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Packing sulfur into carbon framework for high volumetric performance lithium-sulfur batteries
Abstract
Low volumetric performance is a common bottleneck of carbon-based electrode materials for practical applications, owing to the low density of porous carbons caused by the intrinsic void space. Specifically, lithium-sulfur (Li-S) batteries as a hot topic of next-generation energy storage devices face the same dilemma that we have to balance their intrinsically electrochemical performance and volumetric performance. The use of conductive porous carbon materials in the cathode of Li-S batteries, such as mesoporous carbon, carbon nanotube and graphene-derived carbons, can effectively accelerate the reaction kinetics, improve the electrochemical performance of sulfur cathode and promote the practical application of insulting sulfur. However, addition of these materials results in massive void space in the electrode, which cannot meet the requirement of compact structure in real applications. Such electrodes usually deliver relatively low volumetric performance even though their gravimetric performance can reach a high value.
... From above results, we can see that another great advantage of using graphene is also to improve the volumetric energy density of the cathode of Li-S batteries [160]. In Li-S batteries, the cathode consists of porous carbon and sulfur. ...
... Generally, the density of the porous carbon is very low (0.3-0.6 g/cm 3 ), while the density of bulk sulfur is above 2.0 g/cm 3 . Thus, one route to improve the volumetric energy density is to pack sulfur into high density porous carbons [160]. Similar methods are also proposed by other groups in which nanosize sulfur attaches to a densely-packed and conductive graphene framework to improve the volumetric energy density [161]. ...
The demand for high performance electrochemical energy storage devices has significantly increased in recent years and many efforts have been made to develop advanced electrode materials. In this respect, graphene-based materials, considered promising high performance electrode materials, have drawn great attention because they can increase the performance of the currently-used devices, such as the Lithium-ion battery and supercapacitor, and make next generation devices, such as the Lithium–sulfur battery, Lithium–O2 battery and Sodium-ion battery, more practical. This review summarizes the current uses of graphene-based materials in these devices and demonstrates their advances. It also discusses the opportunities for graphene in high performance electrode material preparation and device configuration, and more importantly, the challenges of graphene for practical use in these devices. Finally, perspectives and possible breakthroughs for future graphene-based materials are also briefly discussed.
... However, further improvement in the energy density of LIBs is restricted due to the low theoretical specific capacity of graphite anode [1][2][3][4][5][6][7]. Because of its lowest reduction potential, extremely high theoretical specific capacity, and suitability to couple with potential cathode materials, such as O 2 , S 8 , and layered oxides, Li metal is an ideal anode candidate for constructing the next generation high energy density secondary batteries, e.g., Li metal batteries (LMBs) [8][9][10]. However, there are issues that restrict the practical application of Li anode, mainly originating from the high activity of Li metal [11][12][13]. ...
Lithium (Li) metal is the most promising anode for improving the energy density of currently commercialized Li-ion batteries. However, its practical application is limited due to its high reactivity to electrolytes, which induces severe electrolyte decomposition and Li-dendrite growth. Interphases are usually constructed on Li anode to address the above issue. Meanwhile, it is a big challenge to balance the stability and plating/stripping overpotential of Li anode. In this work, we report a novel strategy for constructing a highly stable and lowly polarized surface film on Li anode. A chemically and structurally unique film is formed by simply dropping a zinc trifluoromethanesulfonate [Zn(OTF)2] and fluoroethylene carbonate (FEC)-containing solution onto Li anode. This unique film consists of inner nucleation sites and outer protection textures, mainly containing Li-Zn alloy and LiF/polymer, respectively. The former results from the preferential reduction of Zn(OTF)2, providing nucleation sites with low polarization for Li plating/stripping. In contrast, the latter arises from the subsequent reduction of FEC, providing protection for the underneath Li-Zn alloy and Li metal and ensuring the stability of Li anode. The Li anode with such a unique surface film exhibits excellent cycling stability and low plating/stripping overpotentials, which have been demonstrated using Li//Li symmetric and Li//LiFePO4 full cells.
... Intrinsically, the low W V of Li-S battery stems from the low density of sulfur (2.07 g cm −3 ), in comparison with the intercalation compounds as cathode in LIBs. The light-weight sulfur makes it hard to construct high density sulfur cathode (S-cathode), and this situation is further aggravated when a large amount of carbon nanomaterials with a low density are used as sulfur host [34]. Recently, some attempts have been made to increase the S-cathode density, including increasing the sulfur content [35], employing compact carbon [36][37][38], and fabricating dense cathode structure [39][40][41][42]. ...
Although the gravimetric energy density of lithium sulfur battery is very encouraging, the volumetric energy density still remains a challenge for the practical application. To achieve the high volumetric energy density of battery, much attention should be paid to the sulfur cathode. Herein, we introduce heavy lithium cobalt oxide (LiCoO 2) nanofibers as sulfur host to enhance the volumetric capacity of cathode, maintaining the high gravimetric capacity simuta-neously. With the high tap density of 2.26 g cm −3 , LiCoO 2 nanofibers can be used to fabricate a really compact sulfur cathode, with a density and porosity of 0.85 g cm −3 and 61.2%, respectively. More importantly, LiCoO 2 nanofibers could act as an efficient electrocatalyst for enhancing the redox kinetics of sulfur species, ensuring the cathode electroactivity and alleviating the shuttle effect of polysulfides. Therefore, a balance between compact structure and high electrochemical activity is obtained for the sulfur cathode. At the sulfur loading of 5.1 mg cm −2 , high volumetric and gravimetric capacities of 724 mA h cm −3 cathode and 848 mA h g −1 cathode could be achieved based on the cathode volume and weight, respectively. Moreover , with this efficient S/LiCoO 2 cathode, the lithium corrosion by polysulfides is supressed, leading to a more stable lithium anode.
... However, using conventional strategies [9][10][11], it is hard to obtain materials with both high density and large surface area. Our previous study has demonstrated that graphene oxide (GO) can be used to produce highly porous but dense carbon monoliths, which are promising materials for compact energy storage highlighting volumetric performance [7,[12][13][14][15][16][17][18]. GO as the oxidative exfoliation product of graphite is an amphiphile, due to its hydrophobic benzene-ring basal plane and the hydrophilic oxygen-containing group-decorated edges [19,20]. ...
Conventional carbon materials cannot combine high density and high porosity, which are required in many applications, typically for energy storage under a limited space. A novel highly dense yet porous carbon has previously been produced from a three-dimensional (3D) reduced graphene oxide (r-GO) hydrogel by evaporation-induced drying. Here the mechanism of such a network shrinkage in r-GO hydrogel is specifically illustrated by the use of water and 1,4-dioxane, which have a sole difference in surface tension. As a result, the surface tension of the evaporating solvent determines the capillary forces in the nanochannels, which causes shrinkage of the r-GO network. More promisingly, the selection of a solvent with a known surface tension can precisely tune the microstructure associated with the density and porosity of the resulting porous carbon, rendering the porous carbon materials great potential in practical devices with high volumetric performance.
... Sponge assembled by graphene flakes possesses high electrical conductivity, excellent mechanical and thermal properties, which holds great promise as a sensing material for piezoresistive pressure sensors with high sensitivity and excellent stability. The graphene sponge (GS) can be fabricated by chemical vapor deposition on metal template [28][29][30], and chemical reduction and hydrothermal reduction of graphene oxide (GO) [31][32][33][34][35][36]. However, GS fabricated through these methods usually presented a high lower limit of pressure detection and low gauge factor, which are determined by the elastic modulus and electrical conductivity of the sponge. ...
Graphene sponge (GS) with microscale size, high mechanical elasticity and electrical conductivity has attracted great interest as a sensing material for piezoresistive pressure sensor. However, GS offering a lower limit of pressure detection with high gauge factor, which is closely dependent on the mechanical and electrical properties and determined by the fabrication process, is still demanded. Here, γ-ray irradiation reduced GS is reported to possess a gauge factor of 1.03 kPa–1 with pressure detection limit of 10 Pa and high stress retention of 76% after 800 cycles of compressing/relaxation at strain of 50%. Compared with the carbon nanotube (CNT) reinforced GS, the improved lower limit of pressure detection and gauge factor of the GS prepared by γ-ray irradiation is due to the low compression stress (0.9 kPa at stain of 50%). These excellent physical properties of the GS are ascribed to the mild, residual free, and controllable reduction process offered by γ-ray irradiation.
