A nitrogen-rich hyperbranched polymer as cathode encapsulated material for superior long-cycling lithium-sulfur batteries

Journal Pre-proof A nitrogen-rich hyperbranched polymer as cathode encapsulated material for superior long-cycling lithium-sulfur batteries Xu Liu, Pingping Chen, Jie Chen, Qinghui Zeng, Zhinan Wang, Zengxi Li, Liaoyun Zhang PII:

S0013-4686(19)32209-1

DOI:

https://doi.org/10.1016/j.electacta.2019.135337

Reference:

EA 135337

To appear in:

Electrochimica Acta

Received Date: 17 September 2019 Revised Date:

31 October 2019

Accepted Date: 18 November 2019

Please cite this article as: X. Liu, P. Chen, J. Chen, Q. Zeng, Z. Wang, Z. Li, L. Zhang, A nitrogenrich hyperbranched polymer as cathode encapsulated material for superior long-cycling lithium-sulfur batteries, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.135337. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

A Nitrogen-rich Hyperbranched Polymer as Cathode Encapsulated Material for Superior Long-cycling Lithium-sulfur Batteries Xu Liu a, Pingping Chen a, Jie Chen a, Qinghui Zeng a, Zhinan Wang a, Zengxi Li a *and Liaoyun Zhang a *

a. School of chemical sciences, University of Chinese Academy of Sciences, Beijing 100049, China E-mail: [email protected]; [email protected]

Abstract The shuttle effect of Lithium-sulfur (Li-S) batteries often leads to poor long-cycling performance. Finding a more effective way to suppress the shuttle effect of lithium sulfur battery is still a huge challenge. Herein, a novel polymer encapsulated layer (HPEIGA)

was

synthesized

by

a

one-step

reaction

of

hyperbranched

polyethyleneimine (HPEI) and Glutaraldehyde. The HPEIGA can not only like the traditional encapsulated layer that has the physical barrier effect for inhibiting the dissolution of the polysulfide. More importantly, large amount of nitrogen atoms in HPEIGA can adsorb polysulfide anions by chemical interaction, which can effectively suppress polysulfide shuttling effect. As a result, the shuttling current of the cell based on the encapsulated carbon-nanotube/sulfur (CNT-S) electrode (HPEIGA@CNT-S) is merely 1/8 that of CNT-S electrode without encapsulated layer. Furthermore, the capacity retention of the HPEIGA@CNT-S cell can reach up 73.68% with a small decay of 0.043% per cycle at the current of 2C even after 600 cycles. Thus, nitrogen-contained hyperbranched polymer has a great potential as encapsulated layer of sulfur electrode for high performance Li-S batteries.

Keywords: hyperbranched polymer, encapsulated layer, shuttle effect, lithium-sulfur battery

1. Introduction

Lithium-sulfur batteries (Li-S batteries) which use sulfur as the cathode and lithium metal as the anode have been widely concerned as the next energy storage due to their high energy density (2567 mW h g-1) and high specific capacity (1675 mAh g-1)[1-7]. However, the problem of shuttle effect often makes Li-S batteries suffer a fast faded cycle life, which greatly hinders the practical application of Li-S batteries[8-12]. To solve the shuttling effect of Li-S batteries, a variety of strategies have been employed, such as physical adsorption of sulfur by porous material[13-17], chemical adsorption of sulfur by carbon material doped with polar atoms such as nitrogen-atom[3, 16, 18-20], covalent connection between sulfur and copolymer. Indeed, these strategies play an important role in improving cycling stability and electrochemical properties of Li-S batteries. Unfortunately, it is not enough to effectively prevent the dissolution of polysulfide owing to the direct contacting with liquid electrolyte. Therefore, rationally designing sulfur cathode is necessary to limit polysulfide dissolution and diffusion for suppressing polysulfide shuttling effect[21, 22]. In recent years, polymers which are used for encapsulated layer of sulfur cathode have aroused great concern[23-28], because they are able to accommodate sulfur volumetric expansion and limit the sulfur–electrolyte direct contact for achieving stable cycling performance[29-33]. For example, Cui et al. studied the most well-known three conductive polymers (PANI, PPy, PEDOT) coating onto monodisperse hollow sulfur nanopsheres, respectively[34]. And these formed materials were used as cathode and the corresponding Li-S battery was assembled. The cells based on PANI-S,

