Porous Organic Polymers for Polysulfide Trapping in ...

FEATURE ARTICLE Polysulfide Trapping

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Porous Organic Polymers for Polysulfide Trapping in Lithium–Sulfur Batteries Zhibin Cheng, Hui Pan, Hong Zhong, Zhubing Xiao, Xiaoju Li,* and Ruihu Wang* a theoretical capacity of 3840 mA h g−1, the conventional Li–S batteries could provide an average battery voltage of 2.2 V and a high theoretical energy density of 2570 W h kg−1, which is 2–3 times higher than practical energy density of the commercial lithium ion batteries (LIBs).[4–6] Moreover, low cost, natural abundance, and environmental friendliness of sulfur endow Li–S batteries with great development potential and space compared with LIBs. Despite the overwhelming advantages, the practical application of Li–S batteries suffers from several technological obstacles, such as (i) poor electrical conductivities of sulfur and solid-state discharging products (Li2S2 and Li2S); (ii) the dissolution of soluble lithium polysulfides (PSs) intermediates in the electrolytes and their free migration between cathode and anode, which results in notorious shuttle effect of PSs; (iii) huge volume fluctuation (≈80%) of the active materials during discharge and charge owing to large density difference between element sulfur and solid-state products. The major disadvantage is the shuttle effect of PSs among the above problems. During discharge and charge cycle, the dissolved high-order PSs generated in the cathode move toward the anode and react with lithium metal to form low-order PSs or a passive layer on the anode surface, low-order PSs diffuse back to the cathode and produce high-order PSs again. This process usually causes irreversible loss of active materials and low Coulombic efficiency, which are associated with fast capacity fading, low energy efficiency, severe self-discharge, and poor cycling stability.[1,7–10] To address these problems, considerable efforts have been devoted to the development of cathodes, current collectors, electrolytes, anodes, and separators in Li–S batteries.[11–17] Motivated by the pioneering work of Nazar and co-workers using a highly ordered mesoporous carbon (CMK-3) to constrain sulfur within its channels,[18] various nanostructured carbon materials, such as carbon nanotubes,[19–21] carbon nanofibers,[22,23] carbon spheres,[24,25] graphene,[26–28] graphene oxide,[29–31] and conductive polymers[32–34] have been tailored to serve as sulfur host materials. Many reviews on the applications of these nanostructured carbon materials in Li–S batteries have been reported, and significant success has made in improving rate performance and cycling stability. Gene rally, the shuttle effect of PSs could be alleviated cooperatively by physical confinement of nanopores and chemical affinity of carbon matrices.[35,36] Despite the fact that the heteroatom

Lithium–sulfur (Li–S) batteries have attracted considerable attentions in electronic energy storage and conversion because of their high theoretical energy density and cost effectiveness. The rapid capacity degradation, mainly caused by the notorious shuttle effect of polysulfides (PSs), remains a great challenge preventing practical application. Porous organic polymers (POPs) are one type of promising carbon materials to confine PSs within the cathode region. Here, the research progress on POPs and POPs-derived carbon materials in Li–S batteries is summarized, and the importance of pore surface chemistry in uniform distribution of sulfur and effective trapping of PSs is highlighted. POPs serve as promising sulfur host materials, interlayers, and separators in Li–S batteries. Their significance and innovation, especially new synthetic methods for promoting sulfur content, reversible capacity, Coulombic efficiency and cycling stability, have been demonstrated. The perspectives and critical challenges that need to be addressed for POPsbased Li–S batteries are also discussed. Some attractive electrode materials and concepts based on POPs have been proposed to improve energy density and electrochemical performance, which are anticipated to shed some light on future development of POPs in advanced Li–S batteries.

1. Introduction The ever-growing demand for energy consumption has driven the development of energy-storage technologies. Lithium–sulfur (Li–S) batteries have been considered as one of promising candidates for the next-generation high-energy-density rechargeable batteries.[1–3] By coupling elemental sulfur cathode with a theoretical capacity of 1675 mA h g−1 and lithium metal anode with Z. B. Cheng, H. Pan, H. Zhong, Z. B. Xiao, Prof. R. H. Wang State Key Laboratory of Structural Chemistry Fujian Institute of Research on the Structure of Matter Chinese Academy of Sciences Fuzhou, Fujian 350002, China E-mail: [email protected] Z. B. Cheng, Z. B. Xiao University of Chinese Academy of Sciences Beijing 100049, China H. Pan, Prof. X. J. Li Fujian Key Laboratory of Polymer Materials College of Chemistry and Materials Science Fujian Normal University Fuzhou, Fujian 350007, China E-mail: [email protected] The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201707597.

