High sulfur loading in activated bamboo-derived porous carbon as a superior cathode for rechargeable Li–S batteries

Accepted Manuscript High Sulfur Loading in Activated Bamboo-Derived Porous Carbon as a Superior Cathode for Rechargeable Li−S Batteries Youmin Cheng, Shaomin Ji, Yuhui Liu, Jun Liu PII: DOI: Reference:

S1878-5352(15)00297-X http://dx.doi.org/10.1016/j.arabjc.2015.10.001 ARABJC 1778

To appear in:

Arabian Journal of Chemistry

Received Date: Accepted Date:

15 July 2015 1 October 2015

Please cite this article as: Y. Cheng, S. Ji, Y. Liu, J. Liu, High Sulfur Loading in Activated Bamboo-Derived Porous Carbon as a Superior Cathode for Rechargeable Li−S Batteries, Arabian Journal of Chemistry (2015), doi: http:// dx.doi.org/10.1016/j.arabjc.2015.10.001

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High Sulfur Loading in Activated Bamboo-Derived Porous Carbon as a Superior Cathode for Rechargeable Li−S Batteries Youmin Cheng,a Shaomin Ji,*a Yuhui Liu,a Jun Liu*b

a

Key Laboratory of Low Dimensional Materials & Application Technology, Ministry

of Education, School of Materials Science and Engineering, Xiangtan University, Xiangtan 411105, China Email: [email protected]

b

School of Materials Science and Engineering, South China University of Technology,

Key Laboratory of Advanced Energy Storage Materials of Guangdong Province Guangzhou 510641, China Email: [email protected]

1

ABSTRACT A novel type of porous carbon material for Li−S batteries was obtained by simple pyrolysis of natural bamboo waste at 850 C. The activated bamboo-derived carbon (A_BC) contains abundant micropores and mesopores, possessing a large surface area of 1565.4 m2 g−1 and total pore volume of 0.95 cm3 g−1, which are larger than vast majority of biomass materials. All these advantages contribute to improving sulfur loading in the A_BC matrix materials, and a high sulfur content of 86 wt% in the A_BC/S composite can be achieved. As the cathode for Li−S batteries, it displayed superior electrochemical properties, with an initial discharge capacity of 1160 mA h g−1 at 0.1 C (1 C = 1675 mA g−1) and 1050 mA h g−1 remained after ten cycles. Further cycled at 0.2 C for one hundred cycles, reversible capacity of 930 and 710 mA h g−1 were reserved for the first and the 100th cycle, respectively. Further increasing to 0.5 C and 1 C, it still showed capacities of 695 and 580 mA h g−1 with coulombic efficiency over 95%, suggesting this porous A_BC could be a superior carbon matrix for high sulfur loading as the cathode of rechargeable Li−S batteries.

Keywords: High sulfur loading, pyrolytic bamboo carbon, cathode, Li−S batteries, biomass

2

Introduction Among various types of Li-ion batteries (LIBs), rechargeable lithium-sulfur (Li−S) batteries have attracted extensive attention due to their high theoretical capacity of 1675 mA h g−1 and theoretical power density of 2600 W h kg−1, which are about five times greater than that of commercialized LIBs currently. Besides, the cathode active material of element sulfur is very abundant in nature, low cost and environmental friendly.1, 2 All of these advantages endow Li−S batteries as one of the most promising candidates for next generation of high energy density rechargeable batteries. However, in spite of these advantages, the practical application of Li−S batteries is still hampered by several problems, including the stability of lithium metal and the insulating nature of pure sulfur with extraordinary low conductivity of ~10−30 s cm−1 at room temperature.3 It is difficult to have sufficient electrochemical contact between the cathode active materials of sulfur and conductive carbon and the current collector, reducing the utilization of sulfur.4 In addition, the dissolution of long chain lithium polysulfides into the organic electrolyte, which causes irreversible capacity loss and low coulombic efficiency is another huge problem impeding the commercialization of Li−S batteries.5 These troubles will lead to poor cycle stability and short cycle life of Li-S batteries. To overcome these issues, considerable strategies have been proposed, of which most are generally focused on novel cell configurations, electrolyte additives and especially sulfur composite cathode materials with preferable nanostructure and morphology.6 For example, much progress had been made to improving sulfur