PPY-S, and PEDOT-S exhibit about 740, 885, and 1004 mAh g-1 at the current of 0.2C, respectively. After 100 cycles, the capacity retentions of the battery could reach 65%, 74%, and 86%, respectively[34]. In addition, to enhance the ionic conductivity of the active material, Wang et al. reported a PEG-CNT/S composites with nest-like structure[35]. Kyung et al. encapsulated the sulfur particles with a dual (ionic and electronic)-conducting polymer, that PEDOT-co-PEG-coated sulfur delivered higher discharge capacity and showed better cycle performance ( capacity retention is 72% after 100 cycles ) than those with pristine sulfur materials[36]. It is clear that these polymers coating on the surface of the sulfur cathode can enhance the electrochemical performance of the electrode. In addition, due to the flexibility of polymers, the volume expansion of the electrode can also be suppressed during cycling. Furthermore, the polymer encapsulating layer can be served as a physical barrier to prevent the dissolution of polysulfide[22, 37, 38]. As a result, the Li-S battery has high capacity and good cycle performance. Nevertheless, polymer coating currently used in sulfur electrode also has a new set of problems. On the one hand, the synthesis of these polymers is normally through time-consuming and complex reaction, which may have some adverse effects on the sulfur cathode, such as the introduction of impurities. On the other hand, almost all reported polymer encapsulated layers only serve as physical barriers, resulting in part dissolution of polysulfides in liquid electrolyte. Therefore, it is still a huge challenge to find a more effective way to further suppress shuttle effect. Developing an effective encapsulating layer which can

directly attract lithium polysulfide by chemical adsorption may be beneficial to solving the above problems. In this study, a novel polymer encapsulated layer (HPEIGA) was synthesized by a one-step reaction of hyperbranched polyethyleneimine (HPEI) and Glutaraldehyde (GLA). The preparation process is fast and without catalyst which can eliminate the introduction of impurities. Furthermore, the HPEIGA can not only be as a physical barrier for ensuring effective restriction of Li2Sx in the cathode. More importantly, large amount of nitrogen atoms in the HPEIGA can adsorb polysulfide anions by chemical interaction, which is ascribed to outstanding complexation between polysulfides and nitrogen[39-41]. Owing to the joint effects of the chemical absorbent and physical barrier, the shuttling effect can be further suppressed. As a result, the shuttling currents of the cell based on the HPEIGA@CNT-S are only 1/8 that of CNT-S electrode without encapsulated layer. Impressively, HPEIGA@CNT-S cell exhibits outstanding long-cycling stability. Its capacity retention can reach up 73.68% with a small decay of 0.043% per cycle at the current of 2C even after 600 cycles. Thus, nitrogen-contained hyperbranched polymer has a great potential as encapsulated layer of sulfur electrode for high performance Li-S batteries.

2. Experimental Section

2.1. Materials. Carbon-nanotube/sulfur (CNT-S) powder is synthesized in melt diffusion strategy (The details can be seen in Supporting Information). The hyperbranched polyethyleneimine (HPEI, Mw = 2.5×104 g mol-1, Mn =1×104 g

mol-1) was purchased from Sigma-Aldrich (Shanghai, China). Glutaraldehyde solution (GLA, 50%) and Methyl alcohol (CH3OH, AR) were purchased from Sinopharm Chemical Regent Co., Ltd., China. N-methyl-2-pyrrolidone (NMP, GC) was purchased from Beijing InnoChem Science&Technology Co. Ltd., and dried with CaH2 and distilled under reduced pressure before used. Polyvinylidene fluoride (PVDF) was purchased Shenzhen kejing electronics Co. Ltd.

