Recent advances in chemical adsorption and catalytic conversion materials for Li–S batteries

Journal of Energy Chemistry 42 (2020) 144–168

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Review

Recent advances in chemical adsorption and catalytic conversion materials for Li–S batteries Xiaodong Hong a, Rui Wang a, Yue Liu a, Jiawei Fu a, Ji Liang b,∗, Shixue Dou b a

College of Materials Science and Engineering, Liaoning Technical University, 47 Zhonghua Road, Fuxin 123000, Liaoning, China Institute for Superconducting & Electronic Materials, Australian Institute of Innovative Materials, University of Wollongong, Innovation Campus, Squires Way, North Wollongong, NSW 2500, Australia

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a r t i c l e

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Article history: Received 15 May 2019 Revised 2 July 2019 Accepted 3 July 2019 Available online 9 July 2019 Keywords: Chemical adsorption Electrocatalysis Li–S batteries Lithium polysulfides Shuttle effect

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a b s t r a c t Owing to their low cost, high energy densities, and superior performance compared with that of Li-ion batteries, Li–S batteries have been recognized as very promising next-generation batteries. However, the commercialization of Li–S batteries has been hindered by the insulation of sulfur, significant volume expansion, shuttling of dissolved lithium polysulfides (LiPSs), and more importantly, sluggish conversion of polysulfide intermediates. To overcome these problems, a state-of-the-art strategy is to use sulfur host materials that feature chemical adsorption and electrocatalytic capabilities for LiPS species. In this review, we comprehensively illustrate the latest progress on the rational design and controllable fabrication of materials with chemical adsorbing and binding capabilities for LiPSs and electrocatalytic activities that allow them to accelerate the conversion of LiPSs for Li–S batteries. Moreover, the current essential challenges encountered when designing these materials are summarized, and possible solutions are proposed. We hope that this review could provide some strategies and theoretical guidance for developing novel chemical anchoring and electrocatalytic materials for high-performance Li–S batteries. © 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

Xiaodong Hong received his Ph.D degree in Materials Physics and Chemistry from Harbin Institute of Technology in 2010. Currently, he works as a professor at the College of Materials Science and Engineering, Liaoning Technical University, China. His-research interests focus on the design of carbon-based composites for supercapacitors and Li–S batteries.

Yue Liu is currently pursuing her master degree at the College of Materials Science and Engineering, Liaoning Technical University, China. Her-research interests focus on the design of cathode materials for Li–S batteries.

Rui Wang is currently pursuing his master degree at the College of Materials Science and Engineering, Liaoning Technical University, China. His-research interests focus on the design of new energy materials.

Jiawei Fu is currently pursuing his master degree at the College of Materials Science and Engineering, Liaoning Technical University, China. His-research interests focus on the design of graphene-based new energy materials for Li–S batteries.

Corresponding author. E-mail address: [email protected] (J. Liang).

https://doi.org/10.1016/j.jechem.2019.07.001 2095-4956/© 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

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Ji Liang received his Ph.D. from the University of Adelaide in 2014. After a two-year T. S. Ke Fellowship in the Institute of Metal Research of the Chinese Academy of Sciences, he was appointed an ARC-DECRA research fellow in the Institute for Superconducting and Electronic Materials of the University of Wollongong, Australia. Hisresearch interests lie in the design of functional carbonbased materials for electrochemical catalysis and energy storage applications.

Prof. Shi Xue Dou is the director of Institute for Superconducting and Electronic Materials and a Distinguished Professor at the University of Wollongong. He received his Ph.D. in chemistry in 1984 at Dalhousie University, Canada and his D.Sc. at the University of New South Wales in 1998. He was elected as a Fellow of the Australian Academy of Technological Science and Engineering in 1994. He is a program leader for the Automotive Corporative Research Center-2020. Fig. 1. Charge–discharge profile and corresponding products of Li–S batteries [3]. Copyright 2015, Elsevier.

1. Introduction Given the rapid development of portable energy storage devices and electric vehicles (EVs), Li-ion batteries (LIB) cannot satisfy the urgent requirements of environmental benignity, cost efficiency, high safety, and high energy density [1]. Therefore, secondary batteries, which are based on alternative electrochemical mechanisms, have been proposed and developed to substitute LIBs. To date, Li–S batteries have been recognized as some of the most promising battery systems for this purpose, and their successful commercialization has been considered to allow to greatly improve the performance of EVs and other devices [2]. 1.1. Principle of Li–S batteries Compared with the reversible insertion and extraction of Li+ ions in the electrode materials of LIBs, Li–S batteries rely on the more complex conversion reactions between metallic Li and elemental sulfur. As illustrated in Fig. 1, during the discharge process, the sulfur (S8 ) species at the cathode is electrochemically reduced to various lithium polysulfides (LiPSs). Commonly, two plateaus can be observed in the discharge profiles of Li–S batteries. The plateau at 2.1–2.4 V corresponds to the transition from S8 into soluble long-chain LiPSs (Li2 Sn , 4 ≤ n ≤ 8), which only delivers approximately 25% of the theoretical capacity of sulfur. The contribution of the second plateau, which occurs at 1.6–2.1 V, to the capacity is much higher: approximately 75% of the total capacity, which is attributed to the transition of soluble LiPSs into short-chain and insoluble Li2 S [3]. A reversible reaction takes place during the charging process: solid Li2 S is first converted into insoluble short-chain LiPSs, then into soluble long-chain LiPSs, and lastly into solid S8 . 1.2. Drawbacks of Li–S batteries Recently, significant progress has been achieved in the development of Li–S batteries [4,5], however, their commercialization is still slow, and is heavily related to the intrinsic features of the sulfur cathode materials, as follows: (1) Volume variation and insulation of sulfur: The conversion from solid S8 into Li2 S discharge product induces ∼80% volume expansion. Therefore, the sulfur host materials should present suitable porosity to accommodate the considerable

volume change and avoid the pulverization of the cathode. Moreover, elemental sulfur and solid Li2 S2 /Li2 S present low ionic and electronic conductivities, and conductive additives or hosts are necessary to maintain the electron transfer at the cathode. (2) Shuttling effect of LiPSs: The dissolved long-chain LiPS intermediates diffuse into the electrolyte and penetrate through the separator into the metallic Li anode region. The “shuttle effect” causes the loss of active sulfur, passivation of metallic Li surface, and self-discharge, which considerably deteriorates the cell performance. (3) Sluggish conversion reaction kinetics of different LiPS species: The conversion of solid elemental S8 into soluble long-chain LiPSs and then into solid Li2 S at the end of the discharging step is a complex and kinetically sluggish chemical process. Therefore, the reaction kinetics of the sulfur species conversion affects the capacity exertion and overall electrochemical performance of Li–S batteries [6–8]. (4) Dendrite formation and other shortcomings of Li anodes: During the charging–discharging process, metallic Li dendrites are formed in Li–S batteries due to the non-uniform deposition of the solid-electrolyte-interface. These sharp dendrites could pierce the separator and could cause internal short circuits and significant safety concerns [9]. In addition to the formation of Li dendrites, the shuttled soluble LiPS intermediates could react with metallic Li and passivate the anode surface [10–12]. The “dead Li” that forms on the anode is unable to provide sufficient Li+ ions for long-term cycling, which leads to the poor Coulombic efficiency of the batteries and even the fast termination of the charging–discharging process. These concerns markedly restrict the electrochemical performance of Li–S batteries, particularly their long-term cycling stability and rate capability. Recently, significant progress has been achieved in the fields of new electrolytes (additives), novel Li anodes, and functional materials for sulfur hosts, separator modifiers, or interlayers which could physically or chemically immobilize the intermediate LiPS species [4]. Compared with physical confinement, chemical binding would anchor the LiPS species effectively, which is, thus, more desirable for achieving long-term cycling performance of Li–S batteries [13]. In addition to the chemical anchoring strategy, the concept of electrocatalysis has been introduced for Li–S batteries. Since 2015, a series of electrocatalytic

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Fig. 2. Schematic illustration of chemical anchoring and electrocatalytic materials for Li–S batteries.

materials have been reported, which could not only modulate the deposition of insoluble Li2 S, but could also accelerate the conversion rate (redox kinetics) of LiPS species and diffusion rate of Li+ ions, thus decreasing the internal resistance and increasing the capacity and rate performance of Li–S batteries [13]. Herein, we comprehensively summarize the recent progress in the development of chemical adsorbing/binding materials and electrocatalytic materials for Li–S batteries [14–16]. As presented in Fig. 2, chemical anchoring materials can be classified into seven categories, i.e., heteroatom-doped carbon materials, conducting and non-conductive polymers, nitrides, oxides, sulfides, hydroxides, MXenes and carbides. According to the electrocatalytic mechanism, electrocatalytic materials can be classified into three types. The first type are defined as polar hosts, including graphitic carbon nitride (g-C3 N4 ), phosphorene nanosheets, metal particles (e.g., Au, Pt, Ni, and Co), metal oxides (e.g., MoO2 /MoO3 , Nb2 O5 , CeO2 , VO2 , and RuO2 ), NiFe-layered double hydroxide (NiFe-LDH), sulfides (e.g., Cox Sy , MoS2- x ,TiS2 , WS2 , and NiS), nitrides (e.g., TiN, InN, VN, and Mo2 N), carbides (e.g., WC and NbC), and some metal compounds, such as FeP, CoPC, and silver-polyoxometalate (AgPOM). The second type include δ -MnO2 , VO2 , CuO, and V2 O5 , which could form thiosulfate groups (S2 O3 2− ) on the host surface to mediate the redox reaction of sulfur species. The last category of materials consists of Te-doped sulfur, which is the only material that can participate in electrocatalytic interactions by accelerating

the lithiation/delithiation reaction and increasing the diffusion rate of Li+ ions. In addition, electrochemical characterization and other characterization techniques of the electrocatalytic interaction are summarized. Lastly, aiming to address the current challenges Li– S batteries, we put forward some possible measures for developing advanced, high-capacity and long-term cycling stability Li–S batteries. 2. Chemical anchoring strategies for Li–S batteries Compared with the traditional physical confinement of soluble LiPSs, the chemical adsorption/binding interaction presents stronger anchoring ability [17–21]. In this section, we summarize the research progress of various materials that were designed to chemically adsorb soluble LiPSs, including heteroatom-doped materials, various polymers, nitrides, oxides, sulfides, hydroxides, MXenes, and carbides. 2.1. Heteroatom-doped carbon materials Heteroatom doping of carbon materials can modify the surface electronic structure of the carbon matrix and change the nonpolarized carbon host materials into polarized ones. This could induce desirable chemical bonding or the adsorption of LiPSs by heteroatoms. Therefore, doping atoms could serve as active sites

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Fig. 3. (a) Schematic illustration of synthesis of N, P co-doped porous carbon/sulfur (NPHPC/S) composite [37]. Copyright 2016, Elsevier. (b) Schematic illustration of fabrication of N, S co-doped hierarchical porous carbon/sulfur (NSHPC/S) composite and interaction with LiPSs in Li–S batteries [40]. Copyright 2017, Wiley-VCH.

for the adsorption of LiPSs, and could, therefore, further improve the stability of Li−S batteries. Various carbon host materials, including porous carbon, carbon nanotubes (CNTs), carbon nanofiber (CNF), and graphene, have been doped with different heteroatoms, such as N, B, P, O, and S, or combinations of two or more of these species, to chemically anchor the soluble LiPSs [22]. N-doped carbon materials are the most commonly used monodoped carbon materials. N-doped graphene [23,24], porous carbon [25], hollow carbon spheres or bowls [26–30], CNTs [31], carbon nanoflowers [32], graphene/CNT hybrids [33], and carbon sphere/CNT hybrids [29] have been reported to present active sites and/or defects that could trap LiPSs, which could further improve the cycling performance of Li–S batteries. The doping configurations of these carbon materials directly impact the anchoring effect of LiPSs. Yin et al. [24] investigated the interactions between LiPSs and different N configurations using density functional theory (DFT) calculations, and reported that pyridinic-N was the best immobilizer for LiPSs among the amino-, pyridinic-, graphitic-, and pyrrolic-N species. Using DFT calculations, Zhang et al. [34] analyzed the dipole–dipole electrostatic interactions between LiPSs and various mono-doping atoms,

including N, O, P, S, B, F, and Cl, and determined that only N or O dopants of the carbon matrix exhibited enhanced interactions, which could prevent LiPSs from shuttling, whereas the capabilities of the other dopants for this task were unsatisfactory. In addition to single-atom doping of carbon substrates, dualdoping is often adopted. For dual-doping, two kinds of heteroatoms play essential roles in improving the electrochemical reactions of LiPSs. Most dual-doped carbon materials contain N together with a secondary element, such as B, P, S, or O. Yuan et al. [35] introduced negatively charged N atoms and positively charged B atoms into a graphene-supported carbon layer to improve the chemisorption of LiPSs. This dual-doped carbon host achieved superior rate performance and long-term cycling stability for Li–S batteries. In another study, N, B co-doped curved graphene nanoribbons were prepared using urea and boric acid as precursors of the dopants [36]. The synergistic effect of the two dopants strengthened the chemical adsorption ability of the material for LiPSs and also improved the conductivity, specific surface area, and sulfur dispersibility over the material. N, P co-doped porous carbon (NPHPC) was prepared via the pyrolysis of melamine polyphosphate, and served as sulfur host (Fig. 3a) [37]. The combination of physisorption and chemical

