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sulfur co-doped carbon as high performance anode materials for lithium-ion batteries
Accepted Manuscript Ultrahigh level nitrogen/sulfur co-doped carbon as high performance anode materials for lithium-ion batteries Zhaozheng Qiu, Yemao Lin, Hailin Xin, Pei Han, Dongzhi Li, Bo Yang, Pengchong Li, Shahid Ullah, Haosen Fan, Caizhen Zhu, Jian Xu PII:
S0008-6223(17)30983-1
DOI:
10.1016/j.carbon.2017.09.100
Reference:
CARBON 12430
To appear in:
Carbon
Received Date: 27 July 2017 Revised Date:
24 September 2017
Accepted Date: 28 September 2017
Please cite this article as: Z. Qiu, Y. Lin, H. Xin, P. Han, D. Li, B. Yang, P. Li, S. Ullah, H. Fan, C. Zhu, J. Xu, Ultrahigh level nitrogen/sulfur co-doped carbon as high performance anode materials for lithium-ion batteries, Carbon (2017), doi: 10.1016/j.carbon.2017.09.100. 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 proof before it is published in its final 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.
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Ultrahigh level nitrogen/sulfur co-doped carbon as high
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performance anode materials for lithium-ion batteries
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Zhaozheng Qiu,a, # Yemao Lin,a, # Hailin Xin,a Pei Han,a Dongzhi Li,a Bo Yang,a Pengchong Li,a
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Shahid Ullah,a Haosen Fan,a Caizhen Zhu,a* and Jian Xub
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* Corresponding author. E-mail address: [email protected] (C. Zhu). #
These authors contributed equally to this work.
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a
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PR China
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b
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Sciences, Beijing, 100190, PR China
College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, 518060,
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Beijing National Laboratory for Molecular Sciences Institute of Chemistry, Chinese Academy of
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Abstract
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Ultrahigh level nitrogen and sulfur co-doped disordered porous carbon (NSDPC) was
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facilely synthesized and applied as anode materials for lithium-ion batteries (LIBs).
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Benefiting from high nitrogen (14.0 wt%) and sulfur (21.1 wt%) doping, electrode
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fabricated from NS1/3 showed a high reversible capacity of 1188 mA h g−1 at 0.1 A g−1
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in the first cycle with a high initial columbic efficiency (>75 %). In addition,
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prolonged life over 500 cycles and excellent rate capability of 463 mA h g−1 at 5 A g−1
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have been realized. The preeminent electrochemical performance is attributed to three
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effects: (1) the high level of sulfur and nitrogen; (2) the synergic effect of dual-doping
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heteroatoms in cooperation with each other; (3) the large quantity of edge defects and
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abundant micropores and mesopores that can provide extra Li storage regions. These
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unique features of NSDPC electrodes suggest that they can serve as a practical
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substitute for graphite as a high performance anode material in LIBs.
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Keywords: Electrode materials; Co-doping; Porous carbon; Anode; Lithium
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1. Introduction
ACCEPTED MANUSCRIPT With the rapid development of portable devices, renewable energy harvesting, and
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electric vehicles[1, 2], safe energy storage devices with improved power and energy
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densities are becoming increasingly important[3]. Among these devices, lithium-ion
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batteries (LIBs) are promising choices for their high energy density and power density,
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additionally with long lifespans[4]. However, application of LIBs in large-scale
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electric energy storage requires further improvements in energy density, rate
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capability, safety, and electrode durability. To a large extent, these issues rely on
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several determinants. One of them is the development of novel anode materials.
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Graphite is the main commercial anode material used in currently available LIBs. But
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the low theoretical specific capacity of 372 mA h g−1 cannot meet the increasing
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demands of rapidly developing markets[5]. Therefore, it’s urgently to develop new
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anode materials with high theoretical capacities and excellent rate and cycling
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performance for the next-generation LIBs.
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Various nanostructured products containing Si[6-8], P[9], or Sn[10-12], as well as
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alloy[13, 14] and some transition metal oxides/sulfide[15-23], have been reported as
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high capacity anode materials for LIBs. Unfortunately, intrinsic problems such as
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huge volume expansion, relatively low conductivity, and large voltage hysteresis,
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exist in these materials during the lithiation process[24]. These deficiencies severely
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limit their commercialization in LIBs. Therefore, novel carbon-based anode materials,
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with enhanced electrochemical performance for lithium storage, are still a major focus
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of research worldwide[25]. Applied them as anode materials, high energy LIBs with
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elevated specific capacity, excellent cycling stability and rate performance have been
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ACCEPTED MANUSCRIPT realized. Significant efforts have been made to design novel nanostructures of
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carbonaceous materials with larger specific capacity for lithium storage [25-27].
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However, for amorphous carbon materials, although activating agents (such as KOH)
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has been applied to increase the specific surface area, inferior stability and poor rate
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performance still occur due to the low degree of graphitization[28]. Therefore, a novel
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strategy of artificial doping with heteroatoms, such as N[26], B[29] or S[30], have
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being proposed. With this method, the electronic properties and electrochemical
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activity of amorphous carbon materials can be effectively improved, leading to
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excellent Li ion storage capacity. Nevertheless, most reported results related to this
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field are based on the single element doping but co-doping and their synergistic
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influence to the electrochemical property is less studied.
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Although nitrogen doped carbon-based materials have been extensively studied, sulfur
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and especially nitrogen/sulfur dual-doping are far less exhaustive[31]. When doped
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with nitrogen the electronic properties of carbon-based materials can be improved[32].
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As a complement to nitrogen, sulfur doping has attracted increasing attention in recent
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carbon materials research. While sulfur has a larger atomic radius, it can enlarged the
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interlayer spacing(d002) of the carbon matrix[31]. The doped sulfur substantially
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increases the charge capacity with the enlarged graphite crystallite size (Lc) and
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creates more micropores, improving the electrochemical properties of the carbon[30].
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Meanwhile, its easily polarizable lone pairs can change the charge state of
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neighboring carbon atoms. Hence, it can be used to tune chemical reactivity and
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catalytic activity of the carbon materials. Therefore, as a doping heteroatom, sulfur
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ACCEPTED MANUSCRIPT can be incorporated into carbon-based anode materials, improving their reversible
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capacity in LIBs[33]. Importantly, when both highly interactive elements of nitrogen
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and sulfur are simultaneously doped into the carbon matrix, synergistic effects can be
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aroused[34]. In addition, it has been reported that the binding state of sulfur and
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nitrogen in the carbon matrix can be tuned when different sulfur and nitrogen sources
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were used[31]. Most studies property on sulfur/nitrogen dual-doped carbon materials
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is about their enhanced electrocatalyst ability[31, 35]. Few reports focus on
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dual-doped carbons for LIBs. Therefore, we were inspired to investigate the favorable
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influences of dual-doped heteroatoms in LIBs systems.
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Herein, we applied a “dual-doping” strategy to synthesize an amorphous porous
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carbon with nitrogen and sulfur at an ultrahigh doping level. In this way, the
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synergistic effects of heteroatom co-doping and a disordered porous structure has
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been achieved. The nitrogen and sulfur co-doped disordered porous carbon (NSDPC)
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was employed as a preeminent anode material for high-performance LIBs. By a facile
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synthesis method, an amorphous porous structure was formed. In the sample of NS1/3,
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micropores and mesopores were homogenously embedded with a nitrogen content of
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14.0 wt% and a sulfur content of 21.1 wt%. Benefitting from the convenient transport
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pathway for Li-ions and electron, as well as the abundant pores for extra Li storage in
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the carbon framework, the NS1/3 electrode exhibits excellent rate performance with
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superior cycling stability. The reversible specific capacity achieved as high as 1188
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mA h g−1 at a current density of 0.1 A g−1, and even after increasing the rate to 5 A g−1
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high capacity of 463 mA h g−1 can still be obtained, the capacity still remains at 653
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2. Experimental Section
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2.1. Synthesis of cystine aggregates
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10 mM L-Cysteine was dissolved in 100 mL ultrapure water (18.0 MΩ cm−1) under
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ultrasonication for about 10 min. The pH value of the L-Cysteine aqueous solution
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was rapidly adjusted to 8.0 using Na2CO3 aqueous solution. 5 mL H2O2 solution (30
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wt%) was added drop by drop slowly to the L-Cysteine solution with a syringe (10
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mL). The whole reaction process was kept at room temperature with stirring. After 10
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min, the resulting solution was incubated at room temperature without interruption for
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24 h. The resulting precipitation was centrifuged and washed several times with water
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and ethanol and finally the white powder of cystine aggregates were obtained after
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dried under oven at 60 °C for 24 h.
