as a Superior Bifunctional Binder for Lithium–Sulfur Batteries of

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Article Cite This: J. Phys. Chem. C 2018, 122, 25917−25929

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Water-Soluble Linear Poly(ethylenimine) as a Superior Bifunctional Binder for Lithium−Sulfur Batteries of Improved Cell Performance Junbin Liao,† Zhen Liu,‡ Xudong Liu,† and Zhibin Ye*,†,§ †

Bharti School of Engineering, Laurentian University, Sudbury, Ontario P3E 2C6, Canada State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Meilong Road 130, Shanghai 200237, China § Department of Chemical and Materials Engineering, Concordia University, Montreal, Quebec H3G 1M8, Canada J. Phys. Chem. C 2018.122:25917-25929. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 11/16/18. For personal use only.

‡

ABSTRACT: To realize the practical application of lithium− sulfur (Li−S) batteries, bifunctional binders featured with the capability of trapping soluble polysulfide species besides the strong binding property are highly desired. Herein, we demonstrate the strong potential of a commercially available, environmentally friendly, water-soluble linear polyethylenimine (PEI) as a superior bifunctional binder for high-sulfurloading cathodes in Li−S batteries. Our investigation shows the significantly improved cathode performance with enhanced sulfur utilization (i.e., higher capacity), reduced capacity decay, and longer cycling life upon the use of PEI as the binder, relative to the traditional polyvinylidene fluoride (PVDF) binder. This arises from the significantly stronger binding strength and valuable polysulfide trapping ability of the linear PEI binder. In particular, its superior polysulfide adsorption capability has been evidenced experimentally with both ex situ and in situ studies, as well as through theoretical density functional theory (DFT) calculations. At a sulfur loading of 2.4 mg cm−2, the capacity decay rate of the cathode with the linear PEI binder is reduced to as low as 0.042% per cycle over 500 cycles at 2 C. In addition, it also enables the fabrication of highsulfur-loading cathodes (as high as 6.5 mg cm−2) with high areal capacity (ca. 4.5 mAh cm−2) and high cycling stability. With its superior performance, linear PEI is promising for fabricating high-sulfur-loading cathodes of significantly lowered capacity decay for practical applications. including micro/mesoporous carbons,8−12 hollow porous carbon spheres,13 graphene,14,15 conductive nanotubes,16 3D fibrous carbon structure,17 etc.; (ii) to use polar sulfur host materials that form chemical interactions with polysulfide anions in lithium polysulfides,18−20 such as conductive carbons doped with heteroatoms (N, O, S, etc.),21−23 stoichiometric metal chalcogenides,24 metal organic frameworks (MOFs) and some crystal faces of nonstoichiometric metal oxides and sulfides,25,26 organic anthraquinone,27 etc.; (iii) to fabricate novel cathode configurations that block polysulfide shuttling through physical confinement/chemical trapping (e.g., carbon/ polymer interlayer modified separators,28,29 polymer coatings on surface of cathodes,30,31 interlayer between cathode and separator,32,33 etc.). Through entrapping polysulfides within the sulfur cathodes, these different strategies help suppress “polysulfide shuttling” and enhance battery performance. Developing bifunctional polymer binders endowed with strong mechanical binding performance and the additional valuable capability of trapping polysulfide species is a new trend for Li−S batteries of improved capacity retention.1,34

1. INTRODUCTION Lithium−sulfur (Li−S) batteries are one of the most promising candidates for next-generation power systems,1,2 as they possess a high theoretical capacity and energy density of 1675 mAh g−1 and 2600 Wh kg−1 or 2800 Wh L−1, respectively, which are much superior to current lithium−ion batteries (e.g., 387 Wh kg−1 for LiCoO2/C battery).3 Additionally, the use of abundant, cost-effective, nontoxic sulfur as the cathode material also makes the Li−S battery system commercially promising.4,5 However, practical applications of Li−S batteries are still hindered by several obstacles. For example, the insulating nature of elemental sulfur and Li2S makes the use of high-sulfur-content cathodes challenging, and the long-known “polysulfide shuttling” leads to pronounced capacity fading and deteriorating Coulombic efficiency over cycling.1,5,6 Meanwhile, sulfur cathodes experience significant volume changes (∼80%), including shrinkage due to polysulfide dissolution into liquid electrolyte and expansion upon formation of Li2S.7 Over the past decade, numerous strategies have been employed to address these issues, with most of them focusing on trapping polysulfides within the sulfur cathodes. Some major elegant strategies include the following: (i) to physically confine lithium polysulfides by using well-designed porous nonpolar carbonaceous materials, © 2018 American Chemical Society

