Reduced Graphene Oxide Interlayer Mitigating Shuttle Effect

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A SnO2/Reduced Graphene Oxide Interlayer Mitigating Shuttle Effect of Li-S Batteries Nana Hu, Xingshuai Lv, Ying Dai, Linlin Fan, Dongbin Xiong, and Xifei Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 17 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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ACS Applied Materials & Interfaces

A SnO2/Reduced Graphene Oxide Interlayer Mitigating Shuttle Effect of Li-S Batteries

Nana Hu,a Xingshuai Lv,b Ying Dai,b Linlin Fan,c Dongbin Xiongc and Xifei Lia,c*

a

Tianjin International Joint Research Centre of Surface Technology for Energy

Storage Materials, College of Physics and Materials Science, Tianjin Normal University, Tianjin 300387, China E-mail: [email protected] b

School of Physics, State Key Laboratory of Crystal Materials, Shandong University,

250100 Jinan, PR China c

Institute of Advanced Electrochemical Energy, Xi’an University of Technology,

Xi’an 710048, China.

Abstract The short cycle life of lithium-sulfur batteries (LSBs) plagues its practical application. In this study, a uniform SnO2/reduced graphene oxide (denoted as SnO2/rGO) composite is successfully designed onto the commercial polypropylene separator for use of interlayer of LSBs to decrease the charge transfer resistance and trap the soluble lithium polysulfides (LPS). As a result, the assembled devices using the separator modified with the functional interlayer (SnO2/rGO) exhibit improved cycle performance, for instance, over 200 cycles at 1C, the discharge capacity of the cells 1

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reaches 734 mAh g-1. The cells also display high rate capability with the average discharge capacity of 541.9 mAh g−1 at 5 C. Additionally, the mechanism of anchoring behavior of the SnO2/rGO interlayer was systematically investigated using the density functional theory calculations. The results demonstrate that the improved performance is related to the ability of SnO2/rGO to effectively absorb S8 cluster and LPS. The strong Li-O/Sn-S/O-S bonds and tight chemical adsorption between LPS and SnO2 mitigate the shuttle effect of LSBs. This study demonstrates that engineering functional interlayer of metal oxide and carbon materials in LSBs may be an easy way to improve their rate capacity and cycling life.

Keywords: lithium-sulfur batteries, shuttle effect, SnO2, reduced graphene oxide, interlayer

1. Introduction As the demands for large-scale electrochemical energy storage systems increase, amounts of effort have been recently focused on the development of the LSBs due to immensely high capacity (~1672 mAh g-1) and specific energy density (~2600 Wh kg-1).1,2 Despite of these promising attributes, the practical application of LSBs is still plagued by short cycle life. Several persistent drawbacks are addressed for limiting LSBs performance: (i) low sulfur utilization because of the poor conductivity of sulfur itself as well as discharge products (Li2S2/Li2S);3 (ii) low Coulombic efficiency and rapid capacity fading because of the diffusion of soluble intermediate LPS (Li2Sn,4 ≤ 2


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n ≤ 8) into the electrolyte;3,4 (iii) the loss of electrical contact with the conductive additives and current collector because of the huge volume change (80%) during the lithiation of sulfur (2.07 g cm−3) to Li2S (1.66 g cm−3).5 All these phenomena result in serious polarization within the cells, hastening degeneration of capacity and reducing the cycle life. To mitigate these issues, considerable efforts have been dedicated to engineer the electrode structures to chemically and/or physically prevent the LPS migration and to increase the electrical conductivity.6 To date, several promising approaches have been developed, including sulfur-porous carbon composites,7 polymer-sulfur

composites,8

electrolyte/binder

additives,9,10

biomimetic

architectures11 as well as metal oxide.12,13 Obviously, these approaches achieved significant improvements in electrical conductivity, cycleability, and capacity, however, it is noted that some challenges, including the loss of active material and a low lithium ion conductivity,14 have to be addressed prior to its commercialization. More recently, various free-standing carbon interlayers such as porous carbon paper,15 carbon nanotube paper16 and natural carbonized leaf,17 have been developed for the absorption of soluble LPS. These carbon interlayers have shown possibility to suppress the diffusion of LPS and enhance cycling life of LSBs. However, some tremendous challenges remain to be resolved: (i) the unacceptable thickness and heavy mass of the interlayers may diminish the overall energy density of the LSBs; (ii) the weak interaction between the nonpolar carbon interlayer and the polar LPS is required to optimize; (iii) the complexity of the fabrication of the carbon interlayers limits the large-scale applications of the LSBs. Moreover, the physical barrier of 3



