Design of structural and functional nanomaterials for lithium-sulfur batteries

Design of structural and functional nanomaterials for lithium-sulfur batteries

G Model ARTICLE IN PRESS NANTOD-640; No. of Pages 30 Nano Today xxx (2018) xxx–xxx Contents lists available at ScienceDirect Nano Today journal h...

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G Model

ARTICLE IN PRESS

NANTOD-640; No. of Pages 30

Nano Today xxx (2018) xxx–xxx

Contents lists available at ScienceDirect

Nano Today journal homepage: www.elsevier.com/locate/nanotoday

Review

Design of structural and functional nanomaterials for lithium-sulfur batteries Jungjin Park a,b,1 , Seung-Ho Yu a,b,∗,2 , Yung-Eun Sung a,b,∗ a b

Center for Nanoparticle Research Institute for Basic Science (IBS), Seoul 151-742, Republic of Korea School of Chemical and Biological Engineering, Seoul National University, Seoul 151-742, Republic of Korea

a r t i c l e

i n f o

Article history: Received 1 September 2017 Received in revised form 30 November 2017 Accepted 23 December 2017 Available online xxx Keywords: Nanotechnology Meso/micropore nanostructure Functionalized/doped carbon Metal chalcogenide Polymeric sulfur

a b s t r a c t Over the last decade, lithium–sulfur (Li–S) batteries have been extensively studied because of the abundance of sulfur, their environmental benignity, and high gravimetric (2600 W h kg−1 ) and volumetric (2800 W h L−1 ) energy densities. However, their unique electrochemical behavior involving the formation of dissolved polysulfide intermediate species and the insulating nature of sulfur and Li2 S are the main drawbacks that must still be overcome. To tackle these limitations, solutions such as appropriate cathode architecture design, electrolyte optimization, and lithium metal stabilization have been proposed. Recently, high areal sulfur loading, a high sulfur content, and a high electrolyte-to-sulfur ratio have also become prominent issues for the commercialization of the Li–S batteries. This paper reviews a wide range of reports on the design of structural and functional nanomaterials for Li–S batteries and suggests future research directions. © 2017 Published by Elsevier Ltd.

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Research trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Reaction mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Nanostructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Mesoporous carbon as a sulfur host . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Microporous carbon as a sulfur host . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Hierarchical micro/mesoporous carbon as a sulfur host . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Hollow carbon nanofiber-encapsulated sulfur cathode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Graphene/sulfur composite cathode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Yolk–shell-structured sulfur cathode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Nano-sulfur (<10 nm) cathodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Surface functionalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Functionalized graphene oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Graphene quantum dots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Nitrogen-doped mesoporous carbon with carbon nanotubes (CNTs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Nitrogen and sulfur co-doped carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Graphitic C3 N4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Nano additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Titanium oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Surface adsorption and diffusion on transition metal oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

∗ Corresponding authors at: Center for Nanoparticle Research Institute for Basic Science (IBS), Seoul 151-742, Republic of Korea. E-mail addresses: [email protected] (S.-H. Yu), [email protected] (Y.-E. Sung). 1 Present address: Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA. 2 Present address: Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, USA. https://doi.org/10.1016/j.nantod.2017.12.010 1748-0132/© 2017 Published by Elsevier Ltd.

Please cite this article in press as: J. Park, et al., Design of structural and functional nanomaterials for lithium-sulfur batteries, Nano Today (2018), https://doi.org/10.1016/j.nantod.2017.12.010

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Surface-bound active redox mediator on metal oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Efficient polysulfide absorber on transition metal sulfides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Catalytic oxidation of Li2 S on the surface of transition metal sulfides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Sulfur nanodomains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Polymeric sulfur-rich compound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Three-dimensionally interconnected sulfur-rich polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Highly microporous and moderately conductive polymeric sulfur . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Nanospace-confined polymeric sulfur with porous carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Polymerization of sulfur with oleylamine-functionalized reduced graphene oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Reaction mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 High sulfur loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 High sulfur utilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Lithium metal anode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Separator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Binder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

Introduction Research trends Since the lithium–sulfur (Li–S) battery concept was reported in the 1960s [1–5], Li–S secondary batteries have been considered promising energy storage systems because of their high gravimetric (2600 W h kg−1 ) and volumetric (2800 W h L−1 ) energy densities [6,7]. However, after the concept of lithium-ion batteries (LIBs) was first reported in the 1990s [8], much more research has focused on them rather than on Li–S batteries, because current lithiumion batteries using graphite as the anode material and LiCoO2 as the cathode material show high electrochemical stability and long-lasting energy capacity owing to the use of the rocking-chair mechanism. In the early 2000s, new concepts for secondary batteries became essential as modern technology developed, leading to the development of electric vehicles (EVs), portable electronic devices, and unmanned aerial vehicles (UAVs) [9–12]. Li–S batteries are attracting significant attention as next-generation rechargeable batteries. Elemental sulfur is environmentally friendly, resourcerich, and inexpensive (Fig. 1a) [13]. In particular, crude oil refinery processes produce abundant sulfur that is available for recycling. Sulfur has a high gravimetric capacity (S: 1675 mA h g−1 ) as an attractive cathode material with a redox potential of approximately 2.1 V when coupled with lithium metal as an anode material (Li: 3860 mA h g−1 ) (Fig. 1b) [14–18]. A recent article, titled “Sulphur back in vogue for batteries” [19], reported that interest in Li–S battery research has been rapidly increasing in recent years (Fig. 1c and d). In addition, the performance of Li–S batteries has been improved remarkably since the mesoporous carbon–sulfur cathode was introduced by the Nazar group in 2009 [20].

Reaction mechanism A Li–S battery is comprised of lithium metal as the anode material, sulfur as the cathode material, a separator, and an electrolyte. Typical Li–S batteries use ether-based electrolytes, while Li-ion batteries generally use carbonate-based electrolytes. Polysulfides (Li2 Sx , 2 ≤ x ≤ 8) commonly show high solubility in polar aprotic media, such as the commonly used mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME), supporting the electrochemical

reaction of S8 (solid) and Li2 S (solid) with extremely low electronic conductivity [7,21,22]. In electrochemical contexts, the Li–S battery has a maximum voltage at the open circuit state (open-circuit voltage, oc) that is directly proportional to the gap between the electrochemical potential of S (␮c ) and the Li anode (␮a ), as shown in Fig. 2a [21]. During discharge, octa-cyclo-sulfur initially reacts with two lithium ions by a two-electron reduction process to form a polysulfide intermediate (Li2 Sx , 2 ≤ x ≤ 8) and is gradually converted into dilithium sulfide (Li2 S) at the end of discharge. In the reverse reaction, electrons are extracted at a certain voltage with the oxidation of Li2 S into Li and S8 . Both discharge and charge voltage profiles comprise two voltage plateaus occurring at about 2.3 V/2.1 V for discharging and 2.3 V/2.4 V for charging, in which the potential difference for discharge/charge arises from polarization [21]. For the upper voltage plateau (dissolution region) in the discharge curves, solid sulfur (S8 ) gradually reduces to soluble polysulfide species and finally converts to Li2 S4 where it has a quarter of the total specific capacity (418 mA h g−1 ). For the lower voltage plateau (precipitation region), soluble polysulfide intermediates eventually reduce to solid Li2 S where it has three-quarters of the total specific capacity (1254 mA h g−1 ) [21]. The overall reaction is expressed as follows. S8 + 16Li ↔ 8Li2 S This scheme shows that soluble polysulfide can react not only electrochemically with electrons and lithium ions but also chemically (by a disproportionation reaction) with soluble polysulfide molecules owing to its diffusion characteristics, as shown in Fig. 2b [23]. This complex and complicated reaction mechanism induces several phenomena. The diffusion of polysulfides into electrolyte leads to dead sulfur, which is no longer involved in the reaction. In addition, inactive floating polysulfide intermediates cause a parasitic shuttle reaction in the electrolyte, or the formation of a lithium-ionimpermeable solid-electrolyte interphase (SEI) layer and passive Li2 S films on the lithium metal, which can deteriorate the reversible battery operation (Fig. 2c) [18]. We examined four issues in this review (Fig. 2d) [24]: 1) The insulating nature of sulfur and Li2 S 2) Irreversible loss of soluble polysulfides in the electrolyte 3) Undesired formation of sulfur/Li2 S upon cycling

Please cite this article in press as: J. Park, et al., Design of structural and functional nanomaterials for lithium-sulfur batteries, Nano Today (2018), https://doi.org/10.1016/j.nantod.2017.12.010

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Fig. 1. (a) Picture of mountains of elemental sulfur from hydrodesulfurization in the petroleum refining process. Reproduced with permission of Nature Publishing Group from Ref. [13]. Copyright 2013. (b) Specific energies of various battery systems. Reproduced with permission of WILEY-VCH from Ref. [18]. Copyright 2017. (c) Ragone plots of various energy conversion and storage systems compared with the internal combustion engine. Reproduced with permission of Nature Publishing Group from Ref. [19]. Copyright 2013. (d) Number of papers on Li–S batteries over the past decade.

4) Use of a metallic lithium anode Recently, the following six problems have been considered practical issues for the commercialization of Li–S batteries (Fig. 2d) [24]. 1) 2) 3) 4) 5) 6)

High sulfur content (≥70 wt%) High areal sulfur loading (≥5 mg cm−2 ) Electrolyte to sulfur (E/S) ratio (≤ 4 ␮L mg−1 ) Electrode size (≥1 cm2 cell−1 ) Discharge cut-off voltage (≥1.7 V) Cycle performance at low C-rate

This review focuses on state-of-the-art perspectives of nanoscience and nanotechnology to solve fundamental scientific problems as well as practical issues for conventional Li–S batteries. In particular, the feasible solutions of physical confinement and surface chemistry have crucial roles in determining the cyclic stability, reaction kinetics, and Coulombic efficiency for current insights into future development in the Li–S battery industry. In addition, we will discuss not only the reaction mechanism, high sulfur loading and high sulfur utilization but also the lithium metal anode, electrolyte, separator, and binder in the Remarks section. Nanostructure As mentioned above, several challenges need to be overcome for the practical use of Li–S batteries. First, sulfur and dilithium sulfide (Li2 S) have an electrically insulating nature. Sec-

ondly, polysulfides—intermediate species formed during charge and discharge—easily dissolve into the electrolyte, which is the main reason for capacity fading during cycling. In addition, dissolved polysulfides diffuse to the anode and inadvertently react with the lithium anode to form shorter-chain polysulfides. These polysulfides diffuse back to the cathode and are oxidized back to longer-chain polysulfides. This continuous movement of polysulfides between two electrodes accompanied by chemical reactions (the so-called “shuttle phenomenon”) causes severe self-discharge. Physical confinement of sulfur in the pores of the conductive material (usually a carbonaceous material) has been considered as one of the most effective approaches to overcome these problems from an early stage. In this section, various cathode/cell structures that improve the electrochemical properties of the system through the physical confinement approach are introduced.

