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Enhanced Cycling Stability of Lithium–Sulfur batteries by Electrostatic-Interaction
Accepted Manuscript Title: Enhanced Cycling Stability of Lithium-Sulfur batteries by Electrostatic-Interaction Author: Zhaoling Ma Xiaobing Huang Qianqian Jiang Jia Huo Shuangyin Wang PII: DOI: Reference:
S0013-4686(15)30590-9 http://dx.doi.org/doi:10.1016/j.electacta.2015.10.009 EA 25812
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
28-7-2015 1-9-2015 2-10-2015
Please cite this article as: Zhaoling Ma, Xiaobing Huang, Qianqian Jiang, Jia Huo, Shuangyin Wang, Enhanced Cycling Stability of Lithium-Sulfur batteries by Electrostatic-Interaction, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.10.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced Cycling Stability of Lithium-Sulfur batteries by Electrostatic-Interaction Zhaoling Maa, Xiaobing Huangb, Qianqian Jianga, Jia Huoa, Shuangyin Wang*a a
State Key Laboratory of Chem/Bio-Sensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, People’s Republic of China b College of Chemistry and Chemical Engineering, Hunan University of Arts and Science, Changde, Hunan Province, People’s Republic of China E-mail: [email protected] Graphical abstract
fx1 Highlights
Electrostatic interaction is utilized to hinder the shuttling of polysulfides. Directly functionalizing SG can better prolong the cycle life of Li-S batteries. SG functionalized showed significantly improved capacity retention.
Abstract Lithiums-sulfur battery is considered as one of the most promising energy storage devices to replace the current Li ion batteries because of its high theoretical capacity of 1675 mA h g-1. However, the poor cycle stability hinders the further development of this battery system. In order to improve the stability of Li-S batteries, the diffusion of polysulfides from electrodes into electrolyte should be suppressed. Herein, we utilize a positively charged polyelectrolyte to functionalize the electrode materials with the aim to hamper the polysulfides dissolution via electrostatic interaction between strong positively charged polyelectrolyte and negatively charged polysulfides anion. The effect of poly(diallyl dimethylammonium) chloride (PDDA) quantity and functionalization sequence on cycling performances is investigated in detail. It is found that the sulfur-graphene composite (SG) directly functionalized with 10 times PDDA exhibited best cycling stability. At a discharge current density of 0.2 C, much higher capacity retention was realized on the functionalized electrodes than the unfunctionalized (81% vs. 47.3%) after 120 cycles. The as-observed results demonstrate that the electrostatic interaction can effectively prolong the cycling life of Li-S batteries, which provides a new promising strategy for improving the electrochemical performance of Li-S batteries. 1
Keywords poly(diallyl dimethylammonium) chloride, electrostatic interaction, cycling stability, graphene, Lithium-sulfur batteries
1. Introduction Li-S battery as advanced electrochemical energy storage devices has brought significant advancement to the current battery technologies, due to its high theoretical capacity of 1675 mAh g-1, abundant storage in the earth, low cost, and low toxicity. Even though more efforts have been conducted to commercialize the high-energy rechargeable Li-S battery system, its practical application was still hindered by several difficulties, such as poor cycle life, low electrode utilization efficiency, the irreversible deposition of Li2S, and low Coulombic efficiency, among which, the poor cycle life of Li-S batteries is a mostly serious problem. The poor stability is mainly because that the generated polysulfides during discharge process tend to dissolve in the ether-based electrolyte leading to the irreversible deposition of Li2S and the loss of active materials [1, 2]. Moreover, a shuttle effect could be initialized by the dissolved polysulfides being oxidized and reduced at the cathode and anode, respectively, which resulted in severely low Coulombic efficiency and the accumulation of solid precipitates on the anode, and finally irreversible performance degradation of Li-S batteries. Various strategies have been proposed to address the poor cycle life of Li-S battery, including the battery membrane treatment [3-5], the sulfur electrode modification[6-10], the electrolyte additives [11-14], and improved anode structure [15]. As for the sulfur electrode modification, there were mainly two approaches to suppress the polysulfide dissolution: physical confinement [16-19] and chemical interaction trapping through nonvalent bonding [20-23]. Graphene with excellent conductivity, high surface area, and modifiable surface structure is usually considered to be an appropriate candidate as the host of sulfur active material. Graphene-enveloped sulfur composite showed acceptable cycling stability of Li-S batteries [24]. Moreover, graphene oxide containing oxygen functional groups was found to be able to anchor sulfur strongly to improve the cycling stability of Li-S batteries [25]. Manthiram and his coworkers successfully prepared hydroxylated graphene-sulfur nanocomposites to enhance the cycling stability of Li-S batteries. Modification of hydrophilic group on the hydroxylated graphene was an effective approach to enhance the discharge capacity and cycle durability. Polysulfides were found to be located well on graphene oxide through the partial conversion of C-OH bonds to C-S bonds, indicating that the hydroxyl functional group of graphene oxide could effectively improve cycle performance of Li-S batteries [26, 27]. Furthermore, amino with two electron-donator in the Lewis-base ethylenediamine (EDA) modified on reduced graphene oxide (rGO) could cross-linked the polar lithium sulfides and nonpolar carbon surface, which effectively blocked the loss of active sulfur and stabilized the discharge product to improve 2
