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Dandelion-derived, nitrogen-doped carbon host with one-dimensional hollow porous architecture achieves impressive rate capacity and superior cycling ...
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Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Dandelion Derived Nitrogen-Doped Hollow Carbon Host for Encapsulating Sulfur in Lithium Sulfur Battery Yan Song, He Wang, Qianli Ma, Dan Li, Wensheng Yu, Guixia Liu, Tingting Wang, Ying Yang, Xiangting Dong,* and Jinxian Wang* School of Chemistry and Environmental Engineering, Changchun University of Science and Technology, 7186 Weixing Road, Changchun 130022, China

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ABSTRACT: Biomass derivative carbonaceous materials are extremely fascinating for developing advanced energy storage devices according to unique architecture. Here, inspired by dandelion with consecutive hollow channel, nitrogen-doped hollow carbon framework (NHCF) as sulfur host was obtained via a simple thermal calcination in ammonia atmosphere for lithium−sulfur battery. The NHCF can effectively confine polysulfides and electrochemical reaction within the hollow channel and expedite lithium ion diffusion and electrode transport through porous nitrogen-doped carbon rampart. With this strategy, a high-level active sulfur loading of 80% and high sulfur utilization can be realized. As a result, the NHCF/S delivered an advanced initial capacity of 950 mAh g−1 at the rate of 0.5 C with small capacity fading rates of 0.049% per cycle after 500 cycles. Additionally, an average reversible areal capacity of nearly 5 mAh cm−2 was realized under raised sulfur mass loading of 5.5 mg cm−2. This superior electrochemical energy storage property endows renewable biomass derived carbon host with promising potential for low-cost rechargeable battery application. KEYWORDS: biomass, hollow carbon host, nitrogen doping, high sulfur content, lithium sulfur battery

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INTRODUCTION

of the batteries at larger current density, which stimulates to settle these intractable problems through effective strategies.8,9 Restraining the migration of soluble polysulfides is a considerable factor for acquiring Li−S batteries with long cycling lifetime and attractive reversible capacity. Metal-based nanomaterials with unique structure have been designed and fabricated to settle these intractable problems.10−12 However, these host materials often exist low conductivity, inferior structural stability during cycling, and high cost, which limits the loading of active sulfur and rate performance.13 Nanotechnology has been exploited to synthesize the metal-based host with porous and hollow architecture.14−16 But the electrodes assembled from these unique structured materials achieve high capacity only in submicrometer thick thin coating.17,18 The thin coating further diminishes the active sulfur content per unit area, which does not satisfy high area specific capacity for practical application. In this regard, the carbon host with a hollow framework as the most ideal electrode candidate has been developed to impregnate the active sulfur.19−21 The hollow carbon framework as host affords sufficient interspace used for high sulfur loading and alleviating the volume change of active material.22 Additionally,

The ever-increasing exploitation for clean and renewable energy sources has stimulated intensive demand on advanced energy storage systems.1,2 Lithium-ion battery, as an existing energy storage device, overcomes the intermittent nature of the renewable new energy source.3,4 However, the limitations of the lithium-ion battery in lower discharge capacity and higher quality proportion have driven the development of rechargeable batteries with advanced energy density storage. The lithium−sulfur (Li−S) battery has recently allured massive attention as a high energy density rechargeable battery for applications in electrical vehicle systems and portable electronic devices.5 Most attractively, the sulfur element with superiorities of lightweight, high abundance, low toxicity, and high effectiveness is beneficial to realize commercial values of Li−S batteries in advance energy storage devices with ultrahigh gravimetric energy density. However, the sulfur cathode has still numerous challenges that plague the utilizable purpose of the Li−S battery, for example, low electronic conductivities of active electrode materials, large volume variation of cathode materials during electrochemical reaction, and shuttle behavior of high soluble intermediate polysulfides (Li2Sx, 4 ≤ x ≤ 8) in the electrolyte.6,7 The disadvantage will induce severe collapse of the sulfur electrode, diminution of reversible capacity, reduction of Coulombic efficiency, unsatisfactory cycle stability © XXXX American Chemical Society

