Confining Sulfur in N-Doped Porous Carbon Microspheres Derived

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Confining Sulfur in N-doped Porous Carbon Microspheres Derived from Microalgaes for Advanced Lithium-Sulfur Batteries Yang Xia, Ruyi Fang, Zhen Xiao, Hui Huang, Yongping Gan, Rongjun Yan, Xianghong Lu, Chu Liang, Jun Zhang, Xinyong Tao, and Wenkui Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05798 • Publication Date (Web): 27 Jun 2017 Downloaded from http://pubs.acs.org on June 29, 2017

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ACS Applied Materials & Interfaces

Confining Sulfur in N-doped Porous Carbon Microspheres Derived from Microalgaes for Advanced Lithium-Sulfur Batteries Yang Xia,1,† Ruyi Fang,1, † Zhen Xiao,2 Hui Huang,1 Yongping Gan,1 Rongjun Yan,3 Xianghong Lu,4 Chu Liang,1 Jun Zhang,1 Xinyong Tao,1,* and Wenkui Zhang1,* 1

College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou,

310014, China 2

College of Materials Science and Engineering, China Jiliang University, Hangzhou, 310018, China

3

Ocean College, Zhejiang University of Technology, Hangzhou, 310014, China

4

College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, 310014, China.

*

Corresponding Authors: [email protected] (X. Tao); [email protected] (W. Zhang)

†

These authors contributed equally to this work.

ABSTRACT Lithium-sulfur (Li-S) battery is one of the most attractive candidates for the next-generation energy storage system. However, the intrinsic insulating nature of sulfur and the notorious polysulfide shuttle are the major obstacles, which hinder the commercial application of Li-S battery. Confining sulfur into conductive porous carbon matrices with designed polarized surfaces is regarded as a promising and effective strategy to overcome above issues. Herein, we propose to use microalgaes (Schizochytrium sp.) as low-cost, renewable carbon/nitrogen precursors and biological templates to synthesize N-doped porous carbon microspheres (NPCMs). These rational designed NPCMs can not only render the sulfur-loaded NPCMs (NPCSMs) composites with high electronic conductivity and sulfur content, but also greatly suppress the diffusion of polysulfides by strongly physical and chemical adsorptions. As a result, NPCSMs cathode demonstrates a superior reversible capacity (1030.7 mA h g-1) and remarkable capacity retention (91%) at 0.1 A g-1 after 100 cycles. Even at an extremely high current density of 5 A g-1, NPCSMs still can deliver a satisfactory

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discharge capacity of 692.3 mAh g-1. This work reveals a sustainable and effective biosynthetic strategy to fabricate N-doped porous carbon matrices for high performance sulfur cathode in Li-S battery, as well as offers a fascinating possibility to rationally design and synthesize novel carbon-based composites. KEYWORDS: Microalgaes; Porous carbon microspheres; Nitrogen doping; Biotemplating method; Lithium-sulfur batteries

1. INTRODUCTION Advanced energy storage system with low cost, long life, excellent safety, high energy density and power density is considered as a promising candidate for addressing intractable challenges of ever-increasing energy consumption and environment pollution.1-4 In recent years, multi-electron reaction is an efficient way for designing and developing high capacity materials and high energy density batteries.5 Particularly, rechargeable Li-S batteries with high energy density (2600 Wh kg-1) exhibit tremendous potential in energy storage applications since sulfur cathode has unrivalled merits including high theoretical specific capacity (1675 mAh g-1), natural abundance and environmental friendliness.6, 7 However, the insulating nature of S and Li2S/Li2S2 as well as the notorious polysulfide shuttle effect are main obstacles to commercialize of Li-S batteries.8-10 In detail, the extremely low conductivity of S (5 × 10−28 S m−1) and Li2S (1 × 10−13 S m−1) results poor electrochemical activity and inferior rate capability.8, 11 Meanwhile, the intermediates of lithium polysulfides are able to dissolve into electrolyte, causing the continuous loss of active materials and the severe capacity fading.12 Furthermore, the dissolved polysulfides shuttled between cathode and anode will reduce Coulombic efficiency and deactivate Li metal anode.6, 13 To address the aforementioned issues, significant efforts and improvements are made. So far, impregnating sulfur into conductive porous carbon matrices to construct carbon-sulfur composites is one of the most popular and effective approaches to


