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The Effect of Boron-Doping to the Graphene Aerogel Used as Cathode for Lithium-Sulfur Battery Yang Xie, Zhen Meng, Tingwei Cai, and Wei-Qiang Han ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08129 • Publication Date (Web): 02 Nov 2015 Downloaded from http://pubs.acs.org on November 2, 2015

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ACS Applied Materials & Interfaces

The Effect of Boron-Doping to the Graphene Aerogel Used as Cathode for Lithium-Sulfur Battery Yang Xie,† Zhen Meng,

†

†

Tingwei Cai,

†

Wei-Qiang Han†,‡*

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of

Sciences, Ningbo 315201, P.R.China ‡ Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China. Keywords Lithium-sulfur batteries, boron-doped graphene aerogel, porous structure, chemical adsorption, electrochemical performance

Abstract A porous interconnected 3D boron-doped graphene aerogel (BGA) was prepared via a one-pot hydrothermal treatment. The BGA material was first loaded with sulfur to serve as cathode in lithium-sulfur batteries. Boron was positively polarized on the graphene framework, allowing for chemical adsorption of negative polysufide species. Compared with nitrogen-doped and undoped graphene aerogel, the BGA-S cathode could deliver a higher capacity of 994 mA h g-1 at 0.2 C after 100 cycles, as well as an outstanding rate capability, which indicated the BGA was an ideal cathode material for lithium-sulfur batteries. 1. Introduction Rechargeable lithium batteries with long cycle life and high energy density are

ACS Paragon Plus Environment


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desired to solve imminent issues1-3. Among them, lithium-sulfur batteries stand out as a desirable choice: sulfur cathode could provide a high theoretical capacity of 1675 mA h g-1; sulfur element is not only natural abundance with a low cost, but environmental friendly as well4-6. Despite its advantages, the development of lithium-sulfur batteries needs to overcome some critical problems, the insulting sulfur cathode, volume expansion and dissolution of lithium polysulfides in the electrolyte, which result in poor rate capacity, serious capacity fading and low coulombic efficiency of lithium-sulfur batteries7-8. The general method to settle those issues is to enhance the conductivity of sulfur cathode and hinder the dissolution of lithium polysulfides. Researchers have already come up with various cathode materials such as carbon nanotube9/carbon nanofiber10, porous carbon11-12, hollow carbon sphere13, and graphene14. Among these, graphene15, a 2D single layer carbon material, has attracted tremendous research effort owing to its unique properties: high conductivity, stable chemical property and strong mechanical strength. Moreover, graphene could be easily modified to enhance its properties, for instance, assembling into porous structure16 or doping with heterogeneous atoms17-22. Zhang16 introduced ascorbic acid as reducing agent to transfer graphene oxide into graphene and self-assemble into graphene aerogels, which not only inhibits the conductivity of 2D graphene sheets but exhibits good porosity as well. Researchers also showed that N/B doped graphene would obtain superior conductivity23, thus they could be applied in Li-ion batteries17-19 and supercapacitors20-22.


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Recently, with the combination of graphene and sulfur in the cathode material, the development of lithium-sulfur batteries has witnessed a huge progress. Cheng24 used a one-step method to anchor sulfur nanocrystals on graphene aerogel (GA), which took advantage of the interconnected 3D porous structure of GA and high conductivity of graphene. The GA-S material was directly used as cathodes in Li-S batteries, which maintained a capacity of ~700 mAh g-1 after running at 0.3 A g-1 for 50 cycles. However, carbon aerogel material has negative effect on the volumetric energy due to the low tap-density of C–S cathodes 25, and the applications of doped graphene aerogel in Li-S batteries were all focused on N-doped graphene aerogel26-28. As the current researches proved, Boron, like its electron counterpart Nitrogen, would enhance the conductivity of pure graphene after doping17,

