A Collaboratively Polar Conductive Interface for Accelerating

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A Collaboratively Polar-Conductive Interface for Accelerating Polysulfides Redox Conversion Bokai Cao, Jiangtao Huang, Yan Mo, Chunyang Xu, Yong Chen, and Hai-Tao Fang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21447 • Publication Date (Web): 14 Mar 2019 Downloaded from http://pubs.acs.org on March 17, 2019

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ACS Applied Materials & Interfaces

A Collaboratively Polar-Conductive Interface for Accelerating Polysulfides Redox Conversion

Bokai Caoa,b, Jiangtao Huangb, Yan Mob, Chunyang Xua, Yong Chenb,*, Haitao Fanga,* a

School of Materials Science and Engineering, Harbin Institute of Technology, 92 West Dazhi

Street, Harbin 150001, China. b

State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan Provincial Key

Laboratory of Research on Utilization of Si-Zr-Ti Resources, College of Materials and Chemical Engineering, Hainan University, 58 Renmin Road, Haikou 570228, China.

Corresponding authors *E-mail: [email protected]. *E-mail: [email protected] (Haitao Fang).

ORCID Bokai Cao: 0000-0002-9193-4284 Yong Chen: 0000-0002-0419-7504

Keywords Lithium sulfur battery, Polysulfides chemisorption, Li-ion diffusion, Phase transformation, Enhanced redox kinetics

Abstract In order to alleviate the inferior cycle stability of sulfur cathode, a self-assembled SnO2-doped manganese silicate nanobubbles (SMN) is designed as sulfur/polysulfides host to immobilize the intermediate Li2Sx, and nitrogen-doped carbon (N-C) is coated on SMN (SMN@C). The exquisite

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N-C conductive network could not only provides sufficient free space for the volume expansion during the phase transition of solid sulfur into lithium sulfide, but also reduces Rct of SMN. During cycling, the soluble polysulfide could be fastened by the silicate with oxygen-rich functional group and hetero nitrogen atoms through chemical bonding, enabling a confined shuttle effect. The synergistic effect between N-C and SMN could also effectively facilitate the interconversion between lithium polysulfides and Li2S, reducing the potential barrier and accelerating the redox kinetics. With an areal sulfur loading of 2 mg/cm2, the S-SMN@C cathodes demonstrate a high initial capacity of 1204 mAh/g, i.e., 72% sulfur utilization at 0.1 C and an outstanding cycle stability with a capacity fade rate of 0.028%, ranging from 2nd cycle to 1000th cycle at 2 C.

Introduction The ever-increasing advanced energy storage demand have promoted the development of lithium-sulfur batteries owing to the overwhelming theoretical specific energy and capacity of 2600 Wh/kg, 1675 mAh/g, and environmental benignity of cathode active elemental sulfur. However, the shuttle effect of polusulfides, irreversible structural damage and uncontrollable precipitation of lithium sulfide induced by the multi-electron redox reaction between lithium and sulfur, which prevents the commercial application of lithium sulfur battery.1-2 Many efforts have been devoted to solving the above problems by tailoring novel structure of carbon and polymer served as sulfur accommodation to improve the electrochemical performance and restrict polysulfides through physical confinement3-4. However, the weak adsorbability of carbon, polymer-based materials towards polysulfides anion derived from different polarity, impeding the efficient interfacial charge transfer and decelerating the electrochemical reaction kinetics5. Therefore, understanding and regulating the interfacial electrochemical reaction of polysulfides and polar matrix are the prerequisites for designing advanced host materials6. Heteroatom doping or co-doping was initially introduced into carbon and polymer electrode7 to enhance the chemisorption of polysulfides8. For instance, nitrogen-9-13, oxygen-14-15 and Co-8,