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enhance electrochemical properties of Li-S battery. For instance, porous carbon14-17 has been successfully verified to ameliorate the electrical conductivity and structural stability of sulfur cathode as well as suppress the dissolution and shuttling of reactive polysulfide intermediates. However, most of porous carbon matrices are hydrophobicity in nature. These conventional carbon matrices only provide physical sequestration and adsorption to slow down the active material loss to some extent because of the weak interactions between nonpolar carbon matrices and polar polysulfide intermediates.18-20 Hence, exploring carbon matrices with polarized surfaces as sulfur hosts to achieve the strong chemical adsorption with polysulfide intermediates for radically overcoming polysulfide shuttling is a crucial step to sulfur cathode in Li-S batteries. Carbon materials doped with heteroatoms will have abundant functional surface groups, exhibiting significant potential for the confinement and adsorption of both sulfur and polysulfides as reported by many literatures.21-25 Particularly, doping nitrogen (N) into carbon framework can form various N-containing groups including pyridinic N, pyrrolic N and graphitic N.26 These nitrogen groups enable carbon matrices to have better hydrophilic ability, resulting dramatically enhanced chemisorption of polysulfides.27, 28 Meanwhile, N-doping significantly improves the electrical conductivity of carbon matrices, which could accelerate the electrochemical reaction.26, 29 Up to now, great efforts are devoted to synthesizing N-doped carbon matrices via direct pyrolysis of N-containing polymers, such as pyridine21, polyaniline30, polypyrrole31, 32, polydopamine33-35, cyanamide36 and melamine26, 37, 38. However, these N-rich precursors often are expensive, unrenewable, and even highly toxic. Moreover, owing to the complex polymer pyrolysis process, the chemical composition, microstructure and morphology of N-doped carbon matrices are difficult to be controlled. Consequently, a green, facile and cost-effective method for fabricating N-doped carbons as sulfur hosts still remains a great challenge. Recently, the rational utilization of biomass to synthesize state-of-the-art carbon-based electrode materials is becoming a hot spot in materials science and



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engineering. Compared to routine methods, biotemplating synthetic strategies have many incomparable merits: i) biomass resources are abundant, inexpensive and renewable; ii) biomaterials have multiple scales, uniform geometries, complex structures, novel morphologies and precise chemical composition; iii) biomaterials have many unique biophysical and biochemical phenomena including metal hyperaccumulation, biological mineralization, and bio-oxidation-reduction. In fact, our group has already successfully established a variety of examples, such as LiFePO4/C39, NiO/C40, MnO/C41, Sn/C42 and Si-O-C43. In the present work, we attempt to fabricate N-doped porous carbon microspheres from microalgaes (Schizochytrium sp.) as sulfur hosts for Li-S batteries (Figure 1). According to previous literature44, Schizochytrium sp. is a kind of unicellular spherical microalgae with an extremely high content of protein (>66% by dry weight), which could provide natural carbon and nitrogen sources. Moreover, Schizochytrium sp. has a vigorous fertility, which is able to increase more than 3 times of its original biomass in 12 h (Figure S1). Consequently, microalgaes served as biotemplates and renewable carbon/nitrogen sources are highly expected to produce N-doped porous carbon microspheres for impregnating sulfur.

Figure 1. Schematic representation of fabricating N-doped porous carbon@sulfur microspheres.

2. EXPERIMENTAL SECTION 2.1 Microalgae Culture


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Schizochytrium sp. (ATCC 20888) was purchased from American Type Culture Collection (ATCC). Firstly, Schizochytrium sp. was grown in sterilized seed medium for 48 h at 20 oC and pH=8.0 with 3000 µmol·m-2·s-1 light intensity. The seed medium (per liter) consisted of 15.0 g seawater crystal, 5.0 g glucose, 1.0 g yeast extract and 1.0 g peptone. After that, Schizochytrium sp. was inoculated into the sterilized fermentation medium and cultivated for 72 h at 20 oC and pH=8.0 with a nature light under bubbling of aseptic air continuously. The inoculative proportion of seed medium and fermentation medium was 6%, the composition of seed medium (per liter) was listed as follows: 22.5 g seawater crystal, 90.0 g glucose, 15.0 g yeast extract and 15.0 g peptone. 2.2 Materials Synthesis N-doped porous carbon microspheres (NPCMs): 100 ml Schizochytrium sp. solution was concentrated at first. The concentrated Schizochytrium sp. solution was rinsed 5 times with deionized water, and dried in an oven at 45 oC for 12 h. Then 2 g Schizochytrium sp. was transferred to a tube furnace and heated to 500 oC for 2 h in a flowing argon (Ar) atmosphere. After that, NPCMs samples were obtained. N-doped porous carbon@sulfur microspheres (NPCSMs): 0.5 g carbon microspheres and 5 g sulfur powder were mixed together with 40 ml CS2 as the solvent medium. The above suspension was then transferred into a Teflon-lined autoclave. After heating at 140 oC for 20 h in an oven, the black precipitation was detached from the solution quickly by a filtering process. Finally, the above precipitation was heated in a tube furnace at 300 oC for 10 min under Ar atmosphere to further eliminate the redundant S that on the sample surface. 2.3 Materials Characterizations X-ray diffraction patterns of NPCMs and NPCSMs samples were recorded on X’Pert Pro diffractometer (Cu Kα radiation, λ = 1.5418 Å). The morphologies were observed on a Hitachi S-4800 Scanning Electron Microscopy (SEM). The microstructure and elemental mappings were performed on a Transmission Electron Microscopy (TEM, FEI, TecnaiTM G2 F30) equipped with an energy dispersion X-ray spectroscopy



