Multifunctional reaction interfaces for capture and boost conversion of polysulfide in lithium-sulfur batteries

Journal Pre-proof Multifunctional reaction interfaces for capture and boost conversion of polysulfide in lithium-sulfur batteries Cheng Huang, Tingting Sun, Hongbo Shu, Manfang Chen, Qianqian Liang, Ying Zhou, Ping Gao, Sheng Xu, Xiukang Yang, Minli Wu, Jian Jian, Xianyou Wang PII:

S0013-4686(20)30049-9

DOI:

https://doi.org/10.1016/j.electacta.2020.135658

Reference:

EA 135658

To appear in:

Electrochimica Acta

Received Date: 4 November 2019 Revised Date:

4 January 2020

Accepted Date: 6 January 2020

Please cite this article as: C. Huang, T. Sun, H. Shu, M. Chen, Q. Liang, Y. Zhou, P. Gao, S. Xu, X. Yang, M. Wu, J. Jian, X. Wang, Multifunctional reaction interfaces for capture and boost conversion of polysulfide in lithium-sulfur batteries, Electrochimica Acta (2020), doi: https://doi.org/10.1016/ j.electacta.2020.135658. 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. © 2020 Published by Elsevier Ltd.

CRediT author statement: Cheng Huang# a: Conceptualization, Methodology, Writing - Original Draft; Tingting Sun#a: Methodology, Validation, Writing - Original Draft; Hongbo Shu*a: Resources, Supervision, Project administration, Funding acquisition, Writing Review & Editing; Manfang Chen a: Visualization; Qianqian Liang a: Data Curation; Ying Zhoua: Investigation; Ping Gaoa: Formal analysis; Sheng Xua: Formal analysis; Xiukang Yanga: Data Curation; Minli Wua: Investigation; Jian Jianb: Software; Xianyou Wang*a: Resources;

Graphical Abstract

1

Multifunctional reaction interfaces for capture and boost

2

conversion of polysulfide in lithium-sulfur batteries

3 4

Cheng Huang# a, Tingting Sun#a, Hongbo Shu*a, Manfang Chen a, Qianqian Liang a, Ying Zhoua, Ping Gaoa, Sheng Xua, Xiukang Yanga, Minli Wua, Jian Jianb, Xianyou Wang*a

5 6 7 8 9

(a Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, National Base for International Science & Technology Cooperation, National Local Joint Engineering Laboratory for Key Materials of New Energy Storage Battery, School of Chemistry, Xiangtan University, Hunan 411105, China; b Hunan University of Science and Technology, Hunan 411105, China)

10

Abstract

11

Lithium-sulfur batteries have gotten a growing number of investigations because of its

12

overwhelming superiority in theoretical energy density and cost. Nevertheless, the

13

application

14

disadvantageous shuttle effect, which arises from the dissolution and migration of

15

intermediate polysulfides and its sluggish conversion kinetics. Herein, we design the

16

conductivity-adsorption-catalysis reaction interface, which is constructed by growing

17

NiCo2S4 nanoparticle on reduced graphene oxide (NiCo2S4@rGO), to afford chemical

18

immobilization and conversion promotion of polysulfides. In this structure, rGO with

19

excellent conductivity can ensure rapid electron transfer and well-distributed NiCo2S4

20

nanoparticles serve as high-efficiency active sites to anchor polysulfides and

21

accelerate its conversion reaction. Thus, lithium-sulfur batteries with this

22

multifunctional reaction interface deliver improved cycling stability with capacity

23

retention rate of 76% after 500 cycles at 1 C. And a good initial capacity of 776 mAh

24

g-1 is gained under high sulfur loading of 3.6 mg cm-2. This work supplies promising

process

of

lithium-sulfur

batteries

is

severely

#

These authors contributed equally to this work

*

Corresponding author: H.B. Shu ([email protected]), X.Y. Wang ([email protected]).

1

obstructed

by

1

interface design strategies to enhance polysulfides redox kinetics and alleviate shuttle

2

effect for high-performance lithium-sulfur batteries.

3

Keywords:

4

Chemical immobilization

5

1. Introduction

Lithium-sulfur batteries; Shuttle effect; Polysulfides conversion;

6

The growing popularity of electric vehicles and small mobile electronic devices

7

continuously raise the requirement toward energy storage system.[1] Among the

8

numerous candidates, lithium-sulfur(Li-S) batteries have become a focus of attention

9

owing to its extremely superior theoretical energy density, low cost and

10

environmentally-friendly of sulfur.[2] Still, its development and commercialization

11

process is seriously impeded by some inherent issues, including a) badly poor

12

conductivity of sulfur and Li2S,[3] b) migration back and forth of soluble polysulfides

13

in organic electrolyte between cathodes and anodes, namely shuttle effect[4] and c)

