Palladium nanocrystals-imbedded mesoporous hollow carbon spheres with enhanced electrochemical kinetics for high performance lithium sulfur batteries

Palladium nanocrystals-imbedded mesoporous hollow carbon spheres with enhanced electrochemical kinetics for high performance lithium sulfur batteries

Carbon 143 (2019) 878e889 Contents lists available at ScienceDirect Carbon journal homepage: www.elsevier.com/locate/carbon Palladium nanocrystals-...

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Carbon 143 (2019) 878e889

Contents lists available at ScienceDirect

Carbon journal homepage: www.elsevier.com/locate/carbon

Palladium nanocrystals-imbedded mesoporous hollow carbon spheres with enhanced electrochemical kinetics for high performance lithium sulfur batteries Shaobo Ma a, Liguang Wang b, Yang Wang a, Pengjian Zuo a, *, Mengxue He a, Han Zhang a, Lu Ma c, Tianpin Wu c, Geping Yin a a

MITT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China Department of Physics, City University of Hong Kong, 83 Tat Chee Ave, Kowloon Tong, Hong Kong, China c X-ray Science Division, Argonne National Laboratory, 9700 South Cass Avenue, IL 60439, USA b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 October 2018 Received in revised form 21 November 2018 Accepted 30 November 2018 Available online 30 November 2018

Lithium sulfur (Li-S) battery is one promising candidate for high energy density electrochemical energy storage system. Accelerating the sluggish reaction kinetics of sulfur cathode and suppressing the lithium polysulfide (LiPS) shuttle are crucial for the high-performance Li-S batteries. Herein, we prepared the catalytic palladium nano-particles (Pd NPs) imbedded in hollow carbon spheres ([email protected]) as sulfur host by a sacrificial template method. Benefitting from the hollow nanostructure and strong chemisorption ability of Pd NPs, [email protected] can effectively mitigate the LiPS shuttling via physical and chemical pathways. Furthermore, the Pd NPs as one electrocatalyst can accelerate the redox reaction kinetics of LiPS. The theoretical calculation and X-ray absorption spectroscopy elucidate that the moderate Pd-S bonding between Pd NPs and sulfur species are beneficial to LiPS conversion. The [email protected]/S electrode delivers a high initial capacity of 1306 mAh g1 and 885 mAh g1 after 100 cycles at 0.2 C, as well as the good cycling stability (a slight capacity decay of 0.068% per cycle over 400 cycles at 1 C). The [email protected]/S cathode with the high sulfur loading of 5.88 mg cm2 delivers an initial capacity of 873 mAh g1 at 0.2 C and good capacity retention of 85% after 100 cycles. © 2018 Elsevier Ltd. All rights reserved.

1. Introduction With the pursuit of high-energy-density rechargeable electrochemical devices, lithium sulfur battery, which possesses an ultrahigh theoretical energy density of 2600 Wh kg1 calculated on the basis of the Li anode (3860 m Ah g1) and the sulfur cathode (1675 mAh g1), has been regarded as one of the most promising next-generation rechargeable batteries [1,2]. Meanwhile, sulfur has obvious merits of low cost, non-toxicity and natural abundance. However, the existing Li-S battery still suffers from a multitude of challenges toward practical application, such as insulating properties of S/Li2S, low utilization of sulfur, significantly capacity fading, self-discharge, low Coulombic efficiency, dissolution/ disproportionation and well-known shutting of soluble lithium

* Corresponding author. E-mail address: [email protected] (P. Zuo). https://doi.org/10.1016/j.carbon.2018.11.086 0008-6223/© 2018 Elsevier Ltd. All rights reserved.

polysulfide (LiPS, S2 x , x  4) intermediates, volumetric changes of electrodes and dendrite/pulverization of lithium anode. To date, tremendous efforts have been devoted to solve the above problems, including the design of cathode host materials with high conductivity [3,4], various porous diameters [5e7], and physical/chemical/ catalytic interaction with LiPS [8,9], the construction of multifunctional separators and interlayer to block LiPS [10,11], and the optimization of electrolyte [12] and lithium anode [13,14]. Among aforementioned strategies, designing rational cathodes is still one main factor to realize the practical application of Li-S batteries. For instance, Nazar et al. have done pioneering work thorough incorporating sulfur into mesoporous carbonaceous materials, which stimulated the research interest in various conductive porous carbon materials as sulfur host matrixes even in practical pouch cells [5]. Nevertheless, the porous carbon/sulfur cathode materials generally still suffer from rapid capacity fading due to the weak interactions between high-polar LiPS and none-polar carbon matrix [15,16]. Then, designing polar host materials such as

