Reliable Interlayer Based on Hybrid Nanocomposites and Carbon

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A Reliable Interlayer Based on Hybrid Nanocomposites and Carbon Nanotubes for Lithium-Sulfur Batteries Tao Liu, Shimei Sun, Jialiang Hao, Wei Song, Quanhai Niu, Xiaolin Sun, Yue Wu, Depeng Song, and Jianfei Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02136 • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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ACS Applied Materials & Interfaces

A Reliable Interlayer Based on Hybrid Nanocomposites and Carbon Nanotubes for Lithium-Sulfur Batteries

Tao Liu, Song,

†

†, ‡

Shimei Sun, † Jialiang Hao,

§

Wei Song, ** Quanhai Niu, † Xiaolin Sun,

†

Yue Wu, † Depeng

Jianfei Wu * †, ‡

†

Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, 266101, PR China

‡

Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049,

PR China §

Ocean University of China, Qingdao, 266100, PR China

**

School of Chemistry, Sun Yat-sen University, Guangzhou, 510275, PR China

ABSTRACT The future energy needs have triggered research interest in finding novel energy storage systems with high energy density. Lithium–sulfur batteries are regarded as one of the most promising options for the nextgeneration energy storage applications because of their high theoretical energy and low cost. However, the electrochemical performances of lithium–sulfur batteries are seriously compromised by the polysulfides (LiPSs) shuttling and the insulating nature of sulfur. To overcome these issues, a novel CoNi1/3Fe2O4 (CNFO) nanoparticles uniformly covered on the carbon nanotubes is now reported as an efficient functional interlayer. Benefiting from the sufficient sulfiphilic sites of the CNFO for chemically bonding with LiPSs, as well as the conductive interconnected skeleton of carbon nanotubes, this composite material showed great enhancement on the rate capability and the cycle stability of Li−S batteries. The Li−S battery using this interlayer exhibited a high initial capacity of 897 mA h g−1 and a low capacity decay of 0.063% per cycle within 250 cycles at 2 C. Meanwhile, an reversible specific capacity of 869 mA h g−1 (at 0.5 C) with high coulombic efficiency could be obtained over 100 cycles at an elevated temperature (60 °C). We speculated that the chemical adsorption of CNFO for polysulfides-anchoring is extremely critical for the performances of Li−S batteries under high temperature.

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KEYWORDS: hybrid nanocomposites, carbon nanotubes, decoration, interlayer, high temperature, Li−S batteries.

INTRODUCTION As a mature technology, the conventional lithium ion batteries have dominated most hand-held electronics and electric vehicles markets, but their energy density limits (less than 400 Wh kg−1) will be unable to meet the demand of future energy1-2. Lithium–sulfur (Li–S) batteries have been regarded as a promising substitute to the current lithium ion batteries, due to their high energy density (2600 Wh kg−1), five times higher than that of the state-of-the-art lithium ion batteries, together with environmental benignity and low cost3-6. Despite these attractive merits, the practical application of Li–S batteries still faces two great challenges: (1) The low conductivities of bulk sulfur and its lithiation products Li2S/Li2S2, which severely restrict the electron transport in the sulfur cathode during cycling, hence resulting in insufficient sulfur utilization and severe polarization7-8. (2) The polysulfides (LiPSs) are highly soluble in the electrolyte and can diffuse throughout the whole cell, leading to unfavorable reactions between LiPSs and lithium metal anodes, which are the primary reasons for fast capacity fading and poor coulombic efficiency of Li–S batteries9. Accordingly, it is crucial to suppress the LiPSs shuttling and enhance the sulfur utilization for boosting the performances of Li–S batteries. Tremendous efforts have been devoted to addressing the aforementioned challenges, including sulfur host designs, LiPSs anchoring binders, functional interlayers and the protection of lithium anodes10-12. Among them, the functional interlayer can not only function as a dam to block the diffusion of LiPSs, but also serve as an upper current collector to recycle the soluble LiPSs. Therefore, the functional interlayers are commonly used approaches for addressing the shuttling issues. Generally, carbon materials were always involved in the functional interlayers, due to their high conductivity and large specific surface area13-14. However, the physical adsorption between nonpolar carbonaceous substrates and polar LiPSs can hardly control the LiPSs diffusion, especially in the case of high sulfur loading and high temperature15-16. Therefore, many metal oxides and sulphides with superior LiPSs adsorptivity were proposed to enhance the anchoring effect of the functional interlayers17-20. Nonetheless, most of these polar metal compounds will hinder the electron transport during cycling because of their poor conductivities, resulting in undesirable compromises in cycling stability and rate performance21. Moreover, by considering that the chemisorption is the ACS Paragon Plus 2 Environment

