Amylose Derived Macro-Hollow Core and Micro-Porous Shell Carbon

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Amylose Derived Macro-Hollow Core and Micro-Porous Shell Carbon Spheres as Sulfur Host for Superior Lithium–Sulfur Battery Cathodes Xiang Li, Xuanbing Cheng, Mingxia Gao, Dawei Ren, Yongfeng Liu, Zhengxiao Guo, Congxiao Shang, Lixian Sun, and Hongge Pan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00672 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on February 27, 2017

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ACS Applied Materials & Interfaces

Amylose Derived Macro-Hollow Core and MicroPorous Shell Carbon Spheres as Sulfur Host for Superior Lithium–Sulfur Battery Cathodes Xiang Li†, Xuanbing Cheng†, Mingxia Gao*,†, Dawei Ren†, Yongfeng Liu†, Zhengxiao Guo‡, Congxiao Shang§, Lixian Sun#, Hongge Pan*,† †

State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and

Applications for Batteries of Zhejiang Province & School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China. ‡

Department of Chemistry, University College London, London, WC1H 0AJ, UK

§

School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK

#

Guangxi Collaborative Innovation Center of Structure and Property for New Energy and

Materials, Guangxi Key Laboratory of Information Material, School of Materials Science and Engineering, Guilin University of Electronic Technology, Guilin, 541004, P. R. China KEYWORDS:

Lithium–ion

batteries,

sulfur

cathode,

biomass

material;

amylose;

Electrochemical performance ABSTRACT: Porous carbon can be tailored to great effect for electrochemical energy storage. In this study, we propose a novel structured spherical carbon with a macrohollow core and a

ACS Paragon Plus Environment

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microporous shell derived from a sustainable biomass, amylose, by a multi-step pyrolysis route without chemical etching. This hierarchically porous carbon shows a particle distribution of 2–10 µm and a surface area of 672 m2 g-1. The structure is an effective host of sulfur for lithium–sulfur battery cathodes, which reduces the dissolution of polysulfides in the electrolyte and offers high electrical conductivity during discharge/charge cycling. The hierarchically porous carbon can hold 48 wt% sulfur in its porous structure. The S@C hybrid shows an initial capacity of 1490 mAh g-1 and retains a capacity of 798 mAh g-1 after 200 cycles at a discharge/charge rate of 0.1 C. A capacity of 487 mAh g-1 is obtained at a rate of 3 C. A one-step pyrolysis and a chemical reagent assisted pyrolysis are also assessed to obtain porous carbon from amylose, but the obtained carbon shows inferior structures for sulfur cathodes. The multi-step pyrolysis and the resulting hierarchically porous carbon offer an effective approach to the engineering of biomass for energy storage. The micron–sized spherical S@C hybrid with different sizes is also favorable for high-tap density and hence the volumetric density of the batteries, opening up a wide scope for practical applications.

INTRODUCTION Under the dual pressure of emission reduction and economic development, developing naturally sustainable, abundant and low-cost materials for energy storage materials is highly desirable. Carbonaceous materials originated from biomass have received considerable attention in the application of energy storage devices with the increasing consciousness of sustainability and environmental benignity.1 Versatile biomass carbon sources in nature offer wide choices in producing carbon of different structures that are required in energy storage, such as the carbonized bean shell used for electric double layer capacitors and lithium-ion battery anodes,2


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peanut shell derived hard carbon as anode materials for lithium and sodium batteries,3 carbonized cotton,4 apricot shell,5 silk cocoon6 and shaddock peel7 as sulfur hosts for lithium-sulfur battery cathodes. Lithium-sulfur batteries (LSBs) have attracted substantial interest in recent years due to their high–energy density.8–10 The electrochemical reactions between lithium metal and sublimed sulfur can be expressed by the following reactions:11 S8+Li++e- → Li2Sx (2.4–2.1V) Li2Sx+Li++e- → Li2S2 and/or Li2S (2.1–1.5V)

(1) (2)

The theoretical capacity of sulfur cathode is 1672 mA h g-1, assuming that a complete reaction product of Li2S is formed.12 However, the long-chain lithium polysulfides (4≤x≤8) generated in reaction (1) is soluble in the electrolyte of the LSBs, subsequently causing shuttling effect with the short–chain sulfides Li2S2/Li2S between cathode and anode, which decreases the utilization of the overall active material during cycling. Moreover, sulfur and Li2Sx (x =1–8) have poor ionic and electronic conductivities, hence the internal resistance of the batteries is large and the reaction kinetics is sluggish. The early formed insoluble insulated layer of Li2S/Li2S2 on the surface of the sulfur particles during discharge impedes the continuous reduction of sulfur, which leads to poor active material utilization.10,13–14 The volume variation of sulfur during cycling is also a problem. The volume expansion from S8 (with a density of 2.07 g cm-3) to Li2S (with a density of 1.66 g cm-3)15 is ca. 79%, which causes pulverization of Li2S, and thus damages the electrical contacts between Li2S particles and hence the integrity of the electrode. Therefore, challenges exist in achieving high cyclic stability and rate capability for LSBs.


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To overcome the aforementioned problems of the dissolution, low conductivity and volume expansion of the sulfur cathode material in LSBs, an effective and widely used method is to incorporate sulfur into porous conductive carbon matrix, such as mesoporous carbon hollow spheres,15-17 microporous carbon spheres,18 hierarchical porous graphene sheets,19 mesomicroporous core-shell carbon.20,21 Alternatively, infiltrating sulfur into the tubes of carbon nanotubes,22 impregnating sulfur in porous microsphere frameworks composed of multi-wall carbon nanotubes,23 wrapping sulfur by graphene,24,25 or coating conductive polymers on the surface of sulfur particles,26,27 are also investigated. The above approaches sequester considerably the direct contact of sulfur and polysulfides with electrolyte, and hence reduce the dissolution of polysulfides in the electrolyte. Those porous carbon hosts and the conductive polymer coatings also improve the electron conductivity of the sulfur cathode and accommodate the volume change of sulfur during cycling. As a result, the electrochemical performance of sulfur cathodes is improved. In addition, other method, such as electrodepositing sulfur nanodots on Ni foam coupling with Li2S8 additive in the electrolyte28 and in situ synthesis of sulfur nanoparticle distributed three-dimensional porous graphitic carbon composites29 also show significance in achieving high performance sulfur cathodes. Compared with engineering carbonaceous materials, biomass derived carbonaceous materials are abundant, renewable and environmentally friendly. Such materials have been considered as carbon hosts for sulfur cathodes for LSBs in recent years, including those from cotton,4 apricot shell,5 silk cocoon,6 shaddock peel,7 bamboo charcoal,30 and pig bone,31 etc. KOH assisted pyrolysis is the commonly used method for carbonization.6,7,31 Hierarchically porous and microporous structures with surface areas of 900–3200 m2 g-1 are achievable.6,7,31 After incorporating sulfur contents of 50–68 wt%, those S@C hybrids show improved capacities


