Nitrogen-Deficient Graphitic Carbon Nitride with Enhanced

Dec 10, 2017 - Brunauer–Emmett–Teller (BET) analysis for ND-g-C3N4 material showed a specific surface area as high as 579.8 m2/g, which is about 1...
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Nitrogen Deficient Graphitic Carbon Nitride with Enhanced Performance for Lithium Ion Battery Anodes Jingjing Chen, Zhiyong Mao, Lexi Zhang, Dajian Wang, Ran Xu, Li-Jian Bie, and Bradley D. Fahlman ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b07116 • Publication Date (Web): 10 Dec 2017 Downloaded from http://pubs.acs.org on December 11, 2017

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ACS Nano

Nitrogen Deficient Graphitic Carbon Nitride with Enhanced Performance for Lithium Ion Battery Anodes Jingjing Chen†, Zhiyong Mao†,*, Lexi Zhang‡, Dajian Wang‡, Ran Xu‡, Lijian Bie‡,*, and Bradley D. Fahlman§ †

Tianjin Key Laboratory for Photoelectric Materials and Devices, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China. ‡

Key Laboratory of Display Materials and Photoelectric Devices, Tianjin University of Technology, Ministry of Education, Tianjin 300384, China. §

Department of Chemistry & Biochemistry and Science of Advanced Materials Program, Central Michigan University, Mount Pleasant, MI USA 48859. ABSTRACT: Graphitic carbon nitride (g-C3N4) behaving a layered feature with graphite was indexed as a high-

content nitrogen doping carbon material, attracting the increasing attentions for the application in energy storage devices. However, poor conductivity and the resultant serious irreversible capacity loss were pronounced for g-C3N4 material due to its high nitrogen content. In this work, magnesiothermic denitriding technology is demonstrated to reduce the nitrogen content of g-C3N4 (especially graphitic nitrogen) for enhanced lithium storage properties as lithium ion battery anodes. The obtained nitrogen deficient g-C3N4 (ND-g-C3N4) exhibits a thinner and porous structure composed of an abundance of relative low nitrogen doping wrinkled graphene nanosheets. A high th reversible lithium storage capacity of 2753 mAh/g was obtained after 300 cycle with an enhanced cycling stability and rate capability. The presented nitrogen deficient g-C3N4 with outstanding electrochemical performances may unambiguously promote the application of g-C3N4 materials in energy storage devices. KEYWORDS: graphitic carbon nitride, lithium ion battery, nitrogen deficiency, magnesiothermic denitriding, nitrogen doped carbon Lithium ion batteries (LIBs) have attracted abundant attention in the scientific and industrial fields during the past decades, driving by the ever-growing requirements for the various portable electronic devices.1-3 Nitrogendoped carbon with extraordinary electrochemical performances provides an attractive perspective in developing high-power and high-energy electrode materials for the next generation high-performance rechargeable LIBs.4-6 Graphitic carbon nitride (g-C3N4), stack of one-atom-thick planar sheets of sp2 hybridized carbon and nitrogen atoms, has drawn growing attention in various fields owing to its promising electric, optical, structural and physiochemical properties as well as its cost-effective availability.7, 8 In view of the layered feature with graphite, bulk g-C3N4 and layer(s) g-C3N4 could be indexed as the high nitrogen doping graphite and graphene respectively, arousing strong curiosity of researchers in the electrochemical energy storage field.8-12 For example, Pan et al. calculated the lithium storage properties of g-C3N4 nanotube by first principle study and revealed that g-C3N4 could react with Li to form Li4C3N4 with a theoretical specific capacity as high as 1165.3 mAh/g, which depicts the potential application of g-C3N4 as a lithium storage materials;13 Jorge et al. characterize

