CNT Microspheres via Ethanol

Subscriber access provided by AUSTRALIAN NATIONAL UNIV

Energy, Environmental, and Catalysis Applications

Facile Synthesis of rGO/g-C3N4/CNT Microspheres via Ethanol-Assisted Spray Drying Method for High Performance Lithium-Sulfur Batteries Jianli Wang, Zhen Meng, Wentao Yang, Xufeng Yan, Rongnan Guo, and Wei-Qiang Han ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17590 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Facile Synthesis of rGO/g-C3N4/CNT Microspheres via Ethanol-Assisted

Spray

Drying

Method

for

High

Performance Lithium-Sulfur Batteries Jianli Wang, Zhen Meng, Wentao Yang, Xufeng Yan, Rongnan Guo, Wei-Qiang Han*

School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China Corresponding authors: *Email: [email protected]

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: rGO/g-C3N4 and rGO/g-C3N4/CNT microspheres are synthesized through the simple ethanol-assisted spray drying method. The ethanol, as the additive, changes the structure of rGO/g-C3N4 or rGO/g-C3N4/CNT composite from sheets clusters to regular microspheres. In the microspheres, the pores formed by rGO, g-C3N4 and CNT stacking provides physical confinement for lithium polysulfides (LiPSs). And enriched nitrogen (N) atoms of g-C3N4 offers strong chemical adhesion to anchor LiPSs. The dual immobilization mechanism can effectively alleviate the notorious “shuttle effect” of lithium sulfur battery. Meanwhile, the cathode with highly cyclic stability can be achieved. The rGO/g-C3N4/CNT/S cathode delivers a discharge capacity of 620 mA h g–1 after 500 cycles with a low capacity fading rate of only 0.03% per cycle at 1 C. Even, the cathode shows a retained capacity of 712 mA h g–1 over 300 cycles with a high sulfur loading (4.2 mg cm–2) at 0.2 C. KEYWORDS: rGO/g-C3N4/CNT microspheres, ethanol-assisted spray drying, high nitrogen content, lithium sulfur battery. INTRODUCTION Lithium-sulfur (Li-S) battery, one of the most promising rechargeable batteries, has been considered as the next generation energy storage system to replace currently commercial lithium ion batteries.1-3 It has attracted remarkable attentions due to high theoretical specific capacity of 1675 mA h g–1 and high energy density of 2600 W h kg–1. In addition, sulfur is low cost, environmentally benign and natural abundant in the earth.4-6 However, Li-S battery still faces some severe challenges on the road of practical application: (1) sluggish reaction kinetics due to intrinsic insulating nature of 2


Page 2 of 31

Page 3 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


sulfur, and the final discharge product (Li2S) covering on the surface of active material lowers the utilization of sulfur; (2) huge volume change (as high as 80%) arising from the density difference of sulfur and Li2S during cycling results in cracking of electrode and demise of active material; (3) the shuttle effect caused by the diffusion of highly soluble lithium polysulfides (LiPSs), which leads to rapid capacity fading and severe self-discharge.7-9 In order to overcome the remained issues above, various efforts have been made, including the design of battery structure,10-11 the introduction of host materials12-14 and the development of all solid state electrolyte.15-17 Among these strategies, employing sulfur host materials is the most popular strategy and has been investigated widely in recently years. Several characteristics should be required as the host materials: (1) excellent electronic conductivity, which can compensate poor conductivity of sulfur and promote the electrochemical kinetics process of electrode, resulting in enhanced rate performance and high sulfur utilization; (2) strong confinement for LiPSs to suppress shuttle effect and realize long cycle life; (3) hollow or porous structure is useful to accommodate sufficient sulfur as well as facilitate good contact between sulfur and host materials.18-19 Graphene-like carbon nitride (g-C3N4) is a two dimensional material, which has continuous tri-s-triazine building blocks linked with planar amino groups in each layer to form a graphene-like structure.20 Due to high nitrogen (N) element content (up to 60 wt%) especially dominant pyridinic-N, it is considered as an effective polar material with strong chemical interaction with LiPSs.21 And g-C3N4 is composed of only carbon 3



and nitrogen element, the light-weight host material can effectively increase the fraction of active sulfur and ensure high energy density.22-24 In addition, g-C3N4 can efficiently catalyze the transformation of soluble LiPSs to insoluble LiPSs and improve the redox reaction kinetics during cycling. Compared with bulk-C3N4, g-C3N4 nanosheets have higher accessible surface and more active sites for LiPSs trapped and conversion.25-26 Based on the above mentioned, g-C3N4 is identified as a promising sulfur host material applied in the Li-S battery. Zhang et al.27 firstly employed oxygenated carbon nitride (OCN) prepared by one step self-supporting solid state pyrosis (OSSP) as the sulfur host material. High N content, oxygen heteroatoms and abundant micropores favored active sulfur utilization and confinement for LiPSs. g-C3N4 was introduced to achieve the effective functionalization of separator at the molecular level by Zhang et al.22 Long cycling Li-S battery was obtained with strong immobilization for LiPSs based on (pyridinic-N)-Li bond formed in the vacancies of g-C3N4 and C-S bond observed between g-C3N4 and Li2S. However, poor electronic conductivity of g-C3N4 is unfavorable as the host material especially at high current density and high sulfur loading. It is a feasible strategy to introduce another material with enhanced conductivity to form a composite with g- C3N4.18 The composite has advantages combining with excellent LiPSs adsorption and conversion capability of g-C3N4 and good conductivity of conducting agent, resulting in good cycling stability and remarkable capacity performance at high current density. Lin et al.28successfully fabricated porous, well-interconnected rGO/gC3N4 composite by hydrothermal reaction. g-C3N4 provided chemical binding with 4


Page 4 of 31

Page 5 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


LiPSs to minimize shuttle effect. The presence of rGO improved overall conductivity of electrode, effectively enhancing charge transfer speed in the electrodes. So, the hybrid/S cathode showed stable cycling performance with low capacity decay rate of 0.09% per cycle over 400 cycles. Very recently, a 3D porous S/graphene@g-C3N4 hybrid sponge was realized through a microemulsion approach by Guo et al.18 This method can not only facilitate coupling between g-C3N4 and graphene, but also guarantee uniform distribution of sulfur in the composite, showing high energy density and stable cycling performance. It should be noted that enriched N-sites and porous structure are vital to make the best of advantages of g-C3N4 composite. Nevertheless, these strategies are generally involved with hydrothermal method to form graphene@gC3N4 inter-linked composite. As we know, the hydrothermal method is not suitable for mass production.29-30 Therefore, it is important to quest an ideal synthesis technique which has advantages of a simple process, low cost and easy scale-up. Herein, for the first time, we propose a simple ethanol-assisted spray drying method. The spray-drying technique, due to the characteristic of simple process and easy scale-up, has been widely used for preparation of various electrode materials.31-32 In the ethanol-assisted spray drying approach, addition of a few ethanol into precursor solution could dramatically change the structure of GO/g-C3N4 and GO/g-C3N4/CNT composites from sheets clusters to hollow microspheres. After low temperature heating treatment, g-C3N4 partially decomposed to realize the reduction of GO,33 meanwhile acted as the template to leave over some pores. Within the rGO/g-C3N4/CNT microspheres, rGO, g-C3N4 and CNT stack with each other in the shell could construct 5



