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Metal/graphene Composites With Strong MetalS Bondings for Sulfur Immobilization in Li-S Batteries Xiaolong Yao, Jijian Xu, Zhanglian Hong, Gaoran Li, Xue-wei Wang, Feng Lu, Wei-Hua Wang, Hui Liu, Chengdu Liang, Zhan Lin, and Weichao Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12063 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Metal/graphene Composites with Strong Metal-S Bondings for Sulfur Immobilization in Li-S Batteries Xiaolong Yao1,§, Jijian Xu2,§, Zhanglian Hong2, Gaoran Li3, Xuewei Wang4, Feng Lu1, Weihua Wang1, Hui Liu1, Chengdu Liang3, Zhan Lin3,* and Weichao Wang1,5,* 1

Department of Electronics and Key Laboratory of Photo-Electronic Thin Film Devices and

Technology of Tianjin, Nankai University, Tianjin 300071, China 2

State Key Laboratory of Silicon Materials, School of Materials Science and Engineering,

Zhejiang University, Hangzhou 310027, China 3

Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture

Technology, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China 4

School of Materials Science and Engineering, Tianjin University of Technology, Tianjin

300384, China 5

Department of Materials Science and Engineering, The University of Texas at Dallas,

Richardson, TX 75080, USA §

These authors contributed equally to this work.

*Email: [email protected] and [email protected]

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Selections of metallic cathode materials and modulations of metal-sulfur bonding strength are crucial for sulfur immobilization in development of high-performance lithium-sulfur (Li-S) batteries with low cost. By combining theoretical calculations and experiments, herein we reveal the relationship between intrinsic electronic structure and metal-S bonding strength, which links to energy density and duribility of Li-S batteries. Through the first-principles calculations, we simulate sulfur clusters (S1, S2, S4 and S8) immobilization on metal (Cu, Ni and Sn) slab surfaces with and without graphene substrate. For sulfur clusters, the metal-Sx (x=1, 2, 4 and 8) bonding strengh is in the sequence of Ni>Cu>Sn without graphene substrate. Nevertheless, sequence changes (Ni>Sn>Cu) in the presence of graphene substrate due to different amounts of charge transfer between these metal clusters and graphene. Guided by these theoretical results, metal (Cu, Ni, Sn)/graphene (G) composites are synthesized and subsequently intergrated into the cathode of Li-S batteries. Among these metal/G systems, the sulfur cathode with Ni/G composites demonstrates the remarkable electrochemical performance, i.e., a discharge capacity of >830 mAh g-1 over 500 cycles with an average coulombic efficiency close to 100%. These findings shed light on theoretical calculations providing insights into the electrode design of Li-S batteries.

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1. Introduction Sulfur (S) is a promising cathode material for rechargeable lithium batteries with a theoretical specific capacity as high as 1672 mAh g-1,1-6 which is nearly ten times as those of traditional cathode materials such as LiCoO2 (∼140 mAh g-1) or LiFePO4 (∼170 mAh g-1). Sulfur material is cost-effective and enviromentally friendly, and thus has drawn great attention.7-14 Nevertheless, the dissolution of intermediate polysulfides into liquid electrolytes results in irreversible loss of active sulfur material,15 which still remains a key impediment to achieve a high specific capacity and a long cycle life of sulfur cathodes. In recent years, extensive works on storing and immobilizing sulfur for cathode materials were performed to improve cycling performance.12, 16-33 Various carbon materials, including mesoporous carbon,6,

16

graphene,9,

13, 33-35

carbon nanotubes,17-19 carbon fibers,36-37 and

carbon nanotiles,38 were used to physically constrain polysulfides within the cathode. However, battery capacity still degradates gradually as cycling number increases. The addition of metals is one promising technique to obtain stable cycling of sulfur cathodes.20-22 The chemical bonding between S clusters and metal surface could effectively immobilize S species.39 Previous reports found that the S clusters tend to separate into smaller ones and spread on metal surface dispersively,40 which benefit sulfur immobilization and subsequent lithium storage on sulfur cathodes. Fudamentally, chemical bonding strength is determined by energy difference between the d-band center of transition metal and the p-band center of S clusters.41-42 In order to tune the d-band center to achieve a stronger p-d orbital coupling, right transition metal should be selected.41 Also, introducing substrates, such as graphene, could further optimize the location of the d-band center. Compared with microporous carbon and -3-



