Direct chitin conversion to N-doped amorphous carbon nanofibers for high-performing full sodium-ion batteries

Author’s Accepted Manuscript Direct Chitin Conversion to N-Doped Amorphous Carbon Nanofibers for High-Performing Full Sodium-Ion Batteries Rui Hao, Yun Yang, Hua Wang, Binbin Jia, Guanshui Ma, Dandan Yu, Lin Guo, Shihe Yang www.elsevier.com/locate/nanoenergy

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S2211-2855(17)30817-0 https://doi.org/10.1016/j.nanoen.2017.12.042 NANOEN2423

To appear in: Nano Energy Received date: 24 September 2017 Revised date: 4 December 2017 Accepted date: 23 December 2017 Cite this article as: Rui Hao, Yun Yang, Hua Wang, Binbin Jia, Guanshui Ma, Dandan Yu, Lin Guo and Shihe Yang, Direct Chitin Conversion to N-Doped Amorphous Carbon Nanofibers for High-Performing Full Sodium-Ion Batteries, Nano Energy, https://doi.org/10.1016/j.nanoen.2017.12.042 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Direct Chitin Conversion to N-Doped Amorphous Carbon Nanofibers for High-Performing Full Sodium-Ion Batteries Rui Haoa, Yun Yanga, Hua Wanga,*, Binbin Jiaa, Guanshui Maa, Dandan Yua, Lin Guo a,*, Shihe Yangb,* [a] School of Chemistry, Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, Beihang University, Beijing 100191, P.R. China [b] Department of Chemistry, The Hong Kong University of Science and Technology Clear Water Bay Kowloon, Hong Kong * Corresponding author at: School of Chemistry, Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, Beihang University, Beijing 100191, P.R. China E-mail addresses: [email protected] (H. Wang), [email protected] (L. Guo), [email protected] (S. Yang).

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Abstract: Currently, renewable and low-cost electrode materials are being intensively pursued to meet the development of sustainable electrochemical energy-storage systems. Chitin, which is the second most abundant biopolymer throughout the natural world and can be sourced cheaply from the exoskeletons of arthropods and shells of cephalopods, has many attractive properties such as renewability, nontoxicity, intrinsically fibrous structure and high nitrogen content. In this study, nitrogen-doped amorphous carbon nanofibers (NACF) fabricated by direct pyrolysis of chitin, were used as the anode material in sodium-ion batteries (SIBs) for the first time. The NACF electrode delivered a high reversible capacity of 320.6 mAh g-1 with excellent rate capability and long cyclability. The superior electrochemical performance can mainly be attributed to synergistic effects of the unique one-dimensional mesoporous nanofibers facilitating the transmission of electrons/electrolyte, and the N-doped amorphous nanostructure increasing electrical conductivity and number of active sites. Furthermore, a sodium-ion full cell was constructed by coupling the NACF electrode with a Prussian blue cathode, and it delivered 115 mAh g-1 while retaining 90% of the capacity after 200 cycles. Our work will hopefully inspire the research community to explore other advanced materials with value-added attributes that can be generated by appropriate treatment of renewable bio-waste.

Keywords: energy storage, renewability, heteroatom doping, carbon nanofibers, synergistic effects, full sodium ion battery

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1. Introduction Sodium-ion batteries (SIBs) have attracted great attention as a promising alternative to Liion batteries for large-scale energy storage systems, owing to the high abundance, low cost and suitable redox potential of sodium [1-5]. Within the already significant body of research on SIBs, metals [6-9], metal oxides/sulfides [10-16], alloys [17-19], and organic compounds [20-22] have all been investigated as potential anode materials. Nevertheless, these types of electrode materials are often burdened with intrinsic limitations during the sodium-ion insertion-extraction reaction, leading to poor cycle stability, low initial coulombic efficiency and large hysteresis. Therefore, it is important to design more suitable electrode materials for SIBs with outstanding electrochemical performance. Owing to their abundance, low price and high thermal stability, carbonaceous materials have been ragarded as promising anode materials for SIBs [23-26]. In the past decades, carbonbased materials from different sources like biomass and with various morphologies, such as hard carbon, fibers, wires, tubes, sheets, spheres and three-dimensional graphene, have been widely investigated [27-38]. Furthermore, in order to dramatically improve the electrical conductivity and electrochemical performance of carbon-based materials, a number of smart strategies have been developed. For example, doping with light-weight heteroatoms (e.g., B, N and S) [39-42] is considered a promising strategy to enhance carbon-based electrode materials by improving their electrical conductivity, increasing the number of active sites, and enlarging their absorption capacity for sodium ions during the charging process. Another approach to improve the electrochemical performance of carbon-based materials is to design unique morphologies with a porous structure, which could provide unobstructed channels for fast diffusion of Na+ ion. However, the manufacturing of such heteroatom-doped carbon materials with special