cross-linked network, consequently confined LiPSs in the physical way. The enriched N-sites provided strong chemical adsorption for LiPSs. The physical-chemical dual immobilization mechanism could effectively restrict the shuttle effect to a large extent, resulting in stable cycling performance of cathode. The presence of rGO and CNT could induce porous morphology and facilitate rapid electron/ion transport in the electrode. Above mentioned features resulted in high electrochemical performance of rGO/gC3N4/CNT/S cathode including superior cycling stability and excellent rate performance. Finally, a specific capacity of 620 mA h g–1 with extremely low capacity fading rate of only 0.03% per cycle at 1 C after 500 cycles was realized. EXPERIMENTAL SECTION Preparation of GO and g-C3N4 Graphene oxide (GO) solution was synthesized by the modified Hummers’ method.34 Graphene-like C3N4 (g-C3N4 ) was prepared via two-step pyrolysis of melamine.35-36 Preparation of rGO/g-C3N4 microspheres and rGO/g-C3N4/CNT microspheres The as-received GO solution was diluted to 4 mg mL–1 by the addition of deionized (DI) water. The obtained g-C3N4 powder was dispersed in DI water and sonicated for 4 h to form a 4 mg mL–1 solution. Then, 100 mL GO solution was mixed with 50 mL gC3N4 solution by ultrasonication for 2 h (mass ratio of GO to g-C3N4 = 2:1), and 7.5 mL ethanol (5 vol%) was added into the mixture. GO/g-C3N4 microspheres was prepared by ethanol-assisted spray drying using spray-dried machine (YC-015, Shanghai Pilotech Co., Ltd). The detailed process was as follow: the spray speed was 500 mL h– 6


Page 6 of 31

Page 7 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1,

the air inlet temperature was 220 oC, and the draught fan frequency was 50 Hz. Next

the composite was annealed in Argon (Ar) atmosphere at 550 oC for 2 h to get rGO/gC3N4 microspheres. The preparation of rGO/g-C3N4/CNT microspheres was same with the above method except for the addition of 90 mg carbon nanotube (CNT). For comparison, GO/g-C3N4 (W) was prepared via the same procedure with GO/g-C3N4 but from the absence of 5 vol% ethanol. Sulfur impregnation Sulfur was infused into composite through a simple melting diffusion method. Generally, 60 mg composite was mixed and ground with sulfur powder. Then, the mixture was treated at 155 oC for 12 h in a sealed 50 mL Teflon-line stainless steel autoclave. Materials characterization Scanning electron microscopy (SEM) image was obtained by Zeiss Utral 55 fieldemission microscopy and energy dispersive x-ray spectroscopy (EDS) with Oxford EDS Inca Energy Coater. Transmission electron microscopy (TEM) of the samples was determined by JEM-2100. X-ray Diffraction (XRD) characterization was carried out by Shimadzu XRD 6000 with filtered Cu Kα radiation from 2θ = 5° to 60 °. Thermogravimetric analysis (TGA) was performed under a nitrogen flow by the Pyris Diamond analyzer to estimate the sulfur content. The specific surface areas and pore volume measurements were taken using Micromeritics ASAP 2020 Plus HD88. The UV-vis spectra were measured in the spectral range of 200-600 nm using a Cary 60 UV-vis variable wavelength spectrophotometer. Surface chemistry and bonding 7



characterizations were conducted with X-ray photoelectron spectroscopy (XPS) in Thermo Fisher Scientific Escalab 250Xi. Electrochemical measurements Electrochemical measurements were estimated by the assembly of 2032 type coin cells. The cathode slurry was made of active materials, conductive additives (super p) and LA132 binder at a mass ratio of 8:1:1, respectively. Firstly, sulfur composites with Super P as the conductive additive and LA132 as the binder were dispersed in DI water to form a homogenous slurry. Then the slurry was uniformly casted on the carboncoated aluminum foil. After being dried in 60 oC for 12 h, the foil was cut into 14 mm diameter circular disk. The sulfur loading of each electrode was ca. 1.5 mg cm–2 , and the higher sulfur loading of 4.2 mg cm–2 was also evaluated. Finally, the cells were assembled in an Ar-filled glove box. Lithium foil was used as the anode, Celgard 2400 membranes was used as the separators and 1M lithium bis (trifluoromethane) sulfonimide (LiTFSI) in 1,3-dioxolane/1,2-dimethoxyethane (DOL/DME, v/v, 1:1) containing 2 wt% LiNO3 was used as the electrolyte to assemble cells. The cyclic voltammetry (CV) was conducted at the scan rate of 0.05 mV s–1 with a potential window of 1.7-2.8 V versus Li+/Li. Electrochemical impedance spectroscopy (EIS) was carried out in the range of 100 kHz to 10 mHz on Salartron 1400 electrochemical workstation. Galvanostatic charge/discharge tests were performed on a LAND instrument at different current densities between 1.8 and 2.8 V. RESULTS AND DISCUSSION

8


Page 8 of 31

Page 9 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1. Schematic illustration of synthesis of rGO/g-C3N4 and rGO/g-C3N4/CNT microspheres by ethanol-assisted spray drying method.

Scheme 1 illustrates the synthetic route of rGO/g-C3N4, rGO/g-C3N4/CNT. Firstly, GO was prepared through the modified Hummers’ method, and g-C3N4 was synthesized via two-step pyrolysis of melamine. Due to the surface oxygenic functional groups, GO nanosheets are negatively charged.37 Then, GO solution was mixed with positively charged g-C3N4 suspension in a weight ratio of 2:1 under sonication to form a homogenous mixture dispersion. Under the electrostatic force, GO and g-C3N4 were preliminary interacted with each other. During the spray drying, the precursor solution was nebulized via the spray nozzle, to form micro-sized droplets.38 GO nanosheets were further incorporated with g-C3N4 as the rapid evaporation of solvent in the droplet. Finally, GO/g-C3N4 (W) and GO/g-C3N4 were synthesized by conventional spray drying and ethanol-assisted spray drying, respectively. To promote conductivity of composite, GO/g-C3N4/CNT was also prepared via the above mentioned strategy with the addition of 13 wt% CNT. The annealing treatment was performed at a relatively low temperature of 550 oC, in which g-C3N4 was partially decomposed to leave over 9



pores and GO was reduced to rGO. After annealing treatment, rGO/g-C3N4 and rGO/gC3N4/CNT were obtained.