Received: September 25, 2018 Revised: October 30, 2018 Published: October 31, 2018 25917

DOI: 10.1021/acs.jpcc.8b09378 J. Phys. Chem. C 2018, 122, 25917−25929

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The Journal of Physical Chemistry C

toluene (HPLC grade), etc., from Fisher Scientific and were dried with 4 Å molecular sieves. 2.2. Ex Situ Polysulfide Adsorption Testing. A lithium polysulfide standard Li2S4 was prepared as per our previous work.47 In an argon-filled glovebox, sulfur was gradually added into a Super-Hydride solution with a Li to S mole ratio of 1:2 under rapid agitation. The solvent in the resulting solution was evaporated under vacuum. The resulting solid was fully washed with hexane and dried, rendering the final yellow solid product, Li2S4. Ex situ adsorption of Li2S4 with PVDF and the linear PEI, respectively, was undertaken at a binder/Li2S4 mass ratio of 20. Typically, the binder at a known mass of 64 mg was added into a known volume of Li2S4 solution (concentration, 0.4 mg mL−1) in mixed DME-DOL solvent (volume ratio, 1:1). The supernatant solution was monitored with UV−vis spectroscopy (Varian Cary 100). UV−vis spectroscopy (Varian Cary 100) was used to monitor the supernatant solution after a prescribed time to monitor the adsorption.47 To obtain spectroscopic evidence confirming the adsorbed Li2S4, X-ray photoelectron spectroscopy (XPS) measurements of the PEI containing the adsorbed Li2S4, pure PEI, and pure Li2S4 were undertaken on a Thermo Scientific Theta Probe XPS spectrometer (Thermo Fisher), respectively. A monochromatic Al Kα X-ray source was used, with a spot area 400 μm. The samples were run in a standard mode, i.e., all angle collected (60° angular acceptance) for the survey spectra and for the region spectra. 2.3. Density Functional Theory (DFT) Calculations. DFT calculations were carried out using the Gaussian 09 program.48 Conformational analysis using the Tinker program was first performed to generate the initial structures for molecular models of PVDF and the linear PEI, as well as for Li2Sn (n = 1, 2, 3, 4, 6, 8) clusters. Subsequently, all the structures were full optimized without any symmetry constraint using B3LYP function in combination with the split valence double-ζ basis set 6-31+G(d,p) for all elements. The D3 version of Grimme’s dispersion was incorporated into the geometry optimization.49 In addition, Truhlar’s SMD solvation model was also incorporated into the geometry optimization by portraying tetrahydrofuran as the model solvent.50 Throughout, harmonic vibrational frequency calculations were undertaken to confirm the proper optimization of structures without showing any imaginary frequency, and to obtain the vibrational free energies as well. The electronic energies of all structures were refined by carrying out singlepoint energy calculations using a large triple-ζ basis set 6-311+ +G (d, p). All the relative energies are in eV and refer to Gibbs free energies at 298.15 K. The binding energies are defined as the energy difference between the separated molecular models plus the Li2Sn (n = 1, 2, 3, 4, 6, 8) clusters, and the corresponding binding structures according to the following equation: Eads = ELi2Sn + Emodel − ELi2Sn model. Thus, a large positive binding energy indicates a strong adsorption of the Li2Sn clusters on the molecular models of PVDF and the linear PEI. 2.4. Sulfur Cathode Fabrication. To ensure the effective contact between sulfur and carbon, elemental sulfur and SuperP carbon at a mass ratio of 3:1 were grinded in a mortar, followed by melt infusion at 155 °C for 12 h in a sealed glass tube. The sulfur content in the resulting sulfur/carbon (S/C) composite was found to be 74.7 wt % by thermogravimetric analysis (TA Instruments Q50 TGA at a heating rate of 10 °C min−1). A known mass (400 mg) of S/C composite was added

