Kinetically Enhanced Electrochemical Redox of Polysulfides on

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Kinetically Enhanced Electrochemical Redox of Polysulphides on Polymeric Carbon Nitrides for Improved Lithium-Sulphur Batteries Ji Liang, Li-Chang Yin, Xiaonan Tang, Huicong Yang, Wensheng Yan, Li Song, Hui-Ming Cheng, and Feng Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05647 • Publication Date (Web): 06 Sep 2016 Downloaded from http://pubs.acs.org on September 8, 2016

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ACS Applied Materials & Interfaces

Kinetically Enhanced Electrochemical Redox of Polysulphides on Polymeric Carbon Nitrides for Improved Lithium-Sulphur Batteries Ji Liang,a Lichang Yin,a Xiaonan Tang,a, b Huicong Yang,a, c Wensheng Yan,d Li Song,d Hui-Ming Cheng a* and Feng Li a* [a] Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Rd., Shenyang, Liaoning 110016, China. [b] School of Chemical Engineering, Shandong University of Technology, 266 Cunxi Rd., Zibo, Shandong 255000, China. [c] School of Physical Science and Technology, Shanghai Tech University, Shanghai 200031, China. [d] National Synchrotron Radiation Laboratory, CAS Center for Excellence in Nanoscience, University of Science and Technology of China, Hefei, Anhui 230029, China.

ACS Paragon Plus Environment

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ABSTRACT. The kinetics and stability of the redox of lithium polysulphides (LiPSs) fundamentally determine the overall performance of lithium-sulphur (Li-S) batteries. Inspired by theoretical predictions, we herein validated the existence of a strong electrostatic affinity between polymeric carbon nitride (p-C3N4) and LiPSs, that can not only stabilize the redox cycling of LiPSs, but also enhance their redox kinetics. As a result, utilization of p-C3N4 in a LiS battery has brought much improved performance in the aspects of high capacity and low capacity fading over prolonged cycling. Especially upon the application of p-C3N4, the kinetic barrier of the LiPS redox reactions has been significantly reduced, which has thus resulted in a better rate performance. Further density functional theory simulations have revealed that the origin of such kinetic enhancement was from the distortion of molecular configurations of the LiPSs anchored on p-C3N4. Therefore, this proof-of-concept study opens up a promising avenue to improve the performance of Li-S batteries by accelerating their fundamental electrochemical redox processes, which also has the potential to be applied in other electrochemical energy storage/conversion systems.

KEYWORDS: lithium polysulphide, carbon nitride, graphene, adsorption, redox kinetics

INTRODUCTION In the pursuit of a high-energy rechargeable battery, alternative energy storage mechanisms to the intercalation of Li ions in a Li ion battery (LIB) have been extensively studied in recent years.1-6 Among them, direct use of elemental sulphur as a cathode to couple with a lithium metal anode is especially attractive due to its remarkably higher theoretical energy density (2600 Wh kg-1) than a LIB (< 500 Wh kg-1).7, 8


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In a lithium-sulphur (Li-S) system, the redox of lithium polysulphides (LiPSs) is the fundamental process that determines its electrochemical performance. During charge and discharge, various soluble or insoluble species are produced and/or involved, and the soluble species tend to migrate between the anode and cathode under the electric field and concentration gradient (i.e. a shuttling effect).4,

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In addition, among these complex and multi-step redox

reactions, those that have sluggish kinetics impede the overall electrochemical reaction and limit the utilization of active materials.10 Consequently, these two issues lead to detrimental effects on the electrochemical performance of Li-S batteries, including poor cycling stability, incomplete sulphur utilization and low efficiency. For the first issue, it is understood that the undesirable shuttling of LiPSs mainly comes from their solubility in electrolyte solvents (typically 1, 3-dioxolane (DOL) and 1, 2-dimethoxyethane (DME)), which results from the strong attraction between LiPSs and DOL/DME due to the polar characters of these molecules.11 Therefore, to achieve a stable redox of LiPSs and immobilize them in the cathode region, the binding energy between the electrode material and LiPSs should be larger than that between the electrolyte solvents and the LiPSs. For example, different oxides (TiOx, MnO2, SiO2, etc.)

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or heteroatoms (N, S, O, etc.)

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have been decorated/doped on

carbon materials to provide strong chemical interactions with soluble LiPSs and to suppress their shuttling. While great effort has been made to stabilize soluble LiPSs, the importance of accelerating their intrinsic sluggish redox reactions has been paid little attention, although it is another factor that impedes the overall electrochemical process.10, 22 It is therefore desirable to discover a multi-functional material that can (1) anchor LiPSs with high strength and (2) kinetically speed up the electrochemical conversion processes for more efficient and easy LiPS


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redox reactions. More importantly, it is also necessary to understand the mechanism of these materials in attracting the LiPSs and influencing their redox reactions. As recently reported, (heavy) doping of carbon hosts by nitrogen can provide an additional binding between soluble LiPSs and carbon, due to the doping-induced local polarization.11, 17-21 Based on these studies, it is reasonable to speculate that polymeric carbon nitride composed of interconnected tri-s-triazine units (Figure 1a), p-C3N4, might also be capable of localizing LiPSs and preventing them from shuttling, because of its very large and abundant intrinsic in-plane polarization arising from the electronegativity difference between C and N atoms. Apart from this, the very high surface polarization of p-C3N4 might also be strong enough to drag the LiPSs towards it and alter the molecular configurations of the adsorbed LiPSs. Consequently, this would also affect the electron transport between them as well as the kinetics of the LiPS redox reactions. As a result of the above considerations, here we have validated, both theoretically and experimentally, the existence of an electrostatic-induced strong affinity between LiPSs and pC3N4. We then hybridized p-C3N4 with graphene to use it in a Li-LiPS or Li-S cell. Compared with pristine graphene, the integration with p-C3N4 has resulted in a superior redox stability of LiPSs due to its electrostatic localization effect. More interestingly, it has also been found that pC3N4 can kinetically facilitate the redox of LiPSs, increase their conversion efficiency, and improve the utilization of active materials at different charge-discharge rates. Density functional theory (DFT) simulations further revealed that this effect originates from the strong attraction between p-C3N4 and LiPSs that alters the spatial and bonding configurations of LiPS molecules. Therefore, this p-C3N4 hybridized material not only represents a promising candidate for high


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performance Li-S systems, but also has the potential to be used in other electrochemical energy storage/conversion systems involving similar redox processes. RESULTS and DISCUSSIONS Density Functional Calculations.