Mesoporous carbon as a sulfur host In 2009, the Nazar group first demonstrated that the capacity and cyclic stability are significantly improved by physical confinement using highly ordered mesoporous carbon [20]. They synthesized highly ordered mesoporous carbon (CMK-3) using a modified method for better access to mesoporous channels. A CMK3/sulfur composite was prepared using a melt-diffusion method. A mixture of CMK-3 and sulfur was heated at 155 ◦ C, and then, the mesoporous channels were filled with sulfur, as illustrated in Fig. 3a. In a scanning electron microscopy (SEM) image of the CMK3/sulfur composite after heat treatment at 155 ◦ C (Fig. 3b), no bulk

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Fig. 2. (a) Electrochemistry of the Li–S cell at open circuit (left) and during the discharge process (middle) and charge process (right). Reproduced with permission of WILEYVCH from Ref. [21]. Copyright 2013. (b) Schematic illustration of the continuous reduction of elemental sulfur to Li2 S. Reproduced with permission of Elsevier from Ref. [23]. Copyright 2014. (c) Overall reaction of typical Li–S cell. Reproduced with permission of WILEY-VCH from Ref. [18]. Copyright 2017. (d) Present challenges and future practical issues for commercial Li–S batteries. Reproduced with permission of WILEY-VCH from Ref. [24]. Copyright 2017. (e–g) Various strategies for the design and engineering of structural and functional nanomaterials for Li–S batteries.

sulfur was observed, while bulk sulfur clearly appeared in the SEM image obtained before the heat treatment. The CMK-3/sulfur composite delivered a specific capacity of 1005 mA h g−1 for the first discharge at a current density of 168 mA g−1 , which is much higher than that of a simple mixture of CMK-3 and sulfur (Fig. 3c). Furthermore, the CMK-3/sulfur composite exhibited a high Coulombic efficiency (99.94%). This confirmed that the mesoporous carbon effectively suppresses polysulfide dissolution. For further trapping of polar polysulfides in the mesoporous carbon, the surface of carbon was modified with polyethylene glycol (PEG) chains. This sample (CMK-3/S-PEG) exhibited an even higher initial discharge capacity (1320 mA h g−1 ) and showed stable cycling for up to 20 cycles after being stabilized at 1100 mA h g−1 (Fig. 3d). The strategies and effects of surface modification will be discussed in detail in next section (Surface functionalization). Following this pioneering work, many researchers have developed sulfur/mesoporous carbon composites [25–40]. For example, Liu et al. prepared three-dimensional (3D) hierarchically ordered porous carbon (HOPC)/sulfur composites [25] that showed a high initial capacity of 1193 mA h g−1 and delivered 884 mA h g−1 even after 50 cycles at 0.1 C. The mesopores (∼9 nm) of HOPC suppressed the diffusion of polysulfides and interconnected micropores (∼300 nm) facilitated the transport of electrolyte.

Microporous carbon as a sulfur host Porous materials can be divided into three categories based on their pore size: microporous materials having pores with diameters of less than 2 nm, macroporous materials having pores with diameters larger than 50 nm, and mesoporous materials having pores with diameters between 2 and 50 nm. As mentioned in the previous section, mesoporous carbon can suppress polysulfide dissolution, although the sulfur in mesoporous carbon still transforms to polysulfide during discharge. However, the electrochemical behavior of sulfur in microporous carbon is different. Wan group demonstrated that confined small sulfur molecules of S2-4 in microporous carbon can completely prevent polysulfide formation [41]. Theoretical calculations of sulfur allotrope sizes showed that small, chainlike sulfur allotropes, from S2 to S4 , have at least one dimension smaller than 0.5 nm, while cyclo-sulfur allotropes, from S5 to S8 , have at least two dimensions larger than 0.5 nm (Fig. 4a). The pore size of microporous-carbon-coated carbon nanotubes (CNT@MPC) is about 0.5 nm, so only small sulfur molecules can be stored in CNT@MPC. In the voltage profiles of S/(CNT@MPC), only a single plateau at about 1.80 V is observed during discharge, whereas two clear plateaus at about 2.3 and 2.0 V are observed in the typical discharge voltage profiles obtained with a glyme-based electrolyte (Fig. 4b). Although a carbonate-based electrolyte is known to be inappropriate for typical sulfur cathodes due to the irreversible

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Fig. 3. (a) Schematic of the sulfur confined in the pore structure of mesoporous carbon. (b) SEM image of CMK-3/sulfur composite heated at 155 ◦ C. (c) Discharge–charge voltage profile of CMK-3/sulfur composite during the first cycle at a current density of 168 mA g−1 . (d) Cycle performance of CMK-3/S-PEG (black) and CMK-3/S (red) at 168 mA g−1 . Reproduced with permission of Nature Publishing Group from Ref. [20]. Copyright 2009.

reaction between the carbonate-based electrolyte and polysulfides, S/(CNT@MPC) showed excellent cyclic stability for 200 cycles with high Coulombic efficiency (Fig. 4c and d), even with a carbonatebased electrolyte [41]. This unique behavior of a short sulfur molecule confined in microporous carbon has been investigated in detail. Li et al. studied the mechanism of small S2-4 molecules by comparing two kinds of sulfur composite electrodes: one electrode consisting of pure small S2-4 molecules in micropores and the other electrode consisting of a mixture of small S2-4 molecules and S8 [42]. They confirmed that small S2-4 molecules confined in micropores react with lithium through a solid–solid mechanism, and both ether- and carbonatebased electrolytes can be used for small S2-4 cathodes, while only ether-based ones are suitable for a S8 cathode. They also suggested that the distribution of small sulfur species becomes ordered owing to decreased polarization upon cycling. Y. Xu et al. studied the reaction mechanism of confined sulfur in porous carbon, mainly using X-ray photoelectron spectroscopy [43]. They proposed a new reaction mechanism for small sulfur species (predominantly S2 ), which involved lithium ion coupling with a charge-sharing matrix of C/2S2− . They suggested that strong association of S2 and carbon is the reason for the relatively low plateau observed in the voltage profiles of small sulfur molecules in micropores. Various sulfur/microporous carbon composites have been developed, and they have generally shown outstanding cyclic stability for long cycles [44–49]. Zheng et al. prepared a C/S composite by infusing sulfur into micro-mesoporous carbon at 850 ◦ C, followed by washing with CS2 [44]. In the voltage profiles of the C/S composite obtained after washing, only one long plateau was observed at about 1.7 V, indicating that only small sulfur molecules remained in the microporous carbon. The C/S composite cathode

showed superior cyclic stability during 500 cycles with a high capacity of over 860 mA h g−1 . Hierarchical micro/mesoporous carbon as a sulfur host As shown in previous sub-sections, mesoporous and microporous carbon can work well as a sulfur-confining host for Li–S battery cathodes. Both meso- and microporous carbon materials facilitate electron transport and suppress/prevent polysulfide dissolution. Compared to sulfur/mesoporous carbon cathodes, sulfur/microporous carbon cathodes have superior properties in terms of cyclic stability and Coulombic efficiency because microporous carbon can completely prevent polysulfide dissolution. However, mesoporous carbon can undergo loading of a high amount of sulfur whereas only limited amount (<40%) of sulfur can be loaded on microporous carbon. In addition, sulfur in microporous carbon shows a lower voltage plateau, which is disadvantageous in terms of energy density. Some researchers have developed hierarchical structures to gain the advantages of both meso- and microporous carbon [50–52]. Li et al. prepared ordered meso-microporous core–shell carbon (MMCS) as a sulfur host [50]. Mesoporous carbon in the core part can load a sufficient amount of sulfur with high utilization, while the microporous carbon shell can act as a strong physical barrier against polysulfide dissolution (Fig. 5a). In the discharge voltage profiles of S/MMCS (Fig. 5b), three obvious plateaus appear: the upper two are due to sulfur in the mesoporous carbon (solid–liquid–solid), and the lower one is due to sulfur in the microporous carbon (solid–solid). The S/MMCS cathode showed high cycle retention (about 81%) during 200 cycles at 0.5 C, maintaining 837 mA h g−1 at the 200th cycle (Fig. 5c). Jung et al. prepared a hierarchical porous carbon (HPC) by ultrasonic spray pyrolysis

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Fig. 4. (a) Theoretically calculated structure and size of sulfur allotropes from S2 to S8 . (b) Discharge–charge voltage profiles of S/(CNT@MPC) composite at 0.1 C. (c) Cycle performance and Coulombic efficiency of S/(CNT@MPC) at 0.1 C. Reproduced with permission of American Chemical Society from Ref. [41]. Copyright 2012.

[51]. In the HPC–S cathode, most of the sulfur was confined in meso- and macropores in the core part, which was surrounded by a microporous carbon shell (Fig. 5d). The HPC–S cathode showed an excellent cycle retention of 77% during 500 cycles at 2.4 C (Fig. 5e).

Hollow carbon nanofiber-encapsulated sulfur cathode A hollow carbon nanofiber can effectively trap polysulfide in the structure when employed in Li–S battery cathodes. Moreover, carbon nanofiber provides a short transport pathway for both electrons and lithium ions. Furthermore, the hollow structure provides sufficient space to accommodate volume expansion during discharge. Zheng et al. fabricated hollow carbon nanofiber arrays using an anodic aluminum oxide (AAO) template [53]. The length of hollow carbon nanofibers was about 60 ␮m and the diameter was about 200 nm (Fig. 6a). Two plateaus were clearly observed in the voltage profiles of the hollow carbon nanofiber/sulfur composite (Fig. 6b), which delivered about 730 mA h g−1 after 150 cycles at 0.2 C (Fig. 6c). Moon et al. also fabricated a hollow carbon nanofiber using an AAO template for Li–S battery applications, with a diameter of about 75 nm and length of 15 ␮m (Fig. 6d) [54]. A monoclinic sulfur phase was detected near the carbon wall in the sulfur/carbon nanofiber composite by transmission electron spectroscopy (TEM) analysis. This was considered very interesting because monoclinic sulfur is stable at high temperatures (>95.3 ◦ C), and monoclinic

sulfur has not been detected in other sulfur/carbon composites, although it is normal to observe monoclinic sulfur after cycling. Another interesting feature of this composite is that only a single plateau is observed in discharge profiles (Fig. 6d). This is abnormal for sulfur in this pore dimension, and even the voltage was slightly higher than those in sulfur/microporous carbon composites. This unique behavior was not fully understood, but the authors explained that this might be mainly attributed to the monoclinic sulfur phase. The sulfur/carbon nanofiber composite showed good cycle retention of 75.8% during 1000 cycles when cycled at 2 C for charge and 5 C for discharge (Fig. 6e).