2.2. HPEIGA coating on carbon-nanotube/sulfur (CNT-S) particles.

For HPEIGA coating on CNT-S particles, in a typical experimental process, the HPEI and the as-prepared CNT-S powder were dispersed in CH3OH in mass ratios of 1:2, 1:3 and 1:4 by sonication for 15 min and stirred for 15 min at room temperature. Then, GLA was slowly dropped into the above aqueous suspension with strong magnetic stirring. After continuous stirred for 30 min, the precipitate was collected by filtration and thoroughly washed with CH3OH several times. Finally, the HPEIGA coated CNT-S powder was obtained after dried in a vacuum oven at 60 oC for 12 h. The mass ratios of HPEI and GLA were 1:1, 2:1 and 3:1, which the corresponding samples

are

referred

as

HPEIGA1@CNT-S,

HPEIGA2@CNT-S

and

HPEIGA3@CNT-S.

2.3. Cathode fabrication and cell assembly

To prepare the sulfur cathode for Li-S batteries, the different HPEIGA@CNT-S, CNT and polyvinylidene fluoride (PVDF) binder were mixed in a weight ratio of 8:1:1 in the NMP solvent to form homogeneous slurry. Next, the mixtures were

spread onto aluminum foil substrates before dried at 50 oC for 30 min and then at 60 o

C for 24 h. Subsequently, the obtained electrodes were cut into circular sheets with

the diameter of 12 mm. For comparison, bare sulfur cathode was also prepared by mixing CNT-S, CNT and PVDF binder in a weight ratio of 8:1:1 in the NMP solvent following a similar procedure as mentioned above. The sulfur content in the cathode based on CNT-S, HPEIGA2: CNT-S (1:2), HPEIGA2: CNT-S (1:3) and HPEIGA2: CNT-S (1:4) is 60%, 48%, 45% and 40%, respectively. Although, the sulfur content is not consistent within different cathodes, the mass loading of sulfur in all electrodes was controlled to be 1.0–1.2 mg cm−2. The electrodes were assembled into CR2025 coin-type cells with lithium metal as counter electrode and Celgard 2300 as the separator in Argon-filled glove box (MB-Labstar 1200/780 with O2 and H2O contents below 0.5 ppm) by using 1, 3-dioxolane (DOL) and dimethoxyethane (DME) (1:1 in volume) with 1 M LiTFSI and 0.2 M LiNO3 dissolved as the electrolyte.

2.4. Characterization.

Fourier transform infrared (FTIR) spectra were carried out by a Bruker Vertex 70 infrared instrument with the attenuated total reflectance (ATR) technique from 4000 to 400 cm-1. The phase identification of the as-prepared samples was identified by powder X-ray Diffractometer Smartlab 9 Kw operating at 45kV and 200 mA with a copper target (λ= 1.54 Å). In the step of scanning rate of 10o min-1, data was collected from 10 to 60o. The morphology was observed by HITACHI SU8010 scanning electron microscope (SEM). The results of element mapping were examined by

energy dispersive spectrometer (EDS) attached to Hitachi scanning electron microscope (Hitachi SEM). Cyclic voltammetry (CV) tests of the coin-type cells were carried out on a CHI660E (Chen Hua) electrochemical working station. The scan rate was 0.1 mV s-1 and the voltage range was from 1.7 to 2.8 V. Electrochemical impedance spectroscopy (EIS) was carried out on ZahnerEnnium electrochemical workstation. The AC amplitude was 5 mV, and the frequency range was from 100 kHz to 10 mHz. On the LANHE CT2001A battery test system (Wuhan), the charge-discharge measurement and cycle capacity were measured at room temperature under a voltage window of 1.7 V to 2.8 V.

3. Results and Discussion

3.1. Synthesis and Structural Characterization of Materials

Considering that the hyperbranced polyethyleneimine (HPEI) contains relatively large amounts of functional nitrogen-containing groups compared with the corresponding linear analogue, the HPEIGA is used to coat CNT-S by in-situ one-step cross-linking reaction of HPEI and GLA. Due to the very fast reaction of HPEI and GLA, so the preparation time can be saved. The schematic diagram for the preparation of HPEIGA@CNT-S is illustrated in Scheme 1.