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2.2. Preparation of NSDPC
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In order to adjust the elemental composition of the obtained NSHPC, varied
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concentrations of melamine and sulfur were added to a constant amount of cystine
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aggregates harvested in front. The nitrogen/sulfur co-doped carbon was synthesized
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by the following synthetic route. In a typical experiment: 8.0 g of cystine aggregates
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and x g (x = 3.0, 2.4, 2.0, 1.6, or 1.0) of melamine and y g (y = 1.0, 1.6, 2.0, 2.4, or 3
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corresponding to x) of sulfur were mixed thoroughly. Followed by annealing at
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500 °C for 2 hours in the atmosphere of Ar with a heating ramp of 2 °C min−1 and the
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obtained sample were labeled according to the following scheme: NSX/Y where X and
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Y denotes the integer proportion of x and y. NS3/1 hence corresponds to an experiment
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ACCEPTED MANUSCRIPT in which 3.0 g of melamine and 1.0 g sulfur were added to 8.0 g of cystine aggregates.
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2.3. Material characterization
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The powder X-ray diffraction (XRD) patterns of all samples were recorded with an
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X-ray diffractometer (D8 Advance of Bruker, Germany) with filtered Cu Kα radiation
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over the 2θ range of 10-60°. Field emission scanning electron microscopy (FE-SEM)
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images were collected on a JSM-7800F scanning electron microscope. The amounts
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of doped nitrogen and sulfur in the synthesized materials were determined by a
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CHNOS Elemental Analyzer (vario EL cube, Germany). Transmission electron
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microscopy (TEM) images were taken on a JEM-2100 transmission electron
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microscope using an accelerating voltage of 200 kV, and high-resolution transmission
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electron microscope (HRTEM) (JEOL-2011) was operated at an acceleration voltage
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of 200 kV. The specific surface area was evaluated at 77 K (Quantachrome
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NOVA1200e) using the Brunauer-Emmett-Teller (BET) method, while the pore
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volume and pore size were calculated according to the Barrett-Joyner-Halenda (BJH)
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formula applied to the adsorption branch. Thermogravimetric analysis (TGA) was
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carried out using a STA409PC from 0 °C to 800 °C at a heating rate of 10 °C min−1
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under Ar. X-ray Photoelectron Spectrum (XPS) was performed on an ESCALAB 250
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X-ray Photoelectron Spectrometer with Al Ka radiation. Raman spectra were obtained
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using a Digilab FTS3500 from Bio-Rad with a laser wavelength of 632.8 nm.
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2.4. Electrochemical measurements
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The electrochemical performance of the NSDPC was examined by using CR 2032
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coin-type cells which assembled in an argon-filled glovebox, using lithium foils as the
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ACCEPTED MANUSCRIPT counter electrodes, Celgard 2500 membrane as the separator, 1 M LiPF6 in a 1:1 (v/v)
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mixture of ethylene carbonate and dimethyl carbonate as the electrolyte. To prepare a
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working electrode, active materials (NSDPCs, 70 wt%), conductive material
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(acetylene black, 20 wt%), and binder (Sodium Alginate (SA), 10 wt%) were milled
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in ultrapure water to form slurries and then coated onto the surface of a copper foil
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current collector. The electrode capacity was measured by a galvanostatic discharge–
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charge method in the voltage range between 0.01 and 3.0 V on a battery test system
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(Land CT2001A, China). Cyclic voltammetry (CV) from 0.01 to 3.0 V (vs. Li/Li+) at
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0.1 mV s−1 and electrochemical impedance spectroscopy (EIS) in the frequency range
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100 MHz to 0.01 Hz and with an amplitude of 5 mV were performed using an
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electrochemical workstation (CHI660A). All electrochemical measurements were
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carried out at room temperature.
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3. Results and Discussion
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3.1. Synthesis and characterizations of the NSDPC
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It has been demonstrated that H2O2 can oxidize the –SH group of cysteine to form
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disulfide bonds and lead to the formation of cystine[36]. The development of cystine
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aggregates is a result of intermolecular interactions, mainly hydrogen bonding and
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electrostatic interactions[37]. The morphology of the obtained cystine aggregates was
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thin slice shape hexagonal crystal (Fig. 1S). Fig. 1 shows the illustration of the
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synthesis of NSDPC. NSDPC was synthesized facilely by annealing the mixture of
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as-prepared cystine aggregates, melamine, and sulfur in a flowing atmosphere of Ar at
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500 °C for 2 h. The cystine aggregates with N content of 11.6 wt% and S content of
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ACCEPTED MANUSCRIPT 26.4% were selected as the substrate material. Melamine with a high N content (66.7
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wt%) and sulfur were used as excess N and S sources, respectively.
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Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of the
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mixture of as-prepared cystine aggregates, melamine, and sulfur (mass ratio of 8: 1: 3)
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were performed in an Ar atmosphere. It indicated that the carbon sulfurization
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temperature was approximately 300 °C (Fig. 2a). When the temperature was increased
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to 500 °C, the carbon sulfurization and carbonization of the as-prepared cystine
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aggregates and melamine were accomplished, and the excess sulfur was evaporated
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off.
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Fig. 1. The schematic of the synthesis of NSDPC composites
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The results obtained from XRD measurements are summarized in Fig. 2b and Table 1.
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Samples of NS3/1, NS3/2, NS3/3, NS2/3, and NS1/3 exhibited broad peaks at around 23.5°
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and weak peaks at 43°, which were readily indexed to the representative (002) and
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(101) planes of amorphous carbon, respectively. According to Bragg’s equation, the
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interlayer spacing (d002) of the five samples was calculated to be approximately 0.38
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nm. The larger d002 values of the NSDPC materials in comparison with that of
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graphite (0.335 nm) implied that the intercalation of sulfur result in an enlarged
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interlayer distance of porous carbon, facilitating the diffusion and insertion/extraction
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of Li+ in carbon matrix[38]. Table 1. Summary of the d-spacing of the NSDPC samples
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NS3/1
NS3/2
NS3/3
NS2/3
NS1/3
23.29
23.66
23.35
23.23
23.66
d002(nm) 0.381
0.376
0.380
0.382
2θ (°)
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0.376
To further examine the structural characteristic, the five samples were measured using
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N2 adsorption–desorption isotherms. As displayed in Fig. 2c and Table S1 (in
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Supplementary Material), the specific Brunauer-Emmett-Teller (BET) surface area of
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the NS3/1 is 34.6 m2 g−1. Whereas the specific surface areas of the NS3/2, NS3/3 and
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NS2/3 are increased significantly to 42.8, 129.2 and 249.6 m2 g−1, respectively. As for
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the sample of NS1/3, it reaches the maximum of 341.0 m2 g−1 at the ratio of
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melamine(X)/sulfur(Y) reach 1: 3. Moreover, NS1/3 possesses the highest total pore
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volume, being 0.474 cm3 g−1 (Table S1). This demonstrates that the effectiveness of a
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suitable ratio of melamine and sulfur in developing more pores in NSDPC skeleton.
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The NSDPC samples showed mesopores are dominant in the samples, which covered
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a range between 2∼10 nm (Fig. 2d).
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Fig. 2. (a) TG and DTA curves of the mixture of cystine aggregates, melamine, and
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sulfur with a mass ratio of 8: 1: 3 in the temperature range of 20–800 °C with a
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heating rate of 10 °C min−1 in an Ar atmosphere; (b) XRD patterns of NSDPC
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samples over the 2θ range of 10–60°; (c) N2 adsorption–desorption isotherm and (d)
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BJH pore-size distribution of NSDPC samples.