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morphology usually involves complex process steps needed to achieve controllable shape fabrication, time-consuming procedures to activate the material, and multiple steps to import heteroatoms from external sources [43-45]. Thus, it is urgent to find a simple and cost-effective approach to prepare new and advanced carbon-based electrode materials that not only incorporate heteroatom doping, but also possess unique, controllable morphologies. Chitin (poly b-(1, 4)-N-acetyl-D-glucosamine), is the second most abundant biopolymer after cellulose and has many fascinating features, such as wide availability, sustainablity, intrinsically interconnected network nanostructure (Scheme 1) [46-49]. Additionally, it is worth noting that a considerable amount of nitrogen is present in the N-acetyl groups of chitin (Figure S1 and S2), and the direct pyrolysis of this precursor can thus result in the homogeneous incorporation of heteroatoms into the carbocyclic rings to obtain high-quality N-doped carbon materials [50-52]. From this point of view, chitin shows great potential as an excellent precursor for the synthesis of nanostructured carbon-based anode materials for SIBs. Here, we propose to use natural chitin, a renewable bio-waste, as a precursor to prepare nitrogen-doped amorphous carbon nanofibers (denoted as NACFs) by a facile pyrolysis process (Scheme 1). The resulting products were used as anodes in SIBs without any additional activation process, and exhibited high reversible capacity, excellent rate performance, high energy density and ultralong cycling stability, which are much more superior than artificial carbonaceous materials (electropolymerization, electrospinning et.al) and other no morphology bio-inspired carbon-based materials [53, 54]. This superior electrochemical performance was found to be significantly affected by the temperature of carbonization, which had a great influence on the synergistic effect of unique 1D mesoporous nanofibers and the N-doped amorphous structure. When coupled with Prussian blue as the cathode, the assembled sodium-ion

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full cell also delivered a considerable electrochemical charge-storage capacity, demonstrating its potential for future practical applications. Experimental section 2.1. Synthesis of the NACF The N-doped amorphous carbon nanofibers were prepared via a facile direct pyrolysis of pure chitin. Firstly, a sample comprising of chitin (Sigma, C9213) was preheated from room temperature to 300 °C in an Ar atmosphere for 1.5 h at a heating rate of 1 °C min-1 to stabilize the nanostructure. Next, the as-prepared precursor was carbonized at the setting temperature for 2 h in a tube furnace under Ar flow at a heating rate of 5 °C min-1. The carbonization temperature was held at 500, 600, 700, 800, and 900 °C, generating carbonized chitin denoted as CC500, CC600, CC700, CC800, and CC900, respectively. 2.2. Synthesis of Prussian Blue High-quality Prussian blue crystals were prepared by a conventional published synthesis method. Briefly, 2 mmol of Na4Fe(CN)6·10H2O and 1 mL of hydrochloric acid (37%) were dissolved in 100 mL of deionized water to obtain a homogeneous solution. The mixture was stirred at 60 °C for 4 h to obtain high-quality Prussian blue nanocubes. The composite was collected by filtration, washed three times with pure water and ethanol, and dried in a vacuum oven at 100 °C for 24 h. 2.3. Material Characterizations Atomic force microscopy (AFM) was measured using a Bruker Dimension Icon Microscope. The morphologies of the samples were characterized by scanning electron microscopy (7500F, JEOL) with a probe current of 10 μA and a 5 kV acceleration voltage. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) were carried out on a JEOL