Figure 1. (a,b) SEM images of GO/g-C3N4 (w). (c-f) SEM images of rGO/g-C3N4 (c,d) and rGO/gC3N4/CNT (e,f). (g-j) TEM images of GO/g-C3N4/CNT (g,h) and rGO/g-C3N4/CNT (i,j). HRTEM images of GO/g-C3N4/CNT (k) and rGO/g-C3N4/CNT (l). (m) SEM image and EDS elemental mapping of rGO/g-C3N4/CNT/S. (n) Elemental line scan of rGO/g-C3N4/CNT/S.

g-C3N4 nanosheets were obtained via further pyrolysis from bulk-C3N4. Compared with bulk-C3N4, g-C3N4 shows sheet-like structure from the SEM images (Figure S1c, d). The volume of g-C3N4 is significantly larger than that of bulk-C3N4 with the same weight, and the color changes to white (Figure S1a), indicating loose structure and successful formation of g-C3N4 nanosheets.39 From the SEM images (Figure 1a, b and Figure S2a-d, supporting information), GO/g-C3N4 (W) exhibits a sheets cluster 10


Page 10 of 31

Page 11 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


structure, like crumpled piece of paper. However, GO/g-C3N4 shows regular spherical structure with a size of 2-10 μm. The morphology transformation clearly demonstrates that ethanol plays an important role in the formation of microspheres. The addition of small amounts of ethanol (5 vol%) changes the morphology of GO/g-C3N4 composite. The reason of microspheres formation can be attributed to surface tension. GO and gC3N4 was in a condition of crumpled sheets (Scheme 1) due to the strong surface tension of water, leading to formation of sheets cluster after the spray drying. However, the surface tension decreases with the addition of ethanol so that GO g-C3N4 was unfolded sheets in the mixed solvent. With the evaporation of solvent, GO and g-C3N4 tended to migrate to the surface of micro-droplets, forming composite microspheres. The spherelike structure was remained with the addition of a certain amount of CNT, which could improve conductivity of the composite. And with the adding of CNT, the specific surface area and pore volume of composite increases (Table S1), indicating that CNT can facilitate the stacking of GO and g-C3N4 in three dimensional direction. The spherelike structure has not changed after heating treatment (Figure 1c-f), showing the structure stability of composite microspheres. The surface of microspheres shows the wrinkle morphology due to rapid evaporation of solvent in the microdroplets, which can be beneficial to the exposure of N-sites and confinement for polysulfides. Figure 1g-l and Figure S3 shows the TEM images of rGO/g-C3N4/CNT and rGO/g-C3N4, respectively, illustrating a crumpled but well-defined hollow sphere-like structure, corresponding to the SEM images. In the part of shell, no visible g-C3N4 agglomerates can be found on account of close interaction between GO, g-C3N4 and 11



CNT. As for GO/g-C3N4/CNT composite, TEM images (Figure 1h, j) reveal the existence of obvious CNT. GO, g-C3N4 and CNT stacked with each other to facilitate the formation of pore structure. The HRTEM images rGO/g-C3N4 and rGO/gC3N4/CNT (Figure 1l and Figure S3f) shows lattice fringes on the GO and g-C3N4, demonstrating g-C3N4 partly decomposed at 550 oC according to previously literature reported.33 After sulfur was embedded, rGO/g-C3N4 and rGO/g-C3N4/CNT still kept sphere-like structure and no obvious sulfur aggregation could be seen outside of composites. Elemental mapping reveals uniform dispersion of sulfur and nitrogen in the microspheres (Figure 1m and Figure S4). To further confirm distribution of these elements, EDS line scan analysis was conducted and shown in Figure 1n. The intensity of sulfur element is smaller than carbon element due to the evaporation of sulfur under the electron beam heating. The image manifests that sulfur is mainly confined within the microspheres. And g-C3N4 is uniformly distributed around the spheres because the nitrogen element is mainly originated form g-C3N4.

Figure 2. (a) XRD patterns of composites. (b) N2 adsorption-desorption isotherm curve of rGO/g12


Page 12 of 31

Page 13 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C3N4/CNT. Inset image: the pore size distribution of rGO/g-C3N4/CNT. (c) XPS survey of rGO/gC3N4/CNT. (d) C 1s and (e) N 1s of XPS spectra of rGO/g-C3N4/CNT.

N2 adsorption/desorption was conducted to test the specific surface area and pore structure. As shown in Figure 2b and Figure S8, the adsorption/desorption isotherms exhibit the typical type IV behavior and significant hysteresis characteristics at a high relative pressure(0.4-1.0), indicating the presence of mesopores and macropores.40 GO/g-C3N4 and GO/g-C3N4/CNT possess Brunauer-Emmett-Teller (BET) surface area of 150 and 145 m2 g–1 and total pore volume of 0.289 and 0.46 cm3 g–1 respectively. With the addition of CNT, the specific surface area of composite remains basically the same, but the pore volume increases obviously, which can be ascribed to positive influence of CNT on pore structure. After heating treatment, the BET surface area and total pore volume were calculated to be 190 m2 g–1, 225 m2 g–1 and 0.62 cm3 g–1, 0.88 cm3 g–1 for rGO/g-C3N4 and rGO/g-C3N4/CNT, which were both higher than that of GO/g-C3N4 and GO/g-C3N4/CNT. The result confirms that g-C3N4 partly decomposed meanwhile leave over some pores in original place. As the source of nitrogen element and pore former, g-C3N4 optimizes the pore structure of rGO/g-C3N4 and rGO/gC3N4/CNT. After the infiltration of sulfur, the specific surface area rapidly reduced to 18.4 and 30.2 m2 g–1, pore volume to 0.04 and 0.06 cm3 g–1 for rGO/g-C3N4-S and rGO/g-C3N4/CNT-S, respectively (Figure S9). The result indicated that sulfur was successfully inserted into pores of microspheres by the simple melting diffusion method. XRD patterns of bulk-C3N4 and g-C3N4 are displayed in Figure S1b. The intensity of peaks at 13.2° and 27.9° of g-C3N4 sharply decreases compared with bulk-C3N4, 13



