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Ketjen Black-MnO Composite Coated Separator For High Performance Rechargeable Lithium-Sulfur Battery
Accepted Manuscript Title: Ketjen Black-MnO Composite Coated Separator For High Performance Rechargeable Lithium-Sulfur Battery Author: Xinye Qian Lina Jin Di Zhao Xiaolong Yang Shanwen Wang Xiangqian Shen Dewei Rao Shanshan Yao Youyuan Zhou Xiaoming Xi PII: DOI: Reference:
S0013-4686(16)30260-2 http://dx.doi.org/doi:10.1016/j.electacta.2016.01.225 EA 26607
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
23-10-2015 24-1-2016 31-1-2016
Please cite this article as: Xinye Qian, Lina Jin, Di Zhao, Xiaolong Yang, Shanwen Wang, Xiangqian Shen, Dewei Rao, Shanshan Yao, Youyuan Zhou, Xiaoming Xi, Ketjen Black-MnO Composite Coated Separator For High Performance Rechargeable Lithium-Sulfur Battery, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.01.225 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.
Ketjen Black-MnO Composite Coated Separator For High Performance Rechargeable Lithium-Sulfur Battery
Xinye Qiana, b*, Lina Jina, Di Zhaoa, Xiaolong Yanga, Shanwen Wanga, Xiangqian Shena*, Dewei Raoa, Shanshan Yaoa, Youyuan Zhouc, Xiaoming Xic
a Institute for Advanced Materials, College of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, P. R. China b Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China. c Hunan Engineering Laboratory of Power Battery Cathode Materials, Changsha Research Institute of Mining and Metallurgy, Changsha 410012, P. R. China
Corresponding author: Dr. Xinye Qian Institute for Advanced Materials, Jiangsu University, Zhenjiang, 212013, P. R. China Tel./fax: +86-511-88791964 E-mail address: [email protected] Pro. Xiangqian Shen Institute for Advanced Materials, Jiangsu University, Zhenjiang, 212013, P. R. China Tel./fax: +86-511-88791964 E-mail address: [email protected]
Highlights
1 MnO-KB compsite was fabricated by wet impregnation in which MnO nanoparticles were well enwrapped by Ketjen Black. 2 Ketjen Black-MnO Composite was firstly used as the modified layer instead of the pure carbon material modified layer for Li-S batteries. 3 Ketjen Black-MnO layer showed excellent polysulfide absorption ability, and the MnO nanoparticles coated by the KB have the catalytic effect which enhanced the redox reactions.
Abstract A Ketjen Black-MnO composite was prepared by wet impregnation and TEM images showed that the MnO nanoparticles were well enwrapped by Ketjen Black (KB). The as-prepared KB-MnO composite were coated on one side of the commercial separators for the improvement of electrochemical performances of Lithium-Sulfur battery. For comparison, conventional separator coated by KB and conventional separator without coating was used in reference Lithium-Sulfur batteries. Electrochemical tests suggested that KB-MnO coated separator could act as the upper current collector and the barrier of lithium polisulfides which consequently achieved higher charge/discharge capacity and more stable cycling stability. Furthermore, TEM elemental map demonstrated that KB-MnO composite was more favorable for the adsorption of lithium polysulfides than KB, therefore the electrochemical performances of Li-S batteries can be dramatically enhanced.