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detector (EDS). Thermogravimetric (TG) analysis was carried out by TA Instruments SDT Q600 in Ar flow with a heating rate of 10 oC min-1. The Raman spectra were conducted on a DXR Raman microscopy (Thermo Fisher Scientific) by using a He-Ne 532 nm laser excitation. The surface area of NPCMs and NPCSMs samples was determined by Brunanuer-Emmett-Teller (BET) method based on nitrogen adsorption-desorption tests (ASAP 2020, Micromeritics Instruments). The pore size distributions

were

calculated

from

the

adsorption

branches

of

N2

adsorption-desorption isotherms. X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD system, Kratos) was used to analyze surface chemical states of samples. In this work, all the peak positions were calibrated using C 1s peak (284.8 eV). The UV-visible absorption spectra of polysulfide adsorption tests were carried out by a SHIMADSU UV-2550 spectrophotometer in a wavelength range from 250 to 600 nm. 2.4 Electrochemical Measurements The working electrode films were prepared by mixing NPCSMs (80 wt.%), polyvinylidene fluoride (PVDF) binder (10 wt.%) and acetylene black (10 wt.%). N-methyl-2-pyrrolidinone (NMP) was employed as dispersant. The cathode slurry was coated onto Al foil by using stainless steel scraper blade, and then dried in a vacuum oven at 60 oC for 12 h. The mass loading of NPCSMs electrode was 1.7~2.0 mg cm-2. As the sulfur content in NPCSMs is 65 wt.%, the areal sulfur loading in each cathode is 1.1~1.3 mg cm-2. For thick electrodes, the average mass loading of NPCSMs was 3.8 mg cm-2, corresponding to a sulfur loading of 2.5 mg cm-2. The cells (CR2025 coin-type) were assembled in an Ar-filled glove box (MIKROUNA, moisture < 1.0 ppm, oxygen < 1.0 ppm). Li metal foil and Celgard 2300 membrane were employed as anode and separator, respectively. The electrolyte was prepared by dissolving 1.0 M LiN(CF3SO2)2 (LiTFSI) salt into a mixed solvent comprised of 1,3-dioxolane (DOL) and dimethoxymethane (DME) by 1:1 in volume. The electrolyte-to-sulfur ratio (E/S ratio) in each cell is controlled as 10 µl mg-1. The galvanostatic charge-discharge tests were performed on Neware battery test system (Shenzhen Neware Technology Co.Ltd.) with the voltage window of 1.5-3.0 V. Cyclic


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voltammograms (CV) and electrochemical impedance spectroscopy (EIS) were carried out by CHI650B electrochemical workstation, respectively.

3. RESULTS AND DISCUSSION

Figure 2. SEM images of NPCMs (a-b) derived from Schizochyrtium sp. cells calcined at 500 oC for 2 h and NPCSMs (c-d) after solvothermal reaction at 140 oC for 20 h and post-heat treatment at 300 oC for 10 min under a flowing Ar atmosphere.

As shown in Figure 2a-b, NPCMs are highly uniform microspheres with a diameter of ca. 2 µm, indicating that microalgaes have excellent thermal stability and structural stability during heat treatment. After the sulfur impregnation, no obvious change in morphology and particle size of NPCSMs can be detected (Figure 2c-d). The detail particle size comparison between NPCMs and NPCSMs also has been presented in Figure S2. Obviously, NPCMs and NPCSMs have a similar particle size distribution, which is centered at 2.0-2.5 µm. Additionally, no discernible sulfur particles or their agglomerations stay at the surface of NPCSMs, suggesting sulfur has been well impregnated into NPCMs.