DOI: 10.1002/adfm.201707597

Adv. Funct. Mater. 2018, 1707597

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doping and/or surface functionalization of the carbon materials promote chemical trapping of PSs and affinity between porous matrix and PSs to a certain degree, the trapping efficiency of PSs is still unsatisfactory due to constrained surface area and low number of the polar sites. In addition, the broad range of pore size and nonuniform distribution of these functional groups have become major impediment for uniform distribution of sulfur and preferential deposition of solid-state discharging products on the carbon matrices. It is highly desirable to develop one type of new host materials with well-defined structure and periodic porous surface. As one class of emerging 2D or 3D porous materials, porous organic polymers (POPs) have captured tremendous interest because of their large surface area, high stability, flexible synthetic strategy, and ready functionality.[37–40] The structures and porous properties of POPs could be elegantly tuned through judiciously selecting building units with different linking groups for a specific purpose. Generally, the shape of the building units determines the topological structure of POPs, while the molecular length of the building units is responsible for pore size of POPs.[41–44] For examples, when diboronic acids with different molecular lengths were applied as building units, 2D square POPs were obtained through the boronate ester formation reaction, their diagonal pore sizes vary from 2.7 to 4.4 nm.[41] The substituents of building units have also exerted important influences on pore chemistry of POPs, we have presented an approach for the porosities modulation of POPs through varying coordination-inert substituents of fluorene-based building units at the molecular level.[44] These synthetic strategies have provided remarkable opportunities to explore the structure-property relationship between host materials and electrochemical performance. In addition, the modular nature of POPs synthesis has allowed for the construction of POPs hosts with suitable pore environment, large specific surface area, high pore volume, and strong PSs affinity by customizing functionalized building blocks at molecular level, which is greatly beneficial for the development of Li–S batteries with high discharging capacity and long cycle life. These unique properties endow POPs with attractive advantages in enhancing sulfur utilization and suppressing the shuttle effect of PSs. Recent research advances have demonstrated that POPs are one type of promising materials as sulfur hosts, interlayers, and separators. However, to our best knowledge, the reviews for POPs are mainly focused on gas storage and separation, sensor, proton conductivity, drug delivery, and heterogeneous catalysis,[41,45–50] POPs applications in Li–S batteries have not been systematically summarized hitherto. In the feature article, we summarize research works on POPs in Li–S batteries for the first time. Depending on the categories of POPs,[37,38,50] the major contents are involved in covalent organic frameworks (COFs), covalent triazine-based organic frameworks (CTFs), porous organic frameworks (POFs), hypercrosslinked polymers (HCPs), and main-chain imidazolium ionic polymers (ImIPs) (Figure 1). These POPs serve as effective sulfur host materials, interlayers, and separators to alleviate the migration of PSs from cathode to anode. Some recent advances for POPs-derived carbon materials are also mentioned. They are validated to be promising polar materials for uniform distribution of the active sulfur species and effective suppression of PSs shuttle effect during cycling.


Zhibin Cheng received his B.S. degree from the School of Chemistry and Chemical Engineering at Shanxi University in 2014. He is currently a Ph.D. candidate in Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences under the supervision of Professor Ruihu Wang. His research interests focus on the construction of porous organic polymers for advanced energy storage/conversion. Xiaoju Li received her Ph.D. in physical chemistry from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences in 2006, and she worked as a postdoctoral fellow in Department of Chemistry, University of Idaho. She joined Fujian Normal University in December 2008. Her research interests mainly focus on porous polymer materials for heterogeneous catalysis and energy storage. Ruihu Wang received his Ph.D. in physical chemistry from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences in 2004. From 2004 to 2008, he worked as a postdoctoral fellow at Department of Chemistry, University of Idaho and Center for of Environmentally Benign Catalysis, University of Kansas. He joined the institute as a professor in September 2008. His research interests mainly focus on porous organic polymers, green catalysis, and energy storage materials.

2. Porous Organic Polymers 2.1. Covalent Organic Frameworks COFs are a class of crystalline POPs, in which organic building blocks are precisely integrated into extended structures with periodic skeletons and ordered pores.[51,52] Depending on the types and dimensions of building blocks, COFs can be

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Figure 1. POPs and their applications in various fields. The COF figure reproduced with permission.[68] Copyright 2018, Elsevier. CTF figure reproduced with permission.[73] Copyright 2014, Royal Society of Chemistry. POF reproduced with permission.[87] Copyright 2014, Royal Society of Chemistry. HCP figure reproduced with permission.[95] Copyright 2017, American Chemical Society. ImIP figure reproduced with permission.[102] Copyright 2017, Wiley-VCH.