furtherly indicating successful synthesis of g-C3N4 nanosheets.41 As shown in Figure 2a, the XRD pattern of GO/g-C3N4 (W) exists sharp peaks at 11.2° and 27.9°. The former corresponds to the (001) of GO, indicating that as-synthesized graphene oxide is a few layers. And the latter is closed to the (002) plane periodic stacking of layers of g-C3N4. Compared with GO/g-C3N4 (W), the intensity of (001) peak of GO and (002) peak of g-C3N4 remarkably diminish and these peaks significantly broader for GO/gC3N4. The change demonstrates that GO and g-C3N4 are in a highly disordered condition on account of closer interaction between GO and g-C3N4 by ethanol-assisted spray drying strategy.42 GO/g-C3N4/CNT has similar shape of GO/g-C3N4 except extra 26°, which corresponds to the (002) diffraction plane of hexagonal graphite (JCPDS card no. 41-1478). The disappearance of diffraction peak at 11.2° and emergence of diffraction peak at 26.2° after annealing indicates that GO was successfully restored to rGO.43 The diffraction peak at 27.9° weakens but not vanish, which confirms g-C3N4 partly decomposed after 550℃. Figure S5 displays XRD pattern of rGO/g-C3N4/S and rGO/g-C3N4/CNT/S composites, all characteristic peaks from 5° to 60° are in accordance with orthorhombic sulfur phase (JCPDS card no. 08-0247), indicating successful sulfur loading. The thermal gravimetric analysis (TGA) was conducted in N2 atmosphere to confirm sulfur content of rGO/g-C3N4/S and rGO/g-C3N4/CNT/S composites, with curves presented in Figure S7. The weight loss between 200-350℃ can be assigned to the evaporation of sulfur. The content of sulfur can be determined to be 70% and 70.8% in rGO/g-C3N4/S and rGO/g-C3N4/CNT/S, respectively. XPS was conducted to identify the element content and condition of composites. 14


Page 14 of 31

Page 15 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


As shown in Figure 2c, rGO/g-C3N4/CNT mainly consists of three elements of C (77.93%), N (14.62%) and O (7.45%). The high resolution N 1s spectrum of rGO/gC3N4/CNT (Figure 2e) can be resolved into three peaks located at 398.7, 400.1 and 401.0 eV, corresponding to pyridinic N, pyrrolic N and graphitic N.44 And pyridinic N is obviously dominated among three types of N, which is considered as the main adsorption sites for LiPSs in N-doped carbon materials.45 In addition, peaks at 404.3 eV and 405.7 eV can be attributed to N-oxide bonding.18 The C 1s spectrum of rGO/gC3N4/CNT is displayed in Figure 2d, the carbon mainly consists of graphitic C (C=C) with binding energy of 284.8 eV, oxygen bridged carbon (C-O) at 286.1 eV and aromatic carbon bonded with N (C-N) at 288.2 eV.46 The result of XPS reveals that high nitrogen content, especially dominated pyridinic N in the composite, which can provide strong restriction for lithium polysulfides. To evaluate electrochemical performance of electrode, 2032-type coin cells were assembled with sulfur composites as the cathodes and lithium plate as the counter. The sulfur loading and amount of electrolyte injected into individual cell are kept 1.5 mg cm–2 and 40 μL respectively to eliminate the influence of external factors on electrochemical properties. Figure 3a and Figure S11 display cycle voltammogram (CV) curves of four cathodes at a scan rate of 0.05 mV s–1. The curves show typical features of Li-S battery with two well-defined reduction peaks and one overlaid oxidation peak.47 The first peak at around 2.3V corresponds to the reduction process from S8 to Li2Sn (4≤n≤8), and the second peak at around 2.05V to transformation of high order Li2Sn to lower order Li2Sn (like Li2S2, Li2S). In the subsequent reversed scan, 15



one broad oxidation peak of around 2.39V can be attributed to the change from lithium sulfide to lithium polysulfide and eventually to sulfur.48-49 Compared with GO/gC3N4/S and GO/g-C3N4/CNT cathodes, the first reduction peak shifts to higher voltage, and the oxidation peak to lower voltage for rGO/g-C3N4/S and rGO/g-C3N4/CNT cathodes. The result confirms that material conductivity can be enhanced with heating treatment, which is related to the reduction of GO and partial decomposition of g-C3N4. In addition, the polarization voltage (the gap of reduction peak and oxidation peak) of GO/g-C3N4/CNT cathode is significantly lower than GO/g-C3N4, indicating CNT can improve electrode conductivity. In the 2nd and 5th cycles, the CV curves can coincide well, manifesting good electrochemical stability and reversibility of electrode.50

Figure 3. (a) Cycle voltammogram curves of rGO/g-C3N4/CNT-S cathode at a scan rate of 0.05 mV

16


Page 16 of 31

Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


s–1. (b) Rate capability of GO/g-C3N4-S, rGO/g-C3N4-S, GO/g-C3N4/CNT-S, rGO/g-C3N4/CNT-S cathodes. (c) Cycle performances of rGO/g-C3N4-S and rGO/g-C3N4/CNT-S cathodes for 200 cycles at 0.2 C. (d) Cycle performances of rGO/g-C3N4-S and rGO/g-C3N4/CNT-S cathodes for long cycles at 1 C with sulfur loading of 1.5 mg cm–2 ( the first three cycles conducted at 0.1 C).

Figure S12 illustrates galvanostatic discharge/charge voltage profiles with the voltage potential of 1.8V-2.8V from 0.05C to 2C. All cathodes have two distinct discharge plateaus (around 2.3V and 2.05V) even at high current density, which is consistent with CV curves.51 rGO/g-C3N4/CNT/S cathode shows the best capacity performance at various current density. Particularly, the discharge capacity of rGO/gC3N4/CNT/S cathode is 1263 mA h g–1 at 0.05C, corresponding to sulfur utilization of 76%. The polarization of electrode under high current density is very serious for traditional host sulfur material, which will lead to the deterioration of electrochemical stability.52 Among four kind of electrodes, rGO/g-C3N4/CNT/S cathode possess smallest polarization voltage at various C-rate, which is consistent with CV curves. With the improvement of current density, the difference of polarization voltage significantly increases for the cathodes. Specifically, the polarization voltages are 0.54V, 0.3V, 0.29V and 0.28V for GO/g-C3N4/S, rGO/g-C3N4/S, GO/g-C3N4/CNT/S and rGO/g-C3N4/CNT/S cathode at 1C. The result confirms that conductivity of material can be promoted by the addition of CNT and reduction of GO. Electrochemical performance was further investigated by the measurement of rate capability. As shown in Figure 3b, the capacity performance of rGO/g-C3N4/S cathode significantly improves compared with GO/g-C3N4/S cathode, especially at high current 17



