Ultrafine TiO2 Decorated Carbon Nanofibers as Multifunctional

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Ultrafine TiO2 decorated carbon nanofibers as multifunctional interlayer for high performance lithium-sulfur battery Gemeng Liang, Junxiong Wu, Xianying Qin, Ming Liu, Qing Li, Yan-Bing He, Jang-Kyo Kim, Baohua Li, and Feiyu Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07487 • Publication Date (Web): 10 Aug 2016 Downloaded from http://pubs.acs.org on August 15, 2016

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ACS Applied Materials & Interfaces

Ultrafine TiO2 decorated carbon nanofibers as multifunctional interlayer for high performance lithiumsulfur battery Gemeng Liang,†,‡ Junxiong Wu,†,‡ Xianying Qin,†, §* Ming Liu,†,‡ Qing Li,†,‡ Yan-Bing He,† JangKyo Kim,§ Baohua Li,†,* and Feiyu Kang†, ‡ †

Engineering Laboratory for Next Generation Power and Energy Storage Batteries, and

Engineering Laboratory for Functionalized Carbon Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China ‡

School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

§

Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science

and Technology, Clear Water Bay, Kowloon, Hong Kong KEYWORDS: TiO2; Carbon nanofibers; Interlayer; Electrochemical properties; Lithium-sulfur batteries

ABSTRACT: Although lithium-sulfur (Li-S) batteries deliver high specific energy densities, lots of intrinsic and fatal obstacles still restrict their practical application. The electrospun carbon nanofibers (CNFs) decorated with ultrafine TiO2 nanoparticles (CNF-T) was prepared and used as multi-functional interlayer to suppress the volume expansion and shuttle effect of Li-S battery.

ACS Paragon Plus Environment

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With this strategy, the CNF network with abundant space and superior conductivity can accommodate and recycle the dissolved polysulfides for the bare sulfur cathode. Meanwhile, the ultrafine TiO2 nanoparticles on CNFs work as anchoring points to capture the polysulfides with the strong interaction, making the battery perform remarkable and stable electrochemical properties. As a result, the Li-S battery with the CNF-T interlayer delivers an initial reversible capacity of 935 mAh g-1 at 1C with a capacity retention of 74.2% after 500 cycles. It is believed that this simple, low-cost and scalable method will definitely bring a novel perspective on the practical utilization of Li-S batteries.

INTRODUCTION Nowadays with the ever-increasing demand of high capacity batteries, more and more attention has been paid on the lithium-sulfur (Li-S) battery for its high energy density.1-2 Sulfur cathode has a high theoretical capacity of 1675 mAh g-1 with a satisfactory operating voltage of 2.1V.3 Furthermore, other merits of environmental friendliness and natural abundance make sulfur the most promising candidate to address the energy storage and sustainable development issues.4 However, the commercialization of Li-S battery is hampered by several severe problems.4-6 First, the inherent poor conductivity of sulfur (5*10-30 S cm-1 at room temperature) 6 inevitably causes the increasing internal resistance and retards the utilization of sulfur in the cathode. Another trouble is the volume change (~80%) occurring in the charge/discharge process, which results in the pulverization of the cathode and loses the electronic contact of sulfur, leading to an even faster capacity fading. Last but not least, there have been increasing concerns and researches on the intermediate polysulfides (Li2Sx, 4≤x≤8) produced during the lithiation procedure, which dissolve in the organic electrolytes and migrate between the anode and cathode known as the


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shuttle effect. The annoying migration eventually results in rapid capacity decay and low Coulombic efficiency. As a result, all these harmful factors lead to the poor performance in either cycle stability or rate capability. Numerous attempts have been made to enhance the electrochemical performance of Li-S batteries. The carbonaceous materials such as porous carbon7-8, carbon nanotube9-10, graphene1113

