Rational Sulfur Cathode Design for LithiumâSulfur Batteries: Sulfur

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A Rational Sulfur Cathode Design for Lithium-Sulfur Batteries: Sulfur-embedded Benzoxazine Polymers Sang Hyun Je, Tae Hoon Hwang, Siddulu Naidu Talapaneni, Onur Buyukcakir, Hyeon Jin Kim, Ji-Sang Yu, Sang-Gil Woo, Min Chul Jang, Byoung Kuk Son, Ali Coskun, and Jang Wook Choi ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00245 • Publication Date (Web): 17 Aug 2016 Downloaded from http://pubs.acs.org on August 19, 2016

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ACS Energy Letters

A Rational Sulfur Cathode Design for LithiumSulfur Batteries: Sulfur-embedded Benzoxazine Polymers Sang Hyun Je,†⊥ Tae Hoon Hwang,†⊥ Siddulu Naidu Talapaneni,† Onur Buyukcakir,† Hyeon Jin Kim,† Ji-Sang Yu,‡ Sang-Gil Woo,‡ Min Chul Jang,ǁ Byoung Kuk Son,ǁ, Ali Coskun*†and Jang Wook Choi*† † Graduate School of Energy, Environment, Water, and Sustainability (EEWS) and KAIST Institute (KI) NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), Daehakro 291, Yuseong-gu, Daejeon 305-701, Republic of Korea. ‡ Advanced Batteries Research Center, Korea Electronics Technology Institute (KETI), 68 Yatap-dong, Bundang-gu, Seongnam, Gyeonggi-do 463-816, Republic of Korea ǁ Future Technology Research Center, Corporate R&D LG Chem Research Park, 188 Munji-ro, Yuseong-gu, Daejeon 305-738, Republic of Korea AUTHOR INFORMATION Corresponding Author *E-mail: (A.C.) [email protected] *E-mail: (J.W.C.) [email protected]

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ABSTRACT: A variety of advanced electrode structures have been lately developed to address the intrinsic drawbacks of lithium-sulfur batteries, such as polysulfide shuttling and low electrical conductivity of elemental sulfur. Nevertheless, it is still desired to find electrode structures that address those issues through an easy synthesis while securing large sulfur contents (i.e., >70 wt%). Here, we report an orthogonal, “one-pot” synthetic approach to prepare a sulfurembedded polybenzoxazine (S-BOP) with a high sulfur content of 72 wt%. This sulfurembedded polymer was achieved via a thermal ring opening polymerization of benzoxazine in the presence of elemental sulfur, and the covalent attachment of sulfur to the polymer was rationally directed through thiol group of benzoxazine. Also, the resulting S-BOP bears homogeneous distribution of sulfur due to in-situ formation of the polymer backbone. This unique internal structure endows S-BOP with high initial Coulombic efficiency (96.5%) and robust cyclability (92.7% retention after 1000 cycles) when tested as a sulfur cathode.

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Advent of rechargeable lithium-ion batteries (LIBs) has caused a paradigm shift in the area of energy storage technologies; LIBs are presently powering a wide range of applications, including portable electronics, vehicles, and stationary electric power storage.1,2 Nonetheless, ever-growing demand on the energy density to support advanced functions of portable electronics and extend driving mileages of electric vehicles (EVs) has stimulated research focusing on various post-LIBs that can store more Li ions per each molecular unit of active material than the conventional counterparts operating based on intercalation mechanism.3-5 In this sense, lithium–sulfur (Li–S) batteries are among the most promising candidates due to the high theoretical gravimetric capacity (1675 mAh g-1) of elemental sulfur along with a theoretical energy density (2600 Wh kg-1) of a cell when paired with Li metal anode, which is about 7 times greater than those (~387 Wh kg-1) of the established LIBs.6-8 Moreover, the utilization of elemental sulfur could provide a useful solution to its global supply surplus arising from the “involuntary” production as a byproduct from the petroleum refinery and natural gas sweetening by a process called hydrodesulfurization. Even after decades of investigation, however, Li-S batteries have yet to enter the commercial markets due to chronic shortcomings originating from the intrinsic properties of sulfur; sulfur and its discharge products (lithium sulfides, Li2S) are electronically insulating. More critically, the dissolution of long-chain polysulfides (Li2Sx, x = 4-8) into the electrolyte, which causes a shuttling process that destabilizes the Li metal interface and triggers self-discharge,9,10 results in a poor cycling stability and low Coulombic efficiency. Tremendous efforts have been devoted to tackle these issues.11-13 For example, elemental sulfur was encapsulated in porous conductive matrices to physically confine polysulfides inside the matrices while buffering the large volume expansion (~90%) of sulfur.14-25 Although these


