Fe3O4-Decorated Porous Graphene Interlayer for High-Performance

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Fe3O4-decorated porous graphene interlayer for high-performance lithium-sulfur batteries Yuanming Liu, Xianying Qin, Shaoqiong Zhang, Gemeng Liang, Feiyu Kang, Guohua Chen, and Baohua Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07316 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 13, 2018

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ACS Applied Materials & Interfaces

1

Fe3O4-Decorated Porous Graphene Interlayer for

2

High-Performance Lithium-Sulfur Batteries a,b

, Xianying Qin

*a,c

3

Yuanming Liu

, Shaoqiong Zhang

4

Kang a,b,Guohua Chen c, Baohua Li *a

a,b

, Gemeng Liang

a,b

, Feiyu

5 6

a

7

Engineering Laboratory for Functionalized Carbon Materials, Graduate School at

8

Shenzhen, Tsinghua University, Shenzhen 518055, China.

9

b

Engineering Laboratory for Next Generation Power and Energy Storage Batteries,

School of Materials Science and Engineering, Tsinghua University, Beijing

10

100084,China.

11

c

12

Hong Kong, China.

13

KEYWORDS: Fe3O4 nanoparticles; porous graphene; interlayer; multi-functional;

14

lithium-sulfur batteries

15

ABSTRACT: Lithium-sulfur (Li-S) batteries are seriously restrained by the shuttling

16

effect of intermediary products and their further reduction on the anode surface.

17

Considerable researches have been devoted to overcoming these issues by introducing

18

carbon-based materials as the sulfur host or interlayer in Li-S systems. Herein, we

19

constructed a multifunctional interlayer on separator by inserting Fe3O4 nano-particles

20

(NPs) in porous graphene (PG) film to immobilize polysulfides effectively. The

21

porous structure of graphene was optimized by controlling the oxidation conditions

22

for facilitating ion transfer. The polar Fe3O4 NPs were employed to trap sulfur species

23

via strong chemical interaction. By exploiting the PG-Fe3O4 interlayer with optimal

24

porous structure and component, the Li-S battery delivered a superior cycling

25

performance and rate capability. The reversible discharge capacity could be

Department of Mechanical Engineering, The Hong Kong Polytechnic University,

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

maintained at 732 mAh g-1 after 500 cycles and 356 mAh g-1 after total 2000 cycles at

2

1 C with a final capacity retention of 49%. Moreover, a capacity of 589 mAh g-1 could

3

also be maintained even at 2 C rate.

4 5

INTRODUCTION

6

With the development of modern society, the demand of rechargeable batteries with

7

high energy density for portable applications and electric vehicles has being more and

8

more urgent1-2. Among the candidates of energy storage systems, lithium-sulfur (Li-S)

9

battery has attracted extensive attention because of its high theoretical specific

10

capacity (1675 mAh g-1) and energy density (2600Wh kg-1)1-3. Apart from above

11

advantages, considering the inherent properties of sulfur, such as easy obtaining, low

12

cost, abundant reserve and environmental benignity, Li-S system would be a

13

promising choice for next generation high performance batteries3-5.

14

However, there are several problems remained to be settled for Li-S system, such as

15

poor electronic conductivity of sulfur, soluble intermediate products (Li2Sx, 4≤ x ≤ 8)

16

and large volume expansion of sulfur during repeated cycle3-4, 6-7. In order to enhance

17

the electrical conductivity of sulfur, suppress the shuttling effect of polysulfides and

18

buffer the volume expansion of sulfur, various efforts have been carried out to solve

19

these problems. Porous and nanoscale carbon materials such as ordered carbon

20

spheres8, carbon nanotubes9-10, graphene11-12, carbon nanofibres13 were employed as

21

host materials to encapsulate or load sulfur forming superior cathode for their superior

22

electrical conductivity and physical adsorption to the dissolved polysulfides. However,


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1

considering the cumbersome preparation procedure for these carbon materials with

2

unique structures, the practical application of Li-S battery is hindered, and the lower

3

sulfur content in porous carbon and lower mass loading of nanomaterials on electrode

4

are also great challenges4, 6.

5

Improving the configuration of Li-S systems by constructing a barrier layer for

6

flexible polysulfides between sulfur cathode and separator (i.e. interlayer) is also a

7

widely used technique to acquire superior Li-S batteries with high capacity, long

8

cycling life and better rate capability3, 14-21. Carbon-based materials, such as carbon

9

nanotubes (CNTs)22, carbon nanofibers23, carbonized leaves24 and graphene oxide

10

(GO)25, are promising candidates as the interlayer for Li-S batteries because of their

11

advantages of electric conductivity, porous structure and chemical stability. Graphene

12

based interlayers have long been studied. For instance, Wang et al. introduced a

13

reduced graphene oxide (rGO), which was obtained via vacuum filtration of rGO

14

dispersion followed by heatment, between the sulfur cathode and separator, acting as a

15

shuttling inhibitor for the polysulfides26. Since the tightly stacked rGO sheets might

16

inhibiting the diffusion of electrolyte, carbon black (CB) particles were introduced

17

into rGO layers to enlarge the gaps between neighboring graphene layers. Zhou et al.

18

designed a unique sandwich structure with pure sulfur between two graphene

19

membranes27. One graphene membrane was used as a current collector with sulfur

20

coated on it as the active material, and the other graphene membrane was coated on a

21

commercial polymer separator. The graphene-separator enables good flexibility and

22

strength of the barrier layer. The sandwich structure of the graphene-separator and



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1

graphene cathode provides excellent electric conductivity, and it can accommodate the

2

large volumetric expansion of sulfur during lithiation.