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Figure 1. Electronic structures of p-C3N4 and Li2S6, and the binding energies of LiPSs with electrolyte solvents, graphene and p-C3N4 based on DFT calculations. (a) Isosurface plot (0.02 e Å-3) of charge density and (b) charge population of p-C3N4. (c) Isosurface plot (0.002 e Å-3) of charge density and (d) charge population of an isolated Li2S6 molecule. (e) The most stable


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adsorbed structure and the charge population of Li2S6 on p-C3N4. (f) The calculated Eb of LiPSs with DOL/DME, and those of LiPSs on graphene and p-C3N4 surface. The electron-rich and electron-deficient regions in the charge density plots are shown in red and green, respectively. The H, C, N, Li, and S atoms are represented by white, purple, cyan, magenta, and yellow balls, respectively.

To discover the interactions between LiPSs and p-C3N4, we calculated their charge density and charge population (Figures 1a, b). It is clear that the electron-rich and electron-deficient regions are centered around the N and C atoms of p-C3N4, showing its in-plane polarization. The charge population analysis further revealed that the N atoms are negatively charged with 1.06 to 1.41 electrons. Taking Li2S6 as a typical example of soluble LiPSs, the electron-rich and electron-deficient regions are centered around the S and Li atoms (Figures 1c, S1), indicating its polarized nature due to the electronegativity difference between S and Li atoms. In this case, the Li atoms are positively charged with 0.86 electrons, while the S atoms are negatively charged with 0.06 to 0.68 electrons, depending on the location of S atoms in the molecule as shown in Figure 1d. Other LiPS molecules have similar charge distributions as shown in Figure S1. As a result, the negatively charged N atoms and the positively charged Li atoms will lead to strong electrostatic attraction between each other. The most stable state of Li2S6 and other LiPS molecules adsorbed on the surface of p-C3N4 was found to be one Li atom bound with two edgeN atoms of p-C3N4 (the –N= sites, Figures 1e, S2). The calculated Li-N distance (2.06 Å) is close to the Li-N bond length (2.10 Å) in α-Li3N.23 It seems that the Li-N bonding formed between Li2S6 and p-C3N4 is a kind of ionic bond. However, the negligible charge transfer from Li2S6 to


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p-C3N4 (0.02 electrons) indicates that the binding between Li in Li2S6 and edge-N in p-C3N4 is an electrostatic interaction without charge transfer between them (Figure 1e). To investigate the strength of this electrostatic interaction between LiPSs and p-C3N4, we then calculated the binding energies (Eb) of different LiPSs with p-C3N4, in comparison with those of LiPSs with graphene or typical electrolyte solvents (e.g. DOL and DME). As shown in Figure 1f, the Eb of LiPSs with p-C3N4 is much larger than those of LiPSs with DME, DOL or graphene. It is interesting to find that the Eb decreases from 1.63 eV for Li2S to 1.47 eV for Li2S2, then increases from 1.45 eV for Li2S3 to 1.74 eV for Li2S8. Compared with Li2S3, the larger binding energies of Li2S(2) with p-C3N4 can be ascribed to the smaller repulsion between the negatively charged S in Li2S(2) and a surrounding edge-N, considering the existence of only one or two S atoms in these molecules. On the other hand, for Li2S3 to Li2S8, the increase of Eb could result from the more significant contribution of the size-dependent van der Waals (vdW) interaction between LiPSs and p-C3N4. In contrast, the values of Eb for LiPSs with DME/DOL are obviously smaller (< 1 eV) and are size-independent due to its nature of the inter-molecular interaction. Compared with p-C3N4 or DME/DOL, graphene has the smallest Eb with LiPSs, and gradually increases with the molecular size due to its nature of the size-dependent vdW interaction. Considering the solubility of LiPSs in the electrolyte and taking Li2S6 as an example, we have also calculated the binding energies between the Li2S6 cluster and p-C3N4, involving an additional electrolyte molecule, DME or DOL, in order to describe the adsorption behavior of LiPSs considering the solvation effect of the electrolyte (Figure S2g-j).24 As we can see, despite that slightly decreased binding energies between Li2S6 and p-C3N4 with additional DME/DOL are obtained (1.091/1.185 eV), they are still larger than those between Li2S6 and DME/DOL molecules (0.896/0.952 eV) or between p-C3N4 and DME/DOL (0.410/0.404 eV). This implies


