School of Materials Science and Engineering, Provincial and Ministerial Co-construction of Collaborative Innovation Center for Non-ferrous Metal new Materials and Advanced Processing Technology, Henan University of Science and Technology, Luoyang 471023, China
b.
Faculty of Engineering, Huanghe Science & Technology University, Zhengzhou 450063, China
c.
State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Materials, Xiamen University, Xiamen 361005, China
Received Date:
02 September 2024 Accepted Date:
19 September 2024 Revised Date:
17 September 2024 Available Online:
15 November 2025
Abstract:
Advanced lithium-chalcogen (S, Se, Te) batteries (LCBs) are among the most promising candidates for next generation energy storage systems because of their high energy density and theoretical capacities. However, they are still facing many challenges, such as expansion of the volume problems of chalcogen elements, the shuttle effect of intermediate products, low Coulombic efficiency and inferior cycling stability, which seriously hinder their commercial applications. The presence of a binder in the cathode causes an uneven distribution of the active substances, and also occupies a part of the electrode's volume, resulting in the unsatisfactory energy density of LCBs. In this regard, binder-free electrodes which do not need binders, conductive materials and even collectors, can be used as electrodes for flexible batteries, effectively solving the above-mentioned problems. In this review, the main methods of fabricating binder-free cathodes and their advantages and disadvantages are discussed. Furthermore, a review of representative works on binder-free cathodes for high-performance LCBs over the last decade is presented. The main binder-free electrode materials include paper cloth (PC), graphene oxide (GO), carbon nanotubes (CNTs), carbon nanofibers (CNFs), carbon cloth (CC), polymers, metallic compounds, and their composites. In addition, we discuss these works from four aspects: Advanced structures, methods of fabrication, electrochemical performance and the potential mechanism of binder-free cathode materials, providing important guidance for further researches. Finally, we propose the current challenges of binder-free LCBs and look forward to breakthroughs in this field through the use of binder-free electrodes.
With the development of portable wearable electronic devices and electric vehicles, people urgently need lightweight and flexible equipment for storing energy with a high energy density to meet the growing energy demand [1,2]. Commercial lithium-ion batteries have been dominating the market since their inception because of their high energy density compared with other battery systems such as lead-acid battery and nickel-cadmium battery [1,2]. However, the intercalation mechanism of lithium-ion batteries puts an upper limit on their energy density, making them unable to meet the ever-increasing demands of the next generation of energy storage devices [3,4]. Therefore, much work has gone into developing high energy-density alternatives to lithium-ion batteries. In this regard, lithium–chalcogen (S, Se, Te) batteries (LCBs) are considered to be the most likely candidates for the next generation of high-energy-density energy storage devices, on account of their high energy density and theoretical capacity, low cost, and environmental friendliness [5,6]. However, their commercial application has been hampered by several challenges. The low conductivity and the shuttle effect of the active cathode materials reduce the utilization rate of the active substance, resulting in a low actual capacity, as well as the growth of lithium dendrites and the large volume expansion of lithium metal anode during the cycling progress, which greatly reduce the safety of LCBs [7].
At present, researchers have mainly used four ways to solve aforementioned problems of LCBs, namely the design of cathode, modification of the separator, optimization of the electrolyte, and protecting the anode [8-11]. Among them, the most promising method is to design a reasonable cathode structure using carbon materials to improve the electrochemical performance of LCBs. The most commonly used carbon materials for cathodes include porous graphite, porous carbon (PC), graphene (G), carbon nanotubes (CNTs), carbon nanofibers (CNFs), metal oxides, and so on, which need to be mixed with conductive agents and binders to make a slurry that is then coated onto the current collector [12]. However, the addition of binder will increase the weight and volume of the electrode, resulting in a decrease in the energy density of the batteries, and the insulation and instability of the binder will reduce the conductivity of the cathodes [13,14]. These problems caused by the binder will ultimately lead to the poor cyclic stability of the battery [15]. In this context, designing and fabricating binder-free electrodes has become an attractive strategy for the preparation of high-performance electrical energy storage devices. It not only makes the binder and current collector unnecessary, greatly saving costs, but provides more space for the active material, further increasing the energy density of the battery [16]. To date, a large number of binder-free electrodes have been designed to improve the performance of lithium–chalcogen batteries [13]. For example, Cao et al. prepared a self-supporting binder-free reduced graphene oxide (rGO)-S film with high flexibility by simultaneously reducing and assembling graphene oxide sheets with nano-sulfur on a Zn metal surface, and the batteries with the rGO-S electrode achieved a high initial capacity and an excellent rate performance because of its unique three-dimensional network that conducted ions and inhibits polysulfide shuttles [17]. Recently, Song and co-works prepared a hyperbranched sulfur-rich polymer (CNT/Ath-PEI@S) based on modified polyethylene-imide (Ath-PEI) by the sulfur polymerization method and used it as a binder-free cathode. The high intrinsic viscosity of Ath-PEI avoided the need of the polyvinylidene fluoride (PVDF) binder, thereby increasing the sulfur content to 75% [18]. The battery with this binder-free electrode delivered an ultra high ultra-high initial discharge capacity (1520.7 mAh/g at 0.1 C), and a high rate capability (804 mAh/g at 2.0 C). Moreover, Mei and co-works designed a molybdenum selenium-enriched graphene aerogel (MoSe2-x@GA/S) and the binder-free cathode supported by graphene networks maximized electronic conductivity and reduced bulk expansion [19]. The nucleation and dissociation of Li2S were accelerated by the defection-rich MoSe2-x, which give the batteries delivered a slow decay rate of 0.024% per cycle after 1000 cycles at 1 C [20]. In addition, Ma et al. combined metal–organic framework (MOF) and CNTs to prepare a binder-free MOF/CNT@S film. The special CNTs interpenetrated the MOFs’ hierarchical porous structures, had excellent electrical conductivity, and could alleviate the expansion in volume during lithiation/delithiation. Therefore, the cathode exhibited a high initial capacity (1263 mAh/g at 0.2 C) and excellent cycling stability with a small fading rate of 0.08% per cycle over 500 cycles at 0.2 C [21]. Furthermore, binder-free lithium-selenium cathode has also been extensively studied, He and co-works [22] fabricated a 3D GO-CNT@Se electrode with a sandwich architecture via a solvothermal method. And the Li-Se battery with GO-CNT@Se electrode delivered high capacity of 632.7 mAh/g, and depicted excellent rate performance. In addition to the works above, other studies on materials such as VO2(P)-NCNT/S film [23], GO/S@polymer film [24], S/graphene@g-C3N4 sponge [25], MXene/rGO@S aerogels [26], CNB-TiC@CNF/S film [27], Se/CoSe2@CNF [28] and Te@CNT [29] cathode have also been reported. These advanced materials and unique structures interact together to improve the electrochemical properties of LCBs.
The last decade has witnessed the rapid development of LCBs based on binder-free cathodes through the reasonable designing of the electrodes’ structure and improving methods of preparing the reasonable electrodes. However, a comprehensive summary of application of binder-free cathodes in high-performance LCBs has been rarely reported. In order to promoting the development of this field, we summarize the application of binder-free cathodes in high-performance lithium–chalcogen (S, Se, Te) batteries. Fig. 1 shows the important achievements in the development of binder-free LCBs [17-19,23,30-37]. The methods of synthesizing the active material host could be divided into two categories: template free methods and template methods. The templates-free methods are discussed according to the category of carbon host (GO, CNT, CNF, common carbon), whereas templates are discussed according to their template category (carbon cloth, metal foil, pore-making agents). Moreover, the main used methods and their characteristics are summarized. The main achievements of the various binder-free compounds are discussed from the aspects of fabrication methods, reasonable structure, potential reaction mechanism and high electrochemistry performance. In addition, the potential use of these binder-free nanostructured electrodes in practical full battery configurations and in flexible and stretchable applications are described. Finally, the challenges and opportunities for future development of binder-free cathodes toward high-performance lithium-chalcogen batteries are proposed.
Figure 1
Figure 1.
Timeline of the major milestones of binder-free LCBs. Reproduced with permission [30]. Copyright 2012, The Royal Society of Chemistry. Reproduced with permission [31]. Copyright 2013, Wiley-VCH. Reproduced with permission [32]. Copyright 2014, Wiley-VCH. Reproduced with permission [33]. Copyright 2015, Wiley-VCH. Reproduced with permission [17]. Copyright 2016, Wiley-VCH. Reproduced with permission [34]. Copyright 2017, Wiley-VCH. Reproduced with permission [35]. Copyright 2018, American Chemical Society. Reproduced with permission [23]. Copyright 2019, Elsevier. Reproduced with permission [36]. Copyright 2021, Wiley-VCH. Reproduced with permission [37]. Copyright 2022, Wiley-VCH. Reproduced with permission [18]. Copyright 2023, Wiley-VCH. Reproduced with permission [19]. Copyright 2024, Wiley-VCH.
2.
Challenges and strategies for lithium–chalcogen batteries
Among lithium–chalcogen (S, Se, Te) batteries, lithium-sulfur batteries (LSBs) are widely studied due to their advantages of low cost and high theoretical specific capacity, as well as high energy densities [18]. Studies have shown that lithium-sulfur batteries are not suitable for carbonate electrolytes because the resulting polysulfide will react nucleophilically with carbonate solvents, resulting in ineffective batteries after the first discharge [15]. In the discharge process, the lithium-sulfur battery exhibits a typical multi-step reaction, first from solid S8 to long chain polysulfide, its corresponding voltage is about 2.3 V; then further lithium intercalation, from long chain polysulfide into Li2S2 and Li2S, corresponding voltage is about 2.1 V and during the discharge progress, solid Li2S2 and Li2S are reduced to long chain polysulfides (Fig. 2a) [38]. LSBs still confront several challenges which significantly hinder their commercialization [39]. These challenges primarily encompass the following aspects: (1) Shuttle effect: During the charging and discharging processes of LSBs, long-chain lithium polysulfides instantaneously dissolves into the electrolyte and these dissolved polysulfides can diffuse from the cathode to the lithium anode and subsequently return to the sulfur cathode driven by concentration gradients, known as “shuttle effect” (Fig. 2b). In diffusion process, soluble polysulfides can be directly reduced to short-chain polysulfides (Li2S2/Li2S) on the Li metal surface without the presence of electrons. Subsequently, these insoluble Li2S2/Li2S deposit on the Li metal, rendering them unavailable for subsequent cycles, thereby leading to severe loss of cathode active material, high self-discharge in the sulfur cathode, and rapid capacity fade. (2) Low active material utilization: The elemental sulfur (S8) exhibits extremely low electrical conductivity at room temperature, amounting to merely 5.0 × 10−28 S/cm, which impedes efficient electron transport between the active material and the current collector. Furthermore, the ultimate discharge products of lithium-sulfur batteries, Li2S2 and Li2S, are also electronically insulating, hindering high-rate performance and limiting the full utilization of active material. Additionally, the severe shuttle effect exacerbates the loss of cathode active material. (3) Severe volume expansion issue: During the reaction processes of sulfur element, new intermediate products are formed, which possess lower densities, leading to significant volume expansion of the original electrodes. For instance, the density disparity between sulfur (S8) and lithium disulfide (Li2S) (2.03 and 1.66 g/cm3, respectively) results in a volume expansion of sulfur by approximately 80% during discharge/charge processes. (4) Notorious lithium dendrites and dead lithium of Li anode side: During the repeated charge-discharge processes, a unstable solid electrolyte interface (SEI) on the surface of the lithium anode can easily lead to uneven lithium deposition, resulting in the formation of lithium dendrites. These dendrites may puncture the separator, causing short circuits and serious safety issues. Moreover, the detachment of lithium dendrites from the anode forms dead lithium, resulting in the severe consumption of active lithium.
