Conformal Polymeric Multilayer Coatings on Sulfur Cathodes via the

Mar 23, 2016 - †The National Creative Research Initiative Center for Intelligent Hybrids, The WCU Program of Chemical Convergence for Energy and Env...
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Conformal Polymeric Multilayer Coatings on Sulfur Cathodes via the Layer-by-Layer Deposition for High Capacity Retention in Li−S Batteries Eui Tae Kim,†,‡ Jungjin Park,‡,§ Chunjoong Kim,∥ Adam G. Simmonds,⊥ Yung-Eun Sung,*,§ Jeffrey Pyun,*,⊥ and Kookheon Char*,† †

The National Creative Research Initiative Center for Intelligent Hybrids, The WCU Program of Chemical Convergence for Energy and Environment, School of Chemical and Biological Engineering, and §Center for Nanoparticle Research, Institute for Basic Science, School of Chemical and Biological Engineering, Seoul National University, Seoul, 08826, Korea ∥ Department of Materials Science and Engineering, Chungnam National University, Daejeon 34134, Korea ⊥ Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States S Supporting Information *

ABSTRACT: We report on the conformal coating of thicknesstunable multilayers directly onto the sulfur (S8) cathodes by the layer-by-layer (LbL) deposition for the significant improvement in the performances of Li−S batteries even without key additives (LiNO3) in the electrolyte. Poly(ethylene oxide) (PEO)/poly(acrylic acid) (PAA) multilayers on a single poly(allylamine hydrochloride) (PAH)/PAA priming bilayer, deposited on the S8 cathodes, effectively protected from the polysulfide leakage, while providing a Li+ ion diffusion channel. As a result, PAH/PAA/ (PEO/PAA)3 multilayer-coated cathodes exhibited the highest capacity retention (806 mAh g−1) after 100 cycles at 0.5 C, as well as the high C-rate capability up to 2.0 C. Furthermore, the multilayer coating effectively mitigated the polysulfide shuttle effect in the absent of LiNO3 additives in the electrolyte.

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Alternatively, the concept of depositing protective coatings comprised of carbonaceous11,12 or polymeric materials13,14 directly onto device components in Li−S batteries has recently been investigated. This approach has been designed to minimize the irreversible loss of the polysulfides through the creation of protective layers on S8 cathodes. The deposition of micron-thick coatings from Nafion,14,15 polydopamine,16 or poly(acrylic acid) (PAA) network gels17 onto the S8 cathodes has been reported as a route to improve the cycling lifetime in Li−S batteries. Kim and Mun et al.18 also reported on the copolymerization approaches to deposit copper-diacrylate complexes as protective coatings onto the S8 cathodes. These elegant reports demonstrate the viability of using polymeric films or coatings to suppress the polysulfide diffusion during cycling. However, there remain opportunities to develop engineered, well-defined thin films particularly, since thicker protective coatings deposited onto cathodes, or separators, can retard the Li+ diffusion between the electrolyte and the cathode surface, resulting in the increased resistance of Li−S cells.15,17 Layer-by-layer (LbL) deposition of associative polymers and/ or electroactive colloidal materials has recently been applied to create a wide range of electrodes,19−21 or functional coatings

ithium−sulfur (Li−S) batteries are considered as one of the most promising candidates for the next generation energy storage systems due to high theoretical specific capacity of sulfur cathode (1,675 mAh g−1). However, it is still a daunting task to achieve its theoretical capacity because of irreversible loss of active sulfur species in the cathode during repeated charge/discharge cycles. Polysulfides, the higher order (dis)charge products, Sn2− (n = 4, 6, and 8), are soluble in the electrolyte medium; thus, they continuously diffuse out from the cathode during the battery operation. The polysulfides in the electrolyte phase migrate onto the Li anode side, forming inactive deposits on the surface of Li metal and/or resulting in the polysulfide shuttle, which leads to the serious loss of capacity and poor Coulombic efficiency.1 In order to solve these critical issues on Li−S batteries, significant efforts have been placed, particularly on tailoring the morphology or the molecular structure of active sulfur species in the cathode. Such improvements have been made by the use of nanosized sulfur entrapped by carbonaceous materials, where polysulfides are preserved within the vicinity of the cathode side with intimate electrical contacts.2−5 Recently, we reported the chemical modification of elemental sulfur (S8) into copolymeric materials, where an organic comonomer acts as a molecular binder, stabilizing the interphases between lithiated and delithiated sulfur products.6−10 © XXXX American Chemical Society