, carbon fibers14, and so on, were firstly thought of for their superior conductivity and physical

adsorption for the dissolved polysulfides. For example, Nazar et al. suggested a way to impregnate sulfur into the ordered mesoporous carbon spheres.7 This exquisite and delicate design endowed the battery with an excellent electrochemical performance even with an increasing sulfur content. Although incorporating sulfur with different kinds of carbon solved the problem of insulation and migration to some extent, the addition of carbon matrix definitely decreases the content of sulfur in the cathode, resulting in relatively low energy density in the practical application.15 Moreover, the complicated and high-cost fabrication of the ingenious carbon matrix would also limit the application range of the sulfur/carbon battery.13 The basic principle for the battery design should always be high mass loading of the sulfur along with a facile fabrication, which is exactly the remarkable benefit of the traditional cathode. Considering on this fundamental, more and more attention has been paid on the bare sulfur cathode in the recent years. Owing to the severe shuttle effect in the bare sulfur battery, a novel battery design is eagerly needed to inhibit the polysulfides from shuttling between the cathode and Li foil.16 Manthiram et al. first raised a novel strategy of inserting a carbon paper interlayer between the separator and cathode, where the interlayer not only improved the conductivity of the integrate cathode but also effectively intercepted the transferring polysulfides.17 Subsequently, the trapped polysulfides could be oxidized and reused by the massive electrons in the carbon interlayer,


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leading to enhanced capacity and rate performance by this simple way. Later on, other than carbon paper, lots of materials such as reduced graphene oxide film18, nickel foam19, MWCNT wafer20, carbon nanofiber21, biomass material22, and so on were continually introduced into the batteries as block interlayers to restrain the immigration via the physical interaction. However, the enhancement derived from the interlayers mentioned above was not good enough for the practical application due to the weak interaction between the non-polar matrix and the polysulfides.23 Surprisingly, it has been recently found that metal oxides (TiO2, MnO2, MgO, et al.) have strong adsorption for the polysulfides, which is attributed to the intense electrostatic attraction between metal-oxygen bond and polysulfides.24-26 Compared with other metal oxides like MnO227 and La2O328, TiO2 is more accessible due to its relatively low cost. Xiao et al. reported an enhanced performance of Li-S battery using a graphene/TiO2 composite interlayer, where polysulfides were blocked and captured by this interlayer with both physical block of graphene and chemical bonding of TiO2.26 Recently, Xu et al. reported that carbon-nanotube paper interlayer combined with the TiO2 could improve the performance of the lithium sulfur battery. Unfortunately, the complex procedure and harsh condition mentioned restrained its wide application.29 More recently, Hwang et al. reported a novel structure of a cathode consisting of sulfur impregnating into the hollow mesoporous TiO2 spheres, along with a porous carbon interlayer at the same time. The performance was also improved to some extent by the respective modification of both cathode and interlayer. However, the use of hydrofluoric acid in the interlayer fabrication was unquestionably unfavorable let alone the complicated and troublesome manufacturing procedure.30 Up to now, the large-scale and low-cost fabrication of interlayers composited with these metal oxides remains to be a question before its wide application.


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Herein, we developed a facile and scalable way to manufacture a multi-functional interlayer by using electrospun carbon nanofibers (CNFs) decorated with a uniform TiO2 coating shell. Figure 1 shows the schematic structure of the battery with/without the interlayer and the TiO2 decorated CNF interlayer (named as CNF-T interlayer, in the following section) as well. The electrospun CNF film with superior conductivity was obtained after carbonization at a higher temperature, which would be employed as conductive skeleton to reduce the internal charge transfer resistance. Meanwhile the non-woven CNFs, unlike the tightly stacked graphene flakes, have abundant channel and space to facilitate ion transfer through the interlayer and to accommodate the volume expansion of sulfur cathode by storing the polysilfides. Furthermore, the ultrafine TiO2 nanoparticles coated on the CNFs act as anchors to capture the dissolved polysulfides in the electrolyte through intense chemical adsorption.26,31,32 This selective permeable interlayer would dramatically suppress the shuttle action of the dissolved polysulfides without sacrificing the fast passing-through of the lithium ion at the same time. As a result, the battery with a CNF-T interlayer can still deliver a high capacity of 694 mAh g-1 at a current density of 1 C even after 500 cycles, which is remarkably enhanced comparing to the bare sulfur battery. This simple, low-cost, and scalable method will definitely bring a novel perspective on the practical utilization of Li-S batteries.