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approaches improved the cycling stability significantly, the synthesis of carbon–sulfur composites usually requires elaborate procedures, such as repeated infusion of molten sulfur and multiple steps involving surface coating that is inevitably accompanied with partial dissolution of sulfur. More critically, when the sulfur loading is higher than ~60 wt%, perfect encapsulation of sulfur becomes non nontrivial, resulting in the dissolution of polysulfides to some level.26 To overcome the above-mentioned limitations, chemical impregnation of sulfur through the strong covalent bond formation has been investigated. Pyun and coworkers introduced inverse vulcanization to synthesize polymeric sulfur materials, which exhibited an improved electrochemical performance compared to the systems based on simple physical encapsulation.2729

Recently, Park et al. prepared triazine-based three-dimensionally interconnected sulfur-rich

polymers, in which triazine units are linked by sulfur chains.30 Although these approaches are promising examples for sulfur cathodes, irregular sulfur domains linked by small organic molecules have the possibility of leaving a certain portion of sulfur unbound from the polymer matrices due to repeated cleavage of S-S bonds in sulfur chains, especially for prolonged cycles. More recently, we also reported elemental sulfur mediated synthesis of a covalent triazine framework, in which in-situ polymerization process facilitated homogenous distribution of sulfur in the porous framework.31 In this structure, however, chemical impregnation of sulfur through radicalic C-H insertion reaction achieved limited sulfur loadings of ~60 wt%. Based on these series of recent findings, we naturally turn to a fundamental question in designing sulfur cathodes: how can we increase sulfur content in the frameworks bearing C-S covalent bonds while sulfur can be confined stably within a conductive matrix even after repeated soluble polysulfides formation?


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In an attempt to find answers to these challenges, herein, we adopted (Figure 1) an orthogonal synthetic approach for the preparation of sulfur-impregnated benzoxazine polymer (S-BOP). In this approach, the polymerization of benzoxazine can be achieved thermally via ring-opening polymerization reaction, while sulfur is incorporated independently via vulcanization reaction utilizing thiol functionality of benzoxazine. This approach enabled a sulfur loading of ~72 wt% along with homogeneous distribution of sulfur within the polymer composite. Even with the high sulfur content, the S-BOP electrode showed an appropriate initial Coulomb efficiency (ICE) of 96.6% and robust cycling performance (92.7% capacity retention after 1000 cycles). Benzoxazines have received considerable attention because of their ability to undergo thermal ring-opening polymerization to form corresponding benzoxazine polymers.32,33 Modularity, ease of synthesis, and high functional group tolerance of benzoxazines provide the resulting polymers with excellent mechanical, thermal, and electrical properties together with diverse functionalities.34 In our approach, in order to increase the sulfur loading amount and warrant the electrochemical reversibility of the resulting sulfur cathode, we resorted to thiolfunctionalized benzoxazine monomer, which was readily synthesized (Figure 1) in a quantitative yield by reacting 4,4-dihydroxy-diphenyldisulfide and 1,3,5-triphenyl-1,3,5-triazine in toluene under reflux condition without any catalyst.35 The resulting thiol-functionalized monomer is then chemically impregnated with elemental sulfur (monomer:sulfur ratio of 1:5 by weight) at 160oC. Following the impregnation of sulfur, ring-opening polymerization was carried out at 180oC, thus leading to homogeneous distribution of sulfur within the polymer matrix. Vulcanization was subsequently performed at 235oC to form short sulfur chains (Sn, n = 2-6). As a control sample, benzoxazine polymer (BOP) without chemical impregnation of sulfur was also synthesized


