CARBON NANOMATERIALS FOR RECHARGEABLE LITHIUM SULFUR BATTERIES
´ 2 and Vinodkumar Etacheri2 A´lvaro Don˜oro1,2, Daniel Cı´ntora-Juarez 1
Universidad Auto´noma de Madrid, Facultad de Ciencias, Cantoblanco, Madrid, Spain 2IMDEA Materials Institute, Madrid, Spain
CHAPTER OUTLINE 11.1 Introduction ............................................................................................................................... 279 11.2 Lithium Sulfur Batteries ............................................................................................................ 280 11.2.1 Electrochemical Characteristics of Li S Batteries ................................................... 280 11.2.2 Drawbacks of Li S Batteries ................................................................................. 282 11.3 Integration of Sulfur in Li S Batteries......................................................................................... 282 11.3.1 Physical Methods ................................................................................................. 283 11.3.2 Chemical Methods................................................................................................ 283 11.4 Carbon sulfur Hybrid Cathodes .................................................................................................. 284 11.4.1 Zero-dimensional Carbon Materials ........................................................................ 284 11.4.2 One-dimensional Carbon Materials ......................................................................... 286 11.4.3 Two-dimensional Carbon Materials ......................................................................... 289 11.4.4 Porous Carbon Materials ....................................................................................... 292 11.4.5 Hierarchical and Hybrid Carbon Materials ............................................................... 294 11.5 Carbon-based Anodes for Lithium Sulfur Batteries ...................................................................... 297 11.5.1 Graphite Anodes................................................................................................... 298 11.5.2 Hard Carbon Anodes ............................................................................................. 301 11.5.3 Composite Anodes of Lithium with Carbon.............................................................. 302 11.6 Conclusions............................................................................................................................... 303 References ......................................................................................................................................... 303 Further Reading .................................................................................................................................. 309
11.1 INTRODUCTION One of the major challenges of modern society is the generation of energy from sustainable sources. Currently, about 70% of the world’s energy supply relies on fossil fuels, and their prolonged exploitation causes global warming associated with increased greenhouse gas emissions . Since the availability of renewable energy sources such as sunlight and wind varies greatly, energy Carbon Based Nanomaterials for Advanced Thermal and Electrochemical Energy Storage and Conversion. DOI: https://doi.org/10.1016/B978-0-12-814083-3.00011-1 © 2019 Elsevier Inc. All rights reserved.
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FIGURE 11.1 Specific energy and power of various energy storage devices, and their applicability in electric vehicles. Reproduced with permission from V. Srinivasan, D. Hafemeister, B. Levi, M. Levine, P. Schwartz, Batteries for vehicular applications, AIP Conf. Proc. 1044 (2008) 283 296 . Copyright 2008, AIP Publishing.
storage is critical for their efficient utilization. Although there are significant advances in the area of renewable energy generation, progress in the area of energy storage is still lagging behind. Despite that recent developments are promising, more advancement in energy and power densities must be achieved for practical applications . This very same reason has aroused a huge interest in developing high performance energy storage devices [1,3,4]. Development of high-energy density and eco-friendly batteries capable of fast-charging is one of the main challenges of modern electrochemistry. Among several types of energy storage devices, Liion batteries are the most common and promising system. Intense studies and constant optimization in the past decades have allowed them to dominate the market of portable devices, electric vehicles, and implantable medical devices [4,5]. Current-generation Li-ion batteries consist of graphite negative electrode (anode) and lithium transition metal compounds (LiCoO2, LiFePO4, LiMn2O4, LiNiMnCoO2, etc.) as positive electrode (cathode) material. Unfortunately, the intercalation electrochemistry of Li1 ions leads to a low gravimetric/volumetric energy and power densities. When comparing the energy and power density of current-generation battery systems (Fig. 11.1), it is clear that Li-ion batteries are far behind from meeting the requirements of future systems including long-range electric vehicles [4,7 10]. This strengthens the necessity of studying recently developed energy storage systems such as lithium sulfur (Li S) and lithium oxygen (Li O2) batteries.
11.2 LITHIUM SULFUR BATTERIES 11.2.1 ELECTROCHEMICAL CHARACTERISTICS OF LI S BATTERIES Rechargeable Li S batteries recently attracted significant interest compared to other secondary batteries [11 14]. The main advantage of this system is the high specific capacity of lithium metal
11.2 LITHIUM SULFUR BATTERIES
(3861 mAh/g) and sulfur cathode (1673 mAh/g), allowing them to reach a theoretical specific energy of 2600 Wh/kg. Although the cell voltage is relatively low (2.2 V), energy density of Li S system clearly exceeds those of commercial Li-ion batteries. This resulted mainly due to the difference in the energy storage mechanism of Li-ion and Li S battery systems. Rechargeable Li S batteries consist of carbon sulfur hybrid cathode and Li-metal anode (Fig. 11.2). Unlike the single-electron transfer intercalation process in conventional Li-ion batteries, charge discharge process of Li S batteries involves a multielectron process that helps to achieve high-energy density. Li-ion battery electrode materials undergo crystal structure change during charge discharge process and first-cycle irreversible capacity loss. Whereas, elemental sulfur of the Li S battery cathode undergo conversion reaction to form lithium sulfide, and lithium dissolution deposition reaction occurs at the anode . Elemental sulfur, which is commonly used as the active cathode material of Li S system, is light-weight and usually encapsulated in a conductive carbon host that can also accommodate the volume change during charge discharge process. Low cost, abundance, and nontoxicity of sulfur are additional advantages for the commercialization of rechargeable Li S batteries [3,4,16 19]. During the discharge of a Li S battery, Li-ions react with sulfur to form Li2S as the final reduction product. In general, two distinct stages can be identified in the discharge voltage profile of Li S batteries. The first plateau at 2.4 V (Fig. 11.3) corresponds to soluble long chain polysulfide (Li2Sx) formation (4 , x , 8). The second plateau at a lower potential of 2.1 V is associated with the formation of short chain polysulfides (Li2S2 and Li2S) in a kinetically slow, liquid solid reaction, where the total conversion is impeded by the solid nature of the final products.
FIGURE 11.2 Schematic of rechargeable Li S battery.
CHAPTER 11 CARBON NANOMATERIALS
FIGURE 11.3 Charge discharge profiles of a Li S battery and corresponding cathode reactions. Reproduced with permission from D.W. Wang, Q. Zeng, G. Zhou, L. Yin, F. Li, H.M. Cheng, et al., Carbon sulfur composites for Li S batteries: status and prospects, J. Mater. Chem. A. 1 (2013) 9382 9394 . Copyright 2013, Royal Society of Chemistry.
