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Improved cycling stability of lithium–sulfur batteries using a polypropylene-supported nitrogen-doped mesoporous carbon hybrid separator as polysulfide adsorbent
Journal of Power Sources 303 (2016) 317e324
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Improved cycling stability of lithiumesulfur batteries using a polypropylene-supported nitrogen-doped mesoporous carbon hybrid separator as polysulfide adsorbent Juan Balach a, *, Tony Jaumann a, Markus Klose a, b, Steffen Oswald a, Jürgen Eckert c, d, Lars Giebeler a, b a
Institute for Complex Materials, Leibniz Institute for Solid State and Materials Research (IFW) Dresden, Helmholtzstraße 20, D-01069 Dresden, Germany €t Dresden, Helmholtzstraße 7, D-01069 Dresden, Germany Institut für Werkstoffwissenschaft, Technische Universita Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstraße 12, A-8700 Leoben, Austria d €t Leoben, Jahnstraße 12, A-8700 Leoben, Austria Department Materials Physics, Montanuniversita b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A functional hybrid separator with a N-doped mesoporous carbon layer was prepared. N-dopants on carbon-coating chemically immobilize sulfur-related species. The functionalized hybrid separator enhanced the overall performance of LieS cells. Excellent cycle stability with ultralow decay rate of 0.037% per cycle was obtained. LieS cells with a hybrid separator delivered a high areal capacity of 3 mAh cm2.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 August 2015 Received in revised form 5 October 2015 Accepted 4 November 2015 Available online xxx
The lithium/sulfur couple is garnering tremendous interest as the next-generation of cost-efficient rechargeable battery systems capable to fulfill emerging energy storage demands. However, the viable commercialization of lithiumesulfur (LieS) batteries is still an obstacle by fast capacity fading and poor cycling stability mostly caused by the polysulfide shuttle and active sulfur material loss. In this contribution, we show that the surface modification of the commercial polypropylene separator with a nitrogen-doped mesoporous carbon enhances the interfacial interaction between the N-dopants on carbon-coating and the sulfur-related species by coupling interactions. These unique physical and interfacial chemical properties of the N-doped mesoporous carbon-coating promote the chemical adsorption and confinement of lithium (poly)sulfide intermediates in the cathode side, improving the active material utilization and hence the overall electrochemical performance of LieS batteries: high initial discharge capacity of 1364 mAh g1 at 0.2C and notable cycling stability with high reversible capacity of 566 mAh g1 and negligible degradation rate of 0.037% after 1200 cycles at 0.5C. Furthermore, despite the use of a simple-mixed sulfur-carbon black cathode with high-sulfur loading of 3.95 mg cm2, the cell with a hybrid separator delivers a high areal capacity of ~3 mAh cm2. © 2015 Published by Elsevier B.V.
Keywords: Nitrogen-doping Mesoporous carbon Functionalized hybrid separator Polysulfide adsorption Lithium-sulfur battery
* Corresponding author. E-mail address: [email protected] (J. Balach). http://dx.doi.org/10.1016/j.jpowsour.2015.11.018 0378-7753/© 2015 Published by Elsevier B.V.
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1. Introduction The development of cost-efficient, high energy storage systems is essential for application in electric vehicles, renewable energy plants and smart grids. In this regard, the lithiumesulfur (LieS) battery has received overwhelming attention in the last few years owing to its outstanding high theoretical energy density of 2.6 kWh kg1, which significantly outperforms the state-of-the-art lithium-ion battery technology based on conventional intercalation oxide cathodes [1e3]. In addition, the wide-spread abundance, low cost and harmless nature of elemental sulfur predestines its use for large-scale applications. In spite of the advantageous features of elemental sulfur as active cathode material, the practical commercialization of LieS batteries is mainly hindered by three inherent issues, related to (i) the poor electrical conductivity of sulfur and its reduced lithium (poly)sulfide products which cause low active material utilization, (ii) the large volumetric expansion of sulfur during lithiation that may lead to the degradation of the sulfur cathode under mechanical strain, and (iii) the even more dramatic shuttle effect [4] that transports soluble high-order polysulfides (Li2Sx, 4 x 8) back and forth between electrodes, causing the corrosion of the lithium anode and the active material loss, finally resulting in a short battery life [5]. The aforementioned technical hurdles have been partially overcome by adopting nano-architectured sulfur/carbon and sulfur/metal oxide composites [6e9], using additives to suppress the polysulfide shuttling and to protect the lithium anode surface [10e12] or designing new cell configurations with polymer or carbon interlayers [13e17]. Among others, the incorporation of a porous carbon layer between the separator and the sulfur cathode, by employing a free-standing interlayer or coated separator, is a promising approach to improve the sulfur utilization and the electrochemical performance of LieS cells. Carbon black [18e20], multiwall carbon nanotubes [21,22], graphene [23,24], carbon nanofibers [25e27] as well as microporous [13,28,29], mesoporous [30e32] and macroporous [33] carbons have all been studied to both increase the cathode conductivity and physically intercept/ confine the dissolved polysulfide intermediates in the cathode region, keeping the active material accessible for further reutilization. Although the major polysulfide-trapping capability of the abovementioned carbon barriers is possible based on the physical trapping, the lack of an effective interaction of the lithium (poly)sulfide intermediates with the carbon network interface eventually leads to the shuttling of the active material and its depletion, especially over long cycling periods. Therefore, a forceful enhancement of the interfacial interaction between functional groups in the carbon barrier and sulfur-related species by coupling interactions is highly desired for improving polysulfide adsorption and, thereby, sulfur utilization and cycle life of LieS batteries. In this regard, carbon materials with nitrogen atoms incorporated into their carbon framework have demonstrated favorable changes in electrical conductivity, surface affinity, electrochemical performance and catalytic activity owing to the alteration of the electronic structure and the chemical activity caused by electron modulation and doping-induced defects [34e38]. Recently, several N-doped carbon materials have been adopted as a conductive host in sulfur cathodes to promote the chemical adsorption capability of the sulfurcontaining guests and thus improve the cycle life of LieS batteries [39e43]. Density functional theory (DFT) and first-principle calculations revealed that pyridinic-N and pyrrolic-N atoms located in the carbon lattice of N-doped carbon hosts can strongly anchor the discharged product Li2S, while the polar polysulfides are preferentially adsorbed on the surface of the quaternary nitrogenneighboring carbon atoms due to the additional delocalization of the p-system, which leads to an improved sulfur utilization and
cycle stability [40e42]. In view of the encouraging chemical features of the nitrogen as a dopant atom, a suitable N-containing carbon barrier can play an essential role on the adsorption of lithium (poly)sulfide species to restrain the shuttle effect and to lengthen the lifespan of LieS batteries. Motivated by the above-mentioned considerations and by our recent findings that a functional large pore volume, mesoporous carbon-coated separator allows for a better electrochemical performance than a conventional LieS cell configuration with a pristine separator and sulfur-infiltrated carbon composite [32], in this contribution, we report a simple modification of the commercial polypropylene separator with a thin layer of melamine-derived Ndoped mesoporous carbon (mesoNC) to improve the interfacial interaction between the N-dopants on carbon-coating and the sulfur-related species by coupling interactions. Despite the use of a simply prepared ball-milled sulfur/carbon black cathode with high sulfur composition, a remarkable electrochemical performance of 3 mAh cm2 and excellent cycle retention is achieved fulfilling the demands for a broad practical application. It will be shown that the unique physical and interfacial chemical properties of the mesoNCcoating are decisive to improve the cycling stability of LieS batteries. To our knowledge, this is the first experimental demonstration that the rational design of modified separators with N-doped mesoporous carbon structures plays an important role on the cycling performance of LieS batteries. The simple processing technique for the preparation of advanced hybrid separators with multifunctionalities to extend the cycling life of the LieS cells is a practical solution which can be straightforwardly incorporated into industrial processes. 