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and capacity retention.6,7,31 In addition, besides the application as porous carbon hosts for sulfur cathodes, biomass derived carbonaceous materials are also reported for other applications in LSBs. For instance, a filamentous fungi derived carbon fiber monolith32 and a bamboo carbon fiber membrane33 are used as conductive porous captor interlayers for lithium polysulfides between sulfur cathode and separator. A carbonized eggshell membrane is also used as a natural polysulfide reservoir for LSBs.34 These are all effective in improving the electrochemical performance of LSBs. However, the electrochemical performance of the biomass derived S@C hybrids still needs to be further improved in order to realize their practical applications. Exploring new biomass carbon sources and structures are important in order to develop high– performance S@C hybrids. Amylose is an abundant biopolymer in nature, with a chemical formula, C12H22O11. In addition, amylose shows a micron-sized (5–15µm) spherical shape, which is desirable in achieving high tap-density. In addition, amylose is originally mesoporous as which will be shown later in the manuscript. It is hopefully that amylose can be used as a novel sustainable and environmental benign raw material to produce porous carbon. In this work, different pyrolysis methods, including multi-step pyrolysis, one-step pyrolysis and KOH (potassium hydroxide) etching assisted pyrolysis, were used to carbonize amylose in order to obtain optimized porous carbons as sulfur hosts for superior LSB cathodes. A micron-sized spherical hierarchical porous carbon with macrohollow core and microporous shell was obtained by the multi-step pyrolysis (the obtained carbon is abbreviated as MHPC) without oxide templates, which is commonly used in the synthesis of spherically hollow carbon.7,17 The carbon shows a high surface area and a high porosity. The hierarchical porous structure can accommodate 48 wt.% S (S8) mostly in the pores after a sulfur infiltration process. The macrohollow core is not fully filled with sulfur after


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the infiltration. This provides extra space for accommodation the volume expansion of sulfur during lithiation and facilities the penetration of electrolyte into the S@C hybrid. The structure is also different from the meso-microporous core-shell carbon hosted S@C composites in which CMK-3 is used as the mesoporous core and an organic derived microporous carbon as shell, where the mesopores are fully filled with sulfur,20,21 without considering that CMK-3 is a high cost material. The obtained S@MHPC hybrid shows superior electrochemical performance for LSB cathodes. While the one-step pyrolytic carbon (abbreviated as OSPC) and the KOH etching assisted pyrolytic carbon (abbreviated as KEAC) show lower surface area and lower porosity, exhibiting inferior electrochemical performance after incorporation with sulfur. The present multi-step pyrolytic spherical hierarchical porous carbon is proposed of great benefit to the development of carbonaceous materials from biomass for LSB cathodes. The micron-sized spherical shape also facilitates high packing density for the cathodes, favoring the volumetric capacity of the batteries, which is important for practical applications. EXPERIMENTAL SECTION Synthesis. From the TG (thermal gravimetric) and MS (mass spectrum) measurements (Hiden Analytical QIC-20) of the amylose (Supporting information, Figure S1a, b), it is known that the main weight loss initiates at 260 °C and completes at 360 °C, showing a value of 57%, which is from the loss of H2O. Negligible amounts of CO2 and CO were released at the temperature range of 270–320 °C. The weight loss from 360 up to 650 °C, including the release of H2 over 450 °C as known from the MS, is only of ca. 7 wt%. Give that amylose has a high concentration of water, controlling the generation rate of steam during carbonization is important to avoid the collapse of the original morphology of amylose. In


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this case, a multi-step pyrolysis method was proposed to carbonize amylose, as shown in Scheme 1a. Amylose was firstly heated to 240 °C at a heating rate of 5 °C min-1 and then to 350 °C at a rate of 0.5 °C min-1, where it was held for 8 h under flowing Ar with a rate of 2 L min-1. After cooling, the initially heat-treated amylose was milled to fine powder in an agate mortar, and then heated to 350 °C at a rate of 10 °C min-1 and then to 600 °C at a rate of 1 °C min-1, holding for 8 h under flowing Ar with a rate of 1 L min-1. The obtained powder was grounded in the agate mortar again, and then heated to 600 °C at a rate of 10 °C min-1 and to 900 °C at a rate of 2 °C min-1, holding for 8 h under flowing Ar with a rate of 1 L min-1. After cooling, the finally obtained powder (MHPC) was loose and highly dispersive. So it was directly used for structural characterization and as sulfur host without further milling. For comparison, a one-step pyrolysis of amylose by heating to 900 °C at a rate of 5 °C min-1 and holding for 8 h under flowing Ar with a rate of 2 L min-1, as shown in Scheme 1b, was also performed. The obtained powder (OSPC) was severely agglomerated. Therefore, it was milled in the agate mortar prior to structural characterization and for sulfur host. In addition, in order to obtain other desirable carbon structures, a chemical etching assisted pyrolysis method, as shown in Scheme 1c, was also used. Typically, 5 g of amylose is mixed with 100 ml 1 mol L-1 KOH aqueous solution by stirring at 90 °C for a better dispersion of KOH. After the water was completely evaporated, the mixture was heated to 900 °C at a rate of 2 °C min-1 and held for 8 h under flowing Ar with a rate of 2 L min-1. After cooling, the product was rinsed by stirring with 250 ml of 0.5 M HCl solution, which was further leached by distilled water at room temperature for several times, until the pH value of the filtrate was 7. The leached product was then dried in vacuum at 160 °C for 24 h. The obtained product (KEAC) was also


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dispersive and used for structural characterization and as sulfur host without further milling. A graphite crucible was used as the container for the pyrolysis of all the samples. For preparation of S@C hybrids, the obtained carbon powder was firstly ball milled with sulfur in a weight ratio of 1: 2, and then diverted into an airtight stainless steel container in vacuum. The container sealed with the mixture was heated at 155 °C for 24 h to infiltrate sulfur into the porous carbon to form S@C hybrids. In order to remove the sulfur covered on the surface of the carbon particles or congregated between the carbon particles, which is considered to be more easily dissolved in the electrolyte and result in poor cyclic stability, the S@C hybrids were further heated at 300 °C for 0.5 h in a tube furnace under flowing Ar. Characterization. Crystal structure of the samples was identified by X–ray diffraction (XRD, X'Pert PRO, PANalytical) using Cu-Ka radiation (λ=1.5418 Å) with a scanning step of 0.04o s-1. Morphologies of the samples were observed by scanning electron microscopy (SEM, Hitachi S4800). The distribution of sulfur in the S@C hybrids was detected by an energy-dispersive spectrometer (EDS, Horiba) attached to the SEM. In order to observe the inner structure of the MHPC and its S@C hybrid, the samples were put into liquid nitrogen and kept for 5 minutes and then mechanically pressed by a pressure of 20 MPa in order to crush the spherical particles. Pore structure of the samples was analyzed by a nitrogen sorption method (Quantachrome Nova 1000e analyzer). A temperature of 77 K was used for the measurement. Specific surface area of the samples was measured by the Brunauer-Emmet-Teller (BET) method. Pore size distribution was calculated from the adsorption branch of N2 isotherm by the density functional theory (DFT). Raman spectra of the samples were recorded by a confocal Raman microscope (ViaReflex) using laser beam with a wavelength of 532 nm. As sulfur is very easy to evaporate during Raman spectrum testing under strong laser excitation, the laser power was kept at less


