Green Chemistry

PAPER

Cite this: Green Chem., 2014, 16,3926

Received 29th April 2014,Accepted 10th June 2014

DOI: 10.1039/c4gc00761a

www.rsc.org/greenchem

Preparation of a macroscopic, robust carbon-fibermonolith from filamentous fungi and itsapplication in Li–S batteries†

Liyuan Zhang,‡a Yangyang Wang,‡a Bing Peng,a,b Wanting Yu,a Haiying Wang,*a,b

Ting Wang,a,b Baiwan Deng,c Liyuan Chai,*a,b Kai Zhangd and Jiexi Wangd

A new sustainable microorganism-based route is reported for the synthesis of carbon-fiber monolith

through using filamentous fungi as feedstock. The fungi are cultured in solution within three days with

biomass as nutrient, and fungi concentration reaches as high as 11 mg mL−1 on an average. Based on the

rational control of fungi filtration and drying, fungi membrane or aerogel was obtained. Through pyrolysis

in an inert atmosphere, intact carbon-fiber monolith (membrane or aerogel) was formed and its conduc-

tivity was more than 1 S cm−1. The carbon-fiber aerogel and membrane synthesized at 800 °C was doped

by N (∼2.4 at%) and O (∼1.3 at%) and displayed a BET surface area of ∼305 and ∼20 m2 g−1, respectively.

Mesopores and macropores were detected in the carbon materials. The carbon-fiber monolith showed

promising capability to improve the cyclability and capacity of lithium–sulphur (Li–S) batteries, and are

expected to be used as versatile electrode in energy storage.

Introduction

Carbon is one of the most attractive materials with outstand-ing prospects in a wide range of applications because of itselectrical conductivity, chemical and thermal stabilities.1–3

Particularly, monolithic carbon materials (e.g., aerogel, mem-brane) composed of uniform microstructures have been exten-sively used in several areas, typically in batteries.4–8

Over the past few years, Li–S batteries have become promis-ing candidates as substantial means of energy storage.9 Suchbatteries are generally comprised a S cathode, insulatedseparator and Li anode, and their theoretical energy densitycan reach 2567 W h kg−1. However, the practical application ofLi–S batteries is limited because of poor cyclability, which ismainly attributed to the so-called shuttling effect of solublepolysulfide (LixSy) to the Li anode.10 In response to this chal-lenge, inserting a carbon layer between a cathode and a separa-tor to block the shuttling of LixSy has displayed exceptionalpromise in improving the cyclability of Li–S batteries. Carbonnanotube, microporous carbon and activated carbon are com-monly used but a nonconductive binder is required to assem-ble these building blocks into a monolithic membrane.11–13

Graphene is an ideal building block because the corres-ponding monolith can be easily fabricated based on π–πand hydrophobic interactions.14 However, the preparation ofreduced graphene oxide is not environmentally friendly andinvolves time-consuming purification procedures.

In this context, a novel method needs to be developedto achieve sustainable synthesis of porous monolithic carbon.Recently, attention has been focused on designing synthesisfrom renewable resources.15–22 Low-temperature hydrothermalsynthesis (<250 °C) is the most popular route for transformingbiomass to highly porous monolithic carbon.23–29 However,after hydrothermal reaction, further pyrolysis is requiredto promote the conductivity of carbon for electrode appli-cations.30,31 More recently, designing and fabricating monolithic

†Electronic supplementary information (ESI) available: ESI-1: Isolation andidentification of the fungi “chai-1”; ESI-2: Microbe growth curve; ESI-3: Separ-ation and qualitative analysis of fungi cell wall; ESI-4: Configuration of Li–S bat-teries; ESI-5: Website of multimedia; ESI-6: TGA-DTA of fungi monolith; ESI-7:Carbon monolith by carbonization of another two fungi; ESI-8: Observation offungi by optical microscope; ESI-9: Carbon monolith by carbonization of cellwall; ESI-10: Monolithic precursors from yeast and bacteria; ESI-11: Raman spec-trum of fungi monolith; ESI-12: N2 adsorption–desorption isotherm and Hgintrusion test; ESI-13: Performance of Li–S batteries using cell-wall-derivedcarbon membrane as conductive interlayer. See DOI: 10.1039/c4gc00761a‡These authors contributed equally to this work.

aDepartment of Environmental Engineering, School of Metallurgy and Environment,

Central South University, 410017 Changsha, China.

