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Letter pubs.acs.org/NanoLett

In Situ Polymerized PAN-Assisted S/C Nanosphere with Enhanced High-Power Performance as Cathode for Lithium/Sulfur Batteries Hao Hu, Haoyan Cheng, Zhengfei Liu, Guojian Li, Qianchen Zhu, and Ying Yu* Institute of Nanoscience and Nanotechnology, College of Physical Science and Technology, Central China Normal University, Wuhan 430079, P. R. China S Supporting Information *

ABSTRACT: Carbonaceous and polymer materials are extensively employed as conductor and container to encapsulate sulfur particles and limit polysulfide dissolution. Even so, high-power performance is still far from satisfaction due to the expansion and collapse of the electrode materials during thousands of charge−discharge process. Herein, it is found that colloidal carbon sphere with high elastic coefficient can be utilized as a framework to load sulfur, which can trap soluble polysulfides species in the pores within the sphere and efficaciously improve the electronic conductivity of the cathode. After modified by polyaniline (PAN) through in situ polymerization, PAN-assisted S/C nanosphere (PSCs-73, with 73 wt % sulfur) effectively minimize polysulfide diffusion, enhance the electron transfer rate and overcome the problem of volume expansion. The fabricated PSCs-73 cell shows outstanding long high-power cycling capability over 2500 charge/discharge cycles with a capacity decay of 0.01% per cycle at 5 C. Substantially, this composite can drive 2.28 W white indicators of LED robustly after minutes of charging by three lithium batteries in series, showing a promising potential application in the future. KEYWORDS: Lithium/sulfur battery, PAN-assisted, vulcanization, high-power, sulfur cathode

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performance of Li−S batteries is still far from satisfaction in comparison with other cathodes, such as LiFePO4.8,11,16 The regrettable high-power capacity is not only connected with inadequate electronic contact to insulating S in the cathode, but also related to the incomplete reduction of polysulfides along discharge path.11,17−19 Moreover, it is noted that soluble lithium polysulfides were the major contributor for cycling performance, while insoluble solids (Li2S2 and Li2S) were incapable of being reduced at a high rate.9 In order to address the above issues, acid activated colloidal carbon sphere was utilized as framework to load S in our work, which could efficaciously trap the soluble polysulfides species in the pores and improve the electronic conductivity of the cathode.20 As for “solid−solid” process (reversible reduction of Li2S2 to Li2S), the content of S is always limited (∼40 wt %) by inadequate electronic contact, leading to low energy density of S-based composites.21,22 From this viewpoint, it is convenient for colloidal carbon sphere framework with large pore volumes to control the content of S through reaction condition and make S particles separate and decentralize in the colloidal carbon sphere.13,23 In this study, the detailed information for the controlled S content is shown in Figure S1 (Supporting Information). Obviously, overloaded sulfur (86 wt %) may bring about dissatisfied electrochemical performance (Figure

ithium−sulfur (Li−S) battery, one of the attractive highenergy-density rechargeable batteries for portable electronics, consumer devices, hybrid electric vehicles, and largescale grid energy storage, is of great importance to cope with the increasingly serious energy depletion.1,2 Based on lightweight element and multielectron reaction, sulfur is acting as a promising cathode material with a high theoretical capacity of 1670 mAh g−1, namely a theoretical specific energy of 2600 Wh/kg, on the assumption of complete reaction of S to form Li2S.3,4 What’s more, S has the character of natural abundance, low cost, and environmental friendliness. Despite the remarkably potential applications in future, there are still some serious problems obstructing the development of Li−S batteries for commercialization.2,5,6 The major limiting factors are the relatively low utilization of active materials and low capacity of cathodes resulted from highly insulating nature of sulfur (5 × 10−30 S cm−1 at 25 °C), high solubility of lithium polysulfides, significant structural and volumetric change (∼80%) during charge and discharge processes.1,2,6 Recently, the strategies of impregnating sulfur in a porous carbon or polymers,7,8 coating the host materials with a polymers,9,10 and fabricating yolk−shell nanostructures with an internal void space for sulfur-storage have been used to improve the electrochemical performance of S-based batteries.11−13 Thereinto, the numerous evidence published newly illustrated that nanostructured porous carbon were beneficial to enhance electrochemical contact within the electrodes and improve the utilization of active materials.9,14,15 However, high-power © XXXX American Chemical Society

