ARTICLE

J. Cent. South Univ. (2019) 26: 1426-1434

DOI: https://doi.org/10.1007/s11771-019-4098-3

Bio-derived N-doped porous carbon as sulfur hosts for high performance lithium sulfur batteries

LIU Yan-yan(刘艳艳), YAN Li-jing(严立京), ZENG Xian-qing(曾显清), LI Ze-heng(李泽珩),

ZHOU Shu-dong(周蜀东), DU Qiao-kun(杜乔昆), MENG Xiang-juan(孟祥娟), ZENG Xiao-min(曾小敏),

LING Min(凌敏), SUN Ming-hao(孙铭浩), QIAN Chao(钱超), LIANG Cheng-du(梁成都)

Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology,

College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract: Shuttle effect, poor conductivity and large volume expansion are the main factors that hinder the practical application of sulfur cathodes. Currently, rational structure designing of carbon-based sulfur hosts is the most effective strategy to address the above issues. However, the preparation process of carbon-based sulfur hosts is usually complex and costly. Therefore, it is necessary to develop an efficient and cost-effective method to fabricate carbon hosts for high-performance sulfur cathodes. Herein, we reported the fabrication of a bio-derived nitrogen doped porous carbon materials (BNPC) via a molten-salt method for high performance sulfur cathodes. The long-range-ordered honeycomb structure of BNPC is favorable for the trapping of polysulfide (PS) species and accommodates the volumetric variation of sulfur during cycling, while the high graphitization degree of BNPC favors the redox kinetics of sulfur cathodes. Moreover, the nitrogen doping content not only enhances the electrical conductivity of BNPC, but also provides ample anchoring sites for the immobilization of PS, which plays a key role in suppressing the shuttle effect. As a result, the S＠BNPC cathode exhibits a high initial specific capacity of 1189.4 mA·h/g at 0.2C. After 300 cycles, S＠BNPC still maintains a capacity of 703.2 mA·h/g which corresponds to a fading rate of 0.13% per cycle after the second cycle. This work offers vast opportunities for the large-scale application of high performance carbon-based sulfur hosts.

Key words: lithium sulfur batteries; high graphitization; nitrogen doping; sulfur hosts; shuttle effect

Cite this article as: LIU Yan-yan, YAN Li-jing, ZENG Xian-qing, LI Ze-heng, ZHOU Shu-dong, DU Qiao-kun, MENG Xiang-juan, ZENG Xiao-min, LING Min, SUN Ming-hao, QIAN Chao, LIANG Cheng-du. Bio-derived N-doped porous carbon as sulfur hosts for high performance lithium sulfur batteries [J]. Journal of Central South University, 2019, 26(6): 1426-1434. DOI: https://doi.org/10.1007/s11771-019-4098-3.