Graphene/sulfur composite cathode Graphene (or reduced graphene oxide, rGO) is a twodimensional carbon with superior electrical conductivity, chemical stability, mechanical strength, and high surface area. It has been widely used in lithium-ion batteries as electrode materials with an anchoring active material. So far, various graphene/sulfur composites have been developed as Li–S battery cathodes, and they have shown much-improved electrochemical properties through physical trapping or chemical absorption of dissolved polysulfides [55–73]. The effects of graphene as a physical barrier are focused on in this section. Wang et al. prepared graphene-wrapped sulfur particles by coating poly(ethylene glycol) (PEG)/sulfur with mildly oxidized

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Fig. 5. (a) Schematic illustration of the structural advantage of the meso-microporous core–shell carbon (MMCS). (b) Voltage profiles and (c) cycle performance of S/MMCS at 0.5 C. Reproduced with permission of American Chemical Society from Ref. [50]. Copyright 2014. (d) Schematic illustration of the electrochemical processes of hierarchical porous carbon with micropores in the outer shell (top) and conventional porous carbon (bottom). (e) Cycle performance and Coulombic efficiency of hierarchical porous carbon (HPC)-S at 2.4 C. The insets show an SEM image and the corresponding energy dispersive X-ray spectroscopy (EDS) mapping of an HPC–S particle after 500 cycles. Reproduced with permission of American Chemical Society from Ref. [51]. Copyright 2014.

graphene oxide sheets [55] (Fig. 7a). The graphene coating increases electrical conductivity and traps polysulfides as well. Moreover, the PEG coating can provide a flexible cushion to accommodate volume changes. Therefore, graphene-wrapped sulfur particles showed a considerably better cyclic stability than composites without a PEG coating or graphene wrapping. The capacity decay from the 10th to the 100th cycle was 13% at a rate of 0.2 C and 9% at 0.5 C (Fig. 7b and c). Xu et al. synthesized graphene-encapsulated sulfur (GES) composites with a core–shell structure through a solution–chemical reaction–deposition method [56]. The sulfur content was 83.3 wt% in the GES composites, which delivered 915 mA h g−1 at a rate of 0.75 C with cycle retention of 86% during 160 cycles. Yolk–shell-structured sulfur cathode The yolk–shell structure has been considered as one of the most ideal structures for high-capacity lithium-ion battery anodes [74,75]. High-capacity electrode materials generally experience large volume changes during cycling, causing pulverization of the active materials and electrical contact loss from the current collector. The void space in the yolk–shell structure effectively accommodates volume expansion during lithiation, which also applies for the Li–S battery because the volume expansion is large during conversion of sulfur to Li2 S. The yolk–shell structure also suppresses polysulfide dissolution from the interior of the structure to the bulk electrolyte. Zhou et al. synthesized sulfur–polyaniline (S-Pani) yolk–shell nanoparticles (NPs) through heat treatment of a core–shell nanostructure (Fig. 8a) [76]. They found that heat treatment is more effective in the preparation of a yolk–shell structure, while chemical etching with toluene/ethanol causes broken structures. They also compared the electrochemical properties of the S-Pani core–shell and yolk–shell structures. The S-Pani yolk–shell structures showed superior capacity retention (69.5%) and Coulombic efficiency during 200 cycles at 0.2 C compared to those of the SPani core–shell structures (Fig. 8b). The S-Pani yolk–shell structures maintained their structure, whereas the S-Pani core–shell structures were destroyed after 5 cycles (Fig. 8c).

Yang et al. prepared yolk–shell-structured sulfur cathodes by ensuring infiltration of sulfur into yolk–shell nanospheres, which showed high cyclic stability with a small capacity decay of 0.05% per cycle during 1000 cycles at 0.5 C [77]. Ding et al. prepared sulfur–carbon yolk–shell particles by oxidizing carboncoated metal sulfides [78]. These particles exhibited a high capacity of about 1400 mA h g−1 (∼560 mA h g−1 per gram of electrode). Yolk–shell structures formed using inorganic materials for the shell instead of carbonaceous materials have also been developed. For example, Cui group prepared sulfur–TiO2 yolk–shell NPs, which exhibited an excellent capacity retention of 67% during 1000 cycles with a high initial discharge capacity of 1030 mA h g−1 [79]. Lodi-Marzano et al. prepared sulfur–silica yolk–shell NPs with a raspberry-like morphology [80]. They delivered a stable capacity of about 750 mA h g−1 with high capacity retention of 93% from the second to 350th cycle. Nano-sulfur (<10 nm) cathodes Apart from polysulfide dissolution, slow charge transfer and low sulfur utilization, which mainly arise from the poor conductivity of sulfur, are the major drawbacks of Li–S batteries for practical application. Ultrafine sulfur NPs have been prepared through chemical synthesis or electrochemical deposition, and they have shown considerably improved electrochemical properties in terms of rate performance and sulfur utilization [81–86]. Chen et al. investigated the effects of sulfur particle size on the electrochemical performance by preparing monodispersed sulfur NPs with different diameters ranging from 5 to 150 nm [81]. The particle size distribution of a series of sulfur NPs was narrow, as confirmed by TEM studies (Fig. 9a–j). Electrochemical characterizations of sulfur NPs with sizes of 5, 10, 20, 40, and 150 nm on rGO confirmed that the smaller sulfur NPs have a lower charge transfer resistance, higher specific capacity, and better rate capability. In particular, 5-nm sulfur NPs showed superior electrochemical performance. The voltage profiles of 5-nm sulfur NPs were close to the theoretical prediction (Fig. 9k): 418 and 1254 mA h g−1 were delivered at the high and low potential plateaus, respectively. The

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Fig. 6. (a) Schematic illustration of hollow carbon nanofiber/sulfur composite. (b) Voltage profiles and (c) cycle performance of the carbon nanofiber-encapsulated sulfur cathode at 0.2 C and 0.5 C. Reproduced with permission of American Chemical Society from Ref. [53]. Copyright 2011. (d) Schematic illustration of sulfur-infiltrated carbon nanotubes (S@C NW). (e) Voltage profiles of the S@C NW electrode at 0.5 C. (f) Cycle performance of the S@C NW electrode with a discharge rate of 0.5 C and a charge rate of 2 C. Reproduced with permission of WILEY-VCH from Ref. [54]. Copyright 2013.

cycle performance of 5-nm sulfur NPs was outstanding with a low capacity decay rate of 0.077% during 500 cycles at 0.5 C (Fig. 9l). Zhao et al. prepared ultrafine sulfur nanodots with a diameter of 2 nm on nickel foam through electrodeposition [82]. The loading level of sulfur on the nickel foam was adjusted from 0.21 to 4.79 mg cm−2 by changing the deposition time. The cathode with 0.45 mg cm−2 of sulfur showed the best performance, with a high initial discharge capacity of 1458 mA h g−1 . It also exhibited excellent cyclic stability (97% of cycle retention) during long cycles. The high reversibility of sulfur NPs was confirmed by in-situ Raman

analysis. TEM images of sulfur NPs obtained after 50 cycles proved that the morphology was well maintained upon cycling, with no agglomeration.

Surface functionalization In the previous section, we reviewed strategies involving physical entrapment mainly using carbonaceous materials. Various binary and multimetallic oxides such as SiO2 , Al2 O3 , and Mg0.6 Ni0.4 O have also been applied to prevent soluble polysul-

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Fig. 7. (a) Schematic illustration of the steps for preparing a graphene-wrapped sulfur particle. (b) Voltage profiles and (c) cycle performance of the graphene–sulfur composite with PEG coating at various C rates. Reproduced with permission of American Chemical Society from Ref. [55]. Copyright 2011.

Fig. 8. (a) Schematic illustration of the S-Pani yolk–shell structure (yellow sphere: sulfur, dark green shell: polyaniline, and black shell: vulcanized polyaniline). (b) Cycle performance of the S-Pani yolk–shell structure. (c) SEM image of the S-Pani yolk–shell structure after 5 cycles. Reproduced with permission of American Chemical Society from Ref. [76]. Copyright 2013.

fides in Li–S batteries [87,88]. However, it is difficult to fabricate an ideal structure with selective impermeability to polysulfide. Moreover, lithium polysulfides are intrinsically polar and do not strongly

interact with typically nonpolar carbon; therefore, physical confinement concept-based carbonaceous materials/metal oxides are only suitable for short-term cycling [89].

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Fig. 9. Transmission electron microscopy (TEM) images of (a) 150, (b) 40, (c) 20, (d) 10, and (e) 5-nm sulfur NPs and (f-j) the corresponding size distributions. (k) Voltage profiles of 5-nm sulfur NPs at 0.1 C. (l) Cycle performance of 5-nm sulfur NPs at 0.5 C and 1 C. Reproduced with permission of American Chemical Society from Ref. [81]. Copyright 2015.

Recently, approaches for trapping soluble polysulfide anion via surface chemical interactions using organic/inorganic materials have been reported to improve the long-term cyclic stability. Surface functionality, intrinsic polarity, and electro-/nucleophilicity are critical parameters that determine the strengthening of the anchoring of soluble and polar polysulfides on the electrochemical active sites during battery operation [89,90]. This section covers research on the surface modification of organic and inorganic materials as additives to increase the rate capability, sulfur loading level, as well as long-term cycle performance. This knowledge about surface chemistry will contribute to the commercialization of Li–S batteries depending on how nanotechnology is used to control the surface properties.

(XPS) at carbon 1 s and sulfur 2p regions in Fig. 10e. The X-ray diffraction (XRD) results indicated that the materials were not crystalline. However, thermal gravimetric analysis (TGA) and energy dispersive X-ray spectroscopy (EDS) confirmed that nano-sulfur was well bound to GO. When this composite was electrochemically tested, it delivered an initial capacity of 1277, 1136, and 815 mA h g−1 at 0.5, 1.0, and 2.0 C, respectively, and after 100 cycles, it still delivered a capacity of 1021, 955, and 647 mA h g−1 (Fig. 10e), respectively. This result showed that the hydroxyl groups of GO were uniformly distributed, indicating that the sulfur was well dispersed, with a shortened electron pathway.

