Oxygen plasma modified separator for lithium sulfur battery

Abstract

To reduce the shuttle effect of lithium sulfur (Li–S) batteries and improve the cycling stability, the surface of the commercial separator in an Li–S battery was modified by the O₂ plasma treatment, which generated lots of electronegative oxygenic functional groups such as –COOH and –OH on the surface of the separator. In order to confirm the existence and the effect of the electronegative oxygenic functional groups on the modified separator, the contact angle measurement and Fourier Transform Infrared Spectrometry (FTIR) were investigated. The charge/discharge and electrochemical impedance spectroscopy (EIS) tests of the Li–S battery assembled with the normal and the O₂ plasma treated separator were analyzed. The surface characterization demonstrated that the oxygenic functional groups on the surface of the separator by the plasma modification played a critical role in improving the wettability and increasing electrical insulating properties. The cycling performance of Li–S batteries with the plasma treated separator had an obvious improvement owing to the electrostatic repulsion between electronegative oxygenic functional groups on the surface of O₂ plasma treated separator and electronegative polysulfide. The battery assembled with O₂ plasma treated separator had a higher capacity retention (48.53%) than that of the normal separator (24.51%).

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1. Introduction

The rechargeable Li–S battery has achieved considerable attention in recent years due to its high theoretical specific capacity (1675 mA h g⁻¹) and high specific energy (2600 W h kg⁻¹),^1,2 which promotes its use as a potential power source for electric vehicles, portable electronics and efficient storage for renewable energy.² Compared with the lithium-ion battery, the Li–S battery is not only of low cost because of the natural abundance of sulfur (almost 3% of the earth's mass) but also environmentally friendly.³ Upon discharging, S reacts with Li through a two-electron reduction process to form polysulfide intermediates (Li₂S_x, 2 < x ≤ 8, eqn (1)), and to generate Li sulfide (Li₂S₂ and/or Li₂S, eqn (2)) at the end of discharge. The reverse reaction occurs when an external electric field with a certain potential difference is produced, leading to the decomposition of Li₂S back to Li and S.^4,5

S₈ + Li⁺ + e⁻ → Li₂S_x(2 < x ≤ 8) (1)

Li₂S_x + Li⁺ + e⁻ → Li₂S₂ and/or Li₂S (2)

However, there are still some disadvantages hindering the application of Li–sulfur battery,³ such as the poor conductivity of sulfur/lithium sulfides,⁶ the volume change during the charge–discharge process,⁷ and the capacity decay induced by the shuttle effect.^3,6,7 The shuttle effect is caused by the diffusion of high order polysulfide to the Li foil anode side, which leads to the loss of sulfur materials and reduces the Coulombic efficiency.⁸

To solve the problems mentioned above, many researchers have focused on sulfur cathode modification. Loading sulfur to various nanomaterial has been widely reported to suppress the shuttle effect of polysulfide intermediates, such as graphene-based composites,^9–11 mesoporous carbon,^12,13 hollow carbon spheres,^14,15 yolk–shell TiO₂ spheres¹⁶ and so on. But there still is lack of the research on modifying separator. Cui and co-workers proposes that coating a conductive layer onto the separator can improve the battery performance of Li–S battery.¹⁷ At the same time, the Li–S battery has high energy density and excellent flexibility owing to an integrated structure of sulfur and graphene coated on a commercial separator.¹⁸ In addition, polydopamine coated separator could form a physical barrier to block the pathway of polysulfides.^19,20 Separator as an important component of battery, is used to separate the cathode and anode,²¹ which should possess an excellent insulativity to avoid short circuit. But, it should possess a favorable ionic conductivity to ensure a better electrochemical property. Furthermore, the separator should be dimensionally stable, and preferably not shrink or swell significantly when immersed into the electrolytic solution.²² Meanwhile, a better wettability to electrolyte solution can provide a better circulation property to the battery.²³

Plasma as one of the four fundamental states of matter, the others being solid, liquid and gas, has very different properties from the other states. Plasma can be created by heating a gas or subjecting it to a strong electromagnetic field by a laser or microwave generator.^24–26 In this work, the plasma-assisted modification on the commercial separator (Celgard 2400, thickness 25 μm) was reported to produce a high-performance separator for practical applications in Li–S batteries. As shown in Fig. 1, the modified separator by the O₂ plasma would introduce oxygenic functional groups onto the surface of separator.²⁶ From the reaction mechanism, plasma induced the crosslinking reaction surface of the polypropylene, due to the side groups of polypropylene (–CH), the space tension generated would weaken the primary C–C bonds in the discharge process. Meanwhile, it tends to break the C–C and C–H bonds and oxygenic functional groups would generate in the broken bond site such as –COOH, –OH and other oxygenic functional groups,²⁷ which could constrain the electronegative polysulfides in the cathode to avoid shuttle effect and improve the cycling stability. This is the first study on the separator modified by the O₂ plasma for rechargeable Li–S batteries. Our research suggests that plasma modification exhibits a promising potential to be used to improve the cycling stability and efficiency for Li–S batteries.

