Heteroatomic SenS8–n

Aug 15, 2016 - S8−n Molecules Confined in. Nitrogen-Doped Mesoporous Carbons as. Reversible Cathode Materials for High-. Performance Lithium Batteri...
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Heteroatomic SenS8−n Molecules Confined in Nitrogen-Doped Mesoporous Carbons as Reversible Cathode Materials for HighPerformance Lithium Batteries Downloaded via TULANE UNIV on January 21, 2019 at 17:40:50 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Fugen Sun,† Hongye Cheng,‡ Jianzhuang Chen,† Nan Zheng,† Yongsheng Li,*,† and Jianlin Shi*,†,§ †

Lab of Low-Dimensional Materials Chemistry, Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, and ‡State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China § State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China S Supporting Information *

ABSTRACT: A reversible cathode material in an ether-based electrolyte for high-energy lithium batteries was successfully fabricated by homogeneously confining heteroatomic SenS8−n molecules into nitrogen-doped mesoporous carbons (NMCs) via a facile melt−impregnation route. The resultant SenS8−n/NMC composites exhibit highly reversible electrochemical behavior, where selenium sulfides are recovered through the reversible conversion of polysulfoselenide intermediates during discharge−charge cycles. The recovery of selenium sulfide molecules endows the SenS8−n/NMC cathodes with the rational integration of S and Se cathodes. Density functional theory calculations further reveal that heteroatomic selenium sulfide molecules with higher polarizability could bind more strongly with NMCs than homoatomic sulfur molecules, which provides more efficient suppression of the shuttling phenomenon. Therefore, with further assistance of mesopore confinement of the nitrogen-doped carbons, the Se2S6/NMC composite with an optimal Se/S mole ratio of 2/6 presents excellent cycle stability with a high initial Coulombic efficiency of 96.5% and a high reversible capacity of 883 mAh g−1 after 100 cycles and 780 mAh g−1 after 200 cycles at 250 mA g−1. These encouraging results suggest that the heteroatomization of chalcogen (such as S, Se, or Te) molecules in mesostructured carbon hosts is a promising strategy in enhancing the electrochemical performances of chalcogen/carbon-based cathodes for Li batteries. KEYWORDS: heteroatomic SenS8−n molecules, nitrogen-doped mesoporous carbons, selenium−sulfur−carbon composites, mixed chalcogen cathodes, lithium batteries suffer from its intrinsically low electrical conductivity (5 × 10−28 S m−1) and relatively poor cycling performance.9,10 In comparison, selenium, the congener of sulfur, has much higher electronic conductivity (1 × 10−3 S m−1) and comparable theoretical volumetric capacity density (3240 mAh cm−3).11 It has also been reported that a Li−Se reaction shows higher utilization rate and better electrochemical activity and reversibility than the Li−S reaction.12 However, Se has a theoretical capacity (675 mAh g−1) much lower than that of S.13 Based on the above opposite but complementary features

T

here has been a steady increase in the demand for highenergy density and long-lasting rechargeable batteries for a wide range of energy-storage applications.1,2 Li/S batteries are considered to be one of the most promising candidates due to their high theoretical capacity (1600 mAh g−1) of sulfur cathodes, which is greater than 5 times that of the existing cathode materials based on transition metal oxides and phosphates.3−5 However, the insulating nature of sulfur and high solubility of polysulfide intermediates have become the major obstacles in promoting Li/S batteries into widespread practical applications.6−8 Although advances have been achieved by using carbon matrixes as hosts for immobilizing sulfur or choosing suitable electrolytes for reducing polysulfide dissolution, the applications of sulfur cathodes nonetheless © 2016 American Chemical Society

Received: April 6, 2016 Accepted: August 14, 2016 Published: August 15, 2016 8289

DOI: 10.1021/acsnano.6b02315 ACS Nano 2016, 10, 8289−8298

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Figure 1. (a) Thermogravimetric curves of SenS8−n/NMC (n = 1−3), S/NMC, and Se/NMC. (b) EDX spectra of SenS8−n/NMC (n = 1−3). Inset in (b) shows the relationship between the Se/S mole ratios in the precursors versus those in the composites determined by EDX analysis.

