Scalable Salt-Templated Synthesis of Nitrogen-Doped Graphene

Scalable Salt-Templated Synthesis of Nitrogen-Doped Graphene Nanosheets toward Printable Energy Storage Downloaded via BUFFALO STATE on July 18, 2019 at 08:30:57 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Nan Wei,†,‡,⊥ Lianghao Yu,†,⊥ Zhongti Sun,†,⊥ Yingze Song,†,⊥ Menglei Wang,† Zhengnan Tian,† Yu Xia,§ Jingsheng Cai,† Ya-yun Li,*,§ Liang Zhao,† Qiucheng Li,† Mark H. Rümmeli,† Jingyu Sun,*,†,‡ and Zhongfan Liu*,†,‡,∥ †

College of Energy, Soochow Institute for Energy and Materials Innovations (SIEMIS), Jiangsu Provincial Key Laboratory for Advanced Carbon Materials and Wearable Energy Technologies, Soochow University, Suzhou 215006, P.R. China ‡ Beijing Graphene Institute (BGI), Beijing 100095, P.R. China § Shenzhen Key Laboratory of Special Functional Materials & Shenzhen Engineering Laboratory for Advance Technology of Ceramics, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, P.R. China ∥ Center for Nanochemistry (CNC), College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P.R. China S Supporting Information *

ABSTRACT: Mass production of graphene powders affording high quality and environmental benignancy serves as a prerequisite for the practical usage of graphene in multiple energy storage applications. Herein, we exploit a salt-templated CVD approach to harness the direct synthesis of nitrogen-doped graphene (NG) nanosheets and related ink dispersions in a scalable, safe, efficient, and green fashion. Thus-fabricated NG accompanying large productivity, excellent electrical conductivity, and favorable solution processability possesses implications in printable energy storage devices. With the NG-based ink in hand, self-standing 3D architectures with programmable patterns can be directly printed over a myriad of substrates. Accordingly, both electrode preparation for flexible supercapacitors and separator modification in Li−S batteries can be enabled via printing by employing our NG-based composite inks. This work thus represents a practical route for mass production of graphene inks with cost-effectiveness and eco-friendliness for emerging energy storage technology. KEYWORDS: salt-templated CVD, nitrogen-doped graphene, printing, green, energy storage

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Nevertheless, these graphitic products in general contain plenty of structural defects and chemical impurities and possess nonuniform layer thickness, thereby making it challenging to prepare homogeneous graphene ink dispersions. Moreover, the production processes typically involve the use of large quantities of concentrated H2SO4 and KMnO4 to ensure sufficient oxidation as well as employ excessive organic solvents to aid the exfoliation and dispersion for hundreds of hours, which is environmentally unfriendly and time-consuming with safety concerns.11−13 Although state-of-the-art syntheses have been updated in terms of large-scale and good-quality production with the aid environmentally friendly solvents,14−19

raphene has by far ignited extensive interest worldwide in both fundamental and technological realms owing to the fascinating physical, chemical and electrical properties.1−3 To enable its versatile potentials in practical scenarios, mass production of graphene with high quality and low cost serves as a key. In general, there are two forms of graphene (powders and films) developed from scalable synthesis, which would guide different application sectors.4−8 In this context, the quest for fulfilling a plethora of energy-storage applications demands rational production of graphene powders, rather than films. The prevailing graphitic powders are normally generated from the graphite precursor by virtue of wet chemistry routes, mainly encompassing reduced graphene oxide (RGO) and liquid-phase exfoliated graphite (LPEG).9,10 Their production capability has reached up to a level of tens of kilotons per year with relatively low cost. © XXXX American Chemical Society

Received: April 24, 2019 Accepted: May 31, 2019 Published: May 31, 2019 A

DOI: 10.1021/acsnano.9b03157 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. Salt-templated CVD synthesis of NG powders. (a) Schematic illustration of salt-templated CVD growth of NG directly on NaCl crystal. (b) Raman spectra of NaCl@NG, RGO, and GO powders. (c) Comparison of the as-grown NG powders with graphene powders derived by prevailing routes. Each type has been evaluated with regard to environmental friendliness (E), quality (Q), cost (C, a low value related to high cost of production), scalability (S), and purity (P). (d) Thus-produced NG aqueous suspensions (0.2 mg mL−1). Inset: Photograph of NG aqueous dispersion showing conspicuous Tyndall effect.

