Sulfur polymerization strategy based on the intrinsic properties of polymers for advanced binder‐free and high‐sulfur‐content Li–S batteries

Abstract

Lithium–sulfur (Li–S) batteries are the promising next‐generation secondary energy storage systems, because of their advantages of high energy density and environmental friendliness. Among numerous cathode materials, organosulfur polymer materials have received extensive attentions because of their controllable structure and uniform sulfur distribution. However, the sulfur content of most organosulfur polymer cathodes is limited (S content <60%) due to the addition of large amounts of conductive agents and binders, which adversely affects the energy density of Li–S batteries. Herein, a hyperbranched sulfur‐rich polymer based on modified polyethyleneimine (Ath‐PEI) named carbon nanotube‐entangled poly (allyl‐terminated hyperbranched ethyleneimine‐random‐sulfur) (CNT/Ath‐PEI@S) was prepared by sulfur polymerization and used as a Li–S battery cathode. The high intrinsic viscosity of Ath‐PEI provided considerable adhesion and avoided the addition of PVDF binder, thereby increasing the sulfur content of cathodes to 75%. Moreover, considering the uniform distribution of elemental sulfur by the polymer, the utilization of sulfur was successfully improved, thus improving the rate capability and discharge capacity of the battery. The binder‐free, sulfur‐rich polymer cathode exhibited ultra‐high initial discharge capacity (1520.7 mAh g⁻¹ at 0.1 C), and high rate capability (804 mAh g⁻¹ at 2.0 C). And cell‐level calculations show that the electrode exhibits an initial capacity of 942.3 mAh g⁻¹_electrode, which is much higher than those of conventional sulfur‐polymer electrodes reported in the literature.

INTRODUCTION

The development of electrochemical energy storage technology is the key for achieving the consumption and large-scale application of renewable energy. The growing demand for renewable energy supplies requires high-performance and low-cost electrochemical energy storage devices.¹ Currently, lithium-ion battery technology cannot fully meet the demand. Therefore, researchers are gradually focusing on next-generation batteries.^2,3 Among many electrode materials, elemental sulfur, as a cathode material, has excellent advantages for rechargeable batteries because of its low price and environmental friendliness. More importantly, Li–S batteries can provide a high theoretical energy density of 2600 Wh kg⁻¹ through a redox reaction.^4,5 Although Li–S batteries have outstanding advantages, their development faces many challenges. First, polysulfide, a soluble electrochemical intermediate product, causes a “shuttle effect,” resulting in low utilization of active materials and rapid capacity decay. In addition, during the charging and discharging, the high-volume change induced by the mutual conversion of sulfur and lithium sulfide destroys the electrode structure.^6–8 Finally, both sulfur and lithium sulfide have poor electrical conductivity and adhesion, thus requiring conductive agents and binders, which reduces the overall energy density.⁹ Among these limitations, the rapid capacity decline of Li–S batteries is mainly caused by the “shuttle effect.”¹⁰ Considering the existing problems of Li–S battery cathodes, previous researches mainly focused on the combination of elemental sulfur and various carbon materials.^11–13 Various carbon host materials with complex nanostructures such as activated carbon, carbon nanotubes (CNTs),^14,15 graphene,¹⁶ and hollow carbon spheres¹⁷ have been designed to suppress side reactions such as the shuttle effect, showing good electrochemical performance.^18,19 Although such methods can alleviate the above-mentioned problems to a certain extent, nonpolar carbon materials can only restrict polysulfides through physical confinement, resulting in a very limited inhibition of the shuttle.²⁰ Especially when the cathodes have high active material content (S content>70%), the gradual dissolution of polysulfides will decrease the overall electrochemical stability during charge–discharge cycles.²¹ Moreover, the uneven distribution of elemental sulfur under high sulfur content aggravates the deterioration of discharge capacity and rate performance. In addition, to suppress the shuttle effect more effectively, researchers have designed various types of improvement strategies (e.g., metal oxides,²² metal selenide,²³ metal sulfides,^24,25 nonmetals-based polar materials such as N, O, P containing carbon materials,^26,27 and various electrocatalysts^28–30) to confine polysulfides by chemical reaction^31,32 and electrocatalysis.^33–35 Although these materials are more efficient than carbon materials, the introduction of inactive materials reduces the content of elemental sulfur in the electrode, thus limiting the energy density of Li–S batteries. Therefore, a novel type of sulfur cathode system capable of combining high sulfur content and good electrochemical performance should be developed for the practical application of Li–S batteries.

Accordingly, organosulfur polymers cathodes formed by strong chemical bonds (CS bond) with high sulfur content and uniform sulfur distribution have shown attractive potential application in Li–S batteries cathode materials.^36–40 Pyun et al.⁴¹ reported a chemically stable polymer material prepared from the direct copolymerization of elemental sulfur with vinyl monomers, and explored its possibility as an active material for Li–S batteries. Afterward, a series of organosulfur polymers cathodes, such as olefin/alkynes derived materials,^42–45 nitriles,⁴⁶ epoxy,⁴⁷ and mercaptan materials.^48,49 These materials contain special functional groups which chemically stabilize the sulfur polymers, inhibit depolymerization, and form stable polymers with high sulfur content.⁴¹ In organosulfur polymers cathodes, elemental sulfur is uniformly dispersed in the nonsulfur organic framework, thus avoiding the aggregation of elemental sulfur.⁴² In addition, the nonsulfur organic framework and CS bond inhibit the shuttle effect and stabilize the internal reaction of batteries by chemical adsorption and covalent bonding.⁴³ Therefore, organosulfur polymers generally show better electrochemical performance than S₈. However, previous studies have mainly focused on the combination of elemental sulfur and small molecular organics.^50–54 Similar to S₈, most small-molecule organosulfur polymers are nonconductive and nonadhesive, requiring the addition of the conductor and the binder when used as cathodes. It decreases the active material (sulfur) content in cathodes, thereby reducing the energy density of batteries.⁵⁰ Therefore, by introducing viscous polymers to prepare novel organosulfur polymers as active materials, the addition of binders can be avoided to increase the sulfur content of the cathode.