11.2.2 DRAWBACKS OF LI S BATTERIES Despite their high theoretical energy density, Li S batteries have the following serious drawbacks that hinder their commercialization and practical application [17,21 24]: 1. Insulating nature of sulfur (electronic conductivity of 5 3 10230 S/cm at 25 C) and low electronic conductivity of lithium polysulfides and Li2S lowers their electrochemical utilization and limits charge discharge rates. 2. Volume changes during charge discharge can cause the pulverization of the active materials and a consequential capacity fading during long cycling. 3. Continuous loss of sulfur due to the formation of soluble high-order polysulfides and their shuttling between the electrodes affect the coulombic efficiency and cycling stability of Li S cells on prolonged cycling. 4. Dendrite growth is usually caused by repeated stripping and deposition on the Li-metal anode in the presence of an organic liquid electrolyte. This could lead to internal short circuit that seriously compromises the safety of Li S battery system.
11.3 INTEGRATION OF SULFUR IN LI S BATTERIES Sulfur can be integrated into the cathode microstructure through various physical and chemical methods. The type of sulfur and its interaction with the carbon conductive additive highly depend on the method employed for electrode preparation. Physical methods usually result in the formation
11.3 INTEGRATION OF SULFUR IN LI S BATTERIES
of sulfur-coated and encapsulated microstructure, whereas, chemical methods form sulfur carbon hybrids containing C S bonds that often result in improved electrochemical performance.
11.3.1 PHYSICAL METHODS 220.127.116.11 Infiltration In this method, elemental sulfur is mixed with porous carbon and heat-treated under inert atmosphere. Sulfur converts to β-phase at 96 C and its transition to λ S occurs at 120 C during the heat treatment. Beyond this temperature, sulfur melts to form high viscosity molecular cyclic structures. Sulfur reaches its minimum viscosity when cyclo-S8 is formed around 155 C. This phasetransformation is essential for the melt diffusion process, given that sulfur needs to easily infiltrate through the pores driven by capillary forces. Nevertheless, complete impregnation is almost impossible because part of the sulfur might deposit on the carbon conductive additive. To solve this issue, some studies suggested vapor phase infiltration at a temperature higher than the boiling point of sulfur (445 C). Benefits of this method include removal of excess sulfur at the carbon surface and improved infiltration. Room temperature solubility of sulfur in nonpolar solvents such as CS2, benzene, and toluene provides another infiltration route by which sulfur crystallizes inside the pores during solvent evaporation.
18.104.22.168 Mechanical intrusion This method involves high-energy mixing of carbon and sulfur, which is one of the most employed synthetic routes to prepare sulfur-based cathode materials. The simplicity of the process makes it a low-cost and reliable synthetic method. Whether it is by high-energy ball milling or stirring, mechanical intrusion leads to physical adsorption of sulfur at the carbon surface, ensuring a highly homogeneous sulfur carbon composite.
11.3.2 CHEMICAL METHODS 22.214.171.124 Surface functionalization Surface modification of carbon with sulfur offers a convenient method for the integration of sulfur into Li S battery cathodes. This method offers the possibility to create chemical bonds with carbon that can directly affect the electrochemical performance of the cathode material. For instance, attachments of functional groups that can absorb polysulfides improve the stability of the cathode. Alternatively, doping with heteroatoms (mainly nitrogen) can also contribute to electronic conductivity enhancement of carbon nanostructures.
126.96.36.199 Chemical reaction deposition This synthetic method involves the treatment of sulfur-containing compounds with reducing or oxidizing agents, transforming sulfur ions to S0 resulting in the formation of sulfur carbon hybrid. Morphology and porosity of the carbon substrate can be varied to tune the electrochemical performance of the final sulfur carbon hybrid cathode.
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11.4 CARBON SULFUR HYBRID CATHODES Fabrication of carbon sulfur hybrid cathodes is a widely used method to improve the electrochemical performance of Li S batteries. In addition to increasing the electronic conductivity of the electrode, rational design of the cathode can be aimed for trapping polysulfides and to accommodate the volume change during charge discharge process [25 27]. Carbonaceous materials of various size and morphology played an important role in the development of high performance Li S battery cathodes [26,28 31]. Unique properties of carbon including high electronic conductivity, natural abundance, low cost, high chemical inertness, and tunable surface area/pore size enabled the fabrication of sulfur cathodes with desired electrochemical performance. Recent advances in the area of carbon nanomaterials such as graphene, fullerenes, carbon quantum dots, and carbon nanotubes (CNT) resulted in Li S battery cathodes with high energy and power density [16,17,20,21,24,32 39]. Based on the type of carbon used for hybrid sulfur cathode fabrication, advances in the area of Li S batteries are summarized below.
11.4.1 ZERO-DIMENSIONAL CARBON MATERIALS Achieving high sulfur loading while maintaining good specific capacity and cycling stability is the main challenge in the designing of carbon sulfur hybrid cathodes. In this regard, zero-dimensional carbon materials are very attractive due to their high specific surface area (SSA) which can provide active sites for sulfur incorporation. Carbon black (CB), a conductive additive in battery electrodes, is an attractive material for fabricating hybrid sulfur cathodes. The semigraphitic/amorphous structure of carbon black consists of spherical primary particles united through van der Waals interactions. Zhang et al. demonstrated the fabrication of carbon black based hybrid sulfur cathode for Li S batteries . While comparing the electrochemical performance of two CB sulfur hybrids, ball-milled composites displayed very poor performance, which can be explained by poor sulfur impregnation into individual carbon particles (Fig. 11.4A). On the other hand, heat treatment of the composite leads to sulfur infiltration into the pores (Fig. 11.4B), resulting in enhanced electrochemical performance (Fig. 11.4C and D). Another interesting 0D carbon material for Li S batteries is carbon nanospheres. Confining and encapsulating sulfur in hollow or solid carbon nanospheres (Fig. 11.5) presents several advantages in terms of controllable permeability and increased packing density. Thus, when used as cathode material, carbon nanospheres not only accommodate high sulfur loadings and withstand its volume expansion during cycling, but also trap polysulfides and avoid their dissolution into the electrolyte. Additionally, Li-ion diffusion and electronic conductivity are significantly improved. Impregnation through infiltration is the most used technique to uniformly load sulfur in 0D carbon nanomaterials [42 50]. Two stage method using melt difussion followed by vapor phase infiltration has also been studied [41,51]. Zhang et al. designed and synthesized sulfur impregnated, double-shell, hollow carbon nanospheres (DHCS S) via vapor phase infiltration . Additionally, a carbon black nanocomposite (CB S) was prepared by the same method to compare the electrochemical performance. DHCS S and CB S electrodes delivered capacities of 690 mAh/g and 250 mAh/g, respectively, after 100
11.4 CARBON SULFUR HYBRID CATHODES
FIGURE 11.4 SEM images of (A) ball milled and (B) thermally treated CB S composites. (C) Charge discharge profiles and (D) galvanostatic cycling performance of thermally treated CB S composite. Reproduced with permission from B. Zhang, C. Lai, Z. Zhou, X.P. Gao, Preparation and electrochemical properties of sulphur acetylene black composites as cathode materials, Electrochim. Acta 54 (2009) 3708 3713 . Copyright 2009, Elsevier.