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The morphologies of the as-prepared NSDPC samples were characterized by
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field-emission scanning electron microscopy (FESEM) as shown in Fig. 3a–f. The
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SEM images of the resulting carbon materials exhibited flake-like morphology. Many
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interconnected pores existed in the carbon sheets, especially for the NS1/3 sample.
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High magnification observation of the sheets revealed that the sheets had numerous
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nanoscale pores within them. This 3D structure has many interpenetrative channels
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with large internal spaces, which are mainly formed by the volatilization of excess
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sulfur, H2O, CO2, H2S, NH3, and CO during the carbonization process[38]. The novel
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structure in this carbon framework ensures facile access of electrolytes, efficient
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ACCEPTED MANUSCRIPT transfer of lithium ions and supply additional lithium storage space, fascinating rapid
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charging/discharging of the electrode. The HRTEM image (Fig. 3g) reveals the rough
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surface of the amorphous carbon with nanopores. TEM images further confirm the
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interconnected porous structure. TEM elemental mapping of the NSDPCs shown in
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Fig. 3g indicates that the distribution of C, N, and S is very homogenous.
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Fig. 3. SEM images of NSDPC samples: (a) NS3/1, (b) NS3/2, (c) NS3/3, (d) NS2/3, and
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(e, f) NS1/3; (g) HRTEM images and TEM-EDS elemental mapping of NS1/3.
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Elemental analysis (EA) was employed to quantify the N and S doping levels in
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carbon bulk. The results demonstrated that the contents of N and S in the NSDPC
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materials could be tailored by varying the ratio of melamine and sulfur (Table 2).
ACCEPTED MANUSCRIPT Pristine NSDPC materials derived from cystine aggregates showed a substantial
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presence of nitrogen and sulfur (10.2 wt% and 7.6 wt%, respectively). N and S
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concentration increased drastically with additional melamine and sulfur. For example,
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the N and S levels reached to 16.3 wt% (N) and 13.1 wt% (S) with the ratio of
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melamine and sulfur to be 3: 1. When the ratio was changed to 1: 3, the content of N
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decreased to 14.0 wt%, while the content of S increased to 21.1 wt%.
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Table 2. Elemental composition of the NSDPC samples Sample C (%) S (%) N (%) O (%) H (%) S/N NS 61.1 7.6 10.2 18.8 2.3 0.75 NS3/1 57.8 13.1 16.3 10.5 2.2 0.80 NS3/2 59.0 13.2 15.6 9.9 2.2 0.85 59.7 14.4 13.4 10.3 2.3 1.07 NS3/3 NS2/3 56.8 18.0 13.5 9.7 2.1 1.33 54.1 21.1 14.0 9.0 1.9 1.51 NS1/3
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Raman spectroscopy was carried out to examine the degree of graphitization and
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imperfections in the obtained carbon samples (Fig. 4a–f). Two characteristic peaks at
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approximately 1403 and 1560 cm−1 correspond to the D and G peaks, respectively.
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The D peak indicate to structure defects and disordered structure in the graphite layer
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due to the co-doping of nitrogen and sulfur. While the G peak is related to the sp2 C–
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C bonds of perfect graphite layer[27]. The intensity ratio of the G to D band (ID/IG) is
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generally employed to evaluate the extent of structural disorder for the carbon
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materials[39]. The ID/IG ratios for NS3/1, NS3/2, NS3/3, NS2/3, and NS1/3 were 1.322,
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1.393, 1.440, 1.477, and 1.558, respectively. Obviously, an increasing trend in the
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ID/IG ratio was observed after more S was doped. The NS1/3 sample showed the
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ACCEPTED MANUSCRIPT highest ID/IG ratio (1.558), indicating that higher S-doping lead to more defect and
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disorder in the carbon framework. The results agree with the previous report on
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sulfur-doped porous carbon[40]. The disordered structure and numerous defects are
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supposed to be benefit to the insertion/extraction and additional storage of Li ions[41].
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As shown in Fig. 4f, G band has a trend of shifted down as the ratio of N and S
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precursor adjust from 3: 1 to 1: 3, which can be attributed to the recovery of the
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conjugated structure[42] and the electron donation of heteroatoms[43].
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Fig. 4. Raman spectra at the D (disordered) band and G (graphitic) band of (a) NS3/1,
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(b) NS3/2, (c) NS3/3, (d) NS2/3, and (e) NS1/3; (f) spectra of the NSDPC samples.
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X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical status
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of the elements and the surface chemistry of the NSDPCs, as shown in Fig. 5. The
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XPS survey spectrum shows peaks for C1s (284.6 eV), N1s (400.5 eV), O1s (531.5
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eV), and S2p (164.5 eV) (Fig. 5a), confirming the existence of elements detected by
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EA and the elemental mapping results (Fig. 3g). The N and S concentrations (at%) on
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the surface of each NSDPC sample are shown in Table S2(in Supplementary Material).
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from the XPS data was generally in concordance with the EA analysis result (21.1
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wt%) (Table 2), indicating uniform distribution of heteroatoms in the bulk materials.
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The high-resolution C1s spectrum was fitted by two sub-peaks, including C–C (ca.
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284.0 eV), and C–S/C–N (ca. 285.5 eV) (Fig. 5b) [26, 27, 44]. The deconvoluted XPS
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spectrum of N1s contains two characteristic sub-peaks at 397.8, and 399.6 eV (Fig.
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5c), corresponding to pyridinic-type, and pyrrolic-type, respectively (Table S4) [45].
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The pyridinic and pyrrolic nitrogen species can improve the conductivity in the
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carbon matrix, thanks to their p-electron pair that donate to the π-conjugated system in
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the graphene layers[46, 47]. The high resolution S2p spectrum is shown in Fig. 5d,
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which exhibits three sub-peaks at binding energies of 163.0, 164.0, and 167.8 eV
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(Table S4). The former two can be attributed to the S2p3/2 and S2p1/2 of the –C–Sx–C–
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(x =1–2) covalent bond of the thiophene-S, while the weak peak at 167.8 eV
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corresponds to C–SOx–C (x = 2–4) groups[38].
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Fig. 5. (a) Full-scan XPS of NSDPCs and (b–d) high resolution C1s, N1s, and S2p
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XPS scans of NS1/3. XPS N1s, and S2p spectra for the other samples are provided in
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the ESI. (High resolution N1s, and S2p XPS scans of the other four samples were
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showed in Fig. S2 in Supplementary Material.)
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3.2. Electrochemical performances
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The cyclic voltammetry (CV) profile of the obtained NS1/3 electrode in the initial
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three cycles at a scan rate of 0.1 mV s−1 is shown in Fig. 6a. In the first cycle, a
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prominent reduction peak appear at 0.65 V, and almost disappeared in the following
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cycles, indicating decomposition of the electrolyte and the formation of a solid
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electrolyte interface (SEI) film on the electrode surface[33]. The peak near 0 V
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corresponds to the insertion of Li-ion into carbonaceous materials, consistent with that
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of a reported graphene anode[48]. The oxidation peak located at 2.34 V can be
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attribute to the transformation of LixS into polysulfides[49]. In the subsequent cycles,
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the CV curves almost overlapped, implying excellent cycling stability of the electrode.
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Correlative plateau regions can be observed in the charge-discharge profiles (Fig. 6b).
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The charge/discharge curves of the NS1/3 electrode was tested using a constant current
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density of 0.1 A g−1. The first discharge and charge profiles of it showed high specific
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capacities of 1572 and 1188 mA h g−1, respectively, with an initial coulombic
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efficiency of 75.6%. It was much higher than that of the 3D NS-GSs derived from
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poly(vinylpyrrolidone) and (NH4)2S2O8 (CE = 43.8%, 4.6 wt% N, and 1.5 wt% S) [50].