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binding was demonstrated to effectively suppress the dissolution of LiPSs. Moreover, the N, P co-doped porous carbon (N, P-HPC) was also fabricated using polyaniline (PANI) aerogels and phytic acid as precursors [38]. The obtained N, P-HPC host presented an interconnected network structure that featured sufficient active sites, and thus, offered superior anchoring effect than the monodoped counterpart. From the polyaniline hydrogel and phytic acid precursors, a N and P co-doped carbon network was designed to support Li2 S nanoparticles as a binder-free Li2 S/N, P-C cathode, and was fabricated using polyaniline hydrogel and phytic acid as precursors [39]. This cathode could not only improve the ionic conductivity of Li2 S but also catalyze the electrochemical redox reactions of LiPSs, which delivered high capacity and superior cycling stability when used in Li–S batteries. Chen et al. reported an N, S co-doped hierarchical porous biomass carbon (NSHPC) with honeycomb-like microstructure (Fig. 3b) [40]. This dualdoped host provided chemical adsorption and active sites for the dissolved LiPSs. Furthermore, the N, S dual-doping also increased the hydrophilic and electronic conductivity of porous carbon, and ultimately improved sulfur utilization and the long-term cycling performance of Li–S batteries (300 cycles) remarkably. Moreover, a N, O co-doped carbon material (NONPCM) was also fabricated at the kilogram scale as a sulfur host [41]. The host achieved high sulfur loading (90 wt%) and provided abundant anchoring sites for LiPSs, which conferred good rate performance and stable cycling performance to Li–S batteries. Only one study reported the use of a N-free dual-dopant: B, O co-doped multi-walled carbon nanotubes (B, O-MWCNTs) as a sulfur host for Li–S batteries [42]. The chemisorption between LiPSs and the carbon matrix was strengthened by numerous B and O adsorptive sites, which improved the rate capability and cycling stability of Li–S batteries. 2.2. Conducting and nonconductive polymers 2.2.1. Conducting polymers Most conducting polymers, including polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh), and poly(3,4ethylenedioxythiophene) (PEDOT), present excellent electron conductivity and could be directly used as sulfur hosts for Li–S batteries. More importantly, positively charged amino-groups on the backbones of conducting polymers could adsorb negatively charged polysulfide anions via electrostatic attraction and confer superior cycling stability to Li–S batteries. In particular, conducting PANI, which comprises abundant N-containing groups on its backbone, has been fabricated into various nanostructures, including hollow spheres [43–46], thin nanofibers [47], and printed layers [48], to immobilize LiPSs more efficiently. Typically, PANI is used as wrapping layer for active sulfur species. For example, S C−1 nanospheres were encapsulated using a PANI layer to inhibit the diffusion of LiPSs [49]. The as-prepared PANI-wrapped S C−1 nanospheres cathode (73 wt% sulfur content) exhibited excellent long-term stability over 2500 cycles at 5 C and the extremely low capacity decay of 0.01% cycle−1 . Using a similar strategy, sulfur/acetylene black (S–C) [50] and ordered mesoporous carbon/sulfur (S-OMC/S) composites [51] were also coated with PANI as adsorbing and conducting agent, and both enhanced the Li–S battery performance, particularly its cycling stability. In addition to porous carbon, graphene and CNTs have also been hybridized with PANI. For instance, a PANI shell layer wrapped on the surface of graphene oxide–sulfur (GO–S) [52] or cetyl trimethylammonium bromide–modified graphene oxide–sulfur (CTAB–GO–S) composite has been demonstrated to effectively trap the dissolved LiPSs [53]. As elastic coating for the adsorption of the dissolved LiPSs, PANI was generated in situ on N-doped graphene sheets separated by nanosulfur (NGNS-S) to form a ternary layered

NGNS-S-PANI cathode [54], which exhibited superior cycling stability and capacity compared with the binary-layered hybrids without PANI. Zhang et al. [55] synthesized bipyramidal sulfur particles and then wrapped them in PANI and GO layers to prepare a double-shell S@PANI/GO composite. The performance of this cathode for Li–S batteries was superior to those of the S GO−1 or S@PANI cathodes, which were covered with single layers. Moreover, PANI could be used to coat single- [56] or multi-walled carbon nanotubes/sulfur (Fig. 4b) [57], and carbon nanofiber/sulfur composites [58] (Fig. 4a, b, and c, respectively) to improve the conductivity and adsorb LiPSs while achieving good rate capability and high specific capacity. To further enhance the chemical interactions between PANI and sulfur, PANI could also be used to prepare sulfur-containing covalent compounds. For example, the H atoms in the benzene rings of PANI could be replaced by Cl atoms (Fig. 5a), which could then be substituted by polysulfide groups [44]. The polysulfide groups were connected to two molecular chains of PANI to prepare a sulfur– polyaniline (SPANI) cathode with interconnected nanopores. This structure encapsulated sulfur perfectly, and the obtained cathode exhibited very good rate capability and cycling stability. In another paper, it was reported that covalent bonds were formed between PANI and sulfur when a cross-linked framework was prepared [59], which was used as a vulcanized polymer cathode for Li–S batteries (Fig. 5b). Not only did PANI act as a conductive matrix in this material, but it also bonded the low-order LiPSs during battery operation, which led to an increased capacity retention of 90% after 300 cycles. Similarly to PANI, other conducting polymers, such as, PPy, PTh, and PEDOT, present superior conductivity, which could compensate the insulation of sulfur, and could also be employed as obstructing layers to prevent the dissolution of LiPSs. Polypyrrole nanofibers for S-PPy nanocomposites were synthesized using methyl orange as a self-degrading template [60]. The enhanced performance of the Li–S battery using these composites was attributed to the adsorption and active interaction of conductive PPy with LiPSs. Li et al. [61] coated sulfur on a PPy nanofiber film to prepare a flexible free-standing S@PPy cathode. Additionally, the PPy-coated separator was used to prevent LiPSs shuttling, and the results indicated that the PPy host and functionalized separator greatly enhanced the electrochemical performance of Li–S batteries. To support sulfur particles, three-dimensional (3D) porous PPy was synthesized using silica (SiO2 ) spheres as templates [62]. The ordered macropores and mesopores/micropores of PPy provided fast transport paths for Li+ ions, and suppressed the shuttling of dissolved LiPSs. As an excellent encapsulation layer, PPy could immobilize LiPSs owing to its special N-containing composition. In particular, Li2 S was wrapped in a PPy layer to prepare a Li2 S cathode [63]. The N–Li interactions between the N atoms of the PPy coating layer and Li2 S strongly immobilized LiPSs and thus, enabled a stable cycling performance to the Li–S battery for more than 400 cycles. Moreover, a PPy-wrapped nanosulfur composite cathode (S@PPy) with a core–shell structure was reported, where the PPy shell layer conferred high conductivity, sufficient space to accommodate volume changes during the charge–discharge process, and suppressed the shuttling of dissolved LiPSs [64]. Consequently, the Li–S battery based on this S@PPy material exhibited high specific energy density (over 400 Wh kg−1 ). In addition to single-layer wraps, a threelayered PPy@S@PPy composite has been developed, where sulfur formed the middle layer and the internal and external PPy layers acted as a conductive matrix and confinement layer for the dissolved LiPSs, respectively [65]. To improve the sulfur immobilization capability of graphene, pyrrole (Py) was deposited on the surface of reduced graphene oxide (rGO) to produce rGO/PPy, and sulfur was then chemically grown on it to prepare an rGO/PPy/S

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Fig. 4. (a) Schematic illustration of preparation of polyaniline–sulfur/single-walled nanotubes (PANI-S/SWNT) [56]. Copyright 2015, Nature Publishing Group. (b) The formation process of multi-walled carbon nanotubes sulfur@polyaniline composite (MWCNTs-S@PANI) [57]. Copyright 2015, Elsevier. (c) Configuration of carbon nanofiber/sulfur/polyaniline (CNF/S/PANI) composite electrode [58]. Copyright 2018, Wiley-VCH.

cathode [66]. The introduction of N- and O-containing groups in rGO/PPy/S significantly facilitated the chemical adsorption of LiPSs on the graphene surface, which conferred superior rate performance and cycling stability to this cathode. In addition to the single-layer PPy coating, sulfur spheres were wrapped in a double shell of PPy and MnO2 (PPy@MnO2 @S). This was used as a composite cathode for Li–S batteries, where PPy served as the second shell on the MnO2 layer to provide the necessary conductivity, additional chemical adsorption, and sulfur restriction [67]. Zeng et al. [68] reported the copolymerization between 3butylthiophene (3BT) and sulfur powder to form S3BT copolymer, which was then wrapped in a conductive poly(3,4ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) layer to fabricate a Li–S battery cathode. The enhanced cell performance was attributed to the chemical bonding between sulfur and 3BT, high conductivity, and sulfur-immobilizing roles of the S3BT copolymer and PEDOT:PSS coating layers. In another study, a high sulfur content (approximately 80 wt%) sulfur-poly(maminothiophenol) (S-PMAT) copolymer was prepared by bonding sulfur to the thiol groups of PMAT [69]. The cross-linked nanostructure of sulfur in S-PMAT effectively confined the shuttling of active sulfur; the initial discharge capacity was 1407.6 mAh g−1 at 0.1 C, and the capacity decay was 0.04% for 10 0 0 cycles at 2 C.

2.2.2. Nonconductive polymers Compared with conducting polymers, some nonconductive polymers, such as polydopamine (PDA)-, poly(ethylene oxide) (PEO)-, poly(acrylic acid) (PAA)-, and phytic acid (PA)-grafted PANI, are commonly used as cladding layers to inhibit the diffusion of dissolved LiPSs by utilizing the abundant hydroxyl, oxygencontaining, or amino groups on their surfaces. Coating the active sulfur species with PDA is a popular strategy for preventing the diffusion of LiPSs. For example, Zhou et al. [70] prepared PDA-coated and N-doped hollow carbon– sulfur composites with double-layered core–shell structure, where the N-doped hollow carbon and PDA coating acted as conductive agent and barrier layer, respectively. This material exhibited superior cycling stability for Li–S batteries, and its capacity was 630 mAh g−1 after 600 cycles at 0.6 C. In addition to single-layer coatings, multi-layered electroactive materials have been reported as more effective barriers for LiPSs. Kim et al. [71] first deposited poly(allylamine hydrochloride)/poly(acrylic acid) (PAH/PAA) double layers on sulfur, and then coated PEO/PAA bilayers to obtain multilayer-coated cathodes. The PAH/PAA/(PEO/PAA)3 multilayers confined the shuttling of LiPSs even in the absence of the LiNO3 additive and enabled the sulfur cathodes with high rate capability and capacity retention.

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Fig. 5. (a) Fabricating routes of sulfur–polyaniline (SPANI) composite [44]. Copyright 2015, Elsevier. (b) Synthesis of sulfur/polyaniline (S@P) and vulcanized polymeric cathode (S@h-P) [59]. Copyright 2018, Elsevier.