1 Introduction

The ever-increasing development of portable electronics, electronic vehicles and power grids urgently calls for batteries with higher energy densities [1-4]. Currently, lithium-ion batteries (LIBs) are finding it very difficult to catch up with such rapid developments due to the bottle-necked cathode materials with limited specific capacities [5-8]. Among various candidates of next- generation battery configurations, Li-sulfur (Li-S) battery, which is featured by high theoretical energy density and low-cost, is gaining world-wide attention [9, 10]. As the cathode material of Li-S batteries, sulfur has an ultra-high theoretical capacity of 1675 mA·h/g [11]. When coupled with lithium metal anodes (3860 mA·h/g theoretical LIU Yan-yan and SUN Ming-hao contributed equally to this work.capacity [12]), Li-S batteries can deliver a specific energy density of 2600 W·h/kg and a volumetric energy density of 2200 W·h/L, both are much higher than that of state-of-the art commercial LIBs (387 W·h/kg or 1015 W·h/L) [13-15]. Among various candidates of next-generation battery configurations, Li-S battery, which is featured by high theoretical energy density and low-cost, is gaining world-wide attention [9, 10]. As the cathode material of Li-S batteries, sulfur has an ultra-high theoretical capacity of 1675 mA·h/g [11]. When coupled with lithium metal anodes (3860 mA·h/g theoretical capacity [12]), Li-S batteries can deliver a specific energy density of 2600 W·h/kg and a volumetric energy density of 2200 W·h/kg, both are much higher than that of state-of-the art commercial LIBs (387 W·h/kg or 1015 W·h/L) [13-15]. However, there are several key issues that prohibit the practical application of Li-S batteries, among which, the notorious “shuttle effect” attracts the most attention [16]. Generally, the reduction process of S₈ molecules to Li₂S₂/Li₂S involves the formation of a series of long-chained PS intermediates [17]. The shuttle effect refers to the dissolution of PS into electrolytes and migrating between the cathode and anode under the driven of concentration gradient and electric field gradient [18, 19], thus leading to massive loss of active materials and poor cycle life of the batteries [20, 21]. Secondly, sulfur undergoes a violent volume variation up to 80% due to the different densities of sulfur (2.03 g/cm³) and Li₂S (1.66 g/cm³) during cycling [22], which only gives rise to the pulverization of sulfur particles and loss of electric contact with the current collector, but may also induce severe safety hazards [13, 23]. Thirdly, sulfur itself is intrinsically an electrical insulator (5×10^-30 S/cm at 25 °C) [11], this deteriorates the kinetic issues of sulfur cathodes which are affected by the large resistance, leading to large polarization and inferior rate capability [24]. In recent years, multiple strategies, including coating a protective membrane onto the separators [25-27], applying of electrolyte additives and fabrication of hosts sulfur [28-31], have been developed to solve the above issues of Li-S batteries. However, the electrolyte additives are usually unstable under working conditions which exhaust rapidly during the charging-discharging process while the protective membranes are inclined to increase the impedance and decrease the volume density of the batteries. Comparing with the above two methods, fabrication of sulfur hosts is more effective in making the most of active materials hence greatly enhancing the electrochemical performance of Li-S batteries, which has attracted the most attention.

Due to its highly porous structure, high electrical conductivity and low density, carbon materials are currently the most researched hosts of sulfur cathodes, which has boosted the electrochemical performances of Li-S batteries greatly [15, 32-35]. However, complex and costly preparation procedures are usually involved to fabricate delicate carbon-hosts for sulfur, which compromises the universality of Li-S batteries [36]. Therefore, a facile and cost-effective method is in urgent need to fabricate carbon host for high performance Li-S batteries.

Herein, based on a molten-salt method, we reported the fabrication of a nitrogen doped porous carbon (BNPC) material from chitosan derivatives as sulfur host for Li-S batteries. The porous structure of BNPC is favorable for the trapping of PS during cycling and accommodates the large volume change of sulfur, while the high graphitization degree of the material is essential for improving the kinetic properties of sulfur cathodes. Moreover, the N content in chitosan leads to N-doping in BNPC, which not only enhances the electrical conductivity of carbon materials, but also provides numerous anchoring sites for PS, thus suppressing the shuttle effect. Consequently, Li-S batteries using S＠BNPC composite cathodes deliver a high initial specific capacity of 1189 mA·h/g at 0.2C and the Coulombic efficiency is as high as 93%, the batteries retain 73% of the specific capacity after 200 cycles. This work will speed up the large-scale application of Li-S batteries.

2 Methods

2.1 Preparation of BNPC

Carboxylation chitosan (C-CTS) was dissolved in deionized water and vacuum dried at 60 °C. Then, the dried C-C-TS was heated at 170 °C under air atmosphere to obtain an cross-linked carboxylation chitosan (C-CTS). For the molten salt synthesis, 11.5 g NaCl, 13.5 g KCl and 0.2 g C-CTS were mixed and ball milling for 20 min. Then the ball milled sample was transferred to a tube furnace and calcined at 1050 °C for 1 h under Ar atmosphere with a heating rate of 5 °C/min. The calcined product was cooled to room temperature followed by washing with a lot of water to dissolve salt and filtered to get BNPC. After being washed with DI water and ethanol rinsing for three times, respectively, the BNPC was ground with mortar and pestle followed by washing again with DI water and ethanol, this procedure was repeated for three times. Finally, the product was dried in vacuum oven for 12 h.