Received: September 12, 2018 Revised: November 26, 2018 Published: December 12, 2018 A

DOI: 10.1021/acssuschemeng.8b04648 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

one-dimensional hollow structure and nitrogen-doping in the carbon framework. Benefiting from aforementioned merits, the hollow carbon framework is demonstrated as a favorable cathode host to encapsulate active sulfur.

the one-dimensional hollow architecture endows hosts with high specific surface and consecutive electron transport pathway for efficient charge transportation between the host and active materials.23−27 In the carbon framework, nitrogen element doping not only can improve the overall electron transportation, but also overcome the concentration gradient force to anchor polysulfides by chemical interactions.28,29 The carbonaceous materials with nanostructure are presently limited due to nonrenewable raw material and complicated and high cost fabrication process. Therefore, the increased attention has recently focus on sustainable and abundant biomass materials which have been dedicated to exploiting carbon-based host with excellent performance for Li−S battery. Nature has created abundant biomass resources and sparked design inspirations of researchers. Biomass compounds are mainly composed of carbohydrates and devoted to fabricate carbon materials with hierarchical microstructures.30 On the basis of this inspiration, renewable sources of biomass with unique structures such as chitin,31 bacterial cellulose,32 watermelon,33 willow catkin,34 rice husk,35 and sweet potato36 have been developed to derive carbonaceous materials recently. Renewable biomass-derived materials afford a facile and convenient strategy to prepare carbonaceous materials combined advantageous mechanical properties and electronic transmission ability.37−40 The approach possesses low cost, efficient, and environmental friendly characteristics. Therefore, in view of the superiority of sustainable biomass source, biomass derived carbonaceous materials with suitable frameworks can be designed as a reliable sulfur host and accomplish large sulfur loading, which will build up new opportunity for achieving high energy density Li−S battery. Herein, we adopted dandelion as template for fabricating nitrogen-doped hollow carbon framework (NHCF) by simple thermal calcination in the ammonia atmosphere. The obtained NHCF was employed as host for depositing active sulfur. The synthetic procedure of NHCF/S composite cathode material is schematically manifested in Figure 1. Benefiting from the