Functionalized graphene oxide

Park et al. applied graphene quantum dots (GQDs) to the Li–S battery as oxygen functional group donors to significantly increase the concentration of the oxygen functional group, which acts as a polysulfide immobilizer additive [91]. They have a much smaller particle size and more oxygen-rich function groups than GO, which can induce a high polarity reaction at very small amounts, as shown in Fig. 11a–c. However, to compensate the relatively low electrical conductivity of GQDs, conductive carbon black was added as a supporting additive. As a result, sulfur was tightly compacted to carbon black with GQDs (Fig. 11d–f), thus facilitating both chemical adsorption as well as physical confinement of soluble polysulfide. These composites delivered ∼1000 mA h g−1 after 100 cycles at 0.5 C and exhibited 540 mA h g−1 at 10 C, respectively (Fig. 11g and h).

At an early stage of developing sulfur cathodes, some researchers introduced graphene oxide (GO). Ji et al. demonstrated that the soluble polysulfide effectively reacted with the GO with a polar–polar interaction when oxygen was functionalized on the basal plane of graphene sheets [57], and therefore this functionalized GO can be used to immobilize soluble polysulfide, as shown in Fig. 10a. The chemical structure of functionalized GO was confirmed by synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) analysis, and chemical structure of the functionalized GO showed high reactivity with the polar polysulfide anion, as supported by density functional theory (DFT) calculations (Fig. 10b). The GO-S nanocomposite showed a high initial and 50th discharge capacity of about 1320 and 954 mA h g−1 , respectively, at 0.1 C (Fig. 10c). In 2013, Zu et al. demonstrated that the hydroxyl group in GO could react with polysulfide anions through a hydrothermal step (Fig. 10d), and nano-sized sulfur eventually bound to GO [60]. This composite was characterized by X-ray photoelectron spectroscopy

Graphene quantum dots

Nitrogen-doped mesoporous carbon with carbon nanotubes (CNTs) Previously, many attempts have been made to enhance the properties of Li–S batteries using oxygen-functionalized or/and

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Fig. 10. (a) SEM image of graphene oxide (GO) with sulfur. (b) Schematic diagram of GO immobilizing S via calculated via density functional theory. (c) Cycle performance of the GO-S at 0.1 C after initial activation processes at 0.02 C for two cycles. Reproduced with permission of American Chemical Society from Ref. [57]. Copyright 2011. (d) Schematic of the synthesis process of the hydroxylated graphene-S nanocomposite. (e) C1s XPS spectra of the GNSOH and GNSOH–S nanocomposite. (f) Cycle performance of GNSOH–S nanocomposite and GNS-S composite. Reproduced with permission of WILEY-VCH from Ref. [60]. Copyright 2013.

oxygen-doped carbon [92]. However, Song et al. reported that the nitrogen-functionalized carbon more effectively caps the soluble polysulfide on the electrochemically active site than oxygenfunctionalized carbon [93], as supported by density functional theory calculations. In this study, the polar–polar interaction between nitrogen-containing carbon and polysulfide was confirmed by UV–visible (UV–vis) spectroscopy and pair distribution function (PDF) analysis. To demonstrate experimentally, carbon nanotubes and mesoporous N-doped carbon sphere composites were synthesized through the evaporation-induced self-assembly (EISA) process shown in Fig. 12a–d. Notably, with a high loading of sulfur of up to 5 mg cm−2 , remarkable cycle performance was obtained, with a delivering capacity of 1200 mA h g−1 after 200 cycles (Fig. 12e and f). Nitrogen and sulfur co-doped carbon Zhou et al. introduced a nitrogen and sulfur co-doped graphene sponge as a cathode material in Li–S batteries [94]. In this study, it was suggested that hetero-doped sites had strong sulfur binding energies relative to that of other mono-doped (S or N) rGO according to the DFT calculations (see Fig. 13a). In addition, the graphene sponge has enough void space, so it can be used for fast charge transfer channel as well as a high sulfur loading reservoir. Remarkably, with a high loading level of sulfur (about 4.6 mg cm−2 ), the materials showed excellent electrochemical performance with

capacities of 1200 mA h g−1 at 0.2 C and 430 mA h g−1 at 2 C after 500 cycles (Fig. 13b–d). Graphitic C3 N4 As noted previously, N-doped carbon can bond strongly with polysulfides. However, it is very difficult to synthesize homogeneously distributed N-doped carbon, especially with a high doping level of nitrogen. Therefore, lightweight nanoporous graphitic carbon nitride (g-C3 N4 ) was introduced as an alternative [95]. The strong chemical coupling reaction between g-C3 N4 and polysulfides was confirmed by Fourier transform infrared spectroscopy. Furthermore, the binding energies of carbon and Li2 S, Li2 S2 , and Li2 S4 were calculated by DFT calculations (Fig. 14a–c) when the doping concentration of nitrogen was changed. g-C3 N4 /S showed 0.04% capacity fading per cycle when cycled at 0.5 C for 1500 cycles. In addition, the sulfur content in the porous structure electrode increased to 5.0 mg cm−2 (Fig. 14d). Nano additives Titanium oxides Evers et al. first reported a mixture of mesoporous titanium dioxide (TiO2 ) and silica colloidal monolith-derived mesoporous carbon (SCM) as a polysulfide reservoir in Li–S batteries [96], as

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Fig. 11. (a and b) High-resolution TEM (HR-TEM) images of graphene quantum dots (GQDs); the inset shows a histogram of the GQD size distribution. (c) Fourier transform infrared (FTIR) spectra of GQDs and CB. (d) HR-TEM image of GQDs–S. (e) SEM image of GQDs–S/CB composite. (f) Raman spectra of GQDs–S and GQDs–S/CB composites. (g) Cycle performance and Coulombic efficiency at 0.5 C. (h) Rate capability of the GQDs–S/CB and S/CB at 0.1 C–10 C. Reproduced with permission of Nature Publishing Group from Ref. [91]. Copyright 2016.

shown in Fig. 15a. It could effectively trap the soluble polysulfide through chemical polar–polar interactions (surface adsorption) and physical confinement of TiO2 , as well as provide short transport paths of electrons and lithium ions. These synergistic effects led to high electrochemical performance, with retention of up to 750 mA h g−1 at 1 C after 200 cycles (Fig. 15a). Sulfur–TiO2 yolk–shell nanostructures were synthesized and employed in Li–S battery cathodes, as shown in Fig. 15b and c. As reported by Seh et al., the internal space effectively accommodates the volumetric expansion of sulfur with no damage to the TiO2 shell, minimizing polysulfide dissolution during battery cycling [79]. The sulfur–TiO2 yolk–shell nanostructures delivered an initial capacity of 1030 mA h g−1 at 0.5 C and showed a high Coulombic efficiency of 98.4% even after 1000 cycles in Fig. 15d. Recently, Liang et al. prepared a hydrogen-reduced titanium dioxide inverse opal structure to first obtain a 3D nanostructured conductive matrix that physically encapsulated the soluble polysulfide [97]. Then, defect-modified TiO2 bound the soluble polysulfide on its surface, thereby suppressing the irreversible

loss of polysulfide. The TiO2 with an open structure was wellreduced by heat treatment with hydrogen, which increased the electronic conductivity (Fig. 15e). Hydrogen-reduced-TiO2 inverse opal/S composites were applied to a Li–S battery cathode. The initial capacity reached was 1100 mA h g−1 , and a capacity of 890 mA h g−1 was maintained even after 200 cycles at 0.2 C (Fig. 15f). It also showed excellent rate properties, with a capacity of 800 mA h g−1 at 1 C (Fig. 15g). In addition, non-stoichiometric Magnéli phase titania (Ti4 O7 ) has been used as a cathode catalyst, and its metallic property increased the rate capability of sulfur cathodes [98,99]. Surface adsorption and diffusion on transition metal oxides A variety of non-conductive oxide additives have been introduced into Li–S batteries to suppress the shuttle effect and control the dilithium sulfide deposits. However, the main roles of nonconductive oxides are still under debate. Tao et al. proposed a new reaction mechanism to unveil the underlying main roles of non-conductive oxides [100]. In order to prove his hypothesis, var-

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Fig. 12. (a) Scheme of the synthesis process for the mesoporous nitrogen-doped carbon sphere (MNCS)/CNT composite. SEM images of the MNCS/CNT composite at different magnifications: (b) low, (c) medium, and (d) high magnification. (e) Charge–discharge profile and (f) cycle performance of MNCS/CNT. Reproduced with permission of WILEY-VCH from Ref. [93]. Copyright 2015.

ious nonconductive oxides (MgO, Al2 O3 , CeO2 , La2 O3 , and CaO) were selected and placed on a carbon flake conducting additive, as shown in Fig. 16. The overall reaction occurred systematically via two steps: surface adsorption and surface diffusion (Fig. 16a). To support this theory, Li2 S8 adsorption experiments were conducted by dispersing non-conductive metal oxides in the polysulfide catholyte. The binding energies of Li2 S and Li2 S8 on the surface of each non-conductive oxide were also theoretically determined, and then, the potential diffusion path was estimated (Fig. 16b and c). In the electrochemical experiments, the initial capacities of Al2 O3 /C, CeO2 /C, La2 O3 /C, MgO/C, CaO/C, and C at 0.1 C were measured to be 1330, 1388, 1345, 1368, 1246, and 1230 mA h g−1 , respectively. After 300 cycles at 0.5 C, CeO2 /C and La2 O3 /C showed better performance than Al2 O3 /C, CaO/C, and C cathodes. Moreover, the MgO/C cathode showed the best electrochemical performance among all samples (Fig. 16d and e). Only CeO2 /C, La2 O3 /C, and MgO/C enhanced the battery performance during electrochemical tests, indicating that if the surface adsorption is too strong, surface diffusion of the non-conductive oxides to conductive additives will

not occur, whereas the polysulfides will not be absorbed if the surface adsorption is too weak. This study reiterates the importance of optimizing polysulfide adsorption and diffusion on the additive surface.

Surface-bound active redox mediator on metal oxides Although various kinds of transition metal oxides have been used as additives in Li–S batteries to improve the electrochemical performance, the exact mechanism remained controversial. In particular, the relation between the polysulfide adsorption and electrochemical behavior was not clear. More precisely, it was not understood how the soluble polysulfide adsorbed onto the transition metal oxide participates in the reversible electrochemical reaction. Thus, Liang et al. proposed a general mechanism: polysulfides are entrapped by the reaction between transition metal oxides and soluble polysulfides to form a surface-bound active redox mediator [101].

