Atomic Iron Catalysis of Polysulfide Conversion in Lithium-Sulfur

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Atomic Iron Catalysis of Polysulfide Conversion in Lithium-Sulfur Batteries Zhenzhen Liu, Lei Zhou, Qi Ge, Renjie Chen, Mei Ni, Wellars Utetiwabo, Xiaoling Zhang, and Wen Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03830 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 26, 2018

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ACS Applied Materials & Interfaces

Atomic Iron Catalysis of Polysulfide Conversion in Lithium-Sulfur Batteries Zhenzhen Liu,a,# Lei Zhou,a,# Qi Ge,a Renjie Chen,b* Mei Ni,a Wellars Utetiwabo,a Xiaoling Zhang,a* Wen Yanga,c*

a

Key Laboratory of Cluster Science of Ministry of Education, Beijing Key

Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China. b

School of Material Science and Engineering, Beijing Institute of Technology, Beijing

100081, P.R. China. c

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials,

Donghua University, Shanghai 200051, P.R. China. #

They contributed equally to this work

* Corresponding author: [email protected]; [email protected]

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Abstract Lithium-sulfur batteries have been regarded as promising candidates for energy storage because of their high energy density and low cost. It is a main challenge to develop long-term cycling stability battery. Here, a catalytic strategy is presented to accelerate reversible transformation of sulfur and its discharge products in lithium-sulfur batteries. This is achieved with single-atomic iron active sites in porous nitrogen-doped carbon, prepared by polymerizing and carbonizing diphenylamine in the presence of iron phthalocyanine and a hard template. The Fe-PNC/S composite electrode exhibited a high discharge capacity (427 mAh g-1) at 0.1C rate after 300 cycles with the Columbic efficiency of above 95.6 %. Besides, the electrode delivers much higher capacity of 557.4 mAh g-1 at 0.5C over 300 cycles. Importantly, the Fe-PCN/S has a smaller phase nucleation overpotential of polysulfides than nitrogen-doped carbon alone for the formation of nanoscale of Li2S as revealed by ex-situ SEM, which enhance lithium-ion diffusion in Li2S, therefore a high rate performance and remarkable cycle life of Li-sulfur batteries were achieved. Our strategy paves a new way for polysulfide conversion

with

atomic

iron

catalysis

to

exploit

high-performance

lithium-sulfur batteries.

Keywords: Iron and nitrogen-doped carbon, single-atomic catalysis, polysulfide conversion, Li2S, Lithium-sulfur batteries

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Introduction Lithium-sulfur (Li/S) batteries have aroused the interest of many researchers because sulfur can be electrochemically activated to accept up to two electrons, resulting in the conversion of S8 to Li2S (S8 + 16Li → 8Li2S).1-6 Therefore, Li/S batteries have a very high theoretical capacity, corresponding to a gravimetic energy density of 2500 Wh kg-1 or volumetric energy density of 2800 Wh L-1. Unfortunately, the commercialization of lithium-sulfur batteries has suffered from several problems such as rapid capacity decay, insufficient cycle life, low Coulombic efficiency, and self-discharge. The lithium polysulfide (LPS, Li2Sn, 4≤ n ≤8) shuttle mechanism has been proposed as the main cause for these performance barriers.7-9 The shuttle mechanism leads to dissolution of LPS in aprotic solvents. The soluble sulfur-based intermediates would thereby leave the cathode, and be transported to the Li anode through the separator generating a passivation layer. Moreover, the intrinsically poor electrical conductivities of sulfur and its discharge product (Li2S/Li2S2) cause low sulfur utilization in lithium-sulfur battery systems. Recently, a catalytic strategy for LPS reduction and oxidation of its discharge products (Li2S2/Li2S) in the lithium-surfur battery has been proposed.5-13 In the process, LPS is rapidly converted to Li2S, and subsequently the Li2S is catalytically oxidized back to S in the reverse reaction in the presence of a nanocatalyst such as metal oxides, metal sulfides, etc. Thus, a Li/S battery with highly reversible capacity and long cycling life can be achieved. Although the effect of nanocatalyst on polysulfide redox conversion is pronounced, a large amount of nanocatalyst is required, possibly leading to decreased surface area and conductivity of carbon hosts, in turn causing low sulfur loading and utilization efficiency. In addition, it is difficult to distinguish the contributions of these metal oxides and sulfides on the capacity of the lithium-sulfur battery. Single-atom catalysts have been considered to be preferential choices for 3