16

modified carbon have shown improved rivet


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function for trapping polysulfides. However, the doping amount in carbon materials could not afford sufficient polar sites, another perspective, polymers that containing plentiful dopant atoms suffered from inferior electrical and ionic conductivity compared with that of carbon4. Recently, polar inorganic materials have been certified to interact with lithium polysulfides via strong chemical bonds6, 17-19, thus restricting the shuttle effect. Significantly, based on oxygen-rich compounds20, such as SiO221, TiO222-25, SnO226, MnO227-29, NiCoO430 and perovskite La0.6Sr0.4CoO3-δ31 have been successfully developed as sulfur/polysulfides captors for enhanced electrochemical stability lithium sulfur batteries. However, the much lower conductivity inevitably reduces the polysulfide conversion efficiency, causing an accumulation on active material surface and discounted rate performance32. Therefore, under the premise of long cycle stability, it’s still a challenge to further improve the capacities at higher current densities, simultaneously. As one of the impactful solutions, but not the only, multiple structural carbon materials was selected as ideal conductive matrix3, for example, carbon nanotube33-35, carbon sheets36-37, carbon nanofiber38, graphene39, hollow indented carbon spheres40 and grid-like multicavity carbon spheres41 as sulfur host material offered high capacity and good cycling retention. The rationally designed nanoarchitecture can reduce the interfacial impendence and relax the strain during repeated discharge/charge process42. Hollow sphere with sufficient inner space has been regard as a popular nanostructures for sulfur and sulfides accommodation43. Benefiting from the significantly increased surface area, providing enough active sites for lithium polysulfide adsorption, multicavity based hollow structures44-46 stand out as attractive materials with improved physicochemical performance. In this study, we report a simple one-pot synthesis of manganese silicate nanobubbles, further self-assembly into hollow nanospheres. In this procedure, the in-situ generated gas acted as soft template to form manganese silicate nanobubbles, meanwhile the monodispersed silica nucleus served both as template and reactant for the deposition of silicate into larger nanospheres. Based on this structure, an N-doped carbon shell was introduced to simultaneously meet the requirement of cycle and structural stability in lithium sulfur batteries. The porous SMN@C with sufficient oxygen



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functional groups enable efficient sulfur utilization and strong chemical adsorption towards polysulfides, further facilitating the electrochemical reaction kinetics. In detail, the polar conductive SMN@C nanospheres were shown to activate the solid-liquid interconversion of Li2S precipitation and decomposition, enabling smooth adsorption-in-situ-conversion of polusulfides. Logically, the sulfur-encapsulated SMN@C cathode exhibits superior electrochemical performances, in brief, 707.2 mAh/g with 72% capacity retention over a thousand cycles at 2 C, suggesting the promising degree for advanced long life Li-S batteries.

Result and discussion A facile surfactant-free approach for fabricating nitrogen-doped carbon coated SnO2-doped manganese silicate nanobubbles (SMN@C) with sulfur impregnated is schematically illustrated in Figure 1. Manganese has been explored as an essential component in numerous catalytic materials47, especially in electron-transfer reactions48-50. In order to achieve the multifunctional composites and the synergetic effects between adsorption-diffusion and conductivity of lithium polysulfides, SnO2 and nitrogen-doped carbon were simultaneously introduced into or on the surface of manganese silicate. The spherical SiO2 beads with monodispersion were first prepared according to the previous reported method51. Then the SiO2 aqueous suspension was mixed with manganese acetate and tin chloride dihydrate followed by hydrothermal process. The in-situ generation of gas-bubbles templates and precipitation of manganese silicate led to a self-assembled hierarchical hollow structure. After coating with nitrogen-doped carbon, the sulfur was encapsulated into SMN@C nanospheres through a modified melt-vapor-infiltrated method. We demonstrate the enhanced electrochemical kinetics of SMN and N-C for both liquid- (Li2S8 → Li2S) and solid-phase reactions (Li2S → Li2S8).


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Figure 1. Fabrication process of the S-SMN@C composites.

The morphology and nanostructure of the as-prepared samples are visually observed in Figure 2, the monodispersed SiO2 beads of 250 nm (Figure 2a) are first prepared as sacrificing template by a modified Stöber method. After the one-pot hydrothermal treatment, manganese silicate hollow nanospheres composed of numerous smaller hollow nanobubbles are formed as shown in Figure 2b, c and f. The bubble-like shell is generated from the deposition of manganese silicate on the soft template of CO2 bubbles (which is attributed to the decomposition of acetate) through the ion-exchange of Mn2+ with H4SiO4 (SiO2 + H2O → H4SiO4) in hydrothermal condition52. This bubbles-into-spheres nanostructure significantly increases the surface area for the suppression of polysulfide dissolution in electrolyte through chemical chemisorption. Subsequently, a thin N-C layer from polydopamine is coated on SMN nanospheres, as shown in Figure 2d, the monodisperse spherical structure still well maintains, and the thickness of N-C layer is presumed to be about 25 nm (Figure 2g). The specific ratio of each element in SMN@C through the EDX (Figure S1) are listed in Table S1. Finally, the sulfur was infused within the SMN@C host by a modified melt-vapor-diffusion method. As shown in Figure 2e, the spherical S-SMN@C composite with smooth surface indicates that no obvious sulfur agglomerates outside the SMN@C nanospheres. The darker area after sulfur loading in TEM image (Figure 2h) and EDX elemental mapping further demonstrate the homogeneous distribution of sulfur inside the SMN@C nanospheres.