Keywords: KB-MnO coated separator; Li-S battery; lithium polysulfides
1. Introduction Lithium-Sulfur battery is one of the most promising candidates of conventional Lithium-ion battery. Based on the reactions of S8+ 16Li ↔ 8Li2S, Lithium-Sulfur battery has a large theoretical energy density of 2600 Wh kg-1. In addition, sulfur as an active cathode material is non-toxic, naturally abundant, and environmentally benign [1-4]. Despite the advantages of Lithium-Sulfur battery, the low conductivity of sulfur, volume expansion of S to Li2S and the soluble intermediate products lithium polysulfide (Li2Sn, 4 < n < 8) which can shuttle between the lithium anode and sulfur cathode are three main problems remain to be solved before the Lithium-Sulfur battery can put into practical use [5-7]. The most popular approaches are to load sulfur in to conductive porous materials, such as meso/micro porous carbon [8,9], carbon nanotubes [10], grapheme [11,12], etc. The combination of carbon materials with sulfur can apparently improve the electronic conductivity of sulfur and accommodate the volume expansion during charge/discharge process. However, the porous structure of carbon materials can only adsorb part of the lithium polysulfides, therefore the high capacity loss induced by the dissolution of lithium polysulfide in organic electrolytes and shuttle effects are still remarkable. Recently, a new approach for the suppression of shuttle effects has been reported by Arumugam Manthiram et al. For example, a bifunctional microporous carbon paper was inserted between the sulfur cathode and separator, the MCP carbon interlayer operates not only as an ‘‘upper current collector’’ to enhance the active material utilization but also as a “polysulfide stockroom’’ to retain the cyclability [13]. Multiwalled carbon nanotubes were also used to fabricate interlayers for Lithium-Sulfur battery which significantly enhances both the specific capacity and cycle stability [14]. Qin et al have reported a cassava-derived carbon sheet as a polysulfide inhibitor for lithium-sulfur battery, The results show that the cell with the CCS interlayer exhibits excellent cycle stability and superior rate capability [15]. These carbon materials interlayers are highly conductive and sufficient in adsorption of lithium polysulfides, therefore the interlayer strategy greatly improves the utilization of sulfur and the cycle stability of Li-S battery. However, the capacity fading was still remarkable due to the limited adsorption
ability of carbon materials for lithium polysulfide. Recently, Nazar et al have reported a quite different chemical approach to polysulfide retention in the sulfur cathode, polysulfide can be sufficiently anchored on the surface of ultra-thin MnO2 nanosheets by the transfer mediator named as polythionate complex [16]. Interestingly, some other metal oxides used as additives or complexes in the sulfur cathode, such as La2O3 and Mg0.6Ni0.4O etc. also achieved superior electrochemical performances compared with the carbon-sulfur cathode materials [17,18]. Researches demonstrated that these metal oxides acted as adsorbents and catalysts of the lithium polysulfides, therefore the capacity, cycle stability and rate capability could be improved obviously. In this work, we have for the first time used the composite of ketjen black (a kind of conductive carbon) and MnO nanoparticles as the barrier layer of lithium polysulfides. KB-MnO composite in which MnO nanoparticles were well wrapped by KB was prepared by wet impregnation and then mechanically coated on one side of conventional separator. Electochemical tests and microstructure characterizations suggested that KB-MnO barrier layer have stronger lithium polysulfides adsorption ability than the pure KB barrier layer as well as a catalytic effect. The easy preparation and cheap raw materials made the KB-MnO coated separator a promising solution for the further improvement of Li–S battery.
2. Experimental
2.1. Synthesis of KB-MnO coated separator and the assemble of Li-S battery
The KB-MnO composite was prepared by a wet impregnation method. In a typical fabrication process, 0.8 g KB and 0.5 g manganese acetate [Mn(CH3COO)2] were dispersed in 100 cm3 ethanol with ultrasound at 500 W for more than 2 h. Then the solution was magnetic stirred for 12 h followed by a drying procedure at 80 °C in a drying oven. At last, the dried sample was heat treated under nitrogen atmosphere at 450 °C for 4 h with the heating rate of 5 °C min-1. The slurry of the membrane coating on one side of the commercial separator Celgard 2400 was prepared by mixing