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(Figure 1). It is noteworthy to mention that these BOPs were synthesized without using any solvent or catalyst under environmentally benign conditions in quantitative yields. In order to demonstrate the importance of covalent impregnation of sulfur, sulfur-impregnated activated carbon (S-AC) was also prepared as a control sample by simple mechanical mixing of activatedcarbon and sulfur in a mass loading 30:70 and thermal annealing at 158oC for 12 h. Physical sulfur impregnation into the pores of AC was verified by Brunauer–Emmett–Teller (BET) surface area analysis before and after sulfur impregnation (Figure S1, Supporting Information); while AC showed high surface area and pore volume of 1295 m2 g-1 and 0.37 cm3 g-1, respectively, these values dropped markedly to 2.27 m2 g-1 and 0.000002 cm3 g-1 after the sulfur impregnation.

Figure 1. Synthetic scheme for the preparation of benzoxazine polymers (BOPs). BOPs were synthesized through thermal ring opening polymerization (ROP) without any catalyst or solvent. For the preparation of sulfur embedded BOP (S-BOP), sulfur impregnation was carried out at 160 oC prior to ROP. Following the polymerization step, the sample was vulcanized at 235 oC to obtain sulfur chains between polymer backbones. BOP, a control polymer without sulfur impregnation, was also synthesized based on the same ROP.

The formation of S-BOP and BOP was verified by carrying out Fourier-transform infrared spectroscopy (FT-IR) analysis (Figure 2a). The complete disappearance of characteristic oxazine 6 Environment ACS Paragon Plus

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peak located at 920 cm-1 in both S-BOP and BOP clearly indicates the formation of the corresponding benzoxazine polymers.32 Evidently, the FT-IR spectrum of S-BOP was found to be in good agreement with that of BOP (Figure S2, Supporting Information), thus proving the viability of the orthogonal synthetic approach, as sulfur does not interfere with the polymerization reaction. The conspicuous peaks at 145236, 127520 and 76037 cm-1 can be assigned to the C-H deformation vibrations from linear aliphatic region of the polymer backbone, C-N stretching and phenyl bending vibrations, respectively. In the FT-IR spectrum of S-BOP, the stretching bands located at 1012 and 645 cm-1 corresponding to C-S bonding were retained, indicating not only successful chemical insertion of sulfur, but also the stability of the polymer backbone. The intensities of these peaks were found to be relatively weak because of the suppression by high amount of sulfur and the intrinsically weak response of those peaks in the IR region.38 The nature of carbon-sulfur bonding within the polymeric matrix of S-BOP was further elucidated by Raman spectroscopy analysis (Figure 2b). S-BOP showed S-S bonding peaks at 152, 220, 473 cm-1 along with the v(C-S) peak at 182 cm-1, which indicates successful chemical insertion of sulfur onto the polymer backbone.38 Moreover, the presence of intense, broad peak at 407 cm-1 arising from the transformation of oxazine ring to an aliphatic chain further proves the formation of S-BOP. We have also obtained (Figure S3, Supporting Information) solid state 13C NMR spectra of BOP and S-BOP. BOP showed broad peaks in the range of 110-160 ppm which were ascribed to the phenyl moieties of BOP. The presence of broad peak at 40 ppm indicates the presence of aliphatic carbon atoms. Importantly, these peaks are mostly retained in the S-BOP, thus proving the retention of polymer backbone even after vulcanization. Slight differences in the aromatic peaks could be attributed to the sulfur insertion onto the phenyl rings. In order to further analyze the form of sulfur within S-BOP, we have also carried out differential scanning


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calorimetry (DSC) analysis of BOP and S-BOP. To avoid any side reactions, we cooled S-BOP down to -80oC, then heated to 150oC at a cooling/heating rate of 10oC/min for 3 cycles. BOP itself showed (Figure S4, Supporting Information) Tc in the range of 0-25oC. Importantly, we did not observe this feature in the DSC spectrum of S-BOP, which further indicates their chemically different nature. Moreover, we also observed two new characteristic peaks, Tc at 52oC, and Tm at 113oC. Since BOP itself did not show this Tc peak, we assigned the former peak at 52oC to the exothermic crystallization of elemental sulfur into monoclinic S839, and the latter peak to the melting of longer S-S bonds in the polymer composite and their crystallization (as also observed by Pyun et al.27). These results clearly indicate co-existence of both elemental sulfur and linear sulfur chains within S-BOP. According to powder X-ray diffraction (PXRD) analysis (Figure 2c), the framework of SBOP turned out to be amorphous, as its overall spectrum exhibited a broad, featureless PXRD pattern in the entire 2θ range of 5-80o. Nonetheless, the PXRD spectrum showed sharp diffraction peaks, reflective of crystalline elemental sulfur trapped within the polymeric matrix even after vulcanization step at elevated temperatures.40,41 As the content and bonding stability of sulfur is critical for the performance of sulfur cathodes, elemental analysis (EA) and thermogravimetric analysis (TGA) were carried out.