Polyethyleneimine (PEI) is a water-soluble polymer with a specific viscosity, and it can be used as a cathode binder in batteries.^55–58 In comparison with common PVDF binder, PEI contains a large number of nitrogen functional groups (e.g., NH and NH₂), which enable PEI to better confine polysulfides in electrochemical reactions.⁵⁷ In addition, the presence of a large number of active functional groups indicates that PEI can introduce different functional groups through surface modification or graft copolymerization and thus polymerize with sulfur as a nonsulfur organic framework. However, polymers such as PEI have beneficial adhesion properties and a large number of active groups, which have only been used in separator modification and modified cathodes, but have not been sufficiently studied in sulfur polymerization.^59–61 In summary, although organosulfur polymer cathodes exhibit impressive electrochemical performance, the introduction of their additional properties and the modulation of their functional groups need to be further developed.

Here, we proposed an idea for polymer design and synthesized an allyl-terminated hyperbranched dendrimer-like PEI (Ath-PEI) with intrinsic viscosity. This structure was used as a nonsulfur organic framework for polymerization with sulfur (Figure 1). During polymerization, a few carboxylated CNTs were added simultaneously. As a result, the CNT-entangled poly (allyl-terminated hyperbranched ethyleneimine-random-sulfur) (CNT/Ath-PEI@S), which has excellent conductivity, was synthesized by a one-pot method. It can make sulfur dispersed well in the framework structure via physical confinement and inhibited the migration of polysulfides by chemisorption and covalent bonding. More importantly, the CNT/Ath-PEI@S can be used as a Li–S battery cathode material without additional binder, which helps to increase the active material content of the cathode materials, and thus increasing the overall energy density of batteries (Figure 1A).^62–66 Moreover, theoretical calculations showed that functional groups such as OH, COC and tertiary amine contained in Ath-PEI can interact with polysulfides, which is beneficial to improving the electrochemical performance (Figure 1B). Considering the well design of the polymer structure, the cathode material exhibits good electrochemical performance, including excellent rate capability (804 mAh g⁻¹ at 2.0 C rate) and cycling stability (decay rate per lap, 0.038% after 400 cycles at 1.0 C). Furthermore, based of evaluation of the binding performance and preparation of cathodes with high sulfur content (75%), this material demonstrates its feasibility for practical applications.

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EXPERIMENTAL

Synthesis of Ath-PEI

First, 2 g of PEI was added to a 20 ml beaker and dissolved in 10 ml of isopropanol (i-PrOH), then the solution was transferred to a two-necked round bottom flask equipped with a magnetic stirrer, and 6.63 g of allyl glycidyl ether (AGE) was added. The reaction was then stirred at 50°C for 24 h to obtain a pale yellow oily crude product. After the reaction was completed, the crude product was slowly transferred to a vacuum oven at 50°C for vacuum drying for 48 h to obtain Ath-PEI.

Synthesis of CNT/Ath-PEI@S and CNT/S

First, 0.2 g of Ath-PEI and 0.8 g of sulfur (>99.5%; Sigma–Aldrich, US) were dissolved in 5 ml of chloroform (CHCl₃) and 5 ml of carbon disulfide (CS₂), respectively, and then the two solutions were mixed uniformly to obtain a pale-yellow homogeneous solution. Subsequently, 0.2 g CNTs was added to the solution and sonicated for 3 h to ensure uniform dispersion of CNT. The mixture was heated and stirred at 60°C for 30 min to ensure complete removal of CS₂ and CHCl₃, and then the solid mixture was kept at 175°C for 6 h to obtain CNT/Ath-PEI@S. H-CNT/Ath-PEI@S with high sulfur content were synthesized in a similar process to CNT/Ath-PEI@S but required 0.9 g sulfur, 0.1 g Ath-PEI, and 0.1 g CNTs. To exclude the influence of the preparation process, CNT/S was prepared by a similar method. Dissolved 0.8 g of sulfur in 8 ml of CHCl₃ and CS₂ (1:1 vol%), then added 0.2 g of CNTs to sonicate to ensure dispersion. The mixture was heated to 60°C for 30 min to ensure complete removal of the solvent. And the sulfur and CNTs were mixed uniformly in a mass ratio of 4 to 1 and heated at 175°C for 6 h.

Materials characterizations

The structural plausibility of the material was analyzed by ¹H-Nuclear magnetic resonance spectroscopy (¹H-NMR; a Bruker AVANCE III 500 device) using chloroform-D as solvent. Elemental composition and possible chemical bond analysis of all materials were done by Fourier transformed infrared (FTIR; a Thermo-Fisher 6700 device) and X-ray photoelectron spectroscope (XPS; an ESCALAB 250 device). In order to determine the crystal structure of the obtained product, tests were carried out using an Intelligent X-ray diffractometer apparatus with Cu K radiation (XRD; a Smart-Lab 9KW device). The degree of defects in the material before and after adding the polymer was tested by DXR Laser Confocal Raman Microscope (Raman, a Thermo-Fisher DXR Microscope device). The sulfur content of different materials was tested by a synchronous thermogravimetric analysis (a Mettler Toledo 1600 device). The microstructures of the synthesized samples and the prepared electrode materials were observed and photographed by scanning electron microscopy (SEM; Nova Nano-SEM 450) and transmission electron microscopy (Tecnai G2F20 U-TWIN), and the distribution of related elements was determined by energy dispersive spectrometer (EDS). The distribution of elemental sulfur in the electrode before and after cycling was observed by a field emission electron probe microanalyzer (EPMA; JEOL JXA-8530F PLUS).