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monolayer adsorption, the adsorption capacities are largely depended on the polar surface areas of the adsorbents. Consequently, the polar compounds with high surface areas and strong binding sites are essential for anchoring effect22-24. Furthermore, as Cui’s group25 reported, the surface diffusion of LiPSs from polar surface to conductive substrate should be simple and efficient during charge-discharge process. Therefore, low diffusion barrier and short diffusion distance of LiPSs on the polar surface are also necessary for poor conductive adsorbents. Despite many encouraging progresses have been made in the functional interlayers and reaction mechanisms in recent years, the influence of functional interlayers on the performance of Li–S batteries at high temperature has rarely been studied. Moreover, most of the high performance Li–S batteries were realized under the thick interlayers or low sulfur loadings, which counteract the high energy density and low cost of Li–S batteries, hence resulting in posing additional challenges to their practical application26. Inspired by the seminal work of Lou and Zhang on the sulfiphilic sites of Cobalt and Nickel–Iron Hydroxide for trapping LiPSs3,

27,

a novel CoNi1/3Fe2O4 (CNFO) nanoparticles anchored on the carbon

nanotubes (denoted as CNFO@CNT) was prepared via simple hydrothermal reactions. For the first time, these composite materials were used to modify the separators of Li−S batteries. This unique functional interlayer shows multiple advantages: Firstly, the carbon nanotubes form a conductive 3D interconnected skeleton for facilitating electron transport and lithium ion diffusion, which results in efficient sulfur redox and high sulfur utilization. Secondly, the CNFO can provide abundant sulfiphilic sites for trapping LiPSs leaked from sulfur cathodes, hence effectively suppressing the shuttle effect. Thirdly, the captured LiPSs on CNFO surfaces can be effectively transferred to conductive substrates, and the electron transport in CNFO is possible due to the well-sized nanoparticles (5−15 nm), resulting in accelerating redox reactions of sulfur and avoiding the accumulation of LiPSs. As a result, the Li−S batteries using CNFO@CNT interlayers showed higher rate performance and better cycling stability compared to that using pure CNT interlayers. Moreover, high specific capacity and acceptable cyclic stability were also obtained under high temperature (60 °C).

EXPERIMENTAL DETAILS The preparation of CNFO@CNT and CNFO@CNT modified separator. Firstly, 500 mg of multi-walled carbon nanotubes (CNTs, Tiannai (zhenjiang) Technology Co., Ltd) were carboxylated by the mixed acid (See Supporting Information for details). Then, 100 mg of the oxidized ACS Paragon Plus 3 Environment


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CNTs (OCNT) were fully dissolved in a mixed solvent of 30 mL deionized water and 30 mL ethylene glycol through magnetic stirring and ultrasonication. Subsequently, 0.6 mmol CoCl2·6H2O, 0.2 mmol NiCl2·6H2O and 1.2 mmol FeCl3·6H2O were completely dissolved in the above mixture, and then 6 mmol urea was added to the mixture with vigorous stirring and ultrasonication. The as-obtained solution was transferred to a 100 mL Teflon-line sealed autoclave and heated at 50 °C for 1 h and 160 °C for 5 h, respectively. The reaction product was washed thoroughly with deionized water and ethanol for several times, and finally dried overnight under vacuum, the mass ratio of CNFO in the CNFO@CNT accounts for 55–60 wt.%. For comparison, CoFe2O4@CNT (CFO@CNT) and Ni1/3Fe2O4@CNT (NFO@CNT) were synthesized using the same method without adding NiCl2·6H2O and CoCl2·6H2O respectively. CNFO@CNT-2 was also prepared by the same method except that doubled the amount of CNFO in CNFO@CNT and added trace dilute H2SO4 solution. The CNFO@CNT modified separator was obtained by coating the slurry of CNFO@CNT, CNT, and polyvinylidene fluoride (PVDF) binder (with mass ratio 8:1:1) onto the pristine separator (celgard 2500), through the method of vacuum filtration, followed by drying at 50 °C for 12 h under vacuum28. The mass loading of the CNFO@CNT composite material on the separator is approximately 1.25 mg cm-2. Preparation of Li2S6 Solution. Li2S6 solution was synthesized according to our previous reports29-30. Firstly, sulfur and lithium sulfide at a stoichiometric mole ratio of 5:1 were added into a vial with electrolyte in a glove box. Afterwards, the mixture was heated at 60 °C with vigorously stirring overnight, and then diluted to 2 mmol L-1 for the adsorption tests. Characterization. The morphology and EDS experiments of samples were characterized by SEM (Hitachi S4800 Japan) equipped with EDS devices (FEI Quanta 650). TEM imaging was carried out on a FEI Tecnai G2 F20 microscope. The X-ray diffraction (XRD) patterns were measured by powder X-ray diffraction (XRD) with Cu Kα radiation (Bruker AXS D8). UV–vis adsorption spectroscopy was conducted using a HITACHI U4100 spectrophotometer. The specific surface area was measured by nitrogen adsorption/desorption measurement (Quantachrome Instruments Quadrasorb EVO). The adsorption performance test was conducted in a biochemical incubator (SHP-100). Electrochemical measurements ACS Paragon Plus 4 Environment