interlayers may block the Li+ transport, raising the cathode resistance.18,19 Therefore, developing a chemically selective and lightweight carbon interlayer has been challenging. As previously reported, the successful encapsulating of sulfur into SnO 2 yolk-shell structures was demonstrated to improve the capacity retention of LSBs.20,21 The results indicate that the SnO2 shells promote the interaction between SnO2 and S owing to the existence of chemical bonding with LPS. The bonding is beneficial for suppressing the shuttle effect, minimizing the sulfur loss, thus achieving superior electrochemical performance. In addition, many researches have substantiated that the oxygen-containing functional groups, including -COOH and -OH, on the surface of rGO, played a significant role in improving the electrochemical performance of LSBs by enhancing LPS’s binding ability and suppressing the shuttle effect.22-25 Inspired by these results, we presented a bifunctional interlayer with a light-weight SnO2/reduced graphene oxide-coated separator (SnO2/rGO-coated separator) for use with pure sulfur cathodes.26 In this work, the sulfur loading in the cathode electrodes is 2.87 mg cm-2. As the key skeleton material in the bifunctional interlayer, rGO sheets can lead to the uniform distribution of SnO2 nanoparticles on rGO surface, reduce the resistance of sulfur cathode, and decrease the deformation of the interlayer. More significantly, the detailed mechanism of mitigating the shuttle effect of SnO2/rGO was systematically investigated using the first-principles density functional theory (DFT) calculations. It is expected that as one promising anchoring material, the SnO2/rGO interlayer can effectively inhibit the shuttle effect in LSBs benefiting by its strong adsorption ability of LPS. 4


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2. Experimental Section 2.1

Synthesis of the SnO2/rGO composite

Graphite oxide was prepared by a modified Hummer's method, as previous studies mentioned.26,27 SnO2/rGO was prepared through a facile hydrothermal method (the resulting mixture was kept at 180 °C for 24h in 50 mL Teflon-lined stainless steel autoclaves) using SnO2 soliquid as a precursor and rGO as a carrier. Pristine rGO was synthesized by the same hydro thermal method without SnO2 soliquid in the preparation process.

2.2

Preparation of SnO2/rGO-coated separator

In the typical experiment, an interlayer was overlaid on one side of commercial polypropylene separator (Celgard 2500) simply by coating with a mixture film. The mixture slurry was prepared via mixing SnO2/rGO composite and polyvinylidene fluoride (PVDF) in the weight ratio of 9:1 with N-methyl-2-pyrrolidinone (NMP) as the dispersant. The SnO2/rGO-coated separator was dried for 12 h at 60 °C in an air oven and then cut into circular disks (16 mm). The rGO-coated separator was prepared by the same process with SnO2/rGO-coated separator.

2.3

Preparation of the pure sulfur cathode

The active material mixture slurry was produced via mixing of 55 wt% sublimed sulfur, 30 wt% carbon black (CB) and 15 wt% PVDF in the NMP solution. Then, the 5



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slurry was coated onto an aluminum foil and dried for 12 h at 80 °C in air. Ultimately, the pure sulfur cathode (CB-S) was cut into circular disks (12 mm).

2.4

Physical characterizations

Crystal-chemical characterization was performed using a D8 Advance X-ray diffracttometer (XRD, Bruker, Germany) with Cu/Ka radiation between 5°and 85°. The surface morphologies of the prepared products were inspected with a Hitachi SU8010 scanning electron microscope (SEM), and JEOLJEM-3000F transmission electron microscope (TEM). The X-ray analysis was employed to collect elemental mapping and signals. Thermo gravimetric analysis (TGA, Pyris Diamond6000 TG/DTA, PerkinElmer Co, America) was performed to confirm the SnO2 content in the composite. Raman spectra were recorded on a LabRAM HR800 system.