catalysis, since the exposed atom efficiency is maximized.14-21 Single iron and nitrogen-doped carbon have been used as nanocatalysts in oxygen reduction, hydrogen evolution, and CO2 reduction. To the best of our knowledge, there have been no reports on single iron- and nitrogen-doped carbon as an activation catalyst to facilitate LPS redox conversion in lithium-sulfur batteries. Herein, we demonstrate that single iron- and nitrogen-doped carbon (Fe-PNC) host materials and catalysts can accelerate LPS redox conversion in the lithium-sulfur battery. The Fe-PNC/S composites with a high sulfur loading (1.3 mg cm-2) show higher capacity, lower overpotential, better rate performance and cycling stability over pristine nitrogen-doped carbon (PNC). The Fe-PNC/S composites without any LPS anchoring materials show an initial specific capacity of 1138.6 mAh g-1 at 0.1 C rate and maintain a discharge capacity of 427.1 mAh g-1 after 300 cycles. The capacity decay rate is 0.2% per cycle and the high Columbic efficiency of 99.0% is maintained during the discharge/charge process.

Results and Discussion The catalyst was synthesized via a simple nanocasting method using iron phthalocyanine as iron precursor, o-diphenylamine (oPD) as nitrogen precursor, and Ludox colloid as hard template in the Scheme S1(seeing supporting information). By annealing in a N2 atmosphere at 900 ºC and subsequent removal of the template, atomic-level iron is highly dispersed in porous nitrogen-doped carbon materials with iron-nitrogen coordinated active sites. The surface morphology of Fe-PNC was studied by transmission electron microscopy (TEM). The TEM images, as shown in Fig. S1, indicate that Fe-PNC contains a large number of interconnected mesopores, and the porous nanostructures of Fe-PNC and PNC are identical. The diameters of the mesopores are close to the size of bare templates. High-resolution TEM with sub-Ångström resolution was performed (Fig.1 a and b). In the carbon matrix, homogeneously dispersed black dots with diameters in the subnanometer range are cleanly resolved. These black dots each correspond to a single iron atom, as 4


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it is a heavy element in comparison with C and N. The presence of iron atoms on the nitrogen-doped carbon materials was further confirmed by elemental mapping in scanning TEM (Fig. S2). Almost all of the iron atoms are highly dispersed in the nitrogen-doped carbon layer, and the signals of the iron and nitrogen overlap, indicating a coherent existence of iron and nitrogen throughout the entire sample. Inductively coupled plasma mass spectrometry was used to analyse the metal contents of the catalyst, and XPS characterization was performed to probe the elemental composition (Fig 1, Fig S3, Fig. S4 and Table S1). Fe-PNC contained Fe 1.0 wt.%, N 6.9 wt.% (from X-ray photoelectron spectroscopy, XPS) and O 5.8 wt.%. The N 1s spectrum (Fig. 1c and Table S2) was deconvoluted into pyridinic-type nitrogen (Binding Energy = 398.8 eV), metal-nitrogen (399.9 eV), graphitic-type nitrogen (401.2 eV), and pyridinic-N-oxide (402.3 eV). The presence of metal-nitrogen bonding in the catalyst further confirmed the presence of metal-coordinated active sites in the carbon structures.

Figure 1. (a) and (b) HRTEM images of Fe-PNC. (c) XPS study of N1s core lever spectra of Fe-PNC. (d) 57Fe Mössbauer transmission spectra at 293 K for Fe-PNC.

