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Enhanced Performance of Lithium Sulfur Batteries with Sulfur Embedded in Sm2O3-Doped Carbon Aerogel as Cathode Material
Accepted Manuscript Title: Enhanced Performance of Lithium Sulfur Batteries with Sulfur Embedded in Sm2 O3 -Doped Carbon Aerogel as Cathode Material Authors: Xueliang Li, Zhongqiang Ding, Luyao Zhang, Ruwen Tang, Yang He PII: DOI: Reference:
S0013-4686(17)30939-8 http://dx.doi.org/doi:10.1016/j.electacta.2017.04.156 EA 29414
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
13-3-2017 26-4-2017 28-4-2017
Please cite this article as: Xueliang Li, Zhongqiang Ding, Luyao Zhang, Ruwen Tang, Yang He, Enhanced Performance of Lithium Sulfur Batteries with Sulfur Embedded in Sm2O3-Doped Carbon Aerogel as Cathode Material, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.04.156 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced Performance of Lithium Sulfur Batteries with Sulfur Embedded in Sm2O3-Doped Carbon Aerogel as Cathode Material Xueliang Li a,b *, Zhongqiang Dinga,b , Luyao Zhanga,b, Ruwen Tang a,b, Yang He a,b
a
School of Chemistry and Chemical Engineering, Hefei University of Technology,
Hefei 230009, PR China b
Anhui Key Laboratory of Controllable Chemical Reaction and Material Chemical
Engineering, Hefei 230009, PR China. *Corresponding author. Tel./Fax: +86-551-62901450. E-mail address: [email protected]
Abstract: A Sm2O3 decorated carbon aerogel (CA-Sm) was successfully synthesized through a sol-gel from resorcinol, phloroglucinol (P), formaldehyde and catalyst of oxalic acid and doping compound with Sm2O3 strategies. The small amount of active reactant phloroglucinol promotes synthesis of carbon aerogel (CA) and formation of uniform and porous carbon spheres. The CA-Sm composites present an interlinked porous structure and high specific surface area. The CA-Sm with sulfur loading (CA/S-Sm) from the composite via molten sulfur technique exhibits an enhanced performance with long-term cycling stability and high rate capacity. In particular, the cathode of CA/S-Sm-2 with Sm/C molar ratio of 2/400 achieved a high initial discharge capacity of 1347 and 976 mAh g-1 at 0.2 C and 2 C, respectively. And the cathode exhibited a nice long-term cycling stability with a high initial discharge capacity of 1175 mAh g-1 and maintained 861 mAh g-1 after 300 cycles at 0.5 C. The excellent electrochemical performance of the battery is attributed to interrelated nanospheres, porous carbon structure, large specific surface area of Sm2O3-doped CA composites and improved electronic and ionic transport at the cathode.
Keywords: Phloroglucinol; Sm2O3 nanoparticles; Adsorption capability; Lithium sulfur battery
1. Introduction Developing green and effective energy storage system is still a formidable challenge, especially for electrical devices and hybrid electric vehicles, and will be a major focus of endeavor for the near future. Therefore, new materials with high energy density and long cycle life have been extensively studied. Among various energy storage technologies, the lithium sulfur battery is one of the most promising candidates for the next generation of rechargeable batteries due to its high theoretical capacity of 1675 mAh g-1 and high energy density of 2600 Wh kg-1 which is much higher than that of conventional Li-ion batteries [1, 2]. Meanwhile, element sulfur gives the superiority of low cost, natural abundance and environmental friendliness [3-5] and the lithium sulfur battery has attracted considerable attention. Despite of these advantages, there are still some problems restricting the practical application of Li-S battery. And it suffers from issues including the low active material utilization, the rapid capacity degradation, the poor cyclability for insulating nature of sulfur and “shuttle effect” from polysulfide dissolution, diffusion [6-8]. Different strategies have been proposed to address these issues, including electrolyte composition optimization, enhancing the electrical conductivity of active materials and the fabrication of carbon/sulfur composites [9-11]. A lot of research works have been made on porous carbon material including carbon nanotubes [12-14], graphenes/graphene oxide [15, 16], hollow carbon nanospheres [17-19], conducting polymers [20-22] to improve electrochemical performance. Meanwhile, separators， electrolytes and lithium anodes play an important in lithium sulfur batteries [23-25].
Recently, doped metal carbon materials have been proven to be effective and facile candidates in restraining the diffusion of lithium polysulfides and provide new opportunity to improve the performance and cycling life of the Li-S batteries. Some doped metal carbon/sulfur composites including metal oxides [26-29], metal nitrides [30, 31] and metal sulfides [32, 33] have been proposed as cathode materials to enhance the cycling stability and rate capability. These researches show that the metal compounds in the cathode present an important effect on Li-S batteries. However, for improving the electrochemical performances, it needs to further explore new materials to extend to other types of metal and to effectively control the porous structure of materials. In this work, the composites of CA/S-Sm with unique structure were obtained via sol-gel approaches and sulfur melt-diffusion as cathode material of lithium sulfur battery. Sm2O3 nanoparticle plays a crucial role in effectively adsorbing polysulfides and ameliorating electrochemical performances. The schematic diagram of the synthesis and fabrication procedure for the CA/S-Sm material is clearly displayed in Scheme 1. The porous CA nanospheres with Sm2O3 were synthesized by sol-gel process, carbonation and sulfur melt-diffusion. And the carbon sulfur electrode with Sm2O3 decoration can effectively adsorb the soluble lithium polysulfides during discharge process. The CA/S-Sm composites showed much better electrochemical performance than the CA0/S and CA/S composites. Moreover, the CA/S-Sm-2 composite delivers the initial discharge capacity of 1347 mAh g−1 and maintains 1090 mAh g−1 after 100 cycles at 0.2 C.
2. Experimental section 2.1. Materials Preparation Phloroglucinol (P) and Sm(NO3)3•6H2O were purchased from Aladdin Chemistry Co., Ltd. (China) and the other reagents were obtained from Sinopharm Chemical Reagent Co., Ltd. All the chemicals were used as an analytical reagent grade and without further purification. 2.2. Synthesis of carbon aerogels with Sm2O3 Carbon aerogels composites with Sm2O3 were synthesized via a sol-gel and carbonization
methods
by
employing
resorcinol
(R),
phloroglucinol
(P),
Sm(NO3)3•6H2O and formaldehyde (F) as the carbon precursor. In a typical experiment, 2.2000 g R, 0.0252 g P, 0.0182 g cetyl trimethyl ammonium bromide (CTAB), 0.0126 g oxalic acid (OA) and 0.1793g Sm(NO3)3•6H2O were dissolved in distilled water of 180 mL with continue stirring for 1h, where the molar ratios of R/P, R/CTAB, R/OA, R/H2O and Sm/C were 100/1, 400/1, 200/1, 1/500 and 1/400, respectively. This solution was heated with vigorous stirring at 85 °C and 5.0 mL F was dissolved in the above solution. After becoming milky white, the solution was poured into a beaker and the obtained hydrogels were further dried in an oven at 85 °C for 36 h and formed red brown intermediate. The as-prepared product was completely carbonized at 850 °C for 2 h with a heating rate of 5 °C min-1 in N2 atmosphere and then CA-Sm composite was obtained. The samples with Sm/C molar ratios of 1/400, 2/400 and 3/400 are denoted as CA-Sm-1, CA-Sm-2 and CA-Sm-3, respectively. Meanwhile, for further comparison other carbon aerogels were prepared
without addition of metal. And samples with phloroglucinol promotion and no promotion were denoted as CA and CA0, respectively. 2.3. Preparation of CA/S-Sm composites CA/S-Sm composites were prepared through a melt-diffusion method. Typically, the CA-Sm and S at a precise mass ratio of 4:6 were put into agate mortar and grind 45 min to form a uniform gray mixture. Then the mixture was put into sealed teflon container in autoclave and then transferred into an oven with keeping 155 °C for 18 h. Finally, the composites of CA/S-Sm-1, CA/S-Sm-2 and CA/S-Sm-3 were obtained. The other sulfur-carbon composites were synthesized for comparison and were respectively labeled as CA0/S, CA/S. 2.4 Material characterization The crystalline phase structures were examined by the X-ray powder diffraction (XRD, X'Pert PRO MPD) using a Cu-Kα radiation in the scan range 2θ = 10-90°. The morphology and structure of the materials were characterized by field emission scanning electron microscopy (FESEM, SU8020) and field emission transmission electron microscopy (FETEM, JEM-2100F). The electronic structures of elements were analyzed by a high resolution X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi). The sulfur contents in the composites were calculated by thermogravimetric analysis (TGA, STA449F3). The nitrogen adsorption/desorption isotherms were obtained by using a Brunauer-Emmett-Teller (BET) strategy and the pore size distributions of the materials were analyzed by the Barrett-Joyner-Halenda (BJH) model.
