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Encapsulating MWNTs into Hollow Porous Carbon Nanotubes: A Tube-in-Tube Carbon Nanostructure for High-Performance Lithium-Sulfur Batteries

Introduction

Generally, capturing energy to utilize it later known as energy storage technology. The electrical energy storage has been one of the hottest topics in past decades with various applications including military power supplies, transportation, portable electronic devices and storage station [1]. There is no doubt that energy storage demand is increasing rapidly and there is a broad research in this area to increase the capacity of energy storage systems [2].

One of the common methods for energy storage is batteries. The battery is a device that can converts chemical energy directly into electrical energy. Some types of batteries are also capable of reverse procedure (converting electrical energy to chemical energy), which are called rechargeable batteries [3]. This types of batteries are commonly used in our portable electronic devices such as cell phones. Due to their rechargeable ability they can be charged/discharged multiple times, which makes them excellent candidate for electrical energy storage applications.

Basically, rechargeable batteries are made up of three different components including anode, cathode and electrolyte in which anode and cathode are separated by electrolyte [3]. The chemical potential difference between the anode and cathode (electrodes) makes the electrons flow out of the anode (oxidation reaction) through an external circuit, which can be collected by cathode (reduction reaction) until battery is fully discharged [4]. By applying external voltage and enforcing electrons flow back to anode the battery is charged aging and preserving some energy. Two main important parameters of rechargeable batteries are energy capacity and life cycle. Energy capacity means how much energy can be saved per unit volume or weight and life cycle refers to number of times that a battery can be charged/discharged.

Lithium-Sulfur Battery Background

There are different types of rechargeable batteries such as lead-acid, nickel-cadmium, Lithium-ion polymer and etc. [5]. One of the recently noticeable rechargeable batteries is Lithium-Sulfur (Li-S) [6], [7]. The Li-S batteries have shown a promising theoretical potential to fulfil electrical energy storage demands due to their high capability of storage capacity. Theoretical calculation based on Li anode and sulfur cathode revealed energy density of 2600 Wh Kg^-1, which is 3-5 times higher than commercial Li-ion batteries [6]. In addition, sulfur is non-toxic abundant material which means it is inexpensive and environmental friendly [8]. By considering all aforementioned qualities, Li-S batteries are a suitable candidate for next generation batteries for high energy storage applications such as electric vehicles.

However, commercial Li-S batteries have not been achieved yet, since they suffer from different practical challenges [8], [9], [10]. The electrochemical performance of Li-S batteries is still poor due to three main problems. First is the low electrical conductivity of sulfur and its discharged products [9], [10]. Good electrode contact is essential for high performance rate. This issue has been partially solved by mixing sulfur with substantial amount of conductive material such as carbon or metals [11]. The second problem corresponds to dissolution of polysulfides intermediates (Li2Sn , 3 ≤ n ≤ 6 ) in organic electrolyte, which can shuttle between cathode and anode [8],[9]. This is called shuttle effect and not only remarkably reduces the discharge/charge capacity but also resulted in internal current and subsequently battery self-discharge [12]. The third obstacle is after complete discharge polysulfides in solution break down to insoluble Li₂S₂/Li₂S that can be deposited on electrode surface and leads to low energy efficiency [8], [9]. General solution for these two last problem is to make polysulfides more stable and preserved within cathode to reduce the shuttle effect. There are different methods to be addressed such as electrolyte additives to inhibit polysulfides shuttle reaction at anode, encapsulation the polysulfides within cathode, utilizing porous material to slow down the diffusion and solid electrolyte that allows lithium pass only [13]. Moreover, the high volumetric change (about 80%) between charged and discharged physical structure makes final product physically unstable with a short life time.

Research Motivation

Since 2009, when Nazar et al reported interesting improvement in Li-S batteries by increasing life cycle, Li-S batteries have been received huge attraction in energy storage research area [6]. Since then the number of research all over the world has been increased and researchers have been working in Li-S batteries to overcome its challenges due to its high potential capacity.

