Sulphur-doped carbon nanosheets derived from biomass as high-performance anode materials for sodium-ion batteries

Journal Pre-proof Sulphur-doped carbon nanosheets derived from biomass as high-performance anode materials for sodium-ion batteries Gongyuan Zhao, Dengfeng Yu, Hong Zhang, Feifei Sun, Jiwei Li, Lin Zhu, Lei Sun, Miao Yu, Flemming Besenbacher, Ye Sun PII:

S2211-2855(19)30926-7

DOI:

https://doi.org/10.1016/j.nanoen.2019.104219

Reference:

NANOEN 104219

To appear in:

Nano Energy

Received Date: 26 July 2019 Revised Date:

21 October 2019

Accepted Date: 21 October 2019

Please cite this article as: G. Zhao, D. Yu, H. Zhang, F. Sun, J. Li, L. Zhu, L. Sun, M. Yu, F. Besenbacher, Y. Sun, Sulphur-doped carbon nanosheets derived from biomass as high-performance anode materials for sodium-ion batteries, Nano Energy, https://doi.org/10.1016/j.nanoen.2019.104219. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Elsevier Ltd. All rights reserved.

Sulphur-doped carbon nanosheets derived from biomass as high-performance anode materials for sodium-ion batteries Gongyuan Zhaoa,c, Dengfeng Yub, Hong Zhangb, Feifei Suna, Jiwei Lib, Lin Zhua, Lei Suna, Miao Yua,*, Flemming Besenbacherc,* and Ye Sunb,* a

State Key Laboratory of Urban Water Resource and Environment, School of Chemistry and Chemical Engineering,

Harbin Institute of Technology, Harbin 150001, China b

Condensed Matter Science and Technology Institute and Department of Physics, School of Science, Harbin Institute of

Technology, Harbin 150080, China c

Interdisciplinary Nanoscience Center (iNANO) and Department of Physics and Astronomy, Aarhus University, Aarhus

8000, Denmark * Corresponding author. E-mail addresses: [email protected], [email protected], [email protected]

Graphical Abstract

Sulphur-Doped Carbon Nanosheets Derived from Biomass as High-Performance Anode Materials for Sodium-Ion Batteries Gongyuan Zhaoa,c, Dengfeng Yub, Hong Zhangb, Feifei Suna, Jiwei Lib, Lin Zhua, Lei Suna, Miao Yua,*, Flemming Besenbacherc,* and Ye Sunb,*

a

State Key Laboratory of Urban Water Resource and Environment, School of Chemistry and Chemical

Engineering, Harbin Institute of Technology, Harbin 150001, China b

Condensed Matter Science and Technology Institute and Department of Physics, School of Instrumentation

Science and Engineering, Harbin Institute of Technology, Harbin 150080, China c

Interdisciplinary Nanoscience Center (iNANO) and Department of Physics and Astronomy, Aarhus University,

Aarhus 8000, Denmark

* Corresponding author. E-mail addresses: [email protected], [email protected], [email protected]

Abstract Sodium-ion batteries (SIBs) have attracted enormous attention as a promising alternative to lithium-ion batteries in recent years due to the richness and low cost of sodium (Na) resource. The SIBs performance is essentially determined by the electrode materials applied. Due to the difficulty of intercalating the large Na ions, developing competent anode materials (AMs) has become particularly fascinating and critical for SIBs. Herein, three-dimensional (3D) scaffolding framework of carbon nanosheets heavily-doped with sulphur (S-CNS) has been fabricated from plant biomass using a facile thermal method. The S-CNS affords an ultrahigh reversible capacity of 605 mAh g−1 at 50 mA g−1, high rate performance 133 mAh g−1 at 10 A g−1, and long-term cycling stability at 5 A g−1 (∼94% retention upon 2000 cycles). These values are among the best results based on AMs of doped carbon derived from biomass ever reported. Moreover, we demonstrate that such S-doped carbon materials with competent electrochemical properties can be easily produced from diverse plant wastes using this method. This work thus introduces a universal strategy and a fertile ground to produce high-performance AMs by using the mixture of biomass and S powder, which may hold considerable potential for scalable production of commercial SIBs.