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that there is still a strong absorption of p-C3N4 for LiPSs, even considering the solvation effect of LiPSs. Based on these DFT calculations, it is clear that p-C3N4 can adsorb LiPS molecules from DME/DOL solvents due to the much higher binding energies of LiPSs with p-C3N4 than those of LiPSs with DOL/DME, while graphene is worst at attracting LiPS molecules due to having the smallest binding energies. These calculations, on the molecular level, have predicted that p-C3N4 can potentially serve as a good immobilizer of the soluble LiPSs to stabilize their redox in a Li-S cell, and possibly influence the redox of LiPSs due to the strong interactions. It should be noted that, the Eb between LiPSs and p-C3N4/graphene shows different trends with the increase of the order of LiPSs. The reason that higher order LiPSs on graphene surface has higher binding energy has been ascribed to the overall result of physical (mainly van der Waals interaction) and chemical interactions between LiPSs and graphene.25 Typically, chemical interaction mainly comes from charge transfer (∆Q) for ionic bonding or charge density overlap for covalent bonding. According to our calculation on the charge population and charge density of the LiPSs adsorbed on p-C3N4, the charge transfer (e.g. 0.02 electrons from Li2S6 cluster to pC3N4) or charge density overlap is insignificant between LiPSs and p-C3N4. Consequently, the interaction between LiPS and p-C3N4 is mainly physical interaction (i.e. electrostatic and vdW interactions), thus leading to the observed trend of Eb with the increase of the order of LiPSs. Similar result was also observed for LiPSs on graphene and N-doped graphene in our recent work. 11 Chemical Adsorption of LiPS on p-C3N4. To verify these predictions, p-C3N4 was prepared by annealing dicyandiamide (DICY) in argon and its adsorbing capability was assessed using Li2S6 to represent typical soluble LiPSs. The chemical composition of p-C3N4 was determined by elemental analysis (EA) to be C3N4.44H2.15 (Table S1), close to the model in Figure 1a. The high


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content of hydrogen (22.4 at.%) in p-C3N4 agrees well with the proposed polymeric melon structure with abundant hydrogen bonds. In a typical solvent containing DOL and DME (1:1 vol.), Li2S6 can be easily dissolved to yield a yellowish color. On the addition of p-C3N4, the color rapidly faded (Figure 2a inset), suggesting the removal of Li2S6 from the solution. This was further confirmed by the UV-Vis absorption spectra of the solutions before and after being treated by p-C3N4 (Figure 2a). The Li2S6 solution possess strong absorption at ca. 350-500 nm and a weak absorption at 610 nm in the spectra, yielding a yellow color. After it was treated by p-C3N4, these characteristic absorptions disappeared and the solution became colorless, indicating the removal of Li2S6 from the solution.

Figure 2. Demonstration of the strong adsorption of LiPSs on p-C3N4. (a) Visible light adsorption of a DOL/DME solvent containing Li2S6 before and after the adsorption treatment by p-C3N4 and the corresponding optical images (inset). (b) XPS survey scan of p-C3N4 before and


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after Li2S6 adsorption and an enlarged area of sulphur region (inset). (c) Elemental mapping of pC3N4 after Li2S6 adsorption showing the distribution of C, N and S on its surface.

We then separated the p-C3N4 from the solution after the adsorption test (product denoted pC3N4-LiPS) to track the whereabouts of the LiPS. The elemental composition of the p-C3N4LiPS, after being thoroughly washed by DOL/DME, was tested by X-ray photoelectron spectroscopy (XPS). Apart from C and N from p-C3N4, sulphur was also detected on the surface of p-C3N4-LiPS. However, no sulphur was found on the pristine p-C3N4, indicating that the sulphur signal on p-C3N4-LiPS was from the LiPS (Figure 2b). To locate the distribution of LiPS on p-C3N4-LiPS, the elemental maps and linear scans of the material were obtained using an Xray energy dispersive spectrometer (Figures 2c, S3). Similar to N and C, a homogeneous distribution of S was observed, suggesting the uniform adsorption of LiPS on the surface of pC3N4 regardless of its morphology. The interactions between the LiPS and p-C3N4 were investigated using high resolution (HR) N1s and C1s XPS spectra and X-ray near-edge structure (XANES) of the p-C3N4-LiPS (K edge of N and C), as shown in Figures S4 and S5. No peak shift compared with pristine p-C3N4 could be observed in both spectra, indicating that there was no electron transfer between p-C3N4 and the adsorbed Li2S6.26 This clearly suggests that the nature of the interaction between them is electrostatic rather than covalent or ionic bonding, and this is consistent with the DFT predictions. Therefore, all of these observations have validated our theoretical prediction of the electrostatic-induced strong adsorption of LiPS on p-C3N4, and suggest a possible influence on


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the redox behavior of LiPS due to this electrostatic interaction, which is similar to the redox reactions in other electrochemical process (e.g. oxygen reductions).27-30 Morphology, Structure and Chemistry of p-C3N4/graphene Hybrid. In order to apply p-C3N4 in the electrochemical redox of LiPSs, we integrated p-C3N4 with reduced graphene oxide to form a porous foam, through a simple cyro-drying and annealing process (denoted as CNG, Figure 3a and S6). For comparison, pristine reduced graphene oxide foam (denoted as r-GO), with an identical pore structure to CNG, was also prepared. The morphology of the materials was investigated using scanning electron microscopy (SEM). Both CNG and r-GO similarly consist of interconnected thin sheets that form large voids of tens of micrometers (Figures 3b, S7). Under transmission electron microscopy (TEM), as shown in Figures 3c and S8, the sheets in CNG were clean, wrinkled and stacked, similar to the typical structure of r-GO.28 The identical macroporous structures of CNG and r-GO thus provide a platform to fairly evaluate the dependence of materials’ electrochemical performance on their surface chemistry. The crystallinity of the materials was studied by X-ray diffraction (Figure S9). p-C3N4 and r-GO gave sharp and narrow diffraction peaks at 2θ of 27.7 and 26.5 °, corresponding to interlayer distances of 0.321 and 0.335 nm, respectively. In contrast, the CNG hybrid showed a wider and less distinct peak centered at ca. 22.1 °, corresponding to an average interlayer distance of ca. 0.400 nm, which is likely to result to the mis-matching of hybridized graphene and p-C3N4 layers.


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Figure 3. Synthesis, structure and chemistry of CNG. (a) Synthesis of CNG material and a model showing the possible structure of the hybrid with p-C3N4 on top of a graphene sheet (dashed square, upper: top view; lower: side view). (b) SEM and (c) TEM images of CNG. (d) XPS survey scan of CNG and the chemical composition obtained by EA and XPS (inset table). The scale bars in (c) and (d) are 50 µm and 200 nm, respectively.