2. Experimental section 2.1. Chemicals Resorcinol (99 wt %), Ludox HS-40 (40 wt. % of 12 nm-indiameter silica), elemental sulfur (S, 99.98 wt. %), lithium sulfide (Li2S, 99.98 wt. %) and N-methyl-2-pyrrolidone (NMP, 99 wt. %) were purchased from Aldrich. Melamine (2,3,6-triamino-1,3,5triazine, 99 wt. %), formaldehyde solution (37 wt. %, containing ~10% (w/w) methanol as stabilizer), hydrofluoric acid (HF, 40 wt. %), Super P carbon (SPC, TIMCAL) and polyvinylidene difluoride (PVDF, Solef 21216) were purchased from Alfa Aesar, Fluka, Merck, BASF and Solvay, respectively. All chemicals were used as received. 2.2. Synthesis of undoped and N-doped mesoporous carbons Melamine-derived mesoNC was produced by hard template method using resorcinol, melamine, and formaldehyde as the carbon precursors, based on previous reports [31,39,44]. Typically, 1.1 g of resorcinol (R) was dissolved in 1.5 ml of formaldehyde (F) solution (37 wt. %) under stirring at 70 C for 5 min. Then, a clear mixture of 0.63 g of melamine (M), 0.75 ml of F solution and 9.98 ml of Ludox HS-40, pre-dissolved at 70 C under stirring for ca. 20 min, was added to the above RF solution and the system was closed. The initial yellowish liquid mixture turned translucent orange and solidified within 5 min under stirring at 70 C, while within the further 30 min the solid became opaque red. This solidification process is due to the R-M-F polycondensation catalyzed by the alkaline medium of the sodium hydroxide-stabilized silica (pH z 9.5). The reactant molar ratio of R:M:F was 1:0.5:3, while the weight ratio of SiO2/R-M was 3. The silica-polymer mixture was subsequently dried for 20 h at 80 C in an oven and further carbonized (without the magnetic stir bar) at 900 C for 2 h under Ar atmosphere. Afterwards, the solid silica template was removed from the carbon sample by etching with 20 wt. % HF and repeatedly
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washing with deionized water. Finally, the obtained mesoNC was dried at 90 C for 20 h and finely ground in a mortar. An undoped mesoporous carbon (mesoC) with similar physical texture but different chemical properties compared to the mesoNC was prepared following our previous report [32]. 2.3. Preparation of hybrid separator A simple modification of the commercial polypropylene (PP) separator (Celgard 2500) was conducted by direct coating of a carbon slurry on one side of the separator using a doctor blade technique. The shaker-milled slurries contain 90 wt. % of mesoNC powder and 10 wt. % of the PVDF binder in NMP. The modified separator was cut into circular disks of 16 mm after drying in an oven at 40 C for 20 h. The mass of the carbon coatings was ~0.5 mg cm2. 2.4. Preparation of sulfur cathode Simple sulfur cathodes were prepared by shaker-milling of elemental sulfur, SPC and PVDF in NMP. The well-mixed slurries were coated onto an aluminum foil (11 mm thickness) current collector by doctor blading. Cathodes with S:SPC:PVDF ratios of 50:40:10 wt. % and 70:20:10 wt. %, respectively, were used for pristine and modified separator cell setups in order to keep similar sulfur/carbon ratios. The obtained cathodes were punched into circular disks of 12 mm after drying in an oven at 50 C for 20 h. The thicknesses measured for cathodes with 50 and 70 wt. % S content were 49 and 38 mm, respectively. The areal sulfur loading of the sulfur cathodes used for the main electrochemical tests was ~1.6 mg cm2. For the evaluation of the areal capacity, a cathode with a S:SPC:PVDF ratio of 70:20:10 wt. % (124 mm thickness) and an areal sulfur loading of 3.95 mg cm2 was used. 2.5. Electrochemical measurements CR2025-type stainless steel coin cells were assembled in a dry Ar-filled glovebox. Lithium metal foil (Chempur, 13 mm, 250 mm thickness) was used as both anode material and reference electrode, and a mixture of 1 M lithium bis(trifluoromethylsulfonyl) imide salt (Li-TFSI, BASF) and 0.25 M lithium nitrate (LiNO3, Merck, >99.995 wt. %) additive in 1,3-dioxolane (DOL, SigmaeAldrich, 99.8 wt. %, anhydrous) and 1,2-dimethoxyethane (DME, SigmaeAldrich, 99.5 wt. %, anhydrous) (1:1 v/v) was used as electrolyte. A commercial microporous PP separator (16 mm) was used as pristine separator. The modified separator was placed into the cell with the carbon coating layer facing to the cathode. The cells were charged and discharged at room temperature in the voltage range of 1.8e2.6 V using a BaSyTec Cell Test System (CTS). Both cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were carried out using a VMP3 potentiostat (Bio-logic). The cyclic voltammograms were recorded in a voltage window of 1.8e2.8 V at a scan rate of 0.1 mV s1 and the EIS between 200 KHz and 100 mHz with an AC voltage amplitude of 5 mV at the open-circuit voltage of the cells. Prior to the start out of the long-term cycling tests at 0.5C, the cells were cycled at 0.2C for the first two cycles as preconditioning step (1C ¼ 1672 mA g1). 2.6. Characterization The morphology of the carbon samples and the modified separator were characterized using a Gemini 1530 scanning electron microscope (SEM) operated at 15 kV acceleration voltage. A FEI Tecnai F30 transmission electron microscope (TEM) equipped with
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a field emission gun (FEG) working at 300 kV acceleration voltage was used for high-resolution imaging. Energy-dispersive X-ray spectrometry (EDXS) measurements were taken on the SEM with a Bruker EDXS spectrometer. Nitrogen sorption experiments were performed using a Quantachrome Quadrasorb SI instrument and Quantachrome Quadrawin 4.0 data analyzer after degassing the carbon samples for 20 h at 150 C. Specific surface areas were calculated at a relative pressure p$p1 0 ¼ 0.05e0.2 using the multipoint BrunauereEmmetteTeller (BET) method while the total pore volume was determined at p$p1 0 ¼ 0.97. The pore size distributions were obtained using the Quenched Solid Density Functional Theory (QSDFT) equilibrium model. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a PHI 5600 CI spectrometer (Physical Electronics) equipped with a hemispherical analyzer operated at a pass energy of 29 eV. Monochromated Al Ka radiation (350 W) was used applying partly low energy electrons for charge compensation. All spectra were calibrated using the C 1s level (284.8 eV corresponding to CeC) as reference. Peaks were fitted with a Gaussian function and basic linear background subtraction. The metallic lithium anodes as well as the PP and hybrid separators were washed after dischargeecharge cycles with a DOL:DME (1:1 v/v) inside a glovebox prior to SEM analysis. Ultravioletevisible (UVeVis) absorption spectroscopy analysis was conducted to investigate the polysulfide adsorption capability of SPC, mesoC and mesoNC. Briefly, the polysulfide Li2S6 solution (1 M) was prepared by chemically reacting elemental sulfur and a stoichiometric amount of Li2S in DOL:DME (1:1 v/v) solution, followed by heating at 50 C with magnetic stirring for 48 h in an Arfilled glove box. Then, 20 mg of each carbon sample was added into a vial containing 4 ml of 0.1 M Li2S6 solution. The vials were sealed and gently shaken for 2 h and then kept under static conditions for 18 h to allow precipitation of the carbon material. Afterwards, the upper remnant Li2S6 solutions were analyzed by UVeVis absorption spectroscopy (SPECORD 250 UVeVis spectrophotometer, Analytik Jena). 0.1 M Li2S6 solution was used as a reference concentration. 3. Results and discussion Recent reports have demonstrated that LieS batteries with sulfur-carbon composite cathodes or protecting carbon-layers consisting of mesoporous carbons with large pore volume are of high importance to trap and retain polysulfide intermediates generated during charge/discharge cycles [31,45,46]. MesoNC with large pore volume was produced by polymerization of melamine and formaldehyde in the presence of a commercial 12 nm-indiameter silica hard template and its subsequent carbonization/ template removal. Note that melamine acts as both carbon precursor and nitrogen source. The texture of the mesoNC powder was studied by nitrogen physisorption experiments (Fig. 1). The nitrogen adsorbed at very low-pressure and the distinct hysteresis loop at a high-pressure region indicates the presence of, respectively, micropores and mesopores in the carbon material (Fig. 1a). The specific surface area derived by the BrunauereEmmetteTeller (BET) method and the total pore volume were calculated to be 836 m2 g1 and 2.96 cm3 g1, respectively. As shown in Fig. 1b, the pore size distribution calculated by the Quenched Solid Density Functional Theory (QSDFT) equilibrium model displays two narrow pore sizes at 1.74 and 12.3 nm. The microporosity of the mesoNC contributes to the specific surface area with 325 m2 g1 (39% of the total surface area) and with 0.137 cm3 g1 to the micropore volume (4.6% of the total pore volume). As expected, the obtained mesopore size is very similar to the average diameter of the silica particles used as hard template, while the microporous structure results from the micropores located in the carbon walls. This dual
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Fig. 1. (a) Nitrogen physisorption isotherm of mesoNC and (b) the corresponding pore size distribution obtained by employing the QSDFT equilibrium model.
