A Bifunctional Binder with Nucleophilic Lithium Polysulfide

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A Bifunctional Binder with Nucleophilic Lithium Polysulfide Immobilization Ability for High-loading, High-thickness Cathodes in Lithium-sulfur Batteries Pauline Han, Sheng-Heng Chung, Chi-Hao Chang, and Arumugam Manthiram ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02399 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 23, 2019

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ACS Applied Materials & Interfaces

A Bifunctional Binder with Nucleophilic Lithium Polysulfide Immobilization Ability for High-loading, High-thickness Cathodes in Lithium-sulfur Batteries Pauline Han, Sheng-Heng Chung, Chi-Hao Chang, Arumugam Manthiram*

Materials Science and Engineering Program & Texas Materials Institute

The University of Texas at Austin, Austin, TX 78712, USA

Corresponding Author * E-mail: [email protected] (A. Manthiram) Tel: +1-512-471-1791; fax: +1-512-471-7681

Keywords: lithium-sulfur batteries, high active-material loading, polysulfide shuttle, polymer, multifunctional binder

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ABSTRACT Lithium-sulfur batteries remain a promising next-generation renewable energy storage device due to their high theoretical energy density over current commercial lithium-ion battery technology. However, to have any practical viability towards reaching the theoretical value, high-loading cathodes with sufficient sulfur content and specifically the effect of nonconductive binders must be investigated. We consider the limitations of conventional binders for highloading, high-thickness cathodes by integrating a bifunctional binder with a linear poly(ethylene) chain and maleate capped ends. The linear polymer allows for flexibility within the high-loading cathode while the maleate ends improve the polysulfide trapping ability with carbon-sulfur binding. With the strong polysulfide immobilization ability due to the nucleophilic binding, the binder achieves high sulfur loadings of 12 mg cm-2 with a high sulfur content of 80 wt.%. The work serves as a proof-of-concept for exploring the incorporation of polymeric materials into sulfur cathodes to realize practical viability.


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Lithium-sulfur (Li-S) batteries are widely regarded as a next-generation energy storage device due to the high theoretical capacity of sulfur (1,675 mA h g-1) and the high theoretical energy density of Li-S batteries (2,600 W h kg-1).1–3 Furthermore, Li-S technology benefits from sulfur being naturally abundant and environmentally benign.1,4 However, due to their conversion chemistry, there are intrinsic challenges within the cell that must be addressed before their implementation and practical viability. First, sulfur and the end discharge products Li2S2 and Li2S are naturally insulating, reducing the electronic or ionic transport within the sulfur cathode and compromising the active material utilization.2,5–7 The charge-discharge process also suffers from a large volume change, resulting in reduced electrochemical utilization due to the loss of electrochemical contact within the cathode.1,8 Additionally, the intermediate lithium polysulfide species (LiPS, Li2Sn, n = 4 – 8) readily dissolve in the organic electrolyte, migrate through the separator to the lithium-metal anode, and shuttle between the cathode and anode during cycling.9,10 The LiPS shuttle results in low electrochemical reutilization and compromised Coulombic efficiency.11–13 These intrinsic challenges are further exaggerated with high-loading sulfur cathodes (> 4 mg cm-2) with a high sulfur content (> 55 wt. %) and are reflected in a reduction of the specific capacity. The electrochemical utilization of sulfur tends to be compromised due to the low internal conductivity brought about by the high insulating sulfur content and loading.6,14 Moreover, the concentration of LiPS species is naturally increased with high sulfur loading, further affecting the redox kinetic capabilities of the Li-S cells.15–19 To overcome these intrinsic