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Figure 3. (a) XRD patterns of NPCMs, NPCSMs and pristine sulfur; (b) TG curves of pristine sulfur and NPCSMs; (c) Raman spectra of NPCMs and NPCSMs; (d) N2 adsorption-desorption isotherms and pore size distributions of NPCMs and NPCSMs.

XRD patterns of sulfur, NPCMs and NPCSMs are presented in Figure 3a. Two broad diffraction peaks are located at 21-27o and 42-45o in NPCMs, indicating the amorphous structure of microalgae carbon in nature. Meanwhile, some sharp and strong diffraction peaks observed in NPCSMs belong to the orthorhombic structure of sulfur (S8, JCPDS no. 08-0247). However, compared with sublimed sulfur sample, the intensity of these diffraction peaks belonged to sulfur in NPCSMs sample is dramatically decreased, demonstrating sulfur has been homogeneously confined in the small pores of NPCMs. The content and thermal stability of sulfur in NPCSMs were determined by TG analysis. As depicted in Figure 3b, a noticeable weight loss of 65 wt.% occurs between 200 and 320 oC in NPCSMs resulting from the evaporation of sulfur, suggesting the actual sulfur content in NPCSMs is ca. 65 wt.%. Meanwhile the evaporating temperature of elemental sulfur starts from 190 oC and ends at 290 oC.


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Apparently, the evaporating temperature of sulfur component in NPCSMs is visibly increased, implying that NPCMs host may have a strong affinity with sulfur and excellent encapsulation ability to sulfur.45 The Raman spectra of NPCMs and NPCSMs are plotted in Figure 3c. Two evident peaks appeared at 1350 cm-1 and 1580 cm-1 in both NPCMs and NPCSMs can be assigned to the disordered sp3 (D band) and graphitic sp2 stretching of graphene (G band).37 The distinct G bands existed in NPCMs and NPCSMs indicate the good electrical conductivity, which will be beneficial to fast electron transfer during the electrochemical reaction process.46 The ratio of ID/IG is an effective indication of the crystallinity degree for carbon materials. The ID/IG values of NPCMs and NPCSMs are 0.82 and 0.95, respectively. The increased ID/IG value indicates that more defects emerge after the sulfur impregnation process. Additionally, the bending and stretching modes of sublimed sulfur often appear below 500 cm-1.11 However, these signals are very weak, further confirming a highly uniform dispersion of sulfur in NPCMs. In order to ascertain the specific surface area and pore size distribution of NPCMs and NPCSMs, nitrogen adsorption-desorption tests are performed. As shown in Figure 3d, a visible hysteresis loop can be observed at a relative pressure > 0.5, corresponding to a typical IV sorption behavior. This result indicates that mesoporous structure is existed in NPCMs. From Barrett-Joyner-Halenda (BJH) analysis, NPCMs exhibits a hierarchically porous feature, in which a narrow pore size distribution in 5 nm, and a wide distribution in range from 10 to 50 nm. These abundant pores and cavities in NPCMs are mainly attributed to the pyrogenic decomposition of Schizochytrium sp. cells. After impregnating sulfur into NPCMs, the small pore centered at 5 nm dramatically decreases in NPCSMs. Meanwhile, the specific surface area/pore volume of NPCMs also sharply decrease from 671 m2 g-1/1.36 cm3 g-1 to 89 m2 g-1/0.17 cm3 g-1. This phenomenon is attributed to the successful impregnation of sulfur into NPCMs, which is similar to our previous work47. In this work, such hierarchically porous characteristic may play multi-roles for achieving high electrochemical performance. On the one hand, the smaller mesopores (< 5 nm) will



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be appropriate to prevent polysulfide dissolution.48, 49 On the other hand, the larger mesopores and cavities can store more sulfur and alleviate the volume expansion.50

Figure 4. (a-c) TEM, STEM and HRTEM images of NPCSMs. (d) High magnification STEM image of NPCSMs taken from the selected area in (b); (e-g) EDS mapping results of C, N and S.

