (b) Nyquist plots of the as-prepared N-doped carbon electrode after 1st and 10th cycle at 0.05 A g−1. (c) Cycling performance of NC-800 and C at the current density of 1 A g−1.
National Natural Science Foundation of China (Nos. 51972220, 51572181).
were successfully prepared with daily onion waste and cheap urea as precursor and additional nitrogen source by simple hydrothermal and annealing reactions. The nitrogen was successfully incorporated into carbon spheres, which improved the electronic conductivity and surface wettability of carbon materials. Thus-derived NC spheres present enlarged d002-spacing and increased defect sites in carbon, greatly enhancing the electrochemical performances of NC in SIBs. The thus-derived NC-800, used as anode material for SIBs, can enable a high reversible specific capacity of 226.7 mAh g−1 at 0.05C. Under a high current density of 1 A g−1, the SIB with NC-800 electrode provides a high initial capacity of 120.1 mAh g−1 and the capacity of 83 mAh g−1 after 2000 cycles, with the high capacity retention of 69%. The NC-800 prepared from onion waste presents considerable capacity and excellent cycling stability. This work demonstrates a simple approach of transforming onion waste to favorable products for SIBs. Such a strategy can be extended to other sustainable biomass resources of wide variety and applied in common energy storage devices.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ssi.2020.115223. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
CRediT authorship contribution statement Majid Khan:Data curation, Writing - original draft.Nazir Ahmad:Investigation, Visualization.Kewei Lu:Methodology.Zhihui Sun:Validation.Chaohui Wei:Writing - review & editing.Xiangjun Zheng:Validation.Ruizhi Yang:Conceptualization, Supervision, Funding acquisition.
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Acknowledgement
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The authors acknowledge the financial support from the National Key Research and Development Program of China (2016YFB0100200), 6
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Nitrogen-doped carbon derived from onion waste as anode material for high performance sodium-ion battery Majid Khan1, Nazir Ahmad1, Kewei Lu, Zhihui Sun, Chaohui Wei, Xiangjun Zheng, Ruizhi Yang
T
⁎
College of Energy, Soochow Institute for Energy and Materials Innovations, Soochow University, Suzhou 215006, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Onion Carbon Doping Sodium ion battery Anode
Biomass-derived carbons are attractive and cost-effective anode materials for sodium ion batteries (SIBs), however, they generally suffer from the low capacity and coulombic efficiency. Foreign atom doping has been proved to be an effective approach to tune the chemistry of carbon material and improve their electrochemical performance. In this contribution, nitrogen (N)-doped carbon spheres (NCs) were successfully prepared from onion waste and urea (CH4N2O) by hydrothermal and annealing process, with urea as cheap and additional nitrogen source. The thus-derived NC was used as anode material for SIBs, the optimal sample (NC-800) can harvest a reversible capacity of 152 mAh g−1 at a current density of 0.05 A g−1 after 200 cycles. Impressively, NC-800 can still offer 83 mAh g−1 capacity after 2000 cycles at 1 A g−1, with a capacity retention rate of 69%. The high performance of N-doped carbon (NC-800) can be attributed to the significant increase of defects and the enlargement of interlayer distance caused by nitrogen doping. This work presents a feasible fashion of converting low-value waste into high-value material for energy storage.