Mössbauer spectroscopy was used to probe iron coordination in Fe-PNC. As given in Fig 1d, the absorption spectrum measured at room temperature was 5



successfully fit with three iron species: (1) D1, corresponding to Fe(II)N4/C (low spin), which is an iron(II) center coordinated to four pyrrolic N-groups connected to the surface of the carbon support; (2) D2, signaling to Fe(II)N2+2/C (intermediate spin), which is a typical iron(II) center in crystalline iron-phthalocyanine; (3) D3, assigned to high-spin FeN4-centers found in different porphyrins (Table S4).20 It should be noted that singlet and sextet components are absent, indicating no Fe or iron carbide or other impure phase in Fe-PNC, and that atomic iron is highly dispersed in nitrogen-doped carbon layers.14 The XRD powder pattern reveals the absence of any Fe-related crystalline phase in the Fe-PNC (Fig S5). Moreover, the O K-edge of the Near-Edge X-ray Absorption Fine Structure Spectrum of Fe-PNC (NEXAFS) also shows that iron oxides such as Fe2O3 are absent in the samples (Fig. S6). These results indicate that atomic iron is homogeneously dispersed in the porous nitrogen carbon materials.22 Carbon K-edge of NEXAFS Spectroscopy was performed to determine the effect of iron doping on the carbon crystalline structure.23-25 As given in Fig. S7a, two main peaks located at 284.5 eV and 298.8 eV, assigned to transitions from C1s core levels to C-C unoccupied π* orbitals and to transitions from C1s core levels to a dispersionless σ* state, respectively. The relative intensities of π* and σ* resonances (Iπ*/Iσ*) is an important parameter for determining the electronic structure of heteroatom-doped carbon materials.22,23 The Iπ*/Iσ of Fe-PNC (0.57) is lower than that of PNC (0.63), indicating that iron-doping contributes to the disruption of π conjugation in the carbon lattice. Fig. S8 shows N K-edge NEXAFS spectra of PNC and Fe-PNC. C1s XPS spectra were also acquired to study the degree of sp2-bonding on the carbon surface, and they confirmed the phenomenon. The C1s spectrum of Fe-PNC is negative shifted compared with the PNC spectrum, indicating that the degree of sp2-bonding of carbon decreases after Fe-doping (Fig. S7b). Raman spectroscopy was further performed. Two distinct bands at 1345 cm-1 and 1590 cm-1 were observed (Fig S9), which can be attributed to the defective/disordered sp3 hybridized carbon (D band) and the crystallized graphitic sp2 carbon (G band), The intensity ratio 6


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of D and G (ID/IG) increased from 0.86 (PNC) to 0.88 (Fe-PNC), indicating that disorder in the carbon lattice increased after Fe-doping. Fig.S10 shows nitrogen adsorption-desorption isotherms and pore size distributions for the Fe-PNC and PNC samples. They are typical type IV isotherms, and the type-H3 hysteresis ring appears when the relative pressure (P/P0) is 0.45-0.95, indicating that both the Fe-PNC and PNC samples have mesoporous structures. Moreover, the specific surface area of the sample was calculated by the BET method,24 and the specific surface areas of Fe-PNC and PNC were 239 m2 g-1 and 237 m2 g-1, respectively (Table S3). The XRD patterns of pristine sulfur, PNC/S and Fe-PNC/S composites were shown in Fig. S11. Fig.S12 shows the thermogravimetric analysis (TGA) thermograms of pure sulfur, PNC/S and Fe-PNC/S composites. Compared with elemental sulfur, the Fe-PNC/S and PNC/S samples show a small mass loss at 30 °C to 180 °C due to evaporation of adsorbed moisture in the sample cell, Fe-PNC/S shows the highest sublimation temperature, probably due to a strong mutual interaction between Fe atoms and elemental sulfur. The total mass loss gives the sulfur content of the carbon-sulfur composite (about 70%). The sulfur loading in Fe-PNC/S is 1.3 mg cm-2.