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Figure 2. Characterization of the nanostructure and elemental distribution of samples. SEM images of (a) SiO2 nanospheres, (b) SiO2@SMN, (c) SMN, (d) SMN@C and (e) S-SMN@C. TEM images of (f) SMN nanobubbles (inset: high resolution TEM image), (g) SMN@C and (h) S-SMN@C. (i-o) EDX mappings of S-SMN@C cathode from Mn to N.

The XRD patterns of SMN, SMN@C and S-SMN@C are shown in Figure 3a, the three weak peaks at 22.5°, 32° and 33° could be attributed to Mn2+Mn63+SiO12, Mn2SiO4, MnSiO3.49, 52 The broad peak between 20° and 30° indicates the (002) crystal face of carbonaceous materials9. After loading sulfur, the S-SMN shows strong but reduced sulfur diffraction peaks9 compared with orthorhombic S8, suggesting the infiltration of sulfur into hollow structure. Thermogravimetric data (Figure 3b and S2) show the sulfur content in S-SMN@C is 70% and the N-C content in SMN@C is 15.3%. The type IV hysteresis loop with 4 nm pore (Figure 3c and inset) demonstrates the mesoporous structure in SMN@C, which is partially beneficial for blocking the overflow of lithium polysulfides. The high specific surface area of 320 m2/g can afford more active sites to adsorb the polysulfides through chemical bonding and accelerate the electrochemical reaction processes.


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Figure 3. Physical characterization of SMN@C and other contrast. (a) XRD patterns of S, S-SMN@C and SMN@C. (b) Thermogravimetric analysis of S-SMN@C in nitrogen. (c) N2 absorption-desorption isotherms of SMN@C and corresponding pore-size distribution (inset). (d) and (e) Digital images of the Li2S6 trapping by carbon, SMN, N-doped carbon and SMN@C nanospheres in DOL/DME solution. (f) UV-vis spectra of Li2S6 solution after being adsorbed.

The strong surface affinity of phenolic resin carbon (C), SMN, N-doped C and SMN@C for lithium polysulfides were observed by visualized adsorption experiments. As shown in Figure 3d, equivalent mass of samples was added into 10 mM Li2S6 solution and stirred for 5 min, after sedimentation the non-polar C mixture remains the same color as the referential solution, indicating a negligible physical adsorption on polysulfides. In contrast, three colorless and transparent vial are conspicuous for polar additives. UV-vis spectrum of the supernatant liquid and blank solvent (DOL/DME) were also employed to quantificationally compare the adsorbability to Li2S6. As shown in Figure 3f, the lower absorbance of SMN demonstrates the stronger interaction towards lithium polysulfides.26, 53 Higher concentration of Li2S6 solution was introduced into an extreme test to visually clarity the upper limit of adsorption efficiency (Figure 3e), specifically, 10 mg SMN and N-doped C were immersed into two separated 50 mM Li2S6 solution, after prolonged contact with



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Li2S6 for 2 h, a significant phenomenon came into sight, the colorless solution for SMN suggests a multiplied interaction against Li2S6 compared with other reference. The precipitate retrieved from the adsorption test was then analyzed by XPS. The Sn 3d and Mn 3d spectra of SMN and SMN-Li2S6 are shown in Figure 4a and b, in which both peaks shift 0.3~0.4 eV towards lower binding energy after contact with Li2S6, suggesting the increased electron density at the metal elements and the formation of chemical bonds between SMN and Li2S64, 54. A similar shift trend is observed in the N-C-Li2S6 system as well, the three emblematic peaks at 397.5, 399.8, and 402.7 eV of N 1s spectrum correspond to the pyridinic, pyrrolic, and quaternary nitrogen atoms in carbon matrix structure9. Exactly, the effective chemical interaction is ascribed to the pyridinic-N along with 0.5 eV drop, which can explain the suppressed adsorption capability. Therefore, the polysulfides adsorption mechanism on the surface of SMN and N-doped C can be illustrated schematically in Figure 4e. Above all, these polar nitrogen and metal cations as adsorption sites can significantly mitigate the shuttle effect, thus conducing to a better electrochemical performance. The actual ability of SMN@C to trap polysulfides in electrode during cycles was investigated in more detail by visible diffusion tests in a transparent H-type simulation electrolytic cell with Li foil anode. To monitor the shuttle behavior of polysulfides, a SMN@C electrode in the cell was cycled at 50 mA/g for one week. As Figure 4d revealed, no distinct color change could be observed after standing 24 h, and only a faint yellow color appeared after one week (Figure S3). These results provide direct evidence in effective blocking the shuttle effect for SMN@C substrate.