Received: April 2, 2015 Revised: June 30, 2015

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DOI: 10.1021/acs.nanolett.5b01294 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters Scheme 1. Schematic of Synthetic Procedure for PAN-Assisted S/C Nanosphere (PSCs)

Figure 1. (a) SEM images of PSCs-73 with low and high (inset) magnifications; (b) TEM and (c) HRTEM images of PSCs-73; (d) DF-STEM analysis of PSCs-73 (lower) and EDX line scan (upper) result showing the element distribution of C, S, and N; (e) DF-STEM image of PSCs-73 and corresponding elemental mapping for C, S, and N; and (f) EDS of PSCs-73.

S2). The activation by acid treatment appears to be an important method to improve interfacial adhesion and introduce polar functional groups and positively charged surface, which can influence nucleation energy and thus regulate the nucleation sites and nucleation density.24 Moreover, the high elastic coefficient and plenitudinous space of colloidal carbon sphere were also beneficial to suppress intrinsic large volume change during charge/discharge process as we reported before.20,24 It is documented that conductive, structurally stable polyaniline (PAN) could inhibit diffusion of soluble lithium polysulfides and fasten resident S by cross-linking of inter- and/ or intrachain bonds.1,9,19 The positive effects in immobilization of S by conductive PAN have motivated us to modify Sembedded colloid carbon spheres by PAN to further inhibit diffuse of polysulfides and improve stability of sulfur cathode during electrochemical cycling. In situ chemical polymerization

at low temperature was an effective approach to prepare small PAN nanoparticles (Figure S3a), which makes it possible to modify the S-embedded colloid carbon spheres and then react with sulfur inside to form a three-dimensional, cross-linked, structurally stable composite with partial PAN on the outer surface. However, as an old saying says “Going too far is as bad as not going far enough”, which means that the thickness of PAN should be appropriate. The TEM images and electrochemical performances of PSCs with different contents of PAN are shown in Figure S4, which indicates that 5 nm PAN on the outer surface of the sphere composites was the best. It is reported that the oxidation kinetics of lithium sulfide to lithium polysulfides was slowed down when PAN was used to retard polysulfide dissolution.9,10 So, if there is more PAN on the outer surface, the tardy process for the oxidation of lithium sulfide to lithium polysulfides may hamper the electrochemical performance. B


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Figure 2. (a) Raman spectra of bare S (i), mSC-73 (mixture of S and C, the sulfur content was designed as 73 wt %) (ii), SCs-73 (iii), and PSCs-73 (iv); (b, c) N2 adsorption−desorption isotherm curves for carbon nanospheres and PSCs-73 (the inset is the BJH pore size distribution); cycle voltammogram (CV) profiles of (d) SCs-73 and (e) PSCs between 1.5 and 3.0 V at a scan rate of 0.3 mV s −1; and (f) voltage profile of SCs-73 and PSCs-73 between 1.5 and 2.8 V at 500 mA g−1 (sulfur).

S strongly. Obviously, parallel red and black curves in Figure 1d represented an even distribution of C and S in the nanospheres of PSCs-73. Moreover, the dark-field scanning transmission electron microscopy (DF-STEM) and corresponding elemental mapping (C, S, and N) depicted in Figure 1e demonstrated the uniform distributions of C, S, and N. Of note, this result is in good agreement with that of line-scanning EDX measurement and FTIR results, which further testifies that in situ chemical polymerization was helpful to make PAN link with S. Additionally, the energy-dispersive X-ray spectroscopy (EDS) was performed to determine the chemical composition of PSCs-73, and the result was displayed in Figure 1f. There was about 66.2 wt % of S, 27.4 wt % of C, 1.3 wt % of N, and 5.1 wt % of O in PSCs. The presence of O in the EDS result was derived from residual functional groups in carbon sphere, adsorbed gas, and water. This is in accordance with the result of TGA (Figure S1a), which manifests the S content in PSCs-73 was 73 wt %. This special sphere with uniform dispensation of C and S can effectively enhance the electrical conductivity by reducing the contact resistance among the particles.11,27 Raman spectroscopy was used to further investigate the structural feature of PSCs-73 (Figure 2a). The strongest peaks centered at 154, 220, and 471 cm −1 matched with corresponding E2, E3 and A1 symmetry species of S−S bonds, respectively.9 As for the disordered D band and G band, they were the intrinsic characteristic peaks for carbon matrix in PSCs-73, which can enhance the electron transportation from/ to the sulfur.28 Noteworthy, the two dissemination peaks located at 400 cm−1 (S−S bond) and 1300 cm−1 (C−C bond) resulted from the vibrational effect of the C−N bond in PAN and S−N bond between PAN and S inside the nanosphere, proving once again that PAN was not only coated on the surface of the composite, but also immobilized S inside the nanosphere as Bein et al. reported that PAN can fill into mesoporous channel through in situ chemical polymerization.29 The three-dimensional, cross-linked, structurally stable PSCs-