... The low material density and the resulting low volume energy density is a bottleneck for nanocarbons. Recently, much more attention has been paid to this [143][144][145][146]. For example, the packing density of solvated graphene films reported by the Li's group can be increased up to ~ 1.33 g cm −3 to achieve high volumetric capacities of 255.5 F cm −3 in an aqueous electrolyte [16]. ...
Carbon is a key component in current electrochemical energy storage (EES) devices and plays a crucial role in the improvement in energy and power densities for the future EES devices. As the simplest carbon and the basic unit of all sp² carbons, graphene is widely used in EES devices because of its fascinating and outstanding physicochemical properties; however, when assembled in the macroscale, graphene-derived materials do not demonstrate their excellence as individual sheets mostly because of unavoidable stacking. This review proposal shows to engineer graphene nanosheets from the nano- to the macroscale in a well-designed and controllable way and discusses how the performance of the graphene-derived carbons depends on the individual graphene sheets, nanostructures, and macrotextures. Graphene-derived carbons in EES applications are comprehensively reviewed with three representative devices, supercapacitors, lithium-ion batteries, and lithium–sulfur batteries. The review concludes with a comment on the opportunities and challenges for graphene-derived carbons in the rapidly growing EES research area.
... Similar to nitrogen, sulfur doping can also improve electron conductivity [22,33]. N and S co-doping has been confirmed to be an effective strategy to significantly promote the electrocatalytic activity of carbon. ...
Hydrothermal carbon (HTC) is typically well-dispersed, but it remains a great challenge for HTC to become conductive. Co-doping with heteroatoms has been confirmed to be an effective strategy to significantly promote the electrical conductivity of carbon. Moreover, there is no simple and green method to construct sensitive HTC based electrochemical biosensors until now. In this paper, N and S dual-doped carbon (NS-C) with ultra-low charge transfer resistance is easily synthesized from L-cysteine and glucose in a hydrothermal reaction system. The morphology, structural properties and electrochemical properties of the as-prepared NS-C are analyzed. In comparison with the undoped hydrothermal (UC) modified glassy carbon electrode (GCE), the charge transfer resistance of UC (476 Ω) is ten times the value of NS-C (46 Ω. The developed biosensor shows a better performance to detect glucose in a wide concentration range (50-2500 μmol L-1) with the detection limit of 1.77 ^mol L-1 (S/N=3) and a high sensitivity (0.0554 μA cm-2 μmol-1 L). The apparent Michaelis-Menten constant value of GCE/NS-C/GOx/nafion modified electrode is 0.769 mmol L-1, indicating a high affinity of glucose oxidase to glucose. These results demonstrate that the hydrothermal method is an effective way for preparing high electrical conductivity carbon with excellent performances in biosensor application.
... For the sulfur cathode, key problems mainly comprise three aspects: the low conductivity of sulfur and its discharge products, the diffusion of polysulfide ions, and the expansion of active material during electrochemical reaction [12][13][14][15][16][17][18][19][20][21][22][23]. To solve the abovementioned problems, one of the most effective methods is loading sulfur into electronically conductive frameworks with good structural stability [24][25][26][27][28][29][30][31][32]. Various nanoporous carbon materials, such as mesoporous carbon [33], porous graphene [34,35], porous carbon nanofibers [36,37], hollow carbon spheres [38], and activated carbon (AC) [39], have been widely studied as loading frameworks for sulfur. ...
Lithium–sulfur batteries have drawn considerable attention because of their extremely high energy density. Activated carbon (AC) is an ideal matrix for sulfur because of its high specific surface area, large pore volume, small-size nanopores, and simple preparation. In this work, through KOH activation, AC materials with different porous structure parameters were prepared using waste rapeseed shells as precursors. Effects of KOH amount, activated temperature, and activated time on pore structure parameters of ACs were studied. AC sample with optimal pore structure parameters was investigated as sulfur host materials. Applied in lithium–sulfur batteries, the AC/S composite (60 wt % sulfur) exhibited a high specific capacity of 1065 mAh g−1 at 200 mA g−1 and a good capacity retention of 49% after 1000 cycles at 1600 mA g−1. The key factor for good cycling stability involves the restraining effect of small-sized nanopores of the AC framework on the diffusion of polysulfides to bulk electrolyte and the loss of the active material sulfur. Results demonstrated that AC materials derived from rapeseed shells are promising materials for sulfur loading.
... In addition, the electronic/ionic insulating nature and complex electrochemical redox of the S species require appropriate host materials for enhancing the interfacial charge transfer, withstanding volume variations, and regulating the coupled reaction/transport of LiPS Nano Res. [25,27,28]. Additionally, the Li metal anode in the Li-S battery system encounters Li dendrite formation and solid-electrolyte interphase (SEI) instability, which lead to serious safety concerns and a low Coulombic efficiency, respectively [29][30][31]. ...
Lithium–sulfur (Li–S) batteries are receiving increasing attention because of their high theoretical energy density and the natural abundance of S. However, their practical applications are impeded by the low areal S loading in the cathode and the fatal Li dendrites in the anode of the Li−S cells, which yield an inferior practical energy density and introduce safety concerns, respectively. In this review, we focus on an emerging approach—the nanostructured current collector—to overcome these two critical challenges for Li−S batteries. We describe the general attributes of nanostructured current collectors and examine how these attributes enhance the S utilization with a high S loading and suppress the Li dendrites by regulating the Li-deposition behavior. We present various assembly blocks that have been used for the construction of advanced nanostructured current collectors to build better S cathodes and Li anodes. Finally, we investigate the current challenges and possible solutions regarding the practical applications of nanostructured current collectors in Li−S batteries.Open image in new window
... The gravimetric energy density, which mainly depends on the intrinsic nature of the electrode materials [20][21][22][23], indicates how much energy can be stored in a unit weight of the assembled cell, whereas the volumetric energy density measures how much energy can be harvested from a unit volume [24]. To improve the volumetric energy density, both the performance of the electrode materials and the configuration of the cell components must be considered [25,26]. Unfortunately, however, the vast majority of published reports have focused mainly on the gravimetric energy density of Li-O 2 batteries and the concept of volumetric energy density has been barely mentioned. ...
... These include severe side reactions, a low packing density and poor scalability of manufacture, which dramatically hinder their practical use. [20][21][22] In the battery industry, a high material density along with a high packing density of the electrode are needed to boost the volumetric energy density, and because of this, the particle size of the active materials must usually be microscale. [23] Thus, for practical use, the assembly of nanosized materials into a microscale secondary structure is key to achieving a high density and retaining the inherent advantages of the nanoscale. ...
Graphene oxide (GO) shows very unique twin functions, both as hard and soft templates, in the production of highly solid Fe2O3 microspheres. The microsphere is a very dense aggregation of nanosized primary particles and represents an important advance in energy storage materials with high volumetric performance. GO as a hard template is responsible for the precursor nucleation in the formation of the primary nanoparticles (NPs), and meanwhile, as a soft template, manages the densification of these NPs to form a secondary solid and compact microsphere. Together with three-dimensional interconnected conductive graphene network formed spontaneously, the highly solid Fe2O3 microspheres exhibit an excellent rate performance and more importantly, a super high volumetric capacity of about 1200 mA h cm⁻³. This work shows a promising approach for the densification of nanostructures and promotes the practical applications of low density nanomaterials in real energy storage devices.
The "shuttle effect" and slow conversion kinetics of lithium polysulfides (LiPSs) are stumbling block for high-energy-density lithium-sulfur batteries (LSBs), which can be effectively evaded by advanced catalytic materials. Transition metal borides possess binary LiPSs interactions sites, aggrandizing the density of chemical anchoring sites. Herein, a novel core-shelled heterostructure consisting of nickel boride nanoparticles on boron-doped graphene (Ni3 B/BG), is synthesized through a graphene spontaneously couple derived spatially confined strategy. The integration of Li2 S precipitation/dissociation experiments and density functional theory computations demonstrate that the favorable interfacial charge state between Ni3 B and BG provides smooth electron/charge transport channel, which promotes the charge transfer between Li2 S4 -Ni3 B/BG and Li2 S-Ni3 B/BG systems. Benefitting from these, the facilitated solid-liquid conversion kinetics of LiPSs and reduced energy barrier of Li2 S decomposition are achieved. Consequently, the LSBs employed the Ni3 B/BG modified PP separator deliver conspicuously improved electrochemical performances with excellent cycling stability (decay of 0.07% per cycle for 600 cycles at 2 C) and remarkable rate capability of 650 mAh g-1 at 10 C. This study provides a facile strategy for transition metal borides and reveals the effect of heterostructure on catalytic and adsorption activity for LiPSs, offering a new viewpoint to apply boride in LSBs.
