One dimensional nanostructures contribute better Li–S and Li–Se batteries: Progress, challenges and perspectives

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One dimensional nanostructures contribute better Li–S and Li–Se batteries: Progress, challenges and perspectives Xingxing Gu a, **, Chao Lai b, * a Engineering Research Center for Waste Oil Recovery Technology and Equipment, Ministry of Education; Chongqing Key Laboratory of Catalysis and New Environmental Materials, College of Environment and Resources, Chongqing Technology and Business University, Chongqing 400067, China b School of Chemistry and Materials Science, Jiangsu Normal University, Xuzhou 221116, China

A R T I C L E I N F O

A B S T R A C T

Keywords: One dimensional Nanostructure Li–S battery Li–Se battery

State-of-the-art rechargeable lithium-ion batteries (LIBs) are approaching their energy densities boundary, but these LIBs still couldn’t meet present energy storage requirements for advanced portable, transportation and residential applications. Lithium-sulfur (Li–S) and lithium-selenium (Li–Se) batteries have attracted tremendous attentions due to their high energy (power) densities in the past ten years. Besides, Li–S and Li–Se batteries are cost-effective and environmentally friendly. However, the commercialization of Li–S and Li–Se batteries is hindered by the low conductivity of S (Se), low utilization and easy dissolution of polysulfides (polyselenides), etc. Recently, one dimensional (1D) nanomaterials, due to their unique advantages, i.e. short lithium ions and electrons transport pathways, good flexibility, has been widely employed in improving the cycling performances and energy densities of Li–S and Li–Se batteries. In this review, we first briefly summarized the synthesis methodologies of 1D nanomaterials, then systematically summarized how 1D nanomaterials modifing the S/Se cathodes “inside” and “outside” to enhance the electrochemical performances, as well as the 1D nanomaterials applications in protecting lithium anode or 1D nanomaterials used as lithium metal free anode for Li–S batteries have also been concluded shortly. What’s more, how to use the 1D nanomaterials to constructe the novel cell configurations (i.e., current collectors, interlayers, separators) for building better Li–S and Li–Se batteries have been systematically and comprehensively summaried as well. Finally, the bottlenecks for Li–S and Li–Se batteries as well as the future development of Li–S and Li–Se batteries are proposed in the conclusions and perspectives.

1. Introduction Since the successful commercialization of Li–ion batteries by Sony Corporation in 1990s, the lithium intercalation batteries have dominated the electronic devices market for more than three decades due to relatively high energy densities (150-250 Wh∙kg
1), high operating potentials and long cycle life [1,2]. However, such monopoly is gone forever. With increasing demands of modern electric devices, i.e. electric vehicles, and the large-scale power grid storage, rechargeable batteries with higher energy densities and lower costs are urgently needed to be commercialized [3,4]. Of them, lithium-sulfur (Li–S) batteries based on reversible redox reactions of element sulfur (i.e., 16Li þ S8 ↔ 8Li2S) can deliver a high theoretical specific capacity of 1675 mAh∙g
1 with a moderate potential of 2.2 V vs. Li/Liþ, yielding an ultra-high gravimetric energy density of

2600 Wh∙kg
1 (almost 6 times higher than that of LiCoO2–graphite battery) and volumetric energy density of 2800 Wh∙L
1 [5,6]. Additionally, element S is cost-effective, non-toxic and abundant in nature [7]. These intriguing advantages have stimulated world-wide focus on Li–S batteries. As for lithium-selenium (Li–Se) batteries, it has similar redox reactions as S (i.e., 16Li þ Se8 ↔ 8Li2Se) [8]. Although its specific capacity (675 mAh∙g
1) is lower than that of Li–S batteries, its volumetric energy density (2600 Wh∙L
1) could be comparable with Li–S batteries [1,9]. Moreover element selenium exhibits higher electrical conductivity (1  10
3 S∙m
1) than that of S (5  10
28 S∙m
1) [8]. These advantages make Se become a prospective cathode material as well for rechargeable lithium batteries. However, the Li–S and Li–Se batteries are still confronted with great challenges as shown in Fig. 1 for their practical applications. The use of nanomaterials for Li–S and Li–Se batteries to enhance the specific

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Gu), [email protected] (C. Lai). https://doi.org/10.1016/j.ensm.2019.05.013 Received 5 March 2019; Received in revised form 9 May 2019; Accepted 9 May 2019 Available online xxxx 2405-8297/© 2019 Published by Elsevier B.V.

Please cite this article as: X. Gu, C. Lai, One dimensional nanostructures contribute better Li–S and Li–Se batteries: Progress, challenges and perspectives, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2019.05.013

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Fig. 1. The challenges of Li–S and Li–Se batteries as well as the corresponding resolution strategies by using the 1D nanostructures in Li–S and Li–Se batteries.

capacity, rate capacity, energy density, cycle life, safety performance, etc., has been the trend in recent years [10]. 1D nanomaterials made from carbon, metal oxide, polymers, ceramics and their composites have been widely utilized for Li–S and Li–Se batteries [10–17]. Firstly, compared with zero dimensional, two dimensional (2D), and even three dimensional (3D) nanomaterials, 1D nanomaterials can provide fast pathways for electrons transport and ions diffusion; secondly, 1D nanomaterials with porous or hollow structures offer more active sites via sufficient contact area between the active materials and the electrolyte; and lastly, they can be easily grown on thin films to form self-standing flexible energy storage devices [18]. Based on the above backgrounds, we systematically summarized the 1D nanomaterials applications in Li–S batteries from the electrodes host for sulfur/Li2S cathode and lithium metal anode, to optimization of cell configurations including 1D carbon-based current collectors, 1D nanostructured interlayers, and 1D nanomaterials-based separators, as shown in Fig. 1. In the case of emerging Li–Se batteries, we briefly summarized the unique 1D Se nanowires (NWs) and nanofibers cathode, the 1D carbon-based nanomaterials (e.g. carbon nanotubes (CNTs), carbon nanofibers (CNFs) and CNT/CNF-based composites) as Se host and Se cathode interlayers. Finally, we conclude the progress and main challenges, along with some future prospects, for the Li–S and Li–Se batteries.

nanostructures, the key point is how to control the dimensions, morphology and uniformity delicately and simultaneously [19]. Here we briefly summarized these methods’ advantages, disadvantages and characteristics.

2. Approaches to construct 1D nanomaterials

The so-called liquid phase growth method is using the hydrothermal or solvothermal techniques, which use the water or organic solvent as the reaction media [22]. When change the reaction conditions, such as temperature increasing, a monomer (i.e., ions or molecules) with a certain concentration will begin aggregation to small clusters through fast and homogenous nucleation [19,22]. Then the clusters serve as seeds for further growth to form a larger 1D nanostructures. The types and ratio

2.1. Vapor phase growth method The vapor phase method is the most popular one for the construction of 1D nanostructures such as nanotubes, nanowires. This method was recognized to synthesize any solid material for 1D nanostructures by controlling the supersaturation at a certain level [19]. Thus, the degree of supersaturation determines the morphology better or not for the 1D nanomaterials. In a typical vapor process, the vapor species are first generated by evaporation, then experiencing chemical reduction or other gaseous reactions, followed by transporting and condensing onto the surface of a substrate at a lower temperature [20]. Even the vapor-phase method has the advantages of simplicity and accessibility [21], it usually requires expensive equipment and the reaction parameters should be controlled accurately in order to obtain the good morphology and reproducible 1D nanomaterials. 2.2. Liquid phase growth method

As the 1D carbon nanomaterials have various advantages mentioned in the introduction, several synthesis strategies, e.g., vapor phase growth, liquid phase growth, electrospinning, electrochemical deposition, template assistance, etching and polymerization, etc., for 1D nanomaterials have been developed as shown in Fig. 2. To synthesize the 1D 2

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Fig. 2. (a) Vapor phase growth methods; (b) template-assisted method; (c) electrospinning method; (d) liquid growth method with capping molecules; (e) electrochemical deposition or electrochemical polymerization method; (f) chemical etching method. Reproduced with the permission from Ref. [19], Copyright 2014, the Royal Society of Chemistry.

mass-productive, controllable, economic and environmentally friendly [25], as well as this method is suitable for fabricated complicated or hybrid 1D nanostructures like electrospinning method. And the drawbacks are similar to the electrospinning method.

of the solvents, temperature change, etc., will result in the failure growth of 1D nanostructures. During liquid phase growth, the self-assembly is a common phenomenon that is induced by the electrostatic interaction, intermolecular interaction, and van der Waals interaction between the nanostructures and substrate [19]. Although the capability and feasibility of nanoscale self-assembly have been demonstrated to a certain extent, there is still a distance to completely self-assembled 0D nanostructures (or molecules) to 1D nanostructures in the liquid phase. Moreover, the yield and purity of the products are usually not high [22].

2.5. Template assistance/chemical etaching method Template assistance method is a straightforward route for the construction of 1D nanostructures. There are two categories of “hard” and “soft” template. During the growth process, the object nanomaterial grows into or around the template and then futher form the 1D nanostructure with a morphology complementary to the template. A large number of templates have been prepared and successfully demonstrated including step edges of a solid substrate surface [26], channels within a porous material [27], biological materials such as crab shells [28]. In addition, the physical templates could be post-treated by chemical etching, which is known as the chemical etaching method. In a chemical etaching process, the template is removed and the well-defined nanostructure could be obtained too. Template-assisted synthesis is a simple, high-throughput and costeffective process. But usually the resulted nanomaterials are polycrystalline and the quantity is relatively limmted. What’s more, if the wanted 1D nanostructures should remove the template by chemical etaching, the operation process should be controlled subtly, which means the process may become complex and the cost may incrase as well.

2.3. Electrospinning method Electrospinning is broadly employed to electrostatically and continuously fabricate fibres of 1D polymers, 1D polymer-inorganic composites and the polymer derived CNFs etc., with diameters ranging from few nanometers to several microns [19,23]. The fabrication process provides unique and flexible capabilities for the continuous production of nanofibers/microfibers with controllable composition and pore structure. Commonly, an electrospinning will provide a high voltage to produce a repulsive force to let the charged polymer solution erupt from the tip of the spinneret. By adjusting the solution viscosity, conductivity, applied voltage, spinneret tip-to-collector distance and humidity, the diameter of the 1D nanostructures can be accurately controlled [23]. It’s easy to set-up the electrospinning machine, and the process is flexibility and consistency. In addition, the syntheszied 1D polymerbased nanostructures own high surface are, tunable porosity, etc. [24], but there is still a big problem for the electrospinning method, which the fabricating process needs long time and the 1D nanomaterials are hard scale-up.

3. 1D nanomaterials applications in Li–S batteries It is widely accepted that there are several aspects affecting the electrochemical performance of Li–S batteries [7]: i) the low electric and Liþ-ionic conductivity of elemental sulfur and discharged end-product (Li2S and Li2S2); ii) polysulfides easily dissolve in the organic electrolyte accompanying with the shuttle phenomena of polysulfides; iii) the low utility efficiency of S and the volume expansion/contraction of active materials; iv) severe lithium dendrites growth. To design Li–S batteries with excellent electrochemical performance, a common approach is to mix the sulfur with various host, including carbon hosts [29–32], polymer host [15,33], inorganic host [34,35], etc. In addition, other methods such as protecting anode [36,37], inserting interlayers [38], modifying current collectors and separators [39,40], can also effectively inhibit the shuttle phenomena of polysulfides and lithium dendrites growth, thus

2.4. Electrochemical deposition method Electrochemical deposition method is coating a metal, oxide or salt on the surface of a conductive substrate through simple electrolysis of a solution containing the desired metal ion or its chemical complex [22]. Various 1D nanostructures (i.e., nanorods, nanotubes and nanoneedles) could be obtained by controlling the electrochemical conditions including the current density, electrodeposition potential and electrolyte composition. The advantages of electrochemical deposition method is simple, 3

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increased to 150  C, the liquefied sulfur was easily infiltrated into CNT. Jin et al. [46], prepared a S-MWCNTs composite by the direct precipitation method, which the sulfur particle was deposited on MWCNT in the Na2S2O3 solution with adding the H2SO4 reductant. Even the sulfur content in this S-MWCNTs composites could reach as high as 80 wt%, the sulfur mainly deposited on the surface of MWCNT. Thus, the cycling performance of S-MWCNT was only given the first 30 cycles. Lai et al., reported how the ball-milling speed and solvents used in ball-milling to affect the morphologies and electrochemical performances of the S/CNT composites [53]. The results showed that with using the ethanol, the morphology of S/CNT composites became loose compared to dry-milling or using the CHCl3. And the S/CNT composites obtained from adding ethanol solvents exhibited the best performances. In addition, the results also showed the high ball-milling speed (1500 rpm) resulted in low sulfur utilization. In addition, combination of two strategies of solution precipitation with melt-diffusion or ball-milling with melt-diffusion is also popular to prepare the core-shell S-CNT composites [54–58]. Apart from the deposition of sulfur on CNT surfaces or using the meltdiffusion strategy to encapsulate sulfur into CNT. A new strategy of vapor infiltration and sulphate carbothermal reduction has been reported. Rachel et al. [59], combined structurally designed hierarchical CNT arrays with site-selective vapor phase sulfur infiltration, to produce thick electrodes with controllable sulfur loading and high areal performance. Compared the melt infiltration strategy, the vapor infiltration of sulfur into CNT could enhance the utilization and areal capacity of active materials in electrode. Cheng’s group applied the sulphate-anodized aluminum (AAO) as the template, taking advantage of the chemical vapor deposition (CVD) to synthesize CNT along with reducing sulfate [14]. Finally, AAO template was removed to obtain the 1D core-shell S-CNT cathode. The whole synthesis process and materials nanostructures have illustrated in Fig. 3a-b.

efficiently improving the cycle stability of Li–S batteries. Hence, in this section, the contents are divided into five categories: 1D nanomaterials host, 1D nanomaterials for anode and anode protection, 1D current collectors, 1D nanomaterials interlayers, 1D nanomaterials for separators and separators modification. 3.1. 1D carbon nanostructures as sulfur hosts 1D carbon nanostructures with the advantages of large surface area, light weight, good electrical conductivity, and high thermal/chemical stability, have been considered as the most important electrode materials of rechargeable batteries [6]. And 1D carbon nanomaterials with a high aspect ratio and good mechanical strength can better construct an electrical conducting network in the electrode to improve the reaction kinetics and rate capability of the sulfur cathode as well as tolerate the volumetric changes during cycling process [6]. 3.1.1. CNT Due to its outstanding electrical conductivities (102–106 S∙cm
1), extremely large aspect ratio (up to 1.3  108) and good mechanical/ chemical stability [2]. 1D CNTs are the most widely employed to build blocks for Li–S batteries. Additionally, its unique wired morphology easily forms cross-linked conducting networks and serve as the conducting phase, thus the CNT was first used as a conductive additive material for sulfur positive electrodes [41]. Following the CNTs has been widely utilized as the sulfur host in Li–S batteries [42]. Early studies were focused on the 1D core-shell sulfur-coated CNT nanostructures. Commonly there are several methods, i.e. melt-diffusion [43–45], solution precipitation [46–48], ball milling [49–52], to synthesis such a structure. As early as 2012, Hagen has applied the melt-diffusion in sulfur incorporation [43]. When the temperature

Fig. 3. (a) Scheme showing the thermal decomposition of C2H2 in a sulphate-containing AAO template and the formation of S-CNTs after AAO removal. (b) SEM images showing the top view of a sulphate containing AAO template, side views of S-CNTs/AAO and S-CNTs. Reproduced with the permission from Ref. [14], Copyright 2012, the Royal Society of Chemistry. (c) Schematic of the process of the formation of HMMCNT and HMMCNT-S materials. Reproduced with the permission from Ref. [30], Copyright 2016, WILEY-VCH. (d) Schematic diagram for preparation of M-CNTP. (e) Cross-sectional SEM image of a cracked M-CNTP. Inset of (c) shows a magnified SEM image of the cracked M-CNTP (scale bar: 1 μm). Reproduced with the permission from Ref. [60], Copyright 2018, American Chemical Society.