Mechanical property test

The continuous shear scanning test and the oscillation frequency scanning test were carried out on the electrode slurry by a rotational rheometer (a Thermo-scientific HAAKE MARS40 device). During the test, the temperature (24°C) and test distance (0.25 mm) remained unchanged, and the test was carried out by changing different shear rates (0.1–100 s⁻¹) and strain amplitudes (0.1–100%). The peel strength test of the electrode sheet was realized by a universal material testing machine. Scotch tape (3M 600) was stuck to the electrode and attached the free end of the tape to the movable end of the instrument, then fixed the other end of the electrode on the immovable end of the instrument. Scotch tape peeled the electrode material 180° upward from the current collector at a speed of 1 mm/s. Optical photos and SEM were taken of the electrode materials remaining on the current collector after stripping.

Electrochemical performance test

The CNT/S-PVDF cathodes were made by mixing CNT/S, Super P, and PVDF in a ratio of 8:1:1 and adding a few drops of N-methyl-2-pyrrolidone (NMP) solution to form uniform slurries. The CNT/SNo Binder cathodes were made by mixing CNT/S and Super P in a ratio of 9:1 without adding the PVDF binder and adding a few drops of NMP solution to form uniform slurries. The CNT/Ath-PEI@S cathodes were prepared by mixing CNT/Ath-PEI@S and Super P in a ratio of 9:1 to prepare slurries without any additional binders. All the above cathodes were cut into small circular pieces with a diameter of 11 mm after vacuum drying at 60°C for 24 h, while ensuring that the area loading of sulfur in each circular electrode piece was maintained at 1.0–1.5 mg/cm². The cathodes were then transferred to a glove box, and a 2032-type button cell was assembled with lithium metal sheets as the anode, single-layer polypropylene (Celgard 2500) as the separator, and ether-based solvent (1.0 M LiTFSI in DOL:DME = 1:1 vol% with 2.0% LiNO₃) as the electrolytes. The ratio of electrolyte to sulfur in each cell was kept around 15 μl/mg. After the batteries were assembled, the galvanostatic charge–discharge (GCD) tests were carried out through the LAND CT2001A system. The rate performances of the batteries were tested at 0.1, 0.2, 0.5, 1.0, and 2.0 C, respectively. At the same time, cyclic voltammetry (CV, 1.5–3.0 V) and electrochemical impedance spectroscopy (EIS; 10 mHz−100 kHz) were performed by Biologic VMP-3 multichannel workstation.^47,78 All the above electrochemical performance tests were carried out in an incubator at 26°C.

Density functional theory calculations

All the calculations were performed in the framework of the density functional theory (DFT) with the projector augmented plane-wave method, as implemented in the Vienna ab initio simulation package. The generalized gradient approximation proposed by Perdew, Burke, and Ernzerhof was selected for the exchange-correlation potential (GGA-PBE). The cut-off energy for the plane wave was set to 500 eV. The energy criterion was set to 5 × 10⁻⁶ eV in the iterative solution of the Kohn-Sham equation. The PVDF and Ath-PEI@S are placed in the cubic cell of 25 Å, which was employed to avoid the interactions between the cells in the x, y, and z directions. All structures were optimized until the forces on the atoms were less than 0.001 eV/Å. The adsorption energy of Li₂S_x was calculated in the equation: $$$\begin{equation*}{E_{{\rm{ads}}}} = {E_{{\rm{L}}{{\rm{i}}_2}{{\rm{S}}_x}{\rm{/sub}}}} - {E_{{\rm{L}}{{\rm{i}}_2}{{\rm{S}}_x}}} - {E_{{\rm{sub}}}}.\end{equation*}$$$

RESULT AND DISCUSSION

The advanced organosulfur polymer was synthesized using Ath-PEI as the nonsulfur organic framework and CNTs as conductive substrate. Two kinds of critical steps are shown in Figure S1. The amine groups in PEI are grafted by the epoxy rings in AGE to prepare the Ath-PEI. Specifically, the primary amine opens epoxy ring to form the secondary amine and alkanol; and the obtained secondary amine reacts with another epoxy ring to form the tertiary amine and alkanol. Then, the CNT/Ath-PEI@S was prepared by a one-pot method with Ath-PEI, CNTs, and sulfur.

The FTIR spectra of the three samples confirmed the sufficient occurrence of the reaction (Figure 2A). In comparison with PEI, the stretching bands of C=C and OH appeared at 1633 and 3450 cm⁻¹ of Ath-PEI, respectively, indicating the successful grafting of AGE. In addition, the disappearance of NH bending bands of primary and secondary amines at 1650 and 1581 cm⁻¹, respectively, indicates that the active hydrogen was entirely consumed, and the reaction proceeded completely. The success of the above reaction was also confirmed by ¹H-NMR (Figure 2B). The signal peaks of -CH=CH₂ and -CHOH- are shown at δ values of 5.1–6.0 and 3.75–3.85 ppm, respectively, while no signal peaks for epoxy groups appeared in the ¹H-NMR of Ath-PEI. It means that the reaction has proceeded to completion without residual reactants. Figure 2C confirms that Ath-PEI has excellent hydrogel-like properties with high viscosity, which are similar to the properties of PEI. It shows that the original structure of PEI was not destroyed after the reaction and maintained the inherently high viscosity characteristics.