The existence of p-C3N4 in CNG was confirmed by HR-XPS (Figure S10). In the deconvoluted HR C1s spectrum of CNG, apart from the typical peaks from the r-GO, characteristic responses from p-C3N4 (N-C=N structures) were observed, and its HR N1s spectrum also showed identical N species as in p-C3N4. In comparison, no such signals were detected on r-GO. These results clearly suggest the existence of p-C3N4 in CNG and their two-dimensional


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integration, especially when considering the TEM observations which showed a clean and sheetlike morphology without obvious particles or agglomerations (Figure 3c, S8).31, 32 The chemistry of the material was studied by elemental analysis (EA) as well as XPS (Figure 3d and Table S1). Interestingly, the XPS survey scan gave a C/N atomic ratio of ca. 6.89, which is larger than the value obtained from EA (5.45). This indicates that a higher p-C3N4 content is detected by EA than by XPS (27.9 vs. 23.1 wt.%). Considering that the XPS detection depth is just a few nanometers beneath the surface of the CNG, this result likely reflects a relatively higher polymerization of p-C3N4 near the surface of the hybrid than there is inside, which is in accordance with the chemical analysis results of p-C3N4. Electrochemical Performance in a LiPS Cell and Kinetics. In order to investigate the influence of the material on the redox reactions of LiPSs, we assembled CNG into a Li-S cell. To be consistent with our theoretical predictions, soluble Li2S6 was used as the cathode material to couple with a lithium metal anode. r-GO was also assessed for comparison. When galvanostatically cycled between 1.7 and 2.8 V vs. Li+/Li, both materials exhibited two plateaus that are commonly believed to result from the redox processes with soluble or insoluble intermediate products (Figure S11). It is worth noting that the CNG material showed a low polarization by giving a small overpotential and this did not change much during the prolonged cycling test, which reflects more rapid and stable electrochemical reactions. In contrast, the polarization of r-GO gradually increased during the cycling, indicating the continuously deteriorating reaction kinetics. The cycling stability of both materials was subsequently tested to evaluate their ability to stabilize the redox of LiPSs (Figures 4a, S12, and S13). After a short period of decreasing in


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capacity during the first 20 cycles, which could be attributed to an adsorption-saturation process of LiPS on p-C3N4, the CNG retained a very stable capacity of over 700 mAh g-1 during the next 280 cycles, with a capacity decay as low as ca. 0.037% per cycle and a coulombic efficiency of 100%, indicating the strong and stable adsorption of LiPS on the surface of CNG. Apart from this, the morphology of the material after cycling was also studied using SEM, showing the similar porous structure as shown before cycling (Figure S14), which indicates the good structural stability of the material. In contrast, the capacity of pristine r-GO was not only significantly lower, but it also quickly and continuously faded at a much higher rate of 0.542% per cycle in 100 cycles. The considerably superior electrochemical performance of the CNG material clearly illustrates its strong ability to immobilize the soluble LiPSs to suppress their shuttling and stabilize their redox due to its integration with p-C3N4. Insights into the effects of CNG and r-GO on the redox efficiency of LiPSs can be obtained by cyclic voltammetry (CV) tests (current normalized to an equal amount of Li2S6, Figure 4b). Typical discharge peaks of both materials at ca. 2.0 and 2.3 V were obtained, corresponding to the two plateaus in their discharge curves. Both peaks for CNG are higher in intensity and more positive in the onset potential (i.e. lower overpotential) than those of r-GO (insets of Figure 4b), which suggests that the reduction of sulphur or LiPSs is more efficiently carried out on CNG than on r-GO. In addition, distinct shoulder peaks (I and I’ in Figure 4b) were observed on CNG, which were absent on the CV loops of r-GO. They have been respectively attributed to the formation of Li2S/Li2S2 (I) or elemental S (I’) from soluble LiPSs, at the end of the discharge or charge process.33-35 These results have explicitly shown that the CNG material can improve the efficiency and completeness of the conversion of Li2S, LiPSs and S, which are intrinsically sluggish on r-GO or other pristine carbons, as reported in previous studies.10


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Figure 4. Electrochemical analysis of the cycling and redox behavior of LiPSs on CNG and rGO. (a) Cycling performance of LiPSs on CNG and r-GO. (b) CV loops and the enlarged onset regions of the discharge peaks (insets) of CNG and r-GO (current normalized to the same amount of Li2S6). (c) EIS Nyquist plots of CNG and r-GO and the corresponding equivalent


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circuit (inset) describing the proposed electrochemical processes shown in (d). (e) The fitted values according to the equivalent circuits of CNG and r-GO.

To justify this, electrochemical impedance spectroscopy (EIS) was performed and Li2S6 was used to represent a typical intermediate state during the redox of LiPSs. Considering the same assembling parameters of the cells with CNG and r-GO electrodes, it is reasonable to attribute the differences of the EIS between these two materials to the two different cathode materials. Nyquist plots of both CNG and r-GO show overlapping and depressed semicircles followed by an inclined line (Figure 4c). In the first semicircles (I and I’), the overall impedance reflects the resistance in the conduction of electrons which consists of the intrinsic electron resistance of the material and the resistance occurring when an electron is relocated from the electrode to the LiPS (R1 and R2 as shown in Figure 4d). On the other hand, for the second semicircles (II and II’) and the inclined lines, the impedance is determined by the rate of lithiation/delithiation of LiPSs in the discharge/charge processes (R3 in Figure 4d) and the rate of mass diffusion in the electrolyte (Warburg). 36, 37 To simulate the above-discussed movements of electrons and charges, an equivalent circuit was proposed (Figure 4c inset). In this circuit, Re is the electrolyte resistance, and constant phase elements in the respective processes (C1, C2 and C3) are adopted to compensate for the capacitance effect on the porous CNG or r-GO. The fitted values of the resistance in the circuit are listed in Figure 4e. The Re, R1 and Warburg values of the two materials are very close due to their similar structures and electron conductivities.31 Noticeably, both R2 and R3 of CNG are significantly smaller than those of r-GO, which suggests much easier electron/charge exchange


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between CNG and LiPS and is in good agreement with the CV and charge-discharge results. Consequently, these observations have clearly shown a lower kinetic barrier in the electrochemical redox of LiPS on CNG and are strong confirmation of our proposed mechanism of the enhanced redox of LiPS on this material.