Keywords: sodium-ion batteries; anode materials; sulfur doping; biomass; high capacity

1. Introduction The foreseeable energy crises due to the scarcity and steeply rising price of the lithium precursors have urged the development of sustainable energy storage devices using materials that are inexpensive, abundant and environmentally friendly [1−3]. Sodium-ion batteries (SIBs) have recently aroused increased level of interest because of the cost-efficiency and richness of sodium (Na) [4,5]. The key factor determining the performance of SIBs is the electrode material applied. In the last decade, a big variety of anode materials (AMs) have been explored, e.g. metals [6,7], metal oxides/sulfides/phosphides [8−12]. However, given the large radius and low diffusion kinetics of Na+, most of these developed AMs are suffered from volume expansion and pulverization during the Na+ insertion/extraction, which lead to poor cycle stability, low initial Coulombic efficiency, and unreliable safety of the resultant SIBs [13,14]. It is, therefore, critical to develop competent AMs remedying these shortcomings. Due to the low price and high thermal stability, diverse carbon nanomaterials with various morphologies, including hard carbon [15], templated carbon [16], carbon fibers/nanotubes [17,18], and graphene [19] have been extensively applied as AMs for SIBs, still facing the same challenges mentioned above. Two effective solutions have been proposed to improve these carbon materials. The first is doping with light-weight heteroatoms, e.g. boron (B), nitrogen (N), phosphor (P), sulphur (S) [20−25]. Superior to introducing B, N, P atoms, S-doping can afford multiple advantages simultaneously, including (1) additional Faradaic reactions to improve the Na+ storage capacity, (2) enlarged specific surface area (SSA) to provide more reaction sites, (3) increased electronegativity and better electrical conductivity [26], and (4) expanded interlayer distance to facilitate Na+ absorption and transfer in the charge/discharge process. The second solution is to construct three-dimentional (3D) framework of hierarchical porous carbons [27−29], where the 3D interconnected skeleton and the pores on the carbon layers can efficiently increase SSA and offer favorable channels for fast diffusion of Na+ [30]. Multifarious S-doped carbons have been synthesized using different methods, such as the gas–solid reaction with H2S as the S source [26], chemical vapor deposition with thiophene as the S source [31], pyrolyzing S-containing polymer [32], and so on. To increase the S-doping level and simplify the synthesis, S powder was lately applied as the S source and heavily-doped disordered carbon was successfully achieved by annealing S powder with polyacrylonitrile [23]. However, it remains one concern, i.e. most organic

compounds and polymers are prone to undergo structural collapse and form discrete nanoparticles in the process of carbonization [33−35], which may compromise the Na+ transfer and the rate performance of SIBs. S-doped carbons with a robust 3D framework is, therefore, highly preferred. As demonstrated in our earlier work [36−38], having a tough skeleton, abundant heteroatoms and inorganic salts, many biomass are inherently desirable precursor to produce 3D framework of hierarchical porous carbons, free from additional requirements for hard templates (e.g. SiO2, MgO) and activator (e.g. KOH, ZnCl2) or time-consuming and high-cost synthesis. New, facile method to ahchieve high-performance S-doped carbons for SIBs remains highly desirable. In this work, we report S-doped 3D scaffolding framework of carbon nanosheets (S-CNS) by one-step pyrolyzation, with a plant biomass (spring onion peel, SOP) and S powder as the precursors (Scheme 1). The resultant S-CNS exhibits an ultrahigh reversible capacity of 605 mAh g−1 at 50 mA g−1, rate performance of 133 mAh g−1 at 10 A g−1, and long-term cycling stability at 5 A g−1 with as low as ∼6% fade upon 2000 cycles. Moreover, we demonstrate that S-doped carbon materials with competent electrochemical properties can be easily achieved from a series of biomass which possess certain features. This work provides a universal and facile route of designing and fabricating multi-heteroatoms doped hierarchical carbon materials for high-performance SIBs.