Figure 1. Schematic configuration of Li-S batteries without/with CNF-T interlayer.


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EXPERIMENTAL SECTION Material Preparation CNFs were obtained based on the electrospinning technique and subsequent heat treatment. In a typical procedure, the polyacrylonitrile (PAN, Mw = 130,000, Donghua University) solution with concentration of 5 wt% for electrospinning was prepared by stirring and dissolving PAN powder in N, N-dimethylformamide (DMF) at 75 oC for 3 h. The electrospinning process was carried out by a high voltage of 18 kV and needle-to-collector distance of 15 cm, together with solution flow rate of 1 mL h-1. After electrospinning, PAN nanofiber mat were carefully peeled off the aluminium foil. Afterwards, the fibers were stabilized through a step-to-step thermal treatment with an ultimate temperature of 280 oC for 1h. To get CNFs with satisfactory conductivity, the stabilized fibers were further carbonized in argon (Ar) atmosphere at 1100 oC for 5 h with a heating rate of 5 oC min−1. TiO2 coating on CNFs was obtained by a simple “dip and dry” method.33 The CNF mat was dipped into a 0.075 M titanium n-butoxide isopropanol solution and subsequently placed into an oven at 60 oC for 4 h to totally remove the isopropanol. The coating process was repeated for 3 times to get a perfect and uniform layer on each filament. Finally, the well-coated mat was annealed in argon atmosphere at 600 oC for 2 h to obtain a fixed decoration of TiO2 shell. The thickness of the CNF-T interlayer was measured about 35 µm with a weight of 0.5-0.6 mg cm-2, while the CNF interlayer is almost the same as the CNF-T interlayer. The conductivity of CNF and CNF-T interlayers was measured by four-point probe method and the conductivity of CNF was around 10.8 S m-1, while the one of the CNF-T interlayer was around 9.8 S m-1.


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Characterization The morphology and subtle structure of the interlayer were investigated by field emission scanning electron microscopy (FE-SEM, ZEISS Supra55) and high-resolution transmission electron microscope (HR-TEM, FEI TECNAIG2 F30). Energy-dispersive X-ray spectroscopy (EDS) was carried out to obtain the elemental mapping results. After 500-cycle test at 1C, the CNF-T and CNF interlayers were carefully recovered from the charged cells for the subsequent SEM and TEM investigations. X-ray diffraction (XRD) patterns of the interlayers were investigated on a Rigaku D/MAX 2500/PC diffractometer using Cu Kα radiation (λ = 0.154 nm). Raman spectra were obtained via a Raman spectroscope (HORIBA Labram HR Evolution) with a 532 nm Ar-ion laser. The TiO2 weight content in the CNF-T interlayer was measured by thermogravimetric analysis (TGA, NETZSCH STA449F3). The pore distribution and Brunauer– Emmett–Teller (BET) surface area of the CNF-T interlayer were measured with Micromeritcs ASAP 2020 analyzer. Electrochemical Measurements The bare sulfur cathode was prepared by the slurry-coating method. In a typical experiment, the slurry composed of 60 wt% sulfur, 30 wt% super P as conductive agent, and 10 wt% polyvinylidene fluoride (PVDF) as binder in N-methyl-2-pyrrolidone (NMP), was coated onto a carbon-coated aluminum foil and dried at 60 oC under vacuum for 12 h. The mass loading of sulfur in the obtained cathode is about 0.8 mg cm−2. CR2032 coin cells were assembled in an Arfilled glove box using Celgard 2250 film as the separator and lithium foil as the counter electrode. For comparison, the cells were modified by inserting CNF and CNF-T interlayers between bare sulfur cathode and separator, respectively. The electrolyte was consisted of 1M bis