Commercial vapor-grown carbon fiber (VGCF), PEO, and carboxymethylated cellulose were mixed uniformly and coated on Al foil that was pre-coated with sulfur, to produce an electrolyte permeating layer and LiPSs adsorption layer [72]. Owing to the shuttle confinement attributed to the active polymer layer, the prepared Al-S-VGCF electrode presented excellent long-term cycling performance in Li–S batteries. To improve the chemical anchoring interaction, P-containing PA was grafted onto the –NH– groups on the backbone of PANI to prepare an N, P-containing polymer with abundant quinonoid imine (–NH+ =/–N=) groups [73]. This polymer was coated onto GO to prepare quinonoid imine-doped graphene. As a sulfur host, the quinonoid imine group strongly adsorbed the dissolved Li2 S8 and also promoted the kinetics of the LiPS redox reaction. When sulfur loading was 3.3 mg cm−2 , the cell still delivered the high capacity of 3.72 mAh cm−2 . 2.3. Nitrides 2.3.1. Graphitic carbon nitride (g-C3 N4 ) Graphitic carbon nitride presents graphite-like structure and abundant N atoms. Owing to the formation of strong N–Li bonds between N-species and LiPS intermediates, g-C3 N4 presents excellent immobilization ability for LiPSs. Therefore, various g-C3 N4 based hosts or separators/interlayers have been developed for

manufacturing high-performance Li–S batteries. In 2015, Zhang et al. [74] reported the use of porous oxygenated carbon nitride (OCN) nanosheets as sulfur host. The S OCN−1 cathode exhibited the high discharge capacity of 1407.6 mAh g−1 at 0.05 C and long cycling (20 0 0 cycles). Afterward, Han et al. also reported g-C3 N4 nanosheets with high surface area (209.8 m2 g−1 ) as sulfur host, which achieved the high initial capacity of 1250 mAh g−1 at 0.05 C and the retained capacity of 578.0 mAh g−1 for 750 cycles at 0.5 C [75]. Nazar et al. [76] prepared nanoporous g-C3 N4 with high surface area to host sulfur (Fig. 6a) and achieved the low capacity decay of 0.04% cycle−1 for 1500 cycles at 0.5 C. Moreover, to improve the conductivity of the semi-conductive g-C3 N4 host, various carbon materials have been adopted to prepare g-C3 N4 -based composites. Zhou et al. [78] prepared g-C3 N4 coated carbon fiber mesh to provide active sites for anchoring LiPSs and confirmed that strong adsorption interactions were induced by the chemical bonds between N and Li. Furthermore, 3D g-C3 N4 /graphene composites [79–82] were also designed for high-performance Li/dissolved polysulfides cells or Li–S batteries. For example, Guo et al. [83] developed 3D graphene@g-C3 N4 (GCN) sponges, which contained sulfur trapped in the macropores, using the microemulsion method. The GCN host ensured the sulfur loading of 82 wt% and presented excellent rate capability and the low capacity decay of 0.017%

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Fig. 6. (a) Schematic illustration of synthesis of graphitic carbon nitride and its interaction with lithium polysulfides [76]. Copyright 2016, American Chemical Society. (b) Boron nitride (BN)–carbon separator and its function during discharging [77]. Copyright 2017, Nature Publishing Group.

cycle−1 over 800 cycles at 0.3 C. In addition to graphene, metal– organic framework (MOF)-derived hollow porous C was used to load g-C3 N4 nanodots to synergistically confine LiPSs both physically and chemically [84]. The g-C3 N4 -functionalized separator was prepared by filtering the g-C3 N4 suspension on a polypropylene (PP) separator [85,86]. Being a superior sulfur immobilizing layer, this g-C3 N4 -functionalized separator achieved superior electrochemical performance compared with the ordinary PP one. 2.3.2. Boron nitride (BN) Similarly to g-C3 N4 , BN nanosheets have been used as a separator modifier or interlayer. Kim et al. [77] designed a BN and carbon (BN–C) dual-coated separator (Fig. 6b). In the cell structure, the carbon and BN layers faced the cathode and anode, respectively, which suppressed the diffusion of LiPSs and protected the metallic Li anode at the same time. The BN–C separator suppressed the growth of dendrites on the Li anode and increased the Coulombic efficiency and cycling stability of Li–S batteries. Furthermore, an ultralight BN nanosheets/graphene interlayer [87] was developed to suppress the shuttling of LiPSs. Coupled with a porous CNT cathode comprising 60 wt% sulfur, the cathode delivered an ultralow capacity decay of 0.0037% cycle−1 for 10 0 0 cycles at 3 C, which was attributed only to the strong electrostatic interactions between the positively charged BN and polysulfide anions. 2.3.3. Metal nitrides Metal nitrides present excellent electrical conductivity, which is even superior to that of carbon materials. Moreover, they exhibit strong chemical binding interactions with LiPSs. Mesoporous titanium nitride (TiN) (Fig. 7a) was directly used to host sulfur as a cathode for Li–S batteries [88]. Owing to the N–S surface bonding between TiN and the dissolved LiPSs, various polysulfide intermediates could be effectively adsorbed. Furthermore, the titanium dioxide (TiO2 ) passivation layer that formed on the surface of TiN, which presented abundant hydrophilic Ti–O groups also formed strong bonds with LiPSs. Therefore, the TiN–S cathode

exhibited superior cycling performance than the TiO2 –S cathode (Fig. 7b), and its capacity decay was 0.07% cycle−1 for 500 cycles. In addition to mesoporous TiN, interconnected mesoporous molybdenum nitride (Mo2 N) was also prepared. Mo2 N presented a high specific surface area of 121 m2 g−1 (Fig. 7c) [89], and served as a conductive skeleton to suppress sulfur shuttling, which increased the capacity and cycling stability of Li–S batteries. The electrical conductivity of vanadium nitride (VN) is high as 1.17 × 106 S m−1 . Sun et al. [90] prepared a free-standing and porous VN nanoribbon/graphene (VN/G) composite using the hydrothermal treatment and calcination method (Fig. 7d). The interconnected graphene network provided transfer paths for electrons and Li+ ions. Furthermore, DFT calculations confirmed the strong polar–polar interaction between VN and Li2 S6 . The VN nanoribbons effectively immobilized the soluble LiPSs and promoted the kinetics of the redox reaction. Consequently, the VN/G host delivered the high specific capacity of 1461 mAh g−1 and retained 1252 mAh g−1 after 100 cycles at 0.2 C. In another study, Co-doped VN yolk–shell nanospheres with N-doped carbon layer (Co-VN@C) were also used as sulfur host [91]. Integrating the excellent conductivity and chemical confinement effect, this Co-VN@C-S cathode exhibited the high initial discharge capacity of 1379.2 mAh g−1 at 0.1 C and good cycling stability. A similar structure of porous-shell VN nanobubbles (VN–NBs) was also prepared to host sulfur [92]. The VN–NBs presented high conductivity and chemical affinity for LiPSs. Moreover, they also promoted the kinetics of the sulfur conversion reaction. Therefore, the S@VN–NBs cathode delivered remarkable capacity and cycling performance. In addition to TiN and VN, mesoporous cobalt nitride (Co4 N) spheres composed of nanosheets were reported as sulfur host [93]. These spheres presented strong LiPS adsorption capability and bifunctional catalytic activities for the redox conversion of sulfur. The Co4 N@S cathode with the sulfur content of 72.3 wt% delivered the high initial capacity of 1659 mAh g−1 at 0.1 C. Even at the high sulfur content of 94.88 wt%, the initial capacity still reached 1259 mAh g−1 . Moreover, the discharge capacity was maintained

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Fig. 7. (a) Microstructure of mesoporous titanium nitride (TiN) and (b) cycling performance of different S hosts [88]. Copyright 2016, Wiley-VCH. (c) Schematic illustration of fabrication of mesoporous molybdenum trioxide (MoO3 ) and mesoporous molybdenum nitride (Mo2 N) for S hosts [89]. Copyright 2018, Elsevier. (d) Schematic illustration of preparation of the porous vanadium nitride nanoribbon/graphene (VN/G) composite [90]. Copyright 2017, Nature Publishing Group.

at 690 mAh g−1 after 800 cycles at 2 C. In a recent study, Mosavati et al. [94] compared the electrochemical activities of various metal nitrides, including tungsten nitride (WN), Mo2 N, and VN, as sulfur hosts for Li–S batteries. They reported that the WN-S cathode delivered higher capacity and presented superior cycling stability than the VN and Mo2 N ones. This was attributed to the strong interactions between WN and LiPSs owing to the formation of S–W– N bonds on the electrode surface. In addition to nitrides being employed as cathode hosts, Zhang et al. [95] reported the use of indium nitride (InN) nanowires as decorated separator for Li–S batteries. During the electrochemical reaction, the N atoms of InN effectively interacted with the Li+ ions, and the In3+ cations were adsorbed by the polysulfide anions. Furthermore, the InN surface also accelerated the LiPS conversion. Therefore, the InN-modified separator could both suppress the shuttling of LiPSs via chemical adsorption and chemically

catalyze the redox reaction of LiPSs. This improved the cycling performance of the Li–S battery, which presented the low capacity decay of 0.015% cycle−1 for 10 0 0 cycles. 2.4. Oxides 2.4.1. Silica (SiO2 ) O atoms in polar O-containing compounds contain lone electron pairs, which interact strongly with polysulfides owing to the high binding energy between O atoms and polysulfide ions [34]. Utilizing the S-anchoring role of SiO2 , Hou et al. [96] fabricated Si/SiO2 @C–S hybrid spheres as a cathode for Li–S batteries, where the Si/SiO2 network was wrapped with porous carbon spheres. Consequently, the positively charged Si/SiO2 and external porous carbon layers interacted strongly with the polysulfide anions via electrostatic interactions and physical adsorption, respectively. In

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Fig. 8. (a) Formation of porous graphene/carbon/silica (G/C/SiO2 ) hollow microsphere for sulfur hosting. (b) Synthesis of sulfur–titanium dioxide yolk–shell nanostructure and its transmission electron microscopy image [100]. Copyright 2014, Nature Publishing Group. (c) Schematic illustration of preparation of titanium monoxide@carbon hollow spheres/sulfur (TiO@C–HS/S) composite [101]. Copyright 2016, Nature Publishing Group.

another study, polar SiO2 was used as the core and was wrapped with a hollow carbon spherical shell to form a yolk–shell SiO2 @HC composite [97]. When this composite was used as sulfur host, its polar SiO2 core and carbon shell acted as a chemical adsorption agent and physical barrier, respectively, to inhibit the dissolution of LiPSs. Consequently, this SiO2 @HC sulfur host with the high sulfur loading of ∼76 wt% achieved a low capacity decay of 0.056% cycle−1 over 400 cycles even at 2 C. Wu et al. [98] prepared porous graphene/carbon/silica hollow micro-spheres (G/C/SiO2 ) as sulfur hosts, where SiO2 nanoparticles were dispersed on the surface of graphene (Fig. 8a). The carbon layer facilitated the formation of the hierarchical porous structure and provided enough space to accommodate the volume expansion during the lithiation of sulfur. Moreover, the SiO2 microspheres effectively anchored sulfur via chemical bonds. In addition, SiO2 could also be used to decorate the PP separator (PP-SiO2 ) [99]. The functionalized PP-SiO2 separator physicochemically interacted with the dissolved LiPSs and increased the electrolyte wettability, which further improved the rate capability and cycling performance of the Li–S batteries. 2.4.2. Metal oxides Metal oxides present abundant hydrophilic metal–oxygen (M–O) groups on their surface, which are capable of adsorbing polar polysulfide anions by forming chemical bonds. As adsorbing additive for trapping LiPSs, magnesium nickel oxide (Mg0.6 Ni0.4 O) was added to the sulfur cathode of a Li–S battery [102], and the higher capacity and capacity retention of the oxide-containing

cathode were higher than those of the additive-free cathode. To further improve the performance of Li–S batteries, metal oxide materials have been designed into various porous structures or cladding/shell layers with high specific surface areas to enhance their adsorption features. For instance, a TiO2 shell was synthesized and subsequently deposited on sulfur particles to form a yolk–shell structure (Fig. 8b) to confine sulfur [100], where the void space between the yolk and shell was able to physically buffer the volume changes during the charge–discharge process, and the TiO2 shell could chemically anchor the dissolved LiPSs. Therefore, this yolk–shell sulfur cathode exhibited the high initial capacity of 1030 mAh g−1 and the low capacity decay of 0.033% cycle−1 after 10 0 0 cycles. Unlike TiO2 , Magnéli-phase tetratitanium heptoxide (Ti4 O7 ) presented superior electronic conductivity, and could be directly used to host sulfur. Furthermore, the chemical interactions between the polar O–Ti–O moieties of Ti4 O7 and LiPS species and the insoluble Li2 S/Li2 S2 ones were strong [103,104]. The capacity and cycling stability of the Ti4 O7 –S composite cathode were higher than those of the TiO2 –S cathode. As for the microstructure design, interconnected porous Ti4 O7 particles were synthesized, and their surface area was high (592 m2 g−1 ) [105]. The porous Ti4 O7 effectively suppressed the loss of LiPSs via physical encapsulation and strong chemical binding effects. In another study, Magnéli Ti4 O7 microspheres with interconnected mesopores were prepared using the carbothermal reduction method [106], and then molten sulfur was infiltrated into the mesopores. The active Ti4 O7 cathode contained 70 wt% sulfur, delivered high capacity and presented low

