C Composites toward High

Research Article www.acsami.org

SnS2- Compared to SnO2‑Stabilized S/C Composites toward HighPerformance Lithium Sulfur Batteries Xiaona Li,† Yue Lu,† Zhiguo Hou, Wanqun Zhang, Yongchun Zhu,* and Yitai Qian* Hefei National Laboratory for Physical Science at Microscale, Department of Chemistry, University of Science and Technology of China, 96 JinZhai Road, 230026, Hefei, China

Jianwen Liang and Yitai Qian School of Chemistry and Chemical Engineering, Shandong University, Jinan, Shandong 250100, P. R. China S Supporting Information *

ABSTRACT: The common sulfur/carbon (S/C) composite cathodes in lithium sulfur batteries suffer gradual capacity fading over long-term cycling incurred by the poor physical confinement of sulfur in a nonpolar carbon host. In this work, these issues are significantly relieved by introducing polar SnO2 or SnS2 species into the S/C composite. SnO2- or SnS2-stabilized sulfur in porous carbon composites (SnO2/S/C and SnS2/S/C) have been obtained through a baked-in-salt or sealed-in-vessel approach at 245 °C, starting from metallic tin (mp 231.89 °C), excess sulfur, and porous carbon. Both of the in situ-formed SnO2 and SnS2 in the two composites could ensure chemical interaction with lithium polysulfide (LiPS) intermediates proven by theoretical calculation. Compared to SnO2/S/C, the SnS2/S/C sample affords a more appropriate binding effect and shows lower charge transfer resistance, which is important for the efficient redox reaction of the adsorbed LiPS intermediates during cycling. When used as cathodes for Li−S batteries, the SnS2/S/C composite with sulfur loading of 78 wt % exhibits superior electrochemical performance. It delivers reversible capacities of 780 mAh g−1 after 300 cycles at 0.5 C. When further coupled with a Ge/C anode, the full cell also shows good cycling stability and efficiency. KEYWORDS: tin disulfide, tin dioxide, lithium sulfur batteries, energy storage, chemical interaction

■

INTRODUCTION Lithium sulfur (Li−S) batteries are considered a promising candidate for next-generation energy storage devices due to their low cost and high theoretical specific capacity (1675 mAh g−1) and energy density (2600 Wh kg−1).1,2 Typically, each sulfur atom could react with two lithium ions in Li−S batteries to form Li2S as the final discharge product, thus allowing for much higher lithium storage capacity compared to that of traditional lithium-ion batteries (LIBs). However, direct conversion from sulfur to Li2S is kinetically sluggish due to their insulating characteristics. Therefore, the use of aprotic electrolytes (usually ether-based) is essential for the reaction in Li−S batteries by dissolving the soluble lithium polysulfide intermediates (LiPSs, Li2Sx, x = 4−8) to connect the two solid phases conversion. This is a merit of the soluble nature of LiPSs. However, it also has disadvantages for Li−S batteries because the dissolution−precipitation transitions of LiPSs are difficult to control during cycling, and the migration of LiPSs to the negative electrode induces a redox shuttle phenomenon in the cell. As a result, the practical use of Li−S batteries will be hindered by severe self-discharge and low Coulombic efficiency accompanied by loss of active material and poor cycling performance.3,4 © 2016 American Chemical Society

Many efforts have been made to overcome the abovementioned challenges faced by Li−S batteries, including the intensive studies on cathode material designs,5−10 functional interlayers,11−13 electrolyte additives,14,15 anode-protective additives,16,17 alternative anode materials,18−20 and new binders.21−23 For the modification and improvement of the cathode materials, research mainly focuses on two aspects. One is on developing functional host materials with strong chemical interactions for sulfur or LiPSs to block the dissolution and diffusion of polysulfides, thus enhancing utilization of the active sulfur and improving the electrochemical performance of Li−S batteries. The other is that shuttling of LiPSs in Li−S batteries should not only be originated from the inevitable diffusion but also influenced by the redox reaction of LiPSs during charge/ discharge. An efficient redox process for transforming longchain LiPSs during cycling means fast reaction kinetics and relatively less diffusion in the electrolyte. For example, Arava et al.24 presented that a sulfur-based cathode with Pt/graphene as host material could deliver a 40% enhancement in specific capacity compared to that of a sulfur/graphene cathode. Received: June 1, 2016 Accepted: July 15, 2016 Published: July 15, 2016 19550

DOI: 10.1021/acsami.6b06565 ACS Appl. Mater. Interfaces 2016, 8, 19550−19557

Research Article

ACS Applied Materials & Interfaces Scheme 1. Synthesis Approaches of (a) SnO2/S/C and (b) SnS2/S/C Composites

Figure 1. Typical (a, b) XRD pattern, (c, d) SEM, and (e, f) TEM images of the SnO2/S/C and SnS2/S/C composites, respectively.

reactivity of LiPSs was due to the strong chemical affinity of CoS2 itself. Intrinsically polar metal oxides, which can interact with polar LiPSs, have been widely used to modify the sulfur-based cathodes, such as representative manganese nickel oxide-,26,27 γalumina-,28 silica-,29 and titania30-based electrodes. Recently, metal sulfides (Ti2S,31,32 CoS2,25 Co9S8,33 and SnS234) have also