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Sulfur polymerization was verified by comparing the crystalline and chemical structures of the elemental sulfur, CNTs, CNT/S, and CNT/Ath-PEI@S. XRD characterizations were first performed to determine the crystal structure of the samples (Figure 3A). For elemental sulfur and CNT/S, the four peaks at 23.07°, 25.87°, 27.75°, and 28.66° correspond to the (2 2 2), (0 2 6), (0 4 0), and (3 1 3) crystal planes of sulfur, respectively, which is consistent with the standard value for the orthorhombic phase of sulfur (S-PDF#08-0247).⁶⁷ For CNT/Ath-PEI@S, the characteristic peaks of elemental sulfur were significantly weakened and partially disappear. Therefore, a considerable part of sulfur was polymerized with Ath-PEI. However, part of elemental sulfur remained in CNT/Ath-PEI@S because of the significant excess of sulfur. Despite this condition, the domain size of sulfur in CNT/Ath-PEI@S (48.12 nm) is significantly smaller than that of sulfur in CNT/S (84.65 nm; Figure 3B). The small domain size of sulfur in CNT/Ath-PEI@S is attributed to the ability of Ath-PEI to stabilize sulfur polymers and inhibit depolymerization, resulting in uniform sulfur distribution.⁵¹ The FTIR spectra demonstrate the complete occurrence of sulfur polymerization as well (Figure 3C). In comparison with the spectra before and after sulfur polymerization, the characteristic absorption peaks of C=C at 910, 1633, and 3080 cm⁻¹ completely disappeared after sulfur polymerization (Table S1), indicating that C=C were entirely consumed. Moreover, the appearance of a strong absorption peak at 1420 cm⁻¹ indicates the formation of CS bonds. To illustrate the sulfur content of the samples, thermal analysis of all the prepared materials was carried out in Figures 3D and S2. The sulfur contents of CNT/S, CNT/Ath-PEI@S and high-sulfur-content CNT/Ath-PEI@S (H-CNT/Ath-PEI@S) are 80, 66.7, and 83.3%, respectively. The internal structures of CNT/S and CNT/Ath-PEI@S were further confirmed based on the XPS spectra. In Figure 3E, the C 1s, O 1s, N 1s, S 2s, and S 2p peaks were contained in the XPS survey spectrum of CNT/Ath-PEI@S. The appearance of N 1s in the CNT/Ath-PEI@S spectrum indicates the existence of Ath-PEI. Unlike CNT/S (Figure S3A), the peak of CN/CS at 285.56 eV can be observed in the high-resolution C 1s XPS spectrum of CNT/Ath-PEI@S (Figure 3F). Meanwhile, the CS splitting peaks located at 162.12 and 163.27 eV can be observed in the S 2p spectra (Figure 3G). These distinct differences in the high-resolution spectra of CNT/ATh-PEI@S and CNT/S (Figure S3) all demonstrate the formation of strong chemical bonds (CS) of Ath-PEI and elemental sulfur.

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Raman test was employed to analyze the interaction between polymers and carbon materials (Figure 3H). The Raman spectra of the three samples show the characteristic bands of CNTs. In carbon materials, these bands are assigned to disordered sp³ stretching at 1350 cm⁻¹ (D bands), ordered graphitic sp² stretching at 1585 cm⁻¹ (G bands), and defective graphite-like materials at 1620 cm⁻¹ (D′ bands), respectively.⁶⁸ The order degree of carbon materials is usually measured by the integral area ratio of D-band to G-band (I_D/I_G). The calculation results show that the I_D/I_G of CNTs, CNT/S, and CNT/Ath-PEI@S are 1.66, 1.68, and 2.03, respectively (Figure S4). It indicates that during the sulfur polymerization process, the order degree of CNT/Ath-PEI@S decreased. Besides, the stretching vibration of amorphous carbon at 1290 cm⁻¹ and the band of nitrogen/oxygen-containing functional groups (A bands) of CNT/Ath-PEI@S at 1480 cm⁻¹ are significantly enhanced by the introduction of Ath-PEI. The integrated area ratio of A-bands to G-bands (I_A/I_G) indicates the proportion of polymers in carbon materials.⁶⁹ As shown in Figure S4, the I_A/I_D of CNTs, CNT/S, and CNT/Ath-PEI@S are 0.14, 0.17, and 1.79, respectively, confirming that the deposition of amorphous carbon structures (CC) and nitrogen/oxygen-containing functional groups (Ath-PEI) in CNT/Ath-PEI@S. The SEM and EDS images of CNT/S and CNT/Ath-PEI@S are shown in Figures S5 and S6. These figures show the tight binding of CNT/Ath-PEI@S and the uniform distribution of organosulfur polymers in the framework structure of CNT/Ath-PEI@S. As a result, Ath-PEI and elemental sulfur were fully polymerized and formed strong interactions with CNTs.