29

; besides that,

Boron-doped carbons could attract polysulfides30. B dopant in the carbon framework is positively polarized, resulting in chemisorption with negatively charged species30-31. Herein, aiming to combine the advantage of boron-doping with graphene aerogel structure, we adopted a one-pot hydrothermal method20,

22, 32

to prepare a 3D

boron-doped graphene aerogel (BGA) framework. Compared with 2D graphene sheets, the aerogel could obtain a porous network which is more suitable to anchor sulfur and lithium polysulfides. Besides, unlike the template-induced method

33

to

synthesis 3D graphene structures, the graphene aerogel could be simply prepared, and its conductivity was enhanced via boron-doping. After loading BGA with sulfur as the cathode material, the electrode showed better electrochemical performance than that



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of N-doped aerogels.

2. Experimental Section Materials synthesis Synthesis of Graphene Oxide. Graphene Oxide (GO) was synthesized from graphite via a modified Hummers method34. Typically, 5 g graphite powder, 2.5 g NaNO3 and 130 ml concentrated H2SO4 solution were mixed together and stirred in an ice-water bath for 2 h. After that, 15 g KMnO4 was drop-wise added into the mixture. During this procedure, the mixture’s temperature was controlled under 20 °C, followed by another agitation at 35° C for 1 h. Then 230 ml H2O was slowly added into the container. The temperature was heated to 98 °C and the mixture was stirred for 30 min. The obtained solution was diluted with 500 ml H2O and treated with 5 ml H2O2 (30 %). The bright yellow solution was washed successively with 5 % HCl and water. The obtained GO was dispersed in diluted water by ultrasonic treatment. Synthesis of GA, NGA and BGA. BGA was synthesized via a typical hydrothermal process

20, 22, 32

. To be specific, 1.5 g H3BO3 was added into 100 ml GO (3 mg/ml)

solution and sonicated for 15 min until dissolution. The mixture was transferred into a Teflon-lined autoclave and reacted at 180 °C for 12 h, resulting in a 3D boron-doped graphene hydrogel, followed by repeated water washing. BGA was obtained by freeze drying of the hydrogel. In order to make a comparison, nitrogen-doped graphene aerogel (NGA) and graphene aerogel (GA) were also prepared via the same process, while the additive was changed from H3BO3 to NH3·H2O21 and ascorbic acid16,


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respectively. Synthesis of BGA-S, NGA-S and GA-S. The BGA-S composites were synthesized through a liquid infiltration process. BGA and elemental nano-sulfur were uniformly mixed by grinding. The mixture was firstly treated at 155 °C for 10 h, then the temperature was increased to 300 °C and kept for 2h. After cooling down to room temperature, the BGA-S composite was obtained. The NGA-S and GA-S were prepared via the same procedure. Materials characterization X-ray diffraction (XRD) measurements were conducted by a D8 advance XRD diffractometer (Cu Kα =0.154056nm) from 2 =5° to 70°. Raman spectra were obtained with a Renishaw inVia Reflex Raman spectrometer using laser excitation at 514.5 nm. Thermogravimetric analysis (TGA) was conducted with Pyris Diamond analyzer in the temperature range of 25-500 °C. The Brunauer-Emmett-Teller (BET) surface area analysis was done with a Micromeritics ASAP 2020M apparatus. Scanning electron microscope (SEM) and energy dispersive x-ray spectroscopy (EDS) images were captured with Hitachi S4800 and FEI Quanta FEG 250 field emission microscopies, respectively. X-ray photoelectron spectroscopy (XPS) measurements were carried out on an Axis Ultra DLD imaging photoelectron spectrometer. The powder samples were pressed into disks at 4MPa, and then their electronic conductivities were measured with a Cresbox four-probe meter and calculated.

