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A polyelectrolyte binder for sulfur cathode to improve the cycle performance and discharge property of Lithium-Sulfur battery Zhixiong Yang, Rengui Li, and ZhengHua Deng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01163 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018
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A polyelectrolyte binder for sulfur cathode to improve the cycle performance and discharge property of Lithium-Sulfur battery Zhixiong Yang , , Rengui Li , and ZhengHua Deng * 1. Chengdu Institute of Organic Chemistry, the Chinese Academy of Sciences, Chengdu, 610041, China
2. University of Chinese Academy of Sciences, Beijing, 100039, China *Correspondence to [
[email protected]]
KEYWORDS: “Lithium-Sulfur battery; Polyelectrolyte; Sulfur Cathode; Aqueous Binder; Discharge”
ABSTRACT:
In order to achieve the higher capacity and the better cycle performance of the lithium-sulfur (L-S) batteries, a copolymer electrolyte prepared via emulsion polymerization was used as the binder for the sulfur cathode in this study. This polyelectrolyte binder has uniform dispersion and good conductivity in the cathode that can improve the kinetics of sulfur electrochemical reactions. As a result, the capacity and cycle performance of the battery are improved evidently when the cell is discharged to 1.8 V. Moreover, when the cell is discharged to 1.5 V, the difficult deposition of will take place easily at 1.75 V, and the difficult transformation from solid to solid will progress smoothly and completely during the 1 / 27
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voltage range of 1.55-1.75 V, too. The capacity of this L-S battery discharged to 1.5 V is as much as 1700ℎ , which is very close to the theoretical value of sulfur cathode. The knowledge acquired in this study is valuable not only for the design of an efficient new polyelectrolyte binder for sulfur cathode, but also the discovery that the discharge degree is the main fact that limits the capacity to reach its theoretical value.
1. Introduction Rechargeable lithium-sulfur (L-S) batteries are very attractive systems because of its highest theoretical capacity of 1675ℎ , and the sulfur used as cathode material has advantages of low cost, high nature abundance, and nontoxicity1-3. However the cycle performance of Li-S batteries is still unsatisfactory because it has hampered mainly by two issues: first, the electrically insulating and the insoluble final reduction products of and , those will impede the cell reaction occurring at the electrode-electrolyte interface4-5; second, the soluble intermediate reduction products, the lithium polysulfides ( , x=4-8), will diffuse into the electrolyte solution and react with the lithium negative electrodes. This continuous reduction of moieties by the anodes avoids their reoxidizing back to elemental sulfur at the cathode side upon charging, known as the notable “shuttle effect”6-8. To address the aforementioned problems, a large number of approaches have been explored to overcome the low conductivity of sulfur and to physically restrain polysulfide dissolution using carbon barrier materials9. These approaches are overwhelmingly devoted to fabricating varieties of sulfur-carbon composite cathodes,
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however, it seems to be not enough because the physical adsorption force between nonpolar carbon substrate and polar lithium polysulfides is weak10-11. Although some unique structures to physically define sulfur and chemically bind with lithium polysulfide were designed and achieved great success in retarding the shuttle effect and presented an improved cycle performance, their experimental processes are often complex and cost-ineffective 10, 12-14 . In our previous work, we employed a lithium bis(trifluoromethane) sulfonamide (LiTFSI) salt as one of the components of the lithium sulfur-salt composite cathode prepared by the simplest ball-milling method. It proves that the ionic environment of cathode plays an important role in improving the electrochemical kinetics performance, resulting in a higher capacity retention15. At the same time, polymer has a promising use in the Li-S battery, including the binders16 So, we here describe a new simple and promise method for the preparation of the sulfur cathode using a polyelectrolyte binder (PEB) with core-shell architecture prepared via emulsion polymerization, which is displayed in Figure 1a. The core of PEB mainly consists of the flexible random terpolymer that has good compatibility with the electrolyte, and the shell mainly consists of sodium benzene sulfonate that has good disassociation in the electrolyte to assist ionic transportation. Therefore, there will be a good conductivity for the PEB soaking in the electrolyte. Besides, because of the flexibility and the compatibility of PEB, the emulsion can undergo a phase transformation from the colloid to homogeneous phase following the water evaporation. Thus the PEB can uniformly disperse in the cathode and tightly adhere 3 / 27
