Porous Coconut Shell Carbon Offering High Retention and Deep

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Porous Coconut Shell Carbon Offering High Retention and Deep Lithiation of Sulfur for Lithium-Sulfur batteries zhaohui chen, Xue-Li Du, Jian-Bo He, Fang Li, Yan Wang, Yu-Lin Li, Bing Li, and Sen Xin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09310 • Publication Date (Web): 14 Sep 2017 Downloaded from http://pubs.acs.org on September 15, 2017

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ACS Applied Materials & Interfaces

Porous Coconut Shell Carbon Offering High Retention and Deep Lithiation of Sulfur for Lithium-Sulfur Batteries Zhao-Hui Chen,† Xue-Li Du,† Jian-Bo He,*,† Fang Li,† Yan Wang,† Yu-Lin Li,† Bing Li,† and Sen Xin*,‡

†

Anhui Province Key Laboratory of Advanced Catalytic Materials and Reaction Engineering,

School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, P. R. China ‡

Materials Science and Engineering Program & Texas Materials Institute, The University of

Texas at Austin, 1 University Station, C2201, Austin, Texas 78712, United States

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ABSTRACT: Retaining soluble polysulfides in the sulfur cathodes and allowing for the deep redox are essential to develop high performance lithium-sulfur batteries. The versatile textures and physicochemical characteristics of abundant biomass offer a great opportunity to prepare biochar materials that can enhance the performance of Li-S batteries in sustainable mode. Here, we exploit micro/mesoporous coconut shell carbon (CSC) with high specific surface areas as a sulfur host for Li-S batteries. The sulfur-infiltrated CSC materials show superior discharge-charge capacity, cycling stability and high-rate capability. High discharge capacities of 1599 and 1500 mA h g-1 were achieved at current rates of 0.5 and 2.0 C, respectively. A high reversible capacity of 517 mA h g-1 was retained at 2.0 C even after 400 cycles. The results demonstrate a high retention and a deep lithiation of the CSC-confined sulfur. The success of this strategy provides insight into seeking high-performance biochar materials for Li-S batteries from abundant bio-resources. KEYWORDS: biomass, coconut shell carbon, sulfur retention, lithium-sulfur battery, sulfur cathode

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INTRODUCTION

In recently years, many research efforts have been devoted to the development of high-capacity energy storage systems for satisfying the ever increasing power and energy demands of electric vehicles, renewable electricity generation, military power supplies, and stationary storage.1-4 The Li-S battery is one of the most promising lightweight energy storage devices, not only because sulfur is highly abundant element, but also due to the fact that it can offer a theoretical capacity of 1675 mA h g-1 (sulfur mass) as the active cathode material,5-9 which is more than five times higher than that of the intercalation cells. However, the application of Li-S batteries is still challenging because of the poor electronic and ionic conductivities of sulfur, and more importantly, the rapid capacity fading due to the formation of soluble polysulfide intermediates (Li2Sn, n = 4–8).1,5,10 Conductive additives, carbons usually, were frequently incorporated into sulfur to facilitate electron transfer to the sulfur. Besides, electrolytes that slightly solubilize sulfur were used to facilitate the combining of sulfur with the Li+ ions to alleviate the low ionic conductivity.2,11 Confining the dissolved polysulfides within the cathodes is fundamental to improve the utilization of active materials and the cycling stability of Li-S batteries. The most studied hosts for the immobilization of sulfur are porous carbon materials prepared via elaborate material designs, such as vertical-aligned carbon nanotubes grown on a nickel foil,11 disordered carbon nanotubes prepared with an anodic aluminum oxide template,12 graphene sheets with interlayer spacers to prevent the stacking of graphene,13 multiwall carbon nanotubes/sulfur composite embedded into the interlayer galleries of graphene sheets,14 boron- or nitrogen-doped porous carbons,15-17 multidimensional nanocarbon-sulfur hybrid materials,18 ordered mesoporous carbons nanocast using mesoporous molecular sieve as a hard template,19 mesoporous carbon decorated with RuO2 nanoparticles,20 hierarchical porous 3 / 25