The performance of conventional polymer binders for sulfur cathodes, such as polyvinylidene fluoride (PVDF) as the mostly commonly used one, is often restricted as they solely serve the single binding role while without the desired affinity to or trapping of the intermediate polysulfide species to ameliorate “polysulfide shuttling”.34−38 To this end, some bifunctional binders showing the valuable capability of trapping/absorbing polysulfides, have been recently reported. These include β-cyclodextrins,39 polyamidoamine dendrimer,40 natural gum Arabic,41 chitosan,36 cross-linked branched polyethylenimine,42,43 poly(vinylpyrrolidone) (PVP),44 polymeric ionic liquids,45 sulfonated polystyrene,46 etc. These bifunctional binders all possess abundant polar groups (such as carboxyl, amine, imine, hydroxyl, imidazolium, and sulfonate) of strong affinity toward the polysulfide species and render sulfur cathodes of significantly improved performance relative to conventional binders. We have recently demonstrated that a quaternary ammonium cationic polymer, polydiallyldimethylammonium with TFSI counteranion (PDADMA-TFSI), behaves well as a bifunctional binder. With its possession of quaternary ammonium cations with weakly associating TFSI counteranions, the polymer shows strong ionic interactions with polysulfide anions and facilitates the effective trapping of polysulfides to render markedly improved battery performance.47 Encouraged by the performance of PDADMA-TFSI, we have been in search of alternative better-performing cationic polymers as bifunctional binders for sulfur cathodes. In this paper, we further demonstrate the use of a commercially available water-soluble linear polyethylenimine (PEI), [−(CH2)2−NH−]n, as a superior bifunctional polymer binder. PEIs, including branched and linear ones, are an important class of water-soluble cationic polymers. With their possession of a high density of valuable polar amino groups capable of trapping polysulfide species,40,42,43 we hypothesize that the linear PEI should render sulfur cathodes of improved performance relative to PVDF as the conventional binder. A systematic study on the performance of the linear PEI as the bifunctional binder has been undertaken, revealing its superior dual functional role in significantly improving cathode performance. We have focused particularly on the fabrication of sulfur cathodes of high sulfur loadings (up to 6.5 mg cm−2) and long cyclic life, which are required for practical applications.

2. EXPERIMENTAL SECTION 2.1. Materials. A commercially available linear PEI (weight-average molecular weight = 250 000 g mol−1; Polysciences Inc.) and poly(vinylidene fluoride) (PVDF, Mw ∼ 534 000 g mol-1 by GPC, Aldrich), were used as received without further purification. Other chemicals and materials, including sulfur (100 mesh particle size, Aldrich), Super-P carbon black (a conductive carbon black, IMERYS Graphite & Carbon, Belgium), Super-Hydride solution (1.0 M lithium triethylborohydride in tetrahydrofuran, Aldrich), 1-methyl-2pyrrolidone (NMP, reagent plus 99%, Aldrich), and lithium nitrite (LiNO3, reagent plus, Aldrich), were all used as received without any additional purification. Bis(trifluoromethane)sulfonimide lithium salt (LiTFSI, 99.95%, Aldrich) was dried under vacuum for over 12 h at room temperature. 1,2Dimethoxyethane (DME, anhydrous, 99.5%, Aldrich) and 1,3dioxolane (DOL, 99%, Aldrich) for electrolyte, and tetrahydrofuran (THF, HPLC grade, > 99%), methanol (>99%), 25918