3

cathode performance with sulfur/porous carbon composite,4,7-11 sulfur/graphene composite12-14 and sulfur/polymer composite materials.15-17 Recently, sulfur/biomass composites have attracted extensive attention for their unique microstructure and morphology, low cost and easy accessibility.5,18-22 Biomass-derived carbon materials have good electronic conductivity, large specific surface area and large pore volume accommodating the volume changes, intensively applied as the anode materials for Li-ion.23-25 These results imply that the biomass-derived carbon materials may be suitable matrix for loading electroactive sulfur for Li-S batteries. Herein, we synthesized a sulfur-loading carbon matrix derived from natural bamboo waste. To further increase the specific surface area (SSA) and pore volume of the raw bamboo, it was firstly ground small and soaked with 1.5 M KOH solution for 5 days. The obtained activated bamboo-derived carbon (A_BC) has a large SSA of 1565.4 m2g−1 and a total pore volume of 0.95 cm3 g−1. With this highly porous carbon material, a kind of A_BC/S composite cathode with 86 wt% sulfur was designed, the obtained A_BC/S

composite

cathode exhibited stable and

superior

electrochemical

performances for Li−S batteries, which may attributed to its abundant porous and pipeline structure.

Experimental procedure Synthesis of porous carbon from natural bamboo waste Typically, the raw bamboo received from local forest was firstly broken into small flakes then treated with 1.5 M KOH solution for 5 days in porcelain crucible at

4

room temperature. After that the liquid was drained from the mixture, the dried product was transferred to a tube furnace with argon flowing and preheated to 300 C for 3 h to remove free and bound water stored in raw bamboo materials, which was then annealed at 850 C for 3 h with at a heating rate of 5 C min−1. After being treated, the residue was washed with HCl solution and distilled water several times to remove KOH and other soluble ions. Finally, the washed product was dried in air condition at 100 C for 10 h and fully ground into small carbon particles standby application.

Preparation of activated bamboo carbon/sulfur composite (A_BC/S) In a typical synthesis, 0.16g as-prepared porous bamboo-derived carbon was firstly scattered in 50 ml of deionized water containing a low concentration of polyvinyl pyrrolidone (PVP, Mw ~ B55,000, 0.02 wt%; at much higher PVP concentrations, hollow sulfur particles are formed instead)26 under vigorous stirring to form a homogeneous solution, then 10 g Na2S2035H2O was added into the solution, after stirring for 3 h, adequate hydrochloric acid was added slowly. The reaction proceeded at room temperature and milky colloidal solution was obtained after reaction for 4 h, the solution was centrifuged to get the product, finally the resultant black powder was heated at 155 C under vacuum for 3 h to fully impregnate the composite with the low-viscosity sulfur.

Materials characterization

5

To evaluate the characteristic of the A_BC/S composite, X-ray diffraction (XRD, GBC MMA, with Cu Ka radiation) was conducted to investigate the compositions of the as-prepared product. The morphology and energy dispersive spectroscopy (EDS) mapping of samples were obtained with field-emission scanning electron microscopy (FESEM, JEOL7500, 15 kV) and JEOL JSM-6610 scanning electron Microscopy (SEM), respectively. Raman spectroscopy was conducted using a JOBIN YVON HR800

Confocal

Raman

system

grating

at

room

temperature.

Nitrogen

adsorption-desorption isotherms for specific surface area and pore diameter analysis were collected with the instrument of TriStar II 3020. The sulfur content in the composite was estimated with a TGA Q50 V20.8 Build 34 under N2 atmosphere with a flow rate of 10 ml min−1 at a heating rate of 10 C min−1 from 30 to 500 C.

Electrochemical measurements The cathode slurry was made by mixing 70 wt% A_BC/S composite with 20wt% acetylene black and 10 wt% polyvinylidene fluoride (PVDF) binder

in

N-methyl-2-pyrrolidinone (NMP) solvent with ethanol as dispersant, after fully grinding, the slurries were spread onto aluminum foil substrates. Then the electrode slices were dried in a vacuum oven at 60 C for 12 h, soon cut into a disk film of 14 mm in diameter. The loading density of active sulfur material in the cathode was calculated as 2.0-2.5 mg cm−2. After that, those electrode films were assembled as Coin-type 2016 cells in a glove box filled with argon with lithium metal as the counter electrode and measured in the range of 1.0 ~ 3.0 V (vs. Li/Li+) at room

6

temperature. Microporous polypropylene film (Celgard 2300) was used as the separator, the electrolyte dissolved 1 M lithium bis(trifluoromethane) sulfonamide (LiTFSI) in 1,3-dioxolane-dimethoxyethane (DOL/DME) (1:1, v/v) prepared in a glove box filled with highly purified argon and the amount of added electrolyte per sulfur mass having an E/S = 10 uL/mg. 27 Galvanostatic charge/discharge cycling tests were used to evaluate the electrochemical capacity and cycle stability of the electrodes by the NEWARE-BTS instrument (Shenzhen, China), cyclic voltammetry (CV) measurement was measured by the CHI 660D electrochemical workstation (Shanghai Chen Hua, China) at a scan rate of 0.1 mV s−1. The EIS was also recorded by the same instrument over the frequency ranging from 100 kHz to 0.1 Hz.