2.2 Electrochemical measurements

The BNPC materials were mixed and ground with sulfur at the mass ratio of 6:4. Then the mixture was sealed and heated for 4 h to melt-infiltrate sulfur into the BNPC to get S＠BNPC composite. For the fabrication of electrodes, a homogeneous slurry containing S＠BNPC, Super P, PVDF (polyvinylidene fluoride) in a mass ratio of 7:2:1 was mixed in a NMP (N-methyl-2-pyrrolidone) solvent and coated on the aluminum foil by a doctor blade and dried in a vacuum oven at 60 °C for 12 h.

The as-prepared electrode was punched into small round pieces with a diameter of 12 mm and assembled into 2025 coin cells in a glove box filled with Ar atmosphere. The metal lithium wafer was used as the counter electrode and Celgard 2400 was used as separator. The electrolyte contains 1 mol/L lithium bis (trifluoromethane sulfonel) imide (LiTFSI), in a binary solvent of DOL and DME (1:1 in volume) with 1.5 wt% lithium nitrate (LiNO₃) as an additive. For comparison, sulfur/ super P composite cathode was also made at a mass ratio of 6:4 with other synthesis conditions been kept equal to that of S＠BNPC. Galvanostatic charge-discharge tests were carried out within a voltage window of 1.8-2.6 V versus Li/Li⁺ at room temperature with a LAND battery cycler. Current densities and specific capacities were calculated based on the mass of sulfur active materials. CV (cyclic voltammetry) of the batteries was carried out on a Solartron 1470E electrochemical workstation at a scanning rate of 0.1 mV/s within a voltage window of 1.8-2.6V (vs Li/Li⁺). Electrochemical impedance spectroscopy (EIS) was conducted using a CHI660E workstation with an amplitude of 5 mV in the frequency range of 0.01-100 kHz. The EIS results in Figure 3(d) are all performed at charged conditions of the corresponding cells.

2.3 Material characterizations

The morphology and structure of the samples were tested using scaning electron microscope (SEM, Hitachi S-4800) and transmission electron microscope (TEM, JEM-2100F). X-Ray diffraction (XRD) pattern was obtained at room temperature using an X’Pert PRO (PANalytical, Netherlands) instrument with Cu K_a radiation. Raman spectroscopy was conducted using a laser confocal Raman microspectroscopy (LabRAM HR Evolution, Horiba Jobin Yvon). XPS analyses were performed on an Escalab 250Xi XPS system with Al K_α (1486.6 eV) source. Brunauer-Emmett-Teller (BET) specific surface area was determined by nitrogen adsorption-desorption isotherm measurements at 77 K (TriStar II).

3 Results and discussion

Electron microscopy was first applied to probe the structure of BNPC. As shown in Figures 1(a) and (b), BNPC exhibits a honeycomb-like ordered macroporous structure under standard temperature and pressure in the scanning electron microscope (SEM) images, the plentiful voids are favorable for the accommodation of volume change of sulfur during cycling and benefits fast transportation of ions [6, 18]. Meanwhile, the porous structure of BNPC was confirmed by Brunauer-Emmett-Teller (BET) analysis. In Figures 1(c) and (d), the typical type-I Nitrogen adsorption-desorption isotherms reveal that the specific surface area of BNPC reaches 353 m²/g, the pore volume analysis further proves the existence of numerous micro- and meso-pores in the carbon skeleton [37], which would play a key role in absorbing PS intermediates.

X-ray diffraction(XRD) measurements were applied to determine the phase and components of BNPC. As displayed in XRD patterns (Figure 2(a)),the broad diffraction peaks around 25.3° are characteristic of carbon materials and weak diffraction peaks around 43.5° indicate the higher degree of graphitization [38, 39]. Raman measurements were further carried out to investigate the structure of BNPC. As shown in Figure 2(b), the two peaks at 1364.2 cm^-1 and 1589.7 cm^-1 correspond to the disordered carbon (D band) and graphitic carbon (G band) [40], respectively, with a peak intensity ratio of 1.13:1 (I_G:I_D). To sum up, both XRD and Raman measurements prove that BNPC is a carbon material of high graphitization degree, which is favorable for achieving a carbon-based sulfur host with fast kinetics for Li-S batteries.