The representative synthetic procedures of the in situ polymerized PAN-assisted S/C nanospheres (PSCs-73, with 73 wt % S, the best sample) electrode is illustrated in Scheme 1. First, acid active sites were formed inside of colloidal carbon spheqre to induce the disproportionation of NaS2O3 (NaS2O3 → NaSO3 + S↓).10 Then, the generated sulfur nucleated and aggregated around the acid active sites. The combination of S and carbon frame was consolidated along with the thermal contraction of carbon sphere during carbonization process. Finally, PAN was used to modify S-embedded colloid carbon spheres (SCs) through an in situ chemical polymerization and vulcanization, which encapsulated and immobilized S in carbon spheres further. The XRD characteristic peaks of PAN in Figure S1b (dark green curve around 28°) and −NquinoidN− stretching band at 1150 cm−1 as well as the out-of-plane vibration of the C−H mode at 780 cm−1 in FTIR spectrum of Figure S2b unambiguously disclosed the existence of PAN.9,19,25 Compared with SCs-73, these new bands shown in PSCs-73 illustrated that S reacted with the unsaturated bonds in the polymer chains for the formation of cross-linked, stereonetwork structures during heat treatment. This process is known as “vulcanization reaction,” availing for chemical confine of S.26 The morphology and microstructure of as-prepared PSCs-73 were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As it can be seen, uniform nanospheres with homogeneous distribution showed smooth surface without naked crystalline sulfur particles (Figure 1a and b), suggesting S was encapsulated in the porous carbon spheres successfully.11,27 A PAN layer with a thickness of approximately 5 nm was clearly distinguished from SCs-73 (Figure 1c), which was further confirmed by the linescanning EDX in Figure 1d. The middle straight line and two sharply peaks of N distribution curve signified that some PAN was uniformly present inside, while some was coating outside of the nanospheres, which is important for PSCs-73 to immobilize C


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Figure 3. (a) Cycling performance of PSCs-73, SCs-73, and mSC-73 electrodes at a current density of 0.5 C (1C = 1670 mA g−1 or 1.34 or 1.34 mA cm−2) within voltage window of 1.5−2.8 V; (b) cycle voltammogram (CV) profiles of PSCs-73 before and after activation (50 cycles); (c) voltage profiles of PSCs-73 and mSC-73 cycled at various C-rates; (d) pictures showing that three lithium batteries in series can light up 12 yellow (d1), green (d2), and blue (d3) indicators of 2835 LED modules (0.96 W), even 57 white (d4) indicators of 2835 LED modules (2.28 W) (inset is the circuit diagram).

multiple reaction mechanism of S, the two cathodic peaks of PSCs-73 at potentials of ∼2.3 (Peak I) and ∼2.0 V (Peak II) (in Figure 2e) corresponded to the formation of polysulfides and Li2S2/Li2S, respectively. Plus, the anodic peaks around 2.4 V were attributed to the reverse process.2,30 It is worth mentioning that the sharp redox peaks of PSCs-73 (Figure 2e) indicated the fast kinetics and less reaction species during the reversible electrochemistry reaction.1,9 In other words, only slight soluble polysulfides diffused during this process before they were reduced. While SCs-73 presented two blunt cathodic peaks, which may be caused by diffusion of varieties of polysulfides. Even worse, two cathodic peaks merged into one in CV curve for mSC-73 compared with PSCs-73 (Figure S6), probably due to incompletely reduction. It is likely because the formation of Li2S on the preexisting sulfur particle surface decreased conductivity and impeded the further reduction of polysulfides.1 This scenario went against the stability of the electrochemical reaction. Figure 2f shows the galvanostatic charge/discharge profile of the SCs-73 and PSCs-73 cathodes at a current rate of at 500 mA g−1 (sulfur). It is consistent with the cyclic voltammetry analysis mentioned before, typical twoplateau behavior was observed during the discharge, which was assigned to the formation of long-chain polysulfides (high

