Insight into the Electrode Mechanism in Lithium-Sulfur Batteries with Ordered Microporous Carbon

Confined Sulfur as Cathode

Zhen Li1, Lixia Yuan*1, Ziqi Yi1, Yang Liu1, Ying Xin2, Zhaoliang Zhang2, Yunhui Huang*1

1Key Laboratory for Advanced Battery Materials and

System (MOE), School of Materials Science and Engineering, Huazhong University of Science and

Technology, Wuhan, Hubei 430074, China 2Shandong Provincial Key Laboratory of Fluorine

Chemistry and Chemical Materials, School of Chemistry and Chemical Engineering, University of Jinan, 106 Jiwei

Road, Jinan, Shandong 250022, China

Introduction Sulfur is one of the most promising cathode candidates

for next generation batteries due to high theoretical capacity (1675 mAh/g), natural abundance, low cost and environmental friendliness. However, the realization of rechargeable Li-S batteries suffers from low sulfur utilization and poor cycle life. Extensive efforts have been done to solve the shortages. Among them, the composites with sulfur embedded within conductive meso / micro-porous carbon framework have been proven promising. However, a precise principle how to choose proper electrolytes for various S cathodes with different supporting matrices and different morphologies is urgently needed, i.e., the criteria for electrolyte selection for Li-S batteries should be set up. Unfortunately, to our knowledge, hitherto no work has been reported on this issue.

In this work, we prepared a highly ordered microporous carbon and confined sulfur within it as cathode for investigating the electrochemical performances of S2-4 and S8 in both ether-based and carbonate-based electrolytes. A preliminary electrolyte selection principle can be further proposed for Li-S batteries based on the experimental results. Results and Discussion

Fig. 1 shows that the diffraction peaks of sulfur still exist in FDU/S-60, indicating that there is redundant sulfur outside the pores of the porous carbon, while FDU/S-40 shows only a broad amorphous peak, demonstrating that the sulfur on the carbon surface has almost been washed off, and the remanent sulfur is well confined within the micropores. From fig. 1 (inset), it can be seen that a certain amount of element sulfur adheres on the carbon particles in the FDU/S-60 sample, while for FDU/S-40, the composite particles almost have the same shape with the pure FDU carbon particles and no extra sulfur is observed.

10 20 30 40 50 60 70 80

FDU

FDU/S-40

FDU/S-60

Inte

nsit

y (

a.u

.)

2 Theta (degree)

S

Fig. 1 XRD patterns of FDU, FDU/S-40, FDU/S-60,

and S; inset: SEM images of FDU/S-60 and FDU/S-40.

Fig. 2 shows the electrochemical behaviors of FDU/S-60 and FDU/S-40 in two typical electrolytes, respectively. For the ether-based electrolyte (fig 2a–d), the discharge

capacities of FDU/S-60 and FDU/S-40 are both higher than 900 mAh/g. The charge-discharge plateaus at different voltages correspond to different electrochemical reactions. For the carbonate-based electrolyte (fig 2e), the FDU/S-40 cathode shows almost the same discharge-charge capacity and plateaus as in the ether-based electrolyte, whereas the situation of FDU/S-60 is totally different. For FDU/S-60, the discharge capacity in the 2nd cycle decreases from 900 mAh/g in the ether-based electrolyte to less than 600 mAh/g in the carbonate-based electrolyte, and the plateaus at about 2.3 V and 2.0 V disappear. From the cycle performance comparison (fig. 2f), it is found that the combination of the small S2-4 molecules (FDU/S-40) and the carbonate-based electrolyte delivers better cyclability.

0 200 400 600 800 10001.0

1.5

2.0

2.5

3.0

Vo

ltag

e (

V,

vs L

i+/L

i)

Specific capacity (mAhg-1)

Li2S4 Li

2S2

Li2S

S4

0 200 400 600 800 10001.0

1.5

2.0

2.5

3.0

Vo

ltag

e (

V,

vs L

i+/L

i)

Specific capacity (mAhg-1)

Li2S8

S4

Li2S4

Li2S2

Li2S

S8

1.0 1.5 2.0 2.5 3.0-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Cu

rren

t (m

A)

Potential (V, vs Li+/Li)

1st cycle

2nd cycle

3rd cycle

1.0 1.5 2.0 2.5 3.0-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Cu

rren

t (m

A)

Potential (V, vs Li+/Li)

1st cycle

2nd cycle

3rd cycle

(a) (b)

(c) (d)

(e)

0 200 400 600 800 1000 12001.0

1.5

2.0

2.5

3.0

Vo

ltag

e (vs

Li+

/Li)

Specific capacity (mAhg-1)

FDU/S-40

FDU/S-60

(f)

0 20 40 60 80 100 120 140 1600

200

400

600

800

1000

1400

1600

FDU/S-60-DME/DOLFDU/S-60-EC/DMC

FDU/S-40 DME/DOL

Sp

ec

ific

Ca

pa

cit

y (m

Ah

/g)

Cycle number

FDU/S-40-EC/DMC

Fig. 2 CV curves and the 2nd charge-discharge profiles

of (a, c) FDU/S-60 and (b, d) FDU/S-40 with ether-based DME/DOL electrolyte. (e)The 2nd charge-discharge

profiles comparison of FDU/S-60 and FDU/S-40 with carbonate-based EC/DMC electrolyte. (f) The comparison

of cycle performance with two kinds electrolytes at a current density of 500 mA/g.

Fig. 3 shows the typical cycle life of FDU/S-40 in the

carbonate-based electrolyte. The specific capacity of FDU/S-40 is as high as 900 mAh/g after 280 cycles. This indicates that the carbonate-based electrolyte can match well with the sulfur-microporous carbon cathode materials, and promises an excellent electrochemical performance.

0 40 80 120 160 200 240 2800

300

600

900

1200

1500

1800

2100

2400

FDU/S-40 Charge

FDU/S-40 Discharge

Sp

ecif

ic c

ap

acit

y (

mA

h/g

)

Cycle number

Current density: 100mA/g

50

60

70

80

90

100

110

Co

ulo

mb

ic e

fficie

ncy (%

) Fig. 3 cycle life of FDU/S-40 in the carbonate-based

electrolyte of 1 M LiPF6 in EC/DMC in the voltage range of 3.0-1.0 V vs Li+/Li at current densities of 100 mA/g

In addition, our work proposes a simple strategy of

electrolyte selection for sulfur cathode. The ether-based electrolyte is proper for open-type composite cathodes, and the carbonate-based electrolyte is only suitable for microporous structured sulfur composite cathodes.

Abstract #42, 224th ECS Meeting, © 2013 The Electrochemical Society