Integrated Polypyrrole@Sulfur@Graphene Aerogel 3D Architecture

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Integrated Polypyrrole@Sulfur@Graphene Aerogel 3D Architecture via Advanced Vapor Polymerization for High Performance Lithium-Sulfur Batteries Hu Tang, Lei You, Jianwen Liu, Shiquan Wang, Pengyu Wang, Chuanqi Feng, and Zaiping Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on April 29, 2019

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ACS Applied Materials & Interfaces

Integrated Polypyrrole@Sulfur@Graphene Aerogel 3D Architecture via Advanced Vapor Polymerization for High Performance Lithium-Sulfur Batteries

Hu Tang,

a,#

Lei You,

a,#

Jianwen Liu,

a,*

Shiquan Wang,

a

Pengyu Wang,

a

Chuanqi Feng

a

and

Zaiping Guo a,b,c,*

a. Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials & Ministry of Educational Key Laboratory for the Synthesis and Application of Organic Functional Molecules & College of Chemistry and Chemical Engineering, Hubei University, Wuhan, 430062, P. R. China. b. Institute for Superconducting and Electronic Materials, Australian Institute for Innovative Materials, University of Wollongong Innovation Campus, North Wollongong, NSW 2522, Australia c. School of Mechanical, Materials, Mechatronic, and Biomedical Engineering, Faculty of Engineering & Information Sciences, University of Wollongong, NSW 2522, Australia

# These

authors contributed equally to this work.

* Corresponding

author: Dr. & A. Prof. Jianwen Liu. E-mail: [email protected] Dr. & Prof. Zaiping Guo. E-mail: [email protected]

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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

Abstract Although lithium-sulfur batteries have been regarded as the most promising candidate for next-generation energy storage devices with high specific capacity, their rapid capacity decay, mainly caused by volume expansion and dissolution of polysulfides, has limited their practical applications. Aiming at these issues, herein, we have designed an ideal three-dimensional (3D)-structured polypyrrole@sulfur@graphene aerogel (PPy@S@GA) as an efficient sulfur host via an advanced pyrrole vapor polymerization. The GA with interconnected 3D porous structure provide an excellent conductive network for electrons and a channel for ion transfer, as well as a physical barrier or absorber for the polysulfides. In addition, the physical confinement and chemical adsorption are further strengthened by the PPy coating layer with polar nitrogen. The electrode with the PPy@S@GA 3D structure delivered a superior initial discharge specific capacity of 1135 mAhg-1 and a capacity of 741 mAhg-1 after 500 cycles at the 0.5 C rate, with capacity fading as low as 0.031% per cycle, superior to both a sulfur electrode and a S@GA electrode. These results demonstrate that the GA as sulfur host further coated with PPy is a promising cathode for pursuing high-performance Li-S batteries.

KYEWORDS: Lithium-sulfur battery, Graphene aerogel, Three-dimensional structure, Polypyrrole, Lithium polysulfides


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Introduction Nowadays lithium-Sulfur (Li-S) batteries are favored by many researchers as the next-generation energy storage device due to the prominent advantages of elemental sulfur, such as high theoretical capacity, low cost, and nontoxicity.1 The mass production and application of Li-S batteries have not yet been achieved, however, which is ascribed to the following issues: 1) poor conductivity of elemental sulfur and the discharge products;2 2) large volume expansion during the charge and discharge processes; 3 3) the shuttle effect between the cathode and anode, resulting in the deposition of solid, low-conductive Li2S/Li2S2 on the surfaces of electrodes.4 The above problems can lead to poor electrochemical performance, including poor coulombic efficiency and fast fading of capacity. Therefore to overcome these issues, many researchers have attempted physical confinement and chemical adsorption methods. The physical confinement methods generally include carbon coating on sulfur or embedding sulfur in porous materials.5 Even though, these methods can improve the electrical conductivity of sulfur, unfortunately, part of the polysulfide can still dissolve into the electrolyte, so that they cannot entirely resolve the problem of sulfur loss in Li-S batteries. In order to further improve the performance, carbon nanotube,6 graphene,7,8 conducting polymer