DOI: 10.1021/acs.jpcc.8b09378 J. Phys. Chem. C 2018, 122, 25917−25929

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cathodes as well as discharge capacity.51 We have initially attempted to employ pure branched PEIs as the binder (at 10 wt % of total cathode materials) for the sulfur cathodes. However, in our experiments, we have found that branched PEIs without modifications render coin cells with high internal resistance infeasible for applications and are thus not suitable binders. We reason that liquid-like branched PEIs nearly completely fill the pores within cathode, obstructing the effective diffusion of the electrolyte to the active materials. On the basis of our finding, we have thus investigated the performance of a linear PEI as a bifunctional binder in this study given its semicrystalline solid morphology which should create porous structures in the cathodes to facilitate electrolyte diffusion. It should be noted that, during our course of study, Chen et al. reported recently that a modified branched PEI formed by cross-linking branched PEIs with diisocyanate or poly(ethylene glycol) diglycidyl ether showed strong affinity for polysulfide absorption and rendered cathodes with significantly improved capacity retention.42,43 Clearly, the modification by covalent cross-linking changes the physical state and performance of branched PEI and makes it more suitable as a superior bifunctional binder. In these two methods, like PVDF, the use of branched PEI therein also requires the use of expensive, toxic, flammable, high-boiling-point dimethylformamide/Nmethyl-2-pyrrolidone as the organic solvents for the cathode fabrication. Meanwhile, Zhang et al. reported recently the fabrication of high sulfur loading cathodes showing high areal capacity over 50 cycles by employing a branched PEI binder (10 wt % of total cathode materials), which differs from our finding with branched PEIs.52 Therein, graphene was used as an additional conducting additive besides the regular conducting carbon, which may be the reason causing the different result. Given their possession of the valuable amino groups in common, these literature works with branched PEIs suggest the strong potential of linear PEIs as superior bifunctional binders, which has not yet been previously reported. With its high solubility in water, the use of linear PEIs enables cathode fabrication simply with water as the green solvent and is thus environmentally friendly. A systematic study on the performance of a commercially available linear PEI (weight-average molecular weight: 250 000 g mol−1) as the binder has been undertaken in our study. To verify the capability of the linear PEI in adsorbing/ trapping polysulfide, we have first studied its interactions with a representative polysulfide species, Li2S4, by investigating polysulfide adsorption on the polymer.42,43,45,47 To a Li2S4 solution in DOL/DME, the linear PEI was added at the polymer/Li2S4 mass ratio of 20. A parallel experiment with PVDF was also conducted for the purpose of comparison. Figure 1a shows the color change of the Li2S4 solutions upon exposure to the corresponding polymers for 5 min, 1 h, 5 h, and 21 h, respectively, relative to the grass green blank Li2S4 solution. It can be noted that the solution exposed to PVDF shows no remarkable change in color throughout the whole adsorption period, even after 21 h. Instead, a very slight darkening in color after 21 h is noticed, which is reasoned to the sedimentation of suspended tiny white PVDF particles over long-standing. This confirms that PVDF does not show apparent affinity toward polysulfides.47 On the contrary, the color of the other solution exposed to the linear PEI gradually lightens with the increase of the adsorption time, with an

