Decoration of Silica Nanoparticles on Polypropylene Separator for

Feb 10, 2017 - Kai YangLei ZhongJiaxiang QinJunchen LiuMin XiaoDongmei HanShan RenLuyi SunShuanjin WangYuezhong Meng. ACS Applied Nano ...
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Decoration of silica nanoparticles on polypropylene separator for lithium-sulfur batteries Jing Li, Yudai Huang, Su Zhang, Wei Jia, Xingchao Wang, Yong Guo, Dianzeng Jia, and Lishi Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00065 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 13, 2017

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Decoration of silica nanoparticles on polypropylene separator for lithium-sulfur batteries Jing Li,1 Yudai Huang,1,* Su Zhang,1 Wei Jia,1 Xingchao Wang,1 Yong Guo,1 Dianzeng Jia,1,* Lishi Wang2 1

Key Laboratory of Energy Materials Chemistry, Ministry of Education; Key

Laboratory of Advanced Functional Materials, Autonomous Region; Institute of Applied Chemistry, Xinjiang University, Urumqi, 830046 Xinjiang, P. R. China 2

Tianjin EV Energies Co., Ltd., Tianjin 300380, Tianjin, P. R. China

KEYWORDS

SiO2 nanoparticles; Decoration; Polypropylene separator; Lithium-sulfur batteries; Electrochemical properties

ABSTRACT

SiO2 nanoparticle decorated polypropylene (PP) separator (PP-SiO2) has been prepared by simply immersing PP separator in the hydrolysis solution of tetraethyl orthosilicate (TEOS) with the assistance of Tween-80. After decoration, the thermal stability and the electrolyte wettability of the PP-SiO2 separator are obviously improved. When the PP-SiO2 separator is used for lithium-sulfur (Li-S) batteries, the

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cyclic stability and rate capability of the batteries are greatly enhanced. The capacity retention ratio of the Li-S battery configured with the PP-SiO2 separator is 64% after 200 cycles at 0.2 C, which is much higher than that configured with the PP separator (45%). Moreover, the rate capacity of the Li-S batteries using the PP-SiO2 separator reaches to 956.3 mAh g-1, 691.5 mAh g-1, 621 mAh g-1, and 567.6 mAh g-1 at the current density of 0.2 C, 0.5 C, 1 C, and 2 C, respectively. The reason could be ascribed to that the polar silica coating not only alleviate the shuttle effect, but also facilitate Li-ion migration.

INTRODUCTION

Li-S batteries are considered as promising secondary batteries because of their extraordinary theoretical capacity and high energy density

1-2

. However, the

long-chain polysulfides (Li2Sn, 4 ≤ n ≤ 8) formed during the reduction of sulfur with lithium tend to dissolve into the electrolyte. The dissolved long-chain polysulfides diffuse through the separator and react with the metallic Li anode, and are further lithiated to short-chain polysulfides (Li2Sn, 1 ≤ n ≤ 4), which will produce insulating Li2S and Li2S2 on the surface of metallic Li. Noticeably this shuttle effect will cause the loss of active materials, the passivation of anodic surface and severe self-discharge 3-7

. But the common PP separator could not relive the shuttle effect. To overcome this

issue, some investigations have been carried out and focus on the modification of PP separators 8-10.

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As the functional inserting layer, the modified separator could cut off the diffusion path of the polysulfides and relive the shuttle effect. For instance, the electrochemical performance of the Li-S batteries could be improved by replacing conventional PP separators to carbon coated PP separators 11-13. It may be attributed to the tunable pore structure and superior conductivity of these carbon materials, which benefit the physical adsorption and reutilization of the polysulfides, thus suppress the shuttle effect. In recent works, inorganic particles (Al2O3, MnO2, V2O5, etc.) oxide

16-17

, heteroatom-doped carbon materials

14-15

, graphene

18-19

, and conducting polymers

20-21

have also been used as coating layers to adsorb polysulfides. But the coating layers are easy to exfoliate, causing a relatively short lifespan 22. As for the other approaches to modify the separators, graft can effectively reduce the exfoliation. The inert surface of separators can be attacked by irradiation treatments to form active position and graft certain functional groups including gamma ray

24

23

. However, these high energy irradiation treatments

, plasma

25

, electron beam

26

, and UV irradiation

27-28

are

costly and difficult to scale up. And excess radiation dose, notably, could cause significant impairment of the polymer matrix 29. Hence, a nonradiation, low-cost and simple method is desired. Based on the experimental evidences and the theoretical simulation results, the researchers proposed that the polysulfides could be effectively adsorbed by metal oxide (SiO2, Al2O3, TiO2, etc.) because of the high binding energy between polysulfides and oxygen atom. Hou et al.