Storing as much energy as possible in as compact a space as possible is an ever-increasing concern to deal with the emerging “space anxiety” in electrochemical energy storage (EES) devices like batteries, which is known as “compact energy storage”. Carbons built from graphene units can be used as active electrodes or inactive key materials acting as porous micro- or even nano-reactors that facilitate battery reactions and play a vital role in optimizing the volumetric performance of the electrode and the battery. In this review, we discuss and clarify the key issues and specific strategies for compact energy storage, especially in batteries. The use of shrinkable carbon networks to produce small yet sufficient reaction space together with smooth charge delivery is highlighted as the simplest structure–function-performance relationship when used in supercapacitors and is then extended to overcome problems in compact rechargeable lithium/sodium/potassium batteries. Special concerns about cycling stability, fast charging and safety in compact batteries are discussed in detail. Strategies for compact energy storage ranging from materials to electrodes to batteries are reviewed here to provide guidance for how to produce a compact high energy battery by densifying the electrodes using customized carbon structures.
Although lithium-sulfur batteries have the advantages of gravimetric energy density, it remains a challenge to achieve high volumetric energy density in lithium-sulfur batteries due to the employment of conductive porous carbon with low tap density and high porosity. To this context, we synthesized a horizontally aligned lamellar porous graphene/nickel composite (LPG-Ni) as a dense sulfur host material to increase the volumetric capacity of lithium-sulfur batteries while retaining high gravimetric capacity. The nickel decorated on the surface of the porous graphene serves as active sites, effectively adsorbing and catalytically converting polysulfides, thus improving the redox kinetics and reducing shuttle effect. More importantly, the abundant pores on the graphene sheet are considered as a shortcut for lithium-ion diffusion, which accelerates the ion transport through the whole electrode. Owing to the synergistic effect, the as-prepared sulfur cathode exhibits both excellent ionic and electronic conductivity, and has a dense-stacked structure. Thus, the LPG-Ni/S cathode provides high gravimetric and volumetric capacity (718.7 mA h g⁻¹cathode and 884.0 mA h cm⁻³cathode at 0.1 C). Meanwhile, the battery exhibits superior rate capability (369.7 mA h cm⁻³cathode at 5 C) and cycling performance (1000 cycles, 0.037% decay per cycle at 1C).
Hierarchically porous carbon (HPC) is considered as one of the promising cathodes of lithium-sulfur batteries (LSBs) owing to its appealing merits. However, it is still preventing the shuttle effect poorly due to the weak interaction between nonpolar carbon and polar polysulfide (LiPS). Herein, we report a facile method to prepare polar hierarchically porous carbon by in-situ loading the different amounts of zinc-based active sites (p-HPC-X) onto the HPC materials. The close combination of a carbon interconnected skeleton and ZnS nanoparticles derived zinc-based active sites forms the strong interaction that suppresses the soluble polysulfide shuttle effect and catalyzes the kinetics redox reactions of lithium polysulfide/sulfide. Moreover, the effect of polar sites introduced in situ on the morphology/porosity of carbon materials is also focused on. Crystallization of zinc-based active sites during the freeze-drying process and self-assembly during carbonization allow the pore size distribution of porous carbon to be customized, which offers fast lithium-ion and electron transport channels. Due to the synergistic effect of the hierarchical pore structure and suitable polarity sites, the p-HPC-X exhibits high capacity and good cycling performance. Especially p-HPC-1, it yields a high reversible capacity of 683.7 mAh g⁻¹ and a low fading rate of 0.041% per cycle over 900 cycles at a current density of 1 C. This work offers novel insights into realizing the effect of in-situ loaded active sites on pore structure and electrochemical performance in LSBs.
Atomic tungsten on graphene with a unique W state of the W-N2O2 structure are first used to decorate the separator of a Li-S battery to promote its electrochemical performance. The unique local coordination structure of W-O2N2-C moiety not only acts as an active site for anchoring lithium polysulfides, but also boosts their kinetic conversion via electrocatalysis.
Abstract
Use of catalytic materials is regarded as the most desirable strategy to cope with sluggish kinetics of lithium polysulfides (LiPSs) transformation and severe shuttle effect in lithium–sulfur batteries (LSBs). Single-atom catalysts (SACs) with 100 % atom-utilization are advantagous in serving as anchoring and electrocatalytic centers for LiPSs. Herein, a novel kind of tungsten (W) SAC immobilized on nitrogen-doped graphene (W/NG) with a unique W-O2N2-C coordination configuration and a high W loading of 8.6 wt % is proposed by a self-template and self-reduction strategy. The local coordination environment of W atom endows the W/NG with elevated LiPSs adsorption ability and catalytic activity. LSBs equipped with W/NG modified separator manifest greatly improved electrochemical performances with high cycling stability over 1000 cycles and ultrahigh rate capability. It indicates high areal capacity of 6.24 mAh cm⁻² with robust cycling life at a high sulfur mass loading of 8.3 mg cm⁻².
The employment of catalytic materials is regarded as the most desirable strategy to cope with sluggish kinetics of lithium polysulfides (LiPSs) transformation and severe shuttle effect within lithium−sulfur batteries (LSBs). Single‐atom catalysts (SACs) with 100% atom‐utilization possess competitive advantages to serve as both anchoring and electrocatalytic centers for LiPSs. However, it’s still a great challenge due to the insufficiency of facile synthetic strategy and ambiguity of the structure‐property relationships. Herein, a novel kind of tungsten (W) SAC immobilized on nitrogen‐doped graphene (W/NG) with a unique W‐O2N2‐C coordination configuration and a high W loading of 8.6 wt% is first proposed via a self‐template and self‐reduction strategy. The special local coordination environment of W atom endows the W/NG with elevated LiPSs adsorption ability and catalytic activity evidenced by combination of experimental and theoretical studies. Impressively, LSBs equipped with W/NG modified separator manifest greatly improved electrochemical performances with high cycling stability over 1000 cycles and ultrahigh rate capability. Moreover, it indicates high areal capacity of 6.24 mAh cm−2 with robust cycling life at a high sulfur mass loading of 8.3 mg cm−2 .
Practical Li-sulfur batteries require the high sulfur loading cathode to meet the large-capacity power demand of electrical equipment. However, the sulfur content in cathode materials is usually unsatisfactory due to the excessive use of carbon for improving the conductivity. Traditional cathode fabrication strategies can hardly realize both high sulfur content and homogeneous sulfur distribution without aggregation. Herein, we designed a cathode material with ultrahigh sulfur content of 88%(mass fraction) by uniformly distributing the water dispersible sulfur nanoparticles on three-dimensionally conductive graphene framework. The water processable fabrication can maximize the homogeneous contact between sulfur nanoparticles and graphene, improving the utilization of the interconnected conductive surface. The obtained cathode material showed a capacity of 500 mA·h/g after 500 cycles at 2.0 A/g with an areal loading of 2 mg/cm2. This strategy provides possibility for the mass production of high-performance electrode materials for high-capacity Li-S battery.
Lithium–sulfur batteries (LSBs) with a high theoretical capacity of 1675 mAh g⁻¹ hold promise in the realm of high‐energy‐density Li–metal batteries. To cope with the shuttle effect and sluggish transformation of soluble lithium polysulfides (LiPSs), varieties of traditional metal‐based materials (such as metal, metal oxides, metal sulfides, metal nitrides, and metal carbides) with unique catalytic activity for accelerating LiPSs redox have been exploited to fundamentally inhibit the shuttle effect and improve the performance of LSBs. Concurrently, some budding catalytic materials also possess enormous potential for facilitating LiPSs redox reaction in LSBs, including metal borides, metal phosphides, metal selenides, single atoms, and defect‐engineered materials. Here, recent advances in these emerging catalytic candidates as well as the evaluation methods and parameters for catalytic materials are comprehensively summarized for the first time. New insights are also given to aid in the design of high‐performance LSBs and satisfy the high expectation in the future, including the exposure of the active sites and adsorption‐catalysis synergy strategies. Finally, the current challenges and prospects for designing highly efficient catalytic materials are highlighted, aiming at providing guidance for configuring catalytic materials to make sure high‐energy and long‐life LSBs.