Actually, these two aspects are not isolated but rather are related to each other, and many host materials play double roles in cells. The redox reactions of LiPSs would be available only if they were adsorbed effectively on the host. Taking half-metallic CoS2 in the CoS2/S/graphene composite reported by Zhang as an example,25 the author declared that the enhanced redox 19551


Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) TGA curves of SnO2/S/C and SnS2/S/C composites under N2. (b) Raman spectra of SnO2/S/C and SnS2/S/C composites.

previous reports35 except that the heating temperature was set at 245 °C considering that the melting point of tin is 231.89 °C (ΔGfθ of SnO2 is approximately −519.65 kJ mol−1). The SnS2/ S/C composite, on the other hand, was obtained by being sealed in a glass tube under vacuum with the same heating process (ΔGfθ of SnS2 is approximately −198.95 kJ mol−1). For the baked-in-salt approach, the oxygen comes from the residual atmosphere reserved in the autoclave, which was enough to oxidize the small amount of tin in the precursor. During the heating process, the melt tin tends to react with oxygen to form SnO2 rather than react with sulfur (the much lower value of ΔGfθSnO2). The porous carbon used here was prepared similarly to the protocol in our previous report,36,37 which is mainly based on thermal conversion from the metal complex. The corresponding characterization of the porous carbon is shown in Figure S1. Demonstrating micro/mesoporous structure, the corresponding Brunauer−Emmett−Teller (BET) surface area is approximately 769.3 m2 g−1, and the pore volume is approximately 2.513 cm3 g−1. The X-ray diffraction (XRD) patterns of the prepared samples obtained are shown in Figure 1a and b. For the sample obtained by the baked-in-salt approach, the XRD pattern could be indexed to tetragonal SnO2 (JCPDF no. 770450, P42/mnm), and the small peaks could be due to sulfur (marked *). For the sample obtained by the sealed-in-vessel under a vacuum approach, the XRD pattern could be indexed to trigonal SnS2 (JCPDF no. 75-0367, P3̅m1), and the peaks of sulfur are more obvious than in the former one. The components of the samples were first analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP), thermogravimetric analysis (TGA), and elemental analysis. The corresponding sulfur, tin, oxygen, and carbon contents are presented in Table S1; the products are labeled as SnO2/S/C and SnS2/S/C. The morphologies of the two samples were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) as shown in Figure 1c−f and Figure S2. The SnO2/S/C sample exhibits a porous structure, and no big SnO2 particles could be observed, which is consistent with the broadened XRD peaks in Figure 1a. The detailed distribution of S, SnO2, and carbon is further revealed by HRTEM and elemental mapping. As shown in Figures S3 and S4, the particles sizes of SnO2 are approximately 6 nm, and both S and SnO2 are mainly distributed in the region of the carbon matrix. On the other hand, SnS2 nanosheets can be clearly observed in Figure 1d and f and Figure S5. TEM and elemental mapping (Figure S6) show that the SnS2 nanosheets

been introduced into the cathode to improve the electrochemical activity of sulfur-based electrodes. Compared to most metal oxides, metal sulfides could not only provide strong interaction with polar LiPSs but also act as activation sites for the redox reactions of those LiPSs during cycling, which is due to their more conductive nature and could ensure the fast transport of electrons during the redox reaction. Furthermore, there should be a tendency of LiPSs to interact with sulfiphilic metal sulfides rather than oxides. However, there is no report on the direct study of the different influences of MOx or MSx (with the same metal atom) on the final lithium-storage performance. In this paper, SnO2- or SnS2-stabilized sulfur in porous carbon composites (SnO2/S/C and SnS2/S/C) were obtained through a baked-in-salt or sealed-in-vessel approach with metallic tin, excess sulfur, and porous carbon as raw materials. In the baked-in-salt approach, the melt tin tends to react with the residual oxygen to form SnO2 rather than react with sulfur due to the much lower value of ΔGfθSnO2 (−519.65 kJ mol−1) than that of ΔGfθSnS2 (−198.95 kJ mol−1). Apart from the solely physical confinement of porous carbon, both of the in situ formed polar SnO2 and SnS2 in the two composites could also provide chemical interactions with lithium polysulfide intermediates. Compared to SnO2/S/C, the SnS2/S/C sample affords a more appropriate binding effect as proven by theoretical calculations, thus ensuring a desirable stabilization effect of the cathodes during cycling. Furthermore, the SnS2/S/ C sample also shows lower charge transfer resistance, which is important for the efficient redox reaction of the adsorbed LiPS intermediates during cycling by providing active sites. When used as cathodes for Li−S batteries, the SnS2/S/C composite with sulfur loading of ∼78 wt % exhibits superior electrochemical performance, delivering a reversible capacity of 780 mAh g−1 after 300 cycles at 0.5 C with Coulombic efficiency near 100%. When further coupled with a Ge/C anode, the full cell also shows good cycling stability and efficiency.