the cycle durability [28]. These results demonstrated that the proper surface modification of support/host carbon could improve the cycling stability of Li-S batteries. Recently, graphene oxide member modified with the oxygen electronegative atoms was used to hinder the transportation of negatively charged polysulfides through the electrostatic interaction. This motivates our interest that utilizing the electrostatic interaction to improve the cycling stability of Li-S batteries. Previously, we have used PDDA (poly(diallyl dimethylammonium) chloride, a positively charged polyelectrolyte shown in Fig. S1) to functionalize graphene and carbon as electrocatalysts for fuel cells and metal-air batteries and found that PDDA could efficiently interact with carbon via either π-π conjugation [29-31] or electrostatic interaction [32-34]. Herein, we firstly utilized 10 times PDDA to directly functionalize sulfur-graphene electrodes (denoted as SG/PDDA) and aimed to enhance the cycling stability of Li-S batteries. It is well-known that a positively charged ion and a positively charged one can easily take place electrostatic adsorption interaction. Therefore, electrophilic nitrogen atoms in the PDDA could offer strong affinity to the nucleophilic polysulfides. It is also reported that nitrogen heteroatom could effectively attract polysulfides and decrease its dissolution [35-37]. Besides, the effect of PDDA quantity and functionalization sequence on cycling performances was also investigated in detail. It is believed that the strong electrostatic interaction between PDDA and polysulfides would result in efficient reserving of polysulfides within electrodes during the charge/discharge process of Li-S batteries. The electrochemical results demonstrated that SG/PDDA functionalized with 10 times PDDA showed significantly cycle durability compared with the samples functionalized with 5 and 40 times of PDDA. On the other hand, directly functionalizing the sulfur-graphene composite (denoted as SG) delivered higher discharge capacity than initially functionalizing rGO. At the discharge current density of 0.2 C, SG/PDDA delivers a high reversible capacity of 734.1 mAh g-1 and the capacity retention of over 81% after 120 charge/discharge cycles. 2. Experimental 2.1. Samples preparation SG was synthesized according to our previous report [38]. Graphene oxide (GO) suspension was obtained by ultrasonicating graphite oxide and then drying in a vacuum oven at 60 °C. Reduced graphene oxide (rGO) was prepared by thermally reduce GO in a quartz tube at 800 oC for 1 h. SG was prepared by mechanically mixing sublimed sulfur and rGO at the mass ratio of 4:1, wetting the mixture with CS2 and then undergoing a vacuum-assisted thermal treatment. Bulk sulfur out the side of SG was thermally removed under 160 oC for 2 h. Preparations of SG/PDDA-5, SG/PDDA, SG/PDDA-40 are as followed. SG composite was suspended and ultrasonicated for 2 h in 100 ml PDDA (Mw ~100,000, 20 wt% aqueous solution) solution followed by stirring overnight for PDDA sufficient adsorption to the surface of SG. The usage of 20 wt% PDDA is based on the stoichiomettric of SG to PDDA. The mass rates of SG to PDDA are 1:5, 1:10, 1:40. The dispersed suspension was stirred overnight and washed for several times. The final SG/PDDA-X (X=5, 10, 40) was obtained after dried at 60 °C. 3
S/G-PDDA was prepared by initially functionalizing rGO with PDDA at mass rate of 1:10 as the above method. The resulted G-PDDA powder was used to the subsequent preparation. The loading sulphur approach of G-PDDA was as same as that of SG by a vacuum-assisted thermal treatment. 2.2 Determination of the sulfur content The measurement approach of the sulfur content of SG/PDDA-5, SG/PDDA, SG/PDDA-40 and S/G-PDDA were as same as SG composite. It was determined by the following method: some quantity of cathode materials were heated to 500 oC under argon with the elevated temperature rate of 10 oC min-1. The percentage of the loss weight to the initial weight is the sulfur content. Due to the limited PDDA adsorption, the sulfur content of SG/PDDA-5, SG/PDDA, SG/PDDA-40 is considered as same as that of SG (76.2%). The sulfur content of S/G-PDDA is 60.2 wt%. 2.3 Characterization SG, SG/PDDA and S/G-PDDA were studied using scanning electronic microscope (SEM, Nova NanoSEM 230). EDS was used for elemental analysis. XRD and Raman were recorded by a Siemens D500 diractometer with a Cu Kα source and a Jobin Yvon Labram-010 micro-Raman system with a 632 nm laser excitation, respectively. X-ray photoelectron spectroscopy (XPS) was recorded on a ThermoFisher-VG Scientific (ESCALAB 250Xi) photoelectron spectrometer. The Brunauer-Emmmett-Teller (BET) specific surface area and porous structure characteristics of the samples were probed by nitrogen adsorption-desorption method at 77K (SSA-4200). 2.4 Electrochemical measurements The cathode was prepared by stirring the slurry of the active materials, Super-P, and polyvinylidene fluoride (PVDF) binder in a weight ratio of 80:10:10. N-methyl-2-pyrrolidone (NMP) was used as the solvent. The slurry was then cast onto a carbon-coating aluminum foil using a doctor blade. After drying in an oven at 60 °C, the dried foil sheet was pressed and cut into a circular disk with a diameter of 12 mm served as a cathode. The areal loading amount of sulfur is about 1 mg. The amount of electrolyte is 40 μl respectively in cathode and anode side. Celgard 2400 was used as the separator. 1M Lithium Bis(trifluoromethanesulfonyl)imide (LiTFSI) and 0.1 M lithium nitrate (LiNO3) dissolved in a mixture of 1,3-Dioxolane and Dimethoxyethane (DOL:DME, 1:1 by volume) was used as the electrolyte. CR2032 type coin cells were assembled in Ar-filled glovebox (Mikvouna). Lithium metal sheet was used as the anode. Cyclic voltammogram (CV) between 1.7 V and 2.8 V at a scan rate of 0.1 mV s-1 and electrochemical impedance spectra (EIS) over the frequency range from 100 kHz to 10 mHz with the amplitude of 5 mV of Li-S coin cell were conducted on a potentiostat galvanostat (Metrohm Autolab PGSTAT302N). The cycle performance was assessed with a LAND-CT2001A battery test system with the charge/discharge voltage range of 1.7-2.8 V at the different current density (1 C equeals to 1675 mA g-1). 4