(PECVD) enables the direct growth of NG on a NaCl crystal at relatively low temperatures (600−700 °C) in a controllable fashion, hence creating a fast process and lowering the production cost.30,31 Benefiting from these merits, the obtained NG accompanying electrical conductivity and solution processability has implications in printable energy storage devices, such as flexible supercapacitors and Li−S batteries.

an alternative fabrication route targeting uniform graphene nanosheets deserves further exploration. An emerging technology to produce high-quality graphene powders relies on the chemical vapor deposition (CVD) formation of graphene over three-dimensional (3D) particulated substrates and postremoval of growth templates.20−23 With the aid of delicate manipulation of CVD parameters, thus-produced graphene powders can be highly crystallized with atomic layer-thickness controllability, structural designability, and fewer noncarbon impurities. Recently, biotemplated growth of graphene was demonstrated by us to ensure the synthesis of graphene powders exploiting CVD techniques.24−26 These graphene architectures even manifested favorable solution processability to be compatible with advanced printing techniques. However, the synthesis inevitably requires a high-temperature procedure, and the templates need to be etched away in a tedious manner by using a large amount of hazardous HF solution, concurrently increasing the environmental pollution and declining the product purity. Along this line, it is imperative to pinpoint a green growth substrate, which could be easily removed and regenerated so that the CVD growth would target lower production costs and afford a positive environmental impact. In this contribution, we report a scalable, safe, efficient, and green route to synthesize nitrogen-doped graphene (NG) nanosheets and related ink dispersions by salt-templated CVD, and study the conformal growth of NG over a NaCl crystal and the readily derived graphene ink via simply applying water. Such a salt-templated CVD process exhibits two key advantages: (i) Green and scalable: NaCl is a naturally abundant crystal with low cost,27−29 thereby enabling the mass production of NG. The water-soluble nature of NaCl renders the facile removal of salt templates after synthesis and direct formation of graphene ink dispersions. (ii) Low-temperature production: The employment of plasma-enahnced CVD

RESULTS AND DISCUSSION Figure 1a schematically illustrates the fabrication process of NG nanosheets by salt-templated CVD. Commercially available NaCl powder was selected as the growth substrate because of its high abundance, low cost, and cubic crystal morphology. Prior to growth, recrystallization of purchased salt was carried out to reduce the crystal sizes (from 250 μm down to 5 μm in average) for boosting the uniform graphene coating (Figure S1, Supporting Information). A PECVD approach was accordingly employed as a reliable route for the lowtemperature (600−700 °C) synthesis of graphene directly on the NaCl crystal, owing to its capability of generating active species in plasma.32−34 To envisage batch production, synthesis was performed in a 4-in. PECVD apparatus with pyridine, which served as both nitrogen and carbon source (Figure S2). During the growth, pyridine precursor is split into CHx and N fragments with the presence of plasma, which accordingly nucleate and diffuse on the surface of NaCl to result in the conformal NG caging (Figure S3). Upon the PECVD reaction, the powder color turns from original white to uniform black to the naked eye, especially for the case of recrystallized sample (Figure S4). Pure NG powders with hollow cubic cages can be readily obtained by simply removing the salt cores of NaCl@NG throughout applying water. Note that such process is facile, green, and ecofriendly, without the use of any harmful reagents (e.g., HF, HCl, or NaOH). Twodimensional NG nanosheets were finally generated with the aid B


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Figure 2. Structural and elemental characterizations of NG. (a) SEM image of an individual NaCl@NG powder. (b) TEM image of NG cage. (c) STEM image and elemental maps of NG cage. (d, e) TEM and HRTEM images of as-obtained NG nanosheet, respectively. (f, g) XPS C 1s and N 1s spectra of NG, respectively. (h, i) AFM image of NG nanosheet on a SiO2 substrate (h) and corresponding height histogram (i). Scale bars: a, d, h, 1 μm; b, 200 nm; c, 100 nm; e, 5 nm.

