S composite cathodes for high cyclability lithium-sulphur batteries

Journal of Alloys and Compounds 704 (2017) 1e6

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Spirulina-derived nitrogen-doped porous carbon as carbon/S composite cathodes for high cyclability lithium-sulphur batteries Jun Wu*, Ji Hu, Kaixin Song, Junming Xu, Huifang Gao Institute of Electron Device & Application, Hangzhou Dianzi University, Hangzhou, Zhejiang 310018, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 October 2016 Received in revised form 31 January 2017 Accepted 6 February 2017 Available online 7 February 2017

Spirulina was used for the first time as a raw nitrogen-doped porous carbon source to synthesize carbon/ sulphur (carbon/S) composite cathodes for lithium-sulphur batteries. The composites consisting of sulphur and spirulina-derived carbon with micro- and meso-pores showed an obvious carbon encapsulating sulphur structure. The carbon/S cathodes exhibited much better electrochemical performance than pristine sulphur, delivering an initial discharge capacity of 662 mAh g
1 with nearly 60% capacity retention after 100 cycles at 1 C current density (1670 mA g
1). The improved electrochemical stability of the composites can be ascribed to the synergistic effects between the encapsulation of sulphur by spirulina-derived carbon and adsorption of polysulfides on N doped carbon lattice. These effects successfully prevented or retarded the shuttle effect of sulphur cathodes. © 2017 Elsevier B.V. All rights reserved.

Keywords: LieS batteries Cathode Composite Spirulina Carbon

1. Introduction The increasing global demand for green and renewable energy is leading to the rapid development of electric vehicles and electronic devices. Among the various energy storage and transfer sources for these devices, lithium-ion batteries (LIBs) currently dominate because of their steady cyclability, high coulombic efficiency, and long cycle life [1e4]. However, LIBs cannot satisfy the requirements of long-distance electricity-driven vehicles or sustainable high power electronic devices, mainly owing to their poor capacity and low voltage window of cathode materials [5,6]. Lithium-sulphur (LieS) batteries have been given increasing attention because of sulphur's super high theoretical specific capacity of 1672 mAh g
1 and specific energy density of 2567 Wh kg
1. Moreover, sulphur exhibits many other advantages, such as abundance in the lithosphere and low price, as well as nontoxic nature and safety. All this enables LieS batteries to be an ideal option for large capacity and high energy density applications [7e10]. However, several challenges including the dissolution of intermediate lithium polysulfides into electrolytes, low conductivity of sulphur, large volumetric expansion of sulphur upon lithiation, and polysulfide shuttle effect impedes the practical application of LieS batteries [11e14].

* Corresponding author. E-mail address: [email protected] (J. Wu). http://dx.doi.org/10.1016/j.jallcom.2017.02.052 0925-8388/© 2017 Elsevier B.V. All rights reserved.

The typical strategy is to encapsulate sulphur cathodes with conductive host materials to improve their conductivity as well as to physically confine lithium polysulfide species within the host during cycling. The most common encapsulation materials used are carbon, such as carbon black [15], graphite [16], carbon nanotube [17,18] or graphene [19e21]. To date, more and more green or ecofriendly organisms have been used as carbon sources, such as fungus [22], protein [23], algae [24], even shells [25] or leaves [26]. Besides their natural carbonous framework, these green hosts are also rich in nitrogen, which is an extra benefit. The doping of pyridinic nitrogen into carbon lattice is significantly beneficial not only for increasing the electrical conductivity but also for strengthening the chemical adsorption between nitrogen atoms and high-order polysulfides. Thus this phenomenon promotes the cycling stability and coulombic efficiency [27e30]. In this report, spirulina as a type of lacustrine algae with enriched protein is first used as a raw nitrogen-doped carbon source, serving as the additive to the sulphur cathodes. After carbonization of spirulina activated by alkaline solution, to obtain nitrogen-doped carbon with numerous pores is expected, where sulphur particles can be encapsulated well. This feasible strategy provides an ecofriendly pathway to synthesize composite cathodes with sulphur encapsulated by composite materials. Additionally, the abundant sustainable natural spirulina resources are available and provide new opportunities to produce cathodes of LieS batteries on a large scale and at a low price.

