Polyimide-polyether binders–diminishing the carbon content in lithiumsulfur batteries

Polyimide-polyether binders–diminishing the carbon content in lithiumsulfur batteries

Materials Today Energy 6 (2017) 264e270 Contents lists available at ScienceDirect Materials Today Energy journal homepage: www.journals.elsevier.com...

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Materials Today Energy 6 (2017) 264e270

Contents lists available at ScienceDirect

Materials Today Energy journal homepage: www.journals.elsevier.com/materials-today-energy/

Polyimide-polyether bindersediminishing the carbon content in lithiumesulfur batteries ndez a, 1, Nerea Lago a, Devaraj Shanmukaraj b, Michel Armand b, Guiomar Herna David Mecerreyes a, c, * n, Spain POLYMAT, University of the Basque Country UPV/EHU, Joxe Mari Korta Center, Avda. Tolosa 72, 20018, Donostia-San Sebastia ~ ano, Alava, Spain CIC EnergiGUNE, Alava Technology Park, Albert Einstein 48, 01510, Min c Ikerbasque, Basque Foundation for Science, E-48011 Bilbao, Spain a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 September 2017 Received in revised form 16 October 2017 Accepted 2 November 2017

Lithium-sulfur batteries are on the run to become the next generation energy storage technology. First of all due to its high theoretical energy density but also for its sustainability and low cost. However, there are still several challenges to take into account such as reducing the shuttle effect, decreasing the amount of conductive carbon to increase the energy density or enhancing the sulfur utilization. Herein, redoxactive binders based on polyimide-polyether copolymers have been proposed as a solution to those drawbacks. These multiblock copolymers combine the ability of poly (ethylene oxide) to act as polysulfide trap and the properties of the imide groups to redox mediate the charge-discharge of sulfur. Thus, poly (ethylene oxide) block helps with the shuttle effect and mass transport in the electrode whereas the polyimide part enhances the charge transfer promoting the sulfur utilization. Sulfur cathodes containing pyromellitic, naphthalene or perylene polyimide-polyether binders featured improved cell performance in comparison with pure PEO binder. Among them, the electrode with naphthalene polyimide-PEO binder showed the best results with an initial capacity of 1300 mA h g1 at C/5, low polarization and 70% capacity retention after 30 cycles. Reducing the amount of carbon black in the cathode to 5 wt%, the cell with the redox-active binder was able to deliver 500 mA h g1 at C/5 with 78% capacity retention after 20 cycles. Our results demonstrate the possibility to reduce the amount of carbon by introducing polyimide-polyether copolymers as redox-active binders, increasing the sulfur utilization, redox kinetics and stability of the cell. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Lithium-sulfur batteries Polyimides Redox mediators Redox-active binders Diminishing carbon

1. Introduction Global energy consumption is increasing at all levels, from small devices, to electric transportation and to the grid scale. Therefore, concerns about environmental pollution coming from the use of fossil fuels supports the search for alternative, sustainable and clean energy storage technologies. Lithium-sulfur batteries fulfill those requirements. Sulfur is an abundant, low cost and environmentally benign element, featuring a high theoretical specific capacity (1675 mA h g1) and specific energy (2500 Wh kg1).

* Corresponding author. Institute for Polymer Materials POLYMAT, Universidad del País Vasco/Euskal Herriko Unvertsitatea (UPV-EHU), 20018 Donostia-San Sebastian, Spain. E-mail address: [email protected] (D. Mecerreyes). 1 €m Laboratory, La €gerhyddsv€ Dept. of Chemistry e Ångstro agen 1, Polacksbacken, 751 21 Uppsala, Sweden. https://doi.org/10.1016/j.mtener.2017.11.001 2468-6069/© 2017 Elsevier Ltd. All rights reserved.