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heteroatom functionalized carbon materials, conductive polymers [17], metal oxides (MnO2 [18], TiOx [19,20]), metal sulfides (Co9S8 [21], TiS2 [22]) and metal nitrides (VN [23], Co4N [24]) has emerged as one effective way to improve the cycling stability of sulfur cathodes via the chemical interaction of polar host materials with LiPS species. Nevertheless, the limited electrical conductivity of many inorganic polar composites would otherwise result in sluggish kinetic of polysulfide redox reactions, inevitably causing poor Li-S specific capacity and rapid capacity decay. Accordingly, designing of sulfur host candidates based on rational structure, high conductivity, moderate polarity and catalytic properties maybe is an effective strategy to improve the performance of Li-S cells. Generally, the hollow spherical materials have shown impressive merits in electrochemical field due to the unique structure and morphology [25]. Hollow spheres have large surface area for active sites’ exposure and high inner hollow space for volume changes of active materials. For instance, Cui and co-workers reported the hollow carbon spheres with Au nanoparticle seeds inside for lithium storage [26], and Jiang et al. synthesized the hollow chevrel-phase NiMo3S4 for hydrogen evolution [27]. The conductive porous hollow carbon spheres are also ideal sulfur host materials, and the large interior void space of hollow carbon can not only allow the higher content of sulfur, but also accommodate the large volumetric expansion of sulfur during cycling [28,29]. Zhou et al. have confirmed that the sulfur preferred diffusing into the hollow carbon rather than aggregating in/on the wall of the carbon [30]. Considering physical confinement of hollow carbon spheres to sulfur species, the introducing of small amount of polar and catalytic materials into the carbon matrix materials is promising, which may significantly enhance the redox kinetics of LiPS conversion. Recently, it has been demonstrated that various transition metals including Pt [31], Mo [32], W [33], Co [34,35], Ru [36], Ni [37] and their composites, could not only immobilize LiPS by chemicaladsorption but also significantly accelerate the kinetics of LiPS redox reactions. For instance, Arava and co-workers reported that the d-orbitals in Pt/XS2 materials (X ¼ Mo, W) and the unsaturated heteroatom (such as sulfur) could form an effective d-band structure for efficient reaction kinetics [28,29]. Despite numbers of reports on electro-catalysis of LiPS conversion reactions, the detailed reaction mechanism of Li-S battery and its kinetics are still unclear. Meanwhile, few electrochemical methods were used to explore the multi-electron redox conversion and multiphase transformation. Palladium catalysts have been widely studied in the electrochemistry fields [38,39], such as ethanol electro-oxidation, oxygen reduction reaction and hydrogen oxidation, mainly due to its special electronic and surface structure effect. Generally, the unoccupied d-orbital and unpaired electrons in palladium will interact with reactant molecule or atom, such as unsaturated sulfur, resulting in the reduction of reaction activation energy. To our knowledge, the effect of palladium on the LiPS conversion reaction kinetics has not been reported. In this contribution, the Pd NPs-imbedded hollow carbon spheres are proposed as sulfur host matrix to immobilize sulfur species and facilitate the kinetics of polysulfide conversion in Li-S battery. The hollow nanostructure and strong chemisorption ability of [email protected] to LiPS can synergistically restrict the LiPS shuttle effects via physical and chemical pathways, and bring a favorable effect that enables the rapid redox reaction kinetics of LiPS, with the function mechanism as shown in Fig. 1. The effects of chemical adsorption and catalysis of Pd NPs on LiPS conversion process were further confirmed by theoretical calculations, adsorption measurement, XAS technique and other electrochemical measurements. The design of the hollow catalytic sulfur host materials and the exploration of interaction mechanism between Pd catalytic

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sites and LiPS species during discharge/charge process are conducive to understanding the multiphase/multielectron cathodic conversion mechanism and constructing stable rechargeable Li-S batteries. 2. Experimental 2.1. Material synthesis 2.1.1. Preparation of Pd NPs Octahedral Pd NPs were synthesized by a modified method based on a protocols reported by Shao et al. [40,41] Specifically, 0.28 g of Na2PdCl4 (Aldrich) was dissolved in 40 mL of water with continuous stirring. Then the prepared Na2PdCl4 aqueous solution was rapidly added into an ethanol solution (15 mL) containing 0.52 g polyvinylpyrrolidone (PVP, K30, Aldrich) and 0.9 g citric acid (Aldrich) in a 250 mL round-bottom glass flask under vigorous stirring. Next, the flask immersed in an oil bath and maintained at 80  C for 3 h with continuous stirring. The resulting Pd NPs suspension cooled to room temperature and maintained in a glass bottle for the following application, noted as solution A. 2.1.2. Synthesis of [email protected] spheres The mono-disperse silica spheres were obtained according to a € modified StOber method [42], and then the surface of silica spheres were amino-modified with (3-aminopropry) triethoxysilane (APTES, Aldrich). Typically, 60 mL of ultrapure water, 18 mL of ethanol and 12 mL concentrated ammonia were mixed under continuous stirring, then a mixture of 5.5 mL of tetraethyl orthosilicate (TEOS, Aldrich) and 90 mL ethanol was added into this solution. After stirring for 6 h at room temperature, the silica spheres solution was centrifuged and washed with ethanol/water/water for three times. The obtained silica particles were then ultrasonically dispersed in 80 mL isopropanol (IPA) solution, and 2 mL of APTES solution was added slowly. The solution was bubbled with nitrogen for 20 min to remove oxygen, and then the reactor was placed in an oil bath at 75  C for 6 h under stirring with nitrogen protection. After centrifuging three times and washing with IPA/water/water, all the particles were dispersed in 20 mL water with gentle sonication, noted as solution B. Typically, 20 mL of colloidal solution (A) was diluted in 90 mL of acetone, and separated by centrifugation and washing. Then, the obtained Pd NPs were dispersed in 150 mL ultrapure water, forming Pd NPs aqueous suspension. The above Pd NPs suspension was added into the modified silica suspension (B) in four batches, and then separated by centrifuging (8500 rpm, 8 min/per time). Finally, the as-prepared [email protected] was washed by centrifugation twice, and dried at 80  C overnight. 2.1.3. Synthesis of hollow [email protected] spheres The resorcinol formaldehyde resin layer (RF) was coated on [email protected] surface by a simple emulsion polymerization method. Initially, all the above [email protected] particles were dispersed in 200 mL of water with gentle sonication. Then, 5.5 mL of 10 mM cetyltrimethylammonium bromide (CTAB, Aldrich) and 0.825 mL of ammonium hydroxide were added, followed by magnetic stirring for 30 min until the surface charge of particles was completely altered by CTAB. After that, 412 mg of resorcinol and 0.58 mL of formaldehyde were added and stirred for 6 h at room temperature. After centrifuged and washed by water for three times, the resulting product was dried in vacuum oven at 80  C for about 12 h. To carbonize the coated resorcinol formaldehyde resin layer, the as-prepared sample was calcined at 850  C (with the heating rate of 2  C min1) for 3 h in Ar atmosphere. Then, the silica core was etched by immersing the sample in 10% of hydrofluoric acid (HF) for