Results and discussion XRD patterns of pure sulfur, A_BC and A_BC/S-86% are displayed in Fig. 1. By comparison, it is obvious that the A_BC has no any sharp peaks, indicating the A_BC is mainly the amorphous carbon. However, the XRD pattern of the A_BC/S-86% includes sharp peaks matching with the standard value (JCPDF NO. 08-0247) of pure sulfur and the broaden peaks matching with the A_BC at the same degree. Furthermore, the pattern of the A_BC/S also reveals that sulfur exists in a mixed state of both amorphous and crystalline phases,28 which may due to the sulfur content in the composite is rather high, and some sulfur particles still retained on the surface of the composite after heated. Previous research shows that when sulfur content in the composite is low, no clear peaks of the sulfur can be observed because sulfur have

7

become amorphous distributed into the matrix.29 TGA was carried out to further confirm the accurate sulfur content in the composite of A_BC/S displayed in Fig. 2. Normally, pure sulfur starts to vaporize at about 200 C and the rate of evaporation becomes particularly fast between 200~300 C, finally finished at about 330 C. The weight loss of A_BC/S composite results from the evaporation of sulfur stored in pores of A_BC matrix. The content of sulfur in the composite is measured to be 86 wt%, exceeding most of the previous related reports.18, 30-34 As we know, a high sulfur loading in the cathode provides higher energy densities and tend to more practical for its commercial application. As shown in Fig. 3a, abundant pores and pristine vascular bundle structure retained from the raw bamboo can be obviously observed, which can facilitate infiltration of the electrolyte and acts as large storage container when sulfur content is very high.5 The morphology of the A_BC/S-86% composite displayed in Fig. 3b is clearly different from Fig. 3a, without any pores can be observed and the surface of the composite becomes smoother. Moreover, some sulfur particles aggregated on the surface of composite as well, which mainly due to the sulfur content in A_BC/S composite is relatively high and the pores in the A_BC matrix are fully filled with sulfur. As illustrated in Fig. 3c, abundant micropores in the A_BC can be observed in the high-resolution TEM (HRTEM) image, which is consistent with the N2 adsorption/desorption results (Fig. 4). Dense of micropores can alleviate the dissolution of long chain lithium polysulfides. At the same time, it can be observed that the sulfur homogeneously dispersed within the pores of A_BC from the TEM

8

image (Fig. 3d), but some large sulfur particles are still observed. In order to further verify the distribution of sulfur in the composite, EDS mapping analysis was carried on the A_BC/S-86% composite, and the corresponding elemental mapping images for sulfur and carbon are shown in Fig. 3f and 3g, respectively. As illustrated in Fig. 3f and 3g, the bright yellow spots stand for the element S and the black spots represent the element C. We can find a uniform coverage of sulfur on the external surface throughout the whole area, with only a small amount of bare carbon. These test results demonstrate that vast majority of sulfur has been well diffused into the pores of A_BC with small portion covering on the surface of the composite. The homogeneously distribution of sulfur in the composite can benefit to improving the performance of Li-S batteries. The specific surface area analysis of the A_BC was performed by nitrogen adsorption measurements, and the corresponding pore size distribution was calculated by the Horvath–Kawazoe method. Fig. 4a shows that A_BC has a typical microporous structure.7 The specific surface area of A_BC is 1565.4 m2 g−1, mainly contributed by micropores and mesopores within the matrix. As we know, high specific surface area and large pore volume can contribute to accommodating high sulfur content, shorten the distance for charge transport and provide more reactive sites to improve the utilization of active sulfur material. 19, 35 As shown in Fig. 4b, the SSA of A_BC/S (86 wt%) composite is only about 11 m2 g−1, and the pore volume decreases from 0.95 cm3g−1 to 0.028 cm3g−1 immediately, which mainly caused by the pores within the A_BC were filled with sulfur particles.5 The curve of the pore size distribution (Fig.