14

repetitious volumetric change between sulfur and Li2S.[5] Among these defects

15

mentioned above, shuttle effect is a major challenge,[6] because it can give rise to

16

irreversible loss of the sulfur species as well as enhancive polarization, thereby

17

triggering terrible cyclic performance and low Coulombic efficiency,[7] which is

18

unbearable in actual application.[8, 9]

19

Many strategies have been proposed to suppress the shuttle of polysulfides.[10]

20

Carbonaceous materials as sulfur hosts[11-15] or interlayers[16-19] for physical

21

confinement are conventional approaches due to their brilliant conductivity and

22

diversiform nanostructures.[20] Polar materials, for example, metal compounds,[21-23]

23

polymer,[24,

24

because it can furnish with chemical interactions[27] to ensnare intermediate

25]

g-C3N4,[26] etc., are developed for polysulfides diffusion restriction

2

1

polysulfides.[28] However, accompanied by continuous generation and accumulation

2

of soluble polysulfides, it is not enough to restrain polysulfides shuttle only by

3

blockage measures of physical confinement and chemical entrapment. In fact, there

4

are complicated multistep polysulfides redox reactions in Li-S batteries, and yet their

5

kinetics are poor,[29] leading to the rising accumulation and inevitable diffusion under

6

concentration gradient.[30] Catalysts such as sulfides[31,32], nitrides[33], and phosphides

7

[34]

8

lower-order polysulfides to decrease dissolution and accumulation as well as

9

residence time of soluble polysulfides[35], resulting in suppressing the shuttle of

10

polysulfides. Therefore, designing a material which can coordinate physical

11

confinement, chemical entrapment and catalysis is an effective way to solve the

12

shuttle effect.

boost the reaction kinetics of propelling conversion of higher-order polysulfides to

13

Transition metal oxides have attracted much attention due to their strong

14

chemical adsorption capabilities, which can effectively suppress the shuttle effect

15

[36]

16

ability, which will greatly limit their electrochemical performance. And materials with

17

too strong adsorption ability will destroy the formation of Li2Sn species, causing the

18

secondary dissolution

19

been widely used in Li-S batteries owing to their catalytic conversion on polysulfides.

20

Compared to single metal sulfides, bimetallic sulfides compounds providing more

21

catalytical active sites can greatly promote polysulfides conversion kinetics

22

NiCo2S4 as a semiconductor, introducing the second metal ion Ni to adjust the

23

electronic structure of cobalt, with complex chemical composition and good synergy

24

of various valence transitions Ni and Co species, possesses relatively high

25

electrochemical activity and good electrical conductivity [41]. However, it is necessary

.However, oxides always suffer from poor electrical conductivity and low catalytic

[37]

. Single metal sulfides including CoS2[38] MoS2[39] etc., have

3

[40]

.

1

to design a rational nanostructure with outstanding conductivity which could assure

2

rapid electro transmission for polysulfides redox reaction.

3

Herein, a novel conductivity-adsorption-catalysis reaction interface, which

4

consists of NiCo2S4 nanoparticles in-situ growth on reduced graphene oxide

5

(NiCo2S4@rGO), is constructed on separators to joint synergy effect of chemical

6

adsorption, catalytic conversion and physical confinement for shuttle restrictions. For

7

the reaction interface, separators with multilayer structure also offer additional

8

physical barriers for confining the migration of polysulfides to the anode. Building

9

conductive net by dense contact between NiCo2S4 nanoparticles and rGO brings about

10

fast electron transfer for polysulfides conversion, and thus efficiency of adsorption

11

and catalyze is further improved. The Li-S batteries with this multifunctional reaction

12

interface achieved enhanced capacity retention rate of 76% after 500 cycles at 1 C,

13

and delivered a good initial capacity of 776 mAh g-1 even under the sulfur loading of

14

3.6 mg cm-2. It is suggesting that integrating blocking of physical/chemical with

15

dredging of catalytic conversion by interface design on separator is a promising

16

strategy to mitigate shuttle effect.

17

2. Experiment

18

2.1 Preparation of NiCo2S4@rGO

19

A typical preparation of NiCo2S4@rGO was synthesized by a one-step

20

solvothermal strategy. In detail, 0.03 g graphene oxide (GO) was dispersed in 30 mL

21

ethylene glycol (EG) to form GO/EG suspension via ultrasonic cell disrupts in

22

ice-water bath for 1.5 h. And then, 0.075 mmol Ni(OAc)2 and 0.15 mmol Co(OAc)2

23

were dissolved in above GO/EG suspension with stirring at 80
for 2 hours. After

24

that, 0.45 mmol thiourea was added to the suspension and dissolved as well. 4

1

Afterwards, the mixed liquor was transferred to a 40 mL autoclave and heated to 200


2

for 6 h in high temperature oven. After natural cooling, the final product was obtained

3

by filtering, washing with plenty of deionized water and freeze drying for 24 hours.