Scheme 1. Schematic illustration of the synthesis of the 3D scaffolding framework of S-doped carbon nanosheets and their application for SIBs.

2. Experimental Section 2.1. Synthesis of S-CNS from SOP SOP washed with deionized water and air-dried before use. 2.0 g of SOP was cut into ∼1.0 cm × 1.0 cm, smashed into powders, and mixed with S power at a mass ratio of 1:2, followed by annealing at 300 °C for 2h and 600 °C for 3h in Argon (Ar) with a heating rate of 5 °C min−1. The obtained powder was washed with dilute HCl solution (10%) and distilled water, and then dried at 100 °C for 12h. As a control group, CNS was produced by heating SOP alone at 300 °C for 2h and 600 °C for 3h in Ar. To demonstrate the universality of this method, a series of products were synthesized following the same S-doping procedure, only replacing SOP with other plant biomass, including onion skin, garlic peel, elm samara and lotus leaf and the final samples were named S-OS, S-GP, S-ES and S-LL, respectively. 2.2. Material characterization The morphology, structure and crystallization nature of the samples were characterized by transmission electron microscopy (TEM, Tecnai G2 F30 microscope), field-emission scanning electron microscopy (SEM, FEI Sirion 200), X-ray diffraction(XRD, PANalytical Multi-Purpose Diffractometer equipped a Cu Kα Radiation, λ=1.5406Å), and Raman spectra (Renishaw Invia spectrometer using laser of 532 nm) at room temperature. The nitrogen adsorption and desorption isotherms were collected at 77 K using a nitrogen adsorption analyzer (Autosorb-IQ2-MP-C system). The PSD and SSA were calculated using the Quench Solid State Density Functional Theory (QSDFT) and BET method, respectively. Chemical structure was analyzed by X-ray photoelectron spectroscopy (XPS) using an ESCALAB-250Xi spectrometer (Thermo Fisher Scientific) with a monochromatic Al Kα X-ray source. Thermogravimetric analysis (TGA) was performed by TA instruments Q500 at a heating rate of 5 °C min−1 under nitrogen atmosphere to evaluate the sulfur content. 2.3. Electrochemical measurements The working electrode was fabricated by mixing the as-obtained carbon product with acetylene black and polyvinylidene difluoride (PVDF) in N-methyl-2-pyrrolidine (NMP) at a mass ratio of 7:2:1. The slurry mixture was pressed onto a copper foil, dried at 100 °C in vacuum for 12h and cut into electrode films with a

mass loading of ~1.0 mg cm−2. CR2025 coin cells were assembled in an Ar-filled glovebox. Na foil and Whatman (GF/D) glass fiber membrane were used as the counter electrode and separator, respectively. The electrolyte was 1 mol L−1 NaClO4 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume ration of 1:1 with 5 wt% fluoroethylene carbonate (FEC). The cyclic voltammogram (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed using an electrochemistry workstation (CHI660E, Shanghai Chenhua), where the former were collected at a scan rate of 0.1–2 mV s−1 at 0.01–3 V, and the latter with the frequency ranging from 0.01 Hz to 100 KHz. The galvanostatic charge−discharge (GCD) tests were carried out on a Neware battery test system at 0.01–3 V. All the electrochemical measurements were carried out under ambient conditions. 3. Results and Discussion S-CNS was synthesized from the mixture of SOP and S powder via one-step pyrolyzation. Pure SOP alone was carbonized as a control group (refer as ‘CNS’). SOP is known for its richness in cellulose and lignin [39], and this kind of epidermis tissue is naturally a robust 3D architecture constructed by thin flakes. CNS was found to maintained the original lamellar morphology of SOP well upon the pyrolyzation, resulting in smooth and solid flakes (Fig. S1). Distinctly, as depicted by the SEM (Fig. 1a and Fig. S2) and TEM images (Fig. 1b−e), S-CNS presented a totally different appearance, showing a 3D scaffolding framework composed of nearly-transparent, roughened flakes full of wrinkles (pointed out by the blue circles) and pores (marked by the red circles). The varied morphology can be primarily attributed to the S volatilization upon sulfurization. Moreover, the average interlayer distance of S-CNS in the high-resolution (HR) TEM image (Fig. 1f) was ∼0.38 nm, much larger than that of graphite (0.335 nm). Both the 3D scaffolding porous framework and the expanded interlayer spacing benefit a large SSA, convenient path for Na+, ample reaction sites, facile insertion/extraction and fast transport between the electrode/electrolyte interfaces.