porosity and the large pore volume of the mesoNC are of high importance for the further preparation of the modified separator because the micropores in the mesoNC-coating can restrain the diffusion of polysulfide intermediates to the anode side by in-situ trapping thereof, while the cavity of the mesopores can buffer the volume change of the confined sulfur species, which is essential for the improvement of the LieS battery performance [32]. The morphology of the mesoNC powder and the mesoNCcoating were investigated by SEM and TEM, as presented in Fig. 2. The SEM image in Fig. 2a shows a disordered porous structure with evenly distributed pores. According to the TEM image (Fig. 2b), the carbon network represents a well-structured replication of the corresponding silica template, recovered in the carbon material with a sphere-like mesoporous structure. The surface chemical composition and the functional groups of the mesoNC were
characterized by X-ray photoelectron spectroscopy (XPS) analysis. The carbon, nitrogen and oxygen content in the mesoNC are, respectively, 82.9, 13.2 and 3.9 at % and the resulting surface N/C ratio of the mesoNC is approximately 0.16, which correlates well with previous reports [39,47]. This result demonstrates that the use of melamine as carbon precursor enables the preparation of an inherent and highly N-doped carbon matrix. Furthermore, the XPS N 1s spectra is fitted by three components with binding energies at 398.3, 400.3 and 401.7 eV, which are assigned to different chemical states of the nitrogen atoms inside the carbon matrix: pyridinic-N (48 at %), pyrrolic-N (35 at %) and quaternary-N (17 at %) (Fig. S1). The major content of pyridinic-/pyrrolic-N functionalities in the mesoNC, which have better affinity to interact with polar lithium (poly)sulfide species than quaternary-N functions [48], could be highly favorable to promote the chemical adsorption of polysulfide
Fig. 2. (a) SEM and (b) TEM images of the pristine mesoNC. (c) Relative polysulfide adsorption of SPC, mesoC and mesoNC and the respective digital image of polysulfide solutions after adsorption. Li2S6 solution was used as a control. (d) Digital image of the modified PP separator showing both the mesoNC-coating and the no-modified sides, (e) top-view and (f) cross-sectional SEM images of the mesoNC-coated separator.
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species. To study the N-doping effect on the polysulfide adsorption capability, the carbon samples mesoNC, mesoC and SPC, with notable difference in physical and chemical properties (Table S1), were dispersed in 0.1 M Li2S6 solution for 20 h to decant the carbon and the retained concentration of Li2S6 (not adsorbed on the carbon surface) in solution was determined by UVeVis absorption spectroscopy. It was found that mesoNC present the most favorable relative polysulfide adsorption with approximately 93% of the initial Li2S6 solution, while mesoC and SPC only adsorbed 65 and 21%, respectively (Fig. 2c). The digital image in Fig. 2c shows a visible decoloration of the Li2S6 solution, highlighting the strong polysulfide adsorption capability of the mesoNC. This finding encouraged us to prepare a mesoNC-coated separator with the aim to improve the interfacial interaction between the N-dopants on carbon and the sulfur-related species and thus the active material utilization. The doctor blade method was used to easily coat one side of the PP separator with a well-adhered layer of mesoNC (Fig. 2c). The SEM image of the mesoNC-coating surface shows a compact layer of homogenously distributed mesoNC particles (Fig. 2d), while the cross-sectional SEM image of the mesoNCcoating reveals a smooth thin layer with a thickness of about 24 mm (Fig. 2e). The successful fabrication of the mesoNC-coated separator and its insertion between the lithium anode and the sulfur cathode (with the mesoNC-coating facing to the cathode side) is expected to enhance the electrochemical performance of the LieS batteries through (i) increasing the electrical conductivity of the mesoNCcoating/sulfur cathode interface and (ii) the early trapping of soluble polysulfides, and thus, effectively reuse the active sulfur material (Scheme 1). To corroborate this promising approach, electrochemical impedance spectroscopy (EIS) measurements were carried out in order to determinate the cathode resistance of the fresh LieS cells with PP separator and mesoNC-coated separator. Fig. 3 shows the Nyquist plots for both types of cells, in which each plot displays a single semicircle at high-to-medium frequency range and an inclined line at low frequency range, related to the charge transfer resistance and the mass transfer process, respectively [31]. The LieS
Scheme 1. Schematic configuration of the conventional LieS cell with a pristine separator (left) and the advanced LieS cell with a mesoNC-coated separator (right).
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cell with the mesoNC-coated separator exhibits a semicircle with a smaller diameter compared to the conventional LieS cell, suggesting a lower charge transfer resistance. The improved electrical conductivity in the sulfur cathode is ascribed to the high electrical conductivity of the mesoNC-coating which acts as a pseudo current collector, reducing the internal impedance of the cell [13]. Note that the mesoNC-coatings were naturally prepared without conductive carbon additives (e.g. carbon black), which highlights the good electrical conductivity of the mesoNC network. The electrochemical cycling performance of the LieS cells were investigated by galvanostatic dis-/charging between 1.8 and 2.6 V vs. Li/Liþ. Fig. 4a and b shows, respectively, the discharge/charge voltage profiles of cells with PP and mesoNC-coated separators acquired during the 2nd, 25th, 50th and 100th cycles at 0.2C (1C ¼ 1672 mA g1). In general terms, both cells exhibit similar voltage profiles associated with the typical two-step sulfur redox reactions consisting of two discharge plateau at ~2.30 and ~2.07 V, which represent the conversion of elemental sulfur (S8) to soluble long-chain polysulfides (Li2Sx, 4 x 8) and their further reduction/precipitation to short-chain polysulfides (Li2Sx, 1 < x < 4) and Li2S, respectively [5]. The two continuous charge plateaus at ~2.28 and ~2.38 V represent the reversible oxidation reactions of Li2S/ Li2S2 to Li2S8/S8. Despite that the two cell set-ups used in this study contain the same amount of carbon material, there are notable differences in the lengths of the voltage plateaus and the voltage polarizations (DE), associated to the reversibility and the redox reaction kinetics of the cell system [24,27,32]. For example, the LieS cell with the mesoNC-coated separator exhibits longer voltage plateaus compared to the LieS cell with PP separator, indicating a better conversion and reutilization of the active sulfur material. Furthermore, the average polarization obtained from the 25th, 50th and 100th cycles at 50% depth of discharge (DOD), reveals a DE of 213 mV for the LieS cell with mesoNC-coated separator, while the DE of the conventional LieS cell present a higher value of 310 mV (Fig. 4a and b, highlighted with blue arrows), demonstrating a faster redox reaction kinetics for those cells with a mesoNC-coated separator. In agreement with the previously described voltage profile study, cyclic voltammograms recorded on cells with PP and hybrid separators exhibited a similar behavior (Fig. S2). Compared to the conventional LieS cell, the cell with a mesoNC-coated separator shows sharper reduction and oxidation peaks with higher peak current, indicating faster conversion kinetics and increased reversibility of the cell reactions in cells with the hybrid
Fig. 3. Nyquist plots of LieS cells with PP separator and mesoNC-coated separator.
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Fig. 4. Discharge/charge voltage profiles acquired during the 2nd, 25th, 50th and, 100th cycles at 0.2C of LieS cells with (a) PP separator and, (b) mesoNC-coated separator. (c) Cycling performance at a current rate of 0.2C and (d) rate performance of LieS cells with PP and mesoNC-coated separator. (e) Long-term cycling performance at a current rate of 0.5C for a LieS cell with a mesoNC-coated separator.