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(trifluoromethane) sulfonamide lithium salt (LiTFSI) dissolved in a mixture of 1, 2-dioxolane (DOL) and dimethoxymethane (DME) (1:1 by volume) with 1 wt% LiNO3. The cells contained. The batteries were charged and discharged between 1.7 V and 2.8 V (vs Li/Li+) on a Land 2001A cell test system (Wuhan, China). The cyclic voltammetry (CV) experiments and electrochemical impedance spectroscopy (EIS) measurement were conducted on a VMP3 electrochemical workstation (Bio Logic Science Instruments). CV tests were carried out between 1.7 V and 2.8 V at a scan rate of 0.1 mV s-1, while EIS measurement was performed over a frequency range of 100 kHz -10 mHz with a disturbance amplitude of 5 mV RESULTS AND DISCUSSION Structure and Morphology The subtle structure and morphology of CNF and CNF-T interlayers were investigated by SEM and TEM as shown in Figure 2. It was shown that the smooth CNF woven with fiber diameter ranging from 100 nm to 300 nm closely stacked up into a porous network, which would buffer the volume expansion of the sulfur cathode during lithiation procedure. The porous CNF skeleton provides intersectional express pathways to allow the fast transit of electrons during the cycling process, which enables the batteries with an excellent rate capability. After coating TiO2, this advantageous structure was still intactly preserved, which is illustrated by Figure 2b. The TiO2 nanoparticles with an average diameter of approximately 15nm were aligned parallel to the smooth surface of CNFs (Figure 2b and c), forming a thin and uniform blocking sheath. The EDS elemental mapping of titanium, oxygen and carbon, as shown in Figure 2d, all presented the shape of regular fibers, certifying the consistent distribution of TiO2 outside the CNFs. It is worth mentioning that the flexibility of the CNF-T interlayer is fairly satisfactory. Even after several times of bending, the CNF-T can still remain integral as shown in Figure S1.This


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selective permeable interlayer, which resembles the cytomembrane, would block and capture polysulfides effectively but endow fast motion of lithium ions. Notably, due to the mini size of TiO2, lots of benefits were achieved. On one hand, the original smooth surface was changed into a slightly fluctuant one, making the CNFs contact with the electrolyte and polysulfides more intimately. On the other hand, the nano-sized TiO2 particle as reacting sites has an extremely high specific surface area, which gifts the interlayer mat superior chemical adsorption of the passing-by polysulfides upon its large specific area and strong chemical interaction with polysulfides.

Figure 2. SEM images of (a) CNFs and (b) CNFs-T; TEM image of (c) CNFs-T and (d) corresponding EDS mapping for elemental carbon, titanium and oxygen.


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The XRD patterns of CNF and CNF-T interlayers are shown in Figure 3a. The broad peak at about 25 degree in the two curves demonstrates the existence of non-graphic carbon known as the CNFs.34 Apart from this, the residual peaks entirely accords with the standard spectrum (JCPDS No.21-1272) and is indexed as the anatase phase of TiO2. The sharp tuber at 25.3 degree in the curve of CNF-T interlayer is the typical and strongest peak of anatase TiO2 and corresponds to its (101) facet, where the lattice fringe spacing is 0.352 nm, shown in the inset photo of Figure 2c.35 The low intensity of the peak demonstrates a small particle size of TiO2. According to the Scherrer equation, the crystal size was calculated at about 16 nm, which was coincident with the observation in both SEM and TEM images. As displayed in the Raman image, the CNF-T interlayer exhibits almost integrated spectral characteristics of CNFs and anatase TiO2. The two typical peaks at around 1346 and 1575 cm-1 are known as D and G band respectively, where D band is linked with the vibrations of the crystal border and G band represents the perfect sp2 vibrations of graphic crystal.36 Before 1000 cm-1, there are five peaks in the curve of CNF-T at 144, 198, 397, 518 and 640 cm-1, respectively. The strongest low Raman shift band at 144 cm-1 is a typical characteristic feature of anatase TiO2 and the other four peaks relate to the Raman active phonon modes of anatase TiO2, corresponding with the conclusion made from the XRD test.37 The TG investigation was carried out to determine the weight content of anatase TiO2 in the CNF-T interlayer. As demonstrated in Figure 3c, the remainder after the heating process was regarded as the anatase TiO2, which accounts for 27.9 wt% of the whole (26.67 / (68.83 + 26.67) * 100% = 27.9 wt%). Notably, the exact content of TiO2 can be easily decided on the definite condition since the addition of TiO2 is simple and facile, undoubtedly making it more appealing. The pore size distribution data (Figure 3d) demonstrates that the CNF-T interlayer, similar to the CNF wafer, owns hierarchical pores. The