Fig. 1 Schematic illustration of O₂ plasma treated separator process.

2. Experimental

2.1. Preparation of O₂ plasma modified separator

Firstly, the commercial separator (Celgard 2400) was put in the plasma generator chamber (K-mate VERG-500). And then O₂ was introduced into the reactor with controlled pressure under 50 Pa. The applied RF power was set at 100 W. The time of treatment was changed to control the extent of modification on the surface of separator from 0.5 to 2 min. Once the treatment time is beyond 2 min, the separator was very easily damaged by plasma and that is over-modification.

2.2. Preparation of sulfur cathode

Graphite oxide (GO) was produced by natural graphite powder using the modified Hummers method.²⁸ Graphene derived from GO by sintering GO at 800 °C for 2 h under Ar atmosphere. The as-prepared graphene and sulfur powder were mixed in mortar with the quality ratio of 1 : 4. In order to confirm sulfur loading into the graphene, the mixture was further sintered in the tube furnace at 155 °C for 6 h under Ar atmosphere.² Electrode material was made of as-prepared G/S composite material, PVDF and Super-P with a mass ratio of 8 : 1 : 1, which was dissolved in the NMP. The homogeneous-dispersed slurries were coated onto aluminum foil substrates followed by being dried at 60 °C for 12 h and then roll-pressed.

2.3. Assembly of Li–S batteries

CR2032 type coin cells were assembled in a glove box. The electrolyte consisted of 1 M LiN(CF₃SO₂)₂ (LiTFSI) and 1% LiNO₃ in a mixed solvent of dimethyl ether (DME) and 1,3-dioxolane (DOL). The cells contained normal separator or as-prepared O₂ plasma treated separator and lithium foils as both the counter and reference electrodes.

2.4. Characterization

The content of sulfur was characterized by thermal gravimetric analysis (TGA, Henven HTG-1) under Ar atmosphere and heated until 500 °C with a heating rate of 10°C min⁻¹. Contact angle measurement was tested by contact angle measuring instrument (KRUSS DSA100). IR was measured by Fourier Transform Infrared Spectrometer (FTIR WQF-410), and the spectral range was 300–4000 cm⁻¹. The galvanostatic charge and discharge tests were carried on battery test instrument (LAND CT2001A) in a voltage range of 1.7–2.8 V (vs. Li/Li⁺). Electrochemical Impedance Spectroscopy (EIS) was carried out by Frequency Response Analyzer (FRA) technique on Autolab Electrochemical Workstation over the frequency range from 100 kHz to 0.01 Hz with the amplitude of 50 mV.

3. Results and discussion

To investigate the chemical change of the separator after the plasma treatment, we performed the FTIR characterizations on the samples. Fig. 2 shows the FTIR spectra of the normal separator and O₂ plasma treated separator. FTIR analysis is intended to confirm the functional groups of the separator, which exhibits the characteristic peak of the commercial separator: a triple peak at 2849.15 cm⁻¹, 2918.57 cm⁻¹ and 2955.54 cm⁻¹, single peak at 1167.68 cm⁻¹, 1376.42 cm⁻¹ and 1461.68 cm⁻¹ and a double peak at 972.71 cm⁻¹ and 997.66 cm⁻¹. However, compared with the normal commercial separator, the O₂ plasma treated separator shows two obvious extra peak in the FTIR spectrum, one of that is at 3422.20 cm⁻¹ and the other is at 1712.01 cm⁻¹. The peak at 3422.20 cm⁻¹ is the characteristic peak of –OH and the peak in 1712 cm⁻¹ is the characteristic peak of –COOH, which further confirms that new oxygen-containing functional groups were generated after the O₂ plasma treatment. This result demonstrates that the oxygenic functional groups like –OH and –COOH have been effectively introduced onto the surface of the separator by the O₂ plasma treatment.

Fig. 2 FTIR spectra of normal and O₂ plasma treated separator.

XPS spectra clarify the chemical composition and surface electronic state in the surface of the normal separator and O₂ plasma treated 1 min separator. XPS survey spectrum of normal separator is shown in Fig. 3a, which shows a distinct peak of C 1s at 284.82 eV. Fig. 3b shows the XPS survey spectrum of O₂ plasma treated separator. Compared with the normal separator, in addition to the distinct peaks of C 1s at 284.8 eV, the O₂ plasma treated separator exhibit a strong peak at 532.5 eV, which corresponding to O 1s. High-resolution XPS for C 1s is shown in Fig. 3c, in which the spectrum can be fitted into four peaks at the binding energy 284.85, 285.40, 286.03 and 287.87 eV, which were assigned to C–C/C–H, C–OR, C=O and COOR groups, respectively. We suppose that under O₂ plasma treatment, some C–C and C–H bonds at the surface of separator tend to be broken and oxygenic functional groups would generate. Through the XPS spectra, we can ensure that oxygenic functional groups such as C–OR, C=O and COOR was effectively introduced onto the surface of the separator via O₂ plasma treatment.