separator. According to the cycle performance carried out at a C rate of 0.2 (Fig. 4c), the LieS cell with the mesoNC-coated separator exhibits an initial discharge capacity of 1364 mAh g1, achieving a sulfur utilization of 82%. After 100 cycles, the cell offers a good capacity retention of 1040 mAh g1 and a high Coulombic efficiency (CE) of 98.5%. In contrast, the conventional LieS cell displays an initial discharge capacity of 1062 mAh g1 (sulfur utilization 64%) which is poorly retained after 100 cycles, reaching a value of only 548 mAh g1 and a CE of 97.4%. These results clearly demonstrate the beneficial effect of the conductive mesoNC-coating, which not only increases the cell conductivity but also ensures the stable cyclability of the cell through the inhibition of inactive sulfur agglomeration and the further reactivation of the entrapped sulfur to an active material. As shown in Fig. 4d, the notable improvements of both the sulfur utilization and the electrochemical performance for LieS cells with mesoNC-coated separator, are also evidently made during the evaluation of the rate capability,
especially at high C-rates. In addition, a long-term cycling test of the LieS cells with the mesoNC-coated separators was carried out at a current density of 0.5C (Fig. 4e). The LieS cell exhibits an initial discharge capacities of 1028 mAh g1 and a notable cycling stability with high reversible capacity of 566 mAh g1, good CE of 96% and, negligible degradation rate of 0.037% after 1200 cycles, one of the lowest reported so far for this kind of cell set-up [3]. Compared with recently reported PP separator modified by carbon black (e.g. SPC) or large pore volume, mesoporous carbon (mesoC) coatings [18,20,32], the cell with a mesoNC-coated separator demonstrates a lower degradation rate upon 500 cycles under similar sulfur content conditions (Table S2), indicating an improved cyclic stability and capacity retention. The absence of a porous structure in the SPC and the lack of nitrogen atoms in the mesoC structure indicate the role of the physisorption and chemisorption of sulfur-related species onto the mesoNC-coating interface, as shown in Fig. 2c. For a better understanding of the mesoNC-coating benefits, cells
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with PP and hybrid separators were disassembled after 100 cycles at 0.5C in the charged state and post-mortem characterizations were performed (Fig. S3). The surface SEM image of the cycled PP separator, which was faced to the cathode, shows a dense solid aggregation of nonconductive material onto the electronically insulating membrane (Fig. S3a), while the surface SEM image of the hybrid separator reveals the absence of agglomerated sulfur species onto the mesoNC-coating (Fig. S3b). Additionally, the carbon/sulfur signals in the corresponding elemental mapping shows active sulfur material uniformly embedded onto the carbon layer, proving that the mesoNC-coating captures the soluble polysulfide intermediates and effectively keeps them as active material on the cathode side. The in-situ chemisorption/confinement of active sulfur species through the mesoNC-coating should restrain the soluble polysulfide migrating towards the anode side and thus prevents further corrosion/degradation of the lithium anode. The above conjecture was corroborated by the surface morphology analysis of the fresh metallic lithium and the corresponding lithium anodes obtained from the cycled LieS cells with PP and mesoNCcoated separator after 100 cycles (Fig. 5). Compared with the smooth and compacted surface of the fresh metallic lithium, the cycled lithium anode from the conventional LieS cell exhibits a highly roughened surface consisting of irregular particles of lithium with an open canyon-like structure, resulting from the strong degradation reaction between dissolved polysulfides and metallic lithium during cycling. In contrast, a significantly smoother surface without apparently cracked particles is observed for the lithium anode when a mesoNC-coated separator is used, evidencing that the early chemisorption/entrapment of polysulfide intermediates through the mesoNC-coating lowers the degradation rate of the lithium anode and hence prolongs its lifetime. In order to demonstrate the potential of the mesoNC-coated separator for practical utilization, the areal sulfur loading of the cathode was increased to 3.95 mg cm2, accounting to a sulfur content in the whole cathode region of 60 wt. %. In general, LieS batteries with high areal capacities and good cycle stability are highly required for their practical application on battery pack for electric and hybrid vehicles. Typically, the increase of sulfur mass loading on the electrode lowers considerably the utilization of active material due to the low electronic conductivity of sulfur, constraining the rise of areal capacity and therewith the energy density. The results of our cell with a hybrid separator and increased sulfur loading are showed in Fig. 6. The cell delivers an initial discharge capacity of 956 mAh g1, which is approximately 30% lower compared to the cell set-up composed of a hybrid separator and sulfur cathode with lower areal sulfur loading (Fig. 4b). However, the cell with higher sulfur loading still shows an improved reversibility, redox reaction kinetics and active sulfur utilization compared to the cell with a pristine separator (Fig. 4a). Importantly, despite the use of simple-mixed sulfur-carbon black
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cathode, the LieS cell with a mesoNC-coated separator demonstrates a good cycle stability and notable high areal capacity of 3 mAh cm2 after 50 cycles, 2- to 6-fold greater than those typical LieS cells with sophisticated sulfur-carbon composite cathodes [49], outperforming current efforts to increase the specific energy density of LieS batteries. The improved performance of the cell with high sulfur loading is ascribed to the early physical trapping of polysulfide intermediates into the highly disordered threedimensional mesoporous network of the mesoNC layer, which restrain the migration of soluble long-chain polysulfides to the anode. In addition, the trapped polysulfides are further effectively immobilized within the electrically conductive porous matrix by chemical adsorption between the N-containing functionalities on mesoNC and the discharged polar lithium (poly)sulfide products. The sum of these beneficial properties of the mesoNC-coating prompts a more effective active material utilization. On the whole, the significantly improved sulfur reutilization, electrochemical performance and cycling stability of the LieS cells with a mesoNC-coated separator is attributed to the distinctive features of the multifunctional mesoNC-coating which (i) effectively intercepts and restrains soluble polysulfides in the cathode side, improving the active material utilization, (ii) decreases the resistance of the cathode and boosts the electron transfer through the intimate contact between the entrapped sulfur-based species and the highly conductive carbon network, (iii) offers a physical space to shockabsorb the sulfur volume expansion/contraction during cycling and prevents carbon interface damages, (iv) chemically adsorbs and reinforces the confinement of lithium (poly)sulfide intermediates through coupling interactions between negatively charged
Fig. 6. Cycling performance of the LieS cell with a mesoNC-coated separator and a simple sulfur cathode cycled at a current rate of 0.2C. Areal sulfur loading: 3.95 mgS cm2.
Fig. 5. Surface SEM images of fresh metallic lithium (a) and cycled lithium anodes from LieS cells with a PP separator (b) and a mesoNC-coated separator (c).
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polysulfides and the polarized nitrogen-neighboring carbon surface that restrains active sulfur material loss. 4. Conclusions We have developed a novel functional hybrid separator by the integration of a nitrogen-doped mesoporous carbon layer onto a commercial polypropylene separator with the aim to enhance the interfacial interaction between the N-induced surface p-system delocalization in the carbon-coating and the polar sulfur-related species. The resulting mesoNC-coated separator reduces the interfacial resistance of the cell at once and its inherent mesoporosity provides an excellent environment to trap soluble polysulfides and buffer the volume change of sulfur. The nitrogen functional groups in the carbon matrix chemically interact with lithium (poly)sulfide intermediates to localize them in the cathode region, attenuating the shuttling effect and prolonging the persistence of the LieS batteries. Consequently, cell setups with mesoNCcoated separator exhibit an outstanding long lifespan of 1200 cycles with a high reversible capacity of 566 mAh g1 at 0.5C and a degradation rate of only 0.037% per cycle. Furthermore, despite to use a simple-mixed sulfur-carbon black cathode with high-sulfur loading of 3.95 mg cm2 and high S/C ratio the cell with a mesoNC-coated separator delivers a high areal capacity of ~3 mAh cm2. The findings of this work can be taken as a benchmark for the development of multifunctional hybrid separators with doped porous carbons and can speed up a feasible commercialization of high-performance LieS batteries. Acknowledgment The authors thank A. Vob, A. Voidel and R. Buckan for their valuable technical support. We gratefully acknowledge financial support from the German Federal Ministry of Education and Research (BMBF) through the Excellent Battery e WING center “Batteries e Mobility in Saxony” (Grant Nos. 03X4637B and 03X4637C). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.11.018. References [1] P.G. Bruce, B. Scrosati, J.-M. Tarascon, Angew. Chem. Int. Ed. 47 (2008) 2930e2946. [2] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Energy Environ. Sci. 4 (2011) 3243e3262. [3] A. Manthiram, S.-H. Chung, C. Zu, Adv. Mater. 27 (2015) 1980e2006. [4] Y.V. Mikhaylik, J.R. Akridge, J. Electrochem. Soc. 151 (2004) A1969eA1976. [5] R. Xu, J. Lu, K. Amine, Adv. Energy Mater. (2015), http://dx.doi.org/10.1002/ aenm.201500408. [6] X. Ji, K.T. Lee, L.F. Nazar, Nat. Mater. 8 (2009) 500e506. [7] N. Jayaprakash, J. Shen, S.S. Moganty, A. Corona, L.A. Archer, Angew. Chem. Int. Ed. 50 (2011) 5904e5908. [8] Z. Wei Seh, W. Li, J.J. Cha, G. Zheng, Y. Yang, M.T. McDowell, P.-C. Hsu, Y. Cui, Nat. Commun. 4 (2013) 1331. [9] C.J. Hart, M. Cuisinier, X. Liang, D. Kundu, A. Garsuch, L.F. Nazar, Chem. Comm. 51 (2015) 2308e2311.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Improved cycling stability of lithiumesulfur batteries using a polypropylene-supported nitrogen-doped mesoporous carbon hybrid separator as polysulfide adsorbent Juan Balach a, *, Tony Jaumann a, Markus Klose a, b, Steffen Oswald a, Jürgen Eckert c, d, Lars Giebeler a, b a