However, there are still several challenges regarding this technology, which are the low electrochemical utilization of sulfur, fast capacity fade and the insulating characteristic of sulfur and the discharge products [1e5]. Research on this technology has been mostly focused on developing different ways to avoid the shuttle effect, to accommodate the volume changes of the active material and to increase the conductivity of sulfur cathodes. The shuttle mechanism results from the dissolution of polysulfides, reducing the coulombic efficiency and involving the loss of active material. Several strategies have been investigated to avoid this problem; for example the addition of a porous polysulfide reservoir matrix [6,7], polymer coatings [8,9], interlayers [10e12] and surface-coated separators [13e15]. Regarding the insulating behavior of sulfur, it has been overcome by the incorporation of sulfur into a conductive matrix (e.g., carbon [8,16,17], polymer [18e20] or metal [21,22]). However, these additives make the cathode processing more complex

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with an inherent decrease in the effective energy density of the battery [4]. The sulfur electrodes usually have three components, elemental sulfur, a conductive carbon additive and a polymeric binder. Both the conductive carbon and the binder are additives without energy contribution to the battery whose content in the electrode composition needs to be minimized. Nowadays, sulfur electrodes require a high amount (aprox. 30 wt%) of carbon additive due to the intrinsic low electrical conductivity of sulfur and slow redox kinetics. The main purpose of the binder is to stabilize the electrode and to ensure a good electrical and physical contact between all the components and the current collector. Conventional binders such as polytetrafluoroethylene (PTFE) or poly (vinylidenedifluoride) (PVDF) are unable to provide any additional advantage [4]. Therefore, alternative binders that can contribute towards improving the performance of the batteries or making the electrode preparation more sustainable are under study. Some examples are poly (ethylene oxide) (PEO) [9,23,24], poly (vinylpyrrolidone) (PVP) [25], other amide-based polymers [26] poly (acrylamide-co-diallyldimethylammonium chloride) (AMAC) [27], gelatin [28] and carboxymethylcellulose:styrene butadiene-rubber (CMC:SBR) [29]. Among them, the most studied one is PEO. Several groups have demonstrated its ability to act as a polysulfide trap and increasing the characteristics of the electrodes such as specific capacity, polarization, cyclability and kinetics [8,9,23,24]. Recently, redox-active additives were recently incorporated to the sulfur cathode composition in order to enhance the active mass utilization [30,31]. Those materials were based on imide groups and studied as redox mediators for the sulfur reactions because both share the same potential range [30,32,33]. Redox mediators are reversible redox couples able to facilitate the electrochemical reaction of the electrode reducing the polarization and enhancing the active material utilization [34e37]. Furthermore, it has been reported that the strong interaction of the oxygen in the carbonyl groups of the imides (NeC¼O groups) with elemental sulfur and lithium sulfides increases the utilization of sulfur and favors the suppression of the shuttle effect [23,25,31]. In comparison with pristine sulfur, they reported a better performance of the cathodes containing redox-active (poly) imides additives [31]. Among the different imide groups, perylenebisimide seems to lead to the highest rate performance of the sulfur batteries with reduced overpotentials [30]. In general, research on lithium-sulfur batteries has been focused on increasing the battery performance at any cost. However, some practical aspects should be considered, such as the ease of cathode preparation or the reduction of carbon content which is a bulky and inactive additive [1,3,38]. Herein, we report the effect of redoxactive binders featuring different redox potentials on sulfur cathodes. A three-in-one strategy was aimed: increase the sulfur utilization, the redox kinetics and the stability by the incorporation of polyimide-polyether copolymers as binders. With a simple electrode processing, the cell performance with these binders was improved even when the amount of carbon was decreased to 5 wt%. 2. Material and methods 2.1. Materials Pyromelliticdianhydride (PMDA) (97%), 1,4,5,8-Naphthalenetetracarboxylic dianhydride (NTDA) (95%) and Perylene3,4,9,10-tetracarboxylic dianhydride (97%) (PTCDA) were obtained from commercial sources (Sigma-Aldrich) and used as received. O,O0 -Bis(2-aminopropyl)polyethylene glycol with a central PEO sequence of 1900 molecular weight (Jeffamine® ED-2003) was purchased also from Sigma-Aldrich and used as received.

