Preparation of Hierarchical Porous Carbon from

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energies Article

Preparation of Hierarchical Porous Carbon from Waterweed and Its Application in Lithium/Sulfur Batteries Chunyong Liang 1 , Xiaomin Zhang 1 , Yan Zhao 1, *, Taizhe Tan 2 , Yongguang Zhang 1, * and Zhihong Chen 3, * 1

2 3

*

School of Materials Science & Engineering, Research Institute for Energy Equipment Materials, Hebei University of Technology, Tianjin 300130, China; [email protected] (C.L.); [email protected] (X.Z.) Synergy Innovation Institute of GDUT, Heyuan 517000, China; [email protected] Shenyang Institute of Automation, Guangzhou, Chinese Academy of Sciences, Guangzhou 511458, China Correspondence: [email protected] (Ya.Z.); [email protected] (Yo.Z.); [email protected] (Z.C.); Tel.: +86-22-60201447 (Yo.Z.)

Received: 20 April 2018; Accepted: 7 June 2018; Published: 11 June 2018

 

Abstract: A nanostructured carbon (NSC) material with a hierarchical porous structure is synthesized through the carbonization of a waterweed, namely Echinodorus amazonicus Rataj. The fabricated NSC is used as an electrode material for sulfur of lithium/sulfur (Li/S) batteries. The NSC provides for a high pore volume (0.19 cm3 g−1 ) and large specific surface area (111.25 m2 g−1 ). Because of the highly hierarchical porous structure of the NSC material, allowing polysulfides to remain in the carbon framework after cycling, the sulfur/NSC composite exhibits an excellent electrochemical performance. Keywords: nanostructured carbon; hierarchical porous structure; lithium/sulfur batteries; polysulfides

1. Introduction With the increasing environmental problems caused by conventional energy sources and the gradual depletion of oil resources, clean energy is becoming an important topic for the whole world. As an electrochemical energy storage device, lithium-ion batteries represent the most advanced energy storage technology and are a key solution for powering portable electronics. Despite this, the current performance of lithium-ion batteries is struggling to meet the market requirements and is approaching its theoretical capacity limit [1–6]. Sulfur, as a cathode of lithium/sulfur (Li/S) batteries, possesses a high theoretical specific capacity (1675 mAh g−1 ) and a high theoretical energy density (2600 Wh kg−1 ), for this reason, it has aroused great interest [7–10]. However, because of the different reaction mechanisms, Li/S batteries need to be developed in a different way with respect to lithium-ion batteries [11]. One of the main limitations comes from the electronic nature of sulfur, which is commonly referred to as a poor electronic conductor (σ = 5 × 10−30 S cm−1 ) requiring the addition of a conductive element such as carbon. The second problem is that the polysulfide ions formed during the discharge/charge processes tend to dissolve into the organic solvent electrolyte [12]. The diffusion of dissolved polysulfides between the electrodes gives rise to a shuttle effect that results in a severe loss of capacity and Coulombic efficiency upon cycling [13–15]. Many previous investigations have pointed out that porous sulfur/carbon composites can efficiently increase the sulfur uptake and the cycle stability [16–22]. Porous carbon can provide for a good electrical conductivity together with a large pore volume that can effectively relieve the Energies 2018, 11, 1535; doi:10.3390/en11061535