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with the active materials. As Figure 1b shows, in the process of discharge, the fixed polyanions soaked in electrolyte can increase the concentration of and improve the conductivity around the sulfur-carbon composite by electrostatic attraction, avoiding the diffusion concentration gradient produced by the consumption of sulfur discharge reactions. At the same time, new discharge behaviors of sulfur cathode were discovered in our cell system, which benefits from this conductivity improvement very much. Those new discharge behaviors are a new clear discharge plateau at 1.75V and a discharge slope between 1.75V and 1.55V, which correspond to the transformations from lithium polysulfide to and from to , respectively. The capacity of this sulfur cathode discharged to 1.5V (1700ℎ ) is very close to the theoretical capacity. This discovery demonstrates that the actual capacity of the sulfur cathode that much lower than the theoretical value is mainly limited by the fact that the elemental sulfur in the cathode cannot be completely reduced to during discharge. 2. Result and discussion The Scanning Electron microscopy (SEM) images of PEB and PVDF sulfur cathodes at initial state and discharged to 1.8V are presented in Figure 2. At the initial state, the PEB uniformly disperses in the cathode (Figure 2a), however, the PVDF undergoes a phase separation to become the thin films in the cathode after the solvent evaporation (Figure 2b), and the films bend due to the volume contraction during drying. After the cathodes have been discharged to 1.8 V, for the PEB cathode (Figure 2c), except for the cubical dilatation caused by the formation of the reduction products, 4 / 27
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nothing else changed obviously. But in the PVDF cathode (Figure 2d), the PVDF films were stretched and even broke by the cubical dilatation of active materials. These results indicate two quite different adhesive mechanisms of these two binders, which have different effects on the battery performances. The specific discharge capacities of PEB and PVDF cathodes are plotted against cycle number in Figure 3a. The initial discharge capacity of PEB cathode is 1400ℎ , and still remains 1008 ℎ after 100 cycles at 200 mA per gram of sulfur, the capacity retention is 72%; however, the cathode using PVDF as binder has an initial capacity of 1280 ℎ , and remains 802 ℎ after 100 cycles at the same current density, the capacity retention is 62.7%. As shown in Figure 3b, the PEB cathode has two higher discharge plateaus and two lower charge plateaus than PVDF cathode in every cycle, indicating the much lower polarization of PEB cathode than that of PVDF cathode. And this polarization is mainly caused by diffusion hysteresis. During the charge, the external voltage forces the reduction products to be oxidized to product that diffuses into the electrolyte. But the diffusion needs to be activated and increases gradually due to the impedance from interfaces and others. So, at the beginning, the rate of the transportation cannot catch up with the rate of the production, resulting in the polarization potential. Therefore, as Figure 3c shows, the voltage of the first charge profile for PVDF cathode has a sudden strongly increase at the very beginning. Then, as the diffusion is activated and accelerates, the accumulated begin and increase to be consumed. As a result, the polarization 5 / 27
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potential drops slowly with this process, the voltage of the first charge profile for PVDF cathode slowly drops down in Figure 3c. We call it interim polarization potential （ ） during this time. Finally, the rate of the diffusion reaches its maximum value, the amount of accumulated stop to decrease, the polarization potential keeps at a minimum value during the following charge. We call it stable polarization potential（ ）. So, the diffusion hysteresis is the key reason for the polarization. Therefore, it is vitally important to improve the transportation between the electrolyte and the active materials for solving this problem. As mentioned in the introduction, the PEB that uniformly coating on the active materials has strong polar groups that well contact with the electrolyte can provide a good conductivity, and the negative charge of the polyanions in it can attract the from the high concentration domain by electrostatic force to eliminate the concentration gradient. As a result the accumulation won’t happen in the PEB cathode during charge, the polarization potential will be eliminated. As shown in Figure 3c, the voltage of PEB cathode during first charge doesn’t suddenly increase at the beginning and then slowly drop like that of PVDF cathode, it increases slowly and steadily and is lower than PVDF cathode. The voltage difference value between these two charge profiles at the beginning is the interim polarization potential （ ）, and when the voltage of PVDF cathode stop to drop, this difference value is the stable polarization potential（ ）. This analysis is also suitable for discharge process shown in Figure 3d, Because of the hindered diffusion, the initial potential of discharge for PVDF cathode is 6 / 27