the electrochemical properties of g-C3N4 and commented that g-C3N4 related materials might provide an idea for the application in electrochemical fields.14 However, Yang et al. investigated the electrochemical performances of gC3N4 as anode material in experimental and found that gC3N4 anode material receive a low initial discharge capacity at 134.9 mAh/g and showed seriously irreversible capacity loss as high as 90% after only 7 cycles.15 In Zhang’s work, the seriously irreversible capacity loss is mainly attributed to the decomposition of g-C3N4 after being electrochemical reduced by Li. At variance, Veith10 and Hankel8,9 reported that the massive amount of graphitic nitrogen in the g-C3N4 should be responsible for the high irreversible capacity loss, and concluded that the g-C3N4 material is not suitable directly applied to LIBs. Thus, composite technology was demonstrated by researchers to enhance the electrochemical performance of g-C3N4 materials. Such as, Hou et al.16 reported a gC3N4/NRGO/MoS2 nanocomposite with a discharge capacity of 855 mAh/g at a current density of 100 mA/g after 100th cycle for lithium ion battery anode material. The mesoporous g-C3N4 nanosheets in this nanocomposite mainly served as an efficient mass

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transport for ions and electrolyte. Shi et al.12 exploited a 2D sandwich-type Fe2O3/g-C3N4-graphene hybrid nanosheets constructed by porous g-C3N4, which was identified to offer readily accessible channels for ionic diffusion. Li et al. developed a Zn2GeO4/g-C3N4 ultrathin nanosheets composite material and obtained a better electrochemical performance than pure Zn2GeO4 material. The large area ultrathin nanosheets g-C3N4 benefit to disperse the decomposed products of Zn2GeO4 after discharge and to accommodate the strain of the volume change of the active material during lithiation/delithiation.17 Although the electrochemical properties of above mentioned g-C3N4-based electrodes are greatly improved by hybriding with other materials, the g-C3N4 itself acts only as the dispersant and buffer agent in these composites. The electrochemical performances of g-C3N4 itself served as an active material were not improved because the compose technology is invalid to substantially change the nitrogen content and nitrogen species, which are pronounced to be the dominant reason for its poor performance.18,19

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magnesium powder were heated to the temperature at 750 °C in argon atmosphere, magnesium vapor led to a magnesiothermic denitriding reaction as expressed in Eq.1. The demonstrated magnesiothermic denitriding route is facile to prepare ND-g-C3N4 material. Photos of the gC3N4 powder before magnesiothermic denitriding and the ND-g-C3N4 after magnesiothermic denitriding exhibits that body color of sample changes obviously from yellow to black to naked eye, indicating the transformation from g-C3N4 to carbon doped with uncertain nitrogen. g-C3N4 + 3x/2 Mg → g-C3N4-x + x/2 Mg3N2

The nitrogen content of g-C3N4 is evaluated to be as high as 57.1 at % and three types of nitrogen species are identified in g-C3N4 structure, including pyridinicnitrogen, graphitic-nitrogen and bridge-nitrogen. Veith et al.10 employed solid-state reaction to simulate the interaction between g-C3N4 and Li ion, and speculated that the Li ions tend to take place irreversible reaction with graphitic-nitrogen preferentially. The irreversible reaction with graphitic-nitrogen will lead to the structural disorder of g-C3N4, which is the dominant reason for the high irreversible capacity loss. Theoretical calculation performed by Hankel et al. suggested that the pyridinic nitrogen is required for high lithium uptake and the graphitic nitrogen is harmful for the stability of the gC3N4 material.9 And two strategies, that reduce the graphitic nitride and introduce graphene backbone, were put forward to improve the lithium storage capacity and cycling stability respectively to enable g-C3N4 materials to be suitable as an anode material. Even though the extremely structure similarity with graphite, g-C3N4 was well identified as a semiconductor with poor conductivity, which is another reason for the unsatisfied performance of g-C3N4 electrodes. Apparently, the serious poor conductivity for g-C3N4 materials is ascribed to the mass amount replacement of carbon by nitrogen in the benzene ring, comparing with the excellent conductive graphite with analogue layered structure.

(1)

Fig. 1 Schematic of the synthesis of ND-g-C3N4 from magnesiothermic denitriding of g-C3N4 and the photos of gC3N4 before magnesiothermic denitriding and ND-g-C3N4 after magnesiothermic denitriding.