amorphous carbon,20-22 graphene has a two dimensional honeycomb lattice and a linear dispersed band structure, showing remarkable electron mobility23 and obvious charge transfer when interacting with metals.24 This p-d orbtial theory can be applied to those metals with p-valence electrons as well. To date, transitional metals and main group metals were used as additives experimentally in sulfur cathodes to investigate their electrochemical performance individually.20-22 The strong bonding between metal and sulfur can prohibit sulfur clustering into soluble polysulfide intermediates. How to link the sulfur immbolization to the fundamental understanding of metallic materials and thus rationally select metal additives and their subtrates for high-performance sulfur cathodes with effective sulfur immobilization, however, still remains unaccessible. Herein, we firstly apply the density functional theory (DFT) calculations to obtain the energetics and bonding configurations of sulfur clusters (S1, S2, S4 and S8) on slab surfaces of metals (Cu, Ni, and Sn), metal/graphene composites, and metal/defect graphene composites. Further electronic structure analysis reveals the impact of electron transfer between metal and graphene on the surface immobilization of sulfur clusters. Secondly, X-ray photoelectron spectroscopy (XPS) and Raman studies verify strong interactions between metal/graphene and sulfur species. The metal/graphene composites enhance electron transport and lithium-ion pathway, while chemical bondings immbolize sulfur species in the cathode for the enhancement in electrochemical performance. Finally, we find that the sulfur cathode based on Ni/G composites provides the best discharge capacity of >830 mAh g-1 over 500 cycles with an average coulombic efficiency close to 100%, which is highly consistent with theoretical predictions. -4-


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2. Theoretical Section Spin-polarized density functional theory calculations are performed using the vasp code,43 with the exchange-correlation potential described by Perdew-Burke-Ernzerhof (PBE) version44 of generalized gradient approximation. Interactions between ions and electrons are described by the projector augmented wave (PAW) approach.45 The plane-wave cutoff is set to 400 eV, and the total energy is converged to 10-5 eV. The metal/graphene composite is modeled by the slab model where five atomic layers (for Cu and Ni) or three bilayers (for Sn) of metal are supported by one layer of graphene. A vacuum region of over 18 Å in the vertical direction is included to minimize the spurious interaction between the adjacent image cells. During strucutral optimazation, the lattice contant of graphene is fixed at the optimized value of 2.46 Å, and all the metal atoms are fully relaxed. For S clusters adsorption, a 4×4 surface unit cell is adopted. The Brillouin zone is sampled by a 7×7×1 Γ-centered k-point mesh. Atomic positions are fully relaxed until the residual Hellmann-Feynman forces on each atom are less than 0.01 eV/Å. 3. Experimental Section 3.1. Synthesis of Metal (Ni, Sn, Cu)/graphene and Related Sulfur Composites In a typical synthesis procedure46, 0.4 g glucose and 0.162 g NH4Cl are mixed with a eutectic composition of NaCl/KCl (11.5 g/13.5 g). Metal chloride (e.g., NiCl2, SnCl2, CuCl2) as the precursor for metal nanoparticles is added into the eutectic composition, and their contents are specifically controlled at 10 at%. The powder mixture is first grinded homogenously, put in an oven at 150 oC overnight, and then loaded into an electric furnace with a continuous nitrogen flow. After flushing with nitrogen for 20 min, the system is ramped -5-



at 50 oC min-1 to carbonization temperature of 900 oC and kept at this temperature for 1 h. The system is finally cooled down to ambient temperature by switching off the power; meanwhile, the nitrogen flow is maintained until the temperature reached below 100 oC. The as-obtained product is washed with dilute HCl solution and a large amount of water is used to remove the salts. The as-prepared samples are further treated with NH3 at 800 oC for 2 h to obtain the final metal (Ni, Sn, Cu)/G composites. Metal/G composites and 70 wt% sulfur are thoroughly mixed in a quartz mortar to yield a black mixture. The mixture is then sealed in a glass container and heated at 155 oC for 4 h to obtain S-metal/G composites. 3.2. Synthesis of Lithium Polysulfides and Li2S4-metal/G Samples Li2S4 is synthesized by reacting elemental sulfur and lithium superhydride (LiEt3BH) in the desired ratio in anhydrous tetrahydrofuran (THF) at room temperature for 1 h inside an Ar-filled glove box. THF is removed in vacuum and the precipitate is washed with toluene and then dried in vacuum. Li2S4 solution is prepared by mixing 10 mg of Li2S4 and 10 mg metal/G in 10 mL of THF in each case. The solutions are further stirred for 1 h. The suspensions are centrifuged and the solids after drying in vacuum overnight are collected for XPS and Raman analyses. All procedures are conducted in an Ar-filled glove box. 3.3. Electrochemical Measurements The active material powder is mixed with carbon black and poly(vinylidene fuoride), PVDF, dissolved in N-methyl pyrrolidinone, NMP, 8 wt%, in a weight ratio 70 : 20 : 10. The slurry is mixed to obtain a homogeneous black paste which is then coated on an aluminum foil. The as-coated aluminum foils are dried in vacuum at 60 oC overnight. The working electrode and Li metal foil counter electrode are assembled into coin cells by using Celgard -6-