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EXPERIMENTAL SECTION

Preparation of NHCF/S. Dandelions were first precarbonized in the air atmosphere under 260 °C for 2 h. The generated black samples were further carbonized at 800 °C for 2 h in an ammonia gas flow. Subsequently, the obtained nitrogen-doped hollow carbon framework (NHCF) and elemental sulfur (4:6 by weight ratio) were mixed in CS2 solution. The mixture was statically soaked for 24 h until the solvent was completely evaporated. The dried sample was transferred to a Teflon-lined autoclave, which continued with thermal treatment of 155 °C for 20 h to obtain the NHCF/S-60 electrode material. In addition, the composites with different sulfur contents (70% and 80%) were also prepared through the same approach, and they are denoted as the NHCF/S-70 and NHCF/S-80, respectively. Material Characterizations. The powder X-ray diffraction (XRD) analyses were performed for identifying the phases of the products by using Bruker D8 FOCUS diffractometer with Cu Kα radiation (λ = 0.15418 nm, 30 kV, and 20 mA). Field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) images were obtained to investigate the morphology features and microstructure by using JEOL JSM-7600F (operating at an accelerating voltage of 10 kV) and JEOL-2010 (operating at 200 kV). The elements distribution was confirmed based on the element mapping of the energy dispersive X-ray spectroscopy (EDX, Oxford X-MaxN80). The specific surface area and pore size distribution of the product were evaluated via a Micromeritics ASAP 2020 instrument. The X-ray photoelectron spectroscopy (XPS) was performed via using PHI 5000 Versa Probe. The content of active sulfur constituent in the NHCF/S electrode material was ascertained on account of thermogravimetric analyses (TGA, SDT 2960) in a N2 atmosphere with a heating rate of 10 °C min−1. Fourier transform infrared (FTIR) analysis of NHCF/S composite was performed with a Shimadzu model 8400s FTIR spectrophotometer. Electrochemical Test. The electrochemical measurements were achieved on coin cells (2032 type) configuration. that were assembled inside an argon-filled glovebox (MIKROUNA, China). The prepared NHCF/S composite (80 wt %) was mixed with acetylene carbon black (10 wt %) and polyvinylidene fluoride (PVDF; 10 wt %) as binder in the N-methyl-2-pyrrolidone (NMP) solvent, which formed slurry and coated onto aluminum foils. As a control sample, the sulfur electrode contained sublimed sulfur, acetylene carbon black, and PVDF (70:20:10 by mass ratio). The standard 2032 type coin cells contained the counter electrode of lithium metal. Both of the pole pieces with cathodic and anodic active substances were separated by using a Celgard 2400 microporous membrane. Additionally, the electrolyte was prepared by dissolving bis(trifluoromethane) sulfonimide lithium (LiTFSI, 1.0 mol L−1) and lithium nitrate (1 wt %) into the 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) cosolvent mixtures with a volume ratio of 1:1. In the assembly process of the cells, the ratio of electrolyte volume to sulfur mass in coin cells was controlled as 50 μL mg−1. The galvanostatic cycling and rate capability tests of cells were conducted on a multichannel battery testing system (NEWARE, BST 4000) in a fixed voltage from 1.7 to 2.8 V. Cyclic voltammetry (CV) tests were performed by a CHI 760E electrochemical workstation (Shanghai Chenhua instrument Co, Ltd., China) in a setting potential range of 1.7−2.8 V with a series of scanning rates (0.1−0.5 mV s−1). Electrochemical impedance spectroscopy (EIS) was researched by using a CHI 760E electrochemical workstation (Shanghai Chenhua instrument Co, Ltd., China) in a scanning frequency range (105−0.01 Hz) with an AC amplitude of 5 mV.

Figure 1. Schematic diagram of the synthetic procedure for NHCF/S electrode material and proposed advantages of the NHCF as sulfur host during cycling process.

channel structure, the electrochemical reaction process of active materials is confined within the hollow channel, and the large space competently realizes high mass loading and buffers volume variation of active sulfur during cycling. Moreover, the NHCF expedites electrode transfer for fast and efficient sulfur electrochemistry reaction, and simultaneously provides strong physical sulfur confinement and the high density anchoring sites for intercepting polysulfides by virtue of the consecutive B

DOI: 10.1021/acssuschemeng.8b04648 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 2. FE-SEM images of (a) dandelion, (b) NHCF, and (c, d) NHCF/S samples; (e−g) EDX mapping for C, S, and N elements.

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RESULTS AND DISCUSSION Structure and Morphology of NHCF Host. The morphological features of dandelion template, NHCF, and NHCF/S were first investigated by a FE-SEM. As displayed in Figure 2a, the dandelion template possesses a well-defined, one-dimensional architecture, and a hollow interior is visible from the fractured interface. Despite suffering from thermal treatment in an ammonia atmosphere, the integral onedimensional hollow architecture is still inherited to obtain NHCF with an open channel, which is easy for electrolyte accessibility and favorable for the filling of the active materials; more importantly, buffers volume changes during repeated cycles. During the carbonization process, the dandelions were first treated by precarbonizing at 260 °C. The precarbonized treatment, including dehydration and decarboxylation processes, can prompt the organic carbon chain to form ring structure and avoid the chain scission under the subsequent high-temperature carbonization process. More interestingly, the NHCF displays a bumpy surface and numerous voids in the magnified SEM images (Figure S1a,b). Hollow architecture and porous structure can be easily witnessed by transmission electron microscope (TEM; Figure S1c,d). The surface and bulk composition of elements in the NHCF are indicated using XPS and EDX characterization (Figure S2). It is confirmed that the elements mainly contain C, N, and O elements for the surface and bulk composition of NHCF. The continuity and porosity of carbon framework are beneficial to rapid transfer of electrons and lithium ions during electrochemical reaction process. The specific surface area and pore structure of NHCF are further investigated via nitrogen isothermal adsorption− desorption measurements at 77 K, as revealed in Figure S3. The evidently distinct hysteresis loop at higher pressure region is ascribed to the existence of the mesopores structure, which demonstrates the magnified SEM and TEM observations. The pore size distribution of the as-prepared NHCF host was determined by virtue of Barrett−Joyner−Halenda method. Figure S3b reveals the presence of numerous mesopores with a