2.5 Adsorption capacity test The adsorption ability of the CA0, CA and CA-Sm-n (n=1, 2, 3) composites to lithium polysulfide (Li2S6) was tested via UV-vis spectroscopy (CARY 5000) at 415 nm [34, 35]. The Li2S6 (2.5mM) solution was prepared from the sublimed sulfur (S) and lithium sulfide (Li2S), where S and Li2S were dissolved in tetrahydrofuran (THF) solution with the molar ratio of 5:1. The synthesis was carried out with slow stirring for two days at room temperature in an argon-filled glove box. To effectively measure adsorption ability, the samples of CA0, CA and CA-Sm-n were dried at 110 °C for 8 h in a vacuum oven. Typically, 50.0 mg sample was put into the Li2S6 solution and stirred for 30 min in an argon-filled glove box. After 12 h adsorption, the solution was separated through a syringe and diluted by a factor of 10 to enable quantitative concentration measurements [36]. The fraction of residual lithium polysulfide in the solution was calculated as follows: fpolysulfides = A415nm (after adsorption)/A415nm (before adsorption). 2.6 Electrochemical Measurements The CA/S-Sm material, acetylene black and polyvinylidene fluoride (PVDF) binder were mixed in a mass ratio of 80:10:10 with N-methylpyrrolidone (NMP) as a dispersant to form a homogeneous slurry. Then, the slurry was evenly coated on an aluminum foil and dried in a vacuum oven at 60 °C for 12 hours. The foils and Celgard 2400 polypropylene (PP) films, acting as cathode and the separators, were punched into disks with a diameter of 10 mm and 12 mm, respectively. Button cells were assembled with lithium foil as anode in an argon-filled glovebox. The electrolyte
solution was prepared by 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (1:1 by volume) with 1 M lithium bis (trifluoromethane) sulfonamide (LiTFSI) and 0.1 M LiNO3. The mass of active materials with sulfur on the electrode was about 1.5-1.8 mg cm-2. Cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) were measured with a CHI 660B electrochemical workstation. CV was conducted at in the potential range of 1.8-3.0 V (Li+/ Li) at a scan rate of 0.1 mV s−1 and EIS was carried out at the frequency range of 0.10 Hz to 100 kHz with the AC perturbation amplitude of 5.0 mV. The galvanostatic charge-discharge test was performed on a Land CT2001A system (Wuhan Jinnuo Electronics Co., Ltd. China) in the voltage range of 1.8-3.0 V. 3. Results and discussion 3.1 XRD phase analysis The X-ray diffraction (XRD) patterns of CA0, CA, CA-Sm-n (n=1, 2, 3), sublimed sulfur and CA/S-Sm-n composites are shown in Fig. 1. The XRD patterns of CA0 and CA only show two broad peaks appeared at about 22°and 44°, respectively (Fig. 1a), indicating the amorphous structure [37, 38] of CA0 and CA composites. The diffraction patterns of CA-Sm-n composites are shown in Figure 1a and the characteristic peaks at 2θ values of 28.3°, 32.8°, 47.0° and 55.8°can be assigned to the facets of (222), (400), (440) and (622) of Sm2O3 nanoparticle (JCPDS No. 15-0813) [39]. The metal Sm has been successfully doped into the CA and with increasing molar ratio of Sm/C, the intensities of diffraction peaks are also increased for the
CA-Sm-n samples. As shown in Fig. 1b, the sharper peaks centered at 2θ = 23.1°and 27.8° correspond to the facets of (222) and (040) of sulfur with the Fddd orthorhombic phase (JCPSD No.08-0247) [40]. The CA/S-Sm-n composites exhibit a similar XRD patterns to sulfur, suggesting a well-defined crystal structure of the chemically deposited sulfur. The peak intensities of orthorhombic sulfur obvious decreased with increasing molar ratio of Sm/C and these results show that the sublimed sulfur has been successfully loaded on CA-Sm-n composites. 3.2 SEM and TEM morphology analysis The surface morphologies of CA0, CA, CA-Sm-n (n=1, 2, 3) composites are shown in Fig. 2, it can be found from Fig. 2a that the CA0 is made up of carbon spheres with smooth surfaces and the diameter of about 300 nm. The morphology of CA in Fig. 2b exhibits uniform structure with the diameter of about 200 nm. The results show that addition of phloroglucinol effective adjusted the size and promoted carbon spheres crosslinking. The SEM images in Fig. 2c-e show the morphologies of CA-Sm-n composites with sizes about 220-350nm. In spite of having some similarity with CA, these samples present evident feature of abundant porous on surface and leads to important role of both enlarging the specific surface area and effectively adsorbing polysulfides. The porous carbon morphology can provide favorable structure for improving the electrochemical performance of Li-S battery. For determining the presence and distribution of the metal and other elements, the CA-Sm-n composites were further investigated by TEM. The TEM images in Fig. 3a-c show a lot of black spots on the surface suggesting that nanoparticles were well
embedded in CA-Sm-n composites. Fig. 3e presents the elemental mappings of the carbon, oxygen and samarium of CA-Sm-2 composite. The EDS mapping images show that elemental Sm and O are uniformly distribution in the carbon matrix and the molar ratio of O and Sm is nearly consistent with Sm2O3. The samarium oxide nanoparticles are ascribed to Sm2O3 from previous XRD analysis. Actually, many Sm2O3 nanoparticles distribute into the internal surfaces of the CA for their growing up in the interior of samples. Fig. 3d clearly shows the HRTEM image of the CA-Sm-2 composite and the nanoparticles present proper sizes of about 6.3 nm. The lattice fringe with spacing of approximately 0.320 nm corresponds to the (222) plane of cubic crystal Sm2O3. 3.3 XPS measurements To further explore the surface chemical composition and chemical bonding state, the CA0, CA, CA-Sm-n (n=1, 2, 3) and CA/S-Sm-n composites have been analyzed by XPS (Fig. 4a-g). As shown in Fig. 4a, it can be found that the CA0 and CA samples show two peaks with C 1s and O 1s. The XPS survey spectra of CA-Sm-n composites exhibit five peaks of C 1s, O 1s Sm 4d5/2, Sm 3d5/2 and Sm 3d3/2 centering at about 284.7 eV, 532.7 eV, 134.1 eV, 1085.1 eV and 1111.3 eV, respectively. Fig. 4b depicts the XPS survey spectra of CA/S-Sm-n (n=1, 2, 3) composites. Besides the former five peaks, other ones corresponding to S 2p and S 2s appeared in the spectra at 164.2 eV and 228.1 eV. Fig. 4c displays the C 1s spectra of the CA/S-Sm-2 composite and the binding energies located at 284.4, 285.1 and 286.0 eV are attributed to sp2 hybridized carbon atoms (sp2 C-C), sp3 hybridized carbon atoms (sp3 C-C) and carbon−sulfur
chains (C-S), respectively [41, 42]. Fig. 4d exhibits the S 2p spectra of the CA/S-Sm-2 composite and two typical component peaks at 163.9 and 165.1 eV are corresponding to S 2p3/2 and S 2p1/2, well matching with the XPS characteristic peak of sublimed sulfur [43]. As shown in Fig. 4e, the S 2p peaks in CA/S-Sm-2 present a small shift of 0.25eV to lower binding energy than that in sulfur for CA/S and are assigned to weaker S−S bond in samarium sulfur chain [44] and the result may benefit for absorbing polysulfides to further enhance electrochemical performance. In the XPS spectra of O 1s in Fig. 4f, the binding energies centered at 531.3, 532.7 and 534.2 eV are ascribed to the oxygen of Sm2+-O groups, Sm3+-O groups and OH groups in the crystal lattice of Sm2O3 nanoparticle, respectively [45]. The XPS spectra of Sm 3d for the CA/S-Sm-2 composite are shown in Fig. 4g. Two obvious peaks at 1085.6 and 1112.8 eV are assigned to Sm 3d5/2 and Sm 3d3/2, respectively. The binding energy peaks indicate Sm presented in oxidation state of Sm3+ and Sm2O3 dopant has been successfully modified on the surface of CA/S composite. 3.4 TGA analysis To ascertain the sulfur contents in samples, the thermal decomposition behavior of CA0/S, CA/S, and CA/S-Sm-n (n=1, 2, 3) composites were investigated under Ar atmosphere with heating rate of 10 °C min−1 by TGA. Meanwhile, TGA was carried out to acquire the Sm2O3 contents in CA-Sm-n composites in air atmosphere with heating rate of 10 °C min−1. Before testing, the samples were dried in a vacuum oven at 110 °C for 12 hours. The TGA curves of CA-Sm-n samples are shown in Fig. S1 in supporting information. The results indicate that the Sm2O3 contents of CA-Sm-1,