Lithium–sulfur (Li–S) batteries hold the promise of the next generation energy storage system beyond state‐of‐the‐art lithium‐ion batteries. Despite the attractive gravimetric energy density (WG), the volumetric energy density (WV) still remains a great challenge for the practical application, based on the primary requirement of Small and Light for Li–S batteries. This review highlights the importance of cathode density, sulfur content, electroactivity in achieving high energy densities. In the first part, key factors are analyzed in a model on negative/positive ratio, cathode design, and electrolyte/sulfur ratio, orientated toward energy densities of 700 Wh L⁻¹/500 Wh kg⁻¹. Subsequently, recent progresses on enhancing WV for coin/pouch cells are reviewed primarily on cathode. Especially, the “Three High One Low” (THOL) (high sulfur fraction, high sulfur loading, high density host, and low electrolyte quantity) is proposed as a feasible strategy for achieving high WV, taking high WG into consideration simultaneously. Meanwhile, host materials with desired catalytic activity should be paid more attention for fabricating high performance cathode. In the last part, key engineering technologies on manipulating the cathode porosity/density are discussed, including calendering and dry electrode coating. Finally, a future outlook is provided for enhancing both WV and WG of the Li–S batteries.
The lithium-sulfur (Li-S) batteries have high theoretical energy density, exceeding that of the lithium-ion batteries. However, their practical applications are hindered by the capacity decay due to lithium polysulfide shuttle effect and sulfur volume expansion. Here, we design a [email protected] carbon with porous shell/MnOx ([email protected]/MnOx) cathode to accommodate and immobilize sulfur and polysulfides, and develop a non-destructive technique X-ray computed tomography (X-ray CT) to in situ visualize the volume expansion of Li-S cathode. The designed cathode achieves a specific capacity of ∼1100 mAh g⁻¹ at 0.2 C with a fade rate of 0.18% per cycle over 300 cycles. The X-ray CT shows that only 16% volume expansion and 70% volume fraction of solid sulfur remaining in the [email protected]/MnOx cathode, superior to the commercial cathode with 40% volume expansion and 5% volume remaining of solid sulfur particles. This is the first reported visualization evidence for the effectiveness of hollow carbon structure in accommodating cathode volume expansion and immobilizing sulfur shuttling. X-ray CT can serve as a powerful in situ tool to trace the active materials and then feedback to the structure design, which helps develop efficient and reliable energy storage systems.
Three‐dimensional (3D) nitrogen‐doped carbon nanofibers (N‐CNFs) which were originating from nitrogen‐containing zeolitic imidazolate framework‐8 (ZIF‐8) were obtained by a combined electrospinning/carbonization technique. The pores uniformly distributed in N‐CNFs result in the improvement of electrical conductivity, increasing of BET surface area (142.82 m² g⁻¹), and high porosity. The as‐synthesized 3D free‐standing N‐CNFs membrane was applied as the current collector and binder free containing Li2S6 catholyte for lithium‐sulfur batteries. As a novel composite cathode, the free‐standing N‐CNFs/Li2S6 membrane shows more stable electrochemical behavior than the CNFs/Li2S6 membrane, exhibiting a high first‐cycle discharge specific capacity of 1175 mAh g⁻¹at 0.1 C and keeping discharge specific capacity of 702 mAh g⁻¹ at higher rate. More importantly, as the sulfur mass in cathodes was increased at 7.11 mg, the N‐CNFs/Li2S6 membrane delivered 467 mAh g⁻¹after 150 cycles at 0.2 C. The excellent electrochemical properties of N‐CNFs/Li2S6 membrane can be ascribed to synergistic effects of high porosity and nitrogen‐doping in N‐CNFs from carbonized ZIF‐8, illustrating collective effects of physisorption and chemisorption for lithium polysulfides in discharge‐charge processes.
Lithium-sulfur (Li-S) batteries are promising next-generation high energy density batteries but their practical application is hindered by several key problems, such as the intermediate polysulfide shuttling and the electrode degradation caused by the sulfur volume changes. Binder acts as one of the most essential components to build the electrodes of Li-S batteries, playing vital roles in improving the performance and maintaining the integrity of the cathode structure during cycling, especially those with high sulfur loadings. To date, tremendous efforts have been devoted to improving the properties of binders, in terms of the viscosity, elasticity, stability, toughness and conductivity, by optimizing the composition and structure of polymer binders. Moreover, the binder modification endows them strong polysulfide trapping ability to suppress the shuttling and decreases the swelling to maintain the porous structure of cathode. In this review, we summarize the recent progress on the binders for Li-S batteries and discuss the various routes, including the binder combination use, functionalization, in-situ polymerization and ion cross-linking, etc., to enhance their performance in stabilizing the cathode, building the high sulfur loading electrode and improving the cyclic stability. At last, the design principles and the problems in further applications are also highlighted. © 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Lithium metal is considered to be the most promising anode material for the next-generation rechargeable batteries. However, the uniform and dendrite-free deposition of Li metal anode is hard to achieve, hindering its practical applications. Herein, a lightweight, free-standing and nitrogen-doped carbon nanofiber-based 3D structured conductive matrix (NCNF), which is characterized by a robust and interconnected 3D network with high doping level of 9.5 at%, is prepared by electrospinning as the current collector for Li metal anode. Uniform Li nucleation with reduced polarization and dendrite-free Li deposition are achieved because the NCNF with high nitrogen-doping level and high conductivity provide abundant and homogenous metallic Li nucleation and deposition sites. Excellent cycling stability with high coulombic efficiency are realized. The Li plated NCNF was paired with LiFePO4 to assemble the full battery, also showing high cyclic stability.
A high areal sulfur loading in a carbon-based cathode together with a high cell capacity is key to the design of lithium-sulfur batteries guaranteeing a superior energy density for use. However, a high sulfur loading produced using traditional blade coating techniques results in many technical issues such as sluggish electron/ion transport kinetics and cracking of the electrodes. Here a well designed two-step electrostatic spray deposition (ESD) technique is proposed to prepare a flexible, multilayer carbon electrode with a high sulfur areal loading, in which different carbon components by a careful selection are used for different functions in each layer. The unique “aerosol deposition” in the ESD creates buffer voids in the electrode, ensuring fast infiltration of the electrolyte and releasing the internal stress of the electrode thus avoiding the cracking of thick electrodes. With such an integrated design, the as-prepared cathode exhibits excellent flexibility, a long cyclic stability with a low capacity decay of 0.064% per cycle at 1 C for 500 cycles and a high rate capability of 736 mAh g⁻¹ at 2 C. Moreover, a high areal sulfur loading of 9.4 mg cm⁻² with an areal capacity of 6.2 mAh cm⁻² at 0.1 C has been achieved.
Lithium‐sulfur (Li‐S) batteries are considered as one of the most promising energy storage systems for next‐generation electric vehicles because of their high‐energy density. However, the poor cyclic stability, especially at a high sulfur loading, is the major obstacles retarding their practical use. Inspired by the nacre structure of an abalone, a similar configuration consisting of layered carbon nanotube (CNT) matrix and compactly embedded sulfur is designed as the cathode for Li‐S batteries, which are realized by a well‐designed unidirectional freeze‐drying approach. The compact and lamellar configuration with closely contacted neighboring CNT layers and the strong interaction between the highly conductive network and polysulfides have realized a high sulfur loading with significantly restrained polysulfide shuttling, resulting in a superior cyclic stability and an excellent rate performance for the produced Li‐S batteries. Typically, with a sulfur loading of 5 mg cm−2, the assembled batteries demonstrate discharge capacities of 1236 mAh g−1 at 0.1 C, 498 mAh g−1 at 2 C and moreover, when the sulfur loading is further increased to 10 mg cm−2 coupling with a carbon‐coated separator, a superhigh areal capacity of 11.0 mAh cm−2 is achieved. Lithium–sulfur batteries are one of the most promising energy storage systems for the next‐generation electric vehicles from the high theoretical energy density. From the unidirectional freeze‐drying approach, a nacre‐like carbon nanotube sheet is prepared and used as the cathode matrix. Such a structure with compact carbon nanotube matrix–sulfur configuration enables a considerably high performance at a high areal sulfur loading.