3

developed a sort of porous-graphene modified separator28. The porous structure of this

4

interlayer could significantly preserve the ion channels within it during long term

5

cycling.

More recently, Zhai et al.

6

However, just the carbon layer working as a physical barrier can’t effectively

7

confine the diffusion and shuttling of polysulfides. It is known that the resisting effect

8

for polysulfide will be improved by increasing the carbon layer thickness. While a

9

thicker carbon layer is certain to lower the diffusion of ions and bring down the

10

volumetric energy density of cells3. Compared to carbon materials, polar metal oxides

11

(V2O5, MoO3 and etc) and sulfides (MoS2, TiS2, VS2 and etc) showed strong binding

12

with polysulfides, which were also used to decorate the carbon-based interlayers29-30.

13

On

14

Cyclized-polyacrylonitrile modified carbon nanofiber31, dense Li4Ti5O12/graphene

15

mixture layer32, ultrafine TiO2 decorated carbon nanofibers33, twinborn TiO2-TiN

16

heterostructures coated on graphene34 have been developed as novel interlayers to

17

enhance the performance of Li-S batteries. More recently, Jiarui He et al. reported that

18

Fe3O4 particles have strong interaction with polysulfides, which showing excellent

19

electrical conductivity among metal oxides35. The research incorporating Fe3O4 with

20

carbon scaffold to modify Li-S batteries has not been carried out up to now.

top

of

these

research,

V2O5

decorated

carbon

nanofiber6,

21

In this article, we report a novel multifunctional interlayer consisting of Fe3O4 nano

22

particles (NPs) and porous graphene (PG) with a holey plane to suppress shuttling


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1

effect of Li-S batteries. Figure 1 shows the schematic structure and working

2

mechanism of Li-S batteries with the PG and PG-Fe3O4 interlayer, as well as without

3

interlayer on polypropylene (PP) separator. In the Li-S cells with PG-Fe3O4 interlayer,

4

the PG layer acts as a physical barrier for polysulfides but promote channels for Li ion

5

transportation due to its holey structure, and Fe3O4 component plays a key role in

6

immobilizing polysulfides via the stronger chemical interaction. The Li-S cells with

7

such the PG-Fe3O4 interlayer deliver a capacity of 732 mAh g-1 after 500 cycles and

8

356 mAh g-1 after total 2000 cycles at 1C (1 C = 1675 mA g-1) with a final capacity

9

retention of 49%, and the corresponding capacity decay rate is 0.02% per cycle. The

10

batteries also show an ultra-high rate capability of 1423 mAh g-1 at 0.1 C and 589

11

mAh g-1 at 2 C. Based on the optimized design of the multifunctional PG-Fe3O4

12

interlayer, we get a deeper understanding on the working mechanism of interlayer of

13

Li-S batteries, which also shows great potential for commercialization.



(a)

(b)

(c)

Shuttle

1

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Shuttle

Shuttle

Fast Li ion transport

Moderate Li ion transport

Li foil

PP (Celgard 2400)

Sulfur cathode

PG interlayer

PG-Fe3O4 interlayer

PG

Fe3O4 nano particles

Li2S4

Li2S6

2

Figure 1. Schematic configuration of Li-S batteries with (a) PP separator; (b) PG @

3

PP; (c) PG-Fe3O4 @ PP interlayer.

4 5

EXPERIMENTAL SECTION

6

Synthesis of Fe3O4 NPs

7

Fe3O4 NPs were synthesized by the modified co-precipitation method reported by

8

Khalafalla et al36. Typically, 0.9925 g of FeCl3 and 1.194 g of FeCl2∙4H2O (equivalent

9

weight ratio is 1:1) were dissolved in 30 mL deionized water and then the mixture was

10

stirred for 30 minutes at 60 oC. Subsequently, 1 M NaOH solution was added into the

11

mixed solution dropwise, until the pH increase slowly to 11 with color changing to

12

black. Then, 0.25 g of Na3C6H5O7·2H2O was added into the above solution followed

13

by stirring for 60 minutes at 80 oC. After cooling down to room temperature, the


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1

obtained black suspension was washed with ethanol and deionized water several times

2

via centrifugation. The obtained Fe3O4 NPs were dried at 60 oC in a vacuum oven for

3

12 hours.

4 5

Synthesis of PG

6

The pristine partially reduced graphene (PPRG) was synthesized by chemical

7

exfoliation37, then the PG with holey planes was fabricated from PPRG under air

8

atmosphere as the literature reported38. Briefly, the PPRG was placed in an alumina

9

crucible and heated in an open-ended tube furnace with a ramp rate of 5 oC/min and

10

held isothermally at 400 oC or 450 oC for 10 hours, respectively. The PG products

11

were directly obtained upon cooling the reaction and denoted as PG-400, PG-450,

12

separately.

13 14

Preparation of PG and PG-450-Fe3O4 interlayers

15

The interlayer was fabricated by vacuum filtrating ethanol solution containing PG

16

or PG-Fe3O4 on PP separator. The PG-450 interlayers with different mass loading

17

(0.159, 0.318, 0.478, 0.637 mg cm-2) were investigated by dispersing diverse amount

18

of PG-450 (2, 4, 6 and 8 mg) in 60 mL ethanol, respectively. Then, two kinds of PG

19

interlayers with the same mass loading of 0.478 mg cm-2 were fabricated by vacuum

20

filtrating 60 mL ethanol solution containing 6 mg PG (PG-400, PG-450) to further

21

explore the effect of holey structure of PG on battery’s performance. Finally, Fe3O4

22

NPs were introduced into the interlayer to further improve the immobilization effect



1

for polysulfides. The PG-450-Fe3O4 (i.e. PG-Fe3O4) interlayer was prepared by

2

filtrating 60 mL ethanol solution containing 3 mg PG-450 and 3 mg Fe3O4 NPs,

3

guaranteeing the same mass loading (0.478 mg cm-2) with PG-450 interlayer.