Electrochemical measurements



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The active material, acetylene black and polyvinylidene fluoride (PVDF) binder were mixed in a weight ratio of 8:1:1 in N-methylpyrrolidone to form a homogeneous slurry. The slurry was uniformly spread onto an aluminium foil and vacuum dried at 60°C for 12 h. The electrode was punched into disks with a diameter of 14 mm to be used as the cathode material. CR2032–type coin cells were assembled in a glove box under argon atmosphere, with lithium metal as the anode. The electrolyte used was 1 M

lithium

bis(trifluoromethane)

sulfonamide

(LiTFSI)

in

1,3-dioxolane/dimethoxymethane (DOL/DME, 1:1 by volume). Galvanostatic charge/discharge tests were performed using a LAND-CT2001 instrument. The cyclic voltammetry (CV) was measured on an Salartron 1400 electrochemistry workstation at a scanning rate of 0.1 mV s-1 in a voltage window of 1.0-3.0 V vs. a Li/Li+ electrode. Electrochemical impedance spectroscopy (EIS)

was performed on the

coin cell via a Salartron 1400 electrochemical workstation with an applied voltage amplitude of 10mV, frequency varying from 100 kHz to 0.1 Hz.

3. Results and discussion The 3D boron doped graphene hydrogel was synthesized through a hydrothermal process of GO solution with the addition of a few boric acid. Here, GO sheets could self-assemble into 3D networks via π - π

interactions, hydrogen bonding,

coordination and electrostatic interactions35, while boric acid served as both boron source and reducing agent20. The change of the mixed solution before and after hydrothermal treatment was shown in Fig.1e. For comparison, the NGA was also


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prepared by the same method. The corresponding SEM images of the NGA and the BGA were shown in Fig.1a-d. It could be seen that the morphologies in the NGA and the BGA both obtained an interconnected, porous 3D network, but the BGA had a more compact porous structure on this scale. It is suggested that under basic conditions, the −COOH groups on the GO exists as −COO and can’t form hydrogen bonds36, thus the cross-link strength between GO sheets were impaired32, resulting in a relatively loose structure in the NGA37. Further XPS analysis in Fig.1f indicated that boron atoms were successfully doped into graphene as the two fitted peaks at 192.6 eV and 191.9 eV showed, which were ascribed to –BC2O,-BCO2 bond and –BC3 bond, respectively17, 38-41. The porosity of the BGA was further measured by N2 adsorption. As shown in Fig.2a, the sample exhibits a typical type IV isotherm with a H3 hysteresis loop, proofing that its structure was mainly composed of macropores and mesopores16, 42-43. The pore size distribution in Fig.2b revealed that the majority of the pores lied in the mesopore (2-50 nm) range with an average diameter of 7.1 nm. The sample had a Brunauer-Emmett-Teller (BET) specific surface area of 302.9 m2g-1 and pore volume of 0.702 cm3g-1. In contrast, the NGA sample obtains a BET specific surface area of 186.3 m2g-1, and its average pore size is 4.7 nm with a pore volume of 0.317 cm3g-1 (pore distribution plot is shown in Fig. S2). Fig.2c showed the XRD spectra of GO and BGA. The diffraction peak of GO was centered at 2 = 11°, corresponding to an interlayer distance of 0.803 nm, which was related to the oxygen-containing groups28. After the hydrothermal treatment, this peak entirely disappeared due to the