9

4c) indicates the A_BC mainly possesses micro-mesopores, displaying a narrow pore size distribution in the range of 1.5 ~ 5 nm, which are consistent with the observation in HRTEM image (Fig. 3c). These abundant micropores with high surface area enable sufficient contact between the insulating S and conductive A_BC. Moreover, they can act as porous reactors and restrict the dissolution of long chain lithium polysulfides into the organic electrolyte.18 However, in the curve of A_BC/S (86 wt%) composite displayed in Fig. 4c, no any pores can be seen, which are highly consistent with the SEM image in Fig. 3b. Due to the strong physical adsorption of capillary force, the sulfur impregnated into the pores of A_BC matrix can maintain stable. The Raman spectrum of A_BC was conducted to further confirm the nature and type of the as-prepared carbon (A_BC), as shown in Fig. 5, the two major Raman bands are located at around 1325 and 1600 cm−1, which are identified as the D band and G band of graphene nanosheets, the peak height ratio of ID/IG is 0.94, indicating the graphitization phase plays a large role in the as-prepared carbon of A_BC,36 which can promote the electrical conductivity of A_BC.33 Cyclic voltammetry testing was conducted to investigate the electrochemical mechanisms of the as-prepared A_BC/S cathode materials. Fig. 6 shows the CV curves of the A_BC/S-86% cathode at a scan rate of 0.1 mV s−1 for the first three cycles. In the first cycle of the cathode reduction process, two peaks at approximately 2.35 V and 2.0 V, respectively. The first reduction peak (2.35 V) corresponding to the reduction process of elemental sulfur to long chain lithium polysulfides (Li2Sx, 4 < x < 8), the other peak (2.0 V) is attributed to the conversion of long chain lithium

10

polysulfides to short chain lithium polysulfides or even to insoluble Li2S.12 In the subsequent anodic scans, only one oxidation peak is seen at around 2.38 V, which can be ascribed to the conversion from short chain lithium polysulfides to long chain lithium polysulfides and sulfur.4, 37 More importantly, from the second and the third cycles onward, the position and areas of the CV peaks remain rather constant and similar shapes upon cycling, indicating the excellent electrochemical stability of the A_BC/S electrode.38,39 As shown in Fig. 7a, the A_BC/S-86% composite cathode cycled at different current rates increased from 0.1 to 1 C and returned back to 0.2 C (1 C = 1675 mA g−1) in the voltage range of 1.0 ~ 3.0 V, the battery exhibited great rate capability and cycling stability. An initial capacity of 1160 mA h g−1 was obtained at 0.1 C, further cycled at 0.2, 0.5 and 1 C rates, it also showed stable reversible capacities about 930, 695 and 580 mA h g−1, respectively. When the rate returned to 0.2 C, 850 mA h g−1 was obtained again. Fig. 7b shows the charge/discharge profiles of the A_BC/S-86% composite cathodes, two obvious plateaus present in the discharge curves, corresponding to the two different reduction processes for the A_BC/S composite cathode. Furthermore, it is observed that the plateaus become shorter and lower along with the growth of rates and cycling number shown in Fig. 7b and 7d. All these discharge curves have a large increased capacity from 1.5 V to 1.0 V, which mainly attributed to the deep reduction from short chain polysulfides to the final products of Li2S and Li2S2. And the long-cycling performance of the A_BC/S-86% cathode at 0.2 C shown in Fig. 7c, an initial capacity of 930 mA h g−1 is obtained and the capacity decays relatively fast for the initial 30 cycles, which mainly due to the

11

loss of active sulfur and the dissolution of long chain polysulfides into electrolyte, then the capacity curves tend to be horizontal and a reversible capacity of over 710 mA h g−1 still remained even after one hundred cycles with a stable coulombic efficiency of about 95%. Fig. 7d displays the charge/discharge profiles of the cathode at 0.2 C, we can see these plateaus are very long and stable, which further confirms the stability and great rate capability of A_BC/S composite cathodes. Moreover, the cycle performance and multi-rate capability comparison of the A_BC/S and pristine sulfur are displayed in Fig. 7e and 7f, respectively. In order to investigate the changes of electrode morphology before and after cycling, the SEM images of lithium fresh, after first and 100 cycles are shown in Fig. 8a, 8b and 8c, respectively. As shown in Fig. 8a, the lithium fresh is very flat and smooth, and the surface of lithium metal after the first cycle also keeps tight and intact shown in Fig. 8b. However, it is obviously observed that some thick passivation layers covered on the surface of lithium metal after 100 cycles displayed in Fig. 8c, which are attributed to the deposition of insoluble reduction products, these passivation layers can protect lithium anode and provide Li−ion pathway, which are beneficial to achieving good rate capability and long-term cycle stability for Li−S batteries.20 Fig. 8d and 8e reveal the morphology of A_BC/S-86% composite before cycling and after 100 cycles, respectively.