4 5

Material characterizations

6

The morphology and subtle structure of PPRG, Fe3O4 NPs, PG-400, PG-450 and

7

PG-Fe3O4 composite interlayer were investigated by Cold Field Emission Scanning

8

Electron Microscopy (SU8010, HITACHI). The morphology and subtle structure of

9

PG-450 was characterized by transmission electron microscopy (T12, Tecnai G2

10

Spirit 120 kV). Energy-dispersive X-ray spectroscopy (EDS) was carried out to obtain

11

the elemental mapping results of PG-450-Fe3O4 hybrid film. X-ray diffraction (XRD)

12

patterns of Fe3O4 NPs were investigated on a Bruker D8 Advance system using Cu

13

Kα radiation (λ=0.154 nm, tube voltage: 40 kV and tube current: 40 mA). The pore

14

distribution and Brunauer-Emmett-Teller (BET) surface area of samples were tested

15

on the Micromeritcs ASAP 2020M+C analyzer.

16

PG-400 and PG-450 were carried out by using the powder directly. The BET test of

17

PG-450-Fe3O4 was conducted using the PG-450-Fe3O4 mixture film, which was

18

scraped off the filter paper after vacuum filtration and drying. X-ray photoelectron

19

spectroscopy (XPS) of PPRG and PG-450 were collected on PHI 5000 VersaProbe II

20

with Al K α irradiation (1486.6 eV). To conduct the adsorption experiment of

21

polysulfides, the 0.02 M Li2S6 solution was prepared by dissolving S8 and Li2S (5:8

22

by molar ratio) in DOL/DME (1:1 by volume) and then diluted to 0.002 M in DME

Typically, the BET tests of PPRG,


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1

and obtain the bright yellow solution.

2 3

Electrochemical measurements

4

The sulfur cathode was prepared by a slurry-coating method. Briefly, 60 wt % nano

5

sulfur powder, 30 wt % super P, 10 wt % poly (vinylidene fluoride) (PVDF) were

6

mixed in N-methyl-pyrrolidone (NMP), and then this suspension was coated onto a

7

carbon-coated aluminum foil and dried at 60 oC under vacuum for 12 h. The mass

8

loading of sulfur in the obtained cathode is 0.6-0.9 mg cm-2. CR2032 coin cells were

9

assembled in an argon-filled glovebox using a lithium foil as the counter electrode, PP

10

(Celgard 2400) as the separator, PG, PG-Fe3O4 filtrated on PP as the interlayers. The

11

electrolyte consisted of 1M bis (trifluoromethane) sulfonamide lithium salt (LiTFSI)

12

dissolved in a mixture of 1, 2-dioxolane (DOL) and dimethoxymethane (DME) (1:1

13

by volume) with 1 wt % LiNO3, 60 µL electrolyte was placed in the two sides of

14

separator equally. The batteries were charged and discharged between 1.7 and 2.8 V

15

(vs Li/Li+) on a LAND 2001 CT battery test system at ambient temperature. The

16

cyclic voltammetry (CV) experiments and electrochemical impedance spectroscopy

17

(EIS) measurements were conducted on a VMP3 electrochemical workstation (Bio

18

Logic Science Instruments). CV test were carried out between 1.7 and 2.8 V at a scan

19

rate of 0.1 mV s-1, while EIS measurement was performed over a frequency range of

20

110 kHz to 10 mHz with a disturbance amplitude of 10 mV. The EIS tests for fresh

21

Li-S batteries with PG-450 and PG-450-Fe3O4 interlayer were carried out by test the

22

cells after standing for 10 hours to let the full infiltration of electrolyte to the sulfur



1

cathode. The EIS tests of cycled Li-S batteries were performed on the cells after 103

2

cycles (3cycles at 0.1 C, 100cycles at 1 C).

3 4

RESULTS AND DISCUSSION

5

Structure and morphology

6

The morphology of PPRG, PG-400 and PG-450 are shown in Figure 2. PPRG

7

shows a wrinkled surface without obvious in-plane holes on the graphene sheet

8

(Figure 2a). After being oxidized at 450 oC under air atmosphere, the PG-450 shows

9

obvious holey structure (Figure 2c, Figure S1), demonstrating nano holes were

10

successfully introduced in the graphene sheets. The inset photos in Figure 2a, b, c are

11

the distribution morphology of relevant graphene powders. During the moderate

12

oxidation process, the defective carbons on the pristine graphene sheets were

13

preferentially oxidized and gasified (converting into CO and / or CO2), leaving behind

14

holes distributed on the graphene lateral surface39. More aggressive conditions such as

15

a higher temperature or a longer duration could incur further gasification of the

16

remaining graphitic carbon and thus enlarged holes38. The XPS analysis results of

17

PPRG and PG-450 were shown in Figure S2. After calcinating process under air

18

atmosphere, the mass ratio of oxygen of PG-450 (13.41%) is higher than that of

19

PPRG (10.67%). The large amount of hydrophilic function group remained in PG-450

20

has been reported in the literature could improve the polysulfide-trapping capacity of

21

carbon substrates40. Figure 2d, e, f show the morphology and structure of

22

PG-450-Fe3O4 interlayer. The smooth morphology of Figure 2d indicates that holey


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1

graphene nanosheets can be homogeneously coated on the PP separator. The Fe3O4

2

NPs can also be uniformly inserted in the graphene layers (Figure 2e). From the

3

cross-section image in Figure 2f, it is noted that a dense stacking construction can be

4

obtained through vacuum filtrating PG-450-Fe3O4 film.