density. The capacity promotion of GO/g-C3N4/CNT/S cathode reveals the conductivity contribution of CNT. rGO/g-C3N4/CNT/S cathode delivers specific capacity of 1263 mA h g–1, 975 mA h g–1, 878 mA h g–1, 709 mA h g–1, 635 mA h g–1 and 554 mA h g–1 at 0.05C, 0.1C, 0.2C, 0.5C, 1C and 2C, respectively, higher than GO/g-C3N4/S, rGO/gC3N4/S and GO/g-C3N4/CNT/S cathode. When the current rate was returned to 0.1C, rGO/g-C3N4/CNT/S cathode still delivers capacity of 966 mA h g–1, without obvious capacity fading, indicating excellent electrochemical stability and good rate capability.53 The cycle performance of rGO/g-C3N4/S and rGO/g-C3N4/CNT/S cathode at 0.2C were tested and shown in Figure 3c. Initial specific capacity of rGO/g-C3N4/CNT/S cathode can reach up to 1030 mA h g–1, better than that of 980 mA h g–1 of rGO/gC3N4/S cathode. And a reversible capacity of 820 mA h g–1 and 710 mA h g–1 was respectively retained for rGO/g-C3N4/S and rGO/g-C3N4/CNT/S cathode after 200 cycles, corresponding to a high capacity retention of 80% and 73%, indicating excellent cycling stability. Cycling performance at high current density is important for high energy density battery.54 Figure 3d shows long-term cycling capability at 1C. The good cycling stability is visible. Particularly, rGO/g-C3N4/CNT/S cathode delivered initial capacity of 730 mA h g–1 and capacity of 620 mA h g–1 was achieved after 500 cycles (high capacity retention of 85%), relating with a very small capacity fading rate of only 0.03%. Excellent cycling capacity can be ascribed to strong immobilization for polysulfides originated from enriched N-sites of g-C3N4 and the porous structure55. To further investigate the electrochemical kinetics of the cathodes, Figure 4a 18


Page 18 of 31

Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


presents electrochemical impedance spectroscopy (EIS) for cathodes after 200 cycles at 0.2C. All curves are composed of two semicircles at high and medium frequency regions and an inclined line at low frequency. The semicircle at high frequency corresponds to charge transfer resistance (Rct). The second semicircle is related to charge transport resistance (Rg), which is associated with resistance of a solid-state layer of accumulated lithium sulfide during discharge.56-57 The inclined line at low frequency reflects ion diffusion resistance (Warburg impedance: W).58-60 It can be seen that both charge transfer and charge transport resistance of GO/g-C3N4/CNT/S are much smaller than that of GO/g-C3N4/S cathode, which reflects the contribution of CNT to electrode conductivity. The Rct and Rg of the rGO/g-C3N4/S and rGO/gC3N4/CNT/S cathodes decreased rapidly compared with that of the GO/g-C3N4/CNT/S are much smaller than GO/g-C3N4/S cathode, confirming the enhanced conductivity of the rGO/g-C3N4/S and rGO/g-C3N4/CNT/S cathodes.

Figure 4. (a) The Nyquist plots of GO/g-C3N4-S, rGO/g-C3N4-S, GO/g-C3N4/CNT-S, rGO/g19



C3N4/CNT-S after 200 cycles at 0.2 C from 100 kHz to 10 mHz. (b) Relative polysulfide adsorption of rGO/g-C3N4, rGO/g-C3N4/CNT and digital images of Li2S6 solution after adsorption (Li2S6 solution was used as the reference). (c) Cycle performance of rGO/g-C3N4/CNT-S cathode for 300 cycles at 0.2 C with sulfur loading of 4.2 mg cm–2.

High sulfur mass loading is of importance to achieve high energy density and commercialization for lithium sulfur battery. Cells with high sulfur loading of 4.2 mg cm–2 were tested for rGO/g-C3N4/CNT/S cathode. As shown in Figure 4c, it delivers an initial specific capacity of 1017 mA h g–1 and a retained capacity of 712 mA h g–1 after 300 cycles at 0.2C with a high coulombic efficiency of up to 99.8%. Good electrochemical performance can be attributed to three points: (1) High specific surface area ensures enough active sulfur infiltration and hollow structure provides sufficient space to accommodate volume expanding; (2) Good conductivity facilitates electron transfer to enhance capacity performance at high current density; (3) High nitrogen content offers strong immobilization for lithium sulfides to achieve superior cycling stability. To confirm strong confinement of the composite microspheres for lithium polysulfides, the visual polysulfides adsorption of rGO/g-C3N4 and rGO/g-C3N4/CNT were tested and shown in Figure 4b. Firstly, the Li2S6 solution of 2.5×10–3 mol L–1 was prepared by the previous reports.61 The details can be found in the supporting information. Then, 5 mg of rGO/g-C3N4 and rGO/g-C3N4/CNT powders were added into 4 mL of as-prepared Li2S6 solution at room temperature for 6 h, respectively. It can be seen that the color of solution changed from deep yellow to near-colorless after 20


Page 20 of 31

Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


adding rGO/g-C3N4 and rGO/g-C3N4/CNT. The result visually shows strong immobilization ability of rGO/g-C3N4 and rGO/g-C3N4/CNT for polysulfides. Furthermore, the relative concentration of Li2S6 solution before and after adding host material can be estimated by the ultraviolet/visible adsorption measurement. As displayed in Figure 4b, the relative polysulfides adsorption is 83% and 88% for rGO/gC3N4 and rGO/g-C3N4/CNT microspheres, respectively. The strong adsorption capability of rGO/g-C3N4 and rGO/g-C3N4/CNT for polysulfides can effectively facilitate long term cycling stability of the batteries. Figure S13 and Figure S14 display the SEM images of rGO/g-C3N4/S and rGO/g-C3N4/CNT/S cathodes before and after cycling, respectively. For the fresh cells, the morphology of both sulfur composites have almost no change, indicating the structure stability of microspheres during the process of cells assembly. After cycling, both composites remain spherical in structure, except that the wrinkles become more obvious, which can be attributed to volume expansion during discharge/charge. CONCLUSION In conclusion, for the first time, rGO/g-C3N4 and rGO/g-C3N4/CNT composite microspheres were successfully prepared via a simple ethanol-assisted spray drying method. The results of SEM and TEM images indicated that ethanol played a vital role in the transformation from sheets clusters to regular sphere-like structure. A few ethanol, as the additive, not only changes the morphology of GO/g-C3N4 and GO/g-C3N4/CNT composite, but also improves the interaction of GO, g-C3N4 and CNT. After low temperature heating treatment, GO was reduced into rGO accompanied with promotion 21