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Fig. 13. (a) Schematic illustration of formation process of the nitrogen/sulfur (N/S) co-doped graphene electrode. (b) Rate properties of the rGO, S-doped graphene, N-doped graphene, and N/S co-doped graphene electrodes (c) Comparison of the potential difference between the charge and discharge plateaus at various current densities. (d) Cycle performance and Coulombic efficiency of Li polysulfide batteries with the rGO, S-doped graphene, N-doped graphene, and N/S-co-doped graphene electrodes. Reproduced with permission of Nature Publishing Group from Ref. [94].

In addition, even better electrochemical properties can be achieved if accompanied by a high active surface area and adsorption. The presence of a reversible redox mediator (thiosulfate/polythionate) on the surface with a polysulfide anion is important. To achieve this, a suitable redox potential should be considered to design metal oxide additives (see Fig. 17a). Furthermore, polarization, which depends on the size and crystallographic structure, should be also considered. When the redox potential of a metal oxide is lower than that of lithium polysulfides—in the case of Co3 O4 (<2.4 V vs. Li/Li+ ) for example—it does not affect the lithium polysulfides. When the redox potential of the metal oxide is higher than that of lithium polysulfides, such as that of CuO additive (2.4–3.05 V vs.

Li/Li+ ), thiosulfate/polythionate groups are chemically bound to the reduced metal oxide surface. These are retained during cycling, because they are ultimately reduced to Li2 S and then regenerated upon charging via a disproportionation reaction. In the case of NiOOH additive (>3.05 V vs. Li/Li+ ), a redox potential too high oxidizes lithium polysulfides to a mixture of sulfate and thiosulfate. The repeated oxidizing of polysulfides to electrochemically inactive sulfate groups leads to lower cycle performance. Sulfate groups block the surface and inhibit full activation of the polythionate (Fig. 17b–p). To complement the previous discussion, a V2 O5 -decorated electrode was separately tested at 1.8–2.5 V (green) and 1.8–3.0 V (orange) (Fig. 17q). When operating at lower than 2.5 V, the capacity

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Fig. 14. Schematic illustration of the most stable binding configurations of two Li2 S2 molecules on (a) pristine carbon, (b) N-doped carbon, (c) g-C3 N4 substrates (see the bottom of the panels for side views) after full relaxation, and their insets on the top left show the 2D deformation charge distributions of the corresponding substrates without Li2 S2 . (d) Cycle performance of a 3.0 mg cm−2 loaded g-C3 N4 /S75 thick electrode at 0.2 C. Inset summarizes the areal discharge capacity of g-C3 N4 /S75 thick electrodes at 0.2 C and 0.05 C. Reproduced with permission of American Chemical Society from Ref. [95]. Copyright 2013.

lasted over long-term cycling, while only short-term stability was achieved above 2.5 V. The electrochemical performance degraded once sulfate was formed, as confirmed by XPS (Fig. 17k–p). This new understanding will help realize long-life Li–S batteries. By designing new materials with appropriate redox potentials, the effects of a thiosulfate/polythionate mediator can be optimized.

Efficient polysulfide absorber on transition metal sulfides Transition metal sulfides/oxides have also attracted significant attention as good polysulfide anion absorbers in the Li–S battery. The polysulfides might be fixed by the polar–polar interaction between the transition metal and the polysulfide anion. The interaction between lithium polysulfide and transition metal has been confirmed by polysulfide adsorption tests and UV–vis spectroscopy. Wang et al. demonstrated that the polysulfide anions can be dominantly formed on the Mo edge site than S-edge or terrace sites in a two-dimensional layered structure of MoS2 (Fig. 18a–d) [102]. Computational calculations showed that the binding energy of bonding between Mo-edge and Li2 S is higher than those of other bonds (Li2 S and S-edge/terrace site), providing evidence of an electrochemical reaction as previously reported. Dirlam et al. reported that MoS2 inclusions, serving as anchors of the polysulfide, can mitigate the dissolution of the lithium polysulfide redox products and contain a sulfur-containing matrix. Excellent electrochemical performance was achieved with these MoS2 sheets [103].

Park et al. prepared two-layer-structured WS2 on a carbon cloth interlayer to modify the non-polar properties of the carbon cloth interlayer, as shown in Fig. 18e and f [104]. This composite showed improved rate capability and cycle performance. The polysulfides trapped by the tungsten and sulfur dangling bonds on the edges of the WS2 particles disproportionated to low-order polysulfides before being reduced to Li2 S by electrons at the anode through the carbon cloth interlayer. This electrode exhibited superior cycle performance (∼1000 mA h g−1 at 0.5 C after 500 cycles) and rate capability (∼750 mA h g−1 at a 5.0 C). Yuan et al. prepared pyrite-type CoS2 with a face-centered cubic crystal structure [105]. The exposed surface of the primary particles is predominantly the (111) plane, which efficiently adsorbs the polysulfide. This indicates that not only transition metal sulfides with 2D-layered crystal structures but also those with different crystal structures can act as good absorbers of polysulfide anions (Fig. 18j). The prepared electrode showed outstanding properties with an initial capacity of 1003 mA h g−1 and 554 mA h g−1 after 2000 cycles at 2 C. Catalytic oxidation of Li2 S on the surface of transition metal sulfides Liu et al. systematically studied transition metal sulfides as catalysts for lowering the activation barrier energy of Li2 S oxidation [106]. Through polysulfide adsorption experiments using each model material (Ni3 S2 –Li2 S, SnS2 –Li2 S, FeS–Li2 S, CoS2 –Li2 S, VS2 –Li2 S, TiS2 –Li2 S, and G/CNT–Li2 S electrodes), the polysulfide

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Fig. 15. (a) Cycle performance of sulfur cathode containing TiO2 with 5 nm pore diameter. Inset shows a schematic illustration of the cell reaction with mesoporous titania additives. Reproduced with permission of American Chemical Society from Ref. [96]. Copyright 2012. (b) Schematic of the synthetic process of sulfur–TiO2 yolk–shell nanostructures. (c) SEM (left) and TEM (right) images of as-synthesized sulfur–TiO2 yolk–shell nanostructures. (d) Capacity retention of sulfur–TiO2 yolk–shell nanostructures, bare sulfur, and sulfur–TiO2 core–shell nanostructures cycled at 0.5 C. Reproduced with permission of Nature Publishing Group from Ref. [79]. Copyright 2013. (e) Schematic and cross-sectional SEM image of the 3D ordered reduced TiO2 with/without sulfur infiltration. (f) Cycle performance (black curve) and Coulombic efficiency (blue curve) of TiO2-x /sulfur composite cathode at a 0.2 C. (g) Rate performance of the composite cathode at different C rates ranging from 0.05 C to 1 C. Reproduced with permission of American Chemical Society from Ref. [97]. Copyright 2014.

adsorption properties were experimentally investigated, which was directly correlated with the activation barrier of the Li2 S oxidation reaction from the Li2 S decomposition peak in the charging profiles in Fig. 19a–c. It was also confirmed that all materials decrease the energy barrier of Li2 S oxidation by theoretical calculations. VS2 , TiS2 , and CoS2 showed capacities of 701, 546, and 581 mA h g−1 at 0.5 C after 300 cycles with high cycle retention, respectively (Fig. 19d). This was in good agreement with the results of adsorption data and theoretical calculations. In summary, strong interactions with lithium polysulfides, intrinsic metal conductivity, Li2 S formation, easy transport of lithium ion, facilitation of surface-mediated redox reaction, and metal sulfide properties are critical factors determining the overall electrochemical performance of Li–S batteries. Sulfur nanodomains Polymeric sulfur-rich compound In 2013, Chung et al. synthesized a polymeric sulfur structure by the inverse vulcanization synthesis method through

free radical polymerization of molten elemental sulfur and 1,3diisopropenylbenzene (DIB) without solvents or catalysts, where sulfur was used as a co-monomer and a small amount of DIB as a co-monomer and crosslinker [13]. When the temperature of elemental molten sulfur was higher than 159 ◦ C, the thermal ring-opening polymerization (ROP) reaction occurred in which the sulfur monomer converted into a linear polysulfane with a di-radical chain end, as shown in Fig. 20a. The carbon double bond in the DIB reacted with the sulfur di-radical chain end to form a C S bond, which then gradually polymerized into high molecular weight polymeric sulfur. Quenching stabilized the di-radical polymeric sulfur, which otherwise leads to depolymerization of monomeric and oligomeric sulfur with semi-crystalline and intractable properties. The polymerization features depend on the amount of DIB (Fig. 20b), which can be confirmed by the endothermic and exothermic peaks of the differential scanning calorimetry (DSC) profile. In the DSC curves (Fig. 20c), endothermic and exothermic peaks appeared for the sulfur copolymer with 10% DIB, while no peaks were observed for the copolymer with 20% DIB. This indicates that

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Fig. 16. (a) Schematic of Li2 Sx adsorption and diffusion on the surface of various non-conductive metal oxides. (b) Experimental and simulated adsorption amount of Li2 S8 on different metal oxides. (c) Lithium diffusion mechanism on the surface of various metal oxides and their potential energy profiles for Li ion diffusion along different adsorption sites on the oxide surface. (d) Charge–discharge voltage profile and (e) cycle performance of the sulfur/metal oxide composites. Reproduced with permission of Nature Publishing Group from Ref. [100]. Copyright 2016.

Fig. 17. (a) Chemical reactivity of different metal oxides with lithium polysulfides (LiPSs) as a function of the redox potential versus Li/Li+ . XPS spectra of the metal oxides and solids recovered from metal oxide-Li2 S4 suspension for (b–d) Co3 O4 , (e–g) CuO, (h–j) NiOOH, (k–m) VO2 and (n–p) V2 O5 . The spectra show (b,e,h,k,n) metal 2p: pure metal oxide/hydroxide, (c,f,i,l,o) metal 2p: metal–Li2 S4 and (d,g,j,m,p) sulfur 2p: metal–Li2 S4 . (q) Cycle performance of sulfur-infiltrated V2 O5 spheres as a function of potential ranges of 1.8–2.5 V (green symbols) and 1.8–3.0 V (orange symbols) in Li–S cell. Reproduced with permission of WILEY-VCH from Ref. [101]. Copyright 2016.

at 20%, the DIB copolymer fully converted into a plasticizer. In Li–S batteries, the amount of DIB strongly affects the capacity, with all the samples (5%, 10%, and 15% DIB) exhibiting improved electrochemical properties (Fig. 20d).