Scheme 1. The Preparation of HPEIGA@CNT-S

The morphologies of CNT, CNT-S and HPEIGA@CNT-S are characterized by SEM. As shown in Figure 1a, the surface of the pristine CNT is smooth and clean with the diameter about 10-15 nm. Compared with the CNT, the surface of CNT-S (Figure 1b) becomes rough, indicating that the sulfurs have deposited on CNT successfully. After in-situ rapidly coating, the diameter of the resulting HPEIGA@CNT-S increases to about 30-40 nm. In addition, Elemental mappings for sulfur and nitrogen by SEM are shown in Figure 1d-f. The uniform distribution of sulfur and nitrogen element demonstrates that HPEIGA were uniformly coated onto the surface of the CNT-S.

Figure 1 (a) SEM images of CNT; (b) SEM image of CNT-S; (c) SEM image of HPEIGA2@CNT-S; (d)-(f) The EDS elemental maps for nitrogen and sulfur of HPEIGA@CNT-S FT-IR technique is adopted for structural characterizations of HPEIGA, CNT-S and HPEIGA@CNT-S. As presented in Figure 2, pure HPEIGA obtained from rapid cross-linking exhibited several significant peaks at the range of 800-3600 cm -1. The peak at 1650 cm-1 which concerns the stretching vibration of C=N suggests that the amine groups of HPEI reacted with aldehyde groups of GLA. The peak at 3270 cm -1 ascribes to the characteristic adsorption bond of N-H. In addition, the C-N stretching vibration occurs at 1545 cm-1, 1043 cm-1 and 1175 cm-1, respectively. These characteristic peaks of HPEIGA also appear in the spectrum of HPEIGA@CNT-S. It can confirm that polymer encapsulated layer HPEIGA on the CNT-S has been successfully achieved. By comparison, there is on obvious peak of uncoated CNT-S at the range of 800-3600 cm -1. The peaks of CNT-S at the range of 400-800 cm

-1

can

be attributed to the sulfur. Furthermore, the peaks of sulfur in HPEIGA@CNT-S did not change, indicating that the rapid cross-linking reaction on CNT-S did not affect the structure of sulfur.

Figure 2 Fourier transform infrared (FT-IR) spectra of HPEIGA, CNT-S and HPEIGA@CNT-S In order to further confirm the phase structures of these composites, Figure 3 presents the X-ray diffraction (XRD) analysis of the HPEIGA, CNT, sulfur, CNT-S and HPEIGA@CNT-S. It can be seen that CNT exhibit typical broad diffraction peaks at 25 and 42o corresponding to a graphite-like diffraction from the walls of CNTs[42]. The patterns of sulfur shows prominent peaks at 23, 26, and 28°, assigned to the orthogonal structure of Fddd[24]. In addition, as can be seen from Figure 3, CNT-S has the same XRD patterns as sulfur except difference in the diffraction intensity. It indicates that sulfur has been successfully loaded on the CNT and no sulfur phase change occurs during the preparation process. Subsequently, even if after in-situ coating of HPEIGA by rapidly crosslinking reaction, there is no obvious difference in the XRD patterns between the CNT-S and the HPEIGA@CNT-S. It suggests that the polymer coating has no effect on the crystal structure of sulfur during the in-situ coating process.