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inset BET result also shows that the CNF-T interlayer has a specific surface area of 217.98 m2 g1

, while the CNF mat only has a value of 180 m2 g-1 (Figure S2). The increased specific surface

area is mainly attributed to that the TiO2 nanoparticles anchoring on the CNFs make the fiber surface scraggly. The hierarchical porous structure and larger specific area are benificial for the infiltration of electrolyte and deposition of polysulfides. The desired improvement is attributed to the ultrafine TiO2 nanoparticles, providing a more intimate contact with the electrolyte and polysulfides.

Figure 3. (a) XRD patterns of CNFs and CNFs-T; (b) Raman spectra of pure anatase TiO2, CNFs and CNFs-T; (c) TGA data of CNFs-T; (d) Pore size distribution and N2 adsorption and desorption isotherms (the inset photo) of CNFs-T.


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Electrochemical Properties Cyclic voltammongrams (CVs) of the Li-S battery with a CNF-T interlayer are shown in Figure 4a. In every cathodic scan, there are two prominent peaks appearing at 2.2 V to 2.3 V (Peak I) and at approximately 2.0 V (Peak II), respectively. The Peak I is known to be associated with the transformation from the cyclo-S to the polysulfides (Li2Sx, 4≤x≤8) which dissolve in the electrolyte. The Peak II usually accounts for the majority of the specific capacity, during which the polysulfides produced in the former procedure are reduced into Li2S2 and Li2S. At 2.3 V to 2.5 V, two typical anodic peaks (Peak III and Peak IV) were presented, where the insoluble Li2S changed into the polysulfides (Peak III) and eventually the element S (Peak IV).38 It was found that the cathodic peaks shift slightly after the first scan. The reason accounting for the location change might be the rearrangement of orthogonal sulfur from the origin place to a more stable site, known as the activation process.22 For the following cycles, no obvious change in the shape and location occurs, indicating a desired superior electrochemical stability. Figure 4b shows the cycle stability for three kinds of Li-S batteries (bare sulfur, with CNF and CNF-T interlayers) measured at a relatively low current density (0.2 C, 1 C = 1675 mA g-1). The first three cycles was set at 0.1 C as an activity process to make full use of the sulfur in the cathode. It is worth emphasizing that the shuttle effect might be severe if tested at the tiny current, due to the enough time for polysulfides to dissolve in the electrolyte and migrate towards the anode. Accordingly, the current of 0.2 C might be a better choice to evaluate the inhibiting effect. It was noted that the bare sulfur electrode without interlayer only delivered an initial discharge capacity of 1054 mAh g-1, meaning a merely 62.9% utilization. While, the batteries