The internal microstructure and element distribution of NPCSMs are depicted in Figure 4. Noticeably, as shown in Figure 4a-b, NPCSMs composite exhibits microspherical shape. The particle size and morphology are similar to NPCMs (Figure S3a-b). Moreover, STEM images (Figure 4b and Figure S3b) vividly describe the porous microstructure in both NPCMs and NPCSMs. Before sulfur impregnation, C and N are homogeneously dispersed in NPCMs (Figure S3e-f). No discernible S signal can be observed (Figure S3g). After sulfur impregnation (Figure 4e-g), C, N and S elements with the similar signal intensities are uniformly distributed in NPCSMs, implying sulfur is well confined within NPCMs host. Furthermore, HRTEM images in Figure 4c and Figure S3c show that there is no orderly lattice fringe of sp2 carbon in both of NPCMs and NPCSMs samples. It is worth noting that the interlayer spacing between two distorted lattice fringes slightly expands from 0.35 to 0.39 nm, indicating some low-molecular sulfur may embed into graphitic layers and form a strong affinity with NPCMs.

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Figure 5. XPS analysis of NPCMs and NPCSMs. (a) XPS survey spectra; (b-d) High-resolution XPS spectra of C 1s, N 1s and S 2p, respectively.

The chemical composition and surface chemical state of NPCMs and NPCSMs have been further examined by XPS tests. As revealed in Figure 5a, NPCMs and NPCSMs samples both have three strong peaks appeared at 284.5, 400.0 and 531.5 eV, which can be assigned to C 1s, N 1s and O 1s, respectively. The atomic percentage of N element is approximately 8.9%. Meanwhile, the existences of S 2p and S 2s peaks in NPCSMs sample reveal sulfur has been successfully incorporated into NPCMs. Moreover, Figure 5b demonstrates that NPCMs and NPCSMs have the similar asymmetric peak profiles in C 1s spectra, which could be assigned to C-C (284.8 eV), C-N/C-S (285.6 eV), C-O (286.6±0.1 eV), C=O (288±0.1 eV) and O-C=O (289.9±0.1 eV), respectively.51, 52 As shown in Figure 5c, N 1s spectra of NPCMs and NPCSMs both show three peaks with binding energies of pyridinic N (398.3±0.2 eV), pyrrolic N (400.2±0.2 eV) and graphitic N (401.4±0.2 eV)49,

51


. According to previous


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literatures49,

53-55

, the appropriate N-containing groups in carbon matrices will

successfully set up an effective chemical gradient for trapping various S species with different polarities. In detail, electron-rich pyridinic/pyrrolic N can bind directly to highly positive charged Li atom in polar polysulfides.54, 55 Meanwhile, graphitic N served as electron donors can provide additional electrons to neighboring delocalized π-systems, resulting higher affinity to polysulfides than graphene plane.54, 55 Therefore, NPCMs derived from microalgaes will render heterogeneous surface for achieving highly amphiphilic affinity to both S (nonpolarity) and polysulfides (polarity), guaranteeing strongly-coupled interfaces between conductive carbon matrices and various S species generated during the charge-discharge process. In addition, no discernible S 2p signal can be observed in NPCMs. However, two peaks sited at 165.2 and 164.0 eV are appeared in the S 2p spectrum of NPCSMs, corresponding to binding energies of S 2p1/2 and S 2p3/2 for S8 molecules. The fitting results indicate the chemical bonds of -C-S-C- (163.7 eV), -C=S- (164.9 eV) and -C-SOx- (168.4 eV) are existed in NPCSMs.51

Figure 6. UV-vis absorption spectra of Li2S6 solution with NPCMs. Inset is the digital images of the polysulfide adsorption test.

To evaluate the adsorption capability of NPCMs, the polysulfide adsorption test was carried out. In this work, the simulative solution is composed of 0.5 mmol L-1 Li2S6 in a 1:1 volume of DOL/DME mixed solvent. As shown in Figure 6, a broad absorption region ranging from 250 to 500 nm can be observed in the polysulfide


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solution.56-60 Two characteristic peaks centered at 260 and 280 nm are ascribed to S62species.58 One broad peak appeared at approximately 420 nm is attributed to S42species.58,

59

After adding NPCMs into Li2S6/DOL-DME solution, the color of

polysulfide solution has a noticeable change. The Li2S6/DOL-DME solution with NPCMs turns dark brown to colorless gradually with 30 min. Correspondingly, a dramatic change can be found in UV-visible absorption spectra as well. The peak intensities of S62- species and S42- species are greatly decreased after adding NPCMs. These results clearly demonstrate that NPCMs have the remarkable adsorption capability for polysulfides.