1−3) molecules into nitrogen-doped mesoporous carbons (NMCs) via a simple melt−impregnation method. The Se/S mole ratios in the eight-atomic SenS8−n ring molecules could be tuned by varying the initial ratios of the mixed precursors. Due to the high similarity of cleavage and formation processes among the S−S, S−Se, and Se−Se bonds, the heteroatomic SenS8−n molecules exhibit highly reversible electrochemical behavior, where the recovery of selenium sulfides has been achieved through the reversible conversion of polysulfoselenide intermediates during discharge−charge cycles. The reversible selenium sulfide molecules endow the SenS8−n/NMC cathodes with the rational integration of S and Se and could bind more strongly with the NMCs than the homoatomic sulfur molecules, providing more efficient suppression of the polychalcogenides shuttling. Therefore, the SenS8−n/NMC composites with an optimal Se/S mole ratio show a cycle stability with a higher reversible capacity superior to that of conventional sulfur−carbon composites. These encouraging results suggest that the heteroatomization of chalcogen (such as S, Se, or Te) molecules in the nitrogen-doped carbon hosts is a promising strategy for improving the electrochemical performance of chalcogen-based cathodes for Li batteries.

between Se and S cathodes, the mixed Se−S cathode materials have been proposed as attractive candidates with advanced comprehensive performances for high-energy lithium batteries.14,15 However, until now, rational combination of S and Se cathodes to obtain optimal performances has been challenging. Because the mixed Se−S cathodes in Li/Se−S batteries also suffer from an intermediate shuttle analogous to that with the sulfur cathodes in the Li/S batteries, the conductive carbon materials that have been used to stabilize S cathodes are expected to also be effective for the mixed Se−S cathodes. Abouimrane et al. conducted pioneering work to couple mixed Se−S materials with carbon nanotubes as cathode materials. They suggested that the Se−S−CNT composites were irreversibly reduced into Li2Se and Li2S through the formation of polyselenide and polysulfide intermediates after the initial discharge, and the discharge products Li2Se and Li2S were, respectively, oxidized to Se and S after subsequent charge akin to that in the S and Se cathodes. As a simple hybrid of the Li/S and Li/Se battery, the Li/Se−S−CNT system only delivered a specific capacity of 833 mAh g−1 at a low current density of 50 mA g−1 after 50 cycles.16 Similarly, the carbonized polyacrylonitrile17 and the porous carbons18 were also used as the hosts for the mixed Se−S cathode materials in the carbonatebased electrolyte. Although cycling stability has improved, these Se−S−C composites suffer from low initial Coulombic efficiency, which could result from the side reaction between sulfides/selenides and the carbonate-based electrolyte. Moreover, given the weak interactions between the bulk Se−S mixed materials and the conductive hosts, it is reasonable that the selenium and sulfur species cannot be effectively restrained on the cathode side. Thus, the shuttle phenomenon would be persistently present, which may inevitably deteriorate the cycling performance of the mixed Se−S cathodes. Therefore, to address these issues, it is reasonably anticipated that further improvement on the reversible capacity of the mixed Se−S cathodes would be achieved by strengthening the interactions of selenium sulfide molecules with carbon hosts in the noncarbonate electrolyte. Meanwhile, it is necessary to understand the inherent synergistic mechanism of Se and S on the electrochemical performance of the mixed Se−S materials. Herein, we provide an integrated strategy to design heteroatomic Se−S molecules as reversible cathode materials in an ether-based electrolyte for high-performance Li batteries, through homogeneously confining heteroatomic SenS8−n (n =

RESULTS AND DISCUSSION The SenS8−n/nitrogen-doped mesoporous carbon (SenS8−n/ NMC; n = 1−3) composites were prepared via a facile melt− impregnation method, in which the melt selenium and sulfur were miscible with each other at 500 °C in the vacuum-sealed vessel19 and then infiltrated into the mesopores of NMCs during the heating procedure. The NMCs, which were obtained via a colloidal silica nanocasting process,20,21 have a large specific surface area of 731 m2 g−1, pore volume of 2.6 cm3 g−1, and nitrogen doping amount of 8.1 wt % (more detailed elemental compositions are given in Figure S1 and Table S1). The Se/S mole ratios and the total SenS8−n content in the SenS8−n/NMC composites could be controlled by tuning the contents of selenium, sulfur, and carbon in the mixed precursors. As shown in Figure 1a, all samples exhibit a weight loss of approximately 60 wt % between 200 and 600 °C in a nitrogen flow, corresponding to the evaporation of SenS8−n with different Se/S ratios. The elemental energy-dispersive X-ray (EDX) results (Figure 1b and Table S2) further verify the composition and the Se/S mole ratios of approximately 1/7, 2/ 6, and 3/5 in these samples. The tunable Se/S mole ratios in the SenS8−n enable the rational integration of S and Se cathodes. To demonstrate the integrated advantage of SenS8−n as a 8290