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than 2.5 mW µm-2 and a short irradiation period of 30 s was used. TG measurement of the S@C hybrids was performed from room temperature (RT) to 600 °C at a heating rate of 5 °C min-1. A pure Ar (99.999%) was used as carrying gas. Electrochemical measurements. Electrochemical properties of the S@C hybrids were measured by using coin cells of CR2025 with Li foil (99.9% Alfa Aesar) as the reference and counter electrode, and a polyethylene membrane (Celgard 2400) as a separator. The working electrode was prepared by mixing the S@C hybrid, Ketjen black, carbon black and polyvinylidene difluoride (PVDF, Alfa Asear) at a weight ratio of 75: 4: 11: 10 in 1-methyl-2-pyrrolidinone (NMP, Aldrich) to form a slurry, which was subsequently pasted onto an aluminum foil by using a blade coater and then dried at 60 °C for 24 h in vacuum. The S loading on each electrode was ca. 1.1 mg cm-2. The electrolyte consists of 1 M bis (trifluoromethane) sulfonamide lithium salt (LiTFSI, Alfa Aesar) in a mixture of 1,3 dioxolane (DOL, Sigma-Aldrich) and dimethoxyethane (DME, Alfa Aesar) (v/v=1:1) with 2 wt% lithium nitrate (LiNO3, Alfa Aesar) as an additive. The cells were assembled in an Ar-filled glove box with H2O and O2 contents less than 0.1 ppm (MBraun, Germany). The cells with electrolyte addition had been stood for 8 h prior to the electrochemical testing. Cycling stability of the S@C hybrids was measured by discharging and charging galvanostatically the cells in a potential rang of 3.0–1.0 V vs. Li/Li+ at a current density of 0.1 C (1 C = 1672 mA g-1) using an electrochemical testing system (Neware Technology Co., China). The rate capability of the S@C hybrids was measured from 0.1 C to 5 C in the same potential range. The specific capacity of the S@C hybrids was calculated on the basis of sulfur mass in the S@C hybrids. Cyclic voltammetry measurements were carried out at a scan rate of 0.1 mV s-1 in a potential rang of 3.0–1.0 V vs. Li/Li+ (MSTAT-1, Arbin). Electrochemical impedance spectra


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(EIS) were measured in a frequency range of 100 kHz to 0.01 Hz and at a potentiostatic signal amplitude of 5 mV by a frequency response analyzer (1255B solarton) equipped with an electrochemical interface (1287, Solartron). In addition, though the S loading of ca. 1.1 mg cm-2 on the electrode used in the present study is a widely used level for electrochemical testing in laboratories,20,21 for testing the specific capacity and cyclic stability of the electrode with high sulfur loading, a sulfur loading of ca. 2.2 mg cm-2 was also used for the hybrids cycled at 0.1 C. All the electrochemical tests were performed at 25 ± 1°C.

RESULTS AND DISCUSSION Structural characterization. XRD analysis (Figure S2) of the pyrolytic amylose by the different methods shows that their diffraction patterns are similar. The broad peak at around 23o and the relatively weak peak at around 43o corresponds to the (002) and (100) reflections of the graphitic domains, respectively,3,7 demonstrating partial graphitization of the carbon. Figure 1a–d shows the SEM images of the raw amylose, its carbonized products by multistep, one-step and KOH etching-assisted pyrolysis, respectively. The insets are the large magnification images of the corresponding samples to show more details of the particles. As seen from Figure 1a, the raw amylose powder has a spherical shape with size ranging from 5 to 15 µm, mainly of ca. 10 µm. Mesopores are observed on the surface of most particles, as shown typically in the inset. The overall morphology of the multi-step pyrolytic product is similar to that of the raw amylose, but shows a reduced particle size of ca. 4–6 µm as seen from Figure 1b. Moreover, from the SEM observation of its crushed particle which was freezed in liquid nitrogen prior to the pressing, shown representatively in the top inset of Figure 1b, it is found that there is


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a macropore in size of ca. 2–3 µm in almost the center of the particle. As the raw amylose particles cannot be crushed even assisted by liquid nitrogen freezing, their inside structure is failed to be characterized. However, most of the mesopores occurring on the surface of the raw amylose particles disappear after the multi-step pyrolysis, instead, the surface becomes rough, as shown in the bottom inset of Figure 1b. In comparison, the spherical morphology of the raw amylose was completely destroyed after the one-step pyrolysis, resulting a severe agglomeration and showing a typically crushed shape after milling in the agate mortar, as shown in Figure 1c and its inset. No pores are observed by SEM. As amylose is rich in water and bio-oil, the onestep quick pyrolysis process cannot preserve the original spherical shape of the amylose. The quickly released steam makes the powder to paste. In addition, other pyrolysis parameters, including: (1) heating the amylose to 350 °C at a rate of 1°C min-1 and maintaining for 3 h and then further heating to 900 °C at a rate of 1°C min-1 and maintaining for 3 h; (2) heating the amylose to 350 °C at 0.5 °C min-1 and maintaining for 3 h and then further heating to 900 °C at 0.5 °C min-1 and maintaining for 3 h; (3) heating the amylose to 350 °C at 1°C min-1 and maintaining for 8 h, and then to 600°C at 1°C min-1 and maintaining for 8 h, and further to 900 °C at 1°C min-1 and maintaining for 8 h, were also tested. The first two processes all result in severely agglomerated pyrolytic carbon. The agglomeration of the third process is evidently reduced, but well dispersed spherical particles still cannot be obtained. Thus those carbonized products are omitted for further structural characterization and electrochemical property testing. The product obtained by KOH etching-assisted pyrolysis shows a fluffy spongy-like structure, as shown in Figure 1d and its inset, which is different from either the multi-step pyrolytic product, or the one-step pyrolytic product. KOH can react with carbon, forming potassium carbonate (K2CO3) at about 600 °C by the following equation,4 and K2CO3 further decomposes into CO2


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and potassium oxide at temperatures higher than 700 °C. 6 KOH+2C → 2K+3H2+2K2CO3

(3)

Figure 2a and b is the N2 adsorption isotherms and the pore size distribution curves of the pyrolytic products, respectively. As seen from Figure 2a, all the samples show type I isotherm adsorption according to the Brunauer classification.33 Steep condensation steps occur only when the P/P0 value is lower than 0.05 and no other condensation steps are observed in the other range. Though the mesopores in the original amylose disappear after the pyrolysis, the N2 adsorption isotherm measurement shows that there are micropores in all the pyrolytic products. Figure 2b shows that the pore size ranges mostly in 1.0–2.0 nm, which is extremely small. It is obtained that the multi-step pyrolytic carbon (MHPC) has a hierarchical porous structure with macrohollow core and microporous shell, and the one-step pyrolytic carbon (OSPC) and KOH etching-assisted pyrolytic carbon (KEAC) have only micropores. Moreover, the MHPC has more amounts of micropores than the other two, and most of the micropores show a size of ca. 1.2 nm (Figure 2b). The surface area and pore volume for the differently pyrolytic carbon are listed in Table 1. As the pores with size larger than 100 nm cannot be measured by the BET method, the macropores inside the MHPC particles are not included in Figure 2 and their volume is not involved in the value listed in Table 1. As shown in Table 1, the MHPC has a specific surface area of 672.6 m2 g-1, which is higher than that of the KEAC (508.2 m2 g-1), and is much higher than that of the OSPC (277.6 m2 g-1). Even without including the macropores, the volume of the micropores of the MHPC, 0.32 cm3 g-1, is still the highest among the three types of carbon, while those of the OSPC and KEAC are only 0.13 and 0.26 cm3 g-1, respectively. As known from the TG curve of the raw amylose (Figure S1), vapor of H2O was highly generated at 260–360 °C. High temperature H2O vapor has high energy, which damages the