73 is likely to provide a sturdy shield to confront the diffusion of polysulfides. Substantially, this result was also in good agreement with that in EDX measurement (Figure 1d and e4). Compared with microporous carbon sphere (Figure 2b), the N2 adsorption−desorption isotherm curves of PSCs-73 (Figure 2c) excitingly revealed a type IV isotherm with a type-H1 hysteresis with a broad pore size distribution at 14, 28, and 73 nm. And the surface area of this composite was 59.6 m2 g−1. To better understand the pore structure of prepared materials, N2 adsorption−desorption isotherms of SCs-73, PSCs-73, PSCs86 (PSCs with 86 wt % S), and mSC-73 (mixture of S and C with 73 wt % S) were measured (Figure S5). Discrete mesoporous distribution of SCs-73 and PSCs-73 indicates that the sulfur was filled into the carbon sphere but not just mixed with carbon sphere. So it can be reasonably speculated that the mesopores of SCs-73 and PSCs-73 were resulted from the expansion of sulfur during heat treatment and vulcanization of PAN and sulfur. However, excess sulfur expansion in PSCs86 narrowed the pore size (Figure S5d) and resulted in the decrease of surface area (Figure S5f). Cyclic voltammograms of SCs-73 (Figure 2d) and PSCs-73 (Figure 2e) cathodes were investigated at a scan rate of 0.3 mV s−1 and potential window of 1.5 and 3.0 V. According to the D


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Nano Letters plateau) and Li2S2/Li2S (low flat plateau). Compared with SCs73, a smaller capacity difference between charge and discharge was observed for PSCs-73, further illustrating the effectiveness of PAN in limiting the diffusion of soluble polysulfides. Besides that, higher and smoother discharge voltage plateau of PSCs-73 than that of SCs-73 mirrored less resistance and better stability.14,24 Cycling performances were conducted to explicitly evaluate the electrochemical performance of the PSCs-73 composite (Figure 3a). Obviously, PSCs-73 cathode assisted with PAN exhibited more steady cycling performance over 550 charge/ discharge cycles at 0.5 C (1 C = 1670 mA g−1 or 1.34 mA cm−2) than that of mSC-73 and SCs-73 cathodes. Actually, the capacity of PSCs-73 increased from initial 809 to 862 mAh g−1 in the first 20th cycle and then maintained at high capacity of 775 mAh g−1 after prolonged 550 cycles. For the mSC-73 and SCs-73 cathodes, the total capacity retentions were just 13% and 56% after only 100 cycles, much worse than that of PSCs73 cathode. Moreover, the initial and average Coulombic efficiency of PSCs-73 were 95.0 and 99.5%, respectively, implying slight polysulfides diffusion in the cyclic process. All of the contrast results illustrate that S-embedded carbon spheres modified with PAN could retard polysulfides diffusion and strengthen the reaction kinetics between lithium and sulfur. Especially, PAN played a more important role in polysulfide diffusion inhibition. Here, the gradual increase of capacity of PSCs-73 during the first several cycles indicated activation process of the electrodes. As shown in Figure 3b, the gap between anodic and cathodic peaks of PSCs-73 became smaller after activation (after 50 cycles), demonstrating that the potential barriers of reactions significantly decreased.32 In addition, after activation, the anodic peaks of PSCs-73 were divided into two peaks, which corresponded to the reverse twostep reduction reaction. This transition was helpful for improving the oxidation kinetics of lithium sulfide to lithium polysulfides. In this case, the activation process could be explained by the increase of available sulfur during the cycling process. Because of the PAN shell, it took some time for sulfur to get contact with the electrolyte and become fully electrochemically active.32−34 Charge/discharge voltage profiles of PSCs-73 and mSC-73 cathodes (Figure 3c) at various Crates (0.2, 0.5, 1, 3, and 5 C) were tested to evaluate their performance. It can be noticed that both the discharge voltage plateaus and the discharge capacity decreased gradually with increasing current rate, which can be ascribed to higher ohmic and kinetic overvoltage at a higher current rate.26 Compared with mSC-73, PSCs-73 showed inconspicuous polarization and capacity difference between the charge and discharge profiles, indicating the overwhelming contribution of sulfur wrapped inside of the carbon matrix and immobilization of sulfur by PAN. To further evaluate as-prepared materials for real high-power application, three half-cells consisting of 1.7 mg of PSCs-73 were assembled in series. After charging for only 10 min (at 5 C) at 6.53 V, the device could power 12 yellow, green, and blue round LED indicators efficiently (nominal voltage is 12 V, and nominal power is 0.96 W; inset is the circuit diagram; Figures 3d1, 3d2 and 3d3). More strikingly, the device drove 57 white indicators of LED modules (nominal voltage is 12 V, nominal power is 2.28 W) robustly after minutes of charging (Figure 3d4), meaning that instantaneous current was 205 A g−1 or an instantaneous power was as high as 1341 W g−1. These results directly and impressively revealed the outstanding high-power