■

RESULTS AND DISCUSSION Scheme 1 illustrates the two synthesis approaches of the SnO2/ S/C and SnS2/S/C composites. The first step in the two approaches is the same: commercial tin, excess sulfur, and porous carbon (molar ratio of S to Sn is 32, and the porous carbon is approximately 10 wt % based on the total mass of the Sn−S−C precursor) were ball-milled with ethanol for 12 h. The difference is in the second step: the SnO2/S/C composite was obtained by a baked-in-salt approach similar to our 19552


Research Article

ACS Applied Materials & Interfaces are anchored on the porous carbon randomly, which could also provide the high affinity of sulfur and polysulfide intermediates for the SnS2 nanosheets and carbon. X-ray photoelectron spectroscopy (XPS) was further employed to reveal the chemical status of Sn, S, O, and C of the two composites, which is displayed in Figure S7. Both of the high-resolution Sn 3d spectra showed two distinct peaks at 487.2 and 495.6 eV for Sn 3d5/2 and Sn 3d3/2 core levels,34 indicating the chemical valence states of Sn in the two samples was +4. On the other hand, for the high-resolution S 2p spectra of the two samples, there were S2− peaks observed around 162 eV8 of the SnS2/S/ C sample in Figure S7b that did not exist in the SnO2/S/C sample. The TGA curves of the two samples also provide information regarding the different interactions between elemental sulfur and the hybrid host (SnO2/C or SnS2/C, Figure 2a). The sulfur loadings of the SnO2/S/C and SnS2/S/C composites are determined to be ∼74 and ∼78 wt %, respectively. The sulfur volatilization temperature of the SnS 2 /S/C sample is approximately 210−400 °C, which is slightly higher than that of the SnO2/S/C sample, indicating stronger restriction and adsorption of sulfur with the hybrid host. The Raman spectra of the two samples in Figure 2b display similar shapes with two main peaks of the D band and the G band (1350 and 1586 cm−1, respectively),38 whereas no strong peaks of SnO2 or SnS2 are observed in the spectra, which should be influenced by the porous carbon in the composites. Furthermore, BET surface area data of the SnS2/S/C and SnO2/S/C composites were also measured, which are only approximately 7 and 31 m2 g−1 (Figure S8), respectively, indicating that most of sulfur is infiltrated into the pores of the carbon during the heating process. Previous reports on polysulfide trapping reagents have shown that polar host materials are expected to interact with lithium polysulfides and, thus, are supposed to improve the cycling stability of sulfur-based electrodes.7,33 Both as intrinsically polar materials, the chemical interactions of SnO2 or SnS2 with polysulfide have to be well-studied. Thus, first-principle calculations based on density functional theory were performed. Here, a simulated 2 × 2 supercell with one adsorbed Li2S4 molecule was used to investigate the interfacial interaction. For the examined structures, the most favorable adsorption configurations are the Li atoms adsorbed on the top sites of the S atoms or O atoms in SnS2 and SnO2, respectively; the corresponding fully relaxed geometries are shown in Figure 3 (the view of the (001) plane of SnS2 and SnO2 are shown in Figure S9). In the relaxed structures, the resultant adsorption energies with respect to SnS2 and SnO2 are 1.26 and 3.25 eV, respectively, with average Li−S and Li−O distances of 2.64 and 1.94 Å, respectively. Here, it should be noted that a proper, but not too strong, binding force is better for host materials, in consideration of not inducing the destruction of Li2Sn species during cycling.39 Therefore, the binding effect of SnO2 here might be too strong compared to that of SnS2. The electrochemical performances of the SnS2/S/C and SnO 2/S/C electrodes were first investigated by cyclic voltammetry (CV) measurements. Panels a and b in Figure 4 show the CV curves of the five initial cycles of the two electrodes between 1.8 and 2.8 V (vs Li+/Li) at 0.1 mV s−1. The two typical peaks for sulfur-based electrodes in the first lithiation scan of the SnS2/S/C electrode shift to higher voltage at 2.27 and 2.05 V in the following cycles, which correspond to a reduction of cyclo-S8 to high-order LiPSs and further

Figure 3. Binding geometries and energies of a Li2S4 molecule on the (001) plane of (a) SnO2 and (b) SnS2 based on DFT. Purple, gray, yellow, and red colored spheres represent lithium, tin, sulfur, and oxygen, respectively.

reduction to low-order lithium sulfides (Li2S or Li2S2). A large peak at the end of the lithiation scan process could also be seen in the first cycle but disappears in the following cycling. During the anodic scan, the peaks at 2.36 and 2.42 V result from the delithiation of low-order lithium sulfides and highorder LiPSs, respectively, reproducing cyclo-S8 in the fully charged state.40 For the SnO2/S/C electrode with the same sulfur loading in the electrode, the peaks appear at a similar voltage but with much low intensity, indicating a less electrochemically active SnO2/S/C electrode, which can clearly be seen in Figure 4c. Panels d and e in Figure 4 present the corresponding discharge/charge profiles of SnS2/S/C and SnO2/S/C electrodes, respectively. In the initial discharge process, apart from the two typical discharge plateaus at 2.3 and 2.1 V, both of the electrodes exhibit a slap voltage around 1.9 V rather than a rapid voltage decrease as seen in previous reports,25,33,34 which might be influenced by the interaction with SnS2 and SnO2 in the hybrid composites. In the initial five cycles at 0.2 C (1 C = 1672 mA g−1), there is a slight capacity decay for SnS2/S/C electrode, whereas the discharge/charge curves in the following cycles at 0.5 C overlap each other well, demonstrating the steady and reversible electrochemical reaction of sulfur in the SnS2/S/C composite. For the SnO2/S/C sample, the capacity is much lower than that of the SnS2/S/C sample, and the cycling stability is also slightly worse, which are further proven in the cycling performance test shown in Figure 4f. The SnS2/ S/C and SnO2/S/C electrodes deliver ∼875 and 545 mAh g−1 after 50 cycles at 0.5 C (based on the sulfur mass in the composite cathodes), respectively, with capacity retention of approximately 90.4 and 84.5% after the activation process at 0.2 C, respectively. Both electrodes show high Coulombic efficiencies near 100%, demonstrating the effectiveness of SnS2/C and SnO2/C in retarding shuttling of LiPSs during cycling. Furthermore, the better cycling stability of the SnS2/S/ C composite suggests a more proper adorption energy of SnS2 within the composite. For the enhanced electrochemical activity of the SnS2/S/C composite to be revealed further, electrochemical impedance spectroscopy (EIS) measurements were performed on the two cells with the two electrodes before cycling test (Figure S10). The EIS data show a smaller semicircle of the SnS2/S/C electrode, indicating its relatively lower charge transfer resistance compared with that of the 19553