Figure 3 XRD patterns of HPEIGA, CNT, sulfur, CNT-S and HPEIGA@CNT-S 3.2 The Electrochemical Properties. To evaluate the electrochemical performance of the HPEIGA@CNT-S, 2025-type coin cells were assembled. Cyclic voltammetry (CV) of Li-S coin cells using HPEIGA2@CNT-S and CNT-S as cathodes were initially performed in a voltage window of 1.7–2.8 V. As exhibited in Figure 4a, the two cathodes display the similar electrochemical conversions. Both the HPEIGA2@CNT-S and CNT-S cathodes possess two obvious cathodic peaks, which can be ascribed to the conversion of sulfur to Li2Sx (4
plateaus at 2.1 and 2.35 V during discharge process are in consistence with the CV results. Furthermore, it can be clearly seen that the initial capacity of the cells which cathodes are CNT-S, HPEIGA2: CNT-S (1:2), HPEIGA2: CNT-S (1:3) and HPEIGA2: CNT-S (1:4) are 1074, 855, 954 and 1052 mAh g-1, respectively. A slight higher initial capacity of CNT-S may be attributed to the lower charge transfer resistance which can be confirmed by electrochemical impedance spectroscopy (EIS) as below.

Figure 4 (a) CV curves of HPEIGA2@CNT-S and CNT-S electrode; (b) The charge/discharge curves for the coin cells with different HPEIGA2@CNT-S and CNT-S Current density: 0.1C; cut off voltages: 1.7–2.8 V; (c) Nyquist plots of

different HPEIGA2@CNT-S cathodes and CNT-S cathodes within the frequency range of 100 kHz to 10 mHz; The equivalent circuit used to simulate the electrochemical models of the Li-S battery: (d) CNT-S (e) HPEIGAs@CNT-S. To obtain further insight of the electrochemical performance, the EIS was also conducted. As shown in Figure 4c, the Nyquist curves of all lithium-sulfur batteries with cathodes coated by polymer consists of two semicircles in the high-intermediate frequency region and one slope line in the low frequency region. In contrast, the CNT-S positive electrode has only one semicircle and one slope line. Figure 4d and Figure 4e display the equivalent circuit used to simulate the electrochemical models of the Li-S battery. In the equivalent circuit, Rs represents the internal resistance. Rp and Rct are related to the encapsulation property of polymer and charge transfer resistance, respectively[35]. The existence of Rp suggests that HPEIGA is successfully coated on the cathode surface, which is consistent with the previous structural characterization. Zw is the Warburg impedance that reflects the diffusion of Li-ion in the solid[34]. Furthermore, the fitted impedance parameters are listed in Table S1 (Supporting Information). Obviously, the sum of Rp and Rct of HPEIGA2: CNT-S (1:4) is the smallest, indicating that it has good electrochemical performance. This also explains why the initial discharge capacity of the composite electrode HPEIGA2: CNT-S (1:4) is better. It is important to assess the lithium ion diffusion coefficient DLi, we calculated DLi by the Randles-Sevick equation related to cyclic voltammetry (CV) measurements at various scan rates (0.1–0.5 mV s−1), as described below.

. .
 = 2.69 × 10 × .   

Where
 indicates the peak current, n is the number of electrons (n = 2 for Li– S batteries), A represents the electrode area,  is the scanning rate and  is the concentration of Li+ in the electrolyte. The CV curve and the linear relationship of
 and  . of HPEIGA@CNT-S are shown in Figure 5a and Figure 5b. These data of CNT-S can be seen in Figure S2. From the linear relationship of
 and  ., lithium  = 5.44×10-9 ion diffusion coefficient DLi is calculated. For HPEIGA@CNT-S,    cm2 S-1,  = 4.94×10-9 cm2 S-1,  = 3.17×10-9 cm2 S-1 can be obtained,

respectively. In contrast, the values of CNT-S are 3.73×10-9 cm2 S-1 (A1), 1.61×10-9 cm2 S-1 (A2), 1.75×10-9 cm2 S-1 (C1) and 8.11×10-10 cm2 S-1 (C2). Obviously, the HPEIGA@CNT-S cathodes have high lithium-ion diffusion coefficient, suggesting that the HPEIGA is beneficial to facilitating fast Li-ion transport.