CA-Sm-2 and CA-Sm-3 are 3.57%, 6.81% and 9.88%, respectively. And the TGA results are in accord with contents of theoretical calculation for the samples of CA-Sm-1, CA-Sm-2 and CA-Sm-3. The TGA curves of CA0/S, CA/S, and CA/S-Sm-n composites are shown in Fig. 4h, and the thermal treatment temperature range from room temperature to 600 °C. The weight loss of sulfur in CA0/S, CA/S and CA/S-Sm-n composites show two-step processes. The first sulfur-loss occurs in the temperature range of 170-310 °C for the loss of sulfur in the macropores and exterior surface of samples. The second weight loses of sulfur mainly take place in temperature range of 310–500 °C, being attributed to sulfur releases in the mesoporous of samples. We can find from Fig. 4h that it needs higher temperature for complete releasing sulfur in the CA/S-Sm-n composites than in the CA0/S and CA/S. The result indicates that the CA/S-Sm-n composites are more thermodynamical stable than the CA0/S and CA/S composites for the influence of the bonding between S and Sm2O3. The TGA results show that the sulfur content of CA0/S, CA/S, CA/S-Sm-1, CA/S-Sm-2 and CA/S-Sm-3 are 57.37%, 56.81%, 55.54%, 55.02% and 54.43%, respectively. 3.5 N2 adsorption-desorption analysis Fig. 5 displays the nitrogen adsorption-desorption isotherms and pore size distribution curves of CA0, CA and CA-Sm-n (n=1, 2, 3) composites. As shown in Fig. 5a, all isotherms of the as-prepared samples exhibit a type IV isotherm with a hysteresis loop according to the IUPAC classification and indicate typical mesoporous structure for the samples [46, 47]. Fig. 5b illustrates the BJH pore size distribution of
these samples and the pores mainly range from 3nm to 6 nm, showing carbon spheres can provide the appropriate pores for cathode. The BET results showed abundant pores with high surface area in a narrow pore size distribution, and the BET surface areas of CA0, CA, CA-Sm-1, CA-Sm-2 and CA-Sm-3 composites are 793, 778, 754, 738 and 711 m2 g-1, respectively. And more information about porosity characteristic can be clearly seen in Table 1. The samples of CA0, CA and CA-Sm-n present large specific surface area and can effectively increase adsorption capacity to polysulfides. Such pores in samples not only provide more room to transport of electronics but also accommodate available volumetric expansion of sulfur in the charge/discharge process. Despite the specific area of CA-Sm-n composites have somewhat decreased, they may provide effective adsorption to polysulfides for the special role of Sm 2O3 being confirmed by subsequent tests and the CA-Sm-n composites with large specific areas can effectively suppress the production of polysulfides. 3.6 Adsorption capability The adsorption capability of the CA0, CA and CA-Sm-n (n=1, 2, 3) composites on polysulfides obtained by UV-vis spectroscopy further confirm the samples immobilizing role to high order polysulfides. Fig. 6 shows the photograph of adsorbing mixture with color changes and the colors gradually become shallow from left to right. It can be found that the color of the polysulfide solution fades greatly with the increase molar ratio of Sm/C. Moreover, the color for the CA-Sm-3 sample gained much lighter corresponding to the nice adsorption capacity on high order polysulfide due to the Sm2O3 modification. Fig. S2 shows the UV-vis curves of the
samples and all samples appeared characteristic absorption peaks at 415nm. Fig. 6 also depicts the contents of residual lithium polysulfide after adsorption and the samples of CA0, CA, CA-Sm-1, CA-Sm-2 and CA-Sm-3 present remaining ratio of 41%, 33%, 20%, 13% and 9% of polysulfides, respectively. The results exhibit excellent adsorption capacity of as-prepared samples and effectively trap soluble polysulfides especially for CA-Sm-3 composites. 3.7 Electrochemical measurements To evaluate the electrochemical performances of the samples, the coin cells assembled with the CA0/S, CA/S and CA/S-Sm-n (n=1, 2, 3) composites as cathode materials and a lithium foil as anode material were further tested by cyclic voltammetry (CV), galvanostatic charge/discharge measurements and electrochemical impedance spectroscopy (EIS). Fig. 7a depicts the CV curves of the CA0/S, CA/S and CA/S-Sm-n composites in the potential range of 1.8–3.0 V at a scanning rate of 0.1 mV s-1 during the second cycle. It can be observed that there are two reduction peaks and one oxidation peak for the CA0/S and CA/S samples, however there are two oxidation peaks and two reduction peaks for CA/S-Sm-n composites. In the cathodic scan, the two reduction peaks are about centered at the potentials of 2.29-2.31 and 2.00-2.01 V for the CA0/S and CA/S electrode. It shows a higher potential of reduction peaks at 2.31-2.33 V and 2.03-2.05 V for the CA/S-Sm-n electrode. Based on the reduction reaction mechanism of sulfur, the right sulfur peaks of the potential in the range of 2.29-2.33 V relate to the reduction of S8 molecule to higher-valence lithium polysulfides (Li2Sx, 4≤x < 8), and the left reduction peak of the potential in
the range of 2.00-2.05 V corresponds to the reduction of higher-valence and soluble lithium polysulfides to lower-valence and insoluble Li2S2/Li2S [48, 49]. During the anodic scan process, there is only one strong oxidation peak can be seen at 2.42-2.45V for CA0/S and CA/S electrode, which is attributed to the transformation of Li2S2 and Li2S to Li2S8 [50, 51]. Two oxidation peaks of CA/S-Sm-n electrode were observed at 2.33-2.38V corresponding to the reversible conversion between low-order polysulfides and high-order polysulfides. It is worth noting that the CA/S-Sm-n electrodes exhibit higher potential of reduction peaks and lower oxidation peaks compared with CA0/S and CA/S electrodes. The CV results demonstrated that the CA/S composites with Sm2O3 modification can help reduce electrochemical polarization and ameliorate electrochemical reversibility. Fig. 7b presents the initial charge/discharge curves of the Li-S batteries with CA0/S, CA/S and CA/S-Sm-n cathodes at a rate of 0.2 C. These curves obviously show two discharge plateaus and one charge plateau, corresponding to the CV results of the samples. The initial discharge capacities of CA0/S, CA/S, CA/S-Sm-1, CA/S-Sm-2 and CA/S-Sm-3 cathodes are 1070, 1118, 1309, 1347 and 1238 mAh g-1, respectively. Compared with CA0/S electrode, the CA/S electrode displays higher initial discharge capacity. Furthermore, the CA/S-Sm-n electrodes show much higher capacities than those of CA0/S and CA/S composites. Especially, the CA/S-Sm-2 electrode exhibits the highest initial discharge capacity of 1347 mAh g-1. This result demonstrates that the Sm2O3-doping CA/S electrodes improve the electrochemical performance.