E-mail: liyuanchaihj@aliyun.com, haiyw25@yahoo.com; Fax: +86 731 88710171;

Tel: +86 731 88710171bThe National Engineering Research Centre for Pollution Control of Heavy Metals,

410017 Changsha, China. E-mail: liyuanchaihj@aliyun.com, haiyw25@yahoo.com;

Fax: +86 731 88710171; Tel: +86 731 88710171cSchool of Metallurgy and Environment, Central South University, 410017 Changsha,

ChinadSchool of Biological Sciences and Engineering at Shaanxi University of Technology,

723001 Hanzhong, China

3926 | Green Chem., 2014, 16, 3926–3934 This journal is © The Royal Society of Chemistry 2014

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carbon materials based on the combination of various subjectshas attracted extensive attention. For instance, cellulose mono-lith as ideal carbonization feedstock can be efficiently producedby the metabolism of bacteria (e.g., Acetobacter xylinus).32–34 Inaddition to metabolites, microorganisms themselves are sus-tainable carbonization feedstock and can reproduce usingcommon biomass as nutrients.35–38 However, to the best of ourknowledge, only a few groups have successfully synthesized theporous monolithic carbon from microorganisms.

Previously, a filamentous fungus was separated by us(ESI-1†). In this paper, we report an original synthesis ofcarbon-fiber monolith from a filamentous fungus based onsimple filtration and subsequent pyrolysis. This sustainableroute produces conductive, porous and intact carbon-fibermonolith (membrane and aerogel), which is doped by nitrogenand sulphur. The functional carbon-fiber monolith was usedas a porous conductive layer to improve the cyclability of Li–Sbatteries. Moreover, the generality of this fungi-based methodis good and can be readily repeated or improved by otherscientists.

ExperimentalCulture of filamentous fungi

The fungus separated by us was identified as Aspergillus aculea-tus by internal transcribed spacers sequence (KF613717)(ESI-1†). Fungi were first cultured in a solid medium. Themedium must contain carbon sources and inorganic salts, andnatural biomass (e.g., potato) commonly satisfies this require-ment. Potato infusion, dextrose (20 g L−1) and agar (15 g L−1)were adopted as nutrients. Potato infusion was prepared byboiling 200 g of sliced potatoes in 1 L of water for 30 minutes,which was then filtrated through a gauze. This infusion, “chai-1”, was incubated on a solid medium at 37 °C for seven days(ESI-1†).

The large-scale production of fungi was made from solutionculture. The nutrients were the same as that for a solidmedium but without agar. The fungi spores on the solidmedium were adopted as microorganism source and trans-ferred to a fluid medium. Temperature and shaking rate werecontrolled at 37 °C and 175 rpm, respectively.

In solution, this fungus reproduced efficiently. Fungi con-centration was close to maximum after 3 days (ESI-2†), whichon an average reached as high as 11 mg mL−1. This fungus canalso be cultured directly in a fermentation tank for industrialscale production. It should be noted that other biomassmaterials (e.g., glucose, starch and maltose) contain suitablenutrients but inorganic salts must be added when using purebiomass.

Preparation of carbon-monolith precursor

Fungi were first dispersed through vigorous stirring(1500 rpm) for 12 h. The dispersed fungi were rinsed withwater and collected by vacuum filtration. Then, fungi were

re-dispersed in a specific amount of water and stored at 0 °Cfor further usage.

To prepare the carbon-membrane precursor, 40 mL of fungidispersion was vacuum filtered by Buchner funnel (diameter∼7 cm) and a thin fungi film was constructed on a filter paper.The fungus film was peeled off from the paper and pressed ontwo slides, and subsequently dried in flowing air at 80 °Cfor 24 h.

To obtain the wet carbon-aerogel precursor, 100 mL offungi dispersion was first gravity filtered with G-2 sand-corefunnel (diameter ∼2 cm). The wet precursor was thenimmersed in 100 mL of specific organic solvent for 1 h, 3times in total, and then dried in vacuum at 80 °C. Specifically,when using ethanol to exchange water in precursor, ∼2 h arerequired to dry the sample. When further exchanging ethanolwith diethyl ether using the same procedure, the drying timeis ∼0.5 h.

Noticeably, for the sake of exhibiting the scale-up fabrica-tion (shown in Scheme 1 below), 300 mL of fungi dispersionwas gravity filtered with sand-core funnel (diameter 3.5 cm or9 cm) to obtain the fungi column. To obtain the membrane,300 mL of fungi solution was vacuum filtered with a Buchnerfunnel with a diameter of 30 cm.