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large-scale mass production, tunable porosity with a surface area of 20–2500 m2∙g
1 and adaptable electrical conductivities from 10
7–103 S∙cm
1, making them ideal candidates as sulfur containers [2]. However, like CNT, CNF was initially used as a conductive additive in the sulfur electrode in order to improve the cycling property [80,81]. Since then various kinds of CNFs, such as porous CNFs [82–86], hollow CNFs [29,87,88], interwoven CNFs [89–92], etc., have been widely applied as the sulfur scaffolds. An ordered mesoporous CNF (OMCF) has been applied as the sulfur host by Guo et al. [85].This OMCF was prepared via the electrospinning method by using resol as the carbon source, copolymer as the template and the mesopores was yielded from removing the silica as shown in Fig. 4a. After incorporating sulfur, the resulting OMCF-S still maintain mesopores (shown in Fig. 4b), which are beneficial for electrolytes infiltration and accommodating volume expansion, accordingly this electrode maintained a stable discharge capacity of 690 mAh∙g
1 at 0.3 C, even after 300 cycles. Later Zheng et al. [93], applied the electrospinning to synthesize another kind of porous CNF (PCNF), where both the pore volume and surface area were tailored by optimizing the sacrificial agent content and the activation temperature (shown in Fig. 4c). At an activation temperature of 550  C, the PCNF showed abundant porous structure (Fig. 4d), which could accommodate 71 wt% sulfur and the resulted PCNF/A550/S demonstrated a high capacity of 945 mAh∙g
1 at 1 C. Recently Kim and his co-workers reported a interwoven CNFs as the sulfur host, which the CNF was also prepared by electrospinning (Fig. 4e-f) [89]. The interwoven CNF-S complex with a high mass loading of 10.5 mg∙cm
2 can deliver a high areal capacity of above 7 mAh∙cm
2. Cui’s group reported a hollow CNF (shown in Fig. 4g) for encapsulating element sulfur, for which this hollow CNF was fabricated using AAO templates, through thermal carbonization of polystyrene [27]. Since the small dimension of these nanofibers provided a large surface area per unit mass for Li2S deposition during cycling and the hollow structure reduced pulverization of electrode materials, a high specific capacity of about 730 mAh∙g
1 (shown in Fig. 4h) was observed at C/5 rate after 150 cycles of charge/discharge. As demonstrated above, the CNFs are commonly prepared via the electrospinning method [94,95]. Actually CNFs derived from the carbonizing fiber-structured materials, such as the bacterial cellulose [96], fiber wood [97], cotton [98,99], alginate fiber [100], etc., are also good candidates as sulfur hosts. Li and her co-workers reported a CNF derived from the bacterial cellulose [96]. This bacterial cellulose derived from CNFs were easily crosslinked to form aerogel with extreme low density, thus it had strong adsorption of catholyte to facilitate the sulfur content reaching 75 wt%. The CNF aerogel-S electrode exhibited excellent cycling stability with 76% of the initial capacity after 200 cycles. Manthiram’s group reported a cotton-based CNF with hierarchical macro-/microporous architecture [98]. The macroporous channels allowed the cotton derived CNF to load and stabilize a large amount of catholyte, while the abundant microporous reaction sites spread throughout the cotton derived CNF facilitated the redox chemistry of Li
S cell system. As a result, even the sulfur loading in carbon cotton as high as 30.7 and 61.4 mg∙cm
2, a reversible capacity of approximately 1000 and 800 mAh∙g
1 have been achieved after 50 cycles at 0.1 C. Additionally, the carbon cloth that are formed by the CNFs have also utilized as the sulfur supporting materials [101,102]. An activated carbon fiber cloth (ACF) with micropores was employed to assemble free-standing electrode by Aurbach [102]. The free-standing ACF-S could exhibit maximum discharge capacity higher than 1050 mAh⋅g
1. And if reducing the diameter of the fibers and the ACF cloth thickness, the sulfur load and utilization would further improve.

Compared with porous carbon materials, CNTs typically have lower specific surface area (< 200 m2∙g
1) and pore volumes. These limitations inspire numerous studies to prepare highly porous CNTs for loading more sulfur [61–64]. Xiong’s et al. [30], reported a hierarchical CNT with a thick microporous wall and inner channel as an efficient scaffold for Li–S batteries. As shown in Fig. 3c, they employed the Te/C nanocables as the template. After experiencing high-temperature calcination in Ar atmosphere and chemical etching, the Te/C nanocables could transform to hierarchical micro/mesoporous CNTs (denoted as HMMCNT). The BET surface area has increased to 596.2 m2∙g
1, but the pore volume is only 0.318 cm3∙g
1, which could only accommodate 50 wt% sulfur content. Thus, seeking other strategies to produce highly pore volume CNT is imperative. Huang’s group reported a highly porous CNT etched by water steam [65]. The BET areas of this porous CNT ranged from 255 to 430 m2∙g
1, but the pore volume could reach as high as 2.16 cm3∙g
1. Thus an ultra-high sulfur content (89 wt%) could be accommodate in porous CNTs, and the resulting CNT-S cathodes could deliver a reversible capacity as high as 1165 mAh∙g
1 at the initial cycle, and 792 mAh∙g
1 after 200 cycles at a current rate of 0.2 C. Recently, Moon, et al. [60], reported a spherical CNT (M-CNTP) prepared by using the polymer spheres as template (Fig. 3d). The M-CNTP illustrated abundant macropores as shown in Fig. 3e, which could encapsulate 70 wt% sulfur. And the corresponding M-CNTP-S cathode exhibited a high reversible capacity of 1034 mAh∙g
1 at a current density of 2 C and capacity retention of 71% over 100 cycles. With the 1D structure and good mechanical characteristic, CNTs are quite easy and facile to form CNT arrays and CNT films. Actually CNT-S flexible electrodes have also been widely explored [14,59,66–69]. Based on the analysis of the reported studies, the free-standing CNT-S electrode can be divided into two categories: (1) self-weaving S-CNT electrodes that are obtained from vacuum filtration homogenous CNT-S solution or vacuum filtration homogenous CNT solution following S incorporation [70–72]; (2) vertical aligned S-CNT electrode—CNT was first grown regularly on the metal catalyst (FeAl) or designed templates (eg. AAO) via CVD method, followed by incorporating S with heating or dropping [59,69,73,74]. For instance, Manthiram’s group has reported a self-weaving S-MWCNT electrode in 2012 [75]. This robust, flexible self-weaving S-MWCNT was synthesized via an in-situ sulfur deposition and a vacuum filtration process. More precisely, this self-weaving S-MWCNT exhibited high capacities of 1352 mAh∙g
1 at 1 C and 1012 mAh∙g
1 at 4 C rate. Following in 2014, both Zhang’s group and Wang’s group prepared the flexible self-weaving S-MWCNT via the vacuum filtration method [76,77]. Recently, Yu et al. [78], reported a free-standing 3D trench-wall CNT sponges as the sulfur host for Li–S batteries. Unexceptionally, the free-standing 3D porous sponge was synthesized by vacuum filtration. While the free-standing sandwich S-CNT electrode was obtained by placing the sulfur between the CNT sponge and being pressed under 50 Mpa. Different from the solution-process procedures that are barely scaled up and require additional purifying steps, the assembly of free-standing S-CNT without filtration and purifying is not rare. As illustrated in ref. [14] and ref. [59], the CNT array was synthesized during the CVD process, and the sulfur incorporation via vapor or carbothermally reduction. The whole process in these two reports didn’t use the solution-process and purify-process. Additionally, D€ orfler et al., reported a combination strategy of solution-process with melt-diffusion to form a binder free vertical aligned CNT/S (VA-CNT/S) electrode [79]. The VA-CNT grew on a catalyst thin film that was dip-coated metal salt solution under the CVD process. Then the sulfur solutions in toluene was dropped on VA-CNT following with heating to get the VA-CNT/S electrodes. The sulfur content reached 70 wt% in the electrode, corresponding an excellent discharge capacity over 800 mAh∙g
1. Following the vertical aligned CNT forest and CNT arrays as the sulfur host have also been explored.

3.1.3. Doped and functionalized CNT/CNF CNTs/CNFs with heteroatoms doping [103,104], can not only enhance the conductivity of the electrode [105], but also offers a strong dipole with lone pair electron on the carbon surface to form strong

3.1.2. CNF CNFs, another typical 1D nanocarbon, hold the merits of feasible 5

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Fig. 4. (a) A schematic illustration of the experimental procedure and the formation of ideal mesoporous structure of mesoporous CNF, (b) pore-size distributions of the OMCF and OMCF-S composite. Reproduced with the permission from Ref. [85], Copyright 2015, Elsevier. (c) Schematic illustration of the synthesis of porous CNF/S composites: porous CNF after etching Fe3C and chemical activation; porous CNF/S by infiltrating sulfur. (d) TEM image of PCNF activated at 550  C. Reproduced with the permission from Ref. [93], Copyright 2017, WILEY-VCH. (e) Fabrication of the CNF-S electrode using electrospun polyacrylonitrile, and (f) the corresponding SEM images of each product. Reproduced with the permission from Ref. [89]. Copyright 2018, American Chemical Society. (g) Hollow carbon nanofiber encapsulated sulfur after etching away AAO template. (h) Cycle life at C/5 and C/2 as compared to a control sample in which the AAO was not etched away. Reproduced with the permission from Ref. [27], Copyright 2011, American Chemical Society.

and MWCNT with O, B, BO and BO2 species. Apparently, the adsorption ability gradually increased with the doping of B and O. While CNTs/CNFs with surface functionalization could accommodate more active materials and affect dynamical interface driven between C and the S-containing guest [117,118]. As known, oxidation sites with oxygen-containing groups on the CNTs result in a negative surface charges that can facilitate CNT dispersion homogenous, which is beneficial for supplying sufficient sulfur-loading sites and alleviating aggregation of sulfur/Li2S and electrode polarization [118]. Pol et al. [116], applied the oxygen and hydrogen to modify the surface of CNTs at 500  C and 800  C, respectively. The CNTs treated by O2 (OCNT) contain abundant oxygen functionalized group and negative charge on the surface (
28 mV), while the H2-treated CNT (HCNT) is lack of oxygen functionalized group and possesses an almost electrically neutral surface (
2.5 mV). It is confirmed by them that the oxygen contents in CNTs highly affected the cycle performance and specific capacity in Li–S batteries as shown in Fig. 5f. The C
O and C
O
C stretching vibration bonds on the oxygen-treated CNT could effective block polysulfides via an electrostatic repulsion as shown in Fig. 5e. Wang et al. employed CO2 atmosphere to modify the CNTs at 900  C, producing the negative charges on the external surface of the tubes [118]. The CO2-treated CNTs are more stable compared with the oxide-treated CNTs, allowing higher sulfur loading and utilization. The resulted free-standing CO2-CNT-S electrode contained 80 wt% sulfur, demonstrating a reversible capacity of 430.5 mAh∙g
1, far higher than O2-treated CNT (159.4 mAh∙g
1)

electrostatic interactions with polar lithium polysulfides [2]. CNTs and CNFs are easily modified by N atoms doping [103]. Zhou et al., first reported N-doped porous CNF webs/sulfur (N-PCNF/S) composites as the cathode materials for an advanced lithium
sulfur battery [106]. The N-PCNF with appropriate N doping were synthesized by pyrolyzation of polypyrrole (PPy) nanofiber and a subsequent KOH activation [106]. Due to the unique 1D porous nanostructure, N-PCNF not only possessed large exposed surface area, but also provided fast and long-distance electron transport that enabled good conductivity. In consequence, the N-PCNF/S composites displayed an excellent electrochemical performance. A 3D hierarchical porous nitrogen-doped aligned CNTs (HPNACNT) as a sulfur scaffold has been reported by Deng et al. [99,107–112]. Due to the synergetic effect of hierarchical porosity and the restraint of shuttle effect via SxLi ….N interactions, the sulfur-HPNACNT cathode with 68.8 wt% sulfur exhibited a high reversible capacity of 817 mAh∙g
1 at high charge/discharge current rate of 5 C. Moreover, dual/tri-doping of N and other elements including S, P, O, B in CNT/CNFs has also been widely reported [113]. N, B, S tri-doped, activated CNT with abundant mesoporous structure enabling fast Liþ transmit and strong polysulfide adsorption was reported recently [114]. According to the first-principle calculation results (Fig. 5a-c), the N, B, S tri-doping showed the highest binding energy on the lithium polysulfides compared to undoping and N, B dual doping. As predicted, the experimental results verified that the rate and cycling performances of N, B, S tri-doped ACNTs/S were higher than N, B co-doped ACNTs/S, N, S co-doped ACNTs/S and ACNTs/S. Tao’s group reported a B,O dual-doped MWCNT as the host materials for sulfur [115]. With introducing the B and O, the MWCNT surfaces existed O, B, BO and BO2 species, and Fig. 5d showed that the adsorption energy of Li2S on optimized pristine MWCNT

3.1.4. CNT/CNF-Carbon hybrids CNT and CNF-based nanostructured cathode with various arrangements and combinations, such as CNT/CNT [31,119], CNT/CNF

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Fig. 5. First-principle calculations presenting the adsorption behavior of the LiSH on the surface of the different heteroatom doping (a) undoping, (b) N, B dual-doping and (c) N, B, S tri-doping. Reproduced with the permission from Ref. [114], Copyright 2018, American Chemical Society. (d) Adsorption energy of Li2S on different surfaces of MWCNTs. Reproduced with the permission from Ref. [115], Copyright 2017, the Royal Society of Chemistry. (e) The cycling performances of CNT/S, HCNT/S and OCNT/S electrodes. (f) Schematic illustrations depict the role of OCNT as sulfur reservoirs during the charge process. Reproduced with the permission from Ref. [116], Copyright 2018, Elsevier.