Figure 7. XPS analysis of pristine Li2S6 and NPCMs-Li2S6. (a) Li 1s spectra. (b) S 2p spectra.

Additionally, the interaction between NPCMs and polysulfide Li2S6 was verified via XPS. Figure 7a depicts that a single symmetric peak at 55.4 eV can be observed in Li 1s spectrum of Li2S6.61 After adding NPCMs, the Li 1s spectrum of NPCMs-Li2S6 sample presents an asymmetric peak that shifts to the high binding energy. An additional peak located at 56.5 eV belongs to the interaction of Li and N (Li-N).28 The S 2p spectrum of NPCMs-Li2S6 sample also has significantly altered compared to pristine Li2S6 (Figure 7b). The high-resolution S 2p spectrum of pristine Li2S6 presents two sulfur contributions at 161.5 and 163.0 eV, corresponding to terminal (S-1T) and bridging sulfur (S B0 ) atoms.62 After coupling with NPCMs, the peak of S-1T dramatically shifts to high binding energy (162.9 eV), suggesting the electron density on S-1T in NPCMs-Li2S6 sample is decreasing.28,

61

This observation confirms that



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NPCMs has strong chemical adsorption to S T . Moreover, an additional S B -C species is detected at 164.3 eV, which is assigned to the disproportionation of Li2S6 into S0.28 It is worth noting that some features of thiosulfate and polythionate species are appeared in NPCMs-Li2S6 sample, which is attribute to the redox reaction between Li2S6 and NPCMs.28 Hence, based on the above XPS analysis, it can be convinced that NPCMs as sulfur host can effectively entrap polysulfides to maintain excellent cyclic stability.

Figure 8. (a) CV curves of NPCSMs electrodes. (b) Charge-discharge profiles of NPCSMs electrodes. (c) Cycling stability and Coulombic efficiency of NPCSMs electrodes. (d) Multi-rate cycling performance of NPCSMs electrodes.

The electrochemical performance of NPCSMs composite was systematically evaluated by CR2025 coin-type cells. As depicted in CV profiles (Figure 8a), the sharp redox peaks indicate the multiple reaction mechanism of sulfur cathode in Li-S battery. Two well-defined reduction peaks are located at 2.32 and 2.04 V during the cathodic scanning. The first reduction peak (2.32 V) is related to the conversion of S8 to higher order Li2Sx (4 ≤ x ≤ 8).56 The second reduction peak located at 2.04 V corresponds to the subsequent transformation of Li2S2 or Li2S.51 Meanwhile, there is


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only one distinct oxidation peak at 2.4 V during the anodic scanning, which is the oxidation process of Li2S/Li2S2 to Li2S8.63 It is noteworthy that the intensity of redox peaks gradually increase during the initial 3 cycles. This phenomenon is attributed to the activation process that the penetration of electrolyte into deeply inner pores and the complete reaction of sulfur will take time. Additionally, all successive cycles of CV profiles overlap well, implying the outstanding electrochemical reversibility of NPCSMs. Figure 8b illustrates the charge-discharge profiles of NPCSMs electrodes. Two well-defined discharge plateaus clearly indicate the multistep reductions of S8 to long-chain Li2Sx (4 ≤ x ≤ 8) at 2.32 V and short-chain Li2S2/Li2S at 2.1 V. Meanwhile, two charge potential plateaus at 2.2 and 2.3 V belong to Li2Sx oxidation process.63 These results match well with above CV tests. The first charge and discharge specific capacities of NPCSMs electrodes are 1207.2 and 1119.5 mAh g-1, resulting an acceptable Coulombic efficiency of 92.7% with a slight overcharge. This slight overcharge phenomenon is attributed to the following reason. Some dissolved Li2Sx derived from the sulfur located in macropores (> 50 nm) may not be completely trapped since the adsorption ability of macropores is weak. Subsequently, the dissolved lithium polysulfides will slightly migrate into electrolyte, resulting the slight overcharge. After 100 cycles, as shown in Figure 8c, the reversible discharge and charge capacities can be retained at 1030.7 and 1032.7 mAh g-1 along with a satisfactory 91% capacity retention. The Coulombic efficiency from the second cycle onward gradually increases to 99 % as well. Here, in order to double check the ability of polysulfides entrapment of NPCSMs electrodes, we tested NPCSMs electrodes in a transparent bottle to survey the color change in electrolyte. As expected, no obvious color changes in electrolyte after 100 cycles, which vividly demonstrates that NPCSMs electrodes have excellent cyclic stability and fascinating capability of trapping polysulfides. The rate capability of NPCSMs was further conducted by multi-rate tests (Figure 8d). The reversible discharge/charge capacities of NPCSMs electrode at various