cycles at a current density of 0.1C (165 mA/g). Rate performance of the composite with hollow structure was substantially better than the carbon black composite. These results clearly demonstrated the advantages of carbon nanospheres over conventional carbon black-based Li S cathodes. Similarly, Jayaprakash et al. reported a sulfur carbon nanocomposite where sulfur was vaporinfused into hollow carbon capsules with a mesoporous shell structure . The large void inner space of the nanospheres provided a SSA of 648 m2/g, allowing a final sulfur loading as high as 70%. This porous hybrid cathode proved to be highly stable, retaining 91% of the initial capacity after 100 cycles at a rate of C/2. Moreover, exceptional rate performance compared to previous reports and a high capacity of 450 mAh/g was obtained at a rate of 3C (5.1 A/g). Nazar et al. proposed an explanation for the effect of porosity and sulfur loading on the electrochemical performance . Their study proved that excess sulfur loading of activated porous hollow nanospheres (a-PCNS) causes performance loss due to pore blocking (Fig. 11.6B). In addition, they demonstrated that a high porosity is needed to adequately accommodate all sulfur. Otherwise, sulfur
CHAPTER 11 CARBON NANOMATERIALS
FIGURE 11.5 (A) SEM and (B) TEM images of sulfur-loaded hollow carbon nanospheres. Reproduced with permission from Y. Qu, Z. Zhang, X. Wang, Y. Lai, Y. Liu, J. Li, A simple SDS-assisted self-assembly method for the synthesis of hollow carbon nanospheres to encapsulate sulfur for advanced lithium sulfur batteries, J. Mater. Chem. A 1 (2013) 14306 14310 . Copyright 2013, Royal Society of Chemistry.
deposition on the external surface results in poor contact and capacity loss on prolonged cycling. Therefore, optimization of both porosity and hollow structure is essential to enhance the electrochemical performance of carbon sulfur hybrid cathode. Porosity and sulfur-loading optimized carbon nanospheres sample p-PCNS-M-70 (Fig. 11.6A) demonstrated an excellent specific capacity of 880 mAh/g at 1C (Fig. 11.6C), rate performance and a capacity retention of 89% after 100 cycles. These results match with their previous results of sulfur-impregnated mesoporous carbon nanoparticles (S-BMC). Moreover, a comparison between nanospheres and bulk carbon was made, restating the fact that the unique properties of nanospherical materials are key to ensure a remarkable electrochemical performance [53,54].
11.4.2 ONE-DIMENSIONAL CARBON MATERIALS One-dimensional materials provide smaller surface area compared to zero-dimensional materials, but offer the possibility of establishing long-range and linearly interconnected networks that can increase the electronic conductivity of the sulfur carbon composite. Two major carbon structures of this category are CNT and carbon nanofibers (CNFs). Although they share a common morphology, CNTs consist of one or more layers of graphene forming a cylindrical structure that permits sulfur incorporation in the external and internal surfaces. On the other hand, CNFs are composed of a wide variety of carbon building blocks such as amorphous carbon, quasi-cylindrical cones, cups, platelets, etc. Han et al. for the first time proposed the idea of using CNTs as an alternative to carbon black in order to improve the electrochemical performance of sulfur carbon hybrid cathode . In this case, electrode microstructure consists of an interconnected multiwall carbon nanotubes (MWCNT) network loaded with sulfur nanoparticles. Despite of the favorable microstructure for superior electronic conductivity and polysulfide adsorption, these MWCNT S hybrid electrodes exhibited only an initial discharge capacity of 500 mAh/g and retained 300 mAh/g after just 50 cycles. After a few
11.4 CARBON SULFUR HYBRID CATHODES
FIGURE 11.6 (A) STEM image of p-PCNS-M-70 (scale bar: 100 nm). (B) Galvanostatic cycling of three C/S cathodes: aPCNS-70 (black), a-PCNS-75 (red), and a-PCNS-80 (blue). (C) Rate capability of p-PCNS-M-70. Reproduced with permission from G. He, S. Evers, X. Liang, M. Cuisinier, A. Garsuch, L.F. Nazar, Tailoring porosity in carbon nanospheres for lithium sulfur battery cathodes, ACS Nano 7 (2013) 10920 10930 . Copyright 2013, American Chemical Society.
years, Yuan et al. reported a PTFE-coated S MWCNT composite with an initial discharge capacity of 1075 mAh/g and good capacity retention on long cycling (670 mAh/g after 60 cycles) . In this case, PTFE coating was found to have a crucial role in controlling the cycling stability by reducing polysulfide formation. Huge capacity fading during initial cycles was found to be a common issue of these composite electrodes, which forced to adopt alternative methodologies to address the problem [57 61]. The idea of sulfur impregnation into pores was also implemented while fabricating CNT and nanofiber-based sulfur carbon hybrid cathodes. For instance, Ji et al. suggested an extended heattreating procedure for the synthesis of carbon nanofiber sulfur (CNF S) composite to ensure complete filling of pores. Despite of the high initial capacity (1000 mAh/g) and reversibility of CNF S hybrid electrodes, these electrodes usually exhibit poor cycling stabilities at low and high current densities . One method to overcome this issue is the deeper sulfur impregnation through vaporphase sulfur infiltration at high temperature.
CHAPTER 11 CARBON NANOMATERIALS
FIGURE 11.7 (A, B) TEM images of sulfur-infused carbon nanofibers and (C) galvanostatic cycling at 200 mA/g. Reproduced with permission from J. Guo, Y. Xu, C. Wang, Sulfur-impregnated disordered carbon nanotubes cathode for lithium sulfur batteries, Nano Lett. 11 (2011) 4288 4294 . Copyright 2011, American Chemical Society.
Qiu’s group studied the effect of the processing temperature and confirmed improved cycling stability of vapor-infused CNF S electrodes (Fig. 11.7A, B) prepared at higher temperatures . Longer cycling stability in this case was attributed to the elimination of polysulfide shuttle (Fig. 11.7C). A different approach was reported by Cheng and coworkers, who claimed that no heat treatment was required to design a CNT/sulfur composite with high sulfur loading (up to 90%) through ball milling method . An areal capacity of 0.893 mAh/cm2 and a volumetric capacity of 1116 mAh/ cm3 were achieved through this method. Electrochemical performances were superior to that of CNT/sulfur composites containing lower sulfur loading. Further investigations were carried out on the surface modification of one-dimensional carbon containing sulfur cathodes. Zheng et al. demonstrated improved cycling stability of carbon nanofiber sulfur hybrid cathode coated with amphiphilic polymer . In this case, the hydrophobic part of the polymer strongly binds to the carbon surface and the hydrophilic part trap lithium polysulfides through oxygen lithium bonding (Fig. 11.8B). This resulted in a high coulombic efficiency and capacity retention of 80% after 300 cycles (Fig. 11.8C). These results are significantly better than the previous results of sulfur-loaded carbon nanofiber electrodes (Fig. 11.8A) .
11.4 CARBON SULFUR HYBRID CATHODES
FIGURE 11.8 Schematic of (A) unmodified and (B) polymer-modified sulfur-impregnated hollow carbon nanofibers before (yellow) and after (orange) discharge. (C) Galvanostatic cycling performance of modified and unmodified sulfur cathodes at C/2 rate. Reproduced with permission from G. Zheng, Q. Zhang, J.J. Cha, Y. Yang, W. Li, Z.W. Seh, et al., Amphiphilic surface modification of hollow carbon nanofibers for improved cycle life of lithium sulfur batteries, Nano Lett. 13 (2013) 1265 1270 . Copyright 2013, American Chemical Society.