Figure 5. DFT simulations of the status of various LiPS molecules on the surface of p-C3N4 (upper) and graphene (lower). The H, C, N, Li, and S atoms are represented by white, purple, cyan, magenta, and yellow balls, respectively.

Molecular Configuration of LiPSs on p-C3N4. To better understand the origin of this kinetic improvement, we carried out DFT calculations to simulate the states of various LiPS molecules localized on the surface of p-C3N4 and graphene (Figure 5). It was found that LiPS molecules are tightly adsorbed on p-C3N4 due to the electrostatic interaction, while they are only loosely adsorbed on the surface of graphene. The smaller interphase distance between LiPSs and p-C3N4


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could reduce the electron transfer barrier and results in a lower R2 value. Because of this strong electrostatic force, the molecular configurations of LiPSs are also altered. As a result, the interaction between the anchored Li atom and the rest of the molecule is weakened as revealed by the elongated Li-S bond length and/or the smaller bond number, which consequently cause easier delithiation for LiPSs. (i.e. smaller R3 during charge). Moreover, the strong electronegativity of N atoms in p-C3N4 would not only attract the Li atoms in LiPSs but also in the electrolyte. In fact, the binding energy of a Li ion on p-C3N4 was calculated to be 2.85 eV, remarkably larger than that between a Li ion and solvent molecules (0.54 and 0.56 eV for DOL and DME, respectively). This could cause a Li-rich environment near the surface of p-C3N4 and facilitate lithiation (i.e. smaller R3 during discharge). However, this is unlikely to exist on graphene due to its much smaller binding energy with Li ions (0.92 eV). Based on these, the increase in the reaction kinetics can be apparently related to the modified molecular structure of LiPS. And the modified molecular structure of LiPS mainly comes from the strong electrostatic interactions between LiPS molecules and p-C3N4. So that it is the strong adsorption of p-C3N4 to polysulfide that has fundamentally resulted in the comprehensive reduction of the kinetic impedance during the electrochemical redox processes of LiPSs, which is further proved by the better performance of CNG at various higher charge-discharge rates (Figure S15). Consequently, these theoretical and experimental results well explain and firmly support our proposed kinetic enhancement of the redox of LiPSs from p-C3N4. CONCLUSIONS In summary, we have discovered an electrostatic-induced interaction between p-C3N4 and LiPSs. It not only improves the stability of LiPSs redox, but also enhances the kinetics of these processes, resulting in a significantly improved battery performance. DFT simulations have


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revealed that this unique kinetic enhancement of the LiPS redox originates from the strong attraction of LiPSs towards p-C3N4 and the consequent distortion of the LiPS molecular configuration. This performance-oriented manipulation of material’s surface functionality not only sheds light on an alternative avenue to improve the performance of Li-S batteries, but also directs the rational design of multifunctional materials for other energy storage/conversion applications that involve the redox of reactants, such as a Li-air battery or a fuel cell. EXPERIMENTAL SECTION Chemicals and Materials Synthesis. Graphite flakes, sulfuric acid (H2SO4, 95-98%), potassium permanganate (KMnO4, 99%), phosphorus pentoxide (P2O5, 98%), potassium persulfate (K2S2O8, 99%), dicyandiamide (99%), sulphur powder (99.5%), lithium sulfide (Li2S, 99.98%), 1,3-Dioxolane (DOL, 99.8%, anhydrous), 1,2-dimethoxyethane (DME, 99.5%, anhydrous), lithium bis(trifluoromethane)sulfonamide (LiTFSI), lithium nitrite (LiNO3, 99%), dicyandiamide (DICY, 99%) were purchased from Sigma-Aldrich and used without further treatment or purification. Graphene oxide (GO) was prepared through the modified Hummers method. GO was dispersed into deionized water to obtain a 5 mg ml-1 suspension. An aqueous solution of DICY (40 mg ml1

) was then added to the GO suspension with a GO:DICY weight ratio of 125:75. The mixture

was stirred for 5 minutes and then frozen at -25 ºC followed by cyro-drying to obtain a browncolored GO foam containing DICY. To obtain the p-C3N4/graphene hybrid (CNG), the foam was placed in a covered crucible and heated at 550 ºC in argon for 4 h. To prepare pristine graphene without p-C3N4, which is denoted as r-GO in this research, a similar protocol was followed, but without adding DICY.


