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Controllably Designed “Vice-electrode” Interlayers Harvesting High Performance Lithium Sulfur Batteries Youchen Hao, Dongbin Xiong, Wen Liu, Linlin Fan, Dejun Li, and Xifei Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12710 • Publication Date (Web): 30 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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ACS Applied Materials & Interfaces

Controllably designed “vice-electrode” interlayers harvesting high performance lithium sulfur batteries

Youchen Hao†,‡, Dongbin Xiong†,‡, Wen Liu†,‡, Linlin Fan‡, Dejun Li†, Xifei Li*†,‡,§ †

Tianjin International Joint Research Centre of Surface Technology for Energy

Storage Materials, College of Physics and Materials Science, Tianjin Normal University, Tianjin 300387, China. ‡

Center for Advanced Energy Materials and Devices, Xi’an University of Technology,

Xi’an 710048, China. §

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education),

Nankai University, Tianjin 300071, China.

KEYWORDS: Lithium sulfur batteries, interlayers, mechanism, vice-electrode, protection of Li metal

ABSTRACT: An interlayer has been regarded as a promising mediator to prolong the life span of lithium sulfur batteries because its excellent absorbability to soluble polysulfide efficiently hinders the shuttle effect. Herein, we design various interlayers, and understand the working mechanism of an interlayer for lithium sulfur batteries in details. It was found that the electrochemical performance of S electrode for an interlayer located in cathode side (marked as S-II) is superior to the pristine one without interlayers (marked as S-I). Surprisingly, the performance of S electrode for an interlayer located in anode side (marked as S-III) is poorer than that of S-I. For 1

ACS Paragon Plus Environment


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comparison, the glass fibers were also studied as a nonconductive interlayer for lithium sulfur batteries. Unlike two interlayers above, these nonconductive interlayers did display significant capacity fading because polysulfides were adsorbed onto insulated interlayers. Thus, the nonconductive interlayers function as a “dead zone” upon cycling. Based on our findings, it was for the first time proposed that a controllably optimized interlayer, with electrical conductivity as well as the absorbability of polysulfides, may function as a “vice-electrode” of anode or cathode upon cycling. Therefore, the cathodic conductive interlayer can enhance lithium sulfur battery performance, and the anodic conductive interlayer may be helpful for the rational design of 3D networks for the protection of lithium metal.

INTRODUCTION Lithium sulfur batteries (LSBs) have been demonstrated to be one of the most potential energy storage devices due to the nontoxic and low cost of sulfur as well as high theoretical energy density (2600 Wh Kg-1) 1. However, some significant challenges, consisting of low conductivity, large volume changes and “shuttle effect” of lithium polysulfide, lead to low utilization and rapid capacity fading of S cathode materials, which hinders the application of LSBs in electric vehicles (EVs) and hybrid electric vehicles (HEVs) 2. Since Nazar et al. succeeded in loading sulfur into CMK-3 in 2009 3, the S based nanocomposites with carbon nanotube 4, porous carbon 5, conducting polymer

6

, and graphene

7

etc. have been regarded as promising

approaches to enhance LSB performance. Furthermore, alternative strategies (coating 2


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8

, heteroatom doping 9, functionalization

10

and constructing 3D structures

11-12

etc.)

were developed to withdraw the shuttle effect of polysulfide. In addition, the binder or electrolyte addictive

13-14

was optimized to enhance electrochemical performance. All

of these measures can improve LSB performance in some degree, but its cycling stability is still required to be further increased for practical applications. In recent years, the introduction of interlayer between cathode and separator is a novel concept to build high performance LSBs

15

. So far, the literature surveys have

shown that the interlayer can obviously promote the electrochemical stability because the dissolved polysulfides can be absorbed oxides or sulfides (such as MnO2

20

16-19

, CoS2

21

. It was reported that transition metal

, TiO2 22 etc.) were employed as the

interlayers, which efficiently encapsulate polysulfide via binding with Li2Sn/Li2S. Particularly, nickel foam has been introduced as an conductive interlayer with performance improvement 23. More interestingly, graphene, an outstanding 2D carbon material, is widely used as a host or addition material of interlayer

24-26

. For instance,

an interlayer consisting of porous graphene oxide and carbon nanotube resulted in an excellent reversible capacity of 671 mA h g-1 in the 300th cycle at 0.2 C 27. As a result, it may be concluded that the interlayers with electrical conductivity and enormous surface area can efficiently absorb polysulfide so as to enhance the reversible capacity and cycling stability for LSBs. However, it is noteworthy that many researchers only focused on the S electrode for an interlayer located in cathode side (marked as S-II), and the results firmly revealed that the interlayers benefit to prolong the life span of LSBs. Unfortunately, the S electrode for an interlayer located in anode side (marked 3