9,10

and

carbon materials doped with heteroatoms (N,11-13 S,14 B,15 and P16) have been employed in Li-S batteries. Among them, the carbon guarantees the conductivity of the composite, while the doped heteroatoms or the functional groups on the conductive polymer provide the active sites for chemical adsorption to anchor the polysulfide. Especially polar nitrogen (pyridinic N and pyrrolic N) has been confirmed to perform strong adsorption to lithium polysulfides.11-13,17-19 In addition, transition metal oxides (MnO2,20,21 Co3O4,22 Fe3O4,23 V2O5,24 TiO225,26 and MgO27), nitrides (WN28 and TiN29) and sulfides (CoS2,30 WS2,31 NiS2,32 and NbS233) have strong adsorption effects towards polysulfide, so



that the loss of sulfur can be further controlled and the electrochemical performance can be correspondingly further improved. Among the various carbon materials, graphene aerogel (GA) have been widely investigated due to their high surface area, good conductivity, good compatibility and lightweight property.34-37 GA have been widely applied in lithium/sodium ion batteries,38-41 whereas only a few workers have introduced GA as a sulfur host in the Li-S batteries, such as by depositing sulfur on gel-like reduced graphene oxide,42 designing a self-supporting three-dimensional (3D) Li2S-doped GA cathode,43 and synthesizing a 3D GA/TiO2/S composite for Li-S batteries.44 However, in the previous published works, the sulfur in GA is mainly loaded on the surface of the graphene layer instead of fully confined in the 3D GA. Herein, we have designed a polypyrrole@sulfur@graphene aerogel (PPy@S@GA) as a 3D porous structured composite in this article, which was prepared by the sol-gel method followed by vapor phase polymerization. Graphene oxide (GO) was first reduced to GA, after transforming Na2Sx into sulfur particles deposited on the surface of GO, and subsequently PPy@S@GA composite was obtained by coating PPy layer on 3D S@GA via pyrrole vapor polymerization. Scheme 1 illustrates the synthesis process for the PPy@S@GA composite, including three main steps: (I) sulfur particles deposition on the surface of GO and the in-situ self-assembly of GO to form the 3D porous graphene-hydrogel-encapsulated sulfur precursor, (II) freeze-drying of the precursor to obtain the S@GA aerogel, and (III) coating a polypyrrole layer on the S@GA aerogel by vapor phase polymerization of pyrrole monomer to obtain the PPy@S@GA composite. In this way, sulfur particles are uniformly distributed within the GA architecture with sulfur loading content of about 55~80%. In addition, the sulfur particles were effectively confined in the 3D matrix with the sulfur


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previous literatures.45,46 sodium sulfide nonahydrate (Na2S·9H2O), hypophosphorous acid (H3PO2), pyrrole monomer, sulfur powders were provided by the company of Macklin and Sigma-Aldrich respectively. First 1.78 g Na2S·9H2O was added into 25 ml deionized water, and stirred to be dissolved, then 0.72 g sulfur powder was added and continued stirring until the solution became transparent orange, thus the Na2Sx solution was obtained. 30 mg GO was dispersed in 10 ml deionized water in a vial, and 3 ml the as-prepared Na2Sx solution was added, followed by a vigorous stirring, then hypophosphorous acid was added with further stirring for 20 min. The vial was placed in the oven at 80 oC for 12 h to obtain the hydrogel precursor, the precursor was finally freeze-dried under vacuum to obtain the S@GA composite. Preparation of PPy@S@GA composite. 5 g the organic ferric salt iron (III) p-toluenesulfonate (FeToS) was dissolved in 45 g butanol solution. The S@GA composite was soaked in ethanol overnight, and then the FeToS butanol solution was dropped into composite until ethanol was completely displaced by FeToS butanol solution. Subsequently, the composite was placed in the oven at 100 oC for 20 min to deposit FeToS. Then the sample was put into a vapor phase polymerization (VPP) chamber with pyrrole monomer vapor in the bottom at 60 °C for 1 h. After that, the sample was transferred to the oven at 50 oC for 30 min, followed by washing with deionized water and ethanol for several times, then the sample was finally freeze-dried under vacuum to obtain the PPy@S@GA composite. Characterization. The components of two composites were determined by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectra (RS) and thermo gravimetric analysis (TGA). The morphologies were investigated by field-emission scanning electron microscopy (FE-SEM), transmission electron microscope (TEM) and the X-ray spectroscopy (EDS) mapping