The phase structures of g-C3N4 and ND-g-C3N4 were investigated by powder XRD technology, as shown in Fig. 2(a). As reported in elsewhere,20,21 g-C3N4 exhibits two diffraction peaks centered at 13.1º(100) and 27.3º (002), which are associated with the in-plane structural packing motif and the inter-planar stacking of conjugated aromatic rings, respectively. After magnesiothermic denitriding, the obtained ND-g-C3N4 exhibits a broad peak at around 23°, indicating the transformation of gC3N4 into nitrogen doped carbon materials with a low degree of graphitization.22 In addition, a sharp diffraction peak around 25.84° that is comparable to the (002) reflection of graphite was recorded, demonstrating a layered structure. It is worth to note that this peak shows a left-shift phenomenon comparing to that of graphite, suggesting a few-layer graphene structure. Raman spectra were further employed to demonstrate the change of g-C3N4 materials undergoing magnesiothermic denitriding, as presented in Fig. 2(b). There are no significant Raman signals observed on g-C3N4 but two clear Raman shift peaks are detected on ND-g-C3N4. The bands at 1350.2 cm-1 and 1571.1 cm-1 corresponds the welldocumented D and G bands pertaining to graphite materials,23 respectively, confirming the transformation of carbon materials from g-C3N4. The high-intensity D band indicates the presence of structural defects in ND-g-C3N4 materials, resulted from the residual nitrogen atoms due to the inadequacy denitriding. These defects could effectively provide more Li+ sites, as well as enhance the kinetics of Li+-ion transport during the chargingdischarging process.24 The decrease of nitrogen and the change of structure could be further declared by the FTIR,

In view of this, nitrogen deficient g-C3N4 (ND-g-C3N4) material was demonstrated by reducing the nitrogen content via a magnesiothermic denitriding route from gC3N4. Enhanced electrochemical performance was obtained for the ND-g-C3N4 materials and the enhanced mechanism was investigated in detail.

RESULTS AND DISCUSSION The ND-g-C3N4 material synthesized through a magnesiothermic denitriding route from g-C3N4 is illustrated in Fig. 1. When the mixture of g-C3N4 and

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ACS Nano

Fig. 2 XRD pattern (a) and Raman spectrum (b) for g-C3N4 and ND-g-C3N4. (c) Nitrogen adsorption isotherms and the pore size distribution for g-C3N4 (c) and ND-g-C3N4 (d).

as depicted in Fig. S1. Several distinct variations related to the nitrogenous component were detected in the FTIR, indicating the decrease of nitrogen level in ND-g-C3N4 in comparison to pristine g-C3N4.

magnification TEM image for g-C3N4 (Fig. 3(a)) shows a sheet-like structure with a high degree of porosity. It was widely reported that the g-C3N4 synthesized by thermal condensation of urea consists of porous nanosheets due to gas evolution at elevated temperatures.28 The ND-gC3N4 (Fig. 3(b)) shows an extremely analogous porous sheet-like with g-C3N4 after magnesiothermic denitriding. In addition, it is found that the pores of ND-g-C3N4 were consisting of abundant wrinkles. High-resolution (TEM) images (Fig. 3(d)) clearly show that these wrinkles are composed of 3-6 layers of graphene. The thin-sheets and porous morphology for the prepared ND-g-C3N4 materials might provide more diffusion channels and shorten the Li+ ions diffusion pass ways while lithiation/delithiation process, leading to promising electrochemical properties.

N2 adsorption-desorption isotherms were carried out (Fig. 2(c)-(d)) to characterize the specific surface area and porosity of the samples, Both g-C3N4 and ND-g-C3N4 exhibit a IUPAC type IV isotherm with a hysteresis loops of H4-type at P/P0 = 0.4-1.0 between the adsorption and desorption branches, indicating the existence of both micro- and meso-pores.11, 25 The pore size distribution (the base of Fig. 2(c) ~ (d)) of g-C3N4 and ND-g-C3N4 were around 3.5 and 3.7 nm, respectively, suggesting that the magnesiothermic denitriding process could enlarge the pore size by remove the nitrogen atom. BrunauerEmmett-Teller (BET) analysis for ND-g-C3N4 material showed a specific surface area as high as 579.8 m2/g which is about 10 times of the g-C3N4 (59.9 m2/g), suggesting more lithium storage sites. Besides, pore volume was also increased from 0.732 cm3/g to 1.173 cm3/g (Fig. 2(c)-(d)). The pores can act as active sites for Li to increase capacity as well. Li reside in the pores contrast to the layers would relatively minimize the volume change during charging/ discharging process and helps enhance the cycle stability8. Since the cross-plane ion diffusion of graphene sheets is limited by their large sheet aspect ratio, the existence of the pores could facilitate high Li mobility through the layers of the material as well, which are necessary for improving high current rate charge/discharge performance.26,27