Figure 2
Figure 2.
(a) The charge and discharge behavior of lithium–sulfur batteries in ether-based electrolytes. Reproduced with permission [38]. Copyright 2016, Royal Society of Chemistry. (b) Schematic diagram of the shuttle effect of LCBs. (c) Schematic diagram of charge and discharge behavior of Li-Se batteries in carbonate and ether-based electrolytes. Reproduced with permission [42]. Copyright 2022, Wiley-VCH. (d) The basic structure of lithium–chalcogen batteries with binder-free cathodes.
Li-Se batteries (LSeBs) have a similar mechanism to that of lithium–sulfur batteries, and possess a high volume specific capacity (3253 mAh/cm3) and a relatively low theoretical weight specific capacity (675 mAh/g). Due to the diverse structures of Se, the mechanism of lithiation is different [40,41]. For the amorphous helical chain α-Se, it is first reduced to Li2Sen (n ≥ 4), corresponding voltage is ~2.0 V, and then further reduced to Li2Se, corresponding voltage is 1.95 V. For crystalline selenium, direct reduction from helical Se to Li2Se. This is because amorphous selenium is more active and the covalent bond energy in selenium is weakened by sufficient electrostatic force and van der Waals force between helical connections (Fig. 2c) [42]. During the charging process, Li2Se is oxidized to Li2Sen (n ≥ 4) and then further oxidized to Se element. Similarly, LSeBs also experience the “shuttle effect” of soluble lithium polyselenide in the cathode and the growth of lithium dendrites in the anode during charge-discharge cycles, significantly degrading battery lifespan and capacity. Although selenium exhibits improved conductivity (1 × 10−3 S/cm) compared to sulfur, it still falls short of requirements and contributes to the loss of selenium as an active material [43]. What is more, the density of elemental selenium and lithium selenide is very different (elemental selenium density is 4.81 g/cm3, lithium selenide density is 2.07 g/cm3), resulting in a volume change of about 2.7 times during charge and discharge. This volume change can easily lead to the pulverization of the electrode structure, which affects the cycle stability and service life of the battery [44].
Compared with S and Se, Te has higher electrical conductivity and therefore higher reaction kinetics [45]. Li-Te batteries undergo a single-phase transition between Te and Li2Te in the absence of soluble polytelenides, but there may be an undefined multi-step lithiation process in the carbonate electrolyte [33]. The Li-Te battery exhibits a high theoretical specific volumetric capacity of 2558 mAh/cm3 and theoretical specific capacity is only 419 mAh/g (lower than LSBs and LSeBs) [46]. Compared with Li-S battery, Te has higher tolerance to carbonate electrolyte, so now Li-Te battery mostly uses carbonate electrolyte. In the electrolyte, Te is first reduced to three-wheeled polytelenide compounds, whose structure is different from the chain structure of polysulfide. Even more pronounced, the tellurium cathode undergoes a volume expansion of roughly 200% during lithiation, rapidly deteriorating long-term cycle stability. This severe volume expansion triggers rapid collapse of the electrode structure, ultimately causing precipitous decay in battery capacity [47,48].
The combined effects of the aforementioned challenges of cathodes for LCBs not only greatly reduce their Coulombic efficiency and cycle life, but also gravely compromise the safe operation of batteries. To better address these challenges, researchers have proposed the following strategies: (1) Cathode material and structural modification: Incorporating high-surface-area carbon materials (such as CNTs, mesoporous carbon, graphene) and nano-metal compounds into cathode materials enhances their conductivity and adsorbs lithium polychalcogenides, thereby reducing their dissoution and diffusion in the electrolyte. (2) Electrolyte optimization: Developing novel electrolyte systems to lower the solubility of lithium polychalcogenides in the electrolyte, thereby mitigating the shuttle effect. (3) Separator modification: Coating the separator with one or more layers of materials capable of adsorbing or blocking the migration of lithium polychalcogenides, such as polymers and inorganic oxides, to reduce the occurrence of the shuttle effect. (4) Lithium anode protection: Constructing an artificial protective layer on the surface of the lithium anode is an effective method to inhibit the side reactions between Li anode and the electrolyte. It can induce uniform deposition of lithium, inhibit the generation of lithium dendrites, and generate a stable solid electrolyte interface (SEI) to accelerate ion transport. Among them, designing and fabricating binder-free cathodes is considered as an effective strategy which can tackle these issues (Fig. 2d). (1) A rationally designed binder-free electrode possesses a unique three-dimensional porous structure. The abundant space within the electrode not only accommodates more active materials, enhancing the loading of LCBs, but also physically and chemically adsorbs soluble intermediates, inhibiting their shuttling between electrodes and effectively suppressing the “shuttle effect”. (2) Binder-free cathodes not only employ carbon materials (CNT, CNF, GO, CC) and metals with excellent conductivity as substrates, significantly improving the electrode's conductivity, but also incorporate polar substances with catalytic and adsorptive properties to facilitate the conversion of polychalcogenides and enhance reaction kinetics. (3) Binder-free cathodes typically exhibit robust self-supporting capabilities and favorable mechanical properties, effectively managing the volume expansion during reactions. Furthermore, the abundant voids in binder-free electrodes provide ample space to mitigate the detrimental effects of volume expansion on the electrode. Therefore, the adoption of binder-free methods for the preparation of composite cathodes can effectively address the challenges faced by LCBs.
3.
The methods of fabricating binder-free cathodes for lithium–chalcogen batteries
To date, various methods have been used to prepare binder-free electrodes for lithium–chalcogen batteries, which can be divided into two types in terms of whether a template is used: template-free methods and template methods. Template-free methods such as self-assembly method [49-51], vacuum filtration method [24], freeze-drying method [52], hydrothermal method [53-55], electrospinning [56,57], and carbonization method [58], have been used to fabricate binder-free electrodes. Among the template methods, the commonly used templates are carbon substrates, including carbon cloth, carbon paper, and carbon foam; metal substrates including nickel foam, aluminum foil, magnesium foil, and their oxides, and other templates such as silica balls, titanium dioxide balls and MOF [59]. These methods endow compounds with desirable features, including excellent flexibility, an abundantly porous structure, excellent conductivity, and outstanding adsorption. The advantages and disadvantages of those methods is shown in Fig. 3.
Figure 3
Figure 3.
(a-f) The advantages and disadvantages of various preparation methods of chalcogen hosts and binder-free electrodes for lithium–chalcogen batteries.
Self-assembly method is one of the most commonly used methods to synthesis binder-free cathodes due to its convenient operation, mild reaction conditions, low cost and the uniform dispersion of each component (Fig. 3a) [25,50]. For instance, Liu and co-workers prepared binder-free and self-supporting GO/rGO/S electrodes via the self-assembly method [51]. As shown in Fig. 4a, the first main step of this method is to suspend GO/rGO and sulfur in a water solution, and then stir the solution for a period of time to disperse all the ingredients evenly, forming a uniform dispersion solution. Finally, the dispersion solution is blade-coated to obtain a composite membrane, and it can be directly used as the electrode of LCBs after drying. Fig. 4b shows that the sulfur and GO are evenly distributed in the electrode, and the binder-free electrode depicts excellent flexibility. The advantages of this method are convenience and low cost, since it does not require complex treatment such as heating. Similarly, the one-pot method and the one-step method are similar to the solution self-assembly method [60]. The active material is dispersed uniformly in the host material, but the combination of these materials is not tight, easily resulting in the loss of sulfur. In order to enhance the bonding of sulfur and carbon materials, researchers usually combine the vacuum filtration method with the self-assembly method to prepare a tightly bonded composite electrode which does not require binders (Fig. 3b). In addition, in order to increase the porosity and specific surface area of the electrode, the freeze-drying technology is sometimes combined with the self-assembly method to prepare a binder free composite electrode with high porosity (Fig. 3c) [52]. The freeze-drying process can remove the solvent under freezing conditions and retain the original three-dimensional structure of the material, forming a porous structure. This porous structure can not only accommodate more active substances and increase the loading capacity, but also could be used to alleviate the volume expansion and limit the shuttling of intermediate products, which is conducive to improving the utilization rate of active substances during the cycling process.
Figure 4
Figure 4.
(a) Schematic illustrations of the process of preparing rGO@S films. (b) Elemental mappings of the rGO@S films. (a, b) Reproduced with permission [51]. Copyright 2019, Elsevier. (c) Synthesis scheme of MoO2/G and MoO2/G–S composites. (d) Scanning electron microscopy (SEM) image of MoO2/G. (c, d) Reproduced with permission [53]. Copyright 2017, The Royal Society of Chemistry. (e) Schematic illustration of the production of freestanding flexible Li2S@NCNF paper electrodes via Ar-protected carbothermal reduction of Li2SO4@PVP fabrics made by electrospinning. (f) An electrode made from multilayered Li2S@NCNF paper and a punched disc. (e, f) Reproduced with permission [63]. Copyright 2017, Wiley-VCH.