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Fig. 1. Schematic illustration of hollow [email protected]/S structure and its working mechanism in discharge process. (A colour version of this figure can be viewed online.)

6 h. Finally, the composites were filtered and washed with water for several times and dried in vacuum oven at 60  C for 12 h. The synthetic method of hollow HCS spheres was similar to that of [email protected] without addition of Pd NPs. 2.1.4. Preparation of [email protected]/S and HCS/S cathodes Sulfur encapsulation was performed through a modified melting-diffusion strategy as reported previously [43]. Typically, 0.2 g of sulfur and 0.8 g of prepared host materials ([email protected] or HCS) were mixed and grinded together. Then, the mixture was dissolved in carbon disulfide under magnetic stirring and sonication (30 min) to form homogeneous slurry. The slurry was stirred overnight to evaporate carbon disulfide in fume cupboard at room temperature. Finally, the obtained powder was grinded and heated at 155  C for 12 h under argon atmosphere in a quartz tube furnace. 2.2. Li-S cell assembly and electrochemical measurements The electrochemical performance of the samples was evaluated using the 2025-type coin cells, which were assembled with the [email protected]/S (or HCS/S) as cathode materials, lithium foil as anode, [email protected] 2400 polypropylene membrane as separator, and 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, Aldrich) with 0.4 M LiNO3 additive (Aldrich) in a solvent of 1,2-dimethoxyethane (DME, Aldrich)/1,3-dioxolane (DOL, Aldrich) with a volume ratio of 1:1 as electrolyte. Typically, 80 wt% of active materials, 10 wt% of Super P, and 10 wt% of polyvinylidene fluride (PVDF) were well mixed in N-methyl-2-pyrrolidnone (NMP, Aldrich) to form a homogeneous slurry. Then the slurry was casted onto the carboncoated aluminum foil and dried in vacuum oven at 50  C for 12 h. The electrode film was punched into round disks with a diameter of 14 mm, and the sulfur area loading on electrode was about 1.2 mg cm2. For the thick cathode (the sulfur loading of 5.88 mg cm2), the weight ratio of [email protected]/S: Super P: conductive fiber: sodium carboxymethyl cellulose (CMC) binder in slurry was adjusted to 7.5: 1: 0.5: 1, and the diameter of circular electrode disc was 10 mm. The conductive fibers with diameter of 5e12 mm were purchased from XFNANO, China. The conductive fibers were washed with acetone and dried in oven to remove surface residue

before further use. The precisely controlled electrolyte/sulfur ratio was around 15 mL mg1. The coin cells were tested in galvanostatic mode within a voltage ranging from 1.7 to 2.6 V (the low voltage was kept over 1.6 V to avoid the decomposition of LiNO3) using Neware battery testing system at 28  C [44]. Cyclic voltammetry (CV) measurements were performed on an AUTOLABPGSTAT302N electrochemical workstation at a scanning rate of 0.1 mV s1 in a voltage range of 1.5e3.0 V. Electrochemical impendence spectroscopy (EIS) curves were recorded on a PARSTAT 2273 workstation in the frequency range between 102 and 105 Hz with an amplitude (sinusoidal voltage) of 10 mV. 2.3. Material characterization The morphology and structure of the as-prepared samples were investigated by field-emission scanning electron microscopy (FESEM, FEI Helios Nanolab 600i) and transmission electron microscopy (TEM, Tecnai G2 F30). A Netherlands' PANalytical X'pert power diffractometer with Cu Ka radiation (l ¼ 1.54 Å) was applied to determine the crystal phases of the samples. The X-ray near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) data were collected at beamline 20-BM, Advanced Photon Source (APS), Argonne National Laboratory. The electronic structure analysis of the samples was performed on a Thermo Fisher Scientific ESCALAB 250XI multifunctional imaging electron spectrometer (XPS) using a monochromic Al X-ray source. The Raman spectra were collected on a Horiba Jobin Yvon LabramHR800 micro-Raman system with a 532 nm YAG laser excitation. N2-physisorption information on BET surface area and pore diameter were obtained on a Beishide 3H-2000PS2 setup. The palladium content of the sample was conducted on the inductively coupled plasma atomic emission spectroscopy (ICP-AES). Thermogravimetric analysis (TGA) was further carried out on Netzsch STA449F3 under argon atmosphere with a linear heating rate of 10  C min1 to determine the sulfur content of composites. 3. Results and discussion Fig. 2a illustrates the preparation process of hollow [email protected]

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Fig. 2. (a) Schematic illustration of the synthesis of hollow [email protected] microspheres, (b, c, d) the TEM images of [email protected] with different magnification, (e, f, g) the SEM images of [email protected] with different magnification, (h, f) the TEM and HRTEM images of hollow [email protected], (g) TEM-EDS of [email protected] (A colour version of this figure can be viewed online.)