pore diameter of about 4.5 nm. Such mesopore characteristics can effectively facilitate the lithium ions transport. Additionally, the specific surface area of the NHCF host was calculated to be 805 m2 g−1 on the basis of Brunauer−Emmett−Teller (BET) theory. High BET specific surface area promises sufficient wetting area between electrolyte and electrode materials. The integration of large reaction area with one-dimensional conductive carbon substrate will facilitate the electrochemical reaction kinetics. Loading sublimed sulfur into NHCF host was performed by combining solution and melt diffusion methods. In the low magnification SEM image (Figure 2c), no bulk sulfur is observed on the outside surface of the hollow framework. Furthermore, we observe the spatial distribution in the carbon tube from the fractured place, confirming uniform penetration of sulfur. The elements distribution was validated using EDX mapping. The result verifies that the element carbon, nitrogen, and sulfur uniformly disperse into the architecture. Homogeneous dispersion of nitrogen and sulfur suggests successful doping nitrogen within the carbon framework and uniform sulfur impregnation into the hollow structure, which is further demonstrated by the TEM image of NHCF/S (Figure S4). Figure S5 reveals the XRD patterns of NHCF and NHCF/S samples to indagate the phase purity and crystal phase change of products. As presented in the XRD pattern of NHCF, an evident diffraction peak around 26° and a weak characteristic peak at 43.8° are ascribed to the (002) and (110) graphitic facet of carbonaceous materials with low graphitization nature, respectively. After loading sulfur into the NHCF host, the XRD pattern of resultant NHCF/S composite reveals the presence of NHCF and sulfur with orthorhombic structure, indicating that diffusion treatment did not cause the transform of the crystal phase of active sulfur. However, the relative intensity of crystalline sulfur peaks become weak, implying good physical confinement of active sulfur by the hollow interior and porous sites of NHCF. The chemical interaction between coated active sulfur and NHCF hosts is further demonstrated on account of the XPS C

DOI: 10.1021/acssuschemeng.8b04648 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 3. High-resolution XPS spectra of NHCF/S electrode at (a) C 1s, (b) O 1s, (c) N 1s, and (d) S 2p regions.

donor, especially pyridinic N, contributes the stronger dipole−dipole interactions between the host and the Li2Sx cluster with stronger electropositivity, which also enhances the inductive and conjugative effects in Li bonds.46 Simultaneously, the presence of nitrogen active sites will improve surface wettability and enhance electrical conductivity. The S 2p spectrum consists of three major binding energy peaks located at 163.7, 164.5, and 168.0 eV, which are ascribed to S 2p1/2, S 2p3/2, and sulfate, respectively. Compared with the binding energy (164.0 eV) of S 2p1/2 for element S, that of the NHCF/S electrode materials slightly reduces. The result further indicates the possible existence of CS bonds.47,48 FTIR spectrum also identifies the chemical bonds structure, as revealed in Figure S7. The absorption bands at 1081, 1391, 1629, and 1719 cm−1 are assigned to corresponding CO, CN/CN, CC, and CO groups, respectively. Furthermore, characteristic stretching vibration peaks of C S can be observed from the FT-IR spectrum. Figure S8 reveals a visual colorimetric assay to evaluate the adsorbability of NHCF for soluble polysulfide. The Li2S6 solution with NHCF becomes light color, suggesting that NHCF presents stronger adsorption capability for soluble polysulfide. The stronger and faster adsorption capability is mainly ascribed to the individual hollow architecture and the surface polarity of NHCF, indicating that the NHCF will effectively advance the cycling performance of Li−S batteries. According to the corresponding TGA results, the actual concentration of sulfur in NHCF/S composite electrode material is confirmed. The active sulfur content can be easily regulated from 58 wt % to 82 wt % by adjusting the concentration of sulfur during the sublimed sulfur