into the solution containing the prescribed mass of binder (linear PEI or PVDF; 50 mg) and Super-P carbon black (50 mg) in water (3 mL, containing 5 drops of 2-propanol) or NMP (3.0 mL), to achieve a final S/C/binder ratio of 60:30:10, followed by thorough mixing with a mechanical stirrer. Sulfur electrodes were fabricated by evenly depositing a known volume of slurry on carbon paper (0.019 mm in thickness, 1.32 cm2 in area, Toray, TGP-H-060) as the current collector. The sulfur loading for all electrodes was controlled at ca. 2.4, 3.5, 5.5, and 6.5 mg cm−2, respectively. The electrodes were dried in an oven at 65 °C for 5 h, then in a vacuum oven at 50 °C for another 12 h prior to assembly. 2.5. Electrochemical Testing. In an Ar-filled glovebox, CR2032-type coin cells were assembled using metallic lithium disk as counter electrode and electrolyte containing a binary solvent of DOL and DME (1:1 in volume) with 2 wt % LiNO3 as additive. The amount of electrolyte was controlled at an electrolyte-to-sulfur ratio of ≈9.0 mL g-1. Microporous polypropylene membranes (Celgard 2500) were used as the separator. Galvanostatic charge−discharge testings of the coin cells were performed on a LAND CT2001A (China) battery testing system at room temperature. Current density and specific capacity were calculated based on the mass of sulfur active material. Cyclic voltammetry (CV) spectra of the electrodes were recorded with a Metrohm Autolab PGSTAT128N electrochemical workstation in the voltage range of 1.7−2.8 V vs. Li+/Li at a scan rate of 0.1 mV s−1. Electrochemical impedance spectroscopy (EIS) measurement was carried out from 0.01 Hz to 100 kHz at room temperature with a potentiostatic signal amplitude of 5 mV. 2.6. In Situ Reflectance UV−Vis Measurements. In situ reflectance UV−vis spectroscopy measurements were undertaken with an Ocean Optics USB2000 spectrophotometer during the cathode discharge (sulfur loading at ca. 2.0 mg cm−2) at room temperature to monitor the concentration of the solubilized polysulfide species leached into the electrolyte from the cathode. For this purpose, the negative CR2032 cases and lithium metal disks with a punched central hole were used for the assembly of coin cells. The negative cases were sealed with a glass window for the transmission of light during online monitoring. In addition, a glass fiber separator was used. An electrolyte (in DOL−DME at 1:1 in volume) volume of 60 μL was employed. The cells were tested at a discharging rate of 0.1 C and in situ reflectance UV−vis spectra were recorded at different voltages. To prepare different polysulfide standard (Li2S2, Li2S4, Li2S6, and Li2S8) solutions for the identification of UV−vis peaks, Li2S and sulfur with molar ratios of 1:1, 1:3, 1:5, and 1:7, respectively, were dissolved in a mixed solvent of DOL and DME (1:1 v/v) containing 1 M LiTFSI under magnetically stirring for 2 days. The whole synthesis procedure was undertaken in an argon filled glovebox. These standard solutions were subsequently measured with UV−vis spectroscopy.

3. RESULTS AND DISCUSSION PEIs are commonly classified into linear and branched ones. While linear PEIs containing exclusively secondary amino groups are semicrystalline solids (melting point: ca. 72 °C) at room temperature, branched PEIs containing primary, secondary, and tertiary amines are liquids at all molecular weights. Previously, the addition of small amounts of branched PEI into PVP as mixed binder systems was shown by Jung and Kim to significantly improve cycling performance of sulfur 25919

DOI: 10.1021/acs.jpcc.8b09378 J. Phys. Chem. C 2018, 122, 25917−25929

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the various time. The solution exposed to PVDF after 5 min and 21 h shows nearly identical overlapping spectra as the blank solution with the strong absorbance within 300−500 nm, verifying the complete nonadsorption of the polysulfide on PVDF. On the contrary, significant reductions in the UV−vis absorbance are observed with the solution exposed to the linear PEI, even shortly after 5 min of exposure. The reduction becomes increasingly pronounced with the increase of exposure time. After exposure for 21 h, the residual polysulfide becomes marginal according to the insignificant UV−vis absorbance. Meanwhile, according to the reductions in peak intensity of the UV−vis absorbance, we can reason that the rate of polysulfide adsorption by the linear PEI is fastest within the first 5 min. This adsorption experiment evidences the strong interactions between the linear PEI and the polysulfide. To obtain further spectroscopic evidence confirming the strong interactions between the linear PEI and lithium polysulfides, XPS characterization was undertaken on the pure linear PEI, pure Li2S4, and the PEI solid containing adsorbed Li2S4, which was recovered from the above adsorption experiment. Figure 2 compares their XPS spectra of Li, S, and N elements. A broad peak with multiple deconvoluted constituting peaks in the range of 160−165 eV is seen in the S2p spectrum of Li2S4 (Figure 2a), arising from its polysulfide anions.45 In particular, the constituting peaks centered at 161.1 and 163.4 eV in S2p spectra can be ascribed to the terminal (ST−1) and bridging sulfur (SB0) atoms in Li− S−S−S−S-Li, respectively, where the sulfur atoms at both ends have a formal charge of (−1) while those in the middle bear a formal charge of (0).47,53,54 The PEI with adsorbed Li2S4 also shows the polysulfide peaks (Figure 2b), while they are absent in the spectrum of pure PEI (Figure 2c). Meanwhile,