5

The morphology of PG-450 and Fe3O4 NPs is also shown in the inset SEM images

6

of Figure 2d. The Fe3O4 NPs with diameter of about 20 nm fabricated via

7

co-precipitation method could be clearly observed in the right inset photo and Figure

8

S3a. The XRD pattern (Figure. S3b) in consistent with JCPDS card No. 88-0866,

9

show an inverse spinel structure of Fe3O4 NPs. As shown in the cross-sectional SEM

10

image (Figure S4c), the thickness of PG-450 interlayer with a mass loading of 0.478

11

mg cm-2 is about 30 µm. While with the same mass loading of 0.478 mg cm-2, the

12

thickness of PG-450-Fe3O4 interlayer is about 15 µm (Figure 2f), which is smaller

13

than the PG-450 one with a mass loading of 0.478 mg cm-2, manifesting that Fe3O4

14

existence can be conductive to further decrease the thickness of PG layer due to the

15

large density of Fe3O4 particles. As a result, the volume energy density of Li-S

16

batteries would be increased by reducing the thickness of PG-450-Fe3O4 interlayer.

17

With ascending of the mass loading of PG-450 interlayer, the interlayer thickness

18

increases gradually from 15 µm (0.159 mg cm-2) to 30 µm (0.478 mg cm-2) (Figure

19

S3). However, when the loading goes up to 0.637 mg cm-2, the PG coating layer has a

20

thickness about 120 µm and could be easily peeled off Celgard 2400 separator,

21

indicating a loose structure for high mass loading PG interlayer. The drastic increase

22

of the thickness from the loading of 0.478 mg cm-2 to 0.637 mg cm-2 may attribute to



1

the loose stack of PG-450 on Celgard 2400 with the ascending of the mass loading.

2

The EDS result and corresponding element distribution of PG-450-Fe3O4 interlayer

3

are shown in Figure S5. As illustrated in Figure S5b, the carbon, oxygen and ferrum

4

elements compose the PG-450-Fe3O4 interlayer. Figure S4c shows the element

5

distribution of carbon, oxygen and ferrum, and it could be clearly seen that the oxygen

6

and ferrum elements distribute uniformly on the surface of interlayer, which further

7

proves the uniform structure of PG-450-Fe3O4 interlayer.

8 9

Figure 2. SEM images of (a) PPRG, (b) PG-400, (c) PG-450 powder with different

10

magnifications; (d) and (e) SEM images surface morphology of PG-450-Fe3O4

11

interlayer at different magnifications; The inset SEM images in (d) correspond to

12

fabricated PG-450 (left) and synthesized Fe3O4 NPs (right); (f) The cross-sectional

13

SEM image of PG-450-Fe3O4 interlayer.

14 15

The pore structures of PPRG, PG-400, PG-450 were further clarified by N2

16

adsorption-desorption isotherm at 77 K (Figure 3a-c). The sharp increase at relative

17

pressure (p/p0) lower than 0.05, the hysteresis of p/p0 between 0.45 and 0.90 and the


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1

significantly nitrogen uptake of p/p0 over 0.90 disclosed the coexistence of

2

micropores, mesopores and macropores of the three kinds of graphene28. This could

3

be verified by the pore distribution of micro pore (Figure 3, HK model, 0.44 nm-4.01

4

nm) and meso, macro pore (Figure S6, BJH model, 1.22 nm-97.45 nm) of these PG

5

samples. Table S1 shows the BET specific surface area, total pore volume and mean

6

pore diameter of PPRG, PG-400, PG-450 and PG-450-Fe3O4 samples. The BET

7

specific surface area of PPRG is only 400.97 m2 g-1. After being incinerated at 400

8

and 450 oC, it increases to 450.84 and 493.84 m2 g-1 respectively, indicating the air

9

oxidation process could boost the pore formation in the plane of graphene nanosheets.

10

The total pore volume of PG-400 is 3.13 cm3 g-1, which is almost the same as the

11

value of PPRG (3.17 cm3 g-1). However, the mean pore diameter of PG-400 (27.8 nm)

12

is smaller than PPRG (31.7 nm). It implies the nucleation of nanopores on the

13

graphene sheets during the heat treatment process, which lowers the mean pore

14

diameter of PG-400. When the processing temperature increases to 450 oC, the mean

15

pore diameter further drops to 26.3 nm, while the total pore volume increases to 3.25

16

cm3 g-1, which indicates the nucleation of nanopores could be further promoted by

17

ascending the calcination temperature. Due to the high specific surface area of 493.84

18

m2 g-1, pore volume of 3.25 m3 g-1 and hierarchical pores with a mean pore diameter

19

of 26.29 nm of PG-450, the ion transport channels in PG-450 interlayer could be well

20

conserved in long term cycling process, and the transport speed of Li ions passing

21

through the PG-450 layer will be largely improved. After decoration with Fe3O4 NPs,

22

the PG-450-Fe3O4 shows a specific surface area of 246.17 m2 g-1, total pore volume of



2

mainly anchored in larger holes of PG-450 (including meso and macro pores) during

3

the vacuum filtration, which can be further verified by the pore distribution curve of

4

PG-450-Fe3O4 in Figure S5d. Nevertheless, the PG-450-Fe3O4 still has sufficient

5

nano-pores, including inherent pores and deuterogenic ones around Fe3O4 NPs to

6

boost the transportation of Li ions. These nano-pores would prohibit the shuttle effect

7

of polysulfides without hampering the motion of Li ions during cycling41.