1

2

3

Figure 2. (a) UV-vis absorption spectra and (b) photograph showing the variation in color of the polysulfide solution (1) after adsorption by PNC (2) and Fe-PNC (3). (c) polarization curves tested at scan rate of 50 mV s-1 and (d) electrochemical impedance 7



spectra of Li2S6-Li2S6 symmetric cells. (e) Schematic illustration of the conversion process of LPS on the Fe-PNC surface with single-atomic iron catalytic sites. It is proposed that the single-atomic iron can suppress the shuttling effect of polysulfides by enhanced interaction in lithium-sulfur batteries. The adsorption abilities of PNC and Fe-PNC were compared in the Li2S6/DOL/DME solution. As shown in the Fig. 2a, the UV-Vis spectra proved that the Fe-PNC host has stronger entrapment ability to Li2S6 due to the synergistic contribution of physical adsorption and chemical interaction. Furthermore, the photographs (Fig. 2b) exhibit a visible decoloration of Li2S6 solution after the adsorption. Previous result showed that the dipole-dipole electrostatic interaction between PS species and nitrogen or O atoms functional groups on the carbon materials could enhance the adsorption of PS species.26 Moreover, Lewis acid-base interaction between PS species and metal organic framework was also reported. Li et al found that the electrophilic Co(II) in the macroporous carbon with Co-Nx site absorbs PS species via a strong interaction between S and Co in S-Co bond.27 Therefore, we believed that Fe-PNC with exclusive iron-nitrogen coordinate active sites, such as Fe(II)N4, have high strong chemical affinity to PS species because the nucleophilic S42- trend to absorb at the electrophilic Fe(II) in the Fe(II)N4.28, 29 The results indicate that Fe-PNC presents better adsorption capability for Li2S6 than PNC The electrocatalytic properties of Fe-PNC and PNC samples for polysulfide redox were investigated by cyclic voltammograms (CV) within a potential window from -0.7 to 0.7 V for PNC/PNC and Fe-PNC/Fe-PNC symmetrical cell. The Fe-PNC exhibits the highest current under identical conditions in Fig. 2c, indicating rapid polysulfide redox conversion on the electrode. The accelerated polysulfide redox reaction is attributed to the catalytic effect of Fe-PNC and the lower charge transfer resistance at the electrode surface, as confirmed by electrochemical impedance spectroscopy (EIS) in Fig. 2d. The rapid conversion of the polysulfide solution further influenced the performance of lithium sulphur batteries, which involve a series of liquid-solid reactions, such as S8 ↔ Li2S8, and Li2S4 ↔ Li2S2/Li2S. Cyclic voltammogram 8


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measurements in lithium-sulphur batteries were also performed to probe the accelerated polysulfide conversion using different host materials. As displayed in Fig.3a, two sharp cathodic peaks at 2.31 (I) and 2.02 (II) V, attributed to S8