Electrode slurries were tested to verify the performance improvement of CNT/Ath-PEI@S as a cathode material for Li–S batteries. The physical properties of slurries were first assessed by rheological testing. Figures 4A and D show the continuous shear scan spectra of the three slurries. The detailed ratio of electrode slurries, are shown in the Electrochemical Test of the Experimental section. The three slurries show significant shear thinning behavior, which is consistent with the properties of non-Newtonian fluids.⁷⁰ Meanwhile, both CNT/S electrode slurry with PVDF binder (CNT/S-PVDF) and CNT/Ath-PEI@S electrode slurry exhibit high apparent viscosity at low shear rates. In particular, the high viscosity of CNT/Ath-PEI@S slurry may also be related to the interaction of oxygen-containing groups (e.g., OH, COC) with Super P.⁷¹ It was evident that both slurries have good sedimentation stability, which is beneficial to the preservation of the slurries. CNT/S slurries without the addition of binders (CNT/SNo Binder) have low apparent viscosity at low shear rates, causing the settling of solid particles (Super P, CNT/S). Considering that the coating process of battery slurries is a high-shear-rate process, its viscosity should not be too large in the high-shear-rate range, otherwise the coating process will be difficult. Compared with the two other slurries, CNT/Ath-PEI@S slurry exhibited lower viscosity at high-shear-rate, which was beneficial for the smooth progress of coating process (Figure 4D). The viscoelastic properties of slurries were further analyzed by oscillation frequency scanning test (Figures 4B, E, and S7). The storage modulus (G′, elastic part) and loss modulus (G″, viscous part) of the three slurries change with the strain amplitude. As the slurries’ state change, the diagram can be divided into two regions. In Region I, the G′ is higher than the G″, and the slurries exhibited gel-like behavior.⁷² With the increase of the strain amplitude, the G′ of the slurries decreased and G″ increased. When G″ was finally higher than G′, the state of slurries entered Region II, showing a more liquid-like behavior. As shown in Figures 4B, E, and S7, the ability of CNT/Ath-PEI@S slurry to maintain the gel state is remarkably better than that of CNT/SNo Binder and CNT/S-PVDF slurry. It also proves that it is easy to form a strong network structure inside the CNT/Ath-PEI@S slurry. Meanwhile, the linear viscoelastic region ranges of CNT/Ath-PEI@S and CNT/S-PVDF slurries are significantly better than that of CNT/SNo Binder slurry, indicating that their internal structure are less prone to fracture and possesses better stability (Figure S8). To further evaluate the possibility of CNT/Ath-PEI@S as a sulfur cathode, the peel strength tests of the electrode materials after coating and drying were carried out. Scotch tape was pressed tightly on the surface of electrode materials, and the scotch tape was slowly peeled off at a constant rate (Figure S9). As shown in Figure 4C, the CNT/Ath-PEI@S electrode exhibits similar peel strengths as the CNT/S-PVDF electrode, which are both superior to the CNT/SNo Binder. The digital photographs during peel off show that the CNT/Ath-PEI@S electrode has more particles (active materials and Super P) retained on the current collector compared with the CNT/S-PVDF electrode (inset in Figure 4C). The morphology of electrode materials on current collector surfaces before and after peeling were further demonstrated in SEM images (Figures 4F–I and S10). Compared with the CNT/S-PVDF electrode, the CNT/Ath-PEI@S electrode was smoother before peeling off because of the uniform distribution of sulfur in the framework structure and the low viscosity of the slurry at high shear rates (Figures 4F and H). After peeling off, the CNT/Ath-PEI@S electrode (Figure 4I) exhibited the most particle residues compared with the CNT/S-PVDF electrode (Figure 4G) and the CNT/SNo Binder electrode (Figure S10). This phenomenon might have been because of the strong interaction between the functional groups on the surface of hydrophilic carbon-coated aluminum foil and in CNT/Ath-PEI@S. As shown in Figure S11, the simple folding experiment also verified the mechanical stability of the CNT/Ath-PEI@S electrode. And the contact angle of the CNT/Ath-PEI@S electrode (18.15°) is slightly reduced compared with that of the CNT/S-PVDF electrode (20.04°) and CNT/SNo Binder electrode (23.01°), possibly because of the improved wettability of the CNT/Ath-PEI@S in the presence of polar functional groups (Figure S12). Moreover, due to the low adhesion of the CNT/SNo Binder electrode, the electrode surface appears wrinkled after dropping the electrolyte, which is not conducive to the stable working of the battery. In comparison with CNT/S-PVDF slurries and electrodes, electrodes and slurries based on CNT/Ath-PEI@S have comparable mechanical properties and stability. The excellent rheological properties of the slurry, the high peel strength and improved wettability of the electrode allow the use of CNT/Ath-PEI@S as a binder-free Li–S battery cathode.