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JEM-2100F microscope. The X-ray diffraction (XRD) spectra of the samples were recorded using a Rigaku Dmax 2200 X-ray diffractometer under Cu Ka radiation (l = 1.5416 Å). The mass fraction of chitin was determined by a Netzsch Sta449F3 thermal analyzer at a heating rate of 10 °C min−1 under N2 flow, using. Nitrogen sorption analysis was tested by an ASAP 2460 accelerated surface area and porosimetry instrument (Micromeritics), using Brunauer-EmmettTeller (BET) calculations at 77 K for the surface area. The pore-size distribution were calculated from the desorption branch of the isotherms based on the Barrett-Joyner-Halenda (BJH) model. Raman spectra were obtained on a LabRAM HR800 system with an excitation wavelength of 514 nm. X-ray photoelectron spectroscopy (XPS) was carried out using an ESCALAB 250 XPS system (Thermo Fisher Scientific). The spectra were referenced to the C 1s binding energy of 284.6 eV and analyzed using X-peak software. The electrical conductivity of the CC samples pre-compressed at 20 MPa was measured using the four-point method on a Kunde KDY-1 system (Guangzhou, P. R. China). 2.4. Electrochemical Measurements of Half Cells The electrochemical performance of the electrodes was evaluated using CR2032 coin-type cells with pure Na metal foil as counter electrode. The NACF electrodes were prepared by coating with a mixture comprising 80 wt % active materials, 10 wt % acetylene black (AB), and 10 wt % polytetrafluoroethylene (PVDF;) on the Cu foil (14 mm), and the PB electrodes were prepared by mixing 70 wt % active materials, 20 wt % AB, and 10 wt % PVDF on the Al foil (14 mm). After drying in vacuum at 60 °C for 12h, the batteries were assembled in an Ar-filled glove box, where both moisture and oxygen levels were kept at less than 1 ppm. 1 M NaClO4 in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) (1:1 v/v) and fluoroethylene carbonate (FEC) (5%) was used as the electrolyte, and a porous glass fiber felt (GF/D, Whatman)

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was used as the separator. Cyclic voltammetry (CV) measurements were performed on a Solartron 1470E instrument (Solartron Analytical, UK) at a scan rate of 0.3 mV s-1 in the range from 0-2.5 V and 2-4 V. Charge/discharge measurements and rate capability tests were carried out on CT 2001A Land battery testing systems (Jinnuo Electronics Co. Ltd., China) under different current densities with the potential window of 0.01 to 2.5 V and 2 to 4 V versus Na/Na+, respectively. The direct current (DC) resistance considered as an estimate of the electronic resistance by the formulas (RDC=VDC/IDC) are measured on a Solartron 1470E instrument with a SOC (state of charge) method. The EIS measurements are performed on an electrochemical 1470E workstation (Solartron) using an open circuit voltage with an amplitude of 10 mV, in the frequency range from 10-1 kHz to 105 Hz. For quasi-open-circuit potential measurements, the NACF cells were discharged and charged for 1 h at 40 mA g-1 with 2 h rest time. All the specific capacity values were calculated based on the mass of active materials, typically, the loading mass of active material was about 0.9~1.3 mg cm-2. 2.4. Assembly and Electrochemical Measurements of Full Cells The full battery was fabricated using the charged NACF anode and Prussian Blue cathode at a mass ratio of 3:1. The as-prepared NACF anode was charged and discharged at a current density of 0.1 A g-1 for several cycles to form a stable SEI membrane. The coin-type cell was disassembled and rinsed with DMC and then dried in Ar. The full cells were reassembled, and galvanostatic measurements were carried out at the current density of 0.1 A g-1. CV curves of the as-assembled full battery were recorded on a 1470E potentiostat electrochemical workstation (Solartron) at a scan rate of 0.3 mV s-1 in the range from 0.01 to 3.5 V. 2. Results and discussion

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We first investigated the general structure, size, and morphology of pure chitin and NACFs (Figure 1). To characterize the surface morphology of pure chitin, an AFM study was conducted as shown in Figure 1A and B. The micrographs revealed that pure chitin consists of fine nanofibers with a uniform diameter of approximately 10-30 nm and a high aspect ratio. SEM images of the NACFs (Figure 1C and Figure S3) show that they can retain the intrinsic morphology of bare chitin fibers, indicating that NACFs have been perfectly fabricated by the pyrolysis of the chitin template. Such a nanofiber structure was also observed by TEM (Figure 1D and Figure S4) and high-resolution TEM (HRTEM) (Figure 1E), indicating that the NACFs contain a disordered carbon phase with a large amount of nanopores. The corresponding selected area electron diffraction (SAED) pattern (inset in Figure 1E) exhibited dispersed diffraction rings as a further demonstration of the amorphous nature of this material, which has a turbostratic microstructure. Furthermore, the X-ray diffraction (XRD) patterns of the as-prepared carbonized chitin that had been annealed at different temperatures (denoted as CC500, CC600, CC700, CC800, CC900) showed broad peaks at 23º (Figure 1F), which were assigned to the crystallographic plane (002). There was no significant change in the peak patterns of different samples, indicating that the material kept its amorphous nature when the carbonization temperature was increased from 500 to 900 °C. The average graphene interlayer spacing (d002) calculated from the peak centers gradually shifted toward lower values (with a minimum of 3.76 Å), and the thickness of the graphitic carbon (Lc) became larger (reaching a maximum of 1.77 nm) with increasing carbonization temperature (Table 1), which showed that all samples satisfied the criteria set forth by the theoretical simulations of sodium-ion insertion/extraction between the graphitic planes.