X-ray photoelectron spectroscopy (XPS) was conducted to gain insight into the chemical composition of g-C3N4 and ND-g-C3N4, as depicted in Fig. 4(a)-(f). There are three sharp peaks at 285.0, 398.4 and 531.8eV in the XPS survey spectra for g-C3N4 and ND-g-C3N4, which are assigned to the binding energies of C 1s, N 1s, and O 1s, respectively29 (Fig. 4 (a), (d)). Quantitative analysis from XPS spectra reveals that the nitrogen content for ND-g-C3N4 was dramatically dropped to 8.84 at.%, which is much lower than the initial g-C3N4 (51.4 at%), implying that the nitrogen content of g-C3N4 was declined significantly by the magnesiothermic denitriding process. From highresolution XPS analysis, the C 1s peak of g-C3N4 (Fig. 4(b)) can be resolved into two individual peaks centered at 285.1 and 288.3eV, related to the C=N (sp2), and the C-(N)3 (sp3) bonds, respectively.30

The morphologies of g-C3N4 and ND-g-C3N4 were characterized by TEM, as shown in Fig. 3(a)-(d). The low-

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Fig. 3 TEM (a) and HRTEM (b) images of g-C3N4; TEM (c) and HRTEM (d) images of ND-g-C3N4.

Fig. 4 (a) XPS survey spectrum; High-resolution XPS analysis of (b) C 1s; and (c) N1s for g-C3N4. (d) XPS survey spectrum; High-resolution XPS analysis of (e) C 1s; and (f) N1s for ND-g-C3N4. After the magnesiothermic denitriding, a new strong peak centered at 284.7 related to the graphite-like sp2 carbon could be identified, indicates that most of the C atoms in the ND-g-C3N4 are arranged in a conjugated honeycomb structure. The high resolution N1s spectrum of g-C3N4 can be deconvoluted into three different peaks with binding energies of 398.4, 400.7 and 404.4eV, which are attributed to sp2 pyridinic N atoms involved in triazine rings, bridging N and graphitic N atoms in N-(C)3, and oxidized N, respectively29. For the ND-g-C3N4, the oxidized N peak disappeared as well as the pyridinic and graphitic N peak decreased, however, a peak with higher intensity for pyrrolic nitrogen emerged (Fig. 4(f)).31,32 Notably, the

graphitic nitrogen content decreased from 23.03 % to 17.8 % after magnesiothermic denitriding treatment. Nitrogen speciation in carbon materials greatly influences its electrochemical performance.8, 10, 18, 33 It has been shown that graphitic nitrogen is more prone to breakage after lithiation than the pyridinic and pyrrolic N.8, 10, 34 The XPS fitting shows less graphitic N moieties than pyridinic/pyrrolic species for the ND-g-C3N4, suggesting the ND-g-C3N4 may deliver promising electrochemical performance in LIBs. To evaluate lithium storage properties of the as prepared ND-g-C3N4 samples, various electrochemical