composites via a sacrificial template method, as described in the experimental section. Uniform silica NPs were synthesized through € a facile modified StOber method as reported previously and further modified with APTES [42]. After immobilizing the Pd NPs on the modified silica spheres’ surface, the products were coated with a layer of resorcinol formaldehyde resin (HCS), which may be beneficial to hinder the Pd NPs aggregation at elevated temperature

[45]. Thereafter, the [email protected]@HCS composites were treated by calcination at 850  C, and the [email protected] was obtained by removing the silica templates through HF etching. The morphology and structure of as-prepared [email protected] and hollow [email protected] microspheres were investigated by means of FESEM and TEM. As shown in Fig. 2bed, the monodisperse [email protected] microspheres with a diameter of ~220 nm were obtained, and the Pd NPs uniformly

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cover in the surface of eNH2 functionalized silica microspheres because of electrostatic interaction. The [email protected] composites maintain regular hollow spherical morphology, with the average diameter of around 230 nm (Fig. 2eeg). No residual Pd NPs are observed on the surface of [email protected], indicating that the Pd NPs were well imbedded into the inlayer of [email protected] during the synthesis process. The energy dispersive spectroscopic (EDS) mapping analysis demonstrates the presence and a uniform distribution of C, O, and Pd elements in the [email protected], with the contents of 77.81 wt%, 20.4 wt% and 1.79 wt%, respectively (Fig. S1). The precise Pd content in [email protected] is 5.49 wt% according to the ICP-OES result. The transmission electron microscopy (TEM) characterization (Fig. 2h) confirms that the [email protected] maintains uniform hollow structure with Pd NPs imbedded inside the carbon shell. Notably, the diameter of [email protected], the thickness of carbon layers and the sizes of Pd particles are about 230 nm, 9 nm and 6 nm, respectively. In the HR-TEM image of [email protected] (Fig. 2i), a lattice fringe with a d-spacing of 2.33 Å is observed, corresponding to the (111) plane of Pd NPs. In Fig. 2j, TEM elemental analysis indicates the distribution of Pd, O, C element in [email protected], corresponding to the red spot region of Fig. 2h. Furthermore, the pristine HCS spheres with a diameter of ~210 nm also display the homogenous hollow structure (Fig. S2). The X-ray diffraction (XRD) patterns confirm the crystalline structure of HCS, [email protected] and [email protected]/S, respectively (Fig. 3a). The broad peak at 25.2 for HCS is assigned to (002) crystal plane of graphitic carbon, indicating that the high-temperature calcination promotes the formation of graphitic carbon with high conductivity. Apart from graphitic carbon peak (002), the characteristic peaks of palladium (JCPDS No. 05-0681) at 40.1 (111), 46.6 (220), and 67.9 (200) are observed in [email protected], indicating that the Pd NPs in [email protected] composites keep well crystalline structure with symmetry space group of Fm3m (225). The main crystal plane (111) of Pd NPs in XRD pattern of [email protected] is also consistent with the d-spacing (2.33 Å) (Fig. 2i). After the incorporation of sulfur into the [email protected], the characteristic peaks of sulfur (JCPDS No. 83-2284) at the 2q degree of 23.1 (222), 25.8 (226) and 27.7 (206) appear, confirming the existence of crystalline S8 in [email protected]/S. Further structural characteristics of the prepared [email protected] and HCS were investigated via Raman spectroscopy (Fig. 3b). Two distinct peaks at around 1342 cm1 (D band) and 1587 cm1 (G band) can be observed, which is attributed the defective/disordered sp [3] hybridized carbon and the in-plane stretching motion of symmetric sp [2] graphitic carbon atoms [46]. The intensity ratio of D and G band (ID/IG) increases from 0.899 (HCS) to 0.931 ([email protected]), implying that the disorder degree of sp [3] domains and the concentration of defects increase after Pd-doping. Many studies demonstrate that the increasing concentration of carbon defects is beneficial to enhance the electro-catalysis activity [47,48]. The wide-scan XPS survey spectra of [email protected] (Fig. S3) implies the coexistence of C and O elements. No obvious signal of Pd is monitored, which is maybe related to the thickness (9 nm) of carbon layer of [email protected] The specific surface area and pore size of hollow [email protected] and HCS were investigated by N2 adsorption/desorption isotherms. As displayed in Fig. 3c, the specific surface areas of [email protected] and HCS are 791.2 m2 g1 and 1054 m2 g1, respectively. Simultaneously, the average pore size distributions of two samples are similar (2.7e3.4 nm). The specific surface area of [email protected] decreases slightly after imbedded with Pd NPs, which is mainly caused by the blocking of the channel structure with the deposition of Pd NPs. Nevertheless, the [email protected] composites still possess high pore volume of 2.33 cm3 g1 and abundant mesoporous channels, which are beneficial to sulfur hosting, electrolyte infiltration, volume fluctuation and ion transporting. The accurate sulfur content in [email protected]/S and HCS/S composites was determined by TGA under Ar atmosphere from room temperature to 800  C, and the ratio of