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In the first cycle, the formation of the SEI layer and the irreversible storage of Li-ions
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into special positions in the porous carbon material make the irreversible capacity
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ACCEPTED MANUSCRIPT inevitable[50]. Exactly, the voltage plateau between 0.6 and 0.8 V in the first
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discharge curve can be attributed to electrolyte decomposition and SEI film formation
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on the electrode surface, agreeing with the CV curve. The cycling performances of the
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obtained carbon materials were evaluated at 0.1 A g−1 over a range of 0.01–3.0 V
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versus Li/Li+. Stable reversible capacities were obtained after the first ten cycles. After
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50 cycles, the NS1/3 electrode still maintained a discharge capacity of 864 mA h g−1
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(Fig. 6c), indicating the excellent cycling stability of the material.
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The cycling performances of the electrodes were also investigated at a larger current
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density of 1A g−1 (Fig. 6d). For the NS1/3 electrode, the reversible capacity is stable at
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653 mA h g−1 after 500 cycles, suggesting extraordinarily stable electrochemical
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performance. The coulombic efficiencies of these anodes remained greater than 98%
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after 10 initial cycles at 0.1 and 1 A g−1, indicating their highly reversible nature for
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efficient Li-ion insertion/extraction. To further understand the electrochemical
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performance of the as-prepared NSDPCs, we examined their rate performances. The
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rate capabilities and cycle performances of the NSDPC electrodes at various current
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densities (from 0.1 to 5 A g−1) are shown in Fig. 6e. At current densities of 0.1, 0.2,
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0.5, 1, 2, and 5 A g−1, stable charge capacities of NS1/3 were 991, 864, 743, 656, 582,
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and 463 mA h g−1, respectively. When the current density was restored to 0.2 A g−1
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and 0.1 A g−1, the capacities rapidly recovered to 873 and 917 mA h g−1, respectively.
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This result demonstrates that the as-prepared N/S co-doped carbon has excellent
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potential as a high-rate anode material for LIBs. As shown in Table 1, N doping level
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for these five samples were comparable, indicating that the enhanced capacity is
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ACCEPTED MANUSCRIPT mainly contributed by S doping. Although many studies report that N doping is an
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efficient way to improving the Li storage capability[51, 52], our results show that S
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doping is more efficient for capacity enhancement. The electrochemical performance
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of dual-doped carbon materials with varying sulfur content was further evaluated. As
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shown in Fig. 6c and d, increasing sulfur content from 7.6 to 21.1 wt% result in the
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continuous increase of reversible capacity. NS1/3 (with N and S content of 14.0% and
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21.1 wt%, respectively) exhibited the best lithium storage performance. In order to
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possess an overall understanding of the recent development of N/S co-doped carbon
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anodes for LIBs, the cycling performance and rate capabilities of these anodes have
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been summarized and listed in Tables S6.
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In addition, electrochemical impedance spectroscopy of LIBs was performed in the
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frequency range from 100 kHz to 0.01 Hz (displayed as Nyquist plots). The superior
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cycle performance of NS1/3 can be attributed to its excellent conductivity (confirmed
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by the results shown in Fig. 6f). The Nyquist plots display a semicircular loop in the
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high-frequency region and a sloped line in the low-frequency region. The diameter of
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the semicircular loop corresponds to the resistance of charge transfer (Rct) at the
337
interface in the LIBs. The sloped line in the low-frequency region represents the
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Warburg impedance, which is related to diffusion of Li-ion in the electrode materials.
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The Nyquist plots were analyzed and fitted by an equivalent circuit model (Fig. 6f).
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The electrode made with NS1/3 exhibited a much lower SEI film resistance (RSEI, 32.0
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Ω) and charge transfer resistance (Rct, 7.9 Ω) in comparison with the other electrodes
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ACCEPTED MANUSCRIPT based on equivalent circuit simulation (Table S5). This result suggest that the NS1/3
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electrode has a thinner SEI film, favoring rapid Li+ insertion/extraction and facile
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charge transfer at the electrode/electrolyte interface. Moreover, the synergistic effect
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of N and S co-doping favors NS1/3 with its enhanced electronic conductivity,
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providing an alternative route for electron transfer and guaranteeing continuous and
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rapid electron transport.
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Fig.
Electrochemical
performance
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voltammograms of NS1/3 at a scan rate of 0.1 mV s−1; (b) charge-discharge curves of
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NS1/3 at 0.1 A g−1; cycling performance at a current density of 0.1 A g−1 (c) and 1 A g−1
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(d); (e) capacity over cycling at different current densities; (f) Nyquist plots of
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NSDPC samples after ten initial cycles.
of
NSDPC
electrodes.
(a)
Cyclic
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6.
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The Li storage mechanism of NSDPC materials is as follows. The relatively large
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specific surface area and hierarchical porous structure, with coexisting micropores
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and mesopores, providing a large quantity of sites for Li-ion storage. Moreover, with
ACCEPTED MANUSCRIPT ultrahigh levels of heteroatom doping, abundant defect and enlarged interlayer
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spacing in the carbon matrix facilitate absorption and insertion/extraction of Li-ion. In
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this case, Li-ion can diffuse through defects perpendicular to the interlayer plane, with
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additional sites in the interlayer space for accommodation of Li-ion[50]. The 3D
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porous networks serves as a reservoir for the storage of Li-ion and reduces the
364
diffusion distance for them, while the mechanical stability of the nanosheets ensures
365
superior
366
performance of the NSDPC materials is the result of their novel porous structure,
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appropriate co-doping of heteroatoms, and the proper degree of graphitization. In
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addition to increasing conductivity, the doped N (in the forms of pyridinic-N,
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pyrrolic-N) act as electrophilic atoms due to their higher electronegativity than C
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atoms, causing the nearby C atoms to be polarized and have more electrochemical
371
activity. Furthermore, in addition to increasing the interlayer spacing and defects in
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the carbon matrix, sulfur atoms (in the forms of thiophenic-S species) covalently
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bonded to the pyrolytic carbon can serve as accommodation sites for Li-ion, leading
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to high capacity even at high rates. As a consequence, dual doping have a synergistic
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effect for the Li-ion storage compare to the doping by single atom[34]. Therefore,
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NSDPC materials are able to accept more charge than undoped carbon materials.
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Recent theoretical studies also prove that doped carbon shows better storage of Li-ion
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due to the increased number of defects in the graphene plane, consistent with our
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results[26, 52].
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Conclusions
Therefore,
the
outstanding
SC
performance.
electrochemical
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ACCEPTED MANUSCRIPT In summary, carbon-based materials are attracting extensive attention in the secondary
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battery industry due to their low-cost and facile preparation. We employed a one-step
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N/S co-doping synthesis process to convert cystine aggregates into heteroatom-doped
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interconnected porous carbon nanosheets. The NS1/3 specimen exhibited a large
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reversible lithium storage capacity of 864 mA h g−1 after 50 cycles at 0.1 A g−1,
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excellent cycle performance (653 mA h g−1 for the 500th cycle at 1 A g−1), and a
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superior rate capability (463 mA h g−1 at 5 A g−1). This obtained carbon materials
388
show great promise as an anode material for high-performance LIBs. The super
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electrochemical performance of NSDPC materials originates from their unique
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interconnected porous structure, appropriate pore distribution, and high inherent N
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and S contents, which shorten the diffusion distance of lithium ions and provide a
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large number of lithium storage sites.
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Acknowledgements
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This work was supported by National Natural Science Foundation of China
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(51673117), National High-tech R&D Program (863) (2015AA03A204), the Science
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and Technology Innovation Commission of Shenzhen (JCYJ20140418091413553,
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JCYJ20150625102750478,
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JCYJ20160520163535684, JCYJ20160422144936457), Foundation for Distinguished
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Young Talents in Higher Education of Guangdong, China(2013LYM_0080), Special
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Program for Applied Research on Super Computation of the NSFC-Guangdong Joint
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Fund (the second phase).
