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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