To highlight the advantages of salt-templated CVD-derived NG powders for energy applications, a systematic comparison on product features between graphene powders fabricated via prevailing routes is drawn (Figure 1c). This includes RGO, LPEG, CVD-grown graphene on silica, and CVD grown NG on NaCl (this work). Each types of graphene powders has been evaluated with regard to environmental friendliness (E), quality (Q), cost aspect (C), scalability (S), and purity (P). Encouragingly, our salt-templated CVD executed at low temperatures marks a green, scalable, and reliable production of graphene powders with relatively high quality and low cost. For instance, this NG product manifests favorable electrical conductivity, which was evaluated by measuring the sheet resistance (Rs) values of the filtrated films by a four-point probe method. In this respect, NG displays a markedly lower Rs value of 29.5 Ω sq−1 (Figure S9), in stark contrast to that of commercially available RGO (4.9 kΩ sq−1). Figure 1d depicts thus-produced printable NG suspensions (0.2 mg mL−1). Note that such a homogeneous dispersion of NG nanosheets ensures the observation of a conspicuous Tyndall effect (Figure 1d inset). Exhaustive electron miscoscopy characterizations were performed to examine the detailed morphologies of samples experiencing CVD reaction. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM)

of ultrasonic treatment, affording the mass production of NG aqueous solution. Raman spectroscopy was employed to inspect the quality of NG directly grown on a NaCl crystal (Figure 1b; Figure S5). Thus-produced products exhibit featured signals of graphene at ∼1349 cm−1 (D band), ∼1590 cm−1 (G band), and ∼2694 cm−1 (2D band). It is evident that a markedly decreased intensity ratio (ID/IG) of the D band to the G band is observed for NaCl@NG powders as compared to those of graphite oxide (GO) and RGO, indicative of less defect density in the atomic structures of NG derived by CVD route. The presence of a noticeable 2D signal for NG is also in stark contrast to the scenarios of GO and RGO, both of which barely present 2D peaks.35,36 The appearance of the 2D peak typically pertains to the formation of thin-layered graphene.37,38 Moreover, Raman statistics were performed on NaCl@NG (Figure S6). In further contexts, the effect of CVD conditions including growth temperature and duration on the quality of NG was probed (Figure S7), implying that both increased temperature and prolonged time were beneficial to improving the crystallinity of NG. In terms of template removal, NG powders show a slightly higher ID/IG ratio as compared to NaCl@NG, implying the water-dissolving process of salt templates exerts a discernible damage on the NG caging (Figure S8). C


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Figure 3. NG-based ink properties and 3D printing of designed patterns. (a) Photograph showing the 3D printing process. The NG-based ink was directly printed on an A4 paper. The inset displays thus-derived homogeneous graphene viscous ink. (b) Rheological behaviors of pure GO dispersion and NG-based composite ink. Inset: Storage/loss modulus as a function of angle frequency for NG ink. (c) Plot of resistance change for ITO/PET and NG/PET over bending cycles. (d) Photograph showing a large-scale printed “Graphene” pattern on PET, presenting favorable flexibility. (e) Photograph of NG-based ink printing with different geometric patterns. (f) Printing the NG-based ink with well-defined shapes directly on a variety of substrates.

X-ray photoelectron spectroscopy (XPS) analysis further reveals the nitrogen-doping information on NG nanosheets. Figure 2f displays the XPS C 1s spectrum, where the main signal at 284.8 eV corresponds to the graphitic sp2 carbon, suggesting most of the carbon atoms in the NG nanosheets are arranged within a conjugated honeycomb lattice. The other peaks at 286.0 and 287.1 eV within weaker intensities reflect different configurations of the C−N bonds, which can be ascribed to the sp2 and sp3 C−N, respectively. The small fraction of CO signal might stem from the adsorbed CO2 during the sample handling stage. Furthermore, the XPS N 1s spectrum (Figure 2g) can be deconvoluted into three contributive signals, corresponding to the pyridinic-N (398.3 eV), pyrrolic-N (400.6 eV), and graphitic-N (402.6 eV), respectively. The atomic percentage of N in the sample was estimated to be at ∼3.5 atom %, with a respective contribution from the pyridinic-N, pyrrolic-N, and graphitic-N signal accounting for 18.9%, 66%, and 15.1%, respectively.39,40 The green and mass production of NG via salt-templated CVD has rendered the fabrication of graphene-based composite inks with the addition of GO, which has great implications in 3D printing targeting costless, rapid, and accurate assembly of arbitrary 3D architectures with wide applications in electronics and energy storage. As shown in Figure 3a, the programmably controlled 3D printer enables direct printing of designed patterns on a A4 paper. The key component, NG-based composite ink, was prepared by mixing NG with GO aqueous dispersions (15 mg mL−1). Note that the composite ink derived patterns/electrodes after printing were subject to a GO reduction process to enhance the electrical conductivity (Figure S15). The digital photo (Figure 3a inset) displays thus-derived homogeneous graphene viscous