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with CNF and CNF-T interlayers could severally reach the values of 1100 and 1328 mAh g-1, corresponding to sulfur utilization of 65.7% and 79.3%, respectively. The remarkable improvement in initial discharge capacity for CNF-T modified battery is attributed to the 3D conductive CNF network decorated TiO2 sheath, which provides accessible electron contact and strong adsorption for the polysulfides drifting away the cathode. It is obvious that the bare sulfur battery suffered a severe capacity decay, fading from 1054 to 110 mAh g-1 just within 500 cycles. After inserting a CNF interlayer, the decrease has been alleviated to some content, with a decay rate of 0.147% for the 500 cycles. However, when the CNF-T interlayer was added, the battery manifested a surprisingly satisfactory performance. Even after 500 cycles at 0.2C, the battery could still retain a capacity of 520 mAh g-1 with a decay rate of only 0.121%. The dramatic improvement can be attributed to as follows: (i) The conductive framework of CNFs solved the problem of the low electrical conductivity of sulfur. (ii) At same time, the CNFs acted like an expansion buffer zone due to its elastic structure with large porosity, preventing the cathode from further fracture. (iii) Most importantly, the ultrafine TiO2 fastened on the surface of CNFs could indeed trap the polysulfides and reuse the dissolved active material during the continual cycling. Meanwhile, the cycle stability of the CNF-T modified batteries at a higher current of 1 C (after activation at 0.1 C for the first three cycles) was shown in Figure 4c. A final reversible capacity of 694 mAh g-1 with a capacity retention of 74.2% was obtained after 500 test cycles, which is high enough for the practical usage. Notably, the Coulombic efficiency has always been around 100% after the 5th cycle, indicating an extraordinary cycle stability. To further present the polysulfide adsorption of the CNF-T interlayer, sulfur cathodes with a much higher mass loading (2 mg cm-2) were used in the batteries with the current density of 1 C, as shown in Figure S3. For bare sulfur cathode, the initial capacity was only 160 mAh g-1, while the


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batteries with interlayers could deliver a much higher capacity above 700 mAh g-1, which is mainly ascribed to the superior conductivity of the interlayer. For subsequent cycles, the capacity of bare sulfur cathode reached the maximum value at 510 mAh g-1 but rapidly declined to 250 mAh g-1 within 200 cycles. Notably, the cell with a CNF-T interlayer could own a steady capacity of 800 mAh g-1 even after 250 cycles at 1 C, while the one with a CNF interlayer decayed gradually from initial 850 to 450 mAh g-1. The high capacity retention after long cycles directly certifies the efficient inhibition of CNF-T interlayers on shuttle effect. Electrochemical impedance spectroscopy (EIS) of the cells (bare sulfur, with CNF and CNF-T interlayers) after two cycle testing at 0.2C are shown in Figure 4d. The three Nyquist curves are all composed of a semicircle in the high-to-medium region and an inclined line at the low frequency, which are common in the Li-S batteries.20, 39 The semicircle is known to be concerned with the charge transfer resistance (Rct), which reflects the difficulty for the charge to transfer the interface between the cathode and electrolyte. The bare sulfur battery exhibited the Rct of 38 ohm, while the batteries with CNF and CNF-T interlayers only had ones of 26 ohm and 30 ohm, respectively. The obvious decrease in the Rct is mainly attributed to the addition of the conductive network made by CNFs, which provides an express pathway for the electrons. The mat effectively reduced the resistance of the insulted sulfur cathode, making a higher utilization of sulfur consist with the former statement. It is worth stating that the slight increase in Rct for the CNF-T battery is due to the outside TiO2 coating when compared with the one of CNF interlayer. The slopping line in the low frequency is linked with the Warburg resistance that is concerned with the lithium ion diffusion within the particle.40,41 For the batteries with an interlayer (CNF and CNF-T), the gradient of the curve is much steeper than that of bare sulfur battery, meaning a faster lithium transfer in the battery. The reason accounting for this may be


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that the porous CNF network has large space to accommodate and trap the polysulfides, meanwhile provides a relatively bigger express for the lithium ions. This assumption is further certified by the steeper slope of CNF-T interlayer battery comparing to the CNF interlayer battery. The ultrafine TiO2 particles distributed outside the CNFs can capture of polysulfide more effectively by the strong chemical adsorption resulting from the intense electrostatic attraction between the Ti-O bond and sulfur-species. With less polysulfides free-dissociating in the channels among CNFs, the lithium ions would obtain a better transfer performance although the number of express is no more increasing. The rate capability is also an important factor to evaluate the batteries as shown in Figure 4e. The rate performance was measured at a step-wise current stage from 0.1 to 0.3, 0.5, 1, eventually to 2 C and then back to 0.1 C. Each current was sustained for 10 cycles. After 10 testing cycles at 0.1 C, the CNF-T battery could still deliver a capacity of 1091 mAh g-1 with a retention of 91%, while for bare and CNF batteries, the capacities could be maintained at 630 and 930 mAh g-1, with retentions of 63% and 82%, respectively. The capacity fade in the first several cycles may result from the dissolution of lithium polysulfides into the electrolyte. The high capacity and large retention at smaller current for CNF-T battery powerfully illustrates that the CNF-T interlayer can serve as an effective line of defense to prevent the shuttle and diffusion of sulfur-species. When the current successively change into 0.3, 0.5, 1 and 2 C, the CNF-T cell exhibited as before the highest capacity of 940, 740 and 620 mAh g-1, respectively. Notably, when the current returned to 0.1 C, a fair stable capacity of 1054 mAh g-1 was obtained, which was nearly the same as the value of the first 10 cycles. This excellent reversibility indicated that CNF-T interlayer could still effectively immobilize polysulfides even at high passing rate of ions. While the bare sulfur battery suffered a severe capacity decay with a recovery capacity of