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Figure 2. High-resolution Se 3d spectra of Se2S6/NMC (a) and Se/NMC (b). High-resolution S 2p and Se 3p spectra of Se2S6/NMC (c) and S/NMC (d).

Figure 3. (a) X-ray diffraction patterns of NMCs and SenS8−n/NMC (n = 1−3). (b) N2 adsorption−desorption isotherms of NMCs and Se2S6/ NMC. Inset in (b) shows Barrett−Joyner−Halenda pore size distributions of NMCs and Se2S6/NMC.

cathode material versus sulfur or selenium, the sulfur- or selenium-incorporated NMC samples (S/NMC or Se/NMC) were also prepared under the same melt−impregnation condition as used for the SenS8−n/NMC series for comparison (Figure 1a). The chemical bonding configurations of SenS8−n confined in the nitrogen-doped carbons were investigated by highresolution X-ray photoelectron spectroscopy (XPS). As shown in Figure 2b, Se 3d5/2 and 3d3/2 peaks located at ∼55.2 and ∼56.1 eV with a spin−orbit splitting of 0.86 eV, which are attributed to the Se−Se homopolar bond, were dedected in Se/NMC.22,23 However, in addition to the Se−Se bond, a doublet peak at a higher binding energy of ∼55.6/ ∼56.0 eV, which corresponds to the Se−S heteropolar bond, were deconvoluted and curve-fitted in Se2S6/NMC (Figure 2a).24 The presence of Se−S and S−S bonds in Se2S6/NMC was also confirmed by the S 2p and Se 3p XPS spectra in Figure 2c,d. In the meantime, all of the SenS8−n/NMC samples with different Se/S ratios exhibited Se−S chemical bonding configurations (Figures 2 and S2), indicating the existence of selenium sulfide compounds. It is known that sulfur can react with selenium to form heterocyclic selenium sulfides at an elevated temperature, and the eight-membered SenS8−n ring molecules are the most stable molecular species found in the sulfur−selenium solid solution.19,25,26 Therefore, it is deduced that SenS8−n exists as heterocyclic molecules in the NMCs.

In order to further confirm the status of SenS8−n confined in the NMCs, X-ray diffraction (XRD) analysis and N2 sorption were conducted. As shown in Figure 3a, no distinct differences were found between the SenS8−n/NMC and the NMC samples, suggesting that the SenS8−n molecules confined in the NMCs are very small nanocrystals or in an amorphous form. In comparison with the pristine NMCs, the specific surface area, pore size, and pore volume of the Se2S6/NMC exhibit the expected decrease, demonstrating the successful incorporation of Se2S6 molecules into the inner surface of the mesoporous structures (Figure 3b). Nevertheless, the composites still retain considerable mesoporosity with a specific surface area of 159 m2 g−1 and pore volume of 0.8 cm3 g−1, allowing the easy penetration of electrolyte and Li ions into the inner mesostructures. The homogeneous distribution of SenS8−n within the NMCs was also confirmed by the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. As shown in Figure 4, no SenS8−n agglomerations can be observed on the external surface of NMCs. The scanning transmission electronic microscopy (STEM) (Figure 4e) and SEM elemental mapping images (Figure S3) further show the well matched spatial distributions of C, Se, and S in the Se2S6/ NMC sample. These observations verify that SenS8−n species have been homogeneously trapped inside the mesoporous channels of NMC matrixes and formed a relatively strong 8291

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Figure 4. (a) SEM and (b) TEM images of NMCs. (c) Schematic model of the Se2S6/NMC structure. (d) SEM image of Se2S6/NMC. (e) STEM elemental mapping images of Se2S6/NMC. Inset in (d) is the STEM image of Se2S6/NMC.