3. Results and discussion Scanning electron microscopy (SEM) images of SG and SG/PDDA are shown in Fig. 1. Porous morphology of SG composite (Fig. 1a) is observed without apparent bulk sulfur particles. After functionalized with PDDA, the 3D porous structure of SG was retained (Fig. 1b), which would facilitate the transportation of the lithium ions and the utilization efficiency of active material sulfur. The corresponding energy-dispersive X-ray spectroscopy (EDS) (Fig. S2) indicates that PDDA was successfully functionalized on the surface SG composite. A high resolution SEM image of SG/PDDA shown in Figure 1c shows the curved and wrinkled rGO surfaces. The sulfur and PDDA were distributed homogenously on rGO, as evidenced by the corresponding element mapping images for C, N, and S in Fig. 1d-f. XRD of SG and SG/PDDA were conducted to characterize the components of material. XRD patterns of both SG and SG/PDDA (Fig. 2a) contain a broad peak at about 26o ascribed to the rGO, other well-resolved diffraction peaks corresponded to the characteristic peaks of orthorhombic S [39, 40] demonstrating that S in SG/PDDA is of high crystallinity, and no new phase was generated during the functionalization process of PDDA [41]. However, PDDA was not observed probably because of its amorphous feature. Raman spectra were used to further demonstrate the existence of PDDA (Fig. 2b). No characteristic peaks of sulfur were observed from the Raman spectra. However, the G band of SG/PDDA has a negative shift compared to that of SG, suggesting the interaction between rGO and PDDA and the successful functionalization SG by PDDA. Furthermore, there is also no observation of chemical bonding between S and PDDA from S 2p XPS spectrum of SG/PDDA (Fig. S3). The porous properties of rGO, SG and SG/PDDA were examined by N2 adsorption-desorption measurement, as shown in Fig. 3. After loading the sulfur, the adsorbed volume of SG decreased obviously in contrast to rGO with higher adsorbed volume (2.64 cm3 g-1 as shown in Table S1), demonstrating sulfur particles were embedded in the mesoporous network of rGO. However, with the PDDA functionalization, both the pore size and pore volume of SG/PDDA increased, as shown in Fig. 3d and Table S1, which is ascribed to the intertwining of PDDA on the SG surface. To prove the enhancement of the cycling performance of Li-S batteries by functionalization of PDDA, various electrochemical measurements on SG and SG/PDDA were conducted. Cyclic voltammetry (CV) was used to investigate the charge-discharge mechanism between 1.7 V and 2.8 V at a sweep rate of 0.1 mV s-1 (Fig. 4a). The CV graphs of both SG and SG/PDDA show two couples of distinct oxidation and reduction peaks. In the discharge process of SG, the first cathodic reduction peak at 2.35 V represents the reduction of sulfur to high-order polysulphides (Li2Sx, x≥4); the second cathodic reduction peak at 2.04 V is assigned to the conversion of higher-order polysulphides to lower-order polysulphides (Li2Sx, 2≤x<4). During the charge process, peaks at approximately 2.3 V and 2.4 V correspond to the conversion of Li2S and/or Li2S2 to sulfur. Note that redox peaks of SG/PDDA shift slightly compared with that of SG. The two anodic peaks positively shift by 0.04 V, while two cathodic peaks negatively shift by 0.03 V. Slight polarization and inner resistance are possibly due to the presence of non-conductive polyelectrolyte, PDDA [42]. 5
In order to investigate the cycling performance of SG/PDDA, the cycling stability of Li-S batteries based on SG and SG/PDDA cathodes was carried out at 0.2 C, as shown in Fig. 4b. The discharge capacity of SG composite decreases from 935.6 mAh g-1 to 442.8 mAh g-1 after 120 cycles and the corresponding capacity retention is only 47.3%. SG/PDDA shows initial discharge capacity of 900 mAh g-1 at 0.2 C, which is slightly lower than that of SG because of the increased resistance of the electrodes with PDDA. However, the reversible capacity of 734.1 mAh g-1 was reserved after 120 cycles and the capacity retention of over 81% is achieved, largely higher than that of SG (47.3%). On the other hand, in contrast to SG, Coulombic efficiency of SG/PDDA is very stable, which is attributed to the effect of LiNO3 additive and the ideal suppression of PDDA for polysulfides dissolution. The improvement in cycling stability of SG/PDDA was also verified at different current rates at 0.1 C, 0.2 C, 0.5 C and 1C, as shown in Fig. 4c. SG/PDDA exhibits better discharge capacity and cycling stability in all cases. After 1C discharge, SG/PDDA retained higher reversible capacity than that of SG at 0.1 C. During the discharge process, inevitable polysulfide dissolution can lead to lower discharge capacity and sulfur utilization. Because of the very slow kinetics and the involved solid state reaction, the second discharge plateau shows more obvious capacity fading while its theoretical discharge capacity of 1256 mAh g-1 is very close to the theoretical discharge capacity of 1675 mAh g-1 in Li-S batteries [43, 44]. In order to assess the loss of polysulfides in the electrode during the prolonged cycling at 0.2 C shown in Fig. 4b, the discharge capacity contribution from the second discharge plateau to the theoretical discharge capacity of Li-S batteries was investigated, as shown in Figure 4d. The initial capacity contribution of the second plateau is 