Carbon‐based flexible self‐supporting cathode for lithium‐sulfur batteries: Progress and perspective

Abstract

The flexible self‐supporting electrode can maintain good mechanical and electrical properties while retaining high specific capacity, which meets the requirements of flexible batteries. Lithium‐sulfur batteries (LSBs), as a new generation of energy storage system, hold much higher theoretical energy density than traditional batteries, and they have attracted extensive attention from both the academic and industrial communities. Selection of a proper substrate material is important for the flexible self‐supporting electrode. Carbon materials, with the advantages of light weight, high conductivity, strong structural plasticity, and low cost, provide the electrode with a large loading space for the active material and a conductive network. This makes the carbon materials meet the mechanical and electrochemical requirements of flexible electrodes. In this paper, the commonly used fabrication methods and recent research progresses of the flexible self‐supporting cathode with a carbon material as the substrate are introduced. Various sulfur loading methods are summarized, which provides useful information for the structural design of the cathode. As the first review article of the carbon‐based flexible self‐supporting LSB cathodes, it provides valuable guidance for the researchers working in the field of LSB.

INTRODUCTION

With the increased demand for portable electronic devices and electric vehicles with long life and high energy density, improving the energy utilization efficiency and storage capacity of the secondary batteries has become the focus of current research work. ^1,2 Compared to the traditional electronic devices, flexible electronic devices are lightweight and wearable, which can greatly increase the capability for structural design and portability of industrial products. Flexible electronic devices need to be equipped with flexible accessories, and batteries, as a crucial part of them, have high requirements in terms of capacity and mechanical properties. In the new energy storage system, lithium-sulfur batteries (LSBs) use sulfur or substances containing sulfur as cathodes and lithium metal as anodes. Compared to other secondary batteries, LSBs have a high theoretical specific capacity (1675 mAh g^–1) and high energy density (2600 Wh kg^–1). The active sulfur species in LSBs are of low cost, no pollution, and nontoxic, which impart them with high potential in the field of flexible electronic devices.

The charging and discharging process of LSBs is a multi-step conversion process of sulfur ions with different valence states. The lithium metal in the anode loses electrons and is oxidized in the process of discharge. Lithium ion migrates to the cathode through the interlayer with the aid of electrochemical potential. The atoms in an S₈ molecule are in the form of eight-membered rings, which obtain electrons and react with lithium ions to form long-chain lithium polysulfides (LiPSs) (Li₂S _n , n ≥ 4). In the following discharge process, long-chain LiPSs will be reduced to form short-chain LiPSs or solid lithium sulfide. The whole charge–discharge cycle is based on the reversible chemical reaction of S₈ + 16Li → 8Li₂S. However, due to the poor electron conductivity of sulfur and the final product lithium sulfide, it is necessary to induce conductive agents to enhance the electrode conductivity. On the contrary, the intermediate product lithium polysulfides are easy to dissolve in the electrolyte, which pass through the separator and migrate to the lithium metal anode under the driving force of concentration gradient. The final insulative product Li₂S would form and deposit on the anode surface to damage the electrode conductivity. The cycle stability and Coulombic efficiency of the battery would be reduced. At the same time, lithium metal itself is active and easy to react with LiPSs to cause self-discharge. In addition, the continuous volumic fluctuation of the sulfur cathode in the process of charge and discharge is easy to destroy the original cathode structure, thus affecting the cycle performance. As the key stone of the LSBs application, sulfur cathode modification is considered an efficient way to improve the electrochemical performance.

To handle these defects, a qualified sulfur cathode should have the following characteristics.

• 1.

  High electrical conductivity to accelerate ion and electron transport capacity.

• 2.

  Adequate pore structure to alleviate volume expansion and improve sulfur content.

• 3.

  Reasonable adsorption capacity to inhibit the shuttle effect.

• 4.

  Stable structure to adapt to the battery cycle process under different working environments. ^3,4

The traditional preparation method of Li-S cathode is to mix the powder of electrode material, conductive agent (carbon black, acetylene black, etc.), and binder (polyvinylidene fluoride, polytetrafluoroethylene, etc.) uniformly and dissolve them in N-methyl pyrrolidone (NMP) and other organic solvents. After stirring, the slurry is formed and coated on the aluminum foil and other collectors. The addition of a conductive agent enhances the conductive property of electrode material to some extent, whereas the conductive agent and binder increase the mass of the electrode and decrease the sulfur content and energy density of the electrode. On the contrary, the binder may block the pores of the electrode material and increase the internal resistance of the electrode, affecting the cycling performance of the electrode. ⁵ In addition, the sulfur utilization is closely related to the contact points between the collector and active materials. After industrial rolling, the adhesion force between the electrode particles under bending state is not uniform, which all results in the incomplete electrolyte permeation, reducing the utilization rate of the active substances. If the content of binder and organic solvents addition is inappropriate, the surface slurry may peel off due to the uneven mechanical properties. Furthermore, the different expansion coefficients of the collector and slurry under a high-temperature environment may also lead to the loss of active species. These issues will reduce the capacity of the electrode (Figure 1). Therefore, one of the important approaches to overcome the above challenges is to design a cathode with high mechanical strength and high specific surface area. ⁶ In contrast, flexible self-supporting electrode does avoid the waste of electrode area without a collector. Even in the bending state, there is a full range of pore infiltration electrolyte, with a larger infiltration area, which improves the utilization rate of electrolyte and active substances. It is worth noting that the flexible self-supporting sulfur cathode avoids the toxicity problems caused by the utilization of NMP in typical coating method. Without a current collector, conductive agent, or binder, it has a higher capacity and weight energy density. This type of electrode uses a flexible paper or film as the substrate material and is coupled with other polar materials to effectively adsorb the polysulfides while integrating the electron and ion conductivity. ⁷ Moreover, the separation between the active material and the collector is avoided in the flexible self-supporting electrode. It can still maintain excellent mechanical and electrical properties even under repeated deformation, which enhances the stability of the battery charge and discharge cycle. In the application, the traditional coated cathode utilizes a metal collector. In the case of overcharge or overdischarge, the metal will react with the hydrofluoric acid produced by the decomposition of electrolyte, resulting in structural damage and short circuit. The self-supporting electrode does not use a collector, thus avoiding this situation.

Figure 1. Factors adversely affecting the performance of lithium-sulfur batteries and the corresponding approaches for improvement

In the past few years, many strategic techniques have been developed on the flexible self-supporting sulfur cathodes to produce high-energy-density LSBs with long cycle life. Carbon materials have been considered as an ideal substrate material for the flexible self-supporting electrode due to the large pore volume and specific surface area, variable dimensional structure, high conductivity and toughness, high abundance, and low cost. ^8-10 However, so far, there are few reviews on the flexible self-supporting cathodes of carbon-based LSBs. It is of great significance to systematically summarize and classify the preparation methods of the carbon-based flexible self-supporting electrodes with various types. To fill this gap, this paper reviews the preparation methods of sulfur cathode materials based on various carbon materials in recent years. The sulfur loading methods in the cathode preparation process are mainly discussed. The relationship between their structural design and electrochemical properties is analyzed. The future research directions and challenges of flexible self-supporting cathode based on carbon substrates for LSBs are discussed.

CARBON SUBSTRATES FOR LSBS FLEXIBLE CATHODES

Carbon materials are often used as the conductive additive or host material in conventional LSBs cathode, and however, these powder-like materials often consist of small particles, which are separated from each other, thus extending the path of electrons and ions. According to such ultra-small size of the discontinuous construction units, the carbon-based conductive additive powder is difficult to be assembled into a flexible self-supporting matrix. Some research work reported that carbon nanotubes (CNTs) and graphene powders at the nanoscale can be assembled to enhance the conductivity of the electrode, or directly serve as the host. Unfortunately, the separation of carbon framework on a microscopic scale leads to the low areal sulfur loading and areal capacity density with even worse cyclic stability. Herein, the flexible carbon substrates are classified into the following three categories based on the morphology and dimension.

• 1.

  One-dimensional (1D) nanocarbon.

• 2.

  Two-dimensional (2D) nanocarbon.

• 3.

  Three-dimensional (3D) skeleton porous carbon.

The first two types of carbon materials have relatively uniform morphology and can be considered to have an ordered structure at the nanoscale. In contrast, the third type of carbon material is commonly obtained by the pyrolysis and carbonization of other organic polymers, which is usually at micron scale. On the basis of this classification, the preparation technologies of each carbon-based flexible self-supporting sulfur cathode are summarized. In the following section, the sulfur loading methods for preparing the carbon-based flexible self-supporting sulfur cathode mentioned above are introduced in detail, and the applicable structures and their impacts are summarized, in which the importance of sulfur loading technology in the preparation of batteries is discussed. In the end, the electrochemical performance of the three carbon-based electrodes is discussed based on the morphology and sulfur loading methods. Review of the research progresses and limitations of carbon-based flexible self-supporting cathodes provide useful guidance for exploring new structural design strategies and novel preparation methods, paving the way to develop high-capacity LSBs applied in different fields.

One-dimensional nanocarbon

Taking advantage of the high aspect ratio, the 1D nanocarbon materials provide excellent tensile strength for the flexibility of the whole cathode, whereas the large specific surface area provides a large number of reactive sites as a highly conductive microscale matrix framework. In addition, a novel 1D carbon structure design can improve the electrochemical reaction kinetics of LSBs. The 1D nanocarbon substrates are mainly CNT and carbon nanofiber (CNF).

Carbon nanotube

The carbon atoms in CNTs are sp²-hybridized, forming a spatial topological structure under a certain degree of bending, with a higher degree of order. ^11-13 Therefore, CNTs have high aspect ratio, modulus, bending degree, and good mechanical properties, which are suitable for the support framework of self-supporting flexible electrodes and meet the requirements of flexible electronic products. Moreover, as the matrix material of cathode, CNTs have excellent conductivity to construct the conductive network of the whole electrode. High specific surface area and the microporous structure improve the compatibility of sulfur and carbon in the microstructure, which provides a physical barrier for the capture of LiPSs. CNTs also meet the requirements of the energy storage devices with light weight and large capacity. In addition, CNTs with various properties can be obtained according to different pretreatments to meet the requirements of the working environment.

Flexible CNT-based electrodes are usually prepared by electrophoretic deposition, vacuum filtration, template method, self-assembly, or composite with other materials to form a membrane or macroscopic paper:

• 1.

  Electrophoretic deposition. Electrophoretic deposition connects the etchable active metal material and inert materials adhesive with CNTs under an external power supply. The charged particles move to the cathode substrate, forming a CNT film on the substrate due to the electric field between two plate materials. The morphology and properties of the CNT film can be tuned by controlling the bath type, reaction temperature and time, current density, electrode spacing, and other technological parameters.

• 2.

  Vacuum filtration. Compared to the electrophoretic deposition, vacuum filtration method holds the advantage of easier operation with controllable morphology and thickness. The CNTs are uniformly dispersed in the aqueous solution containing alcohol, polyvinyl pyrrolidone (PVP), and other organic substances with enhanced dispersion. The suspension is filtered through the filter paper, where the CNT film can be obtained.

• 3.