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Characterization. To test the adsorption property of p-C3N4, firstly, 10 mg of finely milled pC3N4 powder was dispersed in 1 ml of a mixture solvent of DME/DOL (1:1 vol.) in a vial. Then, 40 µl of Li2S6 (equivalent sulphur concentration of 2 M in DME/DOL (1:1 vol.) without LiTFSI or LiNO3) was added into the vial to assess the adsorption of Li2S6 on p-C3N4. The solution firstly gave a yellowish color due to the added Li2S6, and faded shortly. A blank test was carried following the same procedure without adding p-C3N4 in the vial. The yellowish color in this vial remained unchanged even after weeks. The light absorbing behavior of the Li2S6 solution with/without p-C3N4 adsorption was probed on a Jasco V-550 spectrophotometer. 10 µl of the above mentioned solutions, with or without pC3N4 adsorption treatment, was diluted by 2 ml of DME/DOL (1:1 vol.) mixture. The solution was then injected into a sealed quartz vial for the light absorption test. The microstructure of the materials was observed by a scanning electron microscope (SEM, Nova Nano SEM 430) and transmission electron microscope (TEM, FEI T12). Elemental mapping was obtained using the energy dispersive spectrometer on the SEM. X-ray photoelectron spectroscopy (XPS) was conducted on an Escalab 250 using Al Kα X-rays. Elemental analysis of the materials was conducted on a Vario EL cube. The C K-edge and N K-edge X-ray absorption near edge structure (XANES) spectra were collected in a total electron yield mode at the XMCD Beam-line in the Hefei Synchrotron Radiation Facility (NSRL). The photoelectron current of the photon beam measured on an Au grid was used to normalize the raw data. Electrochemical test. To prepare the electrolyte, 0.69 g LiNO3 (10 mmol) and 14.36 g LiTFSI (50 mmol) was dissolved by a mixture solvent of DOL and DME (1:1 vol.). The solution was


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adjusted to a volume of 50 ml in a volumetric flask. This electrolyte contains 0.2 M LiNO3 and 1 M LiTFSI. To prepare the Li2S6 catholyte, 0.31 g of Li2S and 1.07 g of sulphur powder was mixed (molar ration 1:5). Afterward, this mixture was diluted by the above-prepared electrolyte and volume was adjusted to 20 ml. The mixture was vigorously stirred at 60 ºC until all the solid dissolved, giving a dark red color. This process usually takes more than one week. The equivalent sulphur concentration in this catholyte is 2 M. Another Li2S6 solution for the adsorption and light absorption test was also prepared in a similar way, but by dissolving the Li2S/S powder in a mixture solvent containing only DME and DOL (1:1 vol.) without LiNO3 or LiTFSI. To test the electrochemical properties of the materials, a CR2032 coin cell was assembled using a lithium foil anode and a Celgard separator. In a typical test, a p-C3N4/graphene or pristine graphene foam was directly used as the electrode material and a certain amount of Li2S6 catholyte was pipetted onto it. The equivalent sulphur loading was 50 wt.% of CNG or r-GO (i.e. ca. 0.6 mg sulphur on 1.2 mg CNG or r-GO). The cycling stability of different electrode materials was tested at a rate of ca. 330 mA g-1 on a LAND CT2001A galvanostatic chargedischarge instrument. Data in Figures 4, S11, 12 were collected in this test. Elemental sulphur was also used as the active material with a high loading that was four times the weight of the CNG or r-GO. To prepare the cathode, sulphur and the electrode material (CNG or r-GO) were finely ground in a mortar followed by thermal infusion at 155 ºC overnight. Then, a slurry (80 wt.% of the as-prepared composite, 10 wt.% carbon black (Super P) and 10 wt.% PVDF) was cast on an aluminum foil. The electrode was dried at 60 ºC in vacuum, cut into


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a suitable shape and assembled into a cell, using the same anode and separator setup described above. Data in Figure S13 was collected in this test. Electrochemical impedance spectroscopy was obtained on an electrochemical station (Biologic VSP-300) by using Li2S6 (2 M in DOL/DME 1:1 vol. with 1 M LiTFSI) to represent the intermediate state of a Li-S battery during charge or discharge. The test was conducted at an open current with an amplitude of 5.0 mV in the frequency region from 10-2 Hz to 100 kHz. A cyclic voltammetry test of the materials was conducted on the same instrument between 1.7 to 2.8 V with a scan rate of 0.1 mV s-1 for 10 cycles. Computational Methods. Density functional theory calculations were performed using the projector augmented wave method and a plane-wave basis set as implemented in the Vienna abinitio simulation package. The Perdew-Burke-Ernzerhof functional for the exchange-correlation term was used for all calculations. The energy cutoff for the PW basis set was set to be 400 eV. A large poly-aromatic hydrocarbon molecule of C96H24 was constructed to represent the graphene with a "30 Å×30 Å×20 Å" supercell. The lattice constant of monolayer p-C3N4 was calculated to be a=16.9 Å and b =12.7 Å, which is slightly larger than the experimental values (a=16.7 Å and b =12.4 Å). A "2×2×1" supercell of p-C3N4 was used to study the adsorption of LiPSs on p-C3N4 and the binding strength between LiPSs and p-C3N4. Only the Γ point was used to sample the first Brillouin zone of this large supercell for all calculations. For the geometry relaxations and energy calculations, van der Waals interactions were involved at the level of vdW-DF with the optB88 exchange functional. The supercell box was fixed and all atoms were allowed to fully relax until the residual force per atom was less than 0.01 eV Å−1. The charge population was calculated using Bader charge analysis.


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Corresponding Authors * Prof. Feng Li and Prof. Hui-Ming Cheng, Emails: [email protected] and [email protected] Author Contributions Ji Liang and Lichang Yin contributes equally to this paper. Ji Liang planned the research and conducted most of the experiments; Lichang Yin performed the first-principles calculations. Huicong Yang and Xiaonan Tang carried out certain part of the experiments and electrochemical characterizations. Feng Li and Hui-Ming Cheng involved in the research planning and supervised it. Wensheng Yan and Li Song carried out X-ray absorption near edge structure measurement in the Hefei Synchrotron Radiation Facility. All authors discussed and analyzed the data. Ji Liang, Lichang Yin, Feng Li and Hui-Ming Cheng wrote the paper. ACKNOWLEDGMENT This work is supported by the T. S. Kě Research Fellowship Program of Shenyang National Laboratory for Materials Science, China Postdoctoral Science Foundation (No. 2015M571342), Ministry of Science and Technology of China (No. 2016YFA0200100, 2014CB932402 and 2014CB848900), the National Science Foundation of China (No. 51521091, 51525206, 11375198, 11574280, U1401243, U1532112 and 51472249), "Strategic Priority Research Program" of the Chinese Academy of Sciences (XDA01020304) , User with Potential from CAS Hefei Science Center (2015HSC-UP020) and the Key Research Program of the Chinese Academy of Sciences (No. KGZD-EW-T06). The theoretical calculations were performed on TianHe-1 (A) at the National Supercomputer Center in Tianjin. ASSOCIATED CONTENT