4

2.2 Preparation of Modified Separators

5

The slurry was made up with NiCo2S4@rGO and polyvinylidene fluoride (PVDF)

6

with weight ratio of 9:1 and Nmethyl-2-pyrrolidinone (NMP) as the dispersant. After

7

stirring for 4 hours, the homogeneous slurry was coated on one side of Celgard 2400

8

polypropylene(PP) membrane and dried at 45
for 36 hours. In the end,

9

NiCo2S4@rGO modified diaphragm was obtained with a 19 mm puncher. As a

10

comparison, rGO modified separators was also made in the same way.

11

2.3 Preparation of carbon nanotubes/sulfur composites(CNTs/S)

12

0.02 g Polyvinylpyrrolidone (PVP), 0.1 g CNTs and 0.03 M sodium thiosulfate

13

pentahydrate was dispersed in 200 mL deionized water with vigorously stirring for 20

14

minutes. And then, 4 mL concentrated hydrochloric acid was introduced dropwise into

15

the above solution under stirring at 50
for 4 hours. After that, the reaction products

16

was acquired by filtering, washing with plenty of deionized water and dried at 60


17

for 24 hours. Finally, the mixture powders was heated at 155
for 12 h in Ar

18

atmosphere and CNTs/S was obtained.

19

2.4 Preparation of Li2S4 solution

20

Lithium polysulfide (Li2S4) solution was prepared via adding the stoichiometric

21

amounts of S and lithium sulfide (Li2S) with a molar ratio of 1:3 into DME/DOL (v/v

22

= 1/1) solution with magnetically stirring at 60 °C for 48 hours in an argon glove box.

23

2.5 Symmetric cells and electrochemical evaluation

24

The samples (NiCo2S4@rGO or rGO) and poly(vinylidene fluoride) (PVDF)

25

were mixed with a mass ratio of 9:1 in N-methyl pyrrolidone (NMP) to form uniform 5

1

mixture, which was coated on carbon-coated Al foils and dried in the vacuum oven at

2

60 °C overnight. And then, the obtained foils were punched to a circular shape with

3

diameter of 10 mm.

4

Two foils were employed as working and counter electrodes. They were

5

assembled to a 2025-coin cell, where polypropylene (PP) membrane was used as

6

separator and 40 µL Li2S6 electrolyte include 0.5 mol L-1 Li2S6 and 1 mol L-1 LiTFSI

7

dissolved in DOL/DME (v/v = 1/1) was added. The CV measurement of the

8

symmetric cell was conducted at a sacn rate of 10 mV s-1 between -0.8 and 0.8 V.

9

2.6 Materials Characterization

10

The structures and morphologies of the as-synthesized samples were investigated

11

by scanning electron microscopy (SEM, JSM-6610LV), field-emission scanning

12

electron microscopy (FESEM, Hitachi S-4800) and transmission electron microscopy

13

(TEM, JEM-2100F, JEOL, Japan). X-ray diffraction (XRD, Model LabX-6000,

14

Shimadzu, Japan) were recorded in the 2θ range of 10-80° to analyze phase

15

composition of the samples. In order to confirm the carbon content in as-synthesized

16

samples, thermogravimetric analysis (TGA) was implemented on a Series Q500

17

instrument (TA Instruments, USA) under air atmosphere with the heating rate of

18

10 °C min-1 from room temperature to 1000 °C. The specific surface areas and pore

19

structures were determined by nitrogen adsorption/desorption tests (JWBK112). The

20

specific surface areas were computed by conventional Brunauer-Emmett-Teller (BET)

21

means. The pore volumes and the pore size distributions were acquired from the

22

adsorption branch of the isotherm on basis of the density functional theory (DFT)

23

theories calculation. To research surface electronic states of products, X-ray

24

photoelectron spectroscopy (XPS, K-Alpha 1063, Thermo Fisher Scientific) was

25

detected. Fourier transform infrared spectroscopy (FT-IR) was characterized via a 6

1

Spectrum One Infrared spectrometer from the PerkinElmer in a scan range of

2

800-3000 cm-1. The degree of wettability of separators was characterized by static

3

contact angle measuring instrument (SZ-CAMB1) and the identity of the liquid

4

solution was deionized water.