sulfur in hollow [email protected]/S and HCS/S composites are ~75.8 wt% and 74.6 wt% (Fig. 3d), respectively. First-principle calculation based on density functional theory (DFT) was performed to obtain the binding geometries and adsorption energies of LiPS on different substrates, where graphene basal plane and Pd4 cluster were modeled as the as-synthesized hollow HCS and Pd crystal in [email protected] (Fig. 4a). The binding energies between Li2S6 and graphene demonstrate that there is almost no chemisorption of nonpolar carbon to Li2S6 (Fig. S4, Fig. 4a). In contrast, the Pd4 cluster possesses strong partial binding to Li2S6, with a high binding energy of 3.579 eV. The detailed binding geometries, binding energy and Pd-S distance between different Li2Sx and Pd4 cluster are displayed in Fig. S5. It's clear that the Pd-S bonding interaction endows the Pd4 cluster with strong attraction to Li2Sx. Meanwhile, the low order Li2S4 and Li2S2/Li2S displays lower adsorption energy and shorter Pd-S distance (compared with Li2S6), which is associated with the adsorption/ desorption process of active intermediate products on Pd NPs catalytic sites in multi-phase redox conversion. The calculated results suggest that Pd NPs can be efficient to absorb LiPS through chemisorption interactions and facilitate the following LiPS kinetic conversion. The strong interaction between lithium polysulfide and [email protected] is further demonstrated using a visual discrimination test by dispersing an equivalent surface area (10 m2) of [email protected] and HCS in 4 mL of 1.25 mM Li2S6 solution (Fig. 4b). After resting for 12 h, the solution with HCS presented no significant color change compared with the pure Li2S6 solution, indicating the inferior interaction between nonpolar carbon and the Li2S6. In contrast, the lucid coloration of Li2S6 solution with [email protected] suggests a stronger chemical binding interaction of [email protected] to Li2S6. Moreover, XPS is recorded to accurately probe the surface elementary valence state of [email protected] after adsorption of Li2S6 solution. In the wide-scan XPS survey spectra, the typical peaks at around 533.5, 286.8, and 164.2 eV appeared, corresponding to the O1s, C1s, and S2p, respectively (Fig. S6). From Fig. 4c, a doublet of S2p spectrum is identified, which is further divided into four types of sulfur. The S 2p3/2 peaks at 164.3 and 163.8 eV, are ascribed to bridging sulfur (S0B) and terminal S (ST1), respectively. Upon interaction with Pd crystal grains, the ST1 spectrum is generally shifted about 2.3 eV to higher binding energy compared with that of the ST-Li bond reported in previous studies, implying the strong interaction between polysulfide species and Pd crystal grains [49]. The S 2p3/2 peaks at 168.7 and 170.2 eV are assigned to the thiosulfate and polythionate, which can be attributed to the surface redox reaction between Pd NPs and polysulfides [50]. The DFT calculations, visualized adsorption tests and XPS results clearly verify the strong chemisorptions of LiPS on Pd NPs. To further demonstrate the catalytic ability of Pd NPs for LiPS conversion, we conducted CV and EIS tests in the symmetrical coin cell, which include two identical electrodes (without sulfur) and Li2S6-containing electrolyte. CV tests were performed within a voltage window of 0.8 to 0.8 V for symmetrical cells at a scan rate of 20 mV s1 in 0.5 M Li2S6 electrolyte. As shown in Fig. 4d, the redox currents of [email protected] significantly increase in comparison with that of HCS, demonstrating that the exposed Pd active sites in [email protected] electrode can dynamically accelerate the electrochemical redox reactions of lithium polysulfides. For a comparison, no obvious current density displays in the symmetrical cell with the blank electrolyte (without Li2S6) due to the sluggish electrochemical redox reaction during cycling. Moreover, the Pd NPs with good electrical conductivity can also provide abundant channels in Pd/polysulfides interface to accelerate electric charge transfer for LiPS conservation, which was verified by EIS of symmetrical cells (Fig. 4e). From the Nyquist plots of the cells, the intercept in the

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Fig. 3. (a) XRD patterns of hollow HCS, [email protected] and [email protected]/S, (b) Raman spectra of [email protected] and HCS, (c) N2 adsorption-desorption isotherms of [email protected] and HCS (inset: corresponding pore size distributions), (d) TGA curves of [email protected]/S and HCS/S. (A colour version of this figure can be viewed online.)

high-frequency region (Rs), the semicircle in the high-mediumfrequency region (Rct) and an inclined line at low-frequency region (Ws), reflect the bulk electrolyte resistance, the charge transfer resistance at electrode/electrolyte interface and the semi-infinite diffusion process on the electrode surface, respectively [7,51]. The Rct of [email protected] (180.7 U) is obviously much lower than that of HCS (755.4 U), and the detailed equivalent circuit and fitting results are displayed in Fig. S7 and Table S1. Consequently, charge transfer rate at [email protected]fide interface is much faster than that at HCSpolysulfide interface, implying that the incorporation of Pd NPs to carbon can remarkably enhance the redox kinetics of polysulfides conservation in liquid phase. Through the CV and EIS tests in symmetric cells, the Pd NPs are considerable to accelerate the redox reaction of LiPS conversion due to the high conductivity of Pd NPs and the enhanced affinity to LiPS. Critical kinetic parameters such as shuttle current and onset potential were measured by designing different electrochemical cells. In order to elucidate the suppression of [email protected] to LiPS shuttling effect, the shuttle current was measured by using the Li-S cell with LiNO3-free electrolyte. The voltage value of potentiostatic test was chosen to 2.38 V, because the shuttle current is the largest at this state-of-charge [52,53]. As shown in Fig. 5a, the shuttle