Institute for Complex Materials, Leibniz Institute for Solid State and Materials Research (IFW) Dresden, Helmholtzstraße 20, D-01069 Dresden, Germany €t Dresden, Helmholtzstraße 7, D-01069 Dresden, Germany Institut für Werkstoffwissenschaft, Technische Universita Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstraße 12, A-8700 Leoben, Austria d €t Leoben, Jahnstraße 12, A-8700 Leoben, Austria Department Materials Physics, Montanuniversita b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A functional hybrid separator with a N-doped mesoporous carbon layer was prepared. N-dopants on carbon-coating chemically immobilize sulfur-related species. The functionalized hybrid separator enhanced the overall performance of LieS cells. Excellent cycle stability with ultralow decay rate of 0.037% per cycle was obtained. LieS cells with a hybrid separator delivered a high areal capacity of 3 mAh cm2.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 August 2015 Received in revised form 5 October 2015 Accepted 4 November 2015 Available online xxx
The lithium/sulfur couple is garnering tremendous interest as the next-generation of cost-efficient rechargeable battery systems capable to fulfill emerging energy storage demands. However, the viable commercialization of lithiumesulfur (LieS) batteries is still an obstacle by fast capacity fading and poor cycling stability mostly caused by the polysulfide shuttle and active sulfur material loss. In this contribution, we show that the surface modification of the commercial polypropylene separator with a nitrogen-doped mesoporous carbon enhances the interfacial interaction between the N-dopants on carbon-coating and the sulfur-related species by coupling interactions. These unique physical and interfacial chemical properties of the N-doped mesoporous carbon-coating promote the chemical adsorption and confinement of lithium (poly)sulfide intermediates in the cathode side, improving the active material utilization and hence the overall electrochemical performance of LieS batteries: high initial discharge capacity of 1364 mAh g1 at 0.2C and notable cycling stability with high reversible capacity of 566 mAh g1 and negligible degradation rate of 0.037% after 1200 cycles at 0.5C. Furthermore, despite the use of a simple-mixed sulfur-carbon black cathode with high-sulfur loading of 3.95 mg cm2, the cell with a hybrid separator delivers a high areal capacity of ~3 mAh cm2. © 2015 Published by Elsevier B.V.
Keywords: Nitrogen-doping Mesoporous carbon Functionalized hybrid separator Polysulfide adsorption Lithium-sulfur battery
* Corresponding author. E-mail address: [email protected] (J. Balach). http://dx.doi.org/10.1016/j.jpowsour.2015.11.018 0378-7753/© 2015 Published by Elsevier B.V.
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1. Introduction The development of cost-efficient, high energy storage systems is essential for application in electric vehicles, renewable energy plants and smart grids. In this regard, the lithiumesulfur (LieS) battery has received overwhelming attention in the last few years owing to its outstanding high theoretical energy density of 2.6 kWh kg1, which significantly outperforms the state-of-the-art lithium-ion battery technology based on conventional intercalation oxide cathodes [1e3]. In addition, the wide-spread abundance, low cost and harmless nature of elemental sulfur predestines its use for large-scale applications. In spite of the advantageous features of elemental sulfur as active cathode material, the practical commercialization of LieS batteries is mainly hindered by three inherent issues, related to (i) the poor electrical conductivity of sulfur and its reduced lithium (poly)sulfide products which cause low active material utilization, (ii) the large volumetric expansion of sulfur during lithiation that may lead to the degradation of the sulfur cathode under mechanical strain, and (iii) the even more dramatic shuttle effect [4] that transports soluble high-order polysulfides (Li2Sx, 4 x 8) back and forth between electrodes, causing the corrosion of the lithium anode and the active material loss, finally resulting in a short battery life [5]. The aforementioned technical hurdles have been partially overcome by adopting nano-architectured sulfur/carbon and sulfur/metal oxide composites [6e9], using additives to suppress the polysulfide shuttling and to protect the lithium anode surface [10e12] or designing new cell configurations with polymer or carbon interlayers [13e17]. Among others, the incorporation of a porous carbon layer between the separator and the sulfur cathode, by employing a free-standing interlayer or coated separator, is a promising approach to improve the sulfur utilization and the electrochemical performance of LieS cells. Carbon black [18e20], multiwall carbon nanotubes [21,22], graphene [23,24], carbon nanofibers [25e27] as well as microporous [13,28,29], mesoporous [30e32] and macroporous [33] carbons have all been studied to both increase the cathode conductivity and physically intercept/ confine the dissolved polysulfide intermediates in the cathode region, keeping the active material accessible for further reutilization. Although the major polysulfide-trapping capability of the abovementioned carbon barriers is possible based on the physical trapping, the lack of an effective interaction of the lithium (poly)sulfide intermediates with the carbon network interface eventually leads to the shuttling of the active material and its depletion, especially over long cycling periods. Therefore, a forceful enhancement of the interfacial interaction between functional groups in the carbon barrier and sulfur-related species by coupling interactions is highly desired for improving polysulfide adsorption and, thereby, sulfur utilization and cycle life of LieS batteries. In this regard, carbon materials with nitrogen atoms incorporated into their carbon framework have demonstrated favorable changes in electrical conductivity, surface affinity, electrochemical performance and catalytic activity owing to the alteration of the electronic structure and the chemical activity caused by electron modulation and doping-induced defects [34e38]. Recently, several N-doped carbon materials have been adopted as a conductive host in sulfur cathodes to promote the chemical adsorption capability of the sulfurcontaining guests and thus improve the cycle life of LieS batteries [39e43]. Density functional theory (DFT) and first-principle calculations revealed that pyridinic-N and pyrrolic-N atoms located in the carbon lattice of N-doped carbon hosts can strongly anchor the discharged product Li2S, while the polar polysulfides are preferentially adsorbed on the surface of the quaternary nitrogenneighboring carbon atoms due to the additional delocalization of the p-system, which leads to an improved sulfur utilization and
cycle stability [40e42]. In view of the encouraging chemical features of the nitrogen as a dopant atom, a suitable N-containing carbon barrier can play an essential role on the adsorption of lithium (poly)sulfide species to restrain the shuttle effect and to lengthen the lifespan of LieS batteries. Motivated by the above-mentioned considerations and by our recent findings that a functional large pore volume, mesoporous carbon-coated separator allows for a better electrochemical performance than a conventional LieS cell configuration with a pristine separator and sulfur-infiltrated carbon composite [32], in this contribution, we report a simple modification of the commercial polypropylene separator with a thin layer of melamine-derived Ndoped mesoporous carbon (mesoNC) to improve the interfacial interaction between the N-dopants on carbon-coating and the sulfur-related species by coupling interactions. Despite the use of a simply prepared ball-milled sulfur/carbon black cathode with high sulfur composition, a remarkable electrochemical performance of 3 mAh cm2 and excellent cycle retention is achieved fulfilling the demands for a broad practical application. It will be shown that the unique physical and interfacial chemical properties of the mesoNCcoating are decisive to improve the cycling stability of LieS batteries. To our knowledge, this is the first experimental demonstration that the rational design of modified separators with N-doped mesoporous carbon structures plays an important role on the cycling performance of LieS batteries. The simple processing technique for the preparation of advanced hybrid separators with multifunctionalities to extend the cycling life of the LieS cells is a practical solution which can be straightforwardly incorporated into industrial processes. 