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The sulfur cathode materials were prepared by mixing 60 wt.% sulfur powder (S), 30 wt.% Super-P (SP), and 10 wt.% polyvinylidenefluoride (PVDF) into N-methylpyrrolidone (NMP) suspension with vigorous stirring for 12 h. The obtained slurry was coated on the aluminum foil by an automatic coating machine, and the electrode was dried at 60 °C for 12 h in vacuum oven and cut into a disc with a diameter of 12 mm before use. The low sulfur loadings in this work were 1.0−1.2 mg cm –2, and the high sulfur loadings were 3.4−3.6 mg cm –2. 2032 coin cells which consisted of sulfur cathode, counter lithium anode and separator, were assembled in an argon-filled glovebox. 1M lithium bis(trifluoromethanesulphonyl)imide (LiTFSI) dissolved in a mixture of 1,3-dioxolane and dimethoxymethane (1:1 by volume) with 1wt% lithium nitrate (LiNO3) additive was used as the electrolyte. The dosage of electrolyte was 25 μL for the sulfur loading of 1.0−1.2 mg cm –2, and 40 μL for the high sulfur loading of 3.4−3.6 mg cm-2, respectively. The galvanostatic charge/discharge test was performed between 1.7 and 2.8 V (vs. Li+ /Li) by a Land CT2001A instrument where cells were put in the biochemical incubators with constant temperature (25 and 60 °C), and all the specific capacities were calculated based on the mass of sulfur. The electrochemical impedance spectra (EIS) and cyclic voltammetry (CV) were performed on a Bio-Logic SP-150 (France). EIS analysis was carried out with the frequency range from 0.01 to 100000 Hz, and CV was performed at a scan rate of 0.1 mV s–1.

RESULTS AND DISCUSSION

Figure 1. Schematic illustration of CNFO anchored CNT nanocomposite preparation.

The simplified synthesis procedure of CNFO@CNT composites is illustrated in Figure 1. For more uniform dispersion of the magnetic CNFO, carbon nanotubes were selected as the supports for the CNFO nanoparticles31. After modifying the raw CNT with oxygen functional group by the mixed acid, the obtained OCNTs can not only maintain their original interconnected structure, but also enable them fully soluble in the mixed solution of ethylene glycol and water (Figure S1 in Supporting Information). Afterwards, metal ACS Paragon Plus 5 Environment