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challenges, conductive additives and composite cathodes have been developed.20–22 The designed composite cathodes involve tailored carbon materials with heteroatoms,23,24 excess porosity,25,26 conductive dopants,27–29 and one-dimensional materials.30–32 These techniques in manipulating the carbon materials have been vastly studied to improve the cycle life, capacity, and stability, but are unable to be scaled for practical and commercial viability.33 Loss of active material due to the loss of electrochemical contact arising from debonding within high-loading cathodes remains an issue.34,35 Thus, investigation into the polymeric binder is important when dealing with high-loading sulfur cathodes. Polyvinylidene fluoride (PVDF) is the conventional binder used in Li-S technology due to its adaptation from lithium-ion battery technology. However, with a larger amount of sulfur in high loading cathodes, PVDF is inefficient in promoting active material utilization due to its insulating nature itself. As a cathode binder, it is stable with the electrode components but lacks the mechanical properties necessary to support the volume change during cycling.34–37 The poor physical binding abilities of PVDF causes a delamination of the electrode components and largely contributes towards the low electrochemical utilization and fast capacity fade of highenergy-density Li-S batteries.21,38,39 A bifunctional binder that immobilizes LiPS species within a conductive carbon matrix is investigated here for realizing better electrochemical contact resulting in improved cycling performance.


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Cathode preparation and scanning electron microscopy (SEM) of the prepared cathodes The binder was first studied for its stability. The surface interactions of the commercial maleate-polyethylene glycol (PEG) binder were first investigated after being dried and purified. Figure S1 portrays the survey and deconvoluted C1s spectra of the maleate-PEG binder by X-ray photoelectron spectrophotometry (XPS). The C1s spectra demonstrate the typical -COOH peak at 288.3 eV and the C-O peak at 287.4 eV. The N-C amine-bound carbon peak is found at 289.1 eV as expected from the structure.40,41 The deconvoluted oxygen 1s peaks demonstrate a C-O-C interaction at 531.5 eV, typically seen with PEG or other glycols.38,42,43 The elemental surface examination gives rise to the three interested groups of study: maleate, PEG group, and aminecapped glycol. The maleate species and amine-PEG ends act as chemical binders for the excess LiPS species while the linear PEG chain acts as a binder for the physical cathode components.40,44,45 To investigate the benefits of the bifunctional binder in Li-S cells, cathodes were first prepared with sulfur, super P, and binder in a 80:10:10 wt. ratio and slurry-cast onto an aluminum foil current collector. The cathodes were dried, cut out, and tested in a CR2032-coin cell with lithium metal as the anode. The cell prepared with maleate-PEG as the binder will be hereafter referred to as “M80S-cell.” For the control cells, the commonly used binders PVDF and PEG were used in lieu of the maleate-PEG binder. The cells prepared with either PVDF or PEG as the binder will be hereafter referred to as, respectively, “PVDF80S-cell” or “PEG80S-cell.”


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Electrochemical impedance spectroscopy (EIS) was conducted on the conventional Li-S battery cathodes before cycling (Figure 1a). The EIS of the cathodes was observed between a frequency range of 1 MHz and 100 mHz. Using an equivalent circuit model, the charge-transfer resistance (RCT) and information on the intrinsic transport properties of the assembled cells can be extrapolated.46–49 The Nyquist plots are represented in Figure 1a. The M80S-cell exhibits a reduced RCT of 21 Ω, which is lower than those of the PVDF-cell (311 Ω) and the PEG-cell (175 Ω). The PVDF-cell exhibits the typical charge-transfer resistance of ~ 300 Ω due to the low conductivity of PVDF and sulfur. PEG is slightly more ionically and electronically conductive than PVDF, offering a lower internal resistance of 175 Ω in the assembled cell.35,43 The M80Scell offers the lowest resistance among the three cells, indicating good binding interactions with the LiPS species and the conductive carbon LiPS trap. The huge decrease in resistance suggests that the M80S-cell cathode has composited with the LiPS trap, forming a homogeneous and conductive composite cathode upon cell assembly.


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Figure 1. Electrochemical performance analysis of M80S-cells. (a) EIS profiles with the equivalent circuit model below, (b) CV profile at a scan rate of 0.075 mV s-1, (c) cyclability at C/5 rate in comparison to the control cells, and (d) cyclability of the M80S-cell at C/5 and C/10 rates.