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micelle structure of amylose and makes amylose into a paste.35 This is likely the main reason for the low porosity and low surface of the one-step pyrolytic carbon, where a quick heating (5 °C min-1) up to 900 °C was used. Whereas for the multi-pyrolysis, the low heating rate in the temperature range of 240–350 °C is suggested to lower greatly the evaporation rate of H2O. The low heating rate, coupled with the high flow rate of the carrying gas, favours greatly the removal of the moisture, and hence reduces the damage of the high energy H2O vapor on the spherical shape of the original amylose. As a result, the spherical shape of the original amylose is preserved. For the KEAC, due to the strong etching of KOH to carbon, the damaged spherical shape was further carved to a fluffy spongy-like structure. The gases (H2O, CO, CO2 and H2, Figure S1) generated during the pyrolysis process and the original mesopores in the amylose can all be possibly the factors for the formation of the micropores in the differently pyrolyzed carbon. Though the formation mechanism of the macrohollow core inside the MHPC is not clear, it is likely to benefit from the low evaporation rate of H2O vapor and the high flow rate of the carrying gas. The present multi-step pyrolysis method gives a way in fabrication unique porous carbon from biomass materials by only pyrolysis without any chemical reagents. In addition, it is also hopeful that the multi-step carbonization process can be optimized to less stages, such as shortening the maintaining periods, which will be tested in the future work. Raman spectra of the differently pyrolyzed carbon and the different S@C hybrids are shown in Figure 3a and b, respectively. The spectra of the differently pyrolyzed carbon look similar. The peaks centered at ca. 1600 cm-1 and ca. 1350 cm-1 are the G and D bands of carbon, respectively, corresponding to the stretching vibrations in the graphene layer and the disordered graphite lattices. The ratios of the intensity of the D and G bands for the differently pyrolyzed carbon, ID/IG, show a similar value of ca. 1.7. In addition, the spectra all show a G′ band in the range of


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2500–2800 cm-1, corresponding to an overtone of the G band, which occurs commonly in graphitic carbon, but not in amorphous carbon36–38 This confirms that the pyrolytic carbon contains graphitic domains as obtained from the XRD analysis. Such G′ band is also reported in a carbonized shaddock peel, which contains comparatively ordered graphitic structure.39 Graphitization, even partially, is a substantial prerequisite in improving the conductivity of carbonaceous materials,36 which is helpful in facilitating the transportation of electrons of sulfur during electrochemical cycling. The D, G and G′ bands also appear in the spectra of the S@C hybrids. The ratios of ID/IG of the different S@C hybrids shows a close value of ca. 2.1, which is slightly higher than that of the original carbon host, indicating a decreasing level of ordered structure. Increase of ID/IG after the incorporation of sulfur is also reported in other S/C systems, such as the carbonized 1,4-H2NDC and sulfur system, where sulfur is reported to interact with the dangling bonds of carbon, resulting in extended sp2 C-C bonds.40 In addition, sulfur is not detected by the Raman measurement, indicating that individual sulfur should be mostly removed during the extra heating at 300 °C for 0.5 h after the sulfur impregnation process and there is no detectable amount of sulfur remaining on the surface of the S@C particles, indicating that the amount of sulfur absorbed on the surface of the carbon particles is very limited. Though sulfur is not detected by Raman analysis, XRD analysis shows that there are strong sulfur diffraction peaks in all the patterns of the three S@C hybrids (Figure S3), which are highly identical to orthorhombic sulfur (JCPDS 08-0247). No visible difference is observed between the diffraction peaks of sulfur in the hybrids and those of the pristine sulfur. The diffraction peaks of sulfur are all very sharp, indicating a high crystallinity. The hump peaks from the pyrolytic carbon (Figure S2) are not observed in the patterns of the S@C hybrids, which is likely due to


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the lowered crystallinity of carbon after the sulfur incorporation as obtained from the Raman measurement. The SEM morphology of the S@MHPC hybrid is shown in Figure 4a. The inset is a schematic structure of the S@MHPC particles. Comparison of Figure 4a and Figure 1a shows that the spherical shape and the size of the MHPC particles are well retained after the incorporation of sulfur. Congregated sulfur as reported in a sulfur/porous carbon fiber composite41 is not found in the S@MHPC hybrid. It is confirmed that congregated sulfur was mostly removed after the extra heating at 300 °C for 30 min under flowing argon. In addition, different from the rough surface of the MHPC particles, the surface of the S@MHPC particles looks smooth. A thin coating of sulfur absorbed tightly on the rough surface of the MHPC particles is also reasonable. Such micron-sized spherical S@C hybrid with different sizes is hopefully favorable in achieving high-tap density and hence enhancing the volumetric density of the batteries, which is of great benefit for practical applications. Figure 4b and c shows representatively a crushed S@MHPC particle and its EDS mapping of sulfur, respectively. It is seen that the macrohollow core is not fully filled with sulfur after the sulfur immersion process. A macropore still exist in almost the center of the S@MHPC particle. It is likely that the long and narrow channels in the thick shell retard the flow of liquid sulfur into the macrohollow core. Nevertheless, Figure 4c shows that sulfur is evenly distributed in the particle shell, indicating that the micropores are fully filled with sulfur. Figure 4d and e shows the morphologies of the S@OSPC and S@KEAC hybrids, respectively, which are similar to their original carbon (Figure 1c and d), indicating that sulfur exists mostly in the micropores of the OSPC and KEAC also. Further EDS analysis also shows that sulfur is evenly distributed in the S@MHPC, S@OSPC and S@KEA hybrids (Figure S4).


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The TG and the derivative weight loss curves of the S@C hybrids are shown in Figure 5. The weight loss is attributed to the evaporation of sulfur. It is seen that the onset evaporation temperature of sulfur in the hybrids are all higher than that of the individual one. Among them, the S@MHPC hybrid shows the highest one of 190 °C, which is ca. 30 °C higher than that of the individual sulfur. In addition, it is clear that the S@MHPC hybrid shows a two-step evaporation process, indicating different evaporation characteristics. The first step takes place at 250–350 °C and centers at ca. 295 °C, corresponding to a weight loss of 17 wt%. The second step takes place at 350–510 °C and centers at ca. 480 °C, corresponding to a weight loss of 31 wt%. A total weight loss of 48 wt% is obtained. Moreover, the evaporation of sulfur at the high temperature range is much slower than that at the low temperature range. It is probably that sulfur confined in deeper micro-channels of the S@MHPC hybrid is more difficult to be evaporated. While both S@OSPC and S@KEAC hybrids show only one-step evaporation, centering at ca. 250 °C and 290 °C and finishing at ca. 280 °C and 335 °C, respectively. These temperatures are all even lower than those of the first step of the S@MHPC hybrid. As shown in Figure 4, the S@OSPC and S@KEAC hybrids show an overall smaller particle size than the S@MHPC hybrid, corresponding to a shallower depth of the micro-channels of the formers than the later, hence results in the one-step evaporation. The values of the total weight loss of the S@OSPC and S@KEAC hybrids, which corresponds to the sulfur contents of the hybrids, are 24 wt% and 37 wt%, respectively. Both are much lower than that of the S@MHPC. In comparison, individual sulfur evaporates quickly and the evaporation ends at a low temperature of ca. 310 °C. So far, it is obtained that there is effective interaction between the confined sulfur and the carbon substrate for all the hybrids, however, due to the deeply and tightly confined sulfur in the MHPC, the S@MHPC hybrid shows the most difficulty for sulfur evaporation.