It can observed that the surface of cathode becomes unfairness and lots

of corrosive holes attached to it after 100 cycles, which may resulted from the loss of active materials and the dissolution of long chain lithium polysulfides along with cycling.

12

To make a further investigation, electrochemical impedance spectroscopy (EIS) was measured to study the dynamics for lithium insertion and extraction during cycling. As shown in Fig. 9, after the first one cycle, the impedance spectrum is composed of a medium-to-high frequency semicircle and a long inclined line (Warburg impedance) in the low frequency region. The semicircle is attributed to the charge transfer process at the interface between the electrolyte and sulfur electrode, the straight line represents the Li ions diffusion resistance within the cathode.14,36 After 100 cycles, the impedance spectrum added a new small resistance, including two different semicircles followed by a short sloping line. Generally, the high-frequency semicircle results from the interfacial charge transfer process, and the semicircle in the medium frequency range is related to the solid–electrolyte-interface (SEI) film resistance caused by the formation of Li2S or Li2S2 on the surface of cathode, and the straight line is attributed to the diffusion of Li ions in the matrix.14,40 The EIS results show that after the first cycle, the resistance of Rct is about 34 ohm, demonstrating that the composite of A_BC/S has great electrical conductivity, which is beneficial to the rate capability of the battery. However, after 100 cycles, the resistance of the cell is larger than that after the first cycle, which may result from the dissolution of part long chain lithium polysulfides during both discharge and charge processes, reducing the migration speed of Li-ions in the electrolyte. As the activated bamboo carbon has a high surface area of 1565.4 m2 g−1, total pore volume of 0.95 cm3 g−1 and abundant micro-mesopores, so it can store a high content of sulfur and prevent long chain lithium polysulfides from dissolving into the organic electrolyte.41

13

Conclusions In summary, we have successfully synthesized a novel type of carbon from natural bamboo waste through simple activation and pyrolysis routes. The as-prepared A_BC possesses total pore volume of 0.95 cm3 g−1 and high specific surface area up to 1565.4 m2 g−1. With this carbon materials, the cathode of A_BC/S composite with a high sulfur loading (86 wt%) was designed and exhibited good rate capability and cycling stability, with an initial discharge capacity of 1160 mA h g−1 at 0.1 C, even after one hundred cycles at 0.2 C, 710 mA h g−1 is still retained. Further cycling at 0.5 C and 1 C, reversible capacities of 695 mA h g−1 and 580 mA h g−1 can be reserved with a high Coulombic efficiency, respectively.

Acknowledgements This work was financially supported by National Natural Science Foundation of China (11202177, 51202207).

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16

20

30

(357)

(319 )

(044 )

(113)

10

A_BC

(206 )

(222 )

Intensity(a.u.)

A_BC/S-86%

40 50 2degree)

Sulfur JCPDF No. 08-0247

60

70

80

Fig. 1. XRD patterns of the standard data of sulfur (brown), A_BC (blue) and A_BC/S-86% composite (red).

17

100

Pure sulfur A_BC/S-86%

Weight (%)

80 60 40 20 0

0

100

200

300

400

500

Temperature (°C)

Fig. 2. TGA-traces of pure sulfur and A_BC/S-86% composite from room temperature to 500 C at a heating rate of 10 C min−1 under N2 atmosphere.

18

Fig. 3. SEM (a, b) and TEM (c, d) images of A_BC (a, c) and A_BC/S-86% (b, d); (e-g) EDS mapping of the region displayed in Fig. 3e for sulfur mapping (f) and carbon mapping (g). The inset of Fig. 3a shows photo image of the typical bamboos grown in the central south of China (Xiangtan, Hunan province).