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The N2 adsorption-desorption isotherms of the CC samples (Figure 2A) show that all of them exhibited a type-IV sorption isotherm (according to the IUPAC and BDDT classification;) with an increasing slope at a high relative pressure, which is commonly related to capillary condensation effects in mesoporous materials. The calculated Brunauer-Emmett-Teller (BET) surface area decreased with the increase of heat-treatment temperature, while CC500 displayed the largest BET surface area of 531.10 m2 g-1. In addition, according to the Barrett-JoynerHalendar (BJH) model, the pore-size distribution as shown in Figure 2B is indicative of a mesoporous dominant hierarchical structure with an average pore diameter of about 4 nm (Table 1). The high surface area and narrow pore size distribution are advantageous for the cell’s charge-storage capacity. Raman spectra (Figure 2C) showed two separated characteristic peaks of the D-band (disordered sp3 C atoms or defective graphitic structures) and G-band (in-plane bond-stretching motion of pairs of sp2 C atoms or crystalline graphite) at 1324 and 1590 cm-1, respectively, which confirmed the amorphous structure of the as-prepared carbon materials [55, 56]. With increasing heat-treatment temperature, the intensity ratio of the D-band to the G-band (La), corresponding to the graphitization degree, gradually became higher (Table 1), implying an improvement of electrical conductivity [57]. X-ray photoelectron spectroscopy (XPS) was performed to characterize the elemental composition and chemical components of the as-prepared products (Figure S5, Table 1). Judging by the wide-region spectrum of CC700, the atomic percentages of C, O and N were predicted to be 86.15%, 6.56% and 7.29%, respectively (Figure 2D). The high-resolution C 1s core level spectrum of CC700 (Figure 2E) showed five individual component peaks centered at 284.7, 285.4, 286.2, 288.2, and 290 eV, corresponding to C-C, C-N, C-O, C=O and O=C-O groups,

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respectively, further confirming the successful incorporation of N into the NACFs [58, 59]. The high-resolution O1s spectrum in Figure 2F clearly reveals the presence of several oxygen-based groups, including C=O quinone-type as well as (O-I) and C-OH phenol-type groups (O-II), which should have a positive influence on the electrochemical performance of the material [60]. Taken together, these results thus demonstrate the unique features of CC, such as high specific surface area and pore volume, hierarchical porous nanostructure and high-level heteroatom doping. In view of the high cost and complicated process of previously reported templating methods for generating heteroatom-doped porous carbon materials [51-54], our approach with simple steps and renewable material resources is promising to be applied in large-scale. In a further step, the electrochemical performance of the as-prepared samples was investigated by using a half-cell configuration versus Na metal. The potential profiles (Figure 3A and Figure S6, Table 1) varied considerably between the CC samples, and CC700 exhibited the highest initial coulombic efficiency (ICE) of 48.01%, which implies different de-intercalation behavior of sodium ions in the different samples. The curved profile of the CC500 sample that had the largest surface area follows a near-rectangular shape, which is a characteristic of capacitive charge-storage behavior caused by surface adsorption/desorption of ions. By contrast, the other samples displayed a linear voltage drop occurring mainly in the range of 2.5-1.0 V, and a gradual voltage decay around 1.0-0.01 V, which suggests that sodium-ion insertion reactions occurred in the pseudographitic carbon structure [61]. It is also noteworthy that CC700 possessed the largest charge capacity, 75% of which was reached at voltages below 1 V, which was the highest among all the CC specimens tested (Figure S7) [62]. All CC electrodes exhibited a good cycling stability at 50 mAh g-1 for 50 cycles (Figure 3B), with almost no capacity fading during the cycling process. Especially CC700 retained a maximum reversible capacity of 320.6 mAh g-1