  Template method. The template method is applicable to the preparation of highly oriented carbon nanotube films. Generally, hard templates such as anodic aluminum oxide (AAO) membranes are coated or vapor-deposited by precursors and CNT films are obtained through carbonization, etch, and other steps, based on the template materials used. ^14-16

To construct a binder-free, highly conductive, and flexible LSBs cathode, Guo et al. ¹⁷ took the lead in using the template method to fabricate flexible CNT-based electrodes. By carbonizing the polyacrylonitrile (PAN) membrane stuck to the surface and internal of the AAO, etching the AAO, liquid impregnation of sulfur, the sulfur-impregnated disordered carbon nanotubes were obtained. This report sheds light on the subsequent CNT-based self-supporting electrode (Figure 2A). Unfortunately, most of the sulfur carried by the liquid phase impregnation method is absorbed on the carbon surface and the LiPSs formed in the reaction are easily dissolved in the electrolyte, which has limited inhibition on the shuttle effect. The results show that each SCNT has a certain degree of polarization and capacity decay rate.

Figure 2. (A) SEM and transmission electron microscopy images of the DCNTs. Reproduced with permission: Copyright 2011, American Chemical Society.17 (B) Illustration of the synthesis process for an as-synthesized and -assembled carbon nanotube (S-CNT) film and its structure model. (C) SEM image of the front side of the S-CNT film at low magnification. (D) SEM image of the front side of the S-CNT film. (E) Cycling profiles of S-CNT film and S-CNT composite powder at a constant current rate of 0.1 C. Reproduced with permission: Copyright 2013, American Chemical Society.18 (F) Schematic of Li−S batteries using the freestanding double-layer FMC@S cathode. (G) Rate capability of the FMC@S-2, FMC@S-1, FMS-1, and pure S cathodes for Li−S cells at current densities from 0.1 to 2 C. Reproduced with permission: Copyright 2020, American Chemical Society.19 (H) Synthesis process of CNCF film. SEM images of 10/1 ZIF-8/CNT film and CNCF-10/1 film. Reproduced with permission: Copyright 2017, Royal Society of Chemistry.20 CNCF, CNT-threaded N-doped porous carbon film; CNT, carbon nanotube; SEM, scanning electron microscopy

To further enhance the content of sulfur in the electrode, Zhou et al. ²¹ prepared an AAO template with sulfate adsorbed in the film by anodizing method. After chemical vapor deposition and the carbothermal reduction of sulfate (SO₄ ^2– + C → S + CO + CO₂), the as-synthesized and -assembled carbon nanotube films (S-CNTs) contain elemental sulfur. The sulfur content of an electrode can be controlled by changing the concentration of sulfate, so the excellent performance of a flexible electrode after the electrochemical performance test can be attributed to this bottom-up design method. This method enables the sulfur obtained from the reaction to be sealed in the membrane, avoiding the massive dissolution of LiPSs. The membrane cathode with 23 wt% sulfur content S-CNT-23 can deliver a capacity of 1438 mAh g⁻¹, whereas the traditional non-self-supporting electrode has a capacity of 332 mAh g⁻¹ at 0.15 A g⁻¹. After 100 cycles, S-CNT-23 and S-CNT-50 demonstrated a high capacity of 653 and 524 mAh g⁻¹ at 1.5 A g⁻¹, respectively.

In the S-CNTs film, the interaction of carbon atoms on sulfur is dominated by van der Waals force only, which caused weak adsorption of LiPSs. The introduction of functional groups, polymers, metal oxides, heteroatoms, and other redox mediators can enhance the capture of sulfur and its compounds through the interaction with the functional groups, thus improving the electrochemical dynamics. Jin et al. ¹⁸ prepared the slightly oxidized carbon nanotubes film by vacuum filtration method, as shown in Figure 2B. Oxygen-containing functional groups such as hydroxyl group and carbonyl group were obtained in the acidified carbon nanotubes. A strong covalent bond between sulfur and carbon nanotubes is formed, which enables the high adsorption capacity of LiPSs. The S-CNTs film displayed a high capacity of 740 mAh g⁻¹ after 100 cycles (Figure 2E).

To further improve the sulfur storage capacity, Sun et al. ²² calcined the prepared super-aligned carbon nanotubes (SACNT) ^23,24 in the air, took them out when they were reduced to 500°C, and rapidly cooled to room temperature to obtain mesoporous carbon nanotubes (PCNT). PCNT with a micromesoporous structure provides larger storage space for the active sulfur species and the LiPSs produced by the chemical reaction process, which makes it easier to absorb the electrolyte and reduce the electrode polarization. The micropores prevent the shuttling of LiPSs, whereas the mesopores serve as the reservoir, which establish a double barrier for anchoring LiPSs with the acceleration of electron transfer. Moreover, SACNTs have better performance compared to the ordinary carbon nanotubes thanks to the superior orientation, large aspect ratio, clean surface, and solid continuous network. ^25,26 In the solution-based method, sulfur powder and PCNTs were suspended simultaneously in a mixed solution of water-ethanol by ultrasonic treatment to form a uniform suspension. After drying, the self-supporting flexible S-PCNTs cathode could be separated from the container. The control of SACNTs oxidation improved the dispersion and electronic conductivity of PCNTs. On the basis of the chemical stability of the skeleton, the relatively high flexibility and abundant tube interactions were retained. S-PCNTs with various sulfur contents showed excellent capacity retention and coulombic efficiency. To enhance the inhibition of the LiPSs shuttle effect, the researchers mixed PVP as a surface modifier and dispersant with SACNTs and Na₂S₂O₃. After ultrasonic treatment, hydrochloric acid was added under magnetic agitation to precipitate sulfur on SACNTs, and PVP@S-SACNT electrode was obtained after drying. ²⁷ PVP not only promotes the formation of a 3D cross-linked SACNT conductive network with good mechanical properties but also encapsulates sulfur nanoparticles on the SACNT network, abridges the electron migration distance, and prevents the dissolution of LiPSs. Comparing the PVP@S-SACNT self-supporting electrode with the PVP@S-CNT electrode prepared by a traditional slurry coating method and the S-SACNT electrode, PVP@S-SACNT self-supporting electrode exhibits a high capacity of 856 mAh g⁻¹ after 200 cycles and stable cyclic performance at 1 C. At current density up to 20 C, the electrode still has a high capacity of up to 590 mAh g⁻¹.

Polar metal oxides can also compensate for the limited adsorption of nonpolar carbon materials on LiPSs and reduce the shuttle effect. Chen et al. ¹⁹ prepared MnO₃ nanoemulsion by a facile hydrothermal method, mixed it with CNT and sodium dodecyl sulfate (SDS), and obtained the MoO₃/CNT/S film through vacuum filtration (Figure 2F). The whole process is simple and feasible. The MoO₃ nanorods compounded with CNT can absorb LiPSs to alleviate the shuttle effect as polar molecules. In addition, it is worth noting that the XPS spectra of Mo 3d change before and after cycling. Before the cycle, there are only characteristic peaks of Mo⁶⁺, whereas new characteristic peaks corresponding to Mo⁴⁺ and Mo-O-S bonds appear after the cycle, indicating that the valence state of Mo ions changes during the process of charging and discharging when they bond with LiPSs, and this reversible redox reaction makes them an active material for reversible insertion/extraction of lithium ion. As a result, the FMC@S-1 electrode exhibits a capacity of 666 mAh g⁻¹ after 350 cycles at a current density of 0.5 C and a better cycle stability (Figure 2G).

Zhao et al. ²⁸ mixed nitrogen-doped CNTs with nanosulfur uniformly. The S/N-CNT membrane obtained by vacuum filtration has a good conductivity, and the LSBs with this material as cathode have a good cycle and rate ability, which can be attributed to the formation of pyridine nitrogen, pyrrole nitrogen, and graphite nitrogen, reducing the shuttle effect. Pyridine nitrogen makes the lithium end of LiPSs be anchored through dipole–dipole interaction, whereas the π system formed between graphite nitrogen and sulfur end of LiPSs has a better phase. ^29-32 Such a dual interaction enhances the absorption of LiPSs on the CNTs surface and improves the electrochemical activity and sulfur utilization.

Metal-organic framework (MOF) is a coordinate polymer formed by connecting metal ions with polyhedral organic connectors. Taking Lee's work ³³ as an example, MOFs with a solid and multiple core-shell hollow structures were generated by inserting, connecting, and etching metal-organic polyhedra (MOPs). Due to their good stability, high crystallinity, and unique adjustable pore size, MOFs have been widely used as a polar material additive in carbon-based self-supporting electrode materials in recent years. ^34-37

Liu et al. ²⁰ prepared composite films of zinc hydroxide nanocrystals (ZNH) and CNTs by immersing ZNH and CNTs in water–ethanol solution containing 2-methylimidazole. A series of ZIF-8/CNT films were prepared by changing the volume ratio of ZHN to CNTs. After pyrolysis and sulfur loading in the liquid phase, the CNT-threaded N-doped porous carbon film (S@CNCF) was obtained (Figure 2H). The network with the formation of strings of polyhedral carbon has a hierarchical porous structure and a large specific surface area. By adjusting the proportion of precursor ZIF-8, different pore structures can be formed such as micro-macropores and meso-macropores structures. Some of the pores are from the porous precursor ZIF-8 crystal, and some are formed due to the removal of Zn during acid treatment, which increases the contact area of electrolyte ions and contributes to the uniform distribution of sulfur, allowing a high sulfur load to improve energy density. In addition, in the process of mixing ZHN and CNTs, N atoms enter the carbon grid, and the adsorption of LiPSs is enhanced, and meanwhile, the conductivity of carbon is increased through oxidation–reduction reaction. CNTs and polyhedral porous carbon are entangled and connected to form a conductive network integrating physical barrier and chemical adsorption, which has a short ion diffusion path and low diffusion resistance, making that S@CNCF electrode exhibits excellent electrochemical performance. When the weight ratio of ZIF-8 to CNT is 10 to 1, the specimen S@CNCF-10/1 has an outstanding performance, with an initial capacity of 1269.1 mAh g⁻¹ and high capacity retention of 87% after 100 cycles at 0.2 C.

Carbon nanofiber

CNF is also a 1D carbon material which has a similar morphology to CNTs. CNFs can be considered as solid or hollow CNTs with a larger length diameter ratio, of which the radial size is at nanoscale, whereas the axial size is at microscale. Micron-sized carbon fibers modified by nanoparticles can also be defined as CNFs. Due to the hydrophobicity and small size of CNFs, it is difficult to encapsulate a large number of active materials inside the CNFs. Therefore, CNFs with larger scale and stronger integrity can exhibit high specific surface area, high conductivity, and high sulfur content. The hollow CNFs coated with inner sulfur film cathode prepared by Zheng et al. ³⁸ using AAO template have attracted much attention (Figure 3A). After being coated with polystyrene (PS), AAO template was carbonized at high temperature, sulfur loading was performed in the liquid phase, and sulfur was absorbed by the hollow carbon fiber uniformly. Because sulfur mainly exists in the interior of hollow CNFs, the hollow CNF-encapsulated sulfur (S@HCNF) cathodes have a good performance in inhibiting the shuttle effect. The results show that the initial capacity of the electrode is about 1140 mAh g⁻¹ and decreases to 730 mAh g⁻¹ after 150 cycles at 0.2 C.