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Anhydrous N,N-dimethylacetamide (DMAc), anhydrous toluene, diethyl ether, dichloromethane, N-methyl-2-pyrrolidone (NMP), anhydrous acetonitrile (ACN) and anhydrous 2-methyltetrahydrofuran (MeTHF) were purchased from Sigma-Aldrich and used as received. Elemental sulfur (99.5%) and poly (ethylene oxide) (PEO) (Mw ¼ 5  106 g mol1) were purchased from Sigma-Aldrich and Lithium Bis(trifluoromethanesulfonyl)imide (LiTFSI) from Solvionic. 2.2. Polymer synthesis and characterization Polyimide-polyether copolymers (pyromellitic polyimide-PEO, naphthalene polyimide-PEO and perylene polyimide-PEO) were synthesized according to the method described in our previous paper [39]. Briefly, the diamine (Jeffamine® ED-2003) was dissolved in anhydrous N,N-dimethylacetamide under nitrogen atmosphere; followed by the addition of an equimolar amount of dianhydride. The mixture was stirred under reflux and imidized azeotropically with toluene at 160  C. The obtained polymers were precipitated into diethyl ether, washed several times and dried under vacuum at 100  C overnight. The chemical structures of the obtained polymers were confirmed by NMR and ATR-FTIR. 1H NMR was performed using deuterated dimethyl sulfoxide-d6 (DMSO-d6) and a BrukerAvance 500 (500 MHz) spectrometer. ATR-FTIR measurements were conducted on a Bruker ALPHA Spectrometer. Electrochemical characterization of the bare redox-active polymers was performed using CR2032 coin cells with a cathode composition of 85 wt% polymer and 15 wt% KJ600 (Ketjenblack® EC-600JD, AkzoNobel), metallic lithium foil as anode and 1M LiTFSI in MeTHF as liquid electrolyte. Cyclic voltammetry experiments were carried out at a scan rate of 1 mV s1on a VMP3 (Biologic®) electrochemical workstation. 2.3. Electrode preparation and electrochemical characterization of lithium-sulfur cells Two types of sulfur cathodes were prepared with different sulfur/carbon/binder compositions. One of the compositions consisted in mixing 60 wt% of sulfur, 30 wt% of KJ600 and 10 wt% of binder while the other one was 45 wt% of sulfur, 5 wt% of KJ600 and 50 wt% of binder. The sulfur electrodes were prepared by dissolving the corresponding binder in acetonitrile (ACN). Afterwards sulfur and carbon black were added and mixed by planetary ball milling. The obtained slurry was casted over an aluminum carbon coated foil and dried under vacuum at 70  C overnight. The films were punched into 13 mm diameter discs with a sulfur loading of 0.8e1.0 mg cm2 for the 60/30/10 composition and 1.2e1.5 mg cm2 for 45/5/50. The microstructural inspections were carried out with a scanning electron microscope (SEM, TM3030, Hitachi) with an energy dispersive X-ray spectrometer (EDX, Bruker). Coin cells (CR2032) were assembled in an argon-filled glove box using lithium foil (Rockwood lithium) as anode. A glass fiber (Glass fiber GFD/55, Whatman) imbibed with a 1M LiTFSI in MeTHF solution was used as a separator. Cyclic voltammetry (CV) experiments for the lithium-sulfur batteries were carried out at 0.05 mV s1 between 1 and 3.2 V vs. Liþ/Li on a VMP3 (Biologic®) electrochemical workstation. Galvanostatic charge/discharge measurements were carried out on a MACCOR® battery tester between 1 and 3 V vs. Liþ/Li and the current density was based on the sulfur mass (1C ¼ 1675 mA g1). Cells were cycled at room temperature.