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JCYJ20150529164656097, JSGG20160226201833790,
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S0008-6223(17)30983-1
DOI:
10.1016/j.carbon.2017.09.100
Reference:
CARBON 12430
To appear in:
Carbon
Received Date: 27 July 2017 Revised Date:
24 September 2017
Accepted Date: 28 September 2017
Please cite this article as: Z. Qiu, Y. Lin, H. Xin, P. Han, D. Li, B. Yang, P. Li, S. Ullah, H. Fan, C. Zhu, J. Xu, Ultrahigh level nitrogen/sulfur co-doped carbon as high performance anode materials for lithium-ion batteries, Carbon (2017), doi: 10.1016/j.carbon.2017.09.100. 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 proof before it is published in its final 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.
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ACCEPTED MANUSCRIPT
Ultrahigh level nitrogen/sulfur co-doped carbon as high
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performance anode materials for lithium-ion batteries
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Zhaozheng Qiu,a, # Yemao Lin,a, # Hailin Xin,a Pei Han,a Dongzhi Li,a Bo Yang,a Pengchong Li,a
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Shahid Ullah,a Haosen Fan,a Caizhen Zhu,a* and Jian Xub
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* Corresponding author. E-mail address: [email protected] (C. Zhu). #
These authors contributed equally to this work.
ACCEPTED MANUSCRIPT 5
a
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PR China
7
b
8
Sciences, Beijing, 100190, PR China
College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, 518060,
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Beijing National Laboratory for Molecular Sciences Institute of Chemistry, Chinese Academy of
9
Abstract
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Ultrahigh level nitrogen and sulfur co-doped disordered porous carbon (NSDPC) was
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facilely synthesized and applied as anode materials for lithium-ion batteries (LIBs).
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Benefiting from high nitrogen (14.0 wt%) and sulfur (21.1 wt%) doping, electrode
14
fabricated from NS1/3 showed a high reversible capacity of 1188 mA h g−1 at 0.1 A g−1
15
in the first cycle with a high initial columbic efficiency (>75 %). In addition,
16
prolonged life over 500 cycles and excellent rate capability of 463 mA h g−1 at 5 A g−1
17
have been realized. The preeminent electrochemical performance is attributed to three
18
effects: (1) the high level of sulfur and nitrogen; (2) the synergic effect of dual-doping
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heteroatoms in cooperation with each other; (3) the large quantity of edge defects and
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abundant micropores and mesopores that can provide extra Li storage regions. These
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unique features of NSDPC electrodes suggest that they can serve as a practical
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substitute for graphite as a high performance anode material in LIBs.
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Keywords: Electrode materials; Co-doping; Porous carbon; Anode; Lithium
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1. Introduction
ACCEPTED MANUSCRIPT With the rapid development of portable devices, renewable energy harvesting, and
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electric vehicles[1, 2], safe energy storage devices with improved power and energy
28
densities are becoming increasingly important[3]. Among these devices, lithium-ion
29
batteries (LIBs) are promising choices for their high energy density and power density,
30
additionally with long lifespans[4]. However, application of LIBs in large-scale
31
electric energy storage requires further improvements in energy density, rate
32
capability, safety, and electrode durability. To a large extent, these issues rely on
33
several determinants. One of them is the development of novel anode materials.
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Graphite is the main commercial anode material used in currently available LIBs. But
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the low theoretical specific capacity of 372 mA h g−1 cannot meet the increasing
36
demands of rapidly developing markets[5]. Therefore, it’s urgently to develop new
37
anode materials with high theoretical capacities and excellent rate and cycling
38
performance for the next-generation LIBs.
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Various nanostructured products containing Si[6-8], P[9], or Sn[10-12], as well as
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alloy[13, 14] and some transition metal oxides/sulfide[15-23], have been reported as
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high capacity anode materials for LIBs. Unfortunately, intrinsic problems such as
42
huge volume expansion, relatively low conductivity, and large voltage hysteresis,
43
exist in these materials during the lithiation process[24]. These deficiencies severely
44
limit their commercialization in LIBs. Therefore, novel carbon-based anode materials,
45
with enhanced electrochemical performance for lithium storage, are still a major focus
46
of research worldwide[25]. Applied them as anode materials, high energy LIBs with
47
elevated specific capacity, excellent cycling stability and rate performance have been
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ACCEPTED MANUSCRIPT realized. Significant efforts have been made to design novel nanostructures of
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carbonaceous materials with larger specific capacity for lithium storage [25-27].
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However, for amorphous carbon materials, although activating agents (such as KOH)
51
has been applied to increase the specific surface area, inferior stability and poor rate
52
performance still occur due to the low degree of graphitization[28]. Therefore, a novel
53
strategy of artificial doping with heteroatoms, such as N[26], B[29] or S[30], have
54
being proposed. With this method, the electronic properties and electrochemical
55
activity of amorphous carbon materials can be effectively improved, leading to
56
excellent Li ion storage capacity. Nevertheless, most reported results related to this
57
field are based on the single element doping but co-doping and their synergistic
58
influence to the electrochemical property is less studied.
59
Although nitrogen doped carbon-based materials have been extensively studied, sulfur
60
and especially nitrogen/sulfur dual-doping are far less exhaustive[31]. When doped
61
with nitrogen the electronic properties of carbon-based materials can be improved[32].
62
As a complement to nitrogen, sulfur doping has attracted increasing attention in recent
63
carbon materials research. While sulfur has a larger atomic radius, it can enlarged the
64
interlayer spacing(d002) of the carbon matrix[31]. The doped sulfur substantially
65
increases the charge capacity with the enlarged graphite crystallite size (Lc) and
66
creates more micropores, improving the electrochemical properties of the carbon[30].
67
Meanwhile, its easily polarizable lone pairs can change the charge state of
68
neighboring carbon atoms. Hence, it can be used to tune chemical reactivity and
69
catalytic activity of the carbon materials. Therefore, as a doping heteroatom, sulfur
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capacity in LIBs[33]. Importantly, when both highly interactive elements of nitrogen
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and sulfur are simultaneously doped into the carbon matrix, synergistic effects can be
73
aroused[34]. In addition, it has been reported that the binding state of sulfur and
74
nitrogen in the carbon matrix can be tuned when different sulfur and nitrogen sources
75
were used[31]. Most studies property on sulfur/nitrogen dual-doped carbon materials
76
is about their enhanced electrocatalyst ability[31, 35]. Few reports focus on
77
dual-doped carbons for LIBs. Therefore, we were inspired to investigate the favorable
78
influences of dual-doped heteroatoms in LIBs systems.
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Herein, we applied a “dual-doping” strategy to synthesize an amorphous porous
80
carbon with nitrogen and sulfur at an ultrahigh doping level. In this way, the
81
synergistic effects of heteroatom co-doping and a disordered porous structure has
82
been achieved. The nitrogen and sulfur co-doped disordered porous carbon (NSDPC)
83
was employed as a preeminent anode material for high-performance LIBs. By a facile
84
synthesis method, an amorphous porous structure was formed. In the sample of NS1/3,
85
micropores and mesopores were homogenously embedded with a nitrogen content of
86
14.0 wt% and a sulfur content of 21.1 wt%. Benefitting from the convenient transport
87
pathway for Li-ions and electron, as well as the abundant pores for extra Li storage in
88
the carbon framework, the NS1/3 electrode exhibits excellent rate performance with
89
superior cycling stability. The reversible specific capacity achieved as high as 1188
90
mA h g−1 at a current density of 0.1 A g−1, and even after increasing the rate to 5 A g−1
91
high capacity of 463 mA h g−1 can still be obtained, the capacity still remains at 653
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93
2. Experimental Section
94
2.1. Synthesis of cystine aggregates
95
10 mM L-Cysteine was dissolved in 100 mL ultrapure water (18.0 MΩ cm−1) under
96
ultrasonication for about 10 min. The pH value of the L-Cysteine aqueous solution
97
was rapidly adjusted to 8.0 using Na2CO3 aqueous solution. 5 mL H2O2 solution (30
98
wt%) was added drop by drop slowly to the L-Cysteine solution with a syringe (10
99
mL). The whole reaction process was kept at room temperature with stirring. After 10
100
min, the resulting solution was incubated at room temperature without interruption for
101
24 h. The resulting precipitation was centrifuged and washed several times with water
102
and ethanol and finally the white powder of cystine aggregates were obtained after
103
dried under oven at 60 °C for 24 h.