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The calculated amount of sulfur that the micropores in the MHPC particles can accommodate based on the porosity of the MHPC (Table 1), taking the densities of the MHPC and sulfur to be 1.39 and 2.07 g cm-3, respectively, are 40 wt%, which is 8 wt% lower that the sulfur content of the S@MHPC hybrid detected by TG analysis (48 wt%). Most of the extra sulfur should permeate to the macropores inside the MHPC particles, as the amount of sulfur absorbed on the surface of the MHPC particles is very limited, which is lower than the detectable amount for Raman analysis as aforementioned. Though the not full filling of the macropores of the MHPC particles with sulfur reduces the sulfur content of the S@MHPC hybrid, it is supposed that the hollow structure of the hybrid offers effective volume accommodation for the volume expansion of sulfur during lithiation, favoring the integrity of the electrode and hence the electrochemical performance. Nevertheless, as there is still macropore inside the S@ MHPC particle (Figure 4b, c), this offers space to accommodate some more amount of sulfur if a further optimized infiltration technique can be found, to further increase the energy density of the electrode. The sulfur contents that the micropores in the OSPC and KEAC can accommodate, calculated by the same way as that of the MHPC, are 22 wt% and 35 wt%, respectively. The values are only slightly lower than those detected by the TG analysis (24 wt% and 37 wt%, respectively). It is likely that there is an extremely thin sulfur layer absorbed on the surface of the carbon particles. Electrochemical performance

Figure 6a shows the cycling stability and the Coulombic efficiency at 0.1C of the S@MHPC, S@OSPC and S@KEAC hybrids, where the sulfur loading of the electrode is 1.1 mg cm-2. The S@MHPC hybrid exhibits an initial discharge capacity of 1490 mAh g-1 at 0.1 C, which is 88% of the theoretical capacity of sulfur and retains a capacity of 798 mAh g-1 up to 200 cycles,


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corresponding to the capacity retention of 54%. In addition, the Coulombic efficiency decreases from 99% for the first cycle to 95% in the following several cycles, and then maintains at 95% upon cycling to 200 cycles, which is supposed due to that the weakly trapped sulfur on the surface of the S@MHPC hybrid was easily dissolved in the electrolyte at the initial several cycles. The degradation of the capacity and Coulombic efficiency in the first several cycles is severer, however, the capacity fading is stable when the weakly trapped sulfur was consumed. The capacity retention after 200 cycles is 58% with respect to the discharge capacity of the third cycle (1373 mAh g-1). In comparison, the S@OSPC hybrid shows an initial discharge capacity of 1280 mAh g-1, which is 210 mAh g-1 lower than that of the S@MHPC hybrid. The capacity after 200 cycles is only 367 mAh g-1, corresponding to the capacity retention of only 27%, much severer capacity fading than that of the S@MHPC hybrid, especially in the initial several cycles as seen from Figure 6a, implying low sulfur utilization with severe polysulfide dissolution. The initial capacity, initial Coulombic efficiency and the capacity retention after 200 cycles of the S@KEAC hybrid are 1421 mAh g-1, 89% and 33%, respectively, which are all lower than those of the S@MHPC hybrid, but higher than those of the S@OSPC hybrid. The Coulombic efficiencies of the S@OSPC and S@KEAC hybrids upon cycling range in 87–93% as overall, which are all several percentages lower than that of the S@MHPC hybrid, indicating lower utilizations of the active materials. It is obtained that the micron-sized spherical hierarchical porous structure of the MHPC is beneficial in trapping sulfur. In addition, the cycle performance of S@MHPC, S@KEAC and S@OSPC hybrids with a high sulfur loading of 2.2 mg cm-2 is shown in Figure S6. It is found that the capacity and capacity retention of the hybrids are somewhat lowered as expected. The initial capacities of the S@MHPC, S@KEAC and S@OSPC hybrids decrease to 1414, 1249, 1173 mAh g-1, and the capacities after 200 cycles decrease to


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694, 358, 234 mAh g-1, respectively. With respect to the S@MHPC hybrid, the capacity retention after 200 cycles for the high loading of 2.2 mg cm-2 is lowered by less than 9% compared with that for the loading of 1.1 mg cm-2, which is not very large. Figure 6b shows the rate capability of the S@C hybrids with the sulfur loading of 1.1 mg cm-2 for the electrode from 0.1 C up to 5 C. It is seen that the capacities of the S@MHPC hybrid at 0.5 C, 1 C, 3 C and 5 C are 907, 681, 487 and 226 mAh g-1, respectively, which are all much higher than those of the S@OSPC and S@KEAC hybrids. Due to the high ohmic and kinetic overpotential at the high rate, the capacity decreases with increasing current rate. In addition, a capacity of 1000 mAh g-1 is recovered when the current density is returned back to 0.1 C, which is still high, though there is capacity degeneration after cycling at the different rates. In comparison, the S@OSPC hybrid fails to cycle at 1 C and its capacity at 0.5 C is only 380 mAh g-1. The S@KEAC hybrid fails to cycle at 3 C and its capacity at 1 C is only 420 mAh g-1. The superior capacity, cyclic stability and high-rate capability of the S@MHPC hybrid is attributed to its unique structure: the inner marcopore can accommodate the volume change of sulfur during lithiation; the long and narrow micro-channels in the carbon shell not only sequester sulfur/polysulfides from the direct contact with the electrolyte, preventing effectively the dissolution of the polysulfides in the electrolyte, but also offer an intimate contact for sulfur/polysulfides with the carbon substrate, resulting in sufficient electron conductivity for the hybrid; the large surface area and high porosity of the MHPC provide considerable sites for loading insulating Li2S/Li2S2 and favour the lithium-ion transportation; the partial graphitization of MHPC offers high electron conductivity. However, such superiority is insufficient for the S@OSPC and S@KEAC hybrids, hence, inferior electrochemical properties are obtained.


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Although the electrochemical properties of the S@C hybrids from different laboratories cannot be quantitatively compared because of different electrode preparation and testing programs, it is still useful to observe progress made among different research communities. Comparison of the capacity and cyclic performance of the present S@MHPC hybrid with those of the typically reported sulfur cathodes using other carbonized biomass hosts as well as some engineering carbon hosts is shown in Table 2. Compared with those with biomass derived carbon hosts, the initial discharge capacity and the capacity after 200 cycles at 0.1 C of the S@MHPC hybrid are much higher than those of the S@cotton4 and S@apricot shell5 systems which are also cycled for 200 cycles but at 0.2 C (the rate used in the references). Moreover, the capacity retention of the present S@MHPC hybrid at 0.1 C after 200 cycles is very close to those of the S@apricot shell system at 0.2 C after 200 cycles5 and the S@silk cocoon system at 0.5 C after 80 cycles,6 indicating a better cyclic performance for the present hybrid. Though the present capacity retention of the S@MHPC hybrid at 0.1 C after 200 cycles seems lower than that of the S@cotton system after 200 cycles but at 0.2 C,4 it is noted that the initial capacity of the S@cotton system seems relatively low. For example, it is 260 mAh g-1 lower than that of the S@apricot shell system at the same rate of 0.2 C,5 which pretends the high capacity retention. Moreover, the present S@MHPC hybrid shows a capacity of 907 mAh g-1 at 0.5 C after cycled at 0.1 C and 0.3 C, respectively, for 10 cycles (Figure 6b), which is even much higher than the initial capacity of 685 mAh g-1 for the S@bamboo charcoal system at 0.5 C. The capacity of 681 mAh g-1 at 1C of the present S@MHPC hybrid (Figure 6b) is comparable to that of the S@cotton system,4 and is higher than the 640 mAh g-1 for the S@apricot shell system,5 the 500 mAh g-1 for the S@shaddock peel system7 and the 481 mAh g-1 for the S@bamboo charcoal system.30