investigated by Wei and Zhang in Tsinghua University [172–175]. In 2012, they reported graphene/single-walled carbon nanotube (G/SWCNT) hybrids (shown in Fig. 7a) as the sulfur reservoirs [172]. Following they employed nitrogen-doped aligned CNT/graphene sandwiches (ACNT/G) as the sulfur host (shown in Fig. 7b) [173]. In a further work, a interconnected CNT/graphene nanosphere scaffold has been designed for sulfur (shown in Fig. 7c) by them [174]. Recently, a 3D free-standing electrode composed of nitrogen-doped porous graphene/sulfur composite granular and super-long CNT skeleton via calendering process (shown in Fig. 7d) has also been reported by them [175]. Without exception, all the Li–S batteries’ performances have been improved by using the graphene/CNT composites networks. What’s more, the graphene-CNF composites as the sulfur reservoirs are not rare [150–155]. As early as 2013, Lu et al., reported a novel graphene-sulfur-CNFs (G-S-CNFs) nanocomposites as the cathode for Li–S batteries [149]. Such a novel electrode could deliver a reversible capacity of 273 mAh∙g
1 even after 1500 cycles at a high rate of 1 C with an extremely low capacity decay of 0.043%. Lou’s group successfully applied the ethylenediamine-functionalized graphene oxide (EFG) dip-coating on root-like multichannel CNFs (LRC) (as shown in Fig. 7e). The resulting LRC/S@EFG showed enhanced cycling performances compared to the LRC/S electrode (shown in Fig. 7f), and at the current density of 1.2 mA∙cm
2, the LRC/S-ERG area capacities could reach 11 mAh∙cm
2 as shown in Fig. 7g. Nanoporous carbons combining with CNF/CNT will form interconnected nanochannels and electronically conductive walls, which are

[120–123], CNT/graphene [124–148], CNF/graphene [149–155], CNT/porous carbon [156–166], CNF/porous carbon [167], CNT/amorphous carbon [168], N-doped MWCNT/g-C3N4 [169], etc., have been widely fabricated for application in Li–S batteries. The combination of CNT/CNF with carbon hetero-structures into hybrid materials renders their promising application as sulfur cathodes with high-rate performance. [170]. Small-diameter CNTs and a high loading amount (85.2 wt%) of sulfur were encapsulated completely inside a large CNT are shown in Fig. 6a-c [119]. Owing to the presence of these electrically-conductive, highly-flexible and structurally-robust small CNT and large CNT overlayer, the S-CNTs@CNT electrode demonstrated superior high-rate cycling performances: reversible capacities of 1146, 1121 and 954 mAh∙g
1 were obtained at 1, 2 and 5 C shown in Fig. 6d. While recently combination of the porous CNF (PCNF) and CNT to form a 3D interconnected conductive network has been reported by Li and Gao [122]. The interfinger space and abundant pores of PCNFs can buffer volume expansion and the better conductivity of CNTs can provide quicker electronic transfer. As a result, the free-standing S/PCNF/CNT with a high sulfur loading 12.0 mg∙cm
2 showed excellent cycle stability, which the area capacity reached 13.5 mAh∙cm
2 after 50 cycles at 0.6 mA∙cm
2. The robust connection between the CNT/CNF and the graphene facilitated the construction of a high electrical conductive pathway [171]. The internal spaces between the two stacked graphene layers and among CNT/CNF also offered room for the sulfur storage [171]. Many pioneering works on 3D S-CNT-graphene cathodes have been

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Fig. 6. (a) Schematic illustration showing the templated growth of CNTs@CNT and S-CNTs@CNT. TEM image of (b) S-CNTs@CNT and (c) CNTs@CNT. (d) Cycling performance of S-CNTs@CNT at high current rates of 1, 2, 5 C. Reproduced with the permission from Ref. [119], Copyright 2018, American Chemical Society.

characteristically good conducting networks for both Liþ and electrons [177]. For example, a hollow CNF@nitrogen-doped porous carbon (HCNF@NPC) core-shell composite, which was carbonized from HCNF@polyaniline, has been utilized as an improved conductive carbon matrix for encapsulating sulfur by Zhang et al. [167]. The NPC shell with relative high surface area and pore volume, could trap polysulfides and Liþ conductive pathways, while the HCNF cores provided electronic conduction pathways and mechanical support. As a result, the HCNF@NPC-S composite with high sulfur content (77.5 wt%) maintained a reversible capacity of 590 mAh∙g
1 after 200 cycles. Guo’s group as early as 2012 has combined the microporous carbon (MPC) with CNT in order to produce better Li–S batteries [178]. First the CNT was coated with a MPC layer that the pore size was only 0.5 nm, then the small sulfur allotropes S2
4 was confined in MPC to get S/CNT@MPC composite by melt-diffusion strategy. In this composite, the CNT core served as a metallizer to provide e
for the redox reaction between Liþ and S and hence improved the Li electroactivity [178]. While the MPC could supply unperturbed Liþ for the confined S2
4, to enhance the electrode reaction kinetics [178]. Additionally, a CNT-interpenetrated mesoporous nitrogen-doped carbon spheres (MNCS/CNT) was synthesized by two-step approach [179]: i) evaporation-induced self-assembly (EISA), ii) carbonization following template removal. The nitrogen functional groups and spherical structure of the MNCS/CNT synergistically contributed to a high specific capacity of approximately 1100 mAh⋅g
1, high areal capacity (ca. 6 mAh⋅cm
2) with a high sulfur loading of around 5 mg⋅cm
2.

(CNR) with high length/radius aspect ratio that can effectively prevent the dissolution of polysulfides, decrease self-discharge, and confine the volume expansion on cycling has been utilized in Li–S batteries by Wang’s group [180]. The resulted CNR-S nanocomposites cathode delivered a high specific capacity of 1378 mAh⋅g
1 at a 0.1 C current rate and exhibited stable cycling performance. While Qie et al. [32], reported a cobalt-embedded nitrogen-doped hollow carbon nanorods (shown in Fig. 8b) as the scaffold for Li–S batteries, which the cobalt carbonate hydroxide nanorods were applied as the template and polydopamine (PDA) as carbon and nitrogen source as shown in Fig. 8a. The as-made Co@NHCRs and sulfur composite (Co@NHCRs/S) possessed high rate capability and excellent cycling stability. Moreover, a high-quality 1D graphene nanoscroll (GNS) in high yield from GO sheets has been reformed by a fast quenching-combined lyophilization approach as shown in Fig. 8c [183]. The GNS had an ordered 1D CNT-like tubular nanostructure as shown in Fig. 8d, as well as a large specific surface area (545 m2⋅g
1), becoming an idea carrier to incorporate with sulfur. The sulfur loading in the GNS could reach 81 wt %. And the resulted GNS-S electrode illustrated high-rate and long-cycling performances. In summary, 1D carbon nanostructures have been widely employed as the sulfur host for Li–S batteries. The 1D carbon nanostructures can not only provide better electrical connection to the active materials, but also their tunable porous structures and rich surface functionalities are beneficial to hold the sulfur and suppress the polysulfides shuttling. What’s more, 1D carbon nanomaterials with good mechanical robustness and flexibility are easily combined with 0D and 2D carbon nanomaterials to form free-standing 3D nanostructures, which facilitates to fabricate high-energy carbon-S cathode, and the interconnected porous structure contributes to high accessible surface area, large pore space and strong confinement to sulfur species. However, due to the nonpolar nature of

3.1.5. Other 1D Carbon nanostructures In addition to CNTs and CNFs, other 1D carbon materials have also been employed as the sulfur host. Among the nanorods were widely reported [32,180–182]. A 3D hyperbranched hollow carbon nanorod 8

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Fig. 7. (a) Schematic illustration of catalytic CVD of G/SWCNT hybrids on FeMgAl layered double hydroxides flakes. Reproduced with the permission from Ref. [172], Copyright 2012, American Chemical Society. (b) Conceptual scheme of the design of N-ACNT/G hybrids with graphene and aligned CNTs as building blocks. Reproduced with the permission from Ref. [173], Copyright 2014, WILEY-VCH. (c) Schematic of fabrication route for CGS electrode, including the ethanol co-dispersion of super-long CNTs and the GS–S, and the assembly into robust free-standing films through vacuum filtration. Reproduced with the permission from Ref. [174], Copyright 2014, Elsevier. (d) Schematic illustration of the calendering process to tune the structure of high sulfur-loading NG@S-CNT electrode. Red sheets: porous graphene/sulfur composite; Yellow tubes: Ultralong carbon nanotubes. Reproduced with the permission from Ref. [175], Copyright 2016, Elsevier. (e) Illustration of the synthesis process of the LRC/S@EFG electrode: LRC/S electrode were dip-coated with EFG. (f) Cycling performance of LRC/S@EFG in comparison with LRC/S at a current density of 0.2 C. (g) Areal capacities of layer-by-layer structured LRC/S@EFG electrodes during cycling at a current density of 1.2 mA∙cm
2. Reproduced with the permission from Ref. [176], Copyright 2016, Nature Publishing Group.

Fig. 8. (a) Schematic illustration for the fabrication of Co@NHCRs. (b) TEM images of Co@NHCRs. Reproduced with the permission from Ref. [32], Copyright of Elsevier 2016. (c) Schematic diagram of preparation processes for GNS and S@GNS. (d) FESEM image of GNS. Reproduced with the permission from Ref. [183], Copyright 2016, American Chemical Society.

these 1D nanostructures, the ability of anchoring the polar polysulfides is limited.

polyacrylonitrile (PAN) [184,185], PPy [186–189], polyaniline (PANI) [190–195], poly (3,4-ethylenedioxythiophene) (PEDOT) [196], polyethylenimine (PEI) [197], polyacrylamide [198], polypyrrole-polyethylene glycol (PPy-PEG) [199], polyvinylpyrrolidone (PVP) [200,201], poly(methacrylate)/PAN/PVP (PMMA/PAN/PVP) [202], etc., form a strong membrane that can resist large volume

3.2. Polymer-based 1D nanostructures as sulfur hosts Conducting

polymers

or

amphiphilic

polymers,

including 9

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the element sulfur and resident polysulfides. Therefore, the SPANI-NT/S composite cathodes exhibited an outstanding cycling stability, i.e. 432 mAh⋅g
1 after 500 cycles at 1 C. In addition, rice-like sulfur/PANI cathode [209], PANI nanowire coating sulfur cathode [210], has been reported subsequently. A sulfur-based cathode material from commercially available PMMA/ PAN fibers has also been reported [211]. By thermal heating PMMA/PAN fibers with element sulfur, sulfurized PAN (SPAN) with up to 46 wt% covalently bound sulfur as shown in Fig. 9b was produced. The fibrous morphology with cylindrical macropores contributed to form electronic conduction networks in the cathode and provided directed diffusion pathways for ions. As a result, Li/SPAN cells showed low internal resistances, high initial capacities up to 1672 mAh⋅g
1, high rate capabilities up to 8 C, and excellent cycle stabilities over 1200 cycles.

expansion during discharge and avoid sulfur from being in direct contact with electrolytes and, therefore, preventing the dissolution of polysulfides in organic electrolytes [180]. In addition, the polymer with functional groups, such as –NH2, –COOH and so on, has stronger chemical trapping capability on polysulfides than the carbon counterpart. 3.2.1. 1D polymer nanostructures Polymer, as an inherent soft material, has attracted tremendous attentions to form an 1D nanostructure for accommodating sulfur [15, 203]. The first attempt on the synthesis of 1D polymer/sulfur composites was reported by Sun et al. through heating element sulfur and PPy nanowire [204]. The PPy here served as conductive additive, distribution agent and absorbing agents. However, the capacity of sulfur/PPy composite only remained 570 mAh⋅g
1 after 20 cycles. Since then, various sulfur/PPy composites synthesized by different strategies have been reported [33,205–207]. While recently, Li et al. developed a free-standing sulfur-PPy cathode by directly pasting a sulfur coated-PPy (S@PPy) on a flexible PPy film [208]. The PPy nanofiber film with rough surface demonstrated superior mechanical flexibility, which can tolerate volume expansion, enhance active materials as well as trap the polysulfides. Consequently, free-standing sulfur-PPy cathode showed better electrochemical performance than the traditional S@PPy cathode coated on Al foil. Liu’s group reported a soft approach to encapsulate sulfur in PANI nanotube [15]. As depicted in Fig. 9a, the PANI was heated at 280  C with sulfur and then partial sulfur reacted with polymer to form a 3D, cross-linked, structurally-stable sulfur-PANI (SPANI-NT) polymer backbone with both inter-/intra-chain disulfide bonds via in situ vulcanization. The soft polymer and 1D nanostructures allowed the polysulfides in situ deposition reversibly and also accommodated volume change during charge/discharge process. More importantly, this SPANI-NT polymer framework could furnish strong physical and chemical confinement to

3.2.2. Polymer Coated CNT/CNF nanostructures A non-conductive host would restrict the reactivation and reutilization of the trapped active material, which causes continuous capacity fading. There is no doubt that most of the polymers’ conductivities are worse than carbon counterpart [212]. Thus a compromise is to create a polymer-supported CNT/CNF nanostructured host for sulfur, which integrates the physical and chemical polysulfide-trapping agents. Wang’s group first carried out the relative study on polymer supported CNT for Li–S batteries [213], which MWCNT was introduced into PAN by in situ polymerization of acrylonitrile (AN) and itaconic acid (IA) additive on the surface of MWCNT shown in Fig. 10a. MWCNTs in the composites created an electronically conductive network and reinforced the structural stability, while rough and thin PAN layers increased the contacting area between PAN and sulfur and favored a full pyrolysis with a uniform sulfur distribution. Chen’s group reported a S/PPy/MWNT ternary composite [214], which the PPy was in situ polymerized on the surface of MWCNT, and the sulfur particle deposited on PPy/MWCNT

Fig. 9. (a) Schematic illustration of the construction and discharge/charge process of the SPANI-NT/S composite. Reproduced with the permission from Ref. [15], Copyright 2013, WILEY-VCH. (b) Synthesis and structural motifs of (fibrous) SPAN (amended structure). Reproduced with the permission from Ref. [211], Copyright 2017, American Chemical Society. 10

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Fig. 10. (a) Schematic diagram of in situ polymerization and systemization of the PAN-S@MWCNT composite. Reproduced with the permission from Ref. [213], Copyright 2011, the Royal Society of Chemistry. (b) Schematic of the S/PPy/MWNT composite preparation. Reproduced with the permission from Ref. [214], Copyright 2013, Elsevier. (c) Schematic of the MWCNTs-S@PANI composite preparation. Reproduced with the permission from Ref. [215], Copyright 2015, Elsevier. (d) Schematic illustration for the fabrication of CF/S/PEDOT composite. Reproduced with the permission from Ref. [196], Copyright 2017, American Chemical Society. (e) First-principles calculation shows the interaction between the LiS (left), Li2S (right) and carbon surface. (f) Schematic showing the interaction between PVP and carbon surface (upper). First-principles calculation shows the interaction between the discharge products and the functional group on PVP. (g) Schematic showing the interaction between Triton X-100 and carbon surface (upper). First-principles calculation shows the interaction between the discharge products and the functional group on Triton X-100. Reproduced with the permission from Ref. [200], Copyright 2013, American Chemical Society.