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capacity fading (< 1%) over 250 cycles at 0.2 C. Given the conductivity and chemical adsorption features of Ti4 O7 , an interlayer of C@Ti4 O7 composite nanofibers was developed to restrict the diffusion of LiPSs [107]. When this special interlayer was placed between the cathode and separator it ensured the high initial capacity of 1046 mAh g−1 and low capacity decay of 0.09% cycle−1 over 500 cycles at 2 C. The conductivity of titanium monoxide (TiO) is also good and TiO also presented more effectively binding interactions with sulfur than TiO2 [108]. TiO@carbon hollow spheres (TiO@C–HS) (Fig. 8c) were prepared to host sulfur and effectively confine LiPSs, and the spheres effectively promoted the kinetics of the redox reactions of different sulfur species [101]. Manganese dioxide (MnO2 ) has been widely used for confining sulfur. A highly ordered mesoporous β -MnO2 (HMM) was synthesized using the KIT-6 template [109]. The mesoporous HMM could accommodate the sulfur species and further prevent their diffusion. Nazar et al. [110] wrapped sulfur particles with a MnO2 shell, using the redox reaction between KMnO4 and sulfur. The formed nanosized MnO2 shell interacted both physically and chemically with LiPSs and effectively trapped them, which enabled the low capacity decay of 0.039% cycle−1 over 1700 cycles. In addition, MnO2 was used to wrap N-doped hollow porous carbon spheres (NHPC@MnO2 ) [111]. The NHPC@MnO2 /S cathode integrated the chemical adsorption effects of both N-doped carbon and MnO2 nanosheets for LiPSs and presented good rate capability and cycling stability even at the sulfur content of 70 wt%. Based on the self-assembly feature of GO, a 3D porous rGO/MnO2 -S cathode was prepared using the hydrothermal treatment method [112]. The MnO2 nanosheets stacked with rGO sheets, and sulfur nanoparticles were dispersed on the composite matrix. The composite effectively inhibited the diffusion of LiPSs owing to its network structure and chemical adsorption of LiPSs by MnO2 . Moreover, GO and the MnO2 nanoparticles were coated on a CNT film to prepare an ultrathin G M@CNT−1 interlayer [113]. The interlayer placed between the cathode and separator achieved the synergetic effect of GO and MnO2 for chemically binding LiPSs, which enabled the low capacity decay of 0.029% cycle−1 for 2500 cycles at 1 C. In addition, zinc oxide (ZnO) was used as a sulfur anchoring host for Li–S batteries. For example, ZnO was ball-milled with S CNT−1 to prepare a ZnO@S/CNT cathode [114], which exhibited better cycling stability than the pure S CNT−1 cathode owing to the strong chemical adsorption of polysulfides by ZnO. Moreover, ZnO or magnesium oxide (MgO) layers can be coated on graphene aerogel–sulfur composites (G–Ss) using the atomic layer deposition (ALD) method [115]. The oxide coating could effectively anchor the dissolved LiPSs and prevent their diffusion. Therefore, the oxide/G–S cathode exhibited superior cyclability and rate performance when used for Li–S batteries. Furthermore, brush-like ZnO nanowires were loaded onto a porous CNF mat and were used as an interlayer for Li–S batteries [116]. When used together with an S MWCNT−1 composite cathode, the ZnO/C interlayer ensured longterm cycling stability and a low capacity loss of 0.05% cycle−1 for 200 cycles at 1 C. Carbon-coated tin oxide (SnO2 @C) hollow nanospheres were prepared using SiO2 as a template [117], following which sulfur was impregnated into the spheres to obtain the S SnO2 @C−1 cathode. The double layers of SnO2 @C greatly suppressed the shuttling of LiPSs by forming S–Sn–O and S–C bonds, which greatly improved the electrochemical performance of Li–S batteries. Qian et al. [118] introduced SnO2 or SnS2 into S C−1 composites using the baked-in-salt or sealed-in-vessel method, and the obtained SnS2 /S/C cathode provided stronger binding interactions with polysulfides than the SnO2 /S/C one. Moreover, SnO2 nanoparticles were deposited on graphene to prepare an SnO2 @rGO interlayer on a PP separator [119]. Strong chemical adsorption was achieved between SnO2 and LiPSs by forming Li–O/Sn–S/O–S bonds, and

therefore, the SnO2 @rGO interlayer effectively prevented the shuttling of the dissolved LiPSs, and exhibited increased cycling stability to Li–S batteries. Aluminum oxide (Al2 O3 ) nanoparticles have been added to a sulfur cathode to confine the diffusion of LiPSs; this effectively promoted the redox reactions of the sulfur species and induced increased discharge capacity and cycle stability [120]. Moreover, Al2 O3 is often used as a surface coating to improve the electrochemical performance of Li–S batteries. Shi et al. [121] coated a thin Al2 O3 layer on G–S composites via ALD. The Al2 O3 coating acted as both a physical barrier and electrostatic adsorption layer to hinder the shuttling of the dissolved LiPSs. In another study, a Nafion/γ -Al2 O3 mixture was coated on the surface of a sulfur– carbon cathode to form a uniform membrane [122]. The ionselective Nafion film prevented the diffusion of long-chain polysulfides and the γ -Al2 O3 layer effectively adsorbed LiPSs. Therefore, the bifunctional Nafion/γ -Al2 O3 membrane suppressed the shuttle effect and decreased the loss of sulfur species. Moreover, a thin Al2 O3 layer was coated on activated carbon cloth (ACC) via ALD to be used as a functional interlayer [123], which was placed between the cathode and separator to collect the polysulfide intermediates and prevent their diffusion into the anode region. The Al2 O3 –ACC interlayer greatly increased the discharge capacity and cycling performance of Li–S batteries. Furthermore, the Al2 O3 -coated separator was used as an ion-conducting layer for intercepting the dissolved LiPSs and effectively enhanced the cell performance [124]. Tricobalt tetraoxide (Co3 O4 ) has been used to host sulfur in Li– S batteries as well. Hollow Co3 O4 nanotubes were prepared and were impregnated with sulfur [125], and effectively restricted the diffusion of the dissolved LiPSs. In another study, Co3 O4 nanoneedles were wrapped on carbon cloth (CC@Co3 O4 ), and the obtained material was employed as “super-reservoir” for Li–S batteries [126]. Long-chain dissolved LiPSs could be adsorbed on the surface of the Co3 O4 nanoneedles and then changed into solid Li2 S/Li2 S2 . The chemical adsorption capability and catalytic role of Co3 O4 conferred Li–S batteries the low capacity decay of 0.049% cycle−1 for 500 cycles at 2 C. Furthermore, vanadium pentoxide (V2 O5 ) has been confirmed to exhibit strong chemical binding interaction with long-chain LiPS species [127]. V2 O5 was coated on one side of a commercial separator to trap polysulfide anions [128]. The V2 O5 -coated separator delivered excellent cycling performance in a Li–S battery with carbon–sulfur cathode, and no noticeable decrease in the capacity of the sulfur cathode was observed after 300 cycles. Furthermore, a V2 O5 -decorated carbon nanofiber (VCNF) interlayer was developed for Li–S batteries [129]. The VCNF interlayer effectively anchored LiPSs via strong chemical binding. This suppressed the shuttling of the dissolved LiPSs and achieved excellent cycling stability to the cathode, its capacity retention being 70.6% for 10 0 0 cycles at 3 C. 2.5. Sulfides Owing to metal sulfides being strong chemical adsorption agents for LiPSs, they have been widely reported as efficient sulfur hosts or interlayer materials for Li–S batteries. Various cobalt sulfides (Cox Sy ), including CoS2 , Co3 S4 , and Co9 S8 , have been reported to present excellent conductivity, ranging from 6.7 × 105 to 1.36 S m−1 [130], and have been synthesized and used to prevent the diffusion of LiPSs in Li–S batteries. In a pioneering study, Zhang et al. [131] added CoS2 to a carbon/sulfur cathode. CoS2 not only exhibited strong chemical adsorption capability for LiPSs, but also accelerated the redox reactions of sulfur and improved the energy efficiency of the battery by 10%. Consequently, the fabricated Li–S battery achieved good cycling stability up to 20 0 0 cycles, with a capacity decay of 0.034% cycle−1 at 2 C. Numerous studies focused on CoS2 -based

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Fig. 9. (a) Schematic illustration of preparation of S@CNTs/Co3 S4 –NBs [134]. Copyright 2017, American Chemical Society. (b) Synthesis process of Co9 S8 /C–S nanopolyhedra [137]. Copyright 2017, Elsevier. (c) Fabrication process of nickel sulfide dispersed on 3D carbon hollow spheres (NiS@C–HS) [142]. Copyright 2017, Wiley-VCH. (d) MoS2 /Celgard separator for Li–S batteries [146]. Copyright 2017, Wiley-VCH.

sulfur hosts and interlayers for Li–S batteries, including the CoS2 embedded N-doped carbon polyhedrons (CoS2 –NC) host [132], 3D porous graphene framework/CoS2 (CoS2 /rGO) host [80], and hierarchically porous CoS2 decorated hydrophilic carbon paper (CoS2 /CP) interlayer [133]. The strong sulfur immobilization and electrocatalytic interaction of CoS2 effectively suppressed the shuttling of LiPSs, and conferred superior long-term cycling performance to Li–S batteries. Co3 S4 presents a metallic nature, and its conductivity is 3.3 × 103 S cm−1 . Pu et al. [130] prepared conductive Co3 S4 nanotubes for storing sulfur in their inner spaces. The special nanotube structure of Co3 S4 efficiently adsorbed the dissolved polysulfides and catalyzed sulfur conversion. The Co3 S4 @S cathode exhibited a high capacity of 1267 mAh g−1 at 0.05 C and the capacity decay of 0.041% cycle−1 for 10 0 0 cycles. Based on the ZIF67 MOF framework, hollow Co3 S4 nanoboxes inserted into CNTs (CNTs/Co3 S4 –NBs, Fig. 9a) were employed as a sulfur host [134]. The interconnected CNT network improved the conductivity, and the hollow Co3 S4 nanoboxes immobilized the LiPSs via physical

confinement and chemical bonding. The S@CNTs/Co3 S4 –NBs cathode delivered high capacity, rate performance, and cycling stability even at 50 °C. Pang et al. [135] synthesized an interconnected graphene-like Co9 S8 with high specific surface area and used it as S host for Li–S batteries. Their results demonstrated the strong dual interaction between Co9 S8 and polysulfides, including the Li+ → Sδ − and Sn 2− → Coδ + processes. Dai et al. [136] prepared hollow polar Co9 S8 tubes as sulfur host material. The hollow structure of the Co9 S8 tubes effectively accommodated the volume change of sulfur and prevented the diffusion of the dissolved LiPSs owing to strong chemisorption reactions. Furthermore, using experimental data and theoretical calculations, it was concluded that the electrochemical reaction kinetics was accelerated by Co9 S8 , which acted as catalyst. Therefore, the S@Co9 S8 cathode exhibited stable cycling over 600 cycles at 1 C and the low capacity decay of 0.026% cycle−1 . Chen et al. [137] synthesized a polar Co9 S8 /carbon hollow nanopolyhedra (Co9 S8 /C, Fig. 9b) as sulfur host using the ZIF-67 MOF.