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analysis. The electrochemical performance was evaluated the cycling performance and rate capability tests, then cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were also measured. Electrochemical test. The sulfur cathode slurry was prepared by mixing active material, acetylene black (AB) and polyvinylidene fluoride (PVDF) at mass ratio of 8:1:1 in N- Methyl pyrrolidone (NMP) solvent, the slurry was coated onto an Al foil, and dried under vacuum at 55 °C for 12 h, the mass loading of active material was about 1~2 mg/cm2. The electrolyte was prepared by mixing the 1, 2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) at volume ratio of 1:1 to dissolve the lithium trifluoromethanesulfonate (LiCF3SO3, 1 M) and LiNO3 (2% in weight). The lithium metal foil was used as anode and the polypropylene membrane was used as separator. CR2025 coin cells were assembled in an argon-filled glove box. Galvanostatic cycling measurements were carried out from 1.7 to 2.8 V at room temperature by a Land Battery Tester and CV measurement was carried out on an electrochemical workstation (CHI 660d) at a scan rate of 0.1 mVS-1, and EIS was also measured within the range of 0.01 to 105 HZ.

Results and discussion Figure 1a presents the X-ray diffraction (XRD) patterns of the S@GA and PPy@S@GA composites, which are both well matched with the standard diffraction pattern of sulfur (JCPDS No. 08-0247). Meanwhile the pattern for carbon cannot be detected because the humps for GA and the PPy coating layer were completely covered by the pattern of sulfur. The Raman spectra of the S@GA and PPy@S@GA composites are displayed in Figure 1b. The main peaks below 500 cm-1 represent the Raman spectrum of sulfur and the characteristic peaks at 1350 and 1580 cm-1 correspond to the



disorder band and graphite bands respectively. In addition, the band intensity ratio (ID/IG) for the PPy@S@GA composite (ID/IG = 1.17) is smaller than that for the S@GA composite (ID/IG = 1.33) due to the low defect level of PPy, which reveals that the PPy was effectively combined into [email protected],47 The weight ratios of sulfur in the PPy@S@GA and S@GA composites were determined according to thermogravimetric analysis (TGA), as shown in Figure 1c. In the range of 200 to 350 oC, the mass losses of S@GA, PPy@S@GA, and PPy@GA are 79% (from sulfur evaporation), 58% (from sulfur evaporation and PPy decomposition), and 6.5% (from PPy decomposition), respectively. Therefore it can be calculated that the content of sulfur is 56.6% in PPy@S@GA and 79% in S@GA composite, respectively. The lightweight property of the aerogel ensures an ideal sulfur loading, which was demonstrated by the photograph of GA in Figure S1. The Figure S2 in the Supporting Information shows the X-ray photoelectron spectra (XPS) of the S@GA and PPy@S@GA composites. Compared with the S@GA composite, an obvious N 1s peak located at around 400 eV appears in the XPS spectrum of the PPy@S@GA composite, which corresponds to the N element from the PPy with its content of 13.46%. Figure 1d further shows the fitted high-resolution N 1s XPS spectrum of PPy@S@GA composite. The three peaks located at 398.1, 399.8, and 401.2 eV correspond to pyridinic N, pyrrolic N, and graphitic N, respectively. As is well known, pyridinic N and pyrrolic N have been confirmed to play key roles in improving the electrochemistry performance of Li-S batteries due to their strong adsorption effect on polysulfides.11,12


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a

b

PDF#08-0247

20 30 40 50 2theta (degree)

c 100

d

20 0

100

200 300 Temperature (

400 )

500

600 900 1200 1500 1800 Raman shift (cm-1)

Raw data Fitted data

Intensity (a.u.)