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Supporting Information Available: DFT calculation data, characterizations and electrochemical assessment of the materials. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES: (1) Van Noorden, R., The Rechargeable Revolution: A Better Battery. Nature 2014, 507, 26-28. (2) Manthiram, A.; Fu, Y.; Chung, S. H.; Zu, C.; Su, Y. S. Rechargeable Lithium-Sulfur Batteries Chem. Rev. 2014, 114, 11751-11787. (3) Wang, D. W.; Zeng, Q.; Zhou, G.; Yin, L.; Li, F.; Cheng, H. M.; Gentle, I. R.; Lu, G. Q. M. Carbon-Sulfur Composites for Li-S Batteries: Status and Prospects. J. Mater. Chem. A 2013, 1, 9382-9394. (4) Liang, J.; Sun, Z. H.; Li, F.; Cheng, H. M. Carbon Materials for Li-S Batteries: Functional Evolution and Performance Improvement. Energy Storage Mater. 2016, 76-106. (5) Yu, M.; Li, R.; Wu, M.; Shi, G. Graphene Materials for Lithium-Sulfur Batteries. Energy Storage Mater. 2015, 1, 51-73. (6) Huang, J. Q.; Zhang, Q.; Wei, F. Multi-Functional Separator/Interlayer System for HighStable Lithium-Sulfur Batteries: Progress and Prospects.. Energy Storage Mater. 2015, 1, 127145. (7) Ji, X.; Nazar, L. F. Advances in Li-S Batteries. J. Mater. Chem. 2010, 20, 9821-9826. (8) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J. M. Li-O2 and Li-S Batteries with High Energy Storage. Nat. Mater. 2012, 11, 19-29. (9) Lv, W.; Li, Z.; Deng, Y.; Yang, Q. H.; Kang, F. Graphene-Based Materials for Electrochemical Energy Storage Devices: Opportunities and Challenges Energy Storage Mater. 2016, 2, 107-138. (10) Lu, Y. C.; He, Q.; Gasteiger, H. A. Probing the Lithium-Sulfur Redox Reactions: A Rotating-Ring Disk Electrode Study J. Phys. Chem. C 2014, 118, 5733-5741. (11) Yin, L. C.; Liang, J.; Zhou, G. M.; Li, F.; Saito, R.; Cheng, H. M. Understanding the Interactions between Lithium Polysulfides and N-Doped Graphene Using Density Functional Theory Calculations.. Nano Energy 2016, 25, 203-210. (12) Pang, Q.; Kundu, D.; Cuisinier, M.; Nazar, L. F. Surface-Enhanced Redox Chemistry of Polysulphides on a Metallic and Polar Host for Lithium-Sulphur Batteries. Nat. Commun. 2014,


25


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Page 26 of 29

5. (13) Liang, X.; Hart, C.; Pang, Q.; Garsuch, A.; Weiss, T.; Nazar, L. F. A Highly Efficient Polysulfide Mediator for Lithium-Sulfur Batteries Nat. Commun. 2015, 6. (14) Ji, X.; Evers, S.; Black, R.; Nazar, L. F. Stabilizing Lithium-Sulphur Cathodes Using Polysulphide Reservoirs Nat. Commun. 2011, 2. (15) Xiao, Z.; Yang, Z.; Wang, L.; Nie, H.; Zhong, M. E.; Lai, Q.; Xu, X.; Zhang, L.; Huang, S. A Lightweight TiO2/Graphene Interlayer Applied as a Highly Effective Polysulfide Absorbent for Fast, Long-Life Lithium-Sulfur Batteries.. Adv. Mater. 2015, 27, 2891-2898. (16) Tang, C.; Zhang, Q.; Zhao, M. Q.; Huang, J. Q.; Cheng, X. B.; Tian, G. L.; Peng, H. J.; Wei, F. Nitrogen-Doped Aligned Carbon Nanotube/Graphene Sandwiches: Facile Catalytic Growth on Bifunctional Natural Catalysts and Their Applications as Scaffolds for High-Rate Lithium-Sulfur Batteries Adv. Mater. 2014, 26, 6100-6105. (17) Song, J.; Gordin, M. L.; Xu, T.; Chen, S.; Yu, Z.; Sohn, H.; Lu, J.; Ren, Y.; Duan, Y.; Wang, D. Strong Lithium Polysulfide Chemisorption on Electroactive Sites of Nitrogen-Doped Carbon Composites for High-Performance Lithium-Sulfur Battery Cathodes. Angew. Chem. Int. Ed. 2015, 4325-4329. (18) Zhou, G.; Paek, E.; Hwang, G. S.; Manthiram, A. Long-Life Li/Polysulphide Batteries with High Sulphur Loading Enabled by Lightweight Three-Dimensional Nitrogen/Sulphur-Codoped Graphene Sponge. Nat. Commun. 2015, 6. (19) Peng, H. J.; Hou, T. Z.; Zhang, Q.; Huang, J. Q.; Cheng, X. B.; Guo, M. Q.; Yuan, Z.; He, L. Y.; Wei, F. Strongly Coupled Interfaces between a Heterogeneous Carbon Host and a SulfurContaining Guest for Highly Stable Lithium-Sulfur Batteries: Mechanistic Insight into Capacity Degradation. Adv. Mater. Interfaces 2014, 1. (20) Liu, J.; Li, W.; Duan, L.; Li, X.; Ji, L.; Geng, Z.; Huang, K.; Lu, L.; Zhou, L.; Liu, Z.; Chen, W.; Liu, L.; Feng, S.; Zhang, Y. A Graphene-Like Oxygenated Carbon Nitride Material for Improved Cycle-Life Lithium/Sulfur Batteries Nano Lett. 2015, 15, 5137-5142. (21) Li, X.; Xu, C.; Zhao, K.; Wang, Y.; Pan, L. Carbon Nitride Based Mesoporous Materials as Cathode Matrix for High Performance Lithium-Sulfur Batteries. RSC Adv. 2016, 6, 1357213580. (22) Al Salem, H.; Babu, G.; V. Rao, C.; Arava, L. M. R. Electrocatalytic Polysulfide Traps for Controlling Redox Shuttle Process of Li-S Batteries. J. Am. Chem. Soc. 2015, 137, 11542-11545.