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540 mAh g-1 (corresponding to only 70% of the former capacity at first 10 cycles). The desired outstanding rate capability of CNF-T battery results from the superior ionic and electronic conductivity in the CNF-T network, which accords with the analysis in the EIS discussion, together with the strong adsorption of TiO2 component for polysulfides. The corresponding charge/discharge curves at different current densities for CNF-T battery were shown in Figure 4f. At a current density of 0.1 C, the typical common two-plateau behavior of the Li-S battery was gotten at about 2.3 V and 2.1 V , which was consist with the peaks in the CV profiles. Although the curve suffered distortion to a certain extent with the increasing current density, the battery could still deliver a relatively high capacity of 620 mAh g-1 at 2 C along with the two distinct plateaus.


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Figure 4. (a) CV curves of the Li-S battery with CNF-T interlayer; (b) Cycle performance of different batteries at a current density of 0.2 C; (c) Long term cycle performance of the Li-S battery with CNF-T interlayer at 1 C; (d) EIS plots of different batteries; (e) Rate performance of different batteries; (f) Corresponding charge/discharge profiles at different current densities for the Li-S battery with CNF-T interlayer


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In order to further understand the desirable electrochemical performance, the CNF-T interlayer was recovered from the charged cells which underwent 500 cycles at 1C. The morphology and element mapping were characterized by SEM and TEM as shown in Figure 5. In Figure 5a, the stacked and porous structure of the CNF network was well preserved and every filament remained the original feature without any fracture and distortion, demonstrating a robust and stable conductive skeleton of CNFs. As shown in the TEM images (Figure 5b), a large amount of TiO2 nanoparticles can be still clearly distinguished on the CNFs, indicating the tight adhesion between anatase TiO2 and CNFs. It was noted that the cycled interlayer remained integral, almost the same as the fresh-made one (seeing in the Figure S4) and the quantity of the ultrafine TiO2 remained on the cycled CNFs was still fair enough to be recycled. This remarkable advantage directly cut down the cost by cyclic utilization, which is more appealing to the practical application of Li-S battery device. TEM mapping was also applied to detect the distribution of the elements including C, O, Ti and S, as displayed in Figure 5c and 5d. All of the three original elements (C, O, Ti) exhibited the shape of the cross fibers, indicating the uniform distribution and firm combination of TiO2 on the CNFs even after the long-cycle charge and discharge. But the distribution of the element sulfur appeared to be irregular without any recognizable shape. The reason accounting for this might be the low content of sulfur produced in the continuous cycling or exfoliation of sulfur-species from fiber matrix during TEM sample preparation. To further verify these assumptions, the EDS region mapping of sulfur-species was carried out by SEM illustrated in Figure 5e and 5f. These two images represented the two sides of the CNF-T interlayer, respectively. Figure 5e and 5f corresponded to the side towards the separator and another side near to the cathode, respectively. In both photos, sulfur has a uniform dispersion,