67.5%, slightly lower than theoretical capacity contribution of 74.9% (the percentage of 1256 mAh g-1 to 1675 mAh g-1). With increasing cycles, the initial capacity contribution increases gradually and even up to 70% after 118 cycles. The high and stable capacity contribution from the second plateau reveals the efficient conversion of the intermediate polysulfides, further proving the electrostatic interaction between PDDA and polysulfides can effectively enhance the cycling stability of Li-S batteries. Electrochemical impedance spectra were measured to investigate the electronic properties of SG/PDDA and SG in Li-S batteries (Fig. 5a). SG/PDDA showed slightly larger inner resistance than that of SG due to the presence of non-conductive PDDA, whose results is consistent to that of CV. Although SG/PDDA presented slightly larger inner resistance, except for high rates its discharge plateau voltages at 0.1 C and 0.2 C almost agree with SG (shown in Fig. 5b). To confirm the structure benefits of SG/PDDA, we investigated the effect of PDDA quantity on the cycling performance of Li-S batteries (Fig. 6a). We prepared SG/PDDA-5, SG/PDDA, and SG/PDDA-40 in which the quantity of PDDA is 5, 10, and 40 times than the unfunctionalized SG composite, respectively. At 0.2 C, SG/PDDA shows the best cycling stability. On the other hand, it is important to understand how the functionalization sequence affects the cycling performance of the composite. To do so, we first functionalized rGO with PDDA followed by the sulfur loading. In this way, PDDA was positioned between rGO and sulfur particles (denoted as S/G-PDDA). However, it is found that S/G-PDDA also shows better cycling performance than the unfunctionalized SG, but poorer than SG/PDDA, indicating the proper position of PDDA is the 6
outer layer of the SG composite. SG/PDDA and S/G-PDDA have the similar surface areas as evidenced by the N2 isotherms (Table S1), but the pore volume of SG/PDDA is higher than that of S/G-PDDA, resulting in better performance in Li-S batteries. The cycling performance of SG/PDDA at 0.1 C was further investigated by running the charge/discharge process at different discharge rates (Fig. 6b). It is obviously observed that at each discharge rate the cathode electrode maintains stable discharge capacity. The plateau of charge-discharge voltage profiles of SG/PDDA in 1st, 20th, 50th, 70th, 100th and 110th at 0.1 C (Fig. 6c) remain unchanged, indicating the excellent electrochemical stability. Finally, a mechanism of SG/PDDA impeding the dissolution of polysulfides via electrostatic interaction was proposed. During the discharge process, high-order polysulfides with long-chain structure tend to dissolve in the ether-based electrolyte because of its high solubility. In the presence of positively charged PDDA, the negatively charged polysulfides could be trapped tightly via the electrostatic interaction. 4. Conclusion In summary, we have developed an electrostatic interaction strategy to impede the dissolution of polysulfides and thus to enhance the cycling stability of Li-S batteries by functionalizing SG electrodes with positively charged PDDA. The PDDA functionalized SG shows significantly better cycling performance than that of the unfunctionalized SG (capacity retention: 81% vs. 47.3%). The enhanced cycling performance is attributed to the efficient electrostatic interaction between the positively charged PDDA and negatively charged polysulfides. The comprehensive characterizations confirm the successful functionalization of SG by PDDA. More importantly, it is found that the functionalization sequence is very important to obtain ideal performance. The as-developed strategy is very simple and straightforward, and SG/PDDA produced by this simple electrostatic interaction strategy could be a promising cathode material for highly durable Li-S batteries. Acknowledgements The authors acknowledge the support from the National Natural Science Foundation of China (Grant No.: 51402100), the Youth 1000 Talent Program of China, and Inter-discipline Research Programe of Hunan University. References [1] Y.X. Yin, S. Xin, Y.G. Guo, L.J. Wan, Lithium–Sulfur Batteries : Electrochemistry, Materials,and Prospects, Angew Chem. Int. Ed. Engl., 52 (2013) 13186-13200. [2] J. Song, M.L. Gordin, T. Xu, S. Chen, Z. Yu, H. Sohn, J. Lu, Y. Ren, Y. Duan, D. Wang, Strong lithium polysulfide chemisorption on electroactive sites of nitrogen-doped carbon composites for high-performance lithium-sulfur battery cathodes, Angew Chem. Int. Ed. Engl., 54 (2015) 4325-4329. [3] J.Q. Huang, Q. Zhang, H.J. Peng, X.Y. Liu, W.Z. Qian, F. Wei, Ionic shield for polysulfides towards highly-stable lithium-sulfur batteries, Energ. Environ. Sci., 7 (2014) 347-353.
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Fig. 1. SEM images of SG (a) and SG/PDDA (b), (c) is a high resolution image of (b). Elemental mapping of SG/PDDDA showing the homogenous distribution of (d) carbon, (e) nitrogen and (f) sulfur.
Fig. 2. (a) XRD and (b) Raman spectra of SG and SG/PDDA.
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Fig. 3. N2 adsorption/desorption isotherms of rGO (a), SG and SG/PDDA (c) and pore size distributions of rGO (b), SG and SG/PDDA (d).
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Fig. 4. (a) Cyclic voltammograms and (b) cycling performance at 0.2 C of SG and SG/PDDA, (c) cycling stability of SG and SG/PDDA at different rates and (d) discharge capacity contribution of the second discharge plateau to the total discharge capacities at 0.2 C.
Fig. 5. (a) Electrochemical impedance spectra and (b) discharge voltage profiles at different rates of SG and SG/PDDA. 12
Fig. 6. (a) Cycling performances of SG/PDDA-5, SG/PDDA, SG/PDDA-40 and S/GPDDA at 0.2 C. (b) cycling performances of SG/PDDA at different discharge rates. (c) charge-discharge profiles of SG/PDDA at 0.1 C.