Figure 1 SEM images of BNPC (a, b), N₂ adsorption/desorption isotherms (c) and pore size distribution plot (d)

The deconvolution XPS results of C 1s spectra (Figure 2(c)) shows characteristic peaks of C–C/C=C (284.3 eV) and C–O (285.7.0 eV). N-doping of BNPC was proved by the XPS spectra of N 1s region (Figure 2(d)), which suggests the N-doping type of graphitic-N (401.4 V), pyrrolic-N (400.2 V), and pydridinic-N (398.5 V) [41, 42], among which pydridinic-N was claimed to suppress the shuttle effect by providing anchoring points for PS intermediates [43]. To sum up, BNPC is a N-doped porous carbon material with high graphitization degree, which provides an excellent host material for sulfur cathodes.

Li-S batteries were assembled with S＠BNPC cathode and metal lithium anode to evaluate electrochemical performance of S＠BNPC.Figure 3(a) shows the cyclic voltammetry (CV) curves of batteries assembled with S＠BNPC cathode, the two cathodic peaks located at 2.31 and 2.00 V are ascribed to the reduction of sulfur to highly soluble long-chain PS (Li₂S_x, 4≤x≤8) and further reduction of PS to insoluble Li₂S₂ or Li₂S, respectively, while the anodic peaks located at 2.35 and 2.43 V refer to the conversion from Li₂S to PS and finally to sulfer, respectively [44, 45]. The CV curves of S＠BNPC electrode are almost overlapped after the first cycle, suggesting excellent redox kinetics and suppressed shuttle effect in the batteries [22]. Meanwhile, CV curves of batteries assembled with the SP＠S electrode (Figure 3(b)) demonstrate broader redox peaks and lower intensities comparing with that of S＠BNPC, suggesting the superiority of BNPC as a sulfur host. Moreover, the voltage heterolysis between the anodic peaks and cathodic peaks in the CV curves of S＠BNPC (0.04 V) is much smaller than that of SP＠S (0.06 V), further confirming the favored kinetic properties of S＠BNPC cathodes.

Figure 2 XRD pattern of BNPC (a), Raman spectrum of BNPC (b), XPS of BNPC C 1s (c) and N1s (d)

Figure 3 CV of S＠BNPC (a) and S＠SP (b), cycling performance of S＠BNPC and S＠SP at 0.2C for 50 cycles (c) and EIS of S＠BNPC and S＠SP (d)

The cycling stability of S＠BNPC electrode was first evaluated at 0.2C. As shown in Figure 3(c), battery assembled with S＠BNPC exhibits an initial specific capacity of 1189.4 mA·h/g while SP＠S only exhibits 849.2 mA·h/g under the same condition. After 50 cycles, S＠BNPC retained 82% of the initial specific capacity which is higher than that of SP＠S (65%), suggesting BNPC is superior than SP as a sulfur host. The large cycling performance difference between S＠BNPC and SP＠S can be attributed to the fact that porous carbon absorbs PS during cycling while the honeycomb-liked structure can immobilize PS thus giving rise to a high mass loading of active materials of S＠BNPC electrode, which eventually leads to a high specific capacity and stable cycling performance.

Electrochemical impedance spectroscopy (EIS) was applied to determining the kinetic properties of S＠BNPC electrode. As shown in Figure 3(d), both the Nyquist plots of S＠BNPC electrode and SP＠S electrode exhibit a semicircle from high to medium frequency range which refers to the charge-transfer resistance (R_ct) [46]. It is obvious that the R_ct of S＠BNPC electrode is smaller than the SP＠S electrode. The R_ct of S＠BNPC decreases only by 25% after 100 cycles, while the R_ct of SP＠S increases by 25%. This may be attributed to the synergetic effect of enhanced conductivity and large surface area of BNPC which increases the electrical contact of S with the current collector and the electrolyte [47].