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Figure 2. (a) Fourier-transform infrared (FT-IR) spectra of benzoxazine monomer, BOP, and SBOP. (b) Raman spectra of S-BOP and elemental sulfur. (c) Powder X-ray diffraction spectrum of S-BOP and BOP, along with the Bragg peak positions of elemental sulfur (bottom). The spectrum of S-BOP indicates amorphous nature of the polymer backbone along with the presence of crystalline sulfur. (d) Thermogravimetric analysis (TGA) of S-BOP and BOP along with the differential mass loss curve of S-BOP.

While the EA indicates a high sulfur loading of 77 wt%, the TGA under N2 atmosphere reconfirmed this feature by showing a sulfur content of ~72 wt% (Figure 2d). In this TGA analysis, S-BOP did not show any mass loss up to 200oC, indicative of covalent attachment of sulfur to the polymer backbone. The covalent attachment of sulfur was also reflected in the differential weight loss curve (orange line in Figure 2d) that exhibited a peak at 320oC.42 On the other hand, the TGA profile of S-AC revealed (Figure S5, Supporting Information) a sulfur 9 Environment ACS Paragon Plus

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loading of 69 wt%, which is similar to that of S-BOP. Notably, the comparison with the TGA curve of BOP (Figure 2d) implies that the covalent attachment of sulfur further improved the stability of benzoxazine polymer by vulcanization, as BOP began to lose its weight at considerably lower temperature near 150oC. Additional TGA measurements (Figure S6, Supporting Information) that held the temperature at 50oC, 100oC, and 150oC for 3~5 h also demonstrated the enhanced structural stability of S-BOP compared to elemental sulfur. Scanning electron microscopy (SEM) characterization indicates (Figure S7, Supporting Information) that S-BOP has irregular shaped, micron-sized particle morphology. In these particles, sulfur was homogeneously distributed according to energy-dispersive X-ray (EDAX) spectroscopy analysis (Figure S7, Supporting Information). Separately, X-ray photoelectron spectroscopy (XPS) analysis (Figure S8, Supporting Information) confirmed that S-BOP contains only carbon, nitrogen, oxygen and sulfur atoms. The XPS C 1s spectrum was deconvoluted into three peaks located at 284.6, 285.4 and 286.2 eV that are assigned for -C=C-, -C-N-, and -C-S- bonding, respectively. The XPS N 1s spectrum revealed -C-N- bonding peak located at 398.9 eV, thus further verifying the integration of polymer backbone even after vulcanization. The -C-O- bonding was also proven by O 1s spectrum, which showed the corresponding peak at 532.5 eV. As for the nature of sulfur bonding in S-BOP, the XPS S 2p spectrum can be deconvoluted to four distinct peaks at 162.8, 163.5, 163.7 and 164.8 eV. While the peaks at 162.8 and 163.7 eV correspond to the S 2p C-S bonding, the peaks at 163.5 and 164.8 eV are ascribed to S 2p S-S bonding, thus proving both successful covalent impregnation of sulfur onto the polymer backbone and the presence of sulfur chains.38,43-45 In addition, the peak at 162.0 eV reflective of thiol group did not appear in the S-