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3. Results and discussion 3.1. Electrode preparation and electrochemical characterization of lithium-sulfur cells Several polyimide-polyether copolymers were synthesized via polycondensation reaction using different dianhydrides (pyromellitic, naphthalene and perylene dianhydrides) and an oligoether-based diamine (Jeffamine® ED-2003) [39,40]. The chemical structure of the dianhydrides will determine the electrochemical properties of the final polymer whereas the solubility and manufacturing process of the materials will be given by the oligoether diamine block. These reactions were quantitative and yielded polyimides whose chemical structure and physical aspect are depicted in Fig. 1. The weight percent of imide groups in each polyimide-polyether copolymer was 10% for pyromellitic, 12% for naphthalene and 16.4% for perylene polyimides-PEO. The chemical structure and complete imidization of the synthesized polymers was confirmed by NMR and ATR-FTIR experiments. Characteristic signals in the 1H NMR spectra from ether groups appeared at 1 ppm (CH3) and 3.5 ppm (CH2) and the ones corresponding to aromatic protons appeared between 7.4 and 8.6 ppm for all the polymers. Imidization was confirmed by the shift in the carbonyl bands towards lower wavenumber values in the case of polyimides in comparison with the corresponding dianhydride precursor. It is also worth mentioning the decrease of the wavenumber for the stretching carbonyl bands when increasing the conjugation of the imide groups, from pyromellitic (1770 and 1714 cm1) to naphthalene (1705 and 1662 cm1) to perylene (1695 and 1654 cm1) polyimides. Further characterization of the polyimide-polyether copolymers, such as thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), gel permeation chromatography (GPC) and atomic force microscopy (AFM) can be found in a previous article [39]. Electrochemical properties of redox-active polyimides derived from pyromellitic, naphthalene and perylene dianhydrides have already been reported in literature [41,42]. However, insoluble powders were obtained with the consequent requirement to add a binder in the electrode preparation process. As previously published, this drawback was overcome when using polyimidepolyether copolymers due to their solubility in acetonitrile, tetrahydrofuran or chloroform thus facilitating the electrode processing [39]. The three redox-active polyimides synthesized in this work were characterized against metallic lithium with an electrode composition of 85 wt% of polymer and 15 wt% of KJ600. As depicted in Fig. 1, the redox potential of the polyimides shifted to positive

potentials when increasing the conjugation of the imide groups from 2.1 V to 1.8 V vs. Liþ/Li for pyromellitic polyimide to 2.3 V vs. Liþ/Li for naphthalene polyimide and 2.4 V vs. Liþ/Li for perylene polyimide. As depicted in Fig. 2, polyimides present redox potentials similar to the ones of sulfur, therefore, they could facilitate the charge transfer favoring the sulfur utilization. Besides, polyether groups could enhance the stability of sulfur cathodes acting as a polysulfide trap [8,14]. Therefore, polyimide-polyether copolymers were chosen as redox-active binders for lithium-sulfur batteries. 3.2. Electrochemical performance of lithium-sulfur batteries In order to study the effect of the different polyimide-polyether copolymers as redox-active binders, a reference binder of PEO was chosen. For a better understanding of the binder's role in the cathode, a schematic illustration of a sulfur cathode using PEO or redox-active polyimide-polyether copolymers is represented in Fig. 3. In both cases, PEO provides two main features: increasing the conductivity of lithium ions throughout the sulfur cathode enhancing the mass transport [43] as well as acting as a polysulfide trap limiting their diffusion out of the electrode [8,9]. The difference between both binders is the presence of imide groups, which are redox-active species known to act as redox mediators for sulfur cathodes [30,33,37]. Furthermore, polyimides in the reduced state become more conductive [40,44], thus, facilitating the charge transfer across the redox-active binder and sulfur interfaces and, consequently, favoring the active mass utilization [33]. To facilitate the electrode processing and avoid the use of complex sulfur-carbon composites, pristine sulfur and carbon KJ600 were employed. These components were mixed with the corresponding binder by wet ball milling. Since polyimidepolyether copolymers are able to act as binders as well, additional inactive components are not needed in the electrode formulation. Sulfur electrodes were fabricated with a composition of 60 wt% sulfur, 30 wt% KJ600 and 10 wt% of binders (PEO, pyromellitic, naphthalene and perylene polyimides-PEO). The cell performance of these different cathodes was evaluated by galvanostatic cycling at C/5 (Fig. 4). No additives, such as LiNO3, were added to the electrolyte solution in order to evaluate the role of redox-active binders towards the polysulfide shuttling effect. As shown in Fig. 4a, the voltage profiles of the first discharge showed two voltage plateaus for all the polymer binders. The upper discharge plateau corresponds to the reduction reaction from sulfur to longchain polysulfides, which are further reduced to Li2S in the second

Fig. 1. Chemical structure, physical aspect and cyclic voltammogram of polyimide-polyether copolymers.

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Fig. 2. Schematic illustration of the energy levels from sulfur and polyimides.

Fig. 3. Schematic illustration of sulfur cathodes containing poly (ethylene oxide) binder and redox-active polyimide-polyether binder. PEO allows the conduction of lithium ions within both electrodes and retains the polysulfides inside the cathode, whereas polyimide acts as redox mediator favoring the charge transfer.

Fig. 4. (a) Voltage profiles at the first cycle, (b) voltage profiles at the 2nd, 5th and 25th cycle and (c) cycling performances and coulombic efficiency of 60S/30KB/10 pyromellitic polyimide-PEO (green lines/circles), 60S/30KB/10 naphthalene polyimide-PEO (blue lines/rhombi),60S/30KB/10 perylene polyimide-PEO (red lines/triangles) and 60S/30KB/10 PEO (black lines/inverse triangles) at C/5.

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discharge plateau. Sulfur cathodes containing redox-active binders were able to deliver higher capacity than the electrode with pure PEO binder, as shown by the longer discharge plateaus in the voltage profile (Fig. 4a). These results indicated the higher active mass utilization of sulfur with the presence of imide moieties in the cathode. Regarding the different polyimide's structure, sulfur cathode containing naphthalene polyimide-PEO showed the highest initial capacity (1300 mA h g1), followed by pyromellitic polyimide-PEO (1180 mA h g1), perylene polyimide-PEO (1050 mA h g1) and, finally, pure PEO (835 mA h g1). The behavior observed for the sulfur cathodes containing redox-active binders could be attributed to the difference in the redox potential of the imides. As it can be seen in the CV of Fig. 1, the first reduction potential of naphthalene and pyromellitic-based polyimides is closer to the one of sulfur, whereas for perylene polyimide-PEO it is higher. This suggested that the closer the reduction potentials of the imides to the ones of sulfur the higher contribution to the redox reaction of the latter component. Furthermore, the polarization was decreased in the second reduction reaction when using redox-active binders, owing to a greater electrical contact achieved through the imide moieties in the reduced state [40,44]. To gain a better insight into the polysulfide diffusion, the voltage profiles of the consequent cycles were explored (Fig. 4b). The overlapping upper discharge curves indicated that the polysulfide diffusion was limited [14]. These results were confirmed with the similar capacity retention (70%) observed for all cells (Fig. 4c). However, a clear difference was observed in the coulombic efficiency, being 85% for the sulfur cell containing PEO binder and higher for the polyimide binders. As reported by She et al. [25], carbonyl groups have strong interactions with lithium sulfides and that would explain the higher coulombic efficiency of polyimides in comparison with pure PEO. As for the difference between naphthalene polyimide and the other two polyimides, the higher coulombic efficiency for naphthalene polyimide-PEO could be due to a better contact in the electrode and lower resistance. Imides have been reported as supramolecular binders [30] and polymer hosts for sulfur cathodes [31], however, complex sulfur-carbon composites were employed or additional binders needed. Herein, we overcome those drawbacks by the use of polyimide-polyether

copolymers which feature a straightforward synthesis, capability to act as binders and enhancement of sulfur redox reactions. If more practical applications were to be considered, carbon content in the electrode should be diminished since it is bulky and inactive, compromising the energy density of the cell [38]. Although a poorer performance would be expected, it is still something worth considering. Therefore, polyimide-polyether copolymers were studied as possible replacements for carbon. Bearing this in mind, the electrode composition was modified by changing the carbon content from 30 wt% to 5 wt%. Besides, to further study the effect of the redox-active binders in the sulfur cathode, the amount of binder was increased to 50 wt%, leading to a final electrode composition of 45 wt% sulfur, 5 wt% KJ600 and 50 wt % binder. Although the weight percent of binder was increased, the capacity contribution of the polyimides to the sulfur cathode is negligible due to the low amount of imide groups in the polyimidepolyether copolymer. Therefore, sulfur mass in the cathode was still used as active mass of the electrode. For this new composition, the binders chosen were the two best performing polyimides, naphthalene and pyromellitic polyimidesPEO and the reference PEO binder. Morphology of these electrodes was examined with scanning electron microscopy (SEM). The white particles from the images in Fig. 5 correspond to elemental sulfur. From the elemental mapping, a poor dispersion of sulfur particles was observed for the electrode containing PEO as binder (Fig. 5a). Dispersion of sulfur within the cathode was improved when using pyromellitic polyimide-PEO (Fig. 5b) and naphthalene polyimide-PEO (Fig. 5c) binders in the formulation. Besides, polyimide-polyether binders showed a better blended cathode and contact with the sulfur particles in comparison with the PEO cathode. The electrochemical behavior of sulfur cathodes was investigated by cyclic voltammetry (CV) between 1 V and 3.2 V vs. Liþ/Li at a scan rate of 0.05 mV s1 (Fig. 6). All three cells showed two reduction peaks centered at 2.3 V and 2 V vs. Liþ/Li. However, the oxidation peak was polarized from 2.5 V vs. Liþ/Li for the cathodes containing polyimide-polyether binders to 2.6 V vs. Liþ/Li for the reference cell with PEO. This is in agreement with the previous observation that polyimides in the reduced state increase the conductivity, thus, reducing the polarization. Furthermore, sulfur

Fig. 5. SEM images and elemental mapping of (a) 45S/5KB/50 PEO, (b) 45S/5KB/50 pyromellitic polyimide-PEO and (c) 45S/5KB/50 naphthalene polyimide-PEO electrodes.

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Fig. 6. Cyclic voltammograms of the 1st, 2nd and 4th cycle of (a) 45S/5KB/50 PEO, (b) 45S/5KB/50 pyromellitic polyimide-PEO and (c) 45S/5KB/50 naphthalene polyimide-PEO, at 0.05 mV s1.

cathodes containing polyimide-polyether copolymers presented sharper reduction and oxidation peaks in comparison with the reference PEO cathode. Thus, higher reaction kinetics was achieved by the electrodes with the redox-active binders, indicating the enhanced electrochemical reaction of sulfur by the presence of imide groups. Consequent scans in CV showed a rapid disappearance of the redox peaks for the sulfur cathodes containing PEO and pyromellitic polyimide-PEO. In contrast, the overlapping voltammograms of the sulfur cell with Naphthalene polyimide-PEO binder indicated higher electrochemical stability and reversibility, owing to a better electrical contact between naphthalene moieties and sulfur and the similar redox potentials. The different behavior observed for the two polyimides, pyromellitic and naphthalene, could be due to a lower electrochemical stability of the former polymer, as it has been previously reported [39,41]. Galvanostatic charge-discharge experiments at C/5 were carried out for the cells with the low amount of carbon. Fig. 7a shows the voltage profiles of the first cycle. Although the three cells presented similar capacity values (502 mA h g1) in the first cycle, the polarization was slightly decreased when using a redox-active binder. As it was observed in the CV in Fig. 6, oxidation potential of sulfur cells with redox-active binders were lower than with the reference binder. Besides, the overpotential observed at the beginning of the first charge process for the cell with the reference binder was suppressed for the cell with the redox-active binders, suggesting the oxidation of Li2S was favored by the imide groups present in the cathode. As it can be seen from the voltage profile of the 10th cycle (Fig. 7b), a clear detrimental performance was observed for the cell using the reference PEO binder in comparison with the first cycle. In contrast, little changes were observed in the voltage profiles of the cells containing the redox-active binders, owing to a better contact between sulfur and imide moieties of the binder. Besides, stability of the electrodes was improved, as it can be seen from the 78% and

73% capacity retention obtained by the Naphthalene and pyromellitic polyimides-PEO, respectively, in comparison with the12% capacity retention for the reference cell after 20 cycles. The poor coulombic efficiency observed for the sulfur cell with naphthalene polyimide-PEO could be avoided by the incorporation of LiNO3 salt to the electrolyte. In order to emphasize the effect of lowering the amount of carbon, the charge-discharge capacities of the two compositions (60S/30KB/10Binder and 45S/5KB/50Binder) using naphthalene polyimide-PEO as binder and normalized by areal of the electrode are represented in Fig. 7c. Maintaining the same electrode preparation method, higher area capacity can be obtained for the 45S/ 5KB/50 naphthalene polyimide-PEO composition in comparison with the other with higher carbon content. Due to the low density of carbon, it occupies most of the electrode's volume. When it is replaced with sulfur or binder, the amount of these two components increase leading to a higher active mass incorporation in the electrode, which can be reflected by the higher areal capacity. Further optimization of the electrode's manufacturing or the incorporation of additives to the electrolyte would reduce the low coulombic efficiency seen for the 45S/5KB/50 naphthalene polyimide-PEO cell. Even though really low amount of carbon was incorporated in the electrode and the simplicity of the strategy, sulfur cathodes with polyimide-polyether copolymers as binders were able to provide higher reaction kinetics and increased electrochemical stability in comparison with the reference PEO binder. 4. Conclusions Three polyimide-polyether copolymers with different imide structures (pyromellitic, naphthalene and perylene) were proposed as redox-active binders for lithium-sulfur cells. As the redox

Fig. 7. (a) Voltage profiles at the first cycle and (b) voltage profiles at the 10th cycle of 45S/5KB/50 pyromellitic polyimide-PEO (green), 45S/5KB/50 naphthalene polyimide-PEO (blue) and 45S/5KB/50 PEO (black) at C/5. (c) Areal charge-discharge capacities of 60S/30KB/10 naphthalene polyimide-PEO (blue rhombi) and 45S/5KB/50 naphthalene polyimide-PEO (red squares) at C/5.