KB-MnO composite, super-P and polyvinylidene fluoride (PVDF) in the weight ratio of 8 : 1 : 1 with N-methyl-e-pyrrolidinone (NMP) as the dispersant. Then the slurry was coated on one side of Celgard 2400 by an automatic coating machine and dried in vacuum oven for 12 hours at 50 °C. The thickness of the coated layer was about 8 μm by the measurement of micrometer as shown in Fig. 1 (a) and (b), the weight was approximately 0.1 - 0.15 mg cm-2. The cathode material of Li-S battery was prepared by melt diffusion method. KB and sulfur was mixed in the weight ratio of 1 : 3 and ball milled for 4 h, then the mixture was sealed in a reactor and heated at 155 °C for 12 h. The slurry of the cathode electrode for Li-S battery was prepared by mixing KB-sulfur composite, super-P and polyvinylidene fluoride (PVDF) in the weight ratio of 8 : 1 : 1 with N-methyl-e-pyrrolidinone (NMP) as the dispersant. Then the slurry was coated on an aluminum foil by an automatic coating machine and dried in vacuum oven for 12 h at 60 °C. The obtained film was cut in to disks with a diameter of 11 mm, the thickness of the cathode was about 80 μm by the measurement of micrometer as shown in Fig. 1 (c) and (d), the average mass of the cathode (include sulfur-KB composite, super-P and PVDF) was estimated to be 1.5 - 2 mg cm-2. Half-cells schematically shown in Fig. 1 (e) were assembled in a glove box (Mikrouna) filled with Ar atomosphere, KB-MnO coated Celgard 2400 was used as the separator, 1,3-dioxolane (DOL) and dimethoxymethane (DME) mixed by the volume ratio of 1:1 with 1 mol dm-3 LiTFSI and 0.1 mol dm-3 LiNO3 addition was used as the electrolyte. For comparison, KB coated Celgard 2400 and Celgard 2400 without coating were used as the separator in reference samples. The three samples were marked as conventional separator sample, KB coated separator sample and KB-MnO coated separator sample, respectively.
2.2. Characterization
The crystal structure analysis was carried out by power X-ray diffraction (XRD, D/Mmax 2500PC) with CuKα radiation (λ=0.154178 nm). The morphology of the KB-MnO composite was carried out by field emission scanning electron microscopy
(FE-SEM, JSM-7001F) and transmission electron microscopy (TEM, JEM-2100). The MnO content of KB-MnO composite was tested by the thermal gravimetric analysis (TGA, Netzsch, STA449F3) at the scan rate of 5 °C min-1 from room temperature to 900 °C. Nitrogen adsorption isotherm was measured by a pore size and surface area analyzer (Nova 2000) to analyze the specific surface areas and pore size distribution of KB and KB-MnO composite.
2.3. Electrochemical measurement
Galvanostatic charging/discharging tests were carried out in a potential range of 1.7 V - 2.8 V (vs Li/Li+) to measure specific capacity and cycling stability of the sulfur-KB composite cathode by using the LAND-CT2001A instrument (Wuhan Jinnuo, China). Cyclic voltammetry (CV) was employed on the AUTOLAB electrochemical work station in a potential range of 1.7 V - 2.8 V (vs Li/Li+) to investigate the redox peaks at the scan rate of 0.1 mV s-1. The electrochemical impedance spectrometry (EIS) was carried out in the frequency range between 100 kHz and 100 mHz.
3. Results and Discussion
Fig. 2 is the XRD pattern of KB-MnO composite. The broad peaks around 26 º and 42 º marked by black dots belong to the (0 0 2) plane and (1 0 0) plane of the graphite carbon structure, the other peaks around 34.9 º, 40.6 º, 58.7 º, 70.2 º and 73.8 º marked by inverted triangles are well-indexed to (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes of face-centered cubic phase of MnO [19-21]. None of the other diffraction peaks are observed in the XRD pattern. This result suggests that the as-prepared production is the mixture of carbon and MnO phase.
Fig. 3 (a) is the TEM image of KB-MnO composite, MnO nanoparticles display in the morphology of grey dots and black dots are uniformly distributed throughout
KB. Fig. 3 (b) illustrates the HRTEM image of KB-MnO composite, MnO nanoparticle is well enwrapped by KB, and the lattice fringes of MnO nanoparticle is clearly shown in the figure. Moreover, the size of a single lattice fringe is estimated to be 0.25 nm which agreed well with the interplanar spacing of (1 1 1) planes of MnO phase. The TEM and HRTEM image demonstrate that the KB-MnO composite was successfully synthesized. Fig. 3 (c) is the SEM image of KB-MnO composite and Fig. 3 (d) is the SEM EDS curve of Fig. 3 (c). EDS curve suggests that only elemental C, Mn and O exist in the KB-MnO composite, the peak of Si is caused by the silicon substrate and the peak of Au is caused by metal spraying before the SEM EDS analysis.