observations indicate that the cubic shape of NaCl crystal can be well preserved after NG growth at 700 °C (Figure 2a and Figures S10 and S11). Upon gentle washing by applying water, a hollow cubic NG cage can be readily attained, demonstrating perfect replication of the fine structures from NaCl crystals to graphene architectures (Figure 2b and Figures S12 and S13). This stems from the conformal growth of NG directly on the salt template realized by the CVD process. To verify the successful incorporation of nitrogen dopants within the graphene lattices, scanning transmission electron microscopy (STEM) imaging and elemental mapping over a representative area of NG cage were carried out, manifesting homogeneous distribution of nitrogen element (Figure 2c). The obtained NG cages were simply treated by sonication to generate two-dimensional lamellar NG nanosheets ready for the prepation of printable inks. The morphologies of NG nanosheets were further investigated by TEM. It is worth noting that the lateral sizes of most NG sheets were limited to 5 μm (Figure 2d), which is ideal for printing operations because larger-sized flakes might block the nozzle. The highly magnified TEM view in Figure 2e identifies the domain and layer information on NG nanosheets. Obviously, the graphene nanosheets contains five to seven layers with an interlayer spacing of ∼0.34 nm. Atomic force microscopy (AFM) characterizations again substantiate the two-dimensional feature of the NG nanosheets (Figure 2h and Figure S14). Such flakes show an average thickness of 3.27 nm (statistically from 50 nanosheets by AFM height measurement, as shown in Figure 2i), which is approximately 5-fold of the thickness of monolayer graphene detected on a SiO2/Si substrate (0.6−0.8 nm), indicating that the NG nanosheets are dominated by few layers. D


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Figure 4. Electrochemical performances of flexible quasi-solid-state supercapacitor devices with VN + NG electrodes. (a) CV profiles of VN + NG SSC at different scan rates of 10−500 mV s−1. (b) Corresponding galvanostatic charge−discharge curves of SSC at various current densities of 1.5−12 mA cm−2. (c) Capacitance retention plot with different scan rates for VN + NG and bare VN SSC devices. (d) Ragone plot of our SSC device in comparison with other reported SSC systems. (e, f) Photographs of the VN + NG SSC bent at different angles and corresponding CV profiles. (g) Long-term cyclic stability of VN + NG SSC at the current density of 12 mA cm−2.

Figure 3d showcases a large-scale printed “Graphene“ pattern on PET, presenting favorable flexibility. To inspect the microscopic uniformity of printed shapes, optical microscopy (OM) observation was carried out, which shows uniform line with fine resolution (Figure S16). To illustrate the full potential of NG-based composite inks in creating sophisticated 3D architectures, different geometric patterns were printed in a stable manner (Figure 3e). Furthermore, 3D printing employing our NG ink realized broad surface compatibility, with well-defined shapes directly printed on a variety of substrates, including polyimide (PI), paper, and glass (Figure 3f). Along with enabling 3D printing of different designed architectures, our NG ink can act as a conducting additive for any functional inks that are electrochemically active, the combination of which would be favorable in making advanced electrodes for supercapacitor applications. In this respect, vanadium nitride (VN) was selected as the electrode material due to its intrinsic conductivity and excellent capacitive feature.43,44 Nevertheless, the presence of native oxides on the surface of VN indeed impede the electrochemical performance, at least to some extent, owing to the insulating nature of such oxide layers. In this regard, conductive additive

ink. To ensure smooth extrusion-type 3D printing, the rheological properties of our ink were examined prior to the printing process. Figure 3b presents the apparent viscosity of pure GO dispersion and NG-based composite ink as a function of shear rate. With the decrease of shear rates, the viscosity of both inks increases, exhibiting shear-thinning fluid behavior. This was further verified by the frequency sweep testing result shown as the inset. As for NG-based ink, the storage modulus (G′) is observed to be obviously higher than the loss modulus (G′′). This would enable the stable flow of ink in the printing process. The rheological features of our NG-based composite ink are in good agreement with those reported graphene inks for 3D printing technology.41,42 The printed pattern on substrates such as polyethylene terephthalate (PET) readily manifests outstanding electrical conductivity and mechanical robustness, as evaluated by continuous bending tests. As depicted in Figure 3c, the NG/PET film displays no marked increase of electrical resistance over 1000 bending cycles (at a constant bending angle of 45°), which outperforms that of ITO/PET under identical test conditions. Such an advanced bending durability of NG/PET over fragile ITO originates from the tight contact between graphene patterns and the substrates created throughout printing. E