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deoxidization of GO, while a broad peak centering at 2 = 25° for the graphite (002) plane arises, proving the reduction of GO and the recovery of graphite crystal structure. The Raman scattering further characterized the disorder degree of the BGA, NGA and GA samples (Fig.2d). The spectra showed all the samples have a typical D band at around 1350 cm-1 and G band at around 1590 cm-1 for graphite materials. Besides that, the intensity ratio of D band to G band (ID/IG) increased from 0.97 for GA to 1.02 for BGA and 1.07 for NGA, declaring that the structural disorder of graphene increases upon the B-doping and N-doping17, 20, 44. The Raman result above was in accordance with our XPS analysis, which showed the heteroatom content in the samples were 7.6 atom% N in NGA and 1.7 atom% B in BGA. Table S1 is a comparison of the heteroatom content and the corresponding conductivity of GA, BGA, NGA powder. It is clear that the conductivity of graphene aerogel is improved with the introduction of N/B atoms, and the conductivity of NGA is higher than that of BGA. The BGA-S composite was prepared with a liquid infiltrating method at 155 °C. The sulfur content in the BGA-S was 59 % (sulfur loading calculated as 0.135 mg/cm2), as determined from the TG curves in Fig.3. The TG curve of NGA-S sample (Fig S2.) indicates a sulfur content of 47 % (sulfur loading calculated as 0.0705 mg/cm2), much lower than the BGA-S one. This might owe to the relatively low pore volume of small pores in NGA material, which mainly serve to confine sulfur elements. To verify the content in the BGA-S sample, elemental mapping of carbon, sulfur and boron were shown in Fig.4. As revealed, boron was homogeneously doped


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in graphene sheets and sulfur was uniformly infiltrated into the BGA material. Boron-doping is positively polarized, so it could chemically adsorb negative species on the surface of carbon framework30-31. For electron abundant species like sulfur and polysulfide, BGA material would effectively trap them within the graphene framework. To prove that, BGA-S, GA-S and NGA-S composites were synthesized using the same method. Their S 2p XPS spectra were demonstrated in Fig.5. We could see that the binding energy of sulfur in BGA-S was higher than those in GA-S and NGA-S, which indicated that the electron density of sulfur was partly distributed to the BGA framework, testifying the interaction between sulfur and BGA30. For electron negative polysulfides ( = 4 − 8) , such interaction would be even stronger30-31. Therefore, the ( = 4 − 8) species, generated during the discharge process, would be trapped by the BGA. In comparison, no such process occurs in GA-S and NGA-S composite, as a result, boron-doped graphene material offered distinct ability to chemically adsorb and stabilize polysulfides in the Li-S battery. To evaluate the electrochemical properties of the BGA-S material, the composite was assembled into typical 2032 coin cell with 1 M LiTFSI in DOL/DME as the electrolyte. Its cycle voltammogram and galvanostatic discharge/charge voltage profile were displayed in Fig.6a and b. From the CV test, we could see that there were two reduction peaks at around 2.3 V and 2.1 V during the discharge process, corresponding to the reduction of sulfur into polysulfides and the further reduction of polysulfides into /

45-46

. The discharge/charge voltage diagram was in



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good agreement with the CV profile, and the first discharge capacity reached 1290 mAh g-1, which was quite close to the first charge capacity, meaning that the coulombic efficiency of the battery approaches 100 %. Fig.7 showed the cycling performance of the BGA-S, NGA-S and GA-S cathodes. At 0.2 C (shown in Fig.7 a), the BGA-S electrode delivered a specific discharging capacity of 1290 mAh g-1 at the first cycle, upon further cycling, its capacity slightly decayed, but still maintained 994 mAh g-1 after 100 cycle. In contrast, the NGA-S cathode suffered from severe capacity fading during the cycling process (only 572 mAh g-1 upon 100 cycle), though it obtained a high discharge capacity at first cycle (1228 mAh g-1). The GA-S material did not have high capacity performance or good cycling stability neither. The cycling performance at higher rate (2 C) was illustrated in Fig.7b, it clearly implied that the BGA-S cathode obtained a discharge capacity of 882 mAh g-1 after few cycles, which might be ascribed to an activation process. Its capacity after 200 cycles was as high as 601 mAh g-1; such result was far more outstanding comparing with NGA-S (310mAh g-1) and GA-S (307mAh g-1) under the same conduction. The reason for the capacity decline could be ascribed to the loss of sulfur materials, which resulted from the dissolution of polysulfides in the electrolyte and the shuttle of polysulfides to the anode47. However, as for the BGA material, owing to the chemical adsorption ability of positively polarized boron atoms, the electronegative polysulfide anions were adsorbed in the BGA, which could recede the shuttle effect and contribute to the capacity48. Thus, the BGA-S material reveals an improved cyclability upon either low or high rate.