The contents of carbon and MnO in KB-MnO composite were checked by the thermal gravimetric analysis at the scan rate of 5 °C min-1 from room temperature to 900 °C in air atmosphere and TG curve is shown in Fig. 4. The contents of carbon and MnO can be estimated by the mass loss of the as-prepared KB-MnO composite. As shown in the figure, KB-MnO composite remains approximately 20 % of the initial mass, therefore, the MnO content in the KB-MnO composite is around 20 % and the carbon content in the KB-MnO composite is around 80 %. However, part of the MnO may be oxidized to Mn2O3, Mn3O4 and MnO2 etc. in the air atmosphere, which will increase the mass of MnO. Thus, the real content of MnO should be a little less than 20 %.
N2 adsorption–desorption isotherms of KB and KB-MnO composite are illustrated in Fig. 5 (a). Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific surface areas. The BET surface areas of KB and KB-MnO composite are 1200 and 1300 m2 g-1, respectively. It indicates that the specific surface area of KB-MnO composite is even a little larger than that of KB, therefore the adsorption ability of KB-MnO composite is probably better than that of KB. BJH pore size distributions of KB and KB-MnO composite are shown in Fig. 5 (b), most of the pores are below 5 nanometers.
Cyclic voltammograms (CV) of the conventional separator sample, KB coated separator sample and KB-MnO coated separator sample were carried out at the scan rate of 0.1 mV s-1 between 1.7 V and 2.8 V. As shown in the cathodic curve of Fig. 6 (a), (b) and (c), all samples present two reduction peaks at about 2.0 V and 2.3 V, corresponding to the sulfur reduction to soluble lithium polysulfide (Li2Sn, 2 < n < 8), and insoluble short polysulfide (Li2S2/Li2S), respectively. In the anodic scan, one broad oxidation peak at about 2.4 V is discovered which corresponds to the oxidation of short polysulfide (Li2S2/Li2S) to lithium polysulfide and elemental sulfur [22,23]. It should be noticed that at the reduction peaks around 2.0 V, the precise reduction peak (2.04 V) of KB-MnO coated separator sample is a little larger than that of KB coated separator sample and conventional separator sample (2.01 V). The higher reduction cell potential indicates the improved chemical reaction kinetics which implies that the KB-MnO coated layer have a catalytic effect on the sulfur reduction. On account of the KB coated separator sample shows the same cell reduction potential as that of the conventional separator sample, this catalytic effect is most probably induced by the MnO nanoparticles. This conclusion is similar to some previous studies in which metal oxide nanoparticles were found to have a catalytic effect on the sulfur redox reaction, for example La2O3, Mg0.6Ni0.4O and Mg0.8Cu0.2O [17,18,24]. Fig. 7 (a), (b) and (c) represents the charge/discharge curves of Li-S batteries using conventional separator, KB coated separator and KB-MnO coated separator, respectively. All the samples were tested at 1 C and all the charge/discharge capacities were calculated by the mass of elemental sulfur. The two discharge plateaus observed in all samples are consistent with the reduction peaks shown in CV curves, and the charge plateaus observed in all of the curves also matched well with the oxidation peaks shown in CV curves [25].