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The electrochemical behaviors of batteries assembled by the CNT/SNo Binder, CNT/S-PVDF, and CNT/Ath-PEI@S electrode as cathodes were characterized by CV, galvanostatic intermittent titration technique (GITT), and EIS tests. Subsequently, to compare electrochemical rate performances and cycling stabilities of the above-mentioned electrodes and high-sulfur-content CNT/Ath-PEI@S cathode material, H-CNT/Ath-PEI@S with sulfur content up to 75% was prepared and tested. The electrochemical reaction processes were first determined by the CV test. The CV curves of the CNT/Ath-PEI@S and CNT/S-PVDF battery at different scan speeds (from 0.1 to 0.5 mV s⁻¹) are shown in Figures 5A and S13, respectively. At different scan rates, the obtained curves have similar shape with two distinct reduction peaks (downward cathodic peaks) and two oxidation peaks in close proximity (upward anodic peaks). Two typical reduction peaks can be attributed to the transformation of S₈ to long-chain Li₂S_x and the transformation of Li₂S_x to Li₂S₂/Li₂S, respectively. And the two oxidation peaks correspond to the inverse process of Li₂S₂/Li₂S conversion to S₈. The change of CV curves at different scan rates is relatively gentle, indicating the great rate performance of the CNT/Ath-PEI@S battery.⁴⁷ It is also directly reflected by the rate performance test (Figure 5B). Based on the comparison of the specific capacities of CNT/Ath-PEI@S, H-CNT/Ath-PEI@S, and CNT/S-PVDF batteries at different current densities (0.1–2.0 C, 1 C = 1675 mAh g⁻¹), the specific capacities of both CNT/Ath-PEI@S and H-CNT/Ath-PEI@S battery were higher than that of CNT/S-PVDF battery at every current density. Specifically, the CNT/Ath-PEI@S battery exhibited specific capacities of 1325.4, 1071.5, 932.4, 864.7, and 804 mAh g⁻¹ at 0.1, 0.2, 0.5, 1.0, and 2.0 C, respectively. The H-CNT/Ath-PEI@S battery exhibited specific capacities of 1256.4, 991.7, 862.3, 789, and 697.9 mAh g⁻¹ at 0.1, 0.2, 0.5, 1.0, and 2.0 C, respectively. These values are significantly higher than the specific capacities of CNT/S-PVDF battery of 1042.9, 828.6, 720, 612, and 502.4 mAh g⁻¹ at 0.1, 0.2, 0.5, 1.0, and 2.0 C, respectively. Notably, the initial discharge capacity of the CNT/Ath-PEI@S and H- CNT/Ath-PEI@S battery are as high as 1520.7 and 1446.9 mAh g⁻¹ at the current density of 0.1 C, respectively. The good rate capability and high initial specific capacity indicates that the utilization rate of sulfur in CNT/Ath-PEI@S and H-CNT/Ath-PEI@S are higher than that in CNT/S-PVDF, which are closely related to the uniform distribution of sulfur in nonsulfur organic frameworks. It is proved that the introduction of polymer structure can improve the distribution of elemental sulfur and increase the utilization of elemental sulfur. Furthermore, CNT/Ath-PEI@S, H-CNT/Ath-PEI@S, and CNT/S-PVDF electrodes exhibit characteristic double plateaus property during the charging-discharging process of Li–S batteries (Figures 5C and S14). The curves of the CNT/Ath-PEI@S and H-CNT/Ath-PEI@S battery, maintain good stability with increasing current density up to 2.0 C. However, for the CNT/S-PVDF battery, with the increase of the current density, the electrode exhibits a certain voltage hysteresis. The second characteristic plateau of CNT/S-PVDF battery slips due to partial polarization at 1.0 and 2.0 C (Figure S14B). The electrochemical properties of the CNT/Ath-PEI@S electrode were further analyzed by EIS. The Nyquist curves of both fresh CNT/Ath-PEI@S and CNT/S-PVDF batteries consist of semicircles in the high and middle-frequency regions and straight lines in the low-frequency region (Figure 5D). Among them, the diameter of the semicircle in the high- and medium-frequency regions represent the charge transfer resistance (R_ct), and the slope of the straight line in the low-frequency region represents the diffusion impedance (Z_w). The interpretation of the equivalent circuit diagram is shown in Table S2. As shown in Figure S15, the CNT/SNo Binder has an extra semicircle in the high-frequency region, making it differ with the two batteries above. It can be explained that the instability of the electrode sheet after the infiltration of the electrolyte leads to a large contact resistance (R_f = 966.6 Ω). Compared with the CNT/SNo Binder battery (169.4 Ω), both CNT/Ath-PEI@S (29.58 Ω), and CNT/S-PVDF battery (29.88 Ω) exhibit small semicircles, corresponding to lower R_ct (Table S3). The internal resistance changes of CNT/Ath-PEI@S and CNT/S-PVDF battery during charging and discharging processes were analyzed by GITT test. The test principle of GITT is that the closed-circuit voltage (CCV) is obtained after 20 min of constant current charging, and the quasi-open circuit voltage (QOCV) is obtained after standing for 20 min. The whole test is carried out at the current density of 0.1 C. The internal resistance changes were monitored at different reaction nodes by normalizing the time. Li₂S nucleation and activation represent the process of soluble Li₂S_x (x = 4, 6, 8) to insoluble Li₂S and the process of Li₂S conversion to long-chain polysulfides, respectively. During the charging and discharging process, the polarization degree of the CNT/Ath-PEI@S (Figure 5E) battery was significantly lower than that of the CNT/S-PVDF (Figure S16). The specific polarization degree can be accurately quantified according to the change of internal resistance during the GITT test. The internal resistance (ΔR_internal) can be calculated according to the following formula: $$$\begin{equation*}{{\Delta}}R{\;_{{\rm{internal}}}}\;\left( {{\Omega}} \right)\; = \;\left| {{{\Delta}}{V_{{\rm{QOCV}} - {\rm{CCV}}}}} \right|/I\end{equation*}$$$where ΔV represent the voltage difference between QOCV and CCV, and I represent the current applied during the test, as shown in Figure 5E inset. The CNT/Ath-PEI@S electrode exhibited lower ΔR_internal than the CNT/S-PVDF electrode (Figure 5F). In particular, as shown in the inset in Figure 5F, the efficiency of nucleation and activation can be quantified by the magnitude of ΔR_internal. The lower ΔR_internal indicates that the CNT/Ath-PEI@S electrode exhibits more rapid reaction kinetics during the electrochemical reaction (reversible reactions between soluble long-chain lithium polysulfide and insoluble Li₂S).