Figure 5 (a) CV curves of HPEIGA@CNT-S electrode from 0.1 to 0.5 mV S-1; (b) The linear fits of the peak currents for the cells with the HPEIGA@CNT-S electrode, (c) Cycling performances of coin cells with different HPEIGA2@CNT-S cathodes and CNT-S at 1C; (d) The shuttling current of coin cells with different HPEIGA2@CNT-S cathodes and CNT-S. The long-cycling stability of cells is a very crucial criterion to evaluate the property of lithium-sulfur battery with the cathode coated by polymer. The discharging capacity and Coulombic efficiency of these series cells were executed at a current density of 1C. It can be observed from Figure 5c that the initial specific capacity of the cell with HPEIGA2 coating on CNT-S is slight lower than that of CNT-S, which may be ascribed to the poor electronic conductivity of HPEIGA. However, it is impressive that all of the cells after coating by HPEIGA2 have more stable cycling performance than the one without coating. Specifically, the capacity of the CNT-S cell exhibits a faster fade with a capacity retention of 58.20% after 150 cycles, while the corresponding capacity retentions of HPEIGA2: CNT-S (1:2), HPEIGA2: CNT-S (1:3) and HPEIGA2: CNT-S (1:4) are 84.48 %, 80.03% and 80.55%, respectively. It reveals that the polymer encapsulated layer HPEIGA can enhance the cycling stability of the Li-S cells. In addition, the cells with different mass ratios of the HPEIGA and CNT-S powder have different cycle performance, which may be due to the different thickness of encapsulated layer coating on CNT-S (this can be confirmed by SEM as shown in Figure1 and Figure S1). The thicker layer can imprison more polysulfides within the HPEIGA@CNT-S electrode by physical

encapsulation. On the other hand, higher cycling stability of the cell based on HPEIGA2: CNT-S (1:2) may be attributed to the presence of larger amount of polar nitrogen atoms in the cathode, which can suppress the shuttle effect more effectively. In order to better understand the effect of polymer encapsulation on the cycling stability of the Li-S cells, we investigate the shuttling current using the method proposed by Narayanan[43]( The details of shuttling current measurement can be seen in Supporting Information). As shown in Figure 5d, it can be obviously seen that the shuttling currents of HPEIGA@CNT-S cathodes are much lower than that of CNT-S. It implies that most of polysulfides can be imprisoned within the HPEIGA@CNT-S electrode. Therefore, the polymer encapsulated layer HPEIGA can alleviate the problem of polysulfides dissolution and shuttle in liquid electrolyte, so that electrochemical stability of cells with polymer coating can be obviously improved. Furthermore, the degree of cross-linking of HPEIGA is also an important factor that needs to be considered. Subsequently, we adjusted the weight ratio of HPEI and GLA to further optimize the cycle stability of the battery (the weight ratio of polymer to CNT/S at 1:4 ). The mass ratios of HPEI and GLA were 1:1, 2:1 and 3:1, which the corresponding samples are referred as HPEIGA1@CNT-S, HPEIGA2@CNT-S and HPEIGA3@CNT-S. Figure S3 and Figure S4 show the charge-discharge curves and AC impedance spectrum of the battery with these materials as cathodes, respectively. In addition, the fitted impedance parameters of HPEIGA1: CNT-S (1:4), HPEIGA2: CNT-S (1:4) and HPEIGA3: CNT-S (1:4) are listed in Table S2. The Rct of them