Fig. 3 XPS survey spectrum of (a) normal separator and (b) O₂ plasma treated separator, and high-resolution scans of (c) C 1s.

Furthermore, we used the contact angle measurement to investigate the surface properties of the separator before and after the plasma treatment. Fig. 4a shows that the contact angle of water on the normal separator is 125.4°. But from Fig. 4b we can clearly see that the contact angle of water in the O₂ plasma treated separator is only 64.9°. Fig. 4c and d shows that the wetting behavior of electrolyte on the normal separator and O₂ plasma treated separator. The contact angle of water and electrolyte on the O₂ plasma treated separator is also smaller than that on the normal separator. The contact angle results indicate that there is a significant change of the surface property of the separator, and the separator modified by the O₂ plasma has better wettability as compared to the normal separator. The contact angle measurements of the normal commercial separator and the O₂ plasma treated separator are shown to clarify the effect of oxygenic functional groups on the surface property of the separator. This consequence reveals that the surface energy of the O₂ plasma treated separator was changed due to the presence of oxygenic functional groups on the surface of the separator. Therefore, it is expected that the presence of oxygenic functional groups on the surface of the separator makes it possible to have high surface energy to be wetted more sufficiently in the electrolyte solution.²⁹ The wettability of the separator plays a critical role in the battery performance because the separator with excellent wettability can effectively maintain the electrolyte solutions and facilitate the electrolytes to diffuse well into the cell.^29,30 After O₂ plasma treated, the polar component like –OH and –COOH were introduced onto the surface of the separator, which would enhance the polarity of the separator. The improving of polarity and higher surface energy of the O₂ plasma treated separator can enhance the interfacial adhesion between electrodes and separator, and consequently improve the cycling performance of the Li–S battery, as investigated below.

Fig. 4 Contact angle measurement of water on (a) untreated normal separator and (b) O₂ plasma treated separator; wetting behavior of electrolyte on (c) the normal separator and (d) O₂ plasma treated separator.

The microscopic structures of separator were characterized by SEM, as given in Fig. 5. From the images, we can infer that both the normal separator and the plasma-treated separator have a regular porous structure. By comparing the SEM images in Fig. 5a and b, it could be observed that the porous structure of normal separator were more intensive than O₂ plasma treated separator. This phenomenon is resulted from the plasma treatment in a low vacuum environment; and under O₂ plasma treatment, the surface structure of separator has changed, and then the structure of pores in the separator become looser. We consider that the looser porous structure will enhance the ionic mobility in the charge and discharge process of the battery, and this conclusion is corresponding to the result of the contact angle, which became smaller after plasma treatment. From the high-resolution SEM images in Fig. 5c, we can see that there were some superficial cracks on the surface of normal separator, those cracks were generated by the high voltage during the SEM testing. In contrast, the surface of O₂ plasma treated separator, shown in Fig. 5d, is smoother than normal separator. We deduce that the separator will show stronger tenacity after O₂ plasma treatment due to the generation of the oxygenic functional groups and we can infer that the separator after plasma treatment is more stable than normal separator under high voltage.

Fig. 5 SEM image at low magnification of (a) normal separator and (b) O₂ plasma treated separator, at high magnification of (b) normal separator and (d) O₂ plasma treated separator.

In order to examine the performance of the plasma-modified separator in Li–S batteries, we used graphene–sulfur composite as the cathode. Fig. 6a shows that the content of sulfur in the graphene/sulfur composite material was 71.28 wt% by TGA. Fig. 6b presents typical discharge/charge profiles for the initial two cycles at the 0.2C rate (1C = 1675 mA g⁻¹) within the voltage window between 1.7 and 2.8 V. The two plateaus correspond to the formation of long-chain polysulfides (Li₂S_x, 4 < x ≤ 8) at around 2.3 V and short-chain Li₂S₂ and Li₂S near 2.1 V. The charge/discharge curves in Fig. 6b indicate that the initial specific capacity of the battery using plasma-treated separator is higher than using normal separator. At the same time, Fig. 6c and d shows the plasma treated separator also leads to higher specific capacity in Li–S battery than that using normal separator after 50 and 100 cycles, indicating the efficiency of the O₂ plasma treatment to improve the performance of the separator.