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Cyclic voltammetry (CV) profiles were recorded for the Li-S cells in the voltage window of 1.7 – 2.8 V (vs. Li/Li+). Figure 1b depicts the voltage profile for the M80S-cell with a sulfur loading of 6 mg cm-2 at a scan rate of 0.075 mV s-1. The two cathodic peaks are marked as peaks (I) and (II), representing the conversion of sulfur to intermediate LiPS species and to the enddischarge product Li2S2/Li2S, respectively. The anodic peaks at (III) and (IV) demonstrate the oxidation from the reduced species back to elemental sulfur. The good overlap of the reduction peaks demonstrates good reversibility and excellent electrochemical stability. The overlapping anodic peaks (III, IV) shift to lower potentials upon cycling, indicative of good cell reaction kinetics and active material reutilization. The good reversibility of the M80S-cell can be attributed to both the chemical and physical retention of sulfur species by the new binder. To deduce the LiPS mediation ability of the maleate functional group upon cycling, CV profiles were compared between the M80S-cell (with maleate group) and the PEG-cell (without maleate group) at a scan rate of 0.055 mV s-1 (Figure S2). The shifts in the peaks give an indication of LiPS diffusion during cycling. Comparing the two curves, the first cycle demonstrates fairly similar cathodic and anodic curves. Upon cycling, the anodic peaks (III, IV) shift considerably for the PEG-cell than the M80S-cell. This demonstrates that there is no LiPS migration and loss of active material in the M80S-cell, giving evidence that the maleate group has significant LiPS mediation abilities. Galvanostatic cycling was performed on the assembled cells to test their electrochemical properties (Table 1). Figure 1c compares the M80S-cell to the PVDF80S-cell and the PEG80S-


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cell at the same sulfur loading of 6 mg cm-2 at C/5 rate. The cells all use the same sulfur content of 80 wt. %. At a C/5 rate, the M80S-cell attains a high initial capacity of 1,102 mA h g-1, resulting in a sulfur utilization of 66 %. After 200 cycles, the reversible capacity is 522 mA h g-1, which translates to a low capacity-fading rate of 0.26% per cycle. The PVDF80S-cell exhibits the fastest capacity fading rate (0.41 % per cycle) over the other two cells. This can be attributed to the rigid structure of the long-chain PVDF and the loss of electrochemical contact in the sulfur cathode during cycling, demonstrating the necessity for new binders for high-loading sulfur cathodes.34,50 The PEG80S-cell demonstrates a similar capacity fade of 0.38 % per cycle over 200 cycles and lower electrochemical utilization with an initial capacity of 914 mA h g-2 at a C/5 rate. Thus, LiPS mediation ability and homogenous cathodes are vital for realizing good cell performance and capacity retention during cycling. The ability for the binder to trap LiPS through chemical and physical means facilitates good charge transport within the cathode. The utilization of the bifunctional binder offers good cell conductivity and improved rate performance with high-loading sulfur cathodes.


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Table 1. Electrochemical performance data of the M80S-cell compared with the control cells PVDF80S-cell and PEG80S-cell Cycling Rate

Initial Capacity (mA h g-1)

Sulfur Utilization (%)

Reversible Capacitya) (mA h g-1)

Reversible Capacityb) (mA h g-1)

Capacity Retentiona) (RQ, %)

Capacity Fading Rate (% per cycle)

M80S-cell

C/5

1,102

66

926

522

84

0.26

PVDF80S-cell

C/5

1,069

64

838

177

78

0.41

PEG80S-cell

C/5

914

55

801

205

88

0.38

C/10

1,038

62

814

413

78

0.10

Cell

M80S-cell a)

after 50 cycles, b) after 200 cycles

Table 2. Upper and lower discharge plateau capacities and the respective retention rates of the M80S-cell at various cycling rates

M80S-cell

Cycling Rate

QH (Peak Cycle, mA h g-1)

QH (50th Cycle, mA h g-1)