Figure 3. (A) The fabrication process of carbon/sulfur cathode structure. (B) Digital camera images showing the contrast of anodic aluminum oxide (AAO) template before and after carbon coating and sulfur infusion. (C) Cycling performance at 0.2 and 0.5 C. Reproduced with permission: Copyright 2011, American Chemical Society.38 (D) Schematic illustration of the preparation process of the Co-CNF (carbon nanofibers film embedded with cobalt) electrode. (E) The curves of specific capacity and Coulombic efficiency at 0.5 C. Reproduced with permission: Copyright 2019, Elservier.39 (F) Schematic diagram of synthetic procedures of CoNi nanoparticle-embedded porous N-doped carbon fiber (CoNi@PNCFs) electrode and an electronic watch powered by Li-S full cell. (G) Scanning electron microscopy images and (H) elemental mapping images of CoNi@PNCFs. (I) Cycling performance of different electrodes at 0.2 C. Reproduced with permission: Copyright 2020, Wiley40

Compared with the uncontrollable properties of CNFs prepared by the template method and the relatively complex process, the morphology and thickness of the film prepared by the electrospinning method can be controlled. Therefore, electrospinning has been widely used for preparation of carbon nanofibers. A high-pressure electric field was used to atomize polymer solutions such as PAN, polymethyl methacrylate (PMMA), polyvinyl alcohol, and so forth, to generate fluids which were collected on the substrate and carbonized at high temperature to form CNFs. ^41-43 By changing the composition of the precursor, hollow, porous, and hierarchical structures can be fabricated to enhance the sulfur loading. In addition, CNFs prepared by electrospinning are suitable for large-scale industrial production and meet the requirements of modern light and flexible electronic equipment. The commercial carbon fiber materials, such as carbon paper, carbon cloth, and carbon belt can also be used for the preparation of flexible self-supporting electrode materials, which are more convenient than the synthesized carbon fiber. ^44-46

Zeng et al. ⁴⁷ are one of the first researchers to prepare CNFs/S flexible self-supporting cathode for LSBs by electrospinning. CNTs and PAN were uniformly dissolved in dimethylformamide (DMF), and the composite nanofibers prepared by electrospinning were carbonized at high temperatures to form microporous CNFs–CNT (PCNFs-CNT) composite and then fused to load sulfur. During the carbonization process, KOH was added to corrode the carbon wall to form a porous structure, and CNT enhanced the conductivity of the material. ^48,49 The preparation of S@PCNFs-CNT showed good electrochemical performance due to its uniformly distributed pore structure which provided storage space and continuous electron conduction path for sulfur.

As sulfur is mainly attached to the outer wall of carbon nanofibers, the adsorption of LiPSs by nonpolar van der Waals forces between carbon nanofibers is also limited. The additives play an important role in inhibiting the shuttle effect, and the loading approach can be classified into two methods: “bottom-up” and “top-down”. ^50-52

“Bottom-up” refers to the loading of heteroatoms or polar substances during spinning. Li et al. ³⁹ prepared PAN/DMF solution containing Co(CH₃COO)₂·4H₂O as the precursor for spinning. After the carbonization process, it was etched with hydrochloric acid to obtain carbon nanofibers film embedded with cobalt metal (Figure 3D). It is worth noting that the traditional LSBs use elemental sulfur as the active substance to accelerate the cycling speed. ^53-55 The result in Figure 7B shows that the embedded cobalt metal promoted the uniform nucleation of lithium sulfide on the surface of CNTs, and maintained a stable electrode capacity at 700 mAh g⁻¹ after 300 cycles when the sulfur content reached 4.6 mg cm⁻². He et al. ⁴⁰ added Co(Ac)₂ and Ni(Ac)₂ to the mixture of polymer precursors of PVP, PMMA, and PAN using a similar method, as shown in Figure 3E. Due to the difference in pyrolysis temperature and residual carbon content of the three polymers, PMMA is completely decomposed in the high-temperature carbonization process with a low residual carbon content, and it serves as a template for the formation of a hierarchical porous structure. ⁵⁶ PVP promotes the formation of mesoporous structure and provides sufficient N content for the formation of Co-Ni-N active sites. ^57,58 Due to its high residual carbon content, PAN provides a flexible conducting network for the electrode as a carbon source. This multidoped, conductive carbon network of hierarchical porous structure with uniform distribution and integration of adsorbability and catalysis enhances the conversion kinetics of LiPSs while alleviating the shuttle effect. The results show that the battery polarization of the S/CoNi@PNCFs electrode is reduced, and at the current density of 0.2 C, the initial discharge capacity of the electrode is 1245 mAh g⁻¹. After 100 cycles, the capacity of 900 mAh g⁻¹ is still retained.

Chen et al. ⁵⁹ used PAN-Mn(CH₃COO)₂-ethyl silicate (TEOS)/DMF as the precursor, assembled the nanofibers and carbonized them, designed and prepared a freestanding woven paper electrode. In the frame of the carbon nanofibers, Mn(CH₃COO)₂ can be transformed into Mn₃O₄ nanoparticles by one-step pyrolysis only, whereas TEOS is converted to SiO₂ template. After etched by NaOH, porous structure can be obtained. The synergistic effect of physical and chemical capture can effectively inhibit the dissolution of LiPSs, and the Mn₃O₄@CNF/S electrode with a sulfur content of 6 mg cm⁻² has an initial capacity of 1180 mAh g⁻¹ at the current density of 0.2 C. In another report, Li et al. ⁶⁰ added ZIF-8 to the precursor liquid. Chemical vapor deposition (CVD) was used to fabricate CNTs on cube-embed CNFs with high porosity after carbonization, making fibers further connected to each other. The as-produced S-CPZC electrode had a high rate capacity and an extremely low capacity attenuation rate.

“Top-down” refers to the loading of polymers, heteroatoms, or polar substances after the formation of the carbon fiber cloth, mainly by hydrothermal or solvent thermal method and deposition method. Yun et al. ⁶¹ prepared sulfur-containing carbon nanofibers as cathode materials for high-performance LSBs. The CNFs prepared by electrospinning and carbonization are immersed in the mixed slurry of sublimated sulfur and NMP. According to the principle of wetting angle, the sulfur adheres to the junctions of the carbon nanofibers network, which is then used as the cathode of LSBs after drying. The intertwined CNFs matrix provides a large number of active sites, which can physically confine LiPSs within the micropores in the cycling process to inhibit the shuttling of LiPSs. When the sulfur content is up to 10.5 mg cm⁻², the reversible capacity of the CNF-S electrode can still reach 752 mAh g⁻¹.

To increase the elastic modulus of the electrode, a catalyst can be used during carbonization. For example, Chai et al. ⁶² enhanced the conductivity and tensile modulus of CNFs by graphitization of carbon catalyzed by iron oxide. Then, a facile method of hydrothermal was used to grow MoS₂ nanosheets on the surface of graphitized porous carbon nanofibers (G-PCNFs). The intertexture of highly crystallized carbon nanofibers has a blocking effect on LiPSs and MoS₂ nanosheets on the surface show a strong chemical adsorption on LiPSs. At the same time, during the high-temperature carbonization process of PAN, the internal nitrogen element is removed but the residual nitrogen element is converted to pyrrolidine and pyridine nitrogen. ^63,64 This integrated 3D network of electron and ion conduction routes and chemical reaction sites enables the initial capacity of S/MoS₂@G-PCNFs electrode to be 1385 mAh g⁻¹ at a current density of 0.1 C, and 594 mAh g⁻¹ at 1 C after a long-term charge and discharge of 500 cycles. It is reported that sulfide is effective in enhancing polarity. Lei et al. ⁶⁵ deposited polar WS₂ nanosheets on carbon cloth fibers. Deposited polar WS₂ nanosheets on carbon cloth fibers, sulfur is strongly absorbed on the surface through the action of van der Waals force and the polar WS₂ functional groups, reducing the shuttle effect. The as-prepared flexible self-supporting composite material (C@WS₂/S) not only has a high sulfur content of 68%, but also possesses a high specific capacity of 995 mAh g⁻¹ after 500 cycles, with long circulation, excellent rate capacity, and coulombic efficiency. To tune the polarity, the structure design is also very important for the sulfur-bearing capacity. Guo et al. ⁶⁶ used a moderate hydrothermal method to grow an iron-cobalt precursor on commercial carbon cloth. After sulfurization, the solid tubular precursor was transformed into a hollow tubular FeCo₂S₄ nanoarray, making it easier for sulfur to be anchored in the host material in the subsequent liquid-phase sulfur-loading process (Figure 4F,G).

Figure 4. (A) The schematic diagram of the morphological changes at different steps for the prepared S/MoS2@G-PCNFs. Reproduced with permission: Copyright 2020, Elsevier.62 (B) Schematic illustration for the preparation of C@WS2/S. (C) SEM images of the as-prepared free-standing C@WS2 composite. (D) Long-term cycling stability of C@WS2/S. Reproduced with permission: Copyright 2016, Wiley.65 (E) Schematic illustration of the FeCo2S4/CC@S composite. FESEM images of (F) FeCo2S4/CC and (G) FeCo2S4/CC@S. (H) CV curves and (I) discharge/charge curves of the FeCo2S4/CC@S and CC@S. (J) Cycling performances of FeCo2S4/CC@S and CC@S. Reproduced with permission: Copyright 2018, American Chemical Society.66 CV, carbon vapor; FESEM, field emission scanning electron microscopy; PCNF, porous carbon nanofibers; SEM, scanning electron microscopy

Because of the low porosity of the substrate material, Zhong et al. ⁶⁷ also expanded the porosity by adding polar materials. Electrodeposition method was used to deposit Ni(OH)₂ on the surface of carbon fibers of a commercial carbon cloth. After calcination in an argon atmosphere at 900°C, the porous carbon fibers/nickel composite (PCF/Ni) was formed on the carbon fiber surface due to the decomposition of Ni(OH)₂ and the generation of gas. Porous carbon fiber cloth with a high specific surface area was prepared by etching Ni from the material. This porous structure greatly increases the sulfur loading capacity and can buffer the volume expansion problem of active substances in electrochemical reversible reactions. The final product (PCF/vanadium nitride [VN]/S) was obtained by a solvothermal method, followed by heat treatment and supercritical seepage. On the one hand, VN arrays grown on the surface of carbon fiber adsorb LiPSs as polar molecules; on the other hand, the high specific surface area and high conductivity of VN are favorable to the diffusion of electrolyte and ion transport, increasing the reactive sites and accelerating the electrochemical reaction speed. Compared to the carbon cloth without pretreatment and the carbon matrix composites without composite VN, the LSBs with PCF/VN/S flexible self-supporting material as the cathode showed better electrochemical performance with initial specific capacity of 1310.8 and 1052.5 mAh g⁻¹ at 250th cycle.

Regarding the phosphates as polar additives, Wang et al. ⁶⁸ composited sulfur, flexible carbon cloth (CC), and CoP nanoarrays, and prepared CC@CoP/C-S flexible self-supporting electrode, which used MOF as the precursor to improve the interaction between the polar material loaded by the collecting fluid and the carbon material. Both the carbon fibers of carbon cloth and the carbon nanoarrays grown on the surface of CoP contribute to the conductivity enhancement of the electrode. Furthermore, double synergies are realized by using CoP and carbon nanoarrays to enhance the adsorption of LiPSs and the effective mitigation of structural damage caused by volume expansion while increasing the sulfur surface load. The solution method was used to prepare the CC@Co-MOF precursor. After the carbon cloth was immersed in the mixed solution for a period of time, the dried CC@Co-MOF was reduced at high temperature in Ar/H₂ (95/5%) to obtain CC@Co/C. CC@CoP/C was obtained by gas-phase phosphating and loading with sulfur in CS₂ solution. Compared to CC@CoP-S with single chemisorption and CC@C-S with a single physical barrier, CC@CoP/C-S has higher initial specific capacity (1030 mAh g^–1), and Coulombic efficiency.