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as S-III) may not be focused on the performance effect of LSBs. It causes some limitations to well understand the working mechanism of an interlayer for LSBs. In this study, we designed an interlayer compositing hydroquinol with reduced graphene oxide (denote as HQG) for LSBs. To highly avoid any effects of other components like carbon in the aforementioned S based nanocomposites, the pure S powder was focused as cathode materials to address the functions of various interlayers. Differing from the previous studies, we clearly understand working mechanism of interlayers via different location (anode side and cathode side) and conductivity (HQG and glass fiber) of interlayers in LSBs. It indicated that a conductive interlayer working as a “vice-electrode” with electrons boosts the redox reactions of absorbed polysulfides.

EXPERIMENTAL SECTION Chemicals. Natural graphite powder (purity>99.95%) was purchase from the Aladdin Chemistry Co., Ltd. KMnO4 (99%), H2O2 (30%), NaNO3 (99%), HNO3 (65-68%), H2SO4 (95-98%), sublimed sulfur (chemically pure) and ethylene glycol (99.5%) were used as raw material from Tianjin Jiangtian Chemical Research Institute. Synthesis of HQG. As we previously reported

28-30

, natural graphite was utilized to

fabricate graphite oxide (GO) via a modified Hummers’ method. 2 mg mL-1 solution of GO (80 mL) in distilled water was obtained by ultrasonic cell disruption for 30 min. 100 mg hydroquinol was added under ultrasonic for 20 min. The un-exfoliated GO was removed via centrifugation at 10,000 rpm for 30 min. Afterward, the resultant 4


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solution was transferred into Teflon-lined stainless steel autoclave (50 mL), and maintained for 12 h under 180 oC. The reaction mixture was washed by distilled water for several times. As a result, the obtained precipitation was freeze-dried to produce HQG. For comparison, a reduced graphite oxide (rGO) was prepared via the similar process without the additional hyroquinol. Materials characterization. Fourier-transform infrared spectroscopy (FT-IR) was performed with an IRAffinity-1 spetrometer. X-ray diffraction (XRD) spectra were tested by a Bruker AXS D8 Advance diffractometer with Cu/Kα radiation. The Raman spectra of as-prepared samples were performed by Raman spectrometer with LabRAM HR800 system (HORIBA, Korea) in the range of 1000-2000 cm-1. The morphologies of as-obtained samples were measured by field-emission scanning electron microscope (SEM Hitach SU8010) and a MIRA3 TESCAN microscope equipped with an energy dispersive spectrometer (EDS) device. Electrochemical test. Electrochemical performance was studied via CR2032 coin-typed cells assembled in an argon filled glove box. The commercial S powder, acetylene black and polyvinylidene fluoride binder (PVDF) with a mass ratio of 55:30:15 were mixed into N-methyl-2-pyrrolidone (NMP) solvent to obtain the slurry. Then the resulting slurry was cast onto Al foil, and dried in a vacuum oven at 90 oC overnight. The loading mass of S is about 0.35 mg cm-2. And a metallic lithium foil was used as the anode electrode. The thicknesses of HQG and glass fiber are ~20 um and ~0.4 mm, respectively, and they were assembled towards anode or cathode sides to

testify

the

effects

of

interlayers

on

LSB

5


performance.

1

M


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bis(trifluoromethanesulfonyl) imide lithium (LiTFSI) and 1 M LiNO3 were dissolved into dimethoxyethane (DME) and 1,3-dioxolane (DOL) (volume ratio of 1:1) as the electrolyte. Galvanostatic charge/discharge tests were conducted on a LAND CT2001A battery tester. Cyclic voltammetry was carried out using a Princeton Applied Research Versa STAT4 with a scan rate of 0.1 mV s-1. Electrochemical impedance spectroscopy (EIS) was conducted on Princeton Applied Research Versa STAT4 within a frequency range of 0.01~100 kHz with the AC amplitude of 5.0 mV at the full charge state (3.0 V) after 5 cycles. All electrochemical tests were conducted at the potential range of 1.5~3.0 V.