include the two-step conversion of high-order lithium polysulfides (e.g., Li2S8) to low-order lithium polysulfides (Li2Sx, 4 ≤ x < 8) and lithium polysulfides to solid-state Li2S, take place at 2.28 and 1.97 V (the blue arrow in Figure 5a,b);27−29 the two-step selenium reduction including the highorder lithium polyselenides (e.g., Li2Se8) to low-order lithium polyselenides (Li2Sex, 4 ≤ x < 8) and then to solid-state Li2Se are at 2.09 and 1.90 V (the red arrow in Figure 5b,c).30−32 However, in addition to the sulfur and selenium reduction peaks, the additional reduction peak at 2.18 V (the green arrow in Figure 5b) is present in the Se2S6/NMC sample. It has also been reported that the cleavage and formation processes among the S−S, S−Se, and Se−Se bonds in the SenS8−n molecules are highly similar, and the different selenium sulfides could interconverse into each other in solution.33 Therefore, it is deduced that the reduction peak at 2.18 V should be due to the formation of polysulfoselenide species (e.g., Li2SexSy, 4 ≤ x + y < 8). The formation of lithium polychalcogenide intermediates in the ether-based electrolyte was further confirmed by the solubility test on the mixture of S and Li2Se, as shown in Figure

interaction with the nitrogen-doped carbon hosts for immobilizing heteroatomic SenS8−n molecules. The electrochemical behaviors of the SenS8−n/NMC (n = 1− 3) composites were investigated by cyclic voltammetry (CV), galvanostatic charging−discharging tests, and electrochemical impedance spectroscopy (EIS). To avoid the side reaction between the chalcogenides and carbonyl groups in the carbonate-based electrolyte, the noncarbonate electrolyte (ether-based electrolyte) was used. We pay special attention to the integrated advantages of the heterocyclic SenS8−n molecules in comparison with the homocyclic S8 or Se8 molecules confined in the nitrogen-doped mesoporous carbons as cathode materials in the ether-based electrolytes. Figure 5a−e shows the CV curves of SenS8−n/NMC (n = 1− 3), S/NMC, and Se/NMC samples scanned at 0.1 mV s−1 in the ether-based electrolyte. In the first cycle, five well-defined reduction peaks centered at 2.29, 2.18, 2.08, 1.99, and 1.89 V for the Se2S6/NMC sample can be distinctly detected, which are different from that for the S/NMC and the Se/NMC samples. It is well-known that the sulfur reductions, which 8292

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Figure 5. Cyclic voltammograms at 0.1 mV s−1: the first cycle of S/NMC (a), Se2S6/NMC (b), and Se/NMC (c); the first cycle of SeS7/NMC, Se2S6/NMC, and Se3S5/NMC (d); the first, second, and third cycles of Se2S6/NMC (e). The charge−discharge curves of Se2S6/NMC at different cycles (f).

peaks in the CV. The initial discharge capacity of the Se2S6/ NMC sample is 1195 mAh g−1, approaching a theoretical capacity of 1226 mAh g−1 for Se2S6 (all capacity values in this work were based on the mass of active SenS8−n species, and the theoretical capacity was based on the complete reduction of SenS8−n to Li2S and Li2Se). Moreover, the Se2S6/NMC sample exhibits a high initial Coulombic efficiency of 96.5% in the ether-based electrolyte, which is significantly higher than that in the carbonate-based electrolyte (Figure S6). These imply that the SenS8−n molecules confined in the NMCs possess an especially high initial electrochemical activity and electrochemical reversibility in the ether-based electrolyte. The electrochemical reversibility of the SenS8−n/NMC composites, that is, the recovery of selenium sulfides through the reversible conversion of the polysulfoselenide intermediates during electrochemical process in the ether-based electrolyte, was further demonstrated by the ex situ XPS analysis of the electrode at different discharge−charge states. As shown in Figure 6a−c and f−h, the Li−S and Li−Se bonds were found, which are generated from the reaction of Se2S6 with Li to form