As polar materials, transition-metal oxides have shown great potentials to improve the adsorption of lithium polysulfides in lithium-sulfur batteries. Herein, a MoO2-ordered mesoporous carbon (M-OMC) hybrid was designed as the sulfur host, in which MoO2 is inlaid on the surface of ordered mesoporous carbons that can store active materials and provide fast electron transfer channel due to its ordered pore structure. The MoO2 can effectively prevent the migration of polysulfides through the chemical adsorption and promote the conversion of polysulfides towards Li-sulfur battery.
Volumetric performance is highly important for evaluating the potential of supercapacitors, especially for the case where electrode space is limited. To achieve high space utilization, the less pores to be included the better. Along this direction, we showed previously a PANI/graphene composite almost free of porosity by shrinking the composite network to the most compact, which yet exhibited a high volumetric capacitance and a good rate capability. The PANI/graphene solid composite simultaneously enabled maximized space utilization of the electrode volume and achieved unimpeded ion transport, which seems counter to the general design principle of electrode materials where appropriate porous structure is highly desired. Here we propose the proton transport mechanism of PANI in the dense composite, which indicates that PANI is a dual electronic-ionic conductivity polymer that acts not only as a pseudo-capacitive active material for high energy storage but also as a proton conductor that realizes proton transport from the electrode/electrolyte interface to the inner of the dense micro-particles. More importantly, we further propose the design principle of non-porous carbon-based composites to achieve high volumetric performance, in which a good dual electronic-ionic conductor is selected as the best pseudo-capacitive filler. This work inspires new insights into better design and preparation of the composite electrodes for compact energy storage devices.
Lithium sulfur batteries (LSBs) with high theoretical energy density are being pursued as highly promising next-generation large-scale energy storage devices. However, its launch into practical application is still shackled by various challenges. A rational nanostructure of hollow carbon nanoboxes filled with birnessite-type manganese oxide nanosheets (MnO2 @HCB) as a new class of molecularly-designed physical and chemical trap for lithium polysulfides (Li2 Sx (x = 4-8)) is reported. The bifunctional, integrated, hybrid nanoboxes overcome the obstacles of low sulfur loading, poor conductivity, and redox shuttle of LSBs via effective physical confinement and chemical interaction. Benefiting from the synergistic encapsulation, the developed MnO2 @HCB/S hybrid nanoboxes with 67.9 wt% sulfur content deliver high specific capacity of 1042 mAh g(-1) at the current density of 1 A g(-1) with excellent Coulombic efficiency ?100%, and retain improved reversible capacity during long term cycling at higher current densities. The developed strategy paves a new path for employing other metal oxides with unique architectures to boost the performance of LSBs.
Environment and energy are the hot spots for sustainable development of our planet. Air pollution, as a vital environmental issue, has puzzled us for decades. The control techniques are of high significance, and a green and economical approach to control the air pollutants is urgently required. New energy storage techniques emerge as fascinating candidate to reduce the potential environmental problem. Therefore, the combination of air pollutants with energy storage devices is an appealing solution for air pollution. Herein, the principles of recovering air pollutants into energy storage materials are proposed, including the direct use as active materials, the conversion into active materials and as dopants to modify the electrode. Typical air pollutants are selected to demonstrate their potential use in green energy storage. Future prospects of the broad application of air pollutants in energy- related fields are also commented.
A liquid–air interfacial assembly is used to produce a flexible substrate-supported graphene film, which does not need the tedious substrate-transfer procedure that is generally required for many other film-making methods. The graphene film is used as the active working electrode in its planar configuration and an acidic polymer gel is used as the electrolyte, which leads to a flexible planar graphene film supercapacitor with a large areal specific capacitance of 773 µF cm−2 and superior retention performance. In order to demonstrate the versatile application of interfacial assembly in obtaining graphene-based hybrid film electrodes, graphene–manganese oxide and graphene–polyaniline hybrid films are assembled and used to construct planar film supercapacitors with areal specific capacitance of 963 and 2561 µF cm−2, respectively. Compared with the graphene film supercapacitor, the graphene–manganese oxide film supercapacitor, with neutral gel electrolyte, shows no apparent increase of specific capacitance, which is probably due to the larger electrolyte ions and hence the slower ion diffusion rate. Overall, the interfacial assembly is a universal technique for the fabrication of flexible, planar graphene-based film supercapacitors, and this report demonstrates that such supercapacitors are promising as an advanced power system in wearable electronics.
High-tap density electrode materials are greatly desired for Li-ion batteries with high volumetric capacities to fulfill the growing demands of electric vehicles and portable smart devices. TiO2, which is one of the most attractive anode materials, is limited in their application for Li-ion batteries because of its low tap density (usually <1 g cm⁻³) and volumetric capacity. Herein, we report uniform mesoporous TiO2 submicrospheres with a tap density as high as 1.62 g cm⁻³ as a promising anode material. Even with a high mass loading of 24 mg cm⁻², the TiO2 submicrospheres have impressive volumetric capacities that are more than double those of their counterparts. Moreover, they can be synthesized with ~100% yield and within a reaction time of ~6 h by optimizing the experimental conditions and formation mechanism, exhibiting potential for large-scale production for industrial applications. Other mesoporous anode materials, i.e., high-tap density mesoporous Li4Ti5O12 submicrospheres, are fabricated using the generalizedmethod. We believe that our work provides a significant reference for the industrial production of mesoporous materials for Li-ion batteries with a high volumetric performance.
The high solubility of polysulfides in the electrolyte, together with the resulting poor cycling performance, is one of the main obstacles to the industrial production and use of lithium-sulfur (Li-S) batteries. We have developed a novel hybrid and dense separator coating that greatly improves the cycling and rate performance of the battery. The coating is fabricated by mono-dispersed Li4Ti5O12 (LTO) nanospheres uniformly embedded in graphene layers. In this hybrid dense coating, the LTO nanospheres have a high chemical affinity for polysulfides and an excellent ionic conductivity to produce highly efficient ionic conductive channels, while the graphene layers play twin roles as a physical barrier for polysulfides and an upper current collector. The unique hybridization guarantees a very dense coating that does not significantly add the volume of the battery and meanwhile achieves an ideal combination of an effective barrier for polysulfide diffusion with a fast ion transport. For a normal coating, a loose and very thick structure is needed to meet these requirements. Cells using a pure sulfur electrode with the dense coating separator show an ultra-high rate performance (709 mA h g⁻¹ at 2 C and 1408 mA h g⁻¹ at 0.1 C) and an excellent cycling performance (697 mA h g⁻¹ after 500 cycles at 1 C with 85.7% capacity retention). The easy achieving of such excellent performance indicates the possibility of producing an industrially practical Li-S battery.
An interlayer nanostructure of rGO/Sn2Fe-NRs array/rGO was synthesized via a versatile integration of Sn2Fe nanorods (NRs) array in between reduced graphene oxide (rGO) nanosheets. Impressively, as an anode material for lithium ion batteries, the as-prepared nanocomposites deliver a high specific capacity of 690 mA h g−1 at a current density of 0.5 C (500 mA g−1), and 582 mA h g−1at 1 C (1000 mA g−1) with exceptional rate capability and cycling stability over 600 cycles. These significantly improved electrochemical properties benefit from the high structural stability and electrical conductivity of rGO/Sn2Fe-NRs array/rGO interlayer nanostructures. It is demonstrated that the designed interlayer nanostructures are outstanding architectures for lithium ion battery anodes.
The lithium-sulfur (Li-S) batteries are potential candidates for next-generation power sources with very high-energy-density. The rational integration of three dimensional (3D) carbon skeletons into sulfur cathode is an efficient and effective route to full use of conductive scaffolds with chemical and mechanical stability and ability to accommodate large amount of active materials. Herein, a 3D free-standing electrode composed of nitrogen-doped porous graphene/sulfur composite granular and super-long carbon nanotube (CNT) skeleton is proposed for Li-S batteries with high volumetric energy density. With a facile calendering process, the mechanical properties, bulk conductivity, and the pore distribution of the flexible graphene@S-CNT electrodes were tuned. The high rate capability, long-life stability, and high volumetric energy density of Li-S batteries are highly depended on the nanostructure evolution of composite carbon/sulfur electrode. A high rate capability of 40% retention of discharge capacity of 2.0 C against that at 0.05 C, a high cyclic stability of 0.1%/cycle within 180 cycles, and a high volumetric energy density over 850 Wh L⁻¹ are demonstrated with the pressed graphene@S-CNT electrode. This work unfolded a general strategy to modulate the bulk structure of electrodes, which is an efficient route towards Li-S batteries with high volumetric energy density.