RESULTS AND DISCUSSION FT-IR was utilized to measure the functional groups of HQG and rGO in Figure S1. In comparison to rGO, it is obvious that the FT-IR spectra of HQG present four specific absorption peaks at 1600 cm-1, 1500 cm-1, 1450 cm-1 and 1380 cm-1 representing the absorption peak of aromatic hydrocarbon. The peak at 1380 cm-1 indicates the existence of phenyl ether groups, which firmly confirmed that the hydroqinol reacted with oxygen-containing functional groups of GO. The XRD patterns of HQG and rGO are compared in Figure 1a. A typical diffraction peak of rGO at 30.5o is observed for the (002) reflection. Compared with rGO, the characteristic diffraction peak of HQG was obviously changed, and an obvious peak is located at 25.7o. The correlations of lattice spacing to the angle of incidence are related to classical Bragg equation 31-32: 6


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2dsinθ = nλ

(Eq. 1)

Where d is lattice spacing, θ is angle of incidence, n is reflectance, and λ is the wave length. In contrast, the (002) peak of HQG shifted to the lower angle as compared to rGO, which indicates an increase in the c-lattice parameter between the graphene layers after the insertion of hydroqinol. In the Raman spectra of Figure 1b, all of GO, rGO and HQG reveal two distinguishable peaks at about ~1350 cm-1 (D band) and ~1590 cm-1 (G band). The D band is assigned to the structural disorder or defects present in graphitic based materials; while G band results from the stretching motion of sp2 carbon pairs. The ratio of ID/IG is commonly used to evaluating the degree of structural disorder, and a high value demonstrates more disorder along with smaller average graphitic crystalline size 28, 33-34. The ID/IG ratios of GO, rGO and HQG are 1.36, 2.00 and 1.64, respectively, on the basis of the fitting of Raman spectroscopy. Compared with rGO, the decrease of ID/IG ratio of HQG mainly attribute to the hydroquinol bonding with graphene during hydrothermal procedure. The morphologies of rGO and HQG were visualized via SEM, as shown in Figure 1c~f. As previously reported by our group 35, rGO tends to overlap and reveals obvious aggregation due to the stacking of scrolled graphene layers. By contrast, an ultralthin sheet-like HQG with wavy structure was obtained due to the introduction of hydroquinol. As shown in Figure S2, HQG was cast on the surface of separator by a spreader. The surface and cross-section morphologies of interlayer can be seen in Figure 1g~h. The HQG film with the thickness of 20 um, as an interlayer, is attached 7



on the surface of separator, and its porous nature may benefit to the soakage of electrolyte and the transfer of Li+. (a)

(b)

002

D band

G band

100

(i) 002

(iii)

Intensity (a.u.)

Relative Intensity (a.u.)

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100

(ii)

(ii)

(i) 20

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40

50

60

70

1000

2 Theta (deg)

(c)

(d)

1 um

200 nm

(e)

(f)

1 um

300 nm

(g)

(h)

1200

1400 1600 -1 Raman Shift (cm )

1800

2000

500 nm

2 um

20 um

500 nm

Figure 1. (a) XRD and (b) Raman of HQG and rGO: (i) HQG; (ii) rGO; (iii) GO. Low- and high- magnification SEM images of (c, d) rGO and (e, f) HQG. (g) Surface morphology and (h) cross-section of HQG interlayer. 8


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The charge/discharge profiles and cyclic voltammogram (CV) curves of S-I, S-II and S-III between 1.5 and 3.0 V are compared in Figure 2 to effectively elucidate lithium storage process. As shown in Figure 2a~c, three typical dramatic peaks of S cathode are observed in each cycle of CV curves. The reactions occurred during discharge process are given below, and the backward reactions will occur upon charging 36-38. Discharge：S + 2Li+ + 2eLi2S4 + 2Li+ + 2eCharge:

Li2S

Li2S4 Li2S

S + 2Li+ + 2e-

(Eq. 2) (Eq. 3) (Eq. 4)

In the discharge process, two main cathodic peaks are attributed to the S reduction to lithium polysulfides, high-order lithium polysulfides, and lithium sulfides. During the first step, sulfur bonds with Li+, and converses to a long chain of lithium polysulfides (Li2Sx (x=4~8)), which is easily dissolved into electrolyte. Then S-S bonding in long chain polysulfides is broken ulteriorly, resulting in a short chain of lithium sulfides (Li2S or Li2S2), which are insoluble, through bonding with Li+