exhibit similar values, indicating that the cross-linking degree of the HPEIGA has little effect on the electron transport of the electrode. The cycle performance at the current of 1C is shown in Figure 6a, it can be seen that the initial specific capacities of coin cells with the HPEIGA1@CNT-S, HPEIGA2@CNT-S and HPEIGA3@CNT-S cathodes are 625, 665 and 655 mAh g-1, respectively. After 150 cycles, the discharge capacity of these cells decreases in the following order: HPEIGA3 (582 mAh g-1) > HPEIGA2 (535 mAh g-1) > HPEIGA1 (495 mAh g-1). Clearly, the coin cell with cathodes coated by HPEIGA3 exhibits the best cycling stability. Specifically, the capacity of the cell using HPEIGA3@CNT-S as cathode maintains 88.85% after 150 cycles, while the corresponding capacity retentions of the other two cells are only 80.45% and 79.2%, respectively. Higher cycling stability of the cell containing HPEIGA3@CNT-S may be attributed to the presence of large amount of polar nitrogen atoms in the HPEIGA3. This is beneficial to enhance the interactions between lithium polysulfides and N atoms, which has been investigated by DFT calculation in the previous report[41]. Furthermore, we confirmed it through visualization adsorption experiment. As shown in Figure S5, the color fading of the Li2S6 solution after adding HPEIGA3 is the most obvious, indicating the stronger adsorption of polysulfide anion by larger amount of polar nitrogen atoms in the HPEIGA3. Moreover, it is also can be observed that the cell with HPEIGA3@CNT-S cathode has the lowest shuttle current (Figure S6). The coin cell with cathodes coated by HPEIGA3 exhibits the best cycling stability. Therefore, the HPEIGA polymer encapsulated layer can fasten the polysulfides by chemical reaction

with nitrogen atoms, which contributed to low shuttling current and outstanding long-cycling stability of lithium-sulfur batteries.

Figure 6 (a) Cycling performance of different HPEIGAs@CNT-S cathodes coin cells

at

1C;

(b)

HPEIGA3@CNT-S(1:4)

Cycling /Li

performance coin

cell

at

and 2C;

coulombic (c)

efficiency

Capacity

of

of the

HPEIGA3@CNT-S(1;4) /Li coin cells at varied C-rates ranging from 0.1 C to 2 C Moreover, the long-cycle stability even at a higher current density of the cell with HPEIGA3@CNT-S cathode is also investigated. As can be seen from Figure 6b,

the charge/discharge capacity of HPEIGA3@CNT-S/Li cell is conducted at a current density of 2C. The initial capacity of cell using HPEIGA3@CNT-S as cathode can reach up to 563 mAh g-1. Notably, the capacity retention of 73.68% can be obtained after 600 cycles with a low fading rate of only 0.043% per cycle, revealing that the HPEIGA3@CNT-S/Li cell possesses superior long-cycle stability. Figure S8 exhibits the charge/discharge capacity of HPEIGA3@CNT-S/Li cell at a current density of 0.5C. In addition, compared with some previous reports about cathode materials encapsulated by other polymer (Table S3, Supporting Information), the cell with HPEIGA3@CNT-S cathode also has superior capacity retention. We further evaluated the rate capacity of the HPEIGA3@CNT-S/Li cell under different C-rates (Figure 6c). From 0.1 to 2 C, the first discharge capacity was 1051, 965, 790, 698, 605 and 545 mAh g-1 at 0.1, 0.2, 0.5, 1, 1.5 and 2 C, respectively. Especially, once the C-rate is turned from 2 to 0.2 C, the capacity still can be maintained almost at the original level, confirming that HPEIGA3@CNT-S/Li cell also has good C-rate performance. High sulfur area loading mass of electrodes is also important for practical use. Electrodes with sulfur loadings of ∼1.6, ∼2.5 and ∼3.3 mg cm−2 are also prepared. The performance of cells with higher sulfur loading are provided in Figure S9.

In order to further verify the stability of the cell, comparing the change of the morphology of the electrode before and after cell cycle is very important. The disassemble cell pictures of the HPEIGA3@CNT-S cathode and CNT-S cathode after 100 cycles are provided in Figure S7. Furthermore, the surface morphology of the CNT-S and HPEIGA3@CNT-S electrodes both before and after 100 cycles at the current of 1C are evaluated by SEM measurements[44]. It can be seen that the CNT-S (Figure 7a) and HPEIGA3@CNT-S (Figure 7c) uniformly disperses in the electrodes. After 100 cycles, electrodes were dried under low vacuum to remove liquid