1. Introduction The fossil fuel depletion and resultant environmental pollution have forced worldwide attention to the utilization of solar, wind and other clean energies as alternative energy sources. However, their intermittency cannot guarantee long-term and sustainable energy output for practical needs. Such situation has driven the rapid development of energy storage devices via electrochemical process to harvest renewable energies. Among the current electrochemical energy storage (EES) technologies, sodium-ion batteries (SIBs) have drawn enormous attention due to the abundant stock of sodium in nature, and the consequently low cost and high availability [1–4]. Moreover, SIBs are featured by relatively high energy-power densities as the combination of a supercapacitor cathode and battery anode. In turn, the challenges of SIBs lie in the balanced integration of two types of electrodes. Specifically, the insufficient charge transfer in anode, because of the large radius of sodium, can hardly match the fast adsorption/de-sorption process in the cathode. In line with this, enormous effort has been devoted to exploring the material candidates with fast kinetic process and large capacity for high performance SIBs. To date, several types of material have been attempted to be anode material for SIBs, such as metal oxides/sulfides, metals/alloys, and carbonaceous materials [5,6]. Metal-relevant materials own advantages
of high capacity, but poorly afford the volume expansion during repeated sodium diffusions [7]. Contrastingly, carbonaceous materials have come to the stage as an anode material for SIBs, due to the low cost, high intrinsic conductivity, environmental benignity, and good structural stability. Furthermore, carbonaceous material with favorable structure can significantly alleviate the structural collapse, and thus promote the reaction kinetics and cycling stability. Various carbonaceous material, like graphite, hard carbon, carbon black, carbon fibers and graphene, have been applied as the anode material of the SIBs [8]. Unfortunately, the carbonaceous materials suffer from poor Na storage capacity, low initial columbic efficiency and moderate cycling/rate performance [9–13]. To further improve the electrochemical properties of carbon materials, doping with heteroatoms including N [14–17], S [18–20], B [21] and P [22–24] is explored to be an effective strategy. Those dopants can enhance electrical conductivity, increase the active sites, expand interlayer spacing, and hence boost the intercalation/extraction of sodium ions during the electrochemical process [25–27]. Tons of food wastes are produced in modern society, and there are plenty of potential precursors for high-value downstream products. Onion wastes, as one of the common wastes from kitchens, are utilized as the precursor for carbon material in this work. Onion wastes contain N, O atoms binding to the carbon lattice even after carbonization. Although the obtained materials were self-doped with nitrogen, the
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Corresponding author. E-mail address: [email protected] (R. Yang). 1 These two authors contributed equally to this work. https://doi.org/10.1016/j.ssi.2020.115223 Received 24 December 2019; Received in revised form 6 January 2020; Accepted 9 January 2020 0167-2738/ © 2020 Published by Elsevier B.V.
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morphology and element distribution of the optimal NC (NC-800/NC-2) were characterized via scanning electron microscopy (SEM), high-resolution transmission microscopy (HRTEM) and selected area electron diffraction (SAED). The SEM image in Fig. 2a clearly shows the sphere shape of NC-800, indicating the successful synthesis of the spherical structure from onion waste. Both the HR-TEM image in Fig. 2b and the SAED image in Fig. 2c show the lack of the lattice fringes, demonstrating the amorphous carbon structure of NC-800 [28]. As shown in Fig. 2d, the NC-800 is further investigated by element mapping to analyze the element distribution including carbon (C), oxygen (O) and nitrogen (N). It can be seen that nitrogen signals are uniformly and densely spread in the mapping area, implying that homogenous nitrogen doping in NC-800, which is beneficial for the diffusion of Na+ and transport of electrons. The