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Figure 5. Electrochemical performances of Li−S batteries with NG-modified separator. (a) a printing-derived NG/PP sheet with an area of 100 × 80 mm2. (b) Schematic illustration of cell configuration using NG-modified separator. (c) H-shaped permeation devices equipped with NG/PP and bare PP separators. (d) Calculated adsorption configurations and energies of Li2S6 on the NPy-G, NPr-G, and NG-G (the upper panel); partial density of states of N on the above systems (the lower panel). The red line and number indicate the position of the p band center of N. Gray, blue, purple, and yellow balls denote carbon, nitrogen, lithium, and sulfur atoms, respectively. (e) CV profile of the Li−S cell with an NG/PP separator in a potential range from 1.7 to 2.8 V. (f) Rate performances of the batteries with NG/PP and bare PP separators. (g) Galvanostatic charge/discharge curve of the battery with an NG/PP separator. (h) Cyclic performance of the battery with an NG/PP separator at 0.5 C for 300 cycles.

capacitance of 21.7 F g−1 could be achieved at 10 mV s−1. At a scan rate of 500 mV s−1, a capacitance of 15.2 F g−1 can still be delivered. In contrast, the VN + SuperP system without the addition of NG ink displays inferior capacitance performance, which stabilizes between 8.5−12.6 F g−1 at a scan rate ranging between 10 and 500 mV s−1 (Figure S17). The advanced electrochemical performance with the presence of NG might stem from the pseudocapacitive contribution pertaining to the incorporation of foreign nitrogen dopant, along with the excellent conductivity of NG.45,46 Figure 4b displays the galvanostatic charge/discharge performance of the VN + NG based SSC in the voltage window of 0−0.8 V at varied current

would be helpful to ameliorate this situation and sustain the capacitive properties of VN. Along this line, VN/NG/binder composite ink (8:1:1 wt %) was prepared, with VN/SuperP/ binder ink serving as a comparison. Such inks were directly printed with thickness controllability on the Cu foil to prepare the electrode. Accordingly, flexible quasi-solid-state supercapacitors (SSCs) adopting symmetric dual-electrode configuration were constructed by using homemade poly(vinyl alcohol)/KOH gel as electrolyte and polydimethylsiloxane film as the flexible capping layer. Figure 4a presents the CV curves of the VN + NG based SSC device at various scan rates, displaying typical pseudocapacitive behavior. A gravimetric F


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atoms indicate that the order of p band center position lies in NPy-G > NPr-G > NG-G, where NPy-G possesses the highest value at −5.29 eV. It is found that the p band center position is positively correlated to the binding energy toward LiPSs. The electrochemical performances of batteries with NG/PP and bare PP separators were evaluated systematically. The CV profile of the Li−S cell with an NG/PP separator in a potential range of 1.7−2.8 V at a scan rate of 0.05 mV s−1 is presented in Figure 5e. Obviously, two cathode peaks can be observed in the potential window of 2.42−2.27 and 2.08−1.95 V, corresponding to the reduction of S8 to soluble Li2S6/Li2S4 and further reduction toward insoluble Li2S2/Li2S, respectively. In addition, the featured single anodic peak, existing at the potential window of 2.24−2.54 V, can be assigned to the oxidaton of Li2S to S8. The CV result demonstrates that our designed NG/PP separator can well adapt to the evolution of LiPSs in Li−S system. Rate performances were collected by elevating the current density stepwise from 0.2 to 2.0 C (Figure 5f). The discharge capacity of cell with NG/PP separator reaches 1250, 960, 798, and 600 mAh g−1 when cycled at 0.2, 0.5, 1.0, and 2.0 C. The battery retains a high capacity of 1075 mAh g−1 when the current density is shifted back to 0.2 C, manifesting superior rate capability in contrast to that with the bare PP separator. The galvanostatic charge/ discharge curves in Figure 5g discloses that NG/PP cell presents prolonged low-voltage plateau and smaller votage hysteresis as compared with the PP cell, implying the effective role of the NG-modified separator in optimizing the sulfur redox. To further evaluate the cycling lifespan, batteries with NG/PP separators were tested at 0.5 C. As seen in Figure 5h, a capacity of 623 mAh g−1 after 300 cycles can still be delivered, with a favorable capacity retention of 87.4%. These electrochemical characterization results confirm that our printed NG/ PP separator can effectively regulate LiPS behaviors to boost the battery performances. The key roles of NG/PP separator played in Li−S system lie in the fact that (i) the doping of pyridinic- and pyrrolic-N offers abundant polar sites for chemical adsorption of LiPSs and (ii) the superior ion and electron conductive properties enable improved sulfur redox kinetics.

