Boosting sulfur‐based cathode performance via confined reactions in covalent organic frameworks with polarized sites

The widespread promotion of the sulfur‐based cathode is continuously threatened by the dissolution or slow kinetic reactions of polysulfides, resulting in a deterioration of capacity and volume expansion. Herein, a hierarchical imine‐based covalent organic framework (COFs) with AA stacking structure is constructed as a sulfur host matrix. The one‐dimensional channels in COF are decorated with polarized methoxy groups which can confine and immobilize sulfur species through weak interactions, alleviating the shuttle effect during the battery tests. The composite electrode (COF@S) is fabricated through the melt‐diffusion method in one‐pot synthesis and exhibits a slow fading rate (0.20% each cycle) in a 0.5 C stability test. A promising mechanism for constructing the cathode composite is demonstrated in lithium‐sulfur chemistry in this work.


| INTRODUCTION
The burgeoning appetite for advanced energy systems is driving the accelerated development of diverse technologies. 1 Lithium-sulfur (Li-S) chemistry is regarded as the most promising battery technology for future electricity storage applications, due to their remarkable storage capacity, high natural abundance, and high power density (2600 Wh/kg). 2 Unfortunately, in addition to these considerable benefits of the Li-S battery systems, they also suffer from several inherent drawbacks that must be solved.
One of their major challenges is the "shuttle effect," 3 and different forms of polysulfides on the electrode are dissolved in the organo-electrolytes in the period of cycling tests, leading to the exfoliation of sulfur-based cathode and severe self-discharging phenomenon. 4 In addition, the different densities observed for Li 2 S and S (1.66 vs. 2.07 g/cm 3 ) result in the remarkable bulk expansion (80%) of the cathode under lithiation, which gives rise to a severe capacity decay of the cell. 5 Moreover, the relatively poor conductivity of Li 2 S (3.6 × 10 −7 S/cm) or S (5 × 10 −30 S/cm) in a cell leads to the inefficient utilization of S. All of these factors Battery Energy. 2023;2:20230002.
onlinelibrary.wiley.com/r/batteryenergy mentioned above hinder the promotion and commercial application of sulfur-based energy system seriously. 6 To overcome these challenges, various methods have been developed to prepare functional cathode composites or separator materials. An effective strategy is to seal the sulfur with a virous host matrix, restraining the dissolution of different polysulfides in batteries and enhancing cycling performance and capacity retention. Such as various mesoporous carbons, 7 metal oxides, 8 polymers, 9,10 and metal-organic frameworks (MOFs), 11 which are confirmed as an efficient strategy to confine sulfur and polysulfides. Furthermore, the synthesized separator materials with decorated polarized sites and specific pore size is another useful method to alleviate the shuttle effect, 12 which could selectively sieve Li + ions and suppress unexpected transference of polysulfides. In pioneering efforts, Tarascon et al. have first used MIL-100 as a persistent host medium to load S, 13 and found that it was favorable for capturing polysulfides due to its mesoporous cavity (∼25-29 Å) and the small pore apertures (5-9 Å). In another way, Zhou et al. prepared a microporous MOF@GO composite as a specified separator for Li + sieving, 14 exhibiting remarkable cycling stability as well as a low-capacity fading efficiency. However, most MOF or MOF-derived materials are prone to structural collapse during the heat treatment of sulfur doping, resulting in reduced porosity and surface areas, which is not favorable for the polysulfide adsorption.
Covalent organic frameworks (COFs), 15-18 a rising star among porous materials, can be finely tuned through various functional groups and different organic linkers in atomic scale, exhibiting unique properties and excellent stability even at high temperature and pressure. Its ordered nanopores could be used for sulfur encapsulation, which suppresses the diffusive loss of soluble polysulfide intermediates. 19 For example, Kaskel et al. conducted one-dimensional (1D) channel COF with dual active sites designed for intriguing Li-storage in Lithiumorganosulfide batteries. 20 Chen et al. assembled COF with conductive carbon nanotube for high performance in Li-S battery. 21 It should be mentioned that although COFs are with random direction on electrodes, their 1D channels facilitated rapid and uniform Li + transport across the host matrix, 22 and greatly enhanced rate performance and improved the ionic mobility of the pristine matrix in previous reports. [23][24][25] Owing to their chemical stability, tunable pore size and high surface area, COFs are considered to be promising platforms for immobilizing sulfur element and confining the mobile polysulfides.