→ Li2S8, and Li2S4 → Li2S2/Li2S, respectively, are observed on the Fe-PNC/S composites. In contrast, the two cathodic peaks at 2.27, and 1.96 V on the PNC/S composites, correspond to Li2S → S8, are 40 and 60 mV positive compared to the Fe-PNC/S composites, indicating the rapid polysulfide conversion observed on Fe-PNC/S composites. Moreover, two noticeable anodic peaks at 2.41 and 2.44 V, are observed on Fe-PNC/S, which are 30 mV and 30 mV negative compared with the PNC/S composites with anodic peaks at 2.44 and 2.47 V. It should be noted that IpaIII/IpaIV is larger on the Fe-PNS/S composite than the PNS/S composite. Table S5 and Table S6 provides the corresponding onset potentials and peak voltages of asymmetrical Li-S cells using different host materials of PNC and Fe-PNC in the first cycle. Bigger cathodic peaks ， larger IpaIII/IpaIV ratio, and greater peak intensities of Fe-PNC/S electrode confirmed the greatly enhanced polysulfide conversion. Fig S13 shows CV profile of asymmetrical Li-S cells using Fe-PNC as a host material in the second cycle. These results indicate that the single atom catalyst can preferentially adsorb LPS during the discharge process, and rapidly convert LPS into Li2S in the lithium-sulfur battery, while catalysing the oxidation of Li2S back to sulfur in the reverse process. Thus, the entire polysulfide redox conversion process is accelerated in the lithium-sulfur battery, and the inherent shuttle effect of lithium-sulfur battery is controlled in the Fig 2e. The galvanostatic discharge and charge curves in Fig. 3 further confirm the result of above CV. In comparison with PNC/S composites, the discharge voltage plateaus of Fe-PNC/S composites shift positively and the charge voltage plateaus shift negatively, indicating a smaller polarization at charge/discharge plateaus due to the high affinity of Fe-PNC for polysulfide adsorption; the improved kinetics and reversibility of polysulfide redox conversion. As exhibited in Fig.3b, the Fe-PNC/S composites present a higher specific capacity of 1138.6 mAh g-1 compared with 9



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PNC/S composites (1113.7 mAh g-1) at 0.1C. The initial Coulombic efficiencies of Fe-PNC/S and PNC/S composites are 99.0, and 97.4 %, respectively. Furthermore, the specific capacities of these electrodes at various rates were also investigated as displayed in Fig.3c. The Fe-PNC/S composites exhibited a higher rate performance in comparison with PNC/S composites, further confirming the kinetic enhancement of polysulfide conversion on Fe-PNS/S composites. In addition, this catalytic effect is more apparent at higher rate on Fe-PNC/S composites as shown in the Fig. S15. To further confirm the excellent polysulfide conversion, EIS measurements were performed for the Fe-PNS/S and PNS-S composites before and after 30 charge/discharge cycles. As shown in the insect of Fig.3d, the Fe-PNC/S composites show much bigger charge transfer impedance during the first cycling, but they show smaller charge transfer impedance after 30 cycles, revealing rapid conversion of polysulfide intermediates after charge-discharge cycle. (Fig 3d). It is a rational explanation that the atomic iron increases the resistance of nitrogen-doped carbon host materials. After discharge-charge cycles, the resistance of Fe-PNC/S electrode decreased gradually due to catalytic effect, which enhanced polysulfide conversion on the Fe-PNC/S electrode. This phenomenon further confirms that atomic iron catalysis tactics could boost the electrochemical performance of Li-S batteries in the cycle. (a)

(b) 2.41

2.44

Ⅲ Ⅳ 2.44

2.47 0.15 0.17

Ⅱ

Ⅰ 2.31

2.27

1.96 2.02

(c)

(d)

7Ω Ω

28Ω Ω

(e)

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Figure 3. (a) Cyclic voltammograms tested at a scan rate of 0.1 mV s-1 and (b) galvanostatic discharge-charge profiles of PNC/S and Fe-PNC/S composites. (c) Rate properties at different current densities for PNC/S and Fe-PNC/s composites. (d) The electrochemical impedance spectroscopy of PNC/S and Fe-PNC/S composites after 30cycles (the inset is preliminary). (e) Cycling stability of PNC/S and Fe-PNC/S at 0.1C and 0.5C.