40

PPy@S@GA S@GA PPy@GA

G band

S

300

60

80 60

D band PPy@S@GA S@GA

Intensity (a.u.)

Intensity (a.u.)

PPy@S@GA S@GA

10

Weight (%)

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


Graphitic N 401.2

408

Pyrrolic N 399.8

Pyridinic N 397.8

405 402 399 396 Binding Energy (eV)

393

Figure 1. (a) XRD patterns and (b) Raman spectra of S@GA and PPy@S@GA composites (c) The thermo gravimetric (TG) curves of PPy@S@GAs, S@GAs and PPy@GAs at the ramping rate of 10 oC·min-1

between 30 oC to 500 oC, and (d) fitted high-resolution N 1s XPS spectrum of

PPy@S@GA.

The morphological features of S@GA and PPy@S@GA composites were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Figure 2a-d, the morphologies both maintain the same 3D porous GA matrix microstructure, and the compact sulfur particles ~10 nm in size are uniformly distributed on the surface of the GA. Meanwhile the 3D porous structure can be also clearly seen in the TEM images of the S@GA (Figure 2e) and PPy@S@GA (Figure 2g) composites, and the PPy@S@GA electrodes also exhibit a three-dimensional structure in SEM cross view of the PPy@S@GA electrodes in Figure S3. On



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comparing their high resolution transmission electron microscopy images (HRTEM), the PPy coating layer is seen to be evenly coated on the surface of the GA with a thickness of ~20 nm, which can be only seen in the surface structure of PPy@S@GA in Figure 2h. In order to further verify the uniform distribution of the PPy coating layer and the sulfur, the corresponding scanning transmission electron microscope (STEM) and energy dispersive X-ray (EDX) element mapping images of PPy@S@GA composite suggest the presence of carbon, nitrogen, and sulfur components, further demonstrating the homogeneous coverage by carbon and the uniform distribution of yellow sulfur in the 3D structure (Figure 3i-l).

a

b

c

Graphene

d

S 1 =m

e 1 =m

i 500 nm

50 nm

f

1 =m

50 nm

g

Graphene

20 nm

hh

1 =m

20 nm

k

j C

PPy layer

l N

S

Figure 2. SEM images of (a, b) S@GA, and (c, d) PPy@S@GA at different magnifications; TEM and HRTEM images of (e, f) S@GA, and (g, h) PPy@S@GA at different magnifications; (i) STEM image of a single particle of PPy@S@GA composite, and the elemental mapping of (j) carbon, (k) nitrogen, (l) sulfur.

Figure 3 presents the electrochemical performances of the PPy@S@GA and S@GA composites,


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along with sulfur. Here, all specific capacities are calculated based on the weight of sulfur. Figure 3a shows the cyclic voltammetry (CV) curves of the initial five scans for PPy@S@GA composite at a scanning rate of 0.1 mVsM( between 1.7-2.8 V. In the CV curves, the cathodic peak at 2.0 V corresponds to the reduction of sulfur to long-chain lithium polysulfides (Li2Sx, x = 4~8), and the other cathodic peak at 2.23 V is attributed to the further reduction of long-chain lithium polysulfides to short-chain lithium sulfides (Li2S or Li2S2).22,23 In addition, an anodic peak at 2.50 V is ascribed to the transformation of lithium sulfides to lithium polysulfide and sulfur. It is obvious that the CV curves from the 2nd to the 4th cycle are not much different, revealing the excellent cycling stability and capacity retention of the PPy@S@GA composite. Figure 3b-d shows the 1st to the 30th galvanostatic charge-discharge profiles of PPy@S@GA, S@GA, and sulfur at 0.1 C between 1.7 and 2.8 V respectively. The discharge curves both exhibit two typical plateaus at 2.35 and 2.08 V, as is consistent with the corresponding CV results. Concerning their charge/discharge profiles, the PPy@S@GA composite electrode shows better reversibility than the S@GA and sulfur electrodes. When compared to the sulfur and S@GA electrodes, the charge-discharge potential difference NE) (shown in Figure S4) of PPy@S@GA composite electrode remains almost constant and the lowest with cycle number, indicating that the lowest polarization occurs in PPy@S@GA composite. Furthermore, as shown in Figure S5, which shows the upper plateau discharge capacity, the PPy@S@GA composite presents the highest and most stable capacity with 89% capacity retention, revealing that the PPy coating layer has strong polysulfide-trapping capability because the upper plateau corresponds to the reduction from sulfur to the soluble polysulfide.48 In addition, the 3D structure and the coating layer can also enhance the lithium ion conductivity, which is confirmed by the different diffusion coefficients of PPy@S@GA, S@GA, and sulfur electrodes (as shown in