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BOP’s spectra, implying that the thiol groups were reacted with elemental sulfur during the sulfur embedding step.46 In order to evaluate electrochemical performance of S-BOP, CR2032 half-cell measurements were carried out (Figure 3) using Li metal disc as both counter and reference electrode. The sulfur electrode was prepared incorporating S-BOP, PVDF and Super P (active material, binder, and conductive agent) in a mass ratio of 60:10:30 through standard slurry casting and electrode drying processes. Mass loadings of S-BOP electrodes were 1.3 mgtotal cm-2 (= 0.9 mgsulfur cm-2) and 3.5 mgtotal cm-2 (= 2.5 mgsulfur cm-2). BOP and S-AC electrodes with 1.3 mgtotal cm-2 (= 0.9 mgsulfur cm-2) were prepared as controls. For all of the electrodes, we used 1 M LiTFSI in TEGDME:DIOX (0.33:0.67 in volume) as the electrolyte, with 0.2 M LiNO3 for surface protection of Li metal anodes from the shuttling process. If the carbon portion in the electrode is reduced below the current value (30 wt%), for example, by replacing super P with other carbon nanomaterials (i.e., graphene, carbon nanotubes, etc.), it would be more beneficial to enhance the energy density of the Li-S cell. However, the current study focuses more on the structural aspects of sulfur cathodes and has adopted the current electrode composition, following the recent studies30 with similar research motivations. The first discharge-charge voltage profiles of S-BOP and BOP were measured (Figure 3a) at C/20 (= 36 mA gsulfur-1) for both samples with 0.9 mgsulfur cm-2 and at C/30 (= 31 mA gsulfur-1) for S-BOP with 2.5 mgsulfur cm-2. Hereafter, all of the current densities and specific capacities are reported based on the mass of sulfur only. Unlike conventional sulfur composite electrodes showing two clear plateaus at 2.1 and 2.4 V originating from the two-step reduction process of sulfur,14 an unusual discharging profile with steep slope in the range of 2.05~2.4 V was observed. This slopping behavior suggests that the transition from S8 to Li2S8 is largely buried in S-BOP 11 Environment ACS Paragon Plus

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due to its bonding nature containing C-S bonds. Although crystalline S8 was reflected in the XRD spectrum (Figure 2c) of S-BOP, the plateau at 2.3 V reflective of S8 was almost invisible because of the internal structure of S-BOP that S8 is surrounded by linear sulfur chains, which are covalently attached to the polymer backbone. At a relative low C-rate, these linear sulfur chains are accessible to Li ions earlier than S8. This interpretation based on the internal structure is also validated for the voltage profiles at higher C-rates as will be discussed below. In the same line, the specific capacities of S-BOP are somewhat lower than those of sulfur cathodes solely based on S8, once again, due to the co-presence of linear sulfur configurations. S-BOP showed initial discharge/charge capacities of 1149 and 1110 mAh gsulfur-1 corresponding to an ICE of 96.6%. This ICE implies that Li ion trapping in the polymer matrix and polysulfide shuttling are successfully suppressed.


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Figure 3. (a) The 1st discharge and charge curves of S-BOP measured at C/20 (36 mA g-1) for 0.9 mgsulfur cm-2 and at C/30 (31 mA g-1) for 2.5 mgsulfur cm-2, and BOP measured at C/20 (36 mA g-1). (b) Cycling performance and CEs of S-BOP (0.9 mgsulfur cm-2) measured at 1C (720 mA g1 ). (c) Voltage profiles at different cycle numbers during the cycling shown in (b). (d) Rate performance of S-BOP (0.9 mgsulfur cm-2) and (e) corresponding voltage profiles at difference Crates.