Fig. 1. Morphology characterization of the S-CNS sample. (a) SEM and (b–e) TEM images of the S-CNS sample, showing the 3D scaffolding framework of S-CNS and ample wrinkles and pores of the half-transparent flakes. (f) HRTEM image of the S-CNS sample, revealing an expanded interlayer spacing relative to that of graphite.

The porous nature was further investigated from nitrogen adsorption−desorption isotherms (Fig. 2a). Consistent with the TEM and SEM results, the SSA of S-CNS was evidently increased compared with CNS (230.97 m2 g−1 vs. 134.17 m2 g−1). Moreover, the IV-type profile with the obvious hysteresis loop of the isotherm indicated the presence of the abundant mesopores in S-CNS. The plot in Fig. 2b further revealed the coexistence of micropores and mesopores, with the pore size mainly distributed in the range of 1.2–3.3 nm and 10–40 nm. Such hierarchical porous structure further emphasize the promise of S-CNS by affording abundant channels for Na+ storage and diffusion, sufficient electrolyte‒eletrode contact, and lower resistance for ion transfer. XRD was performed to explore the structural varation and crystallization nature of the S-doped product (Fig. 2c). For both CNS and S-CNS, only two weak and broad peaks located at 23.4° and 44.0° corresponding to the graphitic (002) and (100) planes were observed, indicating their amorphous structure. It was noted that the (002) peak of S-CNS was shifted to a lower angle, suggesting that the S doping can enlarge the interlayer spacing. According to Bragg’s formula, the interlayer spacing (002) of S-CNS was calculated to be 0.38 nm, in good agreement with the HRTEM results. The graphitization degree was further evaluated by Raman

spectroscopy (Fig. 2d). Besides the D- and G-band, the Raman spectrum of S-CNS exhibited additional peaks at 350 cm−1, 493 cm−1 and 1434 cm−1, which are related to the C−S, S−S stretching vibration and C=C bond in thiophene rings [40], confirming that S atoms formed covalent bonds in S-CNS. Furthermore, evident red-shift of D-band and blue-shift of G-band were observed from the spectrum of S-CNS. Such shifts are characteristic of n-type doping which can effectively improve the conductivity of the product [41].

Fig. 2. Pore structure and composition of S-CNS and CNS. (a) Nitrogen adsorption-desorption isotherms, (b) pore size distribution, (c) XRD patterns, and (d) Raman spectra of the S-CNS and CNS.

The element content and chemical structure of S-CNS were examined by using TGA and XPS. The survey spectrum (Fig. S3) revealed the pronounced S 2s , S 2p, C 1s, N 1s, O 1s peaks at about 164, 228, 285, 399, and 534 eV, respectively. The high resolution XP spectrum of C 1s (Fig. 3a) can be deconvoluted into five peaks, located at 284.4, 284.8, 285.4, 286.2, and 287.1 eV and attributed to C=C, C–C, C–S, C–N, and C=O groups [42], respectively. The N 1s XP spectrum (Fig. 3b) can be deconvoluted into three components at 398.4, 401.1, and 402.6 eV, corresponding to pyrrolic N (N−5), graphitic N (N−4), and oxidized N (N−X) [43], respectively. The different chemical states of O were further identified in the O 1s spectrum (Fig. 3c), where the three peaks centered at 531.3, 532.1, and 533.7 eV can be assigned to C=O, C−O, and O−C=O groups [44]. Moreover, the two peaks in the S spectrum (Fig. 3d) at 164.0 and 165.2 eV can be assigned to the S 2p3/2 and 2p1/2 peaks of C–S–C covalent bond, while the weak peak at 168.4 eV corresponds to C–SOx–C

groups [31]. According to the results of TGA (Fig. S4), the S content was of ∼23%, which is significantly higher than the S-doping level of carbon materials reported previously [26,31,41,45].