For further investigate the storage capacity and cycling stability of these samples, the cycling performances of the CA0/S, CA/S, and CA/S-Sm-n composites were measured at a low rate of 0.2 C. As shown in Fig. 7c, the discharge capacity of CA0/S electrode delivers 725 mAh g-1 after 100 cycles and it retains about 67.8% of initial capacity. Compared with CA0/S electrode, the CA/S electrode delivers a higher discharge capacity of 816 mAh g-1 after 100 cycles and it retains about 73.0% of initial capacity at 0.2 C. The CA/S-Sm-n electrodes show better cycling performances compared with CA0/S and CA/S electrodes. In particular, the CA/S-Sm-2 electrode exhibits the best discharge capacity of 1090 mAh g-1 after 100 cycles and it retains about 80.9% of initial capacity. The results show that the CA/S-Sm-n electrodes can further help to improve the cycling performances for Li-S battery. Considering the fact of the better adsorbing ability of CA-Sm-n on polysulfides, the improvement of discharge capacity of CA/S-Sm-n can be attributed to restraining polysulfides dissolution. Fig. 7d displays the rate performances of CA0/S, CA/S and CA/S-Sm-n composites at varying rates at 0.2, 0.5, 1 and 2 C. We can see from Fig. 7d that the CA/S-Sm-n electrodes show better rate performances than CA0/S and CA/S ones for different rates. The initial discharge capacities of CA0/S cathode at 0.2, 0.5, 1 and 2 C current rates are 1070, 916, 808 and 695 mAh g-1, respectively. The further results show that the electrodes doped with Sm2O3 can effectively improve the rate capability. Among all cathodes, the CA/S-Sm-2 composite exhibits the best rate capability. The CA/S-Sm-2 cathode at 0.2, 0.5, 1 and 2 C current rates deliver initial discharge
capacities of 1347, 1223, 1119 and 976 mAh g-1, respectively. Moreover, the CA/S-Sm-2 electrode delivers a high recovered capacity of 1231 mAh g-1 with the current rate backing to the 0.2 C. These results further reveal the electrodes modified by Sm2O3 present nice redox stability and can effectively inhibit the “shuttle effect”. Fig. 8 further shows electrochemical performance of CA0/S, CA/S and CA/S-Sm-2 composite for long-term cycle. Fig. 8a show the charge/discharge curves of CA/S-Sm-2 composite and shows a high discharge capacity at 2 C. In spite of capacity decreasing with repeating charge-discharge cycle, it retains discharge capacity of 678 mAh g-1 after 100 cycles. Fig. 8b shows the cycle performance of the CA0/S, CA/S and CA/S-Sm-2 cathodes at 1 C. The CA0/S and CA/S cathodes display initial discharge capacity of 872 and 808 mAh g-1, and remained 577 and 494 mAh g-1 after 100 cycles, respectively. It can be clearly found that the CA/S-Sm-2 cathode shows higher discharge capacity than cathodes of CA0/S and CA/S, and it still remain 825 mAh g-1 after 100 cycles. Fig. 8c displays the long-term cycling curves of the CA0/S and CA/S-Sm-2 cathodes at 0.5 C for 300 cycles. As shown in Fig. 8c, the discharge capacities of samples gradually decrease with increasing cycle test and the CA0/S cathode keeps 509 mAh g-1. But for the CA/S-Sm-2 cathode, it exhibits a nice cycling stability and still maintains 889 mAh g-1 after 300 cycles with a capacity decay of 0.091% per cycle. These results confirm crucial role of Sm2O3 modification on carbon-sulfur cathodes with enhancing the electric conductivity, restraining the dissolution of the polysulfides, increasing sulfur utilization. Fig. 9 shows the EIS profiles of CA0/S, CA/S and CA/S-Sm-n (n=1, 2, 3)
electrodes. For effectively analyzing EIS data, the equivalent circuit was used to be a model of the Li-S battery, where the Rs, Rct and W represent the resistance of electrolyte solution, charge-transfer resistance and the Warburg impedance, respectively. The CPE is related to the constant phase element of double layer capacitance. As shown in Fig. 9, all the EIS curves of the CA0/S, CA/S and CA/S-Sm-n composites before cycling consist of a semicircle and an oblique line. The semicircle at high-frequency region and the oblique line at low-frequency region relate to the charge transfer resistance (Rct) and the Warburg impedance (W), respectively. After EIS data is being fitted with the equivalent circuit, the impedance parameters of the CA0/S, CA/S and CA/S-Sm-n composites were shown in Table 2. Typically, CA/S-Sm-2 electrode presents values of Rs and Rct with 4.4 and 11.6 Ω, but the CA0/S electrode with 6.8 and 22.5 Ω. Compared with the CA0/S and CA/S electrodes, the CA/S-Sm-n electrodes show decreased value of Rs and Rct. In order to highlight the effect of Sm2O3, we also have tested the impedance of CA0/S, CA/S and CA/S-Sm-n samples after 100 cycles. And the EIS curves and impedance parameters of these samples are shown in Fig. S3 and Table S1 (in supporting information.), respectively. The impedance of samples have increased to some degree. However, it is worth mentioning that the CA/S-Sm-2 electrode presents almost unchanged Rs and Rct compared with the values before cycling. The EIS results demonstrate that the Sm2O3 modified carbon-sulfur electrodes can help reduce the surface resistance and further enhance the electrochemical performance for Li-S battery.
4. Conclusions In summary, the novel CA/S-Sm composite materials were successfully prepared by doping metal and loading sulfur in the porous carbon aerogel. The cathode materials modified by Sm2O3 can effectively immobilize the polysulfides and improve the electrochemical performance. The CA/S-Sm composites present a nice cycling stability and rate capability. Moreover, the CA/S-Sm-2 cathode delivers a high initial discharge capacity of 1345 mAh g-1 remaining 1082 mAh g-1 after 100 cycles at 0.2 C, and the CA/S-Sm-2 cathode exhibits the capacity of 971 mAh g-1 at a high rate of 2 C. The excellent electrochemical performance of CA/S-Sm cathode is attributed to the unique interconnected, porous structure and high adsorption ability of CA-Sm composites. This doping strategy provides a facile and efficient method to obtain promising candidate material for Li-S batteries.
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Fig. 1. XRD patterns of (a) CA0, CA, CA-Sm-n; (b) CA/S-Sm-n and sublimed sulfur.
Fig. 2. SEM images of (a) CA0; (b) CA; (c) CA-Sm-1; (d) CA-Sm-2; (e) CA-Sm-3.
Fig. 3. TEM images of (a) CA-Sm-1; (b) CA-Sm-2; (c) CA-Sm-3; (d) HRTEM image of CA-Sm-2; EDS mappings of (e) CA-Sm-2; (f) C,(g) O and (h) Sm.
Fig. 4. XPS survey spectra of (a) CA0, CA and CA-Sm-n composites; (b) CA/S-Sm-n composites; XPS spectra and fitted curves of the CA/S-Sm-2 sample of (c) C 1s; (d)S 2p; (e) S 2p XPS spectra of CA/S and CA/S-Sm-2 composites; XPS spectra of the CA/S-Sm-2 sample (f) O 1s; (g) Sm 3d; (h) TGA curve of CA0/S, CA/S and CA/S-Sm-n composites under Ar atmosphere.
Fig. 5. (a) N2 adsorption–desorption isotherms and (b) BJH pore size distributions of CA0, CA and CA-Sm-n composites.
Fig. 6. The photograph of adsorb polysulfide mixture and the amount of remaining polysulfides for CA0, CA and CA-Sm-n composites.
Fig. 7. (a) Cyclic voltammogram curves of the cell with CA0/S, CA/S and CA/S-Sm-n composites at 0.1 mV s-1 in the potential range of 1.8−3.0 V (Li+/ Li); (b) The initial discharge/charge profiles at a current rate of 0.2 C; (c) Cycling performances at a rate of 0.2 C; and (d) Rate performances at different current densities (0.2-2 C).
Fig. 8. (a) The charge/discharge profiles of the CA/S-Sm-2 composite cathode at 2 C; (b) Cycling performance of the CA0/S, CA/S and CA/S-Sm-2 electrodes at 1 C; (c) Long term performance of the CA0/S and CA/S-Sm-2 electrodes at 0.5 C
Fig. 9. EIS curves of the CA0/S, CA/S and CA/S-Sm-n electrodes before cycling
Scheme 1. Schematic Illustration of the Formation for CA/S-Sm Composite.