crystallization and defect degree of the as-prepared samples are probed by X-ray diffraction (XRD) and Raman spectroscopy. The XRD patterns of C and NCs at different carbonization temperatures like NC700, NC-800, NC-900, NC-1000 and NC-1100 are shown in Fig. 3a. All the spectra display two broad peaks (002) and (100) located at around 23° and 44°, respectively, which are typical characteristics of amorphous carbon [29]. Noticeably, the (002) peaks of NCs occur at a lower diffraction angle than that of C, revealing the increase of the average layer spacing (d002) in NC spheres. According to Bragg's formula, the d002 distances of C and NC can be obtained and shown in Fig. 3b. Distinctly, all the NCs present larger d002-spacing compared to C, and NC-800 owns d002-spacing greater than the rest of the samples. Specially, the d002 distances of C and NC-800 are 0.358 and 0.384 nm, respectively. Similar findings can be found regarding to the comparisons between C and NC-1–NC-4 as shown in Fig.S1a-b. These results indicate that nitrogen doping enhance the expansion of the d002 interlayer distance, benefiting the intercalation/extraction of the large sodium ions [30]. The Raman spectra of C and NCs at different carbonization temperatures are exhibit in Fig. 3c, showing two peaks at 1331.35 and 1585 cm−1, corresponding to D-band (disorder and defect in carbon materials) and G-band (graphite structure), respectively. From Fig. 3d, the ID/IG ratios of NCs (1.16–1.08) are higher than that of C (1.07), manifesting that NCs have more edge defects and structural deformation than C due to N doping [31]. Moreover, the ID/IG value of the NC-800 is higher than the rest of the NCs annealed at different carbonization temperatures (NC-700, NC-900, NC-1000 and NC-1100). There is also a comparison between C and NC-1–NC-4 in Fig. S1c-d, where NC-2 possesses the greatest ID/IG value among the investigated NCs. The findings suggest relatively low-degree graphitization of NCs, consistent with the HRTEM and SAED results. X-ray photoelectron spectroscopy (XPS) was utilized to analyze the element compositions and chemical state of the as-prepared material to disclose the doping mode of nitrogen dopants. As shown in Fig. S2, there are three peaks at around 284.6, 399.9, and 532 eV in the XPS surveys, corresponding to the C 1 s, N 1 s and O 1 s, respectively [32]. The deconvolution of C 1 s of NC-800 are divided into five separate peaks at 284.6, 285.5, 286.1, 289.7 and 292.5 eV, correlated to CeC, CeN, CeO, C]O and O=C-O groups, respectively [33] as shown in Fig. S3. It has been reported that C]O and CeO groups can participate in the surface redox reaction of sodium ions, thus improving the storage capacity [34]. The NeC peak in the C 1s clearly suggests the successful nitrogen doping in the carbon. The high-resolution N 1s spectrum in the C and NCs at different temperatures and prepared with different amount of urea are shown in (Fig. 4a) and (Fig. S4a), respectively. The four peaks are remarked as pyridinic-N (398.3 eV), pyrrolic N (399.4 eV), graphitic or quaternary-N (400.9 eV) and oxidized-N (402.3 eV), demonstrating that nitrogen atoms are bonded to carbon in different binding states [35,36]. As shown in Fig. 4b and Fig. S4b, the content of the graphitic or quaternary nitrogen in NC-800/NC-2 is higher than the rest of NCs. The high content of the quaternary nitrogen in NC-800/NC-2 is advantageous to enhance charge transfer and consequently rate performance [37]. Furthermore, the content of the
doping degree was unsatisfied, thus the urea was adopted to further increase the nitrogen content. The doped carbon spheres with rich nitrogen (NC) were simply synthesized through a two-step approach involving hydrothermal and annealing processes. Different temperatures were investigated, and it was found that the annealing temperature of 800 °C was the optimal. As the anode of sodium ion battery, the thusderived NC-800 could deliver high reversible specific capacity of 226.7 mAh g−1 at 0.05C, and retained 77.9 mAh g−1 of reversible capacity at 2.5C. When the current density dropped back to 0.05C, the SIB can harvest a reversible capacity of 217.1 mAh g−1, demonstrating satisfying electrochemical performance. Moreover, the SIBs with NC800 demonstrated good cycling stability with 73.2% of capacity