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Figure 4. Illustration of the chemisorption of SMN@C towards polysulfides. High-resolution (a) Sn 3d spectra of SnO2 and SMN-Li2S6, (b) Mn 3d spectra of SMN and SMN-Li2S6, (c) N 1s spectra of N-doped carbon and N-C-Li2S6. (d) Visible cell cycled at 50 mA/g. (e-g) Schematic illustration of Li2S6 adsorption on different polar surface.

From the Nyquist plots, after N-doped carbon coating, the impedance of S-SMN@C is much lower than that of S-SMN (Figure 5a), indicating an improved ability to facilitate the electrons transfer55. The enhanced electrochemical kinetics enabled by the SMN@C was investigated by CV at average gradient scanning rates, ranging from 0.1 to 0.5 mV/s54, which can offer the important information about the Li2S deposition and decomposition. As shown in Figure 5b and c, the two-independent decreasing cathodic peaks at around 2.3 V (IC1) and 2.0 V (IC2), corresponding to the lithiation of sulfur (S to Li2Sx then Li2S). The anodic peaks at around 2.4 V are attributed to the reversible transition of Li2S to Li2Sx (4 ≤ x ≤ 8) and sulfur (IA)54. With the increase of scan rate, the S-SMN electrode shows more extreme increase in potential in IC1 and IC2, and more severe decrease in IA than S-SMN@C (Figure 5d-f). It can be clearly seen that the sluggish lithiation/delithiation reaction on SMN surface is quickened on SMN@C, mainly arising from the inherent strong polysulfides adsorption and the elevated conductivity by N-doped carbon layer. The faster electrochemical reaction kinetics accompanied with increased peak currents and lower ACS Paragon Plus Environment


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over-potentials in comparison with S-SMN cathode suggest that the collaboration of SMN and N-doped carbon enables efficient conversion of sulfur-containing species. In order to further understand the positive role of SMN@C in the electrochemical delithiation of Li2S, the solid phase transition barrier of C-Li2S, SMN-Li2S, N-doped C-Li2S and SMN@C-Li2S cathodes during charging process were evaluated by their initial charge potential curves as shown in Figure 5g54-56. The SMN-Li2S cathode exhibits the highest potential barrier about 3.8 V, indicating a passivated electrode progress and large charge-transfer resistance. After introducing N-doped C coating layer, the potential barrier (SMN@C) was significantly reduced to 3.4 V, corresponding to an enhanced conductivity and decreased Rct compared with other electrodes. The combination of SMN and N-C two polar substances, can significantly decrease the polarization for Li2S oxidation, better than each of the two counterparts. The liquid-liquid redox kinetics of lithium polysulfides were investigated by symmetrical cells of sulfur electrodes55-56. The redox current curves under a polarization voltage between -0.8 V and 0.8 V was also in good accordance with the EIS results. The increased redox current of SMN@C (Figure 5h) suggests the improved liquid phase conversion kinetics compared with SMN and C. The EIS of the symmetric cells shown in Figure 5i indicates that, in comparison with the pristine SMN, SMN@C showed a decrease of 50 % in semi-circle impedance at high frequency region, benefiting from the superior conductivity of N-doped C. The impedance comparison between SMN and C implies that the chemical adsorption is not the only reason for accelerating reactions, the reduced Rct of SMN@C by the incorporation of N-C also play a crucial role.