categorized into 2D and 3D COFs. Among them, 2D COFs have captured considerable interests in energy storage,[39,42,53] and have shown interesting applications as sulfur host materials, interlayers, and separators for suppressing the shuttle effect of PSs. An intriguing feature of 2D COFs is that the stacking of 2D planar sheet could form a layered eclipsed or staggered structure, which not only provides highly ordered nanopores for uniform impregnation of sulfur, but also permits charge migration and ionic transport through the frameworks, resulting in enhanced redox kinetics of the active sulfur species. Besides physical confinement of PSs by periodic nanopores, the formed linking groups, such as imines, boroxines, triazines and hydrazones, could interact with sulfur species through lithiophilic and/or sulfiphilic interactions, which cooperatively constrain soluble PSs within the cathode region, thus resulting in high sulfur utilization and stable cycling performance. Schiff-base reaction based on the reversible condensation of an amine and an aldehyde is a popular reaction for the construction of COFs.[39,54] The formed imine group not only serves as an effective linkage of various building blocks, but also imine nitrogen atom could coordinate with lithium ion, the resultant lithiophilic interaction is beneficial for alleviating the migration of PSs between cathode and anode. Wang and co-workers[55] reported a 2D porphyrin-based COF (Por-COF) through the condensation reaction of 5,10,15,20-tetrakis(4-benzaldehyde)porphyrin and p-phenylenediamine under solvothermal conditions (Figure 2a). Por-COF is a microporous crystalline material, and exhibits a 4.1 Å slipped AA stacking mode (Figure 2b). The nitrogen sorption measurement at 77 K shows that Por-COF possesses large Brunauer-Emmett-Teller (BET) surface area of 1095 m2 g−1, relatively high pore volume


of 0.71 cm3 g−1, and narrow pore size distribution of 1.55 nm (Figure 2c). Considering the pore size of Por-COF is more than two times higher than that of S8 molecule (0.7 nm), the aggregation of S8 is restricted in the channel, which favors intimate contact between sulfur and Por-COF. After loading 55 wt% sulfur through conventional melting-diffusion method, sulfur is homogeneously accommodated into the nanopores of PorCOF (Figure 2d). Strong chemical interaction between imine group and PSs cooperatively confines soluble PSs within the cathode region, thus alleviating the shuttle effect of PSs. In addition, π–π stacking of the porphyrin units in the adjacent layers facilitates charge carrier mobility and mass transfer. These advantages endow the Por-COF/S electrode with high sulfur utilization, low polarization, and superior cycling stability. As shown in Figure 2e, the Por-COF/S electrode delivers a reversible discharging capacity of 929 mA h g−1 in the second cycle at 0.5 C, which slowly decreases to 721 mA h g−1 in the 100th cycle and 633 mA h g−1 in the 200th cycle. During extended cycling, the average capacity decay is 0.16% per cycle, and the Coulombic efficiency is above 96% after the first three cycles. It has been confirmed that medium capacity and relatively low Coulombic efficiency originate from an incomplete redox conversion between element sulfur and Li2S during cycling. The Li2S2 and Li2S8 intermediates are formed in the discharge and charge process, respectively. Even though the electrochemical performance of Por-COF/S is not competitive with those in some carbon-based host systems,[56–58] as a porous material with atomically precise periodicity in the skeleton, this research provides an ideal platform to explore the structure-property relationship between host materials and electrochemical performance. The modular nature of COFs synthesis allows for tailorable construction of various 2D COFs with suitable pore sizes and functionalities for sulfur storage in Li–S batteries. A 2D AzoCOF containing multiple polar sites was constructed through the Schiff-base condensation reaction between triformylphloroglucinol and 4,4-azodianiline (Figure 3a).[59] Azo-COF possesses large surface area of 1150 m2 g−1, total pore volume of 0.90 cm3 g−1, and small mesopore at 2.6 nm (Table 1). After 39 wt% sulfur is diffused into nanopores of Azo-COF. AzoCOF/S delivers high initial discharge capacities of 1536 and 1044 mA h g−1 at 0.1 and 1 C, respectively, corresponding to 91.9% and 62.4% of the theoretical capacity, respectively. After 100 cycles, the electrode still presents high discharge capacities of 741 and 602 mA h g−1, respectively.[60] The pore size and surface area of COFs have exerted important effects on sulfur distribution and electrochemical performance. Cai and co-workers[61] used an imine-based 2D COF (TAPB-PDA-COF) from solvothermal reaction of 1,3,5-tris(4aminophenyl)benzene and terephthaldehyde as a sulfur host (Figure 4a).[62] TAPB-PDA-COF possesses relatively low BET surface area of 242 m2 g−1, and pore size distribution is centered on 2.5 nm. After loading 60 wt% sulfur through conventional melting-diffusion method, the inherent pores in TAPB-PDA-COF are not large enough to accommodate so high content of sulfur, part of sulfur is dispersed in the interstitial space of TAPB-PDA-COF particles. Moreover, sulfur is in a crystalline state, which is different from those in Por-COF/S[55] and Azo-COF/S.[60] The interstitial distribution of sulfur in