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salts and urea were introduced into the above solution. Benefiting from the electric attraction and complexation between the oxygen functional groups and the metal-ions31-32, the metal-ions could be uniformly adsorbed onto the CNT surface. With the temperature increasing, OH- ions would gradually generate from the hydrolysis of urea, hence the CNFO was in situ nucleated and uniformly grown on the CNT surface27. The morphological features of the CNFO@CNT were first characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM image (Figure 2a) revealed that the CNFO possessed well-defined nanoparticles uniformly covering the CNT surface, demonstrating that OCNT enabled good size control and uniform dispersion of the CNFO nanoparticles. Furthermore, the CNFO@CNT displayed the interconnected frameworks, similar to the pure CNT structure. This highly nanoporous and interconnected structure is favorable for electrolyte diffusion and electron conduction during electrochemical reactions. The TEM images (Figure 2b–c) further confirmed that the average diameter of CNFO nanoparticles is 5–15 nm, which is much smaller than the previously reported NiFe2O4 and CoFe2O433-34. Moreover, benefiting from the synergistic effect from both cobalt and nickel ions in the oxide structure, the electronic conductivity of the CNFO will be superior to NiFe2O4 and CoFe2O4, respectively35-36, which can facilitate the conversion efficiency of the captured LiPSs. The XRD pattern revealed that the crystal phase of CNFO was the spinel structure with Fd3m space group (Figure 2e), which is consistent with previous report about NiFe2O4 and ZnCo2O421,37. Figure 2d is the representative highresolution TEM (HRTEM) image of CNFO@CNT, the lattice spacing of 0.254 nm can be attributed to the (311) crystallographic planes of the CNFO (Figure 2d)6. The CNFO@CNT composite showed a high BET surface area of 189 m2 g−1 (Figure 2f), which were beneficial for capturing LiPSs and enhancing sulfur utilization. The elemental distribution maps of CNFO@CNT composite showed the high intensity and the well-matched spatial distributions of Co, Ni, and Fe elements, further demonstrating the uniformly distribution of CNFO on the CNT surface (Figure 2g). EDS analysis result further confirmed that the molar ratio of the three metals (Co: Ni: Fe) was 7.3%: 2.8%: 16.2%, similar to the theoretical ratio of CoNi1/3Fe2O4 (Figure S2).


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Figure 2. (a) Scanning electron microscopy (SEM) and (b-c) low-magnification TEM images showing the morphology of CNFO@CNT composites. (d) HRTEM image of the as-prepared CNFO@CNT. (e) XRD pattern, (f) N2 adsorptiondesorption isotherm curves of CNFO@CNT composites. (g) SEM image and corresponding elemental maps of a CNFO@CNT lump.

A comparison of the optical images of LPSs capturing performance between CNT and CNFO@CNT is presented in Figure 3. 25 mg CNT and CNFO@CNT with the same weight were added into the 6 ml 2.0 mmol L-1 Li2S6 solutions separately, followed by standing in 25 °C incubator (Figure 3a). CNFO@CNT composites thoroughly decolored the Li2S6 solutions after 24 h, while the CNT had no remarkable effect on the Li2S6 solution, demonstrating that the CNFO@CNT show much better LPSs capturing performance than the raw CNT. Meanwhile, this visual investigation was also conducted at high temperatures of 60 °C, the Li2S6 solution steeped with the CNFO@CNT composites became almost colorless even after 8 h (Figure 3b), suggesting that the anchoring effect of CNFO@CNT at 60 °C is more efficient than that of 25 °C. This result reveals that the adsorption capacity of CNFO@CNT increases obviously with the increase of temperature, which is consistent with one essential characteristic of chemisorption22. Meanwhile, trace yellow precipitation was found at the bottom of the Li2S6 solutions, which could not be redissolved at 25 °C, suggesting the disproportionation reaction of Li2S6 at high temperature. Simultaneously, UV–vis spectroscopy (Figure 3c–d) reconfirmed the strong polysulfides-anchoring of CNFO@CNT as the much weaker signal of Li2S6 was detected in CNFO@CNT solutions than that of pure CNT and blank samples. ACS Paragon Plus 7 Environment


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These sharp contrasts indicate an excellent chemisorption of CNFO@CNT for bonding with LiPSs from working electrolyte both at room temperature and high temperature 38.

Figure 3. The static adsorption tests of the Li2S6 solution after exposing to the different composites and corresponding UV– vis spectra at (a, c) 25 °C after 24 h and (b, d) 60 °C after 8 h.