5

2.7 Electrochemical measurements

6

The cathode electrodes were prepared by CNTs/S, acetylene black, and

7

polyvinylidene fluoride binder with a weight ratio of 7:2:1 dispersing in N-methyl

8

pyrrolidone (NMP) under stirring to form uniform slurry. The slurry was coated on

9

carbon-coated aluminum foil and dried in the vacuum oven at 60 °C for 12 h. The

10

obtained film was punched to a circular shape with diameter of 10 mm. The sulfur

11

areal mass loading of the electrode is about 1.6, 2.5 and 3.6 mg cm-2. The electrolyte

12

is 1 mol L−1 lithium bis(trifluoromethane) sulfonimide (LiTFSI) in a mixture of equal

13

volumes of 1,3 dioxolane (DOL) and 1,2-dimethoxyethane (DME) with the 2 wt%

14

LiNO3 additives (electrolyte/sulfur=20 µL mg-1). NiCo2S4@rGO modified separators,

15

rGO modified separators and Celgard 2400 polypropylene membrane were applied as

16

the separators, respectively. The areal mass loading of the NiCo2S4@rGO layer on the

17

separator is 0.834 mg cm-2. Metal lithium was employed as the anode. The coin cells

18

were assembled in an argon-filled glove box with O2 and H2O lower than 1 ppm.

19

Galvanostatic discharge/charge cycling tests was performed on Neware multichannel

20

battery testing system BTS-XWJ-6.44S-00052 in a narrow potential range from 1.7 to

21

2.8 V (vs Li+/Li). The CV measurements were carried out by using electrochemical

22

workstation (CHI660E). The electrochemical impedance spectroscopy (EIS) was

23

measured with the same instruments over the frequency ranging from 100 kHz to 10

24

mHz with 5 mV of AC amplitude. All the electrochemical measurements were

25

performed at room temperature and the specific capacities are based on the mass of 7

1

sulfur.

2

3. Results and discussions

3

The conductivity-adsorption-catalysis reaction interface consisted of in-situ

4

growing NiCo2S4 nanoparticles on rGO sheets was synthesized via a simple and

5

convenient one-step solvothermal preparation strategy as shown in Figure 1.

6

Ni(OAc)2, Co(OAc)2 and thiourea is dissolved and dispersed in GO/EG suspension,

7

followed by solvothermal process. Under the solvothermal condition with the

8

reducibility of thiourea, the GO transforms to rGO which can lead to better

9

conductivity. After filtering and freeze-drying, that NiCo2S4 nanoparticles uniformly

10

grow on rGO sheets (NiCo2S4@rGO) is finally obtained.

11 12

Figure 1 Schematic illustration of the preparation of NiCo2S4@rGO.

13

The microstructure of NiCo2S4@rGO was characterized by FESEM and TEM.

14

As show in Figure 2a-c, NiCo2S4 nanoparticles with diameter of about 90 nm are

15

uniformly grown on the pleated rGO sheets which deliver a length of ~20µm, a width

16

of ~18µm (Figure S1). Besides, Figure 2d-e exhibit that the rGO layer is relatively

17

thin and the distribution of NiCo2S4 nanoparticles is dense. In addition, there are two

18

sets of lattice fringes with spacing of 0.28 nm and 0.54 nm in Figure 2f, which is

19

corresponded to the (311) and (111) crystal plane of NiCo2S4,[42] respectively. 8

1

Furthermore, the layered fringes of rGO can be clearly observed in Figure 2f. From

2

the elemental mapping images (Figure 2g-j), the Co, Ni and S elements are uniformly

3

distributed at the position of the NiCo2S4 nanoparticles. Based on the above results,

4

NiCo2S4@rGO composites in which NiCo2S4 nanoparticles were grown in-situ on

5

rGO sheets is successfully prepared.

6 7

Figure 2 (a-c) FESEM, (d-e) TEM, (f) high resolution TEM and (g-j) elemental

8

mapping images of NiCo2S4@rGO.

9

The X-ray diffraction (XRD) is carried out to verify the crystal structures of

10

as-prepared materials. As shown in Figure 3a, there are only two broad peaks

11

belonging to carbon in XRD pattern of rGO. While the diffraction peaks of

12

synthesized NiCo2S4@rGO are well-matched with cubic phase of NiCo2S4 (JCPDS

13

NO. 20-0782), verifying the high purity of the sample. The carbon content of

14

NiCo2S4@rGO was confirmed by TG at air atmosphere, and the TG curve is shown in 9

1

Figure 3b. There is 68% loss which is relative to the oxidation of rGO and NiCo2S4.

2

rGO as carbon was entire loss under high temperature in the air. In order to analyze

3

oxidation of NiCo2S4, NiCo2S4@rGO was calcined at 850
in air, and XRD pattern

4

of oxidation products reveals in the inset of Figure 3b. According to the diffraction

5

peak, the oxidation products are mixture of CoO and NiO. On the basis of the

6

following equation: Carbon weight(%)=1-32%×MNiCo2 S4 /MNiCo2 O3

7

The carbon content is about 57% in as-synthesized NiCo2S4@rGO composites. In

8

addition, as shown in Figure 3c, the absorption peaks of oxygen-containing

9

functional group are significantly weakened in IR spectrum of rGO and

10

NiCo2S4@rGO. Meanwhile, the intensity of surface oxygenic functional group in C

11

1s XPS of NiCo2S4@rGO (Figure 3d) is relatively lower than GO (Figure S3). The

12

IR and XPS results suggest that the GO is reduced to rGO under the preparation

13

condition.

