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Polysulfide Stabilization: A Pivotal Strategy to Achieve High Energy Density Li–S Batteries with Long Cycle Life Yuqing Chen, Hongzhang Zhang,* Wenbin Xu, Xiaofei Yang, Ying Yu, Xianfeng Li, and Huamin Zhang*

The disproportionation of polysulfide (PS) is a long-neglected and vital issue that causes the fast capacity fading of Li–S batteries. Based on the hard and soft acids and bases (HSAB) theory, a large size N-methyl-N-ethyl pyrrolidinium (MEP+) cation is proposed to complex and stabilize the PS in electrolyte. The disproportionation of PS is successfully suppressed by this simple method, thereby avoiding the precipitation of sulfur in the electrolyte and reducing the loss of the active materials. The mutual interaction mechanism between MEP+ and Sn2− in electrolyte is comprehensively investigated and verified for the first time, via both density functional theory (DFT) calculation and experimental characterization. It enables the 5000 mA h Li–S batteries (soft package type) to achieve initial specific energy over 300 Wh kg−1 and maintain over 65% after 100 charge/discharge cycles at 1/20 C, while merely 24% is remained at 59 cycles without MEP+. This interesting finding is believed to shed light on the further development of Li–S batteries.

1. Introduction Recently, Li–S batteries, with their high theoretical energy density (≈2500 Wh kg−1) and low cost, have aroused intense scholarly interest as one of the most promising next-generation power batteries for electric vehicles and portable electronic devices.[1] However, the practical application of Li–S batteries is still hindered by rapid capacity fading during cycling.[2,3] It is mainly caused by the soluble and disproportionation nature of intermediate polysulfide (PS, Sx2−, x ≥ 4), which separately leads to two serious issues: the PS shuttle and sulfur inactivation. Y. Q. Chen, Prof. H. Z. Zhang, W. B. Xu, X. F. Yang, Y. Yu, Prof. X. F. Li, Prof. H. M. Zhang Division of Energy Storage Dalian Institute of Chemical Physics Chinese Academy of Sciences Zhongshan Road 457, Dalian 116023, China E-mail: [email protected]; [email protected] Y. Q. Chen, X. F. Yang, Y. Yu School of Chemical Science University of Chinese Academy of Sciences Beijing 100049, China Prof. H. Z. Zhang, Prof. X. F. Li, Prof. H. M. Zhang Collaborative Innovation Center of Chemistry for Energy Materials (iChEM) Dalian 116023, China The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201704987.

DOI: 10.1002/adfm.201704987

Adv. Funct. Mater. 2018, 1704987

During the past 10 years, researchers focused their most efforts on solving the PS shuttle problem by developing multifunctional and/or multiarchitectural cathodes, separators, electrolytes and anodes.[4] The most eye-catching and accepted result is LiNO3 additive, which has been demonstrated that can effectively suppress PS shuttle via forming a stable solid electrolyte interphase (SEI) layer, resulting in prolonged cycling life as well as improved coulombic efficiency (close to 100%). However, another big issue of Li–S batteries, the PS disproportionation (Equation (1)), is still lack of effective solution[5,6] Li 2Sx Li 2Sy + ( x − y )/8 S8

( x − y > 0) (1)

Zhang and Kim et al. have already pointed out that PS disproportionation is the key reason for capacity attenuation of Li–S batteries.[7] Once disproportionation occurs, the insoluble elemental sulfur or Li2Sx (x < 3) will precipitate immediately, detach from the original conducting matrix, and become “dead sulfur.”[8] Especially for the practical Li–S batteries with specific energy over 300 Wh kg−1, where the PS concentration (or sulfur/electrolyte mass ratio) should be higher than 25%, the disproportionation of PS and capacity decay become more obvious.[9] Therefore, to achieve high energy density of Li–S batteries with long cycle life, an effective strategy must be proposed to stabilize the intermediate PS. Hard and soft acids and bases (HSAB) theory is widely used in chemistry for explaining stability of compounds, reaction mechanisms, and pathways. It states that soft acids react faster and form stronger bonds with soft bases, whereas hard acids react faster and form stronger bonds with hard bases, when all other factors being equal. “Hard” applies to species which are small, have high charge states, and are weakly polarizable. “Soft” applies to species which are big, have low charge states, and are strongly polarizable.[10] This theory has already been successfully applied in Zn/Br liquid flow batteries, by using the large size of N-methyl-N-ethyl pyrrolidinium (MEP+) cation to stabilize the bromonium (Br3− or Br5−) anion with similar size. As should be noted, the PS and polybromide possess very similar properties: (1) they are soft bases; (2) they are soluble in electrolyte; (3) they are both chain like anions with large molecular size. It is worth wondering whether the MEP+ cation would have similar interaction with PS.