retention after 200 cycles at 0.05C. This contribution offers a cost-effective approach to transform the daily wastes into high performance material for SIBs. 2. Experimental 2.1. Preparation of nitrogen-doped carbon The onion wastes collected from kitchen were firstly washed with ultrapure water thoroughly. Then the onion wastes were soaked in antibiotics solution to wash away impurities followed by drying at 130 °C for 24 h. After being grounded into fine powder, the onion wastes were ready for use. NC spheres were synthesized from the above onion powder and urea (CH4N2O) by a two-step reaction process. Typically, 3 g of onion powder and 0.12 g of urea (CH4N2O) were dissolved into 60 mL of deionize water under vigorous stirring overnight, until a uniform solution was formed. Then the obtained mixture was hydrothermally treated for 12 h in a high-pressure autoclave at 180 °C. Subsequently, the material was then collected through the vacuum filtration with distilled water for several times, till the pH value of the filtrate reached about 7, and next dried at 80 °C for 12 h. Thusthrived powder was heated up to 800 °C upon the heating rate of 3 °C min−1 and kept at 800 °C for 3 h in the nitrogen atmosphere. The product was denoted as NC (NC-800/NC-2). For comparisons, NC was carbonized at different temperatures from 700 to 1100 °C, marked as NC-700, NC (NC-800/NC-2), NC-900, NC-1000 and NC-1100, respectively. The mass ratios between onion powder and urea were ranged from (3:0.08), (3:0.12), (3:0.24) to (3:0.36) under the same temperature of 800 °C, thus-obtained materials were marked as NC-1, NC (NC2/NC-800), NC-3 and NC-4, respectively. Moreover, the carbon without additional N-doping carbonized at 800 °C was also prepared to compare with other samples doped with nitrogen. 2.2. Cell assembly The NC, acetylene black, and polyvinyl fluoride (PVDF) were well mixed under a mass ratio of 7:2:1, and then uniformly coated on a copper foil. After the coated foil were dried at 70 °C for 12 h, it was cut into 13 mm diameter discs. The mass loading of active material in all electrodes were between 1.5 and 2.5 mg cm−2. The prepared electrode was used as the working electrode and sodium metal as the reference/ counter electrode, while glass fiber (GF/D, Whatman) as separator. The coin cells (CR2032) were assembled in an argon-filled glove box. The electrolyte used was 1.0 M NaClO4 in diethyl carbonate/ethylene carbonate (v/v, 1:1) with 5% fluoroethylene carbonate added. The LandCT2001A battery test system was used for the cell test at different current densities in the voltage range of 0.01 to 3 V. The cyclic voltammetry and electrochemical impedance spectroscopy (EIS) of the cells were tested with the CHI 770. 3. Results and discussion Fig. 1 schematically illustrate the preparation process of N-doped carbon which involves hydrothermal and carbonization process. The 2
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Fig. 1. Schematic illustration of the synthesis of N-doped carbon sphere.
Fig. 2. (a) SEM image of NC-800. (b) HRTEM image of NC-800. (c) SAED of NC-800. (d) STEM image and the mapping area of NC-800 and the corresponding elemental mapping of O, C and N.
can be divided into two sections: the first section is a smooth plateau below 0.1 V corresponding to the Na intercalation among the carbon layers, and the second section is a slant above 0.1 V matching to Na+ binding with defects [40,41]. The NC-800/NC-2 exhibits higher capacities in both smooth plateau and slant section as compared to the rest of the samples, attributed to the increase of layer spacing and defects caused by N-doping in carbon spheres. The rate performances are also investigated to estimate the kinetic properties of C and NC samples, as illustrated in Fig. 5c. The C provides the rate capability with a reversible capacity of 59.4, 52.3, 44, 34.6, 26.9, 20.1 and 67.5 mAh g−1 at 0.05, 0.1, 0.2, 0.5, 1, 2.5 and 0.05C, respectively. Contrastingly, NC800/NC-2 presents better rate capability with a reversible capacity of 226.7, 167, 144.2, 121.2, 102.4, 77.9 and 217.1 mAh g−1 at 0.05, 0.1, 0.2, 0.5, 1, 2.5 and 0.05C, respectively. When the current density switches back to 0.05C, the capacity can recover to 95.7% of the initial capacity. The marked performance of NC-800/NC-2 can be attributed to the increased conductivity, layer spacing and active sites induced by nitrogen doping, enabling the shortened channel of ion diffusion, enlarged pool ion buffer and consequently additional capacity [42]. The cycling performance of C and NCs at different carbonization temperatures from 700 to 1100 °C at a current density of 0.05 A g−1 for