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2400 as the separator and 1 M LiTFSI in DOL/DME 50:50 as the electrolyte. The typical sulfur loading is 1.0–1.5 mg per electrode, with an electrolyte volume of 50 µl in all cases. 12 Coin cells with each sample (Ni/G-S, Sn/G-S, Cu/G-S) are constructed in an Ar-filled glove box and are tested under the same conditions. 10 out of 12 coin cells perform well for each sample. The specific capacity and current density are calculated based on sulfur mass only. Cyclic voltammetry is recorded in a voltage range of 1.8-2.6 V at a scan rate of 0.1 mV s-1. The electrochemical impedance spectroscopy (EIS) is carried out using an automated electrochemical workstation (CHI760E) and is measured between 0.01 Hz to 105 Hz with an excitation voltage of 0.005 V at various potentials. 3.4. Characterization Powder X-ray diffraction (XRD) is performed by a Rigaku D/max-3B X-ray diffractometer with Cu Kα line as the radiation source (λ = 0.15406 nm, 40 kV, 35 mA). X-ray photoelectron spectroscopy (XPS) analyses are carried out on an Escalab 250Xi XPS system with Al Kα (1486.6 eV) source. The air-sensitive samples are transported to the spectrometer under an Ar atmosphere and transferred into the chamber anaerobically. All spectra are fitted with Gaussian–Lorentzian functions and a Shirley-type background using the CasaXPS software. For S 2p spectra, three constraints are used on the fitting for 2p3/2 and 2p1/2 doublets: (a) a peak position difference of 1.18 eV between them, (b) a peak area ratio of 2:1 for 2p3/2:2p1/2, and (c) equal full-width half maximum. The binding energy values are all calibrated using the C 1s peak at 285.0 eV. A JEOL-1200 transmission electron microscope (TEM) (100 kV) and a Hitachi S-4800 field emission scanning electron microscope (FE-SEM) (5 kV) equipped with an energy-dispersive X-ray (EDX) spectroscope are used to analyze the -7-



material morphology and elemental composition. The thermal stability of electrode materials is measured on a thermal gravimetric analyzer (TGA-DAT-2960 SDT) at a heating rate of 20 o

C min-1 from 25 to 500 oC in Ar. The Brunauer–Emmett–Teller (BET) specific surface areas

of the composites are measured by nitrogen adsorption at 77 K on a surface area and porosity analyzer (Micrometrics ASAP 2020). Before each measurement, a sample of 0.10 g is degassed at 300 oC for 4 h. A pore size distribution plot is derived from the adsorption branch of the isotherm based on density functional theory (DFT). 4. Results and Discussion Theoretically, we adopt the slab model to mimic the interaction between metal and S clusters since the size of metal particles (~5 nm) in the cathode material is far larger than that of adsorbed sulfur clusters (~2 Å).20-22 The previous work showed that the adsorption strength of sulfur clusters on the oxide surface would decrease as the cluster size becomes larger, indicating that large sulfur clusters can hardly be adsorped on the oxide or metal surface.47 Therefore, small sulfur clusters (S1, S2, S4 and S8) are adopted to qualitatively study the binding trend in energy and binding stength difference among various metal substates in our work. We begin with the adsorption of S clusters (S1, S2, S4 and S8) on the surfaces of Cu, Ni, and Sn without graphene substrate. Figure 1 depicts stable adsorption configurations of S1, S2, S4 and S8 on the most stable low index metallic (111) surfaces. For a single atomic adsorption, the S atom prefers high coordinated hollow sites rather than bridge site and the top site on the Cu and Ni surfaces. On the Sn surface, the S atom has a preference for the top site which could attribute to stronger covalency of Sn atom. The bond lengths between the adsorbed S atom and the surface atoms of Ni, Cu, and Sn are 2.12 Å, 2.24 Å and 2.32 Å, respectively, -8-