Received: February 19, 2016 Accepted: March 21, 2016

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DOI: 10.1021/acsmacrolett.6b00144 ACS Macro Lett. 2016, 5, 471−475

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Figure 1. Schematic illustration of the layer-by-layer (LbL) deposition of a single (PAH/PAA) priming bilayer and (PEO/PAA)n multilayers on S8 cathodes.

Figure 2. SEM images and photo images of water droplets on (a) a pristine S8 cathode and (b) a PAH/PAA/(PEO/PAA)5 multilayer-coated cathode. A cross-sectional SEM image of (d) a pristine S8 cathode and (e) a PAH/PAA/(PEO/PAA)5 multilayer-coated cathode. (c) Cross-sectional SEM image of a PAH/PAA/(PEO/PAA)5 multilayer-coated cathode for EDS analysis. (f) Oxygen (orange) elemental map by EDS of the PAH/ PAA/(PEO/PAA)5 multilayer-coated cathode. (g) Changes in contact angle as a function of adsorbed polymer layers on the substrates of S8-only, carbon + binder, and S8 + carbon + binder (i.e., S8 cathode).

onto separators,22 for the use in energy storage devices. Kim et al.23 reported on the use of LbL films from poly(allylamine hydrochloride) (PAH) and PAA deposited on separators to affect the polysulfide diffusion process and improve the Li−S battery performance. However, up to date, the direct modification of S8 cathodes with thin protective coatings via LbL approaches has not yet been explored. Herein, we describe for the first time a facile LbL protocol to deposit conformal thin films based on a single PAH/PAA priming bilayer with overcoated bilayers of (PEO/PAA)n, directly onto S8 cathodes for Li−S batteries (Figure 1). Furthermore, we report on the precise structure−property effects of (PEO/PAA)n bilayer number on cycling performance and observe the effective suppression of irreversible capacity loss owing to polysulfide diffusion while providing Li+ ionic conduction. To further demonstrate the benefit of these LbL coatings on the S8 cathodes, we demonstrate excellent capacity retention without the addition of lithium nitrate (LiNO3), which, in the vast majority of reports, is known to be a key additive required to address the polysulfide shuttling. These advances offer an inexpensive methodology to enhance the performance of Li−S batteries with a negligible increase in its weight. To enable the deposition of stable LbL thin films, the discovery of primer-adhesion layers based on polyelectrolytes

to the surfaces of S8 cathodes was essential. This task was particularly challenging due to the inherent polarity mismatch between water-soluble polymers (polyelectrolytes) and very hydrophobic surfaces of S8 cathodes, which were comprised of elemental sulfur (S8; 60 wt %), conductive carbons (20 wt %), and poly(vinylidene fluoride) (PVDF; 20 wt %). We discovered that a sequentially adsorbed PAH/PAA bilayer on the S8 cathode effectively served as a priming layer for further LbL deposition of (PEO/PAA)n multilayers, when spin-coated from aqueous solutions of PAH (pH 7.5) and PAA (pH 3.5) in the presence of lithium bis(trifluoromethanesulfonyl)-imide (LiTFSI, 0.1 M) salts. It is not easy to unequivocally prove the initial priming effect of the PAH/PAA bilayer on a S8 cathode due to the compositional heterogeneity of the cathode surface; however, the weak positive/negative charges and high ionic strength of these polymer solutions was anticipated to be sufficiently screened from long-range electrostatic repulsions and thereby enhanced the hydrophobic attractions of adsorbed chains to the cathode surface.24 Furthermore, the spin-assisted polyelectrolyte adsorption generated thinner and uniform surfaces compared to those formed by the dipping method due to the shear forces during spin-coating imposed on the adsorbing polymer chains to promote the dense coverage on the cathode surface.25−27 After the deposition of the priming bilayer (PAH/PAA) on the cathode, (PEO/PAA)n multilayers 472