14 15

Figure 3 (a) XRD pattern of rGO and NiCo2S4@rGO; (b) TG curve of NiCo2S4@rGO

16

in air (inset: XRD pattern of NiCo2S4@rGO calcinations at 850
in air); (c) IR 10

1

spectrum of GO, rGO and NiCo2S4@rGO; (d) C 1s XPS of NiCo2S4@rGO.

2

The specific surface and pore structure of composites were analyzed by N2

3

adsorption-desorption measurements. As exhibited in Figure 4a-b, the curves of

4

NiCo2S4@rGO and rGO display the same trend, corresponding to the third type of

5

isothermal adsorption-desorption curve. Specifically, there are low-level hysteresis

6

loops, indicating the existence of mesopores in both materials. Moreover, there are a

7

certain number of macropore, demonstrating by that the adsorption amount rises

8

rapidly at high pressure. According to the test results, the specific surfaces area of

9

rGO and NiCo2S4@rGO is 39 and 37 m2 g-1, respectively. Benefit from the uniform

10

NiCo2S4 nanoparticles on the surface of the rGO, there is no obvious reduction of the

11

specific surface area, which guarantees the sufficient conductive reaction interface. In

12

addition, pore-size distribution is concentrated in mesopores and macropores as

13

displayed in Figure 4c-d. The mesopores could impose physical limitations on the

14

diffusion of polysulfides, and macropores could facilitate the entry of electrolytes and

15

transport of Li ions. Based on the calculation of BJH mode, the pore volumes of rGO

16

and NiCo2S4@rGO were 0.149 and 0.128 cm3 g-1, respectively. In short, such porous

17

structure can provide a relatively high-quality conductive surface, which is beneficial

18

to electron/ion transport and ensures the kinetics basis of the good reaction interface.

11

1 2

Figure 4 N2 adsorption/desorption isotherm of (a) rGO, (b) NiCo2S4@rGO; (c-d)

3

corresponding pore-size distribution.

4

NiCo2S4@rGO composites were coated on one side of Celgard 2400

5

polypropylene (PP) separator to serve as reaction interface for polysulfide conversion.

6

The NiCo2S4@rGO modified separator was cut into a disc with a diameter of 19 mm

7

(Figure 5a). That coating is uniform and full ensures completely contact of the

8

reaction interface with polysulfide. As delivered in Figure 5b, the thickness of the

9

coating layer is about 12 µm with stacked structures, which can act as multi-layer

10

reaction interface to ensure efficient adsorption and conversion of polysulfides, and

11

also favorable layers barrier for polysulfide diffusion. During flexibility tests of the

12

modified separator (Figure 5c-d), it can still be restored after folding without

13

electrode material dropping and electrode cracking, indicating that the modified

14

membrane is good flexible and the combination between composites and separator is

15

very tight. Besides, as exhibited in Figure 5e-f, the contact angle of the

16

NiCo2S4@rGO modified separator is observably smaller than that of the rGO

17

modified separator, suggesting that the introduction of NiCo2S4 nanoparticles 12

1

improves the hydrophilicity and polar of rGO. Based on the compatibility principle,

2

due to the hydrophilicity of electrolyte and the affinitive with hydrophilic functional

3

groups of polysulfide[43,44], it can be speculated that better polar surface with lyophilic

4

property is more conducive to improve the surface wettability and enhance the polar

5

interaction with the electrolyte solution thus promoting infiltration of the electrolyte

6

and adsorption of polysulfide.

7 8

Figure 5 (a) digital photo, (b) cross-sectional SEM and (c-d) flexible test of

9

NiCo2S4@rGO modified separator; contact angle test of (e) NiCo2S4@rGO and (f)

10 11

rGO modified separator. In order to explore the effect of the reaction interface constructed by 13

1

NiCo2S4@rGO on the performance of lithium-sulfur batteries, NiCo2S4@rGO

2

modified separator (NiCo2S4@rGO), rGO modified separator (rGO) and commercial

3

PP separator (PP) were used as battery separators, respectively. And CNTs/S

4

electrodes were employed as cathode materials to assemble batteries. The sulfur

5

content is 89.78% calculated based on the TG results (Figure S1, Supporting

6

Information). A series of electrochemical performance test comparisons were

7

performed. As shown in CV curves of Figure 6a, all the as-synthesized samples

8

reveal two reduction peaks that the high voltage reduction peak represents the

9

conversion of S8 to the soluble long-chain polysulfide Li2Sx (4≤x≤8), and the other

10

peak corresponds to the conversion of long chain polysulfide to Li2S2/Li2S.[45]

11

Conversely, the oxidation peaks belong to the oxidized process of Li2S2/Li2S back to

12

S8.All the redox peaks of battery with NiCo2S4@rGO modified separator is the higher

13

and sharper than that of battery with other two separators, indicating that

14

NiCo2S4@rGO interface induces a lower polarization. In addition, the comparison of

15

the peak potential of the samples is exhibited in Figure 6b. There are a higher

16

reduction peak potential and a lower oxidation peak potential of NiCo2S4@rGO

17

modified separator, which demonstrates that reaction interface constructed by

18

NiCo2S4@rGO can effectively reduce electrochemical polarization and improve the

19

redox kinetics of polysulfide conversion.