11.4.3 TWO-DIMENSIONAL CARBON MATERIALS The family of 2D carbons consists of several materials with tunable functional properties that can be controlled by varying morphology, thickness, and defects . Graphene, one of the most promising 2D nanomaterials, consists of a single-atom thick sheet with hexagonally organized, sp2 hybridized carbon atoms. Based on the number of layers and its lateral dimensions, different graphene-based structures can be distinguished: multilayer, few-layer, nanosheets, nanoplatelets, nanoribbons, etc. Additionally, chemical modifications can be made, leading to graphene oxide (GO) or its reduced version (rGO) that can also be arranged in several structures mentioned above. Due to its unique structural and electronic properties (conductivity, surface area, mechanical robustness, flexibility, etc.), graphene and graphene-based materials are suitable candidates for their application in energy storage and conversion devices [66 69]. Their role in energy storage devices including Li S batteries can be in the form of active material, conducting agent, or structural component, etc., as shown in Fig. 11.9. Consequently, graphene can be considered as a potential alternative to other carbonaceous materials for the fabrication of sulfur carbon hybrid cathodes with excellent electrochemical performance.
CHAPTER 11 CARBON NANOMATERIALS
FIGURE 11.9 Schematic of graphene-based composite architectures. Reproduced with permission from R. Raccichini, A. Varzi, S. Passerini, B. Scrosati, The role of graphene for electrochemical energy storage, Nat. Mater. 14 (2015) 271 279 . Copyright 2014, Springer Nature.
Physical and/or chemical interaction between sulfur and graphene determines the role of graphene in the composite’s structure. While acting as a support, graphene wraps sulfur particles to form a highly conductive electrode. Various strategies have been demonstrated to design graphenebased nanocomposites; impregnation through melt diffusion was one of the first approaches used [70,71]. Other explored methods include sonication-assisted chemical reduction of Na2S2O3, and in situ chemical oxidation of Na2Sx with GO . Nazar and coworkers designed a graphene composite with high sulfur loading (87%) and an initial discharge capacity of 705 mAh/g . Ji et al. also reported a modified version of this method, which included Na2Sx oxidation on GO at 155 C to promote sulfur melt diffusion into the pores . Nanocomposite prepared by this method performed excellently both at low and high charge discharge rates, delivering a capacity of 1247 mAh/g at 0.02C and 550 mAh/g at 1C rate, with large reversibility and high coulombic efficiency. As an alternative to this method, Wang et al. coated sulfur particles with graphene layers and a polyethylene glycol interlayer (Fig. 11.10A, B) that acted as a flexible matrix . This architecture alleviates the strain resulting from volume change during charge discharge process, causing good cycling stability (Fig. 11.10C) at both C/5 and C/2 rates compared to non-PEGcoated graphene sulfur composite (Fig. 11.10D). Cao and coworkers designed a functionalized graphene sheet-sulfur nanocomposite (FGSS) with an additional Nafion polymer coating that was essential to improve the cycling stability . Protective role of this polymer resulted from the presence of sulfonate anions that acted as an electrostatically repulsive barrier against polysulfide anions, and at the same time facilitated insertion of Li-ions into the active material. This effect clearly affected the long cycling stability, considering that the coated electrode was able to retain 80% of the initial capacity after 50 cycles, whereas the uncoated electrode maintained only 52% under similar experimental conditions. Alternative sandwich-like designs, where graphene is not an intrinsic part of the active material were also reported. For instance, Zhou and coworkers fabricated a sandwich structure composed of pure
11.4 CARBON SULFUR HYBRID CATHODES
FIGURE 11.10 (A) Schematic and (B) SEM image of graphene-wrapped PEG-coated sulfur particles. Galvanostatic cycling performance of (C) PEG-coated and (D) non-PEG-coated sulfur-graphene composite. Reproduced with permission from H. Wang, Y. Yang, Y. Liang, J.T. Robinson, Y. Li, A. Jackson, et al., Graphene-wrapped sulfur particles as a rechargeable lithium sulfur battery cathode material with high capacity and cycling stability, Nano Lett. 11 (2011) 2644 2647 . Copyright 2011, American Chemical Society.
sulfur, graphene current collector (GCC/S), and a graphene-coated commercial polymer separator . They tested various cell configurations and the optimized design exhibited very high reversible capacities, rate performance, cycling stability, and coulombic efficiencies under current densities ranging from 0.3 to 6 A/g. Several other groups also developed bifunctional hybrid cathodes in which graphene acts as both conductive additive and polysulfide trap [71,77 79]. Zhao et al. covered mesoporous carbon CMK-3/S composite with a negatively charged layer of rGO (Fig. 11.11A) to improve the cycling stability, achieving specific capacities of 650 mAh/g and 550 mAh/g at 0.1C and 1C, respectively (Fig. 11.11B, C) . Similarly, fabrication of rGO coated graphene sulfur composite electrode was reported by Li and coworkers . Discharge capacities as high as 928 mAh/g after 100 cycles at 0.2 A/g and 667 mAh/g after 200 cycles at 1.6 A/g were observed. In this case, rGO coating was very effective in improving the overall electrochemical performance of graphene sulfur nanocomposite.
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11.4.4 POROUS CARBON MATERIALS Porous materials, in general, exhibit high surface area that can be tuned by controlling the pore size. Porous carbon materials are very attractive for sulfur cathodes due to the efficient utilization of large amount of active sites. Tuning their pore size and effective surface area can control reactivity of porous carbon toward various ions and molecules. Reaction mechanism of sulfur, and therefore the electrochemical performance, usually depends on the open pore size. Through DFT calculations Huang et al. demonstrated that the narrow size of micropores avoids solvent penetration into the carbon sulfur hybrid cathode . This ensures that solid (S)-solid (Li2S2/Li2S) lithiation/delithiation of sulfur molecules (S2-4) are confined inside the micropores, so that polysulfide dissolution can be totally avoided (Fig. 11.12A). On the contrary, mesopores can be easily filled with solvent molecules, leading to solid (S)-liquid (PS ions)-solid (Li2S2/Li2S) lithiation due to Li-ion solvation, which results in the polysulfide formation and shuttle effect (Fig. 11.12B). Similar to other applications, such as electrocatalysis and molecular separation, homogeneity is a key factor for the effectiveness of porous carbon in Li S battery cathodes [81,82]. Therefore uniform distribution of tunable pores is required to control the transport of Li-ions and electrons to obtain the best electrochemical performance. Higher sulfur loading could be obtained in mesoporous carbon compared to microporous counterparts due to the bigger pore size. There has always
FIGURE 11.11 (A) Schematic of synthesis, (B) long cycling performance at 1C, and (C) rate capability of [email protected]
/S. Reproduced with permission from X.Y. Zhao, J.P. Tu, Y. Lu, J.B. Cai, Y.J. Zhang, X.L. Wang, et al., Graphene-coated mesoporous carbon/sulfur cathode with enhanced cycling stability, Electrochim. Acta 113 (2013) 256 262 . Copyright 2013, Elsevier.