Table 1. Structural parameters calculated from nitrogen adsorption isotherms of samples Samples CA0 CA CA-Sm-1 CA-Sm-2 CA-Sm-3
BET Surface Area (m2 g−1) 793 778 754 738 711
Pore Volume (cm3 g−1) 0.92 0.88 0.84 0.82 0.78
Average Pore Diameter (nm) 5.36 5.24 5.22 5.13 5.05
Table 2. Impedance parameters of samples before cycling Samples CA0/S CA/S CA/S-Sm-1 CA/S-Sm-2 CA/S-Sm-3
Rs/Ω 6.8 5.7 4.6 4.4 4.7
Rct/Ω 22.5 18.6 13.4 11.6 14.1
S0013-4686(17)30939-8 http://dx.doi.org/doi:10.1016/j.electacta.2017.04.156 EA 29414
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
13-3-2017 26-4-2017 28-4-2017
Please cite this article as: Xueliang Li, Zhongqiang Ding, Luyao Zhang, Ruwen Tang, Yang He, Enhanced Performance of Lithium Sulfur Batteries with Sulfur Embedded in Sm2O3-Doped Carbon Aerogel as Cathode Material, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.04.156 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced Performance of Lithium Sulfur Batteries with Sulfur Embedded in Sm2O3-Doped Carbon Aerogel as Cathode Material Xueliang Li a,b *, Zhongqiang Dinga,b , Luyao Zhanga,b, Ruwen Tang a,b, Yang He a,b
a
School of Chemistry and Chemical Engineering, Hefei University of Technology,
Hefei 230009, PR China b
Anhui Key Laboratory of Controllable Chemical Reaction and Material Chemical
Engineering, Hefei 230009, PR China. *Corresponding author. Tel./Fax: +86-551-62901450. E-mail address: [email protected]
Abstract: A Sm2O3 decorated carbon aerogel (CA-Sm) was successfully synthesized through a sol-gel from resorcinol, phloroglucinol (P), formaldehyde and catalyst of oxalic acid and doping compound with Sm2O3 strategies. The small amount of active reactant phloroglucinol promotes synthesis of carbon aerogel (CA) and formation of uniform and porous carbon spheres. The CA-Sm composites present an interlinked porous structure and high specific surface area. The CA-Sm with sulfur loading (CA/S-Sm) from the composite via molten sulfur technique exhibits an enhanced performance with long-term cycling stability and high rate capacity. In particular, the cathode of CA/S-Sm-2 with Sm/C molar ratio of 2/400 achieved a high initial discharge capacity of 1347 and 976 mAh g-1 at 0.2 C and 2 C, respectively. And the cathode exhibited a nice long-term cycling stability with a high initial discharge capacity of 1175 mAh g-1 and maintained 861 mAh g-1 after 300 cycles at 0.5 C. The excellent electrochemical performance of the battery is attributed to interrelated nanospheres, porous carbon structure, large specific surface area of Sm2O3-doped CA composites and improved electronic and ionic transport at the cathode.
Keywords: Phloroglucinol; Sm2O3 nanoparticles; Adsorption capability; Lithium sulfur battery
1. Introduction Developing green and effective energy storage system is still a formidable challenge, especially for electrical devices and hybrid electric vehicles, and will be a major focus of endeavor for the near future. Therefore, new materials with high energy density and long cycle life have been extensively studied. Among various energy storage technologies, the lithium sulfur battery is one of the most promising candidates for the next generation of rechargeable batteries due to its high theoretical capacity of 1675 mAh g-1 and high energy density of 2600 Wh kg-1 which is much higher than that of conventional Li-ion batteries [1, 2]. Meanwhile, element sulfur gives the superiority of low cost, natural abundance and environmental friendliness [3-5] and the lithium sulfur battery has attracted considerable attention. Despite of these advantages, there are still some problems restricting the practical application of Li-S battery. And it suffers from issues including the low active material utilization, the rapid capacity degradation, the poor cyclability for insulating nature of sulfur and “shuttle effect” from polysulfide dissolution, diffusion [6-8]. Different strategies have been proposed to address these issues, including electrolyte composition optimization, enhancing the electrical conductivity of active materials and the fabrication of carbon/sulfur composites [9-11]. A lot of research works have been made on porous carbon material including carbon nanotubes [12-14], graphenes/graphene oxide [15, 16], hollow carbon nanospheres [17-19], conducting polymers [20-22] to improve electrochemical performance. Meanwhile, separators， electrolytes and lithium anodes play an important in lithium sulfur batteries [23-25].
Recently, doped metal carbon materials have been proven to be effective and facile candidates in restraining the diffusion of lithium polysulfides and provide new opportunity to improve the performance and cycling life of the Li-S batteries. Some doped metal carbon/sulfur composites including metal oxides [26-29], metal nitrides [30, 31] and metal sulfides [32, 33] have been proposed as cathode materials to enhance the cycling stability and rate capability. These researches show that the metal compounds in the cathode present an important effect on Li-S batteries. However, for improving the electrochemical performances, it needs to further explore new materials to extend to other types of metal and to effectively control the porous structure of materials. In this work, the composites of CA/S-Sm with unique structure were obtained via sol-gel approaches and sulfur melt-diffusion as cathode material of lithium sulfur battery. Sm2O3 nanoparticle plays a crucial role in effectively adsorbing polysulfides and ameliorating electrochemical performances. The schematic diagram of the synthesis and fabrication procedure for the CA/S-Sm material is clearly displayed in Scheme 1. The porous CA nanospheres with Sm2O3 were synthesized by sol-gel process, carbonation and sulfur melt-diffusion. And the carbon sulfur electrode with Sm2O3 decoration can effectively adsorb the soluble lithium polysulfides during discharge process. The CA/S-Sm composites showed much better electrochemical performance than the CA0/S and CA/S composites. Moreover, the CA/S-Sm-2 composite delivers the initial discharge capacity of 1347 mAh g−1 and maintains 1090 mAh g−1 after 100 cycles at 0.2 C.
2. Experimental section 2.1. Materials Preparation Phloroglucinol (P) and Sm(NO3)3•6H2O were purchased from Aladdin Chemistry Co., Ltd. (China) and the other reagents were obtained from Sinopharm Chemical Reagent Co., Ltd. All the chemicals were used as an analytical reagent grade and without further purification. 2.2. Synthesis of carbon aerogels with Sm2O3 Carbon aerogels composites with Sm2O3 were synthesized via a sol-gel and carbonization
methods
by
employing
resorcinol
(R),
phloroglucinol
(P),
Sm(NO3)3•6H2O and formaldehyde (F) as the carbon precursor. In a typical experiment, 2.2000 g R, 0.0252 g P, 0.0182 g cetyl trimethyl ammonium bromide (CTAB), 0.0126 g oxalic acid (OA) and 0.1793g Sm(NO3)3•6H2O were dissolved in distilled water of 180 mL with continue stirring for 1h, where the molar ratios of R/P, R/CTAB, R/OA, R/H2O and Sm/C were 100/1, 400/1, 200/1, 1/500 and 1/400, respectively. This solution was heated with vigorous stirring at 85 °C and 5.0 mL F was dissolved in the above solution. After becoming milky white, the solution was poured into a beaker and the obtained hydrogels were further dried in an oven at 85 °C for 36 h and formed red brown intermediate. The as-prepared product was completely carbonized at 850 °C for 2 h with a heating rate of 5 °C min-1 in N2 atmosphere and then CA-Sm composite was obtained. The samples with Sm/C molar ratios of 1/400, 2/400 and 3/400 are denoted as CA-Sm-1, CA-Sm-2 and CA-Sm-3, respectively. Meanwhile, for further comparison other carbon aerogels were prepared
without addition of metal. And samples with phloroglucinol promotion and no promotion were denoted as CA and CA0, respectively. 2.3. Preparation of CA/S-Sm composites CA/S-Sm composites were prepared through a melt-diffusion method. Typically, the CA-Sm and S at a precise mass ratio of 4:6 were put into agate mortar and grind 45 min to form a uniform gray mixture. Then the mixture was put into sealed teflon container in autoclave and then transferred into an oven with keeping 155 °C for 18 h. Finally, the composites of CA/S-Sm-1, CA/S-Sm-2 and CA/S-Sm-3 were obtained. The other sulfur-carbon composites were synthesized for comparison and were respectively labeled as CA0/S, CA/S. 2.4 Material characterization The crystalline phase structures were examined by the X-ray powder diffraction (XRD, X'Pert PRO MPD) using a Cu-Kα radiation in the scan range 2θ = 10-90°. The morphology and structure of the materials were characterized by field emission scanning electron microscopy (FESEM, SU8020) and field emission transmission electron microscopy (FETEM, JEM-2100F). The electronic structures of elements were analyzed by a high resolution X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi). The sulfur contents in the composites were calculated by thermogravimetric analysis (TGA, STA449F3). The nitrogen adsorption/desorption isotherms were obtained by using a Brunauer-Emmett-Teller (BET) strategy and the pore size distributions of the materials were analyzed by the Barrett-Joyner-Halenda (BJH) model.