Adsorption Desorption

2500 2000 1500 1000 500 0 -500

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure(P/P0)

0.1 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Pore Width(nm)

(d)

0.5 0.4 0.3 0.2

3500 3000


2500 2000 1500 1000 500 0 0.0

0.2

0.4

0.6

0.8

1.0


0.1 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Pore Width(nm)

0.6

Volume(cm3 g-1 STP)

0.2

3000

0.5 0.4 0.3

3500 3000


2500 2000 1500 1000 500 0 0.0

0.2

0.4

0.6

0.8

1.0


0.2 0.1 0.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Pore Width(nm) 0.20 0.16 0.12 0.08

3500

Volume(cm3 g-1 STP)

0.3

Volume(cm3 g-1 STP)

0.4

(b)

3500

3 Differential Pore Volume(cm g-1 nm-1)

(c)

0.5

Volume(cm3 g-1 STP)

(a)


0.62 m3 g-1 and mean pore diameter of 10.0 nm. The results signify that Fe3O4 NPs


1


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3000


2500 2000 1500 1000 500 0 0.0

0.2

0.4

0.6

0.8

1.0


0.04 0.00 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Pore Width(nm)

8 9 10

Figure 3. Pore size distribution (HK model) and N2 adsorption and desorption isotherms (the inset photo) of (a) PPRG; (b) PG-400; (c) PG-450; (d) PG-450-Fe3O4.

11 12 13

Electrochemical properties Figure 4a shows the CV curves of Li-S batteries with a PG-450-Fe3O4 interlayer. In


Page 15 of 33 1 2 3 4 5 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

each cathodic scan, there are two prominent peaks appearing around 2.35 V and 2.10

2

V. The former one is known to be associated with the transformation from cyclo-S to

3

polysulfides (Li2Sx, 4≤x≤8), which are prone to dissolve in the electrolyte33. The

4

other peak is related to the reduction process of the dissoluble polysulfides into

5

insoluble Li2S2 and Li2S, accounting for the majority of discharge capacity. From the

6

2rd to 5th reverse anodic scan, two typical peaks between 2.3 and 2.4 V are presented,

7

corresponding to the initial conversion from insoluble Li2S into the soluble

8

polysulfides and final oxidation into element sulfur, respectively33. However, during

9

the first anodic scan, besides the peak at 2.4 V, a slope also appears ranging from 2.3

10

to 2.35 V, which should be attributed to the transformation from Li2S into polysulfides.

11

This slope is different from the subsequent cycles, suggesting a potential new

12

electrode/interlayer dynamics. It is found that the cathodic peaks shift slightly after

13

the first scan, which can be attributed to the rearrangement of orthogonal sulfur from

14

the original place to a more stable site, known as the activation process26. For the rest

15

cycles, no obvious change in the peak shape and location can be found, indicating a

16

superior electrochemical stability of the Li-S batteries with the PG-450-Fe3O4

17

interlayer.

18

EIS of the Li-S cells without interlayer and with PG-450 and PG-450-Fe3O4

19

interlayer before and after 103 cycles (3 cycles at 0.1 C, 100 cycles at 1 C) are shown

20

in Figure 4b, c respectively. The resulted resistance parameters are listed in Table S2.

21

The EIS curve of fresh cells with PG-450 or PG-450-Fe3O4 interlayers consists of

22

one semicircle and a slope line. The intersection of the semicircle with the real axis



1

refers to a bulk resistance, which mainly reflects the resistance of electrodes and

2

electrolyte. The semicircle and the slope line represent the charge transfer resistance

3

(Rct) and the Warburg impedance (Zw) related to the lithium ion diffusion within the

4

sulfur cathode. For fresh cell without interlayer and cells being cycled, a semicircle

5

emerged in high frequency region, which should ascribed to the formation of solid

6

electrolyte interface (SEI)42. As to the value of each resistance, Rct of the fresh cells

7

with PG-450 and PG-450-Fe3O4 interlayer were 32.18 Ω and 42.21 Ω separately,

8

suggesting the relatively weak initial contact between PG-450-Fe3O4 interlayer and

9

sulfur cathode and inferior electrical conductivity of Fe3O4 metal oxide when

10

compared with PG-450. After 103 cycles, cells with PG-450-Fe3O4 interlayer deliver

11

the lowest Rct (3.869 Ω) when compared with Rct of cell with PG-450 interlayer

12

(5.359 Ω) and without interlayer (12.12 Ω), which ascribe to the sufficient infiltration

13

of electrolyte to the interlayer and sulfur cathode, demonstrating that this hybrid

14

coating layer could not only restrict shuttling of polysuifides effectively and

15

contributes to the smaller impedance but also support the unobstructed transport of

16

lithium ions even for long-term cycling, which lead to excellent electrochemical

17

performances including a high capacity, superior cycling stability and rate capability.

18

As shown in Figure S8, the Warburg factor σ value of the cell without interlayer, and

19

with PG-450 and PG-450-Fe3O4 interlayer is 3.803, 1.547 and 2.162 respectively.

20

Based on the calculation method for lithium-ion coefficient43, it can be concluded

21

qualitatively that the ion diffusions of the cell with PG-450 interlayer is the fastest,

22

which attributed to the sufficient holey structure existed on the surface of PG-450.