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Figure 5. (a) Electrochemical impedance spectroscopy of S-SMN, S-SMN@C and S-N-doped C cathodes before cycling. CV curves of the (b) S-SMN@C and (c) S-SMN electrodes at various scan rates. Plots of oxidation-reduction potentials versus scan rates for the (d) anodic oxidation of Li2S to S (IA), (e) first cathodic reduction of S to Li2Sx (IC1) and (f) second cathodic reduction of Li2Sx to Li2S (IC2). (g) Initial charge voltage curves of SMN-Li2S, C-Li2S, N-C-Li2S and SMN@C-Li2S electrodes. (h) Polarization curves and (i) EIS of the Li2S6 symmetric cell.

It is well understood recently that the capacity of Li-S battery mainly originates from the conversion of short-chain lithium polysulfides into Li2S57. To further demonstrate the nucleation and growth of solid Li2S on different surfaces, the asymmetric cells (Li/electrolyte || Li2S8/cathode) were designed and galvanostatically discharged to 2.05 V53, 56, 58-60. As compared in Figure 6a-d, the voltage plateau attributed to Li2S precipitation decreases in the order of N-doped C (2.097 V) > SMN@C (2.08 V) > C (2.075 V) > SMN (2.07 V and 2.064 V), reflecting the enhanced



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electrochemical kinetics of polysulfides reduction. Figure 6e-h show the potentiostatic discharge curves at 2.04 V. Obviously, the maximum current of Li2S formation on N-doped C and SMN@C are much higher than those on C and SMN. The capacities of Li2S precipitation on N-doped C, SMN@C, C and SMN are 115 mAh/g(S), 63.57 mAh/g(S), 49.26 mAh/g(S) and 49.36 mAh/g(S), respectively, based on the sulfur weight in used Li2S8. These results strongly prove that conductive polar structure could facilitate the conversion process from Li2Sx (4 ≤ x ≤ 8) to Li2S.

Figure 6. The synergy between conductive-polar interfaces on enhanced electrochemical reactions Kinetics. Galvanostatic discharge curves of (a) C, (b) SMN, (c) SMN@C and (d) N-doped C cathode based on Li2S8/tetraglyme catholyte at 200 μA. (e-h) Potentiostatic discharge curves of Li2S8/tetraglyme solution at 2.04 V on different surfaces.

The charge/discharge curves of S-SMN@C cathode under different cycles at 0.5C are displayed in Figure 7a, the stable discharge voltage plateaus around 2.0 V remains highly constant up to 300 cycles. In addition, the almost overlapping discharge curves demonstrates the high electrochemical reversibility and good cycling stability. Figure 7b shows the corresponding cycling performance, a capacity of 761.2 mAh/g retains after 300 cycles with ~100% coulombic efficiency. The capacity retention of 81% with a decay rate of 0.064% implies the assumption that majority of polysulfide intermediates are immobilized by SMN@C. It is well known that the shuttle effect is more prominent at lower current, as a result the cycling performance further clarifies the stronger chemisorption in dual-encapsulation of SMN@C for polysulfides. In contrast, the S-SMN cathode ACS Paragon Plus Environment

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delivers a rapidly decreasing capacity from 730 mAh/g to 475 mAh/g, suggesting the sluggish electrochemical kinetics. To further investigate the improved reaction kinetics of N-doped carbon incorporated SMN/S cathode, the electrochemical performances of S-SMN@C and S-SMN batteries were evaluated by 2016 type coin cells with areal loading around 2 mg/cm2. As shown in Figure 7c, the well-defined charge-discharge plateaus maintain well for S-SMN@C even at high current rates, however, the S-SMN cathode shows sloping charge/discharge process when the current density rises to 2 C rate (Figure 7d). In contrast with S-SMN, the polarization phenomenon of S-SMN@C cathode is much alleviated as the current densities increases, suggesting that the nanobubbles-into-nanospheres hybrid structure with N-C coating is beneficial for improving the electrochemical kinetics of sulfur cathode. As shown in Figure 7d, the reversible capacities of the S-SMN@C electrode decreases gradually from 1071 to 968.2, 881.7, 786.3 and 698 mAh/g with the current increasing from 0.1C to 2C, respectively. In comparison, a drastically declined capacities of 200 mAh/g at 2C was emerged for S-SMN electrode with much poorer rate performance. When back to 0.1C rate again, the S-SMN@C electrode could stabilize the capacity at 920 mAh/g, indicating the excellent rate performance induced by fast charge transfer and tight lithium polysulphides immobilization.