Wang et al. ⁶⁹ prepared hierarchical mesoporous SnS₂ nanosheets on carbon cloth fibers by a hydrothermal method and obtained C@SnO₂ via a simple heat treatment process (Figure 5A). The specific surface area of C@SnS₂ is smaller than that of C@SnO₂, because the sulfur atom in C@SnS₂ is replaced by the oxygen atom with a smaller radius. Therefore, C@SnO₂ with higher specific surface area and more active sites were selected as the sulfur host to improve the electron transfer and electrolyte diffusion rate, which is attributed to the effect of polar substances on the anchoring of LiPSs. As a result, C@SnO₂/S exhibits an initial discharge capacity of 1228 mAh g^–1, which preserves a high capacity retention of 90% after 100 cycles at 0.2 C, as shown in Figure 5D. Wang et al. ⁷⁰ prepared 1T-MoS₂ on carbon cloth fiber by hydrothermal method. Li₂S₈ was utilized as the active material, converted into Li₂S by electrocatalysis and adsorption of 1T-MoS₂, and then directly deposited on the surface of 1T-MoS₂. Instead of the traditional elemental sulfur, LiPSs were utilized as the active material, thus avoiding the capacity loss caused by the shuttle effect caused by the dissolution of sulfur in the electrolyte during the transition from elemental sulfur to LiPSs and the slow kinetic rate of solid–liquid–solid multistep conversion.

Figure 5. (A) Schematic illustration of C@SnO2. (B) SEM images of CNFs and (C) C@SnS2 nanosheets. (D) Cycling at 0.2 C over 100 cycles of CNFs/S, C@SnS2/S, and C@SnO2/S. Reproduced with permission: Copyright 2017, Royal Society of Chemistry.69 (E) Schematic illustration of C@SnO2/TMS. SEM images of (F) C@SnO2, and (G) C@SnO2/TMS. (H) Cycling at 0.5 C over 200 cycles of C@1T-MoS2/S, C@SnO2/S, and C@SnO2/TMS/S composites. Reproduced with permission: Copyright 2018, American Chemical Society.71 CNF, carbon nanofiber; SEM, scanning electron microscopy; TMS, tetramethylsilane

Self-supporting materials can load heterojunction structure catalysts, and the effective interface between the media with strong polarity and less conductivity and the media with strong conductivity and less polarity can realize the effective adsorption and rapid transformation of LiPSs, as well as the rapid nucleation of lithium sulfide. To further improve the structural performance, the heterojunction hierarchical SnO₂@1T-MoS₂ nanoarray (C@SnO₂@TMS [tetramethylsilane]) was designed and constructed on the carbon cloth as the sulfur cathode of LSBs. ⁷¹ After the preparation of C@SnO₂ using the same method as above, a uniform MoS₂ nanoarray was further prepared on the surface of the SnO₂ array (Figure 5E). The dual nanoarrays provide more active sites for the storage of sulfur and the adsorption of LiPSs while increasing the specific surface area of the material. On the contrary, the 1T-MoS₂ nanosheets located outside of the material have high conductivity, which can effectively accelerate the chemical reaction kinetics, as shown in Figure 5F,G. These results show that C@SnO₂@TMS has a higher capacity at a higher rate, indicating that the heterojunction structure can promote the performance of the high-capacity LSBs. As shown in Figure 5H, THE initial capacity of 1261 mAh g^–1 and the retaining capacity of 1175 mAh g^–1 after 200 cycles at 0.5 C of C@SnO₂@TMS is much more superior than that of the samples lacking one of the two nanoarrays. Zhang et al. ⁷² prepared ZIF-67 as precursor on carbon cloth by coprecipitation method. Utilizing ZIF-67 framework structure, the sheathed CC@Co₉S₈ was formed after sulfuration and pyrolysis. In NH₃ atmosphere, CC@Co₉S₈ was further nitrided to form CC@Co₉S₈-Co₄N heterojunction structure. As control experiments, the researchers prepared CC@PCNA (porous carbon nanosheet arrays), CC@Co₉S₈, and CC@Co₄N. Compared with the single nanoarray compounded on carbon cloth fiber, this kind of hollow heterostructure can increase the sulfur loading capacity and alleviate the volume change. More importantly, the heterostructure can inhibit the dissolution of LiPSs and accelerate the transformation of long-chain LiPSs into short-chain Li₂S₂/Li₂S. On the one hand, Co₉S₈ has strong polarity and adsorption to LiPSs. On the other hand, the high conductivity of Co₄N makes up for the low catalytic activity of Co₉S₈ and promotes the nucleation of Li₂S at the interface of heterostructure. To prove this point, potentiostatic Li₂S precipitation experiments were desighed. Compared with pure carbon fiber paper or Co₉S₈-CP, Li₂S nucleated faster on the heterostructure Co₉S₈-Co₄N-CP. As a result of electrochemical performance test, S/CC@Co₉S₈-Co₄N still maintains 1137 mAh g^–1 after 100 cycles at 0.5 C, especially at a high current density of 5 C, and its capacity only decreases by 0.027% from 648 mAh g^–1 after 1000 cycles.

In addition, introducing defects to catalyze the inherent materials is also a way to accelerate the reaction kinetics. Wang et al. ⁷³ prepared a self-supporting electrode which combines the rigid array of polar metal compounds with flexible CNF. Different from the previous carbon nanofiber base, they first prepared CoO _x -TiO₂ precursor fibers by electrospinning. After heat treatment in argon and hydrogen hybrid atmosphere, oxygen vacancies were formed. A flexible CNF was formed on the surface of Co-TiO_{2 − x} by chemical vapor deposition with acetylene as the conductive network of the self-supporting electrode. Compared with pure TiO₂, the introduction of oxygen vacancy can strengthen the Ti-S bond and increase the binding energy with LiPSs, whereas the ionic conductivity of TiO_{2 − x} containing oxygen vacancy is enhanced, which accelerates the electrochemical reaction rate. The Co-S bond is introduced by Co doping, which further increases the polarity, proved by DFT analysis and electrochemical performance test. The preparation of CNF with metal compounds from bottom to up not only ensures the flexibility and conductivity of the material but also greatly increases the polarity and sulfur loading.

To further increase the specific surface area of carbon nanofibers, carbon nanofibers with the hollow or yolk-shell structure were synthesized by electrostatic spinning using nanosphere as the matrix. ^74-76 Li et al. ⁷⁷ used SiO₂ nanospheres as a template to wrap TiO₂ on SiO₂ nanospheres by the coprecipitation method. Composite nanoparticles and PAN were uniformly dissolved in DMF, and SiO₂@TiO₂@PAN fibers were prepared by electrospinning. As a medium, PAN was used to connect SiO₂@TiO₂ nanospheres together. After etch, carbonization, and reduction, the CNFs assembled by the hollow TiO nanospheres in the shape of a grape cluster were formed. The carbon fiber formed by the pyrolysis of PAN and the pores on the surface of the TiO-cladding layer provide a channel for the sublimation of sulfur into the hollow nanospheres through the gas-phase perfusion method, and the solid sulfur is loaded inside after cooling. It is assumed that the dual microporous shell layer inhibits the dissolution of LiPSs so that the electrochemical redox reaction is limited in each nanochamber, and the ion migration distance is reduced, which is favorable to accelerate the sulfur redox reaction. TiO polar cladding accelerates the reaction kinetics in the nanochamber and further enhances the adsorption of LiPSs through chemical action. Overall, the synergetic effect of the nano-scaled reaction chamber and the micron-scaled conductive network is effective in alleviating the volume expansion in the cycling process, ensuring the high sulfur capacity at different current densities, and making the electrode have excellent rate performance and cycling stability.

Pei et al. ⁷⁸ and Lin et al. ^78,79 used a facile one-pot sol-gel method to coat the SiO₂ nanosphere on a PB/SiO₂@SiO₂ bilayer, and then electrospun SiO₂@PB/SiO₂@SiO₂@PAN fiber together with PAN. After the preoxidation-annealing treatment to enhance the toughness of precursor fiber and carbonization, SiO₂ was etched. Due to the high concentration of SiO₂ in the PB/SiO₂ layer, the shell structure of internal support and coating components were lost and collapsed, so the bowl-shaped core hollow carbon shell structure in clusters like grapes was obtained. The hollow bowl-shaped carbon nanosphere (BCN), tightly connected by the carbon frame, further increases the porosity and the number of reaction sites in its internal grooves compared with the spherical one. In addition, the use of a bowl-shaped hollow host avoids the dead sulfur formation in the traditional rigid hollow host due to the existence of sulfur that is not in contact with the outer wall of the host material, which improves the utilization rate of the active material. Each nanosphere can be considered as a reaction chamber at the nanoscale, which plays an important role in preventing sulfur agglomeration and electron ion conduction at the microscale. Because sulfur is sequestered in the lumens, the carbon shell effectively prevents the dissolution of the LiPSs. By integrating the synergies of self-supporting structure, high surface area, nitrogen atom doping, and yolk-shell structure, the BCN@ hollow carbon shell (HCS)/S electrode shows better capacity and cycle stability compared with the HCS/S, traditional powder bowl-shaped carbon spheres electrode (BCN/S), and the traditional powder electrode mixed with carbon spheres and carbon shells (BCN + HCS/S). The BCN@HCS/S-70 cathode demonstrated a high capacity of 1041 mAh g⁻¹ after 100 cycles at 0.2 C (Figure 6).

Figure 6. (A) Schematic of the grape-cluster-inspired design and characterization of intermediate materials. (B) Scanning electron microscopy (SEM) image ofgrape-cluster-like titanium monoxide@carbon hollow fiber (GC-TiO2@CHF). (C) Transmission electron microscopy (TEM) image of GC-TiO@CHF. (D) TEM image of GC-TiO@CHF/S. (E) Cycle properties at 0.1 C of electrodes. Reproduced with permission: Copyright 2017, Elservier.77 (F) Schematic illustration of the templated electrospinning strategy for the fabrication of the yolk-shell carbon fiber network. (G) SEM images of the bowl-shaped carbon nanospheres@ hollow carbon shell (BCN@HCS) fibers. (H) TEM images of the BCN@HCS/S composite. (I) Cycling performances of the four cathodes at 0.2 C. Reproduced with permission: Copyright 2018, Elservier79

Two-dimensional nanocarbon

Graphene, also arranged in sp² hybrid orbital, is a 2D and one-atom-thick layer of carbon atoms arranged in a hexagonal lattice. It has light weight, high conductivity, and excellent mechanical strength, making it widely used in electronic, medicine, energy storage, and other fields. ^80-84 Compared to 1D carbon materials, graphene with a 2D crystal structure, can grow continuously and be self-assembled over a large area, and the bonds formed between carbon atoms are strong, which impart graphene with high tensile strength and flexibility. In general, the commonly used one-step preparation methods of graphene-based flexible materials include evaporation-induced self-assembly, dip-coating, and vacuum filtration.

• 1.

  Evaporation-induced self-assembly. Graphene is dispersed in the solution under heat, and the particles move and deposit on a substrate under the effect of interlayer interaction, van der Waals force, and hydrogen bond. When the solvent completely evaporates, a graphene film is assembled at the bottom of the smooth container or the gap of the cover plate to cover the surface. However, although the operation is simple, it is challenging to effectively control the suspension concentration, temperature, and evaporation rate which can affect the morphology of the film in the preparation process.

• 2.