30

proposed that oxygen atom with

the lone pair electrons could strongly interact with polysulfides by dipole-dipole interaction.

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Herein, we prepared SiO2 decorated polypropylene (PP) separator (PP-SiO2) by simply immersing PP separator in the hydrolysis solution of tetraethyl orthosilicate (TEOS) with the assistance of Tween-80. After decoration, the thermal stability and the electrolyte wettability of the PP-SiO2 separator are obviously improved. When the PP-SiO2 separator is used for Li-S batteries, the cycling capacity and the rate capacity are greatly improved. The reason could be attributed to that the polar silica coating not only relive the shuttle effect by strong physicochemical interaction with the polysulfides, but also facilitate Li-ion migration by favorable electrolyte wettability. Moreover, we observed that the SiO2 nanoparticles are still tightly anchored on the separator after 100 cycling, which ensures the long term usage of the PP-SiO2 separator. This work provided a new way to prepare modified separator for high performance Li-S batteries.

RESULTS AND DISCUSSION

FTIR-ATR analysis of the separators The FTIR-ATR patterns of the PP and the PP-SiO2 separators are shown in Figure 1. The characteristic peaks locate at 2850-3000 cm-1, 1451 cm-1, and 1396 cm-1 in the two curves are attributed to the C-H stretching and bending vibrations

23

. The peaks

located at 1100 cm-1 and 955 cm-1 in the spectrum of the PP-SiO2 separator represent the asymmetrical stretching of the Si-O-Si and the Si-OH groups stretching derived from the cross-linked silica networks

23,33

. A broad peak at ca. 3330 cm-1 could be

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ascribed to the O-H bonds. Notably, the appearance of the shoulder peak at the left side of Si-O-Si peak indicates the formation of Si-O-C bond

34

. We proposed the

hydrophobic groups of the Tween-80 are strongly attached to the surface of the PP separator and the hydrophilic groups tend to react with hydroxyl of the cross-linked silica networks. So the SiO2 nanoparticles are decorated on the PP separator tightly with assistance of Tween-80.

Figure 1. FTIR-ATR spectra of the PP and the PP-SiO2 separators. Characterization of the separators Table 1. Thermal shrinkage and electrolyte uptake of the separators. Samples

PP

PP-SiO2

Thermal shrinkage ( % )

33.3

10

Electrolyte uptake ( wt. % )

93

149

Figure 2 shows the SEM images of the PP and the PP-SiO2 separators. The SiO2 nanoparticles are tightly anchored on the surface of the PP separator. Moreover, the SiO2 nanoparticles are still decorated on the PP separator after 100 cycles at 0.2 C (Figure 2c). From the elemental mapping in Figure 2d-f, the oxygen and silicon are homogeneously distributed on the separator after 100 cycles. It indicates that the

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PP-SiO2 separator is sufficient to long term usage.

Figure 2. SEM images of (a) the PP separator, (b) the PP-SiO2 separator before and (c) after 100 cycles at 0.2C; (d-f) The elemental mapping of the PP-SiO2 separator after 100 cycles at 0.2C. The thermal stability of the separator is considered as a vital aspect for Li-S batteries. The thermal shrinkage of the PP and the PP-SiO2 separators was investigated by annealing at 150 °C in air for 2 h. As shown in Figure 3, the shrinkage of the PP-SiO2 separator is ca. 10%, which is much lower than that of the PP separator (ca. 33.3%) (Table 1). From the FTIR-ATR results of the PP-SiO2 separator after thermal treatment in Figure S1, we found that the characteristic peaks of SiO2 become sharp and the skeleton structure of SiO2 has no change. The broad peak of O-H bonds at ca. 3330 cm-1 is disappeared, which is ascribed to cleavage of Si-OH bond when the temperature is up to 150 °C. The improved thermal stability is attributed to that the inorganic SiO2 layer maintains the integrity of the separator even though the annealing temperature is closed to the softening point of the PP separator (ca. 165 °C). The result suggests that SiO2 has a positive impact on improving thermal stability of

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the separator, which is in agreement with previously results

35-36

. Moreover, the

improved thermal stability of the PP-SiO2 separator could enhance the endurability and safety of the Li-S batteries at high operating temperature.