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2. Experimental 2.1. Synthesis of carbon/S composite cathodes In a typical procedure as shown in Fig. 1, live spirulina was washed several times with deionized water, and then vacuum freeze-dried for 24 h. Up to 15 g dried spirulina was added into 50 ml 1 M KOH solution and dissolved completely through continuous mechanical stirring. After 24 h vacuum freeze-drying, the spirulina precursor was pyrolyzed at 800  C for 2 h under argon atmosphere. Then, the calcination product was washed by 1 M HCl and deionized water several times in sequence until pH 7 and dried for 24 h. The carbonation process was carried out again at 1200  C for 3 h to produce the porous carbon with good crystallinity. The obtained porous carbon and sublimed sulphur (99.9%, Aladdin) were mixed at a weight ratio of 3:7 and ball milled at 600 rpm for 6 h to synthesize carbon/S composite cathodes. The complete mixture was then dried for 12 h at 155  C to obtain the final carbon/S composite materials. As a control sample, pure sublimed sulphur (99.9%, Aladdin) with the same weight similar to that of carbon/S composites was used as active material to prepare pristine S cathodes. 2.2. Material characterization and electrochemical measurements The morphologies of the obtained samples were characterized through field emission scanning electron microscopy (FESEM; Hitachi S4800) and high-resolution transmission electron microscopy (TEM, JEOL-2010 HR). Brunauer-Emmett-Teller instrument (BET, Beishide 3H-BET2000-A) was used to determine the structure type and pore size distribution of porous carbon derived from spirulina. The nitrogen adsorption-desorption process has been executed at 100  C under vacuum condition for 12 h. The specific surface area was calculated using the BET equation for relative pressures in the range of p/p0 ¼ 0.01e0.15. The pore size distribution of the samples was determined using Barrett-JoynerHalenda (BJH) method with a cylindrical pore model from the desorption branch of the hysteresis loop of the isotherm [31,32].

Fig. 1. Schematic of the preparation process from spirulina to carbon/S composite cathodes.