  Dip-coating. A substrate is immersed in the solution with uniformly dispersed graphene and steadily pulled out of the sol at a constant rate. Under the action of viscosity and gravity, a homogeneous gelatinous film is coated on the surface of the substrate, and the pure graphene film is obtained after solvent evaporation and substrate removal. The selection of substrate, lift velocity, and suspension concentration are factors affecting the properties of the film, and it is difficult to control.

• 3.

  Vacuum filtration. Graphene films can also be prepared by vacuum filtration. Compared to 1D CNTs, it is easier to prepare graphene films using the vacuum filtration method because of the interlaminar force existing in graphene. Thin films of graphene can even be obtained in water without adding any polar materials.

It is worth noting that most of the graphene films are prepared using the facile self-assembly method, and the films exhibit excellent performance. The resulting interlayer structure imparts graphene with a super high surface area, provides enough bearing space and chemical sites for sulfur, and generates layers of physical barriers for the dissolution of LiPSs while alleviating volume expansion. Therefore, graphene-based flexible materials generally have high sulfur content and specific capacity. In addition, graphene has a good dispersion performance, which reduces particle agglomeration, facilitates the uniform distribution of active substances, improves the utilization rate, and reduces the risk of electrode damage caused by uneven contact between the electrolyte and the materials.

Jin et al. ⁸⁵ were the first to prepare graphene/sulfur flexible self-supporting electrodes (Figure 7A). The uniform graphene suspension was added to Na₂S₂O₃·5H₂O and HCl. Sulfur was introduced into the carbon by a simple in-situ growth method, and the flexible composite paper of graphene and sulfur was prepared by the vacuum filtration method. The result shows that the reversible capacity is 600 mAh g⁻¹ after 100 cycles at the current density of 0.1 C, which has a high capacity retention rate of 83%. Sun et al. ⁸⁶ used a simple evaporation-induced self-assembly method to prepare a super-aligned carbon nanotube/graphene (CNT/G) composite electrode as shown in Figure 7B. The as-prepared SWCNTs were mixed with graphene and sulfur powder in water-ethanol solvent and treated with deep ultrasound. After drying, the solvent was removed, and the S-CNT/G flexible self-supporting electrode material was separated from the container. As the host with a large area of conductive network and sulfur, graphene provides a large number of adsorption sites for LiPSs. The uniformly distributed sulfur particles expedite the formation of sulfur–carbon bonds, and the binding energy between sulfur and carbon atoms prevents the self-agglomeration of SACNTs. The self-accumulation of large π bonds between graphene allows the 3D conductive network to be re-established. In addition, the strong interaction between SACNTs enhances the adsorption of LiPSs. Self-assembly of graphene also makes the bonding between the atoms of the substrate stronger, which enhances the stability of the electrode before and after the cycle. As a result, the S–CNT/G flexible self-supporting cathode exhibits a specific capacity up to 1048 mAh g⁻¹ at 1 C, whereas a high capacity of 818 mAh g⁻¹ is achieved after 200 cycles at 1 C as shown in Figure 7D.

Figure 7. (A) Schematic illustration and photograph of the flexible self-supporting GS/S paper. Reproduced with permission: Copyright 2013, Royal Society of Chemistry.85 (B) Schematic of the synthesis procedure of the S-CNT/G composite. (C) TEM images of the S-CNT/G composite. (D) Cycle performance at 1 C of S-CNT and S-CNT/G composites. Reproduced with permission: Copyright 2015, Royal Society of Chemistry.86 (E) Schematic illustrations of the synthesis of LF@CNT-GN electrode. (F) Digital photograph and (G) scanning electron microscopy image of the LF@CNT-GN substrate after the drying process (the inset: low magnification). (H) Schematic illustration of the LF@CNT composites before and after the drying process. (I) Cycling performance of cathodes at 1 C. Reproduced with permission: Copyright 2018, Royal Society of Chemistry.87 CNT, carbon nanotube; GS, graphene sheet; LF, lignin fiber; TEM, Transmission electron microscopy

To increase the adsorption of LiPSs, additives are added to graphene. Cao et al. ⁸⁸ uniformly deposited and assembled a composite film of graphene oxide (GO) and sulfur nanoparticles on the surface of the zinc foil by a dip-coating method. The zinc foil was removed by hydrochloric acid to obtain reduced GO (rGO), and a freestanding rGO-S membrane was obtained by cleaning and drying. As a surface-modified derivative of graphene, GO has a large specific surface area and is rich in functional groups, such as hydroxyl group (–OH), carbonyl group (–C═O), and carboxyl group (–COOH). The functional groups can mitigate the agglomeration of graphene caused by the π–π-conjugated structure. In addition, functional groups make a certain contribution to restricting LiPSs. However, the conductivity of graphene can be reduced when the number of functional groups is beyond a limit. By chemical reduction or thermal reduction, functional groups can be decomposed to restore their partially conjugated structures, which can effectively adsorb LiPSs and impart excellent mechanical and electrochemical properties. ^89-91 The rGO-S composite films delivered a high capacity of 1302 mAh g⁻¹ and retained a capacity of 978 mAh g⁻¹ after 200 cycles at 0.1 C.

Xiao et al. ⁹² prepared a homogeneous suspension of GO, nanosulfur, and poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and reduced GO under the condition of heat and ascorbic acid. The nano S-graphene-PEDOT:PSS (SGP) film was obtained via vacuum filtration. Compared with other conductive polymers, PEDOT has better rate properties and cycle stability, whereas its insolubility in water limits its ability to form composites with other materials. After surface grafting of PSS, PEDOT:PSS polymer was formed, which greatly improved the water solubility of PEDOT, ⁹³ enabling its dispersion with GO in water. The SGP film obtained by vacuum filtration method has a compact sandwich structure. The highly conductive PEDOT:PSS and rGO promote the electron transport in this flexible electrode, and meanwhile, the functional groups also provide a limitation for the dissolution of LiPSs. The SGP cathode delivers an initial specific capacity of 1008 mAh g⁻¹, and retains at 806 mAh g⁻¹ after 500 cycles at 1 C. After this study, Liu et al. ⁸⁷ used the vacuum-filtered graphene (GN) membrane as the substrate, on which the lignin fibers (LF)/CNTs mixed slurry was deposited. The slurry passed through the GN membrane and the filter paper under vacuum pressure to form an LF@CNT-GN film in the shape of reinforced concrete. As Figure 7H shows, the pore structure was formed among the LF, CNTs, and the graphene substrate because of the shrinkage of wet LF after drying. This increased the specific surface area of the electrode, offering sufficient space for sulfur loading and the formation of multiple interconnections. Because the reinforced concrete structure is thick but light in weight, it is favorable to electrolyte diffusion and ion migration while maintaining high sulfur content. The LF@CNT-GN/Li₂S₆ electrode based on this substrate shows a good performance with an initial capacity as high as 1632 mAh g⁻¹ at 0.1 C and a retention of 987 mAh g⁻¹ after 500 cycles at 1 C (Figure 7I).

As for polar compounds, Hong et al. ⁹⁴ soaked the filter paper containing branched polyethylenimine (bPEI) coating in GO/Co(AC)₂·4H₂O (CA) suspension and obtained the film after 24 h of treatment by vibrating screen. The CoS₂/rGO film was obtained by using thioacetamide as the sulfur source, hydrothermal treatment, freeze-drying, and heat treatment. CoS₂ has a strong adsorption and activation effect on polar LiPSs, and can effectively accelerate the redox reaction in the cycling process of the electrode. Likewise, the CoS₂ nanoparticles can block the layering of graphene. After the hydrothermal treatment, GO was reduced to form rGO through the process of dissolution and recrystallization, which is favorable to the uniform mixing of Co²⁺, rGO, and S at the atomic and ion levels, forming a continuous coating of the multilayer of these components, and effectively inhibiting the migration of LiPSs through steric hindrance and atomic attraction. ^95-97 The experimental results show that the initial discharge capacity can reach 993 mAh g⁻¹, and the discharge capacity after 110 cycles is 806.7 mAh g⁻¹.

To further improve the properties of graphene films, such as the porosity, adsorption capacity of sulfur and its compounds, and shuttle effect inhibition, the 3D graphene aerogel, combining the characteristics of aerogel and graphene such as high porosity, large specific surface area, and excellent conductivity and flexibility, is developed. ^98,99 After being combined with active substances, flexible graphene aerogel-based films can be obtained by cutting and pressing for electrode materials. Template method and template-free method are commonly used for fabricating graphene aerogel. For the template method, graphene grows on the template surface of nickel foam or other types of sponges and aerogels by CVD or other methods, and the template is etched to obtain graphene aerogels. ^100-102 However, the synthesis process of this method is relatively complex and is generally performed at high temperature. In addition, the process is not scalable, and the materials obtained are usually fragile under low compression. ¹⁰³ Therefore, the template-free method plays a major role in the preparation of graphene aerogels as precursors. In general, the graphene suspension is prepared first. In the hydrothermal environment at high temperature and pressure, the π–π stacking interaction between the graphene layers becomes the driving force for the construction of physical crosslinking, in combination with hydrogen bonds and van der Waals force, enabling the self-assembly of graphene to form hydrogels. ¹⁰⁴ In the later process of freeze-drying, the ice water molecules in the prefrozen hydrogel are sublimated into water vapor and vented by the vacuum pump under low temperature and vacuum conditions. The original water molecules are removed and the pores remained, which supports the original structure of the materials, thereby avoiding the collapse and accumulation of graphene due to the π–π stacking effect. This results in both high porosity and structurally complete graphene aerogels, which can be cut and extruded to obtain graphene films. ^105,106 Wang et al. ¹⁰⁷ prepared a suspension by uniformly mixing with GO and sulfur nanoparticles, obtaining the S-rGO aerogel and film electrode after freeze-drying and heat treatment in the container. Elemental sulfur fused and grown between the graphene sheet layers after heat treatment is confined to the physical barrier with LiPSs through the interaction between the layers, reducing the capacity loss during the cycle. On the other side, due to the abundant hydrophilic functional groups in GO, the residual functional groups after reduction can inhibit the wrinkling caused by the interaction between graphene layers (Figure 8).