39

. These two steps

correspond to the two platforms of ~2.3 V and ~2.0 V in the discharge profiles. In reverse, the charge process encounters a long platform which is related to the short chain Li2S or Li2S2 converting to polysulfides via deintercalation of Li+ at around 2.5 V. Moreover, the following cycles of S-I and S-II reveal a good coincide, while S-III shows a voltage excursion. It seems that the anodic interlayer accelerates the deposition of Li2S onto the anode side, which leads to huge irreversible capacities 9



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upon cycling. In Figures 2d~f, two typical voltage plateaus of S-I, S-II and S-III were clearly observed at around 2.0 and 2.3 V in the galvanostatic discharge curves, and the polarization of S-I, S-II and S-III reveals an obvious distinction of ∆E3>∆E1 >∆E2. It was interesting that the charge status of S-III exhibited much higher capacities than that of discharge throughout the whole cycles. It is due to the existence of anodic interlayer absorbing enormous polysulfides and preventing the conversion reactions of Eq. (3) during discharge process. On charging, Eq. (4) normally occurs on the S-II, however, the situation of S-III varies slightly, and the anodic polysulfides may capture electron and Li+ and facilitate Eq. (3), thereby providing an extra charge capacity. As for S-II, the insertion of interlayer efficiently increases reaction sites, leading to a high level utilization of sulfur. As a result, among three S electrodes, S-III showed the lowest capacities of 615discharge/696.3charge mA h g-1 at a current density of 320 mA g-1 after 5 cycles, even worse than S-I (872.5discharge/919.4charge mA h g-1), and the S-II revealed the highest reversible capacities of 1077discharge/1095.4charge mA h g-1.

10


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3.0

(a)

2

1st 2nd 3rd

1

Voltage (V)

-1

Current Density (mA mg )

3

0

(d)

2.5

E1 2.0 1st 2nd 5th

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-2 1.5

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(e) E2

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1st 2nd 5th

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0 3.0

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(c)

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800 1000 1200 1400 -1

Specific Capacity (mA h g )

(f)

2 1

Voltage (V)

1st 2nd 3rd

0 -1

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E3 2.0

1st 2nd 5th

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(g)

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(h)

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Current Density (mA mg-1)

600

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Voltage (V)

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


2.5

E4

2.0 1st 2nd 5th

1.5 0

200

E5

2.0

400

600

-1

800


1st 2nd 5th

1.5 0

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-1

800


Figure 2. CV curves of (a) S-I, (b) S-II and (c) S-III at selected cycles. Charge/discharge profiles of (d) S-I, (e) S-II and (f) S-III at different cycles under a current density of 320 mA g-1. Charge/discharge profiles of (g) S-IV and (h) S-V at different cycles under a current density of 800 mA g-1. 11



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The cycling performance of S-I, S-II and S-III is compared in Figure 3a in a voltage range of 1.5 to 3.0 V at a current density of 800 mA g-1. It was remarkable that compared with S-I and S-III, S-II displays a higher reversible capacity of 1139.5 mA h g-1. The high sulfur utilization can be largely ascribed to the introducing of cathodic interlayer, which expands the contact area between active materials and electron. However, the unmatched charge/discharge specific capacities of S-III mainly own to the interlayer towards anode side enhancing the unexpected “shuttle effects”. As a result, a fast fading tendency of S-III was maintained with a long cycling. After 150 cycles, the reversible capacities of S-I, S-II and S-III were 387.6 mA h g-1, 531.5 mA h g-1 and 649.1 mA h g-1, respectively. Furthermore, as show in Figure 3b, S-II resulted in enhanced coulombic efficiency among three S electrodes. Interestingly, the coulombic efficiency of S-III was always lower than 90% up to 30 cycles, which is far below than that of S-I and S-II. Figure 3c compared rate capability of three S electrodes at various current densities. Apparently, S-II manifests higher reversible capacities of 1017.5, 873.1, 791.0, 640.5 and 502.5 mA h g-1 at varying current density of 320, 640, 800, 1600, 3200 mA g-1, respectively. As the current density decreases back to 800 mA g-1, the original capacity was nearly restored. On the contrary, at a high current density of 3200 mA g-1, S-III only maintain a reversible specific capacity of 327.8 mA h g-1, which is even worse than S-I (400 mA h g-1). It is reasonable that S-II reveals better electrochemical performance than S-I. Interestingly, the performance of S-III is inferior to S-I. This difference revealed that an interlayer 12