104
2.2. Preparation of NSDPC
105
In order to adjust the elemental composition of the obtained NSHPC, varied
106
concentrations of melamine and sulfur were added to a constant amount of cystine
107
aggregates harvested in front. The nitrogen/sulfur co-doped carbon was synthesized
108
by the following synthetic route. In a typical experiment: 8.0 g of cystine aggregates
109
and x g (x = 3.0, 2.4, 2.0, 1.6, or 1.0) of melamine and y g (y = 1.0, 1.6, 2.0, 2.4, or 3
110
corresponding to x) of sulfur were mixed thoroughly. Followed by annealing at
111
500 °C for 2 hours in the atmosphere of Ar with a heating ramp of 2 °C min−1 and the
112
obtained sample were labeled according to the following scheme: NSX/Y where X and
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Y denotes the integer proportion of x and y. NS3/1 hence corresponds to an experiment
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ACCEPTED MANUSCRIPT in which 3.0 g of melamine and 1.0 g sulfur were added to 8.0 g of cystine aggregates.
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2.3. Material characterization
116
The powder X-ray diffraction (XRD) patterns of all samples were recorded with an
117
X-ray diffractometer (D8 Advance of Bruker, Germany) with filtered Cu Kα radiation
118
over the 2θ range of 10-60°. Field emission scanning electron microscopy (FE-SEM)
119
images were collected on a JSM-7800F scanning electron microscope. The amounts
120
of doped nitrogen and sulfur in the synthesized materials were determined by a
121
CHNOS Elemental Analyzer (vario EL cube, Germany). Transmission electron
122
microscopy (TEM) images were taken on a JEM-2100 transmission electron
123
microscope using an accelerating voltage of 200 kV, and high-resolution transmission
124
electron microscope (HRTEM) (JEOL-2011) was operated at an acceleration voltage
125
of 200 kV. The specific surface area was evaluated at 77 K (Quantachrome
126
NOVA1200e) using the Brunauer-Emmett-Teller (BET) method, while the pore
127
volume and pore size were calculated according to the Barrett-Joyner-Halenda (BJH)
128
formula applied to the adsorption branch. Thermogravimetric analysis (TGA) was
129
carried out using a STA409PC from 0 °C to 800 °C at a heating rate of 10 °C min−1
130
under Ar. X-ray Photoelectron Spectrum (XPS) was performed on an ESCALAB 250
131
X-ray Photoelectron Spectrometer with Al Ka radiation. Raman spectra were obtained
132
using a Digilab FTS3500 from Bio-Rad with a laser wavelength of 632.8 nm.
133
2.4. Electrochemical measurements
134
The electrochemical performance of the NSDPC was examined by using CR 2032
135
coin-type cells which assembled in an argon-filled glovebox, using lithium foils as the
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ACCEPTED MANUSCRIPT counter electrodes, Celgard 2500 membrane as the separator, 1 M LiPF6 in a 1:1 (v/v)
137
mixture of ethylene carbonate and dimethyl carbonate as the electrolyte. To prepare a
138
working electrode, active materials (NSDPCs, 70 wt%), conductive material
139
(acetylene black, 20 wt%), and binder (Sodium Alginate (SA), 10 wt%) were milled
140
in ultrapure water to form slurries and then coated onto the surface of a copper foil
141
current collector. The electrode capacity was measured by a galvanostatic discharge–
142
charge method in the voltage range between 0.01 and 3.0 V on a battery test system
143
(Land CT2001A, China). Cyclic voltammetry (CV) from 0.01 to 3.0 V (vs. Li/Li+) at
144
0.1 mV s−1 and electrochemical impedance spectroscopy (EIS) in the frequency range
145
100 MHz to 0.01 Hz and with an amplitude of 5 mV were performed using an
146
electrochemical workstation (CHI660A). All electrochemical measurements were
147
carried out at room temperature.
148
3. Results and Discussion
149
3.1. Synthesis and characterizations of the NSDPC
150
It has been demonstrated that H2O2 can oxidize the –SH group of cysteine to form
151
disulfide bonds and lead to the formation of cystine[36]. The development of cystine
152
aggregates is a result of intermolecular interactions, mainly hydrogen bonding and
153
electrostatic interactions[37]. The morphology of the obtained cystine aggregates was
154
thin slice shape hexagonal crystal (Fig. 1S). Fig. 1 shows the illustration of the
155
synthesis of NSDPC. NSDPC was synthesized facilely by annealing the mixture of
156
as-prepared cystine aggregates, melamine, and sulfur in a flowing atmosphere of Ar at
157
500 °C for 2 h. The cystine aggregates with N content of 11.6 wt% and S content of
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ACCEPTED MANUSCRIPT 26.4% were selected as the substrate material. Melamine with a high N content (66.7
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wt%) and sulfur were used as excess N and S sources, respectively.
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Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of the
161
mixture of as-prepared cystine aggregates, melamine, and sulfur (mass ratio of 8: 1: 3)
162
were performed in an Ar atmosphere. It indicated that the carbon sulfurization
163
temperature was approximately 300 °C (Fig. 2a). When the temperature was increased
164
to 500 °C, the carbon sulfurization and carbonization of the as-prepared cystine
165
aggregates and melamine were accomplished, and the excess sulfur was evaporated
166
off.
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Fig. 1. The schematic of the synthesis of NSDPC composites
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The results obtained from XRD measurements are summarized in Fig. 2b and Table 1.
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Samples of NS3/1, NS3/2, NS3/3, NS2/3, and NS1/3 exhibited broad peaks at around 23.5°
172
and weak peaks at 43°, which were readily indexed to the representative (002) and
173
(101) planes of amorphous carbon, respectively. According to Bragg’s equation, the
174
interlayer spacing (d002) of the five samples was calculated to be approximately 0.38
175
nm. The larger d002 values of the NSDPC materials in comparison with that of
176
graphite (0.335 nm) implied that the intercalation of sulfur result in an enlarged
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interlayer distance of porous carbon, facilitating the diffusion and insertion/extraction
178
of Li+ in carbon matrix[38]. Table 1. Summary of the d-spacing of the NSDPC samples
180 181 182
NS3/1
NS3/2
NS3/3
NS2/3
NS1/3
23.29
23.66
23.35
23.23
23.66
d002(nm) 0.381
0.376
0.380
0.382
2θ (°)
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0.376
To further examine the structural characteristic, the five samples were measured using
185
N2 adsorption–desorption isotherms. As displayed in Fig. 2c and Table S1 (in
186
Supplementary Material), the specific Brunauer-Emmett-Teller (BET) surface area of
187
the NS3/1 is 34.6 m2 g−1. Whereas the specific surface areas of the NS3/2, NS3/3 and
188
NS2/3 are increased significantly to 42.8, 129.2 and 249.6 m2 g−1, respectively. As for
189
the sample of NS1/3, it reaches the maximum of 341.0 m2 g−1 at the ratio of
190
melamine(X)/sulfur(Y) reach 1: 3. Moreover, NS1/3 possesses the highest total pore
191
volume, being 0.474 cm3 g−1 (Table S1). This demonstrates that the effectiveness of a
192
suitable ratio of melamine and sulfur in developing more pores in NSDPC skeleton.
193
The NSDPC samples showed mesopores are dominant in the samples, which covered
194
a range between 2∼10 nm (Fig. 2d).
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Fig. 2. (a) TG and DTA curves of the mixture of cystine aggregates, melamine, and
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sulfur with a mass ratio of 8: 1: 3 in the temperature range of 20–800 °C with a
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heating rate of 10 °C min−1 in an Ar atmosphere; (b) XRD patterns of NSDPC
199
samples over the 2θ range of 10–60°; (c) N2 adsorption–desorption isotherm and (d)
200
BJH pore-size distribution of NSDPC samples.