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S0013-4686(15)30590-9 http://dx.doi.org/doi:10.1016/j.electacta.2015.10.009 EA 25812
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
28-7-2015 1-9-2015 2-10-2015
Please cite this article as: Zhaoling Ma, Xiaobing Huang, Qianqian Jiang, Jia Huo, Shuangyin Wang, Enhanced Cycling Stability of Lithium-Sulfur batteries by Electrostatic-Interaction, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.10.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced Cycling Stability of Lithium-Sulfur batteries by Electrostatic-Interaction Zhaoling Maa, Xiaobing Huangb, Qianqian Jianga, Jia Huoa, Shuangyin Wang*a a
State Key Laboratory of Chem/Bio-Sensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, People’s Republic of China b College of Chemistry and Chemical Engineering, Hunan University of Arts and Science, Changde, Hunan Province, People’s Republic of China E-mail: [email protected] Graphical abstract
fx1 Highlights
Electrostatic interaction is utilized to hinder the shuttling of polysulfides. Directly functionalizing SG can better prolong the cycle life of Li-S batteries. SG functionalized showed significantly improved capacity retention.
Abstract Lithiums-sulfur battery is considered as one of the most promising energy storage devices to replace the current Li ion batteries because of its high theoretical capacity of 1675 mA h g-1. However, the poor cycle stability hinders the further development of this battery system. In order to improve the stability of Li-S batteries, the diffusion of polysulfides from electrodes into electrolyte should be suppressed. Herein, we utilize a positively charged polyelectrolyte to functionalize the electrode materials with the aim to hamper the polysulfides dissolution via electrostatic interaction between strong positively charged polyelectrolyte and negatively charged polysulfides anion. The effect of poly(diallyl dimethylammonium) chloride (PDDA) quantity and functionalization sequence on cycling performances is investigated in detail. It is found that the sulfur-graphene composite (SG) directly functionalized with 10 times PDDA exhibited best cycling stability. At a discharge current density of 0.2 C, much higher capacity retention was realized on the functionalized electrodes than the unfunctionalized (81% vs. 47.3%) after 120 cycles. The as-observed results demonstrate that the electrostatic interaction can effectively prolong the cycling life of Li-S batteries, which provides a new promising strategy for improving the electrochemical performance of Li-S batteries. 1
Keywords poly(diallyl dimethylammonium) chloride, electrostatic interaction, cycling stability, graphene, Lithium-sulfur batteries
1. Introduction Li-S battery as advanced electrochemical energy storage devices has brought significant advancement to the current battery technologies, due to its high theoretical capacity of 1675 mAh g-1, abundant storage in the earth, low cost, and low toxicity. Even though more efforts have been conducted to commercialize the high-energy rechargeable Li-S battery system, its practical application was still hindered by several difficulties, such as poor cycle life, low electrode utilization efficiency, the irreversible deposition of Li2S, and low Coulombic efficiency, among which, the poor cycle life of Li-S batteries is a mostly serious problem. The poor stability is mainly because that the generated polysulfides during discharge process tend to dissolve in the ether-based electrolyte leading to the irreversible deposition of Li2S and the loss of active materials [1, 2]. Moreover, a shuttle effect could be initialized by the dissolved polysulfides being oxidized and reduced at the cathode and anode, respectively, which resulted in severely low Coulombic efficiency and the accumulation of solid precipitates on the anode, and finally irreversible performance degradation of Li-S batteries. Various strategies have been proposed to address the poor cycle life of Li-S battery, including the battery membrane treatment [3-5], the sulfur electrode modification[6-10], the electrolyte additives [11-14], and improved anode structure [15]. As for the sulfur electrode modification, there were mainly two approaches to suppress the polysulfide dissolution: physical confinement [16-19] and chemical interaction trapping through nonvalent bonding [20-23]. Graphene with excellent conductivity, high surface area, and modifiable surface structure is usually considered to be an appropriate candidate as the host of sulfur active material. Graphene-enveloped sulfur composite showed acceptable cycling stability of Li-S batteries [24]. Moreover, graphene oxide containing oxygen functional groups was found to be able to anchor sulfur strongly to improve the cycling stability of Li-S batteries [25]. Manthiram and his coworkers successfully prepared hydroxylated graphene-sulfur nanocomposites to enhance the cycling stability of Li-S batteries. Modification of hydrophilic group on the hydroxylated graphene was an effective approach to enhance the discharge capacity and cycle durability. Polysulfides were found to be located well on graphene oxide through the partial conversion of C-OH bonds to C-S bonds, indicating that the hydroxyl functional group of graphene oxide could effectively improve cycle performance of Li-S batteries [26, 27]. Furthermore, amino with two electron-donator in the Lewis-base ethylenediamine (EDA) modified on reduced graphene oxide (rGO) could cross-linked the polar lithium sulfides and nonpolar carbon surface, which effectively blocked the loss of active sulfur and stabilized the discharge product to improve 2