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good electrical conductivity together with a large pore volume that can effectively relieve the stress due to the high volume expansion of sulfur cathodes during cycling [23,24]. Current methods for stress dueporous to the high volume expansion of sulfur cathodes during cycling [23,24]. Current preparing carbons are based on laser etching [25], arcs [26], nano-casting [27,28], andmethods others; for preparing carbons are based on laser [25], [26], For nano-casting [27,28], and others; however, mostporous of these methods are costly andetching difficult to arcs upscale. this reason, biomass waste however, of these methods are costly and difficultmaterials to upscale. Fortothis would be most an ideal precursor for Li/S battery electrode due the reason, tunablebiomass physicalwaste and would beproperties, an ideal precursor for Li/Scost, battery due natural, to the tunable physical and chemical low production and electrode by being materials a renewable, and environmental chemicalresource properties, low production cost, and by being a renewable, natural, and environmental friendly [29–33]. friendly resource [29–33]. Aquatic plants, thanks to their luxuriant foliage, rapid growth, low-cost, and high content of Aquatic thanks to their luxuriant foliage, rapid growth, high content carbon speciesplants, are ideal and sustainable precursors for producing porouslow-cost, biochar. and Waterweed, such of Echinodorus carbon speciesamazonicus are ideal and sustainable precursors forpossess producing porousporous biochar. Waterweed, as Rataj (EAR) was shown to a highly structure and such as Echinodorus amazonicus Rataj was shown to possess a highly porous structure and excellent adsorption properties; this has(EAR) motivated us to select the nanostructured EAR as carbon excellent adsorption properties; hasmaterial. motivated us to select nanostructured as carbon precursor to synthesize a porous this carbon Because of its the unique hierarchical EAR nanostructure, the resulting nanostructured carbon (NSC) could Because promoteof good electrical contact and effectively precursor to synthesize a porous carbon material. its unique hierarchical nanostructure, inhibit the dissolution of polysulfides. The electrochemical of the sulfur/NSC (S/NSC) the resulting nanostructured carbon (NSC) could promoteperformance good electrical contact and effectively composite a cathodeof material for Li/SThe batteries has also been investigated. inhibit theas dissolution polysulfides. electrochemical performance of the sulfur/NSC (S/NSC) composite as a cathode material for Li/S batteries has also been investigated. 2. Materials and Methods 2. Materials and Methods 2.1. Sample Preparation 2.1. Sample Preparation The NSC material was synthesized following a simple method based on the carbonization of TheEAR NSC(Lianyungang material was synthesized following a simple method based on the carbonization of the the sole Yuanhai Garden Technology, Lianyungang, China) carbon precursor sole the EARprocess (Lianyungang Yuanhai Garden China) carbon precursor andthe the and was shown in Figure 1. Technology, Firstly, EARLianyungang, was thoroughly cleaned and dried, then process was shown in Figure 1. Firstly, EAR was thoroughly cleaned and dried, then the compound compound was heated at 800 °C with a heating rate of 5 °C min−1 and the temperature maintained was2 heated 800 ◦ Catmosphere. with a heating rateitofwas 5 ◦ Ccooled min−1toand the temperature for 2ground h in an for h in anatargon Then, room temperature,maintained taken out and argonthe atmosphere. it was powder. cooled toThen, room temperature, takendispersed out and ground until thehydroxide formation until formationThen, of a black the powder was in potassium of a black powder. Then, the powder was dispersed in potassium hydroxide (KOH, Tianjin (KOH, Tianjin Guangfu, Tianjin, China, ≥99% purity) solution (1 M) and stirred until theGuangfu, solvent Tianjin, China, ≥ 99% purity) solution (1 M) and stirred until the solvent was completely evaporated; was completely evaporated; then, the powder was heated again at 800 °C for 2 h and cooled down then, the powder wasresiduals heated again 800 ◦ C for h and cooled acid down(HCl, naturally. The KOH residuals naturally. The KOH wereatremoved by2 hydrochloric Tianjin Fuchen, Tianjin, were removed by hydrochloric acid (HCl, Tianjin Fuchen, Tianjin, China, ≥ 99% Purity) distilled China, ≥99% Purity) and distilled water. Finally, the sample was dried in a drying ovenand to obtain a water. Finally, the sample was dried in a drying oven to obtain a chemically activated porous carbon. chemically activated porous carbon. The S/NSC composite material was obtained by mixing carbon The sulfur S/NSC composite material was obtained mixing andratio, sulfur (Tianjin Tianjin, and (Tianjin Kewei, Tianjin, China, ≥98%by purity) in acarbon 1:3 mass placing theKewei, mixture in an ◦ C for China, ≥ 98% purity) in a 1:3 mass ratio, placing the mixture in an autoclave and heating to 155 autoclave and heating to 155 °C for 10 h [29,34]. 10 h [29,34].

Figure Figure 1. 1. Schematic Schematic of of the the preparation preparation of of the the sulfur sulfur nanostructured nanostructured carbon carbon (S/NSC). (S/NSC).