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lower than that of PEB cathode, this voltage difference is the interim polarization potential （ ）, then as diffusion increasing gradually, the voltage of PVDF cathode drops more slowly than PEB cathode. Namely, the voltage difference between the discharge profiles of these two cathodes gets smaller and smaller. Finally the diffusion achieves the fastest speed, the voltage difference reaches minimum value, that is the stable polarization potential（ ）. For the second discharge, the beginning part of the PEB cathode profile has a good overlap with that of first discharge, which indicates the polarization potential is very small. On the contrary, the initial voltage of PVDF cathode at second cycle gets higher than that of first discharge, namely the and become smaller than those at first discharge. This could be considered as the result of an easier diffusion that activated by the first cycle. To get further insight into the effect of the PEB on the cathode, electrochemical impedance spectra （EIS） and SEM of the sulfur cathodes after discharged to 1.8 V and 100 cycles are conducted and presented in Figure 4. In the Figure 4a and b, the Nyquist curves of the sulfur cathodes that first discharged to 1.8V and after 100 cycles all consist of one semicircle in high frequency region, one quarter circle in median frequency region and a straight line in the low frequency region. According to the literature17-20, the resistance of the semicircle at the higher frequency reflects the charge transfer process at interfaces, and the semicircle at median frequency related to the formation of the !" film. The equivalent circuits are shown in the inserts of Figure 4a and b. In the circuit, the #$ represents the impedance of the resistance of the electrolyte, the #%& is the charge transfer resistance at the interface 7 / 27
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of the conductive agent. The CPE1 is used instead of double-layer capacitance, CPE2 describes the space charge capacitance of the or film, and #( is the resistance of the or film. Wo is the Warburg impedance due to the diffusion of the polysulfides in the cathode. The fitting results of the Nyquist Plots are list in Table 1. Whether it is discharged to 1.8 V or after 100 cycles, the #%& values of the PEB cathodes are distinctly smaller than those of PVDF cathodes. This indicates that the PEB cathode has a better conductive network, which is related to its distribution in the cathode. These results are consistent with the facts shown in the SEM images of the cathodes after discharge to 1.8 V (Figure 4c and d). It is very clear that the non-heterogeneous distribution of the PVDF in the cathode decreases the continuity of the conductive network, so that the PVDF cathode has a bigger #%& . As to the resistance of the !" film (#( ), when the cathodes were discharged to 1.8 V, the #( of the PVDF cathode is very close to that of PEB. However, after cycling for 100 times, the R( of the PEB cathode doesn’t increase evidently, which is consistent with the fact that the porous architecture of the PEB cathode remains unchanged after 100 cycles shown in Figure 4e, comparing with the same porous architecture of it discharged to 1.8 V (Figure 4c). But the R( of the PVDF cathode has distinctly increased from 17.41Ω to 51.41Ω. It can be clearly seen by comparing the structures of PEB cathode and PVDF cathode after 100 cycles presented in Figure 4e and f that the porous structure of the PVDF cathode has changed evidently: all gaps in the cathode are fully filled with the depositions. During the repeat cycling, if the transportation cannot catch up with the electrochemical 8 / 27
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reactions, the insufficiently reacted sulfur at the inner-layer will migrate to the electrolyte to continue the next reactions15, 21-22, therefore the reaction region will grow bigger and bigger towards the electrolyte, like what happened in the PVDF cathode (Figure 4f), the porous will be fully filled. The consequence of this structure change is an evident #( increase. But for PEB cathode, benefitting from the improved conductivity by PEB that around the sulfur, enough can be transported from the electrolyte to the sulfur and other intermediate reduction products to finish the reduction reactions in the limited space near the carbon, so they will stay in a fixed region rather than diffusing into the electrolyte, therefore the porous architecture of PEB cathode can