Elsevier

Nano Energy

Volume 37, July 2017, Pages 7-14
Nano Energy

Full paper
Multifunctional Co3S4@sulfur nanotubes for enhanced lithium-sulfur battery performance

https://doi.org/10.1016/j.nanoen.2017.05.009Get rights and content

Highlights

  • A novel Co3S4 nanotube is developed to enhance the electrochemical properties of lithium sulfur batteries.

  • The nanotube structure of Co3S4 is able to accommodate sulfur and adsorb polysulfide species.

  • The metallic conductivity and catalytic properties of Co3S4 further enhance the redox kinetics of polysulfides.

Abstract

Lithium sulfur batteries attract the increasing attentions because of the high energy density. However, sulfur cathodes suffer from several scientific and technical issues which are related to polysulfide ion migration, low conductivity, and volume changes. Many strategies such as porous hosts, polysulfide adsorbents, catalyst, and conductive fillers and so on have been proposed to address these issues, separately. In this study, novel Co3S4 nanotubes are developed to efficiently host sulfur, adsorb polysulfide, and catalyze their conversion. Because of these multifunctional advantages in one structure, the resulting Co3S4@S nanotube electrodes demonstrate superior electrochemical properties for high performance lithium sulfur batteries.

Introduction

Lithium-sulfur (Li-S) batteries are receiving great attentions because of their high theoretical energy density (∼2600 Wh kg−1) [1], [2], low cost, environmental friendliness, and natural abundance of sulfur resources [3], [4], [5], [6]. Although Li-S batteries have many advantages, they suffer from several scientific and technical issues, which impede the practical implementation [7]. First, sulfur is an insulator with a very low conductivity of only 5×10−28 S m−1 [8], which limits the sulfur utilization and reduces the rate capability of Li-S batteries [9]. Second, the “shuttle effect” caused by polysulfides dissolution and diffusion decreases the specific capacity and Coulombic efficiency [10], [11], [12], [13], [14], [15]. Third, the volume changes during sulfur lithiation/delithiation may damage the cathode structure and lower cycling performance of Li-S batteries [16].

Recently, nanostructured carbons, such as meso/micro-porous carbons [17], [18], hollow carbon spheres [19], [20], graphene [21], [22], [23], carbon nanotubes [24], [25], [26], and nanofibers [27], [28], have been proposed to host sulfur materials. Because carbon is able to provide a rapid electron pathway and hollow structures physically trap polysulfides, the resulting carbon/sulfur composites prolong the cycle lives and increase the deliverable capacities [29]. However, carbon matrix is repellent to the polysulfides. During long-term cycling, sulfur species detach from the carbon matrix [30], [31]. The weak interaction between polysulfide species and carbon matrix raises the charge transfer resistance and reduces the redox kinetics of polysulfides [32], [33], [34]. Recently, it was reported that heteroatom doped carbon provides the abundant adsorption sites and strong chemisorption of polysulfides to address the issues [35]. Ti4O7 [30], NiFe2O4 [36], TiO2 [37], MnO2 [38], TinO2n-1 and some other metal oxides [39], [40], demonstrated the strong affinity to polysulfides and the high capacity retention when used in the cathodes of Li-S batteries. However, these metal oxides usually have relatively low electronic conductivity which reduces the electrode kinetics. To explore more conductive polysulfide adsorbents, the research attentions have been turned to transition metal sulfides because some of them usually have the relatively high electronic conductivity. As the absorbent and conducting phase, the sulfides must first have high bulk conductivity to facilitate charge transports through the interfaces and electrodes. More importantly, a continuous electronic network is necessary to improve the overall electrode conductivity. Fiber or whisker-like morphology has a low percolation threshold to form a continuous conducting network. Thus, the conductive absorbents with high aspect ratios and hollow structures are highly desired for sulfur cathodes.