2.2. 2.2. Characterization Characterization The The infrared infrared spectra spectra were were obtained obtained from from Fourier Fourier transform transform infrared infrared spectroscopy spectroscopy (FTIR, (FTIR, Thermo Nicolet 6700, Thermo Fisher Scientific, Waltham, MA, USA), the sulfur loading content was Thermo Nicolet 6700, Thermo Fisher Scientific, Waltham, MA, USA), the sulfur loading content

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was estimated by thermogravimetric analysis (TGA, SDTQ 600) under argon with a heating rate of 5 ◦ C min−1 from 25 to 800 ◦ C, the microscopic morphology of the NSC and S/NSC samples was determined by scanning electron microscopy (SEM, S-4800, Hitachi Limited, Tokyo, Japan, equipped with energy dispersive X-ray spectrometry elemental analysis) and transmission electron microscopy (TEM, JEM-2100F, JEOL, Tokyo, Japan), respectively. Raman spectroscopy analysis was performed on Raman spectroscopy (LabRAM Hr 800, HORIBA Jobin Yvon, Paris, France) with a laser wavelength of 514 nm, crystalline structure of the samples were characterized by X-ray diffraction (XRD, Rigaku-TTRIII, Tokyo, Japan) in the range of 10◦ to 90◦ at a scan rate of 12◦ min−1 . The surface area was calculated using the Brunauer–Emmett–Teller (BET, ASAP 2020, Micromeritics, Aachen, Germany) equation based on adsorption data and the total pore volume was determined from the amount of nitrogen adsorbed at a relative pressure of 0.98. Pore size distributions (PSDs) were calculated by adsorption isotherms for pores of different sizes. X-ray photoelectron spectroscopy (XPS) analysis was performed on an X-ray photoelectron spectroscopy (XPS, VG ESCALAB MK II, VG Scientific, London, England) at room temperature. 2.3. Cell Fabrication and Electrochemical Measurement The S/NSC composite, polyvinylidene fluoride (PVDF, Kynar, HSV900), and acetylene black (Shanghai SIMBATT Energy Technology, Shanghai, China, 99.5% purity) were weighted at a mass ratio of 8:1:1 and dissolved in 1-methyl-2-pyrrolidinone (NMP, Sigma-Aldrich, St. Louis, MO, USA, ≥99.5% purity) to obtain the electrode paste, which was coated on aluminum foil (Guangzhou Zhongtian Aluminum Industry, Guangzhou, China) and then dried off. The electrodes were then cut into 1.5 cm diameter sheets and the sulfur loading of the electrode was 2 mg/cm2 . The S/NSC composite was used as the cathode material in a 2025-type coin cell assembled in a glove box filled with argon. The selected electrolyte was tetraethylene glycol dimethyl ether with 1 M lithium trifluoromethanesulfonamide (LiTFSI) (Shenzhen Tianchenghe Technology, Shenzhen, China) [35,36]. Galvanostatic charge/discharge curves were recorded with a multichannel battery tester (BTS-5V5mA, Neware, Shenzhen Xinweier Electronics, Shenzhen, China) between 1.0 and 3.0 V and the capacity was calculated per gram of sulfur. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were measured on a PARSTAT 4000 electrochemical workstation. The cyclic voltammetry (CV) curves were conducted with a scanning rate 0.1 mV s−1 between 1.0 and 3.0 V. The impedance spectrums were obtained in the frequency range of 100 kHz to 10 mHz. All the electrochemical measurements were performed at room temperature. 3. Results and Discussion The FTIR spectra obtained for the NSC and S/NSC samples are shown in Figure 2a. The interaction between C=C and C=N stretching vibrations was observed at 1615 cm−1 [37]. Nitrogen-doped carbon is commonly observed in biomass materials because they usually contain a certain amount of proteins; this nitrogen doping can effectively improve the electrochemical performance by chemisorbing from intermediate polysulfides. The absorption peaks at 1473 cm−1 and 1428 cm−1 are associated with C=C stretching. Compared with NSC, in the S/NSC, we can clearly observe the additional absorption peaks at 1090 cm−1 and 669 cm−1 associated with the C=S and C-S stretching [38], indicating the chemical bonding between sulfur and NSC. As shown in Figure 2b, there is an abrupt weight loss phase in the TGA curve from the S/NSC composite in the temperature range of 180 to 250 ◦ C. Interestingly, as compared with the initial mass, the weight loss of the S/NSC sample was about 75 wt %, which proves the excellent sulfur incorporation capacity of the NSC, ascribed to its highly porous structure.

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Figure 2. (a) Fourier transform infrared spectroscopy (FTIR) spectra of NSC and S/NSC composites; Figure 2. (a) Fourier transform infrared spectroscopy (FTIR) spectra of NSC and S/NSC composites; (b) thermogravimetric curve of NSC and S/NSC composites. (b) thermogravimetric curve of NSC and S/NSC composites.