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ACS Nano measurements were carried out. Fig. 5(a) shows the first four cyclic voltammogram (CV) curves of the ND-g-C3N4 electrode at room temperature between 0 and 3 V, at a scanning rate of 0.5 mV/s. The ND-g-C3N4 electrode display a typical CV curves for the initial 4 cycles, in agreement with other reports on carbonaceous anodes materials. A strong peak at 0.5 V in the Li-insertion process is attributed to the irreversible formation of solidelectrolyte interphase (SEI) film due to the occurrence of side reactions on the electrode,35 and the incomplete extraction of Li ion as the result of the lithium trap in porous electrodes but cannot release back in the first cycle, which are the main reasons for the initial irreversible capacity.36,37 This peak disappeared during the subsequent discharge. From the second cycle onwards, it is important to note that the CV curves almost overlapped, indicating the stable and superior reversibility of the prepared ND-g-C3N4.38 A peak close to 0 V was also observed in the CV curves resulting from Li+ intercalation into the ND-g-C3N4 electrode.39 During the Li-extraction process, peaks around 0.25 and 1.25 can be find, which are attributed to Li extraction from graphene layers and pores/defects, respectively,40,41 whereas two peaks observed around 2.6 V are due to Li binding with heteroatoms on the surface of anode materials.40, 42 Furthermore, the efficient ionic transport of g-C3N4 and ND-g-C3N4 anode is further supported by the electrochemical impedance spectroscopy (EIS) results (Fig. 5(b)). The Nyquist plot of the g-C3N4 and ND-g-C3N4 anode both reveals a semicircle refers in the high to medium frequency region, related to the charge transfer resistance at the interface of the electrode and the electrolyte. The following sloping line response at low frequencies indicates Warburg type resistance caused by ion diffusion in the electrode.43,44 Apparently, the ND-gC3N4 anode presents a much smaller interfacial chargetransfer and lithium-ion diffusion resistances than g-C3N4 anode, indicating a favorable charge-transfer behavior with low resistance for ND-g-C3N4 anode.12 Note that, even though the extremely structure similarity with graphite, g-C3N4 material delivers a huge charge transfer resistance of the g-C3N4 material due to its semiconductor characters with poor conductivity. A comparison about the electrical conductivity for g-C3N4 and ND-g-C3N4 materials was depicted in Fig. S2. One can see that the electronic conductivity of the ND-g-C3N4 is about 105 times higher than the pristine g-C3N4 due to the partially graphitization after magnesiothermic denitriding, which may further ameliorate the rate capability and the power density of the ND-g-C3N4 material.

result of the lithium trap in porous electrodes but cannot release back in the first cycle and the irreversible formation of SEI. This result also matches the CV results in which the cathodic peaks are present in the first scan, but absent in subsequent scans. From the second cycle onwards, the coulombic efficiency is improved. There was insignificant capacity fading during extended cycling. The average capacities of the ND-g-C3N4 were recorded to be 860 to 359 mAh/g for the current densities ranging from 0.1 to 10 A/g, respectively. Even for a large current density of 20 A/g, the ND-g-C3N4 still delivered 328 mAh/g, and the capacity could back to 1121 mAh/g immediately when the current density returned to 0.1 A/g from 20 A/g, demonstrating excellent reversibility. Amazingly, when the rate is tuned back to 0.1 A/g after rate cycling (190th cycle), the capacity achieved a maximum value of 2753 mAh/g (volumetric capacity of about 177 mAh/cm3) after 300 cycles. Compared to the original g-C3N4 which delivered only 61 mAh/g after 100 cycles, the electrochemical performances were significantly improved. The enhanced electrochemical performances of ND-g-C3N4 application for lithium ion battery anodes are attributed to the increase of intrinsic conductivity (Fig. S2) and the appearance of graphene backbone (Fig. 2b and 3d), which resulted from the reducing of nitrogen content and the transformation of graphitization carbon respectively after magnesiothermic denitriding process.9,11 Moreover, the promotion of the pyridinic and pyrrolic nitrogen ratio could further improve the stability of the ND-g-C3N4 material8-10 as compared to the g-C3N4 material. Notably, the achieved reversible capacity is larger than the theoretical value of graphene (744 mAh/g) and comparable with the optimized N-doped carbon materials summarized in Table S1. The excellent electrochemical performance for the presented ND-gC3N4 materials could be attributed to following reasons. Firstly, the porous structure plus the high specific surface area (Fig. 2(d)) of ND-g-C3N4 materials can shorten the ion diffusion distance by providing defined and large electrode/electrolyte contact area and crossed channels to facilitate the Li ion accession.45 Secondly, topological defects (Fig. 2(b)) and curled edges (Fig. 3(c)) can provide more active sites for Li storage.46 Thirdly, the high pyridinic and pyrrolic nitrogen (Fig. 4(f)) located at edges would be beneficial for adsorption of Li and cause additional Li storage sites.47 Especially, the formation of nitrogen defects and dangling bonds around N dopants could provide higher Li mobility and increase the capacity of Li intercalation.48,49 As a result, these features pertaining to the ND-g-C3N4 material enable the outstanding electrochemical performances for lithium ion battery anodes. Additionally, a full cell (ND-gC3N4/LiFePO4) was assembled to elucidate the cycle performances of the prepared ND-g-C3N4 anode, as depicted in Fig. S3. The first discharge and charge capacities are 161.3 and 133.5 mAh·g-1, respectively, with a first-cycle CE of 82.74%. During the following cycles, the CE gradually increased to 99.5% and a stable capacity of 76 mAh·g-1 can be received after 200 cycles.