DOI: 10.1021/acsmacrolett.6b00144 ACS Macro Lett. 2016, 5, 471−475

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C-rate (1 C = 1675 mA g−1) of multilayer-coated S8 cathodes are shown in Figure 3. The region of the upper plateau at ∼2.4

were successively deposited onto the primed S8 cathodes by alternating dipping depositions from PEO and PAA solutions, both of which were prepared by adding LiTFSI (0.1 M) and adjusting the solution pH at 2.5 to induce hydrogen bondings between ether oxygens of PEO and protonated carboxylic acids of PAA.28,29 The PEO/PAA pairing for the LbL deposition was chosen to enable the sufficiently high ionic conductivity30−32 into the underlying cathode. Hammond et al. reported similar LbL assembled (PEO/PAA)n films, whose ionic conductivity was 10−7 S cm−1, when soaked in LiPF6/propylene carbonate solution.33 It is important to note that the LbL deposition of PEO/PAA onto S8 cathodes was unsuccessful in the absence of the PAH/PAA priming bilayer (Figure S1). The surface and cross-sectional SEM images of a pristine S8 cathode and PAH/ PAA/(PEO/PAA)5 multilayer-coated cathode are compared in Figure 2. Porous and rough surface morphologies are observed on the pristine cathode, in which carbon black particles (consisting of primary particulates of approximately 50 nm in diameter) were predominately presented to the coating surface (Figure 2d). After the PAH/PAA/(PEO/PAA)5 multilayer deposition, the surface was conformally coated by the polymers, as shown in the SEM images (Figure 2b,e), where the polymers could be deeply penetrated inside the cathode through the pores, as evidenced from the EDS elemental map of oxygen in Figure 2c,f. The actual thickness of the multilayers could not be directly determined because of heterogeneity of the interface between the cathode surface and the multilayer. Hence, model study using the same LbL deposition process was conducted on a flat Si substrate to estimate the thickness of the multilayers (Figure S3). A nearly linear relationship between film thickness as a function of bilayer number was observed, revealing a coating thickness of 80 nm for the PAH/PAA/(PEO/PAA)5 multilayer. Due to the compositional heterogeneity of the surface of the S8 cathode, model studies of LbL deposition were performed with different substrates, coated with each of the cathode components, and then compared with the original S8 cathode substrate. The cathode components were separated into two different substrates: “S8-only” (spin-casted on a flat Au plate), and “carbon + binder” (doctor-bladed on an Al foil), on which the multilayer deposition procedure described above was performed (Figure S4). The polymer adsorption behavior on different substrates was investigated by contact angle measurements (Figure 2g). All of the substrates under the test were initially hydrophobic (104.1, 164.9, and 144.4° for S8-only, carbon + binder, and S8 + carbon + binder substrates (i.e., S8 cathode), respectively, but during the sequential deposition of multilayers up to five (PEO/PAA) bilayers on the PAH/PAA priming bilayer, the contact angles of three different initial substrates significantly decreased to 24.7, 13.4, and 16.2°, respectively. Even lower water contact angles of carbon + binder substrates and S8 cathode, when compared to the S8only substrate, could be partly explained by much more porous and rough surface morphologies of them. To interrogate the effect of the LbL coatings on S8 cathodes, Li−S batteries were fabricated and subjected to the galvanostatic charge−discharge cycling. The S8 cathodes with different bilayer numbers of PAH/PAA/(PEO/PAA)n (n = 1, 3, 5) coatings were punched (d = 12 mm) for 2032-type coin cells. The electrolyte system used in these studies were LiTFSI (1.0 M) and LiNO3 (0.1 M) in dioxolane (DOL)/dimethyl ether (DME), and the sulfur loading of each cathode was around 1 mg cm−2. The charge/discharge voltage profiles at 0.5