The scanning electron microscopy analysis was was usedused to characterize the micromorphology of the of The scanning electron microscopy analysis to characterize the micromorphology NSC and S/NSC samples (Figure 3). The 3D connected spongelike structure of the precursor was well the NSC and S/NSC samples (Figure 3). The 3D connected spongelike structure of the precursor preserved without majorwithout damages. Moreover, it is found that the is composed of connected flakes of was well preserved major damages. Moreover, it isNSC found that the NSC is composed with a wrinkled surface. in Figure 3e–g,As sulfur was in uniformly the connected flakes withAsashown wrinkled surface. shown Figure distributed 3e–g, sulfurthroughout was uniformly carbon framework, provingthe thatcarbon sulfur framework, is successfully impregnated into is thesuccessfully pores of theimpregnated NSC. The EDX distributed throughout proving that sulfur into spectrum analysis shown the inset of Figure 3e, showing theinset composite material contains the pores of the is NSC. TheinEDX spectrum analysis is shownthat in the of Figure 3e, showing that both C (30%) and S (70%) elements. the composite material contains both C (30%) and S (70%) elements.

Figure TypicalSEM SEMimages images of of (a,b) composite; (e) (e) SEM image of aof select region Figure 3. 3.Typical (a,b) NSC, NSC, (c,d) (c,d)S/NSC S/NSC composite; SEM image a select of the composite and the element energyenergy spectrum; (f,g) the(f,g) element mappingmapping of the S/NSC region ofS/NSC the S/NSC composite and the element spectrum; the element of sample. the S/NSC sample.

High-magnification transmission electron microscopy images clearly show the presence of High-magnification transmission electron microscopy images clearly show the presence of white white spots in the NSC sample (Figure 4a), testifying to the highly porous material structure spots in the NSC sample (Figure 4a), testifying to the highly porous material structure together together with its large specific surface area. The size of white spots in the S/NSC composite is with its large specific surface area. The size of white spots in the S/NSC composite is significantly significantly reduced (Figure 4b), indicating that sulfur has partially filled the nanopores of the reduced (Figure 4b), indicating that sulfur has partially filled the nanopores of the NSC. NSC.

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Figure 4. TEM images of sample (a) NSC and (b) S/NSC. Figure 4. 4. TEM TEM images images of of sample sample (a) (a) NSC NSC and and (b) (b) S/NSC. S/NSC. Figure