keep unchanged, resulting the much lower #( and better cycle performance. Furthermore, we believe that this good conductivity of PEB may produce a more positive effect on the kinetics of the sulfur electrochemical reactions. To study this effect, the rate capability, cyclic voltammetry and discharge test were chosen. The rate capability of these two PEB and PVDF sulfur cathodes is presented in the Figure 5a, when the current density increases from 0.6C to 1C, the PEB battery has a capacity drop that much lower than that of PVDF battery, and a better capacity recover when the current density decreases from 0.6C to 0.4C and 0.2C. This result shows the kinetic of PEB cathode is better than PVDF cathode. The first cyclic voltammetric curves of these two PEB and PVDF sulfur cathodes with different scan rates and a voltage range of 1-3V are presented in Figure 5b. All cathodes show discharge behaviors that different from many other reports during the 9 / 27
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cathodic scan. Except for the two typical reduction peaks positioned around 1.9 V and 2.2 V, a new reduction peak is shown around 1.6 V in their curves. Benefitting from the improved kinetic, the discharge peaks of the PEB cathode shift to the lower potential much more shortly than those of the PVDF cathode with the increasing scan rates. The shifts of PVDF cathodes are so big that the peaks can be distinguished from each other harder and harder with increasing scan rate. As to the new discharge peaks around 1.6 V, according to its shape and area, it cannot be assigned to the reductions of the additives ()*+ and methylbenzene) in the electrolyte, because their contents in the cell are small, and although they can be reduced at potentials lower than 1.8V vs. / , their main reduction potential are lower23-26, so the weaker current during the scan from 1.4-1.0V can be related to these reductions of additives in the electrolyte. This can also be proved by the discharge test of these two cathodes discharged to 1.5V at the current density of 200mA per gram of sulfur that displayed in Figure 5c. The discharge plateau according to this new peak at ~1.7 V still exist at second and third discharge, while the reductions of the additives are irreversible. The capacity attenuation of the discharge processes during 1.75-1.5V is mainly caused by the irreversible side reactions and polarization. By comparing their first discharge curves below 1.8V, their profiles are different. The PEB cathode has a short discharge plateau at 1.75 V and a discharge slope between 1.75 V and 1.55 V. The capacity of this new discharge process is approximately 300ℎ , and the capacity of the whole discharge process of this cathode discharged to 1.5 V is approximately 1700ℎ , which is very close to the theoretic capacity of sulfur. For the PVDF 10 / 27
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cathode, because of the polarization potential, the new short discharge plateau is lower and the discharge slope is steeper. Thus, the discharge plateau at 1.75V can be assigned to the deposition of - → , and the discharge slope during 1.55-1.75V is related to the reductions of additives and the transformation from to in solid states. The differences of the profiles indicate that these two processes can progress smoothly and completely in the PEB cathode. It is needed to be point out that the additives ()*+ 22-24, 26
and methylbenzene25) in the electrolyte may also have significant impacts on
the electrochemical reactions of the cathodes, so that the new discharge reaction at 1.6V can also took place in the PVDF cathode. Nevertheless, the improved kinetic by the PEB around the active materials to eliminate the polarization is still the dominant factor for these new discharge reactions to progress smoothly and completely in the PEB cathode. This will be proved in the following XPS results of these two cathodes. To fully understand these different discharge processes, the XPS was utilized to study the chemical valence states of sulfur in the cathodes that discharged to 1.8V and 1.5V. The XPS spectra of S2p level for the cathodes is displayed in Figure 6. When the cathodes were discharged to 1.8 V, there are no peaks associate with the formation of
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and
29
in the binding energy range from 161eV to 164eV in both
PVDF cathode (PVDF 1.8V) and PEB cathode (PEB1.8V). However, after the cathodes were discharged to 1.5 V, two peaks at 163.7eV and 162.4eV related to the formation of and 22 are found in the spectrums of PVDF cathode (PVDF1.5V) and PEB cathode (PEB1.5V). Moreover, to compare the relative 11 / 27