of conductivity of composite. Meanwhile, the specific surface area and pore volume of composite can be increased with the partial decomposition of g-C3N4. As the host material in lithium sulfur battery, enriched pore structure offers the accommodation for active sulfur, and good electronic conductivity can effectively promote electrochemical kinetics resulting in excellent capacity performance. Strong chemical confinement provided by abundant N-sites of g-C3N4, especially pyridinic N, can efficiently immobilize polysulfides during charge/discharge. Applied in lithium sulfur battery, both rGO/g-C3N4/S and rGO/g-C3N4/CNT/S cathodes show high electrochemical capability with outstanding capacity performance and good cycling stability. Particularly, rGO/g-C3N4/CNT/S cathode exhibits an initial specific capacity of 1030 mA h g–1 and a reversible capacity of 820 mA h g–1 after 200 cycles at 0.2C. At high C-rate of 1C, rGO/g-C3N4/CNT/S cathode still delivers capacity of 730 mA h g–1 and retains a capacity of 620 mA h g–1 after 500 cycles with an extremely low capacity fading rate of only 0.03%. Even in high sulfur mass loading of 4.2 mg cm–2, the cathode also demonstrated excellent electrochemical performance with a reversible capacity of 712 mA h g–1 after 300 cycles at 0.2C. ASSOCIATED CONTENT Supporting Information SEM and TEM images, XRD patterns, XPS spectra, TGA curves, N2 adsorptiondesorption isotherm curves and pore size distribution curves, UV-vis adsorption spectra, CV curves, the charge/discharge profiles, table of specific surface area and pore volume, table of comparison of the cathode performances in this work with other carbon 22


Page 22 of 31

Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


materials reported recently. AUTHOR INFORMATION Corresponding Author *Email: [email protected] ORCID Wei-Qiang Han: 0000-0001-5525-8277 Notes The authors declare no competing financial interest.

23



REFERENCE (1) Peng, H.-J.; Huang, J.-Q.; Cheng, X.-B.; Zhang, Q. Review on High-Loading and High-Energy Lithium-Sulfur Batteries. Adv. Energy Mater. 2017, 7 (24) , 1700260. (2) Eftekhari, A.; Kim, D.-W. Cathode materials for lithium-sulfur batteries: a practical perspective. J. Mater. Chem. A 2017, 5 (34), 17734-17776. (3) Wei Seh, Z.; Li, W.; Cha, J. J.; Zheng, G.; Yang, Y.; McDowell, M. T.; Hsu, P. C.; Cui, Y. Sulphur-TiO2 yolk-shell nanoarchitecture with internal void space for longcycle lithium-sulphur batteries. Nat. Commun. 2013, 4, 1331. (4) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J.-M. Li-O-2 and Li-S batteries with high energy storage. Nat. Mater. 2012, 11 (1), 19-29. (5) Seh, Z. W.; Sun, Y. M.; Zhang, Q. F.; Cui, Y. Designing high-energy lithium-sulfur batteries. Chem. Soc. Rev. 2016, 45 (20), 5605-5634. (6) Yang, C.-P.; Yin, Y.-X.; Guo, Y.-G.; Wan, L.-J. Electrochemical (De)Lithiation of 1D Sulfur Chains in Li-S Batteries: A Model System Study. J. Am. Chem. Soc. 2015, 137 (6), 2215-2218. (7) Liu, M.; Qin, X.; He, Y.-B.; Li, B.; Kang, F. Recent innovative configurations in high-energy lithium-sulfur batteries. J. Mater. Chem. A 2017, 5 (11), 5222-5234. (8) Lang, J.; Qi, L.; Luo, Y.; Wu, H. High performance lithium metal anode: Progress and prospects. Energy Storage Mater. 2017, 7, 115-129. (9) Xie, Y.; Meng, Z.; Cai, T. W.; Han, W. Q. Effect of Boron-Doping on the Graphene Aerogel Used as Cathode for the Lithium Sulfur Battery. ACS Appl. Mater. Interfaces 2015, 7 (45), 25202-25210. (10) Song, J.; Xu, T.; Gordin, M. L.; Zhu, P.; Lv, D.; Jiang, Y.-B.; Chen, Y.; Duan, Y.; Wang, D. Nitrogen- Doped Mesoporous Carbon Promoted Chemical Adsorption of Sulfur and Fabrication of High- Areal- Capacity Sulfur Cathode with Exceptional Cycling Stability for Lithium- Sulfur Batteries. Adv. Funct. Mater. 2014, 24 (9), 12431250. (11) Su, Y.-S.; Manthiram, A. Lithium-sulphur batteries with a microporous carbon paper as a bifunctional interlayer. Nat. Commun. 2012, 3, 1166. 24


Page 24 of 31

Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(12) Ji, X. L.; Lee, K. T.; Nazar, L. F. A highly ordered nanostructured carbon-sulphur cathode for lithium-sulphur batteries. Nat. Mater. 2009, 8 (6), 500-506. (13) Kaiser, M. R.; Ma, Z.; Wang, X.; Han, F.; Gao, T.; Fan, X.; Wang, J.-Z.; Liu, H. K.; Dou, S.; Wang, C. Reverse Microemulsion Synthesis of Sulfur/Graphene Composite for Lithium/Sulfur Batteries. ACS Nano 2017, 11 (9), 9048-9056. (14) Zhao, J.; Yang, Y.; Katiyar, R. S.; Chen, Z. Phosphorene as a promising anchoring material for lithium-sulfur batteries: a computational study. J. Mater. Chem. A 2016, 4 (16), 6124-6130. (15) Yao, X.; Huang, N.; Han, F.; Zhang, Q.; Wan, H.; Mwizerwa, J. P.; Wang, C.; Xu, X. High-Performance All-Solid-State Lithium-Sulfur Batteries Enabled by Amorphous Sulfur-Coated Reduced Graphene Oxide Cathodes. Adv. Energy Mater. 2017, 7 (17) , 1602923. (16) Manthiram, A.; Yu, X.; Wang, S. Lithium battery chemistries enabled by solidstate electrolytes. Nat. Rev. Mater. 2017, 2 (4) , 16103. (17) Zheng, J.; Fan, X.; Ji, G.; Wang, H.; Hou, S.; DeMella, K. C.; Raghavan, S. R.; Wang, J.; Xu, K.; Wang, C. Manipulating electrolyte and solid electrolyte interphase to enable safe and efficient Li-S batteries. Nano Energy 2018, 50, 431-440. (18) Juan, Z.; Jin‐Yi, L.; Wen‐Peng, W.; Xing‐Hao, Z.; Xing‐Hua, T.; Wei‐Guo, C.; Yu‐Guo, G. Microemulsion Assisted Assembly of 3D Porous S/Graphene@g‐C3N4 Hybrid Sponge as Free‐Standing Cathodes for High Energy Density Li–S Batteries. Adv. Energy Mater. 2018, 8 (14), 1702839. (19) Jung, D. S.; Hwang, T. H.; Lee, J. H.; Koo, H. Y.; Shakoor, R. A.; Kahraman, R.; Jo, Y. N.; Park, M.-S.; Choi, J. W. Hierarchical Porous Carbon by Ultrasonic Spray Pyrolysis Yields Stable Cycling in Lithium-Sulfur Battery. Nano Lett. 2014, 14 (8), 4418-4425. (20) Wang, X.; 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 (1), 76-80. (21) Liao, K.; Mao, P.; Li, N.; Han, M.; Yi, J.; He, P.; Sun, Y.; Zhou, H. Stabilization 25