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It is clear that the capacity, capacity retention and rate capability of the present S@MHPC hybrid are superior to those of many other reported S@C hybrids with biomass derived carbon. Though the special surface area of the present MHPC (672 m2 g-1) is not so high as those of the cotton (1286 m2 g-1),4 apricot shell (2269 m2 g-1),5 silk cocoon (3243 m2 g-1)6 and shaddock peel (937 m2 g-1)7 derived carbon, the long and narrow channels in the spherical shell are likely to confine sulfur effectively, preserving the polysulfides from dissolving in electrolyte, resulting in less shuttle effect and offering an intimate electrical conductive contact for the active material with the carbon substrate. Compared with the S/C composites using meso-microporous core-shell carbon host, where CMK-3 was used as the mesoporous core and an organic carbon source derived microporous carbon as shell,20,21 the present S@MHPC hybrid shows a comparable capacity after 100 cycles with the one reported by Huang et al.,20 as from the data listed in Table 2. The present sulfur content is 12 wt% lower than that in the reported work, however, CMK-3 is a high cost raw material, the present S@MHPC hybrid is hopefully of lower cost than the reported S@C system. In addition, compared with another similar meso-microporous core-shell structured carbon hosted S/C composite where CMK-3 is also used,21 the present S@MHPC hybrid shows a higher capacity and a much better capacity retention if considering the specific capacity with respect to the S/C hybrid, not only sulfur, as known from Table 2 also. The capacity retention after 100 cycles of the present S@MHPC hybrid cycled at 0.1 C is comparable to that of a polydopamine derived carbon hosted sulfur composite further wrapped by graphene cycled at 0.2 C,17 and a S/CNFs composite cycled at 0.06 C,42 indicating a better cyclic performance for the present system. However, further increasing the sulfur is necessary in our future work to get a high energy density for sulfur cathodes.


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Figure 7a–c shows the discharge/charge profiles of the different S@C hybrids with a cut-off potential of 1–3 V (vs. Li/Li+) of selected cycles at 0.1 C. It is seen that all the hybrids show mainly two discharge plateaus and one charge plateau. The discharge plateau at the comparatively high potential range, corresponding to the transition of elemental sulfur to longchain polysulfides (Li2Sx, 4≤x≤8), drops slightly upon the discharge process, which indicates the existence of an overpotential for the reaction. The flat plateau at the comparatively low potential range, corresponding to the further reduction of the polysulfides to Li2S2/Li2S, indicates that the reduction occurs at a considerably stable potential and it is diffusion-independent, especially for the S@MHPC hybrid. The latter process is a major one, generating more capacity than the former. The discharge plateau of the S@MHPC hybrid (Figure 7a) is slightly lowered by ca. 0.05 V after initial several cycles, and turns to recover with the further cycling, maintaining almost stable up to the tested 200 cycles, indicating a very low polarization. However, those of the S@OSPC and S@KEAC hybrids shift gradually to low potentials upon cycling, indicating an increasing reduction polarization. The charge plateau represents the reverse reaction from Li2S2/Li2S to polysulfides and finally to sulfur, or to Li2S8 if the oxidation is insufficient. The initial charge plateaus for all the three S@C hybrids are considerably flat. However, only the S@MHPC hybrid maintains flat and stable charge plateau upon cycling up to 200 cycles, while both S@OSPC and S@KEAC hybrids show increasing sloping charge plateaus upon cycling, indicating an increasing polarization with a sluggish oxidation. The overpotential, the potential gap between charge (oxidation) and the main discharge (reduction) plateaus, ∆V, as shown in Figure 7a–c, of the S@MHPC hybrid at 0.1 C shows an almost stable value of 0.20 V for both the first cycle and the 200th cycle as obtained from the redox peak differences of the differential profiles of capacity to potential of the discharge/charge


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curves (dQ/dV profiles, Figure S7a, b), suggesting high reaction kinetics with small barrier. The overpotentials of the S@OSPC and S@KEAC hybrids in the initial cycle at 0.1 C are 0.22 V and 0.21 V (Figure S7c, e), respectively, which are close to that of the S@MHPC hybrid. However, their corresponding values increase to 0.51 V and 0.43 V, respectively, at the 200th cycle (Figure S7d, f). The polarization becomes severer upon cycling, especially for the S@OSPC hybrid. The above result confirms that the unique hierarchical porous structure of the MHPC alleviates effectively polysulfides from dissolving in the electrolyte during cycling, and hence there are less Li2S/Li2S2 generated on the electrode surface. The high surface area and high porosity of the MHPC also allows more sites for the deposition of the insulating Li2S/Li2S2 and allows facile lithium-ion transportation. In addition, as seen from Figure 7b and c, the increase of the overpotentials of the S@OSPC and S@KEAC hybrids takes place mainly in the initial several cycles, whereas the increase becomes small from the 50th cycle to the 200th cycle, indicating that the electrochemical reaction tends to be stable after the initial consuming of the easily dissolved surface sulfur. Figure 8 illustrates the overpotentials of the S@C hybrids at the different discharge/charge rates, which are obtained from the redox peak differences of the dQ/dV profiles of the first cycle at that rate (Figure S8–10), the discharge capacities of which are shown in Figure 6b. As the S@OSPC hybrid failed to cycle over 0.5 C and the S@KEAC hybrid failed to cycle over 1 C, their overpotentials are only obtained up to 0.5 C and 1 C, respectively. As seen from Figure 8, the overpotential increases evidently with the current rate, and the differences for the hybrids enlarge with the increase of the current rate. The overpotentials of the S@MHPC hybrid at 0.5 C, 1 C, 3 C and 5 C are 0.22, 0.32, 0.62 and 1.0 V, respectively. However, the overpotential of the S@OSPC hybrid at 0.5 C is as high as 1.0 V, which is the same as that of the S@MHPC hybrid


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at 5 C. The values of the overpotentials of the S@KEAC hybrid at the different rates are always between those of the S@OSPC and S@MHPC hybrids. The above result suggests further that the hierarchically porous structured spherical carbon is beneficial in improving the kinetic performance of the sulfur cathode. Figure 9a–c shows the cyclic voltammetry (CV) plots of the initial five cycles of the S@C hybrids. For the S@MHPC hybrid, as seen from Figure 9a, there are two major peaks centering at ca. 2.28 and 2.03 V (vs. Li/Li+) in the first cathodic sweep, corresponding to the transition from elemental sulfur to polysulfides (Li2Sx, 4≤x≤8) and the further reduction of the polysulfides to Li2S2/Li2S as common, respectively, which is in agreement with the two discharge plateaus in Figure 7a. The two minor peaks centering at 1.68 and 1.40 V in the first cathodic sweep are attributed to the irreversible reduction of LiNO3.29,43 Similar weak peaks also appear in the S@OSPC and S@KEAC hybrids, as seen in Figure 9b and c. It seems that LiNO3 decomposed at the low potential range. However, it is reported that a protective surface film formed after Li foil was immerged in an electrolyte with LiNO3 addition.44 The present cells with electrolyte addition had been stood for 8 h prior to the electrochemical testing. It is supposed that a protective surface film is formed during the standing. The stable Coulombic efficiency of 95% of the S@MHPC hybrid after initial several cycles (Figure 6a) attributed partially to such a film. In the first anodic sweep of the S@MHPC hybrid, there is a strong and sharp peak centering at ca. 2.48 V, corresponding to the conversion of Li2S/Li2S2 to Li2S8/S8, which is also consistent with the main charge plateau in Figure 7a. Because of the hysteresis in the CV technique,45 the cathodic peaks shift to a lower potential and the anodic peak shifts to a higher potential compared to the discharge/charge plateau potentials. There is a weak shoulder anodic peak centering at ca. 2.69 V, which is considered from the conversion from Li2S8 to sulfur.46 The cathodic peak corresponding