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This value is also in contrast with previously reported sulfur-rich polymers that usually showed ICEs higher than 100% due to the shuttling process.30 On the contrary, when tested under the same conditions, BOP showed almost no electrochemical activity toward Li-ions, indicating that the redox activity observed with S-BOP originates from its sulfur chains, and the thiol group does not participate in the reaction. Furthermore, S-BOP with a higher sulfur content of 2.5 mgsulfur cm-2 (= 3.5 mgtotal cm-2) showed initial discharge/charge capacities of 1034 and 975 mAh gsulfur-1 (94.3% ICE) at a current density of 31 mA gsulfur-1 (Figure 3a). Importantly, S-BOP exhibited (Figure 3b) excellent cycling performance, as 92.7% of the original capacity (630 mAh gsulfur-1) was preserved after 1000 cycles when cycled at 1C (= 720 mA gsulfur-1). During the early cycling period, the specific capacity of S-BOP increased gradually from 630 mAh gsulfur-1 (2nd cycle) to 705 mAh gsulfur-1 (30th cycle), which might be attributed to a certain electrode activation process. It is anticipated that it takes the activation period to gain a full access to the embedded sulfur, although additional microscopic structural analyses are required for clarification. The good reversibility in each cycle was reflected in the CEs: the CEs in the 2nd, 5th, 50th, 200th, 500th, and 1000th cycle were 94.8%, 99.8%, 100.0%, 100.2%, 100.3%, and 100.4%, respectively. The galvanostatic and cyclic voltammetry (CV) profiles at different cycle numbers are presented in Figure 3c and Figure S9, Supporting Information. The superior cycling performance of S-BOP is ascribed to the suppressed polysulfide dissolution due to their favorable interaction with the heteroatoms on the polymer backbones and the well-confined sulfur distribution.47-49 S-BOP with the higher mass loading of sulfur (2.5 mgsulfur cm-2) also showed good cycling performance, such as 84.1% capacity retention after 100 cycles and 100.8% average CE during the corresponding 100 cycles (Figure S10, Supporting Information).


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S-BOP also exhibited decent rate capability (Figure 3d) when tested at different C-rates (1C = 720 mA gsulfur-1). When the current density was increased by 2, 4, 10, 20, and 40 times from C/20, S-BOP retained 86.2%, 78.8%, 70.2%, 61.5%, and 50.2%, respectively, with respect to the reversible capacity of 1148 mAh gsulfur-1 at C/20. Furthermore, when the current density restored to C/10, 99.0% of the original capacity was recovered. Interestingly, as the C-rate increased, the top discharging plateau (the region before the second flat plateau) was split into two regions more pronouncedly (Figure 3e). The upper and lower regions must originate from the two different atomic configurations of sulfur, crystalline S8 and sulfur chains bonded to the polymer backbone, respectively. As described above, because of the internal structure of S-BOP in which S8 is surrounded by sulfur chains, the sulfur chains are first accessible to incoming Li ions, and S8 is then accessible. Thus, as C-rate increases, the availability of S8 at the beginning of lithiation becomes more significant, and the voltage profiles in the upper region resultantly become more prominent.

Figure 4. Galvanostatic intermittent titration technique (GITT) profiles of S-BOP at (a) C/10, (b) C/5, and (c) C/2. The current duration time was 10 min and the rest time at each point was 1 h.

Our understanding on the structure of S-BOP was further verified by its galvanostatic intermittent titration technique (GITT) analysis (Figure 4). Interestingly, the pseudo-equilibrium


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profiles did not show the slopping profile in 2.1~2.5 V during discharge, but rather exhibited a plateau at near 2.3 V, like the case of typical elemental sulfur. As the C-rate increases, the level of relaxation at each point becomes more significant, and the pre-relaxation profile (connected line of the data point at each measurement spot before relaxation) becomes closer to the original galvanostatic data in Figure 3a that exhibited the slopping profile in 1.7~2.3 V. This observation can be explained by the aforementioned description of the S-BOP structure; S8 and linear sulfur chains are co-present. In kinetics-limiting case, the inner S8 is not first accessible by Li ions, leading to the situation that the surrounding linear sulfurs are accessed earlier than the S8. However, when measured at (pseudo-) equilibrium points, the kinetic limitation does not hold true anymore, and the signature of S8 can be thus revealed. In order to elucidate the effect of covalent attachment of sulfur to the polymer backbone, the cycling performance of S-BOP was compared with S-AC (Figure 5a). When cycled at C/2 (360 mA gsulfur-1), both electrodes showed clearly distinct cycling stability; the specific capacity of S-BOP continuously increased from 806 mAh gsulfur-1 (2nd cycle) to 832 mAh gsulfur-1 (25th cycle) due to the electrode activation process, and finally reached 828 mAh gsulfur-1 at the 50th cycle, corresponding to 102.8% capacity retention compared to the capacity in the 2nd cycle. By contrast, S-AC with the same sulfur loading (0.9 mgsulfur cm-2) started at a higher capacity of 930 mAh gsulfur-1 at the 2nd cycle but dropped severely to 591 mAh gsulfur-1 at the 50th cycle, leading to a much lower 63.5% capacity retention. In particular, the CE of S-AC surpassed 100% at the 8th cycle, reflecting that polysulfide dissolution became amplified with cycling.50,51 The observed specific capacity, cyclability, and CEs of S-BOP are quite remarkable in comparison with those of other notable sulfur electrodes reported recently (Table S1, Supporting Information).