pyrrolic N in NC-800 is comparatively higher than other NCs. The pyrrolic N tends to locate at the edge plane and defect position in the carbon skeleton, increasing the conductivity and layer distance to enhance the diffusion of sodium ion [38]. Overall, NC-800 is considered to be a promising anode candidate for SIB, owing to the considerable amorphous structure and heteroatom doping, which improves the conductivity and active sites of the material [39]. The characteristics of sodium ion storage of C and NC samples were studied by Galvano charge-discharge (GCD) technique. The GCD profiles of C and NC with different carbonization temperatures and different ratios of urea were carried out in the voltage ranging from 0.01 to 3 V at a constant current density of 0.05 A g−1, for the first cycle as shown in Fig. 5a and Fig. 5b, respectively. The NC-800 exhibits the highest reversible capacity of 225.7 mAh g−1, while the capacity of C, NC-700, NC-900, NC-1000 and NC-1100 are 129.5, 183.8, 190.4, 145.9 and 141.5 mAh g−1, respectively, as illustrated in Fig. 5a. The reversible capacities rise with the carbonization temperature from 700 to 800 °C, while descend after 800 °C. In Fig. 5b, C, NC-1, NC-2, NC-3 and NC-4 achieve 129.5, 172.8, 225.7, 190.5 and 177.8 mAh g−1. Clearly, due to the less defects induced by nitrogen source, C shows the lowest capacities as compared to NC. The GCD profile of all NC samples and C 3
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Fig. 3. (a) XRD patterns of C and NC treated at different temperatures. (b) The bar chart of interlayer spacing of NC and C obtained from the XRD patterns in (a). (c) Raman spectra of C and NC treated at different temperatures. (d) The bar chart of ID/IG obtained from the Raman spectra in (c) .
Fig. 4. (a)The deconvolution of N 1s for the NC treated at different temperatures. (b) The content of different N species in the C and NC treated at different temperatures. 4
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Fig. 5. GCD profiles of C and N-doped carbon (NC) at different reaction temperatures (a) and different ratio of onion to urea (b). (c) Rate capability of C and NC-800/ NC-2. (d) Cycling performance of C and NC-700~NC-1100 at 0.05 A g−1.
lines in low-frequency area related to the Warburg impedance [49–51]. The Rct of NC-800 after 1st and 10th cycle are 111.5 and 87.19 Ω, respectively. Interestingly, the Rct decreases slightly after 10 cycles, implying the activation and stability of the electrode was boosted at the initial stage of the cycling. The Nyquist plot of C and NC-800 prior to test was also performed, as shown in Fig. S5. The first value of X axis represents the equivalent circuit resistance (ESR), including the resistivity of the material, electrolyte and contact resistance. The NC-800 shows smaller ESR than that of C as shown in Fig. S5, implying that the conductivity of NC is improved by nitrogen doping, as the ESR difference is caused by the resistance diversity of the electrode material. The Rct of NC is also lower than that of C, probably caused by enriched nitrogen dopants, leading to the increase of conductivity and layer spacing in NC spheres. These result in the enhancement of fast Na+ diffusion and charge transfer at the interface between the electrolyte and electrode. Furthermore, ultra-long cycling performance of NC-800 and C at a high current density of 1 A g−1 is displayed in Fig. 6c. The initial specific capacity of NC-800 is 120.1 mAh g−1, while that of C is 23.9 mAh g−1 at 1 A g−1. Remarkably, NC-800 can still provide a reversible capacity of 83 mAh g−1 with a capacity retention percentage as high as 69% after 2000 cycles. The high capacity and stable cycling performance of NC-800 can be attributed to the following factors: 1) the improved conductivity, enlarged d002 layer distance and increased defects in carbon due to N doping, enhancing the diffusion of sodium ion; 2) stable structure, alleviating the structural deformation during cycling.