Figure 1. Photographs (a) and UV−vis spectra (b) of Li2S4 solutions exposed to linear PEI and PVDF (polymer/Li2S4 mass ratio, 20:1), respectively, after various time.

appreciable change observed even after as short as 5 min. After 21 h, the solution exposed to PEI has become completely colorless, indicating the effective adsorption of the polysulfide species. The strong capability of the linear PEI in adsorbing Li2S4 is further demonstrated quantitatively through ex situ UV−vis spectroscopy. Figure 1b compares the UV−vis spectra of the two solutions exposed to the two polymers after exposure for

Figure 2. XPS study on the interactions between Li2S4 and the linear PEI: (a−c) S2p spectra, (d−f) Li1s spectra, and (g−i) N1s spectra of pure Li2S4, pure PEI, and the PEI containing adsorbed Li2S4, respectively. 25920

DOI: 10.1021/acs.jpcc.8b09378 J. Phys. Chem. C 2018, 122, 25917−25929

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Figure 3. DFT calculation results: (a) the possible interactions between various lithium sulfides (n = 1, 2, 3, 4, 6, 8) and molecular structures of PVDF and the linear PEI, respectively, in form of ball-and-stick model; (b) theoretically calculated adsorption binding energies between the polysulfides and the two different polymers.

Figure 4. (a) Photos of electrodes fabricated with PVDF and linear PEI, respectively, on aluminum foils as current collectors before and after repeated bending (10 times). Sulfur loading density: 3.0 mg cm−2. (b) Digital photographs and (c) SEM surface images of electrodes fabricated with PVDF and linear PEI, respectively, at various sulfur loading densities (3.5, 5.5, 7.0, and 9.0 mg cm−2).

toward a lower binding energy is observed after the capture of Li2S4 by PEI. This can be attributed to the electropositive nature of −NH− on PEI, which forces electrons away from the terminal sulfur and results in a slight decrease in the binding energy. Similar findings have been reported on the interactions

additional strong peaks are observed between 171 and 166 eV in the PEI with adsorbed Li2S4, attributed to the generated sulfate (SO42−) and sulfite (SO32−) like species resulting from the oxidation of the polysulfide (Figure 2b).40,54 Similar results have been seen by Bhattacharya et al.40 A small shift of 0.2 eV 25921

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Figure 5. CV curves of cathodes fabricated with (a, c) PVDF and (b, d) linear PEI as the binder (sulfur loading density, 3.5 mg cm−2; scan rate, 0.1 mV s−1).

between MgO and Li2S4 by Ponraj et al.,55 and between TiO2 and Li2S4 by Nazar et al.56 Correspondingly, the binding energy of lithium atom in Li2S4 in Figure 2d−f decreases from 55.4 to 54.6 eV upon adsorption, possibly, due to the strengthened interaction resulting from the increased polarity of polysulfide. Owing to the electropositive nature, the binding energy of nitrogen atom in PEI shifts to higher values (from 397.4 to 398.1 eV and from 398.6 to 399.2 eV; see Figure 2g− i) after the adsorption of Li2S4. These XPS results further confirm the adsorption of lithium polysulfides on PEI and the interactions between the cationic amino groups on PEI and the polysulfide species, which change the binding energies of the related elements upon the adsorption of the polysulfide on PEI. For further quantitative evaluation, the theoretical binding strengths between the polysulfide species (Li2Sn with n = 1, 2, 3, 4, 6, 8) and the linear PEI have been investigated by DFT calculations, along with PVDF for comparison. Figure 3b shows the calculated binding energy between PEI and various Li2Sn. Featured with a high density of valuable polar amino groups that can effectively anchor Li2Sn, significantly high binding energies (0.654−0.742 eV) between the linear PEI and Li2Sn species are obtained, which are around six times those (