2. Experimental section 2.1. Chemicals Resorcinol (99 wt %), Ludox HS-40 (40 wt. % of 12 nm-indiameter silica), elemental sulfur (S, 99.98 wt. %), lithium sulfide (Li2S, 99.98 wt. %) and N-methyl-2-pyrrolidone (NMP, 99 wt. %) were purchased from Aldrich. Melamine (2,3,6-triamino-1,3,5triazine, 99 wt. %), formaldehyde solution (37 wt. %, containing ~10% (w/w) methanol as stabilizer), hydrofluoric acid (HF, 40 wt. %), Super P carbon (SPC, TIMCAL) and polyvinylidene difluoride (PVDF, Solef 21216) were purchased from Alfa Aesar, Fluka, Merck, BASF and Solvay, respectively. All chemicals were used as received. 2.2. Synthesis of undoped and N-doped mesoporous carbons Melamine-derived mesoNC was produced by hard template method using resorcinol, melamine, and formaldehyde as the carbon precursors, based on previous reports [31,39,44]. Typically, 1.1 g of resorcinol (R) was dissolved in 1.5 ml of formaldehyde (F) solution (37 wt. %) under stirring at 70 C for 5 min. Then, a clear mixture of 0.63 g of melamine (M), 0.75 ml of F solution and 9.98 ml of Ludox HS-40, pre-dissolved at 70 C under stirring for ca. 20 min, was added to the above RF solution and the system was closed. The initial yellowish liquid mixture turned translucent orange and solidified within 5 min under stirring at 70 C, while within the further 30 min the solid became opaque red. This solidification process is due to the R-M-F polycondensation catalyzed by the alkaline medium of the sodium hydroxide-stabilized silica (pH z 9.5). The reactant molar ratio of R:M:F was 1:0.5:3, while the weight ratio of SiO2/R-M was 3. The silica-polymer mixture was subsequently dried for 20 h at 80 C in an oven and further carbonized (without the magnetic stir bar) at 900 C for 2 h under Ar atmosphere. Afterwards, the solid silica template was removed from the carbon sample by etching with 20 wt. % HF and repeatedly
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washing with deionized water. Finally, the obtained mesoNC was dried at 90 C for 20 h and finely ground in a mortar. An undoped mesoporous carbon (mesoC) with similar physical texture but different chemical properties compared to the mesoNC was prepared following our previous report [32]. 2.3. Preparation of hybrid separator A simple modification of the commercial polypropylene (PP) separator (Celgard 2500) was conducted by direct coating of a carbon slurry on one side of the separator using a doctor blade technique. The shaker-milled slurries contain 90 wt. % of mesoNC powder and 10 wt. % of the PVDF binder in NMP. The modified separator was cut into circular disks of 16 mm after drying in an oven at 40 C for 20 h. The mass of the carbon coatings was ~0.5 mg cm2. 2.4. Preparation of sulfur cathode Simple sulfur cathodes were prepared by shaker-milling of elemental sulfur, SPC and PVDF in NMP. The well-mixed slurries were coated onto an aluminum foil (11 mm thickness) current collector by doctor blading. Cathodes with S:SPC:PVDF ratios of 50:40:10 wt. % and 70:20:10 wt. %, respectively, were used for pristine and modified separator cell setups in order to keep similar sulfur/carbon ratios. The obtained cathodes were punched into circular disks of 12 mm after drying in an oven at 50 C for 20 h. The thicknesses measured for cathodes with 50 and 70 wt. % S content were 49 and 38 mm, respectively. The areal sulfur loading of the sulfur cathodes used for the main electrochemical tests was ~1.6 mg cm2. For the evaluation of the areal capacity, a cathode with a S:SPC:PVDF ratio of 70:20:10 wt. % (124 mm thickness) and an areal sulfur loading of 3.95 mg cm2 was used. 2.5. Electrochemical measurements CR2025-type stainless steel coin cells were assembled in a dry Ar-filled glovebox. Lithium metal foil (Chempur, 13 mm, 250 mm thickness) was used as both anode material and reference electrode, and a mixture of 1 M lithium bis(trifluoromethylsulfonyl) imide salt (Li-TFSI, BASF) and 0.25 M lithium nitrate (LiNO3, Merck, >99.995 wt. %) additive in 1,3-dioxolane (DOL, SigmaeAldrich, 99.8 wt. %, anhydrous) and 1,2-dimethoxyethane (DME, SigmaeAldrich, 99.5 wt. %, anhydrous) (1:1 v/v) was used as electrolyte. A commercial microporous PP separator (16 mm) was used as pristine separator. The modified separator was placed into the cell with the carbon coating layer facing to the cathode. The cells were charged and discharged at room temperature in the voltage range of 1.8e2.6 V using a BaSyTec Cell Test System (CTS). Both cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were carried out using a VMP3 potentiostat (Bio-logic). The cyclic voltammograms were recorded in a voltage window of 1.8e2.8 V at a scan rate of 0.1 mV s1 and the EIS between 200 KHz and 100 mHz with an AC voltage amplitude of 5 mV at the open-circuit voltage of the cells. Prior to the start out of the long-term cycling tests at 0.5C, the cells were cycled at 0.2C for the first two cycles as preconditioning step (1C ¼ 1672 mA g1). 2.6. Characterization The morphology of the carbon samples and the modified separator were characterized using a Gemini 1530 scanning electron microscope (SEM) operated at 15 kV acceleration voltage. A FEI Tecnai F30 transmission electron microscope (TEM) equipped with
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a field emission gun (FEG) working at 300 kV acceleration voltage was used for high-resolution imaging. Energy-dispersive X-ray spectrometry (EDXS) measurements were taken on the SEM with a Bruker EDXS spectrometer. Nitrogen sorption experiments were performed using a Quantachrome Quadrasorb SI instrument and Quantachrome Quadrawin 4.0 data analyzer after degassing the carbon samples for 20 h at 150 C. Specific surface areas were calculated at a relative pressure p$p1 0 ¼ 0.05e0.2 using the multipoint BrunauereEmmetteTeller (BET) method while the total pore volume was determined at p$p1 0 ¼ 0.97. The pore size distributions were obtained using the Quenched Solid Density Functional Theory (QSDFT) equilibrium model. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a PHI 5600 CI spectrometer (Physical Electronics) equipped with a hemispherical analyzer operated at a pass energy of 29 eV. Monochromated Al Ka radiation (350 W) was used applying partly low energy electrons for charge compensation. All spectra were calibrated using the C 1s level (284.8 eV corresponding to CeC) as reference. Peaks were fitted with a Gaussian function and basic linear background subtraction. The metallic lithium anodes as well as the PP and hybrid separators were washed after dischargeecharge cycles with a DOL:DME (1:1 v/v) inside a glovebox prior to SEM analysis. Ultravioletevisible (UVeVis) absorption spectroscopy analysis was conducted to investigate the polysulfide adsorption capability of SPC, mesoC and mesoNC. Briefly, the polysulfide Li2S6 solution (1 M) was prepared by chemically reacting elemental sulfur and a stoichiometric amount of Li2S in DOL:DME (1:1 v/v) solution, followed by heating at 50 C with magnetic stirring for 48 h in an Arfilled glove box. Then, 20 mg of each carbon sample was added into a vial containing 4 ml of 0.1 M Li2S6 solution. The vials were sealed and gently shaken for 2 h and then kept under static conditions for 18 h to allow precipitation of the carbon material. Afterwards, the upper remnant Li2S6 solutions were analyzed by UVeVis absorption spectroscopy (SPECORD 250 UVeVis spectrophotometer, Analytik Jena). 0.1 M Li2S6 solution was used as a reference concentration. 3. Results and discussion Recent reports have demonstrated that LieS batteries with sulfur-carbon composite cathodes or protecting carbon-layers consisting of mesoporous carbons with large pore volume are of high importance to trap and retain polysulfide intermediates generated during charge/discharge cycles [31,45,46]. MesoNC with large pore volume was produced by polymerization of melamine and formaldehyde in the presence of a commercial 12 nm-indiameter silica hard template and its subsequent carbonization/ template removal. Note that melamine acts as both carbon precursor and nitrogen source. The texture of the mesoNC powder was studied by nitrogen physisorption experiments (Fig. 1). The nitrogen adsorbed at very low-pressure and the distinct hysteresis loop at a high-pressure region indicates the presence of, respectively, micropores and mesopores in the carbon material (Fig. 1a). The specific surface area derived by the BrunauereEmmetteTeller (BET) method and the total pore volume were calculated to be 836 m2 g1 and 2.96 cm3 g1, respectively. As shown in Fig. 1b, the pore size distribution calculated by the Quenched Solid Density Functional Theory (QSDFT) equilibrium model displays two narrow pore sizes at 1.74 and 12.3 nm. The microporosity of the mesoNC contributes to the specific surface area with 325 m2 g1 (39% of the total surface area) and with 0.137 cm3 g1 to the micropore volume (4.6% of the total pore volume). As expected, the obtained mesopore size is very similar to the average diameter of the silica particles used as hard template, while the microporous structure results from the micropores located in the carbon walls. This dual
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Fig. 1. (a) Nitrogen physisorption isotherm of mesoNC and (b) the corresponding pore size distribution obtained by employing the QSDFT equilibrium model.