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current densities ranging from 0.1 to 5.0 A g-1 are 1098.9/1105.5, 1020.5/1023.7, 940.5/960.1, 803.4/807.7 and 696.6/700.8 mAh g-1, respectively. Notably, during each rate cycling interval, the discharge capacity can always be completely recovered when the current density is abruptly switched back to 0.1 A g-1. Furthermore, the high mass loading of sulfur is a significant criterion for Li-S batteries in recent years. Hence, the thick NPCSMs cathodes (NPCSMs loading = 3.8 mg cm-2, S loading = 2.5 mg cm-2) were evaluated as well. As shown in Figure S4a, the charge/discharge curves of thick NPCSMs electrodes are similar to that of routine ones. No obvious polarization can be detected. Upon cycling at 0.1 A g-1, thick NPCSMs electrodes still deliver high charge/discharge specific capacity of 924.6/831.9 mAh g-1 with a satisfactory Coulombic efficiency of 89.9% in the 1st cycle. Interestingly, the charge specific capacity of thick NPCSMs electrode decreases during the initial 7 cycles, however, the discharge capacity is increasing gradually (Figure S4b). A peak discharge capacity of 892.2 mAh g-1 can be achieved at the 7th cycle. And the Coulombic efficiency is approximate to 99.2 %. This result can be ascribed to the activation of high-loading active materials in Li-S batteries.64 After 30 cycles, a high reversible discharge capacity can be stabilized at 858.3 mAh g-1, indicating good cycling stability of thick NPCSMs electrodes. Additionally, although the specific capacity of routine NPCSMs electrode (1030.7 mAh g-1) is higher than that of thick one (858.3 mAh g-1), the areal capacity greatly increases from 1.23 mAh cm-2 (routine NPCSMs electrode) to 2.12 mAh cm-2 (thick NPCSMs electrode). Hence, the above results evidently prove that NPCMs is an attractive carbon matrix to realize high-energy density and long lifespan of sulfur cathode in Li-S battery. EIS tests were performed at the initial state and after cycling state to better illustrate the superior electrochemical performance of NPCSMs electrode (Table S1, Figure S5 and S6). The charge transfer resistance (Rct) of NPCSMs electrodes sharply decreases from 152.3 to 109.6 Ω after 50 cycles. And the solid electrolyte interface (SEI) layer resistance (Rf) and Warburg factor (σ) of NPCSMs sample are slightly increased after extended cycles. This can be ascribed to the deposition of some


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residual insulating Li2S and the gradual formation of SEI layer on the surface of NPCSMs electrode. However, this change is very small, demonstrating the good electronic and ionic conductivity, fast electrochemical reaction kinetics and remarkable stability of NPCSMs electrode. Hence, the unique porosity and N-doped carbon matrix of NPCSMs may be responsible for the better interactions between sulfur and electrolyte, and few aggregations of non-conductive Li2S during long-term cycling. Table 1. Comparison of various N-doped carbon materials from biomass for Li-S batteries Sulfur Carbon source

Current

content

density

(%)

(mA g-1)

Cellulose

70

837.5

Yeast

65

Silkworm cocoon

Cycle

Discharge

Capacity

capacity

retention

(mAh g-1)

（%））

500

775

67.5

28

167.5

400

725

60.3

46

73

335

60

815

73.3

49

Silk cocoon

48.4

837.5

80

804

55.7

50

Shrimp shell

63

500

100

450

82.0

65

glucosamine

50

167.2

100

505

42.4

66

Fish scale

42

167.5

100

1228

99.5

67

Microalgae

65

100

100

1031

91.0

This work

number

Ref.