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Combining the spatial confinement, chemical binding, and catalytic activity of Co9 S8 , the Co9 S8 /C-S cathode exhibited the low capacity decay of 0.041% cycle−1 over 10 0 0 cycles at 2.0 C. Recently, He et al. [138] reported a MOF-derived Co9 S8 array dispersed on 3D graphene foam as binder-free sulfur host for Li–S batteries. The porous 3D graphene foam enabled the high sulfur content of 86.9 wt% (10.4 mg cm−2 ) and facilitated the transport of electrons, while the Co9 S8 array formed strong chemical bonds with the LiPSs. Therefore, the reversible capacity, rate capability, and cycling stability of the Li–S batteries were increased significantly. Using the superior conductivity and catalytic activity of Cox Sy , sulfur–cobalt sulfide (CoS2 and Co9 S8 ) nanocomposites were also synthesized as sulfur hosts to enhance the performance of Li–S batteries [139]. Similarly to Cox Sy , NiS2 also presents high conductivity (55 S cm−1 ). In addition, NiS2 also exhibits excellent sulfur anchoring properties. Lu et al. [140] reported an S NiS2 –C−1 composite cathode for Li–S batteries. The computational results demonstrated the strong affinity between LiPSs and NiS2 . The S NiS2 –C-1 cathode, which contained 54.9 wt% sulfur exhibited the specific capacities of 730 and 544 mAh g−1 after 200 cycles at 0.5 C and after 500 cycles at 2 C, respectively. Liu et al. [141] used amorphous NiS2 (a-NiS2 ) as sulfur host material, where a-NiS2 could effectively adsorb the dissolved long-chain LiPSs and could accommodate the insoluble sulfur during the charging–discharging process. The S@aNiS2 cathode delivered the high initial capacity of 1540 mAh g−1 (0.1 A g−1 ), and 77% of this initial capacity was retained after 1200 cycles (0.5 A g−1 ). Nanosized NiS dispersed on 3D carbon hollow spheres (NiS@C–HS, Fig. 9c) was also prepared for hosting sulfur [142]. These C–HSs provided physical confinement for LiPSs, while NiS presented strong adsorption capability for LiPSs and facilitated the kinetics of the sulfur species redox reactions. The hybrid host presented the low capacity decay of 0.013% cycle−1 and final capacity of 695 mAh g−1 for 300 cycles at 0.5 C. As sulfur host material, molybdenum sulfide (MoS2 ) was used to prepare a sulfur copolymer composite by polymerizing liquid S8 on MoS2 sheets [143]. For this special cathode, MoS2 served as polysulfides anchoring sites to alleviate the dissolution of polysulfide intermediates. The composite cathode exhibited a high capacity of 500 mAh g−1 even at 5 C, and long-term cycling stability, its low capacity decay being 0.07% cycle−1 for 10 0 0 cycles. In another study, MoS2 -coated and N-doped mesoporous carbon spheres (NMCS@MoS2 ) were employed as sulfur host, along with a carbon nanotubes/chitosan (CNTs/CH)-modified separator to improve the performance of Li–S batteries [144]. The polar MoS2 accelerated the reaction kinetics and provided strong chemical adsorption capability for LiPSs. The CNTs/CH-modified separator improved the ion transfer and cycling stability of the Li–S battery, which delivered the low capacity decay of 0.08% cycle−1 over 500 cycles. In addition, amorphous MoS3 acted as “sulfur equivalent” cathode material [145], which was analogous to organic sulfides, and exhibited sulfur-like electrochemical behavior during the charging–discharging process, but without generating LiPSs. The MoS3 -based battery delivered stable cycling performance, and the use of MoS3 instead of the traditional problematic S8 provided a new strategy for manufacturing high-performance Li–S batteries. In the field of functionalized separators, a MoS2 membrane was coated on a Celgard separator to prevent the loss of LiPSs (Fig. 9d) [146]. The MoS2 -modified separator efficiently depressed the shuttle effect of LiPSs by preventing the migration of the dissolved LiPSs to the anode, without affecting the transport of the Li+ ions, and consequently conferred good cycling stability to Li–S batteries. A lightly reduced graphene oxide@MoS2 interlayer was also fabricated as polysulfide barrier for Li–S batteries [147]. Combining the chemical adsorption of LiPSs by MoS2 with the catalytic activity of MoS2 for the redox reactions of LiPSs, this interlayer

conferred superior electrochemical performance to Li–S batteries, which achieved the low capacity decay of 0.116% for 500 cycles at 1 C. Furthermore, vanadium disulfide (VS2 ) presents metallic features, which could be used to suppress LiPS shuttling and accelerate the kinetics of the sulfur conversion reaction in Li–S batteries [100,148,149]. A sandwiched composite comprising repeating rGOVS2 sheets and pure sulfur layers was fabricated as cathode for Li–S batteries [148]. This sandwich-type structure accommodated the volume expansion and provided surface and space for Li2 S deposition. Moreover, the sulfur immobilization and electrocatalytic effect of VS2 effectively enhanced the electrochemical performance of Li–S batteries. Even at the high sulfur content of 89 wt%, the rGO-VS2 /S cathode still delivered the high volumetric capacity of 1182.1 mAh cm−3 , which was much higher than that of sandwichstructured carbon/sulfur composites. Moreover, VS2 /reduced graphene oxide nanosheets (G-VS2 ) were prepared via an in situ assembling route, and were used as sulfur host [149]. The 2D VS2 nanosheets provided abundant adsorption and catalytic sites for LiPSs to accelerate the sulfur redox conversion process. The G-VS2 /S cathode delivered high rate capability and stable cycling performance, and its capacity was 532 mAh g−1 after 300 cycles. A dual-function tungsten disulfide (WS2 ) cathode has been reported, which consisted of a layered WS2 -containing cathode current collector and WS2 -containing carbon cloth interlayer (WS2 /CCl) [150]. The sulfophilic WS2 in the carbon cloth effectively trapped the dissolved LiPSs, and the WS2 particles also promoted the conversion of long-chain LiPSs into lower-order LiPSs. The dual-function WS2 /S-WS2 /CCl delivered superior cycling stability, its capacity being 10 0 0 mAh g−1 after 500 cycles at 0.5 C, and good rate capability. Tin sulfide (SnS2 ) has also been reported as sulfur host for Li– S batteries. Li et al. [151] generated SnS2 particles inside hollow carbon nanospheres and then used them as sulfur host. Moreover, LiPSs were immobilized by the SnS2 particles via chemical adsorption. The hybrid sulfur cathode which contained 10 wt% SnS2 delivered the high capacity of 1237.5 mAh g−1 at 0.2 C, and its capacity remained 924 mAh g−1 after 200 cycles. Qian et al. [118] fabricated SnS2 /S/C and SnO2 /S/C cathodes by adding polar SnS2 or SnO2 to a sulfur/carbon composite and compared the performances of the obtained batteries. The SnS2 /S/C cathode exhibited lower charge transfer resistance and stronger binding effect than the SnO2 /S/C one, which facilitated the redox reaction of the adsorbed LiPSs, and therefore the SnS2 /S/C cathode presented higher reversible capacity and cycling stability. Pyrite (FeS2 ) was reported to be an efficient LiPS adsorbent for Li–S batteries [152]. During the discharging process, a FeS2 – Li2 Sn complex (Li2 FeS2+ n ) was produced by forming S–S covalent bonds, and the process could be described as follows: FeS2 + Li2 Sn → Li2 FeS2+ n . Therefore, FeS2 presented the exceptional ability to chemically adsorb Li2 Sn . This provided a new strategy for trapping LiPSs. Copper sulfide (CuS) was prepared by mechanically milling Cu crystals and sulfur [153]. The CuS and sulfur combination served as a cathode for assembling all-solid-state Li–S batteries with solid Li2 S–P2 S5 electrolytes. The active sulfur and CuS cathode achieved a discharge capacity of 650 mAh g−1 over 20 cycles. Moreover, hierarchical CuS nanoneedles were grown on an MWCNT backbone, then sulfur was impregnated into CuS@CNT and employed as cathode for Li–S batteries [154]. Owing to the sulfurimmobilization role of CuS and excellent conductivity of CNTs, the S@CuS@CNT cathode exhibited high and reversible capacity and cycling performance. Considering the general role of metal sulfides as sulfur hosts, Chen et al. [155] explored the rational design principles of sulfur cathodes by analyzing first-row transition metal sulfides (TMSs), including ScS, TiS, VS, CrS, MnS, FeS, CoS, NiS, CuS, and ZnS. Their

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Fig. 10. (a) Schematic illustration of synthesis of cobalt hydroxide/layered double hydroxide/sulfur (CH@LDH/S) composite [157]. Copyright 2016, Wiley-VCH. (b) Fabrication of sulfur@nickel hydroxide (S@Ni(OH)2 ) double-shell hollow composite spheres [161]. Copyright 2017, Elsevier. (c) Fabrication process of sulfur@carbon black@nickel nitrate hydroxide (S8 @CB@NNH) hybrids [162]. Copyright 2015, Nature Publishing Group.

DFT calculations indicated that VS presented the highest binding energy of all these TMSs, and exhibited the strongest anchoring effects for Li2 S immobilization as well as a low diffusion barrier of Li+ ions. Furthermore, strong sulfur binding was induced between the transition metal and sulfur atoms, which was preferred to Li binding. Zhou et al. [156] investigated the polysulfide adsorption capabilities of various metal sulfides, including VS2 , TiS2 , CoS2 , FeS, Ni3 S2 , and SnS2 . Their results indicated that the Li2 S decomposition energy barrier was related to the binding between the sulfur in sulfides and isolated Li+ ions. The sulfides induced lower overpotential than carbon materials. By contrast, the VS2 -, TiS2 -, and CoS2 -based cathodes presented lower diffusion and activation energy barriers and higher binding energies, which could effectively improve the capacity and cycling stability of Li–S batteries. 2.6. Hydroxides Recently, two-dimensional (2D) layered double hydroxides (LDHs) have been used as sulfur hosts and interlayer materials for Li–S batteries. Zhang et al. [157] developed a doubleshelled cobalt hydroxide/layered double hydroxide (Co(OH)2 /LDH, CH@LDH) nanocage as sulfur host (Fig. 10a). The hollow polyhedra with LDH shell featured high sulfur content (75 wt%), and also suppressed the outward diffusion of LiPSs by chemically bonding them onto the LDH surface. Therefore, the CH@LDH/S cathode delivered superior electrochemical performance. Moreover, a NiFe-LDH less

than 5 nm thick was grown on mesoporous N-doped graphene (NG) and was used as “sulfiphilic” component [158]. The LDH@NG layer was coated on the separator to confine the shuttling of the soluble LiPSs. The sulfiphilic LDH provided strong chemisorption for LiPSs owing to its functional M–O(H) groups. The N O−1 doping sites on the NG substrate also bound to the Li+ ions of LiPSs. The dual function of the sulfiphilic LDHs and “lithiophilic” NG efficiently inhibited the shuttling of LiPSs and promoted the reaction kinetics. The Li–S battery that used this LDH@NG separator achieved the low capacity decay of 0.060% cycle−1 for 10 0 0 cycles at 2 C, and its final capacity remained 337 mAh g−1 . In addition to LDHs, hydroxides have also been used as coating layers to prevent the migration of LiPSs. Niu et al. [159] used a Co(OH)2 nanosheets-coated sulfur/conductive carbon black (Co(OH)2 @S/CCB) cathode for Li–S batteries. As coating stabilizer, the Co(OH)2 layer prevented the shuttling of LiPSs and the Co(OH)2 @S/CCB presented superior capacity and cycling stability than the S CCB−1 one. Furthermore, a nickel hydroxide-modified sulfur/conductive carbon black composite (Ni(OH)2 @S/CCB) was reported as cathode material [160]. Small Ni(OH)2 nanoparticles (1–2 nm) modified the S CCB−1 surface, which further improved the capacity and cycling performance of Li–S batteries by preventing the shuttling of LiPSs. As efficient LiPS adsorbent, α -Ni(OH)2 hollow spheres constructed from ultrathin nanosheets (Fig. 10b) were synthesized and used as hollow sulfur nanosphere host [161]. The hollow structure achieved the high sulfur loading of 81 wt%.