Synthesis of carbon-fiber monolith

GSL-1500X-50 tube furnace was used. Dried fungi monolith inceramic crucible was put in a quartz tube. Air in the tube wasremoved by vacuum pumping hardware, and then N2 (purity99.999%) was added to the tube. To promise a good inertatmosphere, the gas in the tube was removed again by vacuumpumping the hardware and subsequently N2 (purity 99.999%)was added. The N2 flowing rate was ∼10 mL min−1. The tubewas heated to a desirable temperature and the ramp rate was5 °C min−1. The isothermal step lasted for 1 h and the tubewas cooled to room temperature at ambient conditions. Afterpyrolysis, fungi monolith shrunk obviously, and the size wasonly ∼50% of its original value.

Characterization

Morphology was investigated by JSM-6360 scanning electronmicroscope (SEM). To acquire the original morphology offungi, chemical treatment was performed. Briefly, a fungipellet was solidified with 2 wt% glutaraldehyde and then 1 wt%osmic acid. The solidified fungi pellet was frozen at −80 °Cfor 1 h and then dried in vacuum at −15 °C for 2 days. Finally,the dried solidified fungi pellet was coated with a thin Aulayer. The acceleration voltage for the SEM is 20 kV, and thevacuum freezing machine is an ancillary equipment ofJSM-6360 SEM. For other samples, no special treatment wasrequired. Thermal gravimetric and differential thermal ana-lysis (TGA-DTA) of fungi was acquired through a PTC10ARigaku Thermoflex thermal analyzer with 5 °C min−1 heatingrate in N2 gas (purity 99.999%). LABRAM-HR 800 spectrometerwas employed to measure the Raman scattering spectra offungi and carbon monolith in the range of 3000–500 cm−1

with an He–Le laser excitation at 513 nm and an acquisition

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time of 10 s. For Fourier transform infrared spectroscopy(FTIR, Nicolet IS10), the free-standing precursor or carbonmonolith was directly put into the IR chamber and examinedin the range of 4000–1000 cm−1 at a resolution of 4 cm−1. X-rayphotoelectron spectroscopy (XPS) was used to determine thecomposition of samples. XPS measurements were made onESCALAB 250Xi spectrometer with dual anode (Mg/Al) X-raysource with 400 W power. The pressure in the analysischamber was maintained at 10−10 Torr or lower during eachmeasurement. To compensate for surface charging effects, allbinding energies were referenced to the C1s neutral carbonpeak at 284.6 eV.26 Field-emission electron microprobe (JEOL60 JXA-8530F) was applied to quantitatively detect elementalcontent (atom ratio) on the sample with an acceleratingvoltage of 10 kV. Conductivity was measured using a four-probe method. N2 adsorption–desorption isotherm of carbonmonolith was carried out with Micromeritics ASAP2050 inliquid N2 and the sample was de-gassed in vacuum at 150 °Cfor 24 h prior to the measurement. Mechanical test was per-formed with an MTS Insight® Electromechanical TestingSystem. Mercury (Hg) intrusion test was carried out by Pore-master-60-GT pore size analyzer. The separation and qualitat-ive chemical analysis of cell wall can be found in ESI-3.†

Cell assembly

Configuration of Li–S batteries by inserting carbon-fiber mem-brane interlayer can be found in ESI-4.† The electrolyte wasfirst prepared in an argon filled glove box by dissolving 1 MLiCF3SO3, 0.3 M LiNO3 in the 1,3-dioxolane and 1,2-dimethoxy-ethane mixed solvent (volume ratio of 1 : 1). Sulphur with anaverage size of 150 mesh (100 μm) was used as receivedwithout any pre-treatment. The ratio of sulphur : carbon black :polyvinylidene fluoride was 60 : 30 : 10 by weight. N-Methyl-

2-pyrrolidinone was applied to prepare a homogenized slurry.After homogenization, the slurry was coated on an aluminiumcurrent collector (20 μm, thickness) using a doctor-blade anddried in vacuum at 60 °C for 24 hours.

Electrochemical measurements

The cyclic voltammogram (CV) plots were measured with aCHI-660D electrochemical workstation at a scan rate of 0.1 mVs−1 in the voltage window of 3.0–1.5 V. Galvanostaticcycling was conducted using an LANDCT2001A battery cyclerat 3.0–1.5 V (vs. Li/Li+). For electrochemical impedancespectroscopy tests, the frequency window was between 1 MHzand 0.01 Hz with an amplitude of 5 mV.