Page 16 of 33

Page 17 of 33

1

Moreover, the ion diffusions of the cell with PG-450-Fe3O4 is still faster than that of

2

the cell without interlayer, which may be ascribed to the shuttling effect of the cell

3

without interlayer could damage the porous structure of Celgard 2400 separator thus

4

influence the ion diffusions. (a)

(c)

(b) 140

5

1st cycle 2nd cycle 3rd cycle 4th cycle 5th cycle

1.6 0.8 0.0

30

120

Celgard 2400

100

PG-450-0.478 mg cm-2

Celgard 2400 PG-450-0.478 mg cm-2

-Z''/ohm

2.4

-Z''/ohm

Current density(A g-1)

1 2 3 4 5 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PG-450-Fe3O4-0.478 mg cm-2

80 60 40

-0.8 -1.6

20

PG-450-Fe3O4-0.478 mg cm-2

10

20 1.8

2.0

2.2

2.4

2.6

0

2.8

Potential(V vs.Li+/Li)

0

20

40

60 80 Z'/ohm '

100

120

0

0

5

10

15 20 Z'/ohm '

25

30

6

Figure 4. (a) CV curves of the Li-S battery with PG-450-Fe3O4 interlayer; (b), (c) EIS

7

plots of Li-S batteries without interlayer and with PG-450, PG-450-Fe3O4 interlayer

8

(the same mass loading of 0.478 mg cm-2) before and after 103 cycles (3 cycles at 0.1

9

C and then 100 cycles at 1 C).

10 11

In order to find the optimal mass loading of the PG-450 interlayer, we prepared

12

various PG-450 interlayers with diverse masses from 0.159 to 0.637 mg cm-2 on the

13

PP separator, as shown in Figure S4. The effect of PG-450 interlayer thickness on the

14

performance of Li-S batteries were investigated at a current of 1 C, and comparison

15

results were shown in Figure 5a. The cells were firstly activated at 0.1 C for initial

16

three cycles to make full use of sulfur in the cathode. After the activation procedure,

17

the discharge capacities of Li-S batteries with PG-450 interlayers of different weight:

18

0 (i.e. without interlayer), 0.159, 0.318, 0.478, 0.637 mg cm-2 are 552, 636, 632, 802,

19

730 mAh g-1 in the first cycle, and 251, 260, 321, 659, 544 mAh g-1 after 497 cycles at



1

1 C, corresponding to a capacity retention of 45.5%, 40.9%, 50.7%, 82.2%, 74.5%

2

respectively. From the results, it is noted that the polysulfide-hampered role of

3

PG-450 interlayer is not obvious when the mass of the interlayer is less than 0.318 mg

4

cm-2. However, too thick interlayer with mass loading of 0.637 mg cm-2 will also

5

decrease the cell’s capacity because of the slower lithium transport in thicker

6

graphene barrier. It is obvious that the Li-S cell with a PG-450 interlayer of 0.478 mg

7

cm-2 shows the best electrochemical performance. Therefore, we choose the interlayer

8

weight of 0.478 mg cm-2 as the best reference for further research.

9

Besides the thickness of PG film interlayer, the pore structure parameters of

10

graphene nanosheets, including specific surface area, pore volume and diameter

11

distribution, are also important factors with regard to the immobilization effect for

12

polysulfide. Two kinds of PG (PG-400 and PG-450) obtained under different

13

oxidation temperatures from PPRG were selected to prepare interlayers for Li-S

14

batteries. With the same mass loading of 0.478 mg cm-2 for PG-400 and PG-450

15

interlayers, the batteries were tested at the same current conditions and the

16

performance were shown in Figure 5b. After activation at 0.1 C for three cycles, the

17

discharge capacity of PG-400 Li-S battery is 696 mAh g-1 for the first cycle at 1 C,

18

and 566 mAh g-1 after 497 cycles with corresponding capacity retention of 81.3%,

19

which was significantly inferior to the data of PG-450 Li-S battery. As described in

20

above section (Table S1), PG-450 shows a larger surface area and pore volume, giving

21

rise to more convenient ion transport and stronger confinement for polysulfides.

22

Therefore, this contributes to a higher capacity for Li-S batteries with the PG-450


Page 18 of 33

Page 19 of 33 1 2 3 4 5 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

interlayer. It should be mentioned that we also tried to prepare PG with larger pore

2

volume and specific surface area via this simple oxidation technique by further

3

increasing the oxidation temperature above 450 oC. However, the PPRG could

4

seriously burn out because of the uncontrollable combustion reaction at higher

5

temperatures. Thus the PG-450 was selected to construct the scaffold for a hybrid

6

structure interlayer.

7

For the sake of showing the strong adsorption and interaction of Fe3O4 to

8

polysulfides, a comparison among Li-S batteries without interlayer, with PG-450 and

9

PG-450-Fe3O4 interlayers was carried out under the same testing conditions

10

(activation at 0.1 C for three cycles before cycling at 1 C for a long time), as shown in

11

Figure 5c. It should be emphasized that PG-450 and PG-450-Fe3O4 interlayers had a

12

same mass loading of 0.478 mg cm-2. After the initial activation process, the discharge

13

capacity of PG-450-Fe3O4 Li-S battery was 727 mAh g-1 for the first cycle, then

14

gradually increased up to 833 mAh g-1 following 45 cycles and well maintained 732

15

mAh g-1 after 497 cycles, which was obviously higher than the other two batteries

16

with PG-450 interlayer (659 mAh g-1) and without interlayer (251 mAh g-1)

17

above-mentioned. More importantly, the PG-450-Fe3O4 Li-S battery could still deliver

18

a discharge capacity of 356.1 mAh g-1 after 1997 cycles at 1 C (Figure S9a), with a

19

capacity retention of 49.0% and decay rate of 0.02% per cycle. With the aim of further

20

confirming the strong absorption of Fe3O4, batteries were also tested at a lower

21

current density of 0.3 C (Figure S9b), because the shuttle effect would be more severe

22

during the testing at lower current density. From Figure S9b, after being activated, the



1

discharge capacities of Li-S batteries with only PP separator, PG-450 and

2

PG-450-Fe3O4 interlayer on PP separator were 643, 705, 975 mAh g-1 in the first cycle

3

at 0.3 C, and 362, 475, 631 mAh g-1 after 97 cycles respectively. The discharge

4

capacity of PG-450-Fe3O4 Li-S battery is significantly higher than others, which

5

manifests the multifunctional interlayer could still immobilize the polysulfides and

6

guarantee the Li ion transport even at lower current density.