To evaluate the electrochemical performances of the CNFO@CNT composites, the Li–S batteries were fabricated using them as the functional interlayer in standard 2032 coin cells. The CNFO@CNT composites were uniformly coated onto the polypropylene separator by a simple filtration process, and no irreversible destruction could be observed after folding/unfolding tests (Figure S3a), indicating rather steady structure of the CNFO@CNT interlayer. The thickness of the functional interlayer is approximately 25 μm without overpenetration in the separator (Figure S3b), and the introduction of the CNFO@CNT interlayer could still meet the demand for the fast transport of lithium ion due to their interconnected structures. Figure S4 exhibits the cycling stability of the Li–S batteries using different interlayers at 1.0 C. The CNFO@CNT sample could achieve a high reversible capacity of 1090 mAh g-1, and maintained 84 % capacity retention over 150 cycles, which are much higher than 80 % of CFO@CNT and 72 % of NFO@CNT, suggesting that the synergetic effect of cobalt and nickel ions in CNFO could enhance the cycling stability of Li–S batteries. Figure S5a shows the initial five cycles of the Li−S batteries using CNFO@CNT interlayers in CV measurement. There are two characteristic reduction peaks in the negative scan stage, the first peak at high voltage is derived from the open ring reduction of sulfur to long-chain LiPSs (Li2S4, Li2S6 and Li2S8), while the second peak at low voltage corresponds to the transformation of the LiPSs to subsequent insoluble ACS Paragon Plus 8 Environment

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lithium sulfide (Li2S2 and Li2S)39-40. In the positive scan stage, the broad oxidation peaks are related to the backward reaction from Li2S to LiPSs and finally to S8. What's more, the nearly perfect superimposition of the peaks after the first cycle suggests good reversibility and stability of the cathode reactions in CNFO@CNT sample41-42. Compared to the CNT sample, Figure S5b revealed that the CV of CNFO@CNT sample displayed significantly negative shift in oxidation peak and positive shift in reduction peak, accompanied with an increased peak intensity, demonstrating the critical role of the CNFO nanoparticles in decreasing the polarization effect and accelerating the sulfur redox kinetics43-45. In order to confirm the origin of the capacity in CNT and CNFO@CNT samples, the CNT and CNFO@CNT interlayers without sulfur cathode were investigated by charge-discharge test from 1.7 to 2.8 V voltage range. The CNFO@CNT showed a higher capacity of 0.031 mAh than that of raw CNT (0.010 mAh) at 0.1 C (Figure S6), indicating that CNFO can provide some capacity during charging and discharging. However, the 0.031 mAh is only 1.8% percent of the capacity of sulfur cathode (1.69 mAh) at 1.0 mg cm-2 sulfur loading, and the CNFO@CNT capacity merely accounts for 0.8 % of the total capacity (1.29 mAh) at 2.0 C. Therefore, the capacities arised from the CNFO@CNT could be negligible in this experiment. The rate performances were compared by increasing the current density stepwise from 0.1 to 2 C every 5 cycles. Impressively, As shown in Figure 4a, the CNFO@CNT sample could deliver considerable specific capacity of 1332, 1168, 1061, 978, and 902 mAh g-1 at the rate of 0.1, 0.2, 0.5, 1.0 and 2.0 C, respectively, and the discharge capacity of 1193 mAh g-1 was obtained after recharging back to 0.1 C after 25 cycles, indicating excellent stability of the batteries using CNFO@CNT interlayer at various current rates. Comparably, the CNT sample displayed a capacity of 1283, 1093, 1003, 902 and 781 mA h g-1 for such rates. At the same current rate, CNFO@CNT sample showed the highest specific capacity among the three cells. Even up to the 2.0 C rate (Figure S7), the specific capacity of CNFO@CNT sample was approximately 5 times higher than that of pure PP, confirming the superior electrochemical performance of the CNFO@CNT interlayer. Moreover, large quantities of sulfur elements were detected in the interlayer even after a single charge-discharge cycle (Figure S8), indicating that the CNFO@CNT interlayer served as the current collectors for reusing the S species at the beginning of cycling.



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Figure 4. Electrochemical properties of Li–S batteries using different interlayers. (a) The rate performance from 0.1 to 2 C. (b) Cycling stability at a current density of 2.0 C with the sulfur loading of 1.0 mg cm−2. The fitting capacity contributions of (c) SH at the upper discharge plateaus and (d) SL at the lower discharge plateaus at 2.0 C.