the polymer for blocking the diffusion loss of polysulfides, the as-fabricated composite electrode demonstrated a high initial capacity of 1272 mAh∙g
1, high reversible capacity of 807 mAh∙g
1 after 200 cycles with a high coulombic efficiency of 99%. Besides using the conductive polymer coating on CNTs/CNF, other polymers in favor of stabilizing the polysulfides have been reported as well, such as that the PEG polymer were most widely reported [199, 216–218]. In addition, Cui’s group employed the amphiphilic polymer (PVP) and Triton X-100 to modify the interface between the CNF and sulfur [200]. When the interface was modified by the amphiphilic polymer, a significant increase in binding energy between the lithium sulfide and amphiphilic polymer was observed in Fig. 10e-g. And the binding energy between the lithium sulfide and PVP were the largest, resulting in the S-CNF/PVP cathode exhibited the most stable cycling performances. Archer’s group designed a CNT-PEI/S cathode by introducing the PEI polymer bearing a large amount of amine groups [197]. The CNT-PEI hybrids in a sulfur cathode stabilized the cathode by both kinetic and thermodynamic processes, which contributed to CNT-PEI/S hybrids cathode displayed high capacity at both low and high current rates, with capacity retention rates exceeding 90%. Recently a sulfur-rich

composite in aqueous suspension following heat-treatment as shown in Fig. 10b. PPy in this ternary composite, acted as a binder to connect sulfur and MWCNT, as well as adsorbed polysulfides into its porous structure. MWCNT core supplied a highly conductive and mechanically flexible framework. Accordingly, the ternary composites cathode exhibited a reversible capacity of 960.7 mAh⋅g
1 at 0.1 C after 40 cycles. A sulfur supported by MWCNT and coated with PANI (MWCNTs-S@PANI) composite as cathode materials has been designed by Li et al. [215]. MWCNTs-S was prepared by loading sulfur on MWCNTs via chemical deposition and coated with polyaniline via in situ polymerization under the control of ascorbic acid as shown in Fig. 10c. The resulting product showed a unique sandwich-like structure that was beneficial for preventing the dissolution and diffusion of polysulfides via PANI and improving electronic conductivity of sulfur cathode via MWCNTs. As expected, MWCNTs-S@PANI illustrated enhanced cycling stability and rate capability compared to the MWCNTs-S electrodes. Ai’s group fabricated a novel coaxial three-layered structure, which the sulfur was deposited on CNFs and coated with PEDOT/poly(styrenesulfonate) (PEDOT/PSS) shown in Fig. 10d [196]. Benefiting from the rigid conductive framework of carbon fibers and flexible buffering matrix of 11

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polysulfides fixer have been gradually popular. And the pure 1D inorganic compounds hosts for Li–S batteries have been reported continuously but not many. Wang et al. first reported a hollow Co3O4 nanotube as the sulfur host [220]. However, the sulfur loading in this Co3O4 nanotube was only 10 wt%. Following S-MnO2 nanowire cathode and S-TiO2 nanofiber cathode have been reported by Choi and Jin [34,221], respectively. Though the sulfur loadings in both composites have reached to 60 wt%, the reversible capacities were not ideal, only 500 mAh∙g
1 after 50 cycles at 0.1 C and 0.2 C respectively. Then Ng et al. reported a 1D Mo2N nanorod as the catholyte support [35]. The reversible capacity has improved compared to the previous reports, which could reach approximately 600 mAh∙g
1 after 100 cycles at 0.1 C rate. Recently, all-MXene-based integrated electrode constructed by 3D Ti3C2 framework host has been reported by Dong et al. [222]. The 3D Ti3C2 framework with open interconnected macropores and exposed surface was constructed from 1D Ti3C2 nanoribbon, guarantying high S loading and fast ionic diffusion for prompt lithiation/delithiation kinetics. When this 3D Ti3C2-S electrode matched with the 2D Ti3C2 MXene interlayer, the integrated a-Ti3C2-S/dTi3C2/PP electrode displayed outstanding rate capacity of 288 mAh∙g
1 at 10 C and long-life cyclability. In addition, the 1D inorganic compounds coating with carbon and polymer as the sulfur host were rarely reported. Yu’s group reported a PPy-MnO2 coaxial nanotubes as sulfur hosts for Li–S batteries as shown in Fig. 11a [11]. The MnO2 restrains the shuttle effect of polysulfides greatly through chemisorption. Hence the S/PPy-MnO2 cathode showed enhanced cycling stability compared to the S/PPy cathode as shown in Fig. 11b. Li’s group reported a conductive porous vanadium nitride nanoribbon/graphene (VN/G) composite (Fig. 11c) accommodating the catholyte as the cathode of a Li–S battery [223]. Due to the high conductivity of vanadium nitride, VN/G based cathode exhibited lower polarization and faster redox reaction kinetics than a reduced graphene oxide-based cathode.

copolymer@CNT hybrid cathode was reported by Liu and Li [219]. Sulfur-1,3-diisopropenylbenzene (S-DIB) copolymer was synthesized by inverse vulcanization through direct dissolution and copolymerization of DIB in liquid sulfur, and then the as-prepared S-DIB was infiltrated into the AAO@CNT host by melt diffusion. After removing the AAO template, the S-DIB@CNT hybrid was obtained. Such a hybrid structure owned numerous advantages: i) the CNT hollow cores served as a nanoscale electrochemical reaction vessel that can confine active materials and accommodate volume expansion; ii) the conductive CNT provided paths for fast electron transport, while the π electrons of the aromatic sulfur copolymer rings enhanced Liþ transfer; iii) the strong chemical interaction of sulfur with the carbon frameworks after copolymerization effectively hindered shuttle effect. As a result, the S-DIB@CNT cathode exhibited a high rate capacity of 700 mAh∙g
1 at 2 C and high reversible capacity of 880 mAh∙g
1 after 100 cycles at 1 C. To sum up, the 1D polymers or polymer coated CNT/CNF composites have demonstrated positive effects on improving the electrochemical performances of Li–S batteries. The non-conductive polymers as its strong chemical bonding function with polysulfides could enhance the cycling stability. While in terms of the conductive polymers, not only the chemical bonding functions exists, but also the conductivity could be improved, particularly when combined with the CNT/CNF cores. However, the synthesis process of conductive polymer is usually complex, and whether these polymers dissolve or swelling in the organic electrolyte or not after long-term cycling should further study. 3.3. Inorganic compounds-based 1D nanostructures Almost all inorganic materials, connected by ionic or polar covalent bonds, possess favorable polar surfaces with strong binding affinities to polar polysulfide intermediates and therefore are beneficial for polysulfide adsorption and shuttle inhibition. Inorganic components, such as transition metal oxides, hydroxides, sulfides, carbides, and nitrides, etc., have been proved to play a key role in enhancing sulfur composites cathodes [7].

3.3.2. Inorganic Compounds-supported CNT/CNF nanostructures Quite a few cases have directly used the 1D inorganic materials as the sulfur host, because inorganic materials are normally rigid that is easily

3.3.1. 1D inorganic Compounds nanostructures In recent five years, using the inorganic compounds as the sulfur and

Fig. 11. (a) Illustration of the synthesis of S/PPy-MnO2 ternary composites. (b) Electrochemical performance of S/PPy-MnO2 ternary composites and S/PPy composites at 0.2 C. Reproduced with the permission from Ref. [11], Copyright 2016, American Chemical Society. (c) Schematic of fabrication process of VN/G composite. Reproduced with the permission from Ref. [223], Copyright 2017, Nature Publishing Group. 12

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Fig. 12. (a) Illustration of the Synthesis of MnO2@HCF/S composite. Reproduced with permission from Ref. [229], Copyright 2015, WILEY-VCH. (b) Schematic detailing the concept of the containment of sulfur in HMT and CNTs. Reproduced with permission from Ref. [227], Copyright 2015, WILEY-VCH. (c) Schematic illustration of the synthesis process of the GC-TiO@CHF/S cathode. Reproduced with permission from Ref. [240], Copyright 2017, Cell Press. (d) Illustration of the h-CNT/S/ZrO2 synthesis process. Reproduced with permission from Ref. [238], Copyright 2015, WILEY-VCH. (e) Schematic illustration of the synthesis process of the CNT/NiFe2O4
S ternary material structure. Reproduced with permission from Ref. [241], Copyright 2015, American Chemical Society. (f) Schematic illustration of the fabrication process for CF@CNTs/MgO-S composite. Reproduced with permission from Ref. [242], Copyright 2017, WILEY-VCH.

in Fig. 12c. The TiO polar layer effectively hinders the dissolution of polysulfides, enabling excellent stability over 400 cycles. Zhou et al. [238], incorporated a fine amount of ZrO2 to the holey CNTs/S composites shown in Fig. 12d. The holey CNTs contributed to the good conductivity of the h-CNT/S/ZrO2 cathode, while appropriate ZrO2 loading preserved the permselective channels for Liþ intercalation/deintercalation and trapped the soluble polysulphides. A CNT/NiFe2O4
S ternary hybrid material structure (shown in Fig. 12e) was synthesized by Wang’s group [241]. The porous CNT network provided fast electron conduction paths and structural stability, NiFe2O4 nanosheets afforded strong binding sites for trapping polysulfide intermediates. Dou’s group reported a 3D multifunctional scaffold for sulfur composed of the N-doped carbon foam (CF), CNTs and MgO nanoparticles via a facial liquid phase immersion/adsorption approach as shown in Fig. 12f [242]. The dense and uniform CNT formed a 3D conductive network to boost electrons/ions transport, ultrafine MgO nanoparticles provided strong chemisorption toward polysulfides and N-doped carbon foam supplied abundant surface anchoring sites for trapping and confining polysulfides. Cui’s group synthesized CNFs with tin-doped indium oxide nanoparticles decorating the surface as hybrid three-dimensional electrodes for Li–S batteries [243]. The conductive tin-doped indium oxide (ITO), electrical conductivity up to 104 S∙cm
1, exhibited a hydrophilic surface, which was attractive for addressing the polysulphide deposition issues. Apart from the above detailed examples, there are still many other 1D CNT/CNF supported oxide compounds nanostructures, i.e., CNTs doped SiO2 [244], CeO2-webbed CNTs, [245] ZnO coating CNTs [246], MoO-CNFs [247], V2O5 deposited CNTs [248]. Al2O3 deposited carbon cloth [249], Co3O4-carbon cloth [250], CNT/NiCo2O4 [251], etc., as the hosts for Li–S batteries. Incorporation the oxide compounds with CNT/CNF is not the only approach. The combination of metal nitrides [252,253], metal sulfides

caused the electrode collapse and a lot of them (e.g. metal oxides) are normally electrically insulating [1]. Therefore, in most cases, inorganic active materials are composited with CNT or CNFs forming 1D nanostructures to attain reliable mechanical and electrochemical properties. Some metal oxides materials have stronger binding energy with sulfur species than carbon materials, such as titanium oxide Ti4O7 [224–227], manganese oxide [228–232], tin oxide [233–236], zirconium oxide [237,238], etc., but the electronic conductivity of metal oxides is much worse than carbon materials, especially CNTs and CNFs. Thus, it is a promising strategy to design some hybrid structures that combine the carbon materials with above-mentioned metal oxides as host for sulfur accommodation, which can achieve both good conductivity and strong absorbability for sulfur and lithium polysulfides at the same time. Lou’s group designed a 1D nanostructure consisted of hollow CNFs filled with MnO2 (MnO2@HCF) and employed it as the sulfur support shown in Fig. 12a [229]. The hollow CNFs facilitated electron and ion transfer during the redox reactions, while MnO2 efficiently prevented polysulfide dissolution. As a result, the MnO2@HCF/S electrode, with 71 wt% S content in the composite and an area of sulfur mass loading of 3.5 mg∙cm
2, maintained a reversible capacity of 662 mAh∙g
1 at 0.5 C over 300 cycles. Follow then the Mn3O4@CNF, MnO@CNF hots have been utilized to accommodate sulfur and inhibit polysulfides shutting one after another [230,231,239]. Sun and co-workers embedded sulfur and hollow-mesoporous titania (HMT) within carbon nanotubes (CNT) as shown in Fig. 12b [227]. The pores of TiO2 allowed for high sulfur loadings and accommodation of the volume expansion during charge/discharge. The CNT component provided an overlapping network that improved both the electronic conductivity and mechanical strength of the S-HMT@CNT. While Lou’s group recently reported a novel 1D TiO@CNF/S cathode [240], which the TiO hollow nanospheres as “grape grains” were packed space efficiently and closely connected by a CNF sheath as the sulfur host as shown

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Fig. 13. (a) Schematic illustration of dual blocking effects associated with “physical block and chemical absorption” for polysulfides in the PCF/VN/S electrode. (b) Cycling performance of four electrodes at 0.1 C. Reproduced with permission from Ref. [253], Copyright 2015, WILEY-VCH. (c) The schematic diagram of the three-dimensional interlaced CNT conductive network formed in S@CNTs/Co3S4
NBs. (d) TEM images of CNTs/Co3S4
NBs and (e) S@CNTs/Co3S4-NBs. Reproduced with permission from Ref. [13], Copyright 2017, American Chemical Society.

[13,254–256], metal carbides [257–260], metal fluoride [261], metal organic framework [262], metal nanoparticle [263–265], hydroxide [266,267], etc., with the CNT/CNF as the efficient trapping agents for polysulfides have been repoted. Among due to the better conductivities, the metal nitrides, metal sulfides and metal carbides are receiving more attentions [7]. Vanadium nitride with high conductivity (1.67  106 Ω
1∙m
1), incorporated with porous carbon fibers (PCF), boosted the electrochemical performance of Li–S batteries [253]. As illustrated in Fig. 13a, the PCF with highly porous structure provideed enough space to accommodate active sulfur and possessed cross-linked maze channels to physically immobilize the polysulfide species, while the VN nanobelt arrays demonstrated strong ability for chemically anchoring the polysulfides, thus retarding the shuttle effect. As a result, the designed PCF/VN/S electrode showed a high reversible capacity of 1052.5 mAh∙g
1 after 250 cycles at 0.1 C, more stable than CF/VN/S, PCF/S and CF/S electrodes as shown in Fig. 13b. Cobalt sulfides commonly show unique metallic or half-metallic characteristics, which means they exhibit particularly high room temperature conductivity [268–270]. Therefore the cobalt sulfides could offer efficient electron pathways and high electrocatalytic activity for polysulfide redox reactions in aqueous solutions [268]. Simultaneously cobalt sulfides can significantly enhance the redox reactivity of lithium polysulfides due to their strong chemical affinity [268,269]. Jin’s group reported a novel sulfur host material based on interlaced CNTs threaded hollow Co3S4 nanoboxes (CNTs/Co3S4
NBs, Fig. 13c-e) [13]. Co3S4 exhibits attractive metallic nature with a remarkable room-temperature conductivity of 3.3103 S∙cm
1 [13]. The conductive and polar Co3S4
NBs have large inner space for sulfur storage, and can effectively

suppress the shuttle effect by the chemical binding/absorption with polysulfide, and the interlaced CNTs inserted/threaded into Co3S4
NBs can provide an integrative conductive network that facilitated the electron transport all-over the electrode. Accordingly, the S/CNTs/Co3S4
NBs demonstrated an outstanding cycling performance, i.e., approximately 600 mAh∙g
1 after 500 cycles at 2 C. Mxenes, one of the metal carbides, are inherently highly conductive and possess a highly active 2D surfaces to chemically bond to intermediate polysulphides by metal-sulphur interactions [271]. A S/CNT-MXene ternary composites were investigated as the cathode materials for Li–S batteries by Nazar’s group [259]. Surprisingly, they observed that, prior to Ti-S bond formation by a Lewis acid-base interaction, the terminal hydroxyl groups on the MXene surface were consumed by the polysulfides via thiosulfate formation. To brief summary, adding inorganic compounds into CNT/CNF-S based cathodes has remarkably enhanced the electrochemical performance due to the so-called chemically adsorbing function, but most of the adsorption mechanisms for polysulfides on inorganic compounds are still not clearly and need to be deeply investigated. 3.4. 1D nanostructures as Li2S hosts Since the Li2S cathode has a high theoretical specific capacity of 1166 mAh⋅g
1 and can be combined with lithium-free high capacity anode (e.g., Si and Sn) [272,273], it is considered as an alternative cathode material for Li–S batteries. However, similar to the sulfur cathode, Li2S cathode is also limited by low electronic conductivity (10
13 S⋅cm
1) and polysulfides shuttle phenomenon [272,274]. Furthermore, the 14