The sulphur loading in carbon/S composites was determined by a thermogravimetric analyzer (TGA, METTLER TOLEDO) under nitrogen atmosphere at a heating rate of 5  C/min at 25  Ce500  C. The composition of carbon/S composite materials was obtained by X-ray photoelectron spectroscopy (XPS) (PHI-5400, Mg Ka source, operating power-400 W). The electrochemical performance was investigated after assembling the cells (CR 2032 coin type) in a glove box. In a full LieS cell, the as-prepared carbon/S composites (for carbon/S samples) or pure sublimed sulphur (for pristine S samples) acted as active cathode materials. They were prepared by mixing acetylene black and polyvinylidene difluoride in a weight ratio of 70:20:10 in Nmethyl pyrrolidone, and then was coated on round aluminum foil current collectors with the diameter of 14 mm. The active material loading was approximately 1.8 mg cm
2 for all samples in coin cells. Pure lithium metal was used as anode. Celgard 2500 microporous polypropylene was separator saturated with electrolyte. This electrolyte was the mixture of 1.0 M lithium bis-(trifluoromethanesulfonyl)amide and 1.0 wt% LiNO3 in 1,3-dioxolane/ 1,2-dimethoxyethane (v/v ¼ 1:1). The galvanostatic charge/ discharge tests were performed using a NEWARE battery system with a voltage window of 1.5e2.8 V vs. Liþ/Li. CHI660E electrochemical workstation (CH Instrument, China) was used to carry out cyclic voltammograms (CV) measurements at a scan rate of 0.1 mV s
1 and electrochemical impedance spectroscopy (EIS) measurements ranging from 10 kHz to 100 kHz. 3. Results and discussion The structure of spirulina-derived carbon was investigated by FESEM and TEM, and the results are shown in Fig. 2(a) and (b). The carbon substrate exhibits a porous structure with a pore diameter of approximately 2e5 mm. Furthermore, a hierarchical pore substructure can be observed in the high magnification image shown in Fig. 2(a) (see insert in the right top corner). Many small holes are formed at the walls of large holes. These holes allow to increase the surface area. TEM image of carbonized spirulina in Fig. 2(b) reveals its morphology containing a zigzag margin, derived from the pore structure of carbon surface. From the inset in Fig. 2(b), some diffraction fringes are observed clearly at an interplanar distance of approximately 0.33 nm. XRD pattern shows presence of typical peaks of graphitic carbon, see Fig. 2(c). These results allow to conclude that the carbonized spirulina is crystalline and possesses similar characteristics to those of graphite. BET measurements had been executed to investigate the porosity of spirulina-based carbon network shown in Fig. 2(d) and (e). The nitrogen adsorptionedesorption isotherm [Fig. 2(d)] exhibits a typical I-type curve with a sharp ascending and a long plateau. This indicates presence of microporous structure. Based on calculations of curves' slop and intercept, the porous carbon possesses a large pore volume and a large specific surface area of 0.5616 cm3 g
1 and 1273.8 m2 g
1, respectively. The pore diameter distribution curves are shown in Fig. 2(e). Most pores fall into the size range of 1e5 nm. This observation shows that the carbon network consists of enormous amount of micro- and meso-pores. Based on the obtained from spirulina porous carbon network, sublimed sulphur had been poured into carbon pores using the preparation process illustrated in Fig. 1. After finishing the perfusion of sulphur into carbon pores, the morphology of the obtained carbon/S composites is shown in Fig. 3(a). Most pores of carbon have been filled by sublimed sulphur and the surface becomes smooth. This confirms the formation of carbon/S structure. Some small pores still exist as observed from the inset image of Fig. 3(a). This may be caused by some mixing of air bubbles during the perfusion process.

J. Wu et al. / Journal of Alloys and Compounds 704 (2017) 1e6

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Fig. 2. Structure characterization of porous carbon from spirulina: (a) FESEM images (inset is the magnification of partial region), (b) TEM of carbon margin (inset is the diffraction result), (c) XRD diffraction pattern, (d) nitrogen adsorption-desorption isotherm, and (e) pore size distribution curves.

Fig. 3. SEM images (a, inset is the magnification of partial region) and TGA curves (b) of carbon/S composite cathodes.

After preparing carbon/S composites, thermal gravimetric analysis (TGA) was used to determine the weight ratio of sulphur in the composites, as shown in Fig. 3(b). The weight loss of spirulinaderived porous carbon was investigated to eliminate a probable error caused by the loss weight of porous carbon matrix. Based on the measurement result, the total weight loss of porous carbon from spirulina is approximately 5.2% at temperature ranging from 20  C to 150  C. This may be mainly due to the adsorbed water on carbon surfaces. However, the weight loss ratio of carbon/S composites is up to 73.9% under the same conditions. Thus the sulphur content in total composites is much higher, being approximately 68.7%. This result is in accordance with the mixing ratio of sulphur and carbon in 7:3 during our preparing procedure. From the TGA curve of carbon/S, two evaporation temperature regions of weight loss exist located at 180e275 and 275e425  C, respectively. This finding corresponds to the presence of two types of sulphur located at the surface or in the interior of porous carbon. Obviously, the interior sulphur is more strongly bound to carbon and thus would need higher temperature to evaporate it as compared to that on the surface.