results (Figure 3). It is noted that the high-resolution XPS spectrum of NHCF/S at C 1s region is composed by five characteristic binding energy peaks after Shirley background subtraction and a Gaussian fitting treatment. Apart from the peaks at 284.6, 286.2, 285.5, 287.1, and 288.8 eV that can be observed, corresponding to conjugated CC/CC, CN, CS, CO, and OCO bonds, respectively. The existence of the CS bond validates the bonding interaction of sulfur with the carbon framework. The presence of surface oxygenic functional groups can be confirmed according to the high-resolution O 1s XPS spectrum. These oxygen heteroatoms can induce strong polarization, which will be favorable for suppressing polysulfide shuttling by interfacial polar interactions and formation of strong chemical bond S−O. The corresponding N 1s region XPS spectrum is deconvoluted into three different types of nitrogen defects, which should be ascribed to graphitic N, pyrrolic N, and pyridinic N.41,42 The nitrogen concentration in the NHCF is estimated to be 4.6 at %, according to the XPS peak area analysis, manifesting efficient nitrogen heteroatoms doping in the hollow carbon framework after thermal treatment at ammonia atmosphere. Both of the anchoring sites (pyrrolic N and pyridinic N) are beneficial to trap soluble lithium polysulfides due to the electron delocalization effect. Both kinds of nitrogen atoms deliver N lone-pair electrons which induce the strong ionic attractions between nitrogen and lithium to form SxLi−N bonds.43 More importantly, the carbon matrix with nitrogen and oxygen dopants can show high bonding energies with representative polysulfides due to stronger electrostatic interactions over Li−N/O bonds.44,45 The electron-rich D

DOI: 10.1021/acssuschemeng.8b04648 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 4. (a, b) Cyclic voltammograms curves of NHCF/S and sulfur as cathodes. (c, d) Charge−discharge profiles under different cycling at 0.5 C. (e) Cycling performance test for the NHCF/S and sulfur electrodes at 0.5 C for 500 cycles.

well-defined anodic peak at 2.45 V (A1) is associated with the conversion from insoluble Li2S2/Li2S to soluble Li2Sx and eventually oxidation to the S8.49 The sharp reduction and oxidation peaks, highly overlapping of the first three cycles, and lower potential hysteresis indicate that NHCF as host endows the cells with a remarkable reversibility and accelerated electrochemical redox kinetics.50 Furthermore, the shape and positions of oxidative and reductive peaks are no obvious displacement and weaken during three cyclic scans on the strength of the distinguished adsorbability of NHCF on polysulfides. These results demonstrate that the NHCF/S cathode present the excellent stabile and reversible electrochemical redox. Nevertheless, the CV curves of the pure sulfur electrode reveal two reduction potentials at 2.18 and 1.91 V in the cathodic scan and a prominent oxidation potential at 2.56

loading process. This combination of N doped, high surface area, and plentiful pore structure prompts hollow carbonaceous material as potential host to improve energy storage density and structure stability of Li−S battery. Electrochemical Performance. NHCF as host materials are assembled into the 2032-type coin cells to investigate the electrochemical performance of the electrodes. The corresponding electrochemical behavior of NHCF/S and sulfur electrode materials was first evaluated by CV curves at a scan rate of 0.1 mV s−1. Figure 4a exhibits that the electrodes present distinct cathodic peaks located at around 2.30 V (C1) and 1.95 V (C2), which are attributed to the formation of soluble lithium polysulfides (Li2Sx, x ≥ 4) and subsequent characteristic reduction of long-chain polysulfides to insoluble short-chain lithium sulfides (Li2S2/Li2S), respectively. The E