200 cycles are shown in Fig. 5d. NC-800/NC-2 harvests a reversible capacity of 152 mAh g−1 with the capacity retention of 72.2% after 200 cycles, superior to other samples with respect to cycle stability. The rest of NC samples and C such as NC-700, NC-900, NC-1000, NC-1100 and C deliver a reversible capacity of 138.3, 149.7, 98.2, 108.4 and 51.4 mAh g−1, respectively. The stable cycling performance of NC-800/ NC-2 can be attributed to the nitrogen doping, which facilitates the transport of sodium ion and alleviating the structural deformation during cycling. The cyclic voltammograms (CVs) of NC-800/NC-2 in the first five cycles under the scan rate of 0.1 mV s−1 and voltage ranging from 0.01 to 3.0 V (vs Na+/Na) are shown in Fig. 6a. Of particular note, there are three cathode peaks located at 1, 0, 0.4, and one anodic peak at 0.2 V in the first cycle. The first peak at 1.0 V can be ascribed to the reaction of sodium ions with functional groups in carbon surface [43]. The peak at 1.0 V recur in the following CVs, demonstrating the constant contribution from functional groups in carbon materials to the ultimate capacity in each cycle [44]. The peaks occur at 0.4 V can be attributed to the decomposition of electrolyte and the formation of solid electrolyte interphase (SEI) [45]. After the first charge/discharge process, those irreversible reduction peaks disappear, and the subsequent CVs almost overlap. The cathode peak near 0 V probably resulted from the insertion of sodium ion in carbonaceous materials [46]. Regarding the anodic process, there is a clear and sharp peak observed at 0.2 V, corresponding to the extraction of sodium ion [47,48]. The EIS measurement of the half-cell was conducted after the 1st and 10th cycle to investigate the ohmic resistance and Na+ diffusion in NC anodes. The Nyquist plots obtained at a frequency range of 0.01–100 kHz are shown Fig. 6b, consisting of semicircles at high-frequency region representing charge transfer resistance (Rct), and oblique
4. Conclusion In conclusion, NC spheres with amorphous and disordered structure 5
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Fig. 6. (a) CVs of NC-800 at the scan rate of 0.1 mV s−1 from 0 to 3 V vs. Na+/Na. (b) Nyquist plots of the as-prepared N-doped carbon electrode after 1st and 10th cycle at 0.05 A g−1. (c) Cycling performance of NC-800 and C at the current density of 1 A g−1.
National Natural Science Foundation of China (Nos. 51972220, 51572181).
were successfully prepared with daily onion waste and cheap urea as precursor and additional nitrogen source by simple hydrothermal and annealing reactions. The nitrogen was successfully incorporated into carbon spheres, which improved the electronic conductivity and surface wettability of carbon materials. Thus-derived NC spheres present enlarged d002-spacing and increased defect sites in carbon, greatly enhancing the electrochemical performances of NC in SIBs. The thus-derived NC-800, used as anode material for SIBs, can enable a high reversible specific capacity of 226.7 mAh g−1 at 0.05C. Under a high current density of 1 A g−1, the SIB with NC-800 electrode provides a high initial capacity of 120.1 mAh g−1 and the capacity of 83 mAh g−1 after 2000 cycles, with the high capacity retention of 69%. The NC-800 prepared from onion waste presents considerable capacity and excellent cycling stability. This work demonstrates a simple approach of transforming onion waste to favorable products for SIBs. Such a strategy can be extended to other sustainable biomass resources of wide variety and applied in common energy storage devices.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ssi.2020.115223. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
CRediT authorship contribution statement Majid Khan:Data curation, Writing - original draft.Nazir Ahmad:Investigation, Visualization.Kewei Lu:Methodology.Zhihui Sun:Validation.Chaohui Wei:Writing - review & editing.Xiangjun Zheng:Validation.Ruizhi Yang:Conceptualization, Supervision, Funding acquisition.
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Acknowledgement
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The authors acknowledge the financial support from the National Key Research and Development Program of China (2016YFB0100200), 6
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