XPS had been used to characterise the nitrogen doping derived from protein component of spirulina (Fig. 4). Five characteristic peaks at approximately 164.9, 228.6, 285.6, 400.3, and 533.6 eV can be observed in the total XPS survey spectrum, corresponding to S2p, S2s, C1s, N1s, and O1s, respectively [Fig. 4(a)]. This result indicates the existence of nitrogen in carbon matrix. High-resolution N1s spectrum of carbon/S composites can be divided into several peaks and are ascribed to pyridinic N (398.8 eV), pyrroile N (400.1 eV), quaternary N (401.5 eV) and graphitic N (403.5 eV) [27,28]. The existence of elementary nitrogen in carbon substrate can enhance the affinity of polysulfides and promote the interaction of sulphur atoms with carbon matrix. Fig. 5 shows the electrochemical properties of S based electrodes containing different cathode materials: pristine sulphur or spirulina-derived carbon/S composites. From Fig. 5(a) and (b), the initial discharge specific capacities of pristine S and carbon/S reach a maximum value of 692 and 662 mAh g
1 at 1 C current density (corresponding to 1670 mA g
1 or the surface current density of about 3.04 mA cm
2) being far away from the theoretical values

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Fig. 4. Total (a) and N 1s (b) of XPS spectra of spirulina-based carbon/S composite cathodes.

Fig. 5. Electrochemical properties of pristine sulphur and carbon/S composite cathodes in LieS batteries. (a) cycling performance of pristine S and carbon/S at 1 C, (b) initial dischargeecharge profiles of pristine S and carbon/S (inset is the initial cyclic voltammograms measurement), and (c) rate performance of pristine S and carbon/S at various current densities from 0.5 C to 5 C.

(only approximately 40% of theoretical ones). The reason maybe in the fact that the cathode materials are not activated completely during the initial cycle. Moreover, the interior materials cannot be involved in the electrochemical reaction. Additionally, the carbon/S composite shows a lower plateau than pristine sulphur. This indicates its low interior impendence. This improvement may be mainly caused by the contribution from carbon having an improved conductivity [33e35].

The pristine S cathode only preserves its capacity of 122 mAh g
1 after 100 cycles (ca. 17.6% of initial capacity). By contrast, the carbon/S cathode shows nearly 60% capacity retention at 391 mAh g
1 after 100 cycles. Although carbon/S cathode exhibits a slightly lower initial reversible capacity than pristine S (the doping of carbon into sulphur would inevitably decrease its total capacity in proportion), a high capacity and coulombic efficiency are still available with an ultralow cyclic fading rate of 0.4% and

J. Wu et al. / Journal of Alloys and Compounds 704 (2017) 1e6

almost 100% reversibility for initial 100 cycles. This result indicates that the spirulina-based architecture for carbon/S cathodes counts for the improved electrochemical stability. The rate performance of the LieS batteries with pristine S and carbon/S cathodes is illustrated in Fig. 5(c). For both cathodes, their capacities gradually decrease with the current density varying from 0.5 C to 5 C. However, carbon/S cathodes exhibit much higher capacities than those of pristine S at all current densities. A reversible capacity of ca. 170 mAh g
1 can still be preserved at a very high current density of 5 C. This outcome indicates an excellent rate capability for spirulina-based carbon/S cathodes. The proposed explanation for the improvement of carbon/S composite is a combination between the efficient encapsulation of carbon for sulphur and its mesopore-interconnection architecture. This condition can not only prevent the dissolution of polysulphides but also may improve the immersion and permeation of electrolyte into active materials. Therefore, its excellent rate performance is rendered. When the current density returns to 0.5 C, the capacity of carbon/S can be recovered and even exceeds the initial level (ca. 720 mAh g
1). This result indicates the excellent cyclability of spirulina-derived carbon/S cathodes and their strong defensive ability from being damaged by high current densities. We note that two discharge voltage plateaus exist for carbon/S at approximately 2.13 and 1.92 V (2.26 and 2.06 V for pristine S) respectively corresponding to a two-step reaction of sulphur with lithium as observed from the discharge-charge profile of pristine S and carbon/S cathode in Fig. 5(b). In the inset to Fig. 5(b) about CV curve measured within a potential window of 1.5e2.8 V (vs. Li/Liþ) at a scan rate of 0.1 mV s
1, two reduction peaks in the initial cathodic scan also demonstrate a two-step reaction mechanism for the elemental sulphur. The high plateau or the first peak at approximately 2.13 V can be assigned to the reduction of cyclic S8 to high-order lithium polysulfides (Li2Sn, 4 < n < 8) [10,36], following Eqs. (1) and (2):