Figure 8. Schematic illustration of the preparation of graphene aerogel by template method of (A) freeze-drying and (B) CVD. Reproduced with permission: Copyright 2013, American Chemical Society100 and 2015, Wiley.102 (C) Schematic illustration of the synthesis process and structure of the designed S-rGO paper. (D) S-GO aerogel before and after heat treatment. (E) Cycling performance at 1 C and corresponding Coulombic efficiency (inset, the digital image of electrode films cut from S-GO aerogel). Reproduced with permission: Copyright 2015, Elsevier.107 CVD, chemical vapor deposition; GO, graphene oxide; rGO, reduced GO

Graphene aerogels can also enhance the anchoring of LiPSs by additives, thus improving the electrochemical properties. Shi et al. ¹⁰⁸ prepared G/CNT aerogels in a similar way using a lighter instead of heat treatment, and G/CNT-S films were produced by liquid phase molten sulfur carrier method. Compared to the pure graphene aerogels, the cross-linked carbon nanotubes and graphene provide a network of ions with high electrical conductivity, further inhibiting the shuttle effect of LiPSs. It is worth noting that G/CNT membrane can be used as the interlayer of Li-S battery. The G/CNT-S//G/CNT cathode formed by assembling the G/CNT film and the G/CNT-S membrane has better performance than the G/CNT electrode, which further illustrates the inhibition of G/CNT on shuttle effect. The result of electrochemical cycle shows that the flexible binder-free cathode G/CNT-S//G/CNT achieves a high capacity of 1286 mAh g⁻¹ at 0.5 C and long-term cyclability. In another report, Zhou et al. ¹⁰⁹ added boric acid (H₃BO₃) or dicyandiamide (C₂H₄N₄) in GO solution as boron source and nitrogen source to prepare B or N atoms-doped rGO aerogel via hydrothermal method, which was cut and compressed into pieces to make the host of Li₂S. The addition of hetero-atomic N or B is beneficial to change the electron partial density to produce polarity and improve the adsorption capacity of a nonpolar carbon skeleton to LiPSs. Moreover, Li et al. ¹¹⁰ prepared 3D nitrogen-doped graphene/titanium nitride nanowires (3DNG/TiN), a network of highly porous conductive graphene that provides efficient channels for electrons and ions. The highly conductive TiN nanowires attached to the graphene sheet can not only form the nanopore structure during annealing due to the vacancy caused by anion exchange and phase transition but also have a strong chemical anchoring effect on LiPSs, which improves its redox kinetics. As a result, the initial and final capacity after 200 cycles are 1480 and 957 mAh g⁻¹, respectively, at a high current density of 1 C. The latest graphene-based self-supporting electrodes started working with the structure design of aerogels. Tan et al. ¹¹¹ prepared CNF by electrospinning and obtained a fishing net structure wrapping sulfur nanospheres via in-situ growth, inside P-doped graphene aerogels. The CNF fishing net serves as a bridging network between graphene sheets to inhibit the aggregation and enhance the ion and electron transport capacity of the electrode. It also provides a double barrier for the dissolution of LiPSs as sulfur storage. On the basis of this superior structure, the sulfur loading is as high as 15.8 mg cm⁻² with a high specific capacity of 1360 mAh g⁻¹ and long cycle stability of 600 cycles.

3D skeleton porous carbon

Commercial skeleton materials with high carbon content and large carbonization yield can be used as raw materials to prepare self-supporting carbon structures after heat treatment, such as cotton linen, organic foam, and biomass materials, and so forth. ^112-117 The 3D skeleton material has a wide range of sources with different structures and low cost. During the carbonization process, the internal structures decompose, resulting in the loss of mass and volume, forming a porous structure, which increases the specific surface area of the material. As a huge 3D conductive network, it is favorable to the storage of sulfur elements and the expansion of buffer volume, so that the electrolyte is fully absorbed. In particular, biomass material is renewable; the utilization of biomass wastes can improve energy efficiency, reduce environmental pollution, and facilitate industrial production.

Schneider et al. ¹¹⁸ used glass microfiber filter paper as a template and 1-ethyl-3-methylimidazolium dicyanamide (EMIM-DCA) as a nitrogen-containing carbon precursor. The filter paper was infiltrated by EMIM-DCA through carbonization, followed by potassium hydroxide infiltration, re-carbonization, and sulfur loading. EMIM-DCA deposited on the surface of filter paper was transformed to porous carbon, providing a conductive network and a framework for sulfur storage, which improved the cycle performance with the help of LiPSs shuttle by nitrogen doping.

Melamine foam (MF), as a 3D architectural framework, can provide high specific surface area and high carbon content after pyrolysis, and the nitrogen it contains can effectively inhibit the shuttle of LiPSs, so it is widely used in the preparation of self-supporting electrodes. Xiang et al. ¹¹⁹ carbonized the MF, compounded it with sulfur, and decorated rGO outside of NCF-S by hydrothermal method. Finally, the composite electrode material of NCF-S@rGO was obtained by freeze-drying and reheat treatment (Figure 9A). The surface modification by rGO not only connects the internal carbon foam skeleton to form a large area of the conductive network but also strengthens the electronic transmission, further increasing the specific surface area and serving as the physical barrier to inhibit LiPSs from dissolving in the electrolyte. Therefore, the NCF-S@rGO cathode exhibits a persistent high capacity and cyclic stability with high sulfur content.

Figure 9. (A) Schematic illustration of the preparation process for the NCF–S@rGO composite. Reproduced with permission: Copyright 2017, Royal Society of Chemistry.119 (B) Schematic illustration of the synthesis process for CTNF@CoS2–CNA. Reproduced with permission: Copyright 2019, Springer.120 (C) Schematic of NCF/CNT/PEDOT@S and NCF/CNT@S electrodes. (D) Low- and (E) high-magnification scanning electron microscopy images of NCF/CNT/PEDOT@S. (F) Corresponding elemental mapping in the NCF/CNT/PEDOT@S electrode. (G) Cycling performance of NCF/CNT/PEDOT@S and NCF/CNT@S electrodes at 0.5 C with a sulfur loading of 2.6 mg cm−2. Reproduced with permission: Copyright 2018, Royal Society of Chemistry.121 CNT, carbon nanotube; GO, graphene oxide; NCF, N-doped porous carbon film; PEDOT, poly(3,4-ethylene-dioxythiophene); rGO, reduced GO

To strengthen the adsorption of LiPSs, Xiang et al. ¹²² infiltrated and adhered CNT and Mg(NO₃)₂ to MF. The carbon skeleton structure with CNT and MgO nanoparticles evenly covering the surface is prepared after NH₃ gas diffusion and carbonization. CNT is able to connect the porous carbon skeleton and enhance conductivity, and the dual chemical adsorption of MgO and N atoms is used to alleviate the shuttle effect. Zhang et al. ¹²¹ soaked and coated the PANI nanoarrays outside MF to increase the surface area, which was favorable to the redox kinetics of catalytic ion reaction. In addition, the PANI nanoarrays reduced the infiltration contact angle between the SWCNTs slurry and the foam matrix interface, making the two surfaces more compatible and favorable to the uniform distribution of SWCNTs. As shown in Figure 9C, the flexible and uniform sulfur-loaded NCF/CNT/PEDOT@S self-supporting electrode was obtained by immersing the carbonized 3D carbon foam in the suspension of PEDOT encapsulated with sulfur nanoparticles, which can be supported by Figure 9D−G. As a conductive polymer, PEDOT acts as a shell to store sulfur and prevent the dissolution of LiPSs, and its polar functional groups play a role in further anchoring LiPSs. The cycling performance result shows NCF/CNT/PEDOT@S has an ideal capacity of 802 mA h g⁻¹ after 100 cycles at 0.5 C (Figure 9G). Similarly, after introducing CNTs on MF, Zhang et al. ¹²⁰ prepared CTNF@ZIF-67 composites by coprecipitation and impregnation. After heat treatment, not only porous carbon scaffolds were formed, but also CoS₂ hollow nanospheres were formed on the surface, where sulfur was introduced by liquid phase impregnation, serving as a physical and chemical barrier to inhibit the transition effect.

Biomass is a type of renewable material which is ubiquitous and of low cost. The use of waste biomass materials can reduce environmental pollution and improve the use of energy. Because the skeleton matrix of biomass materials is beneficial to the cycling performance, they are good candidates for the electrode materials. Various biomass materials have been used for fabricating Li-S batteries such as pomelo peel, peanut shells, and luffa, and so forth, ^123-127 which provides a new idea for the application of self-supporting flexible electrode in Li-S batteries.

Wu et al. ¹²⁸ selected a low-cost porous cellulose plate as the 3D carbon skeleton of the self-supporting electrode and introduced Li₂S particles inside by solvent evaporation. After acetylene (C₂H₂) gas deposition and heat treatment, the graphitization degree is improved to enhance the conductivity of the base-derived carbon. The pores in the cellulose derived from the carbon skeleton serve as a reservoir for Li₂S particles, reducing the energy barrier of redox reaction and making the discharge capacity close to the theoretical value. The tempering process of the secondary heat treatment can improve the stability of the structure and reduce the brittleness of the material, thus inhibiting the damage of the electrode caused by the volume change. At a current density of 0.5 C, this flexible electrode can achieve a capacity retention rate of about 97% after 100 cycles. The electrode with different sulfur contents also has a stable cycling performance.

In another report, Zhu et al. ¹²⁹ took the fresh melaleuca bark as raw material to fabricate PCF/S electrodes. Porous carbon was derived by a carbonization process through the corrosion of the carbon wall by KOH, ^130,131 followed by impregnation and melting. PCF has a lamellar structure and high specific surface area, which provides enough space for the encapsulation of sulfur and buffers the volume changes during the cycle. As an electrode preparation strategy, the raw material is economical and easy to obtain, which is convenient for industrial production. The initial discharge capacity of the PCF/S electrode is 1330 mAh g⁻¹. After 100 cycles, it has a good reversible capacity of 880 mAh g⁻¹ and good cycling performance at 0.2 C.

Some scholars have adopted 3D printing technology for microscale 3D carbon-based sulfur cathode materials, which is different from the previous printing used only for carbon. In this study, sulfur/carbon mixture and metal compounds are directly used as the mixed ink to construct the electrode at one time. Cai et al. ¹³² prepared a 3D-printed electrode with the “ink” consisted of super P, sulfur, and LaB₆ electrocatalyst via an equipped programming software, which could anchor LiPSs and promote the redox kinetics. The highlight is that this strategy not only laid out a new self-supporting electrode preparation method but also brought about a high initial capacity and a cycle life of up to 800 cycles at a high current of 6 C. Cai et al. ¹³³ applied the heterostructure to 3D printing electrode, which accelerated the nucleation of lithium sulfide by using V₈C₇-VO₂ heterointerface. Furthermore, Li-S pouch cells with 3DP-V₈C₇-VO₂/S cathode were assembled and tested under different bending states, which laid the foundation and proved the feasibility of the application of flexible self-supporting LSB cathode in wearable devices (Figure 10).

Figure 10. Photograph and Scanning electron microscopy images of (A) the as-prepared melaleuca bark, (B,E) carbonated bark foam, (C,F) porous carbon fiber (PCF), and (D) PCF/S composite, (G,H) transmission electron microscopy images of PCF. Reproduced with permission: Copyright 2018, Elsevier.129 (I) Schematic diagram of the fabrication procedure of 3DP-LaB6/SP@S electrodes (inset: the photographs of printing patterns using LaB6/SP@S composite ink). Reproduced with permission: Copyright 2020, Elsevier132

Sulfur loading methods for flexible self-supporting cathode

As a critical part in the design of LSBs cathode structure, the sulfur loading process has a large impact on the capacity and cycle life of the battery. The traditional coating method is to evenly grind the sulfur powder and the electrode material powder, followed by heat treatment at 155°C to make the molten sulfur into the host material before preparing the electrode. Due to the particular structure of the self-supporting electrode, the material cannot be ground, where sulfur can either be introduced into the material after forming the electrode substrate slice or before assembling the electrode from the precursor material. Therefore, how to integrate sulfur into the self-supporting electrode stably and uniformly while ensuring high performance is one of the challenging problems. On the basis of the previous section, the sulfur loading methods are summarized.

Melt infiltration

Similar to the traditional melting method, sulfur is melted at about 155°C and infiltrated into the thin sections of a substrate. As the material cannot be ground, the substrate prepared is usually immersed in S/CS₂ solution or other sulfur-soluble nonpolar solvents to make the sulfur molecules uniformly adhere to the substrate. After drying, the substrate is heat treated in vacuum or inert gas. When the substrate is thin, it does not require to be impregnated, and the sulfur layer can be directly deposited on top of the substrate. The molten sulfur liquid flows into the material, and after cooling, the solid sulfur is coated inside the pores of the substrate and on the outer surface of the skeleton.