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carbons produced by carbonizing zinc-containing metal-organic frameworks,21 and Kroll-carbons formed by carbochlorination etching of titania nanoparticles inside a dense carbon matrix.22 The above designs of porous carbon-based materials have raised the initial discharge capacities up to 1100-1500 mA h g-1. Most recently, biochar materials derived from biomass have been studied as sulfur hosts for Li-S batteries, due to their facile synthesis, low cost, high availability and sustainability.10 Porous carbon sponges were prepared from sodium alginate, and then doped with ketjen black or carbon nanotubes to form conductive carbon network inside the sulfur-impregnated carbon sponges.7,23 Integrated carbon slices were prepared by carbonizing the natural architecture of plane tree barks, offering an initial discharge capacity of 1159 mA h g-1 at 200 mA g-1 (current rate: 0.12 C).24 The porous carbons from rice husks,25 shaddock peel,26 pomelo peel,27 mandarin peel,28 and bamboo29 can deliver an initial capacity of 834 (0.5 C), 906.5 (0.5 C), 1258 (0.2 C), 886 (0.1 C) and 1262 (0.1 C) mA h g-1, respectively. The particular textures and physicochemical characteristics of biomass can effectively affect the porous structure and surface activity of the resulting biochar.24,30 The diversity in both biomass precursors (species and growing regions) and preparation conditions leads to various types of biochar with particular categories of shapes, pores and surface chemistry properties, affording a great opportunity to seek biochar materials that can naturally enhance the performance of rechargeable Li-S batteries. Generally, the efficiencies of sulfur reduction in Li-S batteries are relatively low (around 75%, 1100-1300 mA h g-1), due to the difficulty in reduction of low order lithium polysulfides, especially lithium disulfide (Li2S2) within the cathodic carbon frameworks.31 Herein, we prepared a biomass-derived porous carbon from coconut shells by a simple carbonization-activation procedure, in order to improve not only the sulfur retention but also 4 / 25


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the reduction depth during the cycling of Li-S batteries. The coconut shell carbon (CSC) has been proved to be able to absorb liquid like “molecular sponge”,32,33 due to the predominance of pores that account for 95% of the available internal surface area to adsorb small molecular weight species.34 This unique property makes CSC an ideal potential sulfur host, although it has been thought to be suitable only as a conducting additive for sulfur electrodes.24 The present work proves that the CSC is a high-efficiency polysulfide trap for lithium-sulfur battery. ■

EXPERIMENTAL SECTION Biochar preparation. All chemicals used in this work were of analytical grade and

solutions were prepared using deionized water with high purity (18.2 MΩ cm-1). Coconut shell biochar was prepared from coconut shells produced in Hainan Island of China. First, the coconut shells were crushed, and then screening to a size of about 2 mm specification for raw materials. The selected coconut shells were cleaned with deionized water and dried in a vacuum oven at 110 °C for 24 h. The dried coconut shells were carbonized in a reactor under argon atmosphere. The carbonizing temperature was raised up to 600 °C at a ramp rate of 20 °C min–1 and then held for 2 h. The carbonized coconut shells were dispersed in KOH (50 wt%) aqueous solution with a KOH/C mass ratio of 4 in a stainless steel reactor for activation. The mixture was heated at 130 °C for 5 h to evaporate water, and then temperature was elevated up to 800 °C at a ramp rate of 5 °C min–1 under argon atmosphere and then held for 1 h. After these, the sample was cooled to room temperature and cleaned with hydrochloric acid followed by deionized water until no more chloride ions can be detected in the wash solution. Finally, the prepared biochar sample was dried in a vacuum oven at 110 °C for 12 h to obtain the coconut shell carbon (CSC) powder.