porosity and the large pore volume of the mesoNC are of high importance for the further preparation of the modified separator because the micropores in the mesoNC-coating can restrain the diffusion of polysulfide intermediates to the anode side by in-situ trapping thereof, while the cavity of the mesopores can buffer the volume change of the confined sulfur species, which is essential for the improvement of the LieS battery performance [32]. The morphology of the mesoNC powder and the mesoNCcoating were investigated by SEM and TEM, as presented in Fig. 2. The SEM image in Fig. 2a shows a disordered porous structure with evenly distributed pores. According to the TEM image (Fig. 2b), the carbon network represents a well-structured replication of the corresponding silica template, recovered in the carbon material with a sphere-like mesoporous structure. The surface chemical composition and the functional groups of the mesoNC were
characterized by X-ray photoelectron spectroscopy (XPS) analysis. The carbon, nitrogen and oxygen content in the mesoNC are, respectively, 82.9, 13.2 and 3.9 at % and the resulting surface N/C ratio of the mesoNC is approximately 0.16, which correlates well with previous reports [39,47]. This result demonstrates that the use of melamine as carbon precursor enables the preparation of an inherent and highly N-doped carbon matrix. Furthermore, the XPS N 1s spectra is fitted by three components with binding energies at 398.3, 400.3 and 401.7 eV, which are assigned to different chemical states of the nitrogen atoms inside the carbon matrix: pyridinic-N (48 at %), pyrrolic-N (35 at %) and quaternary-N (17 at %) (Fig. S1). The major content of pyridinic-/pyrrolic-N functionalities in the mesoNC, which have better affinity to interact with polar lithium (poly)sulfide species than quaternary-N functions [48], could be highly favorable to promote the chemical adsorption of polysulfide
Fig. 2. (a) SEM and (b) TEM images of the pristine mesoNC. (c) Relative polysulfide adsorption of SPC, mesoC and mesoNC and the respective digital image of polysulfide solutions after adsorption. Li2S6 solution was used as a control. (d) Digital image of the modified PP separator showing both the mesoNC-coating and the no-modified sides, (e) top-view and (f) cross-sectional SEM images of the mesoNC-coated separator.
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species. To study the N-doping effect on the polysulfide adsorption capability, the carbon samples mesoNC, mesoC and SPC, with notable difference in physical and chemical properties (Table S1), were dispersed in 0.1 M Li2S6 solution for 20 h to decant the carbon and the retained concentration of Li2S6 (not adsorbed on the carbon surface) in solution was determined by UVeVis absorption spectroscopy. It was found that mesoNC present the most favorable relative polysulfide adsorption with approximately 93% of the initial Li2S6 solution, while mesoC and SPC only adsorbed 65 and 21%, respectively (Fig. 2c). The digital image in Fig. 2c shows a visible decoloration of the Li2S6 solution, highlighting the strong polysulfide adsorption capability of the mesoNC. This finding encouraged us to prepare a mesoNC-coated separator with the aim to improve the interfacial interaction between the N-dopants on carbon and the sulfur-related species and thus the active material utilization. The doctor blade method was used to easily coat one side of the PP separator with a well-adhered layer of mesoNC (Fig. 2c). The SEM image of the mesoNC-coating surface shows a compact layer of homogenously distributed mesoNC particles (Fig. 2d), while the cross-sectional SEM image of the mesoNCcoating reveals a smooth thin layer with a thickness of about 24 mm (Fig. 2e). The successful fabrication of the mesoNC-coated separator and its insertion between the lithium anode and the sulfur cathode (with the mesoNC-coating facing to the cathode side) is expected to enhance the electrochemical performance of the LieS batteries through (i) increasing the electrical conductivity of the mesoNCcoating/sulfur cathode interface and (ii) the early trapping of soluble polysulfides, and thus, effectively reuse the active sulfur material (Scheme 1). To corroborate this promising approach, electrochemical impedance spectroscopy (EIS) measurements were carried out in order to determinate the cathode resistance of the fresh LieS cells with PP separator and mesoNC-coated separator. Fig. 3 shows the Nyquist plots for both types of cells, in which each plot displays a single semicircle at high-to-medium frequency range and an inclined line at low frequency range, related to the charge transfer resistance and the mass transfer process, respectively [31]. The LieS
Scheme 1. Schematic configuration of the conventional LieS cell with a pristine separator (left) and the advanced LieS cell with a mesoNC-coated separator (right).
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cell with the mesoNC-coated separator exhibits a semicircle with a smaller diameter compared to the conventional LieS cell, suggesting a lower charge transfer resistance. The improved electrical conductivity in the sulfur cathode is ascribed to the high electrical conductivity of the mesoNC-coating which acts as a pseudo current collector, reducing the internal impedance of the cell [13]. Note that the mesoNC-coatings were naturally prepared without conductive carbon additives (e.g. carbon black), which highlights the good electrical conductivity of the mesoNC network. The electrochemical cycling performance of the LieS cells were investigated by galvanostatic dis-/charging between 1.8 and 2.6 V vs. Li/Liþ. Fig. 4a and b shows, respectively, the discharge/charge voltage profiles of cells with PP and mesoNC-coated separators acquired during the 2nd, 25th, 50th and 100th cycles at 0.2C (1C ¼ 1672 mA g1). In general terms, both cells exhibit similar voltage profiles associated with the typical two-step sulfur redox reactions consisting of two discharge plateau at ~2.30 and ~2.07 V, which represent the conversion of elemental sulfur (S8) to soluble long-chain polysulfides (Li2Sx, 4 x 8) and their further reduction/precipitation to short-chain polysulfides (Li2Sx, 1 < x < 4) and Li2S, respectively [5]. The two continuous charge plateaus at ~2.28 and ~2.38 V represent the reversible oxidation reactions of Li2S/ Li2S2 to Li2S8/S8. Despite that the two cell set-ups used in this study contain the same amount of carbon material, there are notable differences in the lengths of the voltage plateaus and the voltage polarizations (DE), associated to the reversibility and the redox reaction kinetics of the cell system [24,27,32]. For example, the LieS cell with the mesoNC-coated separator exhibits longer voltage plateaus compared to the LieS cell with PP separator, indicating a better conversion and reutilization of the active sulfur material. Furthermore, the average polarization obtained from the 25th, 50th and 100th cycles at 50% depth of discharge (DOD), reveals a DE of 213 mV for the LieS cell with mesoNC-coated separator, while the DE of the conventional LieS cell present a higher value of 310 mV (Fig. 4a and b, highlighted with blue arrows), demonstrating a faster redox reaction kinetics for those cells with a mesoNC-coated separator. In agreement with the previously described voltage profile study, cyclic voltammograms recorded on cells with PP and hybrid separators exhibited a similar behavior (Fig. S2). Compared to the conventional LieS cell, the cell with a mesoNC-coated separator shows sharper reduction and oxidation peaks with higher peak current, indicating faster conversion kinetics and increased reversibility of the cell reactions in cells with the hybrid
Fig. 3. Nyquist plots of LieS cells with PP separator and mesoNC-coated separator.
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Fig. 4. Discharge/charge voltage profiles acquired during the 2nd, 25th, 50th and, 100th cycles at 0.2C of LieS cells with (a) PP separator and, (b) mesoNC-coated separator. (c) Cycling performance at a current rate of 0.2C and (d) rate performance of LieS cells with PP and mesoNC-coated separator. (e) Long-term cycling performance at a current rate of 0.5C for a LieS cell with a mesoNC-coated separator.