More notably, the-prepared Fe-PNC/S composites also demonstrate excellent stability at 0.1 C and 0.5C rate in Fig. 3e. Specifically, the Fe-PNC/S electrode still maintains a high discharge capacity (427 mAh g-1) at 0.1C rate after 300 cycles with above 95.6 % Columbic efficiency. And, the capacity decay per cycle over 300 cycles is only 0.2 %. The discharge capacity of Fe-PNC/S composites after 300 cycles is almost 1.8 time larger than that of PNC-S composites (maintaining a capacity of 235 mAh g-1 at 0.1C with 94.6 % Columbic efficiency). In addition, the Fe-PNC/S composite even shows much higher capacity of 557.4 mAh g-1 at 0.5C over 300 cycles compared with the capacity of 427.1 mAh g-1 at 0.1C. It is obvious that the Fe-PNC/S composite exhibits even better eventual capacity and capacity retention after cycling 500 cycles at faster C-rate. This is because that atomic iron in Fe-PNC/S electrode could improve the sluggish kinetics of polysulfide reduction for lithium-sulfur battery due to catalytic effect. Meanwhile, electrode reaction at faster C-rate contributed to reduce the dissolution of polysulfide in electrolyte. Therefore, catalytic effect at faster C-rate markedly enhanced the electrochemical performance of Fe-PNC/S cell by promoting polysulfide conversion. Thus, the Fe-PNC/S composite has a huge advantage in rate capability and cycle stability (Fig S14). Although, some novel designs of the matrix to load the sulfur cathode with batter capacity and cycling stability has been reported by some researchers. However, 11



homogeneous dispersed atomic iron in the nitrogen-doped carbon were prepared by us, and we found that the single-atomic iron active sites in porous nitrogen-doped carbon could single-atomic iron could serve as an electrocatalyst, accelerating the polysulfide redox kinetics, especially for the reduction of soluble Li2S6/Li2S4 to insoluble nanoscale of Li2S particles, and subsequently activate of Li2S nanoparticles. The single-atomic iron catalytic sites on porous nitrogen-doped carbon provide a low cost effective avenue for the development of practical Li-S batteries.

Figure 4. The discharge morphology of Fe-PNC/S and PNC/S electrode at 0.1 C rate: (a) the first discharge morphology of Fe-PNC/S electrode; (b) the second discharge morphology of Fe-PNC/S electrode; (c) the first discharge morphology of PNC/S electrode; (d) the second discharge morphology of PNC/S electrode.

The morphology of Li2S in the Li-S batteries is an important issue. It is considered as the final discharge product in the Li-S batteries. Recently, Yang et al, who applied a high charging cutoff voltage to obtain micrometer-sized Li2S particles for activating electrochemical inert Li2S.30-31 The morphology of both Fe-PNC/S and PNC/S after first and second cycles at 0.1 C was observed by SEM. The samples were obtained coin-cells at fully charged or discharged states. As shown in Fig. 4, Li2S nanospheres composed of small nanoparticles with diameters of about 150 nm 12


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observed in Fe-PNC/S at the first cycle, and the diameters of nanospheres further increased to 200 nm while the morphology was retained at the second cycles. In contrast, for the sample of PNC/S, micrometer-sized rods with length of 1.5-2 µm and width of 120-150 nm are formed in the first cycle, and the scatted distributed micrometer-sized rods were evolved into high density of vertically growing microchips. The results show that the Fe-PCN/S has a smaller phase nucleation overpotential for the formation of nanoscale of Li2S, and thus reduce the distance of Li-ion and enhance Li ion transport rates inside Li2S particles through these nano grain boundaries.

Conclusion In summary, a porous nitrogen-doped carbon decorated with single-atomic iron was synthesized via a simple strategy and further applied as a highly efficient cathode host. The novel hybrid structure not only delivered higher discharge capacity at high rates but also enhanced cycling stability due to strongly coupled Fe-polysulfide interactions as well as the physical adsorption of the polysulfides on the mesoporous carbon microspheres. More importantly, single-atomic iron could serve as an electrocatalyst, accelerating the polysulfide redox kinetics, especially for the reduction of soluble Li2S6/Li2S4 to insoluble nanoscale of Li2S particles, and subsequently activate nanoscale of Li2S nanoparticles. The single-atomic iron catalytic sites on porous nitrogen-doped carbon provide a low cost effective avenue for the development of practical Li-S batteries.