11.4 CARBON SULFUR HYBRID CATHODES
FIGURE 11.12 Schematic of the sulfur lithiation mechanism in (A) micropores and (B) mesopores. Reproduced with permission from D.W. Wang, Q. Zeng, G. Zhou, L. Yin, F. Li, H.M. Cheng, et al., Carbon sulfur composites for Li S batteries: status and prospects, J. Mater. Chem. A. 1 (2013) 9382 9394 . Copyright 2013, Royal Society of Chemistry.
FIGURE 11.13 Schematic of (A) CMK-3/S hybrid cathode and (B) structural changes during discharge/charge process. (A, B) Reproduced with permission from X. Ji, K.T. Lee, L.F. Nazar, A highly ordered nanostructured carbon sulphur cathode for lithium sulphur batteries, Nat. Mater. 8 (2009) 500 506 . Copyright 2009, Springer Nature. (C E) Reproduced with permission from X. Li, Y. Cao, W. Qi, L.V. Saraf, J. Xiao, Z. Nie, et al., Optimization of mesoporous carbon structures for lithium sulfur battery applications, J. Mater. Chem. 21 (2011) 16603 16610 . Copyright 2011, Royal Society of Chemistry.
been a debate regarding the optimum sulfur loading and efficient utilization of carbon material. In 2009, Nazar’s group for the first time reported a stable Li S battery based on an ordered mesoporous CMK-3 carbon sulfur hybrid (Fig. 11.13A) cathode . In this work, sulfur was partially filled (60% 70% loading) in the pores through a melt diffusion process, so that sufficient empty space was available to accommodate the volume change during charge discharge process
CHAPTER 11 CARBON NANOMATERIALS
(Fig. 11.13B). A discharge capacity of 1005 mAh/g was initially obtained, which was later increased to 1320 mAh/g through a polymer coating procedure. Wang and coworkers later reported a sucrose-derived, mesoporous carbon-containing sulfur as a high performance cathode material . Although their work was focused on the advantages of using an ionic liquid electrolyte, they claimed that the mesoporous structure is beneficial for uniform sulfur distribution and effective polysulfide trapping. Following Nazar’s work, Liu et al. systematically studied the relevance of tunable pore size by synthesizing three mesoporous carbon composites with different pore sizes (MC22, MC12, and MC7) and comparing them with the above mentioned CMK-3 as carbon host reference . Among these samples, M22 loaded with 50% sulfur exhibited a clear improvement in capacity retention (650 mAh/g after 100 cycles). This was due to the fact that electrochemical process only depends on the reactions taking place at the sulfur carbon interface. Similarly, Li et al. designed a high sulfur-loaded (84%) mesoporous carbon host with interconnected channels . This unique structure allowed a high sulfur loading and mitigated sulfur volume expansion due to void volume available inside individual pores. Compared to completely filled mesoporous carbon (Fig. 11.13C, D), partial filling allowed improved contact with electrolyte solution that led to improved Li-ion diffusion (Fig. 11.13E), also permitting the accommodation of lithium polysulfides . Zhang et al. encapsulated 42 wt.% sulfur into the narrow micropores of carbon nanospheres (Fig. 11.14A), thus restricting the polysulfide formation . They were able to achieve a specific capacity of 890 mAh/g at low current density (200 mA/g), 730 mAh/g at high current density (1200 mA/g) (Fig. 11.14B), and a reversible capacity of 650 mAh/g after 500 cycles at 400 mA/g (Fig. 11.14C). Furthermore, Aurbach et al. carried out a study to evaluate the different routes of sulfur loading into microporous carbon materials, concluding that combining ball milling and melt diffusion was the most effective method to bind sulfur into the micropores of carbon . Their work also included a thorough analysis of the sulfur electrolyte ratio and its influence in the long cycling performance of the cell, claiming that the amount of electrolyte should be optimized in order to balance the ohmic drop. The combination of both mesopores and micropores in the same material was also studied. Since mesopores and micropores have their own unique role during the electrochemical process, bimodal carbon offered the advantage of optimizing both sulfur and carbon structure’s utilization. Mesopores facilitated improved Li-ion transport and polysulfide trapping, whereas micropores act as sulfur containers and provided high surface area active sites for the electrochemical reactions (Fig. 11.15A). Synergy between these effects can lead to high reversible capacity values at high current densities. Nevertheless, the cycling stability seems to be an important issue (Fig. 11.15B), which is evident from the poor capacity retention of these composite electrodes after 30 charge discharge cycles [87 89]. Su et al. provided a different approach by using microporous carbon paper as an interlayer between the separator and the sulfur cathode with the main objective of absorbing and trapping the polysulfides to enhance the long cycling stability .
11.4.5 HIERARCHICAL AND HYBRID CARBON MATERIALS Hierarchical and hybrid carbons loaded with sulfur are recently employed as efficient cathodes in Li S battery system. The use of building blocks with various sizes and morphologies resulted in
(B) Discharge capacity (mAh/g-sulfur)
200 mA/g 400 mA/g 800 mA/g
200 mA/g 1200 mA/g
600 400 200 0
40 60 Cycle number
600 400 200
Current density: 400 mA/g
200 0 0
800 Discharge capacity (mAh/g-sulfur)
40 60 Cycle number
200 300 Cycle number
FIGURE 11.14 (A) TEM image, (B) rate performance, and (C) galvanostatic cycling performance of microporous carbon nanospheres sulfur composite cathode. Reproduced with permission from B. Zhang, X. Qin, G.R. Li, X.P. Gao, Enhancement of long stability of sulfur cathode by encapsulating sulfur into micropores of carbon spheres, Energy Environ. Sci. 3 (2010) 1531 1537 . Copyright 2010, Royal Society of Chemistry.
FIGURE 11.15 (A) Schematic and (B) galvanostatic cycling performance of bimodal porous carbon sulfur hybrid cathode. (A) Reproduced with permission from G. He, X. Ji, L. Nazar, High “C” rate Li S cathodes: sulfur imbibed bimodal porous carbons, Energy Environ. Sci. 4 (2011) 2878 2883 . Copyright 2009, American Chemical Society. (B) Reproduced with permission from A. Rosenman, R. Elazari, G. Salitra, D. Aurbach, A. Garsuch, Li S cathodes with extended cycle life by sulfur encapsulation in disordered micro-porous carbon powders, J. Electrochem. Soc. 161 (2014) A657 A662 . Copyright 2011, Royal Society of Chemistry.