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The morphologies of the as-prepared NSDPC samples were characterized by
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field-emission scanning electron microscopy (FESEM) as shown in Fig. 3a–f. The
204
SEM images of the resulting carbon materials exhibited flake-like morphology. Many
205
interconnected pores existed in the carbon sheets, especially for the NS1/3 sample.
206
High magnification observation of the sheets revealed that the sheets had numerous
207
nanoscale pores within them. This 3D structure has many interpenetrative channels
208
with large internal spaces, which are mainly formed by the volatilization of excess
209
sulfur, H2O, CO2, H2S, NH3, and CO during the carbonization process[38]. The novel
210
structure in this carbon framework ensures facile access of electrolytes, efficient
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ACCEPTED MANUSCRIPT transfer of lithium ions and supply additional lithium storage space, fascinating rapid
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charging/discharging of the electrode. The HRTEM image (Fig. 3g) reveals the rough
213
surface of the amorphous carbon with nanopores. TEM images further confirm the
214
interconnected porous structure. TEM elemental mapping of the NSDPCs shown in
215
Fig. 3g indicates that the distribution of C, N, and S is very homogenous.
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Fig. 3. SEM images of NSDPC samples: (a) NS3/1, (b) NS3/2, (c) NS3/3, (d) NS2/3, and
219
(e, f) NS1/3; (g) HRTEM images and TEM-EDS elemental mapping of NS1/3.
220 221
Elemental analysis (EA) was employed to quantify the N and S doping levels in
222
carbon bulk. The results demonstrated that the contents of N and S in the NSDPC
223
materials could be tailored by varying the ratio of melamine and sulfur (Table 2).
ACCEPTED MANUSCRIPT Pristine NSDPC materials derived from cystine aggregates showed a substantial
225
presence of nitrogen and sulfur (10.2 wt% and 7.6 wt%, respectively). N and S
226
concentration increased drastically with additional melamine and sulfur. For example,
227
the N and S levels reached to 16.3 wt% (N) and 13.1 wt% (S) with the ratio of
228
melamine and sulfur to be 3: 1. When the ratio was changed to 1: 3, the content of N
229
decreased to 14.0 wt%, while the content of S increased to 21.1 wt%.
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Table 2. Elemental composition of the NSDPC samples Sample C (%) S (%) N (%) O (%) H (%) S/N NS 61.1 7.6 10.2 18.8 2.3 0.75 NS3/1 57.8 13.1 16.3 10.5 2.2 0.80 NS3/2 59.0 13.2 15.6 9.9 2.2 0.85 59.7 14.4 13.4 10.3 2.3 1.07 NS3/3 NS2/3 56.8 18.0 13.5 9.7 2.1 1.33 54.1 21.1 14.0 9.0 1.9 1.51 NS1/3
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Raman spectroscopy was carried out to examine the degree of graphitization and
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imperfections in the obtained carbon samples (Fig. 4a–f). Two characteristic peaks at
235
approximately 1403 and 1560 cm−1 correspond to the D and G peaks, respectively.
236
The D peak indicate to structure defects and disordered structure in the graphite layer
237
due to the co-doping of nitrogen and sulfur. While the G peak is related to the sp2 C–
238
C bonds of perfect graphite layer[27]. The intensity ratio of the G to D band (ID/IG) is
239
generally employed to evaluate the extent of structural disorder for the carbon
240
materials[39]. The ID/IG ratios for NS3/1, NS3/2, NS3/3, NS2/3, and NS1/3 were 1.322,
241
1.393, 1.440, 1.477, and 1.558, respectively. Obviously, an increasing trend in the
242
ID/IG ratio was observed after more S was doped. The NS1/3 sample showed the
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244
disorder in the carbon framework. The results agree with the previous report on
245
sulfur-doped porous carbon[40]. The disordered structure and numerous defects are
246
supposed to be benefit to the insertion/extraction and additional storage of Li ions[41].
247
As shown in Fig. 4f, G band has a trend of shifted down as the ratio of N and S
248
precursor adjust from 3: 1 to 1: 3, which can be attributed to the recovery of the
249
conjugated structure[42] and the electron donation of heteroatoms[43].
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Fig. 4. Raman spectra at the D (disordered) band and G (graphitic) band of (a) NS3/1,
252
(b) NS3/2, (c) NS3/3, (d) NS2/3, and (e) NS1/3; (f) spectra of the NSDPC samples.
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X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical status
255
of the elements and the surface chemistry of the NSDPCs, as shown in Fig. 5. The
256
XPS survey spectrum shows peaks for C1s (284.6 eV), N1s (400.5 eV), O1s (531.5
257
eV), and S2p (164.5 eV) (Fig. 5a), confirming the existence of elements detected by
258
EA and the elemental mapping results (Fig. 3g). The N and S concentrations (at%) on
259
the surface of each NSDPC sample are shown in Table S2(in Supplementary Material).
ACCEPTED MANUSCRIPT The sulfur content of 8.1 at% (equivalent to 18.4 wt% in Table S3) in NS1/3 calculate
261
from the XPS data was generally in concordance with the EA analysis result (21.1
262
wt%) (Table 2), indicating uniform distribution of heteroatoms in the bulk materials.
263
The high-resolution C1s spectrum was fitted by two sub-peaks, including C–C (ca.
264
284.0 eV), and C–S/C–N (ca. 285.5 eV) (Fig. 5b) [26, 27, 44]. The deconvoluted XPS
265
spectrum of N1s contains two characteristic sub-peaks at 397.8, and 399.6 eV (Fig.
266
5c), corresponding to pyridinic-type, and pyrrolic-type, respectively (Table S4) [45].
267
The pyridinic and pyrrolic nitrogen species can improve the conductivity in the
268
carbon matrix, thanks to their p-electron pair that donate to the π-conjugated system in
269
the graphene layers[46, 47]. The high resolution S2p spectrum is shown in Fig. 5d,
270
which exhibits three sub-peaks at binding energies of 163.0, 164.0, and 167.8 eV
271
(Table S4). The former two can be attributed to the S2p3/2 and S2p1/2 of the –C–Sx–C–
272
(x =1–2) covalent bond of the thiophene-S, while the weak peak at 167.8 eV
273
corresponds to C–SOx–C (x = 2–4) groups[38].
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274 275
Fig. 5. (a) Full-scan XPS of NSDPCs and (b–d) high resolution C1s, N1s, and S2p
ACCEPTED MANUSCRIPT 276
XPS scans of NS1/3. XPS N1s, and S2p spectra for the other samples are provided in
277
the ESI. (High resolution N1s, and S2p XPS scans of the other four samples were
278
showed in Fig. S2 in Supplementary Material.)
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3.2. Electrochemical performances
281
The cyclic voltammetry (CV) profile of the obtained NS1/3 electrode in the initial
282
three cycles at a scan rate of 0.1 mV s−1 is shown in Fig. 6a. In the first cycle, a
283
prominent reduction peak appear at 0.65 V, and almost disappeared in the following
284
cycles, indicating decomposition of the electrolyte and the formation of a solid
285
electrolyte interface (SEI) film on the electrode surface[33]. The peak near 0 V
286
corresponds to the insertion of Li-ion into carbonaceous materials, consistent with that
287
of a reported graphene anode[48]. The oxidation peak located at 2.34 V can be
288
attribute to the transformation of LixS into polysulfides[49]. In the subsequent cycles,
289
the CV curves almost overlapped, implying excellent cycling stability of the electrode.
290
Correlative plateau regions can be observed in the charge-discharge profiles (Fig. 6b).
291
The charge/discharge curves of the NS1/3 electrode was tested using a constant current
292
density of 0.1 A g−1. The first discharge and charge profiles of it showed high specific
293
capacities of 1572 and 1188 mA h g−1, respectively, with an initial coulombic
294
efficiency of 75.6%. It was much higher than that of the 3D NS-GSs derived from
295
poly(vinylpyrrolidone) and (NH4)2S2O8 (CE = 43.8%, 4.6 wt% N, and 1.5 wt% S) [50].