of polysulfides via lithium bonds for Li-S batteries. J. Mater. Chem. A 2016, 4 (15), 5406-5409. (22) Fan, C.-Y.; Yuan, H.-Y.; Li, H.-H.; Wang, H.-F.; Li, W.-L.; Sun, H.-Z.; Wu, X.L.; Zhang, J.-P. The Effective Design of a Polysulfide-Trapped Separator at the Molecular Level for High Energy Density Li-S Batteries. ACS Appl. Mater. Interfaces 2016, 8 (25), 16108-16115. (23) Meng, Z.; Xie, Y.; Cai, T. W.; Sun, Z. X.; Jiang, K. M.; Han, W. Q. Graphene-like g-C3N4 nanosheets/sulfur as cathode for lithium-sulfur battery. Electrochim. Acta 2016, 210, 829-836. (24) Pang, Q.; Nazar, L. F. Long-Life and High-Areal-Capacity Li S Batteries Enabled by a Light-Weight Polar Host with Intrinsic Polysulfide Adsorption. ACS Nano 2016, 10 (4), 4111-4118. (25) Liang, J.; Yin, L.; Tang, X.; Yang, H.; Yan, W.; Song, L.; Cheng, H.-M.; Li, F. Kinetically Enhanced Electrochemical Redox of Polysulfides on Polymeric Carbon Nitrides for Improved Lithium-Sulfur Batteries. ACS Appl. Mater. Interfaces 2016, 8 (38), 25193-25201. (26) Wang, M.; Liang, Q.; Han, J.; Tao, Y.; Liu, D.; Zhang, C.; Lv, W.; Yang, Q.-H. Catalyzing polysulfide conversion by g-C3N4 in a graphene network for long-life lithium-sulfur batteries. Nano Res. 2018, 11 (6), 3480-3489. (27) Liu, J.; Li, W.; Duan, L.; Li, X.; Ji, L.; Geng, Z.; Huang, K.; Lu, L.; Zhou, L.; Liu, Z.; Chen, W.; Liu, L.; Feng, S.; Zhang, Y. A Graphene-like Oxygenated Carbon Nitride Material for Improved Cycle-Life Lithium/Sulfur Batteries. Nano Lett. 2015, 15 (8), 5137-5142. (28) Song, J.; Feng, S.; Zhu, C.; Lee, J.-I.; Fu, S.; Dong, P.; Song, M.-K.; Lin, Y. Tuning the structure and composition of graphite-phase polymeric carbon nitride/reduced graphene oxide composites towards enhanced lithium-sulfur batteries performance. Electrochim. Acta 2017, 248, 541-546. (29) Rubio-Martinez, M.; Avci-Camur, C.; Thornton, A. W.; Imaz, I.; Maspoch, D.; Hill, M. R. New synthetic routes towards MOF production at scale. Chem. Soc. Rev. 26


Page 26 of 31

Page 27 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2017, 46 (11), 3453-3480. (30) Li, F.; Duan, C.; Zhang, H.; Yan, X.; Li, J.; Xi, H. Hierarchically Porous MetalOrganic Frameworks: Green Synthesis and High Space-Time Yield. Ind. Eng. Chem. Res. 2018, 57 (28), 9136-9143. (31) Sun, Z.; Wang, X.; Ying, H.; Wang, G.; Han, W.-Q. Facial Synthesis of ThreeDimensional Cross-Linked Cage for High-Performance Lithium Storage. ACS Appl. Mater. Interfaces 2016, 8 (24), 15279-15287. (32) Ying, H.; Zhang, S.; Meng, Z.; Sun, Z.; Han, W.-Q. Ultrasmall Sn nanodots embedded inside N-doped carbon microcages as high-performance lithium and sodium ion battery anodes. J. Mater. Chem. A 2017, 5 (18), 8334-8342. (33) Wen, Z.; Wang, X.; Mao, S.; Bo, Z.; Kim, H.; Cui, S.; Lu, G.; Feng, X.; Chen, J. Crumpled Nitrogen-Doped Graphene Nanosheets with Ultrahigh Pore Volume for High-Performance Supercapacitor. Adv. Mater. 2012, 24 (41), 5610-5616. (34) Hummers, W. S.; Offeman, R. E. PREPARATION OF GRAPHITIC OXIDE. J. Am. Chem. Soc. 1958, 80 (6), 1339-1339. (35) Niu, P.; Zhang, L.; Liu, G.; Cheng, H.-M. Graphene-Like Carbon Nitride Nanosheets for Improved Photocatalytic Activities. Adv. Funct. Mater. 2012, 22 (22), 4763-4770. (36) Hou, Y.; Wen, Z.; Cui, S.; Feng, X.; Chen, J. Strongly Coupled Ternary Hybrid Aerogels of N-deficient Porous Graphitic-C3N4 Nanosheets/N-Doped Graphene/NiFeLayered Double Hydroxide for Solar-Driven Photoelectrochemical Water Oxidation. Nano Lett. 2016, 16 (4), 2268-2277. (37) Zhou, G. M.; Zhao, Y. B.; Manthiram, A. Dual-Confined Flexible Sulfur Cathodes Encapsulated in Nitrogen-Doped Double-Shelled Hollow Carbon Spheres and Wrapped with Graphene for Li-S Batteries. Adv. Energy Mater. 2015, 5 (9) , 1402263. (38) Wu, H.; Zheng, G.; Liu, N.; Carney, T. J.; Yang, Y.; Cui, Y. Engineering Empty Space between Si Nanoparticles for Lithium-Ion Battery Anodes. Nano Lett. 2012, 12 (2), 904-909. (39) Gong, Y.; Fu, C.; Zhang, G.; Zhou, H.; Kuang, Y. Three-dimensional Porous C3N4 27