Results and discussion

Scheme 1 illustrates the synthesis route of carbon monolith.As shown, solution culture with biomass as nutrients can beused to efficiently produce the suspension of fungi pellets(Culture of filamentous fungi, Experimental). Various carbonmonolith precursors can be constructed by altering filtrationroutes. For instance, vacuum filtration produces fungi mem-brane (Scheme 1e) while gravity filtration generates fungicolumn (Scheme 1d). Soft monolithic precursors can bereadily peeled-off from the funnel (ESI-5: website of multi-media†). By pyrolysis under an inert atmosphere, intact carbonmonolith is formed and the yield calculated from TGA is27.1 wt% at 800 °C (ESI-6†). From a macroscopic viewpoint(total step in Scheme 1), biomass nutrients are C sources andfungi are bridges to transform C sources to fibers, which canbe further transformed to carbon monolith by filtration andpyrolysis. The synthesis in present research does not require

Scheme 1 Illustration of the synthesis of carbon monolith. (a) culture medium; (b) fungi suspension; (c) SEM image of fungi pellet; (d)–(e) mono-lithic fungi as carbon monolith precursor; (f )–(g) corresponding carbon monolith. From a macroscopic perspective (total step), biomass was trans-formed into fibrous carbon sources with the fungi as a bridge.

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complicated procedures, expensive and toxic agents, which islarge-scale and sustainable.

In further research, other fungi were used to successfullyfabricate the carbon monolith (ESI-7†), which demonstrates arelatively high feasibility of the fungi-based method to obtainthe carbon-fiber monolith. It should be noted that filamentousfungi in environment are separable and some typical ones arecommercially available, which prompts other researchers torepeat and improve this method.

Structural study of fungi before and after pyrolysis

The morphology of the fungi was first investigated. For SEMcharacterization, the living fungi must be chemically solidifiedto avoid obvious morphological variation after drying (Charac-terization, Experimental). As seen in Fig. 1a and b, the fungi arecomposed of ultra-long column-like microfibers with size ofaround 3 μm and aspect ratio of more than 50. Moreover,optical microscope was used to observe the stained livingfungi (ESI-8†). As shown in Fig. 1c, the long fungi fibers fromSEM image are actually hollow microtubes. According to themycology, microtube is the fungi cell and the outermostsection of the microtube is actually the cell wall. Within thecell wall are cytoplasmic membrane, cytoplasm, nucleus andwater. The cell wall possesses high mechanical strength, whichbuilds the entire cell skeleton. However, the cytoplasm andnucleus are tiny particles suspending in water, and even thecytoplasm membrane physically flows all the time.

The carbonized products of the dried fungi monolith(membrane and column) were characterized by SEM. As seenin Fig. 1d and f, the carbon monolith (membrane and aerogel)is composed of uniform and interconnected one-dimensional(1D) microstructures. The high-resolution images (Fig. 1e and

g) reveal that the 1D microstructures of membrane are flatbelts, while that of the aerogel are not completely flat despitewhich most of them collapse. Noticeably, the fungi used tofabricate the carbon monolith were not solidified, and thusthe loss of water after drying and carbonization inevitablyforced the cell wall tubes to collapse.

By taking the above discussion into account, cell wall maybe a very important component in the formation of a carbonmonolith. Actually, the separated cell wall dispersion (ESI-3†)can be directly used to fabricate a flexible membrane precursorby vacuum filtration, which can also be transformed to theintact carbon monolith by pyrolysis (ESI-9†). This experimentsufficiently verifies the irreplaceable role of cell wall on the for-mation of fungi monolith.