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Preparation of S-infiltrated CSC. Sulfur powder (Aldrich) was completely mixed with CSC at a weight ratio of 6:4 to form a mechanical mixture denoted as S&CSC. The mixture was then heated up to 155 °C at a ramp rate of 1 °C min-1 under Ar atmosphere and then held for 12 h, allowing the molten sulfur to fully infiltrate into the pores of CSC. The as-prepared S-infiltrated CSC (denoted as S@CSC) was cooled to room temperature and used for preparation of the cathode of Li-S battery. The preparation process of the S@CSC sample is shown in Scheme 1. Materials characterization. Scanning electron microscopic (SEM) images were collected on a Zeiss Hitachi SU8020 field-emission scanning electron microscope at a voltage of 5 kV. Transmission electron microscopic (TEM) images, including energy-dispersive X-ray spectroscopy (EDX) mapping, were collected on a Hitachi H-7650 at an accelerating voltage of 100 kV. X-ray diffraction (XRD) patterns were recorded on an X’Pert PRO MPD diffractometer using Cu Kα radiation. Raman spectra were detected on LABRAM-HR confocal laser microRaman spectrometer with an excitation wavelength of 532 nm. X-ray photoelectron spectroscopic (XPS) spectra were collected on an ESCALAB250Xi X-ray photoelectron spectrometer equipped with a monochromatic X-ray source. Thermogravimetic analysis (TGA) was performed for the S@CSC samples with TA-SDT Q600 thermal analyzer (heating rate: 10 °C min-1; atmosphere: N2). Nitrogen adsorption/desorption isotherms were measured on ASAP 2020 HD88 physisorption analyzer for calculating the Langmuir surface area, cumulative pore volume (CPV) and incremental pore volume (IPV) of the CSC samples by using Density Functional Theory (DFT). The size of the CSC particles was determined by Mastersizer 2000 laser diffraction particle size analyzer (Malvern Instruments Ltd.).

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Electrochemistry. The S@CSC powder, ketjen black (KB) and poly-(vinyl difluoride) (pVDF) were thoroughly mixed in a mass ratio of 80:10:10. The mixture was added to N,N-dimethyl pyrrolidone (NMP) to form a well-proportioned black slurry. The slurry was uniformly coated on an aluminum foil (Goodfellow), dried overnight, and sliced to yield a sulfur cathode with a diameter of 12 mm and an average areal S loading of 0.78 mg cm-2. The cathode was then coupled with a fresh Li-foil anode in an Ar-filled glove box to form a Swagelok-type Li-S cell. The electrolyte was 1.0 M lithium bis(trifluoromethane) sulfonamide (LiTFSI) in a mixed solvent of 1,3-dioxolane/1,2-dimethoxymethane (v:v = 1:1), with 0.5 wt % lithium nitrate as an additive. A microporous polypropylene membrane from Celgard Inc. was used as the separator. Cyclic voltammograms (CVs) and Nyquist plots were collected

on

an

electrochemical

workstation

(Autolab

PG302N).

Galvanostatic

discharge-charge (GDC) cycling tests of the batteries were performed on an Arbin BT-1 system at a potential range from 1.0 to 3.0 V (vs. Li+/Li). The current rates and specific capacities in this work are calculated on the basis of the mass of sulfur. ■

RESULTS AND DISCUSSION The prepared CSC powder (Figure S1) comprises the particles at the scale of ten micron

(see the SEM image, Figure S2a), with a median particle size of 18.8 µm determined by laser particle size analysis (Figure S3). The biochar mainly consists of the elements C and O, while also contains trace amounts of metals such as Ca, Cu and Mg according to the EDX spectrum (Figure S2b). An XPS test revealed that the atomic percentage of O in the CSC carbon is 11.3%. The thin CSC sheets are sufficiently electron transparent to enable TEM examinations (Figure 1a). The high-resolution TEM (HRTEM) image shows a large number of micropores on the surface of the CSC (Figure 1b), which agrees well with the Type-I N2