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Fig. 14. (a) Synthetic procedure of the 3DCG-Li2S composite. (b) Cyclic performances of 3DG
Li2S and 3DCG
Li2S cathodes at 0.2 C for 300 cycles. Reproduced with permission from Ref. [281], Copyright 2016, American Chemical Society. (c) Schematic illustration of Li2S-FWNT@rGO NBF. (d) Cycling performances of the Li2S/FWNTs@rGO NBF cells and the Li2S/rGO NBS cells at 0.2 C. Reproduced with permission from Ref. [282], Copyright 2017, WILEY-VCH.

potential barrier during the first charge process leads to a utilized capacity loss [272,275]. Therefore, assembling Li2S with a 1D conductive matrix, such as CNTs [40,276–278], CNFs [279,280], can improve the utilization of active materials and accommodate the large volume changes [272]. A Li2S-CNT sandwiched electrode was developed by Manthiram’s group [278], which the Li2S bulk powder was placed into two layers of self-weaving CNTs. The unique sandwiched electrode architecture, facilitated ion and electron transport and trapped cycled products within the electrode. At the rate of 0.2 C, the Li2S-CNT sandwiched electrode exhibited 613 mAh∙g
1 over 100 cycles. Combining the 1D Li2S-CNT with graphene could further improve the performances. He et al. developed a 3D graphene-CNT-Li2S (3DCG-Li2S) electrode as shown in Fig. 14a [281]. With adding the CNT conductive framework in the electrode, the cycling performances of DCG-Li2S were far better than the 3D-graphene-Li2S (3DG
Li2S) composite as shown in Fig. 14b. Wu et al. reported another Li2S-CNT/graphene cathode, which was farbricated by LiSO4, few-walled CNTs (FWNTs) and rGO nanobundle forest (rGO NBF) [282]. As shown in Fig. 14c, FWNTs acted as axial shafts to direct the structure, Li2S served as the internal active material, and GO sheets provided an external coating to minimize the direct contact of Li2S with the electrolyte. Undoubtedly, the Li2S-FWNT@rGO NBF demonstrated better cycling performance than Li2S@rGO NBS. Even after 300 cycles, a reversible capacity of approximately 1200 mAh∙g
1 at 0.2 C as shown in Fig. 14d for Li2S-FWNT@rGO NBF could achieve. These electrodes allows ready use of pristine Li2S powder and lithium-free anodes, which is a critical problem facing the use of rechargeable Li–S batteries with Li metal anode. However, the Li2S

cathode is very sensitive to the oxygen and moisture, which means no matter synthesizing the nanostructured CNT/CNF-Li2S or scraper bladecoating electrode should operate in an Argon glove box, as a result the operation is complex and and the cost become higher. 3.5. 1D nanostructures used in anode Lithium metal is the ideal anode material for Li–S batteries due to its high theoretical specific capacity (3860 mAh⋅g
1), low potential (0.304 V vs. standard hydrogen electrode and very small density (0.53 g⋅cm
3) [283]. At present, the bottleneck for the development of high-energy Li-S batteries is the issues of lithium metal anode. And the most severe issues of Li metal anodes are the instability of the solid-electrolyte interphase (SEI) and lithium dendrites growth [284]. The unstable SEI generated from the side reactions between the Li metal and the electrolyte continuously expose fresh Li to the electrolyte and thus consume and etch the Li metal, will result in the low Coulombic efficiency of Li-S batteries [170]; while the lithium dendrite caused by unstable lithium deposition can pierce the separator, causing the battery to short circuit, increasing the ohm heat, and ultimately resulting in safety issues. To realize high-energy, long-cycle life and high-safety Li-S batteries, both the inhibition of lithium dendrites and the stabilization of the SEI should be strongly considered. Generally, constructing stable SEI could be regulated by engineering solvents [285], lithium salts [286], and electrolyte additives [287]. While the most popular strategy to inhibit the lithium dendrites growth is protecting the lithium anode [288–291]. The 1D nanostructures, particulary the CNT and CNF, due to their 15

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Fig. 15. (a) Schematic illustration of three prototypes of current collectors for Li–S batteries and the corresponding digital photos of current collectors and electrodes. (b) Galvanostatic charge–discharge curves at a current density of 0.2 C, (c) cycling performance at a current density of 0.5 C, and c) EIS after 100 cycles at 0.2 C of CNT/S cathodes (sulfur content: 60 wt%; areal sulfur loading: 3.7 mg∙cm
2) with 2D Al, 2D GF, and 3D CNT current collectors. Reproduced with the permission from Ref. [39], Copyright 2016, WILEY-VCH.

replacing the Li metal anode with a pre-lithiated Si anode, these dendrite problems could be successfully prevented and were not observed in the prolong cycles. To summarize, how to modify the lithium anode to promote the stable deposition of lithium and inhibit the growth and volume expansion of lithium dendrite is particularly important. By using the 1D nanostructures to protect the lithium anode could prevent the lithium dendrites growth and thus enhance the electrochemical performances as well as ensure the safety of Li–S batteries. However, the molten Li infusion and hot-pressing at high temperature in an inert atmosphere are relatively complex. Additionly, the mixed conducting framwork may increase the weight of the composite electrode that may reduce the high-capacity feature of the Li metal anode [296].

excellent mechanical property and good conductivity, have become the best choice for lithium anode protection. For instance, Wei et al. inserting a CNT film at the side of lithium anode to prevent the lithium dendrite growth [37]. By inserting this CNT film, the Li metal anode illustrated an ultra-smooth surface. As a result, the sustainable cycling stability and Coulombic efficiency have improved significantly [37]. In addition, Li et al. used a strategy that electroplated the lithium metal on CNF-graphite foam (CNT-UGF) [292]. The anode with CNF-UGF cycled for more than 800 h without short circuiting, while the Li metal anode happened short circuiting at the 260 h. A Li–S cell with the Li/CNT-UGF anode (20 wt% Li content) delivered a high-rate capacity of 860 mAh∙g
1 at 12 C, corresponding to a remarkable specific power of 8680 W∙kg
1 with a specific energy of 720 Wh∙kg
1 with respect to the mass of the cathode. In addition, employing Li/CNTs composite anode instead of lithium metal was also reported [37,293]. Zhang’s group reported a new 1D nanostructure (3D fibrous Li7B6 matrix) to inhibit the lithium dendrites growth [36]. Compared to the anode of metallic Li contained in fibrous Li7B6, metallic Li anode exhibited an irregular potential fluctuation and high impedance due to the complex reaction between polysulfides and Li metal to form inhomogeneous deposited electron/ion inhibitor of Li2S as high transfer barrier with poor polarization. Additionally, Seeking for appropriate anode materials to replace lithium metal in Li–S batteries could avoid the lithium dendrites problem. The Si nanowires is a good candidate. As early as 2010, Cui’s group has applied the Si nanowires as the anode, mesoporous carbon/Li2S as the cathode to farbricate a cell [12]. This new battery can yield a theoretical specific energy density of 1550 Wh∙kg
1, four times higher than that of the theoretical specific energy density of existing lithium-ion batteries based on LiCoO2 cathodes and graphite anodes (410 Wh∙kg
1). Following Hagen et al. employed the pre-lithiated Si microwire array and Krause et al. employed the pre-lithiated Si nanowire as the anodes, CNT-S as cathodes, to assemble full cells [294,295]. By

3.6. CNT/CNF based current collectors By using CNT nanostructured current collectors, corrosion and passivation issues can be simultaneously addressed without other configurational modifications. Embedding S into CNT-assembled nanostructured current collectors yields a mechanically robust high-S loading electrode [40,297,298]. Wei and Zhang’s group have done tremendous efforts on CNT-based current collectors [170]. For example, they applied the CNT to design a 3D carbonaceous macroporous current collector for Li–S batteries [39]. And then the electrochemical performances (Fig. 15b-c) and electrode intergrity (Fig. 15a) with using the 3D CNT-based current collector have been compared with using the 2D Al current collector and 2D graphene film (GF). As shown in Fig. 15a, after cycling, Al foil was pulverized and the cathode was delaminated. 2D GF exhibited no corrosive behavior due to its high chemical stability and good surface adhesion; however, a passivation layer still formed on the cathode due to the 2D nonporous feature of GF. While 3D CNT flm with highly sp2 carbonaceous texture and abundant macropores inhibited the corrosion and prevented the 16

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current density of 0.5 C after 600 cycles. Though the CNT interlayer can impede the shuttle effects of polysulfides to some extent, the lack of polarized groups in non-polar carbon interlayers have weak affinity with polar lithium polysulfides. In order to enhance the cycling stability in Li–S batteries further, the modification of the CNT interlayer surface by heteroatoms is an effective method. Kim el al. prepared a high sulfur-loading electrode with N-doped MWCNT coating on it [308]. The N-doped MWCNT provided conductive pathways for the cell and had interactions with polysulfides to suppress their migrations. Even at high sulfur loading of 10 mg∙cm
2, the modified high-sulfur loading electrode displayed high capacity retention of 91% at the current density of 1/3 C after 50 cycles. Except for the modification way by heteroatoms doping, the CNT based interlayers can be modulated with other conductive carbon materials or functional parts [309]. Huang et al. prepared the graphene oxide-CNT hybrid films as the interlayer in Li–S batteries by a vacuum filtration method [310]. The graphene oxide acted as a polar section with extensive oxygen-containing groups which have strong chemical bonding interactions with lithium polysulfides. Combination of conductive CNT networks and graphene oxide with polar groups, the graphene oxide-CNT hybrid are beneficial to fast electronic transport and the immobilization of polysulfides. As a result, the hybrid had low capacity degradation and retention capacity of 671 mAh∙g
1 after 300 cycles at the current density of 0.2 C. More importantly, combining the CNT with other carbon contributes to Li–S batteries with high sulfur loading and energy density. Chang et al. designed a free-standing tandem sulfur cathodes with SWCNT/CNF thin films inserting between CNF/S thin films [311]. This configuration of sulfur cathode can achieve high sulfur loading reaching 16 mg∙cm
2. Even at such high sulfur loading mass, it possessed high initial capacity of 771 mAh∙g
1 and areal capacity of 12.3 mAh∙cm
2 at 0.1 C. Yu et al. presented a sandwich electrode to utilize carbon nanotube/nanofibrillated cellulose (CNT/NFC) framework as bottom and top layers wrapping the active layer which consisted of sulfur/N-doped graphene composites [312]. The CNTs were interpenetrated with NFCs fabricating conductive network and the hydroxyl functional groups in NFCs had strong interactions with Li2S. At high areal sulfur loading of 8.1 mg∙cm
2, the electrode exhibited high capacity and stable cycling performance with the initial capacity of 935.6 mAh∙g
1 and the retention capacity of 303.2 mAh∙g
1 after 1000 cycles at 0.5 C as shown in Fig. 16b. Polymers modulated CNT interlayers are also common as the functional groups in polymers that are regarded as chemical confinement sites for lithium polysulfides. Wang et al. fabricated a free-standing poly(3,4ethylenedioxythiophene): polystyrene sulfonate–carbon nanotube (PEDOT: PSS-CNT) interlayer for Li–S batteries [313]. By the synergic effects of physical and chemical adsorption interactions with polysulfides, the PEDOT: PSS-CNT interlayer can achieve high specific capacity and excellent cycling performance. Kim et al. prepared poly(acylic acid)-coated single walled carbon nanotube (PAA-SWNT) film as the layer to impede the migration of polysulfides [314]. The SWNT constructed the conductive network and the carboxyl groups in PAA had strong chemical interactions with polysulfides. Therefore, the PAA-SWNT had better cycling stability compared with pristine SWCNT which only possessed physical confinement with polysulfides. Yoo et al. designed a type of CNT interlayer with in situ growth of covalent organic framework (COF) on it [315]. The authors fabricated the hierarchical porous chemical trap and analyze the influence of COF with different pore size on the capture of lithium polysulfides. The COF with micropore size of 0.7 nm promoted the solid-liquid conversion of Li2S and displayed significant improvement of cycling stability in Li–S batteries. The inorganic materials also have strong affinity with lithium polysulfides and are combined with CNT acting as the interlayer in Li–S batteries [227,232,316–319]. Kong et al. prepared the MnO2/graphene oxide/carbon nanotube (G/M@CNT) interlayer applied in Li–S batteries [318]. Both of the MnO2 nanoparticles sheets and graphene oxide layers had strong chemical interactions with lithium polysulfides and played

embedded cathode from passivation. While Manthiram’s group and Jeong’s group successively applied a strategy that dropped an apporiate amount of catholyte on the free-standing CNT paper [299,300]. CNT not only acted as a support of the catholyte but also a conductive current collector. The Li–S batteries made of the catholyte@CNT film in both works yielded high initial discharge capacities. The CNF as the current collector has also been reported [301,302]. Liu and his-coworkers prepared a hollow carbon fiber foam via a simple, low cost and scalable approach [301,302]. When used as both 3D current collector and MWCNT-S slurry scaffold, it can make the electrode achieve ultrahigh area sulfur loading, ranging from 6.2 to 21. 2 mg∙cm
2. When the loading achieved 16.5 mg∙cm
2, the HCFF-S electrodes exhibited a specific capacity of 800 mAh∙g
1 and area capacity of 12 mAh∙cm
3, and the Coulombic efficiency could also achieve higher than 95%. Wang’s group reported a N,S dual doped carbon paper as the current collector to realize the high performance Li–S batteries with high sulfur loading (9.0 mg∙cm
2) [113]. When the sulfur-doped CMK-3 matched with this N,S-dual doped carbon paper, the cycling stability enhanced sighnificantly with only a fading rate of 0.074% per cycle for 300 cycles. In addition to CNF and CNT current collector, recently, a stainless stell fibers current collector has been reported by Huang’s group [303]. The 316 L SSF metal yarns used as both current collectors and cathode supports for Li–S batteries. The porous fibrous structure allowed the imbition of graphene-sulfur composite by capillary action to form a flexible hybrid fibrous cathode, exhibiting a reversible capacity of 335 mAh⋅g
1 at 0.1 C ater 100 cycles. CNT/CNF current collectors could provide the high electrolyte permeation for active material accessibility, and mechanical robustness without the assistance of a binder [170]. Even due to their nonpolar nature, they show inferior interfacial affinity to polar lithium polysulfides, but they could combine with the metal compounds and polymers as the composites current collectors to overcome this drawback [208,242]. Actually the main problem of CNF and CNT nanostructured current collectors is the poor utilization of inner channels [170]. 3.7. 1D nanostructured interlayers for Li–S batteries Manthiram’s group first proposed a strategy to intercalate an interlayer between the sulfur cathode and separator in Li–S batteries to restrain the diffusion of polysulfides [304,305]. The efficient interlayers play important roles in interacting with lithium polysulfides and are in favor of enhancing the batteries’ cycling stability. Because of this original design, the preparation of functional interlayers in Li–S batteries has attracted extensive attention. 3.7.1. CNT based interlayers CNT based materials are suitable for constructing the interlayers in Li–S batteries owing to its characteristics in high conductivity and surface modifiability. For example, the MWCNT paper can be inserted between the sulfur cathode and the separator as an interlayer to work as pseudoupper current collector as shown in Fig. 16a [304]. This MWCNT interlayer can reduce the charge transfer resistance in sulfur cathode and restrain the migration of polysulfides. Owing to the design of the MWCNT interlayer, the cycling stability of Li–S batteries has been improved with high specific capacity. Furthermore, the performances of Li–S batteries by utilizing a bare MWCNT paper or a self-assembled MWCNT paper have been investigated [306]. In comparison with the bare MWCNT interlayer, the self-assembled MWCNT was beneficial to the interfacial contact between the interlayer and the cathode and promote the fast transport of ions and electrons, facilitating the Li–S batteries a better cycling performance. Sun et al. used cross-stacked CNT film as the interlayer which was composed of super-aligned CNT layers [307]. The construction of cross-stacked CNT film can support the cathode materials and provide conductive network to promote the electron transfer for the cathode. Therefore, the cross-stacked CNT film restrains the loss of the active material with the capacity of 460 mAh∙g
1 at the 17