current density of hollow [email protected] (1 mA cm2) decreases significantly compared with that of hollow HCS (4 mA cm2), demonstrating that the Pd NPs are effective to suppress the soluble Li2Sn (n  4) diffused from cathode to lithium anode during cycling. The liquid LiPS dissolution is crucial for shuttling, and hence, it is anticipated that the Pd NPs can be involved in chemisorptions with soluble LiPS and further in redox reaction activation. To further demonstrate the electro-catalytic activities of Pd NPs for the overall multiphase conversion during discharge/charge process, we conducted the CV tests in semi-liquid cells, using the sulfur-free host materials ([email protected] or HCS) as working electrodes vs lithium as the reference/counter electrode, and 0.5 M Li2S6 and 1 M LiTFSI in DOL/DME (1:1) as electrolyte. As shown in Fig. 5b, two typical cathodic peaks at around 2.3 and 2.0 V can be observed, which is attributed to the solid-to-liquid reaction (sulfur to high order polysulfide, S8/Li2S8/Li2S6) and liquid-to-solid conversion (high order polysulfide to low-order sulphide solid, Li2S4 /Li2S2/ Li2S). The anodic peak at about 2.45 V is assigned to the oxidation process of Li2S/Li2S to sulfur. It is noted that the CV curves of HCS cathode possess much broader current peaks as well as the larger voltage hysteresis than those of [email protected] cathode. In detail, two broad cathodic peaks at 2.13 (I) and 1.74 (II) V, and one anodic peak

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Fig. 4. (a) Binding geometries and energy of Li2S6 molecule on graphene and Pd4 cluster, which is derived from theoretical calculation based on DFT, (b) Visualized adsorption of the samples with Li2S6 as a representative LiPS, (c) Refined XPS spectra of sulfur after adsorption of Li2S6, (d) Polarization curves of symmetric cells, (e) EIS spectra of symmetrical Li2S6Li2S6 cells. (A colour version of this figure can be viewed online.)

at 2.76 (III) V are observed for the Li-S cell employing HCS cathodes. Nevertheless, the incorporation of Pd NPs can obviously mitigate polarization, corresponding to cathodic peaks up-shift and anodic peaks down-shift by approximately 0.2 V. Two sharp cathodic peaks at 2.26 (I) and 1.98 (II) V, and one anodic peak at 2.45 (III) V are displayed in the cell with [email protected] cathode, implying the fast kinetics of LiPS conversion on the [email protected] electrode. Moreover, the first two laps of CV curves coincide well, demonstrating the good cycling stability and catalytic Pd NPs can constantly restrain the electrochemical polarization. The detailed values of cathodic peak voltages, anodic peak voltage and onset potentials are shown in Fig. 5c. The onset potentials around three redox peaks further validate the electrocatalytic effect of palladium for LiPS conversion [54]. The onset potential is selected at a current of 20 mA beyond the baseline current [34], which was also generally employed in electro-catalysis field [55,56]. Three onset potentials of [email protected] are 2.42 (I), 2.08 (II), and 2.17 (III) V, and simultaneously those of HCS are 2.41 (I), 1.98 (II), and 2.18 (III) V, respectively. Compared with the HCS system, the higher cathodic onset potential and lower anodic onset potential indicate that the palladium can promote the conversion reactions of LiPS. Interestingly, the polarization of the HCS increases enormously with the enhancement of scan rate, along with the remarkably decreased redox peaks, while the incorporation of palladium to HCS can generate stable LiPS conversion (Fig. S8). The remarkable decrease of polarization for liquid-solid conversion in semi-liquid Li-S cells at high scan rates is beneficial to the electrochemical rate performances of the cells with [email protected]/ S as cathode. The electrochemical behaviors of cathode materials were investigated by CV tests and galvanostatic discharge/charge experiments. Fig. 6a demonstrates the CV curves of [email protected]/S and HCS/S cathodes in the voltage between 1.5 and 3.0 V at a scan rate of 0.1 mV s1. The typical CV curves of [email protected]/S show two well-

defined cathodic peaks at 2.29 and 2.01 V, corresponding to the reduction of sulfur to soluble long-chain LiPS (Li2Sn, 4  n  8) and the further reduction of long-chain LiPS species to short-chain Li2S2/Li2S, respectively [57,58]. In the subsequent anodic scan, both [email protected]/S and HCS/S display asymmetrical oxidation peaks at around 2.45 V, corresponding to the revisable transformation of Li2S2/Li2S to LiPS species and subsequent oxidation to S8 [59,60]. Compared with [email protected]/S electrode, the two cathodic peaks of HCS/S electrode shift to lower potentials at around 2.24 and 1.92 V, and the anodic peaks shift to higher potentials at about 2.40 and 2.47 V, demonstrating the more serious electrochemical polarization of HCS/S. Besides, the [email protected]/S electrode delivers more positive cathodic onset potentials, more negative anodic onset potentials, and higher peak currents than HCS/S. These results imply that the incorporation of palladium can accelerate the redox kinetics and reduce the polarization, which is consistent with the previous tests of semi-liquid cell (Fig. 5b and c). Additionally, Fig. S9 displays the galvanostatic discharge/charge profiles of [email protected]/S and HCS/S cathodes at 0.2 C. Compared with the HCS/S cathode, the [email protected]CS/S cathode exhibits longer charge/discharge voltage plateaus and a smaller voltage hysteresis, indicating the fast electrochemical reaction and low resistance for the [email protected]/S cathode. Fig. 6b shows the cycling performance of the [email protected]/S and HCS/S cathodes with the sulfur loading of ~1.2 mg cm2 at 0.2 C. The [email protected] electrode delivers an excellent initial capacity of 1306 mAh g1 and high sulfur utilization of 78%. After 100 cycles, the [email protected]/S cathodes still maintain high reversible capacity of 885 mAh g1. Compared with [email protected]/S, the hollow HCS/S electrode displays lower initial capacity of 1206 mAh g1 at 0.2 C and faster capacity fade (650 mAh g1 after 100 cycles), implying that the existence of catalytic Pd can effectively suppress the shuttle of LiPS and accelerate the kinetics of LiPS conversion reaction. Besides the unparalleled cycling duration, the Coulombic efficiency of