The cycling performance of the batteries was testified under the constant current rate of 2.0 C. As shown in Figure 4b, the cells could exhibit high capacity of 853 mAh g-1 after the introduction of the CNT interlayer, while the capacities of CNT sample declined significantly with increasing cycles, and could only maintain at 617 mAh g-1 after a prolonged 250 cycles, indicating that the diffusion of LiPSs could be hardly controlled in a long cycle by the non-polar CNT interlayer with only physical adsorption. Surprisingly, CNFO@CNT sample showed high discharge capacity of 897 mAh g-1, and maintained a stable capacity of 755 mAh g-1 with 84% capacity retention and high coulombic efficiency over 250 cycles. These electrochemical performances are better than that of other previous reports about functional interlayer (Table S1 in Supporting Information), suggesting that the CNFO@CNT interlayer showed effective control of the shuttle effect and recycled the soluble LiPSs during cycling. Figure 4c–d exhibit the capacity contribution of two discharge platforms of the cells with different interlayers at the rate of 2.0 C. The first plateau capacity at high potential (denoted as SH) is associated with the reduction of sulfur to soluble long-chain LiPSs (S8→Li2S4−8) through solid-to-liquid diffusion, which are the main cause of the shuttle effect. The plateau capacity at low potential (denoted as SL) is attributed to the formation of Li2S2/Li2S through liquid-to-solid diffusion (Li2S4−8→Li2S2/Li2S), in which the reaction kinetic reaction is relatively sluggish due to poor conductivity of Li2S2/Li2S, hence resulting in insufficient utilization of LiPSs46. Considering that the capacity fading of Li−S batteries is mainly caused by the loss of the active sulfides and the insufficient redox ACS Paragon Plus 10 Environment

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reaction47, and thus we compared the cycle stability of SH and SL, respectively. As shown in Figure 4c, the CNT sample displayed a high SH of 319 mA h g−1, and maintained a reversible capacity of 228 mA h g−1 after 250 cycles, corresponding to 71 % capacity retention. In contrast, CNFO@CNT sample exhibited an excellent cyclic stability, which showed a high SH of 314 mA h g−1 and obtained 88% capacity retention after 250 cycles, suggesting better active materials retention in CNFO@CNT sample during cycling. Moreover, the same results were observed in the SL (Figure 4d), CNFO@CNT sample shows higher and more stable SL with 83% capacity retention compared to 75 % of CNT after 250 cycles, which confirms that most LiPSs could convert into insoluble Li2S2/Li2S at the low voltage plateau in CNFO@CNT sample. These results clearly demonstrate that the CNFO@CNT interlayer can effectively capture and recycle the soluble LiPSs even at high rate current. Figure S9 shows the electrochemical impedance spectroscopy (EIS) of CNFO@CNT and blank samples under different cycles, the semicircle in high frequency region is related to the charge-transfer resistance, and the inclined line in the low frequency region is ascribed to the warburg diffusion process. The charge-transfer resistance of the CNFO@CNT sample was greatly reduced and maintained after 50 and 100 cycles, indicating significant enhancement of redox kinetics of active materials by CNFO@CNT composite. To bring this CNFO@CNT interlayer one step closer to practical application, we further evaluated the cycling performance of the batteries with high sulfur loading of 3.4 mg cm-2 (Figure S10). The CNFO@CNT interlayer could exhibit a high capacity of 1040 mAh g-1 and maintain 822 mAh g-1 after 100 cycles at 0.5 C, indicating an excellent cycling stability under high sulfur loading. For comparison, we changed the CNFO morphology in CNFO@CNT composites, and doubled the amount of CNFO in CNFO@CNT composites for eliminating the effect of chemisorption area. From here on, CNFO@CNT composites with larger and more CNFO are denoted as CNFO@CNT-2. As shown in Figure S11, there were plenty of CNFO nanoparticles with the diameter of 50–300 nm, indicating cluster formation in CNFO@CNT-2 due to lower pH value during reaction process. Figure 5a shows the rate performance of CNFO@CNT-2 sample, it only delivered a specific capacity of 860 and 767 mA h g-1 at 1.0 and 2.0 C, which were inferior to those of CNT@CNT sample, revealing that the rate performance is closely related to the CNFO morphology in CNFO@CNT. Figure 5b illustrates the long-term cycle performance of CNT 、 CNFO@CNT and CNFO@CNT-2 samples. The CNFO@CNT could deliver an initial capacity of 1090 mAh g-1 and maintain 831 mAh g-1 after 250 cycles. Meanwhile the CNT sample showed a capacity of ACS Paragon Plus 11 Environment