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displaying the strongest interaction between the S atom and the Ni surface. For the S2 cluster case, the most stable adsorption configurations prefer to lie down on the metal surface in order to bond with metal atoms as strong as possible. The average distances between the S2 clusters and the top atoms in Ni, Cu and Sn slabs are 2.20 Å, 2.31 Å and 2.44 Å, respectively, revealing the strongest bonding strength between the S2 cluster and the Ni surface. For the sulfur clusters with larger size (S4 and S8), the adsorption configurations lying down on the metal surfaces are also the most stable. The average distances from the S4 (S8) clusters to the Ni, Cu and Sn surfaces are 2.29 Å, 2.42 Å and 2.50 Å (2.73 Å, 2.87 Å and 3.06 Å), respectively, showing again the strongest bonding strength between the S cluster and the Ni surface. Furthermore, distances from sulfur clusters to metal surfaces increase with sulfur cluster size, displaying chemical bonding is weakened as sulfur cluster becomes larger. Figure 2a illustrates adsorption energies of S clusters on Cu, Ni and Sn surfaces. We calculate adsorption energies of S clusters on metal surfaces in Formula 1:

Ead = EMS − EM − ES

(1)

where EMS, EM, ES are total energies of the metal-S systems, the metal host, and the S cluster, respectively. A larger absolute value of negative adsorption energies indicates a stronger adsorption. For the adsorption on metal surfaces, all sulfur clusters (S1, S2, S4 and S8) manifest the same tendency: the adsorption strength is the strongest on the Ni surface, the intermediate on the Cu surface, and the weakest on the Sn surface. This trend can be explained by the molecular orbital theory, as displayed in Figure 3. The interaction between the sulfur 3p state and the metal 3d band (for Cu and Ni) or 5p band (for Sn) results in a low-lying bonding state and a high-lying antibonding state. The bonding state is below the -9-



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Fermi level and fully occupied, but the extent of filling of antibonding state depends on the location of metal valence band relative to the Fermi level. A higher lying valence band of metal with respect to the Fermi level corresponds to the decrease in filling of the antibonding state, leading to stronger bonding between the metal and the sulfur cluster adsorbate. On the contrary, a lower lying valence band of the metal with respect to the Fermi level causes a weaker bonding. To conduct a quantitative analysis of the relative location of the valence band of metal, we calculate the d-band and p-band centers of Cu, Ni, and Sn (Table 1), which is defined as the mathematical expectation of the d or p density of states (DOS). The d-band center of Ni lies above that of Cu due to less d valence electrons of Ni. As a result, S clusters absorb stronger on the Ni surface than on the Cu surface. As for the transition metal Cu and Ni, the valence d states are highly localized, thus their hybridization with the localized p states of sulfur clusters leads to localized bonding states and antibonding states. However, for the main group metal Sn, its valence p states are delocalized. The coupling between the delocalized p states of Sn and localized p states of sulfur clusters brings about delocalized bonding states and antibonding states. It means that more antibonding states are below the Fermi level and occupied in this situation. Thus, the sulfur clusters bond weaker on the Sn surface than on the Cu and Ni surfaces even though the p-band center of Sn lies higher than the d-band centers of Cu and Ni. Another trend is that the smaller sulfur clusters have lower adsorption

energies

on

all

considered

surfaces

(metal,

metal/graphene

and

metal/defect-graphene) as is shown in Figure 2. Thus, for a given amount of sulfur, to lower the energetics of the system, sulfur tends to form dispersive smaller clusters rather than gathering into larger clusters. In the cathode reaction, the lithium ions in the electrolyte will - 10 -


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bind with these sulfur clusters to form lithium sulfides Li2Sx. Introducing graphene, a zero-gap semiconductor where charge carriers obey the linear dispersion relation25 and have extremely high carrier mobilities23, to support the metal clusters will result in substantial orbital coupling at the metal/graphene interface and thus expect to produce stronger chemical bondings between metals and graphene substrates. In such a way, the sulfur immobilization on the electrode with metallic cluster could be further optimized. Here we introduce graphene as substrate for metal slab. Distances between metal slab and graphene layer are 2.11 Å, 3.07 Å, and 3.92 Å for Ni, Cu and Sn, respectively. The shorter spacing of metal/graphene interface indicates the stronger chemical bonding, namely more charge transfer between metal and graphene. Thus, chemical bonding is the strongest for Ni/graphene, the medium for Cu/graphene, and the weakest for Sn/graphene, which agree with the previous theoretical results that the Ni-graphene interaction is the strong chemisorption and the (Cu or Sn)-graphene interaction is the weak physisorption.32 On the surfaces of these metal/graphene composites, adsorption energies of sulfur clusters present the sequence different from those on the surfaces of freestanding metals. In Figure 2b, all sulfur clusters adsorb more strongely on the surface of Ni/graphene (Ni/G) composite compared with on the surface of freestanding Ni. On the contrast, sulfur clusters bonds weaker on the surface of Cu/graphene (Cu/G) composite than on the surface of freestanding Cu. However, there is no obvious variation of adsorption energy for all sulfur clusters on the surface of Sn/graphene (Sn/G) composite compared with on the surface of freestanding Sn. Calculated valence band centers in Table 1 reveal that the d-band center of Cu in the Cu/G system reduces 1.05 eV than that in the freestanding Cu. The d-band center of Ni in the Ni/G system - 11 -