2.5 Adsorption capacity test The adsorption ability of the CA0, CA and CA-Sm-n (n=1, 2, 3) composites to lithium polysulfide (Li2S6) was tested via UV-vis spectroscopy (CARY 5000) at 415 nm [34, 35]. The Li2S6 (2.5mM) solution was prepared from the sublimed sulfur (S) and lithium sulfide (Li2S), where S and Li2S were dissolved in tetrahydrofuran (THF) solution with the molar ratio of 5:1. The synthesis was carried out with slow stirring for two days at room temperature in an argon-filled glove box. To effectively measure adsorption ability, the samples of CA0, CA and CA-Sm-n were dried at 110 °C for 8 h in a vacuum oven. Typically, 50.0 mg sample was put into the Li2S6 solution and stirred for 30 min in an argon-filled glove box. After 12 h adsorption, the solution was separated through a syringe and diluted by a factor of 10 to enable quantitative concentration measurements [36]. The fraction of residual lithium polysulfide in the solution was calculated as follows: fpolysulfides = A415nm (after adsorption)/A415nm (before adsorption). 2.6 Electrochemical Measurements The CA/S-Sm material, acetylene black and polyvinylidene fluoride (PVDF) binder were mixed in a mass ratio of 80:10:10 with N-methylpyrrolidone (NMP) as a dispersant to form a homogeneous slurry. Then, the slurry was evenly coated on an aluminum foil and dried in a vacuum oven at 60 °C for 12 hours. The foils and Celgard 2400 polypropylene (PP) films, acting as cathode and the separators, were punched into disks with a diameter of 10 mm and 12 mm, respectively. Button cells were assembled with lithium foil as anode in an argon-filled glovebox. The electrolyte
solution was prepared by 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (1:1 by volume) with 1 M lithium bis (trifluoromethane) sulfonamide (LiTFSI) and 0.1 M LiNO3. The mass of active materials with sulfur on the electrode was about 1.5-1.8 mg cm-2. Cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) were measured with a CHI 660B electrochemical workstation. CV was conducted at in the potential range of 1.8-3.0 V (Li+/ Li) at a scan rate of 0.1 mV s−1 and EIS was carried out at the frequency range of 0.10 Hz to 100 kHz with the AC perturbation amplitude of 5.0 mV. The galvanostatic charge-discharge test was performed on a Land CT2001A system (Wuhan Jinnuo Electronics Co., Ltd. China) in the voltage range of 1.8-3.0 V. 3. Results and discussion 3.1 XRD phase analysis The X-ray diffraction (XRD) patterns of CA0, CA, CA-Sm-n (n=1, 2, 3), sublimed sulfur and CA/S-Sm-n composites are shown in Fig. 1. The XRD patterns of CA0 and CA only show two broad peaks appeared at about 22°and 44°, respectively (Fig. 1a), indicating the amorphous structure [37, 38] of CA0 and CA composites. The diffraction patterns of CA-Sm-n composites are shown in Figure 1a and the characteristic peaks at 2θ values of 28.3°, 32.8°, 47.0° and 55.8°can be assigned to the facets of (222), (400), (440) and (622) of Sm2O3 nanoparticle (JCPDS No. 15-0813) [39]. The metal Sm has been successfully doped into the CA and with increasing molar ratio of Sm/C, the intensities of diffraction peaks are also increased for the
CA-Sm-n samples. As shown in Fig. 1b, the sharper peaks centered at 2θ = 23.1°and 27.8° correspond to the facets of (222) and (040) of sulfur with the Fddd orthorhombic phase (JCPSD No.08-0247) [40]. The CA/S-Sm-n composites exhibit a similar XRD patterns to sulfur, suggesting a well-defined crystal structure of the chemically deposited sulfur. The peak intensities of orthorhombic sulfur obvious decreased with increasing molar ratio of Sm/C and these results show that the sublimed sulfur has been successfully loaded on CA-Sm-n composites. 3.2 SEM and TEM morphology analysis The surface morphologies of CA0, CA, CA-Sm-n (n=1, 2, 3) composites are shown in Fig. 2, it can be found from Fig. 2a that the CA0 is made up of carbon spheres with smooth surfaces and the diameter of about 300 nm. The morphology of CA in Fig. 2b exhibits uniform structure with the diameter of about 200 nm. The results show that addition of phloroglucinol effective adjusted the size and promoted carbon spheres crosslinking. The SEM images in Fig. 2c-e show the morphologies of CA-Sm-n composites with sizes about 220-350nm. In spite of having some similarity with CA, these samples present evident feature of abundant porous on surface and leads to important role of both enlarging the specific surface area and effectively adsorbing polysulfides. The porous carbon morphology can provide favorable structure for improving the electrochemical performance of Li-S battery. For determining the presence and distribution of the metal and other elements, the CA-Sm-n composites were further investigated by TEM. The TEM images in Fig. 3a-c show a lot of black spots on the surface suggesting that nanoparticles were well
embedded in CA-Sm-n composites. Fig. 3e presents the elemental mappings of the carbon, oxygen and samarium of CA-Sm-2 composite. The EDS mapping images show that elemental Sm and O are uniformly distribution in the carbon matrix and the molar ratio of O and Sm is nearly consistent with Sm2O3. The samarium oxide nanoparticles are ascribed to Sm2O3 from previous XRD analysis. Actually, many Sm2O3 nanoparticles distribute into the internal surfaces of the CA for their growing up in the interior of samples. Fig. 3d clearly shows the HRTEM image of the CA-Sm-2 composite and the nanoparticles present proper sizes of about 6.3 nm. The lattice fringe with spacing of approximately 0.320 nm corresponds to the (222) plane of cubic crystal Sm2O3. 3.3 XPS measurements To further explore the surface chemical composition and chemical bonding state, the CA0, CA, CA-Sm-n (n=1, 2, 3) and CA/S-Sm-n composites have been analyzed by XPS (Fig. 4a-g). As shown in Fig. 4a, it can be found that the CA0 and CA samples show two peaks with C 1s and O 1s. The XPS survey spectra of CA-Sm-n composites exhibit five peaks of C 1s, O 1s Sm 4d5/2, Sm 3d5/2 and Sm 3d3/2 centering at about 284.7 eV, 532.7 eV, 134.1 eV, 1085.1 eV and 1111.3 eV, respectively. Fig. 4b depicts the XPS survey spectra of CA/S-Sm-n (n=1, 2, 3) composites. Besides the former five peaks, other ones corresponding to S 2p and S 2s appeared in the spectra at 164.2 eV and 228.1 eV. Fig. 4c displays the C 1s spectra of the CA/S-Sm-2 composite and the binding energies located at 284.4, 285.1 and 286.0 eV are attributed to sp2 hybridized carbon atoms (sp2 C-C), sp3 hybridized carbon atoms (sp3 C-C) and carbon−sulfur
chains (C-S), respectively [41, 42]. Fig. 4d exhibits the S 2p spectra of the CA/S-Sm-2 composite and two typical component peaks at 163.9 and 165.1 eV are corresponding to S 2p3/2 and S 2p1/2, well matching with the XPS characteristic peak of sublimed sulfur [43]. As shown in Fig. 4e, the S 2p peaks in CA/S-Sm-2 present a small shift of 0.25eV to lower binding energy than that in sulfur for CA/S and are assigned to weaker S−S bond in samarium sulfur chain [44] and the result may benefit for absorbing polysulfides to further enhance electrochemical performance. In the XPS spectra of O 1s in Fig. 4f, the binding energies centered at 531.3, 532.7 and 534.2 eV are ascribed to the oxygen of Sm2+-O groups, Sm3+-O groups and OH groups in the crystal lattice of Sm2O3 nanoparticle, respectively [45]. The XPS spectra of Sm 3d for the CA/S-Sm-2 composite are shown in Fig. 4g. Two obvious peaks at 1085.6 and 1112.8 eV are assigned to Sm 3d5/2 and Sm 3d3/2, respectively. The binding energy peaks indicate Sm presented in oxidation state of Sm3+ and Sm2O3 dopant has been successfully modified on the surface of CA/S composite. 3.4 TGA analysis To ascertain the sulfur contents in samples, the thermal decomposition behavior of CA0/S, CA/S, and CA/S-Sm-n (n=1, 2, 3) composites were investigated under Ar atmosphere with heating rate of 10 °C min−1 by TGA. Meanwhile, TGA was carried out to acquire the Sm2O3 contents in CA-Sm-n composites in air atmosphere with heating rate of 10 °C min−1. Before testing, the samples were dried in a vacuum oven at 110 °C for 12 hours. The TGA curves of CA-Sm-n samples are shown in Fig. S1 in supporting information. The results indicate that the Sm2O3 contents of CA-Sm-1,