S4. During the charge process, only one broad oxidation peak is observed, suggesting the transformation of all the chalcogenides into the selenium sulfides with the most facile oxidation kinetics. Meanwhile, similar CV curves are also observed for the SeS7/NMC and Se3S5/NMC samples (Figure 5d), indicating the similar electrochemical behaviors in the SenS8−n/NMC (n = 1−3) samples. In the subsequent scans, the overlapped redox onset potential promises a good electrochemical reversibility of the heteroatomic SenS8−n molecules in the NMCs (Figure 5e). The discharge and charge properties of the SenS8−n/NMC (n = 1−3), S/NMC, and Se/NMC samples in the ether-based electrolyte at a current density of 250 mAh g−1 were further investigated. As shown in Figures 5f and S5, five plateaus are observed for the SenS8−n/NMC samples, which are different from that for the S/NMC and Se/NMC samples. These five plateaus could be attributed to the formation of long-chain polysulfides (Li2Sx, 4 ≤ x ≤ 8) at 2.3 V, polysulfoselenides (Li2SexSy, 4 ≤ x + y < 8) at 2.2 V, polyselenides (Li2Sex,4 ≤ x ≤ 8) at 2.1 V, and short-chain Li2S at 2.0 V and Li2Se at 1.9 V, respectively, which agree well with the five apparent reduction 8293

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Figure 6. High-resolution S 2p and Se 3p spectra: (a) Li2Se; (b) Li2S; (c) Se2S6/NMC after the first discharge; (d) Se2S6/NMC after the first charge; (e) pristine Se2S6/NMC. High-resolution Li 1s and Se 3d spectra: (f) Li2Se; (g) Li2S; (h) Se2S6/NMC after the first discharge; (i) Se2S6/NMC after the first charge; (j) pristine Se2S6/NMC.

Li2Se and Li2S after the first discharge to 1.25 V. Furthermore, the S−S, S−Se, and Se−Se bonds were not detected in the discharge product. These indicate that Se2S6 was completely reduced at the end of the first discharge, which ensures the high initial discharge capacity approaching the theoretical value. Similarly, the heteroatomic selenium sulfide molecules are reversibly recovered after the first and subsequent cycles, which can be known from chemical bonding configurations of S−S, S−Se, and Se−Se bonds in the charge product that are similar to those of the pristine Se2S6/NMC sample (Figures 6d−e,i,j and S7). The recovery of selenium sulfide molecules endows the SenS8−n/NMC cathodes with the rational integration of S and Se cathodes, instead of the irreversible formation of a mixture of elemental S and Se as the charged products in the previously reported work on bulk Se−S mixed materials.16 This difference further suggests that only the heteroatomic selenium sulfide molecules confined in the nitrogen-doped mesoporous carbons are more capable of reversibly cleaving and recovering in the reduction and oxidation of an ether-based electrolyte. Compared with homoatomic sulfur or selenium molecules, the heteroatomic selenium sulfide molecules exhibit higher

polarizability due to the heteropolar Se−S bond,34 which is expected to have a stronger interaction with the carbon substrates. Therefore, the cycle performances of the SenS8−n/ NMC (n = 1−3), S/NMC, and Se/NMC samples in the etherbased electrolyte at 250 mA g−1 are further compared in Figures 7a,b and S8. The S/NMC sample delivers considerable discharge capacity of 1236 mAh g−1 at the first cycle and a reversible capacity of 751 mAh g−1 after 100 cycles, which is comparable to the other carbon−sulfur composites.35−37 Such a reversible capacity is probably due to the enhanced electronic conductivity and the effective chemisorption of polysulfides on the carbon matrix promoted by nitrogen doping.38−40 On this basis, the heteroatomic SenS8−n molecules confined into the NMCs further suppress the shuttle effect and facilitate the redox reaction kinetics, resulting in much improved cycling performances. As a result, The Se2S6/NMC sample exhibits the best electrochemical performances with the highest reversible capacity of 883 mAh g−1 after 100 cycles and the highest Coulombic efficiency of 96%. After extensive cycling for 200 cycles, a high reversible capacity of 780 mAh g−1 can still be retained for the Se2S6/NMC sample (Figure 7e). The cycling of 8294