Fig. 3. XPS analysis of the S-CNS sample. (a) C 1s, (b) O 1s, (c) N 1s, and (d) S 2p XP spectra.

The Na+ storage performance of the samples was investigated by the CV and GCD measurements in Na-half cell tests. The initial four cycles of CV curves of S-CNS in the range of 0.01–3 V (vs. Na+/Na) at a scan rate of 0.1 mV s−1 is presented in Fig. 4a, which are obviously different from those of CNS (Fig. S5a). An irreversible cathodic peak was observed from the CNS electrode at 1.08 V in the first cycle, but disappeared in the subsequent cycles, showing a typical behavior of solid electrolyte interface (SEI). In sharp contrast, a pair of redox peaks at 1.85/1.10 V were observed in all CV curves of S-CSN, which are attributed to the redox reactions between S and Na+. Moreover, the CV curves of the second, third, and fourth cycle were nearly identical, suggesting the excellent cycling stability of the S-CSN electrode. The GCD profiles were measured at a current density of 0.05 A g−1 (Fig. 4b). Remarkably, in the first cycle, a discharge capacity as high as ∼1211.3 mA h g−1 and a charge capacity as high as ∼707.9 mA h g−1 were achieved, yielding initial Coulombic efficiency of ∼58%. These values are much higher than those of most carbon anode materials reported recently [46–49]. Consistent with the CV results, an obvious sloping platform appeared around 1.80 and 1.15 V in these GCD curves, also suggesting that the capacity was partially contributed by the Faradaic reactions of Na+ with S.

In addition to the considerable capacity, the S-CNS electrode also delivered high rate capability. As shown in Fig. 4c, the reversible capacities of ∼601.2, 545.2, 505.1, 451.3, 396.8, 326.4, 220.1 and 133.6 mA h g−1 were obtained at 50, 100, 200, 500, 1000, 2000, 5000, and 10000 mA g−1, respectively. Importantly, a high capacity of ∼133.6 mA h g−1 can be retained at a high current density of 10 A g−1. These values are among the best values for SIBs with biomass-derived carbon anode materials reported so far (Table S3). It is worth noting that, when the current density was back to 50 mA g−1 after cycling at different current densities, the specific capacity can still reach ∼605.3 mA h g−1, indicating the reliable cycling stability and good reversibility of S-CNS. The rate performance of the CNS electrode is presented in Fig. S5b. The CNS electrode delivered a first-cycle charge and discharge capacity of ∼169.1 and 611.4 mAh g−1 at 50 mA g−1, leading to the initial Coulombic efficiency of ∼27.7%, similar to the previously reported sodium storage properties for disordered carbons [50,51]. The discharge capacity of the CNS electrode at 0.05, 0.1, 0.2, 0.5, 1.0, 2.0, and 5.0 A g–1 were ∼170.2, 133.4, 110.1, 84.0, 66.2, 45.5, and 23.3 mAh g-1, respectively, which were evidently lower than those of S-CNS. Furthermore, the long-term cycling performance of S-CNS was evaluated at a moderate current density of 5 A g−1. As shown in Fig. 4d, the reversible capacity was maintained as high as ∼211.0 mA h g−1 upon 2000 cycles, and the capacity variation upon the cycling was smaller than 6%. The EIS of the S-CNS electrodes before and after 2000 cycling were measured to gain more insight into the electrochemical performance (Fig. S6). The Nyquist plots showed a large semicircle in the high frequency region and an inclined line in the low frequency region, which are associated with the charge transfer resistance (Rct) and Na+ diffusion in the electrode, respectively. The S-CNS electrodes possessed a small original Rct (∼115.7 Ω) and a fair Rct (∼196.3 Ω) after 2000 cycles. All these results emphasize the high Na+ storage capability and rate performance of S-CNS, which can be attributed to its special features as follows: (i) an appropriate level of S-O-N-doping which improves the electrical conductivity and wettability of the electrode meanwhile provides the Faradaic reactions with Na+; (ii) the 3D scaffolding framework of hierarchical porous structure affords plenty of reaction sites and adequate contact with the electrolyte, and shortens the Na+ diffusion distance; (iii) the large interlayer spacing and tough skeleton further facilitate the insertion/extraction and transfer of Na+, and also reduce the risk of volume expansion and pulverization of the electrode.