The ND-g-C3N4 exhibits excellent rate performances. As shown in Fig. 5(c), electrochemical cycling performances of ND-g-C3N4 was initially tested at 0.1 A/g for 50 cycles, and subsequently at various current densities (from 0.2 to 20 A/g) each for 20 cycles, respectively. The first discharge and charge capacities are 2627 and 1362 mAh/g respectively, with a coulombic efficiency of 45.7%. The irreversible capacity loss in the first cycle can be mainly attributed to the incomplete extraction of Li ion as the

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Fig. 5 (a) CV curve for the 1 ~ 4 cycle. (b) Nyquist plots for g-C3N4 and ND-g-C3N4 electrode (Inset, high frequency region Nyquist plots). (c) Rate performance for ND-g-C3N4 electrode at current density of 0.1, 0.2, 0.5, 1, 2, 5, 10 and 20A/g and galvanostatic discharge property for g-C3N4 electrode.

dilute acetic acid and distilled water for 3 times at room temperature. The nitrogen deficient g-C3N4 (ND-g-C3N4) can be obtained by vacuum drying.

CONCLUSIONS In summary, nitrogen deficient g-C3N4 (ND-g-C3N4) material with enhanced electrochemical performance application for lithium ion battery anodes was demonstrated by reducing the nitrogen content via magnesiothermic denitriding of g-C3N4. The obtained ND-g-C3N4 delivered a high reversible lithium storage capacity of 2753 mAh/g after 300th cycle with an enhanced cycling stability and rate capability. The enhance electrochemical performances, including reversible capacity and cycling stability, for ND-g-C3N4 are attribute to the increase of intrinsic conductivity and occurrence of graphene backbone, which resulted from the reducing of nitrogen content (especially graphitic nitrogen) and the transformation of graphitization carbon induced by magnesiothermic denitriding. The presented ND-g-C3N4 materials with outstanding electrochemical performances may unambiguously promote the development of carbonbased electrode materials for applications in energy conversion and storage devices.

Sample characterization. Powder X-Ray diffraction (XRD) patterns were recorded on a Rigaku (D/Max2500pc) using Cu Kα1 radiation (λ = 1.5418 Å) operated at 40 kV and 150 mA within 10-60°. Transmission electron microscope (TEM) images and high-resolution transmission electron microscope (HRTEM) images were captured on a JEOL JSM-2100 microscope at an acceleration voltage of 200 kV. The specific surface areas were studied using the Brunauer-Emmett-Teller (BET) method using N2 adsorption-desorption at 77 K after treating the samples at 200 °C and 10−4 Pa for 3 hours using a Quadrasorb SI analyser. X-ray photoelectron spectroscopy (XPS) was performed using a Thermol Scientific ESCALAB 250 Xi XPS spectrometer, using a monochromated aluminum (Kα radiation: hν= 1486.6 eV) anode operated at 15 kV and 12.8 mA with a resolution of 0.1 eV (beam spot size: 500 µm2). Raman spectroscopy measurements were performed on a laser Raman spectrometer (LabRAM HR, France) with a He-Ne laser (532 nm) as the excitation source. The electrical conductivity of the powder samples were measured using a four probe resistivity tester (ST2722-SZ, Suzhou Jingge Electronic Co., LTD, China).