Fig. 8 (a) displays the the charge/discharge cycle performance of conventional separator sample, KB coated separator sample and KB-MnO coated separator sample at the rate of 1 C, all the charge/discharge capacities were calculated by the mass of elemental sulfur. The initial discharge capacities of conventional separator sample,
KB coated separator sample and KB-MnO coated separator sample are 723 mAh g-1, 907 mAh g-1 and 1059 mAh g-1, respectively. After 200 cycles, the reversible capacities of the three samples are 420 mAh g-1, 663 mAh g-1 and 901 mAh g-1. The capacity retention ratios of the samples are 58 %, 73 % and 85 % respectively. Moreover, the average coulomb efficiency of the three samples is approximately 97 %, 95 % and 90 %, respectively. The remarkably improved cyclic performance and coulomb efficiency of KB-MnO coated separator sample may be attributed to the strong adsorption ability of KB-MnO structure which enhanced the utilization of elemental sulfur and the suppression of shuttle effect. By rough calculation, the discharge capacity of KB-MnO coated separator sample after 200 cycles is more than two times as that of conventional separator sample, but the total mass and thickness in the cathode side increased by the KB-MnO coated layer are no more than 20 %.
After 200 cycles, Li-S batteries were disassembled. KB and KB-MnO composite coated on the separator were ultrasonic washed and centrifugated for several times in acetonitrile and NMP to remove LiTFSI and PVDF respevtively. Then TEM EDS analysis and elemental map test were carried out to further study the lithium polysulfide adsorption of KB and KB-MnO composite. EDS analysis of KB and KB-MnO composite shown in Fig. 9 (a) and (c) suggest that elemental sulfur exist in KB and KB-MnO composite after 200 cycles. It proves that lithium polysulfides were blocked and absorbed by KB or KB-MnO barriers which could suppress the shuttle effect. Elemental F is found in the EDS curve of KB barrier, it may be induced by PVDF which is not cleaned thoroughly. Fig. 9 (b) is the elemental map of KB barrier, carbon map and sulfur map are uniformly distributed and matched well with each other which indicates that lithium polysulfides are uniformly absorbed by the pores of KB. But in the elemental map of KB-MnO composite, the distribution of carbon and sulfur are not matched so well. Interestingly, the elemental map of sulfur is somehow matched with that of Mn. As shown in Fig. 9 (d) In the area where Mn existed, the orange dots represent elemental sulfur are more intensive than the area without Mn which indicates more lithium polysulfides are adsorbed on the spots of MnO
nanoparticles. Consequently, more lithium polysulfides can be absorbed by KB-MnO hybrid structure than that of KB. Thus, KB-MnO coated separator sample displayed larger discharge capacity and more stable cycle stability than that of KB coated separator sample and conventional separator sample.
Fig. 10 displays the rate capabilities of conventional separator sample, KB coated separator sample and KB-MnO coated separator sample at various rates. With the charge/discharge rates increased from 0.2 C to 2 C, KB-MnO coated separator sample delivers the discharge capacities of about 1200 mAh g-1 and 950 mAh g-1, respectively. KB coated separator sample delivers the discharge capacities of about 1120 mAh g-1 and 800 mAh g-1, respectively. Conventional separator sample delivers the lowest discharge capacities of about 930 mAh g-1 and 490 mAh g-1, respectively. When the charge/discharge rates decreased to 0.2 C, KB-MnO coated separator sample delivers the discharge capacities of about 1140 mAh g-1, thus the discharge capacities can maintain 95 % of the original value after the high rates charge/discharge processes. KB coated separator sample delivers the discharge capacities of about 1020 mAh g-1, thus the discharge capacities can maintain 91 % of the original value. Conventional separator sample delivers the discharge capacities of about 780 mAh g-1 which maintains only 83 % of the original value. The result of rate performance shows that KB-MnO coated separator sample can recover most of the discharge capacity when the charge/discharge rate was set back to 0.2 C. It suggests that lithium polysulfides are most sufficiently blocked between the cathode and the KB-MnO coated separator [26]. Moreover, KB-MnO coated separator sample shows the largest discharge capacity at the rate of 2 C which implies that it possesses the highest conductivity among the three samples, therefore it can effectively promote the electrochemical reactions in cathode electrode and KB-MnO barrier layer and improve the rate capability of Li-S battery [27].