Figure S6). Therefore these calculations suggest that the integrated GA host material with 3D structure and the PPy coating layer can both improve the electrochemical performance of Li-S battery. The cycling performances of PPy@S@GA, S@GA, and sulfur cathode were first measured at a current density of 0.1 C, as shown in Figure 3e. Benefiting from the GA host and PPy coating layer, the PPy@S@GA cathode exhibits a high initial discharge capacity of 1198 mAhg-1 and retains 1018 mAhg-1 after 100 cycles. In contrast, the S@GA and sulfur cathodes only present a discharge capacity of 802 and 440 mAhg-1 after 100 cycles, respectively, owing to the severe dissolution of polysulfides, correspondingly demonstrating the structural advantages of trapping polysulfides. These results also prove that the combination of the 3D GA host and the PPy coating layer can provide effective confinement and chemical adsorption of polysufides, thus guaranteeing remarkable cycling stability. Meanwhile the long-term cycling performance of PPy@S@GA electrode was tested at current density of 0.5 C for 500 cycles, shown in Figure 3f. The PPy@S@GA electrode delivers a discharge specific capacity of 741 mAhg-1 after 500 cycles, representing about 84.6% retention of the initial discharge specific capacity of 876 mAhg-1, while the S@GA composite displays a discharge specific capacity of 415 mAhg-1 after 500 cycles, corresponding to 36.3% capacity retention. Hence, the capacity fading of PPy@S@GA composite electrode is as low as 0.031% per cycle, which is much lower than that of S@GA composite with 0.134%. The long-term cycling performance further confirms the functions of the GA host and PPy coating layer towards improving the performance of sulfur cathode. In addition, the cycling performance of PPy@S@GA elecetrode with different sulfur loading is shown in figure S7, when the sulfur loading is as high as 4.5 mg cm-2, the PPy@S@GA electrode still shows excellent cycling performance.


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A rate capability comparison of PPy@S@GA, S@GA, and sulfur cathodes is shown in Figure 3g. When cycled at 0.1, 0.2, 0.5, 1, 2, and 5 C (1C = 1675 mAhg-1), the PPy@S@GA electrode exhibits excellent discharge capacities of 1186, 978, 879, 778, 755, and 603 capacity can be recovered to 950

O

-1

O

-1,

respectively, and the

when the current density is returned to 0.1 C again. By

comparison, the S@GA and sulfur electrodes exhibit worse rate capabilities than the PPy@S@GA electrode, especially when cycled at current densities over 1 C. When the current density reaches 5 C, the S@GA and sulfur electrodes exhibit the discharge specific capacity of only 480 and 285 respectively, and the capacity remains at only 720 and 534

O

-1

O

-1,

when the current density is

returned to 0.1 C again. The excellent electrochemical performance of PPy@S@GA electrode also benefits from its low resistance (as shown in Figure 3h). In the electrochemical impedance spectroscopy (EIS) plot, there is a semicircular loop in the high frequency region, the diameter of which is attributed to the charge-transfer resistance resulting from the electrochemical reaction itself and the corresponding equivalent circuit model is inserted in Figure 3h.49,50 The results indicate that the charge-transfer resistance of the battery with PPy@S@GA (79.4 P electrode is the lowest compared with S@GA (9 9P and sulfur (;

Integrated Polypyrrole@Sulfur@Graphene Aerogel 3D Architecture

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