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Fig. 16. (a) A schematic cell configuration of rechargeable Li–S batteries: traditional configuration with severe shuttle effect and Li2S poison problems (left) and new configuration with the MWCNT interlayer (right). Reproduced with the permission from Ref. [304], Copyright 2012, the Royal Society of Chemistry. (b) Cycling performances of the electrode with sulfur loading of 8.1 mg∙cm
2. Reproduced with the permission from Ref. [312], Copyright 2016, WILEY-VCH. (c) A schematic configuration of the Li–S cell with a BCFM interlayer for capturing the polysulfides, (d) Cycling performances of the cells with and without the BCFM interlayer at 1 C. Reproduced with the permission from Ref. [335], Copyright 2015, the Royal Society of Chemistry. (e) The schematic diagrams of the role of CNF@VS2/CNT@GN and CNF@CNT@GN interlayers in Li–S batteries, (f) Cycling stability of the cell with CNF@VS2/CNT@GN at high sulfur loading of 5.6 mg∙cm
2. Reproduced with the permission from Ref. [354], Copyright 2018, Elsevier.

the carbonization method to prepare a interlayer of porous bamboo carbon fiber membrane (BCFM) for capturing the lithium polysulfides as shown in Fig. 16c. [335]. The macro/microporous structures in conductive bamboo carbon fiber membrane can promote rapid ionic transport and play the role in confinement of sulfur. As a result, the Li–S batteries with BCFM interlayer delivered high cycling stability (Fig. 16d) with the capacity decay rate of 0.11% per cycle at 1 C after 300 cycles. Lee et al. prepared the graphitic carbon fiber felt (GCFF) interlayer after heat treatment at 2800  C [336]. The electrospinning GCFF achieved low electrical resistivity after graphitization process, which facilitated the charge transfer process and displayed high initial discharge capacity of 1280.14 mAh∙g
1 at the current density of 0.2 C. Singhal et al. studied the influence of thickness, surface area and pore size distribution of the CNF interlayers including carbonized PAN nanofibers (NPCNF), CO2 activated carbonized PAN nanofibers (ACNF), and carbonized PAN-Nafion nanofibers (MCNF) on the cycling performance of Li–S batteries [337]. According to their study, the thickness of the interlayer was the key factor to improve the utilization of the active material and suppress the migration of polysulfides. In addition, the porous structure mainly affected the ability to prevent the diffusion of polysulfides. In order to enhance the adsorption efficiency of lithium polysulfides by CNFs, Li et al. utilized cyclized-polyacrylonitrile modifying the CNF interlayer (CP@CNF) [338]. Because of the existence of

important roles in suppressing the shuttle effects of polysulfides. Meanwhile, the CNT acted as the conductive scaffold to promote the charge transport. Because of the synergic effects of physical shields and chemical confinement in G/M@CNT, the Li–S batteries exhibited stable cycling performance with a low capacity fade rate and high rate performance. Luo et al. presented a multifunctional interlayer consisting of molybdenum diphosphide nanoparticles and a CNT film [319]. The conductive CNT film can support the molybdenum diphosphide nanoparticles which can impede the shuttle phenomenon and increase the redox reactivity of the polysulfides. Because of this design, the electrode displayed an improved cycling stability with the capacity retention of 905 mAh∙g
1 after 100 cycles at 0.2 C. 3.7.2. CNF based interlayers CNFs with high conductivity, tunable porosity, is also a good interlayer candidate that was used in Li–S batteries to suppress the shuttle effects of lithium polysulfides [320–329]. As introduced in 2.1.2, there are two common strategies to obtain the CNFs, among which one of the synthesis way is carbonization [330,331], another one is electrospinning [332,333]. Zhang et al. presented the CNF monolith by carbonizing filamentous fungi as the interlayer [334]. It can be noted that the CNF monolith derived from the biomass material achieved nitrogen doping facilitating the confinement of dissolved lithium polysulfides to improve the cycling performance of Li–S batteries. Our group successfully used 18

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cycles at the current density of 1 C. In comparison with metal oxides, some metal sulfides exhibit metallic or half-metallic conductivity and still have strong affinity with lithium polysulfides. Ma et al. used hierarchically porous CoS2/carbon paper interlayer to trap polysulfides in Li–S batteries [353]. The porous structure of the CoS2/carbon paper interlayer can capture the dissolved polysulfides by physical adsorption and the CoS2 can restrain the shuttle effects by chemical interactions. As a result, the synergic effects of the interlayer can improve the cycling performance of the relevant Li–S batteries. Wang et al. prepared a multicomponent sandwich-type interlayer consisting of VS2/CNT composite, CNF substrate and graphene coating layer (CNF@VS2/CNT@GN) [354]. The interlayer was complex and each component had its unique function in the batteries. The VS2/CNT composite can anchor the lithium polysulfides effectively and restrain the self-discharge phenomenon in Li–S batteries as shown in Fig. 16e. The CNF substrate played a role in increasing the wettability of electrolyte and reducing the diffusion impedance of lithium ions. The GN layer was used as the second collector in the structure. By combining the features of each parts in the interlayer, the Li–S batteries delivered high rate capability with the capacities of 1525, 903, 834, 686, and 621 mAh∙g
1 at the current densities of 0.1, 0.5, 1, 5, and 10 C respectively. Even with the increase of sulfur loading reaching 5.6 mg∙cm
2, the relevant Li–S batteries displayed high capacity of 750 mAh∙g
1 at 0.3 C with the retention capacity of 685 mAh∙g
1 after 45 cycles as shown in Fig. 16f.

cyclized-polyacrylonitrile in CP@CNF, the CP@CNF delivered a stronger pyridinic nitrogen peak than that of CNF. The extensive pyridine groups were beneficial to confine the diffusion of polysulfides and the Li–S batteries with the interlayer of CP@CNF displayed the improved performance with a capacity retention of 74% after 200 cycles at the current density of 0.3 C. Except for nitrogen-containing groups, the oxygen-containing functional groups have strong affinity with lithium polysulfides as well [339]. Wu et al. used excimer ultraviolet radiation to functionalize the surface of CNF interlayer (EUV-CNF) [340]. After the radiation process, the oxygenated functional groups and nanopores were generated on the surface of the interlayer. Therefore, the Li-O interaction can impede the diffusion of polysulfides and the carbon backbone of the interlayer can reduce the resistance of the charge transfer resistance. At the current density of 0.2 C, the Li–S battery with the EUV-CNF interlayer had high retention capacity of 917 mAh∙g
1 after 200 cycles and the average capacity decay rate of 0.16% per cycle. As discussed in 3.3, inorganic materials, for example, transitional metal oxides with strong polar surface could trap polysulfides effectively but lack of high conductivity. As a result, an effective solution is to construct composites with high conductive carbon materials providing conductive pathways and CNF is a suitable carbon substrate to form a CNF supported inorganic compounds interlayer to improve their cycling performance further [341–346]. The CNF-based compounds can supplement physical and chemical shields anchoring lithium polysulfides and prolong the life span of the batteries. Titanic oxide have been widely used as trapping sites in Li–S batteries with strong electrostatic interactions with lithium polysulfides [347–349]. Liang et al. presented the CNF substrate coated with TiO2 shell (CNF-T) as an interlayer in Li–S batteries [349]. The conductive CNF can overcome the high charge transfer resistance drawbacks of TiO2 which acted as chemical adsorbent to impede the diffusion of polysulfides. According to the consequence by four-point probe method, CNF and CNF-T interlayer had comparable conductivity around 10.8 S∙m
1 and 9.8 S∙m
1 respectively, which proved that the existence of high conductive CNF played the role in reducing the charge transfer resistance. Therefore, the Li–S batteries with CNF-T interlayers displayed stable cycling performance with capacity retention of 74.2% after 500 cycles at the current density of 1 C. With the increase of sulfur loading in the cathode, the Li–S batteries will face more serious shuttle effects of polysulfides and capacity decay. Zhao et al. prepared praline-like interlayer composed of TiO2 nanoparticles and CNF conductive backbones with sulfur loading of 3 mg∙cm
2 in the cathode [350]. Because of the synergistic of TiO2 nanoparticles and porous carbon network, the metal oxide composites delivered good capacity and cycling stability with the retention capacity of 770.8 mAh∙g
1 after 300 cycles at 0.2 C. The interaction mechanism of V2O5 or MnO2 with lithium polysulfides is different from that of the TiO2 adsorbent. Owing to the high redox potentials of V2O5 and MnO2, they can oxidize the polysulfides and anchor the products, while TiO2 only displays electrostatic interactions with lithium polysulfides [342]. Liu et al. prepared a V2O5 decorated CNF (VCNF) interlayer exhibiting good rate performance and cycling stability [351]. It can be noticed that the V2O5 can not only play a role in interacting with polysulfides strongly, but also provide extra capacity and voltage regulation function and avoid the self-discharge problem [351]. Owing to the effective design, the Li–S batteries with VCNF-V2O5 interlayer delivered high rate capability with the capacities of 1432, 1059, 953, 849, 757, and 709 mAh∙g
1 at the current densities of 0.1, 0.3, 0.5, 1, 3, and 5 C, respectively. In addition, the VCNF-V2O5 interlayer also made the Li–S batteries display stable cycling performance with the capacity retention of 70.6% after 1000 cycles at 3 C. Liu et al. utilized CNF/MnO2 composites coating on sulfur cathode to restrain the shuttle effects of polysulfides [352]. The MnO2 can anchor the polysulfides by in-situ generated thiosulfate species and the CNF can provide conductive pathways [352]. Taking the advantages of MnO2 and CNF, the interlayer improved the utilization of active materials and the cycling stability of Li–S batteries with the residual capacity of about 600 mAh∙g
1 after 400

3.7.3. Other 1D nanostructured interlayers In addition to the CNT/CNF supported inorganic material interlayers, other 1D inorganic materials as the interlayers for Li–S batteries have been reported too [355–358]. Zhao et al. modified the sulfur cathode with a brush-like interlayer consisting of ZnO nanowires growing on the CNFs [357]. The authors first grew the ZnO nanowires on the surface of Ni foam acting as the interlayer that makes Li–S batteries display stable cycling stability at 2 C. According to the theoretical density functional theory calculation, the ZnO displayed strong affinity with lithium polysulfides by forming Li-O interactions. Inspired by this successful design, they further displaced Ni foam by lighter CNF and synthesized brush-like ZnO/CNF interlayer coating on sulfur cathode. Therefore, the ZnO/CNF interlayer can anchor the lithium polysulfides effectively and the resultant Li–S batteries delivered cycling stability and low capacity decay rate with 0.05% average capacity loss per cycle after 200 cycles at 1 C. Moreover, Peng et al. prepared a type of 1D interlayer coated on the sulfur cathode through electrospinning methods [359]. The nitrogen-doped carbon black particles were co-electrospun with PAN and dispersed uniformly in PAN fibers. In this work, the high conductivity of the interlayer was based on the nitrogen-doped carbon black particles which played a role in interacting with lithium polysulfides. Therefore, the sulfur cathode modified by that interlayer achieved stable cycling performance with the initial capacity of 1279 mAh∙g
1 and the maintaining capacity of 1030 mAh∙g
1 after 100 cycles at 200 mA∙g
1. In conclusion, to slow down the diffusion of polysulfides and prevent the loss of sulfur species in a cell, a interlayer has the capability to alleviate the diffusion of polysulfides re-utilize precipitated sulfur for Li–S cell with long cycle life and high sulfur utilization. However, additional interlayers in the Li–S cells reducing the energy density is a shortcoming that can’t be ignored. 3.8. 1D nanomaterials as the separators for Li–S batteries The separator that provides the pathways for ions transport and divides the direct contact between anode and cathode avoiding the internal short circuit, is an essential component in batteries [360]. However, the generated lithium polysulfides during the electrochemical process in the Li–S batteries can dissolve in the electrolyte and migrate through the separator reacting with lithium anode, which will result in the destruction of anode and the capacity decay. Therefore, the development and 19

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transport pathways. Even at high sulfur loading of 70 wt%, the Li–S batteries with the HNCF@pδ-MnO2 separator presented a stable cycling performance with the capacity of 856.1 mAh g-1 after 200 cycles at the current density of 0.5 C. Jeong et al. delivered an exfoliated 1T MoS2@CNT-based bifunctional separator for Li–S batteries as shown in Fig. 17e [373]. Exfoliated MoS2-based functional separator without CNT web strongly interacts with polysulfides but easily results in the formation of the inhomogeneous aggregates (indicated as dark brown colored chucks, shown in Fig. 17g) of the irreversible discharge products during cycling. While for the MoS2@CNT coated seprators, MoS2 effectively traps polysulfides and CNT web provides fast electron pathway to the trapped polysulfides leading to the prevention of huge aggregation of irreversible discharge products as shown in Fig. 17f. As a result, the MoS2@CNT-based separator presented better performance to block the migration of Li2S6 molecules. And the Li–S batteries with the MoS2@CNT-based separator delivered high cycling stability with the capacity of about 670 mAh∙g
1 after 500 cycles at 1 C.