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Figure 7. (a) Cycling performance and (b) Coulombic efficiency of SenS8−n/NMC (n = 1−3) at 250 mA g−1. (c) Rate performances of SenS8−n/ NMC (n = 1−3) at different current densities. (d) EIS of SenS8−n/NMC (n = 1−3) before the first cycle. (e) Long-term cycling performance of Se2S6/NMC at 250 mA g−1. Inset in (a) shows the theoretical capacity of SenS8−n/NMC (n = 1−3), which is based on the complete reduction of SenS8−n to Li2S and Li2Se.

selenium sulfides with the nitrogen-doped mesoporous carbons were created. To elucidate more information about the integrated advantages of heteroatomic SenS8−n molecules, we modeled and calculated the binding energy of the carbons with Se−Scontaining species by first-principle calculations based on density functional theory (DFT). The NMCs were modeled with a single-layer carbon (typically six carbon rings) doped with pyrrolic, pyridinic, or quaternary N atoms, and the adsorption of Se−S and S−S (to represent heteroatomic selenium sulfide and homoatomic sulfur molecules, respectively) on the carbon layer was investigated, as shown in Figure 8 and Table S4. It is found that binding of the chalcogencontaining species at the N sites is more stable than that at the carbon sites in all cases, which is consistent with the enhanced immobilization effects on the carbon hosts by nitrogen doping. Comparing the different chalcogen-containing guests, the interaction of heteroatomic Se−S species with the N sites was stronger than that of homoatomic S−S species (Table S4). The strongest binding for Se−S species was observed with

the Se2S6/NMC sample is also quite stable at a higher rate of 1 and 3 A g−1, with stable capacities of 562 and 455 mAh g−1 achieved after 200 cycles (Figure S9). When both the reversible capacity and initial Coulombic efficiency are considered, the electrochemical performance of the Se2S6/NMC sample is among the best series of carbon-based sulfur/selenium cathode materials (as listed in Table S3).16−18 In addition, the expected decline in the specific capacity was found by increasing the Se/ S mole ratio from 2/6 to 3/5 in the SenS8−n/NMC samples, undoubtedly due to the limitation of the theoretical capacity, as shown in the inset of Figure 7a. In Figure 7c, it is found that the Se2S6/NMC also presents the highest rate capacity of 525 mAh g−1 after 70 cycles even at the maximum current density of 5 A g−1. The outstanding kinetic behavior should be attributed to the improved electronic transport characteristics of the Se2S6, which could be further supported by their relatively low charge transfer resistances from the EIS results (Figure 7d). These further confirm the high electrochemical activity and reversibility of the Se2S6 molecules confined in the NMC hosts, and thus strong surface interactions of the heteroatomic 8295

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Figure 8. Schematic diagram of the discharge−charge process of (a) S8/NMC and (b) SenS8−n/NMC. Results of first-principle calculations showing the most stable configurations and calculated binding energies of (c) S−S and (d) Se−S species with the carbon layer doped with pyridinic N.

to recover through the reversible conversion of polysulfoselenide intermediates during discharge−charge cycles. The recovery of selenium sulfide molecules endows the SenS8−n/ NMC cathodes with the rational integration of S and Se cathodes. Compared with homoatomic molecules, the heteroatomic selenium sulfide molecules with higher polarizability are more electrochemically stable due to their stronger surface interactions with the nitrogen-doped carbon substrates, which is further confirmed by the DFT calculations. Therefore, with further assistance of mesopore confinement of nitrogen-doped carbon matrixes, the Se2S6/NMC composite at the optimal Se/ S ratio of 2/6 exhibits a high initial Coulombic efficiency of 96.5%, a high reversible capacity of 883 mAh g−1 after 100 cycles and 780 mAh g−1 after 200 cycles at 250 mA g−1, and a high rate capability of 525 mAh g−1 up to 5 A g−1. This work clearly demonstrates that the mesostructured composite of heteroatomized Se−S molecules confined in nitrogen-doped carbon hosts is a highly promising cathode candidate and may also provide some valuable hints for other chalcogen-based cathodes (such as S, Se, or Te) for high-performance lithium batteries.

pyridinic N atoms, and the binding energy is 2.92 eV, which decreases to 2.14 eV for the S−S species. This stronger binding of the Se−S species with NMCs could be attributed to the higher density of polarizable electrons contributed by the larger Se atoms and could explain the higher adsorption capacity of heteroatomic selenium sulfide compared to that of homoatomic sulfur molecules by the NMCs. Therefore, the reversible conversion upon discharge−charge cycles makes the selenium sulfide molecules homogeneously confined, meanwhile binding strongly with the NMCs hosts, resulting in excellent electrochemical activity and cycling stability for high-energy lithium batteries.