increases 0.45 eV than that in the freestanding Ni. For the Sn/G system, the p-band center of Sn changes merely 0.1 eV compared with that in the freestanding Sn. These valence band center shifts of metals in metal/graphene composites are consistent to the adsorption energy changes of sulfur clusters. This rule is easily understood according to the theory of two-levels coupling (Figure 3): when the metal’s valence band centershifts up, the antibonding states of metal-S interaction are pushed to the higher energy and consequently less occupied, leading to the increased bonding strength. On the contrary, when the metal’s valence band center shifts down, the antibonding states of metal-S interaction decrease and are more occupied, hence the bonding strength is weakened. To understand the influence on sulfur clusters adsorption exerted by the interaction between metal slab and graphene layer, we calculate band structures of three metal/G systems (Figure S1). Since the location of the Dirac point is susceptible to electron transfer on graphene, we can inspect the interaction between metal slabs and graphene layer according to the location shifts of the Dirac point. For the Cu/G system, the Dirac point is above the Fermi level (Figure S1a), thus electrons tansfer from graphene to Cu. While for the Ni/G and Sn/G systems, the Dirac points are both below the Fermi level (Figures S1b and S1c). Therefore, the electron transfer is from Ni or Sn to graphene. From these results, we obtain an understanding on the origin of the valence band center variations of metals before and after interacting with graphene. As shown in Figure 3, the electrons of graphene fill the d-band of Cu and subsequently lower its d-band center. While the d electrons of Ni tranfering to graphene give rise to the elevation of its d-band center. Because of more extensive 5p-band near the Fermi level, the p-band center location of Sn isn’t obviously influenced by its p - 12 -


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elelctrons transfering to graphene. Based on the above analysis, we may draw the conclusion that graphene substrate could change metal’s valence band center through electron transfer, thus tuning the metal-S interaction. For transition metal with localized d states, this tuning effect from graphene substrate could be significant. While for main group metal with extented p states, metal-S interaction is hardly affected by graphene substrate. In the realistic chemical synthesis process of graphene, various intrinsic and extrinsic defects will appear on the graphene. For instance, the NH3 treatment as used in our synthesis procedure will incorporate nitrogen into graphene unavoidably.48-49 Besides, many oxygen-containing functional groups (e.g. hydroxyl groups, carboxyl groups and epoxy groups) will attach to the surface of graphene. These defects and functional groups will influence the electronic structure of graphene and the composite. Here, for further investigating the effect on sulfur clusters adsorption exerted by the defects and functional groups, we calculate the adsorption energies of sulfur clusters on surfaces of metal/defect graphene composite systems with three prototype defects: intrinsic carbon vacancy defect (Figure 2c), p-type boron doping (Figure 2d) and n-type nitrogen doping (Figure 2e). The adsorption energies of sulfur clusters all decrease on the surface of the three Cu/defect graphene (Cu/G_V, Cu/G_B and Cu/G_N) systems, compared with the surface of the Cu/G system. On the other hand, For (Ni or Sn)/defect graphene system, the adsorption energies of sulfur clusters all increase compared with the their cooresponding metal/G system. As a result, the bonding strengths of sulfur clusters on metal/defect graphene surface perform a sequence of Ni>Cu>Sn, which is different from that on metal/graphene surfaces but is the same with that on the freestanding metal surfaces. These adsorption energy variations of sulfur clusters - 13 -