the cycle durability [28]. These results demonstrated that the proper surface modification of support/host carbon could improve the cycling stability of Li-S batteries. Recently, graphene oxide member modified with the oxygen electronegative atoms was used to hinder the transportation of negatively charged polysulfides through the electrostatic interaction. This motivates our interest that utilizing the electrostatic interaction to improve the cycling stability of Li-S batteries. Previously, we have used PDDA (poly(diallyl dimethylammonium) chloride, a positively charged polyelectrolyte shown in Fig. S1) to functionalize graphene and carbon as electrocatalysts for fuel cells and metal-air batteries and found that PDDA could efficiently interact with carbon via either π-π conjugation [29-31] or electrostatic interaction [32-34]. Herein, we firstly utilized 10 times PDDA to directly functionalize sulfur-graphene electrodes (denoted as SG/PDDA) and aimed to enhance the cycling stability of Li-S batteries. It is well-known that a positively charged ion and a positively charged one can easily take place electrostatic adsorption interaction. Therefore, electrophilic nitrogen atoms in the PDDA could offer strong affinity to the nucleophilic polysulfides. It is also reported that nitrogen heteroatom could effectively attract polysulfides and decrease its dissolution [35-37]. Besides, the effect of PDDA quantity and functionalization sequence on cycling performances was also investigated in detail. It is believed that the strong electrostatic interaction between PDDA and polysulfides would result in efficient reserving of polysulfides within electrodes during the charge/discharge process of Li-S batteries. The electrochemical results demonstrated that SG/PDDA functionalized with 10 times PDDA showed significantly cycle durability compared with the samples functionalized with 5 and 40 times of PDDA. On the other hand, directly functionalizing the sulfur-graphene composite (denoted as SG) delivered higher discharge capacity than initially functionalizing rGO. At the discharge current density of 0.2 C, SG/PDDA delivers a high reversible capacity of 734.1 mAh g-1 and the capacity retention of over 81% after 120 charge/discharge cycles. 2. Experimental 2.1. Samples preparation SG was synthesized according to our previous report [38]. Graphene oxide (GO) suspension was obtained by ultrasonicating graphite oxide and then drying in a vacuum oven at 60 °C. Reduced graphene oxide (rGO) was prepared by thermally reduce GO in a quartz tube at 800 oC for 1 h. SG was prepared by mechanically mixing sublimed sulfur and rGO at the mass ratio of 4:1, wetting the mixture with CS2 and then undergoing a vacuum-assisted thermal treatment. Bulk sulfur out the side of SG was thermally removed under 160 oC for 2 h. Preparations of SG/PDDA-5, SG/PDDA, SG/PDDA-40 are as followed. SG composite was suspended and ultrasonicated for 2 h in 100 ml PDDA (Mw ~100,000, 20 wt% aqueous solution) solution followed by stirring overnight for PDDA sufficient adsorption to the surface of SG. The usage of 20 wt% PDDA is based on the stoichiomettric of SG to PDDA. The mass rates of SG to PDDA are 1:5, 1:10, 1:40. The dispersed suspension was stirred overnight and washed for several times. The final SG/PDDA-X (X=5, 10, 40) was obtained after dried at 60 °C. 3
S/G-PDDA was prepared by initially functionalizing rGO with PDDA at mass rate of 1:10 as the above method. The resulted G-PDDA powder was used to the subsequent preparation. The loading sulphur approach of G-PDDA was as same as that of SG by a vacuum-assisted thermal treatment. 2.2 Determination of the sulfur content The measurement approach of the sulfur content of SG/PDDA-5, SG/PDDA, SG/PDDA-40 and S/G-PDDA were as same as SG composite. It was determined by the following method: some quantity of cathode materials were heated to 500 oC under argon with the elevated temperature rate of 10 oC min-1. The percentage of the loss weight to the initial weight is the sulfur content. Due to the limited PDDA adsorption, the sulfur content of SG/PDDA-5, SG/PDDA, SG/PDDA-40 is considered as same as that of SG (76.2%). The sulfur content of S/G-PDDA is 60.2 wt%. 2.3 Characterization SG, SG/PDDA and S/G-PDDA were studied using scanning electronic microscope (SEM, Nova NanoSEM 230). EDS was used for elemental analysis. XRD and Raman were recorded by a Siemens D500 diractometer with a Cu Kα source and a Jobin Yvon Labram-010 micro-Raman system with a 632 nm laser excitation, respectively. X-ray photoelectron spectroscopy (XPS) was recorded on a ThermoFisher-VG Scientific (ESCALAB 250Xi) photoelectron spectrometer. The Brunauer-Emmmett-Teller (BET) specific surface area and porous structure characteristics of the samples were probed by nitrogen adsorption-desorption method at 77K (SSA-4200). 2.4 Electrochemical measurements The cathode was prepared by stirring the slurry of the active materials, Super-P, and polyvinylidene fluoride (PVDF) binder in a weight ratio of 80:10:10. N-methyl-2-pyrrolidone (NMP) was used as the solvent. The slurry was then cast onto a carbon-coating aluminum foil using a doctor blade. After drying in an oven at 60 °C, the dried foil sheet was pressed and cut into a circular disk with a diameter of 12 mm served as a cathode. The areal loading amount of sulfur is about 1 mg. The amount of electrolyte is 40 μl respectively in cathode and anode side. Celgard 2400 was used as the separator. 1M Lithium Bis(trifluoromethanesulfonyl)imide (LiTFSI) and 0.1 M lithium nitrate (LiNO3) dissolved in a mixture of 1,3-Dioxolane and Dimethoxyethane (DOL:DME, 1:1 by volume) was used as the electrolyte. CR2032 type coin cells were assembled in Ar-filled glovebox (Mikvouna). Lithium metal sheet was used as the anode. Cyclic voltammogram (CV) between 1.7 V and 2.8 V at a scan rate of 0.1 mV s-1 and electrochemical impedance spectra (EIS) over the frequency range from 100 kHz to 10 mHz with the amplitude of 5 mV of Li-S coin cell were conducted on a potentiostat galvanostat (Metrohm Autolab PGSTAT302N). The cycle performance was assessed with a LAND-CT2001A battery test system with the charge/discharge voltage range of 1.7-2.8 V at the different current density (1 C equeals to 1675 mA g-1). 4