Fig. 4. Electrochemical performance of the S-CNS as anode materials for SIBs. (a) CV curves at a scan rate of 0.1 mV s−1 within a potential window of 0.01–3.0 V. (b) GCD curves from 1st to 50th cycles at a current density of 50 mA g−1. (c) Rate performance at various current densities. (d) Cycling performance at a current density of 5 A g−1.

The capacity of carbon electrodes can be generally rationalized into two resources: i.e. diffusion-controlled intercalation process (‘C1’), and the surface-controlled pseudo-capacitance and electrical double-layer capacitance (‘C2’). To get better understanding of the storage mechanism of S-CNS, its CV curves at different scan rates range from 0.1 to 2.0 mV s−1 were recorded (Fig. 5a). The capacity was qualitatively analyzed by the b-value following the equation of i =avb, where i is the peak current, v is the scan rate, b = 1 means that the capacity is totally originated from ‘C2’, and b = 0.5 suggests a complete ‘C1’ [52]. As shown in Fig. 5b, the b-value of S-CNS was deduced to be ∼0.90 and 0.73 for the reduction and oxidation state, suggesting a combined contribution from both ‘C1’ and ‘C2’. The proportion of ‘C1’ and ‘C2’ was then quantitatively determined by i (V ) = k1v + k2 v1/2 , where k1ν presents the surface-controlled charge, and k2ν1/2 corresponds to diffusion-controlled charge [53,54]. The voltammetry response (Fig. 5c) showed that ‘C2’ (yellow area) was primary (∼72%) at a scan rate of 1 mV s‒1, and the ratio of this contribution can further increase with the scan rate; on the contrary, the ratio of ‘C1’ decreased accordingly. As shown in Fig. 5d, the ‘C2’ for the total charge storage at various scan rates of 0.1, 0.2, 0.5, 1.0 and 2.0 mV s−1 was ∼52%, 57%, 65%, 72% and 77%, respectively, further confirming its dominance in the total capacity. The primary capacitive effect hence the reactions mainly occurred on the electrode surface thus benefited excellent rate performance

and cycling stability by maintaining the S-CNS structure during charge‒discharge cycling. The b value of CNS is calculated to be 0.97 and 0.76 for the reduction and oxidation state (Fig. S5d), indicating that the storage capability of CNS is also composed of ‘C1’ and ‘C2’. As shown in Fig. S5f, the capacitive contribution at 0.1, 0.2, 0.4, 1 and 2 mV s–1 are quantified to be ∼55%, 60%, 68%, 82%, and 95%, respectively. The CNS sample showed higher capacitive contribution than S-CNS, very likely attributed to the mismatch between the relatively small interlayer spacing of CNS and the large size required by Na+ intercalation. In return, the results emphasize that the present sulfur doping can efficiently expand interlayer distance of the carbon materials hence facilitate Na+ absorption and transfer in the charge/discharge process.