The hydrothermal method is one of the most commonly used methods to synthesize binder-free electrodes (Fig. 3d) [53-55]. For example, Sun and colleagues grew hollow MoO2 balls on the surface of GO surface by the hydrothermal method, in which the GO sheet acted as a conductive skeleton and the MoO2 balls were anchored to the graphene oxide's surface to inhibit the dissolution of polysulfides (Fig. 4c) [53]. This method is often used to grow polar materials on the substrate material, so that the polar materials and the matrix are firmly combined to form a stable structure in the composite cathode. The SEM image shows that hollow MnO2 nanospheres with uniform particle size was homogeneously coated with rGO, forming an interconnected 3D network (Fig. 4d). The main advantages of this method are that the synthesized nanomaterials have a uniform particle size distribution and a controllable morphology, and are not easy to aggregate. However, this method also has the disadvantages of a long reaction time and a high reaction temperature. Hydrothermal methods have also been used to grow nanomaterials in situ on carbon substrates, known as in-situ growth methods [61]. The main step of the method is to add the precursor or reactant to the reactor, grow the target product in situ by hydrothermal method, anchor the target product on the carbon matrix (CNT, CNF, GO), and then prepare the binder-free electrode. The advantage of this method is that the target material can be tightly combined with the carbon substrate [62].
In addition to the methods mentioned above, some other methods are also used to synthesize binder-free cathodes, such as electrospinning method, template method and template etching method (Fig. 3e) [56]. For example, Yu et al. successfully prepared binder-free CNF composite electrodes via combining electrospinning spray technology with thermal reduction of carbon, which converted Li2SO4 to Li2S (Fig. 4e) [63]. The electrode prepared by this method exhibited good flexibility and could be bent without cracking, which allowed it to be used directly as a binder-free electrode (Fig. 4f). The binder-free electrode also showed excellent electrochemical performance. CNF films prepared by electrospinning not only have good flexibility, but excellent electrical conductivity, which makes them the best potential candidates for binder-free cathodes [33]. Typically, a carbonization process is used to further carbonize a film prepared by electrospinning to improve the electrical conductivity of the fiber film since polymers generally do not conduct electricity well. However, carbonization process requires high temperatures and vacuum conditions, which would undoubtedly further increase manufacturing costs. Template methods are also used for preparing binder-free cathodes, and these methods can be divided into two types. One is to grow carbon and polar materials on the matrix, known as the matrix method. The main purpose of this method is to increase the adsorption of the host material. Commonly used matrix materials include CC, CP, GO foam, CNF and other metal matrix materials such as nickel foil, aluminum foil, zinc foil and nickel foam. For example, Mao and his colleagues used CC as a substrate to grow polar Mo2C, and after melting the sulfur, they obtained Mo2C/CC@S composite electrodes. Mo2C nanofibers are uniformly coated on the surface of carbon fibers, which can effectively adsorb polysulfides (Figs. 5a and b) [64]. The elemental mappings of Mo2C/CC@S showed that the various elements were evenly distributed, as shown in Fig. 5c. The CC had good mechanical properties, provided good support for the surface material, and had good electrical conductivity, which was conducive to the transportation of lithium ions. The CC interacted with the material growing on the surface, inhibited the shuttling effect and improved the reaction kinetics, which is beneficial for subsequent improvement of batteries’ performance [65]. Another class of template methods is called template etching method, which involves etching the template to customize the structure of the material to achieve structural control. Templates used for etching include SiO2 ball [66,67], TiO2 ball and anodic aluminum oxide (AAO) [34]. The chemical meteorological deposition (CVD) is used to grow carbon materials on the template, and the advantage of this method is high efficiency (Fig. 3f). For example, Hu and colleagues successfully fabricated sulfur-1,3-diisopropenylbenzene@CNT (S-DIB@CNT) binder-free electrodes from CVD and a copolymer using AAO as a template (Figs. 5d-f). The uniform distribution of CNTs in the polymer can improved the conductivity and flexibility of the battery. However, the CVD method requires a processing temperature of 650 ℃, which increases the costs (Fig. 3f).
Figure 5
Figure 5.
(a) Schematic illustration of the process of synthesizing Mo2C/CC@S. (b) SEM and (c) elemental mappings of Mo2C/CC@S. (a-c) Reproduced with permission [64]. Copyright 2021, Elsevier. (d) Schematic showing the process of fabricating the S-DIB@CNT hybrid. (e) Interwoven S-DIB@CNT hybrid network in a membrane and an optical photograph of the membrane (inset). (f) XRD patterns of the S-DIB copolymer, S@CNT, and S-DIB@CNT materials. (d-f) Reproduced with permission [34]. Copyright 2017, Wiley-VCH.
4.
Application of binder-free cathodes in Li-S batteries
4.1
Binder-free sulfur cathodes based on template-free methods
4.1.1
Graphene-based sulfur cathodes
Graphene is considered as a new star in the material world and has been a favorite of researchers since it was first reported [68,69]. Graphene's strictly 2D single-atom carbon sheet gives it unique physical and chemical properties, such as its high specific surface area (2630 m2/g), good electrochemical stability, good mechanical strength (~1 TPa) and excellent electrical conductivity, which gives it great advantages in the field of electrochemical energy storage [70]. Graphene oxide (GO) and reduced graphene oxide (rGO) are two widely used kinds of graphene. These types of graphene can form different binder-free three-dimensional structures, such as films [71], aerogels [55,72], foams [73] and sponges [74] after treatment. The combination of graphene with other materials such as polymers [24], CNT [50], CNF [74], and polar materials including nitrides [25], sulfides [52], oxides [53,75], MXene [26] and ferroelectric materials [76] is regarded as an effective way to improve the electrochemical performance of binder-free electrodes.
Although graphene has excellent conductivity and mechanical flexibility, it cannot effectively suppress the shuttling of polysulfides, and researchers need to oxidize and reduce graphene to obtain defective graphene oxide (GO) and reduce graphene oxide (rGO), which are used as binder-free host materials [77]. The oxidation process can produce a large number of oxygen-containing groups on the graphenes’ surface to increase the adsorption of polysulfide. However, the oxygen-containing groups on the surface of GO will cause structural instability. Further reduction of graphene oxide can remove the oxygen-containing groups on the surface of GO to obtain rGO, making its structure more stable [72,78]. Moreover, researchers can also improve its electrochemical performance by rationally designing the structure of graphene [79]. For example, Papandrea et al. [80] successfully prepared a freestanding 3D framework with a high sulfur weight content via a one-pot synthesis, which could be directly used as a cathode without binders and conductive additives. The Thermogravimetric Analysis (TGA) showed that this unique porous foam structure could have the highest loading of sulfur (95 wt%). Therefore, the LSBs with freestanding cathodes exhibited superior capacity retention (90% after 500 cycles at 1 C, corresponding to a decline in capacity of 0.052% per cycle). The excellent electrochemical performance of the electrodes could be ascribed to how the unique 3D GO skeleton could accommodate sulfur and also be used as a conductive skeleton to provide an efficient channel for transmitting lithium-ion [80]. In addition to the graphene films mentioned above, binder-free S/GO aerogels [55], porous rGO/Li2S8 aerogel cathodes [72] and wire-shaped rGO/sulfur cathodes [81] have also been prepared to be used as binder-free electrodes, due to the good electrochemical properties of graphene and their reasonable structural design.
In order to effectively prevent the aggregation of graphene, other carbon materials, such as CNTs [82], polymers [24,83], CNFs and polar materials (metal oxides, metal sulfides [54] and metal nitrides [25]), can be added to fabricate binder-free electrodes with excellent electrochemical properties. Combining CNTs with GO can greatly improve the conductivity and flexibility of composite materials and these materials can be directly applied to lithium–sulfur batteries as a binder-free electrode [84]. Subsequently, other binder-free CNT/G/S electrodes with different structures, such as 3D CNT/graphene-Li2S aerogels [85], 3D S-GO/MWCNT papers [50], GO/CNT@S aerogels [86], 3D N-GO/CNT@S microspheres [87], have also been prepared. Through the combination of these special structures and CNT, the electrochemistry of GO composite electrodes has been improved. Polymer@GO composites are synthesized by coating a layer of polymers on the surface of a graphene network [86,88]. Commonly used polymers coated to the surface of graphene including 3,4-ethylene-dioxythiophene, styrenesulfonate (PEDOT:PSS) [24], polyacrylonitrile (PAN) [89], polyvinyl pyrrolidone (PVP) [90], and polypyrrole (PPy) [73], which require only a one-step reaction in aqueous solution. The polymer can be made into CNF by electrospinning to improve its mechanical properties, and combining CNF with graphene forms binder-free CNF@G composites, including pie-like CNF/amino-G@S [91], CNF/rGO [92], 2D CNF/S/rGO [93], and P-do-G/CNF/S aerogesl [74,92,94], which is regarded as an efficient strategy to improve the performance of lithium–sulfur batteries. For instance, Li et al. combined CNF with GO to prepare a pie-like free-standing CNF/amino-G@S electrode [91]. Three-dimensional CNF coated with sulfur forms a carbon fiber electrode, forming an excellent conductive network, and the high parallel channel is conducive to the combination of carbon and polysulfide. With the combined effects of CNF and GO, the CNF/amino-G@S cathode delivered high capacity of 1314 mAh/g at 0.1 C [91].
In addition to the above mentioned materials with GO, some polar materials such as MoSe2-x [19], g-C3N4 [25], Mxene [26], MoO2 [53], CoS2 [54], and TiO [75] have been combined with graphene to form binder-free electrodes. These polar materials can effectively adsorb and restrict polysulfide lithium and promote the conversion of polysulfides. Recently, MXene has attracted widespread attention because of its their highly polar 2D structure and excellent conductivity [26,95]. For example, Song et al. [26] prepared a unique 3D porous Ti3C2Tx (T stands for the surface terminal functional groups, such as –O, –OH, and/or –F) MXene/rGO (MX/GO) hybrid aerogel as a polysulfide reservoir (Figs. 6a-c). The MXene/rGO skeleton had polar active surfaces, and thus provided high electronic conductivity and exhibited strong polysulfide-trapping abilities. As a result, the MXene/rGO-Li2SX cathode delivered a high initial capacity of 946 mAh/g at 1 C and stable cycling performance with a reversible capacity of 596 mAh/g over 500 cycles (Fig. 6c). In addition, a 3D porous binder-free S/Graphene@g-C3N4 sponge electrode was successfully assembled using a flexible microemulsion-assisted method of assembly. The S/graphene@g-C3N4 composite electrode exhibited excellent electrochemical properties, including superior high-rate capability (612 mAh/g at 10 C) and an extremely low capacity fading rate (0.017% per cycle over 800 cycles at 0.3 C). This excellent performances was a result of the spongy porous structure providing abundant spaces for sulfur, and the pyridine nitrogen site in g-C3N4 had a strong adsorption and catalytic capacity for polysulfide lithium, which effectively improved the utilization rate of sulfur [26]. Recently, Zhai et al. successfully prepared molybdenum selenide-modified graphene aerogels with abundant Se vacancies (MoSe2-x@GA) compound through a combination of freeze-drying and the hydrothermal method [19]. The gel had good self-supporting ability and could be directly used as a binder-free sulfur host material (Fig. 6d). High-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) images showed that molybdenum diselenide was attached to the GO'S surface and had many defects (Figs. 6e and f). The (MoSe2-x@GA/S electrode showed a high discharge capacity (1256.9 mAh/g at 0.2 C) and a low rate of decay (0.024% per cycle at 1 C after 1000 cycles), as shown in Fig. 6g [19].