DOI: 10.1021/acssuschemeng.8b04648 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 5. CV curves at a series of scan rates and corresponding plots of electrochemical reaction current vs the square root of the scan rates of the coin cells with (a, c) NHCF/S and (b, d) pure sulfur cathode.

per cycle can be achieved. These results imply the effective charge and discharge process resulting from stronger adsorbability of NHCF to efficiently stabilize polysulfides within the cathode region.52 To further understand the potential advantages of NHCF/S electrode on the electrochemical performance, the detailed charge storage process is investigated by the CV measurements at a series of sweep rates from 0.2 to 0.5 mV s−1 (Figure 5). CV curves of NHCF/S cathode exhibit that the shape of cathodic and anodic current peaks barely changes with increasing the scan rates, manifesting that the NHCF endows the electrode with mass transportation and electron conduction. However, the CV curves of pure sulfur electrode display that the shape of cathodic and anodic peak gradually broadens and the potential shift is observed with the increasing scan rate, which is ascribed to the weak polysulfide adsorbability and deposition of Li2S on the electrochemical reaction interface.53 The redox peak currents obeys a linear relationship with square root of scan rates, which is devoted to confirm diffusion-controlled process in the redox reaction.54 In this kind of reaction, lithium ions diffusion rate is evaluated on the basis of the classical Randles− Sevcik equation: Ip = 2.69 × 105n1.5SDLi0.5CLiν0.5, where Ip represents the corresponding redox peaks current, n is the numbers of transfer charge, S represents the active electrode area, DLi characterizes lithium ions diffusion coefficient, CLi corresponds to concentration of lithium ions, and ν stands for the sweep rate. Figure 5c,d expressly reveals the lithium ion diffusion rate in the cells with NHCF/S and sulfur by comparing the corresponding slopes. It can be observed that the slopes of cathodic and anodic peaks for NHCF/S are higher than those of sulfur, indicating that fast lithium ion diffusion is accomplished in the NHCF/S electrode. The

V in the following anodic scan. It is rather remarkable that the separation value between cathodic and anodic potential is higher, suggesting accrescent polarization during cathodic and anodic scan. Furthermore, there are peak shifts during the initial three cycles on account of the deposition of insoluble dead Li2S on the electrochemical reaction interface. Compared with the sulfur electrode, NHCF/S electrode shows higher cathode potential during the negative scan and lower anode potential during the positive scan due to the fast electron transfer and efficient utilization of sulfur. The cycling stability was evaluated at the 0.5 C rate in the voltage range from 1.7 to 2.8 V based on an areal sulfur mass loading as 1.6 mg cm−2. The characteristic galvanostatic charge−discharge voltage profiles of NHCF/S (Figure 4c) and sulfur (Figure 4d) cathodes demonstrate two discharging platform and a long charging platform, consistent with the multistep electrochemical redox reaction among solid S8, soluble Li2Sx and Li2S2/Li2S in the CV curves.51 The sulfur cathode only affords a reduced capacity retention because of the inevitable dissolution and shuttle effect of intermediate products (soluble Li2Sx) inducing the decrease of active sulfur in the cathode. In addition, a low capacity retention of 13.0% and fast capacity attenuation rates of 0.17% per cycle are observed (Figure 4e). Fast capacity attenuation of pure sulfur electrode is concerned in the major loss of active sulfur component derived from the continuously pulverized particles and the lack of effective anchoring structure for dissolution polysulfides. By contrast, NHCF/S electrode sustains higher reversible capacity of approximately 700 mAh g−1 along with an approved Coulombic efficiency of over 98% in 500 cycles at 0.5 C. Simultaneously, an advantageous capacity retention of 75.6% and remarkable average capacity fading rates of 0.049% F