In this study, we constructed a novel composite marked as COF@S elaborately, which exhibited excellent electrochemical capacity, cycling stability and enhanced rate performance. The imine-based TPB-DMTP-COF, which shows great stability in water, strong acids, and bases, was used as a starting matrix to introduce sulfur and conductive carbon into its channels via one-pot synthesis. 26 Moreover, the channels decorated with polar methoxy groups in COF would trap and immobilize sulfur species through the weak interaction, alleviating the shuttle effect during battery cycling. Furthermore, because of their 1D channels and polarized sites, this composite cathode displays improved electrochemical capability and enhanced rate performance among other comparable battery systems. On the other side, on the basis of the theoretical calculation, we found that methoxy moieties have a possibility to attract polysulfides, and then suppress the "shuttle effect." This kind of sulfur-based composite sets a new trend for further research in advanced power technologies.

| RESULTS AND DISCUSSION
The COF used within this work is constructed through a Schiff-base reaction under 120°C for 72 h and the detailed synthesis procedures can be accessed in the supporting information, which shows the simulated AA stacking structure ( Figure 1A). Then, the composite was obtained by heating blends of COF and S (labeled as COF@S) ( Figure 1A). 27,28 To verify and illustrate the successful synthesis of COF and COF@S, various techniques including powder X-ray diffraction (PXRD), transmission electron microscopic (TEM), N 2 adsorptiondesorption experiments, and X-ray photoelectron spectra (XPS) were investigated.
PXRD pattern of COF ( Figure 1B) reveals a crystalline structure with diffraction peaks at 2θ = 2.76°, 4.82°, 5.60°, and 7.42°, which assigned to (100), (110), (200), (210) and (220) facets, separately. The simulated result is consistent with the laboratory measurement, identifying the perfect crystal among COF and without apparent impurity phase. In addition, COF still retains partial crystalline after being impregnated with sulfur element through its PXRD pattern ( Figure 1B), indicating its outstanding stability. Next, we measured the N 2 adsorption-desorption curves (Figures 1C and S1) of COF together with COF@S at 77 K. The Brunauer-Emmett-Teller areas of COF and COF@S were acquired as 1836.46 and 210.97 m 2 /g. The ultralow surface area of COF@S demonstrates the polarized channels of COF are filled with S successfully. Thermogravimetric analysis (TGA) was also employed to illustrate the relative content of S in COF@S ( Figure 1D). Compared with COF, the TGA curve of COF@S showed two weight loss processes that corresponded to the removal of sulfur (63.5 wt%) and COF (36.5 wt%).
Moreover, the binding analysis of COF@S through XPS was explored. As illustrated by Figure S2, the whole scanned spectrum of COF@S identifies the appearance of each constituent element (C, N, O, S) at the range of 0-800 eV. The C 1s spectrum (Figure 2A) reveals several characteristic peaks for C-C/C═C, O-CH 3 , and C-NH 2 . The multiple peaks of N 1s are plotted in Figure 2B, corresponding to C-NH 2 (398.85 eV) and C═N (399.88 eV), respectively. The detected signals of sulfur at 164.45 and 165.68 eV are recognized as the sulfur (S-S) bond ( Figure 2C). 29,30 Furthermore, the TEM images of COF@S ( Figure 3) exhibit lengths of ca. 750 nm with the homogeneous mapping patterns of individual elements. In addition, compared with the flake-like COF in the size of ca. 100 nm in Figure S3, COF@S shows rod-like and more compact after sulfur loading.
More importantly, the interlayer interaction is reinforced after introducing two electron-donating methoxy groups, improving its stabilization through π-π packing. This is also demonstrated by the noncovalent interaction analysis, which examines weak interactions in chemical systems ( Figure 4A). 31 The π-π packing has   been identified as the green isosurface in the reduced density gradient among layers, which is a strong drive to stabilize the framework. Another intermolecular interaction demonstrated in Figure 4A is the red isosurface in the center of the benzene rings, arising from the ring stretch or interlayer repulsive interactions.