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Furthermore, the functionalized surface of the α -Ni(OH)2 nanosheets effectively trapped LiPSs in the hollow spheres via chemical bonds. The double-shell S@Ni(OH)2 structure achieved the low capacity decay of ∼0.04% cycle−1 for 10 0 0 cycles at 1 C. To prevent the loss of soluble LiPS intermediates, Jian et al. [162] prepared a thin-layered α -Ni(OH)2 wrapped S8 @carbon black cathode (S8 @CB) (Fig. 10c). In this core–shell hybrid, the abundant polar groups on the thin hydroxide layers irreversibly reacted with Li to form protective layers. This greatly improved the long-term cycling stability of the Li–S battery, which presented negligible capacity decay over 500 cycles. 2.7. MXenes and carbides 2.7.1. MXenes The molecular structure of MXenes is Mn +1 AXn , where M is an early transition metal, A is an element from group 13 or 14 of the periodic table, and X is carbon or nitrogen [163–165]. MXenes include a large family of 2D carbide materials, which can be prepared by extracting the A element from layered ternary carbides, such as Ti3 AlC2 and other MAX phases. Exfoliated 2D MXene nanosheets are commonly hydrophilic owing to their abundant surface functional groups, such as –OH, –O, or –F. Moreover, MXenes also present high conductivity which ranges from 60 0 0 to 80 0 0 S cm−1 . Similarly to graphene or graphite, various MXenes have been widely reported as promising sulfur hosts or functionalized interlayers for Li–S batteries. In 2015, Liang et al. [166] reported that the “acid” Ti sites on exfoliated MXenes strongly anchored polysulfide intermediates by forming chemical bonds (Fig. 11a). The initially adsorbed LiPSs were transformed into Li2 S, which then acted as nucleation sites for the further deposition of sulfur. The excellent sulfur anchoring effect of the Ti2 C host that contained 70 wt% sulfur delivered superior cycling performance, and its capacity retention was 80% after 400 cycles at 0.5 C. Additionally, the surface reactivity of the Ti3 C2 and Ti3 CN MXene phases with polysulfides species was reported [167]. The –OH groups on the MXene surface reacted with polysulfides and produced thiosulfates by forming strong Ti–S bonds. MXenes trapped the dissolved LiPSs using a double mechanism, which included the Lewis acid-base interaction and thiosulfate/polythionate conversion. Zhao et al. [168] prepared a layered Ti3 C2 (L–Ti3 C2 ) material with accordionlike structure and 57.6 wt% sulfur loading as cathode for Li–S batteries. The S L–Ti3 C2 −1 cathode delivered the initial capacity of 1291 mAh g−1 , and the retained capacity of 970 mAh g−1 after 100 cycles at 200 mA g−1 . Bao et al. [169] prepared a 3D metal carbide@mesoporous carbon hybrid (Ti3 C2 Tx @Meso-C) as sulfur host by carbonizing Ti3 C2 Xx @MOF-5. The hierarchical porous structure of Meso-C physically confined sulfur, and the hydrophilic surface of Ti3 C2 Tx could adsorb LiPSs. The Ti3 C2 Tx @Meso-C/S cathode delivered the initial capacity of 1225.8 mAh g−1 , which remained 704.6 mAh g−1 after 300 cycles at 0.5 C. In addition, S@Ti3 C2 Tx viscous ink (Fig. 11b) was also used to prepare a binder-free conductive film cathode using the facile slurry-casting or filtration method [170]. The Ti3 C2 Tx chemically adsorbed the dissolved LiPSs and converted them into thiosulfates/sulfates. In addition, Ti atoms were exposed and efficiently anchored the LiPSs during long-term cycling, which improved the reversible capacity and cycling performance of Li–S batteries. Therefore, the S@Ti3 C2 Tx film cathode, which contained 70 wt% sulfur, delivered the high capacity of 1244 mAh g−1 and presented superior cycling performance (capacity decay of 0.048% cycle−1 for 800 cycles). Recently, Wu et al. [171] reported an “all-MXene-based” Li–S battery, which consisted of a 3D alkalized Ti3 C2 MXene nanoribbon/S cathode (a-Ti3 C2 -S) and PP separator coated with delaminated Ti3 C2 (d-Ti3 C2 ) nanosheets, as presented in Fig. 11(c and d).

The macroporous framework of the a-Ti3 C2 host facilitated the ion diffusion and high sulfur loading, and the d-Ti3 C2 interlayer inhibited the shuttling of LiPSs via physical blockage and chemical adsorption. This all-MXene-based cathode delivered high rate capability, and the high capacity of 288 mAh g−1 even at 10 C. Furthermore, Lin et al. [172] prepared a glass fiber separator covered by a few-layered Ti3 C2 nanosheets using vacuum filtration. The Ti3 C2 nanosheets effectively suppressed the shuttle effect by strongly interacting with LiPSs. Moreover, the high conductivity of Ti3 C2 also reduced the internal resistance of the cathode. Therefore, this unique separator was used to obtain a traditional sulfur cathode (70 wt% sulfur loading) with the high discharge capacity of 820 mAh g−1 , and the capacity remained 721 mAh g−1 after 100 cycles (0.5 A g−1 ). 2.7.2. Carbides Unlike 2D layered MXenes, various metal carbides exhibit high conductivities and polar features, which allow them to chemically anchor LiPSs owing to their unique surface reactivity and special electron distributions. Zhou et al. [173] developed titanium carbide nanoparticles (TiC NPs)-decorated CNFs as sulfur host. The conductive CNFs provided pathways for electron transfer, and the TiC NPs strongly bonded with the S8 molecule. Consequently, the S TiC–CNFs−1 cathode delivered superior cycling stability and rate performance. Salem et al. [174] synthesized nanostructured tungsten monocarbide (WC) and TiC as sulfur hosts, and these cathodes improved the cycle performance and rate capability of Li– S batteries. In addition, DFT calculations demonstrated that metal carbides strongly interacted with Li2 S8 . Furthermore, TiC and WC promoted the redox processes of the polysulfide intermediates by chemically adsorbing the high-order LiPSs and reversibly binding the low-order LiPSs. The S–TiC cathode achieved the stable capacity of 860 mAh g−1 for 100 cycles. Zhou et al. [175] synthesized ditungsten carbide (W2 C), molybdenum carbide (Mo2 C), and TiC NPs-decorated CNFs (MC NPsCNFs) and compared their abilities to improve the electrochemical performance of sulfur cathodes. The MC NPs also exhibited electrocatalytic effect and promoted the electrochemical redox reactions of different LiPSs. Among the three cathodes, the W2 C NPs-CNFs host delivered the highest capacity of 1200 mAh g−1 at 0.2 C and excellent long-term cycling stability. The conductivity of niobium carbide (NbC) is high (2.9 × 106 S m−1 ), and its polar molecular structure enabled strong chemical interactions with polysulfides anions. Therefore, an NbC coated separator [176] was prepared and used as interlayer to improve the performance of Li–S batteries. The NbC separator successfully prevented the shuttling of LiPSs and Li anode corrosion, which enabled the sulfur/carbon cathode to achieve stable cycling performance and the capacity decay of 0.037% cycle−1 for 1500 cycles at 2 C. For the functionalized separator, a heterostructure of graphene and TiC (G–TiC) was coated on the separator to confine the LiPSs [177]. The bound interface between graphene and TiC reduced the diffusion barrier of Li+ ions and electrons. The highly conductive TiC also presented strong affinity to LiPSs, suppressed the shuttle effect, and improved the interface behavior. Therefore, the G–TiC separator greatly increased the capacity and rate capability of Li–S batteries, and presented superior performance compared with the coating layers that consisted of doped graphene and other carbon–noncarbon hybrids. 3. Electrocatalytic strategy in Li–S batteries 3.1. Electrocatalysis mechanism and corresponding materials During the reversible redox reaction of active sulfur species, the loss of dissolved long-chain LiPSs would lead to the shuttling

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Fig. 11. (a) Replacement of Ti–OH bond on MXene surface with S–Ti–C bond via heat treatment or contact with polysulfides [166]. Copyright 2015, Wiley-VCH. (b) Schematic illustration of preparation of S@Ti3 C2 Tx ink and its viscous consistency [170]. Copyright 2018, Wiley-VCH. Schematic illustration of preparation of (c) integrated alkalized Ti3 C2 nanoribbon/delaminated Ti3 C2 /polypropylene (a-Ti3 C2 –S/d-Ti3 C2 /PP) electrode for Li–S batteries and (d) d-Ti3 C2 nanosheets and a-Ti3 C2 –S hybrid [171]. Copyright 2018, American Chemical Society.

effect, which would deteriorate the long-term cycling performance of Li–S batteries. Moreover, the deposited solid Li2 S would not participate in the reversible cycle of S8 , and the sluggish reaction kinetics would also affect the reversible conversion from S8 to Li2 S, which would decrease the conversion efficiency of sulfur species and compromise the electrochemical performance of Li–S batteries [178]. To solve the reversible conversion of S8 to Li2 S and corresponding sluggish reaction kinetics concerns, the electrocatalysis concept has been introduced for Li–S batteries [179–181]. The primary purpose of electrocatalysis is to achieve the rapid conversion of LiPS intermediates by reducing the irreversible deposition of solid Li2 S and ensure the reversibility of the entire reactions by adding suitable electrocatalysts to the cathode hosts or separators/interlayers. Recently, various electrocatalytic materials have been reported to improve the electrochemical performance of Li–S batteries

[2,6,182–184]. The current electrocatalytic materials could be classified into three categories based on their different electrocatalytic mechanisms. The first type of electrocatalytic materials could promote the kinetics of the polysulfide intermediates reactions by forming chemical bonds or surface defects, which would reduce the deposition of solid Li2 S on the electrode surface. Most polar sulfur hosts, including g-C3 N4 , metal particles, metal oxides, hydroxides, sulfides, nitrides, and carbides, belong to this category. The second type of electrocatalytic materials could enhance the LiPS redox process by forming S2 O3 2– on the host surface, and the S2 O3 2– groups, could then effectively anchor the LiPS intermediates and mediate the redox reaction of sulfur species. This category of electrocatalytic materials includes some transition metal oxides, such as δ -MnO2 , VO2 , CuO, and V2 O5 . The third type of electrocatalytic materials can accelerate the lithiation/delithiation reaction and increase the diffusion rate of Li+ ions. To date, Te-doped sulfur

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(Te–S) [185] and Se-doped sulfurized polyacrylonitrile (Sex SPAN) [186] have been reported to present such functions. In the field of Li–S batteries electrolytes, lithium thiophosphate (Li3 PS4 ) has been reported to be a redox mediator generator and presented catalytic effect on reducing the first charge overpotential of Li2 S, which prolonged the cycling life of the battery [187]. In this section, we summarize the recent advances on these three types of electrocatalytic materials. 3.1.1. Polar sulfur hosts Carbon nitride and phosphorene: Polymeric carbon nitride (pC3 N4 ) presents sheet-like structure with abundant N atoms and has been validated to present strong electrostatic affinity for LiPSs [79,188], which could effectively overcome the kinetic barrier of LiPS redox reactions by modifying the molecular structure of LiPSs. Moreover, p-C3 N4 decreased the kinetic impedance and promoted the kinetics of the LiPS redox reaction. Therefore, the p-C3 N4 /graphene host exhibited superior performance than pure graphene for Li–S batteries. Furthermore, framework porphyrin (POF) was synthesized to improve the kinetics of the LiPS conversion reaction [189]. Phosphorene is a kind of thin nanosheet similar to graphene, where each P atom bonds with three neighboring atoms and forms a puckered honeycomb structure [190], which was reported as efficient electrocatalyst and LiPS immobilizer for Li–S batteries [191]. Exfoliated few-layer phosphorene (FLP) nanosheets were loaded on a CNF network and used as host for the dissolved Li2 S6 catholyte. Owing to FLP strong affinity for LiPSs and its superior electrical conductivity (450 S m–1 ), the polarization of the internal chemical reaction was reduced, and the redox reaction between LiPS species was accelerated, which further enhanced the sulfur utilization and cycling performance (over 500 cycles) of the Li–S battery. Metal particles, oxides, and hydroxides: Recently, Pt, Au, or Ni nanoparticles were loaded on an Al foil and employed as electrocatalytic current collector to improve the redox kinetics and cycling performance of Li–S batteries [179]. The special current collector also served as electrode in a “metal/polysulfides/metal” battery, which enabled the Li–S battery featuring Li2 S8 catholyte to achieve negligible capacity decay over 100 cycles. To further improve their catalytic activity, Pt and Ni nanocatalysts were loaded on graphene to host LiPSs (Fig. 12a) [180]. These metallic nanocatalysts facilitated the transformation of insoluble Li2 S2 /Li2 S into long-chain LiPSs via redox processes, which prevented the deposition of Li2 S2 /Li2 S on the electrode and promoted the reaction kinetics. As sulfur host, graphene coated with metal nanocatalysts achieved higher capacity (140%) than pure graphene, along with the higher Coulombic efficiency of 99.3% at 0.2 C and stable cycling for 100 cycles. Co also strongly interacts with Li2 S [192]. Li2 S can be deposited on Co with a lower decomposition barrier. The catalytic activity of Co particles promoted the decomposition of Li2 S and reduced the polarization. Consequently, the S Co@GC–PC−1 cathode delivered the high capacity of 790 mAh g−1 after 220 cycles at 0.2 C, which was much higher than that of the carbon-based cathode. In addition to Co particles, Co atoms in nitrogen-doped graphene (Co–N/G) have also been reported as electrocatalysts to trigger the surface-mediated reaction of LiPSs [193]. Furthermore, a nanostructured Li2 S cathode containing a single-Fe atom as catalyst also exhibited catalytic activity for enhancing the electrochemical conversion reactions in Li–S batteries [194]. In addition to metal nanoparticles, some metal oxides and hydroxides were also adopted as electrocatalytic materials for improving the cycling performance of Li–S batteries. Molybdenum dioxide (MoO2 ) presents the high conductivity of 190 S cm−1 . Mesoporous MoO2 was prepared as sulfur hosts to effectively encapsulate sulfur via the S–O bonds that formed between the oxygenated MoO2 and LiPSs [196]. Owing to the fast reaction