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adsorption/desorption isotherms collected for the CSC sample (Figure 1c). The DFT pore size distribution reveals a dominant size range of 1 to 3 nm (Figure 1d). The micro/mesopores within the CSC particles leads to a high specific surface area of 2160 m2 g-1 (Langmuir Surface Area). A cumulative pore volume of 0.68 cm3 g-1 was reached (Figure 1d), which allows a theoretical sulfur loading of about 55.3 wt% based on the density of liquidized sulfur (1.82 g cm-3).19 Infiltration of sulfur into CSC was determined by SEM, XRD and Raman spectroscopy. The CSC particles are in a shape of irregular lumps as shown in the SEM image (Figure 2a). The bulk sulfur in the sulfur-CSC mixtures was examined before and after melting-infiltrating treatment. The crystal sulfur particles were present in the initial mixture S&CSC as seen by SEM (Figure 2b), but completely disappeared in the sample of S@CSC after infiltrating (Figure 2c). The sulfur-infiltrating process was also followed by XRD and Raman spectroscopy. The CSC showed a broad XRD peak between 16 and 28°, corresponding to the diffraction from amorphous carbon phase (Figure 2d). Several additional peaks from the crystalline sulfur occurred in the XRD pattern of S&CSC, but vanished in that of S@CSC. This difference indicates that almost all of the crystalline sulfur was converted to amorphous sulfur, through the dispersal of sulfur into micro/mesopores of the amorphous CSC. Similar results were obtained from the Raman spectra of CSC, S&CSC and S@CSC (Figure 2e). The characteristic peak located at 1580 cm-1 (G band) corresponds to graphite-like sp2 carbon and another peak at 1360 cm-1 (D band) corresponds to disordered sp2 carbon linking with sp3 carbon atoms.35 The Raman peaks of sulfur occurred below 500 cm-1 in S&CSC but disappeared in S@CSC, resulting from the interference of the carbon micro/mesopores on the Raman signals of the encapsulated sulfur.36 All of the above results

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indicate the absence of bulk sulfur in S@CSC, due to the effective trapping of sulfur into the micro/mesopores of CSC. The amorphous sulfur after infiltrating into CSC was further characterized by TGA, XPS and TEM-EDX mapping. The TGA thermogram of S@CSC shows a weight loss of approximately 60 wt % between 200 and 450 °C (Figure 3a), which can be attributed to the evaporation of sulfur from [email protected] This weight loss ratio is in well accordance with the content of sulfur initially added for the preparation of S@CSC. There is only one sulfur environment in S@CSC, as indicated by the single 2p doublet in the XPS spectrum (Figure 3b). The positions of the S 2p3/2 peak at 164.05 eV and the S 2p1/2 peak at 165.22 eV correspond well with the elemental sulfur.36 The elemental mappings further confirm the uniform distribution of sulfur in CSC (Figure 3c-e), with the sulfur map (red) matching well with the carbon map (green). This result supports the encapsulation of sulfur in the uniformly distributed micro/mesopores in CSC. The micro/mesopores, serving as ion transportation channels, can hinder polysulfide from dissolving into bulk electrolyte, and so to improve the cyclic stability and the power performance.37 In addition, oxygen atoms were also found to distribute uniformly in CSC (Figure 3f), which may increase the wettability of porous surface,38 and enhance polar-polar interaction with lithium polysulfides.10 The S@CSC material was assembled into Li-S batteries to evaluate the electrochemical performance of S@CSC as cathode materials. Figure 4a shows the CVs of the S@CSC electrode at a low scan rate of 0.1 mV s-1. A reduction peak at 2.28 V (vs. Li+/Li) followed by a larger one at 2.07 V corresponds to the stepwise reduction of cyclooctasulfur (cyclo-S8) to lithium polysulfide intermediates (Sn2-, 4 ≤ n ≤ 8) and then to lower order polysulfides, respectively.5,20,31 A very broad and low reduction wave appeared below 1.9 V, due to the reduction of the low order polysulfides to Li2S.31,39 Only one oxidation peak occurred in the 9 / 25