CHAPTER 11 CARBON NANOMATERIALS
FIGURE 11.16 (A) Schematic and (B) SEM image of a porous three-dimensional nitrogen-doped graphene electrode. (C) Schematic and (D) SEM image of a freestanding 3D graphene composite. (A, B) Reproduced with permission from X. Yu, J. Zhao, R. Lv, Q. Liang, C. Zhan, Y. Bai, et al., Facile synthesis of nitrogen-doped carbon nanosheets with hierarchical porosity for high performance supercapacitors and lithium sulfur batteries, J. Mater. Chem. A. 3 (2015) 18400 18405 . Copyright 2014, Royal Society of Chemistry. (C, D) Reproduced with permission from W. Deng, A. Hu, X. Chen, S. Zhang, Q. Tang, Z. Liu, et al., Sulfur-impregnated 3D hierarchical porous nitrogen-doped aligned carbon nanotubes as high-performance cathode for lithium sulfur batteries, J. Power Sources 322 (2016) 138 146 . Copyright 2016, Springer Nature.
the unique physical and chemical properties of the hierarchical materials. These hierarchical hybrid structures are attractive due to their improved electrochemical performance that arises from the highly conductive architecture for electron and ion transport, with enough inner void space to trap lithium polysulfides and to accommodate high sulfur loadings/volume expansion. These hierarchical nanocomposites require a great order within the structure to attain excellent electrochemical performance. Thus, tuning pore size distribution was an established method to produce hierarchical carbon structures for improved electrochemical performance where micropores, mesopores, and macropores capable of performing different roles in Li S batteries are part of the same hybrid electrode [91 94]. Although there are only a few studies on hierarchical electrode materials composed of a single type of building blocks, most of the reports were focused on 3D macroscopic graphene structures [95 97]. The design of a conductive scaffold integrated by graphene sheets created a highly porous interconnected framework (Fig. 11.16) that not only served as host for high sulfur loadings, encapsulating it into a three-dimensional structure, but also highly enhanced electron conductivity through directed conductive electron pathways. Alternatively, Zhang et al. designed a highly ordered hierarchically porous carbon in which mesopores were uniformly loaded with elemental sulfur through melt diffusion . In this electrode containing 50% sulfur, mesopores were in charge of containing sulfur/polysulfides; and
11.5 CARBON-BASED ANODES FOR LITHIUM SULFUR BATTERIES
empty interconnected macropores facilitate superior Li-ion diffusion. An initial capacity of 1193 mAh/g was obtained at 0.1C rate with a 96% coulombic efficiency and 75% capacity retention after 50 cycles. As expected, lower specific capacities were observed at higher current densities, and this hierarchical electrode recovered to a capacity of 750 mAh/g, even after cycling at a high rate of 2C. But it was Wei et al. who further improved the performance of Li S batteries by integrating multiple carbon materials in a single cathode [99,100]. They reported an all carbon hybrid nanostructure ([email protected]
) in which single-walled carbon nanotubes (SWCNT) and graphene were hybridized through chemical vapor deposition (CVD). This highly conductive network was additionally coated with activated pyrolytic carbon, introducing micropores and mesopores into the structure to accommodate a sulfur loading of 50% . Excellent rate performance and cycling stability (877 mAh/g after 250 cycles at 1C) was observed for this hierarchical electrode in comparison to graphene/SWCNT sulfur (GSH S) and nonactivated pyrolytic carbon GSH sulfur ([email protected]
S) nanocomposites. These results clearly demonstrated the advantages of hierarchical carbon electrodes in comparison to other nanocarbon-based Li S battery cathodes.
11.5 CARBON-BASED ANODES FOR LITHIUM SULFUR BATTERIES In the previous sections we reviewed the crucial role of different types of carbons used in sulfur/ carbon composite cathodes, mainly for improving the electrical conductivity, limiting the polysulfide dissolution, and buffering the volume expansion of the cathode of Li S batteries. Most S/C composite cathodes contain a considerable amount of carbon, typically in the range of 30 60 wt.%, which reduces the practical energy density of Li S battery system. In order to make an effective use of the practical capacity of sulfur-based cathodes, they must be paired with high capacity anodes to fabricate high-energy density batteries. Metallic lithium is one of the most attractive active materials for high-energy density batteries, including Li S and Li O2 systems, due to its high theoretical capacity (3860 mAh/g), low electrochemical potential (23.04 V vs Normal Hydrogen Electrode (NHE)), and low density (0.534 g/cm3). Nevertheless, the use of lithium metal in rechargeable batteries is hampered due to the safety risks associated with dendrite growth and related internal short circuit [101 103]. In the case of Li S batteries, irregular deposition of lithium upon charging and its reactivity with liquid electrolytes also causes: (1) irreversible capacity loss; (2) formation of inhomogeneous, unstable SEI; and (3) continuous reduction of polysulfide intermediates [104,105]. Combined approaches for lithium metal protection have been explored in recent years based on the understanding of SEI chemical composition and its formation mechanisms in the most common Li S battery electrolytes. The most general approaches consist on the ex situ formation of protecting coatings [106,107], controlled growth of SEI through the use of novel electrolytes and additives , and the application of solid electrolytes or interlayers able to block polysulfide diffusion to the anode [108 110]. Increasing efforts have been devoted in recent years for replacing lithium metal in order to realize Li-ion sulfur batteries. Carbon is considered as the promising alternative to metallic lithium for developing practical Li-ion sulfur batteries due to its low cost, abundance, and ease of fabrication methods.
CHAPTER 11 CARBON NANOMATERIALS
11.5.1 GRAPHITE ANODES Graphitic carbons were initially explored for replacing lithium and lithium alloys in rechargeable batteries, and today’s commercial Li-ion batteries contain graphite-based anodes (Qtheo: 372 mAh/ g). Graphite can undergo reversible Li-ion intercalation below 0.2 V in the most commonly used alkyl carbonate-based Li-ion battery electrolyte solutions. Irreversible capacity loss and volume expansion of graphite are markedly lower compared to alloying type anodes. These effects can be further minimized by tuning the morphology/porosity of the active material, and by using electrolyte additives . It is worth noting that the replacement of lithium by graphite causes an energy density reduction due to the higher redox potential and lower volumetric capacity of graphite (850 mAh/cm3). It has been also reported that graphite suffers exfoliation due to cointercalation of ether-based solvents widely used for Li S batteries . Solvents such as ethylene carbonate (EC) and dimethyl carbonate (DMC) that can prevent exfoliation of graphite cannot be used in Li S batteries with S/C cathodes due to the reactivity of carbonyl moieties with polysulfides and/ or Sx radicals . Nevertheless, initial reports of Li-ion sulfur batteries with carbon-based anodes considered standard LiPF6/carbonate-based electrolytes inherited from Li-ion battery technology. Prelithiation of battery anodes is a common strategy for providing a source of lithium when the battery is assembled in the charged state. This preconditioning step also alleviates the electrolyte consumption due to SEI formation on the anode that would affect the ionic conductivity of the electrolyte and disrupt the redox balance of the electrodes. A Li-ion sulfur battery using graphite anode in the presence of LiPF6 EC/DEC electrolyte was presented by He et al. . This battery consists of a lithium foil between the graphite anode and polypropylene separator for the in situ prelithiation of graphite. In this configuration, the battery provided an initial capacity of 1.75 mAh at a current density 0.25 mA/cm2, and retained 70% of its initial capacity after 60 galvanostatic cycles. Prelithiation of carbon sulfur hybrid cathode produces Li2S (1170 mAh/g), which incorporated otherwise would entail handling and production difficulties at atmospheric conditions due to the high reactivity of Li2S. Synthesis of Li2S/C hybrid cathode through prelithiation of a mesoporous carbon sulfur hybrid (S/MC) using stabilized lithium metal powder (SLMP, Fig. 11.17A, B) was reported by Zheng et al. . This Li-ion sulfur battery containing Li2S/C cathode and graphite anode in a conventional LiPF6-EC/DEC electrolyte provided a discharge capacity of 650 mAh/g (Fig. 11.17C), with capacity loss of 8% after 150 cycles. Although this work was mostly focused on the synthesis of Li2S/C cathode through prelithiation, a practical Li-ion sulfur battery with excellent electrochemical performance enabled by graphite anode in alkyl carbonate-based electrolyte solution was also demonstrated (Fig. 11.17D). Implementation of solid-state electrolytes is reported as an efficient approach for overcoming polysulfide shuttle, and also for enabling high temperature (70 C 100 C) operation of Li S batteries. It is also beneficial to improve the safety of Li S batteries by reducing the possibility of internal short circuit. An all solid-state Li-ion sulfur battery consisting of graphite anode, Li7P3S11 solid electrolyte, and Li2S/C composite cathode was proposed by Takeuchi et al. . Deposition of the solid electrolyte on the graphite anode was achieved by spark plasma sintering (SPS). A simple blending strategy was preferred for preparing the Li2S/C cathode containing Li7P3S11. Discharge and charge capacities of the full-cell reached c.715 and 600 mAh/g, at C/5 and C/2 rates, respectively. Solid-state electrolyte-coated graphite anode exhibited high specific capacities, low irreversible capacity, and improved cycle life compared to a blended graphite/Li7P3S11 anode.