Figure 3. Charge/discharge voltage profiles at 0.5 C, during the initial 10 cycles of (a) a pristine S8 cathode, PAH/PAA/(PEO/PAA)n multilayer-coated S8 cathodes with (b) n = 1, (c) 3, and (d) 5.

V in the discharge profile is believed to originate from the dissolution reaction,34 where solid sulfur is reduced to soluble polysulfides (S8 to Sn2−, n = 8−4) upon lithiation. In this region, the decrease in the specific capacity during initial 10 cycles is denoted as “ΔQ”, which mainly results from the irreversible loss of active sulfur due to the formation and diffusion of soluble polysulfides into the electrolyte medium. Hence, the values of ΔQ can be directly correlated to the suppression of polysulfide diffusion due to the LbL overcoating of S8 cathodes (Figure S7). The ΔQ value of the pristine cathode without LbL protective coating is the largest (164 mAh g−1), while ΔQ values were observed to systematically decrease (ΔQ = 106, 92, and 81 mAh g−1 with PAH/PAA/(PEO/PAA)n multilayer coating at n = 1, 3, and 5, respectively). The surfaces of Li anodes disassembled after 10 cycles also confirm the effects of the LbL coatings, as shown in Figure S9. With the bare S8 cathode, yellowish contaminants on the Li anode appear after cycling, presumably due to the polysulfide migration and subsequent deposition onto the Li anode side, which is absent with the multilayer coated cathodes. Meanwhile, in Figure 3d, more pronounced polarization (ΔV = 0.141 V) between charge and discharge voltages, and more oblique voltage drops below ∼2.1 V at the end of discharge were found with five (PEO/PAA) bilayer-coated cathodes when compared with the pristine cathode. These are believed to result from the reduced Li+ diffusion rate through the thicker multilayer deposited on the cathode. In this regard, it is noted that there is a trade-off point between ΔQ and the voltage drop due to the slow Li+ diffusion, suggesting that there exists an optimum number of (PEO/PAA) bilayers for optimal electrochemical performance, as will be discussed shortly. The suppression of polysulfide dissolution was also confirmed in the experiment with excess electrolytes of beaker cells, assembled with a pristine S8 cathode or a multilayer-coated cathode (Figure S11), where the extent of polysulfide dissolution was found to be significantly lowered with the multilayer-coating, as noted by optical absorbance spectroscopy (with absorptions at 430 nm (Sx2− (x = 4−8)) and 620 nm (S3•−)). 473

DOI: 10.1021/acsmacrolett.6b00144 ACS Macro Lett. 2016, 5, 471−475

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confirmed in comparison with the pristine S8 cathode using electrochemical impedance spectroscopy (EIS; Figure S13) and SEM (Figure S14). The significant effects of the bilayer numbers on the battery performance of the cathodes were further confirmed by the additional cycling experiments at 0.5 C without lithium nitrate (LiNO3) additives in the electrolyte (Figure 5). It has been

The cycling performance of multilayer-coated S8 cathodes was observed to impart significantly enhanced specific capacity retention up to 100 cycles (at C-rate = 0.5; Figure 4a), while

Figure 5. Cyclic performance and Coulombic efficiency (CE) of a pristine S8 cathode and PAH/PAA/(PEO/PAA)n multilayer-coated S8 cathodes (n = 1, 3, and 5) at 0.5 C without LiNO3 salts in the electrolytes (CE = discharge capacity/charge capacity).