Figure 6
Figure 6.
(a) The process of preparing an MX/G aerogel electrode. (b) High resolution TEM (HRTEM) images. (c) The long-term cycling performance of binder-free MX/G-30 electrodes. (a-c) Reproduced with permission [26]. Copyright 2019, The Royal Society of Chemistry. (d) Schematic illustration of the process of preparing 3D MoSe2-x@GA/S electrodes. (e) HRTEM, (f) SEM images of and (g) the long-term cycling performance of 3D MoSe2-x@GA/S electrode. (d-g) Reproduced with permission [19]. Copyright 2024, Wiley-VCH.
According to the analysis above, 2D graphene is widely used as an ideal binder-free material for lithium–sulfur batteries due to its high conductivity and mechanical properties and the good flexibility and self-assemble, which allow it to form self-supporting electrodes without the need for binders and conductive agent, improving the energy density of batteries. What is more, oxidation and reduction treatment of graphene increase sites of defect on the surface of graphene, which not only improves the electrical conductivity of graphene but also increases its adsorption capacity to absorb polysulfide. In order to further improve the performance of LCBs, a variety of materials including polymers, CNF, CNTs and polar materials, are used to combined with graphene to form binder-free composite electrodes. The addition of these materials effectively prevents the aggregation of GO, and also increases to absorb polysulfides. Herein, we summarized the synthesis methods, sulfur loading and the electrochemical performance of graphene-based binder-free composite electrodes in lithium-sulfur batteries, as shown in Table 1 [96,97].
Table 1
Table 1.
Comparison of sulfur loading/content, methods of synthesis and electrochemical performance of different kinds of non-templated binder-free electrode materials for Li-S batteries.
After curling, graphene can form materials known as CNTs with excellent properties, which have the same excellent physical and chemical properties as graphene [70]. The conductivity and stable chemical properties of CNTs are superior to other nanostructured carbon materials because of their special nanostructure [98]. CNTs are reported to be one of the most promising cathode materials due to their excellent conductivity and mechanical properties. Similar to GO, pure CNTs also aggregates easily so the specific surface area of CNTs is relatively low [99]. In order to effectively inhibit the aggregation of CNTs, combining CNTs with other materials (porous carbon [100-102], polymers [103], CNF [104,105], metallized forms) to prepare composite materials for the cathodes of lithium–sulfur batteries is attractive.
Considering the high specific surface area of porous carbon, the combination of porous carbon and CNTs can effectively inhibit the aggregation of CNTs [100]. Recently, Jia et al. prepared a 3D super-aligned carbon nanotube (SACNT) matrix reinforced with a multi-functionalized sulfur/nitrogen co-doped carbon (SNC) [102]. SNC acts a dual role as both a binder and a catalyst, which helps the SACNT to be more stable. At 1 C, the battery achieved a very high initial capacity of 1079 mAh/g. Impressively, the rate of capacity decay in each cycle was only 0.037% after 1500 cycles at 2 C.
Polymers with rich functional groups on the surface of CNTS can effectively adsorb polysulfide and inhibit the shuttling effect [98,106,107]. On the basis of this knowledge, researchers have fabricated a series of binder-free CNT-based compound electrodes, such as CNT/Ath-PEI@S [18], sulfurized polyacrylonitrile/CNT films [108], and sulfurized polyacrylonitrile (SPAN)@CNT [103]. For example, Zhang's team compounded a layer of polypropylene coating on the surface of CNT, and grew poly(3,4-ethylenedioxythiophene) (PEDOT) spheres on the surface to form self-supporting composite electrodes [109]. The organic groups on the surface of the electrode could form chemical bonds with polysulfides, effectively inhibiting the shuttling effect. Exception PEDOT, Wang et al. [103] combined sulfurized polyacrylonitrile (PAN) with a small amount of CNTs (SPAN/CNT) to produce a 3D conductive nanofiber network (Figs. 7a and b). Sulfur atoms exist in the form of short S2 and S3 chains, covalently bonded to the main chain of pyrolytic PAN main chain, and the electrochemical reduction of SPAN by Li+ was a single-phase solid–solid reaction with Li2S as the only discharge product. The capacity of lithium–sulfur batteries barely decayed after 800 cycles in Fig. 7c, which means the material is valuable for commercial purposes because of its ultra-high stability [103]. Recently, Song and co-works prepared a hyperbranched sulfur-rich polymer based on modified polyethylene-imine (CNT/Ath-PEI@S) by polymerization of sulfur, and used it as a binder-free cathode for LSBs (Fig. 7d) [18]. Modified polyethylene imine (Ath-PEI) was prepared by grafting the amine group in polyethylene imine with the epoxide ring by 1H nuclear magnetic resonance (NMR) (Fig. 7e). The high characteristic viscosity of Ath-PEI provided considerable adhesion and avoided the addition of PVDF binder, increasing the sulfur content of the cathode to 75%. The binder-free CNT/Ath-PEI@S electrode revealed high initial capacity and a long cycling life (Figs. 7f and g). According to these reports, sulfurizing a polymer and using it as the cathode of LSBs is an effective strategy to achieve a high energy density.
Figure 7
Figure 7.
(a) Schematic illustration of the process of synthesizing SPAN/CNT electrodes. (b) TEM images of a single SPAN/CNT-12 fiber. (c) The long cycling performance of SPAN/CNT electrodes. (a-c) Reproduced with permission [103]. Copyright 2019, Wiley-VCH. (d) Schematic of CNT/Ath-PEI@S as a binder-free cathode. (e) H-NMR spectrum of Ath-PEI and PEI. (e) Typical the galvanostatic charge–discharge (GCD) profiles of the CNT/S-PVDF, CNT/Ath-PEI@S and H-CNT/Ath-PEI@S at 0.2 C. (f) Cycling performance and Coulombic efficiency at a rate of 1.0 C of CNT/Ath-PEI@S and CNT/S-PVDF electrodes. (d-g) Reproduced with permission [18]. Copyright 2023, Wiley-VCH.
In order to improve the adsorption capacity to absorb lithium polysulfide, polar metallides including CoS nanostraws [35], hollow VOx nanospheres [110], CoFe2O4 nanoparticles [111], MoS2 nanosheets [112], AlF3 [113] and MOF-74-Ni [114] have been combined with CNTs as the catchers via forming polar sites to adsorb lithium polysulfide. Metal sulfides are another kinds of polar material and are widely used as electrode materials in lithium–sulfur batteries, since metal sulfides can adsorb soluble polysulfide lithium well, and also provide a certain capacity as an active substance. For instance, Ma et al. [35] prepared a crisscrossed network of CNTs reinforced CoS nanostraws (CNTs/CoS-NSs) compounds via electrostatic spinning spray technology. The S@CNTs/CoS-NSs electrodes were assembled into a soft-pack battery and exhibited excellent cycling performance. The electrode exhibited excellent electrochemical properties because the inner CNTs could be used as transport channels for lithium ions, and the outer CoS nanostraws can inhibited the shuttling of polysulfides through chemisorption and physical confinement.
Based on the results obtained above, the self-weaving behavior of CNTs is the basis of their high flexibility and mechanical properties, which are suitable for flexible electrodes without binders. In order to inhibit the aggregation of CNTs, various materials such as porous carbon, graphene, CNF, polymers and polar materials are added to CNTs. The introduction of these materials not only inhibits the accumulation of CNTs, but also provides a abundant spaces to accommodate sulfur and improve the active load of the electrode. In addition, the polar metal oxides can enhance the adsorption of CNTs to polysulfide, and can inhibit the shuttling effect. Adding a small amount of CNTs into a polymer significantly improves the strength and toughness of the material, and the CNTs can act as a conductive network, improving the electrical conductivity and mechanical properties of the electrode. Herein, we summarized the synthesis methods, sulfur loading and the electrochemical performance of CNTs-based binder-free composite electrodes in lithium-sulfur batteries, as shown in Table 1 [115-117].
4.1.3
Carbon nanofiber-based sulfur cathodes
CNF is another widely used material due to its structural designability, excellent mechanical properties and flexibility [118,119]. The common method of preparing carbon nanofibers is electrospinning, which makes the fiber's structure have good structural designability [120]. The defects, active sites and electrical conductivity of CNF can be effectively improved by adding polar materials to the compound with CNF [121,122]. The synthesis methods, sulfur loading and the electrochemical performance of CNFs-based binder-free composite electrodes in lithium-sulfur batteries were summarized in Table 1 [123-128].
Electrospinning technology is one of the most commonly used techniques to prepare CNFs [122]. For example, Li et al. [56] also prepared a surface-functionalized CNF matrix by electrospinning technology, and then obtained a binder-free electrode with a high sulfur content by melting sulfur on the surface of the CNF in Figs. 8a and b. The independent and highly conductive network of CNF could be used directly as a binder-free electrode and current collector. As a result of the adsorption of polysulfide by the functional groups on the surface of CNF and the fast ion pathway of CNF, the batteries with binder-free CNF electrode extended their lifetime up to 400 cycles with a high Coulombic efficiency of ≈100% (Fig. 8c) [56]. The removal of binders and current collectors allowed the electrodes more space to load sulfur and reduce the expansion of volume during reaction, which increases the energy density of the battery to 450 Wh/kg.
Figure 8
Figure 8.
(a) Schematic diagram and (b) TEM image of CNF-SiO2 prepared by electrostatic spinning. (c) The long cycle performance of CNF-SiO2/S electrodes. (a-c) Reproduced with permission [56]. Copyright 2019, Wiley-VCH. (d) Schematic illustration of the process of preparing FeSA-PCNF. (e) SEM images, (f) rate performance and (g) long-term cycling performance of FeSA-PCNF. (d-g) Reproduced with permission [131]. Copyright 2022, The Royal Society of Chemistry.
Polymers like poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) [57], polyaniline [129], and polyacrylonitrile [130] are widely combined with CNF to prepare binder-free electrodes, mainly because of their low cost and low preparation temperature. On the one hand, the combination of a polymer and CNF can reduce the use of binders, saving costs, and on the other hand the abundant functional groups of oxygen and nitrogen and abundant pores in the polymer can effectively adsorb polysulfide, inhibiting the shuttling effect.