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Fig. 5. (a) Potentiostatic charge curves of different [email protected]/S and HCS/S for shuttle current tests, (b) CV profiles of semi-liquid Li-S cells with Li2S6 as the active material, where the solid lines and short dotted lines refer to the first cycle and second cycle, respectively, (c) Corresponding peak voltages and onset potentials of asymmetrical Li-S cells. (A colour version of this figure can be viewed online.)

[email protected]/S is above 99% even after 100 cycles, indicating the higher LiPS conversion efficiency and less loss of active sulfur species. The rate capacities of [email protected]/S and HCS/S electrodes were evaluated at various current densities from 0.1 to 2 C, and then recovered back to 0.1 C (Fig. 6c). Compared with HCS/S electrode, the [email protected]/S electrode shows the remarkably improved rate capacity. The [email protected]/S electrode demonstrates excellent capacities of 1423, 1087, 950, 843, 578 mAh g1 at 0.1, 0.2, 0.5, 1, 2 C, respectively. In contrast, the HCS/S electrode shows an inferior rate performance, suggesting sluggish reaction kinetics and a significant loss of active sulfur by LiPS shuttling. After switching the current density back to low rate, the cell with [email protected]/S cathode still remains a stable high capacity of 1102 mAh g1 at 0.1 C, demonstrating the high stability and excellent rate capacity. The corresponding discharge/charge profiles of [email protected]/S electrode at various rates are displayed in Fig. 6d. It's obvious that all of the discharge curves contain two typical plateaus, corresponding to the multi-phase transformation process, which is well consistent with CV curves. Although the polarization deteriorated seriously with the increase of current rate, an evident discharge plateau was achieved at around 1.88 V at a high rate of 2 C. In addition, a large polarization of the [email protected]/S cathode during the initial charging

stage at 0.1 C can be found, which is mainly related to the reduction degree of lithium polysulfides during the first discharging process. The hollow [email protected]/S cathode can facilitate the conversion of lithium polysulfides into Li2S during the first discharging process, and the plentiful Li2S in cathode will result in a larger polarization at the initial stage of the first charging process. To investigate the long term cycling stability of [email protected]/S cathode, a prolonged galvanostatic discharge/charge test is carried out at a high rate of 1 C for 400 cycles (Fig. 6e). An initial discharge capacity of 805 mAh g1 is achieved for the [email protected]/S cathode at 1 C, and 72.9% of capacity (587 mAh g1) is maintained after extensive 400 cycles with a high Coulombic efficiency above 99%, corresponding to a slight average capacity decay of 0.068% per cycle. Constructing high-performance positive electrode with high sulfur loading is crucial to facilitate the practical application of Li-S batteries [61,62]. To validate the superiority of the hollow [email protected] sulfur host material, high areal sulfur loading cathode (~5.88 mg cm2) based on the composite with a sulfur content of 75.8% were fabricated and cycled at 0.2 C. To obtain stable thick electrodes, small amount conductive fiber was added into the slurry, which can hinder the cracking of thick electrode and improve the conductivity and electrolyte soaking for cathode. As

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Fig. 6. (a) Typical CV profiles of [email protected]/S and HCS/S electrodes, (b) Cycling performance of different cathodes at 0.2 C rate for 100 cycles, regarding the CE (upper) and specific capacity (bottom), (c) Rate capabilities of different cathodes at various current densities from 0.1 C to 2 C, (d) Representative voltage profiles of the [email protected]/S cathode at various C rate, (e) Long-term cycling stability of [email protected]/S cathode at 1 C over 400 cycles, (f) Cycling performance of [email protected]/S electrode with high sulfur loading of 5.88 mg cm2 at 0.2 C. (A colour version of this figure can be viewed online.)

shown in Fig. 6f, the [email protected]/S electrode with an area sulfur loading of 5.88 mg cm2 achieves an initial discharge capacity of 873 mAh g1 (corresponding to a specific areal capacity of 5.13 mAh cm2), and maintains 742 mAh g1 after 100 cycles at 0.2 C with an average capacity decay rate of ~0.15% per cycle. Fig. S10 shows the galvanostatic charge/discharge curves of [email protected]/S with different cycles at 0.2 C. The voltage hysteresis increases obviously due to high resistance and the improved area current density for the Li-S

cell with the high sulfur loading electrode. Nevertheless, the second discharge voltage plateau appears at around 1.95 V and remains highly constant during cycling, implying the high electrochemical reversibility and good stability of the cell. Despite the designed hollow [email protected]/S cathodes display outstanding performance at high sulfur area loading, the lithium metal anode and electrolyte are still critical toward practical application of Li-S battery [63,64]. Especially, the lithium dendrites, lithium