previously observed that Li−S batteries fabricated in the absence of LiNO3 salts were prone to very rapid capacity fading due to the self-discharging reaction between polysulfides and Li anode (i.e., polysulfide shuttling).37 The S8 cathode without LbL coatings showed a drastic decrease in the specific capacity (141.32 mAh g−1) with poor Coulombic efficiency (CE = 28.2% at 84th cycle) due to the severe “polysulfide shuttling”, while superior retention of the capacity up to 100 cycles (618.5, 744.9, and 715.3 mAh g−1 with 1, 3, and 5 bilayer coatings, respectively) were observed with the multilayer coatings. Moreover, significantly improved CE of the multilayer-coated cathodes (77.1, 83.8, and 86.7% after 100 cycles for 1, 3, and 5 bilayers, respectively) was directly attributed to the effective suppression of polysulfide migration by the conformal coating layers on the cathodes. It is important to note that the LbL coating of S8 cathodes shows enhanced performance relative to the LbL coating of a separator by Kim et al.,23 because the conformal contacts without gaps between the multilayers and the cathode surfaces in our system are more beneficial to minimize the polysulfide losses. In conclusion, PAH/PAA/(PEO/PAA)n multilayers deposited on S8 cathodes significantly improve the capacity retention of Li−S batteries and effectively mitigate the polysulfide shuttle effect even without LiNO3 salts in the electrolytes, by the suppression of polysulfide diffusion during electrochemical cycles. Moreover, the (PEO/PAA) bilayer numbers provides an opportunity to optimize the battery performance by the finetuning of multilayer thickness during the LbL deposition process.

Figure 4. Cyclic performance of a pristine S8 cathode and PAH/PAA/ (PEO/PAA)n (n = 1, 3, and 5) multilayer-coated S8 cathodes: (a) at 0.5 C, and (b) with different C-rates from 0.1 to 2.0 C, with 10 cycles for each C-rate.

the pristine S8 cathode showed the rapid capacity fading even after first 10 cycles. The three (PEO/PAA) bilayer-coated cathode exhibited the best capacity retention (806 mAh g−1) after 100 cycles, in comparison to the 728 mAh g−1 capacity obtained with the five bilayer-coated cathode. The reason is presumably that the limited Li+ diffusion through the thicker coating layer dominantly affect the loss of its capacity, despite its better capability of the protection from the irreversible polysulfide loss, as previously discussed. The influence of C-rate (0.1 to 2.0 C, 10 cycles for each) on the capacity retention behavior of the multilayer-coated cathodes was investigated in Figure 4b. The abrupt decreases in the capacities were observed over 1.0 C with the pristine S8 cathode, and at 2.0 C with one bilayer-coated cathode. The polysulfide diffusion with insufficient protection on the surfaces of these cathodes results in the gradual decomposition of electrical contacts between the carbon framework and sulfur products during repeated cycles, and that leads to the limited utilization of active sulfur, particularly, at high C-rate.35,36 On the other hand, three and five bilayer-coated cathodes retained high capacity retention, even at 2 C, which was clear evidence of the effective protection from the polysulfide diffusion by the sufficient multilayer coatings on S8 cathodes. The structural stability of multilayer-coated cathodes during cycles was also

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00144. 474

DOI: 10.1021/acsmacrolett.6b00144 ACS Macro Lett. 2016, 5, 471−475

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Experimental details, surface morphologies, elemental analysis, and additional electrochemical characterizations on multilayer-coated cathodes, including XPS, SEMEDS, UV−vis spectroscopy, and electrochemical impedance spectroscopy (EIS) (PDF).

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions ‡

These authors contributed equally (E.T.K. and J.P.).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Ministry of Science, ICT and Future Planning (MSIP), The National Creative Research Initiative Program for “Intelligent Hybrids Research Center” (No. 2010-0018290), the BK21 Plus Program in SNU Chemical Engineering, the Institute for Basic Science (IBS-R006-G1), and the NSF (NSF-CHE 1305773).

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DOI: 10.1021/acsmacrolett.6b00144 ACS Macro Lett. 2016, 5, 471−475