Raman spectroscopy was used to analyze the molecular structure of the carbon material Raman spectroscopy was used to analyze the molecular structure of the carbon material −1 carbon Raman to analyze the molecular the material 5a). andstructure 1597.4 of cm (G-band) can (Figure be clearly (Figure 5a).spectroscopy Two peakswas atused 1360.5 cm−1 (D-band) and 1597.4 cm−1 (G-band) can be clearly (Figure 5a). Two peaks at 1360.5 cm−1 (D-band) − 1 − 1 distinguished: The cm D-band represents of the whileThe the D-band G-band Two peaks at 1360.5 (D-band) and disordered 1597.4 cm carbon (G-band) can graphite be clearlystructure, distinguished: distinguished: The D-band represents disordered carbon of the graphite structure, while the G-band corresponds to the carbon E2g mode vibrations) crystalline graphite [39]. The represents disordered of the (stretching graphite structure, while of the the G-band corresponds to the E 2g mode corresponds to the E2g mode (stretching vibrations) of the crystalline graphite [39]. The graphitization degreeofofthethe carbon materials wasThe evaluated based degree on theofintensity ratio of the (stretching vibrations) crystalline graphite [39]. graphitization the carbon materials graphitization degree of the carbon materials was evaluated based on the intensity ratio of the D-band to the based G-band D/IG ) [7]: theratio smaller the ratio, to thethe higher the(Idegree of ordering in the the was evaluated on (I the intensity of the D-band G-band the smaller D /IG ) [7]: D-band to the G-band (ID/IG) [7]: the smaller the ratio, the higher the degree of ordering in the carbon [40].degree In theofS/NSC composite, ID/IG ismaterial almost [40]. the same in the pure porousIDcarbon ratio, thematerial higher the ordering in the carbon In theasS/NSC composite, /IG is carbon material [40]. In the S/NSC composite, ID/IG is almost the same as in the pure porous carbon −1, where samplethe except sulfur peak.sample We could find in sulfur the range of 700-450 cmWe almost samefor as the in the purediffraction porous carbon except foritthe diffraction peak. could sample except for the sulfur diffraction peak. We could find it in the range of 700-450 cm−1, where − 1 −1 −1 sulfur could obviously be identified. Thesulfur peak could at 486obviously cm−1 couldbecorrespond to the S-Satbond [41]. find it in the range of 700-450 cm , where identified. The peak 486 cm sulfur could obviously be identified. The peak at 486 cm could correspond to the S-S bond [41]. This also shows to that sulfur was dispersed evenly in the composite. In Figure 5b an could correspond theelemental S-S bond [41]. This also shows that elemental sulfur was dispersed evenly in This also shows that elemental sulfur was dispersed evenly in the composite. In Figure 5b an exemplary curve fit for the SNC sample was Pöschl al. proved that in the interpretation of the composite. In Figure 5b an exemplary curveshown. fit for the SNCetsample was shown. Pöschl et al. proved exemplary curve fit for the SNC sample was shown. Pöschl et al. proved that in the interpretation of −1 should soot in Raman spectra, the range 1000–2000 cm−1−1 should be performed with bands: be G, D1 (D), D2, that the interpretation of soot Raman spectra, the range 1000–2000 cmfive performed soot Raman spectra, the range 1000–2000 cm should be performed with five bands: G, D1 (D), D2, D3, and The G-band is ascribed theD4. presence of an ideal graphitic the D, D2, and D4 with fiveD4. bands: G, D1 (D), D2, D3, to and The G-band is ascribed to lattice, the presence of an ideal D3, and D4. The G-band is ascribed to the presence of an ideal graphitic lattice, the D, D2, and D4 bands confirm of D4 a disordered graphitic lattice while the D3-band originates from the graphitic lattice,the thepresence D, D2, and bands confirm the presence of a disordered graphitic lattice while bands confirm the presence of a disordered graphitic lattice while the D3-band originates from the amorphous phase X-ray diffraction from the diffraction NSC and patterns S/NSC composite the D3-band carbon originates from[42]. the amorphous carbonpatterns phase [42]. X-ray from the amorphous carbon phase [42]. X-ray diffraction patterns from the NSC and S/NSC composite samples shown in Figure 5c, there broad halos in range 20–30 2 theta, NSC and are S/NSC composite samples are are shown in Figure 5c,the there are of broad halos in theindicating range of samples are shown in Figure 5c, there are broad halos in the range of 20–30 2 theta, indicating carbon with indicating an amorphous [43]. The reduction of these crystalline peaks 20–30 2 theta, carbonstructure with an amorphous structure [43]. The sulfur-related reduction of these sulfur-related carbon with an amorphous structure [43]. The reduction of these sulfur-related crystalline peaks (marked inpeaks the picture) upon sulfur uptake indicates that most of the sulfur has of impregnated the crystalline (marked in the picture) upon sulfur uptake indicates that most the sulfur has (marked in the picture) upon sulfur uptake indicates that most of the sulfur has impregnated the porous carbon; severalhowever, diffraction peaksdiffraction related topeaks sulfurrelated on thetocarbon which impregnated thehowever, porous carbon; several sulfur appear, on the carbon porous carbon; however, several diffraction peaks related to sulfur on the carbon appear, which demonstrates that some sulfur a strong crystalline form remainsform in theremains composite. appear, which demonstrates thatwith some sulfur with a strong crystalline in the composite. demonstrates that some sulfur with a strong crystalline form remains in the composite.

Figure 5. 5. (a) (b)(b) curve fit fit forfor the the NSCNSC sample and and (c) XRD patterns of NSC S/NSC Figure (a)Raman Ramanspectra spectra curve sample (c) XRD patterns ofand NSC and Figure 5. (a) Raman spectra (b) curve fit for the NSC sample and (c) XRD patterns of NSC and S/NSC composites. S/NSC composites. composites.