11.5 CARBON-BASED ANODES FOR LITHIUM SULFUR BATTERIES
FIGURE 11.17 SEM image of (A) pristine S/MC electrode, (B) SLMP modified S/MC electrode, (C) discharge charge voltage profiles, and (D) galvanostatic cycling performance of full-cell containing Li2S/MC cathode and a graphite anode. Reproduced with permission from S. Zheng, Y. Chen, Y. Xu, F. Yi, Y. Zhu, Y. Liu, et al., In situ formed lithium sulfide/microporous carbon cathodes for lithium ion batteries, ACS Nano 7 (2013) 10995 11003 . Copyright 2013, American Chemical Society.
Based on the impedance and extended X-ray absorption fine structure (EXAFS) measurements of the solid electrolyte coated anode, it was concluded that a lower interfacial resistance between graphite and the solid electrolyte, as well as higher ion mobility of Li7P3S11 are responsible for the enhanced electrochemical performance. Wang et al. demonstrated a Li-ion sulfur battery composed of graphite anode, commercial Li2S cathode, and Li11x1yAlxTi2 xSiyP3 yO12 (LTAP) solid-state electrolyte membrane to separate two different electrolyte solutions (Fig. 11.18A) in contact with electrodes . The electrolyte used in the anode compartment consisted of 1 M LiPF6 in EC/DMC/DEC solvents (1:1:1 by volume), whereas the Li2S cathode was in contact with 1 M LiClO4 dissolved in tetrahydrofuran. This fullcell configuration delivered a specific capacity of 700 mAh/g after 20 cycles at 0.05C rate, with good coulombic efficiency, and low charge discharge polarization (Fig. 11.18B). Solvent cointercalation and exfoliation of unprotected graphite in a Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) - 1,3‑dioxolane (DOL) / 1,2‑dimethoxyethane (DME) electrolyte solution has been considered as a barrier for the application of this low-cost anode material in practical Li-ion sulfur batteries . Jeschull et al. identified polyacrylic acid sodium salt (PAA-Na) as an effective binder for stabilizing graphite electrodes in LiTFSI-DOL/DME electrolyte containing LiNO3 additive . The full-cell containing electrochemically lithiated graphite anode, PAA-Na binder, and sulfur carbon hybrid cathode delivered an initial capacity of 1080 mAh/g at C/10 rate; and 27%
CHAPTER 11 CARBON NANOMATERIALS
FIGURE 11.18 (A) Schematic of Li S battery composed of Li anode, organic electrolyte, LATP separator, and Li2S carbon cathode. (B) Charge discharge profiles of Li-ion sulfur battery containing Li2S cathode and graphite anode. Reproduced with permission from L. Wang, Y. Wang, Y. Xia, A high performance lithium-ion sulfur battery based on a Li2S cathode using a dual-phase electrolyte, Energy Environ. Sci. 8 (2015) 1551 1558 . Copyright 2012, American Chemical Society.
capacity was retained after 130 charge/discharge cycles. Performance of the Li-ion sulfur cell was superior to that of a cathode-limited Li S cell using a similar S/C hybrid electrode and lithium metal foil. Furthermore, application of PAA-Na binder in combination with LiNO3 additive promoted high coulombic efficiency and low self-discharge rate. Stable cycling of mechanically lithiated graphite in Li-ion sulfur batteries by applying the concept of highly concentrated electrolytes has been reported by Lv et al. . This was an extension of an idea originally proposed for preventing the cointercalation of propylene carbonate into layered ZrS2 cathode in a LiAsF6-based electrolyte . Full-cell assembled with mechanically lithiated graphite, carbon sulfur hybrid cathode, and 5 M LiTFSI/DOL electrolyte delivered an initial reversible capacity of 980 mAh/g, 81.3% of capacity retention and 97% coulombic efficiency after 100 cycles at 0.5C rate. More recently, electrochemically prelithiated mesocarbon microbeads (MCMB)-based anode has been tested in a full-cell with 5 M LiTFSI DOL/DME electrolyte and carbon sulfur hybrid cathode . This Li-ion sulfur cell delivered an initial capacity of 1031 mAh/g at 0.1C rate with 33% capacity fading after 105 charge discharge cycles. XPS studies confirmed the formation of a stable SEI layer on the graphite anode predominantly consisting of TFSI decomposition products, which might not provide effective protection toward a reaction with polysulfides. This was also evidenced by the considerable capacity fade and the coulombic efficiency exceeding 100%. It is worth noting that both electrochemical prelithiation in half-cell configuration and the use of high salt concentration electrolyte are impractical for lithium-ion sulfur cells due to the need of additional processing steps and expensive nature of LiTFSI salt. Ionic liquids (ILs) composed of ligands that coordinate strongly with the cations and/or anions of salts constitute another class of concentrated electrolytes applied in high-energy density batteries . This type of concentrated electrolytes offers an alternative to conventional nonaqueous electrolytes for Li-ion sulfur batteries due to their high electrochemical stability, high thermal stability, and excellent flame resistance. Although ionic liquids have been applied to Li-ion and Li S batteries, only a recent report by Li et al. describes the application of ionic liquid diluted by a nonpolar, ether-based cosolvent as a single-phase electrolyte for a Li-ion sulfur battery . The proposed
11.5 CARBON-BASED ANODES FOR LITHIUM SULFUR BATTERIES
FIGURE 11.19 (A) Schematic illustration (B) galvanostatic charge discharge curves, and (C) galvanostatic cycling performance of Li2S/graphite full-cell using solvate ionic liquid (IL) electrolytes. Reproduced with permission from Z. Li, S. Zhang, S. Terada, X. Ma, K. Ikeda, Y. Kamei, et al., Promising cell configuration for nextgeneration energy storage: Li2S/graphite battery enabled by a solvate ionic liquid electrolyte, ACS Appl. Mater. Interfaces 8 (2016) 16053 16062 . Copyright 2016, American Chemical Society.