Next, long-term cycling performance of S＠BNPC electrode was compared with S＠SP at 0.2C. As shown in Figure 4(b), S＠BNPC electrode exhibits an initial discharge specific capacity of 1189.4 mA·h/g, after 300 cycles, S＠BNPC still maintains a capacity of 703.2 mA·h/g with a capacity fading rate of 0.13% per cycle after the second cycle. In the charge/discharge profiles of S＠BNPC and S＠SP (Figures 4(b) and (c)), the two typical plateaus of 2.35 and 2.05 V correspond to the reduction of S₈ to S₄^2- and S₄^2- to S₂^2-. Impressively, the plateaus of S＠BNPC are flat and stable with a relatively low polarization of 150 mV and the charge-discharge voltage gaps are almost identical after 300 cycles, suggesting excellent capacity retention and fast kinetics of SBNPC electrodes. This further confirmed the efficiency of BNPC in suppressing the shuttle effect and a fast kinetic of the batteries thus favoring a stable cycling performance. Rate performance of S＠BNPC and S＠SP electrodes were evaluated at different rates from 0.2C to 5C. As shown in Figure 4(d), S＠BNPC delivers a reversible specific capacity of 1310.6, 947.4, 784.6, 594.6, 412. 4 and 299.0 mA·h/g, respectively, at 0.2C, 0.5C, 1C, 2C, 3C and 5C. A discharge capacity of 1109.0 mA·h/g is retained, which is much higher than that of S＠SP electrode (Figure 4(e)), when the high rate tests are finished and the rate is switched back to 0.2C.

The excellent electrochemical performance of S＠BNPC electrode can be ascribed to the following reasons: 1) The honeycomb structure of BNPC plays a key role in hosting sulfur and suppressing the shuttle effect; 2) The high graphitization degree of BNPC facilitates fast electron transfer in the electrodes, which is favorable for the efficient utilization of sulfur active materials. 3) The incorporation of nitrogen in this carbon material not only traps sulfur through chemical bonds, but also attracts Li⁺ because of its negative charge that facilitates the transport of Li⁺, which favors a long-term cycling performance of the batteries by suppressing the shuttle effect. The above merits all contribute to the excellent electrochemical performances of S＠BNPC electrodes.

4 Conclusions

In summary, we reported a facile and cost-effective method for preparing a nitrogen doped porous carbon (BNPC) material from chitosan derivatives as sulfur host for Li-S batteries through molten-salt method. The BNPC material effectively traps PS through highly porous structure and plentiful N anchoring site, thus suppressing the shuttle effect, while the high graphitization degree of the material facilitates the kinetic properties of sulfur cathodes. This work paves a new way for the large-scale application of high performance carbon- based sulfur hosts.

Figure 4 Cycling performance of S＠BNPC at 0.2C for 500 cycles (a), charge/discharge profiles of S＠BNPC (b), charge/discharge profiles of S＠SP (c), rate performance of S＠BNPC (d) and rate performance of S＠SP (e)

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(Edited by FANG Jing-hua)

中文导读

生物衍生的氮掺杂多孔碳材料作为高能锂硫电池的固硫材料

摘要：穿梭效应，低导电性，巨大的体积膨胀是阻碍硫正极实际应用的主要原因。目前解决上述问题最有效的措施是合理设计固硫碳材料结构，但是，碳基固硫材料的制备通常很复杂并且成本较高。因此，发展一种有效且低成本的方法来制备高能硫电极的碳基材料十分必要。在此，本文提出一种基于熔融盐法的生物衍生的氮掺杂多孔碳材料(BNPC)。BNPC有序的蜂窝结构有利于硫和固定多硫化物并且容纳循环过程中的体积膨胀，高度石墨化促进了硫正极氧化还原反应动力学。此外，掺杂的氮不仅可以提高BNPC的导电性，还为硫和多硫化物提供固定位点，这在抑制穿梭效应方面起到关键作用。因此，S＠BNPC 电极呈现出1189.4 mA·h/g的高初始比容量，在0.2C下循环300圈后仍然有703.2 mA·h/g的容量，衰减速率每圈只有0.13%。这项工作为碳基固硫材料的大规模应用提供了很大的可能性。

关键词：锂硫电池；高度石墨化；氮掺杂；固硫；穿梭效应

Foundation item: Project(2018YFB0104300) supported by the National Key R&D Program of China; Project(51774150) supported by the National Natural Science Foundation of China

Received date: 2018-10-31; Accepted date: 2019-03-12

Corresponding authors: LING Min, PhD, Researcher; Tel: +86-571-87953150; E-mail: minling＠zju.edu.cn; ORCID: 0000-0003-4428- 5370; LIANG Cheng-du, PhD, Professor; Tel: +86-571-87953150; E-mail: cdliang＠zju.edu.cn; ORCID: 0000-0002-3017-7785