CONCLUSIONS In summary, we devise a green, scalable, facile and low-cost route to synthesize NG nanosheets by employing salttemplated PECVD, where homogeneous NG ink dispersions can be readily obtained throughout applying water to rapidly remove the salt cores. Thus-fabricated NG nanosheets accompanying large productivity, excellent electrical conductivity and favorable solution processability are versatile enough for multifunctional energy storage applications with the aid of printing technology. When shaped as flexible electrode using NG-based VN ink for quasi-solid-state supercapacitor, the device delivers high power density of 10144.3 W kg−1 with good cyclic stability. When utilized as printing-derived NG/PP separator of Li−S battery, the corresponding cells afford high rate capability from 0.2 to 2.0 C and favorable cycling performance, with 87.4% capacity retention after 300 cycles at 0.5 C. Our work offers a possible route for scalable production of graphene ink targeting emerging printable energy storage devices. G


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EXPERIMENTAL SECTION

Experimental setup, sample characterizations and electrochemical performances (PDF)

Preparation of Recrystallized NaCl Powders. Commercial NaCl powders were dissolved in deionized water to form a saturated aqueous solution. Ethanol was then added to the saturated solution to promote the recrystallization of NaCl crystals. After further filtration and drying treatment, NaCl crystals with an average size of 5 μm were obtained. CVD Synthesis of NG Powders. The recrystallized NaCl powders were loaded into a 4-in. tube furnace for the CVD process. The CVD system was pumped with a base pressure of 1 Pa and then purged with 60 sccm H2 and 2000 sccm Ar as carrier gases. When the desired growth temperature was reached, plasma (80 W) was initiated and evaporated pyridine precursor was introduced into the system, which was placed in a steel cylinder connected to the growth tube. After synthesis, the system was cooled to room temperature in H2 and Ar atmosphere. The obtained NaCl@NG powders were washed by water to remove the salt templates. Pure NG powders were obtained via vacuum filtration and freeze-drying. Printing Process. The direct printing was carried out by employing a multiaxis printing/dispensing system (QZ-NC0903) with 3D programming capability. In this work, the graphene inks for 3D printing were prepared by dispersing as-fabricated NG and GO (1:1, weight ratio) into deionized water, followed by sonication/ stirring for a few hours until a specified concentration was obtained. The preprogrammed pattern was printed on various substrates (e.g., PET, paper, PI) with a typical printing pressure of 20 psi and a processing speed of 2 mm s−1. After printing, the printed structures/ patterns were freeze-dried to remove the water solvent, followed by a simple GO reduction process with the aid of HI vapor. Characterization. As-prepared NG powders were systematically characterized with the aid of SEM (Hitachi SU8010), AFM (Veeco Nanoscope IIIa, tapping mode), Raman spectroscopy (Horiba, HR Evolution, 532 nm), TEM (FEI Titan Themis G2, with an acceleration voltage 30−300 kV), XPS (Escalab 250Xi spectrophotometer using a monochromatic Al Kα X-ray source), and four-probe resistance measuring system (Guangzhou 4-probe Tech Co., Ltd., RTS-4). The CV profiles were acquired on an Autolab PGSTAT302N potentiostat. Electrochemical measurements of supercapacitors were carried out using a CHI 660 electrochemical workstation. A Land CT2001A battery testing system was utilized to test the electrochemical performances of Li−S batteries including rate and cycling performances. Theoretical Simulation. First-principles simulations were performed with spin-unrestricted density functional theory, implemented by Vienna Ab Initio Simulation Package (VASP)57 using projector augmented wave (PAW) method58 as pseudopotential. The exchange-correlation interactions were treated by the generalized gradient approximation parametrized by Perdew, Burke, and Ernzerhof (GGA-PBE).59 The cutoff energy with plane wave basis set was 400 eV. All of the structures were relaxed fully with a fixed lattice until the total energy and force per atom was less than 10−5 eV and −0.01 eV/Å. The zero damping D3 method of Grimme60 was applied to correct the weak interactions between LiPSs and graphene with and without N-doping. Due to the low N doping concentration according to XPS data, a supercell with the size of 8 × 8 × 1 was built, and the first Brillouin zone integrations were sampled with 2 × 2 × 1 and 4 × 4 × 1 by a Monkhorst−Packing61 method for the geometry optimization and static electronic property calculations, respectively. The binding energy of LiPSs including S8, Li2S8, Li2S6, Li2S4, Li2S2, and Li2S on the graphene with and without N doping were calculated using this equation: Eb = Etotal − Esurf − ELiPS, where Etotal and Esurf are the total energy of graphene system with and without LiPSs, respectively, and ELiPS is the energy of the LiPS species.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: zfl[email protected]. *E-mail: [email protected]. ORCID