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strength of the peaks of and in the spectrums, it is clear that the PVDF cathode contains appreciable quantities of , although it contains more . However, the amount of in the PEB cathode is very small, almost all product is consisted of . This shows the final reduction products of sulfur only form when the cathode is discharged to 1.5 V in this system, and the PEB cathode achieves a deeper degree of the sulfur reduction reactions for almost all final product is rather than . So, the short discharge plateau at 1.75V in Figure 5 b could be assigned to the reaction of - → , and the discharge slope between 1.75V and 1.55V could be assigned to the transformation from to in solid states. The HRTEM images of the cathodes of PEB and PVDF discharged to 1.8V and 1.5V shown in Figure 7 provide a directive insight of the differences of this new discharge process between them. When the cell is discharged to 1.8V (Figure 7a and 7b), there are some bigger lumps on the carbon particle clusters. In the Figure 7c and 7d, the fine structure near the carbon particles is displayed. For the PEB cathode (Figure 7c), two different crystal can be seen, there are with broader crystal plane spacing, and the smaller crystal nucleus of close to the surface of crystals. This demonstrates that the deposition of has already begun close to the carbon when the cathode discharged to 1.8V, and at the same time, the transformation from to has also happened on the surface. For the PVDF cathode (Figure 7d), quite different from what happened in the PEB cathode, only the crystal of can be found close to the carbon. This can also be explained 12 / 27
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by the polarization in the cathode: as the conduction is hindered by the polarization, during the constant-current discharge, the newly formed subsequently turned into to keep the constant current output. However, these only took space on the surface of the carbon in a very small scale, not in the whole cathode. Besides, this area is covered by other materials, so this cannot be detected evidently by XPS shown in Figure 6 when the cells discharged to 1.8V. Discharging the cathode to the lower voltage of 1.5V, the structure of the cathode will change distinctly. For PEB cathode (Figure 7e), there are a lot of crystals with rice shape fully and uniformly dispersed on the carbon and the lumps on the carbon particle clusters (Figure 7a) disappeared, on the contrast, there are very little this kind of crystals in the PVDF cathode (Figure 7f), though the lumps on the carbon particle clusters (Figure 7b) disappeared too. The fine structure of their crystal is displayed in Figure 7g and 7h respectively. To compare with the crystal structures of the cathodes those discharged to 1.8V shown in Figure 7c and 7d, the crystals grew much bigger and no crystal is observed in the PEB cathode, on the other hand, the crystal structure in the PVDF cathode (Figure 7h) is not different from that in the Figure 7d. These results are corresponding with the results of XPS. As discussed above, the improvement of the ionic conductivity of the PEB achieves a better kinetics that results in a deeper electrochemical reaction degree, so that the crucial rate determine step (the transformation from insoluble to insoluble 6) that is difficult to carry on in kinetics can progress smoothly and completely. 13 / 27
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The actual capacity of sulfur cathode is highly limited on the basis of the depth of the sulfur reduction reactions, namely the ratio of the final product of formed in the cathode.
3. Conclusion In this work, a new type of polyelectrolyte binder for sulfur cathode was prepared via emulsion polymerization. Compared with the traditional PVDF, this anionic copolymer binder has a better adhesion. Besides, this polyelectrolyte around the sulfur can evidently improve the ionic conductivity and eliminate the polarization potential, as a result, the kinetics of the sulfur electrochemical reactions are significantly improved. By this way, the capacity and the cycle performance of L-S battery are improved evidently. At the same time, benefitting from this improved kinetics, the discharge behaviors of sulfur cathode have also been remarkably improved, the difficult processes of the deposition of and the transformation from to can progress smoothly and completely, which makes the higher capacity of the sulfur that reaches the theoretical value. The reason the actual capacity of sulfur cathode lower than the theoretical value is due to the poor reaction depth of the sulfur reduction to form . 4. Experimental Procedures The preparation of Polyelectrolyte Binder (PEB) 3 g N, N-bis(2-cryano-ethyl)-acrylamide(the preparation is introduced in our previous work30), 3 g N-Vinyl Pyrrolidone (TCI), 3 g 2-methoxyethyl acrylate 14 / 27
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(Macklin, China), 1 g sodium p-styrenesulfonate (Huaxia Reagent, AR, China) and 30 g distilled water were added into a 100 mL flask, without other emulsifier was added. Then heated to 72 ℃, and 0.2 g Ammonium persulfate initiator was added into it. After 6 hours, the polymer emulsion was obtained.