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powdering and dead lithium triggered by large current density in Li-S pouch cell are current bottleneck for their practical application [65]. To understand the kinetic characteristics of [email protected]/S and HCS/ S cells, electrochemical impedance spectroscopy (EIS) tests were further performed (Fig. S11). The equivalent circuit model and detailed fitting values are displayed in Fig. S12 and Table S2. The charge transfer impedance (26.03 U) of [email protected]/S cathode exhibits a noticeable decrease in comparison with that of HCS/S cathode (31.86 U), implying the favorable electrochemical kinetics at [email protected]/S cathode interfacial region. In other hand, the significant reduction of slop value in low-frequency region is observed for the [email protected]/S, demonstrating that high conductive Pd NPs can promote the lithium ion diffusion into the electrode materials. The significant decrease of Rct at liquid-solid interface and increase of Liþ diffusion coefficient in [email protected]/S electrode demonstrate that the [email protected] with polarity and high intrinsic conductivity can speed up the electron transfer and the polysulfide conversion at the electrode. X-ray adsorption spectroscopy (XAS) measurements are further conducted to investigate the local structure of catalytic Pd NPs at atomic scale during cycling. Fig. 7a shows the selected discharge/ charge states for XAS experiments. Fig. 7b displays the Pd K-edge of the X-ray absorption near edge structure (XANES) spectra for Pd foil standard along with those of [email protected]/S cathodes at different charge/discharge stages. The K-edge for fresh [email protected]/S shows a distortion and distinct shift toward higher energy compared with the reference spectrum of Pd foil, indicating that the Pd NPs in [email protected]/S exists in oxidized state [66]. Generally, this kind of distortion and upshift is related to a change of Pd electronic structure, implying that the abundant sulfur with high electronegativity in the [email protected]/S causes the interface interaction and electron transfer from Pd to S atoms. However, the Pd-S bond is moderate in [email protected]/S cathode and is not strong enough to form PdS crystalline phase which maybe be harmful for electrochemical catalysis. As the binding energy of Pd XANES in [email protected]/S is significantly lower than that in PdS, and no characteristic XRD diffraction peaks of PdS crystalline are monitored in the fresh [email protected]/S sample (Fig. 3a) [67]. As observed in the inset image of Fig. 7b, the Pd K-edge shows an obvious shift towards higher energy with the increasing depth of discharge (DOD) in the cells, implying that the valence of Pd increases, and the interaction between catalytic Pd NPs and short chain LiPS (or Li2S/Li2S2) becomes stronger. Interestingly, the adsorption edge for charged [email protected]/S cathode displays minimal change compared with that of fresh cathode, indicating that the Pd catalyst possesses high reversibility during

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discharge/charge process. Fig. 7c shows the Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) of the samples, which will reflect the atomic coordination environment and bond lengths of the Pd-sorbent. It is obvious that the Pd-Pd bond is the main binding state in the Pd foil, and the dominant peak is at around 2.44 Å. As a contrast, the dominant peaks for [email protected]/Sbased materials at different discharge/charge stages are observed at about 1.75 Å, which are located between Pd-Pd and Pd-O bonds [68]. It can be deduced that the above bond is one moderate Pd-S interaction in accordance with our DFT calculation results and the previous reports of Pd-S distance (1.659 Å) [69,70]. The Pd-S bond distance in the samples deceases from 1.795 to 1.733 Å with the increasing DOD, and increases back to 1.793 Å at fully charged state. No significant change can be observed for EXAFS in the samples at different discharge/charge stages, and it is believed that the Pd catalytic active sites are very stable during cycling, which is consistent with the XANES results. From the XAS, the moderate PdS bonds between Pd NPs are beneficial to the adsorption/desorption of reactant molecules on Pd active sites, and can accelerate the electrochemical reaction on Pd NPs catalytic cites.

4. Conclusion In summary, we proposed a rational strategy to construct the palladium-imbedded hollow carbon spheres as sulfur host materials via a sacrificial template method. The prepared [email protected]/S cathode delivers an ultrahigh initial specific capacity of 1306 mAh g1 and 885 mAh g1 after 100 cycles at 0.2 C, excellent Coulombic efficiency above 99%, impressive long cycling stability (a sluggish capacity decay of 0.068% per cycle over 400 cycles at 1 C). For the electrode with sulfur mass loading of 5.88 mg cm2, it still shows a high initial specific capacity of 873 mAh g1 at 0.2 C and high capacity retention of 85% over 100 cycles. Through theoretical calculation and electrochemical measurements, the well-designed hollow [email protected] can effectively mitigate the lithium polysulfide (LiPS) shuttling via physical and chemical confinement, and facilitate the redox reaction kinetics of LiPS. The XAS technique demonstrates that the enhanced electrochemical properties are contributed to the special electronic structure of Pd NPs and moderate Pd-S bonds between Pd NPs and reactant molecules. The presented work affords an innovative strategy to immobilize the LiPS and enhance polysulfide redox reaction kinetics by constructing catalytic hollow sulfur host materials, and the detailed kinetics studies of Li-S redox reaction are critical for constructing stable rechargeable Li-S batteries.

Fig. 7. (a) The galvanostatic discharge/charge profile of the first cycle. As the colored dots indicated [email protected]/S cathode at different discharge/charge states monitored by XAS, (b) Normalized XANES spectra at the Pd K-edge of the Pd foil and [email protected]/S at different discharge/charge stages, (c) Pd K-edge EXAFS Fourier transform spectra of the samples. (A colour version of this figure can be viewed online.)

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Conflicts of interest There are no conflicts to declare. Acknowledgments Thanks for financially supported by the National Natural Science Foundation of China (no. 51772068). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC0206CH11357.

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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2018.11.086.

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