Research Article


Figure 4. Cyclic voltammetry (CV) curves of (a) SnS2/S/C and (b) SnO2/S/C electrodes between 1.8 and 2.8 V (vs Li+/Li) at 0.1 mV s−1. (c) Comparison of CV curves between the SnS2/S/C and SnO2/S/C electrodes. Discharge/charge profiles of (d) SnS2/S/C and (e) SnO2/S/C electrodes in the initial 10 cycles at 1.8−2.8 V (0.2 C for cycles 1−5 and 0.5 C for cycles 6−10). (f) Cycling performance of SnS2/S/C and SnO2/S/ C electrodes at 0.5 C over 50 cycles (activated at 0.2 C for 5 cycles).

Figure 5. (a) Rate capability of the SnS2/S/C cathode. (b) Capacity retention of the SnS2/S/C cathode at 0.5 C over 300 cycles.

SnS2/S/C cathode releasing the volume change during cycling. These results indicate that the introduction of both SnS2 and porous carbon are important for the good electrochemical performance of the SnS2/S/C composite, and such improvement should be due to synergetic effects of all of the components in the composite rather than just the sum of the individual components. Figure 5a shows that the SnS2/S/C cathode has a good rate capability when lithium metal is used as the anode for both charge and discharge, retaining 51.5% of its capacity when the discharge rate increases from 0.1 to 2 C. Moreover, the discharge capacity could be recovered to 1040 mAh g−1 when the rate was set back to 0.2 C, indicating the stable structure of the reversible reaction with lithium of SnS2/S/C even after being subjected to high current densities. At 0.5 C, the SnS2/S/ C cathode still could deliver 750 mAh g−1 after 300 cycles with a high Coulombic efficiency of approximately 100% (Figure 5b). Even when the electrode was subjected to cycling at a rate of 1 C for 1000 cycles, the cell could still retain approximately

SnO2/S/C electrode. This means faster electron transfer within the SnS2/S/C electrode, which is important for the efficient redox reaction of the adsorbed LiPSs intermediates during cycling, thus inducing the facilitated solid−liquid/liquid−solid electrochemical reaction in Li−S battery systems.25 When further compared to S/C composites, it could be seen in Figure S11 that, among the three samples, the SnS2/S/C cathode still exhibits the highest reversible capacity and best capacity stability. Moreover, though the capacity of SnO2/S/C cathode in the initial period is not as high as that of S/C cathode, it could deliver comparative capacity thereafter with better capacity retention than that of S/C cathode. The lower reversible capacity of the SnO2/S/C cathode might be due to the two strong binding forces between the SnO2 and Li2Sn species. As a comparison, we also provided the electrochemical performance of SnS2/S with similar sulfur contents to those of the SnS2/S/C composite (Figure S12), from which one can see that the SnS2/S/C cathode still exhibits better capacity retention, which might be due to the porous carbon in the 19554


Research Article


Figure 6. (a) The charge/discharge galvanostatic curves of the SnS2/S/C−Li half cell, SnS2/S/C−Ge/C full cell and Ge/C−Li half cell. (b) The cycling performance of SnS2/S/C−Ge/C full cell for the first 50 cycles at 1 C in the range of 2.6−1.2 V. (c) Charge/discharge curves of a SnS2/S/ C−Ge/C full cell for the first 50 cycles.