The nitrogen adsorption-desorption isotherms isotherms of of Figure Figure 6 are acquired to evaluate the The nitrogen adsorption-desorption adsorption-desorption isotherms of Figure 6 are acquired to evaluate the Brunauer–Emmett–Teller (BET) specific surface area and pore size distribution (PSD) curves of the Brunauer–Emmett–Teller area and pore sizesize distribution (PSD) curves of theof NSC Brunauer–Emmett–Teller(BET) (BET)specific specificsurface surface area and pore distribution (PSD) curves the NSC and S/NSC samples. The adsorption curves correspond to type IV-isotherms, indicating the and samples. The adsorption curves curves correspond to type IV-isotherms, indicating the presence NSCS/NSC and S/NSC samples. The adsorption correspond to type IV-isotherms, indicating the 2 g−1 and 0.19 cm3 presence of mesopores; the BET surface area and pore volume in NSC is 111.25 m 2 − 1 3 −13, 2 g−10.19 of mesopores; the BET surface andarea poreand volume in NSC in is NSC 111.25is m g m and presence of mesopores; the BETarea surface pore volume 111.25 and cm 0.19 gcm −1, respectively. The decrease following the sulfur uptake is due to the filling of mesopores of the g respectively. TheThe decrease following thethe sulfur uptake g−1, respectively. decrease following sulfur uptakeisisdue duetotothe thefilling fillingof of mesopores mesopores of the carbon-based material. Figure 6b indicates that the size of the pores is in the order of a few to tens carbon-based material. Figure 6b order ofof a few to to tens of 6b indicates indicates that thatthe thesize sizeofofthe thepores poresisisininthe the order a few tens of nanometers, it also shows that the activation produced both mesopores and micropores, with a of nanometers, it also shows that the activation produced both mesopores and micropores, with a

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nanometers, it also shows the activation both mesopores and micropores, a meansulfur mean size of less than 2 nmthat (micropores) in produced the S/NSC sample, confirming that thewith elemental size of less than 2 nm (micropores) in the S/NSC sample, confirming that the elemental sulfur has has filled the pores. filled the pores.

Figure 6. (a) N2 adsorption–desorption isotherms and (b) pore diameter distribution of the NSC and the S/NSC composite.

The Survey XPS spectra of S/NSC are shown in Figure 7. The C 1s spectrum of S/NSC Figure 6. (a) pore diameter distribution of theofNSC Figure 6. (a) N2Nadsorption–desorption isothermsand and(b)(b) pore diameter distribution the and NSC and 2 adsorption–desorption isotherms d with four peaks centered at 284.4, 285.2, 286.4, and 288.9 eV binding energy, attribut the S/NSC composite. the S/NSC composite. /C=C, sp3-C, C-O, and O-C=O bonds [44], respectively (Figure 7c). Among them, the C-S Survey XPSspectra spectra Figure 7. and The Csulfur. 1sCspectrum S/NSC was TheThe Survey XPS of S/NSC S/NSC are areshown shown in Figure 7. The 1s spectrum of S/NSC was pared to Figure 7b, shows the combination of incarbon Asofshown in Figure 7d, fitted with four peaks centered at 284.4, 285.2, 286.4, and 288.9 eV binding energy, attributed to fitted with four peaks centered at 284.4, 285.2, 286.4, and 288.9 eV binding energy, attributed to 3 -C, C-O, andpeaks pectrum shows five attributed to S 2P(Figure 3/2 (164.2 eV), S-O eV), S 2p1/2 ( C-C/C=C, spadjacent O-C=Obonds bonds [44], respectively 7c).7c). Among them, the (164.6 C-S C-C/C=C, sp3-C, C-O, and O-C=O [44], respectively(Figure Among them, thebond, C-S bond, compared Figure(169.2 7b, shows the combination of carbon and sulfur. As shown in Figure 7d, the S 2p S-O (165.8 eV), and S-O eV) compared toto Figure 7b, shows the bonds. combination of carbon and sulfur. As shown in Figure 7d, the S spectrum shows five adjacent peaks attributed to S 2P

(164.2 eV), S-O (164.6 eV), S 2p

(165.4 eV),

3/2 2p spectrum shows five adjacent peaks attributed to S 2P3/2 (164.2 eV), S-O (164.61/2 eV), S 2p1/2 (165.4 S-O (165.8 eV), and S-O (169.2 eV) bonds. eV), S-O (165.8 eV), and S-O (169.2 eV) bonds.

Figure 7. (a) Survey XPS spectrum of S/NSC; (b) C 1s spectra of NSC composite; (c,d) C 1s and S 2p spectra of S/NSC sample, respectively.