7

To further show the multifunction of PG-450-Fe3O4 interlayer, the cathode with

8

sulfur mass loading of 2 mg cm-2 were tested at 0.2 C, as shown in Figure S10. The

9

high mass-loading Li-S battery with PG-450-Fe3O4 interlayer showed the discharge

10

capacity of 615 mAh g-1 at the first cycle and 799 mAh g-1 at the ninth cycle, which

11

was attributed to the insufficient usage of sulfur in the cathode during the initial

12

several cycles. More profoundly, it still delivered the discharge capacity of 503 mAh

13

g-1 after 400 cycles. Therefore, it is worth emphasizing that the high capacity and

14

superior cycling performance should be assigned to the strong interaction between

15

polysulfides and Fe3O4 NPs anchored on PG-450, leading to robust adsorption and

16

immobilization for polysulfides, and the porous and conductive graphene layer with

17

stable construction serving as an upper current collector and assistant host material for

18

the active cathode, which could not only transport ions but also encapsulate the whole

19

cathode, accommodate the volume expansion of electrode, as well as increase the

20

reutilization of sulfur species during cycling3, 33.

21

Furthermore, the rate capability of different batteries was investigated as displayed

22

in Figure 5d. The rate performance was measured at 0.1 C (5 cycles), 0.3 C (10


Page 20 of 33

Page 21 of 33 1 2 3 4 5 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

cycles), 0.5 C (10 cycles), 1 C (10 cycles), 2 C (10 cycles) and ten back to 0.2 C (10

2

cycles). After 5 cycles at 0.1 C, the discharge capacities of Li-S batteries with only PP

3

separator, PG-450 and PG-450-Fe3O4 interlayer on PP separator are 674, 958, 1148

4

mAh g-1 respectively. When the current density successively changed to 0.3, 0.5, 1,

5

and 2 C, the PG-450-Fe3O4 Li-S cell exhibited as before the highest capacity of 887,

6

789, 673, 589 mAh g-1, respectively. Notably, when the current density returned to 0.2

7

C, a fair stable capacity of 865 mAh g-1 was obtained at the first cycle, and then went

8

up to 911 mAh g-1 at the second cycle. This excellent reversibility indicated that

9

PG-450-Fe3O4 interlayer could still effectively immobilize polysulfides even at a high

10

passing rate of ions. The associated charge/discharge curves at different current

11

densities for the cells with PG-450-Fe3O4 interlayer are shown in Figure S11. At a

12

current density of 0.1 C, the typical common two-plateau (2.3 and 2.1V) could be

13

obtained for PG-450-Fe3O4 interlayer Li-S batteries, which was consistent with the

14

peaks in the CV profiles. It is worthy to be mentioned that the long slop of the first

15

discharge curve might be derived from the lithiation of Fe3O4 NPs and PG. The

16

PG-Fe3O4 interlayer is in closely contact with sulfur cathode, thus the Fe3O4 NPss and

17

PG would be undergo lithiation reaction within the voltage from 2.8 to 1.7 V,

18

especially for the first cycle, due to their large irreversible capacity44-46. Although

19

suffered from distortion to some extent, the voltage profile could still retain the two

20

distinct plateaus at a high passing rate of ions. The high capacity and excellent

21

capacity retention at larger current densities of the Li-S battery with PG-450-Fe3O4

22

interlayer powerfully illustrates that the abundant nano pores contributed to the



1

superior Li ion transport, while the polysulfides were effectively confined by the

2

physical obstructing of graphene layer and chemical bonding of Fe3O4 NPs.

3

4

0.637 mg cm-2

800

40

400

20 0

100

2000

200 300 Cycle

400

0 500 120

1C 100

1600 PG-450-Fe3O4 PG-450 Celgard 2400

1200

80 60

800

40

400 0

20

0

100

200

300

400

0 500

-1

100

1600 PG-450-0.478 mg cm-2 PG-400-0.478 mg cm-2

1200

(d)

80 60

1C

800

40

400 0

20 0

100

200

300

0 500

400

Cycle

1600

120 100

0.1 C

1200

PG-450-Fe3O4 PG-450 Celgard 2400

0.3 C

800

1C

0.2 C

80 60

2C

40 400 20

0.5 C

0

0

10

Cycle

20

30

Coulombic Efficiency (%)

1200

120

2000

40

50


Cellgard 2400 0.318 mg cm-2


100 0.159 mg cm-2 80 0.478 mg cm-2 60

1600

Discharge Capacity (mAh g )

1C

0

(b)

120


(c)

2000

Discharge Capacity (mAh g-1)

-1 Discharge Capacity (mAh g )

(a)

Discharge Capacity (mAh g-1)

1 2 3 4 5 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 33

0

Cycle

5

Figure 5. (a) Cycling performance of Li-S batteries without interlayer and with

6

PG-450 interlayer of different loading at 1 C after activation at 0.1 C for 3 cycles; (b)

7

Cycling performance of PG-400, PG-450 interlayer modified Li-S batteries with

8

loading of 0.478 mg cm-2 at 1 C after activation at 0.1 C for 3 cycles; (c), (d) Cycling

9

performance (tested at 1 C after activation at 0.1 C for 3 cycles) and rate capability

10

(from 0.1 C to 2 C) of Li-S batteries without interlayer and with PG-450,

11

PG-450-Fe3O4 interlayer (the same mass loading of 0.478 mg cm-2).