modification of separators to restrain the shuttle effects of lithium polysulfides is an effective way to deliver a Li–S battery with high cycling performance. 3.8.1. Separators modification by 1D nanostructures The separator can be modified by CNTs and CNFs which act as an upper current collector to enhance the electrical conductivity and play a role in suppressing the migration of lithium polysulfides [361–364]. Chung et al. prepared a series of activated-carbon-nanofiber-filter-coated separator applied in Li–S batteries [324]. The carbon nanofibers were prepared by electrospinning technology and activated by potassium hydroxide which can permeated the carbon substrate and adjust the surface area and porous structure. As a result, the activated carbon nanofibers displayed higher microporosity and micropore sizes range from 0.4-1.2 nm, which can capture the dissolved polysulfide effectively. The relevant Li–S batteries displayed capacity decay rate of 0.13% per cycle after 200 cycles at the current density of 0.2 C. Though the 1D carbon-coating separators can improve the cycling performance of the Li–S batteries, the polarized surface of 1D carbon materials can interact with lithium sulfides stronger [365–367]. One of the preparation methods to polarize the surface of the carbon materials is by heteroatom doping process. Chung et al. applied a boron-doped MWCNT coated separator in Li–S batteries [368]. The boron-doping of the CNT can overcome the drawback of non-polar surface on the carbon materials and provide chemical adsorbing sites to restrain the diffusion of lithium polysulfides. Ponraj et al. modified the separator by using hydroxyl-functionalized carbon nanotubes (CNTOH) [369]. The CNTOH were prepared by hydrothermal method with the oxygen content of 9.4 at %. The conductive CNTOH facilitated the rapid transport of electrons and the hydroxyl groups in CNTOH had strong interactions with lithium polysulfides (shown in Fig. 17a). Therefore, the modified separator can mitigate the migration of lithium polysulfides and improve the cycling performance of Li–S batteries effectively. The relevant Li–S batteries displayed the initial capacity of 1056 mAh∙g
1 with a capacity decay rate of 0.11% per cycle adter 400 cycles at 0.5 C as shown in Fig. 17b, far less than that of pristine separator and CNT-coated seprator. Polymers consisting of polar functional groups are beneficial to restrain the migration of lithium polysulfides [370]. Luo et al. prepared the modified separator decorated by the layer-by-layer MWCNT/PEG [371]. The MWCNT/PEG coating layers acted as an upper current collector and combined the physical and chemical interactions to impede the diffusion of lithium polysulfides. Chang et al. utilized an ultra-light weight polyaniline nanofibers/MWCNT (PANiNF/MWCNT) coated separator in Li–S batteries [372]. The conductive polymer PANiNF restrained the migration of polysulfides effectively deriving from the chemical interactions between the imine group of the quinoid ring and the sulfur-containing species as shown in Fig. 17c [372]. In addition, the MWCNT was used as the conductive skeleton of the coating layer, leading to higher utilization of the active materials. As a result, the Li–S batteries with the functionalized separator displayed superior stabling cycling performances and ultra-small self-discharge as shown in Fig. 17d. The inorganic materials with polar bonds acting as trapping sites for lithium polysulfides can modify the separator in Li–S batteries with high cycling performance [374,375]. Yang et al. prepared the TiO2 decorated carbon nanotube composite (CNT@TiO2) by hydrolytic method and coated the separator by the CNT@TiO2 composite [376]. The CNT@TiO2 composite provided strong chemisorption with lithium polysufldies and facilitated to enhance the reutilization of the active material. In comparison with the simple CNT coating layer, the CNT@TiO2 coated separator can improve the cycling performance of the Li–S battery and display 12.6% more discharge capacity improvement. Lai et al. delivered a carbon nanofiber@mesoporous δ-MnO2 nanosheet-coated (HNCF@pδ-MnO2) separator for Li–S batteries [377]. The δ-MnO2 nanosheets can restrain the migration of long-chain polysulfides and facilitate the sediment of short-chain polysulfides [377,378]. The conductive framework of carbon nanofibers provided the rapid electron

3.8.2. New separators with 1D nanostructures A separator is a key component of battery technology. It is well recognized that the porous structure and thermal dimensional stability of separators considerably affect the Li-ion battery performance [379]. For Li–S batteries, the appropriate separators should also have the functionalities of inhibiting the polysulfides shuttle and lithium dendrite growth [360,380]. Until now, the research on developing new type separators for current Li–S batteries is still on its early stages [379]. And there were few reports on new 1D nanostructured separators. Wang et al. prepared a separator with hybrid structure composed of a conventional polypropylene/polyethylene (PP/PE) membrane and a commercial glassy fiber (GF) paper [379]. In comparison with PP/PE membrane, the porosity of GF paper is higher which means than the GF paper is beneficial to the fast ionic transport. In the meantime, the porous GF paper can suppress the diffusion of polysulfides mitigating the shuttle effects more effectively compared with the PP/PE membrane. Zhu et al. utilized GF membrane as the separator directly without using commercial polypropylene membrane [381]. The excess utilization of polypropylene membrane can increase the resistance and decrease the energy density of the batteries. The GF membrane with high porosity structure had higher electrolyte wettability and thermal stability. As a result, the Li–S batteries based on a simple GF membrane delivered better cycling stability and rate performance. These works have introduced novel separators and provided new design proposal for the configurations of Li–S batteries. Li et al. not only utilized the GF membrane as a separator, but modified the GF membrane by coating porous carbon nanofibers (PCNF) derived from immiscible polyacrylonitrile/poly(methyl methacrylate) [382]. The PCNF coating layers can act as the upper current collector facilitating the electrical transport and capture the dissolved lithium polysulfides. Because of this modification method, the performance of resultant Li–S batteries was improved further delivering the initial capacity of 1499 mAh∙g
1 and a retention capacity of 808 mAh∙g
1 after 200 cycles at the current density of 0.2 C. According to the above analysis and summary, the multi-component 1D separators are more effectively for lithium polysulfide capture to achieve advanced Li–S batteries [383–391]. While the new type 1D separators that are suitable for Li–S batteries are limited, which required further developed. And many 1D nanostructures-coating separators will cause the problem of adding the extra weight for the battery, reduce the energy density and increase the internal resistances. 4. Li–Se batteries The electrochemical behavior of Li–Se batteries is similar to that of Li–S batteries, hence the drawbacks hindering Se cathode’s practical use are very similar to Li–S batteries: (1) bulk Se has low reactivity with lithium metal [392]; (2) element Se easily dissolve in the ether-based 20

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Fig. 17. (a) Schematic illustration of trapping lithium polysulfides by the CNTOH-coated separator, (b) Discharge capacities of the of Li–S cells with different separators at 0.5 C rate as a function of cycle number. Reproduced with the permission from Ref. [369]. Copyright of American Chemical Society 2017. (c) A schematic of the Li–S cell employing the PANiNF/MWCNT-functionalized separator, (d) The statically electrochemical stability of Li–S cell with the PANiNF/MWCNT-functionalized separator. Reproduced with the permission from Ref. [372]. Copyright of the Royal Society of Chemistry 2015. (e) Schematic diagram of an exfoliated MoS2 based functional separator for lithium sulfur batteries, (f) 1T-phase exfoliated MoS2@CNT-based bifunctional separator, (g) Exfoliated MoS2-based functional separator without CNT web. Reproduced with the permission from Ref. [373]. Copyright 2017, the Royal Society of Chemistry.

of Li–Se batteries. Hence, in this section, the contents are mainly introduced to 1D nano selenium cathode, 1D carbon nanomaterials-based hosts, and 1D nanostructured interlayer for Li–Se batteries.

electrolytes, occuring serious shuttle phenomenon, which leads to capacity fading on discharge/charge cycling [393–395]; (3) the volume expansion during charge/discharge process results in the electrode collapse [8,396]. What’s more, Se has a lower abundance than S, leading to a significantly higher cost almost 10 times as many as that of S [8,394]. To realize high-performance Li–Se batteries, several strategies including converting bulk Se to nano-sized Se [397,398], encapsulating element selenium into various hosts [399–401], inserting interlayers or coating separators [402–404], developing new electrolytes [405,406], etc., have been proposed to reduce the polyselenides dissolving and shuttling, thus efficaciously improving the electrochemical performances

4.1. Se nanowires and nanofibers as the cathodes Selenium nanowires/nanofibers are extremely attractive since this special 1D character not only can accommodate volume expansion without pulverization, but also can facilitate axial charge transport and short radial Li-ion diffusion distances [397]. Gao and co-workers investigated the amorphous selenium (a-Se) 21

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Fig. 18. Initial charge/discharge curves of the (a) a-Se, (b) c/a-Se, and (c) c-Se NWs (600 rpm, 10 h) at a current density of 0.1 C between 0.01 and 3.0 V. Reproduced with permission from Ref. [397], Copyright 2010, American Chemical Society. (d) Schematic illustration for the formation of G@Se/PANI and the corresponding TEM images of Se NWs (e), Se/PANI (f), G@Se/PANI (g). Reproduced with permission from Ref. [407], Copyright 2015, Elsevier.

(G@Se/PANI) was designed by Qian et al. as shown in Fig. 18d [407]. The selenium NWs (Fig. 18e) were well-sealed in the PANI layer (Fig. 18f) with a thickness of 25 nm forming a core/shell structure and then encapsulated in graphene nanosheets (Fig. 18g). With such a unique 1D nanostructure, the G@Se/PANI nanocomposite exhibited a superior high reversible discharge capacity of 510.9 mAh∙g
1 at 2 C after 200 cycles. Zhang et al., reported a 1D selenium/carbon-rich core-shell nanostructures by using the Se NWs as the template and ascorbic acid as the carbon source [410]. This 1D Se/carbon-rich composites exhibited a discharge capacity of 558 mAh∙g
1 in the first cycle and maintained a capacity of 181 mAh∙g
1 after 80 cycles at a rate of 0.5 C, better than that of the pristine selenium. Even dealing elemental selenium to nanosize could enahnce the electrochemical performances, but such improvement is not enough due to the inherent low conductivity of element selenium.

nanowires (NWs), crystalline (c) and crystalline/amorphous (c/a) selenium NWs that have been successfully prepared through a facile highenergy ball-milling method, as the cathode materials for Li–Se batteries [397]. Different crystallization of Se NWs showed different chemical reaction process with Liþ. As shown in Fig. 18a-c, the galvanostatic charge/discharge voltage performances of a-Se, c/a-Se, and c-Se NWs were revealed. For amorphous Se NWs and crystalline/amorphous Se NWs, the initial discharge involved three well-defined plateaus (at 1.55 V, 1.6 and 2.04 V) indicative of the Li ion insertion, while that of c-Se NWs presented only one platform (at 1.55 V). Such differences could be attributed to: i) in a-Se NWs lithiation process, Se helical chains are first reduced to Li2Sen (n 4) and then further reduced to Li2Se2 and Li2Se, respectively; ii) comparatively, Se helical chains in c-Se NWs are reduced fast to Li2Se, which is due to its active state and the weakened covalence band in Sen helical chains coming from the sufficient electronic and van der Waals forces between the Sen helical chain. Following Wang et al. injected molten Se into the AAO template to form NWs and used as the cathode materials for Li–Se batteries [398]. The Se NWs illustrated a higher storage capacity of 1425.6 mAh∙g
1 compared with the capacity of bulk Se cathode with 454 mAh∙g
1 at a current density of 150 mA∙g
1. The conductivity of Se is indeed not good, thus combined the Se NWs and Se nanofibers with conducting polymers and conductive carbon have been reported [407–410]. For example, a novel nanocomposite of graphene–encapsulated selenium/polyaniline core–shell NWs

4.2. 1D Carbon-Se composites cathodes Compared to pure Se with nanowire/nanofibrous structure, a composite of Se and a conductive agent can work easily and stably because of the low risk that polyselenides will be dissolved in popular carbonatebased or even ether-based electrolytes [395,411]. 4.2.1. CNT-Se Composites Cathodes Amine et al. brought the concept of Se-CNTs composites for 22

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Fig. 19. (a) Formation mechanism of bimodal porous nitrogen-doped carbon nanofiber homogeneously filled with chain-like Se. Reproduced with the permission from Ref. [400], Copyright 2018, the Royal Society of Chemistry. (b) Schematic illustration of the synthesis process of the Se@PCNFs electrode. Reproduced with the permission from Ref. [416], Copyright 2014, WILEY-VCH.

composite cathode delivered an initial discharge capacity of 563.9 mAh∙g
1 and a reversible capacity of 414.5 mAh∙g
1 after 100 cycles at 0.2 C. The unique interconnected porous nanofiber structure of PCNFWs contributed to the facilitating of charge transport, the incorporation of selenium and the trapping of polyselenides and cycled products. Recently, a novel porous CNFs with bimodal pores (micro/meso), as efficient Se hosts for Li–Se batteries, has been successfully synthesized by carbonization of electrospun zeolitic imidazole framework-8/PAN nanofibers and further chemical activation as shown in Fig. 19a [400]. The mesopores contributed to easy access of electrolyte and the micropores facilitated the high utilization of chain-like selenium with low range ordering as well as the 1D N-doped CNF provided excellent electronic conductivity. As a result, a discharge capacity of the Se-BP-CNF could achieve 588 mAh∙g
1 at the 300th cycle at a current density of 0.5 C. In order to further improve the volumetric energy and power density of Li–Se batteries, it is ideal to design a flexible, free-standing, and binder-free electrode. Yu’s group have done good works [416,417], and the earliest work could be dated back to 2014. A flexible and free-standing porous CNFs/selenium composite electrode (Se@PCNFs) was prepared by infiltrating Se into mesoporous carbon nanofibers (PCNFs) as shown in Fig. 19b [416]. Se@PCNFs electrodes delivered a reversible capacity of 516 mAh∙g
1 after 900 cycles without any capacity loss at current of 500 mA∙g
1 as shown in Fig. 19c. Such excellent results could be attributed to i) mesoporous CNFs allowed most Se molecules to react with Li ions; ii) 3D interconnected CNFs provided

rechargeable lithium batteries [16]. They mainly focused on the electrochemical reaction mechanisms of Se cathode in carbonate electrolyte and ether-based electrolyte. In carbonate electrolyte, Se was redced to Li2Se in one step during discharge process, while in ether-based electrolyte, Se was first reduced to polyselenides (Li2Sen, n4), then reduced to Li2Se. The reversible capacity and selenium loading in this work is not enough high. Therefore, following works were devoted to improve the cycling stability and selenium loading by modifying the CNTs [399,412, 413]. Dutta et al. developed a pressure-induced capillary encapsulation strategy to confine the selenium in an open-ended MWCNTs [399]. The loading of selenium inside of MWCNTs could reach as high as 85%. Zhang et al. prepared a MWCNT/Se composite via a solution-based process (denoted MWCNT/Se-S) with using the ethylenediamine modified the MWCNT. [412]. Compared to the MWCNT/Se composite synthesized by melt-diffusion method (denoted MWCNT/Se-M), the MWCNT/Se-S exhibited a higher initial discharge capacity (645.7 mAh∙g
1 vs. 440.3 mAh∙g
1) and reversible capacity (355.5 mAh∙g
1 vs. 128.1mAh∙g
1) after 100 cycles. 4.2.2. CNF-Se Composites Cathodes CNF with outstanding physical and chemical properties can satisfy the requirements as Se host [414]. Zhang et al. fabricated a porous carbon nanofiber webs/selenium (PCNFW/Se) composites via heating the selenium with interconnected porous carbon nanofiber webs derived from the PPy nanofiber in Ar atmosphere [415]. The PCNFW/Se

23

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Fig. 20. (a) Schematic illustration of the synthesis procedure of the 3DG-CNT@Se composite. (b) Cyclic stability of the CNT@Se and 3DGCNT@Se cathode at 0.2 C for 150 cycles. (c) Rate performance at various C rates of the CNT@Se and 3DG-CNT@Se cathode. Reproduced with permission from Ref. [17], Copyright 2016, American Chemical Society. (d) Schematic diagram showing the preparation of Se/(CNT@MPC). Reproduced with permission from Ref. [392], Copyright 2016, American Chemical Society.