Up to now, many natural resources also have been employed as both carbon and nitrogen sources to prepare N-doped carbon materials for sulfur hosts in the application of Li-S batteries, such as cellulose28, yeast46, silk cocoon49, 50, shrimp shell65, glucosamine

66

and fish scale67. Herein, we summarized the electrochemical

performance of these N-doped carbon matrices from various natural biomass. As shown in Table 1, N-doped carbon synthesized from microalgaes has comparative mass loading of sulfur and specific capacity with that from other biomass materials. Particularly, NPCSMs exhibits an outstanding cycling stability corresponding to the high capacity retention rate of 91%. Moreover, in order to better demonstrate the advantages of using microalgaes as N-doped carbon precursor to serve as high-performance sulfur hosts, non-biomass derived N-doped carbon materials have also been made a careful comparison as listed in Table S2. The results clearly indicate



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that the sulfur content, reversible capacity and cyclic stability of NPCSMs composite are better than that most of non-biomass derived N-doped carbon materials. Based on the above results, this enhanced electrochemical performance of NPCSMs could be mainly explained by following: Firstly, uniformly dispersed sulfur in NPCMs hosts can shorten the diffusion distance for electrons/ions to reaction sites, leading to an improved reactivity and rate capability. Secondly, NPCMs hosts with rich porous structure will facilitate better contact of sulfur with a moderate amount of electrolyte and promise fast electron transportation. Thirdly, according to previous literatures21,

33-36, 68

, N-doping in carbon matrices will greatly enhance the

chemisorption between polysulfides and N-containing groups on carbon surfaces, which could efficiently suppress the polysulfides shuttle.

4. CONCLUSIONS In this work, a new kind of N-doped porous carbon microspheres has been successfully prepared by using low-cost and renewable microalgaes as carbon and nitrogen sources to encapsulate sulfur for advanced Li-S batteries. The unique hierarchically porous structure of NPCMs host can not only offer enough space to achieve a high content of sulfur, but also facilitate a better accessibility of sulfur to Li ions. More importantly, such multimodal porous structure guarantees a highly homogenous dispersion of sulfur in carbon matrix, and further retards the dissolution and migration of sulfur and its intermediate polysulfides. Meanwhile, doping nitrogen atoms into carbon to form some N-containing groups can realize a strongly chemical adsorption to polysulfides, and simultaneously suppress the polysulfide shuttle effect, resulting a high utilization of active materials. Owing to above merits, NPCSMs electrodes exhibit the significantly enhanced electrochemical performance with high reversible specific capacity, superior rate capability and remarkable cyclic stability. This green and facile biosynthetic strategy may open new avenues for the rational design and controllable fabrication of advanced Li-S batteries.


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■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.xx00xxx. Growth curve of Schizochytrium sp., particle size distribution histograms of NPCMs and NPCSMs, TEM, STEM, HRTEM and EDS mapping of NPCMs, cycling performance of thick NPCSMs electrode, Nyquist plots, Warburg plots and EIS results of NPCSMs electrodes at 1st and 50th cycle, comparison of N-doped carbon materials from non-biomass for Li-S batteries. ■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. Author contributions Y. Xia, X.Y. Tao and W.K. Zhang conceived the idea. R.J. Yan and X.H. Lu cultivated microalgaes. Y. Xia and R.Y. Fang prepared materials and conducted electrochemical tests. Z. Xiao, H. Huang and Y.P. Gan performed materials characterizations. C. Liang and J. Zhang prepared figures and tables. All authors discussed the results and commented on the whole paper. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21403196, 51572240 and 51677170), Natural Science Foundation of Zhejiang Province

(LY17E020010,

LQ14E020005,

LY16E070004),

and

Science

&

Technology Department of Zhejiang Province (2016C31012 and 2016C33009). ■ REFRENCES (1) Armand, M.; Tarascon, J. M. Building Better Batteries. Nature 2008, 451, 652-657.



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(61) Yang, J.; Chen, F.; Li, C.; Bai, T.; Long, B.; Zhou, X. A Free-Standing Sulfur-Doped Microporous Carbon Interlayer Derived from Luffa Sponge for High Performance Lithium-Sulfur Batteries. J. Mater. Chem. A 2016, 4, 14324-14333. (62) Peng, H. J.; Zhang, Z. W.; Huang, J. Q.; Zhang, G.; Xie, J.; Xu, W.T.; Shi, J. L.; Chen, X.; Cheng, X. B.; Zhang, Q. A Cooperative Interface for Highly Efficient Lithium-Sulfur Batteries. Adv. Mater. 2016, 28, 9551-9558. (63) Sun, Y.; Seh, Z. W.; Li, W.; Yao, H.; Zheng, G.; Cui, Y. In-Operando Optical Imaging of Temporal and Spatial Distribution of Polysulfides in Lithium-Sulfur Batteries. Nano Energy 2015, 11, 579-586. (64) Chung, S. H.; Chang, C. H.; Manthiram, A. A Carbon-Cotton Cathode with Ultrahigh-Loading

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Confining Sulfur in N-Doped Porous Carbon Microspheres Derived

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