Serious shuttle effect of polysulfides is the main obstacle preventing the practical application of lithium sulfur batteries. Herein, we report an ultrathin yet multifunctional polysulfide blocking layer (MPBL) coated on the cathode to effectively restrain the polysulfides shuttling. This MPBL, which was prepared by a simple, one-step electrostatic spray deposition (ESD) technique, was tightly and compactly coated on the electrode with controllable thickness, realizing the full protection of the whole cathode. It is noted that the MPBL can be very thin that guarantees the fast ion diffusion but still keeps a high efficiency of blocking polysulfides due to a combination of physical (from carbon) and chemical confinement (from conductive polymer). In addition, the MPBL with good conductivity can act as the upper current collector to reuse the captured polysulfides, and thus improve the sulfur utilization during cycling. With such multifunctional design, the MPBL-coated carbon-sulfur cathode exhibits long cyclic stability and high rate capability, which has only 0.042% capacity decay per cycle at 1 C for 1000 cycles and a capacity of 615 mAh g⁻¹ even at a high rate of 3 C.
Lithium-sulfur (Li-S) batteries have been strongly regarded as next-generation energy storage devices for their very high theoretical energy density (2600 Wh kg⁻¹), low cost, and non-toxicity. Li polysulfide (LiPS) shuttle in the cathode and Li dendrite growth in the anode are among the toughest issues on the practical applications of Li-S batteries. By efficient cathode and membrane design, LiPS can be effectively trapped in the cathode side and the coin cells deliver a superior cycling performance of 2000 cycles. When the coin cell strategy is transplanted to a pouch cell, the possibility to achieve a long-lifespan Li-S pouch cell should be further explored. Herein, the gaps between the coin and pouch cell were probed and the failure mechanism of a Li-S pouch cell was explored. Compared with LiPS shuttle issue, Li metal powdering and the induced polarization are more responsible for pouch cell failure. Electrochemical cycling performance and ex-situ morphology characterization indicated that the continuously striping/plating of Li ions results in Li dendrite growth during cycling test, which exposes more fresh Li to electrolyte and induces the formation of dead Li powder. Therefore, the Li ion diffusion resistance is increased and the Coulombic efficiency is reduced gradually. When the cycled Li anode was updated by a fresh Li metal, the renaissance cell with cycled cathode vs fresh anode exhibited an improved discharge capacity from 314 to 1030 mAh g⁻¹ at 0.1 C. Relatively to a neglected role in coin cells in most cases, Li metal anode tremendously affects long-term cycling performance of Li-S pouch cells. More attentions should be concentrated on efficiently protecting Li metal anode to achieve large capacity and safe Li-S cells with high energy density.
A controllable drying strategy is proposed for the precise and non-destructive control of the structure of a 3D graphene assembly. Such an assembly is used as a model carbon material to investigate the pore structure-dependent shuttle effect and cycling performance of the cathode of a Li-S battery.
Two-dimensional (2D) nanomaterials such as transition metal dichalcogenides (TMDs) and graphene have attracted extensive interest as emergent materials, owing to their excellent properties that favor their future use in electronic devices, catalysis, optics, and biological- or energy-relevant areas. However, 2D nanosheets tend to easily restack and condense, which weakens their performance in many of these applications. Assembling these 2D nanosheets as building blocks for three-dimensional (3D) architectures not only maintains the intrinsic performances of the 2D nanostructures but also synergistically makes use of the advantages of the 3D microstructures to improve the overall material properties. In this critical review, we will highlight recent developments of sundry 2D nanosheet-assembled 3D architectures, including their design, synthesis, and potential applications. Their controllable syntheses, novel structures, and potential applications will be systematically explained, analyzed, and summarized. In the end, we will offer some perspective on the challenges facing future advancement of this field. © 2015 Science China Press and Springer-Verlag Berlin Heidelberg.
A sheet-like carbon sandwich, which contains a graphene layer as the conductive filling with N-doped porous carbon layers uniformly coated on both sides, is designed as a novel sulfur reservoir for lithium–sulfur batteries and experimentally obtained by a hydrothermal process of a mixture of graphene oxide, glucose and pyrrole, followed by KOH activation. In the hydrothermal process, graphene oxide is both employed as the precursor for the central graphene filling and a sheet-like template for both-side formation of N-doped porous carbon layers, resulting in an N-doped carbon sandwich structure (N-CS). This carbon sandwich is about 50–70 nm in thickness and has a high specific surface area (∼2677 m2 g−1) and a large pore volume (∼1.8 cm3 g−1), making it a promising high capacity reservoir for sulfur and polysulfide in a lithium–sulfur cell. The sheet-like morphology and the interconnected pore structure of the N-CS, together with a nitrogen content of 2.2%, are transformed to the assembled N-CS/sulfur cell with a high rate performance and excellent cycling stability because of fast ion diffusion and electron transfer. At a 2C rate, the reversible capacity is up to 625 mA h g−1 and remains at 461 mA h g−1 after 200 cycles with only 0.13% capacity fading per cycle. More interestingly, the sheet-like structure helps the N-CS materials form a tightly stacked coating on an electrode sheet, guaranteeing a volumetric capacity as high as 350 mA h cm−3.
The volumetric performance of electrochemical energy storage (EES) devices, other than gravimetric performance, is attracting increasing attention due to the fast development of electric vehicles and smart devices. Carbon-based electrodes have advanced the fast development of EES devices while being limited by their low volumetric performance because of their porous structure and the resulting low density. This paper aims to clarify the importance of the volumetric performance and review the most recent progress on advanced EES devices with a high volumetric performance. Strategies for improving the volumetric performance, particularly with carbon-based materials are also proposed here. The transformation of normal low-density carbons to high-density ones through the assembly of different building blocks is highlighted as a promising remedy for this issue, and their applications in next-generation EES devices (Li-S, Li-air, Na-ion, etc.) are also discussed.
This work reports the highly dense graphene/sulfur assembly for compact Li-S batteries with high volumetric capacity, while retaining the good structural stability and conductivity. This dense assembly was prepared by a reduction-triggered self-assembly of graphene oxide with simultaneously deposition of sulfur followed by a unique evaporation-induced spatially volume shrinkage. Such a novel assembly has an ultrahigh density, delivering an unprecedented high volumetric capacity that is much higher than common carbon/sulfur cathodes. In particular the unique spatial confinement derived from the shrinkage of graphene/sulfur assembly is favorable for the stabilization of sulfur cathodes.
A small volumetric capacitance resulting from a low packing density is one of the major limitations for novel nanocarbons finding real applications in commercial electrochemical energy storage devices. Here we report a carbon with a density of 1.58 g cm(-3), 70% of the density of graphite, constructed of compactly interlinked graphene nanosheets, which is produced by an evaporation-induced drying of a graphene hydrogel. Such a carbon balances two seemingly incompatible characteristics: a porous microstructure and a high density, and therefore has a volumetric capacitance for electrochemical capacitors (ECs) up to 376 F cm(-3), which is the highest value so far reported for carbon materials in an aqueous electrolyte. More promising, the carbon is conductive and moldable, and thus could be used directly as a well-shaped electrode sheet for the assembly of a supercapacitor device free of any additives, resulting in device-level high energy density ECs.
We review the development of carbon???sulfur composites and the application for Li???S batteries. Discussions are devoted to the synthesis approach of the various carbon???sulfur composites, the structural transformation of sulfur, the carbon???sulfur interaction and the impacts on electrochemical performances. Perspectives are summarized regarding the synthesis chemistry, electrochemistry and industrial production with particular emphasis on the structural optimization of carbon???sulfur composites.