2.5

Electrochemical characterizations

The CR2032-type coin cells were assembled in an Ar-filled glove box. The cell consisted of working electrode (pure sulfur cathodes) as the counter electrode and lithium

metal

as

the

bis-trifluoromethanesulfonylimide

reference (LiTFSI)

electrode. in

1.0M

1,3-dioxolane

lithium

(DOL)

and

1,2-dimethoxyethane (DME) at a volume ratio of 1: 1 with 1 wt% LiNO3 additive was employed as electrolyte. A commercial polypropylene separator was used as pristine separator. The modified separator including SnO2/rGO-coated separator and rGO-coated separator were placed into the cells with the functional interlayer facing 6


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to the sulfur cathode. The cells with commercial polypropylene separator, rGO-coated separator and SnO2/rGO-coated separator are referred to here as CB-S, CB-S@ SnO2/rGO and CB-S@SnO2/rGO, respectively. The discharge/charge performance was tested with a Land battery test system (LANHE CT2001A) over a voltage interval ranging from 1.5 V to 3.0 V. Different current densities were applied to test the rate performance of the cells. Cyclic voltammo grams (CV) was collected using Princeton Applied Research VersaSTAT4 electrochemical work station at a scan rate of 0.1 mVs-1. Electrochemical impedance spectra (EIS) data was collected within the frequency range from 0.01 Hz to 100 kHz with an AC voltage amplitude of 5 mV. 2.6

Computational method

Spin-polarized density functional theory (DFT) calculations were performed in the Vienna ab initio simulation package (VASP).28,29 The exchange-correlation were described using Perdew-Burke-Ernzerhof (PBE)30 in the scheme of generalized gradient approximation (GGA).31 We treated Sn 5s, 5p together with O 2s and 2p orbitals as valence states, and the rest were considered as core. Van der Waals (vdW) interactions were considered by employing Grimme (DFT+D2) scheme.32 The energy cut-off for plane waves was set to 450 eV. Geometric configurations were fully relaxed until the maximum force on each atom was less than 0.02 eV/Å-1 and the energy break criterion was to 10-5 eV. The Brillouin zone was sampled using a 3×3×1 and 9×9×1 grid generated by Monkhorst-Pack scheme for geometry optimization and electronic properties calculations, respectively. The vacuum space in the z direction is large than 15 Å between two adjacent images to prevent spurious interactions. 7



The anchoring performance of SnO2/rGO substrate was explored by comparing the binding energies (𝐸𝑏 ) between LPS and substrate with those between LPS and electrolyte molecules. The 𝐸𝑏 of LPS with the substrate was calculated according to the equation 𝐸𝑏 = 𝐸𝑡𝑜𝑡𝑎𝑙 − 𝐸𝐿𝑖𝑃𝑆𝑠 − 𝐸𝑠𝑢𝑏 The 𝐸𝑏 of various LPS with the electrolytes were calculated by 𝐸𝑏 = 𝐸𝑡𝑜𝑡𝑎𝑙 − 𝐸𝐿𝑖𝑃𝑆𝑠 − 𝐸𝑒𝑙𝑒𝑐 where 𝐸𝑡𝑜𝑡𝑎𝑙 ,𝐸𝐿𝑖𝑃𝑆𝑠 ,𝐸𝑠𝑢𝑏 , and 𝐸𝑒𝑙𝑒𝑐 represent the total energy of the adsorption configuration, the free-standing LPS, the optimized SnO2/rGO sheet, and the electrolyte molecule, respectively.