It has been well-demonstrated that the cell wall of filamen-tous fungi is primarily constructed by polysaccharides (chitinand glucan) and proteins, although their relative contentchanges for different genus.39,40 In fact, both of these compo-sitions are essential for the formation of flexible monolithicprecursors because high mechanical strength and fungi fibersinter-adhesion mainly arise from them.41–43 Moreover, pro-teins and polysaccharides are common cell wall compositionfor all microorganisms,44 including filamentous fungi. Thisallows the generic fabrication of monolithic precursor frommicroorganisms. For instance, membrane precursor can beproduced by vacuum filtering the dispersion of yeast (S. cerevi-siae) and bacteria (E. coli) (ESI-10†). However, as comparedwith the filamentous fungi, the primary difference is that themembrane precursor fabricated from yeast or bacteria cannotbe peeled-off. In fact, in contrast with the yeast (S. cerevisiae)and bacteria (E. coli), filamentous fungi possess obviously highaspect ratio. It is possible that for each fiber there are plenty of

Fig. 1 (a)–(b) SEM images of fungi treated by chemical cross-linking, (d)–(e) SEM images of membrane, (f )–(g) SEM images of aerogel, (c) opticalmicroscopy image of fungi and (h) illustration of the multiple contacts of fungi fibers.

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sites on cell wall to allow other fibers to adhere to (Fig. 1h),which generates multiple inter-adhesion along the fibers. Inother words, filamentous fungi with high aspect ratio are poss-ibly advantageous for the fabrication of monolithic carbon.

Surface analysis techniques (Electron Microprobe, FTIR,Raman and XPS) were adopted to investigate the variation incomposition of fungi before and after pyrolysis.

Fig. 2 presents the atomic ratio (at%) of element in varioussamples. For fungi (Fig. 2a), C, N and O are the three mainelements. C content is ∼85 at% and the total content of N andO is ∼14 at%. C, N and O should mainly originate from poly-saccharides and proteins. After treatment at 400 °C, the rela-tive content of C, N and O does not change clearly. However,when the fungi were treated at 800 °C, the C content increasedto 95.9 at% while the total content of N and O reducedto ∼3.8 at%. This indicates that the extent of carbonizationimproves with an increase in pyrolysis temperature.

FTIR spectra are given in Fig. 3a. The fungi before pyrolysisdisplay strong IR adsorption. The sharp peaks in the range of

3100–2800 cm−1 and at 1745 cm−1 are caused by lipids.45 Thepeaks between 1700–1300 cm−1 are attributed to the amide ofproteins and chitin.46 Sharp peaks at 1160 and 1031 cm−1 aretypical adsorption peaks of polysaccharides.47 A weak peak at873 cm−1 is an indicative of β-linked glycosidic bonds fromglucan.47 Moreover, the qualitative chemical detection of cellwall confirms the presence of proteins and polysaccharides onthe fungi (ESI-3†), which is in line with the IR results. Whenthe fungi were carbonized at 400 °C, the vibration of quinoid(1610 cm−1), C–H3 (1407 cm−1), C–O–C (1237 and 1121 cm−1)are measured.48 The functional groups of fungi completely dis-appear after treatment at 800 °C, which proves that the N andO detected by Electron Microprobe are doped on the carbon.Fig. 3b and ESI† provide the Raman spectra. In comparisonwith the fungi (ESI-11†), the carbonized products exhibit twoapparent scattering peaks. For the carbon prepared at 400 °C,1673 and 1421 cm−1 are related to the G and D bands, respecti-vely. However, for the carbon prepared at 800 °C, the graphi-tized band shifted to 1600 cm−1 with an increased intensity.This demonstrates the promoted graphitization of carbon athigh temperature, and is in line with the results of ElectronMicroprobe and FTIR.

XPS (C 1s, N 1s and O 1s) experiments were carried out todiscuss the variation in composition according to bindingenergies (Fig. 4). Composition content relative to the totalcontent of corresponding element was calculated from thepeak area. The C 1s spectrum of fungi (Fig. 4a) reveals that thecontent of C–O and C–N (∼286.2 eV: superposition of C–O andC–N) accounts for 50.9% of total C content, against only 14.8%for OvC–N–. These chemical bonds resulted from poly-saccharides and proteins. However, the predominant O chemicalbonds are C–O (71.8%) from O 1s spectrum of fungi (Fig. 4c)and the content of CvO is only 21.1%, which indicates thatthe prominent components of cell wall are polysaccharides.