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Figure 1. Photographs of a 0.1 m Li2S6 salt in a) M-0 and b) M-5 electrolytes after shelving at room temperature for 0–12 d. c) The schematic diagram of PS disproportionation in M-0 electrolyte. d) The schematic diagram of the complexation between MEP+ and Sx2− in M-5 electrolyte. e) Product configuration and Gibbs free energy for the complexing reaction between MEP+ and Sx2− (take S42−, S62− and S82− for examples).

In this paper, MEP-Br was introduced into the Li–S batteries to stabilize the soluble PS away from disproportionation. The complexation behavior and mechanism between MEP cations and PS anions was originally verified by DFT calculation and direct experiment observation. The effect of MEP+ on the cathode, electrolyte, and anode was comprehensively investigated to explore the reason of battery performance improvement. In addition, 5000 mA h and 300 Wh kg−1 scaled soft package Li–S batteries were carried out to verify the practical value of this pivotal strategy.

2. Results and Discussion 2.1. Disproportionation of PS in Electrolyte The disproportionation nature of PS in electrolyte was verified by the typical shelving experiment. First, the same amount of 1 m nominal Li2S6 solution was respectively added into the conventional ether-based electrolyte with and without 5 wt% MEP-Br in the argon filled glove box. And then, both the electrolytes were sealed and laid at room temperature and atmospheric


pressure for 2 weeks. From Figure 1a,b, it was obvious that the color of both solutions was dark brown when the Li2S6 solution was just added. Afterward, the precipitation began to yield in the conventional ether-electrolyte (M-0) after 1 day and accumulate after 12 days (Figure 1a). Such precipitation was filtered out and analyzed with X-ray diffraction (XRD) measurement. As shown in Figure S4 (Supporting Information), characteristic peaks of elemental sulfur were clearly detected. Meanwhile, the color of M-0 electrolyte turned lighter after 12 days, due to the reduced PS content. Therefore, it is concluded that the sulfur precipitation attributes to continuous disproportionation of PS, coinciding with Equation (1) (as shown in Figure 1c).[6] On the contrary, PS in the electrolyte with 5 wt% MEP-Br (M-5) exhibited high stability and negligible change for 12 days shelving (Figure 1b). This satisfying result owed to the complexation between MEP+ and Sx2−, as shown in Figure 1d. Based on the HSAB theory, the hard acids are more easily to combine with hard bases via electrostatic attraction due to the strong electron accepting and donating ability; the soft acids and soft bases prefer to combine as well and form covalent coordinating bonds due to the strong overlapping ability of large electron clouds.[10,11] In the M-5 electrolyte, both of the MEP+ and Sx2−

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have large ionic radius and diffused charge clouds, thus the complexation can form through the deformation and overlap of electron clouds. 2.2. Complexation between MEP+ and PS In order to verify the complexation between Sx2− and MEP+, DFT calculation was conducted to thermodynamically predict the spontaneity of the complexing reaction. For simplicity, only single cation of MEP+ and single anion of Sx2− were considered for the simulation, regardless of other interference factors in the electrolyte. Taking S42−, S62−, and S82− for examples, the values of Gibbs free energy for the reaction between MEP+ with these Sx2− were −593, −505, and −508 kJ mol−1 (as shown in Figure 1e), respectively. The binding force of S42− with MEP+ is obviously higher than that of S62− and S82− due to the highest charge density of S42−. Moreover, the longer chain of S82− can make terminal negatively charged sulfur much closer to positively charged nitrogen through the rotation of SS single bond than shorter-chain S62−, thus exhibiting slight higher binding energy with MEP+. Even though the Gibbs free energy differs a lot between S42−, S62−, and S82−, they are negative enough for spontaneous complexing process between Sx2− and MEP+. The Gibbs free energy of the reaction between MEP+ with other anions in the electrolyte was also calculated. As shown in Figure S5 (Supporting Information), the values for Br−, NO3−, and TFSI− are −331, −309, and −252 kJ mol−1, respectively, which are much lower than that for Sx2−. It can be concluded that MEP+ thermodynamically prefer to complex Sx2− rather than other anions. Nevertheless, the actual interaction between the MEP+ and Sx2− still needs to be verified by experimental characterization.

To this end, the adsorption measurement was conducted to confirm the complexation between MEP+ and Sx2−, excluding the interference of TFSI−. After adding MEP-Br salt, the color of Li2S8 solution (0.1 m, dimethyl ether, DME, as solvent) significantly faded (Figure S6, Supporting Information). It should be noted that MEP-Br, as a kind of strong polar salt, is unable to be dissolved in week polar ether solvent, without the existence of strong polar salt lithium bis(trifluoromethane-sulfonyl) imide (LiTFSI). Further characterize the existence of PS in supernatant via UV–vis spectroscopy, and both supernatants were diluted five times due to detection limits (Figure 2a). As shown in Figure 2b, the nominal Li2S8 exhibited strong absorbance at the wavelength range of 350–500 nm, coinciding with the recent reports, while a sharp drop of absorbance took place in that region after adding MEP-Br salt, which confirms the strong PS absorbability of MEP-Br.[12,13] The complexation between MEP and PS was directly detected by liquid phase NMR. Given the extremely week response of 15N and none response of S and O, only 1H and 13C NMR spectrograms are available. The liquid state NMR of MEP-Br dissolved in deuterated DMSO is consistent with the reported lecture, as shown in Figure S7 (Supporting Information).[14] After introducing PS, the positions of peaks both in 1H and 13C NMR spectrograms have a slight shift. Besides, the division of the 1H peak at 1.19 ppm also changed from triplet to multiplet. It demonstrates that the electron cloud density distribution of MEP+ can be influenced by PS. Beyond that, FTIR was also conducted to investigate the interaction between MEP+ and Sx2−. The MEP-Br sediment after absorbed Li2S6 was filtered out and dried at 50 °C in glove box overnight. The PS sample was prepared by adding a drop of Li2S6 solution and drying at 50 °C in glove box for 2 h. The transmission FTIR curves of pure MEPBr, MEP-Br absorbed Li2S6, and pristine Li2S6 were displayed

Figure 2. a) Photograph and b) UV−vis spectra of the nominal Li2S8 solution before and after absorption by MEP-Br. c) UV−vis spectra of nominal Li2S8, Li2S6, and Li2S4 in M-5 electrolyte and M-0 electrolyte. d) Photograph and e) dissolved sulfur content of nominal Li2S8 and Li2S4 solution in electrolytes.


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in Figure S8 (Supporting Information). There is a new peak at 510 cm−1 appeared on the FTIR curves of MEP-Br absorbed Li2S6, corresponding to stretching vibration of SS bond.[15] Comparing with pristine Li2S6, the peak position of SS bond had a blueshift from 480 cm−1, regardless of several peaks at 600–2000 cm−1 due to the residual DME solvent. Based on the study of Li et al. and Biswas et al., the change mainly attributed to the different conformation of Sx2−.[16] The SS bonds of Sx2− can rotate and distort to form a stable conformation with the effect of MEP+, coinciding with the theoretical calculation results (Figure 1e). These results indicate that the complexation between MEP+ and Sx2− is not strong as covalent bond, but can still induce the deformation of Sx2− and redistribution of MEP+ electron cloud density to form stable complex. By the way, the moderate intensity of such complexation is also beneficial to avoid steady chemical bond obstructing the release of Sx2− and increasing the loss of active material. Account for the interference of other ions, the complexation between MEP+ and Sx2− in M-0 and M-5 electrolyte was further investigated by UV–vis spectroscopy (as shown in Figure 2c). It is known that PS in the electrolyte is composed of various chain-length PS anions due to a series of disproportionation process. Many researchers have reported that longer-chained PS anion can undergo a disproportionation reaction to produce shorter-chained PS anion and elemental S8, following Equations (2) and (3), and S62− can also dissociate to produce the radical anion S3•− along with Equation (4) as well.[6,17,18] Consequently, these species, S82−, S62−, S42−, and S3•−, will coexist in electrolyte solution with a dynamic equilibrium 1 S82− S62− + S8 4

(2)

1 S62− S42− + S8 4

(3)

S62− 2S3•−

(4)

In the M-0 electrolyte, maximum absorption wavelengths (λmax) of Li2S4, Li2S6, and Li2S8 were 318, 319 and 320 nm respectively, which attributed to strong absorbance of S42− (420, 320 nm) and weak absorbance of S3•− (617 nm).[12,18,19] Quite intriguingly, the λmax of Li2S4, Li2S6, and Li2S8 in M-5 electrolyte shifted up to 327 nm (9 nm higher than M-0), 332 nm (13 nm higher than M-0), and 337 nm (17 nm higher than M-0), respectively. This redshift mainly attributes to the higher content of long-chain PS like S62− (470, 350 nm) and S82− (560 nm).[19] It is the strong combination and stable complexing products between MEP+ and Sx2− that significantly drive the balance of Equations (2)–(4) toward the reverse direction of the PS disproportionation, thus improving the stability of S62− and S82−. On this basis, it can be concluded that the addition of MEP+ would affect the distribution of PS composition in electrolyte during charge/discharge process, which is confirmed via ex-situ dissolving test. This experiment was carried out by adding the same mole of nominal Li2S8 (mixing S8 and Li2S with molar ratio of 7:8) and Li2S4 (mixing S8 and Li2S with molar ratio of Adv. Funct. Mater. 2018, 1704987

3:8) in M-0 and M-5 electrolyte respectively, and then stirred at 55 °C for 60 h. The reaction equation is shown below 7 Li 2S + S8 → Li 2S8 8

(5)

3 Li 2S + S8 → Li 2S4 8

(6)

As shown in Figure 2d, M-5 electrolyte with Li2S8 exihibited less precipitation residual than that of M-0 electrolyte. However, negligible discrepancy could be recognized for both electrolytes with Li2S4. The sulfur concentration (Figure 2e) was detected by oxidizing all the dissolved sulfur species into insoluble sulfate.[20] For the nominal Li2S8 solution, the dissolved sulfur content in M-5 electrolyte is 10.88%, which was 34.5% higher than the value of 8.09% in M-0 electrolyte. According to the previous theoretical calculation and experimental results, the strong combination between MEP+ and Sx2− can effectively improve the stability of long-chain PS, thus the balance of Eqution (5)–(6) will be drived toward the long-chain PS products, which leading to high sulfur content. As for the short-chained PS, Li2S4 as example, the sulfur content in both electrolytes sharply reduced, while the value of M-5 electrolyte (6.34%) still gained 8% more than the value of M-0 electrolyte (5.86%).

2.3. Electrochemical Performance of MEP-Contained Electrolyte The electrochemical performance of MEP-contained electrolyte on anode side was first studied by using LiǀLi symmetric cells, where the lithium plate simultaneously served as work electrode and counter electrode. Figure 3a–d showed that the impedence of lithium symmetric cells exhibited good stability after standing for 4, 8, 12, and 24 h at room temperature, indicating high interficial stability of lithium metal surface. By fitting the Nyquist plots to a equivalent circuit modal (shown in Figure S9, Supporting Information), the charge transfer impedance (Rct), which corresponding to the diameter of semicircle at high frequency, is associated with the Li+ transfer resistance through the interface on lithium anode (SEI layer). Notebly, the Rct shows the trend of first decreasing and then raising with the MEP content increasing and the lowest value is exhibited for M-5 electolyte. That means the lowest Li+ transfer resistance through SEI layer can be achieved with the M-5 electolyte, which is beneficial to promote the migration of Li+ through electrolyte/anode interface. The further cyclic stability of LiǀLi symmetric cells was compared in Figure 3e–h, where the current density and plating/stripping capacity were set as 1 mA cm−2 and 1 mA h cm−2, respectively. Comparably stable voltage profile can sustain until the MEP content rise to 5 wt% during 100 cycles. Nevertheless, a significant increasing trend of plating/stripping voltage was observed as the MEP content reached to 10 wt%. Based on the SEM images of lithium surface after 100 cycles, the addition of MEP can lead to lower Li nucleation radius and looser SEI layer (as shown in Figure S10, Supporting Information). The loose structure facilitate the permeation of Li+ through the SEI layer and lower the transfer

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Figure 3. The EIS curves of LiǀLi cells with a) M-0, b) M-2, c) M-5, and d) M-10 electrolyte. Galvanostatic plating/stripping profile of LiǀLi cells with e) M-0, f) M-2, g) M-5, and h) M-10 electrolyte. The current density is 1 mA cm−2 and the stripping/plating capacity is 1.0 mA h cm–2.

resistance to some extent. However, too high MEP content (10 wt%) leads to excessively thick SEI layer on the lithium surface (as shown in Figure S11, Supporting Information), thus increasing the Li+ conducting resistance. So as to investigate the SEI composition and the effect of MEP-Br on Li anode, X-ray spectra (XPS) measurement was conducted on Li surface after 40 cycles in LiǀLi cells with M-0 and M-5 electrolyte. Figure S12 (Supporting Information) shows the C1s, N1s, F1s, and S2p spectra of the Li anode, and Table S1 (Supporting Information) shows the detailed results and analysis of peak deconvolution and assignments. It is not hard to see that all of the solvents, conducting salt and additives in electrolyte can react with lithium to form a complicated SEI film. For C1s spectra, the organic species including alkyllithium (284.8 eV), alcoholates (285 eV), and carbonate ester (288.7 eV) generate by the polymerization and reduction of ether solvents.[21,22] The inorganic species mainly compose of residual and decomposition of conducting salt and additives, such as imide group (399 eV), SO2CF3 (688.9, 169 eV), LixSOy (167 eV), and LiF (685.2 eV) deriving from LiTFSI, LiNO2 (407.4 eV), and Li2N2O2 (404 eV) deriving from LiNO3.[21,23] After introducing MEP-Br in the electrolyte, ammonium group (402.5 eV) appears in the N1s spectra, which belongs to part of MEP+ that embedded in the reduction products of anions to keep charge balance.[24] Besides, the


content of some inorganic species attributed to the deposition of LiNO3 and LiTFSI exhibits an apparent decline with the addition of MEP-Br, while the content of some organic species like carbonate ester trend to increase. The increasing organic species will induce the construction of SEI film much looser and facilitate Li+ transmission in the SEI layer, which is consistent with the SEM images of Li surface morphology and cyclic performance of LiǀLi cells. The electrochemical performance of MEP-contained electrolytes on cathode side were tested with the CR2016 Li–S cells composing of C/S composite cathodes, Celgard 2325 seperators, Li anodes, and electrolytes. There is an uncommon but reasonable phenomenon shown in CV curves. During the reduction process of S cathode, two peaks were clearly observed at about 1.9 and 2.3 V with all the electrolytes, corresponding to the reduction of sulfur to long-chain PS (Li2Sx, 4 ≤ x ≤ 8) and the reduction of long-chain PS to Li2S, respectively (Figure 4a).[3,25] However, the onset potential at high voltage region improved with the increasing MEP+ content (shown in Figure 4c), while the opposite trend appeared at low voltage region (shown in Figure 4b). According to Nernst equation, the electrochemical reaction potential is closely related to the concentration and existence form of reactants and products. During the discharge process, the electrochemical reaction and Nernst equation of cathode is described as below:

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Figure 4. a) CV curves of Li–S batteries assembled by M-0 electrolyte with various MEP content. Enlarged view of the CV curves at the b) low voltage region and c) high voltage region near the onset reduction potentials. The scan rate is 0.1 mV s−1 and voltage range is 1.7–2.8V.

Electrochemical reaction − x 1e S8 + mMEP + n → [Sx (MEP)m ]m −2 I 8 xLi + + n2e − +2  → xLi 2S + mMEP + II

(7)

Nernst equation E 1 = E 10 +

E 2 = E 20 +

RT  (CM )m  RT RT 0 ln  ln β − ln CPS  = E1 − n 1F  C C  n 1F n 1F

RT  CC ⋅ (CLi )x ln  n2F  (CM )m

(8)

 RT RT 0 ln β + ln [CPS ⋅ (CLi )x ] (9)  = E2 + n 2F n 2F 

where

β=

CC CPS ⋅ (CM )m

(10)

Equations (8) and (9) correspond to the Nernst equation of electrochemical reactions I and II, respectively, where the concentration of solid can be regarded as 1. E1, E2, E10, and E20 stand for the actual and standard potential of reactions I and II, respectively. CM, CPS, CC, and CLi correspond to the concentration of MEP+, uncoordinated PS, complex compound, and Li+, respectively. n1 and n2 refer to the electron transfer number of reactions I and II, respectively. β is the equilibrium constant of complexing reaction between PS and MEP+. With the increasing content of MEP+, PS is more easier to be complexed thus acquiring reduced concentration of uncoordinated PS, which leads to the increaing E1 and decreasing E2. Similarly, the onset potential change was also reflected in the oxidation process of cathode. It demonstrates that the complexation of Sx2− and MEP+ changes the electrochemical reaction mechanism and potentioal of the sulfur cathode, and such effect is reversible. Given the influence of ion transport, the half peak width and potential difference between the oxidation and


reduction peaks were significantly increased when the content of MEP anions rose up to 10 wt%, which mainly caused by the higher ionic strength and lower ionic conductivity (as shown in Figure S13, Supporting Information). Galvanostatic charge/discharge measurement was subsequently carried out to detect the cyclic and rate performance of CR2016 Li–S batteries. As shown in Figure 5a, the highest reversible capacity of Li–S battery with optimized M-5 electrolyte was reserved over 930 mA h g−1 after 100 cycles at 0.2 C, which was much higher than that of Li–S battery with M-0 electrolyte (744 mA h g−1). Based on the sulfur loading (≈1.5 mg cm−2), electrode area (1.539 cm−2), electrolyte volume (45 µL) and electrolyte density (≈1.16 g mL−1) of coin Li–S cells, the molar ratios of MEP+ and sulfur for M-2, M-5, and M-10 cells are 13.4, 5.3, and 2.7, respectively. During the charge–discharge process, the concentration of PS can reach maximum value when the sulfur species are total transform to S42−. In this case, 30% and 75% S42− can be stabilized by MEP+ in M-2 and M-5 electrolyte, respectively, while all the S42− can be stabilized in M-10 electrolyte. Even for the longestchained PS S82−, the MEP+ in M-2 electrolyte can only stabilize 59.7% PS. Excessive MEP+ like M-10 electrolyte enhances not only the stability of PS but also its solubility in electrolyte, which may bring a risk of shuttle effect and active materials loss. On the other hand, insufficient stabilizing ability like M-2 electrolyte is not enough to effectively suppress the PS disproportionation. M-5 electrolyte is no doubt the most optimized among these three, owing to its appropriate stabilizing ability and superior battery performance. As shown in Figure 5c,d, the initial discharge capacity of Li–S batteries was almost the same. However, after 100 cycles, the capacity output with M-10 electrolyte took obvious reduction at low potential paltform compared with M-5 electrolyte, which mainly caused by the serious polarization, consistent with the CV results. Besides that, M-5 electrolyte also owned the highest ionic conductivity of 10.29 mS cm−1 (Figure S13, Supporting Information) and lowest Li+ transfer resistance on Li anode surface, leading to superior rate performance as shown in Figure 5b, which is

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Figure 5. The a) 0.2 C cyclic performance and b) rate performance of Li–S batteries with various kinds of electrolyte. c) The initial and d) the 100th charging/discharging curves of (a).

827 mA h g−1 at 1C rate (maintained 58.5% of the discarge capacity at 0.1 C). Long cyclic performance of Li–S batteries with and without MEP-Br electrolyte additive was also test at the rate of 1 C, except that the first cycle was test at the rate of 0.1 C to activate the cathode. As shown in Figure S14a (Supporting Information), batteries with MEP+ electrolyte additive exhibited higher cyclic stability than these without MEP+. The discharge capacity of Li–S batteries with M-5 electrolyte is able to maintain at 868.9 mA h g−1 after 100 cycles and 748.5 mA h g−1 after 300 cycles, which were 32% and 31% higher than that of batteries with M-0 electrolyte after 100 and 300 cycles, respectively. It convincingly verified that the introducing of MEP+ can effectively complex PS to avoid fast capacity fade caused by disproportionation PS, even at high rate. However, M-5 batteries suffered rapid capacity loss after 400 cycles and M-0 batteries failed after 372 cycles. It mainly blames on the exhausted LiNO3, drying electrolyte and lithium dendrites, which needs to address in the future research. The morphology of cathode was characterized by SEM measurement before and after 70 cycles. As shown in Figure S15a,b (Supporting Information), the distribution of sulfur on origin cathode is uniform and there is no bulk sulfur appeared. During the charge–discharge process, part of active materials will be dissolved into electrolyte as form of PS and detach from the porous carbon matrix. The soluble PS is not stable in electrolyte and will take a series disproportionation to generate bulk sulfur precipitates (as shown in Figure S15c,d, Supporting Information). The bulk sulfur precipitates are hardly possible to return to electrochemical process because of its poor conductivity, thus come into being “dead sulfur.” In this way, active materials continuously depart from conducting network and become “dead sulfur,” which results in increasing polarization


and rapid capacity fading. After adding MEP+, the PS stability was significantly improved and no bulk sulfur precipitates existed on the cathode (as shown in Figure S15e,f, Supporting Information). It demonstrates that introducing a PS stabilizer is an effective strategy and shouldn’t be overlooked on the way of pursuing high energy density and long cycle life of Li–S batteries. Aiming at pratical application, the 5000 mA h soft-package Li–S batteries were assembled with M-0 and M-5 electrolyte. Both of the electrolyte can achieve 300 Wh kg−1 of initial energy density (Figure 6). However, batteries with M-0 electrolyte suffered rapid capacity fading and short lifetime (58 stable cyles before the sudden decrease). It is mainly caused by the serious PS disproportionation in practical soft package batteries, where the sulfur/electrolyte ratio is extremely high (1/4 or higher) to ensure high specific density.[26] After introducing large size MEP+, the PS stability in electrolyte was significantly improved, and over 65% capacity still remaind after 100 cycles at 1/20 C. It demonstrates that PS stabilization is a pivotal strategy to achieve high energy density Li–S batteries with long cycle life.

3. Conclusion In summary, this work introduced a kind of large size cation (MEP+) as a PS stabilizer to suppress the disproportionation of PS and improve the performance of high energy-density Li–S batteries. The complexation between MEP+ and Sx2− was verified by ex-situ UV–vis spectroscopy, liquid state NMR, FTIR, and DFT theoretical calculation. Further investigating the complexation effect on the electrolyte, cathode, and anode of Li–S batteries, it was found that the PS stability in electrolyte was significantly improved, the electrochemical reaction

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Figure 6. a) Cyclic performance of soft-package Li–S batteries at 1/20 C. The insert picture is the digital photograph of soft-package Li–S battery. b) The initial and c) the 100th charge–discharge curve of soft-package Li–S batteries.

mechanism and potential of cathode were reversibly changed, and Li+ transfer resistance through the SEI layer was also improved with the addition of MEP+. Thanks to these achievements, the reversible capacity of coin Li–S cells remained 827 mA h g−1 at 1 C and 930 mA h g−1 after 100 cycles at 0.2 C, when the optimized MEP content in the electrolyte was 5 wt%. Moreover, 5000 mA h soft-package Li–S batteries with high energy density of 300 Wh kg−1 also realized long cycle life of 100 cycles (58 cycles without MEP+), indicating the great value of PS stabilization strategy in practical application.

4. Experimental Section Materials Preparation: M-0, which stands for electrolyte including 1 mol L−1 LiTFSI and 5 wt% LiNO3 in 1,3-dioxolane/DME (v/v = 1:1), was purchased from Guotai-Huarong New Chemical Material Corp., and the water content was below 25 ppm. MEP-Br (Shanghai Chengjie Chemical Corp.) was dried in vacuum oven at 100 °C for 12 h. The various MEP-contained electrolytes were prepared by directly mixing M-0 with 2, 5, or 10 wt% MEP-Br in an Ar-filled glove box, and respectively corresponding to M-2, M-5, or M-10 electrolyte. Physicochemical Characteristic: XRD (DX-2700) tested at 30 mA and 40 kV with Cu-Ka radiation (λ = 0.154 nm) and the XRD dates were collected from 10° to 90° in 2θ at a scanning rate of 14° min−1. UV– vis spectrometer (JASCO, FT-IR 4100) was used to characterize PS with different chain length in electrolytes. Fourier transform infrared (FTIR) spectra were performed on an FT-IR Spectrometer (Nicolet iS5, Thermo Scientic). 1H and 13C NMR spectra were recorded on a 400 MHz spectrometer (Bruker AvanceIII). XPS were performed on a photoelectron spectrometer (ESCALAB 250Xi, Thermofisher). Monochromatic Al-Ka (hν = 1486.6 eV, 15 kV, 10.8 mA) was used as excitation. The working


pressure for the analysis chamber was 7.1 × 10−5 Pa. Electrochemical impedance spectroscopy (EIS) was scanned from 1.0 × 106 to 0.1 Hz at open-circuit voltage with an amplitude of 10 mV on Solartron 1287 electrochemical station. The cross sections and surface morphologies of cycled lithium anodes and cathodes were recorded by SEM (JEOL 6360LV, Japan). The conductivity of electrolyte was directly tested by conductivity meter (Mettler Toledo, SevenGo SG3). Theoretical Calculation Details: DFT calculations were performed using Becke’s three parameter functional and the correlation function of Lee et al. (B3LYP).[27] The geometry optimization and frequency calculation were carried out for all the relevant species at the B3LYP/6311+G(d,p) level of theory, which has been proved to be efficient at previous works.[28] Vibrational frequencies were computed for yielding zero-point energy and thermal corrections, Gibbs free energies were calculated at 298.15 K. The Self-consistent calculation convergence was chosen as 1 × 10−6 Hartree, and the convergences of maximum force and RMS force were set to 0.00045 and 0.0003 Hartree Å−1, respectively, with the default setting of cutoff energy depending on the atom type. All these calculations were implemented in the Gaussian 09 suite of programs.[29] The free energy change ΔG was expressed as ∆G = G complex − nGMEP − GLi x S y , and n represents the numbers of MEP additive cations. Electrochemical Characteristic: The C/S composites were synthesized according to Wang et al.,[30] and the sulfur content is 75 wt%. Mix 80 wt% as-prepared S/C composites, 10 wt% carbon black (Super P), and 10 wt% polyvinylidene difluoride in N-methyl-2-pyrrolidone to form slurry. Then pasted it on Al foil and dried at 60 °C under vacuum overnight. The sulfur mass loading was about 1.5 ± 0.2 mg cm−2. The coin cells 2016 (CR2016) were assembled with the as-prepared S/C cathode, lithium anode, Celgard 2325 separator, and aforementioned electrolytes for further electrochemical measurement. The volume of electrolyte in coin Li–S cells is 45 µL and the volume/mass ratios of electrolyte and sulfur is 19. The galvanostatic charge–discharge test was conducted between 1.7 and 2.8 V by using a LAND CT-2001A system

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at 25 °C. Cyclic voltammetry measurements were performed on a CHI611E electrochemical workstation at a scan rate of 0.1 mV s−1. All capacity values were calculated on the basis of sulfur mass. The voltage mentioned in this article was respected to Li+/Li (vs Li+/Li).

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements The authors acknowledge the financial support from DICP (Grant Nos. DICP ZZBS201708 and ZZBS201615), National Natural Science Foundation of China (Grant Nos. 51403209 and 21476224), Youth Innovation Promotion Association of CAS (Grant No. 2015148), and Dalian Science and Technology Star Program (Grant No. 2016RQ026).

Conflict of Interest The authors declare no conflict of interest.

Keywords complexing agent, high energy density, HSAB theory, Li–S battery, polysulfide stabilization Received: August 30, 2017 Revised: November 9, 2017 Published online:

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