In order to further demonstrate the improved conductivity of KB-MnO coated separator sample. EIS of the three samples were tested in the frequency region of 100
kHz - 100 mHz. The Nyquist plots of the samples obtained at the original state and after 200 charge/discharge cycles are shown in Fig. 11 (a), (b) and (c). These Nyquist plots are are consisted of an intercept at high frequency on the real axis which represents the resistance of the electrolyte (Rs), a depressed semicircle at high frequency region which corresponds to the charge transfer resistance (Rct) of the sulfur electrode, and an inclined line at low frequency region which reflects the Li+ diffusion into the active mass. As listed in Tab. 1, the Rct values of conventional separator sample, KB coated separator sample and KB-MnO coated separator sample at the original state are 75 Ω, 21 Ω and 14 Ω respectively. The relatively low Rct values of KB coated separator sample and KB-MnO coated separator sample are attributed to the high conductance of KB and KB-MnO coated layer which can act as the upper current collector. After 200 cycles, the Rct values of the samples are approximately 194 Ω, 35 Ω and 24.5 Ω respectively. The considerable charge transfer resistance increasement of conventional separator sample may be induced by the sulfur agglomeration on the surface of cathode during the charge/discharge process. By contrast, KB barrier layer and KB-MnO barrier layer can sufficiently adsorb the lithium polysulfides on the surface of the cathode, therefore the sulfur agglomeration is accommodated and the increase of charge transfer resistance is suppressed. As KB-MnO coated separator sample shows the lowest charge transfer resistance before and after 200cycles, it is supposed to have the fastest charge transportation speed, therefore the electrochemical reactions and rate capability can be dramatically improved [28]. The inclined line is attributed to the diffusion of the lithium ions into the bulk of the electrode material named as Warburg diffusion. The Warburg diffusion coefficient σw can be obtained by Eq. (1) [29,30]: Zre=Rs+Rct+σwω-1/2
(1)
σw are the slopes for the plots of Zre vs. the reciprocal root square of the lower angular frequencies (ω−1/2), which are presented in Fig. 11 (e). The diffusion coefficient values of the lithium ions (D) for their diffusion into the bulk electrode material have been calculated using Eq. (2) and are listed in Tab. 1 too. D=½ (RT/AF2σwC)2
(2)
where R is the gas constant (8.314 J mol−1 K−1), T is the room temperature (298.5 K), A is the area of the electrode surface (0.95 cm2), F is the Faraday’s constant (9.65×104 C mol−1) and C is the molar concentration of Li+ ions (1.1×10-3 mol cm-3). The D value of KB-MnO coated separator sample before and after cycling is about two orders of magnitude larger than that of the conventional separator sample, proving the superior Li+ diffusion mobility which is beneficial for the redox process. The exchange current density is given by Eq. (3): i°= RT/nFRct
(3)
n is the number of electrons transferred in the electrochemical reaction which is 2 for the charge/discharge reaction of S and Li. The i° of KB-MnO coated separator sample before and after cycling is higher than that of the other samples, therefore the redox reactions are also stronger. The double layer capacitance Cdl is determined by Eq. (4) [30]: Cdl = 1/2πfRct
(4)
Where f is the frequency corresponds to the maximum value of vertical axis on the semicircle. It is observed that the Cdl value of KB-MnO coated separator sample is lower than that of the other two samples, indicating the lower polarization during the charge/discharge process.
4. Conclusions In summary, KB-MnO composite was successively fabricated by wet impregnation and TEM images showed that the Manganese oxide nano particles were well coated by KB. BET results showed that the specific surface area of KB-MnO composite was approximately the same as that of KB. KB-MnO coated separator was prepared by coating the slurry on one side of the conventional separator for Li-S battery. TEM elemental map analysis demonstrated that KB-MnO composite could adsorb more lithium polysulfides than pure KB, consequently, Li-S battery using KB-MnO coated separator displayed higher discharge capacity and coulomb efficiency after 200cycles. Additionally, the highly conductive KB-MnO coated layer also acted as an upper current collector which speed up the electron transportation and
therefore enhanced the rate capability. However, the intrinsic chemical or physical reactions between KB-MnO and lithium polysifides which induced the better polysulfide adsorption ability still remain to be studied. Furthermore, other metal oxides such as transitional metal oxides and rare metal oxides with excellent adsorption ability and catalytic effect are existed to be researched for the application in Li-S battery.