In order to further improve the reaction kinetics and conductivity of the electrode, the polar materials such as Fe single atom decorated porous CNFs (FeSA-PCNF) [20,131], TiS2@N,S co-doped PC/CF [132], g-C3N4@PCNF [58] and MoS2@N-CNFs [133] are usually compounded with CNF to produce high-performance sulfur host compounds. These materials are usually prepared by combining electrostatic spinning with pyrolysis to make particles of the polar particles grow on the carbon fiber. Recently, Zhang and colleges carefully designed Fe single-atom modified porous carbon nanofibers (FeSA-PCNF) for self-supporting binder-free cathodes of lithium–sulfur batteries via electrospinning in Fig. 8d [131]. The unique structure of FeSA-PCNF, with its interconnected fiber network and layered porous structure, guaranteed fast charge transfer dynamics as well as rich active interfaces for the conversion of LiPS conversion (Fig. 8e). In addition, the highly active FeN4 group was embedded in the porous CNFs with the surrounding graphite N-doped, ensuring strong chemisorption and superior electrocatalytic conversion to LiPSs. The Li-S batteries assembled with FeSA-PCNF delivered excellent electrochemical performance with a high rate capacity of 791 mAh/g under 5 C, and the rate of capacity decay per cycle was as low as 0.048% after 500 cycles at 2 C (Figs. 8f and g) [131]. In these kind of CNF composite electrodes, the 3D carbon fiber serves as a fast transport channel for electrons and ions, and its multi-space structure can also effectively inhibits the transport of polysulfide. The polar materials can enhance the adsorption capacity of polysulfide by forming strong chemical bonds with polysulfide during the reaction progress.
According to the works mentioned above, CNFs prepared by electrospinning technology have structural designability, abundant defects or active sites, excellent conductivity, flexibility and mechanical properties, which gives it potential as a material for self-supporting, cheap, and environmentally friendly binder-free cathodes. The porous 3D CNF network can not only be used as a conductive network to quickly transport ions and electrons, but also has abundant space to accommodate sulfur and improve the loading of sulfur. The addition of polar functional groups and polar materials on the surface of the polymer further improves the polarity of the surface of the CNF complex, and enhances the adsorption and catalytic ability of soluble polysulfides.
4.1.4
Others
In addition to the above-mentioned carbon-based materials, other carbon hosts mainly include carbon materials with a simple structure, unlike CNTs, GO and CNF, including 3-aminopropyltriethoxysilane copolymer sulfur (APS) [134], N,P co-doped carbon frameworks [135], boron-doped carbon aerogels [136], Ni@N-doped carbon nanospheres (Ni,N-CNSs) [137], and surface-modified graphite paper [138,139]. In general, these materials are porous, which gives them a high specific surface area and porosity [140,141]. The high specific surface area and porosity provide abundant space for the absorption of sulfur in the active material. For instance, Yu et al. prepared a nitrogen and phosphorus co-doped carbon (N,P-C) framework form a phytic acid-doped polyaniline hydrogel and used it as a binder-free electrode [135]. The 3D porous carbon structure doped with P act as an the ion transport channel, inhibiting the shuttling effect and improved the reaction kinetics of the electrode. Therefore, Li2S/N, P-C carbon composite electrodes showed excellent coulomb efficiency and a high specific capacity. In order to achieve better electrochemical performance, Liu and colleagues then used the method of hydrothermal atomic layer deposition method for the first time to assemble novel structurally binder-free Ni@N-carbon nanosphere films as sulfur hosts for the sulfur [137]. The Ni layer on the surface improved the conductivity of the electrode, and inhibited the shuttling of soluble sulfide. The Ni@N-CNSs/S e electrode obtained good rate performance(816 mAh/g at 2 C) and a long cycling life (87% after 200 cycles at 0.1 C).
4.2
Binder-free sulfur cathodes based on template-assisted strategies
4.2.1
Carbon substrates
Carbon-based substrates have been widely used as binder-free electrode materials because of their excellent conductivity, electrochemical stability and flexibility. CC is considered to be an ideal substrate for preparing flexible, binder-free electrodes [142].
Recently, CC has been extensively studied as a sulfur host due to its high electrical conductivity and flexibility [143-145]. It is an effective strategy to grow polar materials with different nanostructures on the surface of carbon cloth to adsorb lithium polysulfide. A variety of binder-free electrodes including MoO2/MoS2@CC [62], FeCo2S4/CC@S [65], MoO2/CC–Li2S6 electrodes [146], S/CC@Co4 N-porous carbon nanosheet arrays [147], CF/FeP@CC@S [148], NiCo2O4/CC@S [149], CC/VN/Co@NCNTs/S [150], 3D porous g-C3N4/CC/S [151] electrodes and S/CC@Co9S8-Co4N [152] have been assembled. These materials on the surface of CC can adsorb polysulfide, but promote the transformation of polysulfide and improve the reaction kinetics during the charge/discharge progress. Among these materials, Zhang et al. synthesized a unique compound of Co4N nanoparticles embedded in porous carbon nanosheet arrays on CC (Fig. 9a) [147]. The nanosheet increased the surface area of the electrode and the Co4N nanoparticles significantly improved the reaction kinetics, accelerate the conversion of substances (Figs. 9b and c). Therefore, the electrode exhibited good rate performance (746 mAh/g at 5 C) and excellent cyclic stability (0.035% per cycle over 500 cycles at 5 C) (Fig. 9d). In addition, composite CC with a 3D hierarchical structure has excellent electrochemical properties due to its complex composition, with a large specific surface area and abundant active sites. This unique structure is compounded with CC can greatly improved the chemical properties of CC. Recently, Cai and co-workers developed a multifunctional binder-free sulfur host consisting of a porous VN array grown on a CC coupled with MOF-derived N-doped carbon nanotubes embedded with tiny Co nanoparticles (CC/VN/Co@NCNTs) (Fig. 9e) [150]. SEM and TEM images clearly showed that CNTS were attached to CC, and the Co sites were evenly dispersed in CNT (Figs. 9f and g). The binder-free CC/VN/Co@NCNTs cathodes exhibited excellent cycling performance (the rate of capacity decay was only 0.063% per cycle after 500 cycles at 1 C), as shown in Fig. 9h.
Figure 9
Figure 9.
(a) Schematic illustration, (b) SEM image, (c) HRTEM image and (d) long-term cycling performance of the S/CC@Co4N-PCNA compound. The inset is a digital photo of the electrodes before and after cycling. (a-d) Reproduced with permission [147]. Copyright 2019, Elsevier. (e) Schematic illustration of synthesizing a self-supported CC/VN/Co@NCNTs/S cathode. (f) SEM image (the inset shows the photograph) and (g) HRTEM images of the self-supporting CC/VN/Co@NCNTs/S. (h) Long-term cycling stability at 1 C for the CC/VN/Co@NCNTs/S and CC/VN/S cells. (e-h) Reproduced with permission [150]. Copyright 2022, Elsevier. (i) Schematic illustration for synthesizing and (j) rate performance of CP-x/S electrodes. (i, j) Reproduced with permission [159]. Copyright 2023, Elsevier.
In addition to CC, carbon paper (CP) [153], 3D CNT backbones [154,155], 3D GO foams [156,157] and polymers [158] can also be used as substrates for the synthesis of binder-free electrodes. For instance, Zhang and co-workers produced self-supporting binder-free CP/S composite electrodes by manipulating chemical defects on the surface of carbon paper (Fig. 9i) [159]. The porous 3D structure provided abundant space to hold sulfur, and the electrode obtained a high sulfur loading. These rich active sites can be used as ion and electron transport channels, and can efficiently adsorb polysulfide and inhibit the shuttling effect. Therefore, the CP-X/S electrodes presented high excellent magnification performance and high capacity retention (Fig. 9j).
According to the results discussed above, CC is commonly used as a substrate for the preparation of binder-free electrodes due to its excellent self-support, electrical conductivity and flexibility. The regularly arranged carbon fibers in CC can be used as channels to quickly transport ions and electrons. In order to improve the polarity of CC and the adsorption capacity of polysulfide, the combination of oxides, nitrides, sulfides and selenides with different polar and catalytic functions with CC has been proven to be an effective strategy to improve the electrochemical performance of electrodes. In addition to CC, CP, CNT foam, and GO foam can also be used as substrates for the preparation of binder-free electrodes, because of their abundant porous, good electrical conductivity and excellent mechanical properties. The used carbon templates, sulfur loading, and electrochemical performance of different kinds of binder-free electrodes of lithium-sulfur batteries are summarized and compared in Table 2 [160-164].
Table 2
Table 2.
Comparison of sulfur loading/content, templates and electrochemical performance of different kinds of templated binder-free electrode materials based on templates for Li-S batteries.
Metal materials can also be used as substrates to synthesize binder-free cathodes since metal templates have good electrical conductivity and flexibility. The metal materials also have a certain catalytic function and carbon materials can be grown in-situ of on their surface [17,165]. The used metal templates, sulfur loading, and electrochemical performance of different kinds of binder-free electrodes of lithium-sulfur batteries are summarized and compared in Table 2 [166-169].
Among these metals, nickel has been widely studied due to its excellent properties and growing carbon on Ni foam is also a promising method for preparing electrode materials. Ni foam can not only provide a 3D skeleton for the growth of carbon materials, but also could be used as a 3D conductive network to transport electrons and ions [157,170,171]. Recently, Yu et al. deposited MoS3/PPy nanowires on the surface of nickel foam by an electrochemical method (3D MoS3/PPy/NF) and used it as the cathode of binder-free lithium–sulfur batteries (Fig. 10a) [172]. Figs. 10b-e show MoS3 was evenly coated on the surface of PPy surface as an active material. The composite electrode showed good electrochemical performance (an initial capacity of 450 mAh/g and a capacity retention of ~70% with a coulombic efficiency of ~100% at 0.9 A/g for 400 cycles) (Fig. 10f). This was mainly caused by its unique 3D skeleton structure, which could accommodate sulfur, and also eased the expansion of the reaction (Fig. 10g). The number of active electrochemical sites increased significantly, and the diffusion length of ions in nanocomposite structures decreased, whereas a flat structure caused MoS3 to fall off the surface of the nickel foam, resulting in rapid capacity decay [172]. Moreover, Lu et al. [173] developed a novel method to grow microporous GO foam on nickel foam reduced by nickel chloride powder. The prepared GO foam had the advantages of low density and a large specific surface area, which can provide an attractive site for the adhesion of polysulfides.
Figure 10
Figure 10.
(a) Method of fabrication, (b) SEM and (c) TEM images of the sulfur-equivalent MoS3/PPy/NF cathode. SEM images of the 3D-MoS3/PPy/NF cathodes (d) before and (e) after 100 charge/discharge cycles. (f) Long-term cyclic performance and (g) schematic representation of the morphological change of 3D-MoS3/PPy/NF. Reproduced with permission [172]. Copyright 2023, Wiley-VCH.