3. Results and Discussion Figure 1a shows the cell configuration modified with the SnO2/rGO-coated separator, and the configuration for a conventional LSBs suffering from the severe LPS diffusion issue is shown in Figure 1b for comparison. The SnO2/rGO-coated side facing the sulfur-based cathode electrode, captures and localizes the diffusing LPS, which benefits from the strong Li-O/Sn-S/O-S bonds and tight chemical adsorption between LPS and SnO2. It can also be regarded as upper current collector to enhance the conductivity of sulfur cathode and assist the reutilization of the active material.33 Moreover, the insulating polypropylene facing lithium anode electrode remains highly electronically resistive. Figures 1c and 1d show scanning electron microscopy (SEM) images of the commercial polypropylene separator and fresh separator with SnO2/rGO 8


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coating film. After modification, the commercial polypropylene separator was uniformly covered with the SnO2/rGO coating layer, which displays tight conglutination due to the interfacial compatibility. Figure 1e exhibits a typical cross-sectional SEM image of the SnO2/rGO-coated separator. The thin-film SnO2/rGO coating layer with a thickness of 20 µm weighs only 0.15 mg cm-2, which overcomes the drawbacks of the added weight of the modified or extra components applied in cell modifications.34,35 After coating with SnO2/rGO slurry by a spreader, a smooth surface of the separator was observed. The XRD pattern in Figure 2a indicates the formation of rutile SnO2 (JCPDS card no.41–1445).36 Further structural characteristics of the synthesized products were examined via Raman spectroscopy (Figure 2b). The D band corresponds to the indication of the disorder in the graphitic layers, and the G band due to the in-plane stretching motion of symmetric sp2 carbon atoms. The intensity ratio of ID/IG is commonly used to determine the disorder degree of the sp2 domains.37,38 It can be observed that the ID/IG values of pristine reduced graphene oxide and SnO2/rGO are 2.26, and 2.16, respectively, indicating a decreasing size of the sp2 carbon domains with the SnO2 nanoparticles anchoring on reduced graphene oxide sheet. In addition, the rutile SnO2 particles accounted for 27 wt% in the SnO2/rGO composite (Figure 2c). Figures 3a and 3b present the morphological features of pristine gossamer rGO and SnO2/rGO composite. Clearly, the SnO2 nanoparticles are homogeneously anchored onto the folded and wrinkled rGO sheets. The particle size is about 5 nm, 9



which can provide the high adsorption activity between SnO2 and LBS. The existence of SnO2 nanoparticles can prevent rGO sheets from stacking, while rGO sheets can prevent the aggregation of SnO2 nanoparticles and improve the conductivity of the composite. Simultaneously, the TEM images of the SnO2/rGO (Figure 3c) can also confirm the above phenomenon. Figure 3d displays the HRTEM image of SnO2/rGO composite. Lattice-resolved image displays the lattice spacing of 0.265 nm and 0.335 nm, which is appropriately assigned to the spacing of (101) and (110) crystal planes of rutile SnO2,27,35 respectively. Figure 3e illustrates the SEM image of SnO2/rGO interlayer, and the corresponding EDX data is shown in Figures 3f-i. It is clearly observed that the carbon, oxygen, and tin element are uniformly distributed in the SnO2/rGO interlayer. The SnO2/rGO sample presents a hierarchical sponge-like structure which is highly conductive, allowing for direct redox and utilization of adsorbed LPS. Moreover, due to the tight chemical adsorption between LPS and SnO2, the interlayer can function as a filter for obstructing the diffusion of LPS. Thus, it is difficult for the dissolved LPS to penetrate through a SnO2/rGO interlayer and escape from it. Figure 4a depicts cyclic voltammetry (CV) plots for CB-S, CB-S@rGO, CB-S@ SnO2/rGO in the first cycle at a scan rate of 0.1 mV s-1 in the potential range of 1.5-3.0 V (vs Li+/Li). Both of the cells display the typical reduction/oxidation reactions of sulfur in LSBs with two pronounced cathodic peaks and an anodic peak. The two cathodic peaks correspond to the reduction of sublimed sulfur (S8) to intermediate LPS (Li2Sx, 4≤x≤8) and then to insoluble Li2S2/ Li2S. When scanning 10