cell consisted of an artificial graphite anode, Li2S/graphene cathode in solvate ionic liquid [Li (G4)x][TFSA]/HFE electrolyte; G4: tetraglyme, TFSA: bis(trifluoromethanesulfonyl)amide, HFE: 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, x 5 0.6, 0.8, 1.0 (Fig. 11.19A). The solvate ionic liquid [Li(G4)0.8][TFSA]/HFE provided the most stable performance in full-cell configuration. Thus, after initial activation to 4.2 V at C/48 rate (Fig. 11.19B), initial discharge capacity reached 809 mAh/g at C/12 rate. Despite the compatibility of ionic liquid electrolyte with both electrodes, Li-ion sulfur battery containing graphite anodes lost 50% of its initial capacity after 100 charge/discharge cycles (Fig. 11.19C).
11.5.2 HARD CARBON ANODES Hard carbons constitute another type of carbon material used as electrodes in rechargeable batteries. Despite a higher degree of turbostratic disorder compared to graphitic carbons, increased porosity of hard carbons translates into improved Li-ion storage capacity at the expense of lower volumetric
CHAPTER 11 CARBON NANOMATERIALS
capacity and sluggish ion diffusion. Additionally, hard carbons have a relatively larger interlayer distance than graphite, which is beneficial for buffering volumetric expansion and for preventing interlayer delamination during Li-ion insertion. Therefore, hard carbon anodes have been considered for matching the high capacity of sulfur-based cathodes and for avoiding the electrolyte decomposition associated to the lithium anode. Bru¨ckner et al. reported an “all carbon” anode composed of commercial hard carbon with MWCNTs, both deposited on carbon fibers . A full-cell composed of the prelithiated anode and hollow carbon spheres sulfur hybrid cathode in LiTFSI-LiNO3/DOL: DME electrolyte provided a discharge capacity of 1355 mAh/g after 10 cycles. Although a high coulombic efficiency of 99.1% was achieved for the 550th cycle, capacity faded to 60% of the initial value. Despite the moderate performance, authors have taken into account the commonly ignored design aspects such as limiting the excess of lithium in the anode to only 10%, as well as controlling the active material to electrolyte ratio. The same hard carbon-based anode was applied by Thieme et al. in a Li-ion sulfur battery with sulfur-carbide derived carbon hybrid cathode and Li2S6 polysulfide electrolyte additive . A high and stable capacity of 1134 mAh/g was obtained after 24 cycles at 0.1C. Notably, the Li-ion sulfur cell was operational for 4100 cycles with a discharge capacity fade of 50% and a considerable decay in voltage, yet outperforming the control battery of common configuration with lithium anode and polysulfide-free electrolyte.
11.5.3 COMPOSITE ANODES OF LITHIUM WITH CARBON Several types of composite lithium anodes have been proposed for lithium metal batteries, and they are rarely applied for Li S cells . Cheng et al. applied a graphene framework for controlling the deposition of lithium exposed to polysulfide in a half-cell configuration . Graphene powder was deposited over cooper current collector, and lithium was incorporated by galvanostatic plating in an ether-based electrolyte containing Li2S8 and LiNO3 additive. Lithium graphene hybrid anode demonstrated lower polarization, reduced impedance, less voltage fluctuations, and higher coulombic efficiency as compared to Li/Cu or Li/Li cells for up to 2000 extended plating/stripping tests. This notable half-cell performance was attributed to the porous structure of the graphene framework, which could limit irregular lithium deposition by distributing high current densities over a large surface area with extended electrical contacts. Availability of stabilized lithium metal powder (SLMP) not only facilitates the prelithiation of Li-free anodes, but also allows the fabrication of composite lithium anodes. Fan and coworkers reported a composite anode with stabilized lithium metal powder and hard carbon held together by polyvinylidene fluoride (PVDF) binder . The best results in terms of specific capacity (1300 mAh/g at 50 mA/g), rate capability, and cycle life (70% retention after 80 cycles at 200 mA/ g) were obtained by using 20 wt.% hard carbon in the Li C composite anode in a LiClO4 DOL: DME electrolyte. Although it was claimed that the higher anode surface area with SLMP would improve the charge transfer and reduce the shuttle effect compared to lithium foil, high surface area lithium powder without an SEI forming additive such as LiNO3 could cause its degradation upon extended cycling. A simple drop-casting method for coating graphene oxide (GO) on lithium using DMC as solvent was reported by Zhang et al. . Li S batteries were built with the modified anode, carbon sulfur hybrid cathode in LiTFSI-LiNO3/DOL-DME electrolyte. Charge discharge profiles of a Li S battery containing modified lithium anode demonstrated
similar features as typically found in the case of bare lithium anode. The use of GO-modified Li metal anode resulted in a discharge capacity of 707 mAh/g after 200 cycles. Plating/stripping and cycling tests of symmetric cells containing LiPF6-based electrolyte showed an improved stability and smooth surface after cycling in comparison to bare lithium.
11.6 CONCLUSIONS Without a doubt, carbon nanomaterials revolutionize the advancement of Li S battery research. The main application of various carbon nanostructures has been in the fabrication of sulfur carbon hybrid cathodes. It was demonstrated that carbon nanomaterials have multiple roles in deciding the electrochemical performance of Li S battery cathode. Increased surface area facilitates better integration of sulfur through physical or chemical interactions. Various morphologies such as hollow spheres, graphene, and hierarchical morphology resulted in the efficient protection of the active material sulfur from electrolyte attack. Tunable porosity of carbon nanomaterials aided the protection of sulfur in micropores, improved loading, and accommodation of volume expansion in mesopores and enhanced Li-ion diffusion through macropores. In addition to the fabrication of high-performance cathode materials, carbonaceous materials were also used for replacing Li-metal anodes in Li S batteries. Furthermore, the combined use of carbon sulfur hybrid cathodes and lithiated carbon anodes provide a cost-effective method for the development of high performance, safer, and long-lasting Li-ion sulfur batteries.
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FURTHER READING T. Tao, S. Lu, Y. Fan, W. Lei, S. Huang, Y. Chen, Anode improvement in rechargeable lithium sulfur batteries, Adv. Mater. 29 (2017) 1700542. M.Q. Zhao, X.F. Liu, Q. Zhang, G.L. Tian, J.Q. Huang, W. Zhu, et al., Graphene/single-walled carbon nanotube hybrids: one-step catalytic growth and applications for high-rate Li S batteries, ACS Nano 6 (2012) 10759 10769.