Fig. 4d–f provide the XPS spectra of carbon obtained at400 °C. In Fig. 4d, the C 1s spectrum can be well-resolved intofour components, including sp2 hybridized C in aromaticdomain (284.3 eV), sp3 C (284.8 eV), C–N/C–O (285.4 eV: super-position of C–O and C–N) and CvO (286.7 eV). The content ofsp2 C is only 39.6%. From N 1s spectrum (Fig. 4e), the originalN groups of fungi (Fig. 4b) transform to pyridinic N, amine orimine, pyrrolic N and quaternary N. Pyridinic, pyrrolic andquaternary N is completely conjugated with aromatic C andtheir relative content reaches 91.1%, which is beneficial forpromoting the conductivity of carbon. Based on the deconvolu-tion of O 1s spectrum (Fig. 4f),49 the content of CvOdecreases while that of C–O increases. When the temperatureis increased to 800 °C, the content of sp2 C clearly increases to54.1% (C 1s, Fig. 4g). Interestingly, pyridinic-N-oxide wasobserved (Fig. 4h), which can be explained by the partial oxi-dation of N species by O species at high temperature.31

Apart from chemical surface analysis, the properties ofcarbon materials were investigated to understand their poten-tial for applications. The conductivity of carbon-fiber mem-brane and aerogel prepared at 800 °C is 1.45 and 1.23 S cm−1

(Table 1), respectively, against less than 1 S cm−1 reported

Fig. 3 FTIR (a) and Raman (b) spectra of fungi treated at differenttemperatures.

Fig. 2 Elemental atomic ratio of fungi (a) treated at 80 °C, productscarbonized at 400 (b) and 800 (c) °C. Data were obtained by electronmicroprobe.

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previously.31,32 Noticeably, the conductivity of carbon-fibermembrane is higher than that of aerogel, which is because ofthe closely packed structure of carbon fibers in the membrane.The specific surface area of carbon monolith was testedthrough N2 adsorption–desorption isotherm (ESI-12†). Thesurface areas of the membrane and aerogel are around 20 and305 m2 g−1, respectively. It should be mentioned that carbonaerogel reported in previous studies commonly displayssurface area in the range of 300–650 m2 g−1,27,28,31 which indi-cates a relatively high surface area of the carbon monolith incurrent research. Moreover, the pore size distribution byN2 adsorption–desorption isotherm and Hg intrusion testsuggests that the mesopores and macropores are created inboth the membrane and aerogel (ESI-12†). Such pore structurefacilitates the transportation of ions/molecules transportation.

The mechanical strength of carbon monolith by com-pression test is given in Fig. 5 and the aerogel is considered to

be a prototype. As seen in Fig. 5a and b, the aerogel syn-thesized at 400 and 800 °C begin to crack at strains of ∼0.086and ∼0.096 mm mm−1, respectively. With further com-pression, the stress–strain curve becomes coarse, indicating amild crush. The Young’s modulus of carbon aerogel is around3.5 (400 °C) and 14.1 (800 °C) MPa. In contrast, the com-pression of fungi aerogel demonstrates that the precursor isductile (Fig. 5c). Thus, the fungi became rigid after carboniz-ation. Moreover, in previous reports the mechanical strengthof various carbon aerogel is commonly less than 10 MPa,50–52

which in-turn indicates the superior mechanical property ofcarbon-fiber monolith in the present study.

Carbon monolith for Li–S batteries

Electrochemical measurements were first applied to character-ize the Li–S batteries with carbon monolith interlayer. InFig. 6a, the CV of Li–S batteries displays one anodic peak at

Fig. 4 XPS spectra of fungi treated at different conditions. The first line: treated at 80 °C; the second line: treated at 400 °C; the third line: treatedat 800 °C. The first column: C 1s; the second column: N 1s; the third column: O 1s (N1: pyridinic; N2: amine or imine; N3: pyrrolic; N4: quaternary;N5: pyridinic-N-oxide).

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∼2.45 V and two cathodic peaks at ∼2.3 and ∼2.0 V, which aretypical of Li–S batteries with carbon-based cathode.12 The firstdischarge–charge voltage profiles of improved Li–S batteriesare presented in Fig. 6b. The plateau at ∼2.3 V and a sub-sequent slope from 2.3 V to 2.0 V could be attributed to theconversion of S8 to soluble Li2S4. The discharge plateau at∼2.0 V was because of the reduction of Li2S4 to Li2S. Theformed LixSy is soluble in electrolyte and can transportthrough a separator to weaken the cycle performance ofbatteries.

The rate capability of the Li–S batteries is shown in Fig. 6c.The Li–S batteries without inserting the carbon monolith

interlayer could only be cycled up to 0.5 C, while maintaininga capacity of ∼230 mA h g−1. At 1 C, the capacity decreased tonearly 0 mA h g−1. Upon inserting carbon monolith interlayer,the capacity could reach up to ∼620 mA h g−1 even at 2 C. Asexhibited in Fig. 6d, the cyclability at 0.5 C was retained over650 mA h g−1 after 100 cycles, which was at least one foldmore than that without inserting an interlayer. Thissufficiently verifies the superior capability of carbon monolithon improving the performance of Li–S batteries.