DOI: 10.1021/acssuschemeng.8b04648 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 6. (a) Rate capability of NHCF/S and sulfur electrodes from 0.1 to 5 C. (b, c) EIS data for the two cathodes before and after 500 cycles at 0.5 C. (d) Self-discharge behavior of NHCF/S and sulfur electrode. (e) Discharge voltage profiles of the NHCF/S and sulfur cathodes before and after rest for 40 h. (f) Areal capacity of NHCF/S cathode with high sulfur loadings at different charge−discharge current rates.

transport and negligible sulfur loss.55 Likewise, in the second stage (Figure S11b), the higher plateau position and preponderant discharge capacity are achieved in the cell with a NHCF host, which is unanimously ascribed to NHCF that can avoid the shuttle of solution Li2Sx in the electrolyte and is beneficial to electrochemical redox kinetics in the reaction regime. The narrow voltage gap between two plateaus elucidates that the battery with a NHCF/S cathode reveals the lower polarization during charge−discharge cycles. EIS analysis also supports fast transfer and effective entrapment for polysulfides in the NHCF electrode. As displayed in Figure 6b, the charge transfer resistance (Rct) of the NHCF/S electrode is obviously smaller compared with the Rct of a pure sulfur electrode before cycle because the NHCF supplies continuous and fast electron transfer pathway. After cycles, the Nyquist plots of two electrodes present two obvious semicircles (Figure 6c) corresponding to Rct and interface resistance (Ri) derived from the deposition of insulative Li2S2/ Li2S onto surface of active materials.56,57 The Ri and Rct of NHCF/S significantly reduce in comparison with that of the sulfur electrode, which signifies that NHCF/S can restrain the migration of soluble polysulfides and decrease the polarization effect in cycling process. The corresponding equivalent circuit models of the battery before and after charge−discharge cycle are presented in Figure S12. Whereafter, self-discharge behavior is evaluated by resting for 40 h after charge (Figure 6d). It can be observed that NHCF/S electrode is stabilized at a higher voltage in comparison to pure sulfur electrode. Meanwhile, according to the discharge curves before and after rest (Figure 6e), two apparent discharge plateaus and less capacity loss demonstrate low self-discharge rate of NHCF/S electrode. The average self-discharge rate of the battery with sulfur electrode is around 27% when the battery is rested for 40

accelerated lithium ions diffusion, enhanced charge transfer, and better reaction kinetics originate from the unique structural of NHCF host, which is able to restrain the migration of soluble polysulfides and provide convenient pathways for lithium ions diffusion and electronic conductivity. The rate performances of NHCF/S and sulfur cathodes are recorded at a series of rates with galvanostatic mode, as presented in Figure 6a. It is obvious that NHCF/S cathode presents prominent rate capability and higher capacity retention rate than the pure sulfur cathode. Even though the rates are as high as 2 and 5 C, the NHCF/S cathode still delivers the relatively stable specific capacity of 616 and 500 mAh g−1, respectively. Furthermore, a stabilized reversible discharge capacity (around 977 mAh g−1) is still delivered after returning to 0.1 C again, without abrupt capacity degradation. The high capacities and rapid capacity recovery manifest that the stabilized architecture and prominent conductive framework of NHCF can maintain integrity of electrode during the larger current rate. Corresponding charge and discharge curves are available in Figure S10. As the charge−discharge current is increased, the NHCF/S cathode still reveals relatively stable voltage plateau located at around 2.3 and 2.1 V. Additionally, the voltage platforms are really apparent even current rate as high as 5 C. This result demonstrates that NHCF host is conducive to accelerate mass transport and electrochemical redox kinetics.51 Meanwhile, compared with pure sulfur electrode, NHCF/S electrode merely shows the slight increase of voltage gap between charge and discharge platforms at various rates. And then we indagate the reaction phenomena in detail based on discharge process of each region. In the first discharge platform and following a slanted section (Figure S11a), the NHCF/S cathode displays a larger onset voltage and a high discharge capacity, indicating accelerative electron G