on metal/defect graphene surfaces indicate that the defect on graphene, such as nitrogen dopants incorporated by NH3 treatment in our synthesis procedure, will decrease the sulfur imobilization capabilities of (Ni or Sn)/defect graphene composites, accordingly damaging their electrochemical cycling performances. While for the Cu/graphene composite, the defect incorporation can inrease its sulfur imobilization capability thus impove the electrochemical cycling stability. The calculated valence band centers (as listed in Table 1 and shown in Figure 3) show that the d-band center of Cu in the Cu/defect graphene system shifts up and the d-band center of Ni in the Ni/defect graphehe system shifts down after these three kinds of defects formation. The p-band center of Sn in the Sn/defect graphene system still vary little under the impact of these defects. These results can be explained by that filling the defect states reduces electron transfer between metal slab and graphene substrate, thus the metal-S interaction in the metal/defect graphene system is similar with that in free standing metal slab. Furthermore, the calculated band structures of metal (Cu, Ni, and Sn)/defect graphene systems (Figure S2) indicate that the stronger p-p hybridization between the Sn slab and the defect graphene substrate, compared with the p-d hybridization between the Cu (Ni) slab and the defect graphene substrate, partly compensates the defect states. In order to validate simulations in choosing suitable metal/graphene composites for high-performance sulfur cathodes and in turn to design better electrode, we firstly synthesize Ni/G, Sn/G, and Cu/G cathode composites with the amount of metal specifically controlled at 10 at%. The X-ray diffraction (XRD) patterns (Figure 4) of the composites indicate that pure Ni/G, Sn/G, and Cu/G phases are successfully synthesized. The microstructures of Ni/G, Sn/G, and Cu/G are

characterized by high resolution transmission electron microscopy (HRTEM) - 14 -


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and selected area electron diffraction (SAED), as shown in Figure 5. Figure 5a displays the typical morphology of Ni/G composite, in which the nanoparticles of ~30 nm for Ni metal are uniformly dispersed and tightly adhered on the surface of graphene. The measured interplanar spacing of 0.23 nm is in agreement with the d-spacing of (010) lattice plane of Ni in Figure 5b and confirmed by the SAED pattern inserted. Similarly, Figures 5c and 5e describes the uniform dispersion of Sn nanoparticles with particle size of ~50 nm and Cu nanoparticles with particle size of ~10 nm on the surface of graphene. The measured interplanar spacings of 0.29 nm (Figure 5d) and 0.21nm (Figure 5f) are corresponding to (200) lattice plane of Sn and (111) lattice plane of Cu, which are consistent with the XRD results. X-ray photoelectron spectroscopy (XPS) and Raman spectra are used to demonstrate chemical bondings between metal (Ni, Sn, Cu)/graphene composites and sulfur clusters in Figures 6 and 7. As is well known, polysulfides are always consisted of a series of sulfur species (Sn) with different clusters (n=1, 2, 4 and 8), thus we prepare the representative lithium sulfide Li2S4 to verify the theoretical results mentioned above. The XPS spectrum of Li2S4 in Figure 6d shows two 2p3/2 contributions at 161.5 eV and 162.9 eV (the ratio is 1:1), respectively, which are attributed to different sulfur species of terminal sulfur (S-1T ) and bridging sulfur (S0B ). It should be noted that only the 2p3/2 component from the 2p3/2/2p1/2 doublet is quoted here to follow the convention of previous results.50-51 The assignment is in accord well with chain-like structure of lithium polysulfides, where ab initio calculations show that negative charge is localized at the termini.52 The XPS spectra of metal (Ni, Sn, Cu)/graphene composites (Figures 6a, 6b and 6c) are compared with the host material of Li2S4 to verify chemical bondings between metal (Ni, Sn, Cu)/graphene composites and sulfur - 15 -



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species. The spectra of Li2S4 binding with metal/graphene composites exhibit blue shifts, i.e., 163.8 eV for Ni/G composite, 163.7 eV for Sn/G composite, and 162.9 eV for Cu/G composite, respectively, which are consistent with the sequence of Ni>Sn>Cu from the above theoretical simulations. The shifts of binding energies are associated with the changes in electron distribution of sulfur atoms that adsorp on the surfaces of metal/graphene composites. Note that the minor broad background contribution between 166 eV and 169.5 eV on S 2p spectra is the characteristic of sulfite or sulfate arising from a trace amount of water in the solvent. Besides, a very small amount of nitrogen content (approximate 5 at.%) is demonstrated in the three metal (Cu, Ni and Sn)/graphene composites