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In order to further characterize the fast reaction kinetics of electrodes, the GCD curves and the symmetric batteries CV curves were analyzed. As shown in Figure 6A, the CNT/S-PVDF, CNT/Ath-PEI@S, and H-CNT/Ath-PEI@S electrodes all exhibit two distinct discharge plateaus: at around 2.3 V, the solid S₈ is converted to liquid polysulfide, producing a short plateau, and the capacity contributed by this high-voltage discharge plateau is noted as Q_H; at around 2.1 V, the liquid polysulfide is converted to solid Li₂S₂/Li₂S, and the capacity contributed by this low-voltage discharge plateau is noted as Q_L. The ratio of Q_L to Q_H is the solid–liquid–solid conversion coefficient inside the Li–S battery, and the theoretical value is about 3.⁷³ The higher the Q_L/Q_H value, the more complete the conversion of the polysulfide. As seen in Figure S17, the Q_L/Q_H of CNT/Ath-PEI@S and H-CNT/Ath-PEI@S are 2.19 and 2.18, respectively. In comparison, the Q_L/Q_H of CNT/S-PVDF is 1.99, which is much lower than CNT/Ath-PEI@S and H-CNT/Ath-PEI@S. It indicates that the nonpolar fluorine-containing groups of PVDF binders cannot provide a strong anchoring site for polysulfides, resulting in lower conversion efficiency and poorer electrochemical performance.⁷⁴ And the introduction of Ath-PEI is beneficial to anchor the polysulfide, improve the effective conversion rate and enhance the utilization of elemental sulfur. In addition, the CV tests of Li₂S₆ symmetric batteries also showed that CNT/Ath-PEI symmetric batteries exhibited larger polarization current compared with CNT/PVDF symmetric batteries, indicating a faster response of polysulfides.^75,76 And the overpotential between the nucleation point and the potential plateau tangent (ΔV) was also used to evaluate the electrochemical kinetics. Specifically, the CNT/Ath-PEI@S and H-CNT/Ath-PEI@S electrodes exhibit faster electrochemical kinetics (phase transition between soluble polysulfide and insoluble Li₂S) during discharge (Figure 6C) and charging (Figure 6D) compared with the CNT/S-PVDF electrodes, which proves that the functional groups in Ath-PEI facilitate the acceleration of electrochemical reactions.⁷⁷ Meanwhile, the GCD curves in the Figure 6A show that the CNT/Ath-PEI@S and H-CNT/Ath-PEI@S electrodes have lower polarization and smaller voltage gap between the anode and cathode platforms (denoted as ΔE) than the CNT/S-PVDF electrodes (Figure 4D), which indicates that the CNT/Ath-PEI@S and H-CNT/Ath-PEI@S electrodes have better conductivity. In conclusion, strong anchoring force, excellent electrochemical kinetics and good conductivity improve the electrochemical performance of CNT/Ath-PEI@S and H-CNT/Ath-PEI@S electrodes.

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The stabilities of the batteries were further tested. Figure 7A shows the CV curves of the CNT/Ath-PEI@S battery at different cycle numbers. The oxidation and reduction peak positions and intensities in the CV curves of CNT/Ath-PEI@S battery remain basically unchanged after several charge–discharge cycles, reflecting the high reversibility of the redox reaction and excellent electrochemical stability. However, the oxidation and reduction peak positions and intensities in the CV curves of the CNT/S-PVDF battery resulted in changes with the increase of the number of cycles, indicating the instability of the internal chemical reactions during the charge–discharge processes (Figure S18). Compared with batteries with CNT/S-PVDF as cathodes, the batteries with CNT/Ath-PEI@S as cathodes had highly reversible redox reactions, ensuring more stable batteries operation. In addition, the oxidation and reduction peaks of the CNT/Ath-PEI@S battery are sharper, and the voltage differences between the peaks are more minor, as well, than those of the CNT/S-PVDF battery. These findings indicate the excellent reaction kinetics of the CNT/Ath-PEI@S battery.⁷¹ Figure 7B presents the cycling stability of the CNT/Ath-PEI@S and CNT/S-PVDF electrode at the current density of 0.5 C. After 400 cycles, the reversible discharge capacity of the CNT/Ath-PEI@S electrode can be still maintained at 773.9 mAh g⁻¹, and the ultimate Coulombic efficiency (CE) is 97.48%. The CE of the CNT/Ath-PEI@S battery remains above 97% throughout the entire cycling process. As a reference, the CNT/S-PVDF electrode exhibited a capacity of only 499.8 mAh g⁻¹ with the CE of 89.28% after 400 cycles at 0.5 C. Throughout the cycling processes, the CE of the CNT/S-PVDF battery gradually decreased from 98 to 89%. Based on the test of the cycle performance of the CNT/SNo Binder electrode at 0.5 C (Figure S19), the battery could not perform normal charge–discharge cycles due to the instability caused by low adhesion, indicating that binders need to be added to common Li–S batteries. The cycling stability of the CNT/Ath-PEI@S battery tested at 1.0 C was also better than that of the CNT/S-PVDF battery (Figure 7D). After 400 cycles, the specific capacities of the CNT/Ath-PEI@S and CNT/S-PVDF battery are 623 (CE, 95.30%) and 419 mAh g⁻¹ (CE, 94.65%), respectively. The uniform distribution of sulfur and the adsorption property from nonsulfur frameworks of the CNT/Ath-PEI@S electrode further enhanced its electrochemical stability. The adsorption capacities of the functional groups (e.g., COC and OH) of the nonsulfur organic framework of CNT/Ath-PEI@S and the binder PVDF for polysulfides were analyzed and compared by calculating their adsorption energies (E_ads) by the first-principles calculations based on the DFT. The adsorption capacity was assessed by calculating the E_ads to measure the bonding strength of Li₂S_x (x = 4, 6, 8) on PVDF and Ath-PEI@S (Figures 7C and S20). The interaction of Ath-PEI@S with three lithium polysulfides (Li₂S₄, Li₂S₆, Li₂S₈) were higher than that of PVDF because of the presence of polar functional groups. The results can be intuitively reflected by the adsorption energy values (Figure 7C). Furthermore, adsorption tests further demonstrated the strong interaction between functional groups and polysulfides. Li₂S₆ was chosen as a representative of typical soluble polysulfides and the optical photographs after adsorption were compared. It can be seen by the color change that Ath-PEI has a stronger adsorption capability for Li₂S₆ compared with PVDF (inset of Figure S21). The supernatant after adsorption was further tested by UV–vis, and the results showed that the absorption intensity of Li₂S₆ was consistent with the adsorption results, indicating that the functional groups in Ath-PEI could effectively immobilize the polysulfides (Figure S21). The higher interaction can improve the anchor of polysulfides and enhance the cycling stability of the battery. The outstanding performance of CNT/Ath-PEI@S electrodes can be attributed to the stability of physical properties, and the reasonable design of its chemical structure. To further verify the advancement and practicability of the CNT/Ath-PEI@S electrode, the H-CNT/Ath-PEI@S electrode with 75% sulfur content was subjected to cycling tests under high current (Figure 7E). After 300 cycles at 2.0 C, the H-CNT/Ath-PEI@S battery can still maintain a specific capacity of 627 mAh g⁻¹ (CE, 98.54%). The 300 cycled H-CNT/Ath-PEI@S battery can still light up the LED screen (Figure S22), indicating its potential for practical applications. Furthermore, the good cycling and rate performance for different mass loadings of cathode active materials and electrolyte-to-sulfur ratios (E/S ratios) also demonstrate the universal applicability of H-CNT/Ath-PEI@S batteries. The H-CNT/Ath-PEI@S batteries showed good discharge capacity and cycling stability even at sulfur contents as high as 3 to 4.5 mg cm⁻² and E/S ratios of 15, 10, and 7.5 μl mg⁻¹ (Figure S23). Overall, based on various chemical tests, the rheological properties of the slurries, the electrochemical properties of the electrode materials, and DFT theoretical calculations confirm that CNT/Ath-PEI@S can be used as an advanced Li–S battery cathode without binder addition. Figure 7F summarizes the representative literature of sulfur polymerized cathode materials in recent years and compares them with CNT/Ath-EPI@S. In order to reflect the superiority of CNT/Ath-PEI@S more intuitively, the initial specific capacity of the whole electrode (including active materials, binders, and conductive agents) was calculated. Benefiting from the high sulfur content and uniform distribution of elemental sulfur, the initial specific capacity based on the electrode is greatly improved (942.3 mAh g⁻¹_electrode at 0.1 C). It is 40% higher than that of the organosulfur polymer electrode reported in the previous literature (generally lower than 700 mAh g⁻¹_electrode at 0.1 C).