■

536 mAh g−1 (Figure S13) with a capacity fading rate of only approximately 0.058% per cycle, further indicating good cycling stability. On the other hand, there are some problems when metallic lithium is used as anode in Li−S batteries, such as the dendrites formed arising from unstable solid electrolyte interphase (SEI) and possible reaction between lithium metal with LiPSs in the electrolyte.41,42 Thus, rechargeable Li−S full cells are also fabricated with a charged state SnS2/S/C cathode and discharged state Ge/C anode in our work. The Ge/C composite was prepared through the reduction and carbonization of germanium chelate method according to our previous study.43 Ge/C electrodes were first prelithiated in Ge/C−Li half cells through discharge the cell in 0.01−1.5 V at 0.5 C and using LiTFSI in DOL/DME (2 wt % LiNO3) as the electrolyte. Even some reports pointed out that LiNO3 should not be used to make sure no other extra lithium source in the cells, while the Ge/C electrodes could not work well in the LiTFSI in DOL/DME electrolyte without LiNO3, as shown in Figure S14. On the other hand, the cycling performance of Ge/C electrodes with 2 wt % LiNO3 addition was also presented (Figure S14), which exhibited good cycling stability. The capacities of SnS2/S/C and Ge/C electrodes are 0.85 and 1.02 mAh for each 2016-type coin cell, respectively, with negative electrode capacity approximately 1.2 times of that of positive electrode, as shown in Figure 6a. As expected, we obtained a SnS2/S/C−Ge/C full cell with two discharge voltages around 2.0 and 1.7 V, delivering a capacity approximately 0.78 mAh (Figure 6a). On the basis of the mass of elemental sulfur in SnS2/S/C composite, the highest capacity of the SnS2/S/C−Ge/C full cell is approximately 743.5 mAh g−1 at 0.5 C, and remains at 613 mAh g−1after 50 cycles (Figure 6b).

CONCLUSIONS In summary, SnO2- or SnS2-stabilized sulfur in porous carbon composites (SnO2/S/C and SnS2/S/C) have been synthesized through baked-in-salt or sealed-in-vessel approaches with metallic tin, excess sulfur, and porous carbon as raw materials. Taking advantage of the high surface area and micro/ mesoporous structure of the porous carbon, apart from the in situ formed SnO2 or SnS2, the excess sulfur would be impregnated into the pores of the porous carbon. Theoretical calculations indicate that the in situ-formed polar SnO2 and SnS2 could also provide chemical interactions with lithium polysulfide intermediates. Compared to SnO2/S/C, the SnS2/ S/C sample affords a more appropriate binding effect and shows lower charge transfer resistance, which are important for the efficient redox reaction of the adsorbed LiPS intermediates during cycling. When used as cathodes for Li−S batteries, the SnS2/S/C composite with sulfur loading of 78 wt % could deliver a reversible capacity of 780 mAh g−1 after 300 cycles at 0.5 C with a Coulombic efficiency near 100%. When further coupled with a Ge/C anode, the full cell also shows good cycling stability and efficiency. We hope that the current work helps provide insight into the different roles or effects of MOx and MSx (with the same metal atom) in corresponding sulfurbased cathodes.

■

EXPERIMENTAL SECTION

Materials. Commercial sublimed sulfur (99.5%) and tin powder (99%, 200 mesh) were purchased from Sigma-Aldrich and used without further purification. Sodium chloride was acquired from Sinopharm Chemical Reagent Co., Ltd. (China). Preparation of Sn/S/C Precursor. To obtain the Sn/S/C precursor, we ball-milled commercial sulfur, tin powder (200 mesh), and porous carbon with the addition of C2H5OH for 12 h at 300 rpm. The molar ratio of S to Sn was 32, and the porous carbon used here was approximately 10 wt % based on the total mass of the Sn/S/C 19555


Research Article


metal as the anode. The cycling performance of the cells was evaluated under galvanostatic conditions using a LAND-CT2001A instrument, and electrochemical processes in the cells were studied by cyclic voltammetry (CV) using a CHI600E electrochemical workstation. AC impedance spectra were also carried out using an electrochemical workstation (CHI660E) by applying an AC voltage of 5 mV in amplitude in the frequency range of 0.01 Hz to 100 kHz.