3. Results and discussion Scanning electron microscopy (SEM) images of SG and SG/PDDA are shown in Fig. 1. Porous morphology of SG composite (Fig. 1a) is observed without apparent bulk sulfur particles. After functionalized with PDDA, the 3D porous structure of SG was retained (Fig. 1b), which would facilitate the transportation of the lithium ions and the utilization efficiency of active material sulfur. The corresponding energy-dispersive X-ray spectroscopy (EDS) (Fig. S2) indicates that PDDA was successfully functionalized on the surface SG composite. A high resolution SEM image of SG/PDDA shown in Figure 1c shows the curved and wrinkled rGO surfaces. The sulfur and PDDA were distributed homogenously on rGO, as evidenced by the corresponding element mapping images for C, N, and S in Fig. 1d-f. XRD of SG and SG/PDDA were conducted to characterize the components of material. XRD patterns of both SG and SG/PDDA (Fig. 2a) contain a broad peak at about 26o ascribed to the rGO, other well-resolved diffraction peaks corresponded to the characteristic peaks of orthorhombic S [39, 40] demonstrating that S in SG/PDDA is of high crystallinity, and no new phase was generated during the functionalization process of PDDA [41]. However, PDDA was not observed probably because of its amorphous feature. Raman spectra were used to further demonstrate the existence of PDDA (Fig. 2b). No characteristic peaks of sulfur were observed from the Raman spectra. However, the G band of SG/PDDA has a negative shift compared to that of SG, suggesting the interaction between rGO and PDDA and the successful functionalization SG by PDDA. Furthermore, there is also no observation of chemical bonding between S and PDDA from S 2p XPS spectrum of SG/PDDA (Fig. S3). The porous properties of rGO, SG and SG/PDDA were examined by N2 adsorption-desorption measurement, as shown in Fig. 3. After loading the sulfur, the adsorbed volume of SG decreased obviously in contrast to rGO with higher adsorbed volume (2.64 cm3 g-1 as shown in Table S1), demonstrating sulfur particles were embedded in the mesoporous network of rGO. However, with the PDDA functionalization, both the pore size and pore volume of SG/PDDA increased, as shown in Fig. 3d and Table S1, which is ascribed to the intertwining of PDDA on the SG surface. To prove the enhancement of the cycling performance of Li-S batteries by functionalization of PDDA, various electrochemical measurements on SG and SG/PDDA were conducted. Cyclic voltammetry (CV) was used to investigate the charge-discharge mechanism between 1.7 V and 2.8 V at a sweep rate of 0.1 mV s-1 (Fig. 4a). The CV graphs of both SG and SG/PDDA show two couples of distinct oxidation and reduction peaks. In the discharge process of SG, the first cathodic reduction peak at 2.35 V represents the reduction of sulfur to high-order polysulphides (Li2Sx, x≥4); the second cathodic reduction peak at 2.04 V is assigned to the conversion of higher-order polysulphides to lower-order polysulphides (Li2Sx, 2≤x<4). During the charge process, peaks at approximately 2.3 V and 2.4 V correspond to the conversion of Li2S and/or Li2S2 to sulfur. Note that redox peaks of SG/PDDA shift slightly compared with that of SG. The two anodic peaks positively shift by 0.04 V, while two cathodic peaks negatively shift by 0.03 V. Slight polarization and inner resistance are possibly due to the presence of non-conductive polyelectrolyte, PDDA [42]. 5
In order to investigate the cycling performance of SG/PDDA, the cycling stability of Li-S batteries based on SG and SG/PDDA cathodes was carried out at 0.2 C, as shown in Fig. 4b. The discharge capacity of SG composite decreases from 935.6 mAh g-1 to 442.8 mAh g-1 after 120 cycles and the corresponding capacity retention is only 47.3%. SG/PDDA shows initial discharge capacity of 900 mAh g-1 at 0.2 C, which is slightly lower than that of SG because of the increased resistance of the electrodes with PDDA. However, the reversible capacity of 734.1 mAh g-1 was reserved after 120 cycles and the capacity retention of over 81% is achieved, largely higher than that of SG (47.3%). On the other hand, in contrast to SG, Coulombic efficiency of SG/PDDA is very stable, which is attributed to the effect of LiNO3 additive and the ideal suppression of PDDA for polysulfides dissolution. The improvement in cycling stability of SG/PDDA was also verified at different current rates at 0.1 C, 0.2 C, 0.5 C and 1C, as shown in Fig. 4c. SG/PDDA exhibits better discharge capacity and cycling stability in all cases. After 1C discharge, SG/PDDA retained higher reversible capacity than that of SG at 0.1 C. During the discharge process, inevitable polysulfide dissolution can lead to lower discharge capacity and sulfur utilization. Because of the very slow kinetics and the involved solid state reaction, the second discharge plateau shows more obvious capacity fading while its theoretical discharge capacity of 1256 mAh g-1 is very close to the theoretical discharge capacity of 1675 mAh g-1 in Li-S batteries [43, 44]. In order to assess the loss of polysulfides in the electrode during the prolonged cycling at 0.2 C shown in Fig. 4b, the discharge capacity contribution from the second discharge plateau to the theoretical discharge capacity of Li-S batteries was investigated, as shown in Figure 4d. The initial capacity contribution of the second plateau is 67.5%, slightly lower than theoretical capacity contribution of 74.9% (the percentage of 1256 mAh g-1 to 1675 mAh