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produces some significant effects on both sides of electrodes. It was previously reported that some 3D conductive network materials were normally constructed as the host of lithium metal to mitigate the lithium metal surface due to the hindered of dendrite growth. In this study, an anodic interlayer directly mitigates the contact between polysulfides and lithium metal, thereby leading to a less corrosion of lithium metal. Coulombic Efficiency (%)

(a)

-1


1200 900

800 mA g-1

600 S-I S-II S-III

300 0 0

30

60

90

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150

(b)

120

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-1

S-III

400 -1

unit: mA g

0 0

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S-V

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Cycle Number Specific Capacity (mA h g )

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80

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(d)

S-I

S-IV

S-V

600 400

800 mA h g

-1

200 0 0

30

60

90

120

150

Cycle Number

Cycle Number

Figure 3. (a) Cycle performance of S-I, S-II and S-III at the current density of 800 mA g-1. (b) Coulombic efficiency of S-I, S-II, S-III, S-IV and S-V at the current density of 800 mA g-1. (c) Rate capability of S-I, S-II and S-III at various current densities. (d) Cycle performance of S-I, S-IV and S-V at the current density of 800 mA g-1.

Based on the aforementioned points, a schematic illustration (see Scheme 1) was 13



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proposed to illustrate the working mechanism of interlayers in LSBs. It may explain why the interlayer in the cathode location results in higher LSB performance than the one towards anode. Briefly, the electrode area can be expanded when a conductive interlayer is employed. For a cathodic interlayer, it not only hinders the escape of polysulfide during discharge process, but also provides addition deposition sites of polysulfides. Therefore, an interlayer towards cathode side affords some extra reaction sites of active materials, and efficiently increases the S utilization with performance improvement of LSBs. However, an anodic interlayer can accelerate the “shuttle effect” of polysulfides due to the absorbability of interlayer, which easily cut off the redox reactions of Eq. (3) during discharge process. Moreover, these anodic polysulfides might be further converted to Li2S on charging, and enhance the irreversible of sulfides with low coulombic efficiency.

(a)

eAnode area

Cathode area

interlayer on the cathode side

electron

interlayer on the anode side

polysulfide

(b)

e-

interlayer

(c)

e-

Scheme 1. The schematic of effects of interlayer location on lithium sulfur batteries: 14


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(a) without interlayer, (b) towards cathode side, and (c) towards anode side.

It was noteworthy that the discussion above is based on the conductive interlayer. Clearly, via a conductive interlayer, the electrons may smoothly transfer from electrode to interlayer area, and realize the occurrence of redox reactions. As a result, the electrical conductivity of interlayer is the key factor since a “conductive bridge” connects interlayer and electrode in the “vice-electrode” area. To further confirm this important point, a glass fiber was introduced as a non-conductive interlayer. For comparison, the S electrodes for a glass fiber towards cathode side and anode side were denoted as S-IV, and S-V, respectively. The charge/discharge profiles of S-IV and S-V are show in Figures 2g and h. Both S-IV and S-V reveal obvious polarization potentials of 0.262 and 0.279 V, respectively, which are higher than those of S-I, S-II and S-III. As previously reported

40

, the soluble polysulfides are successively

infiltrated into interlayers, and are re-utilized during charge/discharge processes. Unfortunately, these processes may be cut off due to the nonconductive nature of interlayer, as a result, a nonconductive interlayer functioned as a “dead zone” due to the absent of electrons. However, compared with anodic interlayer, the cathodic interlayer was tightly attached to the cathode electrodes, and some absorbed polysulfides on its surface may be utilized upon cycling. Therefore, S-IV delivered a relative higher specific capacity than that of S-V at the initial cycles. The reversible capacities of S-IV and S-V were less than 800 mA h g-1 in the first several cycles, and both electrodes presented a fading tendency throughout the entire cycles (see Figure 15



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3d). Both S-IV and S-V retained capacities of only 132.5 mA h g-1 and 144.5 mA h g-1 at a current density of 800 mA g-1 after 150 cycles, respectively, which were much worse than S-I. The unstable and coincident performance of S-IV and S-V corresponds to the above model we proposed, suggesting that the glass fiber failed to utilize the absorbed sulfides functioning as a “dead zone”. In addition, the coulombic efficiency of S-IV and S-V reached nearly 100% after several cycles, as shown in Figure 3b, showing a robust bonding between polysulfides and glass fiber. Figure 4a~c compare the SEM images of pristine glass fiber and the one towards cathode side under full charge and full discharge states after 100 cycles. A 3D hierarchical structure of un-conductive interlayer mainly consists of glass fibers. After 100 cycles, this cathodic interlayer at full charge and discharge states exhibits similar morphologies of wrapped glass fibers in Figures 4b and c, further confirming that the “dead zone” existed in cycled S-IV. A strong sulfur signal can be found in Figure 4d, showing that polysulfides were well captured by the glass fiber, and may not re-back to sulfur due to no electron transfer into un-conductive interlayer.