precursor. The porous carbon used here was prepared based on our previous report.36,37 Synthesis of Hybrid SnS2/S/C Composites. The Sn/S/C precursor was sealed in a glass tube covered with NaCl on its surface under a vacuum. Heat treatment at 245 °C for 24 h at 2 °C min−1 was conducted considering the melting point of tin (231.89 °C). The aim of using NaCl here was to reduce the loss of S during the heat treatment process. Synthesis of Hybrid SnO2/S/C Composites. The SnO2/S/C composites were obtained through a baked-in-salt approach similar to that of our previous report.35 Typically, the Sn/S/C precursor was sandwiched between compacted NaCl in a 2 mL Teflon-lined stainless steel autoclave and heated at 245 °C for 24 h at 2 °C min−1. The obtained products were washed with distilled water and ethanol several times to remove NaCl and surface sulfur. Finally, the products were dried in a vacuum oven at 60 °C overnight. Material Characterization. X-ray diffraction (XRD) patterns were recorded by a Philips X’Pert Super diffractometer using Cu Kα radiation (λ = 1.54178 Å) operated from 2θ = 10−80°. Scanning electron microscopy (SEM) images were taken using a JEOL JSM6700F field-emission scanning electron microscope (FE-SEM). Transmission electron microscopy (TEM) images were taken by a JEM-ARM (Japan) 200F FE-TEM. XPS analysis was measured on a PerkinElmer ESCALAB MK II XPS system with a resolution of 0.9 eV from an aluminum/magnesium anode X-ray source. The Raman spectra were generated using a LabRamHR Raman microscope with 514.5 nm excitation laser. The sulfur content was determined by thermogravimetric analysis using TGA equipment (SDT Q600, TA Instruments, USA) at a heating rate of 5 °C min−1 from 10 to 800 °C under a nitrogen atmosphere. Computational Methods. All density functional theory (DFT) calculations were performed in Materials Studio (CASTEP program package44) with projector augmented waves (PAW) pseudopotentials45 to describe the ion−electron interaction and the functional of Perdew, Burke, and Ernzerhof (PBE)46 to describe the exchangecorrelation functional. The convergence criterion of optimal geometry was performed by using the conjugated gradient method for the total energy of our system to converge to within 5.0 × 10−6 eV atom−1, kinetic energy cutoff of 330 eV for the plane-wave basis set, 0.01 eV Å−1 for the maximum final force, and 5.0 × 10−4 Å for the maximum final displacement. Considering the calculated accuracy and efficiency, the vacuum layers between the repeat slabs are both 15 Å to avoid interactions between different slabs. We modeled the SnS2 (001) and SnO2 (001) surfaces by a 2 × 2 unit cell along with the slab. One Li2S4 molecule was adsorbed on only one side of the exposed surfaces. The binding energy (Eb) was computed to measure the binding effect between SnS2 or SnO2 and Li2S4. It is defined as the energy difference between the adsorbed system (Etotal = ESnS2+Li2S4 or Etotal = ESnO2+Li2S4) and the summation of pure SnS2 or SnO2, and pure Li2S4 could be expressed as Eb= ESnS2 (or ESnO2) + ELi2S4 − Etotal. Electrochemical Measurements. The electrochemical properties of SnS2/S/C−Li, SnO2/S/C−Li half-cells, and SnS2/S/C−Ge/C full cell were evaluated with CR2016-type coin cells. To prepare the cathodes, SnS2/S/C or SnO2/S/C composites were first mixed with carbon black and PVDF binder with a weight ratio of 80:10:10. The mixture was then spread uniformly on the carbon-coated aluminum foil and heated at 60 °C for 15 h under vacuum. The mass loading of the electrode is controlled at ∼1.5 mg cm−2 (based on the mass of sulfur in the composites) unless otherwise stated. For assembling the full cell, the sulfur loading was slightly less with the consideration of avoiding diffusion resistance. 2016-type coin cells were fabricated in an argon-filled glovebox using lithium foil as the anode and lithium bistrifluoromethanesulfonylimide LiTFSI (1 M in 1:1 v/v DOL/DME) containing LiNO3 (2 wt %) as the electrolyte. The electrolyte used for the pre-cell is approximately 40 μL. For assembling full cells, Ge/C electrodes43 were first prelithiated in the Ge/C−Li half cells through discharge to 0.05 V at 0.2 A g−1 for several cycles using LiTFSI in DOL/DME with 2 wt % LiNO3 addition as the electrolyte and lithium

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b06565. TEM image and pore size distributions of the asprepared porous carbon, EIS plots of SnS2/S/C and SnO2/S/C electrodes, long cycling stability of the SnS2/ S/C electrode, and cycling performance of the Ge/C electrode in LiTFSI electrolyte (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions †

X.N.L. and Y.L. contributed equally to this work.

Author Contributions

Y.C.Z. and Y.T.Q. supervised the project. X.N.L. and Y.L. conceived the idea and wrote the manuscript. X.N.L., Y.L., J.W.L., Z.G.H., and W.Q.Z. carried out the examples, characterization, and data analysis. All of the authors discussed the results, commented, and revised the manuscript. Funding

This work was financially supported by the National Natural Science Fund of China (Nos. 91022033 and 21201158) and the China Postdoctoral Science Foundation (Grants 2015M580548 and 2015M582083). Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Bresser, D.; Passerini, S.; Scrosati, B. Recent Progress and Remaining Challenges in Sulfur-Based Lithium Secondary Batteries-A Review. Chem. Commun. 2013, 49, 10545. (2) Manthiram, A.; Fu, Y.; Su, Y. Challenges and Prospects of Lithium-Sulfur Batteries. Acc. Chem. Res. 2013, 46, 1125. (3) Yin, Y.; Xin, S.; Guo, Y.; Wan, L. Lithium-Sulfur Batteries: Electrochemistry, Materials, and Prospects. Angew. Chem., Int. Ed. 2013, 52, 13186. (4) Barghamadi, M.; Kapoor, A.; Wen, C. A Review on Li-S Batteries as a High Efficiency Rechargeable Lithium Battery. J. Electrochem. Soc. 2013, 160, A1256. (5) Li, G.; Sun, J.; Hou, W.; Jiang, S.; Huang, Y.; Geng, J. ThreeDimensional Porous Carbon Composites Containing High Sulfur Nanoparticle Content for High-Performance Lithium-Sulfur Batteries. Nat. Commun. 2016, 7, 10601. (6) Pope, M. A.; Aksay, I. A. Structural Design of Cathodes for Li-S Batteries. Adv. Energy Mater. 2015, 5, 1500124. (7) Peng, H. J.; Zhang, Q. Designing Host Materials for Sulfur Cathodes: From Physical Confinement to Surface Chemistry. Angew. Chem., Int. Ed. 2015, 54, 11018. (8) Zeng, L.; Jiang, Y.; Xu, J.; Wang, M.; Li, W.; Yu, Y. Flexible Copper-Stabilized Sulfur-Carbon Nanofibers with Excellent Electrochemical Performance for Li−S Batteries. Nanoscale 2015, 7, 10940. (9) Niu, X.; Wang, X.; Wang, D.; Li, Y.; Zhang, Y.; Zhang, Y.; Yang, T.; Yu, T.; Tu, J. Metal Hydroxide - A New Stabilizer for the