Fig. 5. Mechanism analysis of Na+ storage behavior of the S-CNS sample. (a) CV curves at various scan rates ( from 0.1 to 2.0 mV s−1). (b) Relationship between the peak current and scan rate in logarithmic format. (c) Capacitive contribution (yellow) to charge storage at a scan rate of 1 mV s−1. (d) The contribution ratio of the capacitive and intercalated charge to the capacity at different scan rates.

Besides the easiness, cost and yield, the universality of a synthetic method is crucial for commercial mass production, broadening the choices of raw resources. We demonstrated that the 3D hierarchical porous S-doped carbon framework can be successfully produced by one-step pyrolyzing the mixture of S powder with a series of various plant biomass, including onion skin, garlic peel, elm samara, and lotus leaf (Fig. 6a−d). These biomass precursors were selected as they are all typical pliable and tough plant epidermal tissues like SOP, rich in cellulose, lignin, protein, and carbohydrate. As revealed in Fig. 6e−h, these biomass precursors

did share the common microscale morphologies with SOP, i.e. being a 3D framework primarily constructed by parallel thin flakes, despite the additional features, e.g. the porous tubes between the flakes of elm samara and the rough surface of lotus leaf. Thanks to the robustness of these epidermal tissues, the S-doped carbon materials derived from these precursors (denoted as ‘S-OS’, ‘S-GP’, ‘S-ES’, and ‘S-LL’, respectively) also preserved their inherent morphology and formed 3D scaffolding framework of thin flakes (Fig. 6i‒p), exactly like S-CNS. XPS was also employed to characterize the composition of these products. As shown in Fig. S7a and Table S1, the biomass precursors contained three main elements, i.e. C, O, and N, besides the trace amount of P or/and S. After the sulfurization treatment, all the four samples resulted in a similar composition, i.e. ∼70% of C, ∼15% of O, and ∼15% of S (Table S2 and Fig. S7b).

Fig. 6. (a–d) Digital photographs, (e–h) SEM images of onion skin, garlic peel, elm samara and lotus leaf. (i–p) SEM images the of the S-OS, S-GP, S-ES, S-LL samples.

The Na+ storage performance of all these samples was also investigated. According to the CV and GCD measurements (Fig. 7a‒h), similar electrochemical activities comparable to that of S-CNS derived from SOP were observed. The first discharge (charge) capacities of S-OS, S-GP, S-ES and S-LL anodes were 1038.3

(555.2), 884.5 (500.0), 1194.0 (692.3), and 1105.1 (600.8) mA h g−1, respectively, leading to initial Coulombic efficiency of 53.5%, 56.5%, 58.0%, and 54.4%. When the current density was increased to 5 A g−1, the values of specific capacity were recorded as high as 178.4, 155.6, 145.7, 176.1 mA h g−1, respectively, indicating the excellent rate performance of these carbon products. Furthermore, the long-term cycling performance of S-OS, S-GP, S-ES and S-LL was evaluated at a current density of 5 A g−1 (Fig. 7i‒l). The capacity retention were maintained to ∼94%, 93%, 93%, 94% upon 1500 cycles, confirming the long-term cyclic stability of these S-doped electrodes.

Fig. 7. Electrochemical performance of S-OS, S-GP, S-ES, and S-LL as the anode materials for SIBs. (a‒d) Rate performance at various current densities. (e‒h) CV curves at various scan rates (from 0.1 to 2.0 mV s−1). (i‒l) Cycling performance at a current density of 5 A g−1.

In addition, based on the fitted plot of log (scan rate)–log (peak current) curve (Fig. 8a‒d), the b-values of S-OS, S-GP, S-ES and S-LL samples were deduced to be ∼0.92, 0.97, 0.96, 0.93 for the reduction and ∼0.74, 0.75, 0.69, 0.74 for the oxidation state, respectively. The capacitive contribution to the total charge storage of four S-doped carbon materials under the scan rates of 1 mV s−1 (Fig. 8e‒h) were ∼70%, 78%, 67% and 70%, respectively. Moreover, the ratio of capacitive contribution can further increase with the scan rate. For instance, it could achieve ∼86% for S-GP at the scan rates of 2 mV s−1. The results indicate that the surface-controlled capacity takes a large proportion in the total capacity, consistent with the case of S-CNS.