RQH (%)

QL (Peak Cycle, mA h g-1)

QL (50th Cycle, mA h g-1)

RQL (%)

C/5

332

292

88

770

736

96

C/10

319

246

77

719

571

79

The electrochemical capability of M80S-cell was also examined at different cycling rates of C/5 and C/10 (Figure 1d). The higher amount of LiPS species formed directly contributes to the differences found in active material utilization. This highlights the need for better electrochemical contact in high-loading sulfur cathodes. The upper discharge (QH) and lower discharge plateaus (QL) are observed for the M80S-cell at various cycling rates (Figure S3, Table 2), explaining the details of the cell’s reaction kinetics during cycling. The upper-discharge plateau provides insight for the initial reduction from sulfur to intermediate LiPS species while


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the lower-discharge plateau serves as evidence for the conversion from intermediate LiPS species to the end-discharge product. At a faster rate (C/5 rate), the RQH and RQL both exhibit high retention rates of, respectively, 88 % and 96 %, indicating a conservation of redox kinetics during the first 50 cycles. The RQH and RQL fall to, respectively, 77 % and 79 % at a slower cycling rate (C/10 rate), indicating the loss of active-material utilization. The loss is typically seen in high-loading cathodes as LiPS easily form and participate in the LiPS shuttle. Thus, the bifunctional maleate-PEG binder offers good chemical and physical LiPS mediation and boosts the capacity in high-loading sulfur cathodes.

Post-mortem examination of sulfur cathode assembled with the binder To elucidate the binding interactions of LiPS species and the maleate-capped ends of the maleate-PEG binder, the cathodes were examined by X-ray photoelectron spectroscopy (XPS) before and after cycling (Figure 2). The cycled M80S-cell cathode exhibits a new deconvoluted C1s peak after cycling, representing the S-C displacement at the α-position of maleate. This demonstrates that the retention of LiPS species and active material was abetted by the maleate ends. To further inspect the cathode binding ability of the short-chain PEG, morphological studies were carried out with the cross-section of the cycled cathodes (Figure S4). The crosssectional SEM and EDX examination of the cathode after cycling demonstrates good sulfur redistribution among the cathode components, representing a good LiPS mediation ability by the


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maleate ends. The cathode integrity after cycling supports the cathode binding abilities when utilizing the maleate-PEG binder. The sulfur is immobilized within the electrochemically active materials, allowing for good reutilization and capacity retention as seen previously in the electrochemical performance. Figure S5 demonstrates the morphological examination of the surface of the electrode after cycling. The homogeneous cathode after cycling demonstrates good stability of the binder in the electrolyte and among the cathode components. The lack of large pores or cracks demonstrates good electrochemical contact after cycling, indicating the bifunctional binder is able to strengthen the durability of the cathode.

Figure 2. Comparison of the XPS analysis of the C1s peaks of the maleate-PEG binder and a cycled M80S-cell.


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State-of-the-art Ultrahigh-loading Sulfur Cathodes To enhance the electrochemical reaction kinetics of the high-loading sulfur cathodes, the previously used cathode composite was applied directly to interconnected carbon networks (Figure 3a, Figure S6). In pursuit of moving beyond the many intrinsic challenges involving an Al foil as the current collector for ultrahigh-loading sulfur cathodes, compressed carbon paper was tested for improved conductivity and physical trapping ability of LiPS species. The compressed carbon paper was applied due to (i) the interconnected one-dimensional carbon network for flexibility and (ii) the rough surface and increased surface area that could provide better slurry binding ability. Because of the bending ability and homogeneous properties, the free-standing, high-loading cathodes with the maleate-PEG binder were used to create layered cathodes (L-M80S-cathode). The slurry was mixed in a 80:10:10 wt. ratio of sulfur, Super P, and maleate-PEG binder and tape-casted onto compressed carbon paper. The motivation behind rollpressing the carbon paper was to match the homogeneity and thickness of Al-foil. After drying, the cathodes were cut out and they had a loading of 6 mg cm-2. The cathodes were stacked to create the layer-by-layer tandem cathode cells (L-M80S-cell), achieving an ultrahigh sulfur loading of 12 mg cm-2. Figure 3a depicts the schematic of assembling the L-M80S-cell.