16


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original

(a)

10 um

50 um discharge

(c)

charge

(b)

(d)

20 um

50 um

S

C

O

F

20 um

20 um

20 um

20 um

Figure 4. SEM image of glass fibers at different states: (a) before cycling, and (b) full discharge / (c) full charge states towards cathode side after 100 cycles. The insets are corresponding digital images of glass fibers. (d) In-plane SEM inspection and elemental analysis of glass fibers at full charge state after 100 cycles.

Taking into account such possibility of involvement of interlayer in electrochemical reaction, one can assume that the relative lithium metal surface of S-III may be etched softly than S-I during cycling because soluble polysulfides are absorbed by the interlayers. The surface of lithium anode was observed to study the effect of interlayers upon cycling. Figure 5 compares the Li metal surface of S-I, S-II and S-III. The surface of pristine lithium is relatively glossy. After 100 cycles, as expected, the relative lithium anode of S-I was strongly etched due to the deposition of soluble polysufildes on its surface. Interestingly, the correlative lithium metal 17



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

surface of S-III is less corrosion than those of S-I and S-II. Therefore, one may deduce that the interlayer may mitigate etching issue of lithium anode due to the shift of repeated polysulfide deposition into the interlayers upon cycling.

(a)

(b)

50um

50um

(c)

(d)

50um

50um

Figure 5. SEM images of (a) pristine lithium metal surface, and the paired lithium metal surface of (b) S-I; (c) S-II and (d) S-III after 100 cycles.

The electrochemical impedance spectra (EIS) of all samples were compared in Figure 6 to further understand the function of interlayer. As shown in Figure 6, two semicircles in the high and medium frequency region and a straight line in the low frequency region were involved in the spectra. The two semicircles may be related to the formation of solid electrolyte interface (SEI) films as well as the interface reaction between electrolyte and electrode. Particularly, though interlayer was introduced to 18


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the cathode side, it functions as “vice-electrode” to provide an addition conductivity and absorbability to polysulfides, and there were no additional microcosmic interface between electrode and interlayer. The impedance spectra were fitted using the equivalent circuit model shown in Figure 6f. R0 and Rct represent ohmic resistance and charge transfer resistance, respectively; CPE1 and CPE2 are the equivalent link of double-layer capacitance in solid electrolyte interface (SEI) film and electrodes, respectively, which coordinate with the resistance of SEI films (Rs) and Rct relative to the semicircle at medium and high frequency region; W0 represents the diffusion-controlled Warburg impedance related to the diffusion of Li+ in the electrode 10-11, 27

. Figures 6a~c show the Nyquist plots of S-I, S-II and S-III at different states

and cycles. In comparison, the relative low and overlapped Rs of S-II can be ascribed to the introduction of conductive cathodic interlayer, which effectively hinders the cracked of sulfur particles and guarantees steady surface situations. On the contrary, the anodic interlayer enhances the escaping of polysulfides, thereby leading to an unstable Rs of S-III. Besides, the fitting values of Rct were presented in Table 1, both S-I and S-III endowed higher Rct values, and its increasing trends resulted from the pulverization and re-deposition of polysulfides onto S cathode upon cycling. In comparison to S-I, the lower Rct value of S-III stemmed from some insulated polysulfides adsorbed onto anodic interlayers. As expected, S-II showed the lowest Rct values among three electrodes, which is due that the uniform distribution of polysulfides onto cathodic conductive interlayers as “vice-electrode” decreased electrochemical polarization. As show in Table 1, the lower Rct value of S-II (54.4 19



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

ohm at 10th cycles) was observed than that of S-I (166.8 ohm at 10th cycles). Apparently, the introducing of cathodic interlayer effectively reduce the resistance of work electrode due to part of polysulfides were absorbed by interlayer and spread on a wider surface. As a result, different from the sulfides in S-I, the sulfides of S-II was more likely to refine and re-deposition on a wilder electrode area due to its special solid-liquid-solid conversion process and the functions of cathodic conductive interlayer. Therefore, the lower Rct value of S-II stemmed from the re-arranged active materials, and the role of interlayer can be regarded as a host materials to provide a fast transfer of electrons and a wider specific surface for polysulfides. The Nyquist plots and corresponding Rct value of S-IV and S-V were as show in Figures 6d~e and Table S1. Both S-IV and S-V revealed similar Rct values throughout the whole cycles. More surprisingly, their values were less than those of S-I, S-II, and S-III. It indicated that the glass fibers as interlayers in the S-IV and S-V revealed strong absorbability of polysulfides upon cycling. These findings demonstrated that to further optimize conductive interlayers with strong absorbability may increase battery performance for LSBs.