Keeping Electrolytes in Porous Electrodes
Electrochemical capacitors (ECs) can rapidly charge and discharge, but generally store less energy per unit volume than batteries. One approach for improving on the EC electrodes made from porous carbon materials is to use materials such as chemically converted graphene (CCG, or reduced graphene oxide), in which intrinsic corrugation of the sheets should maintain high surface areas. In many cases, however, these materials do not pack into compact electrodes, and any ECs containing them have low energy densities. Yang et al. (p. 534 ) now show that capillary compression of gels of CCG containing both a volatile and nonvolatile electrolyte produced electrodes with a high packing density. The intersheet spacing creates a continuous ion network and leads to high energy densities in prototype ECs.
Many applications proposed for graphene require multiple sheets be assembled into a monolithic structure. The ability to maintain structural integrity upon large deformation is essential to ensure a macroscopic material which functions reliably. However, it has remained a great challenge to achieve high elasticity in three-dimensional graphene networks. Here we report that the marriage of graphene chemistry with ice physics can lead to the formation of ultralight and superelastic graphene-based cellular monoliths. Mimicking the hierarchical structure of natural cork, the resulting materials can sustain their structural integrity under a load of >50,000 times their own weight and can rapidly recover from >80% compression. The unique biomimetic hierarchical structure also provides this new class of elastomers with exceptionally high energy absorption capability and good electrical conductivity. The successful synthesis of such fascinating materials paves the way to explore the application of graphene in a self-supporting, structurally adaptive and 3D macroscopic form.
Exceptional performance claims for electrodes used in batteries and electrochemical capacitors often fail to hold up when all device components are included.
Three-dimensional (3D) architectures of graphene are of interest in applications in electronics, catalysis devices, and sensors. However, it is still a challenge to fabricate macroscopic all-graphene 3D architectures under mild conditions. Here, a simple method for the preparation of 3D architectures of graphene is developed via the in situ self-assembly of graphene prepared by mild chemical reduction at 95 °C under atmospheric pressure without stirring. No chemical or physical cross-linkers or high pressures are required. The reducing agents include NaHSO(3), Na(2)S, Vitamin C, HI, and hydroquinone. Both graphene hydrogels and aerogels can be prepared by this method, and the shapes of the 3D architectures can be controlled by changing the type of reactor. The 3D architectures of graphene have low densities, high mechanical properties, thermal stability, high electrical conductivity, and high specific capacitance, which make them candidates for potential applications in supercapacitors, hydrogen storage and as supports for catalysts.
Lithium-ion batteries (LIBs) are extensively used in numerous portable devices such as smart-phones and laptops. However, current LIBs based on the conventional intercalation mechanism cannot meet the requirements of the electronics industry and electric vehicles although they are approaching their theoretical capacity. Therefore, it is extremely urgent to seek for systems with higher energy densities. Among various promising candidates, lithium-sulfur (Li-S) batteries with a high theoretical capacity are very attractive. However, the commercial use of Li-S batteries still faces obstacles such as the low electrical conductivity of sulfur and lithium sulfide and the dissolution of polysulfides. The introduction of nanocarbon materials into Li-S batteries sheds light on the efficient utilization of sulfur by improving the conductivity of the composites and restraining the shuttle effect of polysulfides. Here, we give a brief review of recent progress on carbon/sulfur composites, especially carbon nanotube-, graphene-and porous carbon-based hybrids, new insights on the relationships between the structure and the electrochemical performance, and propose some important aspects for the future development of Li-S batteries.
Three-dimensional (3D) graphene-assembled monoliths (GAs), especially ones prepared by self-assembly in the liquid phase, represent promising forms to realize the practical applications of graphene due to their high surface utilization and operability. However, the understanding of the assembly process and structure control of 3D GAs, as a new class of carbon materials, is quite inadequate. In this Perspective, we give a demonstration of the assembly process and discuss the key factors involved in the structure control of 3D GAs to pave the way for their future applications. It is shown that the assembly process starts with the phase separation, which is responsible for the formation of the 3D networked structure and liquid phase as the spacers avoid the parallel overlap of graphene layers and help form an interlinked pore system. Well-tailored graphene sheets and selected assembly media must be a precondition for a well-controlled assembly process and microstructure of a 3D GA. The potential applications in energy storage featuring high rate and high volumetric energy density demonstrate advantages of 3D GAs in real applications.
With the advent of flexible electronics, flexible lithium-ion batteries have attracted great attention as a promising power source in the emerging field of flexible and wearable electronic devices such as roll-up displays, touch screens, conformable active radio-frequency identification tags, wearable sensors and implantable medical devices. In this review, we summarize the recent research progress of flexible lithium-ion batteries, with special emphasis on electrode material selectivity and battery structural design. We begin with a brief introduction of flexible lithium-ion batteries and the current development of flexible solid-state electrolytes for applications in this field. This is followed by a detailed overview of the recent progress on flexible electrode materials based on carbon nanotubes, graphene, carbon cloth, conductive paper (cellulose), textiles and some other low-dimensional nanostructured materials. Then recently proposed prototypes of flexible cable/wire type, transparent and stretchable lithium-ion batteries are highlighted. The latest advances in the exploration of other flexible battery systems such as lithium-sulfur, Zn-C (MnO2) and sodium-ion batteries, as well as related electrode materials are included. Finally, the prospects and challenges toward the practical uses of flexible lithium-ion batteries in electronic devices are discussed.
An effective strategy was developed to obtain flexible Li-S battery electrodes with high energy density, high power density, and long cyclic life by adopting graphene foam.•The graphene foam can provide a highly electrical conductive network, mechanical support and enough space for high sulfur loading.•The electrode with 10.1 mg cm−2 sulfur loading could deliver extremely high areal capacity of 13.4 mAh cm−2 at 300 mA g−1 and 9.3 mAh cm−2 at 1500 mA g−1.•Stable cyclic performance with ~0.07% capacity decay per cycle over 1000 cycles at 1500 mA g−1 was obtained.
Hollow nanostructures afford intriguing structural features ranging from large surface area and fully exposed active sites to kinetically favorable mass transportation and tunable surface permeability. The unique properties and potential applications of graphene nanoshells with well-defined small cavities and delicately designed graphene shells are strongly considered. Herein, a mesoscale approach to fabricate graphene nanoshells with a single or few graphene layers and quite small diameters through a catalytic self-limited assembly of nanographene on in situ formed nanoparticles was proposed. The graphene nanoshells with a diameter of ca. 10-30 nm and a pore volume of 1.98 cm(3) g(-1) were employed as hosts to accommodate the sulfur for high-rate lithium-sulfur batteries. A very high initial discharge capacity of 1520 mAh g(-1), corresponding to 91% sulfur utilization rate at 0.1 C, was achieved on a graphene nanoshell/sulfur composite with 62 wt % loading. A very high retention of 70% was maintained when the current density increased from 0.1 C to 2.0 C, and an ultraslow decay rate of 0.06% per cycle during 1000 cycles was detected.
A study was conducted to propose the concept of in-situ chemical vapor deposition (CVD) self-assembly of hierarchical vine-tree-like CNTs (VT-CNTs) and evaluate their applications for lithium-sulfur batteries. CNTs were selected as the of the model system is due to the fact that they were one of the most typical low dimensional building blocks, which demonstrated impressive properties and commercial applications in lithium ion batteries and nanocomposites. Single-walled CNTs (SWCNTs) were selected as the flexible ?vine? and multi-walled CNTs (MWCNTs) were employed as the rigid ?tree? for the fabrication of VT-CNT nanostructures. Catalyst nanoparticles (NPs) with bimodal size distribution were required to achieve the direct growth of such VT-CNTs.
As an important form of graphene assembled in macroscale, the graphene-based membrane attracts much attention due to its easy manipulation and various potential applications. However, tailoring the microstructure of these membranes is hard to achieve and the surface utilization of graphene layers is low. By analyzing the drying process for the wet graphene oxide membrane (GOM), it is found that the trapped water in freshly formed GOM actually provides potential forces to tune its microstructure. According to the phase diagram of pure water, with a reduced pressure, the trapped water boils seriously and then transforms into ice crystal instantaneously around the triple point. This sudden phase change across the triple point provides strong forces to change and fix the microstructure of GOM. In this study, the ordinary evaporation drying process for the wet GOM is replaced with a two-stage drying process and the tightly layered structure of graphene membrane is turned into an open and grade structure. The obtained membrane shows high surface utilization. Thus, after reduction, the membrane possesses high adsorption capability towards various molecules, especially for heavy oil and lithium polysulfide products in the cathode of Li–S battery. Furthermore, the membrane shows high rate performance as the electrodes for supercapacitors.