Supporting information Experimental Section; Schematic illustration of the preparation process of Fe-PNC; SEM images of the PNC and Fe-PNC; TEM image of the Fe-PNC and corresponding element mappings; XPS spectra of PNC and Fe-PNC; XRD patterns of PNC and Fe-PNC; O, C and N K-edge NEXAFS spectra of PNC and Fe-PNC; Raman spectra of Fe-PNC and PNC; N2 adsorption and desorption isotherms and corresponding pore 13



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distributions; The XRD patterns of pristine sulfur, PNC/S and Fe-PNC/S composites; CV profiles of asymmetrical Li-S cells using Fe-PNC as a host material in the first cycle and the second cycle; Cycling performance at different current densities of 0.1 C, 0.2 C and 0.5 C of Fe-PNC/S composites; Summary of elemental compositions of PNC and Fe-PNC; Contents of N-species (at.%) in PNC and Fe-PNC; Textural features of PNC and Fe-PNC; Mössbauer parameters at room temperature; The peak voltages of asymmetrical Li-S cells using different host materials of PNC and Fe-PNC; The onset voltages of asymmetrical Li-S cells using different host materials of PNC and Fe-PNC.

Acknowledgments This work was supported by the Nature Science Foundation of Beijing Municipality (No. 2172051), State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, the Open Funds of the State Key Laboratory of Electroanalytical Chemistry (SKLEAC201708). National Natural Science Foundation of China (No. 21575015, 51772030), National Key Research and Development Program of China “New Energy Project

for

Electric

Vehicle”

(2016YFB0100204),

Thanks

to

the

1W1B@Beijing Synchrotron Radiation Facility for providing measurement time. We appreciated help from Dr. Jiaou Wang (4B9B@Beijing Synchrotron Radiation Facility) for XAS measurements. We appreciated Professor Dr. Shaojun Guo (College of Engineering, Peking University) for huge help. We also appreciated Dr. Yi Xing (College of Engineering, Peking University) for ex-situ SEM measurements and discussion.

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Table of Contents (TOC) Graphic

19



111x55mm (150 x 150 DPI)


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Figure 1. (a) and (b) HRTEM images of Fe-PNC. (c) XPS study of N1s core lever spectra of Fe-PNC. (d) 57Fe Mössbauer transmission spectra at 293 K for Fe-PNC. 158x127mm (96 x 96 DPI)



Figure 3. (a) Cyclic voltammograms tested at a scan rate of 0.1 mV s-1 and (b) galvanostatic dischargecharge profiles of PNC/S and Fe-PNC/S composites. (c) Rate properties at different current densities for PNC/S and Fe-PNC/s composites. (d) The electrochemical impedance spectroscopy of PNC/S and Fe-PNC/S composites after 30cycles (the inset is preliminary). (e) Cycling stability of PNC/S and Fe-PNC/S at 0.1C and 0.5C. 119x130mm (150 x 150 DPI)


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Figure 2. (a) UV-vis absorption spectra and (b) photograph showing the variation in color of the polysulfide solution (1) after adsorption by PNC (2) and Fe-PNC (3). (c) polarization curves tested at scan rate of 50 mV s-1 and (d) electrochemical impedance spectra of Li2S6-Li2S6 symmetric cells. (e) Schematic illustration of the conversion process of LPS on the Fe-PNC surface with single-atomic iron catalytic sites. 136x74mm (150 x 150 DPI)



Figure 4. The discharge morphology of Fe-PNC/S and PNC/S electrode at 0.1 C rate: (a) the first discharge morphology of Fe-PNC/S electrode; (b) the second discharge morphology of Fe-PNC/S electrode; (c) the first discharge morphology of PNC/S electrode; (d) the second discharge morphology of PNC/S electrode. 122x83mm (150 x 150 DPI)


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Atomic Iron Catalysis of Polysulfide Conversion in Lithium-Sulfur

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