discharge performance of PSCs-73, which indicates that the standpoint of rapid electronic/ionic transport and improved electrochemical kinetics can be achieved by a poriferous carbon matrix assisted with conductive polymer as we expected. Figure 4a presents the rate performance of PSCs-73 cathode cycled at

Figure 4. (a) Charge/discharge capacity of PSCs-73 cycled at various C-rates; (b) cyclic voltammograms of activated PSCs-73 (after 50 cycle) at different scan rates; (c) cycling performance of PSCs-73 at 5 C; and (d) the sulfur content in PSCs-73 after different electrochemical cycle.

various C-rates. The average reversible capacities presented to be 1040 (760 for composite mass), 806 (588), 667 (487), and 384 (280) mAh g−1 at discharge current density of 0.2, 0.5, 1, 3, and 5 C (1 C = 1670 mAg−1), respectively. When the C-rate was switched abruptly from 5 to 0.5 C again, the original capacity was largely recovered, indicating unexceptionable stability of the cathode materials. Evidently, the CV curves of activated PSCs-73 (Figure 4b) kept well when the scan rate increased from 0.3 to 3.0 mV s−1. A new cathodic peak emerged around 2.1 V due to the incomplete reduction of polysulfides when the scan rate increased to 10.0 mV s−1, the cathodic peak was still sharp. To be sure, the PSCs-73 possessed superior rate performance, which could be ascribed to the restraint of polysulfide diffusion as it is reported that soluble lithium polysulfides was the main contributor for capacity at high rate.14 Figure 4c displayed the cycling characteristic of PSCs-73 battery device as many as 2500 times, which have rarely been demonstrated for half-cells based on liquid electrolyte. The increased capacity during first 700 cycles was also likely due to an “activation process”.3,8 When the cycling approached 2500 times, the capacity was still kept at 345 (252 for composite mass) mAh g−1 with a retention of 75%, corresponding to a very small capacity decay of 0.01% per cycle. It has represented a wonderful high-power performance for long-cycle lithium− sulfur batteries so far. It still can be argued that the PSCs-73 with the help of PAN obviously impeded polysulfide diffusion E


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Figure 5. (a) TEM images of PSCs-73 before, (b) after 2500 cycles at 5 C, and (c) their corresponding diameter distribution; (d) Nyquist plots for the SCs-73, PSCs-73, PSC-86, and mSC-73 in the frequency range of 10 mHz to 100 kHz before and (e) after 50 cycles at the current density of 0.5 C; and (f) schematic illustration for the properties of mSC-73, SCs-73, SCs-73-SiO2-20 nm, and PSCs-73 during lithium insertion and extraction.

fracturation of electrode materials. In particular, the conductive PAN-assisted procedure made further efforts to prevent polysulfides from dissolution and diffusion into the electrolyte. Moreover, no obvious volume change can be seen from the TEM images (Figure 5a and b), diameter distribution (Figure 5c) and cross-sectional SEM images (Figure S7a and b) of PSCs-73 before and after 2500 cycles at 5 C, which further confirmed the advantage of PSCs-73 in accommodation of S volume expansion. We further carried out electrochemical impedance spectroscopy (EIS) measurements of the SCs-73, PSCs-73, PSC-86, and mSC-73 before and after 50 cycles. As shown in Figure 5d and e, the charge transfer resistance of PSCs-73 was much lower than that of mSC-73 before and after 50 cycles, indicating that the carbon matrix could effectively reduce the charge transfer resistance and increase stability of PSCs-73. Compared with SCs-73, the low resistance of PSCs73 means that the vulcanization in PSCs-73 would consolidate the link of sulfur and carbon sphere and also reduce the resistance of PSCs-73. The impedance of PSCs-86, one composite with 86 wt % of sulfur, was found to be relatively high when compared with PSCs-73. That is to say, over sulfur loading led to resistance increase. Notably, after 50 cycles, the resistance change of PSCs-73 was inconspicuous, showing the