CA-Sm-2 and CA-Sm-3 are 3.57%, 6.81% and 9.88%, respectively. And the TGA results are in accord with contents of theoretical calculation for the samples of CA-Sm-1, CA-Sm-2 and CA-Sm-3. The TGA curves of CA0/S, CA/S, and CA/S-Sm-n composites are shown in Fig. 4h, and the thermal treatment temperature range from room temperature to 600 °C. The weight loss of sulfur in CA0/S, CA/S and CA/S-Sm-n composites show two-step processes. The first sulfur-loss occurs in the temperature range of 170-310 °C for the loss of sulfur in the macropores and exterior surface of samples. The second weight loses of sulfur mainly take place in temperature range of 310–500 °C, being attributed to sulfur releases in the mesoporous of samples. We can find from Fig. 4h that it needs higher temperature for complete releasing sulfur in the CA/S-Sm-n composites than in the CA0/S and CA/S. The result indicates that the CA/S-Sm-n composites are more thermodynamical stable than the CA0/S and CA/S composites for the influence of the bonding between S and Sm2O3. The TGA results show that the sulfur content of CA0/S, CA/S, CA/S-Sm-1, CA/S-Sm-2 and CA/S-Sm-3 are 57.37%, 56.81%, 55.54%, 55.02% and 54.43%, respectively. 3.5 N2 adsorption-desorption analysis Fig. 5 displays the nitrogen adsorption-desorption isotherms and pore size distribution curves of CA0, CA and CA-Sm-n (n=1, 2, 3) composites. As shown in Fig. 5a, all isotherms of the as-prepared samples exhibit a type IV isotherm with a hysteresis loop according to the IUPAC classification and indicate typical mesoporous structure for the samples [46, 47]. Fig. 5b illustrates the BJH pore size distribution of
these samples and the pores mainly range from 3nm to 6 nm, showing carbon spheres can provide the appropriate pores for cathode. The BET results showed abundant pores with high surface area in a narrow pore size distribution, and the BET surface areas of CA0, CA, CA-Sm-1, CA-Sm-2 and CA-Sm-3 composites are 793, 778, 754, 738 and 711 m2 g-1, respectively. And more information about porosity characteristic can be clearly seen in Table 1. The samples of CA0, CA and CA-Sm-n present large specific surface area and can effectively increase adsorption capacity to polysulfides. Such pores in samples not only provide more room to transport of electronics but also accommodate available volumetric expansion of sulfur in the charge/discharge process. Despite the specific area of CA-Sm-n composites have somewhat decreased, they may provide effective adsorption to polysulfides for the special role of Sm 2O3 being confirmed by subsequent tests and the CA-Sm-n composites with large specific areas can effectively suppress the production of polysulfides. 3.6 Adsorption capability The adsorption capability of the CA0, CA and CA-Sm-n (n=1, 2, 3) composites on polysulfides obtained by UV-vis spectroscopy further confirm the samples immobilizing role to high order polysulfides. Fig. 6 shows the photograph of adsorbing mixture with color changes and the colors gradually become shallow from left to right. It can be found that the color of the polysulfide solution fades greatly with the increase molar ratio of Sm/C. Moreover, the color for the CA-Sm-3 sample gained much lighter corresponding to the nice adsorption capacity on high order polysulfide due to the Sm2O3 modification. Fig. S2 shows the UV-vis curves of the
samples and all samples appeared characteristic absorption peaks at 415nm. Fig. 6 also depicts the contents of residual lithium polysulfide after adsorption and the samples of CA0, CA, CA-Sm-1, CA-Sm-2 and CA-Sm-3 present remaining ratio of 41%, 33%, 20%, 13% and 9% of polysulfides, respectively. The results exhibit excellent adsorption capacity of as-prepared samples and effectively trap soluble polysulfides especially for CA-Sm-3 composites. 3.7 Electrochemical measurements To evaluate the electrochemical performances of the samples, the coin cells assembled with the CA0/S, CA/S and CA/S-Sm-n (n=1, 2, 3) composites as cathode materials and a lithium foil as anode material were further tested by cyclic voltammetry (CV), galvanostatic charge/discharge measurements and electrochemical impedance spectroscopy (EIS). Fig. 7a depicts the CV curves of the CA0/S, CA/S and CA/S-Sm-n composites in the potential range of 1.8–3.0 V at a scanning rate of 0.1 mV s-1 during the second cycle. It can be observed that there are two reduction peaks and one oxidation peak for the CA0/S and CA/S samples, however there are two oxidation peaks and two reduction peaks for CA/S-Sm-n composites. In the cathodic scan, the two reduction peaks are about centered at the potentials of 2.29-2.31 and 2.00-2.01 V for the CA0/S and CA/S electrode. It shows a higher potential of reduction peaks at 2.31-2.33 V and 2.03-2.05 V for the CA/S-Sm-n electrode. Based on the reduction reaction mechanism of sulfur, the right sulfur peaks of the potential in the range of 2.29-2.33 V relate to the reduction of S8 molecule to higher-valence lithium polysulfides (Li2Sx, 4≤x < 8), and the left reduction peak of the potential in
the range of 2.00-2.05 V corresponds to the reduction of higher-valence and soluble lithium polysulfides to lower-valence and insoluble Li2S2/Li2S [48, 49]. During the anodic scan process, there is only one strong oxidation peak can be seen at 2.42-2.45V for CA0/S and CA/S electrode, which is attributed to the transformation of Li2S2 and Li2S to Li2S8 [50, 51]. Two oxidation peaks of CA/S-Sm-n electrode were observed at 2.33-2.38V corresponding to the reversible conversion between low-order polysulfides and high-order polysulfides. It is worth noting that the CA/S-Sm-n electrodes exhibit higher potential of reduction peaks and lower oxidation peaks compared with CA0/S and CA/S electrodes. The CV results demonstrated that the CA/S composites with Sm2O3 modification can help reduce electrochemical polarization and ameliorate electrochemical reversibility. Fig. 7b presents the initial charge/discharge curves of the Li-S batteries with CA0/S, CA/S and CA/S-Sm-n cathodes at a rate of 0.2 C. These curves obviously show two discharge plateaus and one charge plateau, corresponding to the CV results of the samples. The initial discharge capacities of CA0/S, CA/S, CA/S-Sm-1, CA/S-Sm-2 and CA/S-Sm-3 cathodes are 1070, 1118, 1309, 1347 and 1238 mAh g-1, respectively. Compared with CA0/S electrode, the CA/S electrode displays higher initial discharge capacity. Furthermore, the CA/S-Sm-n electrodes show much higher capacities than those of CA0/S and CA/S composites. Especially, the CA/S-Sm-2 electrode exhibits the highest initial discharge capacity of 1347 mAh g-1. This result demonstrates that the Sm2O3-doping CA/S electrodes improve the electrochemical performance.