Nitrogen-doped aligned CNT/graphene sandwiches are rationally designed and in-situ fabricated by a facile catalytic growth on bifunctional natural catalysts that exhibit high-rate performances as scaffolds for lithium-sulfur batteries, with a high initial capacity of 1152 mAh g(-1) at 1.0 C and a remarkable capacity of 770 mAh g(-1) can be achieved at 5.0 C. Such design strategy for materials opens up new perspectives to novel advanced functional composites, especially interface-modified hierarchical nanocarbons for broad applications.
N-doped activated carbons for electric double-layer capacitors were prepared from waste medium density fiberboard using K2CO3 activation. The effects of carbonization temperature, activation temperature, activation time and K2CO3/coke mass ratios on surface chemical compositions, pore structure and electrochemical performance of the resulting activated carbons were investigated. Results indicated that the activated carbons had nitrogen contents from 0.93-2.86%, Brunauer-Emmett-Teller specific surface areas from 569 to 1 027 m2/g and specific capacitances from 147 to 223 F/g, depending on carbonization and activation conditions. The maximum specific capacitance was attributed to a high surface area, optimum pore size, large pore volume and high N-5 (the pyrrolic nitrogen and pyridinic nitrogen in association with oxygen functionality) content.
Preventing the stacking of graphene is essential to exploiting its full potential in energy-storage applications. The introduction of spacers into graphene layers always results in a change in the intrinsic properties of graphene and/or induces complexity at the interfaces. Here we show the synthesis of an intrinsically unstacked double-layer templated graphene via template-directed chemical vapour deposition. The as-obtained graphene is composed of two unstacked graphene layers separated by a large amount of mesosized protuberances and can be used for high-power lithium-sulphur batteries with excellent high-rate performance. Even after 1,000 cycles, high reversible capacities of ca. 530 mA h g(-1) and 380 mA h g(-1) are retained at 5 C and 10 C, respectively. This type of double-layer graphene is expected to be an important platform that will enable the investigation of stabilized three-dimensional topological porous systems and demonstrate the potential of unstacked graphene materials for advanced energy storage, environmental protection, nanocomposite and healthcare applications.
The sp2-hybridized nanocarbon (e.g., carbon nanotubes (CNTs) and graphene) exhibits extraordinary mechanical strength and electrical conductivity but limited external accessible surface area and a small amount of pores, while nanostructured porous carbon affords a huge surface area and abundant pore structures but very poor electrical conductance. Herein the rational hybridization of the sp2 nanocarbon and nanostructured porous carbon into hierarchical all-carbon nanoarchitectures is demonstrated, with full inherited advantages of the component materials. The sp2 graphene/CNT interlinked networks give the composites good electrical conductivity and a robust framework, while the meso-/microporous carbon and the interlamellar compartment between the opposite graphene accommodate sulfur and polysulfides. The strong confinement induced by micro-/mesopores of all-carbon nanoarchitectures renders the transformation of S8 crystal into amorphous cyclo-S8 molecular clusters, restraining the shuttle phenomenon for high capacity retention of a lithium-sulfur cell. Therefore, the composite cathode with an ultrahigh specific capacity of 1121 mAh g−1 at 0.5 C, a favorable high-rate capability of 809 mAh g−1 at 10 C, a very low capacity decay of 0.12% per cycle, and an impressive cycling stability of 877 mAh g−1 after 150 cycles at 1 C. As sulfur loading increases from 50 wt% to 77 wt%, high capacities of 970, 914, and 613 mAh g−1 are still available at current densities of 0.5, 1, and 5 C, respectively. Based on the total mass of packaged devices, gravimetric energy density of GSH@APC-S//Li cell is expected to be 400 Wh kg−1 at a power density of 10 000 W kg−1, matching the level of engine driven systems.
Graphene oxide is effectively reduced by H2S and a graphene/sulfur hybrid is simultaneously obtained. The resulting graphene sheets interlink, forming a curly and porous structure that accompanies a uniform distribution of sulfur on the graphene sheets. The product is a promising electrode for Li-S batteries and provides a novel strategy for the removal of H2S.
Graphene-sulfur (G-S) hybrid materials with sulfur nanocrystals anchored on interconnected fibrous graphene are obtained by a facile one pot strategy using a sulfur/carbon disulfide/alcohol mixed solution. The reduction of graphene oxide and the formation/binding of sulfur nanocrystals were integrated. The G-S hybrids exhibit a highly porous network structure constructed by fibrous graphene, many electrically conducting pathways and easily tunable sulfur content, which can be cut and pressed to pellets to be directly used as lithium-sulfur battery cathodes without using metal current-collector, binder and conductive additive. The porous network and sulfur nanocrystals enable rapid ion transport and short Li+ diffuse distance, the interconnected fibrous graphene provides highly conductive electron transport pathways, and the oxygen-containing (mainly hydroxyl/epoxide) groups show strong binding with polysulfides preventing their dissolution into electrolyte based on first-principles calculations. As a result, the G-S hybrids show a high capacity, an excellent high-rate performance and a long life over 100 cycles. These results demonstrate the great potential of this unique hybrid structure as cathodes for high performance lithium-sulfur batteries.
Rechargeable Li/S batteries have attracted significant attention lately due to their high specific energy and low cost. They are promising candidates for applications, including portable electronics, electric vehicles and grid-level energy storage. However, poor cycle life and low power capability are major technical obstacles. Various nanostructured sulfur cathodes have been developed to address these issues, as they provide greater resistance to pulverization, faster reaction kinetics and better trapping of soluble polysulfides. In this review, recent developments on nanostructured sulfur cathodes and mechanisms behind their operation are presented and discussed. Moreover, progress on novel characterization of sulfur cathodes is also summarized, as it has deepened the understanding of sulfur cathodes and will guide further rational design of sulfur electrodes.
The innate character of graphene, defined as the basic unit of sp(2) carbon materials, provides opportunities for the design and construction of carbon nanostructures with tuned properties. Here, we report a novel three-dimensional graphene macroassembly with a core-shell structure starting from reduced graphene oxides (RGO) by a one-pot self-assembly process under very mild conditions. In the presence of KMnO(4), such an assembly process is initiated by low-temperature heating below 100 degrees C at atmospheric pressure and is totally free from a severe hydrothermal process. Such a core-shell structure is characterized by a porous core and layered membrane shell, and the macro-morphology and infrastructure of the macroassembly are well controllable and tunable. The macroassembly presented here and its self-assembly preparation directly from graphene nanosheets present the first example of the simultaneous formation of a coaxial hybrid infrastructure in a graphene-based nanostructured material, and such a core-shell structured macroassembly shows potential for use in energy storage and other applications.
Securing our energy future is the most important problem that humanity faces in this century. Burning fossil fuels is not sustainable, and wide use of renewable energy sources will require a drastically increased ability to store electrical energy. In the move toward an electrical economy, chemical (batteries) and capacitive energy storage (electrochemical capacitors or supercapacitors) devices are expected to play an important role. This Account summarizes research in the field of electrochemical capacitors conducted over the past decade.
Li-ion batteries have transformed portable electronics and will play a key role in the electrification of transport. However, the highest energy storage possible for Li-ion batteries is insufficient for the long-term needs of society, for example, extended-range electric vehicles. To go beyond the horizon of Li-ion batteries is a formidable challenge; there are few options. Here we consider two: Li-air (O(2)) and Li-S. The energy that can be stored in Li-air (based on aqueous or non-aqueous electrolytes) and Li-S cells is compared with Li-ion; the operation of the cells is discussed, as are the significant hurdles that will have to be overcome if such batteries are to succeed. Fundamental scientific advances in understanding the reactions occurring in the cells as well as new materials are key to overcoming these obstacles. The potential benefits of Li-air and Li-S justify the continued research effort that will be needed.
Aligned carbon nanotube/sulfur composite cathodes with high sulfur content for lithium-sulfur batteries
- X B Cheng
- J Q Huang
- Q Zhang
- XB Cheng
A graphene foam electrode with high sulfur loading for flexible and high energy Li-S batteries
- G M Zhou
- L Li
- C Q Ma
- GM Zhou
Carbon-sulfur composites for Li-S batteries: status and prospects
- D W Wang
- Q C Zeng
- G M Zhou
- DW Wang