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reverse anodic scans, corresponding to reversible oxidation of Li2S back to sulfur chains/small sulfur clusters. This oxidation peak was located at 2.47 V in the first scan and was shifted to 2.39 V with an increased peak current in the subsequent cycles, indicating an electrochemical activation effect in the initial oxidation.40 After several cyclic scans, the CVs showed little change, tending to a stabilized sulfur redox cycling. The electrochemical activation effect was also confirmed by electrochemical impedance spectroscopy (Figure 4b). The charge transfer resistances can be estimated from the semicircle diameters at the high frequencies of the Nyquist plots. Two values of 47.8 and 18.1 Ω were obtained for the S@CSC before and after the discharge-charge cycling, respectively, indicating a fast charge transfer kinetics especially after activation. In addition, a very low ionic transfer resistance of about 3.3 Ω was obtained for the S@CSC material. The GDC voltage profiles show a discharge plateau around 2.28 V followed by a longer one around 2.08 V (Figure 4c and d), corresponding to the two reduction peaks in the CVs. Only one charge plateau occurred around 2.34 V, besides the additional overpotential required at the beginning stage of the 1st charging process. This additional plateau vanished in the subsequent charge processes due to the activation effect in the first charge. An initial discharge (charge) capacity of 1599 (1484) mA h g-1 with a Coulombic efficiency (CE) of 92.8% was achieved at a current rate of 0.5 C (i.e., 837.5 mA g-1, Figure 4c). This discharge capacity equals to 95.5% of the theoretical capacity of sulfur (1675 mA h g-1), which indicates a deep lithiation of sulfur in the S@CSC cathode. It should be noticed that the gentle slope below 2.0 V in Figure 4c (the third discharge plateau) is a part of the theoretical discharge curve of Li-S cells, which is attributed to the reduction of low order lithium polysulfides.31 Specifically, the reductions of Li2S3 to Li2S plus Li2S2 and of Li2S2 to Li2S occurred in the ranges of 2.0-1.8 V and the lower, offering a theoretical capacity of 209.4 and 10 / 25


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418.8 mA h g–1, respectively. The conversion from Li2S2 to Li2S requires additional overpotentials to overcome the sluggish kinetics of the solid-solid reaction.10 The insoluble products probably block the carbon framework of the sulfur electrode and then decrease the efficiency of sulfur reduction usually down to about 75% (corresponding to the absence of the third discharge plateau).31 The third discharge plateau present in Figure 4c indicates a deep reduction of low order lithium polysulfides within the micro/mesopores of S@CSC, due to the superior pore structure and distribution of CSC. The GDC voltage profiles were quickly stabilized after three cycles, with a third cycle discharge capacity of 1332 mA h g-1 (CE = 103%). At a high rate of 2.0 C (i.e., 3350 mA g-1), S@SCS still exhibited the two discharge plateaus at almost the same polarization degrees as at 0.5 C, with an initial capacity of 1500 mA·h g-1 (CE = 87.8%) (Figure 4d). Generally, the increase of the applied current rate would cause a significant increase in polarization overpotentials of the discharge and charge plateaus.5,13,41 The little rate-dependent plateau voltages shown in Figure 4d indicate a good electrical conductivity of CSC and a fast transfer of electrons across the internal surface of the micro/mesopores and of Li+ ions in the microchannels. The cycling performance of S@CSC was studied at three current rates as shown in Figure 4e. A significant discharge capacity fade (CF) occurred between the first and the second cycles, probably because of the solution loss of some amount of sulfur unavoidably present on the outer surface of the CSC particles. At a current rate of 0.5 C, the discharge capacity decreased from 1364 to 1030 mA h g-1 between the third and the 100th cycles (CF = 24%). At a larger rate of 1.0 C, the discharge capacity decreased from 1305 to 1029 mA h g-1 between the third and the 100th cycles (CF = 21%), and then to 854 mA h g-1 at the 200th cycle (CF = 34%). When the rate was further increased to 2.0 C, a third-cycle capacity of 11 / 25



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1138 mA h g-1 was achieved, and it decreased to 875 mA h g-1 at the 200th cycle (CF = 23%) and then to 517 mA h g-1 at the 400th cycle (CF = 55%). It can be seen that the cycling stability of the S@CSC cathode was improved with increasing current rate. In addition, the coulombic efficiency of the S@CSC electrodes remained close to 100% at different current rates. We have also tested the rate capability of the S@CSC cathode. Upon gradually increasing the rate from 0.5 C to 5 C, the S@CSC cathode shows a favorable performance at each rate, as demonstrated by an insignificant capacity loss upon raising the rate from 2 C to 5 C (Figure S6). After reducing the discharge-charge rate back to 0.5 C, the cathode capacity restores to ~1200 mA h g-1 (Figure S6). The stable electrochemistry of the S@CSC cathode is also demonstrated by ex-situ SEM observations. It can be seen from Figures S4 and S5 that, the cycled cathode largely preserves its pristine morphology, with a rich amount of sulfur species detected on the cathode. This finding gives a reliable evidence to support the excellent electrochemical stability and structural integrity of the S cathode. The superior electrochemical performance of the S@CSC cathode is attributed to the unique structural and compositional advantages of the carbon substrate. The micro/mesopores throughout the carbon substrate enables homogeneous distribution and confinement of amorphous sulfur inside, which greatly prevents polysulfide intermediates from dissolving into the electrolyte and thus mitigates the shuttling effect during cycling. The micro/mesopores also facilitate the migration of Li+, which helps to improve the kinetics of reaction between Li and S and enables a deep lithiation of S. On the other hand, the high content of oxygen in the CSC means the carbon substrate is rich in oxygen-containing functional groups such as hydroxyl group or carboxyl group, which may show strong interactions with the in-situ formed polysulfides and immobilize them onto the carbon according to the former work by Zhang et al.42. In this way, the CSC acts as a “molecular 12 / 25