Figure 3. Photographs of the PP and the PP-SiO2 separators (a) before and (b) after thermal treatment at 150 °C for 2h. We also found that the hydrophilic SiO2 nanoparticles could facilitate the surface wettability and the electrolyte uptake of the separators. The PP-SiO2 separator could be fully wetted by the electrolyte, while the contact angle of the PP separator towards the electrolyte is ca. 35º (Figure 4). The electrolyte uptake of the two separators was measured by immersing both separators in the electrolyte for 2 h. The PP-SiO2 separator shows a higher electrolyte uptake (149 %) than the PP separator (93%) in Table 1. The good wettability and high electrolyte uptake could benefit interfacial compatibility, reduce the electrolyte filling time, and facilitate Li-ion migration, thus the rate capability and the cycling stability of Li-S batteries could be greatly improved 37-38

.

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Figure 4. Spreading of the electrolyte droplet and the corresponding contact angles on the PP and the PP-SiO2 separators. Electrochemical properties Figure 5a shows the cycle performance of the Li-S batteries assembled with the PP and the PP-SiO2 separators at 0.2 C. The discharge capacity increases from 852.3 to 937 mAh g-1 when the battery assembled with the PP-SiO2 separator instead of the PP separator. This is probably due to the good electrolyte wettability could facilitate Li-ion migration

38

. Though rapid capacity fading could be observed in both of the

batteries in the first 20 cycles, in the following cycles, however, the capacity fading of the batteries assembled with the PP-SiO2 separator is much slower than those assembled with the PP separators. This is attributed to the PP-SiO2 separator could reduce the irreversible loss of active materials

39-40

. The battery configured with the

PP-SiO2 separator still performs the specific capacity of 603.5 mAh g-1 with the decay rate of only 0.05% per cycle after 200 cycles. For the battery using the PP separator, capacity declines to 383.6 mAh g-1 with the decay rate of 0.16% per cycle. The cyclic stability of the Li-S battery using the PP-SiO2 separator is much better than that using the PP separator. Additionally, both batteries show coulombic efficiency of ca. 100 %.

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It may be because the passivation separator formed with LiNO3 could protect Li anode 41. The loading of SiO2 may be an important factor that determines the electrochemical performance of Li-S batteries

42

. According to the TG analysis in Figure S2, the

loading of SiO2 nanoparticles on PP separator is ca. 12.4%, which is an appropriate value for the separator because the extra high SiO2 content will block the ionic pathway of separator and the extremely low SiO2 content is unable to effectively relieve the shuttle effect 42. From Figure 5b, the discharge capacity of the Li-S battery using the PP-SiO2 separator reaches to 956.3 mAh g-1, 691.5 mAh g-1, 621 mAh g-1, and 567.6 mAh g-1 at the current density of 0.2 C, 0.5 C, 1 C, and 2 C, respectively. However, the discharge capacity of the Li-S battery using the PP separator is only 888.6 mAh g-1, 608.6 mAh g-1, 531.9 mAh g-1, and 421.6 mAh g-1. The introduction of SiO2 nanoparticles could enhance rate performance of the Li-S battery 25,33. It may be ascribed to the presence of SiO2 not only alleviate the shuttling effect by chemical confinement, but also benefit the interfacial compatibility and facilitate Li-ion migration. Figure 5c and d show the discharge/charge curves of the second circle at different rates from 0.2 C to 2 C. For the battery with PP separator, the difference of voltage plateaus at 0.2 C, 0.5 C, 1 C, and 2 C is 356 mV, 458 mV, 548 mV, and 687 mV, while the potential difference of the battery with PP-SiO2 separator is 307 mV, 352 mV, 440 mV, and 565 mV, respectively. It is obvious that the polarization of the battery with PP-SiO2 separator is lower than that with PP separator. According to that, we could

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deduce that the PP-SiO2 separator may relieve the shuttle of long-chain polysulfides and reduce the generation of insulating Li2S and Li2S2 coating on the surface of Li anode. EIS was carried out to investigate the kinetics of the Li-S batteries using different separators. The results are shown in Figure 5e and f. The semicircle at high frequency region corresponds to the charge transfer resistance and the straight line in the low frequency region represents the Warburg impedance

21, 43

. For the Li-S batteries

assembled with the PP and the PP-SiO2 separators, the charge transfer resistance decreases largely after 2 cycles. It is due to the electrochemical activation and rearrangement of active material 20. Notably, the resistance of Li-S batteries using the PP-SiO2 separator is lower than that of using the PP separator both before and after cycling because the PP-SiO2 separator could hold a lot of electrolyte and facilitate Li-ion migration 15.