In addition to nickel metal as a substrate, zinc [17], aluminum oxide [34], and stain less [174] can be used. Thin films can also be used as a substrate for synthesizing carbon-based materials. For example, Yan and colleges used aluminum foil as a substrate to prepare binder-free cathodes via self-assembly method [175]. AAO films are usually used as filters to deposit carbon films via filtration [34]. In addition, Mao et al. [21] filtered a solution with a mixture of CNTs on AAO film and obtained a MOF/CNTs composite film as a host for for sulfur after heat treatment. The batteries delivered a high initial capacity of 1263 mAh/g and a low fading rate of 0.08% per cycle over 500 cycles at 0.2 C. What is more, metallic zinc is also a common substrate for carbon deposition. For example, Gao et al. [17] deposited a rGO-S composite film on Zn plate by self-assembly method, and then removed the Zn plate to obtain the self-supporting rGO-S film. The film was directly used as the cathode of LSBs without any binders, which greatly increased the energy density. SEM and TEM images showed that the sulfur nanoparticles were uniformly dispersed in the graphene sheet, and the stress-strain curve was as high as 25 MPa, indicating that the composite film had excellent unloading strength. As a proof of the concept of flexible batteries, the team successfully assembled cable-type batteries, using this prepared GO-S film. The cable-type LSBs not only achieved a high initial capacity of 1360 mAh/g at 0.1 C, and also exhibited excellent electrochemical performance under bending conditions [17].
In the above mentioned works, the commonly used metal templates including nickel, zinc, aluminum, magnesium and their oxides, exhibited excellent conductivity and catalytic performance, and they can be used as catalysts for the growth of carbon materials such as CNTs and GO. The metal composites have unique 3D porous structures, and outstanding mechanical properties. Furthermore, the excellent conductivity of the compounds greatly accelerates the transport of ions and electrons, and the metal particles in the composite can catalyze the transformation of sulfide, improving the reaction kinetics. In addition, the composite materials have a structure of hierarchical structure, which can effectively inhibit the shuttling of polysulfides and improve the utilization of active substances. Therefore, the composite electrodes assembled by this methods exhibits good cycle stability and long cycle life. Using metal as a substrate to synthesize electrode material is an effective method to improve the performance of lithium–sulfur batteries.
4.2.3
Others
Due to its advantages of low density, a high specific surface area and the high pore volume, porous carbon materials are considered to be among the promising materials for the preparation of high-performance LSBs [176]. The commonly used templates for the preparation of porous carbon materials include inorganic materials such as spherical SiO2, TiO2, and MOF, as well as organic materials such as polymers and biomass materials [177,178].
Porous carbon synthesized from silica spheres has the advantages of high specific surface area, an adjustable structure and high conductivity [179,180]. For instance, Zhou et al. [33] successfully prepared double-shelled nitrogen-doped hollow carbon spheres using hollow SiO2 spheres as templates, and then encapsulated sulfur into the GO wrapped hollow carbon spheres. Compared with common carbon spheres, the capacity and cycling stability of batteries with nitrogen-doped hollow carbon spheres were greatly improved. What is more, Cao's group [67] designed and prepared an electrode with a cuprous sulfide@void@carbon wrapped in a graphene nanosheet electrode. In this unique structure, the Cu2S tube was encased in conductive carbon tubes to form a novel barrel-tube nanostructure with a high content of 80 wt%. It is well known that polar Cu2S has a strong chemical adsorption effect on polysulfide. The battery has a long cycling life and capacity retention of 74.3% at 1 C and 69.5% at 2 C respectively after 800 cycles.
Combining carbon spheres with CNTs is also a method of preparing high performance binder-free electrodes. For example, Kang's team synthesized nitrogen-doped porous carbon (NPC) using the TiO2 sphere template method and compounded it with CNT to produce a dual-layer, free-standing cathode [181]. During the reaction, NPC particles act in the compound as donors to the active site through their high surface area and strong absorbance of dissolved polysulfides, while the dense layer of CNTs at the top of the cathode inhibits the migration of polysulfides from the cathode to the anode and accelerates the transport of ions and electrons. Therefore, the cathode with NPC spheres/MWCNT@S composites demonstrated high cycling stability and a long cycle life (83.4% at 1 C after 500 cycles).
In addition to TiO2 sphere templates, MOFs are often used as a template to prepare hollow carbon because the hollow carbon derived from MOFs has advantages such as a high specific surface area, high porosity, and controllable pore size and structure [125]. For instance, Yao and colleges prepared CNTs compound the ZIF-8 derived nitrogen-doped porous carbon films by combining a simple vacuum filtration with in situ reaction and subsequent pyrolysis method as sulfur hosts [182,183]. The interconnected CNTs enhances the flexibility of the composite, and act as a conductive network to improve the conductivity of the electrode. The battery delivered an extremely long life over 1800 cycles with a low rate of capacity decay rate of 0.02% at 1 C.
As discussed above, porous carbon has a unique porous structure, a high specific surface area, and high porosity, so it is widely used to prepare high-performance LSBs. Commonly used porous carbon templates are SiO2, TiO2, MOFs, polymers and biomass materials. SiO2, and TiO2 are popular with researchers because they are cheap and easy to prepare, making them a good choice for preparing porous carbon. Compared with porous carbon prepared with SiO2, hollow carbon derived from MOF has the advantages of an adjustable structure. In order to further improve the performance of the battery, researchers usually combine hollow carbon with other materials with high conductivity, such as CNT, and GO, to prepare composite materials with excellent performance.
5.
Application of binder-free cathodes in Li-Se batteries
Selenium is a non-metallic element in the same main group as sulfur that is located below sulfur and has similar advantages to sulfur. In addition, the electronic conductivity of selenium (1 × 10−3 S/m) is better than that of sulfur (5 × 10−28 S/m), and the reactivity and electron conductivity of elemental selenium and lithium ion are significantly better than that of elemental sulfur [42]. Moreover, although the theoretical specific capacity of the lithium-selenium battery (675 mAh/g) is lower than that of the lithium-sulfur battery (1675 mAh/g), it has a higher theoretical volume specific capacity (3253 mAh/cm3), which meets the needs of the volume-limited power battery. Therefore, binder-free lithium-selenium batteries have also attracted the attention of researchers and have been rapidly developed [184]. The basic information of binder-free Li-Se batteries is summarized in Table 3.
Table 3
Table 3.
Comparison of loading/content, methods of synthesis and electrochemical performance of Li-Se batteries and Li-Te batteries with binder-free electrode materials.
For Se, Graphene is an ideal host material and has been widely used in the preparation of binder-free electrodes due to its excellent electrical conductivity and mechanical properties [185]. For instance, Han et al. first reported a binder-free graphene-Se@CNT electrode made by a simple two-step assembly method. Subsequently, in order to realize a high selenium loading, they synthesized a 3D mesoporous carbon/graphene electrode coated with selenium particles (Se/MCN-rGO) to produce a binder-free cathode with high selenium content (Figs. 11a and b) [186]. Fig. 11c shows the initial capacity of this high-selenium cathode could reach 97% of the theoretical capacity at 0.1 C, and the battery had a long cycle life and excellent cycle stability (at 1 C, the battery cycled 1300 times, the average rate of capacity attenuation per cycle was 0.008%), as shown in Fig. 11d. The excellent electrochemical performance of the electrode may be attributed to the fact that MCN provides a space to accommodate selenium while also limiting the dissolution of polyselenide. The conductive network constructed by 2D graphene and MCN provides the electrode with excellent conductivity. The flexibility of the graphene sheet is conducive to the adjustment of changes in volume changes during long-term cycling and inhibits breakage of the cathode structure [187].
Figure 11
Figure 11.
(a) Schematic of the synthesis route of Se/MCN-rGO paper. (b) TEM images of Se/MCN-rGO at different magnifications. (c) Digital photos of the free-standing Se/MCN-rGO paper electrode and bended with tweezers (inset). (d) Long-term cycling stability of the Se/MCN-rGO electrodes (0.1 C for the first five cycles and 1 C for the next 1300 cycles). (a-d) Reproduced with permission [186]. Copyright 2016, Wiley-VCH. (e) Schematic illustration of the synthesis of CoSe2@CNF catalytic electrode. (f) Photograph of a Se/CoSe2@CNF free-standing electrode. (g) HRTEM images of the Se/CoSe2@CNF composite. (h) Cycling test of Se/CNF and Se/CoSe2@CNF free-standing electrodes at 0.5 C. (e-h) Reproduced with permission [28]. Copyright 2023, Wiley-VCH.
Moreover, carbon nanotubes have outstanding electrical conductivity and flexibility, and are often used to prepare binder-free electrodes [188]. For instance, Cai et al. [189] encapsulated selenium into the inner shell of hollow nuclear nitrogen doped carbon nanotubes (CNx) to prepare a double shell with a definite volume in the inner cavity volume. Later, Cui et al. successfully prepared a structure of selenium-CNT without binder by chemical method. They investigated the effect of different Se loads on the electrochemical performance of the cell and found that the electrode thickness had an important effect on the distribution of selenium [190,191]. Recently, Wu et al. [28] successfully prepared Se/CoSe2 modified carbon nanofibers (Se/CoSe2@CNF) via electrospinning and carbonization treatment, and obtained self-supporting binder-free electrodes after selenization treatment (Fig. 11e). The SEM images showed that CoSe2 nanoparticles were uniformly attached on the surface of CNF, which can effectively adsorbed and catalyzed the conversion of selenium polysulfide (Figs. 11f and g). The electrode demonstrated excellent cycle performance (the rate of capacity decay of each cycle was only 0.032% over 800 cycles at 0.5 C) in Fig. 11h.
Hitherto, binder-free Li–Se electrodes have been developed rapidly due to their high theoretical capacity and high reaction kinetics. Graphene, CNT, CNF, etc., are considered to be the best materials for binder-free electrodes due to their excellent electrochemical and mechanical properties. However, single carbon material still cannot solve the problems faced by Li–Se batteries, so the combination of polar materials with specific adsorption and catalytic properties and carbon materials could solve the problem of polyselenides shuttling and expansion of the volume.
6.
Application of binder-free cathodes in Li-Te batteries
Lithium-tellurium batteries are also regarded as promising candidates to replace lithium-ion batteries as the next generation of energy storage systems because of their similar volumetric capacity to LSBs [39,47]. Compared with sulfur, tellurium has a higher electrical conductivity, which gives it the advantage of strong reaction dynamics. Therefore, Li–Te batteries have the advantages of a high utilization rate of Te [45]. Binder-free energy storage devices are widely used to realize high performance and energy density, Li–Te batteries are no exception. The basic information of binder-free Li-Te batteries is summarized in Table 3.