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Figure 2. a) Schematic illustration for the synthesis of Por-COF. b) Simulated eclipsed structure in an AA stacking mode for Por-COF. c) N2 absorption/ desorption isotherm of Por-COF and inset is the corresponding pore size distribution curve. d) Schematic illustration for sulfur impregnation and the charge/discharge process of the Por-COF/S composite. e) Cycling performance of Por-COF/S at 0.5 C. Reproduced with permission.[55] Copyright 2016, Royal Society of Chemistry.

TAPB-PDA-COF is unfavorable for utilization efficiency and trapping of sulfur. Therefore, TAPB-PDA-COF/S exhibits relatively low discharge capacity and fast capacity decay when compared with Por-COF/S and Azo-COF/S. When different conductive additives, such as acetylene black (A-B) and super-P (S-P), are used in the cathode materials. The use of S-P (TAPB-PDACOF/S@S-P) provides higher electrochemical performance in terms of discharging capacity, rate capability, and cycling stability than the use of A-B (TAPB-PDA-COF/S@A-B) under the same conditions, which is mainly ascribed to the different morphology of A-B and S-P (Figure 4c). A-B forms a branch-like conductive network, while S-P is fine powder and is dispersed around the active materials to form a more uniform conductive network, which facilitates ionic/electronic transport and reduces the surface resistance of electrolyte-electrode, thus improving sulfur utilization and cycling stability of the electrodes. The application of 2D imine-based COFs with smaller pore size has also been explored in Li–S batteries.[63] DMTA-COF was prepared through the Schiff-based reaction of 2,5-dimethoxy-1,4-dicarboxaldehyde and 4,4′,4″,4′″-(ethene-1,1,2,2-tetrayl)tetraaniline (Figure 5a).[62] An AB-stacking mode in DMTA-COF results


in low BET surface area of 305 m2 g−1 and small pore size of 0.56 nm. Strikingly, the pore size is smaller than that of PSs, which allows lithium ions to pass through DMTA-COF, while blocks PSs migration between cathode and anode (Figure 5b). This holds a great promise to use as a separator for improving cycling stability of Li–S batteries. Cai and co-workers have coated DMTA-COF onto a ceramic separator, the electrochemical performance of the resultant electrode is much better than that of the pristine ceramic separator and the super-P coated ceramic separator (Figure 5c). This is first report to use COFs as a coating layer of the ceramic separator, which holds the potential to arouse significant interest in further development of Li–S batteries and even other electrochemical devices. The 2D imine-based COFs allow for uniform distribution of sulfur in the nanopores, but PSs are trapped indirectly by interacting with lithium ions, it is not as efficient as direct trapping of PSs anions. Yaghi and co-workers[64] reported a B,O doubledoped COF-1 through the condensation reaction of phenyldiboronic acid (Figure 6a). The positively polarized boron atoms and negatively polarized oxygen atoms in boroxine units could guarantee simultaneous adsorption of PS anions and lithium

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Figure 3. Schematic illustration for the preparation of a) Azo-COF and b) Azo-COF/S. c) Cycling performance of the S/Azo-COF electrode at c) 0.1 C and d) 1 C. Reproduced with permission.[60] Copyright 2015, Royal Society of Chemistry.

ions in soluble PSs. COF-1 shows an AA stacking mode, BET surface area and the pore size are 430 m2 g−1 and 1.5 nm, respectively. Tang and co-workers[65] used COF-1 as a sulfur host material and explored the effects of the electroactive boroxine units on PSs trapping and electrochemical performance. Table 1. Porous properties of POPs calculated from nitrogen adsorption analysis. POPs

BET-specific surface area [m2 g−1]

Pore size [nm]

Total pore volume [cm3 g−1]

References

Por-COF

1095

1.55

0.71

[55]

Azo-COF

1150

2.6

0.9

[60]

TAPB-PDA-COF

242

2.5

–

[61]

DMTA-COF

305

0.56

–

[63]

COF-1

430

1.5

–

[65]

–

COF-5

1103

2.7

COF-10

1073

3.4

–

TB-COF

708

1.2

0.28

[68]

CTF-1

789

1.23

0.37

[73]

PAF

2023

1.6

1.73

[84]

POP-A

1362