The preparation of sulfur-carbon composite First, 8.4 g sublimed sulfur (Kemiou 99.5%) and 7.6 g supper carbon (supper P, Timcal) were mixed by ball milling for 2 minutes at 300 round per minute, then, added into a 50 mL digestion tank. The mixture then was heated in a close roaster, the temperature was increased from room temperature to 200 ℃ in 30 minutes and kept for 3 hours. After cooling down to room temperature, the sulfur-carbon composite that contains 56 % of sulfur was obtained. All processes were conducted in the air.
The preparation of sulfur cathode The cathode slurry in water solvent was composed of 90 % sulfur-carbon composite, 8 % supper P, and 2 % PEB. Then the slurry was coated on the aluminum current collector and dried in an air-circulating oven at 75 ℃. Last, this cathode was dried at 55 ℃ under a low pressure of 5 kPa for 10 hours again. The sulfur content in the cathode is 50.4 %, and the sulfur loading of the as-prepared cathode is approximately 1/ .
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Cell assembly and electrochemical characterizations The coin-type (CR2032) cells were assembled in an argon-filled glove box. Lithium metal (Sigma Aldrich) was used as the anode, and the microporous polypropylene was selected as the separator. The electrolyte was composed of 2 g 1,3-dioxolane (DOL), 4g 1,2-dimethoxyethane (DME), 2.95g bis(trifluoromethane sulfony) imide (LiTFSI, 1 !01 ), 0.5g )*+ (can be dissolved in 2g DOL + 4g DME mixed solvent), and 0.5g methylbenzene. The amount of electrolyte used in the cells was 18 uL. The cycle voltammetry (CV) was carried out using an electrochemical workstation (Arbin instrument), with metal lithium for the reference electrode and the counter electrode (scan rate 0.223 , voltage range 1 -3 V). Electrochemical impedance spectroscopy was collected by an electrochemical impedance spectrometer using a Solartron 1260 frequency response analyzer over a frequency range from 50 mHz to 100 kHz with an amplitude of 5 mV. The cycle performance of cells was carried out on a Neware BTS-5V/5mA battery tester in voltage range of 1.8-2.5 V. The capacity was normalized by the weight of sulfur with current density of 200mA per gram of sulfur.
Physical characterization The morphologies of the sulfur cathodes were observed by scanning electron microscopy (SEM) using a HITACHI SU8010. The X-ray photoelectron spectroscopy
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(XPS) measurements of sulfur in the cathodes were collected with a XSAM800. The cathode samples used for SEM and XPS were took out from the disassembled cells in a dry room and then dried in a vacuum oven under conditions of 50℃ and 5kPa for 10 hours. The crystal structure of cathodes was observed by high resolution transmission electron microscope (HRTEM) using a ZEISS Libra 200 FE, the cathode materials for HRTEM were took out from the disassembled cells in a dry room and then immediately dispersed by ultrasonic soaked in 1, 2-dimethoxyethane (DME), where the LiTFSI can be washed away and the discharge products of sulfur cannot be dissolved.
Reference (1) Fang, X.; Peng, H. A revolution in electrodes: recent progress in rechargeable lithium-sulfur batteries. Small 2015, 11 (13), 1488-511. (2) Manthiram, A.; Chung, S. H.; Zu, C. Lithium-sulfur batteries: progress and prospects. Adv Mater 2015, 27 (12), 1980-2006. (3) Yin, Y. X.; Xin, S.; Guo, Y. G.; Wan, L. J. Lithium-sulfur batteries: electrochemistry, materials, and prospects. Angew Chem Int Ed Engl 2013, 52 (50), 13186-200. (4) Jiang, J.; Zhu, J.; Ai, W.; Wang, X.; Wang, Y.; Zou, C.; Huang, W.; Yu, T. Encapsulation of sulfur with thin-layered nickel-based hydroxides for long-cyclic lithium-sulfur cells. Nat Commun 2015, 