296
In the first cycle, the formation of the SEI layer and the irreversible storage of Li-ions
297
into special positions in the porous carbon material make the irreversible capacity
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299
discharge curve can be attributed to electrolyte decomposition and SEI film formation
300
on the electrode surface, agreeing with the CV curve. The cycling performances of the
301
obtained carbon materials were evaluated at 0.1 A g−1 over a range of 0.01–3.0 V
302
versus Li/Li+. Stable reversible capacities were obtained after the first ten cycles. After
303
50 cycles, the NS1/3 electrode still maintained a discharge capacity of 864 mA h g−1
304
(Fig. 6c), indicating the excellent cycling stability of the material.
305
The cycling performances of the electrodes were also investigated at a larger current
306
density of 1A g−1 (Fig. 6d). For the NS1/3 electrode, the reversible capacity is stable at
307
653 mA h g−1 after 500 cycles, suggesting extraordinarily stable electrochemical
308
performance. The coulombic efficiencies of these anodes remained greater than 98%
309
after 10 initial cycles at 0.1 and 1 A g−1, indicating their highly reversible nature for
310
efficient Li-ion insertion/extraction. To further understand the electrochemical
311
performance of the as-prepared NSDPCs, we examined their rate performances. The
312
rate capabilities and cycle performances of the NSDPC electrodes at various current
313
densities (from 0.1 to 5 A g−1) are shown in Fig. 6e. At current densities of 0.1, 0.2,
314
0.5, 1, 2, and 5 A g−1, stable charge capacities of NS1/3 were 991, 864, 743, 656, 582,
315
and 463 mA h g−1, respectively. When the current density was restored to 0.2 A g−1
316
and 0.1 A g−1, the capacities rapidly recovered to 873 and 917 mA h g−1, respectively.
317
This result demonstrates that the as-prepared N/S co-doped carbon has excellent
318
potential as a high-rate anode material for LIBs. As shown in Table 1, N doping level
319
for these five samples were comparable, indicating that the enhanced capacity is
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321
efficient way to improving the Li storage capability[51, 52], our results show that S
322
doping is more efficient for capacity enhancement. The electrochemical performance
323
of dual-doped carbon materials with varying sulfur content was further evaluated. As
324
shown in Fig. 6c and d, increasing sulfur content from 7.6 to 21.1 wt% result in the
325
continuous increase of reversible capacity. NS1/3 (with N and S content of 14.0% and
326
21.1 wt%, respectively) exhibited the best lithium storage performance. In order to
327
possess an overall understanding of the recent development of N/S co-doped carbon
328
anodes for LIBs, the cycling performance and rate capabilities of these anodes have
329
been summarized and listed in Tables S6.
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In addition, electrochemical impedance spectroscopy of LIBs was performed in the
332
frequency range from 100 kHz to 0.01 Hz (displayed as Nyquist plots). The superior
333
cycle performance of NS1/3 can be attributed to its excellent conductivity (confirmed
334
by the results shown in Fig. 6f). The Nyquist plots display a semicircular loop in the
335
high-frequency region and a sloped line in the low-frequency region. The diameter of
336
the semicircular loop corresponds to the resistance of charge transfer (Rct) at the
337
interface in the LIBs. The sloped line in the low-frequency region represents the
338
Warburg impedance, which is related to diffusion of Li-ion in the electrode materials.
339
The Nyquist plots were analyzed and fitted by an equivalent circuit model (Fig. 6f).
340
The electrode made with NS1/3 exhibited a much lower SEI film resistance (RSEI, 32.0
341
Ω) and charge transfer resistance (Rct, 7.9 Ω) in comparison with the other electrodes
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ACCEPTED MANUSCRIPT based on equivalent circuit simulation (Table S5). This result suggest that the NS1/3
343
electrode has a thinner SEI film, favoring rapid Li+ insertion/extraction and facile
344
charge transfer at the electrode/electrolyte interface. Moreover, the synergistic effect
345
of N and S co-doping favors NS1/3 with its enhanced electronic conductivity,
346
providing an alternative route for electron transfer and guaranteeing continuous and
347
rapid electron transport.
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349
Fig.
Electrochemical
performance
351
voltammograms of NS1/3 at a scan rate of 0.1 mV s−1; (b) charge-discharge curves of
352
NS1/3 at 0.1 A g−1; cycling performance at a current density of 0.1 A g−1 (c) and 1 A g−1
353
(d); (e) capacity over cycling at different current densities; (f) Nyquist plots of
354
NSDPC samples after ten initial cycles.
of
NSDPC
electrodes.
(a)
Cyclic
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The Li storage mechanism of NSDPC materials is as follows. The relatively large
357
specific surface area and hierarchical porous structure, with coexisting micropores
358
and mesopores, providing a large quantity of sites for Li-ion storage. Moreover, with
ACCEPTED MANUSCRIPT ultrahigh levels of heteroatom doping, abundant defect and enlarged interlayer
360
spacing in the carbon matrix facilitate absorption and insertion/extraction of Li-ion. In
361
this case, Li-ion can diffuse through defects perpendicular to the interlayer plane, with
362
additional sites in the interlayer space for accommodation of Li-ion[50]. The 3D
363
porous networks serves as a reservoir for the storage of Li-ion and reduces the
364
diffusion distance for them, while the mechanical stability of the nanosheets ensures
365
superior
366
performance of the NSDPC materials is the result of their novel porous structure,
367
appropriate co-doping of heteroatoms, and the proper degree of graphitization. In
368
addition to increasing conductivity, the doped N (in the forms of pyridinic-N,
369
pyrrolic-N) act as electrophilic atoms due to their higher electronegativity than C
370
atoms, causing the nearby C atoms to be polarized and have more electrochemical
371
activity. Furthermore, in addition to increasing the interlayer spacing and defects in
372
the carbon matrix, sulfur atoms (in the forms of thiophenic-S species) covalently
373
bonded to the pyrolytic carbon can serve as accommodation sites for Li-ion, leading
374
to high capacity even at high rates. As a consequence, dual doping have a synergistic
375
effect for the Li-ion storage compare to the doping by single atom[34]. Therefore,
376
NSDPC materials are able to accept more charge than undoped carbon materials.
377
Recent theoretical studies also prove that doped carbon shows better storage of Li-ion
378
due to the increased number of defects in the graphene plane, consistent with our
379
results[26, 52].
380
Conclusions
Therefore,
the
outstanding
SC
performance.
electrochemical
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ACCEPTED MANUSCRIPT In summary, carbon-based materials are attracting extensive attention in the secondary
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battery industry due to their low-cost and facile preparation. We employed a one-step
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N/S co-doping synthesis process to convert cystine aggregates into heteroatom-doped
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interconnected porous carbon nanosheets. The NS1/3 specimen exhibited a large
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reversible lithium storage capacity of 864 mA h g−1 after 50 cycles at 0.1 A g−1,
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excellent cycle performance (653 mA h g−1 for the 500th cycle at 1 A g−1), and a
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superior rate capability (463 mA h g−1 at 5 A g−1). This obtained carbon materials
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show great promise as an anode material for high-performance LIBs. The super
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electrochemical performance of NSDPC materials originates from their unique
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interconnected porous structure, appropriate pore distribution, and high inherent N
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and S contents, which shorten the diffusion distance of lithium ions and provide a
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large number of lithium storage sites.
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Acknowledgements
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This work was supported by National Natural Science Foundation of China
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(51673117), National High-tech R&D Program (863) (2015AA03A204), the Science
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and Technology Innovation Commission of Shenzhen (JCYJ20140418091413553,
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JCYJ20150625102750478,
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JCYJ20160520163535684, JCYJ20160422144936457), Foundation for Distinguished
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Young Talents in Higher Education of Guangdong, China(2013LYM_0080), Special
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Program for Applied Research on Super Computation of the NSFC-Guangdong Joint
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Fund (the second phase).
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JCYJ20150529164656097, JSGG20160226201833790,
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