separator. According to the cycle performance carried out at a C rate of 0.2 (Fig. 4c), the LieS cell with the mesoNC-coated separator exhibits an initial discharge capacity of 1364 mAh g1, achieving a sulfur utilization of 82%. After 100 cycles, the cell offers a good capacity retention of 1040 mAh g1 and a high Coulombic efficiency (CE) of 98.5%. In contrast, the conventional LieS cell displays an initial discharge capacity of 1062 mAh g1 (sulfur utilization 64%) which is poorly retained after 100 cycles, reaching a value of only 548 mAh g1 and a CE of 97.4%. These results clearly demonstrate the beneficial effect of the conductive mesoNC-coating, which not only increases the cell conductivity but also ensures the stable cyclability of the cell through the inhibition of inactive sulfur agglomeration and the further reactivation of the entrapped sulfur to an active material. As shown in Fig. 4d, the notable improvements of both the sulfur utilization and the electrochemical performance for LieS cells with mesoNC-coated separator, are also evidently made during the evaluation of the rate capability,
especially at high C-rates. In addition, a long-term cycling test of the LieS cells with the mesoNC-coated separators was carried out at a current density of 0.5C (Fig. 4e). The LieS cell exhibits an initial discharge capacities of 1028 mAh g1 and a notable cycling stability with high reversible capacity of 566 mAh g1, good CE of 96% and, negligible degradation rate of 0.037% after 1200 cycles, one of the lowest reported so far for this kind of cell set-up [3]. Compared with recently reported PP separator modified by carbon black (e.g. SPC) or large pore volume, mesoporous carbon (mesoC) coatings [18,20,32], the cell with a mesoNC-coated separator demonstrates a lower degradation rate upon 500 cycles under similar sulfur content conditions (Table S2), indicating an improved cyclic stability and capacity retention. The absence of a porous structure in the SPC and the lack of nitrogen atoms in the mesoC structure indicate the role of the physisorption and chemisorption of sulfur-related species onto the mesoNC-coating interface, as shown in Fig. 2c. For a better understanding of the mesoNC-coating benefits, cells
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with PP and hybrid separators were disassembled after 100 cycles at 0.5C in the charged state and post-mortem characterizations were performed (Fig. S3). The surface SEM image of the cycled PP separator, which was faced to the cathode, shows a dense solid aggregation of nonconductive material onto the electronically insulating membrane (Fig. S3a), while the surface SEM image of the hybrid separator reveals the absence of agglomerated sulfur species onto the mesoNC-coating (Fig. S3b). Additionally, the carbon/sulfur signals in the corresponding elemental mapping shows active sulfur material uniformly embedded onto the carbon layer, proving that the mesoNC-coating captures the soluble polysulfide intermediates and effectively keeps them as active material on the cathode side. The in-situ chemisorption/confinement of active sulfur species through the mesoNC-coating should restrain the soluble polysulfide migrating towards the anode side and thus prevents further corrosion/degradation of the lithium anode. The above conjecture was corroborated by the surface morphology analysis of the fresh metallic lithium and the corresponding lithium anodes obtained from the cycled LieS cells with PP and mesoNCcoated separator after 100 cycles (Fig. 5). Compared with the smooth and compacted surface of the fresh metallic lithium, the cycled lithium anode from the conventional LieS cell exhibits a highly roughened surface consisting of irregular particles of lithium with an open canyon-like structure, resulting from the strong degradation reaction between dissolved polysulfides and metallic lithium during cycling. In contrast, a significantly smoother surface without apparently cracked particles is observed for the lithium anode when a mesoNC-coated separator is used, evidencing that the early chemisorption/entrapment of polysulfide intermediates through the mesoNC-coating lowers the degradation rate of the lithium anode and hence prolongs its lifetime. In order to demonstrate the potential of the mesoNC-coated separator for practical utilization, the areal sulfur loading of the cathode was increased to 3.95 mg cm2, accounting to a sulfur content in the whole cathode region of 60 wt. %. In general, LieS batteries with high areal capacities and good cycle stability are highly required for their practical application on battery pack for electric and hybrid vehicles. Typically, the increase of sulfur mass loading on the electrode lowers considerably the utilization of active material due to the low electronic conductivity of sulfur, constraining the rise of areal capacity and therewith the energy density. The results of our cell with a hybrid separator and increased sulfur loading are showed in Fig. 6. The cell delivers an initial discharge capacity of 956 mAh g1, which is approximately 30% lower compared to the cell set-up composed of a hybrid separator and sulfur cathode with lower areal sulfur loading (Fig. 4b). However, the cell with higher sulfur loading still shows an improved reversibility, redox reaction kinetics and active sulfur utilization compared to the cell with a pristine separator (Fig. 4a). Importantly, despite the use of simple-mixed sulfur-carbon black
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cathode, the LieS cell with a mesoNC-coated separator demonstrates a good cycle stability and notable high areal capacity of 3 mAh cm2 after 50 cycles, 2- to 6-fold greater than those typical LieS cells with sophisticated sulfur-carbon composite cathodes [49], outperforming current efforts to increase the specific energy density of LieS batteries. The improved performance of the cell with high sulfur loading is ascribed to the early physical trapping of polysulfide intermediates into the highly disordered threedimensional mesoporous network of the mesoNC layer, which restrain the migration of soluble long-chain polysulfides to the anode. In addition, the trapped polysulfides are further effectively immobilized within the electrically conductive porous matrix by chemical adsorption between the N-containing functionalities on mesoNC and the discharged polar lithium (poly)sulfide products. The sum of these beneficial properties of the mesoNC-coating prompts a more effective active material utilization. On the whole, the significantly improved sulfur reutilization, electrochemical performance and cycling stability of the LieS cells with a mesoNC-coated separator is attributed to the distinctive features of the multifunctional mesoNC-coating which (i) effectively intercepts and restrains soluble polysulfides in the cathode side, improving the active material utilization, (ii) decreases the resistance of the cathode and boosts the electron transfer through the intimate contact between the entrapped sulfur-based species and the highly conductive carbon network, (iii) offers a physical space to shockabsorb the sulfur volume expansion/contraction during cycling and prevents carbon interface damages, (iv) chemically adsorbs and reinforces the confinement of lithium (poly)sulfide intermediates through coupling interactions between negatively charged
Fig. 6. Cycling performance of the LieS cell with a mesoNC-coated separator and a simple sulfur cathode cycled at a current rate of 0.2C. Areal sulfur loading: 3.95 mgS cm2.
Fig. 5. Surface SEM images of fresh metallic lithium (a) and cycled lithium anodes from LieS cells with a PP separator (b) and a mesoNC-coated separator (c).
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polysulfides and the polarized nitrogen-neighboring carbon surface that restrains active sulfur material loss. 4. Conclusions We have developed a novel functional hybrid separator by the integration of a nitrogen-doped mesoporous carbon layer onto a commercial polypropylene separator with the aim to enhance the interfacial interaction between the N-induced surface p-system delocalization in the carbon-coating and the polar sulfur-related species. The resulting mesoNC-coated separator reduces the interfacial resistance of the cell at once and its inherent mesoporosity provides an excellent environment to trap soluble polysulfides and buffer the volume change of sulfur. The nitrogen functional groups in the carbon matrix chemically interact with lithium (poly)sulfide intermediates to localize them in the cathode region, attenuating the shuttling effect and prolonging the persistence of the LieS batteries. Consequently, cell setups with mesoNCcoated separator exhibit an outstanding long lifespan of 1200 cycles with a high reversible capacity of 566 mAh g1 at 0.5C and a degradation rate of only 0.037% per cycle. Furthermore, despite to use a simple-mixed sulfur-carbon black cathode with high-sulfur loading of 3.95 mg cm2 and high S/C ratio the cell with a mesoNC-coated separator delivers a high areal capacity of ~3 mAh cm2. The findings of this work can be taken as a benchmark for the development of multifunctional hybrid separators with doped porous carbons and can speed up a feasible commercialization of high-performance LieS batteries. Acknowledgment The authors thank A. Vob, A. Voidel and R. Buckan for their valuable technical support. We gratefully acknowledge financial support from the German Federal Ministry of Education and Research (BMBF) through the Excellent Battery e WING center “Batteries e Mobility in Saxony” (Grant Nos. 03X4637B and 03X4637C). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.11.018. References [1] P.G. Bruce, B. Scrosati, J.-M. Tarascon, Angew. Chem. Int. Ed. 47 (2008) 2930e2946. [2] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Energy Environ. Sci. 4 (2011) 3243e3262. [3] A. Manthiram, S.-H. Chung, C. Zu, Adv. Mater. 27 (2015) 1980e2006. [4] Y.V. Mikhaylik, J.R. Akridge, J. Electrochem. Soc. 151 (2004) A1969eA1976. [5] R. Xu, J. Lu, K. Amine, Adv. Energy Mater. (2015), http://dx.doi.org/10.1002/ aenm.201500408. [6] X. Ji, K.T. Lee, L.F. Nazar, Nat. Mater. 8 (2009) 500e506. [7] N. Jayaprakash, J. Shen, S.S. Moganty, A. Corona, L.A. Archer, Angew. Chem. Int. Ed. 50 (2011) 5904e5908. [8] Z. Wei Seh, W. Li, J.J. Cha, G. Zheng, Y. Yang, M.T. McDowell, P.-C. Hsu, Y. Cui, Nat. Commun. 4 (2013) 1331. [9] C.J. Hart, M. Cuisinier, X. Liang, D. Kundu, A. Garsuch, L.F. Nazar, Chem. Comm. 51 (2015) 2308e2311.
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