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(Table 1), which was larger than those of other recently reported carbon-based materials (Table S1). Electrochemical impedance spectroscopy (EIS) was carried out to investigate the charge transfer and transport properties of the CC samples after 10 cycles (Figure 3C). It comprised a wide semicircle at the high frequencies, representing the charge-transfer kinetic-controlled area, and a Warburg straight line at the low frequencies, standing for the semi-infinite diffusion of sodium ions within the electrodes. Obviously, due to the smallest semicircle diameter and largest straight slope, the charge-transfer resistance and Warburg impedance of CC700 was remarkably lower than in the other samples, indicating that this material possesses favorable electron/ion charge-transfer kinetics [63]. Furthermore, cyclic voltammetry (CV) and quasi-open-circuit potential (QOCP) measurements were performed on CC700 to characterize its sodium ion insertion/extraction properties (Figure 3D). In the first cycle, a small irreversible peak appeared at 0.9 V versus Na+/Na, corresponding to the decomposition of the electrolyte and formation of the SEI layer, while a broad redox peak was observed in a wide potential range of 0.01-1 V, which can be attributed to the reaction between sodium and functional groups at the carbon surface. During the subsequent cycles, this irreversible peak disappeared and the CV curves showed a continuous increase in the cathodic current (sodium-ion insertion), mainly at voltages below 1 V, and remained almost unchanged, which demonstrated a stable chemisorption/desorption behavior. The gradual decrease of voltage was likely due to the range of local environments available for sodium ions in CC700, which contains areas of pseudographitic carbon. Moreover, QOCP measurements showed that the discharge profiles can be divided into a linear segment from 2.5 to 1 V and a curved segment from 1 to 0.01 V, which was consistent with the CV result of CC700, suggesting that a sodium-ion insertion reaction may occur in the pseudographitic carbon

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structure [61,64]. The rate performance of CC700 was evaluated by continuously changing the current density, as shown in Figure 3E, and it can be seen that the discharge capacity varied from 323.8 to 120.6 mAh g-1 when the rate increased from 0.05 to 1 A g-1. As expected, the average capacity was able to recover to 320 mAh g-1 when the current finally returned to 0.05 A g-1, illustrating the material’s superior rate capability, which can be ascribed to its structural stability and high conductivity. Energy density is one of the most important factors for evaluating the applicability of energy-storage devices. According to a published computational formula [33], the as-prepared NCAF electrode showed an energy density as high as 193.75 Wh kg-1 (Figure 3F), which was much higher than in most other sodium-ion batteries, such as the ones based on nitrogen-doped carbon sheets (146.1 Wh kg-1) [33], nitrogen-doped carbon spheres (120 Wh kg1

) [34], conventional hard carbon (109.3 Wh kg-1) [35], carbon nanofibers (87.1 Wh kg-1) [36],

and reduced graphene oxide (87.2 Wh kg-1) [37]. Impressively, even after 8000 cycles at a current density of 1 A g-1, the CC700 electrode still delivered a reversible capacity of 105 mA h g−1,(Figure 3 G), corresponding to a coulombic efficiency of the anode remained at nearly 100% and capacity retention ratio of 85%. Morphology observation has been made on the NACFs at different cycling state (Figure S8). It appears that the NACFs maintain the 1D structure with slight collapse and fracture after long cycle. This result indicates that the NACFs could keep the sustaining and interconnected structure to provide short electron /ion transporting channel during long-term cycling. To reveal the mechanisms responsible for such good electrochemical performance of CC700, SEM and XPS were used to characterize the evolution of the surface morphology and the elemental state of the incorporated N in samples obtained at different annealing temperatures. It can be clearly seen that the surface of CC500 is smooth, so that single carbon fibers cannot be