Nanosheets@Reduced Graphene Oxide Network as Sulfur Hosts for High Performance Lithium-Sulfur Batteries. Electrochim. Acta 2017, 256, 1-9. (40) Xie, X.; Zhao, M.-Q.; Anasori, B.; Maleski, K.; Ren, C. E.; Li, J.; Byles, B. W.; Pomerantseva, E.; Wang, G.; Gogotsi, Y. Porous heterostructured MXene/carbon nanotube composite paper with high volumetric capacity for sodium-based energy storage devices. Nano Energy 2016, 26, 513-523. (41) Yang, S.; Gong, Y.; Zhang, J.; Zhan, L.; Ma, L.; Fang, Z.; Vajtai, R.; Wang, X.; Ajayan, P. M. Exfoliated Graphitic Carbon Nitride Nanosheets as Efficient Catalysts for Hydrogen Evolution Under Visible Light. Adv. Mater. 2013, 25 (17), 2452-2456. (42) Mei, R.; Song, X.; Hu, Y.; Yang, Y.; Zhang, J. Hollow reduced graphene oxide microspheres as a high-performance anode material for Li-ion batteries. Electrochim. Acta 2015, 153, 540-545. (43) Tang, K.; Fu, L.; White, R. J.; Yu, L.; Titirici, M.-M.; Antonietti, M.; Maier, J. Hollow Carbon Nanospheres with Superior Rate Capability for Sodium-Based Batteries. Adv. Energy Mater. 2012, 2 (7), 873-877. (44) Qiao, Y.; Hu, X.; Liu, Y.; Chen, C.; Xu, H.; Hou, D.; Hu, P.; Huang, Y. Conformal N-doped carbon on nanoporous TiO2 spheres as a high-performance anode material for lithium-ion batteries. J. Mater. Chem. A 2013, 1 (35), 10375-10381. (45) Qiu, Y.; Li, W.; Zhao, W.; Li, G.; Hou, Y.; Liu, M.; Zhou, L.; Ye, F.; Li, H.; Wei, Z.; Yang, S.; Duan, W.; Ye, Y.; Guo, J.; Zhang, Y. High-Rate, Ultra long Cycle-Life Lithium/Sulfur Batteries Enabled by Nitrogen-Doped Graphene. Nano Lett. 2014, 14 (8), 4821-4827. (46) Zeng, X.; Ding, Z.; Ma, C.; Wu, L.; Liu, J.; Chen, L.; Ivey, D. G.; Wei, W. Hierarchical Nanocomposite of Hollow N-Doped Carbon Spheres Decorated with Ultrathin WS2 Nanosheets for High-Performance Lithium-Ion Battery Anode. ACS Appl. Mater. Interfaces 2016, 8 (29), 18841-18848. (47) Li, X.; Wolden, C. A.; Ban, C.; Yang, Y. Facile Synthesis of Lithium Sulfide Nanocrystals for Use in Advanced Rechargeable Batteries. ACS Appl. Mater. Interfaces 2015, 7 (51), 28444-28451. 28


Page 28 of 31

Page 29 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(48) Zhang, Z.; Kong, L.-L.; Liu, S.; Li, G.-R.; Gao, X.-P. A High-Efficiency Sulfur/Carbon Composite Based on 3D Graphene Nanosheet@Carbon Nanotube Matrix as Cathode for Lithium-Sulfur Battery. Adv. Energy Mater. 2017, 7 (11) , 1602543. (49) Zheng, D.; Liu, D.; Harris, J. B.; Ding, T.; Si, J.; Andrew, S.; Qu, D.; Yang, X.Q.; Qu, D. Investigation of the Li-S Battery Mechanism by Real-Time Monitoring of the Changes of Sulfur and Polysulfide Species during the Discharge and Charge. ACS Appl. Mater. Interfaces 2017, 9 (5), 4326-4332. (50) Zhu, P.; Zang, J.; Zhu, J.; Lu, Y.; Chen, C.; Jiang, M.; Yan, C.; Dirican, M.; Selvan, R. K.; Kim, D.; Zhang, X. Effect of reduced graphene oxide reduction degree on the performance of polysulfide rejection in lithium-sulfur batteries. Carbon 2018, 126, 594-600. (51) Zheng, G.; Yang, Y.; Cha, J. J.; Hong, S. S.; Cui, Y. Hollow Carbon NanofiberEncapsulated Sulfur Cathodes for High Specific Capacity Rechargeable Lithium Batteries. Nano Lett. 2011, 11 (10), 4462-4467. (52) Zhou, W.; Xiao, X.; Cai, M.; Yang, L. Polydopamine-Coated, Nitrogen-Doped, Hollow Carbon Sulfur Double-Layered Core-Shell Structure for Improving Lithium Sulfur Batteries. Nano Lett. 2014, 14 (9), 5250-5256. (53) Ni, L.; Wu, Z.; Zhao, G.; Sun, C.; Zhou, C.; Gong, X.; Diao, G. Core-Shell Structure and Interaction Mechanism of gamma-MnO2 Coated Sulfur for Improved Lithium-Sulfur Batteries. Small 2017, 13 (14). (54) Liu, X.; Huang, W.; Wang, D.; Tian, J.; Shan, Z. A nitrogen-doped 3D hierarchical carbon/sulfur composite for advanced lithium sulfur batteries. J. Power Sources 2017, 355, 211-218. (55) Ding, Y.-L.; Kopold, P.; Hahn, K.; van Aken, P. A.; Maier, J.; Yu, Y. Facile SolidState Growth of 3D Well-Interconnected Nitrogen-Rich Carbon Nanotube-Graphene Hybrid Architectures for Lithium-Sulfur Batteries. Adv. Funct. Mater. 2016, 26 (7), 1112-1119. (56) Meng, Z.; Li, S. J.; Ying, H. J.; Xu, X.; Zhu, X. L.; Han, W. Q. From Silica Sphere 29



to Hollow Carbon Nitride-Based Sphere: Rational Design of Sulfur Host with Both Chemisorption and Physical Confinement. Adv. Mater. Interfaces 2017, 4 (11) , 1601195. (57) Li, W.; Zhang, Q.; Zheng, G.; Seh, Z. W.; Yao, H.; Cui, Y. Understanding the Role of Different Conductive Polymers in Improving the Nanostructured Sulfur Cathode Performance. Nano Lett. 2013, 13 (11), 5534-5540. (58) Chung, S.-H.; Manthiram, A. High-Performance Li-S Batteries with an Ultralightweight MWCNT-Coated Separator. J. Phys. Chem. Lett. 2014, 5 (11), 1978-1983. (59) Zu, C.; Klein, M.; Manthiram, A. Activated Li2S as a High-Performance Cathode for Rechargeable Lithium-Sulfur Batteries. J. Phys. Chem. Lett. 2014, 5 (22), 39863991. (60) Yu, X.; Manthiram, A. Highly Reversible Room-Temperature Sulfur/Long-Chain Sodium Polysulfide Batteries. J. Phys. Chem. Lett. 2014, 5 (11), 1943-1947. (61) Xu, T.; Song, J. X.; Gordin, M. L.; Sohn, H.; Yu, Z. X.; Chen, S. R.; Wang, D. H. Mesoporous Carbon-Carbon Nanotube-Sulfur Composite Microspheres for HighAreal-Capacity Lithium-Sulfur Battery Cathodes. ACS Appl. Mater. Interfaces 2013, 5 (21), 11355-11362.

30


Page 30 of 31

Page 31 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC:

31


CNT Microspheres via Ethanol

Recommend Documents