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To further explore the changes of the H-CNT/Ath-PEI@S during charge–discharge cycles, EPMA was used to perform the electrode surface scanning tests before and after cycling. When the beam of electrons was scanned on the electrodes, the concentration distribution of elements on the electrode can be reflected in the form of different colors, in which the gradient from blue to red represents the gradual increase of element concentration. Therefore, the enrichment degrees of the element can be observed through the EPMA surface scan tests. The distributions of elemental sulfur before cycling and after 30 cycles of the H-CNT/Ath-PEI@S electrode and CNT/S-PVDF electrode are clearly shown in Figures 8A, B, D, and E. Based on the comparison of the distribution of elemental sulfur before cycling, the CNT/S-PVDF electrode show a more obvious sulfur enrichment, while the distribution of elemental sulfur on the H-CNT/Ath-PEI@S electrode is more uniform. It indicates that the sulfur polymerization process contributed to solving the problem of the uneven distribution of elemental sulfur on electrodes. Even for the organosulfur polymer electrode with a high-sulfur content of 75%, the uniformity of elemental sulfur was significantly better than that of the traditional CNT/S-PVDF electrode with a sulfur content of 60% (Figures 8A and B). After 30 cycles, the sulfur distribution of the CNT/S-PVDF electrode was slightly uniform, but sulfur enrichment was still observed (Figure 8E). However, for the H-CNT/Ath-PEI@S electrode, elemental sulfur can be better dispersed in the electrode after 30 cycles (Figure 8D). The above results show that the distribution of elemental sulfur in the H-CNT/Ath-PEI@S electrode is consistently more uniform than that of the CNT/S-PVDF electrode during cycling. Before cycling, the enrichment of elemental sulfur was effectively mitigated by terminating linear sulfur chains with Ath-PEI. During cycling, the inherent polymer framework structure of Ath-PEI can produce a physical confinement effect, thereby promoting the uniform distribution of elemental sulfur (Figure 8F). Therefore, sulfur polymer electrodes (CNT/Ath-PEI@S and H-CNT/Ath-PEI@S electrodes) exhibit higher specific capacity and better rate capability than CNT/Ath-PEI@S electrodes (Figures 8C and B). In brief, the introduction of Ath-PEI polymer promotes the uniform distribution of elemental sulfur, thus improving the utilization of elemental sulfur, and then improving the electrochemical behavior inside the Li–S battery.

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CONCLUSION

In summary, a sulfur-containing polymer with Ath-PEI as an organic framework was synthesized and used as a Li–S battery cathode without requiring any binder. Unlike small-molecule organosulfur polymers, the Ath-PEI with high intrinsic viscosity is fully utilized as a binder to provide room for the addition of more sulfur in a limited volume, thus obtaining organosulfur polymer electrodes with sulfur content as high as 75%. Simultaneously, the well-designed polymer framework structure of Ath-PEI enabled uniform distribution of elemental sulfur, and achieved an ultra-high initial discharge capacity of 1520.7 mAh g⁻¹ at 0.1 C and high rate capability of 804 mAh g⁻¹ at 2.0 C. The strategy not only increased the sulfur content of organosulfur polymer cathodes, but also improved the sulfur inhomogeneity of common carbon–sulfur composite cathodes at high sulfur content. Benefiting from the above advantages, the initial discharge capacity based on the electrode can reach 942.3 mAh g⁻¹_electrode, which is nearly 40% higher than those of organosulfur polymer electrodes reported in the previous literature. Moreover, EPAM test further demonstrated the uniform distribution of elemental sulfur during cycling. Therefore, the customized design and rational application of polymer framework structure could play an important role in the construction of high-performance Li–S batteries.

ACKNOWLEDGMENTS

The authors acknowledge the support from National Outstanding Youth Science Fund (52222314), CNPC Innovation Found (2021DQ02-1001), Liao Ning Revitalization Talents Program (XLYC1907144), Xinghai Talent Cultivation Plan (X20200303), and Fundamental Research Funds for the Central Universities (DUT22JC02, DUT22LAB605).

CONFLICT OF INTEREST

The authors declare no conflict of interest.

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