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back to 3.0 V, the anodic peak is related to the conversion of Li2S/Li2S2 to LPS and sulfur.39,40 It can be observed that the redox peak in the CB-S@SnO2/rGO were much narrower than those in the CB-S@rGO and CB–S, which suggests a more homogeneous distribution of sulfur-species in CB-S@SnO2/rGO. Furthermore, compared with the other two samples, the CB-S@SnO2/rGO exhibited a higher intensity peak. It can be speculated that, during the lithiation process, the strong interaction between LPS and SnO2/rGO may have enhanced interphase electronic contact of the particles.41 To demonstrate the effect of SnO2/rGO interlayer and strong affinity of SnO2 nanoparticles to LPS, the initial five CV curves of CB-S@SnO2/rGO were performed (Figure 4b). Particularly, no obvious potential or current changes occurred in the CV peaks during subsequent CV scans, which confirms the high reversibility facilitated by the SnO2/rGO interlayer. The initial galvanostatic discharge-charge profiles of all cells were compared at 1 C (1 C=1675 mA g-1) in Figure 4c. The CB-S delivered the discharge/charge capacities of 558/545 mAh g-1. Commendably, the charge capacity of the CB-S@rGO and CB-S@SnO2/rGO increased to 838 and 992.7 mAh g-1, respectively, while the discharge capacities increased to 848.2 and 1026 mAh g-1, respectively. In the discharge curves, the upper discharge plateau of CB-S@SnO2/rGO at 2.24 V represents the reduction of S8 to LPS. The lower discharge plateau at 2.03 V represents transformation of soluble LPS to insoluble Li2S2/Li2S. Meanwhile, the experimental QH and QL values of the CB-S@SnO2/rGO were much higher than that of CB-S and CB-S@rGO. Moreover, the first cycle voltage hysteresis (ΔE) of the 11



CB-S@SnO2/rGO was much lower than those of the CB-S and CB-S@rGO, and no apparent voltage hysteresis changes occurred in the during subsequent galvanostatic charge-discharge profiles of the CB-S@SnO2/rGO at 1 C, as shown in Figure S1a of the Supporting Information, indicating the CB-S@SnO2/rGO displays lower polarization. As demonstrated in Figure 4d, the CB-S@SnO2/rGO displayed a discharge capacity of 734 mAh g-1 after being continuously cycled for 200 cycles at a rate of 1 C and a very high Coulombic efficiency of approximately 100% was obtained during the cycles. A gradual capacity increase is observed in the first 10 cycles, which results from an electrochemical activation process, in which, the electrolyte gradually diffuses through the cathode and SnO2/rGO coating layer.41 By contrast, after 200 cycles, the retained capacities of CB-S and CB-S@rGO were only 417 and 552 mAh g-1, respectively. On account of a fierce electrochemical kinetic barrier, the initial discharge capacities of CB-S and CB-S@rGO were only 838 and 723 mAh g-1, respectively. The rate capability of CB-S, CB-S@rGO and CB-S@SnO2/rGO were further studied to confirm the contribution of the SnO2/rGO interlayer. As shown in Figure 4e. the CB-S@SnO2/rGO displayed much higher rate capability, and the average discharge capacity at 5 C reached 541.9 mAh g−1. Importantly, when the current rate was returned to 2 C, the discharge capacity could recover to 623.5 mAh g−1, which is close to the initial value at 2 C. The charge-discharge curves of the CB-S@SnO2/rGO at various current rates (0.5-5 C) were also illustrated in Figure S1b. Specific capacity values of 1076, 992, 477, and 734 mAh g-1 were measured at 0.5, 1, 2 and 5 C respectively. The cycling 12


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performance of the CB-S@SnO2/rGO was also tested at 2 C and 3 C (Figure 4f). The discharge capacities of the LSBs approached 602 mAh g-1 and 501 mAh g-1 at 2 C and 3 C after 200 cycles, with capacity retention rates of 82.17% and 80.33%, respectively. The improved cycling performance and rate capability of the cells can be attributed to the following reasons: First, the SnO2/rGO interlayer can enhance the conductivity of the pure sulphur cathode, deriving from the improved interactions at the S/electrolyte/SnO2/rGO interphase and accelerating the electrochemical conversion reaction. Second, the SnO2/rGO interlayer shows better polysulfide-trapping capability due to the strong Li-O/Sn-S/O-S bonds and tight chemical adsorption between LPS and SnO2. Thus, the SnO2/rGO interlayer can reduce or even avoid the shuttle effect, leading to complete reaction in cathode. To ascertain the effect of the SnO2/rGO interlayer on improving the electrochemical performance, electrochemical impedance spectroscopy was carried out at full-charged state after 1st and 30th cycles at frequencies of 0.01 Hz to 100 kHz with an amplitude of 5 mV, respectively, as shown in Figure 5a. Simultaneously, all EIS spectra presented two depressed semicircles followed by a linear section.42 An equivalent circuit model in Figure 5b was employed to fit the EIS curves, where Re represents electrolyte ohmic resistance, and Zw denotes Warburg resistance associated with the solid state diffusion of Li+, which is in the low frequency region and depicted as a sloping line. The two depressed semicircles are relevant to the charge transfer resistance (Rct) between the cathode electrode and the electrolyte, as well as the interface contact resistance (Rsf) between cathode electrode and soluble LPS, 13