20


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350

350

(a) 1st C3.0V 2nd D2.1V 3rd D2.1V 10th D2.1V 50th D2.1V

200 150 100

200 150 100

50

50

0

0 0

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1st D2.1V 1st C2.5V 5th C2.5V 10th C2.5V

150

350

(f)

(e)

200

300

Z' (ohm)

R0

Rs

R ct

Ԝ0

100 CPE1

50

0

0

Z' (ohm)

250

-Z'' (ohm)


1st C3.0V 2nd D2.1V 3rd D2.1V 10th D2.1V 50th D2.1V

250

0 50

300

(d)

250

(c)

300

50 0

350

Z' (ohm)

300


250

-Z'' (ohm)

-Z'' (ohm)

250

350

(b)

300

-Z'' (ohm)

300

-Z'' (ohm)

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


CPE2

0 0

50

100

150

200

250

300

0

50

Z' (ohm)

100

150

200

250

300

Z' (ohm)

Figure 6. Nyquist plots of (a) S-I; (b) S-II; (c) S-III; (d) S-IV and (e) S-V at different states and cycles. (f) Possible equivalent circuit.

Table 1. The comparison of Rct values of S-I, S-II and S-III. Samples

3rd

10th

50th

S-I

126.2 99.94 102

166.8 54.4 108

196.1 71.88 166.7

S-II S-III

CONCLUSION In summary, we successfully designed various interlayers, and clearly understood the working mechanism of interlayer in LSBs. It was demonstrated that the electrical conductivity is a crucial factor to guarantee the functions of interlayer. Typically, a conductive interlayer towards cathode side can highly enhance the battery performance of electrodes. On the contrary, an insulated interlayer absorbing polysulfides is regarded as a “death zone” without transferred electron upon cycling. 21



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As for the interlayer towards anode side, the fast fading capacity mainly contributed to an enhanced “shuttle effect”. Moreover, optimizing polysulfide absorbability of conductive interlayers may further enhance electrochemical performance of S electrodes. Therefore, as “vice-electrode”, three virtues of an interlayer are required to deliver excellent performance of S electrodes, that is, electrical conductivity, location, and polysulfide absorbability. It is noteworthy that in this study the obtained electrochemical performance was based on commercial S powder with the designed interlayers. It is inevitable that higher battery performance may be achieved when the S based nanocomposites with carbon nanotube, porous carbon, conducting polymer, and graphene, as mentioned in Introduction, are involved in our study in the future.

ASSOCIATED CONTENT Supporting Information The additional FT-IR of HQG and rGO; Schematics of interlayers cast on the surface of separator; the comparison of Rct values of S-IV and S-V are provided. These materials are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email addresses: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS 22


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This research was supported by the National Natural Science Foundation of China (51572194), Key Projects of Tianjin science and technology support plan (14JCZDJC32200), and Academic Innovation Funding of Tianjin Normal University (52XC1404). REFERENCES (1) Wang, L.; Zhang, T.; Yang, S.; Cheng, F.; Liang, J.; Chen, J. A quantum-chemical study on the discharge reaction mechanism of lithium-sulfur batteries. J. Energy Chem. 2013, 22, 72-77. (2) Bresser, D.; Passerini, S.; Scrosati, B. Recent progress and remaining challenges in sulfur-based lithium secondary batteries--a review. Chem. Commun. (Camb) 2013, 49, 10545-10562. (3) Ji, X.; Lee, K. T.; Nazar, L. F. A highly ordered nanostructured carbon-sulphur cathode for lithium-sulphur batteries. Nat. Mater. 2009, 8, 500-506. (4) Zheng, G.; Yang, Y.; Cha, J. J.; Hong, S. S.; Cui, Y. Hollow carbon nanofiber-encapsulated sulfur cathodes for high specific capacity rechargeable lithium batteries. Nano Lett. 2011, 11, 4462-4467. (5) Zhang, B.; Qin, X.; Li, G. R.; Gao, X. P. Enhancement of long stability of sulfur cathode by encapsulating sulfur into micropores of carbon spheres. Energy Environ. Sci. 2010, 3, 1531-1537. (6) Wang, L.; Dong, Z.; Wang, D.; Zhang, F.; Jin, J. Covalent bond glued sulfur nanosheet-based cathode integration for long-cycle-life Li-S batteries. Nano Lett. 2013, 13, 6244-6250. (7) Huang, J.-Q.; Liu, X.-F.; Zhang, Q.; Chen, C.-M.; Zhao, M.-Q.; Zhang, S.-M.; Zhu, W.; Qian, W.-Z.; Wei, F. Entrapment of sulfur in hierarchical porous graphene for lithium–sulfur batteries with high rate performance from −40 to 60°C. Nano Energy 2013, 2, 314-321. (8) Fang, X.; Weng, W.; Ren, J.; Peng, H. A Cable-Shaped Lithium Sulfur Battery. Adv. Mater. 2016, 28, 491-496. 23