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Figure 5. (a) Cycle performance of the Li-S batteries configured with different separators at 0.2 C; (b) Rate capability of the Li-S batteries with the PP and PP-SiO2 separators at various current rate. Charge-discharge curves of the second circle at different rates (0.2 C, 0.5 C, 1 C, and 2 C) of (c) PP and (d) PP-SiO2. EIS of the Li-S batteries with different separators (e) before cycling and (f) after 2 cycles. Confinement of polysulfides According to the previous reports

44-45

, oxides are believed to form S-O bond with

polysulfides by theoretical calculation with density functional theory. In order to investigate the strong interaction between SiO2 and polysulfides (Li2Sn), XPS was

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carried out on the PP-SiO2 separator after immersing in Li2S8 solution. As shown in Figure 6a, the two peaks appeared at ~162.5 and ~164 eV are ascribed to the terminal and bridging sulfur atoms in accordance with the chain structure of the Li2Sn 46. The peaks at ~167.2 and ~169.1 eV are assigned to the thiosulphate and the polythionate, respectively

47

. These two strong peaks are also considered as the S-O bonds,

revealing that the polysulfides could be strongly trapped by the decorated SiO2 nanoparticles during charge-discharge process. We deduced that the strong physicochemical interaction between the decorated SiO2 nanoparticles and the polysulfides could effectively relieve the shuttle effect, reduce the contact between polysulfides and metallic Li, improve the utilization of active material, and enhance the cyclic stability of the Li-S batteries 16,48. To further prove this point of view, the diffusion of polysulfides through the PP and the PP-SiO2 separators was investigated by means of V-shaped permeating matrix shown in Figure 6b and c. The polysulfides permeate through the PP separator immediately and the clean electrolyte at the right side turns to brown in 1 h. But for the PP-SiO2 separator, the clean electrolyte at the right side only turns to pale yellow. The slow diffusion rate of the polysulfides indicates that the PP-SiO2 separator could alleviate the shuttle effect.

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Figure 6. (a) S 2p XPS spectra of the PP-SiO2 separator after immersion in Li2S8 solution. Diffusion test of polysulfides in V-shaped permeating matrix with (b) the PP and (c) the PP-SiO2 separator in 1h. Based on the mentioned above, the as-prepared PP-SiO2 separator could significantly improve the electrochemical performance of the Li-S battery. The reason is ascribed to: (I) The strong physicochemical interaction between SiO2 and the polysulfides could relieve the shuttle effect, thus improve the electrochemical performance of Li-S batteries. (II) The superior electrolyte wettability and uptake of the PP-SiO2 separators benefit the interfacial compatibility, reduce the electrolyte filling time, and facilitate Li-ion migration.

CONCLUSION

In this work, a simple route is developed to decorate PP separator with SiO2 nanoparticles. The PP-SiO2 separator exhibits improved thermal stability and electrolyte wettability. Besides, the capacity of the Li-S batteries configured with the PP-SiO2 separators is 937 mAh g-1 at the first cycle and 603.5 mAh g-1 at 200 cycles at

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0.2 C, which is much higher than those configured with the PP separators. The reason for the improved performance is due to that the hydrophilic SiO2 not only traps polysulfides by chemical interaction, but also facilitates Li-ion migration by increasing the electrolyte wettability of the separator. This work provided a simple method to prepare advanced separators for high performance Li-S batteries.

ASSOCIATED CONTENT

Supporting Information

Experimental details. FTIR-ATR spectra of the PP and the PP-SiO2 separators after thermal treatment. TG analysis of the PP-SiO2 separator.

AUTHOR INFORMATION

Corresponding Author * [email protected]; [email protected] Funding Sources This work was financially supported by the National Natural Science Foundations of China (21301147 and 21466036), the High-Tech Project of Xinjiang Province (201515105), the Scientific and Technological Innovation Leading Talent Reserve of Xinjiang Province (wr2015cx02) and the Science and Technology Assistance Foundation of Xinjiang Province (201491128)

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