DOI: 10.1021/acssuschemeng.8b04648 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering h. In sharp contrast, the NHCF/S electrode presents a relative high discharge capacity with a lower self-discharge rate of 11% after resting for 40 h. Obviously, the NHCF/S electrode can significantly inhibit the self-discharge issue, which is attributed to effective confinement for polysulfides in the NHCF. For satisfying the practical application, the capacity and cyclability of NHCF/S electrodes were evaluated with an area loading content increasing to 3.2 and 5.5 mg cm−2. In the case of higher area loading (5.5 mg cm−2), the NHCF/S cathode exhibits an average areal specific capacity of nearly 5 mAh cm−2 at a rate of 0.1 C and encouraging specific capacities at a series of rates. Based on homologous galvanostatic charge−discharge curves (Figure S10), voltage hysteresis slightly expands with increasing of sulfur mass loading, which is consistent with the polarized potential seen in the CV date (Figure S13). Besides, a stabilized discharge capacity of nearly 600 mAh g−1 is delivered along with high Coulombic efficiency of almost 100% under the NHCF/S with areal loading as high as 5.0 mg cm−2 at a larger current density of 2 C in 300 cycles, representing superior capacity and excellent cyclability among the previous carbon-based cathodes with high sulfur mass loading (Figure S14). Superior capability and cyclability are mainly attributed to the unique one-dimensional carbon architecture (Figure S15). NHCF provides sufficient free space for loading high sulfur and buffering volumetric expansion of encapsulated active sulfur in cathodes during cycling. Under the electrochemical cycling processes, a one-dimensional hollow structure also endows the electrode with a large infiltration area between active materials with electrolyte, strong sulfur confinement, consecutive electron transport, and short lithium ion transport pathways. These results strongly manifest the competitive capacity of the cathode structural design for practical applications.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Guixia Liu: 0000-0002-4100-5639 Xiangting Dong: 0000-0002-3368-7457 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (51573023, 50972020), the Natural Science Foundation of Jilin Province (20170101101JC, 20180520011JH), Industrial Technology Research and Development Project of Jilin Province Development and Reform Commission (2017C052-4), Science and Technology Research Planning Project of the Education Department of Jilin Province during the 13th five-year plan period (JJKH20170608KJ, JJKH20181122KJ), Innovative Foundation (XJJLG-2017-04) and Youth Foundation (XQNJJ-2016-01, XQNJJ-2017-17) of Changchun University of Science and Technology.

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CONCLUSIONS In summary, we employed dandelion as the biological template to derive a nitrogen-doped, carbon-based host with a onedimensional hollow porous architecture for Li−S batteries. Benefiting from the unique architecture, resultant NHCF/S electrodes achieve impressive rate capacity and superior cycling stability due to strong sulfur confinement, consecutive fast mass transportation, and electron conduction. Meanwhile, sufficient free space enables the NHCF host to realize high sulfur loading. As a consequence, the NHCF/S delivers a higher initial specific capacity of nearly 950 mAh g−1 at 0.5 C rate along with an average Coulombic efficiency over 96% and a low capacity decline rate of 0.059% in 500 cycles. More importantly, an average reversible areal capacity of nearly 5 mAh cm−2 is achieved at 0.1 C with a sulfur mass loading as high as 5.5 mg cm−2. Based on the widespread biotemplate and facile synthesis route, the NHCF/S electrode will serve as an advantageous cathode material to develop high energy density batteries at a low cost.

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TR, XRD, and TGA of NHCF/S electrode material, photographs of the Li2S6 adsorption test, along with other characterization results from charge−discharge profiles at various rate for sulfur and NHCF/S electrode with different sulfur loading, and CVs and cycling durability tests of NHCF/S with high sulfur loading (PDF).

REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b04648. SEM and TEM image, N2 adsorption/desorption isotherms, XPS and EDX analysis of NHCF, TEM, FIH

DOI: 10.1021/acssuschemeng.8b04648 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

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DOI: 10.1021/acssuschemeng.8b04648 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX