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An Alternative Approach to Enhance the Performance of High Sulfur-loading Electrodes for Li-S Batteries Hee Min Kim, Ho-Hyun Sun, Ilias Belharouak, Arumugam Manthiram, and Yang-Kook Sun ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00104 • Publication Date (Web): 18 May 2016 Downloaded from http://pubs.acs.org on May 18, 2016

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ACS Energy Letters

An Alternative Approach to Enhance the Performance of High Sulfur-loading Electrodes for Li-S Batteries Hee Min Kim,† Ho-Hyun Sun,‡ Ilias Belharouak,§ Arumugam Manthiram,*,‡ Yang-Kook Sun*,†

†

Department of Energy Engineering, Hanyang University, Seoul, 133-791, South Korea,

‡Materials

Science and Engineering Program & Texas Materials Institute, The University of Texas at Austin, Austin, Texas, 78712, United States, and

§Qatar

Environment and Energy Research Institute, HBKU, Qatar Foundation, P.O. Box 5825, Doha, Qatar

AUTHOR INFORMATION Corresponding Author *Tel: +82-2-2220-0524. E-mail: [email protected] (Sun, Y.-K.) *Tel: +1-512-471-1791. E-mail: [email protected] (Manthiram, A.)

1 Environment ACS Paragon Plus

ACS Energy Letters

ABSTRACT

Due to lithium-sulfur battery's high theoretical capacity and energy density, Li-S has been considered as a promising candidate for next-generation Li battery. Despite this, Li-S batteries suffer from poor electrical conductivity and shuttle effect, which result in loss of active material and active material loading limitation, thus hindering Li-S's practical application. This paper introduces the modified high-sulfur loading electrode (MHSE) with a loading of 10 mg cm-2 which directly addresses these two drawbacks and employs a simple production process suitable for mass production through the use of elemental sulfur. The MHSE consists of three distinct components which provides additional conductivity, mechanical support, and polysulfide adsorption ability on each level to enhance electrochemical performance. The electrode manifested an initial discharge capacity of 1332 mAh g-1 with a 91% cycle retention at the end of 50 cycles and cycled with stability from 0.1 C to 2 C during rate capability test.

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In the search for next-generation energy-storage materials, the lithium-sulfur battery has drawn much attention due to its immensely high theoretical specific capacity of 1675 mAh g-1 and energy density of 2600 Wh kg-1.1,2 Its added benefit of being a low-cost, copiously available, and environmentally benign material has also prioritized the Li-S battery as an auspicious candidate. Despite such auspiciousness, the lithium-sulfur battery has inherent disadvantages that hinder its commercialization: (i) elemental sulfur and Li2S suffer from poor electrical conductivity which makes the use of high-sulfur content electrodes challenging and (ii) polysulfide migration to and reverse reaction at the negative electrode during the charge/discharge process, known as shuttle effect, result in a loss of active material and degeneration of capacity and Coulombic efficiency.3,4 A plethora of efforts has been made to resolve these issues. A traditional approach taken by researchers is the use of sulfur mixed with, confined by, and impregnated in all kinds of carbons to augment conductivity.5-9 Other approaches have focused on increasing polysulfide adsorption by doping carbons with nitrogen10,11 and boron12, employing metal-oxides13-15, and using new binders16,17. The concept of a free-standing carbon interlayer between the cathode and the separator has also been explored to suppress the shuttle effect.18,19 Efforts to incorporate the above three concepts to produce a synergetic composite of webbed carbon nanotube-TiO2 coupled with a dual carbon-paper interlayer have also been explored.20 However, the novelty of these ideas does not compensate for the fact that many of these processes are convoluted and expensive. We present here an advanced cathode that is able to accommodate high-sulfur-loading for maintaining high energy density by addressing the problems of Li-S batteries. The modified high sulfur-loading electrode (MHSE) does exactly this with its advanced yet simple construction and


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achieves an enormously high-sulfur-loading of 10 mg cm-2 through the usage of elemental sulfur. The MHSE is divided into three separate components, each of which plays a central role in enhancing the electrochemical performance: (i) the scaffold’s uniform mixture of N-doped acetylene black (AB) and N-doped multi-walled carbon nanotube (MWCNT), (ii) chitosanbinder, and (iii) nitrogen-doped MWCNT of netting carbon film (NCF). Additionally, elemental sulfur was used in the process to raise the sulfur loading and simplify the production process; elemental sulfur, conducting agent, and binder are thoroughly mixed and coated onto an aluminum foil with NCF being established through the distribution of N-doped MWCNT directly onto the cathode before electrolyte addition. The combined ramification of these structures is an exceptional specific capacity and cycle life with high efficiency. As such, MHSE differentiates itself from previous strategies with its simple construction and electrochemical performance despite a massively high-sulfur content of 10 mg cm-2.

Figure 1. (a) Schematic design of the modified high sulfur-loading electrode (MHSE). SEM image of (b) NCF and (c) mixed N-doped MWCNT and N-doped AB by mixer-milling.


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Figure 1 illustrates the overall schematic of the MHSE. It is divided into three entities: the scaffold, chitosan-binder, and NCF. The scaffold consists of elemental sulfur, MWCNT, and AB amalgamated in a uniform structure. Elemental sulfur is favorable in that it is extremely costeffective and increases the sulfur loading in the electrode. Specially treated sulfur such as impregnated sulfur or metal-oxide encapsulated sulfur requires elaborate and often expensive production processes and the use of materials other than sulfur lowers the sulfur loading in limited space. However, sulfur is a well-known insulator with a conductivity of 5×10-30 S m-1 2,21 and additional conducting agents are needed to properly operate a lithium-sulfur cell; this is achieved through the use of carbons such as AB and MWCNT. Structurally, AB is composed of dot-like structures while MWCNT is made up of strands of hair-like thin tubes (Figure S1a and b). While AB allows for coating of thick electrodes due to stacking of dot-like structures, it results in an uneven distribution of sulfur in the electrode. In addition, AB based high-sulfurloading electrodes are prone to structural degradation during cycling. MWCNT, on the other hand, features structural stability during cycling for high-sulfur-loading electrodes. The fibershaped carbon also has the benefit of having a high conductivity of 107 S m-1. MHSE merges the strengths of the two materials to compensate the shortcomings of each carbon by making the coating of high-sulfur-loading electrodes possible and providing structural stability during cycling. However, both AB and MWCNT have a tendency to congregate with the same species and when mixed together, results in areas where empty voids emerge, which defeats the purpose of combining the two species together (Figure S1c and S2b). In order to overcome this, mixing methods were varied through mixer-milling to achieve a more homogeneous electrode structure and the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the results are displayed in Figure 1c and S2a. Unlike the SEM and TEM images of


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standard mortar-and-pestle mixed carbon electrodes, which revealed disparate regions of AB and MWCNT, the mixer-milled carbon electrode SEM and TEM images displayed homogeneous mixtures of carbons as desired. The resulting arrangement is a network of woven fibers with dots of AB evenly distributed across the MWCNT net. Such a structure promotes structural integrity and conductivity as the overlapping network of MWCNT supplies the much needed supporting mechanical strength against stress and provides channels for electron transport.22 Moreover, the scaffold structure improves sulfur utilization as dots of AB, distributed along the fibers of MWCNT, fill in the voids between the fibers, and increase the contact interface with sulfur, allowing more sulfur to participate in the reaction to increase the reversible capacity.

Figure 2. UV/VIS spectra and photographs of (a) lithium-polysulfide electrolyte with and without MWCNT/N-doped MWCNT and (b) lithium-polysulfide electrolyte with deionized water, lithium-polysulfide electrolyte with PVdF-binder, and lithium-polysulfide electrolyte with chitosan-binder. (Lithium-polysulfide UV/VIS peak wavelength (green dotted line): 232 nm, 264 nm.)


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Additionally, high-sulfur-loading electrodes suffer from extensive shuttle effect and as a countermeasure, both AB and MWCNT were nitrogen-doped to suppress the polysulfide shuttle through the formation of bonds between sulfur and oxygen functional groups on carbon10,11,23,24; this ability was confirmed by a simple test, in which polysulfide was mixed with AB and MWCNT with and without N-doping; the resulting solutions were analyzed with UV/VIS spectra (Figure 2a). In accordance with the literature, the lithium-polysulfide peak appeared at 232 nm and 264 nm, confirming the presence of lithium-polysulfide.25 While MWCNT without Ndoping did have an affinity for polysulfide, it was only 20% lower than the UV/Vis peak of pure lithium-polysulfide. The N-doped MWCNT in polysulfide had a substantially lower peak (61%) compared to that in the pure polysulfide peak, indicating that N-doped MWCNT efficaciously binds to polysulfide and hinders its migration. Note that the intensity of the filtered solution color also corresponded to the UV/VIS peak sizes. The N-doping of MWCNT was affirmed through X-ray photoelectron spectroscopy (XPS) in Figure S3a. While N-doped MWCNT exhibited a N1s peak, MWCNT without N-doping did not show a N1s peak as anticipated. Figure S3b separates the N1s XPS peak into various N-group peaks and four distinct N-group configurations were verified: pyridinic N (N-6, 398.5 eV), pyrrolic N (N-5, 399.8 eV), quaternary N (N-Q, 400.5 eV), and oxidized N (N-X, 402.3 eV). N-6 and N-Q groups were more abundant than N-5 and N-X groups and the respective groups appeared in accordance with the literature.26 Similarly, AB with and without N-doping were analyzed with UV/VIS spectroscopy in Figure S4a. The AB sample without N-doping filtrate displayed a strong dark red color not much different from the color of pure lithium-polysulfide. Its level of absorbance was also merely 7% lower than that of pure lithium-polysulfide, affirming that AB without N-doping is not adqequate for adsorbing lithium-polysulfide. On the contrary, the N-doped AB solution, which confirmed N-doping


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through XPS (Figure S4b), was significantly lighter compared to the dark red of pure lithiumpolysulfide, and its UV/VIS peak was 20% lower than the UV/VIS peak of AB without Ndoping. As such, the UV/VIS spectra of the N-doped samples verify that N-doping in AB increases polysulfide adsorption property slightly, which positively impacts the efficiency and cycle life of a cell. Another defining characteristic of the MHSE is the introduction of chitosan-binder. As a linear polysaccharide composed of randomly distributed β-(1-4)-linked D-glucosamine and N-acetylD-glucosamine, chitosan is abundant in –OH and -NH2 functional groups. –OH and –NH2 groups are known to be proficient in binding to polysulfide and suppressing polysulfide dissolution and aid in sulfur dispersion.27 Chitosan’s ability to bind to polysulfide was tested similar to the Ndoped carbons in which polysulfide was mixed thoroughly with chitosan-binder and PVdFbinder. The resulting solutions are displayed in Figure 2b. The inherently dark red color of pure polysylfide can be observed in solutions of polysulfide mixed with PVdF-binder. On the other hand, polysulfide mixed with chitosan-binder exhibited a distinctly clear light orange color, indicating that much of the existing polysulfide was attached to chitosan and left behind with it. This phenomenon is further affirmed by the UV/VIS spectra (Figure 2b), which reveals that the polysulfide-PVdF-binder peak has absorbance values similar to that of pure polysulfide peak while polysulfide-chitosan-binder peak has only half as much absorbance values. These results clearly confirmed chitosan’s polysulfide adsorption ability. Another advantage of using chitosan is the homogeneous distribution of sulfur with conducting agent in binder as demonstrated by the zeta potential analysis (Figure S5a). Slurries of sulfur mixed with chitosan-binder with DI-water and PVdF-binder with NMP were measured and the chitosan-binder slurry demonstrated a much higher zeta potential compared to the PVdF-binder


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ACS Energy Letters

slurry. The images of the two slurries (Figure S5b) also indicate that chitosan slurry has a characteristically homogenous mixture while PVdF slurry has two separate layers where sulfur is mainly predominant in one layer and PVdF-binder is predominant in the other layer. Even when sulfur and conducting agent were added, chitosan slurry had better material dispersion than PVdF slurry as confirmed in the photograph and zeta potential data. In optical microscopy image (Figure S5a and S5c), chitosan-binder mixtures possessed well-dispersion morphology than the PVdF-binder mixtures. These results confirmed that chitosan-binder-based slurry achieves a uniform cathode scaffold by discouraging aggregation of materials. A homogenous distribution is paramount as well-dispered sulfur and conducting agent particles result in better active material utilization and minimize isolated chunks of sulfur dead-sites through higher sulfur surface area exposure.28 Not to mention, during charge/discharge, the cathode structure may degrade due to voids engendered by sulfur dissolution. This degradation can be alleviated through well-dispered sulfur use, which would leave behind smaller voids following sulfur dissolution. Furthermore, the use of an aqueous binder derived from the exoskeleton of crustaceans instead of the NMP based binder makes the MHSE more environmentally friendly and cost-effective.


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Figure 3. (a) TEM image of N-doped MWCNT of NCF and EDX data of NCF of (b) carbon, (c) sulfur, and (d) nitrogen after 50 cycles. The third component of the MHSE is the NCF. The NCF consists of N-doped MWCNT and encompasses the scaffold and chitosan-binder. As explained above (Figure 3), MWCNT’s affinity for polysulfide, coupled with the nitrogen-group on the MWCNT, acts similar to the scaffold and chitosan-binder to trap polysulfide.18,29 The densely packed nitrogen-doped MWCNTs of NCF (Figure 1b) scavenges the leftover polysulfide and acts as the final barrier in inhibiting polysulfide migration. As such, the combination of N-doped scaffold, chitosan-binder, and N-doped NCF greatly increases the MHSE’s polysulfide adsorption property.


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Figure 4. (a) Comparison of the cycling performances at 1/3C rate of a blank-electrode cell with othercells. (Binder: PVdF/chitosan, NCF: MWCNT with and without N-doping, conducting agent: mixed MWCNT and AB without N-doping by mixer-milling.) (b) Initial charge/discharge curves, (c) cycle performance at 1/3C rate, and (d) C-rate capability (0.1C, 0.2C, 1/3C, 0.5C, 1C, 1.5C and 2C) of the MHSE cell (sulfur loading: 10 mg cm-2). (mAh g-1(s) - specific capacity by sulfur in MHSE) Despite these component modifications being auspicious, the improvements would be futile if the actual cell performance is mediocre. In order to assess the efficacy of the modifications, electrodes with different component modifications were tested along with a blank-electrode (conducting agent without N-doping, PVdF-binder, and NCF without N-doping) and their respective electrochemical performances are displayed in Figure 4a and S6b. Not surprisingly,


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the blank-electrode failed in all its evaluations. It had an initial discharge capacity of 950 mAh g1

with a horrid 1st charge/discharge efficiency of 67% and showed unstable and rapidly

deteriorating cycle life with a 1.8% capacity fade per cycle at 1/3C rate. Such abysmal performances can be ascribed to the lack of polysulfide adsorption ability. To address such a problem, electrodes were modified and tested. The first modification made to the blank-electrode is the replacement of the standard PVdF-binder with chitosan-binder. As demonstrated in Figures 2b and S5, the chitosan-binder offers better polysulfide adsorption and uniform dispersion properties. The result is an improved initial discharge capacity of 1030 mAh g-1 and a considerable deceleration in capacity degeneration to achieve a cycle life of 67% at the end of 50 cycles. Despite the improvements, the binder modified electrode is still poor in performance and failed to prevent polysulfide shuttle. An alternative modification method is nitrogen-doping of MWCNT comprising the NCF. Figure S4a validated that N-doped MWCNT is proficient in adsorbing lithium-polysulfide to boost cycle life and efficiency. N-doped NCF-based cells exhibited a slightly higher discharge capacity of 1002 mAh g-1, better cycling efficiency, and a higher cycle life of 77% due to the suppression of polysulfide migration, improving overall performance. However, due to the use of PVdF-binder, polysulfide adsorption on the binder level was absent and insufficient dispersion of materials resulted in an unstable electrode structure, ensuing in fluctuating cycling capacity and efficiency. Naturally, the two modification methods were combined to further enhance the electrochemical performance. Cell test results showed that electrode with both chitosan-binder and N-doped NCF had an initial capacity of 1241 mAh g-1 with a cycling efficiency of 95% and a cycle life of 85%. These results are dramatic improvements over the blank-electrode’s performance and the oscillations in efficiency and cycling capacity vanished.


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Lastly, MWCNT and AB conducting agents were both N-doped. When all the modifications were culminated into the MHSE, its electrochemical performance, despite a high-sulfur content of 10 mg cm-2, came out to be phenomenal, owing to the synergetic effects of the scaffold, chitosan-binder, and NCF. Figure 4b-d delineate the cell test outcomes of the MHSE cell. The MHSE cell was first discharged to 1332 mAh g-1 at 0.1C. It had a stable efficiency of 98% throughout cycling and only had 1.9 mAh g-1 capacity fade per cycle, culminating in a cycle retention of 91% at the end of 50 cycles at 1/3C rate. We also showed cell capacity and specific capacity of total cathode used 10 mg cm-2 sulfur loading electrode in Figure S7. Nitrogen-doping of conducting agents further boosted the polysulfide adsorption ability of the modified electrode and increased the conductivity at higher sulfur loadings. The rate capability test also showed outstanding results (Figure 4d). The cells were cycled at rates between 0.1C and 2C and even at high currents, capacities did not plummet (2C discharge capacity: 845 mAh g-1, 2C/0.1C: 63%). Despite it being diffcult to confirm the long-term cyclability of MHSE cell due to corrosion and non-uniform growth of lithium-metal caused by high current density (Figure S8),30 the modified cathode on Al-foil successfully enhanced electrochemical performances even at high sulfur loadings. As such, the electrochemical evaluations of the MHSE validated that each of the modifications was substantial and demonstrated that the MHSE is a superb electrode designed for easy mass production process with a high-sulfur-loading through the use of elemental sulfur. In summary, extensive structural modifications were made to the major components of the cathode with a focus on enhancing the electrochemical performance for high-sulfur-loading electrodes. The resulting MHSE has a high sulfur loading of 10 mg cm-2 through the use of elemental sulfur and comprises of the scaffold, chitosan-binder, and NCF where each component plays a paramount role in addressing lithium-sulfur battery’s shortcomings. The scaffold’s


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uniform mixture of sulfur, N-doped AB, and N-doped MWCNT increases the structural strength of the cathode, inhibiting the dissolution of polysulfide and degradation in electrical conductivity. The chitosan-binder’s -NH2 and -OH functional groups bind firmly to polysulfide and induce a more homogenous distribution of sulfur in the conducting agent and the NCF’s Ndoped MWCNT retards polysulfide shuttle effect further. Cell tests were performed to evaluate its electrochemical performance and the MHSE demonstrated a stable 1st discharge capacity of 1332 mAh g-1. It also had a noteworthy cycle retention of 91% at 50 cycles and maintained a stable efficiency on cycling of 98%. As such, the MHSE's characteristic of a high loading elemental sulfur cathode on Al-foil is an important step in realizing the dream of lithium-sulfur battery development.

Experimental Methods Details are provided in the Supporting Information.


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ASSOCIATED CONTENT Supporting Information. Experimental Methods. SEM images of N-doped AB and MWCNT. TEM images of mixed Ndoped AB and MWCNT. XPS of MWCNT with and without N-doping. UV/VIS Spectra of polysulfide and carbon with/without N-doping. Zeta potential of binder comparison. Initial charge/discharge curves. Cycled lithium metal images. Additional figures. AUTHOR INFORMATION Corresponding Authors * Phone: +82-2-2220-0524. E-mail: [email protected] (Sun, Y.-K.) * Phone: +1-512-471-1791. E-mail: [email protected] (Manthiram, A.) Authors Contributions Hee Min Kim and Ho-Hyun Sun contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Global Frontier R&D Program (2013M3A6B1078875) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning and the Human Resources Development program (No. 20154010200840) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy. The work at the University of Texas at Austin was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy,


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Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DEEE007218.

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ACS Energy Letters

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