Acknowledgements This work was financially supported by the Start-up Fund of Jiangsu University (Grant No. 14JDG060, 14JDG058), the Postdoctoral Fund of Jiangsu Province (Grant No. 1402196C), open fund of the Laboratory of Solid State Microstructures, Nanjing University (M28035), the National Natural Science Foundation of China (Grant No. 51274106, 51474113, 51474037), the Natural Science Foundation of Jiangsu Provincial Higher Education of China (Grant No. 12KJA430001).
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Fig. 1 (a) thickness of conventional separator, (b) thickness of KB-MnO coated separator, (c) thickness of aluminum foil, (d) thickness of aluminum foil coated with cathode materials, (e) schematic diagram of KB-MnO coated separator
Fig. 2 XRD pattern of KB-MnO composite
Fig. 3 (a) TEM image of KB-MnO composite, (b) HRTEM of a single MnO nanodot, (c) SEM image of KB-MnO composite and (d) SEM EDS curve of KB-MnO composite.
Fig. (a)
Fig. (b)
Fig. (c)
Fig. (d)
Fig. 4 TG curves of KB-MnO composite.
Fig. 5 (a) N2 adsorption–desorption isotherms of KB and KB-MnO composite and (b) BJH pore size distribution of KB and KB-MnO composite.
Fig. 6 CV curves of (a) conventional separator sample, (b) KB coated separator sample and (c) KB-MnO coated separator sample carried out at the scan rate of 0.1 mV s-1.
Fig. 7 charge/discharge curves of Li-S batteries using (a) conventional separator, (b) KB coated separator and (c) KB-MnO coated separator.
Fig. 8 charge/discharge cycle performance of conventional separator sample, KB coated separator sample and KB-MnO coated separator sample.
Fig. 9 (a) EDS curve of KB barrier layer after 200cycles, (b) Elemental map of KB barrier layer after 200cycles, (c) EDS curve of KB-MnO barrier layer after 200cycles, (d) Elemental map of KB-MnO barrier layer after 200cycles.
(a)
(b)
(c)
(d)
Fig. 10 Rate capability of conventional separator sample, KB coated separator sample and KB-MnO coated separator sample.
Fig. 11 EIS of (a) conventional separator sample, (b) KB coated separator sample and (c) KB-MnO coated separator sample at original state and after 200 cycles, (d) schematic diagram of equivalent circuit, (e) the relationship of the samples between Zre and ω−1/2 at low frequencies.
(d)
(e)
Table 1 Electrochemical impedance parameters of the samples Rs Rct σw D Cdl samples (Ω) (Ω) (Ω s−1/2) (cm2 s−1) (F) C-original 1.7 75 22.8 6.3×10-11 5.3×10-6 C-200 cycles 5 194 22.08 6.7×10-11 1.39×10-6 KB-original 1.5 21 14.34 1.6×10-10 1.78×10-6 KB-200 cycles 4 35 11.57 2.4×10-10 2.35×10-6 KB-MnO-original 1.7 14 17.8 1.0×10-10 1.21×10-6 KB-MnO-200 cycles 3.5 24.5 3.05 3.5×10-9 1.03×10-6
i° (mA cm-2) 1.7×10-4 6.6×10-5 6.1×10-4 3.7×10-4 9.2×10-4 5.2×10-4
S0013-4686(16)30260-2 http://dx.doi.org/doi:10.1016/j.electacta.2016.01.225 EA 26607
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
23-10-2015 24-1-2016 31-1-2016
Please cite this article as: Xinye Qian, Lina Jin, Di Zhao, Xiaolong Yang, Shanwen Wang, Xiangqian Shen, Dewei Rao, Shanshan Yao, Youyuan Zhou, Xiaoming Xi, Ketjen Black-MnO Composite Coated Separator For High Performance Rechargeable Lithium-Sulfur Battery, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.01.225 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.