and thus reduced the loss of active materials, which is directly supported by corresponding sulfur content in Figure 4d. It can be seen that the content of sulfur decreased slowly with the increase of cycle time, but the amount change was slight. Before cycling, about 71.8 wt % was found in PSCs-73 composite, which was in accordance with TG results shown in Figure S1a. After 2500 cycles, the content of sulfur still reached 63 wt %, namely, only 12.3% sulfur loss. To evidence the stability of PSCs-73, the SEM images, DFSTEM image, and corresponding elemental mapping images of PSCs-73 after the 2500th cycle at 5 C were collected, and the results are shown in Figure S7. As it can be seen, the nanosphere structure of PSCs-73 electrode was basically maintained after 2500 cycles, and the elements of C, S, and N displayed very similar intensity distributions to that before cycling (Figure 1). Although, the distribution of sulfur was shifted to one side of the nanosphere (compared with that for PSCs-73 before cyclic process), and finally intercepted by the PAN layer as shown in Figure S7d. This scenario indicates that such elastic spherical structure, which could ease the volume expansion during charge/discharge process,20,24 is of great significance for overcoming the polysulfide diffusion problem in polymer-coated S batteries, resulting in the crack and F


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research funds of CCNU from the colleges’ basic research and operation of MOE (Nos. CCNU15ZD007 and CCNU15KFY005).

stability of electron transfer. This may relate to insignificant structure change and little diffusion of polysulfide. Consequently, based on the discussion above, it is easy for us to draw the conclusion that polysulfide diffusion can alter electrical conductivity and then affect the stability and cycling performance. In order to testify this statement, SiO2, one material, which is widely used to impede electron transfer in photocatalytic reaction,35,36 was coated on the surface of SCs73. The XRD patterns in Figure S9a unambiguously disclosed the peak of amorphous SiO2,37 and the SEM images (Figure S9b and c) reveals that the thickness of SiO2 layers were 8 and 20 nm, respectively. As seen in impedance curves in Figure S9d, SCs-73-SiO2 showed poorer conductivity than SCs-73, and the resistance increased with the increase of SiO2 layer thickness. Notwithstanding, the capacity of SCs-73-SiO2 (Figure S9e) was still much higher than SCs-73, for the reason that SiO2 outside prevented the diffusion of polysulfide. As described in the mechanism diagram (Figure 5f), the fatal dissolution of polysulfide and the crack of mSC-73 lead to the capacity decrease despairingly after 100 cycles.11 Compared with PSCs73, SCs-73 without of the protection shell displayed fast capacity fade, indicating that restraining polysulfide dissolution is vital for Li/S batteries. That is why SCs-73-SiO2 showed poor conductivity but still possessed better capacity than SCs-73. In conclusion, we have first successfully synthesized in situ polymerized PAN-assisted S/C nanosphere with high S content (PSCs-73) electrode, which had a long high-power cycling capability over 2500 charge/discharge cycles with a capacity decay of 0.01% per cycle. Strikingly, this composite can drive 57 white indicators of LED modules (2.28 W) robustly after minutes of charging by three lithium batteries in series. This novel composite can effectively minimize polysulfide diffusion. Moreover, the inherited advantages of both colloidal carbon sphere and in situ polymerizated PAN may enhance electron transfer rate and remit the volume expansion of S. The design of electrode material with special structure and temperate scaleproducible technology here can be applicable for other electrode materials with high electrochemical performance. This work will shed light on the development of highperformance rechargeable batteries as well.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, additional Supporting Information and figures. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.nanolett.5b01294.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

H.H. and H.C. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Science Foundation of China (No. 21377044), the Key Project of Natural Science Foundation of Hubei Province (No. 2015CFA037), Wuhan Planning Project of Science and Technology (No. 2014010101010023), and self-determined G


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