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sponge” that shows affinity to and retains active S molecules on the cathode side. The above point has been demonstrated by a simple polysulfide adsorption test, in which the addition of CSC carbon into the tetrahydrofuran solution of Li2S4 leads to a significant change in solution color from orange to almost clear (Figure S7). ■

CONCLUSION In summary, CSC prepared from coconut shells was used as the cathode material for

Li-S batteries after the encapsulation of sulfur in the micro/mesopores. The huge internal surface of the micro/mesopores provides effective contact between the encapsulated sulfur and the carbon inner walls. The as-fabricated Li-S batteries with the S@CSC cathode exhibited superior discharge-charge performance because of the high sulfur retention in the micro/mesopores and the deep lithiation of CSC-confined sulfur. High discharge capacities of 1599 and 1500 mA h g-1 were achieved at current rates of 0.5 and 2.0 C, respectively. Even after 400 cycles, a high reversible capacity of ca. 517 mA h g-1 was retained at 2.0 C, with a Coulombic efficiency close to 100%. As for the practicality, the CSC powder can be easily produced at a factory scale by a simple carbonization-activation procedure, without the need of precise control of operating conditions. The cost of the CSC material is estimated to be about 8.3 USD per kilogram from the cost of coconut shells (~1 USD per kilogram) and the average product yield of ~12 wt-%. This work confirms the potential to seek high-performance biochar materials used in but not limited to Li-S batteries from extremely rich plant resources in the world. ■

ASSOCIATED CONTENT

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: Optical image, SEM and EDX, particle size distribution of CSC; SEM, EDX, elemental mappings of both the initial and the cycled S@CSC; rate performance of S@CSC; and ability of CSC to absorb Li2S4 (PDF) ■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.-B.H.). *E-mail: [email protected] (S.X.). Author Contributions

Z.-H.C. and X.-L.D. contributed equally. Notes

The authors declare no competing financial interest. ■

ACKNOWLEDGMENTS

This work was supported by National Natural Science Foundation of China (Grants 21576063, 21403050). ■

REFERENCES

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Scheme 1. Schematic diagram for the preparation of S@CSC.

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Figure 1. Pore structure characterizations of CSC by (a) TEM; (b) HRTEM; (c) Nitrogen adsorption and desorption isotherms; (d) Incremental and cumulative pore volumes.

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Figure 2. Morphological and structural characterizations for determining the infiltration of sulfur into CSC. (a-c) SEM images, (d) XRD patterns, and (e) Raman spectra of CSC, S&CSC and S@CSC.

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Figure 3. Sulfur characterizations for the sample S@CSC by (a) TGA; (b) XPS; (c) TEM; (d-f) EDX mapping for the elements C, S and O.

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Figure 4. Electrochemical performance of Li-S batteries with the S@CSC cathode. (a) CVs of the initial five cycles at 0.1 mV s−1; (b) Nyquist plots before and after cycling at 0.5 C, frequency range: 10-1-105 Hz (The inset shows the local enlargement); (c) GDC voltage profiles of the initial five cycles at 0.5 C; (d) GDC voltage profiles of the first cycle at 0.5, 1.0, 2.0 C; (e) Cycling performance at 0.5, 1.0, 2.0 C. 24 / 25


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Table of contents (TOC)

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Porous Coconut Shell Carbon Offering High Retention and Deep

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