Three-dimensionally interconnected sulfur-rich polymers Kim et al. prepared triazine-based 3D-interconnected sulfurrich compound as the cathode material of Li–S battery [107]. Porous

trithiocyanuric acid crystals were used as soft templates, wherein the ROP of elemental sulfur occurred along the thiol surface to form three-dimensionally interconnected sulfur-rich phases, as shown in Fig. 21a. This synthesis method can control the shape and morphology, in contrast with the conventional inverse vulcanization synthesis method. In addition, the vulcanization of sulfur can occur uniformly throughout the homogeneous structure, making chemical bonding homogeneous. This results in the formation of amorphous sulfur with an indeterminable structure, as confirmed

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Fig. 18. (a) Schematic of horizontally aligned MoS2 (H-MoS2 ) nanosheets on glassy carbon (GC) substrate with Li2 S electrodeposition. (b) SEM image of H-MoS2 nanosheet with Li2 S NPs decorated along the edges, as indicated by red arrows. (c) Schematic of H-MoS2 nanosheet on edge-terminated vertically aligned MoS2 (V-MoS2 ) nanofilm. (d) SEM image of H-MoS2 on V-MoS2 nanofilm without obvious edge effects, as shown in (b), due to the competition from the edge sites on the substrate. Reproduced with permission of American Chemical Society from Ref. [102]. Copyright 2014. Schematic illustration of (e) kinetics of WS2 supported on the carbon cloth interlayer (CCI) (WS2 /CCI) and (f) four cathode configurations: (i) S, (ii) WS2 /S, (iii) S–CCI, and (iv) WS2 /S–WS2 /CCI. (g) The TEM image with selected area electron diffraction (SAED) of WS2 with the [001] zone axis. (h) SEM image of WS2 /CCI. (i) X-ray diffraction (XRD) patterns of S (blue), WS2 (green), CCI (grey), and WS2 /CCI (red). Reproduced with permission of WILEY-VCH from Ref. [104]. Copyright 2017. (j) Schematic illustration of the discharge process in sulfur cathodes with/without CoS2 -incorporated carbon. Pure carbon/sulfur cathode where the polysulfide reduction is rate controlled and polysulfide diffusion is dominant (left), and CoS2 -incorporated carbon/sulfur cathode where polysulfide reduction is accelerated, and polysulfide diffusion is weakened (right). Reproduced with permission of American Chemical Society from Ref. [105]. Copyright 2015.

Fig. 19. (a) Photograph of lithium polysulfide (Li2 S6 , 0.005 M) adsorption properties by carbon and metal sulfides mixture in DOL/DME solution and (b) corresponding simulation of Li2 S6 adsorbed on the surface of metal sulfides. Atomic conformations and binding energy for Li2 S6 species adsorption on Ni3 S2 , SnS2 , FeS, CoS2 , VS2 , and TiS2 . (c) Schematic illustration of the sulfur conversion process and the Li2 S catalytic oxidation on the surface of the substrate. (d) Cycle performance and Coulombic efficiency of the different composites at 0.5 C for 300 cycles. Reproduced with permission of National Academy of Sciences from Ref. [106]. Copyright 2017.

by X-ray diffraction spectra, which can facilitate the feasibility of conducting lithium ions in the sulfur copolymer (see Fig. 21b). These three-dimensionally interconnected sulfur-rich phases had excellent properties at 1 C (872 mA h g−1 ), 3 C (803 mA h g−1 ), and 5 C (730 mA h g−1 ). In addition, they demonstrated an excellent electrochemical performance with 850 mA h g−1 after 450 cycles, as shown in Fig. 21c. These results indicate that the impermeable structure prevents the soluble polysulfide anions from diffusing out.

Highly microporous and moderately conductive polymeric sulfur Talapaneni et al. synthesized covalent triazine framework (CTF) with embedded polymeric sulfur with a high sulfur content (62 wt%) via inverse vulcanization of 1,4-dicyanobenzene and elemental sulfur to effectively improve the electrical conductivity of sulfur-containing polymers as well as to uniformly bind sulfur with polymer matrices [108]. In-situ formation of CTF with chemical bonding and uniform framework leads to a porous sulfur struc-

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Fig. 20. (a) Synthetic scheme for copolymerization of S8 with DIB to form chemically stable sulfur copolymers. (b) Digital image of elemental sulfur and poly(S-r-DIB) copolymers with varying weight percentages of DIB. (c) Differential scanning calorimetry (DSC) thermograms of 2nd cycles of sulfur, S-DIB copolymers prepared with different weight ratios of DIB 10 wt%, 20 wt%, 30 wt% and 50 wt%. Reproduced with permission of Nature Publishing Group from Ref. [13]. Copyright 2013.

Fig. 21. (a) Schematic illustration of the procedure to synthesize sulfur-rich polymers with controllable morphology (b) Powder XRD spectra of the vulcanized STrithiocyanuric acid (TTCA)-I and S-TTCA-II. (c) The cycle performance and Coulombic efficiency of the Li/S-TTCA-I and Li/S-TTCA-II cells for 300 cycles at different C rates. Reproduced with permission of Nature Publishing Group from Ref. [107]. Copyright 2015.

ture with well-distributed pores, which facilitates electron/lithium ion transfer in the structure and suppresses the irreversible loss of soluble polysulfide intermediates effectively, as shown in Fig. 22a. Notably, the crystallinity of sulfur greatly decreased after synthe-

sis, as confirmed by XRD analysis. The lower crystallinity after synthesis indicates a smaller sulfur domain size compared to the conventional ones, which suggests that lithium ion conductivity can be facilitated (Fig. 22b). With the introduction of nanotech-

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Fig. 22. (a) Synthesis scheme and optical image of S-CTF-1 from elemental sulfur. (b) Comparative powder XRD patterns of S-CTF-1, monomer 1,4-dicyanobenzene and elemental sulfur (c) Cycle performance of S-CTF-1 measured at 1 C and 2 C. (d) Rate capability of S-CTF-1 measured at various C rates. Reproduced with permission of WILEY-VCH from Ref. [108]. Copyright 2016.

nology to regulate the sulfur domains, excellent electrochemical performance could be achieved. The synthesized CTF had cycle retentions of 85.8% and 81% after 300 cycles at 1 C and 2 C, respectively, with ∼100% Coulombic efficiency, as shown in Fig. 22c and d. Nanospace-confined polymeric sulfur with porous carbon Inverse vulcanization using various cross-linkers is advantageous with respect to cycle retention and sulfur content ratio. However, such inverse-vulcanized sulfur polymers tend to have low electrical conductivities and thus poor rate capabilities. There was an attempt to increase the conductivity by directly using the conductive polymer crosslinker, or by polymerization of the conductive polymer and the crosslinker, separately. However, the polymer had insufficient electrical properties for use as a commercial Li–S battery. In order to overcome these limitations, many researchers have tried to synthesize in-situ polymerized sulfur by adding carbonaceous additives (rGO, multi-wall carbon nanotube, porous carbon, and so forth). Ding et al. reported a nanospace-confinement copolymerization strategy to encapsulate polymeric sulfur in a porous carbon matrix, as shown in Fig. 23a [109]. The material was synthesized by melt diffusion of sulfur into mesoporous carbon, followed by the insertion of DIB additives as crosslinkers (Fig. 23b). The morphology of the composite could be readily controlled by varying the time of thermal polymerization (Fig. 23c–e). The confined polymeric sulfur in both the porous carbon structure and plentiful interparticle paths facilitates harmonic electronic/ionic accessibility to active materials, which mitigates the irreversible loss of soluble poly-

sulfide intermediates and increases the electrochemical kinetics. Remarkably, insulating dilithium sulfide (Li2 S) electrochemically forms on the carbon frame with homogeneous dispersion, supporting the reversible reaction site during battery operations. The initial specific capacity was about 1105 mA h g−1 . Interestingly, the capacity during the initial five cycles gradually increased and remained at 889 mA h g−1 after 100 cycles at 0.5 C (Fig. 23f). Even at a high C-rate (1C), the carbon/polymeric sulfur electrode exhibited superior cyclic stability (703 mA h g−1 after 100 cycles) with high Coulombic efficiency. The capacities were 1094, 905, 815, 751, and 681 mA h g−1 at the rates of 0.2, 0.5, 1, 2, and 5 C, respectively (Fig. 23g). Polymerization of sulfur with oleylamine-functionalized reduced graphene oxide Recently, it was reported that the melt diffusion of carbon with sulfur is not effective due to its low surface affinity. Often, with the progress of the infiltration reaction, solid sulfur tends to aggregate more on the outer surface of carbon than in the carbon pores. Park et al. reported a rational design in which the polysulfide chains are covalently bonded to the conductive medium and the sulfur nano-domains are uniformly distributed within the powder [110]. The structure of this polymer first covalently bonds to DIB through the inverse vulcanization reaction of the linear vulcanization chain. Then, the assembled copolymer (poly S) additionally covalently bonds to the oleylamine (OLA)-functionalized rGO (OrGO), as shown in Fig. 24a. From the structural design standpoint, incorporation of poly S-O-rGO improves the electrical conductivity by providing a conductive path, and covalent bonding of the

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Fig. 23. (a) Schematic of preparation procedure for C/PS composites and electrochemical reaction of polymeric sulfur in confined nanopores. (b) Branched and linear units in polymeric sulfur. The mass ratio of DIB to elemental sulfur depends on the reaction time. SEM images of C/PS composites prepared at different polymerization times: (c) 10 min, (d) 20 min, and (e) 30 min. Electrochemical performances of C/PS (20 min) electrode: (f) cycle performances of C/S and C/PS electrodes at a C rate of 0.5 C and (g) rate performance of the C/PS electrode. Reproduced with permission of American Chemical Society from Ref. [109]. Copyright 2015.

linear sulfur chain to O-rGO effectively decreases the irreversible polysulfide dissolution as well. The XRD patterns and DSC peaks of the poly S-O-rGO structure indicated that the nano-domains were well formed and uniformly polymerized. In addition, the near edge X-ray absorption fine structure (NEXAFS) spectra confirmed that a new carbon and sulfur bonding existed. Moreover, it was confirmed using galvanostatic intermittent titration technique (GITT) that the polarization of poly S-O-rGO was smaller than that of conventional sulfur. Systematic electrochemical analysis showed that the homogeneous distribution of the sulfur nano-domains and the enhanced conductivity derived from O-rGO contributed synergistically to the enhanced electrochemical performance of the Li–S battery; the initial capacity was 1265 mA h g−1 and the cycle retention maintained at 79.2%, even after 500 cycles at 0.5 C (Fig. 24b). In addition, it maintained a capacity of over 900 mA h g−1 at 1.0 C (Fig. 24c). Interestingly, scanning photoelectron microscopy (SPEM) revealed that the poly

S-O-rGO electrode had a relatively low loss of sulfur, as observed by studying the L2,3 -edge energy of sulfur in Fig. 24d. In another approach, Ghosh et al. reported the concept of polymeric sulfur with rGO using cardanol benzoxazine as a crosslinker. Cardanol-based benzoxazine with a high loading of sulfur (90 wt%) was used as a cathode material for Li–S battery [111]. Cardanol derived from renewable raw material is responsible for the molecular designing of benzoxazine, facilitating melt polymerization with sulfur.