EXPERIMENTAL SECTION Sample preparation. Nitrogen deficient g-C3N4 (ND-gC3N4) material was synthesized by a magnesiothermic denitriding route from pristine g-C3N4. The pristine gC3N4 was firstly prepared by the conventional thermal condensation of urea.20 Typically, urea was placed into a covered crucible, and heated at 550 °C for 6 h in static air in a muffle furnace, with a heating rate of 2.5 °C /min. Then, 10 g pristine g-C3N4 and 5g magnesium powder were mixed together and loaded into a stainless-steel crucible with a stainless-steel lid. The mixture was then heated to 750 °C in a tube furnace under a flowing Ar atmosphere, and was preserved at this temperature for 2 h. After cooling, the residual magnesium and the generated byproducts were removed successively with

Electrochemical measurements. Working electrodes were prepared by mixing the prepared ND-g-C3N4 material with acetylene black carbon and polyvinylidene fluoride (PVDF) binder at a weight ratio of 8:1:1, in the solvent of N-methyl-2-pyrrolidone (NMP). The obtained slurry was cast onto a Cu foil with a wet film thickness of about 150 μm, and dried at 100 °C in vacuum for 12 h then compressed at 10 MPa pressure. Subsequently, CR 2032 coin-type cells were assembled in an argon-filled glove box using ND-g-C3N4 as the working electrode, Li discs as both the counter and reference electrode, microporous