This loading sulfur method is common, where sulfur can easily enter the host material. As the molecular diameter of the S₈ molecule is at nanoscale, this sulfur loading method is more appropriate for the host material, especially the one with a micromesoporous structure. For the macroporous structure, the coating performance of sulfur is limited, which may lead to the dissolution of LiPSs. Therefore, for the carbon-based materials described in this paper, the carbon structure is stable, and it is difficult to decompose during the melting and infiltrating of sulfur. Most carbon-based self-supporting electrodes have a large specific surface area and high porosity, which is favorable to sulfur fusion. To summarize, the appropriate material for melt infiltration method should have the following properties: stable electrode substrate material with microporous structure, high decomposition temperature of host material and additives, and insolubility in organic solvents.

However, under the influence of basement infiltration or top layer deposition, a large number of sulfur molecules adhere to the surface, which not only leads to the formation of agglomeration, capacity loss, and penetration of lithium dendrites but also results in the formation of LiPSs that dissolve easily in the electrolyte. To avoid the influence caused by shuttle effect, after the melt infiltration, the electrode material can be treated at about 300°C, above the evaporation temperature of sulfur, and heated again to evaporate the residual sulfur molecules on the outer surface of the electrode. However, it is challenging to control the temperature and time of volatilization, which can easily lead to excess sulfur volatilization and result in capacity loss. Therefore, it is difficult to find a balance between inhibiting shuttle effect and increasing sulfur content for the melt penetration method.

Gaseous diffusion

It is difficult for the liquid sulfur to flow into the bottom of the material when using the melt infiltration method. When the porosity of the material is low and the base material is thick, it is easier to be penetrated by gaseous sulfur. A substrate material is placed above an open container where the bottom is coated with sulfur powder, or it can be placed above a porous partition layer which separates the sulfur layer inside a closed container, and the sulfur vapor diffuses to the substrate at an inert gas atmosphere above 300°C. The decomposition temperature of the host materials and additives is higher than that in the melt infiltration method. However, in the evaporation process, it is necessary to use the flowing inert gas to help the diffusion of sulfur, which will cause a certain loss of sulfur.

In addition, it is difficult for the substrate material to be completely diffused by sulfur vapor. At the bottom of the substrate material, which is close to the sulfur powder, the sulfur content will be higher than that on the top, resulting in an uneven distribution of sulfur and correspondingly causing the uneven electrochemical effect during charging and discharging. Under these circumstances, inconsistent expansion and contraction of the upper and lower parts of the electrode can occur, making it bent and cracked, which leads to a reduction in the battery life. Moreover, the longer the gas diffusion time is, the higher tendency the sulfur adsorbed by the substrate material flows out with the inert gas. Hence, it is important to strategically control the gas flow speed and time.

Wetting adhesion

In the first two methods discussed, sulfur loading is generally carried out after the preparation of the substrate electrode plates. In contrast, for the wetting adhesion method, the precursor powder with high porosity on the surface and the 3D porous electrode precursor is generally selected as the sulfur-loading material, which is mixed with sulfur nanoparticles in water or other solvents. Under magnetic stirring or ultrasonic oscillation, the sulfur nanoparticles adhere to and get stuck on the inner wall of the pores on the host material due to the action of wetting angle. Compared with the melt infiltration and gaseous diffusion methods, this method is relatively simple, and however, sulfur content of the material is low due to the weak physical adhesion of the sulfur particles to the material. Therefore, a large amount of sulfur remains in the solvent. In addition, sulfur mainly presents on the external surface, which can cause the shuttle effect.

In-situ deposition

For in-situ deposition, two types of precursor materials can be selected: (1) materials containing sulfur compounds such as sulfate and (2) materials without sulfur, which can be dispersed or dissolved in the solution containing sulfur ions or placed in the sulfur-containing reaction gas. In the case of chemical reactions, the precursor materials react with the sulfur ions in the solvent or the sulfur-containing gas to generate sulfur elemental substances, which grow and deposit on the electrode under internal intrinsic or external adhesion effect. In general, in-situ deposition requires less high-temperature treatment and can be used to obtain nanoscaled sulfur which can be introduced into the material by simple stirring or ultrasonic treatment, suitable for powdery and thin precursor materials. Compared with the wetting adhesion method, the strong chemical bonds make the sulfur grow tightly in the material, restraining the LiPSs from dissolving. Compared with the melt infiltration and gaseous diffusion methods, the spatially refined active material reduces the migration path of ions and electrons, which is favorable to the redox kinetics of the catalytic cycle and reduces sulfur elemental agglomeration, capacity loss, and electrode damage. However, because the materials used in this method are usually at nanoscale, they have a low sulfur-bearing capacity and a low sulfur content. In addition, in the course of various chemical reactions, it is possible to change the mechanical properties of the material, making it prone to fracture when assembled into electrodes.

On the basis of the discussion above, each sulfur loading method has advantages and disadvantages. It is important to control the process parameters to overcome the limitations. A mathematical model can be established to simulate the sulfur loading process, such as calculating the relationship between the diffusion rate of the gas-phase, time, and sulfur evaporation, to better design the experiment parameters and predict the experimental results. In addition, a single or traditional sulfur loading method has a limited loading capacity for sulfur, and therefore, multiple sulfur loading methods may be combined to improve the material performance such as the sulfur content, sulfur capacity, and cycling performance. For instance, Yuan et al. ¹³⁴ prepared two hierarchical MWCNT–VACNT-S carbon nanotube papers by means of “top-down” and “bottom-up” design approaches. Here “top-down” can be considered as the preparation of electrode substrate layer before the modification of its structural composition: multi-wall CNTs (MWCNTs) and vertically aligned CNTs (VACNTs) were mixed in proportion and filtered into a hierarchical paper followed by the evaporation of sulfur via liquid permeation. The “bottom-up” method is to manipulate the unit materials in a controlled manner and then integrate them into an electrode. After the sulfur was infiltrated in the MWCNT network by the melting method, the composite was mixed with VACNTs to prepare a suspension, followed by filtration to obtain the MWCNT–VACNT-S paper. In such an electrode, short MWCNTs act as short-range conducting networks and sulfur–carbon binding sites, and long VACNTs serve as long-range conducting networks and crosslinking agents. This 3D conductive structure can offer sufficient spaces for sulfur loading as well as volume fluctuation. As a result, the free-standing electrode made by the bottom-up method exhibits better initial discharge capacity and cycle life than the one produced by the top-down method. When the electrode made by the bottom-up method is treated by liquid phase sulfurization, a large amount of sulfur attaches to the surface of the carbon nanotubes and accumulates with its recycled products to block the ion channels. The initial discharge capacity of the free-standing electrode made by the bottom-up method reaches 995 mAh g⁻¹, which is 60% of the theoretical value of the elemental sulfur, and 700 mAh g⁻¹ can be maintained after 150 cycles. In addition, shuttle effect was better inhibited and the polarization of the electrode was alleviated in the electrode made by the bottom-up method, compared to the one treated by liquid phase sulfurization (Figure 11).

Figure 11. (A) Schematic illustration of the free-standing electrode made via a bottom-up method. Scanning electron microscopy images of the free-standing electrode produced by (B) a top-down method and (C) a bottom-up method. (D) Cycling performance at a current density of 0.05 C. (E) Charge–discharge curves of bottom-up free-standing electrodes at different cycles. Reproduced with permission: Copyright 2014, Wiley.134 MWCNT, multi-wall carbon nanotube; VACNT, vertically aligned carbon nanotube

Brief summary

To compare the performance of each electrode more conveniently, values of the selected sulfur loading, current density, and initial capacity of different electrodes are listed and compared in Table 1 and a 3D diagram (Figure 12) is drawn to interpret the relationships among these parameters. Reflected from the values of the initial capacity, as shown in Figure 12, 1D nanocarbon-based cathodes have relatively uniform dispersion and stable electrochemical performance; 2D nanocarbon-based cathodes show great promise under high sulfur content and current density; current research work on 3D skeleton porous carbon-based cathodes is limited, which reveals there exists great potential for researchers to explore in this field. The carbon-based flexible self-supporting cathode materials of LSBs show great potential and a review of the processes of these materials provides valuable guidance for future research.

Table 1 Comparison of the electrochemical performance of the flexible self-supporting cathodes in lithium-sulfur batteries

Category	Flexible self-supporting cathode	Initial capacity (mAh g−1)	Final capacity (mAh g−1)	Cycle number	Current density (C)	Sulfur dontent (wt%)	Sulfur loading (mg cm−2)	Reference
One-dimensional nanocarbon	S-CNT-23	1438	653	100	1.0	23	3	[21]
	S-CNT	1104	740	100	0.1	65	5	[18]
	MWCNT-VACNT-S	995	700	150	0.05	60	6.3	[134]
	S-PCNT	776	528	100	0.1	70	1.9	[22]
	PVP@S-SACNT	1175	856	200	1.0	47	1	[27]
	FMC@S-1	1074	666	350	0.5	28.8	1	[19]
	S/N-CNT	1267	807	100	0.2	61	–	[28]
	S@CNCF-10/1	1269.1	1104.1	100	0.2	70	3	[20]
	S@HCNF	1140	730	150	0.2	75	1	[38]
	S@PCNFs–CNT	880	637	100	0.05	40	0.8	[47]
	Co-CNF/Li2S6	820	700	300	0.5	55	4.6	[39]
	S/CoNi@PNCFs	1245	900	100	0.2	69.7	1.5	[40]
	Mn3O4@CNF/S	1180	700	100	0.2	50	6	[59]
	S-CPZC	1286.9	1059.8	50	0.2	75	5.1	[60]
	CNF-S	1139	847	100	0.1	67.85	4.4	[61]
	S/MoS2@G-PCNFs	1385	857	100	0.1	54	1	[62]
	C@WS2/S	1180	995	500	0.5	11	2	[65]
	FeCo2S4/CC@S	969	729	300	0.5	17.6	3.2	[66]
	PCF/VN/S	1310.8	1052.5	250	0.1	60.1	8.1	[67]
	CC@CoP/C-S	1030	998	500	1.0	60.74	4.17	[68]
	C@SnO2/S	1228	1105	100	0.2	11.4	2.05	[69]
	C@SnO2/TMS/S	1261	1175	200	0.5	13.32	2.75	[71]
	GC-TiO@CHF/S	1172	988	150	0.1	73	5	[77]
	BCN@HCS/S-70	1083	1041	100	0.2	70	4	[79]
Two-dimensional nanocarbon	GS/S	722.9	600	100	0.1	67	0.5	[85]
	S-CNT/G (1:1)	1048	818	200	1.0	50	—	[86]
	rGO-S	1302	978	200	0.1	56	2.2	[88]
	SGP	1008	806	500	1.0	59	2	[92]
	LF@CNT-GN/Li2S6	1140.4	986.6	500	1.0	—	4.3	[87]
	CoS2/rGO	993	806.7	110	0.5	60	7.2	[94]
	S-rGO	657	592	160	1.0	66	1	[107]
	G/CNT-S//G/CNT	1286	970	100	0.5	50	2.46	[108]
	Li2S/N-doped G	657	403	300	0.5	55	2	[109]
	3DNG/TiN	1480	957	200	1	68.5	9.6	[110]
3D skeleton porous carbon	N-doped C/S	1375	1150	30	0.05	25	2.5	[118]
	NCF-S@rGO	918	640	100	0.2	61	3.2	[119]
	CF@CNTs/MgO-S	961	769	100	0.2	49	3.8	[122]
	NCF/CNT/PEDOT@S	1395 (0.2 C)	802	100	0.5	80	2.6	[121]
	CTNF@CoS2–CNA/S	1013	750	100	1	51.79	3.2	[120]
	PCF/S	1330	880	100	0.2	51	6	[129]
	Free-standing Li2S	1262	1100	200	0.5	—	1.3	[128]

Figure 12. The sulfur loading, current density, and initial capacity of carbon-based flexible self-supporting cathode materials

To further illustrate the relationship between the structural design of each carbon-based electrode and its electrochemical properties, Figure 13 shows the distribution of sulfur on four substrates and the propagation routes of ions and electrons.