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Moreover, we tested its rate capability at varying rates (Fig.7c), at 0.1 C, 0.2 C, 0.5 C, 1 C and 2 C, the BGA-S electrode affords a capacity of 1243, 1028, 938, 864,and 808 mAh g-1, respectively. After the current density recovered to 0.1 C, a capacity as high as 1036 mAh g-1 was obtained, surpassing the NGA-S one. In order to further testify the effect of boron doping on the batteries, we conducted the electrochemical impedance spectroscopy (EIS) measurement taken at 2.8 V at the pristine state (before cycling) and the cycled state (after cycled for 15 times at 0.2 C). The plot was fitted according to an equivalent circuit (shown as inserted in Fig.8) which simulated the electrochemical models of Li-S battery49-50. The Re represented the resistance of the electrolyte and W was the Warburg impedance displayed as a slash in the low frequency range, while the diameter of the semicircle in the high frequency range showed the interface resistance Ri (combining charge transfer resistance Rct and SEI film resistance RSEI). From the fitted diagram, it was obvious that at the pristine state, the Rct of BGA-S was a bit bigger than that of NGA-S, which could be attributed to the better conductivity of NGA material (in accordance with the data in Table S1). After 15 discharge/charge cycles, the Rct of both BGA-S and NGA-S cathode witnessed a slight decrease, which implied the reduction of interface resistance resulting from the activation of cathode material. Upon the discharge/charge procedure, the polysulfides would slowly dissolve into the electrolyte, leading to the increase of Warburg impedance. On the one hand, for NGA-S cathode, the dissolution of polysulfides would bring about the increase of W shown in Fig.8; on the other hand, however, as boron atoms could effectively trap the



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polysulfides, its W kept almost steady during the circulation. The analysis of Warburg impedance consisted with the cycling performance of BGA-S composite, confirming the confinement of polysulfieds by BGA material.

Conclusions A boron-doped graphene aerogel material was successfully synthesized via a simple hydrothermal approach. The aerogel demonstrated a macroporous & mesoporous interconnected 3D structure with a boron content of 1.76 atom %. The BGA material was loaded with 59 % sulfur to serve as cathode in lithium-sulfur batteries. It showed a discharge capacity of 994 mAh g-1 after cycling at 0.2 C for 100 cycles. The capacity still reached 600 mAh g-1 at 2 C after 200 cycles. Considering the unique advantage of boron-doped graphene aerogel, we believe that the practical application of high energy density S cathode will be relived via optimization of the aerogel structure and amount sulfur loaded51-52.

Author Information Corresponding Author *Email: [email protected]. Phone: (+86) 574-87615697

Supporting Information Pore size distribution plot of NGA, TG curve of NGA and NGA-S, Content of heteroatoms in GA, BGA and NGA and their corresponding electric conductivity.


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Acknowledgements The authors are grateful for the financial support by the Ningbo 3315 International Team of Advanced Energy Storage Materials, Zhejiang Province Key Science and Technology Innovation Team (2013TD16), the National Natural Science Foundation of China (Grant No. 51371186), the “Strategic Priority Research Program” of the Chinese Project Academy of Science (Grant No. XDA09010201).

(a)

(b)

(c)

(d)

(e)

(f) B 1s

-BC2O,-BCO2

Intensity / a.u.

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


-BC3

200

198

196

194

192

190

Binding Energy / eV


188

186

184


Figure 1. SEM image of the microstructure of the NGA (a) and (b) and the BGA (c) and (d) at different magnifications. Photograph of the GO & H3BO3 mixture before (left) and after (right) hydrothermal treatment (e). (f) B 1s peak of XPS spectrum of BGA.