As mentioned Li-S batteries suffer from sulfur low conductivity and dissolution of polysulfide intermediates. In order to overcome these issues so many methods and techniques have been investigated. One of the most common techniques is to encapsulate sulfur into porous carbons to not only prevent the dissolution of intermediates but also increase conductivity [14], [15]. In this strategy porous carbon acts as a host material for sulfur adsorbing. Due to its high specific surface area, it can provide good transportation between sulfur and the host material while can prevent polysulfide dissolving in electrolyte. High storage capacity and good life cycle can be achieved by performing this technique [16]. There has been some research on carbon nanotubes (CNTs) and graphene as a host material but they relatively small specific surface area does not provide good impregnation and also can not prevent dissolution of polysulfide due to the poor porosity of CNTs.

In this method, two factors are necessary to be consider for host material designing in order to obtain high-performance hybrid sulfur-carbon electrode.  One is highly conductive carbon martials and the other one is a good porosity. In 2012 Xin et al. group applied a new version of this technique by synthesizing a hybrid carbon nanomaterial on a multi-walled carbon nanotubes (MWNTs) which resulted in considerable improvement in Li-S batteries with good cycling stability [17]. However, energy density of this hybrid system was not high enough due to low sulfur content (40–60 wt %). Increasing sulfur content is highly desirable in this kind of S-hybrid systems.

Experiment Design

In this report a novel technique has been introduced by Guan research group from China. They have reported carbon based tube-in-tube structure (TTCN) as a host material for sulfur cathode. They have claimed this specific structure can potentially overcome all the challenges that Li-S batteries are faced to them by good conductivity between sulfur and host materials, prevention of polysulfides intermediates dissolution and improvement of sulfur content impregnation by providing a large accommodation space.

Figure 1 illustrates the synthesis schematic of sulfur impregnated TTCN (S-TTCN) composite as a cathode electrode. Frist the MWNTs sample with diameter of 20-50 nm was provided (purchased). Synthesis process initiated by nitric acid treatment. Afterwards, MWNTs were coated by SiO₂ layer followed by porous SiO₂ layer deposition on top of it. Next octadecyltrimethoxysilane (C18TMS) was introduced to the composite to be trapped within porous structure that is shown in 1th steps in the Figure 1. Afterwards, composite were chemically treated and then high temperature calcinations were preformed, to convert the C18TMS into carbon, 2th step in figure 1.  Then SiO2 layers was etched by NaOH solution,  to achieve the TTCN composite, with MWNTs surrounded by porous carbon nanotubes, 3th step in figure 1.  Finally, sulfur was impregnated to the composite by melt infiltration method, step number 4.

Result and Discussion 

The evolution of composite’s morphology and structure has been done at different synthesis steps in order to make sure composite processed in the right way and also to measure some important parameters such as porous thickness and hollow space between MWNT/S-TTCN. First evaluation was performed by SEM and TEM on TTCN composite. Figure 2a illustrates the SEM image of initial MWNTs with diameters around 20–50 nm with several micrometer lengths. Comparing it with Figure 2b, which is SEM image of TTCN, shows same morphology as MWNT with thicker diameter of 80-140 nm. High resolution TEM (Figure 2c and 2d) inspection determined MWNT encapsulation and hollow space formation between porous carbon shell and MWNT core. The porous carbon layer thickness was measured with approximately 10-13 nm. An interesting capability of this method is that internal hollow space and porous carbon shell thickness can be adjusted by changing the thickness of coated SiO2 and amount of C18TMS respectively. In this work internal hollow space with 0.34 nm radial thickness was obtained as shown in Figure 2e. A large hollow space is desirable in these method. The hollow space act as an accommodation at sulfur impregnation, lager space result in more sulfur content loading and subsequently improvement in energy density of Li-S batteries. Figure 2f shows TTCN after sulfur impregnation, and final S-TTCN composition formation.

Comparing Figure 2f (S-TTCN TEM image) and Figure 2c (TTCN TEM image) revealed morphology was kept same after sulfur encapsulation and all the impregnated sulfur was fully diffused into porous carbon shell. This is really important that there are not any sulfur particles outside of the composition since they can be easily contact with electrolyte, dissolve and shuttle between electrodes, which will be significantly affect the battery cycling performance. Scanning transmission electron microscopy (STEM) with corresponding element mapping inspection was performed for further investigation on sulfur distribution.  Figure 2h and Figure 2i show carbon and sulfur element mapping respectively. These images imply sulfur homogeneous distribution within the TTCN host and not outside of it.