g-1). With increasing cycles, the initial capacity contribution increases gradually and even up to 70% after 118 cycles. The high and stable capacity contribution from the second plateau reveals the efficient conversion of the intermediate polysulfides, further proving the electrostatic interaction between PDDA and polysulfides can effectively enhance the cycling stability of Li-S batteries. Electrochemical impedance spectra were measured to investigate the electronic properties of SG/PDDA and SG in Li-S batteries (Fig. 5a). SG/PDDA showed slightly larger inner resistance than that of SG due to the presence of non-conductive PDDA, whose results is consistent to that of CV. Although SG/PDDA presented slightly larger inner resistance, except for high rates its discharge plateau voltages at 0.1 C and 0.2 C almost agree with SG (shown in Fig. 5b). To confirm the structure benefits of SG/PDDA, we investigated the effect of PDDA quantity on the cycling performance of Li-S batteries (Fig. 6a). We prepared SG/PDDA-5, SG/PDDA, and SG/PDDA-40 in which the quantity of PDDA is 5, 10, and 40 times than the unfunctionalized SG composite, respectively. At 0.2 C, SG/PDDA shows the best cycling stability. On the other hand, it is important to understand how the functionalization sequence affects the cycling performance of the composite. To do so, we first functionalized rGO with PDDA followed by the sulfur loading. In this way, PDDA was positioned between rGO and sulfur particles (denoted as S/G-PDDA). However, it is found that S/G-PDDA also shows better cycling performance than the unfunctionalized SG, but poorer than SG/PDDA, indicating the proper position of PDDA is the 6
outer layer of the SG composite. SG/PDDA and S/G-PDDA have the similar surface areas as evidenced by the N2 isotherms (Table S1), but the pore volume of SG/PDDA is higher than that of S/G-PDDA, resulting in better performance in Li-S batteries. The cycling performance of SG/PDDA at 0.1 C was further investigated by running the charge/discharge process at different discharge rates (Fig. 6b). It is obviously observed that at each discharge rate the cathode electrode maintains stable discharge capacity. The plateau of charge-discharge voltage profiles of SG/PDDA in 1st, 20th, 50th, 70th, 100th and 110th at 0.1 C (Fig. 6c) remain unchanged, indicating the excellent electrochemical stability. Finally, a mechanism of SG/PDDA impeding the dissolution of polysulfides via electrostatic interaction was proposed. During the discharge process, high-order polysulfides with long-chain structure tend to dissolve in the ether-based electrolyte because of its high solubility. In the presence of positively charged PDDA, the negatively charged polysulfides could be trapped tightly via the electrostatic interaction. 4. Conclusion In summary, we have developed an electrostatic interaction strategy to impede the dissolution of polysulfides and thus to enhance the cycling stability of Li-S batteries by functionalizing SG electrodes with positively charged PDDA. The PDDA functionalized SG shows significantly better cycling performance than that of the unfunctionalized SG (capacity retention: 81% vs. 47.3%). The enhanced cycling performance is attributed to the efficient electrostatic interaction between the positively charged PDDA and negatively charged polysulfides. The comprehensive characterizations confirm the successful functionalization of SG by PDDA. More importantly, it is found that the functionalization sequence is very important to obtain ideal performance. The as-developed strategy is very simple and straightforward, and SG/PDDA produced by this simple electrostatic interaction strategy could be a promising cathode material for highly durable Li-S batteries. Acknowledgements The authors acknowledge the support from the National Natural Science Foundation of China (Grant No.: 51402100), the Youth 1000 Talent Program of China, and Inter-discipline Research Programe of Hunan University. References [1] Y.X. Yin, S. Xin, Y.G. Guo, L.J. Wan, Lithium–Sulfur Batteries : Electrochemistry, Materials,and Prospects, Angew Chem. Int. Ed. Engl., 52 (2013) 13186-13200. [2] J. Song, M.L. Gordin, T. Xu, S. Chen, Z. Yu, H. Sohn, J. Lu, Y. Ren, Y. Duan, D. Wang, Strong lithium polysulfide chemisorption on electroactive sites of nitrogen-doped carbon composites for high-performance lithium-sulfur battery cathodes, Angew Chem. Int. Ed. Engl., 54 (2015) 4325-4329. [3] J.Q. Huang, Q. Zhang, H.J. Peng, X.Y. Liu, W.Z. Qian, F. Wei, Ionic shield for polysulfides towards highly-stable lithium-sulfur batteries, Energ. Environ. Sci., 7 (2014) 347-353.
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Fig. 1. SEM images of SG (a) and SG/PDDA (b), (c) is a high resolution image of (b). Elemental mapping of SG/PDDDA showing the homogenous distribution of (d) carbon, (e) nitrogen and (f) sulfur.
Fig. 2. (a) XRD and (b) Raman spectra of SG and SG/PDDA.
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Fig. 3. N2 adsorption/desorption isotherms of rGO (a), SG and SG/PDDA (c) and pore size distributions of rGO (b), SG and SG/PDDA (d).
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Fig. 4. (a) Cyclic voltammograms and (b) cycling performance at 0.2 C of SG and SG/PDDA, (c) cycling stability of SG and SG/PDDA at different rates and (d) discharge capacity contribution of the second discharge plateau to the total discharge capacities at 0.2 C.
Fig. 5. (a) Electrochemical impedance spectra and (b) discharge voltage profiles at different rates of SG and SG/PDDA. 12
Fig. 6. (a) Cycling performances of SG/PDDA-5, SG/PDDA, SG/PDDA-40 and S/GPDDA at 0.2 C. (b) cycling performances of SG/PDDA at different discharge rates. (c) charge-discharge profiles of SG/PDDA at 0.1 C.
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