Multifunctional behaviour of graphite in lithium–sulfur batteries☆
Graphical abstract
Introduction
Rechargeable lithium-ion batteries (LIBs) have received ever-increasing attention as one of the most promising energy storage systems since their introduction in 1991 [1]. LIBs have been widely used in portable and smart devices and are the most talented candidates for large-scale applications in electric and hybrid vehicles (EVs and HEVs) [2]. However, even when fully developed, the highest specific energy storage that LIBs can deliver is low, around 300 Wh kg−1 [3]. Additionally, the cathode in LIBs, typically with cobalt, is costly [4]. Rechargeable lithium-sulfur batteries (LSBs) [5,6] as one of the most promising battery systems will play an increasingly important role as future battery systems in terms of the high theoretical capacity of 1672 mAh g−1, which is an order of magnitude higher than LIBs cathode materials such as lithium-nickel-manganese-cobalt-oxide (NMC) cathodes [3]. On the other side, sulfur (S) is abundant and a low-cost material [7], and in particular, the electrochemical reduction of S yields a high theoretical specific energy of 2561 Wh kg−1, compared to today's LIBs, 387 Wh kg−1, based on the reactions (1) and (2), respectively [8]. Therefore, as shown in Fig. 1a, the LSBs have very high specific energies compared to other rechargeable battery systems.
Although the theoretical specific energy of LSBs is significantly high, the projected practical energy is anticipated to be more than 50% alone that of state-of-the-art LIBs. The optimized cell can potentially have a specific energy of 500–600 Wh kg−1 [7]. Therefore, the LSBs have the opportunity to provide high specific energy at lower electrode mass loading (Fig. 1b) and also at a lower cost. This makes LSBs attractive for aviation and heavy tracks.
Research on LSBs began in the early 1960s by Herbert Danuta and Ulam Juliusz [9]. However, in 2009 Nazar group [10] achieved a significant innovation by introducing a mesoporous carbon host to encapsulate S, and then significant progress has been achieved over the last decade in suggesting a variety of LSBs systems [4,7,[11], [12], [13], [14], [15], [16], [17], [18]]. It is well-known that the success of commercial batteries is greatly dependent on the functional components of a battery, including the cathode and anode electrode materials, the electrolyte, and the separator. Regarding LSBs, the anode and cathode materials are usually lithium (Li) and S, respectively (Fig. 1c). The metallic Li with the lightweight (density = 0.53 g cm−3) high specific capacity (3860 mAh g−1) [4] and high volumetric capacity (2042 mAh cm−3) [19], is a well-known and common material as an anode for LSBs. However, Li metal anode in non-aqueous electrolytes forms dendritic deposits, leading to low Coulombic efficiency (CE), short cycle life, and safety concerns [20]. Although some strategies have been used to solve the aforementioned challenges, but several alternative materials such as intercalation anode, Gr [21,22], and alloy anodes, silicon (Si) [23] and tin (Sn) [24] have been studied as anodes of LSBs.
In the LSBs cathodes, S is electrically insulating (electronic conductivity: 5 × 10−30 S cm−1 at 25 °C) [25], and also not a Li-ion conductor, and since both electronic and ionic conductivities are critical in the electrochemical reaction, typically S is mixed with a conducting material such as carbon materials [[26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36]], graphene [37,38], polymers [[39], [40], [41]], metal-organic frameworks (MOFs) [42], and metal oxides [43], in order to increase conductivities and to maximize the amount of S in the electrodes. The Gr [44] can also be directly used with S (without any change in the structure) [45,46], or it can be expanded, EGr, as a well-organized host (consisting of graphene nano-sheets) and worthy electronic conductor of the S cathode [47,48].
As shown in Fig. 1d, the shuttle effect of soluble lithium polysulfides, LiPSs, (Li2Sx: 3 < x < 8), through separators to the anode side is well-known to be major problem in the LSBs [[49], [50], [51]]. Recently, some strategies have been suggested to enhance the electrochemical performance of LSBs by modifying the separator or introducing an interlayer between the cathode and the separator [[52], [53], [54]]. Besides many other interlayer materials [55], and separator modifiers [[56], [57], [58]], Gr also can be used as an interlayer and separator modifier of LSBs [[59], [60], [61]].