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distinguished clearly due to the low carbonization temperature (Figure 4A). When the annealing temperature was increased from 500 to 700 °C, the carbon fibers became slenderer and displayed a rougher appearance. Moreover, it can be clearly seen that the structure of the fiber network began to melt and collapse with the continuous increase of annealing temperature from 800 to 900°C. This result was in agreement with the thermogravimetric curve (Figure S9) [65]. The direct current (DC) resistance of the batteries based on the CC samples is small, indicating all the samples has high electric conductivity, while it shows a little reduction with the increasing fabrication temperature due to the change of structure (Table S2). The percentages of nitrogen in CC500, CC600, CC700, CC800 and CC900 were determined from the XPS survey spectra to be around 10.21, 8.25, 7.29, 6.64 and 5.31, respectively. The N1s spectra of the samples (Figures 4B) can be divided into three individual peaks centering at 398.4, 399.8 and 401.0 eV, corresponding to pyridinic nitrogen (N-6), pyrrolic nitrogen (N-5), and quaternary nitrogen (N-Q), respectively. For CC500 in Figure 4 B1, the N-6 peak becomes much stronger than N-5 and N-Q peaks due to a low carbonization. Upon further increasing the carbonization for CC600 and CC700 (Figure 4 B2 and B3), the intensity of the N5 peak became much weaker sequentially, whereas the proportion of N-Q rose promptly. At even higher temperatures, the N-5 peak is negligible, and the N-6 and N-Q peaks are found in the case of CC800 and CC900 (Figure 4 B4 and B5). Additionally, the values of n (N-Q)/n (N-6) in CC samples (Table S3) are 0.62, 1.15, 1.87, 2.78 and 4.25, respectively, indicating that the formed N-Q is much more stable than N-6 at high temperatures. These results demonstrate that N atoms within acylamino of chitin are progressively converted to two types of nitrogen-containing moieties (N-6 and N-Q) in the carbonization process (Figure 4C). Hence, the carbonization of chitin plays a key role in conferring surface functionalities on the carbon materials, without the

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need to introduce external nitrogen sources. Moreover, pyridinic and pyrrolic N, providing more active sites for fast ions/electrons transfer in the electrode. The abundant accessible N-containing species would provide chemically active sites to improve the electrochemical energy-storage performances, accompanied by an increase in electrical conductivity. Quaternary N could enhance the conductivity, however, high N-Q content may result in bad cycling performance (Table 1), which was in accordance with the Raman spectra [66]. Therefore, the excellent capacitance and rate capability of CC700 can be mainly attributed to the following reasons (Figure 4D): (i) The nano-sized fibers formed from the chitin template can serve as channels for continuous electron transport, while the large surface area provides a sufficiently large electrode/electrolyte interface to absorb sodium ions and promote rapid chargetransfer reactions. At the same time, the mesoporous structure facilitates the fast diffusion of the electrolyte, and may act as a reservoir for the storage of sodium ions [67-70]. (ii) Nitrogen doping can improve the electron conductivity of the material and provide more electrochemically active sites, which additionally contributes to its exceptional electrochemical performance. (iii) The carbon in the amorphous phase is randomly oriented and its large interlayer distance can provide more sodium storage space compared with graphitic carbon [71-73]. The high capacitance, excellent cycling stability, and good rate capability of the NACF half-cell encouraged us to further measure its performance in a full cell. Recently, a number of promising cathode materials such as FePO4, NaTi2(PO4)3, NaV6O15 and Na3V2(PO4)3 [74-78] have been studied in detail, and Prussian Blue (PB, Na0.61Fe(Fe(CN)6)0.94) [79] was chosen as the cathode material for the fabrication of the full cell, as illustrated schematically in Figure 5A. The PB was prepared by a conventional synthesis method reported in the literature [80] (Figure S10), and its electrochemical performance was investigated (Figure S11). The CV curves of the full