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(23) Schulz, H.; Schwarz, K. Is There an N3- Ion in the Crystal Structure of the Ionic Conductor Lithium Nitride (Li3N)? Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1978, 34, 999-1005. (24) Kamphaus, E. P.; Balbuena, P. B. Long-Chain Polysulfide Retention at the Cathode of Li-S Batteries J. Phys. Chem. C 2016, 120, 4296-4305. (25) Zhang, Q.; Wang, Y.; Seh, Z. W.; Fu, Z.; Zhang, R.; Cui, Y. Understanding the Anchoring Effect of Two-Dimensional Layered Materials for Lithium-Sulfur Batteries Nano Lett. 2015, 15, 3780-3786. (26) Montoro, L. A.; Abbate, M.; Rosolen, J. M. Changes in the Electronic Structure of Chemically Deintercalated LiCoO2. Electrochem. Solid-State Lett. 2000, 3, 410-412. (27) Liang, J.; Du, X.; Gibson, C.; Du, X. W.; Qiao, S. Z. N-Doped Graphene Natively Grown on Hierarchical Ordered Porous Carbon for Enhanced Oxygen Reduction. Adv. Mater. 2013, 25, 6226-6231. (28) Liang, J.; Jiao, Y.; Jaroniec, M.; Qiao, S. Z. Sulfur and Nitrogen Dual-Doped Mesoporous Graphene Electrocatalyst for Oxygen Reduction with Synergistically Enhanced Performance Angew. Chem. Int. Ed. 2012, 51, 11496-11500. (29) Liang, J.; Zheng, Y.; Chen, J.; Liu, J.; Hulicova-Jurcakova, D.; Jaroniec, M.; Qiao, S. Z. Facile Oxygen Reduction on a Three-Dimensionally Ordered Macroporous Graphitic C3N4/Carbon Composite Electrocatalyst Angew. Chem. Int. Ed. 2012, 51, 3892-3896. (30) Liang, J.; Zhou, R. F.; Chen, X. M.; Tang, Y. H.; Qiao, S. Z. Fe-N Decorated Hybrids of CNTs Grown on Hierarchically Porous Carbon for High-Performance Oxygen Reduction Adv. Mater. 2014, 26, 6074-6079. (31) Zheng, Y.; Jiao, Y.; Zhu, Y.; Li, L. H.; Han, Y.; Chen, Y.; Du, A.; Jaroniec, M.; Qiao, S. Z. Hydrogen Evolution by a Metal-Free Electrocatalyst. Nat. Commun. 2014, 5. (32) Zhao, Y.; Zhao, F.; Wang, X.; Xu, C.; Zhang, Z.; Shi, G.; Qu, L. Graphitic Carbon Nitride Nanoribbons: Graphene-Assisted Formation and Synergic Function for Highly Efficient Hydrogen Evolution. Angew. Chem. Int. Ed. 2014, 53, 13934-13939. (33) Guo, J.; Xu, Y.; Wang, C. Sulfur-Impregnated Disordered Carbon Nanotubes Cathode for Lithium–Sulfur Batteries. Nano Lett. 2011, 11, 4288-4294. (34) Cheon, S. E.; Ko, K. S.; Cho, J. H.; Kim, S. W.; Chin, E. Y.; Kim, H. T. Rechargeable Lithium Sulfur Battery: I. Structural Change of Sulfur Cathode During Discharge and Charge J.


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Electrochem. Soc. 2003, 150, A796-A799. (35) Elazari, R.; Salitra, G.; Garsuch, A.; Panchenko, A.; Aurbach, D. Sulfur-Impregnated Activated Carbon Fiber Cloth as a Binder-Free Cathode for Rechargeable Li-S Batteries Adv. Mater. 2011, 23, 5641-5644. (36) Deng, Z.; Zhang, Z.; Lai, Y.; Liu, J.; Li, J.; Liu, Y. Electrochemical Impedance Spectroscopy Study of a Lithium/Sulfur Battery: Modeling and Analysis of Capacity Fading. J. Electrochem. Soc. 2013, 160, A553-A558. (37) Ahn, W.; Kim, K. B.; Jung, K. N.; Shin, K. H.; Jin, C. S. Synthesis and Electrochemical Properties of a Sulfur-Multi Walled Carbon Nanotubes Composite as a Cathode Material for Lithium Sulfur Batteries. J. Power Sources 2012, 202, 394-399.


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Table of Content of “Kinetically Enhanced Electrochemical Redox of Polysulphides on Polymeric Carbon Nitrides for Improved Li-Sulphur Batteries” by J. Liang and L. Yin, et al.

A strong electrostatic affinity exists between the polymeric carbon nitride and lithium polysulphides molecules. This inter-species attraction is able to stabilize the redox cycling of lithium polysulfides, modify their molecular configurations and significantly improve their redox kinetics.


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Kinetically Enhanced Electrochemical Redox of Polysulfides on

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