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potentials of polyimides coincide with the ones of sulfur, they have been used to enhance the electrochemical reactions of sulfur. The cell with Naphthalene polyimide-PEO binder, whose redox voltage is closer to the ones of sulfur, showed the highest sulfur utilization and lowest polarization in comparison with the other polyimides and the reference PEO binder. The cathode with 60 wt% sulfur, 30 wt% KJ600 and 10% naphthalene-based polyimide cycled at C/5 showed an initial capacity of 1300 mA h g1 and 70% capacity retention after 30 cycles. Moving towards practical applications the amount of carbon was reduced, and the cathode composition was modified to 45 wt% sulfur, 5 wt% KJ600 and 50 wt% of binder. Cyclic voltammetry of the sulfur cells containing polyimide-polyether copolymers featured lower polarization and higher reaction kinetics than pure PEO. Despite the low amount of carbon, the stability of the cell with naphthalene and pyromellitic polyimides was improved to 78% and 73% capacity retention, respectively, after 20 cycles. In this work, polyimide-polyether copolymers used as redox-active binders proved the three-in-one strategy: Improved sulfur utilization, enhanced redox kinetics and stability. Further optimization of the electrode preparation or incorporation of more elaborate redoxactive binders and cathode structures could lead to higher specific capacities. Acknowledgements Authors thank Dr. G. Patricia Leal and Dr. Maitane Salsamendi for their contribution with the SEM characterization. This work was financially supported by the Starting Grant Innovative Polymers for Energy Storage (iPes) 306250 from the European Research Council (ERC). Guiomar Hern andez thanks Spanish Ministry of Education, Culture and Sport for the predoctoral FPU fellowship. References [1] A. Manthiram, S.-H. Chung, C. Zu, LithiumeSulfur batteries: progress and prospects, Adv. Mater 27 (2015) 1980e2006. k, Progress towards commercially viable LieS [2] S. Urbonaite, T. Poux, P. Nova battery cells, Adv. Energy Mater 5 (2015) 1500118. [3] A. Manthiram, Y. Fu, S.-H. Chung, C. Zu, Y.-S. Su, Rechargeable lithiumesulfur batteries, Chem. Rev. 114 (2014) 11751e11787. [4] P.T. Dirlam, R.S. Glass, K. Char, J. Pyun, The use of polymers in Li-S batteries: a review, J. Polym. Sci. Part A Polym. Chem. 55 (2017) 1635e1668. [5] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.-M. Tarascon, Li-O2 and Li-S batteries with high energy storage, Nat. Mater 11 (2012) 19e29. [6] X. Ji, S. Evers, R. Black, L.F. Nazar, Stabilizing lithiumesulphur cathodes using polysulphide reservoirs, Nat. Commun. 2 (2011) 325. [7] S. Wei, H. Zhang, Y. Huang, W. Wang, Y. Xia, Z. Yu, Pig bone derived hierarchical porous carbon and its enhanced cycling performance of lithium-sulfur batteries, Energy Environ. Sci. 4 (2011) 736e740. [8] X. Ji, K.T. Lee, L.F. Nazar, A highly ordered nanostructured carbon-sulphur cathode for lithium-sulphur batteries, Nat. Mater 8 (2009) 500e506. [9] Y. Fu, Y.-S. Su, A. Manthiram, Sulfurecarbon nanocomposite cathodes improved by an amphiphilic block copolymer for high-rate lithiumesulfur batteries, ACS Appl. Mater. Interfaces 4 (2012) 6046e6052. [10] Y.-S. Su, A. Manthiram, A new approach to improve cycle performance of rechargeable lithium-sulfur batteries by inserting a free-standing MWCNT interlayer, Chem. Commun. 48 (2012) 8817e8819. [11] G. Ma, Z. Wen, J. Jin, M. Wu, X. Wu, J. Zhang, Enhanced cycle performance of LieS battery with a polypyrrole functional interlayer, J. Power Sources 267 (2014) 542e546. [12] K. Zhang, F. Qin, J. Fang, Q. Li, M. Jia, Y. Lai, Z. Zhang, J. Li, Nickel foam as interlayer to improve the performance of lithiumesulfur battery, J. Solid State Electrochem 18 (2014) 1025e1029. [13] J.-Q. Huang, Q. Zhang, H.-J. Peng, X.-Y. Liu, W.-Z. Qian, F. Wei, Ionic shield for polysulfides towards highly-stable lithium-sulfur batteries, Energy Environ. Sci. 7 (2014) 347e353. [14] S.-H. Chung, A. Manthiram, A polyethylene glycol-supported microporous carbon coating as a polysulfide trap for utilizing pure sulfur cathodes in lithiumesulfur batteries, Adv. Mater 26 (2014) 7352e7357. [15] Z. Zhang, Y. Lai, Z. Zhang, K. Zhang, J. Li, Al2O3-coated porous separator for enhanced electrochemical performance of lithium sulfur batteries, Electrochim. Acta 129 (2014) 55e61.

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