Figure 7. Electrochemical properties of S-SMN@C. (a) Galvanostatic charge/discharge voltage curves of S-SMN@C electrode at different cycles under 0.5 C. (b) Cycle performance and ACS Paragon Plus Environment


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Coulombic Efficiency of S-SMN@C and S-SMN electrodes at 0.5 C. (c) The initial galvanostatic charge/discharge voltage curves of S-SMN@C electrode from 0.1 to 2 C and (d) the corresponding rate performance of S-SMN@C and S-SMN electrodes. (e) Prolonged cycling stability of the S-SMN@C electrode at 1 C and 2 C.

To examine the potential for high power storage, the S-SMN@C electrode was tested at 1C and 2 C. As shown in Figure 7e, after 1000 cycles, the S-SMN@C electrode remains a stable capacity of 587.5 mAh/g and 511.2 mAh/g with a capacity fading rate of 0.027% and 0.0277%, respectively. The cycle performances of S-SMN@C electrodes with different areal densities are shown in Figure S4, a capacity of 450 mAh/g for an electrode of 6.2 mg/cm2 remains after 500 cycles, demonstrating the potential of this electrode design for practical use. To further confirm the confined shuttle effect and the protected lithium anode from corrosion, SEM images as well as EDS mapping of lithium anode and S-SMN@C electrode before and after cycling (1 C for 100 cycles) are provided (Figure S5 to S8). The uniformly dispersed sulfur and relatively smooth lithium-metal surface indicates the effectively restricted diffusion of polysulfides into electrolyte, thereby alleviating the formation of lithium sulfide aggregation and the consumption of Li foil electrode. The comparison of electrochemical properties with other metal oxide and carbon-based sulfur cathodes from literatures has been listed in Table S2. The significantly improved properties can be attributed to promotion of sulfur utilization, benefitting from the multi-confined nanostructure, where (i) the SMN@C can provide sufficient space for sulfur accommodation and expansion during cycles; (ii) by refining the nanosized sulfur particles into nanobubbles, the utilization of S has been improved; (iii) the N-C coating can effectively reduce Rct of SMN; (iv) the soluble lithium polysulfides intermediates can be captured by SMN with abundant surface and N-doped porous carbon layer through chemical interaction; (v) the synergy of dual-component towards the phase transition can remarkably lower the polarization and enhance the electrochemical kinetics in Li-S battery; and (vi) the intimate contact of nanospheres can greatly improve the energy density of the


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S-SMN@C cathode.

Experimental section Preparation of SMN and SMN@C The optimal doping content of SnO2 is controlled below 10%. The SnO2 doped manganese silica nanobubbles (MSN) were synthesized through a hydrothermal method by dissolving 0.32 mmol SnCl2·2H2O into Mn(CH3COO)2·4H2O aqueous solution (40 mM) with 400 mg SiO2 nanospheres.52 For N-doped carbon coating, 50 mg of dopamine hydrochloride and 100 mg of Tris were added into 150 ml well-dispersed MSN suspension (0.66 mg/ml) and stirred for 24 h. The obtained polydopamine-coated MSN were then heated at 800 °C in argon for 2 h to carbonize the polydopamine (SMN@C). The residual SiO2 was removed by 1 mol/L NaOH at 60 °C for 1 h to form a reserved space to accommodate abundant sulfur (SMN@C).

Materials characterization The microstructural characterizations and EDS (Energy Dispersive Spectra) of samples were performed on ZEISS Merlin Compact SEM (Scanning Electron Microscopy) and JEOL 2100 field emission TEM (Transmission Electron Microscopy). Other physical properties were characterized by XRD (X-ray Diffraction, Panalytical Empyrean), TGA (Thermogravimetric Analysis, NETZSCH STA449C), AutoSorb iQ2, XPS (X-ray Photoelectron Spectra, ThermoFisher ESCLAB 250Xi) and UV-vis (SHIMADZU UV-1800), details in literatures.9, 26,

Electrochemical measurements Primarily, the S-MSN@C composite was prepared by heating the sulfur/SMN@C mixture (7:3) at 155 °C for 10 h then 300 °C for 2 h in a modified 304 stainless steel reaction chamber through a facile melt-vapor-infiltrated method to ensure the sulfur was all impregnated into the SMN@C. The CR2016 coin cells were assembled and measured as described detailedly in our previous work.26



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The morphology of the cycled cathode was achieved through disassembling in Ar-filled glove box, followed by washing with DME.