CONCLUSION In conclusion, the mixed Se−S materials in the form of heterocyclic SenS8−n molecules have been homogeneously confined within the mesostructures of the NMCs by a facile melt−impregnation route and used as cathode materials in an ether-based electrolyte for high-energy lithium batteries. The electrochemical tests and ex situ study clearly demonstrate that the selenium sulfide molecules confined in the NMCs are able 8296

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ACS Nano

separator was a microporous membrane (Celgard 2400). The cell was assembled in an argon-filled glovebox. Galvanostatic charge−discharge tests and CV measurements were conducted using an Arbin battery cycler (BT2000, USA). Electrochemical impedance spectroscopy was performed with a PCI4/300 electrochemical working station (Gamry Instrument, Warminster, PA, USA). The sinusoidal excitation voltage applied to the coin cells was 5 mV, with a frequency range from 100 kHz to 0.01 Hz. All electrochemical tests were performed at room temperature. DFT Calculations. First-principle calculations were conducted using the Perdew−Burke−Ernzerhof exchange-correlation functional in the framework of general gradient approximation implemented in the DMol3 package in Materials Studio (version 7.0) of Accelrys Inc. An all-electron double numerical basis set with polarization functions was used in this work. The convergence criteria applied for geometry optimizations were 1.0 × 10−5 au, 2.0 × 10−3 au Å−1, and 5.0 × 10−3 Å for energy change, maximum force, and maximum displacement, respectively. The threshold for self-consistent-field density convergence was set to 1.0 × 10−6 eV, and the global cutoff was set to fine. To quantitatively measure the interaction between the NMCs and chalcogen-containing species, we defined the binding energy Eb as follows: Eb = Ecarbon + Echalcogen − Etotal, where Ecarbon, Echalcogen, and Etotal represent the total energies of a polyaromatic molecule, an isolated chalcogen-containing species, and a polyaromatic molecule binding to a chalcogen-containing species, respectively.