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ACS Nano 8. Hankel, M.; Searles, D. J., Lithium Storage on Carbon Nitride, Graphenylene and Inorganic Graphenylene. Phys. Chem. Chem. Phys. 2016, 18, 14205-14215. 9. Hankel, M.; Ye, D. L.; Wang, L. Z.; Searles, D. J., Lithium and Sodium Storage on Graphitic Carbon Nitride. J. Phys. Chem. C 2015, 119, 21921-21927. 10. Veith, G. M.; Baggetto, L.; Adamczyk, L. A.; Guo, B. K.; Brown, S. S.; Sun, X. G.; Albert, A. A.; Humble, J. R.; Barnes, C. E.; Bojdys, M. J.; Dai, S.; Dudney, N. J., Electrochemical and SolidState Lithiation of Graphitic C3N4. Chem. Mater. 2013, 25, 503508. 11. Fu, Y. S.; Zhu, J. W.; Hu, C.; Wu, X. D.; Wang, X., Covalently Coupled Hybrid of Graphitic Carbon Nitride with Reduced Graphene Oxide as a Superior Performance LithiumIon Battery Anode. Nanoscale 2014, 6, 12555-12564. 12. Shi, M. J.; Wu, T. H.; Song, X. F.; Liu, J.; Zhao, L. P.; Zhang, P.; Gao, L., Active Fe2O3 Nanoparticles Encapsulated in Porous gC3N4/Graphene Sandwich-Type Nnanosheets as a Superior Anode for High-Performance Lithium-Ion Batteries. J. Mater. Chem. A 2016, 4, 10666-10672. 13. Pan, H., Graphitic Carbon Nitride Nanotubes as Li-Ion Battery Materials: a First-Principles Study. J. Phys. Chem. C 2014, 118, 9318-9323. 14. Jorge, A. B.; Cora, F.; Sella, A.; McMillan, P. F.; Brett, D. J. L., Electrochemical Properties of Graphitic Carbon Nitrides. Int. J. Nanotechnol. 2014, 11, 737-746. 15. Yang, X. H.; Wang, H. J.; Lu, X. F.; Cui, D. L.; Zhang, S. Y., Solid-state Synthesis of Graphite-like C3N4 and Its Reversible Li+ Intercalation. Acta Chim. Sinica 2009, 67, 1166-1170. 16. Hou, Y.; Li, J. Y.; Wen, Z. H.; Cui, S. M.; Yuan, C.; Chen, J. H., N-doped Graphene/Porous g-C3N4 Nanosheets Supported Layered-MoS2 Hybrid as Robust Anode Materials for LithiumIon Batteries. Nano Energy 2014, 8, 157-164. 17. Li, W.; Li, C.; Chen, B.; Jiao, X.; Chen, D., Facile Synthesis of Sheet-Like N-TiO2/g-C3N4 Heterojunctions with Gighly Enhanced and Stable Visible-Light Photocatalytic Activities. Rsc Adv. 2015, 5, 34281-34291. 18. Ma, C. C.; Shao, X. H.; Cao, D. P., Nitrogen-Doped Graphene Nanosheets as Anode Materials for Lithium ion Batteries: a First-Principles Study. J. Mater. Chem. 2012, 22, 89118915. 19. Ren, W.; Li, D. J.; Liu, H.; Mi, R.; Zhang, Y.; Dong, L.; Dong, L., Lithium Storage Performance of Carbon Nanotubes with Different Nitrogen Contents as Anodes in Lithium Ions Batteries. Electrochim. Acta 2013, 105, 75-82. 20. Wang, X. C.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M., A Metal-Free Polymeric Photocatalyst for Hydrogen Production from Water under Visible Light. Nat. mater. 2009, 8, 76-80. 21. Ho, W. K.; Zhang, Z. Z.; Lin, W.; Huang, S. P.; Zhang, X. W.; Wang, X. X.; Huang, Y., Copolymerization with 2,4,6Triaminopyrimidine for the Rolling-Up the Layer Structure, Tunable Electronic Properties, and Photocatalysis of g-C3N4. ACS Appl. Mater. Inter. 2015, 7, 5497-5505. 22. Ju, H. M.; Choi, S. H.; Huh, S. H., X-ray Diffraction Patterns of Thermally-Reduced Graphenes. J. Korean Phys. Soc. 2010, 57, 1649-1652. 23. Li, X. H.; Kurasch, S.; Kaiser, U.; Antonietti, M., Synthesis of Monolayer-Patched Graphene from Glucose. Angew. Chem. Int. Edit. 2012, 51, 9689-9692. 24. Lin, J.; He, J. R.; Chen, Y. F.; Li, Q.; Yu, B.; Xu, C.; Zhang, W. L., Pomegranate-Like Silicon/Nitrogen-Doped Graphene Microspheres as Superior-Capacity Anode for Lithium-Ion Batteries. Electrochim. Acta 2016, 215, 667-673. 25. Thommes, M., Physical Adsorption Characterization of Nanoporous Materials. Chem.-Ing.-Tech. 2010, 82, 1059-1073.

polypropylene film (Celgard 2325) as separator, and 1 M LiPF6 dissolved in a 1:1 mixture of ethylene carbonate and diethyl carbonate as the electrolyte. The cells were galvanostatically charged and discharged at various current densities in the voltage range of 0~3.0 V vs. Li/Li+ at current densities of 0.1, 0.2, 0.5, 1, 2, 5, 10, and 20 A/g using an automatic battery tester system (Land CT2001A, China). Cyclic voltammetry (CV) was carried out in the potential range of 0~3.0 V (vs. Li/Li+) with a scanning rate of 0.5 mV·s-1. Electrochemical impedance spectroscopy (EIS) measurements were carried out over a frequency range from 100 kHz to 10 mHz with an AC signal of 5 mV in amplitude as the perturbation using a CHI660E electrochemical workstation.

ASSOCIATED CONTENT Supporting Information. FTIR and electrical conductivity for the prepared ND-g-C3N4 and pristine g-C3N4, summarization about the N-doped carbon materials application for lithium ion batteries reported in references, assembling details and cycle performance of full cell. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Corresponding authors: E-mail address: Prof. Mao, [email protected]. E-mail address: Prof. Bie, [email protected].

ACKNOWLEDGMENT We gratefully acknowledge the financial support by the National Natural Science Foundation of China (Nos. 51777138, 21401139 and 21271139), Tianjin Natural Science Council (No. 15JCQNJC02900) and “Foreign Experts” Thousand Talents Program (Tianjin, China).

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