Figure 8a shows the cyclic voltammograms (CV) of a S/NSC composite electrode for the first two cycles. Two plateaus are of observed atC2.36 and 2.10 V in the reduction related to Figure 7. (a)typical Survey XPS spectrum S/NSC; (b) of NSC composite; (c,d) C 1sprocess, and S 2p Figure 7. (a) Survey XPS spectrum of S/NSC; (b) C1s1sspectra spectra of NSC composite; (c,d) C 1s and S 2p the reduction sulfur to soluble higher-order lithium polysulfide (Li2Sn, 4 ≤ n ≤ 8) and soluble spectra ofofS/NSC sample, respectively. spectra oflithium S/NSCpolysulfide sample, respectively. to insoluble Li2S2 and Li2S, respectively. The charge profiles show the oxidation plateau at 2.45 V, which relates to the reverse reaction during the charge process. Moreover, the first cycles of the cyclic CV curve almost completely coincided, the good cycle performance of for the Figure 8atwo shows the voltammograms (CV) ofindicating a S/NSC composite electrode the battery. cycles. Two typical plateaus arecurve observed at 2.36 2.10 Vthe in platforms the reduction process, rela In the discharge-charge (Figure 8b), it canand be seen that were consistent position the peak in this figure. A significant specific discharge reductionwith of the sulfur to ofsoluble higher-order lithiuminitial polysulfide (Li2Scapacity n, 4 ≤ of n 1387.8 ≤ 8) and so

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Figure 8a shows the cyclic voltammograms (CV) of a S/NSC composite electrode for the first two cycles. Two typical plateaus are observed at 2.36 and 2.10 V in the reduction process, related to the reduction of sulfur to soluble higher-order lithium polysulfide (Li2 Sn , 4 ≤ n ≤ 8) and soluble lithium polysulfide to insoluble Li2 S2 and Li2 S, respectively. The charge profiles show the oxidation plateau at 2.45 V, which relates to the reverse reaction during the charge process. Moreover, the first two cycles of the CV curve almost completely coincided, indicating the good cycle performance of the battery. In the discharge-charge curve (Figure 8b), it can be seen that the platforms were consistent with the position of the peak in this figure. A significant initial specific discharge capacity of 1387.8 mAh g−1 at 0.1 C was measured, and the capacity reduced to 938.2 mAh g−1 by the 50th cycle, this is due to the loss of sulfur deposited on the surface and the escape of some intermediate soluble polysulfides from the nanopores of the carbon structure [34]. Afterwards, however, the capacity fading rate is considerably lower, there was still 898.2 mAh g−1 at 100th cycle, and after 150 cycles, the S/NSC electrode still show a capacity of 823.5 mAh g−1 . As shown in Figure 8c, the coulombic efficiency and cycling performance at 0.1 C of S/NSC composite and carbon black-sulfur composite. Coulombic efficiency is high for both samples, 99% and 98%, respectively, which shows the excellent cycle stability of both batteries. However, there was a large difference in the cycle performance. Compared with the carbon black-sulfur sample, the cycle performance of the S/NSC sample has a great advantage from the initial cycle to the 150th cycle. This result intuitively shows that the addition of the NSC carbon matrix significantly improves the electrochemical performance of the Li-S battery. This is due to the porous structure of the NSC material and its ability to immobilize sulfur and sulfur polysulfides. Figure 8d shows the rate performance of the S/NSC electrode at the current density from 0.1 C to 1 C (1 C = 1675 mAh g−1 ). The discharge capacity of the S/NSC is 725.4 mAh g−1 at 1 C and it reaches 1043.2 mAh g−1 when the current density goes back to 0.1 C; this shows that the battery has a great reversibility and rate performance. In a related study of biomass materials as a carrier of sulfur for cathode materials of lithium-sulfur batteries, this result is very competitive. For example, in the study of Yang’s group, they used an apricot shell as a carrier of sulfur, and the initial discharge specific capacity of the battery was 1277 mAh g−1 [45]. Moreover, the rate performance of S/NSC is also significantly better than that of Qu’s group. They used biomass waste (gelatin) as a carbon-based material for lithium-sulfur battery cathodes [46]. Obviously, the porous structure of NSC material provides for a high encapsulation ability of sulfur particles, which confers to the battery its good electrochemical performance. As shown in Figure 8e, the EIS spectra of the S/NSC electrode after first and forth cycles show typical Nyquist plots, comprising a compressed semicircle in the high to medium frequency range and an inclined line in the low frequency range; the semicircle at high-middle frequency is attributed to the charge-transfer resistance. The inclined line in the low-frequency region refers to the Warburg impedance [47]. It can be seen that after four cycles the Nyquist plot of the cell has transformed from a single compressed semicircle type to two compressed semicircles. This transition indicates that an interfacial change occurred during the discharge/charging process, which protected the cathode from further decomposition, indicating the good cycle stability of the battery [32].