Fig. 8. Mechanism analysis of Na+ storage behavior of S-OS, S-GP, S-ES, and S-LL. (a–d) Relationship between the peak current and scan rate in logarithmic format. (e–h) Capacitive contribution to charge storage at a scan rate of 1 mV s−1. (i–l) The contribution ratio of the capacitive and intercalated charge to the capacity at different scan rates.

The spring onion, onion, garlic and lotus leaf were purchased from supermarket in Harbin and the elm samara was collected from campus of Harbin Institute of Technology. The different batches of S-doped carbon products of each plant precursor purchased from different supermarket or collected from different locations on

campus were compared. The results were well reproducible, with differences in their electrochemical capabilities practically negligible.

4. Conclusion In summary, this work reports a green, simple and efficient method for synthesizing high-performance S-O-N co-doped 3D scaffolding framework of hierarchical porous carbon sheets from biomass precursors as the anode materials for SIBs. The high heteroatoms-doping level, expanded interlayer spacing, tough 3D framework, hierarchical porous structure hence large assessable SSA have afforded ample reaction sites, improved electrical conductivity and wettability, additional Faradaic reactions, convenient path for Na+ diffusion, and robust host for Na+ insertion/extraction. Consequently, the resultant S-CNS product delivers an ultrahigh reversible capacity of 605 mA h g−1 at 0.05 A g−1, initial Coulombic efficiency of 58%, rate performance of 133 mA h g−1 at 10 A g−1, and long-term cycling stability (211 mA h g−1 at a current density of 5 A g−1 after 2000 cycles with a capacity retention of 94%). We demonstrate that, S-doped carbon products with competent electrochemical properties for SIBs can not only be derived from spring onion peel by this simple doping method, but also from a variety of other plant epidermal tissues, including onion skin, garlic peel, elm samara and lotus leaf. These biomass share the common characteristics: 3D scaffolding framework of thin lamellar structures, rich in cellulose, lignin, protein, and carbohydrate. This work thus introduces a fertile ground to produce powerful electrode materials by using the mixture of biomass and S powder, which may hold considerable potential for scalable production of commercial SIBs. Appendix A. Supplementary material

Additional information as noted in the text. This material is available free of charge via the internet at https://www.journals.elsevier.com/nano-energy. *Address correspondence to [email protected], [email protected], and [email protected]. Acknowledgments This work is financially supported by the National Natural Science Foundation of China (51772066, 21473045), Natural Science Foundation of Heilongjiang Province, China (E2015003), and State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (2018DX04).

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Sulphur-doped carbon nanosheets derived from biomass as high-performance anode materials for sodium-ion batteries Gongyuan Zhaoa,c, Dengfeng Yub, Hong Zhangb, Feifei Suna, Jiwei Lib, Lin Zhua, Lei Suna, Miao Yua,*, Flemming Besenbacherc,* and Ye Sunb,* a

State Key Laboratory of Urban Water Resource and Environment, School of Chemistry and Chemical Engineering,

Harbin Institute of Technology, Harbin 150001, China b

Condensed Matter Science and Technology Institute and Department of Physics, School of Science, Harbin Institute of

Technology, Harbin 150080, China c

Interdisciplinary Nanoscience Center (iNANO) and Department of Physics and Astronomy, Aarhus University, Aarhus

8000, Denmark * Corresponding author. E-mail addresses: [email protected], [email protected], [email protected]

Highlights ● 3D scaffolding S-doped carbon nanosheets are produced from biomass for Na ion batteries. ● An ultrahigh reversible capacity of 605 mAh g−1 at 50 mA g−1 is achieved. ● High rate performance of 133 mAh g−1 at 10 A g−1 and long-term cycling stability are shown. ● The universality of this synthesis is demonstrated by using various plant biomass.