respectively.43,44 Both of the Rct and Rsf of the CB-S were larger than those in the CB-S@rGO and CB-S@SnO2/rGO because the rGO and SnO2/rGO interlayer possess the commendable conductive performance and wrinkled and folded construction, which provided additional electron/Li+ pathways and continuous electrolyte channels of sulfur cathodes. Compared to pristine rGO, the SnO2/rGO has stronger binding ability of LPS, which is beneficial for trapping the LPS in the charge/discharge process. Thus, the Rsf values of CB-S@SnO2/rGO almost unchanged with increasing cycles, while that of CB-S@rGO obviously increased attributed to the immigration of LPS onto the anode side. The adsorption of LPS on SnO2/rGO composite was theoretically investigated to better understand the anchoring behavior. First, the relaxed structural parameters of graphene and SnO2 monolayers are explored to be 2.46 Å and 3.23 Å, which are consistent with previous results.45,46 When exploring the equilibrium configuration, the √7×√7 supercell of graphene and 2×2 SnO2 attached to each other result in a lattice mismatch of 0.8%, which is facile of forming heterostructures. Several different stacking patterns are considered, and the most stable arrangement is obtained as schematically shown in Figure S2a of the Supporting Information. In addition, various optimized configurations of LPSs, including Li2S, Li2S2, Li2S4, Li2S6, Li2S8, are illustrated in Figure S2b. For S8, the bond length and bond angel are 2.09 Å and 109.3°, respectively. During the lithiation process, the final products (Li2S and Li2S2) exhibit C2v symmetry while Li2S4, Li2S6 and Li2S8 possess C2 symmetry. The relaxed structures of Li2Sn and S8 in this work are consistent with recent researches.47 14


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Fully structure relaxation was performed to determine the most stable adsorption configurations of LPS on the surface of SnO2/rGO. The optimized configurations of LPS anchored on SnO2/rGO are shown in Figure 6. It can be seen that after deposition on SnO2/rGO, the LPS can be stably anchored on the substrate, and SnO2/rGO may maintain the original structure commendable. The dependence of binding strength between LPS and SnO2/rGO can be determined from Li-O distances. More specifically, for unlithiated S8, the S8 plane is parallel to the SnO2/rGO with the distance of 2.86 Å, and the configurations of S8 and the substrate are nearly unchanged. As the lithiation process starts, the binding strength between LPS and SnO2/rGO become stronger to different extents. After the adsorption, Li2S on SnO2/rGO causes the smallest Li-O distance (1.83 Å) among all cases considered, which is consistent with the trend of binding energy in the next paragraph. As shown in Figure 6a, the robust chemical interaction results in the deformation of substrate, however, both LPS and the substrate maintain the configuration intact, indicating the excellent anchoring performance of SnO2/rGO. The binding energies of LPS and sulfur cluster on the substrate were investigated to further estimate whether SnO2/rGO is a suitable host material. As listed in Table 1, the calculated binding energy of S8 cluster is about -1.65 eV. As the lithiation process starts, the binding strength of LPS gradually becomes stronger. For the final lithiation stage Li2S, the binding strength is the strongest with the binding energy of -3.46 eV. As the dominating intermediate products during the charge/discharge process, Li2Sn (3≤ n ≤8) are soluble into electrolyte because of their strong interaction with the DME 15