kinetics induced by the mesoporous MoO2 hosts, the MoO2 /S cathode presented excellent cycling performance for LI–S batteries. Nb2 O5 nanocrystals were grown on a mesoporous carbon framework (Fig. 12b) and were used as effective electrocatalyst for Li– S batteries [195]. The Nb2 O5 nanocrystals accelerated the kinetics of the polysulfides species redox reactions, decreased the active energy of the sulfur lithiation reactions, and thus, promoted the reduction of dissolved Li2 S6 /Li2 S4 to insoluble Li2 S2 /Li2 S. The niobium pentoxide (Nb2 O5 )-containing cathode exhibited high reversible capacity, low polarization, and superior rate capability for Li–S batteries. Furthermore, cerium oxide (CeO2 ) nanocrystals were implanted on micro/mesoporous N-rich carbon nanospheres (CeO2 /MMNC), and were used as sulfur host [197]. The polar CeO2 nanocrystals exhibited efficient electrocatalytic activity for promoting the redox reactions of LiPS species. Furthermore, the Ndoped carbon spheres and 3D porous nanochannels interacted both chemically and physically with LiPSs. The CeO2 /MMNC-S cathode presented high capacity and stable long-term cycling performance. The retained capacity was 836 and 721 mAh g−1 after 500 cycles at 1 C and after 10 0 0 cycles at 2 C, respectively. Vanadium dioxide (VO2 ) nanobelts were employed as host materials as well as catalytic additive for Li–S batteries [198]. As typical electrocatalyst, VO2 effectively promoted the kinetics of the sulfur species redox reactions during the charging–discharging process. Therefore, the graphene–sulfur cathode that contained few additives (∼4 wt% VO2 ) delivered the high discharge capacity of 1405 mAh g−1 at 0.2 C and presented good cycling stability for 200 cycles. Based on the concept of “electrocatalysis on separator”, ruthenium dioxide nanoparticle-decorated mesoporous carbon (RuO2 −MPC) was coated on the separator to improve the kinetics of the LiPS redox reactions [199]. The conductive RuO2 nanoparticles accelerated the redox reaction of LiPSs and effectively inhibited the active sulfur migration, owing to their high adsorption capability and superior trapping features. The RuO2 −MPC separator conferred the sulfur cathode the high discharge capacity of 1276 mAh g−1 at 0.1 C and stable cycling performance over 300 cycles. Furthermore, a lamellar α -MoO3 nanobelt, as catalyst for the redox reaction of LiPS species, was used to coat the separator [181]. When used as separator in Li–S batteries, MoO3 conferred them the high discharge capacity of 1377 mAh g−1 and low capacity decay of 0.251% cycle−1 for 200 cycles at 0.5 C. Peng et al. [158] researched metal hydroxides and reported a novel concept that integrated lithiophilic nitrogendoped graphene (NG) and sulfiphilic NiFe-LDH to improve the electrochemical performance of Li–S batteries (Fig. 12c). The LDH@NG coated separator strongly bound to LiPSs, which further accelerated the nucleation of Li2 S and improved the kinetics of the sulfur species redox conversion. When used in a carbon nanotubes/sulfur cathode, the LDH@NG coated separator achieved the low capacity decay rate of 0.060% over 10 0 0 cycles, which was superior to that obtained using the traditional PP separator. Metal sulfides and nitrides: Cobalt disulfide (CoS2 ) strongly interacts with LiPSs. When used as catalytic additive for the carbon/sulfur cathode [131], CoS2 effectively accelerated the redox reactions of the liquid LiPS intermediates (Fig. 13a and b), and weakened the deposition of insoluble Li2 S2 /Li2 S. Therefore, the CoS2 -containing carbon/sulfur cathode achieved the high capacity of 1368 mAh g–1 at 0.5 C and stable cycling performance up to 20 0 0 cycles. Moreover, CNT-reinforced hollow cobalt CoS nanostraws were prepared as freestanding and flexible sulfur host [200]. The CoS nanostraws presented efficient electrocatalytic effect and accelerated the redox reaction of sulfur species. Furthermore, the special hollow structure also provided enough space for loading sulfur. The hybrid structure also conferred high conductivity, strong chemisorption, and capillary effect for the soluble LiPSs. In addition to CoS2 and CoS, the MoS2− x /reduced graphene oxide

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Fig. 12. (a) Pt nanoparticles loaded on graphene to improve the kinetics of sulfur species redox reactions [180]. Copyright 2015, American Chemical Society. (b) Niobium pentoxide (Nb2 O5 ) nanocrystals loaded on mesoporous carbon framework as sulfur host [195]. Copyright 2016, Royal Society of Chemistry. (c) Cooperative interface of sulfiphilic NiFe layered hydroxide (LDH) coated on nitrogen-doped graphene (LDH@NG) accelerated adsorption and redox reactions of polysulfides by forming both ∗ Li and ∗ S species [158]. Copyright 2016, Wiley-VCH.

(MoS2− x /rGO) composite was also developed to catalyze the redox reaction of LiPSs (Fig. 13c) [201]. The sulfur deficiency of MoS2− x promoted the kinetics of the LiPS conversion. The fast conversion of sulfur species reduced the accumulation of insoluble Li2 S2 /Li2 S, and confined the diffusion and shuttling of soluble LiPSs. Therefore, only 4 wt% MoS2− x /rGO in the sulfur cathode could lead to the stable cycling performance of the Li–S battery, which featured the capacity decay of 0.083% cycle−1 for 600 cycles at 0.5 C. Moreover, vanadium disulfide (VS2 )/graphene composites have been reported as efficient electrocatalysts for Li–S batteries [148,149]. The high conductivity and polarity of VS2 facilitated the transformation of sulfur species and accelerated the kinetics of different LiPS redox reactions. Other sulfides, such as Co9 S8 [135], TiS2 [202,182], WS2 [150] and NiS [142], also presented similar electrocatalytic features when used for Li–S batteries. These sulfides have been employed as catalysts to promote the reversible transformation between Li2 S and sulfur. Among the sulfide-based cathodes reported in the literature, VS2 -, TiS2 -, and CoS2 -based cathodes were the most effective for increasing the capacity and cycling stability of Li–S

batteries (Fig. 13d and e), which has been demonstrated by using both experimental data and theoretical calculations [156]. In addition, an ultra-strong bonds exists between titanium nitride (TiN) surfaces and sulfur, which led to TiN surfaces presenting catalytic activity for the conversion of LiPSs as well as a significant anchoring effect. Therefore, TiN could be used as a catalyst for Li–S batteries [88]. Moreover, TiN could also help overcome the sluggish kinetics of the LiPS chemical disproportionation reaction. Consequently, TiN catalysts greatly improved the capacity and rate capability of Li–S batteries, and the discharge capacity even reached 700 mAh g−1 at 5 C. In addition to TiN, highly conductive InN [95], VN [90–92], WN, Mo2 N [94], and Ni3 FeN [6] present similar electrocatalytic activities and have also been used to accelerate the LiPS conversion reaction. Metal carbides and other metal compounds: The surface of WC is sulfiphilic, and WC could be used as effective catalyst to improve the electrode reaction kinetics by modulating the LiPS fragmentation [203]. As presented in Fig. 14(a), the long-chain LiPSs could be chemically anchored on the surface of WC. During the

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Fig. 13. (a) and (b) Schematic of CoS2 particles for improving the redox kinetics of sulfur species [141]. Copyright 2016, American Chemical Society. (c) Sulfur-deficient MoS2− x electrocatalyst/reduced graphene oxide (MoS2− x /rGO) host material [201]. Copyright 2017, Royal Society of Chemistry. (d,e) Sulfur conversion principle and Li2 S catalytic oxidation on substrate surface [156]. Copyright 2017, National Academy of Science.

electrochemical reduction of LiPSs, WC promoted the chemical disproportionation of the short-chain LiPSs and thus accelerated their dissociation. Therefore, the cathode that contained WC additives presented superior reversible capacity and rate capability, which resulted in the high discharge capacity of 780 mAh g−1 even at 5 C. Moreover, W2 C nanoparticles were decorated on CNFs and were used as sulfur host [175]. Cyclic voltammetry (CV) tests indicated that the electrocatalytic role of W2 C accelerated the redox reactions of different LiPSs, which conferred high rate performance and long-term cycling stability to Li–S batteries. Furthermore, WC/W2 C, TiC [174] and NbC [176] have also been reported to catalyze Li–S battery reactions. Moreover, iron phosphide (FeP) was reported to be an efficient electrocatalyst for Li–S batteries as well [205]. The FeP nanocrystals loaded onto a rGO–CNT framework could not only chemically bind the LiPSs by forming Li–P and Fe–S bonds, but could also effectively decrease the Li2 S nucleation energy by providing abundant adsorptive interfaces (Fig. 14c), which comprehensively promoted the nucleation and growth of solid Li2 S and facilitated the redox reactions between the long-chain LiPSs and Li2 S. Owing to the

electrocatalytic activity of the FeP nanocrystals, the FeP/rGO/CNT-S cathode presented a high capacity, and rate performance, and the ultra-low capacity decay of 0.04% cycle−1 for (400 cycles, at 1 C). Similarly to FeP, molybdenum phosphide (MoP) nanoparticles have also been confirmed to present the electrocatalytic effect in sulfur cathodes, which improved the kinetics of the LiPS conversion, even at a low electrolyte/active material ratios [184]. Huang et al. [206] were the first to report the electrocatalytic role of cobalt phthalocyanine (CoPc) for improving the electrochemical performance of Li–S batteries. The electrocatalytic mechanism of CoPc was attributed to the formation of the unique Co-N4 unit, which effectively anchored the long-chain soluble LiPSs and enhanced their reduction to insoluble Li2 S2 /Li2 S (Fig. 14d). The sulfur cathode that contained 3.8 wt% CoPc exhibited the capacity of 719.6 mAh g−1 after 400 cycles at 0.2 C, which was much higher than that of the control material. In addition, Fe–N–C molecular catalysts, which were based on iron(II) phthalocyanine [207], were synthesized and added to mesoporous carbon microspheres to serve as sulfur hosts. The small amount of 0.33 wt% Fe–N–C in the host effectively promoted the redox kinetics of sulfur species.

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Fig. 14. (a) Schematic illustration of tungsten carbide (WC) promoting the redox kinetics of lithium polysulfides (LiPSs) [203]. Copyright 2018, American Chemical Society. (b) Adsorbance of soluble long-chain Li2 Sn by dual-functional electrode materials [204]. Copyright 2018, American Chemical Society. (c) Polysulfides immobilization and regulation principle using iron phosphide (FeP) nanocrystals [205]. Copyright 2018, Elsevier. (d) Li2 S catalytic oxidation on substrate surface [206]. Copyright 2018, Royal Society of Chemistry.