19556


Research Article


(29) Ji, X.; Evers, S.; Black, R.; Nazar, L. F. Stabilizing LithiumSulphur Cathodes Using Polysulphide Reservoirs. Nat. Commun. 2011, 2, 325. (30) Seh, Z. W.; Li, W.; Cha, J. J.; Zheng, G.; Yang, Y.; McDowell, M. T.; Hsu, P. C.; Cui, Y. Sulphur-TiO2 Yolk-Shell Nanoarchitecture with Internal Void Space for Long-Cycle Lithium-Sulphur Batteries. Nat. Commun. 2013, 4, 1331. (31) Garsuch, A.; Herzog, S.; Montag, L.; Krebs, A.; Leitner, K. Performance of Blended TiS2/Sulfur/Carbon Cathodes in LithiumSulfur Cells. ECS Electrochem. Lett. 2012, 1, A24. (32) Seh, Z. W.; Yu, J. H.; Li, W.; Hsu, P. C.; Wang, H.; Sun, Y.; Yao, H.; Zhang, Q.; Cui, Y. Two-Dimensional Layered Transition Metal Disulphides for Effective Encapsulation of High-Capacity Lithium Sulphide Cathodes. Nat. Commun. 2014, 5, 5017. (33) Pang, Q.; Kundu, D.; Nazar, L. F. A Graphene-Like Metallic Cathode Host for Long-Life and High-Loading Lithium-Sulfur Batteries. Mater. Horiz. 2016, 3, 130−136. (34) Li, X.; Chu, L.; Wang, Y.; Pan, L. Anchoring Function for Polysulfide Ions of Ultrasmall SnS2 in Hollow Carbon Nanospheres for High Performance Lithium-Sulfur Batteries. Mater. Sci. Eng., B 2016, 205, 46. (35) Li, X.; Liang, J.; Hou, Z.; Zhang, W.; Wang, Y.; Zhu, Y.; Qian, Y. A New Salt-Baked Approach for Confining Selenium in Metal Complex-Derived Porous Carbon with Superior Lithium Storage Properties. Adv. Funct. Mater. 2015, 25, 5229. (36) Li, X.; Zhu, Y.; Zhang, X.; Liang, J.; Qian, Y. MnO@1-D Carbon Composites from the Precursor C4H4MnO6 and their High-Performance in Lithium Batteries. RSC Adv. 2013, 3, 10001. (37) Li, X.; Zhu, X.; Zhu, Y.; Yuan, Z.; Si, L.; Qian, Y. Porous Nitrogen-Doped Carbon Vegetable-Sponges with Enhanced Lithium Storage Performance. Carbon 2014, 69, 515. (38) Wang, Y.; Alsmeyer, D. C.; McCreery, R. L. Raman Spectroscopy of Carbon Materials: Structural Basis of Observed Spectra. Chem. Mater. 1990, 2, 557. (39) Zhang, Q.; Wang, Y.; Seh, Z. W.; Fu, Z.; Zhang, R.; Cui, Y. Understanding the Anchoring Effect of Two-Dimensional Layered Materials for Lithium-Sulfur Batteries. Nano Lett. 2015, 15, 3780. (40) Yu, M.; Ma, J.; Song, H.; Wang, A.; Tian, F.; Wang, Y.; Qiu, H.; Wang, R. Atomic Layer Deposited TiO2 on a Nitrogen-Doped Graphene/Sulfur Electrode for High Performance Lithium-Sulfur Batteries. Energy Environ. Sci. 2016, 9, 1495. (41) Fang, X.; Peng, H. A Revolution in Electrodes: Recent Progress in Rechargeable Lithium-Sulfur Batteries. Small 2015, 11, 1488. (42) Cao, R.; Xu, W.; Lv, D.; Xiao, J.; Zhang, J. G. Anodes for Rechargeable Lithium-Sulfur Batteries. Adv. Energy Mater. 2015, 5, 1402273. (43) Li, X.; Liang, J.; Hou, Z.; Zhu, Y.; Wang, Y.; Qian, Y. A Synchronous Approach for Facile Production of Ge−Carbon Hybrid Nanoparticles for High-Performance Lithium Batteries. Chem. Commun. 2015, 51, 3882. (44) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. I.; Refson, K.; Payne, M. C. First Principles Methods Using CASTEP. Z. Kristallogr. - Cryst. Mater. 2005, 220, 567. (45) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953. (46) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865.