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Figure 3. (a) Coin-cell configuration schematic of the L-M80S-cell. (b) EIS profile of the LM80S-cell upon assembly. The cell was rested for 3 h before the EIS measurements. (c) Cycling profile of the L-M80S-cells at different cycling rates. (d) Images of the cathodes before and after folding and bending to test the strength of the slurry coating.


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The Nyquist plot of the L-M80S-cathode is shown in Figure 3b. The EIS of the M80S-cell and the tandem cathodes exhibit similar charge-transfer resistance (RCT = 22 Ω). This is similar to the EIS previously seen for cells assembled with an Al-foil current collector (M80S-cell). Although the L-M80S-cathodes were assembled with a layer-by-layer structure containing a high amount of sulfur, there is still good electrochemical contact and low impedance within the network. Thus, the electrochemical cycling profile was conducted on the L-M80S-cells (Figure 3c, Figure S7). At C/5 and C/10 rates, the cell achieves a high peak capacity of, respectively, 850 and 855 mA h g-1. At C/5 rate, this corresponds to a practically viable areal capacity of 10.2 mA h cm-2. After 50 and 100 cycles, the L-M80S-cell maintains high specific capacities of, respectively, 700 and 488 mA h g-1. The capacity retention (RQ) falls from 82% at 50 cycles to 58 % at 100 cycles. The electrochemical performances of the L-M80S-cells are summarized in Table S1. These cathodes serve here as a novel proof-of-concept-cell for high-loading sulfur cathodes with high specific capacity to move past Al-foil current collectors. Figure 3d depicts the single layered cathodes after coating. After bending, the cathode remains homogeneous and welladhered onto the carbon current collector. The layer-by-layer sandwiched cells still offer good malleability with a high-loading cathode and good contact among the active material, carbon network, and Li-ions at a high sulfur content of 80 wt. %. The maleate-LiPS binding interactions readily keeps the active material in redox available spaces, allowing for a homogeneous, practically viable sulfur cathode.


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Post-mortem Examination Figure 4 demonstrates the morphological examination of the cycled ultrahigh-loading sulfur cathode by SEM. There is a good binding between the carbon current collector and the cathode components. The notable binding between the cathode and the carbon substrate emphasizes the ability for the layer-by-layer cells to achieve good electrochemical contact in ultrahigh-loading cathodes during cycling. The cathode and binder show good stability with the electrolyte and electrode components despite the higher volume of LiPS species presented.

Figure 4. Morphological examination of L-M80S-cell cathode surface after cycling: (a) lower magnification and (b) higher magnification.

In conclusion, this study applied a binder with bifunctional LiPS mediation properties to achieve better electrochemical performance with high-loading sulfur cathodes. The good LiPS binding efficacy by the maleate and amine groups allows for better initial conductivity and


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active-material utilization. The short linear chain of the PEG offers flexible binding abilities to bolster the stability of the cathode components during cycling, compensating for severe delamination typically seen during cycling. The binder is also composited with an interconnected carbon network and then stacked layer-by-layer to achieve an ultrahigh loading of 12 mg cm-2 with an initial areal capacity of 10.2 mA h cm-2. Thus, the investigation into bifunctional binders with both LiPS chemical trapping ability supported by an interconnected conductive network is a scalable and viable strategy to stabilize good electrochemical contact within high-loading sulfur cathodes.