Polyoxometalates (POMs) are well-defined anionic cluster frameworks connected by shared O atoms [208,209]. Recently, nanostructured POMs (Ag(I)-substituted Keggin K3 H3 AgI PW11 O39 ) were reported as a bifunctional Lewis acid and base catalyst (Fig. 14b). In addition to strongly adsorbing the sulfur moieties of LiPSs, Ag-POM also facilitated the redox reversibility of LiPSs and sulfur, which further increased sulfur utilization [204]. Therefore, the AgPW11 /S cathode delivered a high initial capacity of 1580 mAh g−1 and retained a capacity of 810 mAh g−1 after 300 cycles at 2 C. In addition, ultra-light magnesium diboride (MgB2 ) was reported as an effective catalyst for mediating the redox reaction of LiPSs by bonding polysulfide anions on the B and Mg surfaces [183]. 3.1.2. Some transition metal oxides Some transition metal oxides, including δ -manganese dioxide (δ -MnO2 ), vanadium dioxide/vanadium pentoxide (VO2 /V2 O5 ), and copper oxide (CuO), could produce S2 O3 2– groups on the host

surface to promote the redox kinetics of sulfur species in Li–S batteries. For example, δ -MnO2 nanosheets have been employed to enhance the LiPS redox by mediating the redox reaction of sulfur species [210–212]. Mn4+ ions could react with Sx 2– anions and form S2 O3 2– groups on the surface, which would further connect with long-chain Sx 2– (x > 4) species to produce (O3 S2 -(S)x − 2 -S2 O3 )2– . Consequently, LiPS intermediates could be anchored (Fig. 15a–c), and that could provide the efficient interface for the deposition of Li2 S2 /Li2 S [210]. Moreover, the resultant S2 O3 2– mediator also promoted stable redox activity during the charging–discharging process. Therefore, the S@δ -MnO2 cathode achieved long-term stable cycling up to 20 0 0 cycles. Other transition metal oxides, including VO2 , CuO, and V2 O5 , presented similar catalytic function to δ -MnO2 . Liang et al. [211] verified that the formation of S2 O3 2– groups was relative to the redox potentials of the metal oxides. The potentials of VO2 and CuO ranged between 2.4 and 3.05 V. If the redox potential windows of soluble LiPSs lay in that range, the oxide would promote the formation of

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Fig. 15. (a) Transmission electron microscopy and (b) scanning electron microscopy images of sulfur/δ –manganese dioxide (S/δ –MnO2 ) composite. (c) Schematic of oxidation of initially formed polysulfide using δ -MnO2 , which formed thiosulfate groups on the cathode surface [210]. Copyright 2015, Nature Publishing Group. (d) Forming mechanism of sulfate complex protective barrier on Ti3 C2 Tx [170]. Copyright 2018, Wiley-VCH. (e) Schematic illustration of possible Li migration pathways in Te–Li2 S, energy profiles for Li migration, and corresponding promotion mechanism [185]. Copyright 2018, American Chemical Society.

polythionate. However, the redox potential of V2 O5 is high (3.5 V), which would lead to the formation of inactive sulfate species that block the surface of the cathode. Therefore, to achieve the best catalytic features for Li–S batteries, tuning the potential down to the suitable voltage window is essential.

reaction kinetics owing to the increase in the diffusion coefficient of the Li+ ions, and decrease in the internal polarization [186].

3.1.3. Te- or Se-doped sulfur Te-doped sulfur (Te–S) increased the electrical conductivity of sulfur and induced low formation energy values during the lithiation process (Fig. 15e), which reduced the diffusion energy barrier of the Li+ ions and promoted the lithiation/delithiation reactions. Therefore, the kinetics of the Li–S redox processes was promoted using Te–S as cathode [185]. Compared with the sulfur/Ketjenblack (S/KB) cathode, the Te-3-S/KB cathode that contained 3 wt% Te exhibited superior rate capability, a capacity of 579 mAh g−1 at 20 A g−1 (compared with only 287 mAh g−1 for the S KB−1 cathode), long-term cycling stability, and a low capacity decay of 0.026% cycle−1 over 400 cycles at 5 A g−1 . Therefore, compared with the aforementioned polar hosts and transition metal oxides, Te-doping provided a new feasible strategy to accelerate the redox kinetics of Li–S batteries. Furthermore, Sex SPAN presented a similar electrocatalytic mechanism when used as a cathode for Li–S batteries, and Se-doping resulted in accelerated

Electrochemical characterization is essential to verify the electrocatalytic role of functional hosts. The electrocatalytic effect of catalysts at the cathode can be detected using the galvanostatic charge–discharge curves of Li–S cells. A long low-voltage plateau would reflect the nucleation and successive growth of Li2 S, which implies smaller polarization and charging barriers [213]. Furthermore, the potentiostatic discharge method [189] has been used to monitor the kinetics of Li2 S nucleation from liquid LiPSs. An early occurrence of the Li2 S nucleation peak in the current-time profile would indicate a reduced overpotential and the distinct electrocatalytic role of the engaged electrocatalyst, which would result in accelerated kinetics for Li2 S nucleation and high capacity values during long-term cycling. Moreover, Zhou et al. [156] adopted the classical Randles–Sevcik equation to describe the Li diffusion process. The adsorption features of LiPSs and catalyzing conversion capability of Li2 S could be obtained by analyzing the CV curves at different potential scan rates.

3.2. Electrocatalytic characterization

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Fig. 16. (a) Polarization curves and (b) electrochemical impedance spectra of symmetrical Li2 S6 –Li2 S6 cells [148]. Copyright 2018, Wiley-VCH.

In addition to describing the electrocatalytic behavior in the typical Li–S cell, symmetrical cells, which were assembled using two identical sulfur hosts and soluble catholyte (no Li anode) were also used to evaluate the electrocatalysis role. Herein, we describe the preparation and electrochemical analysis of a symmetrical cell. During the preparation of the symmetrical cell, pristine host materials are coated on the current collector (Al foil) to prepare sulfur-free electrode disks. The symmetrical cell did not include a Li anode, only two identical sulfur host electrodes, which were separated using a traditional PP separator, and Li2 S6 as catholyte was added to the two electrodes. Both CV and electrochemical impedance spectra (EIS) tests were necessary to elucidate the electrocatalytic features of the host materials. Considering rGO-VS2 and rGO as examples of host materials [148], the electrochemical performances of the symmetrical cells is illustrated in Fig. 16(a). The current density of the rGO-VS2 cell was much higher than that of the rGO one, which confirmed that the electrochemical reaction of LiPSs was accelerated in the rGO-VS2 cell [131]. In addition, the diameter of the semicircles in the EIS Nyquist plots in Fig. 16(b) reflected the charge transfer resistance. The diameter of the rGO-VS2 composite was smaller than that of rGO electrode, which indicated that faster charge transfer occurred at the rGO-VS2 /LiPS interface than that at the rGO/LiPS one. Comparing the CV and EIS curves, it was concluded that the electrochemical reaction at the rGO-VS2 /S cathode occurred faster. In addition to electrochemical characterization, electrocatalytic interactions could also be assessed using the mutual transformation of different polysulfide species. The ex situ analysis of the microstructure and composition of host materials is critical for evaluating the transformation of sulfur species. The catalytic effect of sulfur hosts could be assessed by observing the layer of deposited insoluble Li2 S product [148], particularly after long-term cycling tests. Moreover, some in situ observation techniques including Xray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, and computational simulation methods could be used to verify the electrocatalytic mechanism and conversion process of sulfur species. 4. Challenges, perspectives, and summary

of active hosts species, sluggish reaction kinetics of sulfur species, and unclear electrocatalytic mechanism. These unsolved problems pose great challenges and prospects for the further improvement of the performance of Li–S batteries. 4.1.1. Difficulty in rational selection of functional species The shuttling effect and irreversible redox reaction of sulfur species led to the capacity decay of Li–S batteries. To some extent, the selection of suitable sulfur host materials is the main strategy for overcoming this problem. Compared with the physical confinement of sulfur, chemical adsorption presents some advantages by inhibiting the diffusion of LiPSs via strong chemical bonding. However, while a wide range of chemical adsorption materials is available, from polymers to inorganic compounds and metal compounds, there are still no guiding principles for selecting the most suitable ones. Therefore, the performance improvement of Li– S batteries is a difficult task from the host materials selection perspective. 4.1.2. Sluggish reaction kinetics of sulfur species The electrochemical performance of Li–S batteries is determined by the reversible redox reaction of sulfur species. During the complicated lithiation–delithiation process, the sluggish reaction kinetics would affect the conversion rate of sulfur species, particularly the irreversible deposition of solid Li2 S, which could often increase the internal resistance and polarization of the electrode and result in fast capacity decay. Therefore, achieving rapid reaction kinetics is an important task in Li–S battery research. 4.1.3. Unclear electrocatalytic mechanisms The chemical interaction between the host material and LiPS species is complicated and involves the formation of chemical bonds and chemical adsorption processes. For metal compounds in particular, both chemical anchoring and electrocatalytic interaction occur, and therefore, it is very difficult to confirm that sulfur confinement was achieved via chemical adsorption or electrocatalytic interaction. Moreover, the existing electrocatalytic mechanisms have not been elucidated yet. Therefore, theories on the chemical confinement of sulfur and electrocatalytic mechanism should be further explored.

4.1. Challenges 4.2. Perspectives Recently, great progress has been achieved for sulfur host materials, separator modifiers, and interlayers via anchoring the LiPS and accelerating their conversion. Thus, the capacity and long-term cycling performance of Li–S batteries have been increased significantly. However, several concerns still remain for these Li–S battery materials [214], such as the difficulty in the rational selection

These challenges of Li–S batteries can be regarded as potential research directions for future studies. Therefore, we propose the following strategies, which might provide deeper insights and pathways to promote the rapid development of sulfur host materials for Li–S batteries.

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4.2.1. Selection and structure design of host materials To chemically prevent the shuttling of soluble LiPSs, ideal host materials, separator modifiers, and interlayers should integrate the physical and chemical confinement along with electrocatalytic interactions, which could achieve the synergistic effect of multiple functions. Some metal oxides, nitrides, and sulfides should be considered preferentially as host materials, owing to their superior conductivity, chemical anchoring effect, and electrocatalytic properties. The structure design of target materials is also important to effectively confine the LiPSs, and this mainly involves the design of the conducting carbon matrix and morphology control of the dispersed phase.

4.2.2. Accelerating the kinetics of sulfur conversion reaction The sluggish kinetics of the sulfur conversion reaction could induce the deposition of insulating Li2 S2 /Li2 S on the cathode active surface, which would increase the overpotential and internal resistance of the cell. Consequently, the Coulombic efficiency and reversible capacity of the cell would be reduced. To overcome the sluggish redox kinetics, finding suitable electrocatalytic materials should be the first task. Thus far, transition metal oxides, sulfides, nitrides, and carbides have served as excellent electrocatalysts for Li–S batteries, which could accelerate the redox kinetics of sulfur species via electrocatalytic interaction.

4.2.3. Disclosing the mechanism of chemical anchoring and electrocatalytic interaction Recently, abundant chemical anchoring and electrocatalytic materials have been developed to immobilize LiPSs and improve the kinetics of redox processed in Li–S batteries. However, owing to the insufficient studies of the Li–S battery chemistry mechanism, various chemical interactions have only been attributed to the formation of chemical bonds or chemical adsorption processes. So far, only three types of mechanisms have been proposed for the electrocatalytic interaction. Therefore, more in-depth understanding of the chemical anchoring and electrocatalytic interaction should be sought by combining electrochemical tests with advanced in situ characterization methods, such as XRD, XPS, Raman spectroscopy, and microscopy techniques.

4.3. Summary In conclusion, some Li–S batteries technologies have significantly progressed recently. The deeper understanding of the Li–S batteries chemistry could provide future research direction on the chemical anchoring theory and electrocatalytic mechanism. Moreover, the development of advanced electrochemical tests, in situ characterization techniques, and computational materials science procedures should help overcome the current theory barriers. The selection and design of sulfur hosts should be essential for improving the sulfur loading and electrochemical performance of Li–S batteries. Some lightweight, free-standing porous carbon-based materials should be considered as conducting matrix, and could preferentially introduce some polar materials to achieve the combination of physical and chemical confinement, and electrocatalysis.

Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51403094), Program of Liaoning Education Department of China (No. LJ2017FBL002), and Australian Research Council through the Discovery Early Career Researcher Award (DECRA, No. DE170100871) Program.

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