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to the transition from sulfur to the polysulfides shifts slight from 2.28 V to 2.32 V after 5 cycles. Moreover, the main anodic peak shows an ignorable shift from 2.48 to 2.47 V after 5 cycles. In addition, the peak position relating to the reduction of polysulfides to Li2S2/Li2S almost does not change upon cycling. Overall, the S@MHPC hybrid shows a relatively low level of polarization in the tested cycles, which is attributed to the well encapsulated sulfur in the long micro-channels of the spherical shells and hence less amount of polysulfides dissolving in the electrolyte. The less amount of insulating Li2S/Li2S2 depositing on the surface of the active material and the intimate contact of sulfur/polysulfides to the carbon substrate favour both electron transfer and lithium-ion diffusion. In comparison, as seen from Figure 9b, there are also two major cathodic peaks in the CV curves of the S@OSPC hybrid, but center at lower potentials (1.98 and 2.21 V) and are much broader than those of the S@MHPC hybrid. There is also only one anodic peak for the S@OSPC hybrid, but centers at a higher value of ca. 2.67 V than that of the S@MHPC hybrid. The peak is broader than that of the S@MHPC hybrid. The larger redox overpotential and broader peak indicate a larger redox polarization and a lower redox kinetics, which are in agreement with those obtained from the discharge/charge plateaus (Figure 7b). The weak shoulder peak occurring in the S@MHPC hybrid is not observed in Figure 9b. It is probably overlapped by the broad peak or there is no such peak. One anodic peak without coupling a weak shoulder peak is also commonly found in other S@C systems, such as the graphene oxide/sulfur cathode47 and the porous carbon nanofibers/sulfur cathode.48 In addition, one main anodic peak, coupled with a weak shoulder peak at a higher potential range of 2.6–2.8 V, is also observed in some another S@C systems, such as the S@porous hollow carbon cathode,17 the dual coaxial nanocable sulfur cathode46 and the nanostructured sulfur/carbon nanotube cathode.49 The weak shoulder peak after


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the main peak is suggested due to the further oxidation of Li2S8 to elemental sulfur after the main oxidation of Li2S/Li2S2 to Li2S8, when a highly conductive carbon matrix is applied. In this case, it further confirms that the unique structure of the MHPC facilitates electron transfer due to the homogenous distribution of sulfur in the microporous shell and the intimate contact of sulfur with the carbon substrate. Moreover, the overpotential of the redox peaks of the S@OSPC hybrid increases evidently during cycling, indicating a severe increasing polarization, consistent with the result from the discharge/charge profiles (Figure 7b). For the S@KEAC hybrid (Figure 9c), the two main cathodic peaks also locate at higher potentials and the main anodic peak locates at a lower potential than those of the S@MHPC hybrid, but the shifts are slightly smaller than those of the S@OSPC hybrid. The weak shoulder peak corresponds to the reduction of Li2S8 to S8 also occurs (2.72 V). It is obtained that the spongy-like porous structure of the KEAC is superior to the microporous bulk structure of the OSPC in reaction kinetics, but is inferior to the hierarchically porous structure of the MHPC. Figure 10 is the Nyquist plots of the three S@C hybrids. The solid symbols denote the experimental data, and the lines represent the fitting results from the calculation based on the equivalent circuit inserted in Figure 10. Figure 10 shows that every spectrum is composed of a semicircle at the high-to-medium frequency region and an inclined line in the low frequency region. The intercept of the semicircle on the real axis at the high frequency is ascribed to the ohmic resistance resulting from electrolyte, electrode and separator, which is denoted as Rs. The semicircle correlates to the charge transfer resistance (Rct) at the interface of the active material and electrolyte. The inclined line represents the Warburg impedance, associated with lithium-ion diffusion in the electrode particles.50 Obtained from the fitting result, the S@MHPC hybrid has a Rct value of 19 Ω, which is lower than the 136 Ω for the S@OSPC hybrid and the 30 Ω for the


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S@KEAC hybrid. The much smaller Rct value of the S@MHPC hybrid indicates a much higher electronic conductivity and reaction kinetics. The result further demonstrates that the macrohollow core and microporous shell structure of the spherical carbon provides better electronic contact of the incorporated sulfur with the carbon substrate than the structures of the other two types of carbon.

Conclusions A micron-sized spherical carbon with a hierarchical porous structure of marcohollow core and microporous shell and a surface area of 672 m2 g-1 is successfully synthesized from a sustainable biomass material of amylose by a multi-step pyrolysis. The synthesis process does not need any assisted chemical solution, which is considered environmentally friendly. After incorporating sulfur by a melting infiltration method, sulfur is well confined in the long and narrow channels in the shell, and the marcohollow core is mostly maintained. This S@C hybrid with a sulfur content of 48 wt% shows a high initial capacity of 1490 mAh g-1 and a capacity of 798 mAh g-1 at 0.1 C after 200 cycles as cathode material for lithium–sulfur batteries. Capacities of 681 and 487 mAh g-1 are achieved at 1 C and 3 C rates, respectively. The S@C hybrid shows low polarization and high redox kinetics during cycling. The confinement of sulfur alleviates greatly the dissolution of the polysulfides to electrolyte and offers intimate contact with the carbon substrate. The large surface area provides sufficient sites for the deposition of the insulating Li2S2/Li2S. The marcohollow core can accommodate the volume expansion of sulfur during lithiation. This unique structure offers both facile electron-transfer and lithium-ion diffusion. While the one-step pyrolysis and the chemical reagent assisted pyrolysis methods only produce inferior structured porous carbon, which offer inferior electrochemical properties after


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incorporating sulfur. The multi-step pyrolysis method and the synthesized hierarchically porous spherical carbon is considered having potential application in the development of carbonaceous materials from biomass materials for highly efficient energy storage.


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(a)

(b)

(c)

Scheme 1. Schematic routes of the preparation process for MHPC (a), OSPC (b) and KEAC (c).

(a)

(b)

(c)

(d)

Figure 1. SEM images of the raw amylose (a) and after multi-step (b), one-step pyrolysis (c) and KOH etching-assisted (d) pyrolysis. The insets are their corresponding large magnification morphologies.


29

3 -1

-1

3

-1

(b)

0.6

3

(a)

Differential Pore Volume (cm g )

250

Volume Adsorbed (cm g STP)

200 150

100

MHPC KEAC OSPC

50 0 0.0

0.5 0.4 0.3 0.2 0.1

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0.7

MHPC OSPC KEAC

0.6 0.5 0.4 0.3 0.2 0.1 0.0 1.0

1.5

2.0 2.5 Pore Width (nm)

3.0

0.0 0.2

0.4

0.6

0.8

1.0

0

5

10

Relative pressure (P/P0)

15 20 25 Pore Width (nm)

30

35

Figure 2. N2 sorption isotherms (a) and pore size distribution (b) of the differently pyrolytic carbon. MHPC

G (1592.7)

S@MHPC

(a)

G (1592.7)

(b)

(1367.3) D

(1366.5 ) D

(2707.3) G'

OSPC

G (1590.1)

(1362.6) D

Intensity (a.u.)

(2695.7) G'

Intensity (a.u.)