Furthermore, the 1D channels decorated with polar methoxy groups would capture and immobilize sulfur species through weak interactions and then alleviates the shuttle effect during battery cycling. To verify this prediction, the electrostatic potential (ESP) of the fragment in COF was calculated ( Figure 4B). 32,33 The results show that it has a positively polarized region around the methyl group (17.98 kcal/mol), benzene rings (12.25 kcal/mol) and the negatively polarized region around O atoms (−13.12 kcal/mol) as well as N atoms (−25.44 kcal/mol), ensuring the adsorption of sulfur anions and Li + cations simultaneously during the cycling process. 34,35 Thus, inhibition of polysulfide dissolution in organic electrolytes would allow more uniform redeposition of sulfur. Furthermore, the CHELPG atomic charge on the fragment was calculated to determine the partial charge of each atom ( Figure 4C). 36 The charges of -CH  Table S1).
Such a unique feature distinguishes this imine-based COF ( Figure S4) from other single-doped materials designed to adsorb only Li + cations or S n 2− (4 ≤ n ≤ 8) anions. This work emphasized the regulation of polarized sites in pores for the polysulfide trapping and reveals the mechanism of interaction between electroactive species and porous matrix, which is profitable for the establishment of a battery system with enhanced rate capability, long cycling life, and high capacity retention.
To get a deep understanding of the electrochemical performance of the COF@S cathode, the cyclic voltammogram (CV) measurement was measured over a region of 1.5-2.8 V (0.1 mV/s) ( Figure 5A). Obvious reductive signals are observed at both 2.3 and 2.0 V, standing for the typical transformation of elemental S to different kinds of soluble polysulfides, before reducing to the insoluble products (Li 2 S 2 /Li 2 S) subsequently. In the following period of oxidative procedure, two overlapped oxidative signals are found at around 2.40-2.60 V, assigning to the period of low (Li 2 S, Li 2 S 4 ) or highdegree polysulfides converting to the S.
Similarly, the nature of charging and discharging behavior is evaluated for the composite electrode under different cycling numbers in 0.5 C ( Figure 5B), which shows excellent reversibility compared with the electrode without carbon black ( Figure S5). As observed, the COF@S cathode exhibits two distinct plateaus where a short plateau starting with the higher voltage of around 2.3 V Li+/Li and a long platform originating from the potential of about 2.0 V Li+/Li , corresponding to the dual continuous reductive peaks in the CV test. 37 The upper plateau represents the conversion process of S to high-order soluble Li 2 S n (n ≥ 4), and the other plateau represents the formation of insoluble low-order compounds. 38 Moreover, the cycling stability test of the COF@S composite cathode is applied with a 0.5 C charging density ( Figure 5C). Assembled coin cells display a primary capacity of 1409 mAh/g compared with other reports recently (Table S2), and also after 200 cycles, a considerable storage capability of up to about 819 mAh/g is held, which calculating for 0.20% capacity decay each cycle. Meanwhile, the cycling reliability of COF@S cathode is demonstrated at different rates, and specific capacities of 1086 (0.1 C), 893 (0.2 C), 751 (0.5 C), and 679 (1.0 C) mAh/ g are achieved. Furthermore, a total reversible capacity of 892 mAh/g is captured upon switching back the rate from 1.0 to 0.1 C (Figure S6), suggesting a highly enhanced rate capability. Furthermore, as shown in Figure S7, compared with the pristine PP separator, the COF@PP show a great effect in inhibiting the migration of polysulfides, even after 12 h. These better electrochemical performances could be owed particularly to the introduction of polarized methoxy group, and the π-π stacking structure among the hierarchical layers is also beneficial to the conductivity. In summary, a high-quality Li-S battery with advanced cathode composite has been successfully established by using imine-based 2D COF as a host matrix, which is decorated with positively and negatively polarized sites for the confinement of lithium polysulfides in 1D channels synergistically. In addition, this unique host matrix can inhibit the dissolution of versatile and soluble polysulfide intermediates and then effectively discourage the "shuttle effect." In particular, this effort will provide a unique approach for the precise preparation of a new porous host matrix at the atomic scale for nextgeneration batteries.