EXPERIMENTAL SECTION Preparation of NMCs. The NMCs were synthesized via a colloidal silica nanocasting, using phenol, melamine, and formaldehyde as carbon precursors.20 Typically, 3.67 g of phenol (39 mmol) and 6.33 g of formaldehyde (37 wt %, 78 mmol) were dissolved in 50 mL of 0.2 M NaOH solution (10 mmol). The mixture was stirred at 70 °C for 40 min. Then, 4.92 g of melamine (39 mmol) and 9.50 g of formaldehyde (107 mmol) were added to the above solution to react for 30 min with consecutive agitation until the solution became clear. Next, 50 g of Ludox SM-30 sol (30 wt % SiO2) was added to the above solution under stirring. The mixture was transferred to a sealed bottle and heated at 80 °C for 3 days. The obtained mixture was directly dried at 80 °C in an ambient conditions, followed by carbonization at 800 °C for 3 h with a heating rate of 5 °C min−1 in a nitrogen flow. The silica/ carbon composite was ground into a powder before the silica template was removed by NaOH etching. The NMCs were obtained by filtration, washed with distilled water until the pH did not change, and dried at 100 °C. Preparation of SenS8−n/NMC Composites. The SenS8−n/NMC nanocomposites were prepared following a facile melt−diffusion strategy. In a typical synthesis procedure, 0.49 g of sulfur (15 mmol) and 0.41 g of selenium (5 mmol) were mixed homogeneously. The mixture was degassed in a vessel and then sealed under vacuum, followed by heating at 500 °C for 3 h. The obtained Se−S solid solution was homogeneously mixed with 0.6 g of as-prepared NMCs. The melt infiltration was further carried out in a vacuum-sealed vessel at 260 °C for 12 h. In this work, by controlling the composition of selenium, sulfur, and carbon in the precursors, three composites with different Se/S mole ratios and a fixed Se−S solid solution content (60 wt %) were prepared, which are denoted as SenS8−n/NMC, where n/ (8−n) represents the Se/S mole ratios in the composites. Material Characterization. Thermogravimetric analysis (TA Instrument Q600 Analyzer) of samples was carried out in a nitrogen flow. The samples were heated to 700 °C with a rate of 10 °C min−1. Elemental analysis was carried out using Elemental Vario EL III. The nitrogen doping contents of the NMCs were determined directly using a thermal conductivity detector. The surface chemistry of the samples was analyzed using an Axis Ultra DLD X-ray photoelectron spectroscopy. The X-ray source was operated at 15 kV and 10 mA. The working pressure was less than 2 × 10−8 Torr (1 Torr = 133.3 Pa). The Li 1s, C 1s, N 1s, S 2p, Se 3p, and Se 3d XPS spectra were measured at a step size of 0.1 eV. The binding energies were calibrated taking C 1s as a standard with a measured typical value of 284.6 eV. The Li 1s, N 1s, S 2p, Se 3p, and Se 3d XPS signals were fitted with mixed Lorentzian−Gaussian curves, and a Shirley function was used to subtract the background using a XPS peak processing software. The XRD patterns were acquired on a Rigaku D/max 2550 diffractometer operating at 40 kV and 20 mA using Cu Kα radiation (λ = 1.5406 Å). Nitrogen adsorption/desorption isotherms were measured at 77 K with a Quadrasorb SI analyzer. The Brunauer− Emmett−Teller method was utilized to calculate the specific surface area. The total pore volume was calculated using a single point at a relative pressure of 0.985. The pore size distributions were derived from the desorption branch by using the Barrett−Joyner−Halenda model. The morphologies of the samples were observed under SEM (JEOL 7100F) and TEM (JEOL 2100F). The SEM mapping and EDX spectroscopy analyses were performed under scanning electron microscopy (FEI Q-300). STEM was conducted on a Tecnai G2 F30. Electrochemical Tests. The SenS8−n/NMC samples were slurrycast onto an aluminum current collector. Typically, 80 wt % of SenS8−n/NMC, 10 wt % of carbon black (Super P conductive carbon black), and 10 wt % of sodium alga acid binder were mixed in water solvent. The slurries were coated on aluminum current collectors and dried at 60 °C overnight. Electrochemical tests of these electrode materials were performed using coin cells with the SenS8−n/NMC cathodes and lithium metal as the counter electrodes. The electrolyte was 1 M bis(trifluoromethane)sulfonimide lithium salt dissolved in a mixture of 1.3-dioxolane and dimethoxymethane (1:1 by volume). The

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b02315. More results including SEM images, EDX, XPS, DFT results, and electrochemical performances (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by National Science Foundation of China (No. 51502090), China Postdoctoral Science Foundation (Nos. 2014M560306 and 2015T80407), and grants from the Science and Technology Commission of Shanghai Municipality (No. 15YF1402800) and Fundamental Research Funds for the Central Universities (No. 222201414033). REFERENCES (1) Choi, N. S.; Chen, Z.; Freunberger, S. A.; Ji, X.; Sun, Y. K.; Amine, K.; Yushin, G.; Nazar, L. F.; Cho, J.; Bruce, P. G. Challenges Facing Lithium Batteries and Electrical Double-Layer Capacitors. Angew. Chem., Int. Ed. 2012, 51, 9994−10024. (2) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J. M. Li-O2 and Li-S Batteries with High Energy Storage. Nat. Mater. 2012, 11, 19−29. (3) Ji, X.; Lee, K. T.; Nazar, L. F. A Highly Ordered Nanostructured Carbon-Sulphur Cathode for Lithium-Sulphur Batteries. Nat. Mater. 2009, 8, 500−506. (4) Yang, Y.; Zheng, G.; Cui, Y. Nanostructured Sulfur Cathodes. Chem. Soc. Rev. 2013, 42, 3018−3032. (5) Li, N.; Wang, Y.; Tang, D.; Zhou, H. Integrating a Photocatalyst into a Hybrid Lithium-Sulfur Battery for Direct Storage of Solar Energy. Angew. Chem. 2015, 127, 9403−9406. 8297

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