due to the NH3

treatment in the synthesis procedure (Figure S3a). And the Ni/graphene composites affords more graphitic N incorporated into graphene than the (Sn and Cu)/graphene composites (Figure S3b-d). We further verified the shifts in binding energy between metal/graphene composites and Li2S4 with the same sequence of Ni>Sn>Cu by the Raman spectra. The observed peaks at the range of 100-500 cm-1 are typical signals for sulfur (Figure 7a).33 The shifts in the range between 120-200 cm-1 in the Raman spectra arise from chemical bondings between metal/graphene composites and Li2S4,53 which are in good accordance with immobilization of sulfur clusters on metal/graphene substrate calculated above. The electrochemical properties of metal/G-S composites are evaluated with 70 wt% sulfur in the electrode cycled from 1.8 to 2.6 V (vs. Li/Li+), as shown in Figures 8. The metal/G is impregnated with 70 wt% sulfur by melt-diffusion at 155 oC, which perform almost identically for all the composites (see details in the supplementary experiment). The sulfur content of 69 wt% in metal/G/S composites is confirmed by TGA (Figure S4). Cyclic - 16 -


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voltammetry (CV) curves of metal/G-S composites with different metals (Cu, Ni and Sn) at the first cycle represent the typical redox sulfur chemistry in Li-S batteries, which is consistent with cycling performance of sulfur cathodes during charge-discharge (Figure S5). As is illustrated in the electrochemical impedance spectroscopy (EIS) (Figure S6), which displays Nyquist plots of the metal/G-S electrodes, the comparable AC impedances indicate similar electron conductivities. Figure 8a shows the electrochemical performances of metal/G-S electrodes in comparison with that of bare G-S electrode. All metal/G-S cathodes show a remarkably high capacity retention at C/5, which are clearly observed as the sequence of Ni>Sn>Cu. The rate capabilities of metal/G-S composites with different metals (Cu, Ni and Sn) from C/5 to 5C are also in the same sequence of Ni>Sn>Cu (Figure 8c). These are consistent with the theoretically predicted results. These electrochemical performance characterizations also manifest that the slight amounts nitrogen incorporated into graphene by the NH3 treatment during synthesis procedure don’t have apparently impact on the electrochemical performances of metal/G composites, which would otherwise present the sequence of Ni>Cu>Sn as that of freestanding metals according to the theoretical calculation results. Compared with bare G-S cathode, all metal/G-S cathodes show better capacity retentions upon long-term cycling. The Ni/G-S electrode delivers the highest capacity retention after 120 cycles among metal/G-S systems (Figure 8a) as well as the best rate performance (Figure 8c). Improved long-term cycling performance is further achieved for the Ni/G-S cathode (Figure 8b). At a current rate of C/5, an initial capacity of 1092 mAh g-1 is attained. And a capacity of 832 mAh g-1 is obtained After 500 cycles, meaning the capacity decay rate of only 0.05%. A rather high average coulombic efficiency (~98.5%) is also - 17 -



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observed in the whole cycle life for the Ni/G-S cathode. This ascribes to the essential role of sulfur immobilization by metal/G composites, which are in a full accordence with the findings from our theoretical simulations as well as experimental XPS and Raman data. In addition, the electrochemical performances of metal-S cathodes without graphene are measured. Metal-S cathodes with 70 wt% sulfur loading display serious capacity decay compared to that with 50 wt% sulfur loading (Figure S7), indicating that the sulfur loading is limited without graphene due to the insulating nature of sulfur species and aggregation of metal nanoparticles. As a comparison, metal/graphene composites show high surface area (Figure S8) and uniform dispersion of metal nanoparticles anchored on graphene without agglomeration, benefiting for electrolyte

infiltration

and

thus

enhancing

lithium-ion

pathway

with

improved

performance.54-55 Overall, these experiments provide solid evidence for the presence of chemical interactions between metal/G composites and sulfur species. Our work thus sheds light on the importance of combination of theoretical simulations and experiments towards rational design of high performance electrode materials in Li-S battery. 5. Conclusions In summary, metal/graphene with electronic conductivity and chemical bonding with polysulfides are theoretically and experimentally explored to design the electrode for the Li-S with high and stable specific capacities up to 832 mAh/g over 500 cycles. We utilize the first principles calculations to investigate sulfur immobilization on the surfaces of Cu, Ni, and Sn and their corresponding metal/graphene and metal/defect graphene systems. We find that graphene have an important impact on the sulfur cluster adsorption on the metal surface through electron transfer between graphene and the metal. For transtion metals of Cu and Ni, - 18 -


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the localized d states are sensetive to electron transfer, leading adsorption energy variation of sulfur clusters. While for main group metal of Sn, electron transfer has less impact on extended p states and hardly alters sulfur clusters’ adsorption energy. In addition, carbon vacancy, boron doping and nitrogen doping on graphene substrate reduces charge transfer and makes the trend of metal-S interaction strength the same as that on free standing metal surfaces. XPS and Raman studies verify strong chemical interactions between metal/graphene composites and sulfur species. Metal/graphene composites enhance electron transport and lithium-ion pathway, while chemical bondings immbolize sulfur species in the cathode. The resulting sulfur cathode based on Ni/G composites demonstrates the remarkable cycling performance. These findings demonstrate the importance of combination of theoretical calcualtions and experiments to design superior electrode materials for Li-S batteries as well as other energy storage systems.