Noticeably, the performance of Li–S batteries can also beimproved by directly modifying the sulphur cathode (e.g.,incorporation of sulphur into nanostructured porouscarbon).53–57 The capacity of Li–S batteries by this method isvery good but the preparation of electrode is complicated,expensive, and difficult to accomplish in the large-scale assem-bly of Li–S batteries. In the current research, it was easier tofabricate the electrode by selecting commercial sulphur with asize of around 100 μm, and the performance of Li–S batterieswas good when inserting the carbon monolith.

Nyquist plots of Li–S batteries were investigated to analysethe influence of carbon monolith interlayer on Li–S batteries(Fig. 6e). The impedance reduces sharply by ∼78% uponinserting interlayer in the batteries, indicating a dramaticdecrease in charge transfer resistance.58 Moreover, doping byN and O elements on the carbon monolith promotes the capa-bility of materials to interact with active components (e.g.,LixSy) in Li–S batteries.59,60 As a consequence, the shuttling ofLixSy was effectively prevented by carbon monolith interlayer.It must be noted that Li+ transportation should be un-obstructed when blocking the shuttling of LixSy by carbon mono-lith. Therefore, the porosity of carbon materials is substantialand as revealed by SEM (Fig. 1), the space between carbonfibers is large, which promises he efficient transportation ofLi+. In contrast, when using the carbon membrane with com-pacted microstructure fabricated by the flat cell wall (ESI-9†),the highest capacity of Li–S batteries is less than 100 mA h g−1

at 0.5 C (ESI-13†).

Conclusions

Herein, we have presented a new method for the sustainablesynthesis of highly porous carbon-fiber monolith (e.g., mem-brane, aerogel) by using filamentous fungi. The fungi cell wallwith high aspect ratio possesses superior mechanical strengthand affords strong adhesion between the contacted fungifibers, which are indispensable features for the synthesis ofcarbon-fiber monolith. The carbon monolith is doped byN and O, and possesses high conductivity, mechanicalstrength and specific surface area. The prospect of carbonmonolith as conductive interlayer toward promoting thecyclability of Li–S batteries are highlighted. Li–S batteriesdisplay an obviously improved performance in the charge/discharge process. After 100 cycles at 0.5 C, the capacity ofbatteries can still reach 650 mA h g−1.

Fig. 5 Compression test of carbon-fiber aerogel prepared at 400 (a),800 (b) °C and fungi aerogel (c).

Table 1 Surface area and conductivity of carbon-fiber monolith

Carbon-x-ya Surface areab (m2 g−1) Conductivity (S cm−1)

Membrane-800 20.8 1.45Aerogel-800 305.4 1.23

a x represents the type of monolith (e.g., membrane, aerogel); yrepresents the pyrolysis temperature. b Surface area is investigatedbased on N2 adsorption–desorption isotherm.

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Acknowledgements

Acknowledgements are made to Prof. Jianyu Zhu(Central South University) for his help on fungicomposition. Haiying and Liyuan would like to thankProf. Jian Luo and Dr Yiming Chen (Georgia Institute ofTechnology) for revising the manuscript. We thank financialsupports from the National Science Found for DistinguishedYoung Scholars of China (50925417), Chang JiangScholars Program (T2011116), Young Scholarship Awardfor Doctoral Candidate Issued by Ministry of Education(1343-76140000018) and Shanghai Tongji Gao TingyaoEnvironmental Science & Technology DevelopmentFoundation.

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Fig. 6 (a) CV of the first cycle of Li–S batteries with carbon monolith interlayer between 3.0 and 1.5 V (scanning rate 0.2 mV s−1). (b) Discharge–charge voltage profiles of the Li–S batteries with carbon monolith interlayer in the first cycle at 0.2 C rate. (c) Rate capability of Li–S batteries withand without carbon monolith interlayer. (d) Cycle performances of Li–S batteries with and without carbon monolith interlayer at 0.2 and 0.5 C rates.(e) Nyquist plots of the Li–S batteries with and without carbon monolith interlayer.

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Paper Green Chemistry

3934 | Green Chem., 2014, 16, 3926–3934 This journal is © The Royal Society of Chemistry 2014

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