20

The cycle performance of batteries with different modified separator was carried

21

out. As shown in Figure 6c, the initial discharge capacity of the battery with

22

NiCo2S4@rGO modified separator is 1264 mAh g-1 at 0.2 C, after 200 cycles its

23

discharge capacity still remains 1086 mAh g-1 with high capacity retention of 85%. As

24

a contrast, the initial capacity of the batteries using the rGO modified separator and

25

PP separator is only 1093 and 1010 mAh g-1, respectively, and the capacity retention 14

1

is as low as 66% and 55% after 200 cycles, respectively. It is obvious that the

2

discharge capacity and capacity retention of batteries was improved by

3

NiCo2S4@rGO modified separator. Moreover, the coulombic efficiency of battery

4

with NiCo2S4@rGO modified separator is relatively higher and more stable than the

5

other two. Especially, as listed in Table 1, this work displays an outstanding discharge

6

capacity and capacity retention compared to previous reports with similar parameter.

7

From the charge-discharge curves as displayed in Figure 6d, it can be found that the

8

difference between charge and discharge platform (△E1) of NiCo2S4@rGO modified

9

separator is smaller than that of rGO modified separator (△E2) and PP separator(△E3),

10

suggesting a better electrochemical reaction kinetics and more complete conversion of

11

polysulfides at NiCo2S4@rGO interface. To further investigate the influence of

12

modified interface toward electrochemical kinetics, EIS are performed after 10 cycles

13

at 0.2 C. As exhibited in Figure 6e, EIS of batteries with three kinds of separators

14

delivers a similar shape consisting of two semicircles and one line, in which the

15

semicircle at high-frequency is assigned to diffusion resistance of solid electrolyte

16

interface (Rf), the another semicircle in middle-frequency belongs to charge-transfer

17

resistance (Rct), and the line at low-frequency stands for Warburg impedance (Wo).

18

The inset of Figure 5e is equivalent circuit for fitting the impedance spectra, in which

19

the Rs means ohmic resistance and CPE represents constant phase element. The Rs, Rf

20

and Rct of battery with NiCo2S4@rGO modified separator is smaller than those of

21

other two (Figure 6e-f), implying that efficient reaction interface constructed by

22

NiCo2S4@rGO provides better electrical contact and ensures rapid and smooth

23

conversion of polysulfides to promotes the utilization of active materials, resulting in

24

a relatively high discharge capacity and good cycling stability.

15

1 2

Figure 6 (a) CV curves, (b) the comparison of peak voltages, (c) cycling performance

3

and (d) discharge-charge curves at 0.2 C, (e) EIS after 10 cycles at 0.2 C, (f) the fitted

4

impedance value of batteries with three kinds of separator.

5 6

Table 1 Comparison of the electrochemical performance of previous literatures with our work. Sample

S content / areal mass loading (% / mg cm-2)

This work

89.78% (1.6 mg cm -2)

Co3O4[36]

-

AB-CoS2[38]

77.8%

ZnO/CNT/rGO[46]

1.7mg cm -2

Initial capacity (mAh g−1 at n C)

Final capacity (mAhg−1 after cycles)

Capacity retention rate (% after cycles at n C)

1264  mAh g−1 (0.2C) 1205  mAh g−1 (0.1C) 1108  mAh g−1 (0.1C) 1061 mAh g−1 (0.2C)

1086  mAh g−1 (200 th) 706 mAh g−1 (100 th) 650 mAh g−1 (150 th) 768 mAh g−1 (150 th)

85% (200 th, 0.2C) 85% (100 th, 0.5C) 59% (150 th, 0.2C) 72.38% (150 th, 0.2C)

16

[47]

PP-rGO

1.6 mg cm

rGO @ MoS2[39]

70%

NiO / rGO-Sn[48]

80%

-2

1103 mAh g−1 (0.1C) 1121 mAh g−1 (0.2C) 1058 mAh g−1 (0.1 A g-1)

786 mAh g−1 (100 th) 671 mAh g−1 (200 th) 868 mAh g−1 (150 th)

71.26% (100 th, 0.1C) 59.86% (200 th, 0.2C) 82.04% (150 th, 0.1 A g-1)

1 2

Rate performance closely relating to the conduction of electron/ions and the

3

kinetics of polysulfide reactions is an important consideration for the performance of

4

lithium-sulfur batteries. As shown in Figure 7a, the battery with NiCo2S4@rGO

5

modified separator exhibits better rate performance. To be specific, at a rate of 0.1, 0.2,

6

0.5, 1 and 2 C, its discharge capacities are 1357, 1253, 1081, 892 and 680 mAh g-1,

7

respectively, which is significantly higher than that of rGO modified separator (1228,

8

1105, 905, 662, 442 mAh g-1) and PP separator (1120, 964, 700, 458, 221 mAh g-1).