Since the charged and discharged products of sulfur are insoluble in the non-aqueous electrolytes and only the soluble polysulfide intermediates are mobile between cathodes and anodes, a rapid and catalytic conversion of sulfur species may have the same consequence as suppressing the shuttle effects and confining the sulfides inside cathodes by using hollow hosts. Nickel sulfide prepared by ball milling nickel and sulfur were first found to be catalytic for Li-S redox reactions [41]. Pt, Al, Ni, metal oxides, and heteratom-doped carbon have been explored to catalyze the Li-S redox reactions [42], [43], [44], [45], [46], [47]. To further enhance the conversion kinetics of redox shuttles by electrocatalysts, the polysulfide anions must at best be chemically entrapped by the functional groups of catalyst materials and physically confined by structured hosts.

Recent reports about sulfides absorbents found that cobalt sulfides (CoS2 and Co9S8) have the strong affinity to sulfur species [48], [49]. Especially, CoS2 exhibits the good catalytic properties for the sulfur species conversion [48], [50], [51]. There are five intermediate phases (Co4S3±y, Co9S8, Co1−yS, Co3S4, and CoS2) in the Co-S binary systems [52]. Co4S3±y and Co1−yS are stable only at high temperatures. CoS2 (6.7×105 S m−1) [53] and Co3S4 (3.3×105 S m−1) [54] have much higher conductivity than Co9S8 (1.36 S m−1) [55]. Earlier research reported that Co3S4 has 2–3 times the electrocatalytic capability of CoS2 for oxygen reduction reactions [56]. Co3S4 has not been studied for catalyzing the conversion of sulfur species which is in the same group as oxygen in the periodic table. It intrigues us to tentatively explore what influences the catalytic, morphologic, and conducting properties of spinel Co3S4 have on Li-S batteries.

In this contribution, we developed a facile route to produce Co3S4@S nanotubes for high-performance Li-S batteries. As shown in Fig. 1a, the catalytic, morphologic, and conducting properties of polysulfide adsorbents/hosts are considered together to enhance the electrochemical properties of Li-S batteries. Nanostructured Co3S4 aims to absorb and catalyze the sulfur species by the relatively large surface area. The nanotube morphology helps to host sulfur species. The metallic conductivity of Co3S4 accelerates the kinetics. Due to these designs, the Co3S4@S nanotubes cathode is able to deliver a capacity of 1267 mA h g−1 AS (active sulfur basis) at 0.05 C. It shows a slow capacity decay rate of 0.041% per cycle through 1000 cycles, which significantly improves the electrochemical properties of Li-S batteries.

Section snippets

Synthesis of Co3S4 nanotubes

All the chemicals were purchased from Sinopharm Chemical Reagent Corporation and used without further purification. About 4 mmol of CoCl2·6H2O and 20 mmol of CO(NH2)2 were dissolved in 50 mL deionized water. The solution obtained was transferred to a Teflon-lined stainless-steel autoclave and heated at 95 °C for 8 h. After cooling down to room temperature, the precursor precipitates were filtered, rinsed, and dried in vacuum. The dry powder (~0.072 g) obtained was added into 1 M thioacetamide solution

Results and discussion

The fabrication procedure is illustrated in Fig. 1b. The hydrothermal treatment of an aqueous urea and CoCl2 solution leads to the intermediate precipitates with the nano-needle morphology. Fig. 2a and b show that the nano-needles diameter is about 80–90 nm. The XRD pattern in Fig. 2g identifies them as Co(CO3)0.35Cl0.20(OH)1.10 (JCPDS card no. 38–547). After a second hydrothermal treatment with thioacetamide (Fig. 1b), the nano-needles are converted to nanotubes due to the Kirkendall effect.

Conclusions

We have proposed the use of multifunctional Co3S4 nanotubes as sulfur host with a high sulfur loading and good electrochemical properties. Due to the high electron conductivity and polysulfide affinity of Co3S4, the specific capacity and rate capabilities of Co3S4@S nanotube electrodes are significantly improved. The high affinity between Co3S4 and polysulfides minimize the polysulfide ions dissolution and increase the cyclability of the sulfur composite electrodes. In addition, Co3S4 nanotubes

Acknowledgements

This work is supported by the Thousand Youth Talents Plan (No. 128010), Jiangsu Outstanding Youth Funds (BK20160012), “Jiangsu Shuangchuang” Program, and Nantong Fundamental Research Funds (GY12016040). The numerical calculations in this paper have been done on the computing facilities in the High Performance Computing Center (HPCC) of Nanjing University.

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