S8 þ 2Li0 Li2 S8 ;

(1)

Li2 S8 þ 2Li0 2Li2 S4 :

(2)

The low or second plateau at approximately 1.92 V corresponds to the further reduction of these polysulfides to the low-order lithium polysulfides (such as Li2S2, Li2S) [11,30], according to Eqs. (3) and (4):

Li2 S4 þ 2Li0 2Li2 S2 ;

(3)

Li2 S2 þ 2Li0 2Li2 S:

(4)

The single peak located at approximately 2.38 V in the following anodic scan is related to the reverse process, i.e., to the conversion of Li2S into the high-order lithium polysulfide and S8 [7]. The high-order polysulfides are prone to diffuse out of cathode, which leads to an irreversible loss of active materials and deterioration of cyclability. Thus, the dominate contribution to the rechargeable capacity of LieS batteries mainly originates from the second plateau related to the formation of insoluble low-order polysulfides. Obviously, the low plateau for carbon/S is more flat and longer than that of pristine S. This finding indicates the high capacity and improved reversibility for spirulina-based carbon/S cathode. The improvement of dischargeecharge properties for LieS batteries using spirulina-derived carbon/S cathode implied that carbon from spirulina can act as the hosts for sulphur particles. This feature can also effectively prevent the dissolution of polysulfides. EIS was conducted to analyze the conductivity of pristine S and carbon/S composite cathode. All spectra show a semicircle in a high

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Fig. 6. Electrochemical impedance spectroscopy of carbon/S composite cathodes of the LieS batteries.

frequency region and an inclined line in low frequency region as shown in Fig. 6. As known, the diameter of the semicircle corresponds to the contact resistance and charge transfer resistance at the interface. Importantly, carbon/S exhibits a lower semicircle diameter than pristine S. This phenomenon indicates an improved lithium ionic conductivity in carbon/S framework owing to the presence of spirulina-derived and N-doped carbon. The improved cycle performance of the spirulina-derived carbon/S cathodes is mainly attributed not only to the encapsulation by carbon of sulphur particles but also importantly to the presence of nitrogen atoms in carbon lattice. Nitrogen, especially pyridinic-N, can provide a strong binding with polysulfides through NeS bonds for trapping polysulfide species [29,30,37e39]. The tight adsorption of Li2Sn on carbon surface caused by the N doping in carbon lattice can help avoiding its dissolution into electrolyte thus promoting a more significant participation of active materials into the redox reaction [37e39]. This phenomenon can effectively prevent or at least alleviate the shuttle effect of polysulfides between the cathode and anode. Therefore, the cyclability and coulombic efficiency of sulphur cathodes can be significantly increased with the aid of spirulina-derived carbon. 4. Conclusions A unique carbon/sulphur composite cathode was first synthesized for lithium-sulphur batteries based on a green type spirulina, where spirulina was carbonized as a porous and nitrogen-rich carbon source for encapsulating sulphur. The carbon/S composites showed a better electrochemical performance than pristine sulphur, delivering the initial discharge capacity of 662 mAh g
1 and achieved nearly 60% capacity retention after 100 cycles at a current density of 1 C (1670 mA g
1). The synergistic effect between the encapsulation of carbon for sulphur and strong binding through the doped N and lithium polysulfides probably decreases or prevents the shuttle effect of sulphur cathodes. This feasible strategy provides an ecofriendly pathway to produce LieS batteries on a large scale and at a low cost. Acknowledgments This research was financially supported by the Zhejiang Province Public Welfare Projects (Grant No. 2016C31108) and the National Natural Science Foundation of China (Grant No. NSFC 61376005, 51672063). The authors also gratefully acknowledge research funding supported by the Zhejiang Top Priority Discipline

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