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cell (Figure 5B) contained a pair of well-defined redox peaks at 1.3/1.5 V, indicating wellbehaved kinetics of both electrodes, was well as an exceptional sodium-ion storage ability. The cell delivered a stable capacity of 120 mA h g-1 even after 200 cycles, and the coulombic efficiency of the full cell approached more than 95%, illustrating its excellent reversibility (Figure 5C). The capacities of a full sodium ion batteries are 120.2, 104.8, 90.4, 74.1, 54.4 mAh g-1 at varied current densities of 0.05, 0.1, 0.2, 0.5, 1 A g-1, respectively, indicating an excellent rate capability (Figure S12). To demonstrate the potential practical use of the sodium-ion full battery, two full cells were connected in series and were used to power a small electric fan (Figure 5D). Such full sodium-ion battery endow their potential for practical applications as large-scale energy storage devices 3. Conclusion In summary, highly N-doped amorphous carbon nanofibers have been successfully prepared by a facile pyrolysis of the naturally abundant bio-waste chitin. When the pyrolysis products were directly applied as anode materials of SIBs without any activation process, the electrode materials exhibited a high reversible specific capacity of up to 320.6 mAh g-1, high energy density (192 Wh kg-1), excellent rate capability and extremely long life (exceeding 8000 cycles), with a coulombic efficiency of nearly 100%. The superior electrochemical performance can mainly be attributed to synergistic effects of the unique one-dimensional porous nanofibers facilitating the transmission of electrons/electrolyte, and the N-doped amorphous nanostructure increasing electrical conductivity and number of active sites. Moreover, when a sodium-ion fullcell was assembled with NACF as the anode and PB as the cathode, it achieved a high reversible capacity of 120 mAh g-1 and a high coulombic efficiency of 95 %. Finally, a commercial electrofan was powered by two cells connected in series, indicating their potential for practical

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application. Significantly, our work not only serves as a good model to better understand the factors guiding the influence of heteroatom-doped carbon-based materials on the electrochemical performance of SIBs, but also sheds light on a general strategy for designing inexpensive and environmentally friendly electrode materials using natural biomaterials for future large-scale electrochemical energy-storage applications.

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Scheme 1. Schematic illustration for the synthesis of nitrogen-doped amorphous carbon nanofibers (NACF) derived from bio-waste chitin.

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Figure 1. The surface morphology of pure chitin: A) AFM height image; B) AFM phase image. The characterization of NACF: C) SEM image; D) TEM image; E) High-resolution TEM image; inset: SAED pattern; F) XRD patterns of the CC samples synthesized at different carbonization temperatures.

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Figure 2. Characterization of the CC samples synthesized by carbonization at different temperatures. A) Nitrogen adsorption-desorption isothermal curves; B) The corresponding BJH pore size distribution; C) Raman spectra. XPS spectra for CC700: D) The survey; E) the narrow C 1s peak; F) the narrow O 1s peak.

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Figure 3. A) The 2nd cycle potential profiles of the CC samples at a current density of 50 mA g-1 in a range from 0.01 to 2.5 V versus Na+/Na. B) Comparison of the cycling performance of the CC electrodes at a current density 50 mA g-1 for 50 cycles. C) Impedance plots used for EIS analysis of the CC electrodes. Inset: Equivalent electronic circuits. R1 is the sum of resistances of the electrical connections, R3 is the charge transfer resistance, and W1 represents the Warburg impedance of sodium diffusion within the carbon-based material. D) CV and QOCP data of the CC700 electrode. E) The rate capability of the CC700 electrode at current densities from 0.05 to 1 A g-1. F) Comparison of the energy densities of various carbon-based materials. G) Ultra-longterm cycling performance at a current density of 1 A g-1.

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Figure 4. A) The SEM images of the CC samples obtained by annealing chitin at different temperatures (scale bar: 200 nm). B) The corresponding narrow N 1s spectra of the CC samples. C) Distribution of different nitrogen types in the CC samples. D) Schematic representation of electron transmission and sodium ion storage in NACFs.

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Figure 5. A) Schematic of the as-assembled full cell with PB and NACF as cathode and anode materials, respectively. B) CV curves of a sodium-ion full battery tested in the voltage window of 0.01-3.5 V at a current of 0.3 mV s-1. C) The cycling stability of the full cell at a current density of 0.1 A g-1. D) Demonstration of a smart electric fan powered by two SIB full cells in series.

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Graphical Abstract N-doped amorphous carbon nanofibers can be fabricated via facile direct pyrolysis of the biowaste chitin. The as-prepared unique porous structure provides stable channels for electron/electrolyte transmission and more active sites for sodium-ion adsorption and storage, leading to a high specific capacity, energy density and extremely long cycle life. The renewable chitin-derived high-performance electrodes will have an important place in low-cost energystorage devices.