Remarks This chapter reviews recent reaction mechanism studies on Li–S batteries using up-to-date analysis methods such as the in-situ transmission X-ray microscopy (TXM) [112], 7 Li nuclear magnetic resonance (NMR) [113,114], X-ray diffraction [115–118], and X-ray absorption spectroscopy [117,119,120] to determine the poly-

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Fig. 24. (a) Synthetic reaction schemes from graphene oxide (GO) to the final nanocomposite (poly S-O-rGO) where linear sulfur chains are uniformly distributed via the termination with 1,3-diisopropylbenzene (DIB) and subsequent covalent linking to oleylamine (OLA). (b) Cycle performances of poly S, S-O-rGO, and poly S-O-rGO when measured at 0.5 C for 500 cycles. (c) The rate performances of the same three electrodes. (d) Sulfur distribution mappings of the S-O-rGO and poly S-O-rGO electrodes based on the intensities at 163.9 eV of scanning photoelectron microscopy (SPEM) analysis when measured after 10 cycles at 0.5 C. Reproduced with permission of WILEY-VCH from Ref. [110]. Copyright 2017.

sulfide intermediates. In addition, this section covers the latest researches on the cell configuration, although they are not limited to nanomaterials and nanotechnology, such as lithium metal anode [121], electrolyte [122–125], separator [126–131], binder [132] as well as the cathode designs as the high sulfur loading [22,133–139]/high sulfur utilization [140–142].

Reaction mechanism Sulfur and Li2 S exist in crystalline solid states, while long-chain polysulfides are soluble in the electrolyte during battery operations. In addition, the upper plateau in the discharge curve is related to the reduction reaction sulfur with Li ions and subsequent dissolution reaction, while the lower plateau is known to be from precipitate Li2 S solid phase by reducing the intermediate polysulfide with Li ions. In 2012, Nelson et al. reported that no crystalline Li2 S was formed at the end of discharge (Fig. 25a), and some soluble polysulfides remain trapped within the cathode matrix, which disagrees to previous results [112]. A similar phenomenon has been reported by See et al. (Fig. 25b). Their results using in-situ 7 Li NMR spectroscopy suggested that sulfur was gradually reduced to a soluble polysulfide species simultaneously with the formation of a solid component (Li2 S) formed near the beginning of the first plateau in the cell structure being studied [113]. In addition, NMR data confirmed that the second plateau was defined by the reduction of the residual soluble species to the solid product (Li2 S). This is

not in agreement with earlier claims that the formation of Li2 S dominates lower plateau capacity fading. In 2017, Conder et al. firstly detected the adsorbed polysulfide on the surface of the fiberglass separator and monitored their evolution during cycling using operando X-ray diffraction (Fig. 25c) [116]. They also demonstrated that adsorption of the polysulfide to SiO2 could be used to buffer the polysulfide redox shuttle. Furthermore, the additional use of fumed silica as an electrolyte additive significantly improves the cycle performance and rate capability of Li–S batteries. The detailed reaction mechanisms of Li–S batteries are not yet fully understood, and new approaches have been reported to investigate these unknown phenomena through new analytical methods.

High sulfur loading The high areal sulfur loading is one of the most important issues for the commercialization of Li–S batteries. In order to increase the loading, Manthiram group proposed a novel cell configuration to insert a carbon-based interlayer between a sulfur cathode and a separator [22,133–139]. The schematic presents novel cell configurations for high sulfur loadings, including carbon interlayers (Fig. 26a). Recently, Chung et al. reported the ultrahigh-loading capability by inserting carbon cotton into the electrode [139]. Comparisons of the carbon-cotton cathodes with those of the reported Li–S batteries in aspect of high-loading cathodes and self-discharge effect are shown in Fig. 26b and c, respectively. Fig. 26d shows

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Fig. 25. (a) Operando transmission X-ray microscopy (TXM) micrographs of a sulfur/carbon composite, where the letters correspond to points along the electrochemical discharge–charge profile. Reproduced with permission of American Chemical Society from Ref. [112]. Copyright 2012. (b) In-situ 7 Li NMR signal overlaid on the electrochemical discharge curve. Reproduced with permission of American Chemical Society from Ref. [113]. Copyright 2014. (c) Discharge curve recorded during the first lithiation process of the Li–S cell with fumed SiO2 electrolyte additive at a rate of 0.02C and (d) the corresponding XRD contour plot. Labels PS1 and PS2 denote the peaks attributed to the PSs–SiO2 interactions. Li2 S refers to the solid end-of-lithiation product. Reproduced with permission of Nature Publishing Group from Ref. [116]. Copyright 2017.

schematic of the fabrication process of the carbon-cotton cathode. Digital and SEM images of cotton and carbon cotton are shown Fig. 26e and f. They successfully improved the cyclic stability using carbon cotton with 70% good dynamic capacity retention after 100 cycles, with high sulfur loading (61.4 mg cm−2 ) and sulfur content (80 wt%). Storage stability is also improved with a high capacitance maintenance of 93% or more and a low time-dependent self-discharge rate of 0.12% per day. High sulfur utilization High-surface-area carbon is widely used for encapsulating sulfur and increasing electrochemical reaction site. However, the high carbon content and low packing density decrease the energy density of cells. In 2017, Pan et al. reported a novel approach that does not rely on sulfur encapsulation (Fig. 27) [141]. This paper reported

the use of low-surface-area carbon fibers to manipulate carbon surface and solvent properties such as donor number, and Li ion diffusion to control the nucleation and growth of sulfur species. The schematic illustration (Fig. 27a) shows that in the traditional meltdiffusion encapsulation approach (MD-Encap), the sulfur species are involved in a 2D surface electrochemical reaction that produces a continuous insulating S/Li2 S film. For the non-encapsulation approach, the sulfur species are deposited onto the carbon fibers by an electrochemical precipitation process from the Li2 S8 catholyte (Fig. 27b). Charge (red) and discharge (black) curves for MD-EncapS/CF and Non-Encap-S/CF in the second cycle are shown in Fig. 27c and d, respectively. SEM images of MD-Encap-S/CF and Non-EncapS/CF at discharge state in the first cycle (Fig. 27e–h) clearly show that this approach promotes the formation of large open sphere and prevents the formation of undesirable insulating sulfur-containing films on the carbon surface. This mechanism leads to ∼100% sulfur

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Fig. 26. (a) Schematic of novel cell configurations including carbon interlayers between the sulfur cathode and the separator for a high sulfur loading. Reproduced with permission of American Chemical Society from Ref. [22]. Copyright 2014. Comparisons of the carbon-cotton cathodes with those of the reported Li–S batteries based on (b) high-loading cathodes and (c) self-discharge effect. (d) Schematic of the preparation of the carbon-cotton cathode. Photographs (top) and SEM images (bottom) of (e) cotton and (f) carbon cotton. Reproduced with permission of American Chemical Society from Ref. [139]. Copyright 2016.

utilization, almost no capacity fading, over 99% Coulombic efficiency, and high energy density (1835 W h kg−1 and 2317 W h L−1 ). This approach provides insights into designing high-energy and low-cost Li–S batteries by controlling the sulfur reaction to lowsurface-area carbons. Lithium metal anode Lithium metal is a promising anode candidate for nextgeneration secondary batteries. Although lithium metal has a very high specific capacity (3860 mA h−1 ), low Coulombic efficiency and the formation of dendritic lithium hinder its application to practical cells. Li et al. reported a self-forming hybrid solid-electrolyte intermediate layer through co-deposition of an organosulfide/organopolysulfide and an inorganic lithium salt using a sulfur-containing polymer as an additive in the electrolyte, which serves as a “plasticizer” in the SEI layer to improve its mechanical flexibility and toughness [121]. Fig. 28a illustrates the inorganic/organic hybrid SEI layer, which provides organic units (organosulfide/organopolysulfide) and inorganic units (Li2 S/Li2 S2 ) in the electrolyte. These lead the formation of hybrid SEI layer (Fig. 28b), which can prevent lithium dendrite growth (Fig. 28c). Cycle performances of the PST-90-electrolyte at a current density of 2 mA cm−2 with various deposition capacity (1–3 mA h cm−2 ) are shown in Fig. 28d–f. The electrochemical performance of the Li–S batteries using electrolytes containing different additives at a rate of 1 C in Fig. 28g. The as-formed SEI layers enable dendrite-free lithium deposition and significantly improve the Coulombic efficiency (99% over 400 cycles at a current density of 2 mA cm−2 ). A Li–S battery based on this strategy exhibited long cycling life (1000 cycles) and good capacity retention. This study reveals an avenue to fabricate a stable SEI layer as a solution to issues associated with lithium metal anodes. Based on this strategy, Li–S batteries exhibit long cycling life (1000 cycles) and excellent capacity retention. Electrolytes With lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as a salt, DME and other ethylene glycol ethers are common electrolytes in Li–S batteries, where they are usually mixed with 1,3-dioxolane

(DOL) owing to their low viscosity and SEI-forming properties. However, these organic solvents greatly suffer from their high volatility, which limits their practical use. Recent, many efforts have been made to solve previous problems through the design of optimized electrolytes for Li–S systems. The electrolytes for Li–S batteries need to meet not only traditional battery electrolyte requirements such as a wide voltage window and sufficient Li ion conductivity to ensure satisfactory rate capability, but also specific requirements for Li–S batteries such as resistance to chemical attack from the Li2 Sn species that are formed upon discharge. In 2014, Cuisinier et al. reported that the combination of a solvent–salt complex [acetonitrile(ACN)2 –LiTFSI] with a hydrofluoroether (HFE) co-solvent represented a new class of Li–S battery electrolytes (Fig. 29a) [122]. They possess stability against Li metal and a viscosity close to those of conventional ethers, but they have the benefit of low volatility and minimal solubility for lithium polysulfides while exhibiting an uncharacteristic sloping voltage profile. In the optimal system, cells can be discharged to full theoretical capacity under quasi-equilibrium conditions while sustaining high reversible capacities (1300–1400 mA h g−1 ) at moderate rates, and capacities of 1000 mA h g−1 with almost no capacity fading at fast discharge rates under selected cycling protocols (Fig. 29b). Their report presented an outlook for sparingly solvating electrolytes as a key path forward for long-lived, high energy density Li/S batteries, including an overview of this promising new concept and some strategies for accomplishing it. Meanwhile, Lin et al. suggested that novel additive, phosphorus pentasulfide (P2 S5 ) in organic electrolyte, is reported to boost the cycle performance of Li–S batteries [143]. P2 S5 promotes the dissolution of Li2 S and alleviates the loss of capacity caused by the precipitation of Li2 S. Also, P2 S5 can passivate the surface of lithium metal and therefore eliminates the polysulfide shuttle phenomenon. The optical image shows that the turbid mixture in Li2 S is insoluble in tetraethylene glycol dimethyl ether (TEGDME). However, the second vial with 1:1 molar ratio of Li2 S/P2 S5 informs the mixture was completely dissolved in TEGDME. (Fig. 29c). Cycle performance with P2 S5 was superior to control cells without P2 S5 (Fig. 29d). This paper concluded that P2 S5 has the role of forming a passivation layer on the surface of lithium metal anode so that converts to Li3 PS4 solid-electrolyte interphase with a dense structure, which can conduct lithium ions while preventing access of the polysul-