Fig. 6 (a) TGA of graphene/sulfur, the sulfur in graphene with the content of 71.28%; the charge/discharge voltage profiles of the Li–S batteries with normal and O₂ plasma treated separator after (b) 1st; (c) 50th and (d) 100th cycle.

In order to further study the electrochemical performance of the Li–S battery with the O₂ plasma-modified separator, galvanostatic cycling curves are shown in Fig. 7a, which shows the cycling performances of the batteries with normal and O₂ plasma separator at the 0.2C rate. The battery with normal separator delivers an initial discharge specific capacity of 973.8 mA h g⁻¹ with capacity retention of 24.51% after 105 cycles. Compared with the normal separator, the battery with plasma treated separator has a better cycling performance and its initial discharge specific capacity is 1028.2 mA h g⁻¹ with capacity retention of 48.53% due to the presence of –OH and –COOH introduced onto the surface of the separator by plasma.

Fig. 7 (a) The cycling performance comparison of the Li–S battery with normal separator and O₂ plasma treated separator for 1 min; (b) the cycling performance of Li–S battery assembled by the separator treated by O₂ plasma with different time.

During the charge/discharge cycling, the soluble polysulfide was generated and it could dissolve in the electrolyte and migrate to the Li anode, which was called as “shuttle effect” of polysulfide. However, when the oxygenic functional groups were introduced onto the surface of separator, the penetration of the electronegative polysulfide through the separator becomes difficult due to electrostatic repulsion by the electronegative oxygenic functional groups on the surface of separator, which results in the enhanced stability of the Li–S battery with the plasma-modified separator. We also investigate the effect of the different plasma treatment time as shown in Fig. 7b, we can clearly find that the separator treated by O₂ plasma for 1 min has the best cycling performance. The shorter treatment time could not induce enough oxygenic functional groups, while the longer treatment time can damage the structure of the separator. A suitably O₂ plasma treated separator can obviously enhance the cycling performance of the Li–S battery.

The electrochemical impedance spectroscopy is an efficient method to investigate the electrochemical behavior of batteries. Fig. 8a shows that the EIS curves of the batteries with normal and O₂ plasma treated separator. The resistance of the battery with normal separator is larger than O₂ plasma treated separator, which reflects that the ionic mobility property of O₂ plasma treated separator is better than normal one. After the O₂ plasma-induced process, the structure of pore in the surface of separator become looser. The looser porous structure will affect the transfer of ion, thus the ionic conductivity will increase. Furthermore, electrochemical impedance spectra were also measured after 105 cycles and show it in Fig. 8b, there shows two obvious semicircles and the both of two semicircles of the battery with O₂ plasma treated separator is still smaller than the normal one. The semicircle at high frequencies is reflects the resistance of the solid-state and insulating interface layer which formed on the surface of the electrodes, the semicircle at medium frequencies is reflects the faradaic charge transfer resistance and its double-layer capacitance.³¹ During charge and discharge process, some insulating discharge products (Li₂S and Li₂S₂) will accumulate in the cathode and formed a passivation layer. Fig. 7b have shown that the passivation layer is more less when assembled battery with O₂ plasma treated separator, this is due to the electronegative oxygenic functional groups on the surface of separator can suppression the rally of Li₂S and Li₂S₂. Besides, a better ionic conductivity property of separator will enhance the cycle performance of the battery.

Fig. 8 EIS of the Li–S battery with the normal separator and O₂ plasma treated separator (a) before cycling and (b) after cycling.

4. Conclusions

In summary, the separator has successfully modified by O₂ plasma, and the cycling performance of Li–S battery with the as-modified separator has been significantly improved. The surface modification of the separator by the O₂ plasma which induces oxygenic functional groups on the surface of separator plays a critical role in improving the performance of Li–S battery. The O₂ plasma treated separator exhibits good wettability with electrolyte and enhancement of the ionic conductivity property. The battery assembled with separator treated by O₂ plasma also shows an obvious improving of cycling performance by the electrostatic repulsion between electronegative oxygenic functional groups and electronegative polysulfides. Consequently, the Li–S battery assembled with the O₂ plasma separator exhibits better cycle performance compared to the normal separator. This study suggests that the performance of the Li–S battery can be prominently enhanced by the plasma technology. The separator modified by the plasma treatment has an enormous potential to be used as a high performance separator for Li–S battery.

Acknowledgements

This work was supported by National Natural Science Foundation of China (Grant No. 21573066, 51402100, 50702020 and 81171461), Youth 1000 talent Program of China (Grant No. 531109020032), and Inter-discipline Program (Grant No. 531107040755), Natural Science Foundation of Hunan (NSFH, Project No. 11JJ4013), Provincial Science & Technology Project of Hunan (Project No. 2013GK3155) and the Fundamental Research Funds for the Central Universities.

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