6, 8622. (5) Zhang, S. S.; Read, J. A. A new direction for the performance improvement of rechargeable lithium/sulfur batteries. Journal of Power Sources 2012, 200, 77-82. (6) Barchasz, C.; Molton, F.; Duboc, C.; Lepretre, J. C.; Patoux, S.; Alloin, F. Lithium/sulfur cell discharge mechanism: an original approach for intermediate species identification. Anal Chem 2012, 84 (9), 3973-80. (7) Cui, Y.; Fu, Y. Polysulfide transport through separators measured by a linear voltage sweep method. Journal of Power Sources 2015, 286, 557-560. (8) Gao, J.; Lowe, M. A.; Kiya, Y.; Abruña, H. D. Effects of Liquid Electrolytes on the Charge–Discharge Performance of Rechargeable Lithium/Sulfur Batteries: Electrochemical and in-Situ X-ray Absorption Spectroscopic Studies. The Journal of Physical Chemistry C 2011, 115 (50), 25132-25137. (9) Xi, K.; Chen, B.; Li, H.; Xie, R.; Gao, C.; Zhang, C.; Kumar, R. V.; Robertson, J. Soluble polysulphide sorption using carbon nanotube forest for enhancing cycle performance in a lithium–sulphur battery. Nano Energy 2015, 12, 538-546. (10) Pang, Q.; Nazar, L. F. Long-Life and High-Areal-Capacity Li-S Batteries Enabled by a Light-Weight Polar Host with Intrinsic Polysulfide Adsorption. ACS Nano 2016, 10 (4), 4111-8. (11) Zhang, J.; Hu, H.; Li, Z.; Lou, X. W. Double-Shelled Nanocages with Cobalt Hydroxide Inner Shell 17 / 27
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and Layered Double Hydroxides Outer Shell as High-Efficiency Polysulfide Mediator for Lithium-Sulfur Batteries. Angew Chem Int Ed Engl 2016, 55 (12), 3982-6. (12) Liang, X.; Nazar, L. F. In Situ Reactive Assembly of Scalable Core-Shell Sulfur-MnO2 Composite Cathodes. ACS Nano 2016, 10 (4), 4192-8. (13) Niu, X.-q.; Wang, X.-l.; Wang, D.-h.; Li, Y.; Zhang, Y.-j.; Zhang, Y.-d.; Yang, T.; Yu, T.; Tu, J.-p. Metal hydroxide – a new stabilizer for the construction of sulfur/carbon composites as high-performance cathode materials for lithium–sulfur batteries. J. Mater. Chem. A 2015, 3 (33), 17106-17112. (14) Lee, J. T.; Zhao, Y.; Kim, H.; Cho, W. I.; Yushin, G. Sulfur infiltrated activated carbon cathodes for lithium sulfur cells: The combined effects of pore size distribution and electrolyte molarity. Journal of Power Sources 2014, 248, 752-761. (15) Peng, Z.; Li, R.; Gao, J.; Yang, Z.; Deng, Z.; Suo, J. Effective sulfur-salt composite cathode containing lithium bis(trifluoromethane) sulfonamide for lithium sulfur batteries. Electrochimica Acta 2016, 220, 130-136. (16) Dirlam, P. T.; Glass, R. S.; Char, K.; Pyun, J. The use of polymers in Li-S batteries: A review. Journal of Polymer Science Part A: Polymer Chemistry 2017, 55 (10), 1635-1668. (17) Wang, J.; Yang, Y.; Kang, F. Porous carbon nanofiber paper as an effective interlayer for high-performance lithium-sulfur batteries. Electrochimica Acta 2015, 168, 271-276. (18) Zhang, C.; Wang, W.; Wang, A.; Yuan, K.; Yang, Y.; Lu, Y. Effect of Carbon Core Grafting on the Properties of Carbon-Sulfur Composite for Lithium/Sulfur Battery. Journal of the Electrochemical Society 2015, 162 (6), A1067-A1071. (19) He, M.; Yuan, L. X.; Zhang, W. X.; Hu, X. L.; Huang, Y. H. Enhanced Cyclability for Sulfur Cathode Achieved by a Water-Soluble Binder. J Phys Chem C 2011, 115 (31), 15703-15709. (20) Yuan, L. X.; Qiu, X. P.; Chen, L. Q.; Zhu, W. T. New insight into the discharge process of sulfur cathode by electrochemical impedance spectroscopy. Journal of Power Sources 2009, 189 (1), 127-132. (21) Deng, Z.; Zhang, Z.; Lai, Y.; Liu, J.; Li, J.; Liu, Y. Electrochemical Impedance Spectroscopy Study of a Lithium/Sulfur Battery: Modeling and Analysis of Capacity Fading. Journal of the Electrochemical Society 2013, 160 (4), A553-A558. (22) Xiong, S.; Xie, K.; Diao, Y.; Hong, X. Characterization of the solid electrolyte interphase on lithium anode for preventing the shuttle mechanism in lithium–sulfur batteries. Journal of Power Sources 2014, 246, 840-845. (23) Zhang, S. S. Role of LiNO3 in rechargeable lithium/sulfur battery. Electrochimica Acta 2012, 70, 344-348. (24) Aurbach, D.; Pollak, E.; Elazari, R.; Salitra, G.; Kelley, C. S.; Affinito, J. On the Surface Chemical Aspects of Very High Energy Density, Rechargeable Li–Sulfur Batteries. Journal of The Electrochemical Society 2009, 156 (8), A694. (25) Choi, J.