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In an effort to further elaborate on the origin of cycling stability of S-BOP, ex-situ XPS (Figure 5b) and cross-sectional SEM analyses (Figure 5c and d) were carried out. From the XPS profiles of S-BOP, it is first notable that S 2p peaks were reversibly shifted in each cycle during cycling (for example, at the 25th and 50th cycle). While the peaks at 163.7 and 164.8 eV corresponding to the C-S and S-S bonding reconfirm the chemical identity of S-BOP, their stable appearance after 50 cycles, which is consistent with the as-synthesized state, supports the reversible and stable reaction of S-BOP. In particular, suppressed polysulfide dissolution is indicated by sulfide (161.5 eV) and polysulfide (162.3~163.9 eV) peaks that remained consistent throughout cycling.52 The electrode thickness change monitored by cross-sectional SEM analysis also unveils the discharge-charge behavior of both S-BOP and S-AC (Figure 5c and d). In the case of S-BOP, the thickness started at 20.9 µm before cycling and increased to 24.7 µm after 25 cycles, reflecting the swelling of the electrode during the early period of cycling. Since the 25th cycle, the thickness stayed persistent near 24.7 µm, indicating stabilized nature of the electrode during repeated discharge-charge. On the other hand, in the case of S-AC, the thickness started at a higher value of 30.1 µm before cycling and was maintained at the similar level until the 25th cycle. However, the thickness increased to 35.1 µm at the 50th cycle. This trend can be explained by the fact that in the early period of cycling, the electrode swelling originating from volume expansion of the active material and unstable electrode-electrolyte interface related to polysulfide dissolution was accommodated by the void spaces in the electrode.53 However, after a certain number of cycles, the uncontrolled electrode swelling continued to increase the electrode thickness. Because of the rough surface morphology, the thickness was measured at multiple spots of each electrode and the average values were adopted.


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Figure 5. (a) Comparison of cycling performance and Coulombic efficiencies of S-BOP and SAC. The mass loadings of S-BOP and S-AC are both 0.9 mgsulfur cm-2. (b) Ex-situ XPS analysis of S-BOP in S 2p branch at different cycles. Ex-situ cross-sectional SEM images of (c) S-BOP and (d) S-AC electrodes at different cycles.


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In conclusion, we have adopted an orthogonal synthetic approach for the preparation of a sulfur-embedded polymer used as a cathode active material for Li-S batteries. The sulfur chains are covalently linked to the polymer matrix through thiol groups present in the benzoxazine monomer. The homogeneous distribution of sulfur, together with the well-confined sulfur via CS bonds, endows S-BOP with desired physical and electrochemical properties all at once: high sulfur content (~72 wt%), high ICE (96.6%), and stable cyclability (92.7% retention after 1000 cycles). The outstanding and balanced properties of S-BOP reveal the usefulness of sulfurmediated polymer synthesis in which appropriate monomer design can lead to final polymers with desired properties for cathodes in Li-S batteries. In a broader perspective, our approach offers one useful direction of utilizing elemental sulfur, which is a low value commodity massively produced as a by-product from petroleum refinery processes, for high-value applications, as high energy density rechargeable batteries have an enormous economical impact.

ASSOCIATED CONTENT Supporting Information. Detailed materials, synthetic procedures, structural and mechanical characterization, and comparison with other reported sulfur-embedded polymers.

AUTHOR INFORMATION Corresponding Author *E-mail: (A.C.) [email protected] *E-mail: (J.W.C.) [email protected]


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AUTHOR CONTRIBUTION ⊥

These authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), which is granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (20152020104870). J. W. C. acknowledges the financial support of the National Research Foundation of Korea grants (NRF-2014R1A4A1003712 and NRF-2015R1A2A1A05001737).

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