9

When the current is restored to 0.1 C, the battery with the NiCo2S4@rGO modified

10

separator still retains a better recovery capacity of 1230 mAh g-1, compared to that of

11

the battery with the rGO modified separator (1014 mAh g-1) and PP separator (837

12

mAh g-1). Furthermore, as exhibited in Figure 7b, discharge-charge curves with

13

NiCo2S4@rGO modified separator at various rates always maintain two stable

14

electrochemical platforms, even at a high current density of 2 C. Combining with the

15

above rate performance analysis, there is a fast electron/ion conduction and

16

polysulfide conversion kinetics on the NiCo2S4@rGO interface.

17

In order to further study the advantages of the NiCo2S4@rGO interface,

18

long-cycle performance tests were also performed. As shown in Figure 7c, battery

19

with NiCo2S4@rGO modified separator delivers an initial discharge of 872 mAh g-1

20

and a remarkable reversible capacity of 664 mAh g-1 corresponding to capacity

21

retention rate of 76% after 500 cycles at 1 C. The capacity decay rate is as low as

22

0.047% and the average coulombic efficiency is around 99%. Such an impressive 17

1

long-cycle stability is mainly attributed to the reason that polysulfide can be well

2

adsorbed and rapidly converted on the interface constructed by NiCo2S4@rGO, which

3

limits the diffusion of soluble polysulfide and thereby alleviating the shuttle effect. In

4

addition, the cycle performance of NiCo2S4@rGO modified separator under high

5

sulfur loading was also carried out. As exhibited in Figure 7d, the initial capacity of

6

NiCo2S4@rGO modified separator is 820 mAh g-1 under sulfur loading of 2.5 mg cm-2

7

at 1 C and its reversible capacity still reaches up to 713 mAh g-1 after 100 cycles.

8

When the sulfur loading further increase to 3.6 mg cm-2, it can deliver a initial

9

capacity of 776 mAh g-1 and a reversible capacity of 588 mAh g-1 after 100 cycles,

10

suggesting that NiCo2S4@rGO interface posses a sufficient ability to absorb more

11

polysulfides and promote their conversion even under high sulfur loading.

18

1 2

Figure 7 (a) Rate performance of batteries with various separator, (b)

3

discharge-charge curves of battery with NiCo2S4@rGO modified separator at different

4

rates; (c) long-term cycling performances and (d) high sulfur-loading cycling

5

performances of batteries with NiCo2S4@rGO modified separator at 1 C.

6 7

In order to clarify the effect of the reaction interface constructed by

8

NiCo2S4@rGO on the polysulfide, static adsorption experiment and XPS analysis

9

were carried out. As shown in Figure 8a, in comparison with blank Li2S4 solution, the

10

color of Li2S4 solution mixed with rGO did not obviously change, but the color of

11

Li2S4 solution fades visibly by the introduction of NiCo2S4@rGO, indicating the

12

high-efficiency adsorption of polysulfide by NiCo2S4@rGO. NiCo2S4 contain

13

expensive Ni3+ and Co3+, and the conductivity can be improved to increase the 19

1

exposure of the active site metal, thereby facilitating the catalytic process

2

displayed in the Ni and Co 2p3/2 XPS of NiCo2S4@rGO (Figure 8b-c), the peaks of

3

binding energy at 852.8, 855.1, 778.8 and 779.2 eV are corresponded to Ni2+, Ni3+,

4

Co3+ and Co2+, respectively.[50-52] After adsorbing Li2S4, the peaks of both Ni 2p3/2 and

5

Co 2p3/2 shift toward low binding energy. Also, the intensity of the Ni2+ and Co2+

6

peaks increases and while intensity of the Ni3+ and Co3+ peaks decreases. The above

7

results illustrate that electrons transfers from polysulfides to Ni and Co of NiCo2S4,

8

resulting in promoting the conversion of polysulfides [53]. The similar mechanism also

9

has been presented by previous works

[54, 55]

[49]

. As

. Furthermore, in order to explain the

10

promotion of polysulfide conversion by NiCo2S4@rGO, CV tests were carried out

11

with assembling NiCo2S4@rGO and rGO as electrodes into symmetrical batteries,

12

respectively. As shown in Figure 8d, the NiCo2S4@rGO electrode exhibits a much

13

higher redox current, demonstrating that it can effectively boost kinetics of

14

polysulfide conversion reaction[56]. Based on the above analysis, NiCo2S4

15

nanoparticles can efficiently capture polysulfides through strong chemical interactions,

16

and can also serve as active sites to accelerate the conversion of polysulfides.