the Warburg impedance [47]. It can be seen that after four cycles the Nyquist plot of the cell has transformed from a single compressed semicircle type to two compressed semicircles. This transition indicates that an interfacial change occurred during the discharge/charging process, which protected the cathode from further decomposition, indicating the good cycle stability of the Energies 2018, 8 of 11 battery [32].11, 1535

Figure scan rate rate of of0.1 0.1mV mVs− s−11 ; (b) (b) discharge/charge discharge/charge performance Figure 8. 8. (a) (a) CV CV curves curves of battery at aa scan performanceof of battery batteryat at0.1 0.1 C; C; (c) (c) cycling cycling performance performance and and coulombic coulombic efficiency efficiency at at 0.1 0.1 C C of of S/NSC S/NSCcomposite compositeand and carbon black-sulfur composite; composite;(d)(d) performance ofS/NSC the S/NSC at current various densities; current carbon black-sulfur raterate performance of the batterybattery at various (e) Nyquist of theplots S/NSC electrode first andatforth densities; (e)plots Nyquist of the S/NSCatelectrode firstcycle. and forth cycle.

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In order order to to determine determinethe thedistribution distributionofofthethe electrode material elements cycle, In electrode material elements afterafter the the cycle, we we scanned the electrode after 150 charge/discharge cycles. As shown in Figure 9, after the charge scanned the electrode after 150 charge/discharge cycles. As shown in Figure 9, after the charge and and discharge cycles, the sulfur and carbon element remained in a uniformly distributed discharge cycles, the sulfur and carbon element remained in a uniformly distributed state. state. This This phenomenon supports the argument that the recycled sulfur remains in the carbon structure, phenomenon supports the argument that the recycled sulfur remains in the carbon structure, indicating that the NSC porous carbon material we prepared indeed has a good sulfur-retaining capacity. It provides strong capacity. strong support support for for its its good good electrochemical electrochemical performance. performance.

Figure 9. (a) SEM images of the S/NSC composite electrode after 150 cycles and (b,c) the element Figure 9. (a) SEM images of the S/NSC composite electrode after 150 cycles and (b,c) the element mapping of the selected region of electrode. mapping of the selected region of electrode.

4. Conclusions In summary, we used a low-cost biomass material, processed by a simple method, as the cathode material for lithium-sulfur batteries and achieved good results. The nanostructured carbon was synthesized by carbonizing Echinodorus amazonicus Rataj and the S/NSC composite was prepared by the melt diffusion method. The hierarchical porous structure of the NSC balanced the volume expansion of sulfur in electrochemical reactions, which confers to the material a strong polysulfide constraining ability; the S/NSC composite was applied as a cathode material for lithium/sulfur batteries showing a high discharge capacity and excellent cycling stability.

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4. Conclusions In summary, we used a low-cost biomass material, processed by a simple method, as the cathode material for lithium-sulfur batteries and achieved good results. The nanostructured carbon was synthesized by carbonizing Echinodorus amazonicus Rataj and the S/NSC composite was prepared by the melt diffusion method. The hierarchical porous structure of the NSC balanced the volume expansion of sulfur in electrochemical reactions, which confers to the material a strong polysulfide constraining ability; the S/NSC composite was applied as a cathode material for lithium/sulfur batteries showing a high discharge capacity and excellent cycling stability. Author Contributions: Formal analysis, C.L. and T.T.; Investigation, X.Z.; Project administration, Y.Z. (Yan Zhao) and Y.Z. (Yongguang Zhang); Supervision, Y.Z. (Yan Zhao), Y.Z. (Yongguang Zhang) and Z.C.; Writing-original draft, C.L.; Writing-review & editing, Y.Z. (Yan Zhao) and Z.C. Acknowledgments: This work was supported by the Program for the Outstanding Young Talents of Hebei Province; Guangdong Provincial Science and Technology Project (2017A050506009). Conflicts of Interest: The authors declare no conflict of interest.

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