(a)

(b) 0.07

500

-1 3 -1

dV/dD / cm g nm

Quantity Adsorbed / cm g

2 -1

0.06 400

300

200

0.05 0.04 0.03 0.02 0.01

100

0.00 0 0.0

0.2

0.4

0.6

0.8

1.0

10

Relative Pressure / (P/P0)

100

Pore Size / nm

(d)

(c)

G

D

BGA Intensity / a.u.

BGA

Intensity / a.u.

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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NGA

GA

GO

10

20

30

40

2θ /

50

60

70

500

1000

o

1500

2000 -1

Raman Shift / cm


2500

3000

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Figure 2. (a) N2-sorption isotherm of BGA. (b) The pore size distribution plot of BGA. (c) XRD pattern of GO and BGA. (d) Raman spectra of GA, BGA and NGA.

BGA BGA-S

100

80

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


60

40

20 100

200

300 o

Temperature / C

Figure 3. TG curve of BGA and BGA-S from 25°C to 500°C.


400

500


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(a)

(b)

(c)

(d)

Figure 4. (a) SEM image of BGA-S (b-d) Corresponding elemental mapping of carbon, sulfur and boron.


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BGA-S

S 2p3/2

Intensity / a.u.

NGA-S GA-S

S 2p1/2

167

166

165

164

163

162

Figure 5. S 2p XPS spectrum of BGA-S, GA-S and NGA-S.

(b)

(a) 1.5

1 st 2 nd 3 rd

3.0

1 st 2 nd 3 rd

2.5

+

Voltage / V vs. Li /Li

1.0

Current / mA

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


0.5

0.0

-0.5

-1.0

2.0

1.5

1.0 1.0

1.5

2.0

2.5

3.0

0

200

+

Voltage / V vs. Li /Li

400

600

800

Capacity / mAh g

1000

1200

1400

-1

Figure 6. (a) Cycle voltammogram of BGA-S scanning at 0.1 mV s-1 (b) galvanostatic discharge/charge voltage profile of BGA-S.



(a) 1600

100 90

-1

Capacity / mAh g

80

BGA-S

1200

70 60

NGA-S 800

50

GA-S

40 30

400

20 10 0

Coulombic Efficiency / %

0 0

50

100

150

200

Cycle Number

(b) BGA-S NGA-S GA-S

1200

Capacity / mAh g

-1

1600

800

400

0 0

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Cycle Number

(c)

-1

1600

Capacity / mAh g

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BGA-S NGA-S

0.1C

1200

0.2C

0.5C

0.1C 1C

2C

800

400

0 0

10

20

30

40

50

60

Cycle Number

Figure 7. (a) Discharge capacities of BGA-S, NGA-S and GA-S at 0.2 C (b) Discharge capacities of BGA-S, NGA-S and GA-S at 2 C (c) Discharge capacities of BGA-S and NGA-S at varying rates from 0.1 C to 2 C.


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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


200

BGA-S 0 cycle BGA-S 15 cycle NGA-S 0 cycle NGA-S 15 cycle

100

0 0

100

200

300

Z' / ohm

Figure 8. Electrochemical impedance spectra (EIS) of BGA-S and NGA-S cathodes before and after 15 cycling at 0.2 C, charging to 2.8 V (plots are the experimental data, lines are the fitted data).



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Goodenough, J. B.; Kim, Y., Challenges for Rechargeable Li Batteries. Chem. Mater. 2010, 22 (3),

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Wang, D.-W.; Zeng, Q.; Zhou, G.; Yin, L.; Li, F.; Cheng, H.-M.; Gentle, I. R.; Lu, G. Q. M.,

Carbon-Sulfur Composites for Li-S Batteries: Status and Prospects. J. Mater. Chem. A 2013, 1 (33), 9382-9394. 5.

Yang, Y.; Zheng, G.; Cui, Y., Nanostructured Sulfur Cathodes. Chem. Soc. Rev. 2013, 42 (7),

3018-3032. 6.

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1600

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20 10 0

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Li ion

S ion

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Effect of Boron-Doping on the Graphene Aerogel ... - ACS Publications

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