17 18

Figure 8 (a) photographs of static adsorption experiments of blank Li2S4 solution, 20

1

rGO and NiCo2S4@rGO; refined XPS of (b) Ni 2p3/2 and (c) Co 2p3/2 of

2

NiCo2S4@rGO before and after contact with Li2S4 solution; (d) CV curves of

3

symmetric cells.

4

Finally, there is a comparative graphical illustration on efficient reaction

5

interface constructed by NiCo2S4@rGO to adsorb polysulfides and catalyze

6

polysulfides conversion. The common commercial PP separator neither inhibits the

7

diffusion of polysulfide nor promotes the conversion of polysulfide, because of its

8

large pore size and inactive interface, resulting in a very serious shuttle effect (Figure

9

9a). Also, the interface constructed by rGO can only limit the diffusion of polysulfide

10

to a certain extent via role of physical barriers, but it is difficult to maintain efficiently.

11

Moreover, although it possesses good electrical conductivity, no catalytic activity

12

center which can not accelerate the conversion polysulfide leading to relatively severe

13

shuttle

14

conductivity-adsorption-catalysis

15

NiCo2S4@rGO. In the NiCo2S4@rGO interface, NiCo2S4 nanoparticles grown

16

uniformly and densely on surface of rGO to form good conductive net, which can

17

ensure rapid electron transport during polysulfide conversion process, and provide

18

deposition surface for Li2S. More importantly, NiCo2S4 as an active center not only

19

can efficiently adsorb polysulfides by strong chemical interactions to restrain shuttle

20

effect, but also boost the kinetics of polysulfide conversion reactions via catalyze

21

action. Especially, from the longitudinal direction, the modified separator with a

22

multi-layer structure can bring more efficient reaction interfaces to fully exert its

23

functions and furnish multiple physical barriers for blocking the diffusion of

24

polysulfides to the anode. Combination with the above advantages of NiCo2S4@rGO

25

interface construct conductivity-adsorption-catalysis reaction interface to achieve

26

excellent electrochemical performance for lithium-sulfur battery.

effect

(Figure

9b).

As reaction

21

shown interface

in

Figure was

9c,

constructed

the by

1 2

Figure 9 Schematic illustration of polysulfide diffusion behavior on different

3

interface of (a) PP, (b) rGO and (c) NiCo2S4@rGO.

4

4. Conclusion

5

The conductivity-adsorption-catalysis reaction interface was successfully

6

designed and constructed by NiCo2S4@rGO to regulate the behavior of polysulfides

7

alleviating shuttle effect and accelerating the polysulfides conversion. In the

8

NiCo2S4@rGO interface, NiCo2S4 nanoparticles grown uniformly and densely on

9

surface of rGO to form good conductive net, which can ensure rapid electron transport

10

during polysulfide conversion process, and provide deposition surface for Li2S.

11

Consequently, Li-S battery with NiCo2S4@rGO modified separator delivers an

12

ameliorative electrochemical performance. An impressive long-cycle stability with

13

capacity retention rate of 76% is accomplished after 500 cycles at 1 C. Also, a good

14

initial capacity of 776 mAh g-1 can be obtained even the sulfur loading as high as 3.6

15

mg cm-2. This multifunctional interface could offer a promising strategy to boost

16

polysulfide conversion and mitigate the shuttle for high-performance Li-S batteries.

17

Acknowledgments

18

This work is support financially by the National Natural Science Foundation of

19

China (No. 51502256), Hunan Provincial Natural Scientific Foundation of China (No.

20

2017JJ3297), Scientific Research Projects of Hunan Provincial Strategic Emerging 22

1

Industries (No. 2016GK4030), China Postdoctoral Science Foundation (No.

2

2014M552142), Hunan Provincial Education Office Foundation of China (No.

3

17C1523) and Scientific Research Fund of Xiangtan University (No. 2018HJYH08;

4

2015SEP03; 13QDZ30; 2014XZX07; 2018ZKKF03).

5

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6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

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26

Research highlights: 1. A conductivity-adsorption-catalyze reaction interface is demonstrated. 2. The lyophilic property and polar surface is more conducive to the electrolyte infiltration and polysulfide adsorption. 3. NiCo2S4@rGO can effectively boost kinetics of polysulfide conversion reaction. 4. Using NiCo2S4@rGO in the interlayer exhibits excellent electrochemical performances especially under high sulfur loading.

Conflict of Interest： The authors declare no conflict of interest.