Liang Zhao: 0000-0001-8572-3547 Jingyu Sun: 0000-0002-9812-3046 Zhongfan Liu: 0000-0003-0065-7988 Author Contributions ⊥

N.W., L.Y., Z.S., and Y.S. contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51702225, 51702218), National Key Research and Development Program (2016YFA0200103), Jiangsu Youth Science Foundation (BK20170336), and Beijing Municipal Science and Technology Commission (Z161100002116020). N.W., L.Y., Z.S., Y.S., M.W., Z.T., J.C., L.Z., M.H.R., J.S., and Z.L. acknowledge support from the Suzhou Key Laboratory for Advanced Carbon Materials and Wearable Energy Technologies, Suzhou, China. REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. TwoDimensional Ggas of Massless Dirac Fermions in Graphene. Nature 2005, 438, 197−200. (3) Novoselov, K. S.; Fal’ko, V. I.; Colombo, L.; Gellert, P. R.; Schwab, M. G.; Kim, K. A Roadmap for Graphene. Nature 2012, 490, 192−200. (4) Eda, G.; Fanchini, G.; Chhowalla, M. Large-Area Ultrathin Films of Reduced Graphene Oxide as a Transparent and Flexible Electronic Material. Nat. Nanotechnol. 2008, 3, 270−274. (5) Moon, I. K.; Lee, J.; Ruoff, R. S.; Lee, H. Reduced Graphene Oxide by Chemical Graphitization. Nat. Commun. 2010, 1, 1−6. (6) Zhu, B. Y.; Murali, S.; Cai, W. W.; Li, X. S.; Suk, J. W.; Potts, J. R.; Ruoff, R. S. Graphene and Graphene Oxide: Synthesis, Properties, and Applications. Adv. Mater. 2010, 22, 3906−3924. (7) Hao, Y. F.; Bharathi, M. S.; Wang, L.; Liu, Y. Y.; Chen, H.; Nie, S.; Wang, X. H.; Chou, H.; Tan, C.; Fallahazad, B.; Ramanarayan, H.; Magnuson, C. W.; Tutuc, E.; Yakobson, B. I.; McCarty, K. F.; Zhang, Y. W.; Kim, P.; Hone, J.; Colombo, L.; Ruoff, R. S. The Role of Surface Oxygen in the Growth of Large Single-Crystal Graphene on Copper. Science 2013, 342, 720−723. (8) Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J. S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Kim, H. R.; Song, Y. I.; Kim, Y. J.; Kim, K. S.; Ozyilmaz, B.; Ahn, J. H.; Hong, B. H.; Iijima, S. Roll-to-Roll Production of 30-Inch Graphene Films for Transparent Electrodes. Nat. Nanotechnol. 2010, 5, 574−578. (9) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The Chemistry of Graphene Oxide. Chem. Soc. Rev. 2010, 39, 228−240. (10) Paton, K. R.; Varrla, E.; Backes, C.; Smith, R. J.; Khan, U.; O’Neill, A.; Boland, C.; Lotya, M.; Istrate, O. M.; King, P.; Higgins, T.; Barwich, S.; May, P.; Puczkarski, P.; Ahmed, I.; Moebius, M.; Pettersson, H.; Long, E.; Coelho, J.; O’Brien, S. E.; et al. Scalable

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Scalable Salt-Templated Synthesis of Nitrogen-Doped Graphene

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