12 13

The structure and morphology of PG-450-Fe3O4 @ PP, PG-450 @ PP, PP separator

14

after 103 cycles (3 cycles at 0.1 C, 100 cycles at 1 C, close to lithium anode) are


Page 23 of 33 1 2 3 4 5 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

shown in Figure 6a, b, c. The surface of PG-450-Fe3O4 @ PP shows apparent pores

2

and channels for lithium ion transportation (Figure. 6a). The inset is an elemental map

3

for carbon and sulfur, and the sulfur content is only 1.27%. In contrast, the surfaces of

4

PG-450 @ PP and PP separator both show a blurred pore structure, and the sulfur

5

contents are 3.75% and 10.77%, respectively. Though the pore structure of

6

PG-450-Fe3O4 @ PP after 2000 cycles (3 cycles at 0.1 C, 1997 cycles at 1 C, close to

7

lithium anode) suffers from collapsing in some area, the sulfur content is only 2.63%,

8

which is significantly lower than PG-450 @ PP and PP separator after 103 cycles.

9

Meanwhile, the adsorption experiment of Fe3O4 to polysulfides was conducted to

10

characterize the strong interaction between Fe3O4 and sulfur species, as shown in

11

Figure S12. After adding 20 mg Fe3O4 NPs to Li2S6 solution and stored for 48 h in Ar

12

atmosphere, it could be seen that the color of the solution changed from bright yellow

13

to nearly transparent after absorbing, indicating the strong bonding interaction of

14

Fe3O4 to Li2S6. The analysis of morphology and sulfur content, and the adsorb

15

property of Fe3O4 to polysulfides could demonstrate the superiority of multifunctional

16

interlayer design directly.



1 2

Figure 6. SEM images, corresponding mapping and EDS results of sulfur element on

3

the separator’s surface (close to the lithium anode). The separators are obtained from

4

the Li-S batteries with (a) PG-450-Fe3O4 @ PP, (b) PG-450 @ PP, (c) PP separators

5

after 103 cycles (3 cycles at 0.1 C and then 100 cycles at 1 C); (d) The separator is

6

from PG-450-Fe3O4 modified Li-S battery after 2000 cycles (3 cycles at 0.1 C and

7

then 1997 cycles at 1 C).

8 9

CONCLUSION

10

In conclusion, a multifunctional interlayer composed of Fe3O4 NPs and porous

11

graphene (PG) was designed in this article. The PG layer acts as the physical barrier

12

of polysulfides and guarantees Li ion transport, while the Fe3O4 NPs play the key role

13

of chemical immobilization for the shuttling effect of polysulfides. With the

14

modification of PG-450-Fe3O4 interlayer, the Li-S batteries show an excellent cycling


Page 24 of 33

Page 25 of 33 1 2 3 4 5 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

performance (732 mAh g-1 after 500 cycles and 356 mAh g-1 after totally 2000 cycles

2

at 1 C with a 49.0% capacity retention and capacity decay rate of 0.02%) and superior

3

rate capability (589 mAh g-1 at 2 C and 1423 mAh g-1 at 0.1 C). The whole fabrication

4

process is non-toxic to environment, which meets the slogan of “green and

5

environmental” in the 21st century. More profoundly, this contribution sheds light on

6

the exploration of dual function interlayer with various absorbent of polysulfides and

7

porous conductive scaffold and bring new sights and ideas to the design of interlayer

8

with dual, triple, and multiple functions.

9 10

ASSOCIATED CONTENT

11 12

Supporting Information

13

This material is available free of charge via the Internet at http://pubs.acs.org.

14

TEM image of PG-450; XPS data of PPRG and PG-450; SEM image and XRD

15

pattern of Fe3O4 nanoparticles; Cross-sectional SEM images of PG-450 interlayer

16

with different thickness; SEM image and EDS result of PG-450-Fe3O4 interlayer;

17

BET data of PPRG, PG-400, PG-450 and PG-450-Fe3O4 composite; EIS fitting results

18

of Li-S batteries; Fitting results between Z ′ and ߱ ିଵ/ଶ ; Cycling performances of

19

Li-S batteries; Discharge and charge curves; Digital photo of the adsorption property

20

of Fe3O4.

21 22

AUTHOR INFORMATION



1

Corresponding Authors

2

*E-mail: [email protected] (B. Li);

3

*E-mail: [email protected] (X. Qin).

4 5

Notes

6

The authors declare no competing financial interest.

7 8

ACKNOWLEDGEMENT

9

This work was supported by National Key Basic Research Program of China (No.

10

2014CB932400), Joint Fund of the National Natural Science Foundation of China (No.

11

U1401243), National Nature Science Foundation of China (No. 51232005), Shenzhen

12

Technical Plan Project (No. JCYJ20150529164918735, KQJSCX20160226191136

13

and JCYJ20170412170911187), and Guangdong Technical Plan Project (No.

14

2015TX01N011).

15 16

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Table of Contents Graphic

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247x85mm (150 x 150 DPI)


Fe3O4-Decorated Porous Graphene Interlayer for High-Performance

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