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Table 1. Physical parameters and electrochemical properties of nitrogen-doped carbon materials prepared by direct pyrolysis of chitin. Sample

HTT a) (°C)

d002b) (Å)

Lc c) (nm)

SBET (m2 g-1)

APD d) (nm)

I e)

Atomic f) C (%)

O (%)

N (%)

Kg) (S cm-1)

ICE h) (%)

RC i) (mAh g-1)

Conductivity (S m-1)

CC500

500

3.97

1.56

531.10

3.97

0.87

81.62

8.1

10.21

5.16

27.30

64.5

5.16

CC600

600

3.92

1.60

446.26

4.26

0.97

85.06

6.69

8.25

8.23

37.19

175.9

8.23

CC700

700

3.87

1.65

369.48

4.28

1.03

86.15

6.56

7.29

15.80

48.01

320.6

15.80

CC800

800

3.82

1.71

330.16

4.30

1.08

88.46

4.9

6.64

18.51

47.05

253

18.51

CC900

900

3.76

1.77

285.26

4.42

1.13

90.26

4.42

5.31

22.01

44.14

215.5

22.01

a)

Heat treatment temperature; b) d002 is calculated based on Bragg’s equation, using the FWHM values of (002) at 2θ ∼ 23º; c) Lc is calculated based on Scherrer‘s equation; d) BJH desorption of the average pore diameter; e) The calculated intensity ratio of the D-band versus the G-band from the Raman spectra; f) Statistic calculated using XPS data; g) Electrical conductivity; h) Initial coulombic efficiency (ICE); i) Reversible capacity after 50 cycles (RC).

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Rui Hao obtained his BS degree at Inner Mongolia University in 2011.He is currently a Ph.D. student in School of Chemistry at Beihang university under the supervision of Prof. Lin Guo. His research interets are inorganic amorphous nanomaterials and low-dimensional nanosystems for energy storage and photoelectrochemistry.

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Yun Yang received her Ph.D. degree in Chemical and Biomedical Engineering from Nanyang Technological University in 2015. She is currently a postdoctor in Prof. Lin Guo’s group in Beihang University. Her research interests focus on microbial fuel cells and nature-inspired energy-storage systems.

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Hua Wang is an associate professor in the School of Chemistry and Environment at Beihang University. He received his Ph.D. degree in Materials Science and Engineering from Beihang University in 2012. Then, he worked as a research fellow at Nanyang Technological University (NTU) for 2 years. His research interest is development of advanced nanomaterials for energy conversion and storage, including lithium/sodium ion batteries, photo-catalytic water splitting and water purification.

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Binbin Jia is currently a Ph.D. student at Beihang University supervised by Prof. Lin Guo. He received his Bachelor degree and Master degree in 2011 and 2015, respectively. His main research interests include amorphous materials for lithium-ion batteries and electrocatalysis.

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Guanshui Ma is a Ph.D. candidate at BeiHang University under the supervision of Prof. Lin Guo. His research interests are surface-enhanced Raman scattering of nano-materials and electrooptic properties of liquid crystal.

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Dandan Yu received her B.E. degree from Inner Mongolia University of Technology. She is currently working on her Ph.D. degree at Beihang University under the supervision of Associate Prof. Hua Wang. Her research focuses on the synthesis of two-dimensional materials for energy storage and conversion.

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Lin Guo is a professor and vice dean of School of Chemistry and Environment in Beihang University. He received his Ph.D. in Materials Science and Engineering from Beijing University of Institute of Technology (BIT) in 1997. He worked as a visiting scholar in Hong Kong University of Science and Technology (HKUST) in 1999. He worked in Dresden Technology University with Humboldt Fellowship for 2 years. His research interests include synthesis and characterization of sophisticated nanomaterials, high-strength nanomaterials with light weight, and functional nanomaterials for energy storage

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Shihe Yang is a full Professor at The Hong Kong University of Science and Technology and Peking University Shenzhen Graduate school. His long-standing interest spans chemistry, physics, and functions of multiscale material systems encom-passing, inter alia, molecular, nanoscopic, and mesoscopic regimes. His current research interest is focused on energy material science, technology and physical chemistry by drawing on the understanding, manipulation and applications of low-dimensional materials.

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Highlights A renewable and low-cost electrode material of nitrogen-doped amorphous carbon nanofibers are fabricated by direct pyrolysis of bio-waste chitin. The nitrogen-doped amorphous carbon nanofibers as SIBs anode exhibits outstanding reversible capacity, excellent rate capability and long cyclability benefitting from its unique nanostructure. A sodium-ion full cell constructed by coupling the nitrogen-doped amorphous carbon nanofibers electrode with a Prussian blue cathode delivers superior electrochemical performance.

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