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(a)

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G band

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Relative Intensity (a.u.)


(ii)

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500 nm

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Figure 1. (a) XRD and (b) Raman of HQG and rGO: (i) HQG; (ii) rGO; (iii) GO. Low- and high- magnification SEM images of (c, d) rGO and (e, f) HQG. (g) Surface morphology and (h) cross-section of HQG interlayer.

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3.0

(a)

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Figure 2. CV curves of (a) S-I, (b) S-II and (c) S-III at selected cycles. Charge/discharge profiles of (d) S-I, (e) S-II and (f) S-III at different cycles under a current density of 320 mA g-1. Charge/discharge profiles of (g) S-IV and (h) S-V at different cycles under a current density of 800 mA g-1. 29



Coulombic Efficiency (%)

(a)

-1


1200 900

800 mA g-1

600 S-I S-II S-III

300 0 0

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S-I

S-IV

S-V

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800 mA h g

-1

200 0 0

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Cycle Number

Cycle Number

Figure 3. (a) Cycle performance of S-I, S-II and S-III at the current density of 800 mA g-1. (b) Coulombic efficiency of S-I, S-II, S-III, S-IV and S-V at the current density of 800 mA g-1. (c) Rate capability of S-I, S-II and S-III at various current densities. (d) Cycle performance of S-I, S-IV and S-V at the current density of 800 mA g-1.

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(a)

eAnode area

Cathode area

interlayer on the cathode side

electron

interlayer on the anode side

polysulfide

(b)

e-

interlayer

(c)

e-

Scheme 1. The schematic of effects of interlayer location on lithium sulfur batteries: (a) without interlayer, (b) towards cathode side, and (c) towards anode side.

31



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

(a)

original

charge

(b)

10 um

(c)

Page 32 of 36

50 um discharge

(d)

20 um

50 um

S

C

O

F

20 um

20 um

20 um

20 um

Figure 4. SEM image of glass fibers at different states: (a) before cycling, and (b) full discharge / (c) full charge states towards cathode side after 100 cycles. The insets are corresponding digital images of glass fibers. (d) In-plane SEM inspection and elemental analysis of glass fibers at full charge state after 100 cycles.

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(a)

(b)

50um

50um

(c)

(d)

50um

50um

Figure 5. SEM images of (a) pristine lithium metal surface, and the paired lithium metal surface of (b) S-I; (c) S-II and (d) S-III after 100 cycles.

33



350

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(a) 1st C3.0V 2nd D2.1V 3rd D2.1V 10th D2.1V 50th D2.1V

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R0

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Ԝ0

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0

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250

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250

0 50

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(d)

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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

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CPE2

0 0

50

100

150

200

Z' (ohm)

250

300

0

50

100

150

200

250

300

Z' (ohm)

Figure 6. Nyquist plots of (a) S-I; (b) S-II; (c) S-III; (d) S-IV and (e) S-V at different states and cycles. (f) Possible equivalent circuit.

34


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Table 1. The comparison of Rct values of S-I, S-II and S-III. Samples

3rd

10th

50th

S-I

126.2 99.94 102

166.8 54.4 108

196.1 71.88 166.7

S-II S-III

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Page 36 of 36

Graphical Abstract

e-

electron polysulfide e-

interlayer


e-

Controllably Designed âVice-electrodeâ Interlayers ... - ACS Publications

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