Experimental Methods Conventional Tape-casted Cathodes: Conventional cathodes were prepared by combining sulfur (Millipore Sigma), Super P (TIMCAL), and maleate-PEG binder (Olaplex)51 in an 80:10:10 weight ratio in a 5:1 vol. ratio of 1-methyl, 3-pyrrolidone (NMP, Millipore Sigma) and isopropanol (IPA, Fisher). The solution was magnetically stirred overnight until a homogeneous black slurry was attained. The resulting slurry was tape-casted onto a carbon-coated aluminum foil current collector (MTI) and dried in a vacuum oven at 50 oC overnight. The resulting cathode was roll-pressed and cut into the desired cathodes. The control cells with polyvinylidene fluoride (PVDF, Millipore Sigma) and polyethylene glycol (PEG, Millipore Sigma) were


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prepared in the same manner. The slurry employing PVDF used NMP as the solvent. The slurry employing PEG used a 1:1 ratio of NMP and IPA. Tape-casted Tandem Cathodes: Tandem cathodes were prepared by tape-casting the previously prepared slurry onto a sheet of carbon paper (Nanotech labs, Bucky Paper) with a doctor blade. The carbon paper had been previously dried and roll-pressed for homogeneity. The cathodes were then dried in a vacuum oven at 50 oC overnight. The, the cathodes were roll-pressed, punched out, and weighed for cell assembly. Materials Characterization: Scanning electron microscopy (SEM, FEI Quanta 650 ESEM) was employed to study the morphological characteristics of the conventional cathodes. The SEM was equipped with an energy-dispersive x-ray spectrometer (EDS) for elemental mapping. To investigate the elemental bonding and surface analysis, x-ray photoelectron spectroscopy (XPS analyzer, Kratos AXIS Ultra) was performed with monochromatic Al kα x-rays.

Electrochemical Testing: To prepare the cells for electrochemical evaluation, the current collector side of the cathode was placed directly onto the bottom cell cap of a CR2032 coin cell inside an Argon-filled glovebox. In the traditional cathodes, Al foil was placed directly onto the cell cap. To create a tandem cathode, the first cathode was placed with the carbon paper current collector facing the cell cap. The next sheet of cathode was stacked on top of the previous cathode with the coating sides facing up. A carbon paper (Nanotech labs, 2.2 mg cm-2) as a polysulfide trap was placed over the cathodes, followed by a polypropylene separator (Celgard


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2500, Celgard). A prepared sheet of lithium foil (Millipore Sigma), which was previously scraped off on the surface, was placed over the separator followed by a nickel foam space and the cap to conclude the cell. To make the organic electrolyte, a 1:1 vol. ratio of 1,2 dimethoxyethane (DME, Millipore Sigma) and 1,3 dioxolane (DOL, Millipore Sigma) was prepared with lithium trifluoromethanesulfonate (1.85 M, LiCF3SO3, Millipore Sigma) and lithium nitrate (0.1 M, LiNO3, Acros Organics) as dissolved salt additives. The electrolyte composition was the same for the conventional cathodes and the tandem cathodes. An electrolyte/sulfur ratio of 10 was employed in the cells. The cells were sealed with a cell crimping press and rested for 3 h after assembly. Electrochemical Measurements: Electrochemical impedance spectroscopy (EIS) was performed on a potentiostat (Biologic Instruments, VMP3) between a frequency range of 1 MHz and 100 mHz. An equivalent circuit model was used to extract the charge-transfer resistance based on the Nyquist plot. Cyclic voltammetry (CV) profiles were also performed on a Biologic potentiostat within a voltage window of 1.7 – 2.8 V (vs. Li/Li+) at specified scan rates. Galvanostatic cycling was carried out with a battery testing system (Arbin, BT-2000) at a voltage window of 1.7 – 2.8 V (vs. Li/Li+). The specific capacities were calculated using the theoretical capacity and the sulfur loading of each cathode.


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ASSOCIATED CONTENT

Supporting Information. The supporting information provides XPS, SEM, and galvanostatic cycling data of the M80S-cell and L-M80S-cells.

AUTHOR INFORMATION

The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by ExxonMobil through its membership in the University of Texas at Austin Energy Institute.

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A Bifunctional Binder with Nucleophilic Lithium Polysulfide

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