Supporting Information. The DFT calculated band structures, XPS survey spectra and N 1s spectra, TGA curves, cyclic voltammetry (CV) curves, EIS spectra, cycling performances and N2 adsorption-desorption isotherm profiles of the metal/graphene composites.

Acknowledgements This work is supported by National Key Research and Development Program (Grant No. 2016YFB0901600), National Natural Science Foundation of China

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(11304161, 11104148, 51171082, 21573117 and 11404172) the National Basic Research Program of China (973 Program with No. 2014CB931703), Fundamental Research Funds for the Central Universities. We thank the technology support from the Texas Advanced Computing Center (TACC) at the University of Texas at Austin (http://www.tacc.utexas.edu) for providing grid resources that have contributed to the research results reported within this paper. This work was financially supported by Chinese government under the "Thousand Youth Talents Program", Zhejiang Province Science Fund for Distinguished Young Scholars (Project LR16B060001), and Key Technology and Supporting Platform of Genetic Engineering of Materials under State's Key Project of Research and Development Plan (Project 2016YFB0700600).

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Table 1. d-band or p-band Centers of Surface Metal Atoms (Cu, Ni, and Sn) in Corresponding Metal, Metal/graphene (M/G) and Metal/defect-graphene (M/G_V, M/G_B and M/G_N) Systemsa

a

metal

M/G

M/G_V

M/G_B

M/G_N

Cu (d-band)

-2.04

-3.09

-2.40

-2.48

-2.51

Ni (d-band)

-1.02

-0.57

-0.88

-0.79

-0.75

Sn (p-band)

-0.49

-0.39

-0.55

-0.53

-0.56

The band centers are calculated with respective to the Fermi level. And the unit is eV.

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Figure 1. (a), (b) and (c) represent the side views of the stable configurations of S1, S2, S4 and S8 adsorbed on surfaces of Cu, Ni and Sn. Cu, Ni, Sn and S atoms are depicted by blue, gray, lavender and yellow balls, respectively.

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Figure 2. Calculated adsorption energies of sulfur clusters Sx (x=1, 2, 4 and 8) on surfaces of (a) metals (M), (b) metal/graphene (M/G) systems and (c-e) metal/defect graphene (MG_V, M/C_B, and M/G_N) systems.

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Figure 3. Energy levels interactions between metal surfaces and sulfur clusters. The blue, red and green bars denote the d-band or p-band centers of Ni d-band, Cu d-band and Sn p-band. The yellow bar denotes the p orbitals of sulfur clusters.

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Figure 4. X-ray diffraction patterns of Ni/G, Cu/G and Sn/G composites, respectively.

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Figure 5. TEM and HRTEM images of Ni/G composites (a and b), Sn/G composites (c and d) and Cu/G (e and f) composites, respectively.

- 29 -



Figure 6. Demonstration of the strong interaction of lithium polysulfides with metal/G by using XPS. High-resolution XPS S 2p spectra of (a) Ni/G-Li2S4, (b) Sn/G-Li2S4, (c) Cu/G-Li2S4 and (d) Li2S4 (black line = experimental data, red line = overall fitted data, solid lines in other colors = fitted individual components).

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Figure 7. Demonstration of strong interaction of lithium polysulfides with metal/G composites by using Raman. (a) Raman spectra of Ni/G-Li2S4, Sn/G-Li2S4, Cu/G-Li2S4, and Li2S4, respectively. (b) The magnified spectra at the range of 120-200 cm-1 demonstrate the Raman shift for Ni/G-Li2S4, Sn/G-Li2S4, Cu/G-Li2S4, and Li2S4, respectively.

- 31 -



Figure 8. (a) The cycling performances of metal (Cu, Ni or Sn)/G-S composites and G-S composites at C/5 over 120 cycles; (b) the long-term stabilities of Ni/G-S composites at C/5 over 500 cycles with corresponding coulombic efficiencies; (c) the rate capabilities of metal - 32 -


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(Cu, Ni or Sn)/G-S composites from C/5 to 5C.

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TOC Graphic:

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