254 mAh∙g
1 at 10 C rate and an insignificant decay of 0.01% per cycle at 1 C rate for 4000 cycles. A microporous carbon (MPC) coated CNT as the Se host was reported by Xin et al. [392], which the Se was loaded into the carbon substrate via a simple mixing-heating route as shown in Fig. 20d. Confined the Sen chains in microporous slits, the Sen chains were converted to Li2Se in a single-step reaction, leading to high capacities and excellent cycling performance in the Li
Se batteries, i.e. a high gravimetric specific capacity of 596 mAh∙g
1 remained after 100 cycles (>88% retention of the theoretical capacity, corresponding to a volumetric capacity density of 2536 mAh∙cm
3. To briefly summarized, the 1D CNFs, CNTs, and their hybrids are effectively scaffolds for selenium cathode. The modification of CNT/CNF with functional groups (e.g. carboxyl groups) and heteratoms (e.g. nitrogen doping) can further improve the electrochemical performances of Li–Se batteries.

continuous electron transportation path. In addition, a graphite platelets nanofiber (GPNFs) as the selenium host has also been reported [418]. Interestingly, the Se/GPNFs showed higher capacity and better cycling stability compared to conventional Se/carbon material composites (with GO, reduced GO and CNTs). That was because the GPNFs with conductive nanoribbons of graphitic structures that interlinked the Se particles, inhibiting Se agglomeration, and the abundant pores on GPNFs benefited for electrolyte penetration through the bulk active materials, thus enabling facile reaction of Se with Liþ ions and electrons and high Se utilization during discharge [418]. 4.2.3. CNT/CNF-Carbon-Se Composites Cathodes As mentioned in 3.1.4, the combination of CNT/CNF with carbon hetero-structures into hybrid materials renders sulfur cathodes with high-rate performance, which also works in Li–Se batteries. Graphene-CNT-Se composites electrodes are still the most popular [17,419,420]. For instance, Chen and co-workers reported a free-standing 3D hierarchical graphene-CNT@Se electrodes [17], which was prepared via a two-step of melt diffusion and solvothermal reaction as shown in Fig. 20a. The unique 3D mesoporous, conductive network not only offered highly efficient channels for electron transfer and ionic diffusion, but also prevented polyselenides shuttling and volume expansion of Se during charge/discharge. As expected, the graphene-CNT@Se composites showed better electrochemical performances than CNT@Se (Fig. 20b-c). While Yu et al. reported a free-standing Se@CNF-CNT composite electrode [417]. The Se molecules could be chemically bonded and physical encapsulated by carbonized PAN-CNT composite. Accordingly, the Se@CNFs-CNT electrode illustrated improved cycling capacity of 517 mAh∙g
1 after 500 cycles at 0.5 A∙g
1 and rate capability, i.e. 485 mAh∙g
1 at 1 A∙g
1 rate. Combining the porous carbon with CNT and CNF was not rare. Balakumar and Kalaiselvi reported a tube-in tube carbon (TTC, MWCNT encapsulated by highly porous carbon), acted as an efficient host for Se cathode [401]. With a high pore volume (2.167 cc∙g
1), the selenium loading can reach 70 wt%. But Se@TTC with 50 wt% Se content, demonstrated best cycling performance that the reversible capacity was

4.3. 1D nanostructured interlayers for Li–Se batteries As the Li–Se batteries face a similar challenge as Li–S batteries: discharged intermediates (polyselenides) dissolution and shuttling. Thus, using the interlayers or separators coatings to prevent the polyselenides shuttling should be highly efficient. Work to date, various interlayers and separator coatings for Li–Se batteries have been reported as well [9,335, 403,404,421–426]. However, the 1D nanostructured-interlayer for Li–Se batteries is infrequent. Zhang et al. reported a carbon fiber interlayer for Li–Se batteries in 2014 [402]. This free-standing carbon interlayer was fabricated easily by carbonizing cellulose based filter paper. With this carbon fiber interlayer, the Li–Se cells showed a higher initial capacity of 656.8 mAh∙g
1 and reversible capacity of 520 mAh∙g
1 after 20 cycles, compared to raw Li–Se cells. Li et al. reported a CNT interlayer for Se-CNT-graphene cathode [420]. With introducing the CNT layer into this cathode side, the charge transfer of the cathode was improved significantly. In addition, a CNTs layer acted as a good barrier to inhibit the polyselenides shuttle effect. The 1D nanostructured interlayers could improve the reversible 24

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Fig. 21. (a) Dark-field TEM image of PAN-SeS2, and corresponding elemental mappings of C(b), S(c), Se(d); (e) Cycle life of PAN-SeS2 cathode at 4.0 A∙g
1. Reproduced with permission from Ref. [432], Copyright 2018, American Association for the Advancement of Science.

Amine and co-workers in 2012 [16]. Since then, various kind of SexSy-based cathodes for Li–SexSy batteries have been reported [427–430]. As the SexSy cathodes face the same issue as pure S and Se cathodes, which the polysulfides and polyselenides are easily dissolved into the organic electrolytes with a rapid capacity fading, thus the 1D carbon nanostructures has also been introduced as SexSy host materials to inhibit the polysulfides and polyselenides shuttling. For example, Pandey et al. reported a CNFs/SexS1-x cathode that by simply heating the CNF with sulfur and selenium with the ratio of 1:2:1 [431]. The CNFs/S1-xSex composites combined the higher conductivity and higher density of SeS2 with high specific capacity of sulfur. Therefore, CNFs/S1-xSex electrode demonstrated more than 600 mA∙h∙g
1 specific capacity after 50 cycles at 0.5 C rate, much higher compared to the CNFs/S cathodes. In addition, Lou’s group reported a pyrolyzed PAN fiber as the SeS2 host [432]. As shown in the dark-field TEM image (Fig. 21a) and correspodning maipping images (Fig. 21b-d), the SeS2 has been emmbedded

capacity to some extent and thus the battery’s energy density could enhance. However, the energy density of Se cathode is not high due to its low theoretical specific capacity, and the electrode weight has great effect on the energy density of the whole battery. Thus, the adding weight of interlayers may cause reducing energy density more compared to enhancing the energy density by improving the reversible capacity. 5. Li–SexSy batteries In general, Se shows a quicker reaction kinetic but lower reversible capacity, while S promises a high reversible capacity but slower reaction kinetic [427]. In addition, Se has higher cost than S. Thus, the combination of Se and S, which utilizes the advantages of both Se and S, is a promising strategy to maximize the electrochemical performance of Se and S electrodes for rechargeable lithium batteries. Pioneering work on SexSy (SeS2) based cathodes for Li–SexSy batteries was published by 25

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Fig. 22. Schematic illustration of the advanced Li–S or Li–Se battery with 3D cathode and anode that formed by 1D nanomaterials.

batteries. It is known that the active materials loading in whole cells are usually less than 3 mg∙cm
2, which is far less than the requirement of 5.0 mg∙cm
2 for practical cells with high energy density of more than 350 Wh∙kg
1 [433–436]. These novel cell configurations can be coupled with a pure sulfur or selenium cathode with high loadings. The weight/volume of these alternative 1D nanomaterials should not sacrifice the overall energy density. A lightweight interlayer or thin layer coatings on the separators could potentially alleviate the energy density concern [4]. The consistent progress by unique 1D nanostructures has brought this technology close to practical applications, particularly for Li–S batteries. And it can be clearly seen that this review is devoted to focusing on the modifications for the S/Se cathode inside and outside by using various 1D nanomaterials. However, it can’t be ignored that there are some crucial techniques hindering their commercialization, such as the safety issues caused by dendrites of lithium anode [291]. The 1D nanomaterials, such as the pre-lithiated Si NWs [294], Li7B6 [36], used as the anode could completely eradicate the growth of dendrites of lithium, which definitely appeares using the lithium metal as the anode. Besides, utilizing the 1D nanomaterials, i.e., SCNT [292,293], silver nanowires [291], etc., to protect the lithium anode could efficiently prevent the lithium dendrites growing. As summarized above, the 1D nanomaterials are not only beneficial for constructing a 3D cathode with high active materials loading, but also the 1D nanomaterials are good candidates for building a safety 3D anode, thus a high-energy and -power density, high Coulombie efficiecny, long cycle life, high-safety and environmentally friendly Li–S and Li–Se battery, with 3D nanostructured electrodes that are assembled from 1D nanomaterials (as shown in Fig. 22), could meet the practical applications in the near future. All in all, there is no doubt that Li–S/Se batteries are receiving increasing attentions due to their high theoretical capacity and energy

into the porous pyrolyzed PAN and distributed homogenously. As a result, the pyrolyzed PAN-SeS2 composite delivered high capacity over 1100 mA∙h∙g
1 at 0.2 A∙g
1 for Li storage with extremely stable cycle life over 2000 cycles at 4.0 A∙g
1 as shown in Fig. 21e. In summary, to compensate for the shortfalls of both S and Se, it is believed that the composite SexSy cathode material might have a great potential for building better batteries beyond traditional lithium ion batteries. 6. Conclusions and perspectives In this review, we systematically provide the recent progress of 1D nanomaterials applications in Li–S and Li–Se batteries. Advanced design of cathode materials and novel structure of battery are the key factors to construct Li–S/Se batteries with excellent electrochemical performance. Of them nanostructured 1D carbon play the most important roles in fabricating high-performance Li–S/Se batteries. For example, designing porous CNT/CNF matrix with unique porous structures, doping heteroatoms into CNT/CNF or functionalizing CNT/CNF with enhanced electronic conductivity, controlling dimensionality from 1D to 3D for effectively charge collection and transport as well as channeling electrolyte, all significantly inhibit the shuttle phenomena of polysulfides (polyselenides) and enhance the active materials utilization. Besides, 1D polymer and inorganic compounds or combining proper polymer and inorganic compounds with 1D carbon matrix could further improve the electrochemical performance of Li–S and Li–Se batteries, as these polymers and inorganic compounds commonly exhibit stronger chemical adsorption ability towards polysulfides (polyselenides) compared to the pure 1D carbon frameworks [395,433]. With regard to constructing novel cell structures (i.e. carbon-based current collectors, interlayers and coating separators), the 1D nanomaterials contribute to the impressive performances of Li–S and Li–Se 26

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density in the past two decades. So far, lots of progress in sulfur-based and selenium-based 1D nanostructured electrodes have been achieved. However, it should be realized that some of previous studies sacrifice the sulfur/selenium content in the electrode for higher specific capacity and longer cycle life. For the practical application, the energy density of the Li–S/Se battery is the prior advantage, thus more emphasis should be laid on enhancing the total energy density of Li–S/Se batteries [437]. Except the sulfur/selenium content has a huge effect on the energy density, electrolyte/sulfur and electrolyte/selenium ratio is another important factor that minimizes the energy density of Li–S/Se batteries, which was usually neglected in the past [2]. The understanding in this filed is still at its infant stage, while developing new electrolyte systems, such as gel polymers, seems to promising [2]. From a practical application ponit of view, a high areal/volumetric capacity of the sulfur cathode and a low amount of electrolyte are primary requirements to allow high energy/power densities of Li–S/Se batteries. Last but not least, cost-effective and scalable fabrication techniques are yet to be developed for practical applications of these 1D nanostructures for use in Li–S/Se batteries.

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Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51871113, 51572116 and 51808080), the Venture & Innovation Support Program for Chongqing Overseas Returnees (Grant No. CX2018129), the Science and Technology Research Program of Chongqing Municipal Education Commission (Grant No. KJQN201800808), the Start-up Foundation of High-level Talents in Chongqing Technology and Business University (Grant No.1856008) and Open Research Fund of Chongqing Key Laboratory of Catalysis and New Environmental Materials (Grant No. KFJJ2018082). We want to express our sincerely thanks to Prof. Min Fu and Prof. Jianmin Ma for their help in the process of writing and publishing this work as well. References [1] H.J. Peng, J.Q. Huang, Q. Zhang, A review of flexible lithium-sulfur and analogous alkali metal-chalcogen rechargeable batteries, Chem. Soc. Rev. 46 (2017) 5237–5288. [2] Z.L. Xu, J.K. Kim, K. Kang, Carbon nanomaterials for advanced lithium sulfur batteries, Nano Today 19 (2018) 84–107. [3] Y.X. Yin, S. Xin, Y.G. Guo, L.J. Wan, Lithium-sulfur batteries: electrochemistry, materials, and prospects, Angew. Chem. Int. Ed. 52 (2013) 13186–13200. [4] A. Manthiram, Y. Fu, S.H. Chung, C. Zu, Y.S. Su, Rechargeable lithium-sulfur batteries, Chem. Rev. 114 (2014) 11751–11787. [5] X. Gu, L. Hencz, S. Zhang, Recent development of carbonaceous materials for lithium–sulphur batteries, Batteries 2 (2016) 33–67. [6] Z. Li, H.B. Wu, X.W. Lou, Rational designs and engineering of hollow micro-/ nanostructures as sulfur hosts for advanced lithium–sulfur batteries, Energy Environ. Sci. 9 (2016) 3061–3070. [7] X. Gu, C. Lai, Recent development of metal compound applications in lithium–sulphur batteries, J. Mater. Res. 33 (2017) 16–31. [8] J. Jin, X. Tian, N. Srikanth, L.B. Kong, K. Zhou, Advances and challenges of nanostructured electrodes for Li–Se batteries, J. Mater. Chem. A 5 (2017) 10110–10126. [9] X. Gu, L. Xin, Y. Li, F. Dong, M. Fu, Y. Hou, Highly reversible Li–Se batteries with ultra-lightweight N,S-codoped graphene blocking layer, Nano-Micro Lett. 10 (2018) 59. [10] V.A. Agubra, L. Zuniga, D. Flores, J. Villareal, M. Alcoutlabi, Composite nanofibers as advanced materials for Li-ion, Li-O2 and Li-S batteries, Electrochim. Acta 192 (2016) 529–550. [11] J. Zhang, Y. Shi, Y. Ding, W. Zhang, G. Yu, In situ reactive synthesis of polypyrrole-MnO2 coaxial nanotubes as sulfur hosts for high-performance lithium–sulfur battery, Nano Lett. 16 (2016) 7276–7281. [12] Y. Yang, M.T. McDowell, A. Jackson, J.J. Cha, S.S. Hong, Y. Cui, New nanostructured Li2S/silicon rechargeable battery with high specific energy, Nano Lett. 10 (2010) 1486–1491. [13] T. Chen, Z. Zhang, B. Cheng, R. Chen, Y. Hu, L. Ma, G. Zhu, J. Liu, Z. Jin, Selftemplated formation of interlaced carbon nanotubes threaded hollow Co3S4 nanoboxes for high-rate and heat-resistant lithium–sulfur batteries, J. Am. Chem. Soc. 139 (2017) 12710–12715. [14] G. Zhou, D.W. Wang, F. Li, P.X. Hou, L. Yin, C. Liu, G.Q. Lu, I.R. Gentle, H.M. Cheng, A flexible nanostructured sulphur–carbon nanotube cathode with high rate performance for Li-S batteries, Energy Environ. Sci. 5 (2012) 8901–8906.

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