Figure 13. The relationship between the structures of carbon-based materials and the particles transport of lithium-sulfur batteries

(1) CNF. The flexible self-supporting electrode of CNF substrate is generally made from a thin film prepared by electrostatic spinning followed by carbonization, so the sulfur loading is usually through a top-down method for melting and diffusion. As most CNFs have solid structure with some pores on the treated surface, sulfur is mainly distributed on the fiber surface and a small portion is anchored in the pores. In electrochemical reactions, carbon fibers with complete structure and long length can facilitate the transfer of ions and electrons, enabling them to move more orderly and rapidly compared to the ones that migrate between the free carbon atoms. On the contrary, as sulfur mainly exists on the surface, the LiPSs formed after lithium ions are transported to the sulfur location will dissolve in the electrolyte, and the wrapping of pores on the fibers has a certain inhibitory effect on the dissolution of LiPSs.

(2) CNT. The commonly used sulfur loading methods for CNT substrates include in-situ formation or melting. The former method can prevent sulfur agglomeration on the surface to some extent, whereas the latter can increase the sulfur loading capacity. For both of the two methods, most of the sulfur molecules are distributed on the surface and a small portion is directly generated inside the CNTs or melted into them, which is dependent on the structure of CNTs. The hexagonal carbon lattice formed by sp² hybridization on the surface can bond with sulfur atoms. After sulfur elemental transformation, the nonpolar forces at the sulfur end have a certain repelling effect on the LiPSs. Inside, the hexagonal carbon lattice acts more like a barrier and bonds between the atoms make the trapped LiPSs more difficult to dissolve.

(3) Graphene. The graphene substrate is loaded with sulfur in the same way as the CNTs, by in-situ formation or melting. Due to the large 2D continuous structure of graphene, the loaded sulfur mainly exists on the surface and between the sheets. Also, because of the sp²-hybridized structure, the barrier and nonpolar forces of carbon lattice can inhibit the dissolution of LiPSs. In addition, the stacking of graphene as a result of π–π force further prevents the LiPSs dissolution. For the migration of ions and electrons, a large area of layered structure with slight undulation in graphene makes them move more freely than those in the 1D materials.

(4) 3D skeleton porous carbon. In general, the feasibility of carbon substrates derived from 3D skeletal materials as the electrode material should be evaluated before the active substances loading. The most commonly used sulfur loading methods of carbon substrates derived from most the skeleton materials are melting or adhesion. Compared to the nanomaterials discussed above, most of the carbon substrates derived from skeleton materials are micromaterials, and therefore, they provide more adequate space for sulfur loading. However, LiPSs dissolution may occur when the pore size is too large. When the sulfur is evenly distributed on the surface, the substrates with microporous, mesoporous, and macroporous structures can load and anchor different amounts of sulfur.

Currently, the flexible self-supporting cathode materials based on carbon substrates for LSBs are still under development, which faces great challenges. As listed in Table 1, the performance of batteries prepared with different carbon-based materials is different, which is related to the structure of each carbon-based material. As shown in Figure 13, carbon nanotubes and carbon nanofibers have small inner diameters, limited bearing capacity for sulfur, and poor dispersion, which makes sulfur easy to aggregate on the outer surface and causes uneven distribution. In addition, for the preparation of carbon nanofibers by electrostatic spinning, it has harsh requirements for air humidity. Improper control of air humidity can cause poor film-forming properties and inefficient utilization of internal pores. As shown in Table 1, it is difficult for an electrode to reach high sulfur content, high capacity, and high efficiency simultaneously. For the 2D graphene material, after being compounded with modified functional groups or other polar materials, the accumulation characteristics caused by π–π-conjugated structure can be alleviated and self-aggregation phenomenon can be inhibited, but the conductivity also decreases accordingly. As a lamellar material, it forms a film under the action of stacking. It is not conductive in the longitudinal direction and has little porosity, and sulfur is difficult to integrate, which wastes a large amount of specific surface area. If the activator, such as potassium hydroxide, is used to corrode the carbon wall to form pores, the process is complicated. Therefore, it is critical to explore the optimum proportion of additives and simplify the pore construction process in graphene. In addition, carbon nanotubes, carbon nanofibers, graphene, and other nanoscale materials are of high cost, and the preparation process of this matrix is complicated, and therefore, it is not economical for industrial production. The research on carbon-based materials derived from other skeleton materials is still limited, and few materials were reported, most of which are prone to fracture and have poor adsorption performance for LiPSs. Comparison of flexible self-supporting cathode materials based on carbon substrates for LSBs discussed above is summarized in Figure 14.

Figure 14. Comparison of flexible self-supporting cathode materials based on carbon substrates for lithium-sulfur batteries. (A) SEM image and (B) transmission electron microscopy image of the PVP@S-SACNT composite. Reproduced with permission: Copyright 2016, Elsevier.27 (C) SEM image of the S-PCNT (70%) composites. Reproduced with permission: Copyright 2016, American Chemical Society.22 (D) SEM images of SGP films and (E) corresponding mapping of element S. Reproduced with permission: Copyright 2017, Wiley.92 (F) SEM images of the rGO-S composite film. Reproduced with permission: Copyright 2016, Wiley.88 (G) SEM images of porous carbon fiber and (H) the corresponding elemental mapping images of sulfur. Reproduced with permission: Copyright 2018, Elsevier.129 (I) SEM images of the NCF-S@rGO composites. Reproduced with permission: Copyright 2017, Royal Society of Chemistry.119 NCF, N-doped porous carbon film; PCNT, mesoporous carbon nanotubes; PVP, polyvinyl pyrrolidone; rGO, reduced graphene oxide; SACNT, super-aligned carbon nanotubes; SEM, scanning electron microscopy; SGP, S-graphene-poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate)

CONCLUSIONS AND OUTLOOK

In this paper, the recent research progresses in the material selection, structural design, and fabrication process of carbon-based flexible self-supporting cathode structure for LSBs are reviewed and their relationships to the battery performance are discussed. It is assured that the preparation of cathode-based flexible self-supporting cathode has become popularized, and relatively stable mechanical and electrochemical properties can be achieved, which paves the way for the development of flexible LSB. However, the development of carbon-based flexible self-supporting cathodes for LSBs is under exploration and it still faces great challenges.

On the basis of the current research progress of the flexible self-supporting carbon-based cathodes for LSBs, the following aspects need to be considered for the rational design of an electrode.

• 1.

  Material selection. Both the carbon substrate and its decoration need to have a good affinity to the active substance sulfur or its compounds, to increase the sulfur carrying capacity and the ability to capture LiPSs while maintaining a good electrical conductivity. The reason is that, as a nonpolar conductor, adsorption of LiPSs on the carbon substrate is mainly dominated by the intermolecular force, which is insufficient for inhibiting the shuttle effect. In addition, given the consideration of maintaining or improving the conductivity, polar modifiers such as metal or conductive metallic oxide and polymer are sometimes introduced. However, there is a difference in surface energy between the polar modifiers and the nonpolar carbon matrix, causing that the polar substances have a stronger attraction to LiPSs than the matrix, which increases the migration path of ions and electrons and reduces the chemical reaction kinetics. Considering that the affinity of carbon substrate to the polar decorations is enhanced, the functional groups can be grafted on its surface to form a "hand in hand" structure with the polar conductive materials, in addition to generating dipoles by heteroatom doping.

• 2.

  To evaluate the performance of the flexible devices, in addition to the study on their electrochemical properties, the changes in energy and the mechanical properties under deformation are also important indicators for the flexible devices’ performance. Only a few researchers tested the stress and strain of the flexible device, as well as the electrical conductivity and power supply effect of the material under bending. In contrast with the button cell capacity, rate, and cycle data collected from the rigid testing, the mechanical and electrochemical performance of the button cell under different deformation states should be conducted to demonstrate its flexibility. At present, little attention is paid to these studies of the flexible self-supporting electrodes in LSBs, and this can be considered as a direction for future work in this field. It should be worth noting that only testing the electrode with different curvature is insufficient to realize the practical application and the influence of different environmental conditions should also be considered. The comprehensive performance of the electrode materials can be evaluated by changing the temperature and humidity.

• 3.

  In terms of cost, nanocarbon-based materials are expensive, and the preparation process is generally time-consuming, which further increases the cost. As for flexible films, they have not been widely industrialized yet. Therefore, while exploring the industrialization of nanomaterials, it is critical to develop cost-effective micron-scaled carbon-based materials with a long lifetime and high energy storage capacity. Natural biomass materials or wastes with complete structure and good mechanical properties are good candidates for producing flexible electrodes to improve energy efficiency and reduce pollution to the environment. In addition, mathematical modeling can be performed to optimize the structural design by tuning the surface binding energy, chemical bond formation/break, and electron transfer, and so forth, of the electrode.

• 4.

  In terms of structural design, it is important to fabricate an ideal flexible self-supporting cathode and optimize other parameters of the battery to achieve high capacity and long cycle life. For the electrolyte, the traditional electrolyte is in a liquid state, whereas the gel electrolyte meets the requirement of a flexible battery. However, when preparing a solid electrolyte, the ratio of polymer to solvent affects its mechanical properties and electrical conductivity, which requires rational structural design. For the anode, lithium dendrite growth of flexible anodes under the condition of deformation is ignored at present. The influence of deformation such as bending, folding, stretching, and contraction deformation on lithium dendrite and transfer direction of lithium ions or electrons can be considered as a future research direction. This will provide guidance for selecting an appropriate flexible substrate with mechanical properties compatible with that of the cathode, avoiding generating uneven stress. Moreover, it is favorable to design a pore structure to reduce the weight and increase the loading content of lithium.

• 5.

  When utilized in practice, heating effect of the battery should be taken into consideration. The safety of batteries can be increased by reducing the usage of ethers electrolyte or using a sensor to test the battery under the working condition of thermal expansion, and researching the influence of electron and ion activities on battery sensitivity to prepare a thermal management system with stable voltage and heat production rate.

In summary, the use of flexible self-supporting carbon-based electrodes will lead to significant improvements in the capacity and efficiency of LSBs, facilitating the development of flexible batteries and devices in various fields.

ACKNOWLEDGMENTS

The authors acknowledge the financial support from the National Natural Science Foundation of China (Nos. 21978110 and 51772126), the Natural Science Foundation of Beijing Municipal (No. L182062), the Talents Project of Beijing Municipal Committee Organization Department (No. 2018000021223ZK21), the Yue Qi Young Scholar Project of China University of Mining & Technology (Beijing) (No. 2017QN17), the Fundamental Research Funds for the Central Universities (No. 2020XJJD01 and 2020YJSJD01), Jilin Province Science and Technology Department Program (Nos. 20200201187JC and 20190101009JH), the "13th five-year" Science and Technology Project of Jilin Provincial Education Department (No. JJKH20200407KJ), Jilin Province Development and Reform Commission Program (No. 2020C026-3), and Jilin Province Fund for Talent Development Program (No. [2019] 874).

CONFLICT OF INTERESTS

The authors declare that there are no conflict of interests.