-W.; Cheruvally, G.; Kim, D.-S.; Ahn, J.-H.; Kim, K.-W.; Ahn, H.-J. Rechargeable lithium/sulfur battery with liquid electrolytes containing toluene as additive. Journal of Power Sources 2008, 183 (1), 441-445. (26) Xiong, S.; Xie, K.; Diao, Y.; Hong, X. Properties of surface film on lithium anode with LiNO3 as lithium salt in electrolyte solution for lithium–sulfur batteries. Electrochimica Acta 2012, 83, 78-86. (27) Single, N. H. T. a. A. M. Determination of Peak Positions and Areas from Wide-scan XPS Spectra. SURFACE AND INTERFACE ANALYSIS 1990, 15. (28) Chaturvedi, S.; Katz, R.; Guevremont, J.; Schoonen, M. A. A.; Strongin, D. R. XPS and LEED study of 18 / 27
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a single-crystal surface of pyrite, 1996; Vol. 81, p 261-264. (29) Chaudhri, K. M. A. a. S. M. The Lithium Surface Film in the Li/SO2 Cell. Electrochimical Science and Technology 1986, 133 (7). (30) Yang, Z.; Li, R.; Deng, Z. A deep study of the protection of Lithium Cobalt Oxide with polymer surface modification at 4.5 V high voltage. Sci Rep 2018, 8 (1), 863.
Figures
Figure 1. The structure of the core-shell architecture polyelectrolyte binder (a), and the diagram of the mechanism of the polyelectrolyte binder (b).
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Figure 2. The Scanning Electron microscopy (SEM) images of PEB and PVDF sulfur cathodes at initial state (a, b) and discharged to 1.8V (c, d).
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Figure 3. The properties of the L-S batteries using PEB and PVDF as cathode binders: cycle performance (a) and discharge-charge curves (b) at different cycles; the beginning part of the charge (c) and discharge (d) profiles at first and second cycle.
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Figure 4. The EIS spectra of sulfur cathodes discharge to 1.8V (a), and after 100 cycle (b); the SEM images of PEB and PVDF cathodes discharged to 1.8 V (c, d) and after 100 cycles (e, f)
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Figure 5. The rate capacity (a), the discharge profiles discharge to 1.5 V at 200mA per gram of sulfur (b) and the first cyclic voltammetric curves at different scan rates (c) of Li-S batteries with PEB cathode and PVDF cathode.
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Figure 6. The XPS spectra of S2p level for different sulfur cathodes discharge to 1.8 V and 1.5 V 24 / 27
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Figure 7. The High Resolution Transmission Electron Microscopy (HRTEM) images of cathodes: (a) and (c), the PEB cathode discharged to 1.8V; (b) and (d), the PVDF cathode discharged to 1.8V; (e) and (g), the PEB cathode discharged to 1.5V; (f) and (h), the PVDF cathode discharged to 1.5V. 25 / 27
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Tables Resistance(Ω)
First discharged to 1.8V
After 100cycles
PEB
PVDF
PEB
PVDF
Re
5.40
4.51
6.27
7.73
Rf
15.52
17.41
15.22
51.41
Rct
36.54
48.39
34.83
53.79
Table 1. The fitting results of the quivalent circuite for PEB and PVDF Lithium-Sulfur battery.
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Table of Contents
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