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1000 mAh g-1 at the first few cycles and exhibited 677 mAh g-1 over 250 cycles. Most interestingly, despite high anchoring effect of CNFO nanoparticles, the CNFO@CNT-2 sample showed relatively poor cyclic stability and only maintained a low discharge capacity of 581 mAh g-1, as well as low coulombic efficiency after 250 cycles, suggesting that even high anchoring effect in CNFO@CNT-2 was unable to guarantee good cycling stability due to the change in morphology of CNFO. Based on the above results, the excellent electrochemical performances of the CNFO@CNT interlayers are not only relied on strong anchoring effect of the CNFO, but also mainly associated with their unique nanostructures. As shown in Figure 5c, electron might pass through the ultrasmall CNFO nanoparticles (515 nm) due to nano effect48, to effectively recycle the captured LiPSs. Moreover, as the “trapping–diffusion– conversion of LiPSs” theories reported by Cui22 and Zhang49, the diffusion of captured LiPSs on the CNFO surface to the closest conductive substrates was fast and effective due to the short diffusion distance on the CNFO surface, hence resulting in realizing high anchoring efficiency and fast conversion of LiPSs in the CNFO@CNT interlayer. In contrast, the CNFO@CNT-2 with larger size CNFO could also effectively capture the LiPSs via metal−sulfur interactions. While the captured LiPSs can not be fully recycled due to their poor conductivity and long diffusion distance (Figure 5d), leading to the accumulation of LiPSs and large polarization effect during cycling. These are also the reasons for the unfavorable performance of CNFO@CNT-2 with respect to poor rate capability, rapid capacity fading and low coulombic efficiency.

Figure 5. (a) The rate performance from 0.1 to 2 C of CNFO@CNT-2 sample. (b) Cycling stability of Li–S batteries using different interlayers at a current density of 1.0 C with the sulfur loading of 1.0 mg cm−2. Schematic of adsorption and ACS Paragon Plus 12 Environment

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diffusion behaviors of the soluble LiPSs on the surface of CNFO. (c) CNFO with the diameter of 5–15nm, and (d) CNFO-2 with the diameter of more than 50 nm.

From a practical point of view, a critical requirement of batteries in portable electronics and electric vehicles is safe operation with a wide temperature window50. The heat resistance is also an important factor for the safety and stability of Li−S batteries. Hence, we further evaluated the performance of the CNFO@CNT interlayer at elevated temperature. High temperature can aggravate the side reactions between LiPSs and lithium metal anodes, accelerating thermal motion of LPSs and weaken physical adsorption to some extent, but some chemisorption capacities will increase slightly with the rise of temperature. At an elevated temperature (60 °C), the cells with CNFO@CNT and CNT interlayers showed an initial discharge capacity of 1118 and 1083 mAh g−1 at 0.5 C (Figure 6a), which are much higher than these of 25 °C, indicating higher conversion kinetics at elevated temperature, which is ascribed to that high temperature can facilitate the lithium-ion transport and enhance electrochemical reaction kinetics. With the cycle increased to 100, CNFO@CNT could maintain the capacity of 869 mAh g−1 with a high coulombic efficiency of 98 %, demonstrating remarkable suppression of shuttle effect and good cycling stability at elevated temperature. In contrast, the CNT interlayer showed a much lower discharge capacity of 719 mAh g−1 as well as lower coulombic efficiency of 90 % after 100 cycles, suggesting that the physical adsorption can hardly suppress the LiPSs shutting at elevated temperature, which is consistent with the above results of adsorption tests. Furthermore, even up to the sulfur loading of 3.5 mg cm-2, the CNT interlayer could exhibit only 679 mAh g-1 with less than 80% coulombic efficiency at 0.5 C after 50 cycles (Figure 6b). In contrast, the CNFO@CNT sample still maintained a highly reversible capacity of 822 mAh g-1 with more than 90% coulombic efficiency after 50 cycles, further demonstrating strong chemisorption of CNFO@CNT at high temperature. The much improved cycling performance at elevated temperature confirmed the critical role of the CNFO component. The enhanced cycling performance by the CNFO@CNT is strongly associated with its sufficient binding sites for chemically bonding with the soluble LiPSs, which greatly curtails the shuttling behavior of LiPSs, as well as confines and recycle them in the interlayer.



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Figure 6. Cycling performance of the Li–S batteries with (a) 1.0 mg cm-2 and (b) 3.5 mg cm-2 sulfur loading at 0.5 C rate at high temperature of 60 °C.