In order to improve the performance of Li–Te batteries, Zong et al. used a traditional melting method to integrate the components into ordered mesoporous carbon to prepare binder-free and conductive electrodes [192]. At a high temperature of 500 ℃, the Te was evenly distributed in the mesoporous carbon. The ordered porous carbon not only accommodated the active material, but also alleviated the problem of expanding volume in the reaction. Therefore, the battery with the electrode exhibited a high volume capacity of 900 mAh/cm3 and energy density of 1800 mAh/cm3 with a low rate of capacity decay of 0.02% per cycle. Later, in order to further improve the electrode's conductivity, Chen et al. successfully prepared a Te electrode with porous carbon sponges by doping nitrogen atoms into porous carbon [193]. The doping of nitrogen atoms was conducive to the transport of electrons and ions, and further improved the conductivity of the electrode.
Graphene has good electrical conductivity and mechanical properties, and it is widely used in this field. For example, He and colleagues successfully assembled a composite aerogel electrode comprising 3D rGO-coated horseshoe-shaped nanowires using a hydrothermal method, and directly used it as the positive electrode of Li–Te batteries (Fig. 12a) [194]. TEM image shows that rGO is uniformly coated on the surface of Te nanowires (Fig. 12b). The electrode delivered a excellent rate performance (1083 mAh/cm3 at 10 C) and long cyclic performance (1685 mAh/cm3 at 1 C for 500 cycles) (Figs. 12c and d). This may be because the rGO in the electrode acted as a skeleton, not only supporting the active material and acting as a conductive network to transport the electrons of the ions. In addition, the 3D structure of the multi-grade system effectively alleviated the problem of expanding volume during the process of charging and discharging. In addition to GO, CNTs also have excellent electrical conductivity, and Yu et al. used CNTs as a conductive network compounded with the Te to prepare binder-free electrodes [29]. In addition to serving as a support network, CNTs can also effectively limit the shuttling of multi-shoe compounds and improve the utilization rate of active substances. The battery with this composite electrode exhibited a long cycling life and excellent rate capability.
Figure 12
Figure 12.
(a) Schematic diagram of preparing 3D binder-free G/Te electrodes. (b) TEM image of 3DGT sample. (c) Rate performance and (d) long cyclic performance of the cell with 3DGT cathode. Reproduced with permission [194]. Copyright 2016, Wiley-VCH.
Template are also commonly used for preparing binder-free Te composite electrodes. Nickel foam has good electrical conductivity and catalytic properties, which is conducive to the growth of active material, and can also effectively conduct ions. Using nickel foam as a template is regarded as an efficient strategy to fabricate binder-free Te electrode. For instance, Shen et al. used an electrical substitution method to uniform nanorods on a nickel foam [195]. With nickel foam as a template, tellurium nanorods were grown on nickel foam via a simple galvanic replacement method on nickle foam and Te@Ni materials can be used as cathode without any binders and carbon additives.
In addition, CC is also a common substrate because of its excellent electrical conductivity and mechanical properties. For example, Zhu and colleagues used the CVD method to grow horse hoe-shaped nanotubes uniformly on a CC to create binder-free electrodes [196]. The composite electrode was combined with a solid polymer electrolyte to obtain high volume-specific energy with a long cycling life. In 2021, Li and colleagues successfully prepared tellurium nanotubes through a simple hydrothermal method, and combined them with nanocellulose to assemble a flexible composite tellurium film [197]. The nanostructure had good adsorption activity and can anchored the polytelluride, thus improving the cycling life of the battery.
7.
Conclusions and outlook
In conclusion, we have systematically discussed the advantages and disadvantages of the common methods used to prepare binder-free electrodes and presented the research progress of binder-free LCBs over the last decades. Binder-free electrodes usually use porous carbon, CNT, CNF, GO, CC, metal substrates and polymers as the host materials for the active materials. These materials have the advantage of being used as binder-free electrodes: for example, porous carbon has the characteristics of a high specific surface area, high porosity and low density, and can be made into sponges, foams and other forms. The high porosity of the porous carbon gives it more space to accommodate the active material, which usually carries a high loading. CNTs have high electrical conductivity, and excellent mechanical properties. Their self-assembling properties are often used to prepare self-supporting films for use as binder-free electrodes. The CNT network can be used to contain active substances and also act as a conductive network, providing a fast transport route for ions and electrons. Like CNTs, GO has high electrical conductivity and excellent mechanical properties. In addition, GO also has thermal stability. GO can be made into a variety of forms, including films, papers, foams, and sponge. These GO complexes can not only increase the load of the battery, and can also be used directly as a binder-free electrode in a battery. CNF is usually prepared by electrostatic spinning spray technology. CNF films and fabrics have excellent mechanical properties and structural integrity. Carbon-based materials such as GO, CNT, and CC, generally have the advantages of high electrical conductivity, excellent flexibility, light weight, and electrochemical stability. These advantages give the electrodes prepared with these material good flexibility, and achieve a high loading, so that the electrochemical performance of the battery is improved. The disadvantage of carbon-based materials is that the process of is more complicated. Among them, CNT, GO and CNF have similar functions to 2D nanomaterials, which have a large specific surface area, and also have good electrical conductivity and self-assembly properties. Combined with active materials, they can form a film with good flexibility without additional binders, which can be directly used as the electrode of battery. CC has a 3D structure formed by the regular arrangement of CNF and has good electrical conductivity and flexibility. It can be directly used as the electrode of battery without the addition of binder.
Compared with traditional LCBs, binder-free batteries exhibit several following advantages: (1) Low cost. Binder-free electrode does not need binders and additional conductive agent, which makes it unnecessary to add a current collector, because the electrode can be directly used as a current collection, which saves the production cost of the battery. (2) High energy density. Without binders and conductive agents, electrodes have more space to improve the content and load of active materials, which is an effective way to improve the battery capacity and energy density. (3) Simple operation. There is no need for blade-coating in the preparation of binder-free electrodes, which simplifies the process of preparing batteries and greatly saves costs. (4) Excellent mechanical properties. The electrode material has excellent mechanical properties and flexibility, making it an ideal material for flexible electrodes. (5) Excellent chemical properties. A binder-free electrode can combine the advantages of a variety of materials, for example, the CC substrate has good electrical conductivity and flexibility, and after polar substances grow on its surface, the composite material not only has good electrical conductivity, but also can effectively adsorb polysulfide and inhibit the shuttling effect. These advantages make the commercialization of binder-free batteries a big step closer.
Although the preparation of binder-free electrodes can improve the electrochemical performance of LCBs, there are still some problems to be solved, which are also common problems of such electrodes, including their low reversible capacity, the low utilization rate of active substances, poor cycling stability and short cycling life. These factors hinder their commercial application. The following points will provide some ideas for solving these problems (Fig. 13).
Figure 13
Figure 13.
Summary and prospects of binder-free cathodes for high-performance LCBs.
(1) New materials. The materials of the binder-free carbon matrix and its decorations should have good electrical conductivity, which is conductive to the transfer of electrons, and have abundant pores, providing space to accommodate active materials and alleviate volume expanding during the charge–discharge process. Most importantly, the selected material should have outstanding self-assembly ability and be able to form a self-supporting film without binders. In light of these requirements, carbon materials such as CNT, GO, CNF, and CC can form various 3D structures through certain treatment methods, and are regarded as the most promising candidates for the preparation of binder-free electrodes. In addition to these carbon materials for binder-free electrodes, the combination of polar materials such as metal oxides, sulfides, nitrides, borides, MOF, layer double hydroxides, and MXene with carbon-based materials can also improve the electrochemical performance of the electrodes, because these materials have strong polarity and catalytic functions, and can effectively adsorb polysulfides and promote the transformation of polysulfides. In addition to the selection of suitable host materials, designing a reasonable structure is also one of the important factors affecting the electrochemical performance of the electrode. Among these structures, there is a kind of nanocore-shell structure worth discussing. The core–shell structure, also known as the yolk structure, consists of a layer of shell and internal particles, which is rich in porous. It was found that the core–shell structure can enclose the active material well, and there is enough space to alleviate the expanding volume. The shell structure can adsorb soluble intermediates under the action of physical and chemical double layers, and inhibit the shuttling effect. The application of the core-shell structure to binder-free electrode may effectively improve the electrochemical performance of the battery.
(2) Mechanism exploration. Binder-free electrodes have a unique structure, which is conducive to improving the electrochemical performance of the battery. However, charge transfer, chemical changes in the surface and interface, the dynamic evolution of solid electrolyte interface films, and various mechanisms of interaction remain unclear. Therefore, in-situ characterization techniques (such as in-situ TEM, in-situ XRD, in-situ SEM, in-situ Raman spectroscopy, in-situ atomic force microscope (AFM,) in-situ NMR) can be applied to study the electrochemical transformation of binder-free electrodes to better reveal the reaction's mechanisms. Through real-time monitoring of electrodes or electrolyte and their interface, the evolution process of its microstructure, composition, elements and chemical distribution is linked to the real-time electrochemical signal of the battery, which provides an analytical analysis basis for in-depth and systematic studies of the operation and attenuation in the performance of the battery. In addition, with the deep integration of computer technology and materials science, using computer simulations technology to explore the reaction mechanism of batteries is considered to be an effective strategy. Reasonable use of in situ technology and computer simulations technology can provide guidance for the design of low-cost, highly flexible high-performance binder-free electrodes with a high load, which would lay a foundation for the commercial application of LCBs.
(3) Efficient synthesis methods. At present, the methods used to prepare binder-free electrodes are more complex, usually involving two to three steps, and the methods such as electrostatic spinning, hydrothermal treatment, pyrolysis, carbonization, etc. require high temperature and high pressure. These steps increase the cost, and also waste time, so the preparation of binder-free electrodes has not been industrialized as yet. In order to facilitate large-scale production, researchers should choose a test method with fewer steps, under ambient temperature and pressure when preparing composite electrodes, and the solution-based self-assembly method and the one-pot method could meet this requirement. However, the preparation of specific binder-free materials also requires specific methods, for example, the fabrication of binder-free carbon fiber films needs to use electro-spinning technology and the best choice for growing polar materials on the surface of carbon materials is hydrothermal method. Therefore, when preparing for binder-free electrode materials, we should fully consider the advantages and disadvantages of the different methods and choose the best method.