In order to evaluate TTCN and S-TTCN composites more in details, nitrogen adsorption-desorption isotherm measurement was performed, which revealed TTCN surface area of 822.8 m2g1 with total pore volume of 1.77 m3g1 as Figure 3a shows. This values were higher than sulfur hybrid structure formed by other methods. Also has been observed micropores at 1.8 nm, mesopores at 3.6 nm, Figure 3b, with pore distribution from 20 to 60 nm. This size of pores can potentially hamper polysulfide dissolution and improve battery’s cycle life. Same measurement was done on S-TTNC revealed 5.6 m2g1 and 0.19 m3g1 for surface area and pore volume, respectively with mesoporous peak at 20 nm approximately. It was a solid indication that sulfur fully accommodated into TTCN’s pores, while some hollow space between MTWN core and TTCN shell was empty. This remaining empty spaces can be beneficial to tolerate structure volume change during charge and discharge cycle.

The sulfur total weight content was determined by thermogravimetric analysis (TGA) at temperature 30-700 C with 10 C increment per minute in N2 environment. High value of 71%wt was obtained as a sulfur loading ratio, Figure 3c. In addition, sulfur evaporation was occurred from 200 ºC to 400 C, which  indicates a strong absorption between sulfur and TTCN host. Figure 3d illustrates the X-ray diffraction (XRD) analysis on both TTCN and S-TTCN composites. Comparing both results determined that sulfur accommodated within internal hollow space in crystalline state. After S-TTCN composite characterization, its performance was evaluated by electrochemical measurement as a cathode for Li-S batteries. The 80 wt % of final cathode component was formed by S-TTCN. Measurement was done at 500 mA g1 between 1.95 V and 2.7, figure 4a shows the first normal discharge/charge profile. There are two obvious plateaus in the first discharge graph, which is typical behavior for Li-S battery’s stepwise discharge. The first plateau was related to S8 reduction to electrolyte-soluble polysulfide Li2S4-8 at approximately 2.3. The second plateau was related to the transformation of Li2S4 to insoluble Li2S2 and finally Li2S when battery was completely discharged. The long plateau in charge process graph was related to the reverse reaction from Li2Sn (S 2) to the final S8 formation. The specific capacity of 1274 and 1264 mAh g1 was obtained in first discharge and charge cycles respectively, which are approximately 75 % of theoretical specific capacity of 1672 mAh g1. The life cycle performance of S-TTCN cathode at current density of 500 mA g1 is shown in Figure 4b. The charge/discharge process was done for cycling stability investigation and the S-TTCN electrode showed excellent performance with discharge capacity of 918 mAh g1 after 50 cycles, which was higher than to values from other s-hybrid formation methods such as microprous/mesoporous carbon [18], hollow carbon spheres [19], carbon nanofibers/nanotubes and hybrid carbons [20]. This can actually be contributed to the polysulfide dissolution prevention by porous carbon shell.

Figure 4c shows the discharge/charge performance of the S-TTCN electrode under different current densities from 0.5 A g⁻¹ to 6 A g⁻¹. As can be observed all the profiles showed similar behavior, even the typical reaction plateaus can be observed at high rates. The rate performance of the S-TTCN electrode was investigated by stabilizing discharge capacity approximately at 800, 750, 650, and 550 mAh g⁻¹ for different cycles at 1, 2, 4, and 6 A g⁻¹ respectively, as Figure 4d shows. The discharged capacity of 850 mAh g⁻¹ was returned back when the current density was dropped back to 500 mA g⁻¹, which is good demonstrates of excellent rate performance of the S-TTCN composite due to its good stability.

Long-term discharge/charge cycle was tested over 200 times at high current densities of 2 A g⁻¹ in order to investigate S-TTCN composite stability. Figure 4e shows the discharge/charge profiles as it can be seen there was small capacity loss, but overly demonstrated stable voltage. Figure 4f illustrates long term cycling performance for 200 cycles, high capacity of 647 mAh g⁻¹ was obtained after 200 cycles which is good indication of Li-S performance improvement.