The cycled batteries were disassembled to gain insights into the strong absorption capacity of the CNFO@CNT composites. The structure and morphology of the CNFO@CNT composites after 100 cycles were investigated, the morphology of CNFO@CNT interlayer could be maintained well after cycling process (Figure S12a–b), and a thin layer of active S-related species were observed on their surfaces (Figure 7a). Moreover, the well-maintained spinel structure of the CNFO@CNT interlayer shown in the XRD patterns, suggesting no change in the crystal structure of CNFO during cycling (Figure S13). The corresponding sulfur element mapping (Figure 7b) further demonstrated the uniform distribution of sulfides deposition in the CNFO@CNT interlayer, clearly suggesting that the interlayers can capture and retard LiPSs in the interlayers. Moreover, related sulfur mappings of the cycled separators facing the Li metal side were performed and compared as well (Figure 7c–d). The strong EDX signals and the homogeneously distributions of sulfur elements can be observed on the cycled separators, while the sulfur signal of CNFO@CNT/PP is much weaker than that of CNT/PP, indicating that the CNFO@CNT/PP has effectively suppressed the shuttling behavior of the soluble LiPSs, and restrained them within the cathode side. The surface morphologies of the cycled lithium anodes were also investigated by SEM and EDS spectra after 100 cycles. As shown in Figure 7e, rough surface and obvious corrosion could be observed on the cycled Li metal surface of the CNT/PP sample, revealing severe side reactions between the soluble LiPSs and Li metal during cycling. In contrast, relatively smooth and dense surface without obvious corrosion was observed on the Li metal for the CNFO@CNT/PP configuration (Figure 7f). EDS spectra results could confirm less sulfides on the surface of Li metal for the CNFO@CNT compared to that of CNT sample, and the presence of the F, O and C elements is attributed to the solid electrolyte interface, which contains LiF, Li2CO3, Li2O


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and some organic lithium salt (Figure S14). These results further provide evidence for proving the strong anchoring effect of polar CNFO nanoparticles.

Figure 7. (a) SEM images and (b) Elemental mapping images of the CNFO@CNT interlayers on the cathode side (inset: corresponding SEM images) after 100 cycles. SEM images and corresponding sulfur element mapping of (c) CNT/PP and (d) CNFO@CNT/PP on the lithium metal side. SEM images of the cycled metallic lithium anode of the batteries using (e) CNT/PP and (f) CNFO@CNT/PP after 100 cycles.

CONCLUSION In summary, a rational design of the CNFO nanoparticles uniformly covered on the CNT surface was proposed to serve as functional interlayer for boosting the Li–S batteries performance. Such the composite material combines the merits of anchoring effect of the hybrid nanocomposites with conducting CNT to enhance the immobilization and conversion of LiPSs. Using the CNFO@CNT interlayer, high capacity and excellent cycling stability were achieved both under high sulfur loading and high temperature. Furthermore, we revealed that CNFO with large size would enhance the accumulation of LiPSs, hence compromising the cycling stability and coulombic efficiency of Li–S batteries. In the light of these results, the desirable interlayer should not only form strong interaction with LiPSs, but also exhibit excellent conductivity for recycling the captured LiPSs. The same strategy could be helpful to explore and develop new hybrid nanocomposites for high performance of Li–S batteries. ACS Paragon Plus 15 Environment


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ASSOCIATED CONTENT Supporting Information Digital photograph of OCNT solution, EDS spectra of CNFO@CNT, digital image and SEM observation of CNFO@CNT/PP, cycle stability, CV profiles and discharge–charge curves of cells, capacitive contribution of CNT and CNFO@CNT, SEM images and EDS of CNFO@CNT interlayer, EIS curves for different cycles, cycling performance of cell with a high sulfur loading, SEM and TEM images of CNFO@CNT-2, XRD patterns and SEM images of CNFO@CNT after cycling, SEM images and EDS spectra of lithiummetal after cycling, cycling performance of cells reported previously

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Jianfei Wu: 0000-0002-1420-3947 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (No.21673267) ， Shandong Provincial National Science Foundation, China (No.ZR2017BEM031), CAS Pioneer Hundred Talents Program, DICP&QIBEBT (Grant: DICP&QIBEBT UN201702)，and Dalian National Laboratory For Clean Energy (DNL，CAS), and China Postdoctoral Science Foundation.

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Reliable Interlayer Based on Hybrid Nanocomposites and Carbon

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