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

Differential Pore Volume (cm g )


S@OSPC

G (1582.4)

(1366.6) D

(2702.6) G'

(2727.2) G'

G

KEAC (1365.5 )

S@KEAC

(1592.3)

G(1581.1)

(1366.4) D

D

(2701.5) G' (2719.4) G'

0

500

1000 1500 2000 2500 3000 -1

Raman Shift (cm )

0

500

1000 1500 2000 2500 3000 -1

Raman Shift (cm )

Figure 3. Raman spectra of the three types of carbon (a) and the three S@C hybrids (b).


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(a)

(b)

(d)

(e)

(c)

Figure 4. SEM morphologies of the S@MHPC hybrid (a), a representative crushed S@MHPC particle (b) and its EDS mapping of sulfur (c), S@OSPC (d) and S@KEAC (e) hybrids. The insert of (a) is a schematic structure of a S@MHPC hybrid particle.

Weight Loss (%)

80

37% S

48% S

90

24% S

100

70 60 50

S@MHPC S@OSPC S@KEAC Sulfur

40 30

o

Derivative Weight Loss (% C )

20

-1

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


10 0 0

100

200

300

400

500

600

o

Temperature ( C)

Figure 5. TG and the derivative weight loss curves of the S@C hybrids prepared from the differently pyrolyzed carbon.


31


(a) Specific Capacity (mAh g-1)

100 90

1600 1400 1200 1000 800 600 400 200 0

S@MHPC

S@OSPC

Coulombic Efficiency (%)

80

S@KEAC

70 60

0.1 C

50 40 30 20 10 0

20

40

60

80

100

120

140

160

180

0

200

Cycle Number

1600

-1

Specific Capacity (mAh g )

(b)

S@MHPC S@OSPC S@KEAC

0.1 C

1400 1200

0.3 C 0.5 C

1000 800

0.1 C

1C 3C

600 400

5C

200 0 0

10

20

30

40

50

60

70

Cycle Number

Figure 6. Cycling stability at 0.1C (a) and rate capability (b) of the S@MHPC, S@OSPC and S@KEAC hybrids.

3.25

Voltage vs.Li/Li Li/Li+ ) Voltage(V / vs.

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

Page 32 of 43

3.00

(a)

200

100 50 10 5 1

(b)

200 100 50

10 5

(c)

1

200 100

50

1

10 5

2.75 2.50 2.25

∆V200th

∆V200th

∆V1st

∆V200th

∆V1st

∆V1st

2.00 1.75 1.50 1.25

S@MHPC

300

S@KEAC

S@OSPC

1.00 600

900

1200

1500

SpecificCapacity Capacity/(mAh mAh gg-1-1) Specific

300

600

900

1200

1500

Specific Capacity (mAh g-1)

300

600

900

1200

1500

Specific CapacitymAh (mAh Specific Capacity/ g-1g-1)

Figure 7. Charge-discharge profiles of the S@MHPC (a), S@OSPC (b) and S@KEAC (c) hybrids of selected cycles at 0.1 C.


32

Page 33 of 43

Figure 8. Overpotentials of the S@MHPC, S@OSPC and S@KEAC hybrids (with respect to the peak differences of the main delithiation and lithiation of polysulfides to Li2S2/Li2S) at different current rates.

-1

Current (A g )

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


3.0 2.48 V 1st (a) 2.5 2nd 2.47 V 2.0 3rd 2.69 V 1.5 4th 5th 1.0 0.5 0.0 -0.5 2.32 V -1.0 1.40 V -1.5 1.68 V -2.0 2.28 V -2.5 S@MHPC 2.03 V -3.0 1.0 1.5 2.0 2.5 3.0

1st 2nd 3rd 4th 5th

1st 2nd 3rd 4th 5th

2.67 V (b)

2.55 V (c) 2.72 V

2.53 V

2.26 V 1.65 V 1.89 V 2.21 V

S@OSPC 1.0

1.5

2.25 V

S@KEAC

1.98 V 2.0

2.29 V 1.41 V 1.69 V

2.5

3.0

1.0

1.5

2.02 V 2.0 V 2.0

2.5

3.0

+

Potential (V vs. Li/ Li )

Figure 9. CV profiles of the S@MHPC (a), S@OSPC (b) and S@KEAC (c) hybrids.


33


120

S@MHPC S@OSPC S@KEAC

100

80

Rs

CPE

W0

Rct

60

''

- Z (ohm)

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

Page 34 of 43

40

20

0 0

20

40

60 ' Z (ohm)

80

100

120

Figure 10. Nyquist plots of the different S@C hybrids.

Table 1 Surface area and pore volume of the differently pyrolyzed carbon and the S@C hybrids. Surface area (m2 g-1)

Pore volume (cm3 g-1)

MHPC

672.6

0.32*

OSPC

277.6

0.13

KEAC

508.2

0.26

S@MHPC

5.3

0.01

S@OSPC

5.4

0.01

S@KEAC

4.6

0.01

Samples

* The volume of the marcopores of the MHPC particles is not involved.


34

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


Table 2 Electrochemical performances of the S@C cathodes based on different biomass derived porous carbon. Carbon hosts

Current Initial Cycle Sulfur Capacity at Capacity rate (C)* capacity number** content the noted retention (mAh g-1) (%) cycle (%) (mAh g-1)

Refs.

MHPC

0.1

1490

200

48

798

54

Our work

MHPC

0.1

1490

100

48

1015

68

Our work

Carbonized cotton

0.2

1017

200

68

760

75

4

Carbonized apricot shell

0.2

1277

200

53

710

56

5

Carbonized

silk cocoon

0.5

1443

80

48

804

56

6

Carbonized shaddock peel

0.1

1190

100

62

779

65

7

Carbonized polydopamine /Graphene

0.2

1360

100

62

940

69

17

CMK-3@ carbonized glucose

0.1

1610

100

60

1014

63

20

CMK-3@ carbonized sucrose

0.1

1422

36

54

398

30

21

Carbonized bamboo charcoal

0.5

685

500

58

414

60

30

Carbon nanofiber

0.06

1152

100

56

800

70

42

** The cycle rates in the Table are those used for testing cycling performance in the references. *The cycle numbers in the Table are the reported maximum numbers in the references.


35


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Page 36 of 43

ASSOCIATED CONTENT

Supporting Information. TG/MS curves of amylase; XRD patterns of the differently carbonized amylose and the S@C hybrids; EDS analysis of the S@C hybrids; TG curves of the sulfur impregnated S@C mixtures; Cyclic performance of the S@C hybrids with a sulfur loading of 2.2 mg cm-2 in each electrode; Differential profiles of capacity to potential (dQ/dV) of the S@C hybrids for the first and 200th cycles at 0.1 C, and for the initial cycles at different rates. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Authors *

Address: State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and

Applications for Batteries of Zhejiang Province and School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Tel/Fax: +86 571 87952615. Email: [email protected] (M Gao); [email protected] (H Pan).

ACKNOWLEDGMENTS

The work was supported by the National Natural Science Foundation of PR China (Nos. 51571178,

51571175

and

51371158),

National

Materials

Genome

Project

(No.

2016YFB0700600), Natural Science Foundation of Zhejiang Province (No. LY14E010004), Pao Yu-Kong International Fund, Zhejiang University and the UK EPSRC (Nos. EP/K002252/1 and EP/K021192/1).


36

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ToC Graphic


43

Amylose Derived Macro-Hollow Core and Micro-Porous Shell Carbon

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