electrolyte completely before analysis. The surface morphology of the CNT-S electrode drastically changes after discharging as shown in Figure 7b, the sulfur particles coalesced into a large solid mass owing to the shuttle effect[45]. In contrast, after 100 cycles, the surface morphology of the HPEIGA3@CNT-S electrode has hardly no change (Figure 7d). This observation demonstrates that HPEIGA is helpful for anchoring sulfur and suppressing the shuttle effect of sulfur. Moreover, the structure of the electrode is not destroyed because of the existence of HPEIGA with large inner space, which can alleviate volume expansion during the long cycling process. In addition, the EIS of the CNT-S and HPEIGA3@CNT-S electrodes both before and after 5 cycles are evaluated (Figure S10 in the Supporting Information). It can be clearly seen that the EIS of CNT-S reveals two distinct semicircles after 5 cycles, which implies the formation of Li2S in the Li anode owing to the shuttle effect[46]. In contrast, after 5 cycles, the EIS of the HPEIGA3@CNT-S electrode has hardly no change. These further showed that the HPEIGA can greatly reduce shuttle effect of polysulfides to improve the stability of the cell.

Figure 7. SEM images of the CNT-S electrode (a) before(b) after 100 cycles and SEM images of the HPEIGA3@CNT-S electrode (c) before (d) after 100 cycles In general, the cell with HPEIGA encapsulated CNT/sulfur cathode exhibits superior long-cycle stability. The excellent cell performance can be largely attributed to some advantages of HPEIGA used as encapsulated layer. The HPEIGA with large inner space can inhibit the volume change of sulfur during the cell cycle process, and thus improve the structural stability of electrode. More importantly, the positive synergism of physical barrier and chemical constraints of nitrogen-rich cross-linked HPEIGA ensures effective restriction of Li2Sx in the cathode to inhibit the shuttling effect of Li-S batteries. As a result, the long cyclic stability of Li-S cell is greatly improved after CNT/sulfur cathode encapsulated by HPEIGA. 4. Conclusions

In summary, a novel polymer encapsulated layer (HPEIGA) was synthesized by a one-step cross-link reaction of HPEI and GLA. Simultaneously, the coin cells with the HPEIGA@CNT-S composite electrode have been successfully fabricated. The HPEIGA in the electrode greatly improved the cycling stability of the Li-S cells. After 150 cycles, the discharged capacity of the HPEIGA3@CNT-S/Li cell still can remain 582 mAh g-1 while there is the 88.85% maintained based on the first discharge capacity of 655 mAh g-1 at the current of 1C. In contrast, the capacity retention of CNT-S is only 58.20% after 150 cycles. Furthermore, the capacity retention of the HPEIGA@CNT-S cell can reach up 73.68% with a small decay of 0.043% per cycle at the current of 2C even after 600 cycles. Impressively, the preparation process of polymer encapsulated layer is not only fast, but also without catalyst which can eliminate the introduction of impurities. It can provide a new method for rapid formation of sulfur coating. In addition, the HPEIGA has joint effects of the containing polar nitrogen atom as chemical absorbent and the stable cross-linking network as physical barrier. Thus, nitrogen-contained hyperbranched polymer has a great potential as encapsulated layer of sulfur electrode and wide application in high performance Li-S batteries. Supporting Information. Supplementary data associated with this article can be found, in the online version. Author Information Corresponding Author Zengxi Li a *: [email protected]

Liaoyun Zhang a *: [email protected]; Notes The authors declare no competing financial interest.

Acknowledgements

The authors express thanks for the analysis and test center of University of Chinese Academy of Science. We thanks to the supports of the National Natural Science Foundation of China (No.51073170), the NSFC-Key project of Shanxi Coal Based Low Carbon Jiont Foundation (No.U1610222) and the Innova-tion Program of CAS Combination of Molecular Science and Education.

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1. 2. 3. 4.

A hyperbranced polymer encapsulated layer for sulfur cathode was synthesized The preparation process of encapsulated layer is rapid and without catalyst This encapsulated layer effectively retards shuttle effect of polysulfide The resulted cell exhibited capacity retention of 73.68% after 600 cycles

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: