Blocking polysulfides and facilitating lithium ion transport: polystyrene

Subscriber access provided by University of South Dakota

Energy, Environmental, and Catalysis Applications

Blocking polysulfides and facilitating lithium ion transport: polystyrene sulfonate@HKUST-1 membrane for lithium-sulfur batteries Yi Guo, Minghao Sun, Hongqing Liang, Wen Ying, Xianqing Zeng, Yulong Ying, Shudong Zhou, Chengdu Liang, Zhan Lin, and Xinsheng Peng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11042 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 18, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Blocking polysulfides and facilitating lithium ion transport:

polystyrene

sulfonate@HKUST-1

membrane for lithium-sulfur batteries ‡

‡

Yi Guo,a Minghao Sun,b Hongqing Liang,c Wen Ying,a Xianqing Zeng,b Yulong Ying,a Shudong Zhou,b Chengdu Liang,b Zhan Linb* and Xinsheng Penga* a

State Key Laboratory of Silicon Materials, School of Materials Science and Engineering,

Zhejiang University, Hangzhou, 310027 b

Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering of Manufacture

Technology, College of Chemical and Biochemical Engineering, Zhejiang University, Hangzhou 310027, China c

Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio,

Texas 78249-0698, USA KEYWORDS fast lithium transportation; low polysulfide permeability; PSS@HKUST-1 separator; high efficient lithium-sulfur batteries; high loading electrode

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 30

ABSTRACT

Minimizing the shuttle effect of polysulfides is crucial for practical application of lithiumsulfur (Li-S) batteries. However, the trade-off between effective suppression of the shuttle effect and fast redox reaction kinetics is inevitable for separator-based Li-S batteries. Herein, via a selfconfined solid-conversion process, we develop a polystyrene sulfonate threaded well-intergrown HKUST-1 (Cu3(BTC)2) (BTC: 1,3,5-Benzenetricarboxylic acid) coated Celgard separator (PSS@HKUST-1/Celgard, PHC) for high performance Li-S batteries. The PHC membrane favors the interception and accommodation of long-chain polysulfides. Notably, enormous sulfonate groups of the three-dimensional PSS networks in PSS@HKUST-1 membrane significantly facilitate lithium ion transport, which guarantee fast redox kinetics. The PHC separator demonstrates efficient inhibition of PS (i.e., 4 orders of magnitude lower in PS permeation rate) with fast Li+ transportation (i.e., 71% higher in ionic conductivity) than the Celgard separator. When applied the PHC membrane in Li-S batteries with conventional sulfur/super P carbon cathode, highly reversible capacity with an average fading rate of 0.05% per cycle is maintained for 500 cycles at 0.5 C, excellent rate performance up to 5 C and high areal capacity over 7 mAh cm-2 are also achieved. This work paves a new way for addressing the trade-off between suppressing the PS shuttle effect and fast kinetic reaction for separator-based Li-S batteries.


2

Page 3 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION The rapidly developing portable electronic devices and electric vehicles have raised high request for energy storage systems1, 2. Due to high theoretical energy density (2600 Wh kg-1, 5 times higher than lithium-ions batteries) and natural abundance of sulfur3, lithium-sulfur (Li-S) batteries are one of the most promising candidates in this regard. The commercialization of Li-S batteries has been hampered by several impediments4, among which the shuttle effect is the most challenging one, which refers to the dissolution of long-chain polysulfides (PS) (Sn2-, 4≤n≤8) in electrolytes and the migration between the cathode and the anode, resulting in severe capacity fading and low Coulombic efficiency. Notably, encapsulation and block of the dissolution of polysulfides, and enforcement of sieving of lithium ions is the golden rule for developing Li-S batteries which has been and will be widely used as the guiding principle in this very important and active field. Rational design of composite sulfur cathodes is the main approach, which not only suppresses the shuttle effect but also improves electronic conductivity and holds volume variation at the same time5-8. However, the complex synthesis procedures of these composite cathodes offset good feasibility and high energy density of sulfur, even without consideration of the PS shuttling in the electrolyte9. As comparison, separators with feasible surface modifications were proposed to mediate the shuttle effect10. In this aspect, the separator goes beyond its established role of preventing shortcircuiting in traditional batteries. Furthermore, the compromise between high Li+ conductivity/electrolyte infiltration and efficient interception of PS is inevitable. Therefore, a separator with densely-packed


3


Page 4 of 30

microporous building units is necessary for effectively blocking PS while maintaining fast Li+ transport for high-performance Li-S batteries. Metal-organic frameworks (MOFs), known as new microporous crystalline materials composed of organic ligands and metal nodes11, 12, have been well developed for selective gas separation on molecular level13, 14. Featured by tunable, highly ordered pore structures and large pore volumes, MOFs are light materials with high surface areas, which make them promising candidates for electrochemical applications15-19. A pioneering work on MOFs based separator for Li-S batteries was reported recently, in which graphene oxide layers were used to block the dissolution of polysulfides20. Unfortunately, MOFs typically are ionically insulating with little contribution to Li+ transport. Very recently, we realized a highly Li+ conductive polystyrene sulfonate threaded HKUST-1 (PSS@HKUST-1) membrane by encapsulating function molecules such as polystyrene sulfonate (PSS) into its cavities via a self-confined solid-conversion method. Its Li+ conductivity was five orders’ magnitude higher than that of pristine HKUST-121. Taking advantage of its both microporous structure and high Li+ conductivity, this PSS@HKUST-1 membrane is expected to be promising for the modification of separator and thus for highefficiency Li-S batteries. As a proof of concept, we herein modified the commercial Celgard separator by directly growing a continuous, PSS threaded well-intergrown HKUST-1 (Cu3(BTC)2) layer on it (PSS@HKUST-1/Celgard, PHC) for Li-S batteries. Due to the incorporation of PSS, the pore entrance (0.9 nm) of HKUST-1 is narrowed down to be smaller than the diameters of polysulfides (0.51-0.68 nm)22 but still large enough for Li+ transport (0.12 nm)23. Based on steric sieving effect, PHC effectively intercepts PS while allows Li+ to permeate through it. Negatively charged -SO3- groups on the surface of PHC reject polysulfides by electrostatic repulsion. Ample


4

Page 5 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


open copper sites and highly porous HKUST-1 enhance the interception and accommodation of PHC towards escaped PS as well as benefiting cycling performance. Moreover, enormous sulfonate groups of PSS network in PHC facilitate Li+ transport and improve redox kinetics of Li-S batteries. It is not only a novel methodology to enforce sieving of lithium ions but also provides the bright promising to rationally tune the pores by the implementation of guest molecules and/or MOF materials collaboratively to fulfill the “golden role” of Li-S batteries. When a PHC is assembled into a Li-S battery, it exhibits extraordinary effectiveness in suppressing the shuttle effect while maintaining outstanding electrochemical performance in the conventional sulfur/super P carbon cathode. The resulting sulfur cathode delivers an initial discharge capacity of 1278 mAh g-1 at 0.5 C and a reversible capacity of 775 mAh g-1 after 500 cycles, corresponding to a capacity decay of only 0.05% per cycle. Excellent rate performances up to 5 C and high areal capacity of 7.67 mAh cm-2 are also achieved. To the best of our knowledge, these results are superior to most separator-based Li-S batteries reported (Table S1), and comparable to those with complex cathode designs (Table S2).

Scheme 1. The illustration of the synthesis of PSS@HKSUT-1 layer on Celgard separator. 2. RESULTS AND DISCUSSION The PHC membrane was prepared via a self-confined growing process21, as shown in Scheme


5


Page 6 of 30

1. Copper hydroxide nanostrands (CHNs) were employed as metal source to prepare HKUST-1. Negatively charged PSS (zeta potential, -33.8 mV) would firmly anchor onto positively charged CHNs (zeta potential, 36.4 mV). The composite was filtered onto a commercial Celgard separator to prepare PSS@CHNs membranes. The HKUST-1 initially formed from CHNs and H3BTC on the top surface after immersing the PSS@CHNs membrane into H3BTC water/ethanol solution (volume ratio of 1/1). The top HKUST-1 layer could block the PSS from dissolving into the solution. Therefore, PSS molecules were well encapsulated into the HKUST1 membrane21, and an intact PHC was readily prepared. As shown in Figures 1a and 1b, PHC7.5% is a continuous and crack-free membrane with the thickness of ca. 9.2 µm. One would readily speculate that the channels in HKUST-1 were most likely to be the pathway for the PS shuttle. In fact, it’s difficult for polysulfides to diffuse through PHC-7.5%, because the pore aperture of HKUST-1 is narrowed by the incorporation of PSS21. Moreover, pore volume became even smaller after PSS was uniformly threaded the HKUST-1 layer in PHC-7.5%, which was proved by the EDS mapping results (Figures 1c and 1d) and BET results (Figure 1e and Figure S1). The XRD pattern of PHC-7.5% is consistent with HKUST-1, indicating the crystal structure of HKUST-1 remained after incorporated PSS (Figure 1f). In FTIR spectra, the characteristic peaks of sulfonate groups on PHC-7.5% located at 1188 and 1130 cm-1, respectively, are consistent with those of free PSS molecules (Figure 1g). The peak of S2p was also observed at 168.8 eV in the X-ray photoelectron spectroscopy (XPS) in Figure S3c. PHC with different PSS contents, namely 2.5%, 5%, 7.5%, and 10% were prepared via the similar process, denoted as PHC-2.5%, PHC-5%, PHC-7.5%, and PHC-10%, respectively. The corresponding SEM images, XRD results, and FTIR spectra are shown in Figures S2 and S3. All the results indicate PSS are incorporated into the HKUST-1 crystal layers. Except for PHC-10%, other PSS@HKUST-1


6

Page 7 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


layers are also well intergrown and continuous.

Figure 1. The SEM images of (a) surface and (b) cross-section of PHC-7.5%. The element mapping images of (c) S and (d) Cu of the marked section zone in (b). (e) The specific surface area and microporous volume of PHC with different PSS contents. (f) The XRD patterns of HKUST-1 and PHC-7.5%. (g) The FTIR spectra of PSS, HKSUT-1, and PHC-7.5%. h) The Nyquist plots of cells assembled by PHC with different PSS contents.

As reported in our previous work21, the PSS@HKUST-1 membrane demonstrated high Li+ conductivity, attributed to three-dimensional (3D) network of PSS in HKUST-1 membrane acting as the Li+ transport pathway. The electrochemical impedance spectra (EIS) of the Li-S


7


Page 8 of 30

cells fabricated with different PHC are presented in Figure 1h. The depressed semicircles in the high-to-medium frequency range of the Nyquist plots are ascribed to charge-transfer resistance (Rct) of the cells24, 25. The Rct and ionic conductivity of HKUST-1 coated Celgard (HC), PHC, and Celgard separators are shown in Table S3. HKUST-1 was known as a poor ionic conductor, thus the cell assembled with HC presented higher Rct than those assembled with PHC. The Rct decreased for cells with increased PSS contents from 2.5% to 7.5%, benefiting from excellent Li+ delivering ability of sulfonate groups of PSS21 in PHC. The ionic conductivity of PHC-7.5% is up to 1.05×10-5 S cm-1, which is 71% higher than that of Celgard. However, extra PSS (10%) would result in obvious cracks in the PSS@HKUST-1 layer of PHC-10% (Figure S2). As a result, polysulfides could diffuse through the cracks easily and lead to poor Coulombic efficiency of the cell. Surprisingly, the Rct of the cell with PHC-7.5% was much lower than that of Celgard. This could be attributed to synergetic effect of improved Li+ transport and rough surface of PHC7.5% for enhanced interfacial contact with electrolyte. Based on the above results, PHC-7.5% was chosen as the modified separator for Li-S batteries to carry out subsequent electrochemical measurements. The permeation test is a direct evidence that visualizes the blocking effect of the separator toward the PS shuttle26. In this work, the permeation experiment was conducted using an Hshape glass cell separated by a PHC-7.5% or a Celgard separator. As shown in Figure 2, two sides of the glass cell were filled with electrolyte containing yellow-brown PS (e.g., Li2S6, 0.1 M) and blank electrolyte, respectively. For the PHC-7.5% separator, the blank electrolyte in the right side of the glass cell remained colorless for as long as 60 h, indicative of successful interception of PS permeation to the left side (Figure 2a). In contrast, the permeation of PS through the Celgard separator is clearly observed within 30 minutes under the same condition


8

Page 9 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(Figure 2b)27, and more and more PS permeated to the left side with time elongation. The PS permeation rate is as low as 9×10-8 mol cm-2 h-1 for PHC-7.5%, which is 4 orders of magnitude lower than that of Celgard (2.07×10-4 mol cm-2 h-1, Figure S4). These results suggest that PHC7.5% can effectively mediate the crossover of PS within the electrodes.

Figure 2. (a) H-type permeation device with PHC-7.5% separator. The PHC-7.5% separator retained its resistance towards PS over 60 h. No obvious PS permeation through the PHC-7.5% separator was observed. (b) H-type permeation device with Celgard separator. The PS molecules gradually permeate through Celgard separator over time and fill the blank electrolyte after 12 h.

Apart from well-intergrown, continuous morphology of PHC membrane, the zeta potentials of PHC in THF decreased with the increase of the PSS content since there are plenty of -SO3groups on PSS (Figure S3d). The plentiful negatively charged sulfonate groups and uncoordinated BTC on the surface of HKUST-1 layer also contribute to the interception of PS by electrostatic repulsion. Although the encapsulation of PSS into PHC-7.5% inevitably leads to decrease in the porosity of HKUST-1, it still reached up to 1291.8 m2 g-1 (Figure 1e) for effective


9


Page 10 of 30

adsorption of PS and fast Li+ transport28. The abundant open Cu sites in the cavities of HKUST-1 also provide strong interaction with PS29. All the above reasons are beneficial for efficient adsorption of PS and fast Li+ transport for high-performance Li-S batteries. The adsorption ability of PHC-7.5% was confirmed by soaking the electrolyte containing PS (e.g., Li2S6, 2 mM). Figure S5a presents UV-vis absorption spectra of PS-containing electrolytes before and after soaking for PHC-7.5%, HKUST-1/Celgard (HC) and Celgard separators. The peak located at 280 nm was attributed to S82-/S62-.7 The sharp contrast of peak intensities before and after soaking suggests partial elimination of PS, further confirming the affinity of PHC-7.5% towards PS. XPS was performed to probe the chemical environment of element S. The peaks at 169.3 and 168.2 eV are attributed to S atoms in -SO3- from PSS, while the peaks at 163.0 and 161.7 eV are attributed to bridging and terminal S atoms in PS absorbed by PHC-7.5% (Figures S5b and S5c). There were no obvious peaks of S atoms in PS of Celgard separator, which means PS were barely absorbed by Celgard. The interaction between sulfur and open Cu2+ sites is not as strong as chemical bonds29. The lower bonding energy of S atoms in PS proved the interaction between sulfur and open Cu2+ sites. This further proving the affinity of PHC-7.5% with PS. In general, PHC-7.5% exhibited outstanding efficiency for the interception of PS owing to the synergic effect of steric sieving, electrostatic repulsion, and high porosity with open metal sites of PSS@HKUST-1 layer. These are desirable for high performance Li-S batteries. Cycling performances of sulfur electrodes with PHC-7.5% and Celgard separators were measured at 0.2 C using sulfur/super P carbon cathodes, respectively (Figure 3a). The cell with PHC-7.5% delivered an initial discharge capacity of 1330.8 mAh g-1, corresponding to 79.5% of the theoretical maximum (1675 mAh g-1) and much higher than that of Celgard (937 mAh g-1). After 50 cycles, a discharge capacity of 1111 mAh g-1 was maintained for the cell with PHC-


10

Page 11 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


7.5%, indicating a fading rate of only 0.04% per cycle after the second cycle. In contrast, discharge capacities of the cell with Celgard separator decrease rapidly to 587 mAh g-1. The average Coulombic efficiency of the cell with PCH-7.5% is also much higher than that of Celgard (99.0% vs. 97.2% for 50 cycles), suggesting effective suppression of the shuttle effect and higher sulfur utilization. Cells with HC were also assembled for comparison, which exhibited comparable cycling stability with that of PHC-7.5% but inferior specific capacity (Figure S6a). This is attributed to improved Li+ delivering ability of PSS in PHC-7.5% than those in Celgard and HC.

Figure 3. (a) Cycling performance of cells with Celgard and PHC-7.5% at C/5. (b) Nyquist plots and (c) CV profiles of fresh cells with Celgard and PHC-7.5%. (d) Rate capabilities of cells with Celgard and PHC-7.5% from C/5 to 5 C.


11


Page 12 of 30

Figure 3b shows the EIS of fresh cells with PHC-7.5% and Celgard separator, respectively. The Rct decreased after replacing Celgard separator with PHC-7.5% separator for the fresh cells, reconfirming that PSS@HKUST-1 layer of PHC-7.5% facilitates Li+ transport. Besides, rough surface of PHC-7.5% enhances interfacial contact between cathode and electrolyte which further reduce internal resistance30. After 50 cycles, Rct of the cells with PHC decreased 25% (Figure S6b), suggesting the improvement of electrolyte infiltration31, while that of Celgard increased by over 85%. To get a better understanding of the evolution in Nyquist plots, SEM and XRD were applied to probe the counter side morphologies of PHC-7.5% and Celgard after 50 cycles (anode side), respectively. As shown in Figure S7, the XRD pattern proved that the PHC was stable during the cycling-life of cells. Meanwhile, insoluble active material agglomerates are clearly observed on the surface of Celgard separator, which block the diffusion pathway of Li+. This indicates unrestricted shuttling of PS through Celgard separator followed by deposition on the counter side during cycling32. In contrast, pristine interwoven porous structure of Celgard separator was maintained on the counter side of PHC-7.5% even after 50 cycles without apparent depositions, indicating effective blocking the crossover of PS and keeping them within cathode region. This is in good accordance with the results of the permeation test (Figure 2). Cyclic voltammetry (CV) profiles of the cell with PHC and Celgard separator were recorded in a voltage range from 1.8 to 2.6 V for the first 20 cycles (Figure 3c and Figure S8). The cathodic peaks centered around 2.3 and 2.0 V are typical in carbon-based sulfur electrodes33. The former is related to transition from elemental sulfur to highly soluble long-chain PS (Li2Sx, 4≤x3 C, Figure S9a). It is worth to note that the rate performance of cell with PHC-7.5% is better than that reported by Nature energy20 (Table S4). A further comparison revealed that complete discharge plateaus were maintained at all rates for the cells with PHC-7.5% (Figure S10a). In contrast, the discharge plateaus shortened and even disappeared at high rates (Figure 10b), reconfirming excellent Li+ delivering ability of PHC7.5% and consistent with the CV/EIS results (Figure S6b and Figure S8). The capacity recovered to 1170 mAh g-1 for the cell with PHC-7.5% when high-rate tests were completed and switched back to 0.2 C. This should be attributed to efficient reutilization of the PS intercepted by PHC7.5%.


14

Page 15 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (a)Long-term cycling performance of cells with Celgard and PHC-7.5%, with sulfur loading of 1.3 mg cm-2. (b)Long-term cycling performance of cells with Celgard and PHC-7.5%, with sulfur loading of 4.3 mg cm-2. (c)Areal capacities of the cells assembled with PHC-7.5% at high sulfur loadings at 0.5 C. (d)Areal capacity comparison of the cells assembled with PHC7.5% and representative interlayer/modified separator-based works (see details in Table S5, Supplementary references).


15


Page 16 of 30

Long-term cycling capability of the cells using PHC and Celgard separators were evaluated at 0.5 C for 500 cycles. As shown in Figure 4a, the cell with PHC-7.5% delivered an initial discharge capacity of 1278 mAh g-1 and maintained at 775 mAh g-1 after 500 cycles. The capacity decay was as low as 0.05% per cycle after the second cycle while the Coulombic efficiency kept consistently above 99%. This performance is the best among those reported for Li-S batteries with only separator modifications (Table S1), and competitive to Li-S batteries with complex cathode designs (Table S2). In contrast, the discharge capacity of cell with Celgard separator faded rapidly from 993 mAh g-1 to

Blocking polysulfides and facilitating lithium ion transport: polystyrene

Recommend Documents