19

(b) 20 Adsorption Desorption

600

3 Quantity Adsorbed (cm /g STP)

3 Quantity Adsorbed (cm /g STP)

(a) 650

550 500 450 400 350 300 250

Adsorption Desorption 15

10

5

0 0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

Relative Pressure (P/P0)

(c)

0.6

0.8

1.0

0.35 A_BC A_BC/S (86 wt%)

0.30

3 -1 -1 dV/dD (cm nm g )

0.4

Relative Pressure (P/P0)

0.25 0.20 0.15 0.10 0.05 0.00 2

4

6

8

10

Pore Diameter (nm)

Fig. 4. N2 adsorption/desorption isotherms of A_BC (a), the A_BC/S-86 wt% (b), and the corresponding pore size distribution calculated by the Barrett-Joyner-Halenda method (c).

20

Table 1. Physical characterization of A_BC and A_BC/S composites Samples

SBET

Total pore volume

(m2g−1)

(cm3g−1)

A_BC

1565.4

0.95

A_BC/S-86%

10.92

0.028

21

G band

Intensity(a.u.)

D band

500

1000

Raman shift/ cm

1500

2000

-1

Fig. 5. Raman Spectrum of the activated bamboo-derived carbon (A_BC).

22

0.8

Current (mA)

1st 2nd 3rd 0.4

0.0

-0.4 1.0

1.5

2.0

2.5

3.0

Voltage (V)

Fig. 6. Cyclic voltammograms of the A_BC/S-86% composite cathode cycled between 1.0 and 3.0 V (vs. Li/Li+), recorded at a potential scanning rate of 0.1 mV s−1.

23

100

0.2 C

60

0.2 C

800

0.5 C

40

1C

600

20

400 200 10

20 30 Cycle number

40

1.5

0

Charge Discharge 80 Efficiency

1000

60

0.2 C

800

40

600

20

400 200 0

20

40 60 Cycle number

1200 A_BC/S S

0.2 C

800 600 400

400 600 800 1000 1200 1400 Capacity (mAh/g) 100th,50th,25th,10th, 1st

2.5

2.0

1.5

1.0 0

(f)

200

400 600 Capacity (mAh/g)

1200 1100

Capacity (mAh/g)

1000

3.0

0 100

80

200

(d)

Voltage (V)

Capacity (mA h/g)

2.0

1.0

100

1200

0.1 C 0.2 C 0.5 C 1C

50

(c) 1400

Capacity (mAh/g)

2.5

0 0

(e)

(V)

1000

3.0

+

0.1 C

Voltage vs Li/Li

Charge 80 Discharge Efficiency

Coulombic efficiency (%)

Capacity (mAh/g)

1200

(b) Coulombic efficiency (%)

(a) 1400

800

1000

A_BC/S S

0.1 C

1000 0.2 C

900

0.2 C

800 0.5 C

700

1C

600 500

200

400 0 0

10

20

30

40 50 60 70 Cycle number

80

0

90

10

20 30 Cycle number

40

50

Fig. 7. Electrochemical performances of A_BC/S-86% cathode for Li−S batteries cycled between 1.0 and 3.0 V versus Li/Li+: (a) rate capability; (b) charge-discharge profiles at different current densities; (c) cycle performance of the A_BC/S-86% cathode at 0.2 C; (d) charge-discharge profiles of the battery at 0.2 C; (e,f) cycling performance and multi-rate capability of A_BC/S and pristine sulfur cathodes.

24

Fig. 8. Surface morphologies of Li fresh anode (a), after the first one cycle (c), and after 100 cycles (c); surface morphologies of A_BC/S-86% cathode before (d) and after 100 cycles (e).

25

60 1st 50

100th

-Z"/

40 30 20 10 0 20

40

60

80

100 120 140 160 180 200

Z'/

Fig. 9.

Nyquist plots of the A_BC/S after the first one cycle (green) and the 100th

cycle (red), measured with the frequency ranged from 100 kHz to 0.1 Hz.

Table 2. Impedance parameters simulated from the equivalent circuits

Cycle number

Resistance

1st

100th

26

(ohm)

Re

2.5

Rct

34

Re

5.5

Rct

69.4

Rs

23.8

TOC Biomass bamboo-derived porous carbon with a surface area up to 1565.4 m2 g−1 and total pore volume of 0.95 cm3 g−1 was synthesized via simple activation and pyrolysis routes. This porous carbon matrix can load electroactive sulfur as high as 86 wt%, which still delivered very stable cycling performance and great rate capability.

100

0.1C

1200

Capacity (mAh/g)

Charge Discharge Efficiency

80

0.2C

60

0.2C

900 0.5C 1C

40

600

300

20

Biomass bamboo-derived carbon/S (86 wt%) cathode

0

0 0

10

20 30 Cycle number

27

40

50

Coulombic efficiency (%)

1500