and DOL. It can be seen that DOL and DME show similar binding strength for the LPS with Eb ranging from -0.80 to -0.74 eV, which is consistent with previous works.48 For each reduction step of the LPS, the binding strength between LPS and SnO2/rGO is stronger than that of other anchoring materials.49 Importantly, the Eb of the LPS with DOL/DME are far smaller than that with SnO2/rGO, indicating that these LPS can be stably anchored on the surface of SnO2/rGO. Therefore, the LPS will not be dissolved into the electrolyte from the energetic perspective.48 For simplicity, our results revealed that the trend of LPS on the substrate is various, depending on the different adsorption configuration of LPS. Since the binding strength of SnO2/rGO is much stronger than those on pristine rGO47 and phosphorene,49 SnO2/rGO sheet holds great promise to act as an efficient anchoring material for LPS, leading to complete reaction and reduce or even avoid the shuttle effect.6,50 Next, we analyzed the nature of chemical bonding between LPS and SnO2/rGO through the partial density of states (PDOS). As shown in Figure 7, after LPS adsorbed on SnO2/rGO heterostructure, the composite systems still maintain their metallic properties, indicating the commendable conductive anchoring performance of SnO2/rGO. In addition, Figure 7 demonstrates a strong hybrid orbital between LPS and SnO2 near the Fermi level, which further proves the strong Li-O/Sn-S/O-S bonds and tight chemical adsorption between LPS and SnO2. On the contrary, for S8 cluster adsorption on SnO2/rGO, the orbital hybridization degree of S8-SnO2 is weak, explaining the larger distance between S8 and SnO2 compared with other LPS. 16


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4. Conclusions In summary, SnO2/rGO-coated separator was successfully fabricated to mitigate the shuttle effect in LSBs. With the pure sulfur cathode, the cells using the SnO2/rGO interlayer exhibited excellent electrochemical performance. It delivered a reversible capacity of 734 mAh g-1 at 1 C after 200 cycles. More strikingly, when the current densities were increased to 2 C and 3 C, after 200 cycles, the discharge capacities of the cells approached 602 and 501 mAh g-1, with capacity retentions of 82.17% and 80.33%, respectively. Furthermore, the spin-polarized density functional theory calculations

profoundly

explain

the

possible

mechanism:

(i)

the

strong

chemical/physical interaction between LPS and SnO2/rGO effectively intercepts the migrating LPS. (ii) the commendable conductive performance of SnO2/rGO allows direct redox and utilization of sulfur and adsorbed LPS. As a result, this study may open up new possibilities for the development of a facile and light-weight interlayer configuration with strong chemical bonds and tight chemical adsorption between LPS and the interlayer.

Acknowledgements This research was supported by the National Natural Science Foundation of China (51572194),

Academic

Innovation Funding of Tianjin

Normal

University

(52XC1404), Tianjin Major Program of New Materials Science and Technology (No. 16ZXCLGX00070), and Training Plan of Leader Talent of University in Tianjin. 17



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Page 26 of 36

Table 1 The binding energies (𝐸𝑏 ) of LPS on SnO2/rGO with electrolyte molecules. 𝐸𝑏 (eV)

LPS SnO2/rGO

DOL

DME

Li2S

-3.46

–

–

Li2S2

-2.54

–

–

Li2S4

-2.23

-0.75

-0.74

Li2S6

-2.01

-0.78

-0.78

Li2S8

-1.92

-0.78

-0.80

S8

-1.65

–

–

26


Page 27 of 36 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


Figure Captions Figure 1 The comparison of the schematic cell configuration of the lithium-sulfur batteries: (a) with the functional interlayer of SnO2/rGO and (b) without functional interlayer; Typical SEM images of (c) the commercial polypropylene separator and (d) the modified polypropylene separator with SnO2/rGO (the insets are digital images of the top view of the separators); (e) Typical cross-sectional SEM image of the SnO2/rGO interlayer onto separator.

Figure 2 (a) Thermo gravimetric analysis curve of SnO2/rGO; XRD patterns (b) and Raman spectra (c) of SnO2/rGO and rGO.

Figure 3 High magnification SEM images of (a) rGO and (b) SnO2/rGO (the insets are low magnification SEM images); (c) TEM and (d) HRTEM images of SnO2 rGO; (e) SEM image, and (f) element mapping images of (g) C, (h) O, and (i) Sn of SnO2/rGO.