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appearing as the random dot fulfilling the whole region. The reason accounting for the nonfibrous morphology of sulfur mapping is due to the overlapping of the multi-layer stacked fibers. The contents of sulfur were measured at about 1.6 wt% and 2.7 wt% corresponding to the separator side and cathode side, respectively, which accords with the little content of TEM investigation. In contrast, the sulfur contents for the comparing CNF interlayer, derived from Figure S5, were about 3.31 wt% and 4.26 wt%, corresponding to the separator side and cathode side. It is also worth mentioning that the difference of sulfur content between two sides does make sense. The side away from the cathode has only 1.6 wt% of sulfur, which is merely 59% of the one towards the sulfur cathode. While the corresponding value for CNF interlayer is up to 78%. The larger mass discrepancy for CNF-T is mainly ascribed to the more powerful capture ability and resulted high-effective reutilization for polysulfides, comparing to the insufficient adsorption of the CNF mat. Once the dissolved sulfur-species attempted to go through the CNFT interlayer, they would be captured by the TiO2 gatekeepers and further converted into immotile low-order species, leading to a high sulfur utilization in the subsequent procedure.


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Figure 5. SEM and TEM images of the cycled CNF-T interlayer (a, b); TEM and corresponding EDS mapping images of the cycled CNF-T interlayer (c, d); EDS results and corresponding sulfur distribution based on SEM image for the cycled CNF-T interlayer according to the separator side (e) and cathode side (f).

To present more intuitional results on the inhibition of polysulfide shuttling for various interlayers, the color and surface morphology of the separators from different batteries which experienced 100-cycle tests at 1C have been investigated, shown in Figure S6. It can be directly distinguished from Figure S6a that after 100 cycles, the CNF-T separator remains as white as the


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original PP separator, while the CNF separator under the same testing condition becomes faint yellow. And the separator in the bare sulfur battery turns to be a yellow one, indicating more polysulfides deposited on the surface of the separator. According to the SEM images, the surface porosity of different separators towards cathodes could be used to determine the block action of interlayers. The separator in the cell with a CNF-T interlayer still owns quantities of nano pores on the surface (Figure S6b) without distinct difference from the initial PP separator (Figure S6e). However, the separators in the other two batteries only has few pores left with the majority filled by polysulfides, as shown in Figure S6c and d. All these results identify the high efficiency of the CNF-T interlayer in prohibiting the shuttle effect of polysulfides in Li-S batteries.

CONCLUSION In summary, we developed a facile and scalable way to fabricate an ultrafine TiO2 decorated CNF (CNF-T) interlayer for Li-S battery, which could significantly improve the cell’s cycle and rate performance. The stacked conductive CNF network could not only provide an express electronic way, solving the intrinsic insulation of sulfur in the cathode, but also relieve the volume expansion of active material during the lithiation process. Meanwhile, the numerous TiO2 nanoparticles working as anchoring points were able to obviously suppress the migrating of the dissolved polysulfides through strong chemical adsorption. Moreover, the well-preserved interlayer after long cycling can still be recycled, which meets the slogan of “green and environmental” in the 21th century. The simple fabrication and low cost would make it achievable in the industrial application of Li-S batteries to produce the next generation batteries with a higher energy density.


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ASSOCIATED CONTENT Supporting Information Digital photos of the CNF-T interlayer under bending conditions; TGA data and pore size distribution and N2 adsorption and desorption isotherms of CNF interlayer; Cycling performances of different cells with a mass loading of 2 mg cm-2 at 1C; Digital photo of the cycled CNF-T interlayer; EDS results and corresponding sulfur distribution based on SEM image for the cycled CNF interlayer according to the separator side and cathode side; Digital images of different separators after 100-cycle test at 1C; SEM images of the toward-cathode surface of the separators in batteries with CNF-T interlayer, CNF interlayer, without interlayer and the origin PP separator; This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (X. Qin); *E-mail: [email protected] (B. Li). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This

work

was

supported

by National

Key Basic Research

Program

of China

(No.2014CB932400), National Natural Science Foundation of China (Nos. 51202121 and 51232005), NSAF (Nos. U1330123 and U1401243) and Shenzhen Technical Plan Project (JCYJ 20150529164918735).


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Ultrafine TiO2 Decorated Carbon Nanofibers as Multifunctional

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