(4) Promising applications. Usually, in order to pursue higher energy density, researchers will reduce the proportion of inactive substances in the electrode. Binder-free lithium batteries are in line with this trend. Binder-free electrodes make binders and even current collectors unnecessary, saving the cost of materials, and improving the energy density of the battery. At present, solid electrolytes have attracted much research interest due to its high safety. A binder-free solid-state battery prepared by combining binder-free electrodes with solid electrolyte could achieve high energy density and high safety, which will be a promising research direction. Looking forward to the future, binder-free batteries are one of the more effective ways to achieve energy storage with a high energy density, and will have a wide range of prospects for application in the future industry, including electric vehicles, daily electronics, drones and so on. At the same time, mature technologies for preparing binder-free electrodes will also be applied in other energy storage and conversion fields, such as Li-ion batteries, electrocatalysis, super-capacitors, fuel cells, and so on. The progress of binder-free technology would greatly promote the development of new energy storage and conversion technologies, and further the realization of energy storage systems with a high energy density energy storage systems.
Overall, this review provides a comprehensive understanding of the methods of preparing and recent progress of binder-free cathodes for high-performance LCBs. We believe that the above-mentioned issues, including developing new materials, exploring mechanism, finding efficient methods of synthesis and promising applications could be solved in the future. We also hope that these ideas in this review will provide useful guidance for commercial applications of LCBs and other related areas.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
This work was supported by the Frontier Exploration Projects of Longmen Laboratory (No. LMQYTSKT008), the Natural Science Foundation of Henan Province (No. 242300420021), the Open Fund of State Key Laboratory of Advanced Refractories (No. SKLAR202210), the Student Research Training Plan of Henan University of Science and Technology (No. 2024054), and the Undergraduate Innovation and Entrepreneurship Training Program of Henan Province (No. S202310464012). The Innovation Fund of Henan University of Science and Technology (No. 2023-S01).
Figure 1
Timeline of the major milestones of binder-free LCBs. Reproduced with permission [30]. Copyright 2012, The Royal Society of Chemistry. Reproduced with permission [31]. Copyright 2013, Wiley-VCH. Reproduced with permission [32]. Copyright 2014, Wiley-VCH. Reproduced with permission [33]. Copyright 2015, Wiley-VCH. Reproduced with permission [17]. Copyright 2016, Wiley-VCH. Reproduced with permission [34]. Copyright 2017, Wiley-VCH. Reproduced with permission [35]. Copyright 2018, American Chemical Society. Reproduced with permission [23]. Copyright 2019, Elsevier. Reproduced with permission [36]. Copyright 2021, Wiley-VCH. Reproduced with permission [37]. Copyright 2022, Wiley-VCH. Reproduced with permission [18]. Copyright 2023, Wiley-VCH. Reproduced with permission [19]. Copyright 2024, Wiley-VCH.
Figure 2
(a) The charge and discharge behavior of lithium–sulfur batteries in ether-based electrolytes. Reproduced with permission [38]. Copyright 2016, Royal Society of Chemistry. (b) Schematic diagram of the shuttle effect of LCBs. (c) Schematic diagram of charge and discharge behavior of Li-Se batteries in carbonate and ether-based electrolytes. Reproduced with permission [42]. Copyright 2022, Wiley-VCH. (d) The basic structure of lithium–chalcogen batteries with binder-free cathodes.
Figure 3
(a-f) The advantages and disadvantages of various preparation methods of chalcogen hosts and binder-free electrodes for lithium–chalcogen batteries.
Figure 4
(a) Schematic illustrations of the process of preparing rGO@S films. (b) Elemental mappings of the rGO@S films. (a, b) Reproduced with permission [51]. Copyright 2019, Elsevier. (c) Synthesis scheme of MoO2/G and MoO2/G–S composites. (d) Scanning electron microscopy (SEM) image of MoO2/G. (c, d) Reproduced with permission [53]. Copyright 2017, The Royal Society of Chemistry. (e) Schematic illustration of the production of freestanding flexible Li2S@NCNF paper electrodes via Ar-protected carbothermal reduction of Li2SO4@PVP fabrics made by electrospinning. (f) An electrode made from multilayered Li2S@NCNF paper and a punched disc. (e, f) Reproduced with permission [63]. Copyright 2017, Wiley-VCH.
Figure 5
(a) Schematic illustration of the process of synthesizing Mo2C/CC@S. (b) SEM and (c) elemental mappings of Mo2C/CC@S. (a-c) Reproduced with permission [64]. Copyright 2021, Elsevier. (d) Schematic showing the process of fabricating the S-DIB@CNT hybrid. (e) Interwoven S-DIB@CNT hybrid network in a membrane and an optical photograph of the membrane (inset). (f) XRD patterns of the S-DIB copolymer, S@CNT, and S-DIB@CNT materials. (d-f) Reproduced with permission [34]. Copyright 2017, Wiley-VCH.
Figure 6
(a) The process of preparing an MX/G aerogel electrode. (b) High resolution TEM (HRTEM) images. (c) The long-term cycling performance of binder-free MX/G-30 electrodes. (a-c) Reproduced with permission [26]. Copyright 2019, The Royal Society of Chemistry. (d) Schematic illustration of the process of preparing 3D MoSe2-x@GA/S electrodes. (e) HRTEM, (f) SEM images of and (g) the long-term cycling performance of 3D MoSe2-x@GA/S electrode. (d-g) Reproduced with permission [19]. Copyright 2024, Wiley-VCH.
Figure 7
(a) Schematic illustration of the process of synthesizing SPAN/CNT electrodes. (b) TEM images of a single SPAN/CNT-12 fiber. (c) The long cycling performance of SPAN/CNT electrodes. (a-c) Reproduced with permission [103]. Copyright 2019, Wiley-VCH. (d) Schematic of CNT/Ath-PEI@S as a binder-free cathode. (e) H-NMR spectrum of Ath-PEI and PEI. (e) Typical the galvanostatic charge–discharge (GCD) profiles of the CNT/S-PVDF, CNT/Ath-PEI@S and H-CNT/Ath-PEI@S at 0.2 C. (f) Cycling performance and Coulombic efficiency at a rate of 1.0 C of CNT/Ath-PEI@S and CNT/S-PVDF electrodes. (d-g) Reproduced with permission [18]. Copyright 2023, Wiley-VCH.
Figure 8
(a) Schematic diagram and (b) TEM image of CNF-SiO2 prepared by electrostatic spinning. (c) The long cycle performance of CNF-SiO2/S electrodes. (a-c) Reproduced with permission [56]. Copyright 2019, Wiley-VCH. (d) Schematic illustration of the process of preparing FeSA-PCNF. (e) SEM images, (f) rate performance and (g) long-term cycling performance of FeSA-PCNF. (d-g) Reproduced with permission [131]. Copyright 2022, The Royal Society of Chemistry.
Figure 9
(a) Schematic illustration, (b) SEM image, (c) HRTEM image and (d) long-term cycling performance of the S/CC@Co4N-PCNA compound. The inset is a digital photo of the electrodes before and after cycling. (a-d) Reproduced with permission [147]. Copyright 2019, Elsevier. (e) Schematic illustration of synthesizing a self-supported CC/VN/Co@NCNTs/S cathode. (f) SEM image (the inset shows the photograph) and (g) HRTEM images of the self-supporting CC/VN/Co@NCNTs/S. (h) Long-term cycling stability at 1 C for the CC/VN/Co@NCNTs/S and CC/VN/S cells. (e-h) Reproduced with permission [150]. Copyright 2022, Elsevier. (i) Schematic illustration for synthesizing and (j) rate performance of CP-x/S electrodes. (i, j) Reproduced with permission [159]. Copyright 2023, Elsevier.
Figure 10
(a) Method of fabrication, (b) SEM and (c) TEM images of the sulfur-equivalent MoS3/PPy/NF cathode. SEM images of the 3D-MoS3/PPy/NF cathodes (d) before and (e) after 100 charge/discharge cycles. (f) Long-term cyclic performance and (g) schematic representation of the morphological change of 3D-MoS3/PPy/NF. Reproduced with permission [172]. Copyright 2023, Wiley-VCH.
Figure 11
(a) Schematic of the synthesis route of Se/MCN-rGO paper. (b) TEM images of Se/MCN-rGO at different magnifications. (c) Digital photos of the free-standing Se/MCN-rGO paper electrode and bended with tweezers (inset). (d) Long-term cycling stability of the Se/MCN-rGO electrodes (0.1 C for the first five cycles and 1 C for the next 1300 cycles). (a-d) Reproduced with permission [186]. Copyright 2016, Wiley-VCH. (e) Schematic illustration of the synthesis of CoSe2@CNF catalytic electrode. (f) Photograph of a Se/CoSe2@CNF free-standing electrode. (g) HRTEM images of the Se/CoSe2@CNF composite. (h) Cycling test of Se/CNF and Se/CoSe2@CNF free-standing electrodes at 0.5 C. (e-h) Reproduced with permission [28]. Copyright 2023, Wiley-VCH.
Figure 12
(a) Schematic diagram of preparing 3D binder-free G/Te electrodes. (b) TEM image of 3DGT sample. (c) Rate performance and (d) long cyclic performance of the cell with 3DGT cathode. Reproduced with permission [194]. Copyright 2016, Wiley-VCH.
Table 1.
Comparison of sulfur loading/content, methods of synthesis and electrochemical performance of different kinds of non-templated binder-free electrode materials for Li-S batteries.
Host material
Sulfur content/loading
Method
Electrochemical performance (Current density, cycle number, finial capacity of long cycle, average decay rate per cycle)
Ref.
GO host materials
GO@(PEDOT:PSS)
56.4 wt%/2.0 mg/cm2
Vacuum filtration
806 mAh/g at 1 C after 2200 cycles, 0.040% decay per cycle
Table 2.
Comparison of sulfur loading/content, templates and electrochemical performance of different kinds of templated binder-free electrode materials based on templates for Li-S batteries.
Sample
Sulfur content/loading
Template
Electrochemical performance (Current density, cycle number, finial capacity of long cycle, average decay rate per cycle)
Ref.
MoO2/MoS2-CC
-/4 mg/cm2
CC
640 mAh/g at 1.0 C after 140 cycles, 0.21% decay per cycle
Table 3.
Comparison of loading/content, methods of synthesis and electrochemical performance of Li-Se batteries and Li-Te batteries with binder-free electrode materials.
Binder-free cathode
Se/Te content/loading
method
Electrochemical performance (Current density, cycle number, finial capacity of long cycle, average decay rate per cycle)
Ref.
Li-Se batteries
3DG-CNT@Se
51 wt%/-
Solvothermal
504.3 mAh/g at 0.2 C after 150 cycles, 0.135% decay per cycle
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