Construction of Sulfur/Carbon Composites as High Performance Cathode Materials for Lithium−Sulfur Batteries. J. Mater. Chem. A 2015, 3, 17106. (10) Niu, X.; Wang, X.; Xie, D.; Wang, D.; Zhang, Y.; Li, Y.; Yu, T.; Tu, J.-P. Nickel Hydroxide-Modified Sulfur/Carbon Composite as a High-Performance Cathode Material for Lithium Sulfur Battery. ACS Appl. Mater. Interfaces 2015, 7, 16715. (11) Su, Y.; Manthiram, A. A New Approach to Improve Cycle Performance of Rechargeable Lithium-Sulfur Batteries by Inserting a Free-Standing MWCNT Interlayer. Chem. Commun. 2012, 48, 8817. (12) Wang, X.; Wang, Z.; Chen, L. Reduced Graphene Oxide Film as a Shuttle-Inhibiting Interlayer in a Lithium-Sulfur Battery. J. Power Sources 2013, 242, 65. (13) Zhou, G.; Li, L.; Wang, D.; Shan, X.; Pei, S.; Li, F.; Cheng, H. A Flexible Sulfur-Graphene-Polypropylene Separator Integrated Electrode for Advanced Li-S Batteries. Adv. Mater. 2015, 27, 641. (14) Gordin, M. L.; Dai, F.; Chen, S.; Xu, T.; Song, J.; Tang, D.; Azimi, N.; Zhang, Z.; Wang, D. Bis(2,2,2-Trifluoroethyl) Ether as an Electrolyte Co-solvent for Mitigating Self-Discharge in Lithium-Sulfur Batteries. ACS Appl. Mater. Interfaces 2014, 6, 8006. (15) Agostini, M.; Scrosati, B.; Hassoun, J. An Advanced Lithium-Ion Sulfur Battery for High Energy Storage. Adv. Energy Mater. 2015, 5, 1500481. (16) Lin, Z.; Liu, Z.; Fu, W.; Dudney, N. J.; Liang, C. Phosphorous Pentasulfide as a Novel Additive for High-Performance Lithium-Sulfur Batteries. Adv. Funct. Mater. 2013, 23, 1064. (17) Xiong, S.; Kai, X.; Hong, X.; Diao, Y. Effect of LiBOB as Additive on Electrochemical Properties of Lithium-Sulfur Batteries. Ionics 2012, 18, 249. (18) Brückner, J.; Thieme, S.; Bö ttger-Hiller, F.; Bauer, I.; Grossmann, H. T.; Strubel, P.; Althues, H.; Spange, S.; Kaskel, S. Carbon-Based Anodes for Lithium-Sulfur Full Cells with High Cycle Stability. Adv. Funct. Mater. 2014, 24, 1284. (19) Lv, D.; Yan, P.; Shao, Y.; Li, Q.; Ferrara, S.; Pan, H.; Graff, G. L.; Polzin, B.; Wang, C.; Zhang, J. High Performance Li-Ion Sulfur Batteries Enabled by Intercalation Chemistry. Chem. Commun. 2015, 51, 13454. (20) Shen, C.; Ge, M.; Zhang, A.; Fang, X.; Liu, Y.; Rong, J.; Zhou, C. Silicon(Lithiated)-Sulfur Full Cells with Porous Silicon Anode Shielded by Nafion Against Polysulfides to Achieve High Capacity and Energy Density. Nano Energy 2016, 19, 68. (21) Li, G.; Ling, M.; Ye, Y.; Li, Z.; Guo, J.; Yao, Y.; Zhu, J.; Lin, Z.; Zhang, S. Acacia Senegal-Inspired Bifunctional Binder for Longevity of Lithium-Sulfur Batteries. Adv. Energy Mater. 2015, 5, 1500878. (22) Lacey, M. J.; Jeschull, F.; Edström, K.; Brandell, D. Functional, Water-Soluble Binders for Improved Capacity and Stability of LithiumSulfur Batteries. J. Power Sources 2014, 264, 8. (23) Li, G.; Cai, W.; Liu, B.; Li, Z. A Multi Functional Binder with Lithium Ion Conductive Polymer and Polysulfide Absorbents to Improve Cycleability of Lithium-Sulfur Batteries. J. Power Sources 2015, 294, 187. (24) Salem, H.; Babu, G.; Rao, C. V.; Arava, L. M. R. Electrocatalytic Polysulfide-Traps for Controlling Redox Shuttle Process of Li-S Battery. J. Am. Chem. Soc. 2015, 137, 11542. (25) Yuan, Z.; Peng, H. J.; Hou, T. Z.; Huang, J. Q.; Chen, C. M.; Wang, D. W.; Cheng, X. B.; Wei, F.; Zhang, Q. Powering LithiumSulphur Battery Performance by Propelling Polysulphide Redox at Sulphiphilic Hosts. Nano Lett. 2016, 16, 519. (26) Zhang, Y.; Zhao, Y.; Yermukhambetova, A.; Bakenov, Z.; Chen, P. Effect of Mg0.6Ni0.4O on Cyclability of Sulfur Composite Cathode in Lithium-Sulfur Battery. J. Mater. Chem. A 2013, 1, 295. (27) Zhang, Y.; Bakenov, Z.; Zhao, Y.; Konarov, A.; Doan, T.; Sun, K.; Yermukhambetova, A.; Chen, P. Effect of Nanosized Mg0.6Ni0.4O Prepared by Self-Propagating High Temperature Synthesis on Sulfur Cathode Performance in Li/S Batteries. Powder Technol. 2013, 235, 248. (28) Choi, Y.; Jung, B.; Lee, D.; Jeong, J.; Kim, K.; Ahn, H.; Cho, K.; Gu, H. Electrochemical Properties of Sulfur Electrode Containing Nano Al2O3 for Lithium/Sulfur Cell. Phys. Scr. 2007, T129, 62. 19557


C Composites toward High

Recommend Documents