Polysulfide adsorption study The concentrated (1 M) Li2S6 solution was prepared according to the previous report.26,55 10 mg of carbon nanospheres, N-doped carbon, SMN and SMN@C were added into 3 mL of diluted lithium polysulfide solution (10 mM), respectively, a blank glass vial was also filled with the same lithium polysulfide solution as a control. In order to observe the diffusion of lithium polysulfide more intuitively, a transparent two-electrode battery was assembled using sulfur-based cathode with a diameter of 12 mm, then discharged to 2.05 V at 50 mA/g, turning solid state sulfur into soluble lithium polysulfides.

Assembly of Li2S6 symmetric cells The electrodes were prepared by dissolving active materials (carbon nanospheres, N-doped carbon, SMN and SMN@C) (70%) with conductive carbon black (20%) and polytetrafluoroethylene (PTFE) (10%) in ethanol to form a slurry then coated on aluminum mesh. The 0.1 M Li2S6-rich electrolyte was prepared by diluting 1 M Li2S6 concentrated solution with electrolyte (for testing). 2016-coin cells were encapsulated with two identical electrodes (10 mm) separated by Celgard 2400 polypropylene membrane, containing 40 µL Li2S6-rich electrolyte. Polarization curves were measured between -0.8 V and 0.8 V at 50 mV/s.

Li2S nucleation measurements The 0.5 M Li2S8 based catholyte was prepared by stirring 1.12 g sulfur and 0.23 g Li2S corresponding to the stoichiometric ratio of 7:1 in 10 ml of tetraglyme, along with 2.87 g LiTFSI additive at 80 °C for 24 h. The cathode materials were prepared through mixing carbon nanospheres, N-doped carbon, SMN and SMN@C with graphene aqueous dispersion, followed by vacuum


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filtration into films. The membrane was then punched into 10 mm wafers with an equal mass loading of 0.5 mg/cm2. Asymmetric electrolyte was added in each side of battery separator, 20 μL Li2S8 based catholyte was dropped onto the cathode and 20 μL blank electrolytes was added to the lithium anode. The cell was galvanostatic discharged to 2.05 V at 200 µA for Li2S to nucleate and grow. As a supporting experiment, the current changes were also recorded by firstly galvanostatic discharged to 2.08 V at 200 µA and then kept at 2.04 V for Li2S sedimentation over 12 h.

Li2S decomposition measurements The Li2S decomposition over-potential was detected by Li-Li2S cells. Specifically, the active materials were prepared by vigorous grinding carbon nanospheres, N-doped carbon, SMN and SMN@C together with commercial Li2S at a mass ratio of 1:1. Then the pole pieces were synthesized according to the sulfur-based electrode, the composition and dosage of the electrolyte are also consistent. The cells were initially charged to 4.0 V at 200 mA/g based on Li2S.

Conclusion In summary, the well-designed N-doped carbon hollow nanospheres with SMN nanobubbles filled in were developed as an accommodation for sulfur to improve the reaction kinetics then electrochemical performance of Li-S batteries. Combined the features of adsorbability with conductivity, where lithium polysulfides are strongly trapped by SMN meanwhile the N-doped carbon coating reduces Rct of SMN, the hierarchical polar conductive nanostructure facilitates both the solid-liquid transformation of Li2S→polysulfides and liquid-solid nucleation/growth of Li2S. With these characters, the S-SMN@C electrode shows high cycling stability and rate performance. A 73% capacity retention after 1000 cycles was achieved at 1 C with a sulfur loadings of 2 mg/cm2. The proved synergistic effect between N-C and SMN in S-SMN@C electrode offers an efficient policy in designing sulfur composite cathodes.



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Conflicts of interest The authors declare no competing financial interests.

Acknowledgments This study was financially supported by Hainan Provincial Natural Science Foundation of China (2018CXTD332), Science and technology development special fund project (ZY2018HN09-3), National Natural Science Foundation of China (No. 21603048) and Innovation Project (Hdcxcyxm201706).

Supporting Information Supporting Information Available: EDS and the ingredients SMN@C; Thermogravimetric analysis of SMN@C in air; Visible cell cycled at 50 mA/g; Cycle performances of S-SMN@C electrodes with different mass loading; SEM images and EDS of lithium-metal surface; SEM images and EDS mapping of S-SMN@C before and after cycling. The comparison of electrochemical properties with other metal oxide and carbon-based sulfur cathodes from literatures.

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