Figure 4 (a) CV profiles of the CB–S, CB-S@rGO, and CB–S@SnO2/rGO for the first cycle; (b) CV profiles of the CB-S@SnO2/rGO; (c) The initial galvanostatic charge–discharge profiles of the CB–S, CB-S@rGO, and CB–S@S/rGO at 1 C; (d) Cycling stability of CB–S, CB-S@rGO, and CB–S@SnO2/rGO at 1 C; (e) Rate capability of the CB–S, CB-S@rGO, and CB–S@SnO2/rGO; (f) Cycling stability of CB–S@SnO2/rGO cathode at 2 C and 3 C.

27



Figure 5 (a) The electrochemical impedance spectroscopy of CB-S, CB–S@rGO, and CB–S@SnO2/rGO in the first and 30th cycles; (b) The equivalent circuit used to simulate EIS curves.

Figure 6 The optimized lowest-energy adsorption configurations of (a)-(e) Li2Sn (n=1, 2, 4, 6, 8) and (f) S8 on the SnO2/rGO surface, both top view (top) and side view (down) are shown.

Figure 7 Atomic partial density of states (PDOS) near the fermi level of (a)-(e) Li2Sn (n=1, 2, 4, 6, 8) and (f) S8 adsorption on SnO2/rGO at different lithiation stages. The fermi level is set to zero.

28


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Page 29 of 36 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


Figure 1

29



(a)(002)

(b) D band

(110)

(101)

(211)

(200)

Intensity (a.u.)

(100) rGO (301)

SnO2/ rGO

G band

SnO2/rGO

rGO

20

30

40 50 2theta (degree) 100

Weight Loss (%)

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

Page 30 of 36

60

1200 1300 1400 1500 1600 1700 1800 -1 Raman Shift (cm )

70

(c)

80 73%

60 SnO2/rGO

40 20 0

0

100 200 300 400 500 600 700 800 Temperature (°C)

Figure 2

30


Page 31 of 36 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


(a)

(b) (b)

(a)

(b)

1μm

250nm

1μm

250nm

0.1um (d) (d) (d)

0.5um (c) (c) (c)

0.265 nm (101)

0.335 nm

0.5um

0.1um(110)

20nm

(e)

5nm

(f)

(a) (a)

1μm

(g)

(h)

(i)

C

O Figure 3

31


Sn


(a)

CB-S CB-S@rGO CB-S@SnO2/rGO

0.002

0.003

-1 Current (A.g )

0.001 0.000

-0.001

(b)

0.002 0.001 0.000 -0.001 -0.002

-0.002

1600

CB-S@SnO2/rGO

-1 Discharge capacity /mAh g

(c)

1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 + Voltage (V) VS Li /Li

(d)

1400

CB-S@rGO CB-S

100 CB-S CB-S@rGO CB-S@SnO2/rGO

1200 1000

∆E1 ∆E2 ∆E3

QH

QL 200 400 600 800 1000 1200 -1 Specific capacity (mAh g )

(e)

1400 1200

CB-S CB-S@rGO CB-S@SnO2/rGO

0.5 C

1000

1C 2C

800

5C

600

2C

400 200 0

10

20 30 Cycle number

40

-1

0

1C

Discharge capacity /mAh g

Voltage (V) vs Li+/Li

1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 + Voltage (V) vs Li /Li 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4

the 1st cycle the 2nd cycle the 3rdcycle the 4th cycle the 5th cycle

800

80 60 40

600

20

400 200 0

0 50

100 Cycle number

150

200

(f)

1200 1000

2C 3C

800 600 400 200

50

0

50

Figure 4

32


100 150 Cycle number

200

Coulombic efficiency

-1 Current (A.g )

0.003

-1 Discharge capacity /mAh g

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

Page 32 of 36

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(a)

(b)

RSEI

Rct

CPE1

CPE2

Re

Figure 5

33



Figure 6

34


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Figure 7

35



Table of Contents (TOC) graphic

36


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Reduced Graphene Oxide Interlayer Mitigating Shuttle Effect

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