Comments

The Guan research group has been shown a great improvement in Li-S batteries performance and there is no doubt about the merit of this work and the novelty that has been introduced in this work. However, there are some issues about this research and its procedure that should have been taken into account in order to make more solid evidence about the potential capability of this work for new generation of commercial Li-S batteries.

One of the main issues that can be addressed here is the conductivity improvement, which final results of capacity measurements indicated that there was a good conductivity between sulfur and TTCN composite. The MWNT structure has been chosen since it shown good improvement in conductivity by other groups. However, in this work no any specific investigation was performed to measure actual conductivity for synthesized S-TTCN composite. This could have been shown a solid determination about good conductivity of such a structure which is necessary for high performance Li-S batteries.

Another issue that should have been consider is the porous carbon thikness and hollow space sizes. As it was mentioned by researcher in this work one the benefits of this novel method is capability of changing the thickness of porous carbon shell and void space between MWNT core and porous carbon shell by adjusting thickness of coated SiO2 and amount of C18TMS respectively. Although they have mentioned this capability but there is no any experiment or investigation in this regard. They have shown one single final structure without any shell and void space optimization. The porous carbon shell played an important role in this work that act as an encapsulation layer to prevent polysulfide dissolving into electrolyte. So its thickness and pores size might have a noticeable effect on shuttle effect elimination for instance thicker shell with smaller pores size might resulted in better sulfur encapsulation or might block Li ion to meet the sulfur for electrochemical reaction. In addition, the internal hollow space thickness can directly affect on amount of sulfur content that can be impregnated to the TTCN composite, thicker first SiO2 coating leads to larger void space and subsequently higher sulfur loading. Higher sulfur content is desirable as it will increase battery energy capacity. They could have done an experimental investigation on sulfur impregnation by coating different thicknesses of SiO2 layer to figure out how high sulfur could be loaded to the composite. Likewise, the minimum required void space that could tolerate volumetric change during discharge/charge cycles. The porous carbon and SiO2 thickness optimization could have been done in this work.

The last issue that worth to mention is stability performance. High cycling stability is one the crucial parameters for energy storage applications and Li-S batteries are suffering from it. High and stable life cycle with capability of thousands time of discharge/charge cycles is required in order to produce commercial Li-S batteries (specially as a storage for electrical vehicles). In contradiction, life cycle investigation was done up to 200 times. Even though it was a good improvement in comparison to pervious repots but they could have evaluated their novel S-hybrid composite for longer period to find falling point. Also, In-Situ monitoring during discharge/charge cycles could have been used for better understating of the internal reactions between Li and sulfur particularly in this kind of structure.

Conclusion

In conclusion, a carbon nanostructure with a core of multi-walled nano tube surrounding with porous carbon nanotubes shell was synthesized and impregnated with sulfur. The final composite was investigated by different techniques and finally its electrochemical performance was tested in order to evaluate its practical potential to be used as a cathode for commercial Li-S batteries. This novel structure has been shown an excellent performance with capability of reserving of 918 mAh g⁻¹ at 500 mA g⁻¹ and 647 mAh g⁻¹ at 2 A g⁻¹ after 50 and 200 cycles respectively. Even the capacity of 550 mAh g⁻¹ was obtained at high rate current density of 6 A g⁻¹. This excellent electrochemical performance of S-TTCN composite was attributed to three main reasons. First, great electrical conductivity of sulfur was achieved as result of using encapsulating sulfur between one dimensional MWNTs core and pours carbon shell and resulted in good rate capability. The second reason was the presence of porous carbon layer that behaved as a perfect outer shell to prevent soluble lithium polysulfide product from dissolving and subsequently prevented polysulfides from shuttling between electrodes. This specific encapsulation feature that was provided by this new sulfur hybrid structure resulted in life cycle improvement with a high efficiency. The last reason was sulfur load content was increased due to the large hollow space that was provided by TTCN composite structure, which led to energy density improvement. In addition, sulfur was not fully accommodated and some void space was remained that resulted in more physical stability of S-TTCN since high volumetric changes of sulfur during discharge/charge cycle could be tolerated. This work generally introduced a novel method to synthesize sulfur hybrid cathode based on porous carbon nanotube with higher sulfur content and better polysulfides dissolving prevention. The outstanding results of the work proved the potential of this promising hybrid structure to achieve high performance Li-S batteries. 

References 

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