Therefore, according to the abovementioned discussions, Gr can be used as a multifunctional material in the LSBs as illustrated in Fig. 2. Mainly Gr can be used in three following parts of LSBs: (i) as cathode: Gr and its expanded form (EGr) along with S, (ii) as anode: for instance, when Li2S is employed as cathode, or besides with Li metal as a hybrid anode when the cathode is S/(Carbon or (E)Gr) composite, (iii) Gr can be employed as an interlayer between cathode and separator, and also it could modify separators for suppressing polysulfides. The progress of using Gr as a multifunctional material in the LSBs will be addressed in the following sections after a short introduction to Gr properties.
Section snippets
Graphite (gr) properties
Gr is a crystalline form and natural allotrope of carbon, has a layer structure [62] in which the atoms are arranged in a hexagonal pattern within each layer [63], and it has a crystal density of about 2.26 gr cm3 [64]. In Gr, carbon atoms are bonded, involved sp2 (trigonal) [65], to three other carbon atoms to form strong, two-dimensional layers (called namely as graphene) [66,67], that are enormously stable. However, each layer is linked by a weak van der Waals interaction produced by a
Graphite in the cathode materials of LSBs
As the main application, using Gr in the LSB cathodes has been getting more attention from studies at the beginning of LSBs discovery to date. In a patent, as one of the first studies, Cheng and West [80] mixed Gr with S as cathode materials of LSBs, and then Sion Power Corporation [45,46], as a pioneer company for developing LSBs, used cathodes containing Gr and S. The Gr/S cathode composite [45] demonstrated a 5C discharge rate with good retention of energy (60%) and capacity (80%) at −40 °C,
Graphite as anode materials of LSBs
Metallic Li is the most common anode material for LSBs, and it is gradually becoming the bottleneck for the commercialization of these batteries. The anode materials play a vital role in battery performance, and more attention is required to promote the anode performance. However, the metallic Li is very unstable, has high activity in the electrolyte, and can react easily with the shuttled intermediate LiPSs, facilitating the formation of a solid electrolyte interface (SEI) layer [134], and
Graphite as interlayer and separator modifier of LSBs
Many approaches have been projected to optimize the electrochemical performance of LSBs by modifying the separator or introducing an interlayer between the cathode and the separator [[52], [53], [54], [55],61,[156], [157], [158], [159], [160], [161], [162], [163], [164], [165], [166], [167], [168]]. The functionalization of a separator is crucial owing to the nonpolar feature of conventional separators containing PP/PE membranes, which can barely mitigate the diffusion of polar LiPSs [55].
Conclusions and outlook
LSBs are next-generation battery technology and have witnessed the vivifying interest in advanced materials that offer benefits for future mobility. Unfortunately, the commercialization of LSBs is obstructed primarily due to the shuttling effect of LiPSs, and the sluggish conversion kinetics, causing significant capacity fading. Moreover, limited power density due to the poor electronic conductivity of S, and the significant volume change of the S cathode during the cycling performance are
Credit author statement
Mozaffar Abdollahifar: Conceptualization, Methodology, Investigation, Supervision, Writing - Original Draft, Writing - Review & Editing, Palanivel Molaiyan: Investigation, Writing - Original Draft, Writing - Review & Editing. Ulla Lassi: Writing - Review & Editing, Funding acquisition. Nae-Lih Wu: Writing - Review & Editing, Funding acquisition. Arno Kwade: Project administration, Supervision, Funding acquisition, Writing - Review & Editing. Mozaffar Abdollahifar and Palanivel Molaiyan have the
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors thank the German Federal Ministry for Education and Research (BMBF) for the funding of the research project EVanBatter (Reference No. 03XP0340B) from Competence Cluster Recycling & Green Battery (greenBatt). This work was also supported and funded by EU/EURF (PASS, A76178) and EU/Interreg Nord (SolBat, grant no. 20202885) projects are acknowledged. N.W. acknowledge the funding from the Ministry of Education (110L9006) and Ministry of Science and Technology in Taiwan (
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Arno Kwade reports financial support was provided by German Federal Ministry for Education and Research (BMBF).