For further investigate the storage capacity and cycling stability of these samples, the cycling performances of the CA0/S, CA/S, and CA/S-Sm-n composites were measured at a low rate of 0.2 C. As shown in Fig. 7c, the discharge capacity of CA0/S electrode delivers 725 mAh g-1 after 100 cycles and it retains about 67.8% of initial capacity. Compared with CA0/S electrode, the CA/S electrode delivers a higher discharge capacity of 816 mAh g-1 after 100 cycles and it retains about 73.0% of initial capacity at 0.2 C. The CA/S-Sm-n electrodes show better cycling performances compared with CA0/S and CA/S electrodes. In particular, the CA/S-Sm-2 electrode exhibits the best discharge capacity of 1090 mAh g-1 after 100 cycles and it retains about 80.9% of initial capacity. The results show that the CA/S-Sm-n electrodes can further help to improve the cycling performances for Li-S battery. Considering the fact of the better adsorbing ability of CA-Sm-n on polysulfides, the improvement of discharge capacity of CA/S-Sm-n can be attributed to restraining polysulfides dissolution. Fig. 7d displays the rate performances of CA0/S, CA/S and CA/S-Sm-n composites at varying rates at 0.2, 0.5, 1 and 2 C. We can see from Fig. 7d that the CA/S-Sm-n electrodes show better rate performances than CA0/S and CA/S ones for different rates. The initial discharge capacities of CA0/S cathode at 0.2, 0.5, 1 and 2 C current rates are 1070, 916, 808 and 695 mAh g-1, respectively. The further results show that the electrodes doped with Sm2O3 can effectively improve the rate capability. Among all cathodes, the CA/S-Sm-2 composite exhibits the best rate capability. The CA/S-Sm-2 cathode at 0.2, 0.5, 1 and 2 C current rates deliver initial discharge
capacities of 1347, 1223, 1119 and 976 mAh g-1, respectively. Moreover, the CA/S-Sm-2 electrode delivers a high recovered capacity of 1231 mAh g-1 with the current rate backing to the 0.2 C. These results further reveal the electrodes modified by Sm2O3 present nice redox stability and can effectively inhibit the “shuttle effect”. Fig. 8 further shows electrochemical performance of CA0/S, CA/S and CA/S-Sm-2 composite for long-term cycle. Fig. 8a show the charge/discharge curves of CA/S-Sm-2 composite and shows a high discharge capacity at 2 C. In spite of capacity decreasing with repeating charge-discharge cycle, it retains discharge capacity of 678 mAh g-1 after 100 cycles. Fig. 8b shows the cycle performance of the CA0/S, CA/S and CA/S-Sm-2 cathodes at 1 C. The CA0/S and CA/S cathodes display initial discharge capacity of 872 and 808 mAh g-1, and remained 577 and 494 mAh g-1 after 100 cycles, respectively. It can be clearly found that the CA/S-Sm-2 cathode shows higher discharge capacity than cathodes of CA0/S and CA/S, and it still remain 825 mAh g-1 after 100 cycles. Fig. 8c displays the long-term cycling curves of the CA0/S and CA/S-Sm-2 cathodes at 0.5 C for 300 cycles. As shown in Fig. 8c, the discharge capacities of samples gradually decrease with increasing cycle test and the CA0/S cathode keeps 509 mAh g-1. But for the CA/S-Sm-2 cathode, it exhibits a nice cycling stability and still maintains 889 mAh g-1 after 300 cycles with a capacity decay of 0.091% per cycle. These results confirm crucial role of Sm2O3 modification on carbon-sulfur cathodes with enhancing the electric conductivity, restraining the dissolution of the polysulfides, increasing sulfur utilization. Fig. 9 shows the EIS profiles of CA0/S, CA/S and CA/S-Sm-n (n=1, 2, 3)
electrodes. For effectively analyzing EIS data, the equivalent circuit was used to be a model of the Li-S battery, where the Rs, Rct and W represent the resistance of electrolyte solution, charge-transfer resistance and the Warburg impedance, respectively. The CPE is related to the constant phase element of double layer capacitance. As shown in Fig. 9, all the EIS curves of the CA0/S, CA/S and CA/S-Sm-n composites before cycling consist of a semicircle and an oblique line. The semicircle at high-frequency region and the oblique line at low-frequency region relate to the charge transfer resistance (Rct) and the Warburg impedance (W), respectively. After EIS data is being fitted with the equivalent circuit, the impedance parameters of the CA0/S, CA/S and CA/S-Sm-n composites were shown in Table 2. Typically, CA/S-Sm-2 electrode presents values of Rs and Rct with 4.4 and 11.6 Ω, but the CA0/S electrode with 6.8 and 22.5 Ω. Compared with the CA0/S and CA/S electrodes, the CA/S-Sm-n electrodes show decreased value of Rs and Rct. In order to highlight the effect of Sm2O3, we also have tested the impedance of CA0/S, CA/S and CA/S-Sm-n samples after 100 cycles. And the EIS curves and impedance parameters of these samples are shown in Fig. S3 and Table S1 (in supporting information.), respectively. The impedance of samples have increased to some degree. However, it is worth mentioning that the CA/S-Sm-2 electrode presents almost unchanged Rs and Rct compared with the values before cycling. The EIS results demonstrate that the Sm2O3 modified carbon-sulfur electrodes can help reduce the surface resistance and further enhance the electrochemical performance for Li-S battery.
4. Conclusions In summary, the novel CA/S-Sm composite materials were successfully prepared by doping metal and loading sulfur in the porous carbon aerogel. The cathode materials modified by Sm2O3 can effectively immobilize the polysulfides and improve the electrochemical performance. The CA/S-Sm composites present a nice cycling stability and rate capability. Moreover, the CA/S-Sm-2 cathode delivers a high initial discharge capacity of 1345 mAh g-1 remaining 1082 mAh g-1 after 100 cycles at 0.2 C, and the CA/S-Sm-2 cathode exhibits the capacity of 971 mAh g-1 at a high rate of 2 C. The excellent electrochemical performance of CA/S-Sm cathode is attributed to the unique interconnected, porous structure and high adsorption ability of CA-Sm composites. This doping strategy provides a facile and efficient method to obtain promising candidate material for Li-S batteries.
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Fig. 1. XRD patterns of (a) CA0, CA, CA-Sm-n; (b) CA/S-Sm-n and sublimed sulfur.
Fig. 2. SEM images of (a) CA0; (b) CA; (c) CA-Sm-1; (d) CA-Sm-2; (e) CA-Sm-3.
Fig. 3. TEM images of (a) CA-Sm-1; (b) CA-Sm-2; (c) CA-Sm-3; (d) HRTEM image of CA-Sm-2; EDS mappings of (e) CA-Sm-2; (f) C,(g) O and (h) Sm.
Fig. 4. XPS survey spectra of (a) CA0, CA and CA-Sm-n composites; (b) CA/S-Sm-n composites; XPS spectra and fitted curves of the CA/S-Sm-2 sample of (c) C 1s; (d)S 2p; (e) S 2p XPS spectra of CA/S and CA/S-Sm-2 composites; XPS spectra of the CA/S-Sm-2 sample (f) O 1s; (g) Sm 3d; (h) TGA curve of CA0/S, CA/S and CA/S-Sm-n composites under Ar atmosphere.
Fig. 5. (a) N2 adsorption–desorption isotherms and (b) BJH pore size distributions of CA0, CA and CA-Sm-n composites.
Fig. 6. The photograph of adsorb polysulfide mixture and the amount of remaining polysulfides for CA0, CA and CA-Sm-n composites.
Fig. 7. (a) Cyclic voltammogram curves of the cell with CA0/S, CA/S and CA/S-Sm-n composites at 0.1 mV s-1 in the potential range of 1.8−3.0 V (Li+/ Li); (b) The initial discharge/charge profiles at a current rate of 0.2 C; (c) Cycling performances at a rate of 0.2 C; and (d) Rate performances at different current densities (0.2-2 C).
Fig. 8. (a) The charge/discharge profiles of the CA/S-Sm-2 composite cathode at 2 C; (b) Cycling performance of the CA0/S, CA/S and CA/S-Sm-2 electrodes at 1 C; (c) Long term performance of the CA0/S and CA/S-Sm-2 electrodes at 0.5 C
Fig. 9. EIS curves of the CA0/S, CA/S and CA/S-Sm-n electrodes before cycling
Scheme 1. Schematic Illustration of the Formation for CA/S-Sm Composite.
Table 1. Structural parameters calculated from nitrogen adsorption isotherms of samples Samples CA0 CA CA-Sm-1 CA-Sm-2 CA-Sm-3
BET Surface Area (m2 g−1) 793 778 754 738 711
Pore Volume (cm3 g−1) 0.92 0.88 0.84 0.82 0.78
Average Pore Diameter (nm) 5.36 5.24 5.22 5.13 5.05
Table 2. Impedance parameters of samples before cycling Samples CA0/S CA/S CA/S-Sm-1 CA/S-Sm-2 CA/S-Sm-3
Rs/Ω 6.8 5.7 4.6 4.4 4.7
Rct/Ω 22.5 18.6 13.4 11.6 14.1