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Fig. 27. Two different growth pathways for sulfur species during the electrochemical process in Li–S batteries. (a) Schematic of traditional melt-diffusion encapsulation approach (MD-encapsulation approach). (b) Schematic of non-encapsulation approach: the sulfur species are deposited onto the carbon fibers via an electrochemical precipitation process from the Li2 S8 catholyte. Charge (red) and discharge (black) curves for the second cycle with (c) MD-Encap-S/CF and (d) Non-Encap-S/CF. SEM images obtained from (e,f) MD-Encap-S/CF and (g,h) Non-Encap-S/CF at discharge state at the first cycle. Reproduced with permission of Nature Publishing Group from Ref. [141]. Copyright 2017.

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Fig. 28. (a) Schematic of the formation of stable inorganic/organic hybrid SEI layer, which provides organic units (organosulfide/organopolysulfide) and inorganic units (Li2 S/Li2 S2 ) in the electrolyte. Schematics of (b) the formation of organosulfide/organopolysulfide-Li2 S/Li2 S2 hybrid SEI layer and (c) the protection of the Li metal by the stable inorganic/organic hybrid SEI layer. Cycle performances of the cells using PST-90-electrolyte (magenta symbols) (d) at a current density of 2 mA cm−2 with a deposition capacity of 1 mA h cm−2 ; (e) at a current density of 2 mA cm−2 with a deposition capacity of 2 mA h cm−2 ; (f) at a current density of 2 mA cm−2 with a deposition capacity of 3 mA h cm−2 . (g) The electrochemical performance of the Li–S batteries using electrolytes containing different additives at a rate of 1 C. Reproduced with permission of Nature Publishing Group from Ref. [121]. Copyright 2016.

fide to the surface of the metallic lithium. The protection of the lithium surface prevents the polysulfide shuttle and thus results in a high Coulombic efficiency for battery cycling. The long-term stability of the passivation layer is still under investigation, which will be extremely valuable to this research field. Separator While the dissolution of polysulfides in organic electrolytes appears to be inevitable, a useful approach to address this issue is to block the shuttle pathway of polysulfides. If a porous material with properly tuned pore sizes can act as a sieve to separate targeted ions from an ionic solution on the basis of their sizes and shapes, it may well be considered an ionic sieve. Metal–organic framework (MOF)-based materials that have a large surface area and highly ordered pores with tunable porosity would be appropriate candidates as ionic sieves to mitigate these shuttling polysulfide ions. In addition, MOF-based materials may also satisfy the electrical requirement in terms of their naturally insulating property when used as separators in batteries. In 2016, Bai et al. reported that the MOF-based separator acts as an ionic sieve in Li–S batteries, which selectively sieves Li ions while efficiently suppressing undesired polysulfides migrating to the anode side [126]. Fig. 30a presents schematic of MOF@GO separators in Li–S batteries. The MOF@GO separator acts as an ionic sieve toward the soluble polysulfides. The enlarged image illustrates the MOF pore size (approximately

9 Å), which is smaller than that of the polysulfides (Li2 Sn , 4 < n ≤ 8). Electrochemical performance of Li–S batteries. Discharge capacity and Coulombic efficiency at a rate of 0.5 C over 500 cycles with MOF@GO separators are shown in Fig. 30b. Cycle performance at a rate of 1 C over 1500 cycles with MOF@GO separators and over 1000 cycles with GO separators are shown in Fig. 30c. When a S-containing mesoporous carbon material (approximately 70 wt% sulfur content) is used as a cathode composite without elaborate synthesis or surface modification, a Li–S battery with a MOF-based separator exhibits a low capacity decay rate (0.019% per cycle over 1500 cycles). Moreover, there is negligible capacity fading after the initial 100 cycles. On the other hand, Li et al. achieved size- and ion-selective transport using membranes fabricated from polymers of intrinsic microporosity (PIMs) so that the PIM membrane dramatically reduced polysulfide crossover (and shuttling at the anode) in Li–S batteries, as shown in Fig. 30d [131]. Volumetric energy density as a function of cycle performance and rate capabilities of cells with Celgard membrane (black circles), PIM-1 membrane (light green circles), and PIM-1 membrane with LiNO3 additive (dark green circles) were reported in Fig. 30e and f, respectively. The paper reported that the cells with the PIM-1 membrane achieved high transport selectivity for LiTFSI by decreasing the membrane pore dimensions to subnanometer regimes, which shuts down polysulfide crossover via a sieving mechanism.

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Fig. 29. (a) Concept of a nonsolvent for polysulfides in a Li–S battery via combination of a solvent–salt complex [acetonitrile(ACN)2 –LiTFSI] with a hydrofluoroether (HFE) cosolvent, representing a new class of Li–S battery electrolytes. (b) Galvanostatic intermittent titration technique (GITT) experiments in ACN–HFE (1:1) and DOL–DME (1:1) without LiNO3 additive. The curve in red is constructed from the last data point of each open circuit voltage (OCV) period. The vertical dashed lines highlight the overcharge ascribed to redox shuttle; the horizontal dashed line highlights that the equilibrium reduction potential in ACN–HFE (1:1) decreases during discharge. Reproduced with permission of Royal Society of Chemistry from Ref. [122]. Copyright 2014. A novel additive, phosphorus pentasulfide (P2 S5 ) in organic electrolyte, is reported to boost the cycle performance of Li–S batteries. (c) Photograph of insoluble Li2 S and dissolved Li2 Sx /P2 S5 (1 ≤ X ≤ 8) complexes in TEGDME. (d) The electrochemical performance of Li–S batteries with/without P2 S5 additives. Reproduced with permission of WILEY-VCH from Ref. [143]. Copyright 2013.

Binder

Conclusions and outlook

In 2017, Ling reported that functional groups on binder can unexpectedly immobilize the polysulfides through a nucleophilic substitution reaction [132]. The polymers with chemical leaving groups can react with polysulfide. The molecular structures of poly(vinyl sulfate) potassium salt (PVS) and natural product of carrageenan and their replacement reactions with polysulfide to immobilize polysulfides on the polymer backbones are shown in Fig. 31a. Optical images show the effects of the polysulfide solution exposed to different binders over 24 h (Fig. 31b). Cycle performance of high-loading sulfur electrodes based on PVDF, PVS, carrageenan at 0.05 C. (Fig. 31c and d). PVDF shows the worst efficiency during the early cycles due to excessive polysulfide shuttles (Fig. 31e). PVS has a very good Coulombic efficiency due to the initial fast reaction of PVS with polysulfide. Carrageenan leads to a high cycling efficiency. Discharge–charge voltage profiles of the 10th cycle in the range between 1.8 and 2.6 V are shown in Fig. 31f. The two-plateau discharge behavior indicates the formation of long-chain polysulfide in the charge state. This enhanced performance is due to the decreased shuttle effect by covalently binding of the polysulfide with the polymer binder.

We reviewed the recent problems and various solutions for the commercialization of Li–S batteries. In particular, we focused on the following issues: 1) irreversible loss of soluble polysulfide species, 2) volume expansion during battery operation, 3) low electronic/ionic conductivities of sulfur and Li2 S, 4) reaction kinetics, and 5) high sulfur loading. In the unique reaction mechanism in Li–S battery, unless the inherent insulating properties of sulfur and Li2 S are solved, the use of soluble polysulfide is mandatory to facilitate electron/Li ion transfer. From this point of view, we categorized the carbon-based frameworks as confinement of sulfur, nano-sulfur particle/crystalline domains, and surface chemistry studies. Although the commercialization of Li–S battery remains challenging, the nanotechnological approach can be a viable solution to make this possible.

Acknowledgements This work was supported by the Institute for Basic Science (IBSR006-G1) in South Korea.

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Fig. 30. (a) Shematic of MOF@GO separators in Li–S batteries. The MOF@GO separator acts as an ionic sieve toward the soluble polysulfides. (b) Discharge capacity and Coulombic efficiency at a rate of 0.5 C over 500 cycles with MOF@GO separators. (c) Cycle performance at a rate of 1 C over 1500 cycles with MOF@GO separators and over 1000 cycles with GO separators. Reproduced with permission of Nature Publishing Group from Ref. [126]. Copyright 2016. (d) The scheme and optical image of ion-selective transport across membranes fabricated from polymers of intrinsic microporosity (PIM-1). For Li–S batteries, both stationary and hybrid flow, blocking Li2 Sx (where x ≥ 4) crossover is critical to sustaining peak battery performance. (e) Volumetric energy density as a function of cycle number for Celgard membrane with no LiNO3 (black circles), PIM-1 membrane with no LiNO3 (light green circles), and PIM-1 membrane with LiNO3 additive (dark green circles). (f) Rate performance of PIM-1 membrane with LiNO3 additive. Reproduced with permission of American Chemical Society from Ref. [131]. Copyright 2016.

Fig. 31. Molecular structures of poly(vinyl sulfate) potassium salt (PVS) and natural product of carrageenan and their replacement reactions with polysulfide to form immobilized polysulfides on the polymer backbones. (b) Optical image of adsorption properties of the polysulfide solution exposed to different binders over 24 h. Cycle performance as a function of (c) specific capacity (mA h g−1 ) and (d) areal capacity (mA h cm−2 ), (e) Coulombic efficiency and (f) charge–discharge profile of PVDF, PVS, carrageenan at 0.05 C. Reproduced with permission of Elsevier from Ref. [132]. Copyright 2017.

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