Ketjen Black-MnO Composite Coated Separator For High Performance Rechargeable Lithium-Sulfur Battery
Xinye Qiana, b*, Lina Jina, Di Zhaoa, Xiaolong Yanga, Shanwen Wanga, Xiangqian Shena*, Dewei Raoa, Shanshan Yaoa, Youyuan Zhouc, Xiaoming Xic
a Institute for Advanced Materials, College of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, P. R. China b Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China. c Hunan Engineering Laboratory of Power Battery Cathode Materials, Changsha Research Institute of Mining and Metallurgy, Changsha 410012, P. R. China
Corresponding author: Dr. Xinye Qian Institute for Advanced Materials, Jiangsu University, Zhenjiang, 212013, P. R. China Tel./fax: +86-511-88791964 E-mail address: [email protected] Pro. Xiangqian Shen Institute for Advanced Materials, Jiangsu University, Zhenjiang, 212013, P. R. China Tel./fax: +86-511-88791964 E-mail address: [email protected]
Highlights
1 MnO-KB compsite was fabricated by wet impregnation in which MnO nanoparticles were well enwrapped by Ketjen Black. 2 Ketjen Black-MnO Composite was firstly used as the modified layer instead of the pure carbon material modified layer for Li-S batteries. 3 Ketjen Black-MnO layer showed excellent polysulfide absorption ability, and the MnO nanoparticles coated by the KB have the catalytic effect which enhanced the redox reactions.
Abstract A Ketjen Black-MnO composite was prepared by wet impregnation and TEM images showed that the MnO nanoparticles were well enwrapped by Ketjen Black (KB). The as-prepared KB-MnO composite were coated on one side of the commercial separators for the improvement of electrochemical performances of Lithium-Sulfur battery. For comparison, conventional separator coated by KB and conventional separator without coating was used in reference Lithium-Sulfur batteries. Electrochemical tests suggested that KB-MnO coated separator could act as the upper current collector and the barrier of lithium polisulfides which consequently achieved higher charge/discharge capacity and more stable cycling stability. Furthermore, TEM elemental map demonstrated that KB-MnO composite was more favorable for the adsorption of lithium polysulfides than KB, therefore the electrochemical performances of Li-S batteries can be dramatically enhanced.
Keywords: KB-MnO coated separator; Li-S battery; lithium polysulfides
1. Introduction Lithium-Sulfur battery is one of the most promising candidates of conventional Lithium-ion battery. Based on the reactions of S8+ 16Li ↔ 8Li2S, Lithium-Sulfur battery has a large theoretical energy density of 2600 Wh kg-1. In addition, sulfur as an active cathode material is non-toxic, naturally abundant, and environmentally benign [1-4]. Despite the advantages of Lithium-Sulfur battery, the low conductivity of sulfur, volume expansion of S to Li2S and the soluble intermediate products lithium polysulfide (Li2Sn, 4 < n < 8) which can shuttle between the lithium anode and sulfur cathode are three main problems remain to be solved before the Lithium-Sulfur battery can put into practical use [5-7]. The most popular approaches are to load sulfur in to conductive porous materials, such as meso/micro porous carbon [8,9], carbon nanotubes [10], grapheme [11,12], etc. The combination of carbon materials with sulfur can apparently improve the electronic conductivity of sulfur and accommodate the volume expansion during charge/discharge process. However, the porous structure of carbon materials can only adsorb part of the lithium polysulfides, therefore the high capacity loss induced by the dissolution of lithium polysulfide in organic electrolytes and shuttle effects are still remarkable. Recently, a new approach for the suppression of shuttle effects has been reported by Arumugam Manthiram et al. For example, a bifunctional microporous carbon paper was inserted between the sulfur cathode and separator, the MCP carbon interlayer operates not only as an ‘‘upper current collector’’ to enhance the active material utilization but also as a “polysulfide stockroom’’ to retain the cyclability [13]. Multiwalled carbon nanotubes were also used to fabricate interlayers for Lithium-Sulfur battery which significantly enhances both the specific capacity and cycle stability [14]. Qin et al have reported a cassava-derived carbon sheet as a polysulfide inhibitor for lithium-sulfur battery, The results show that the cell with the CCS interlayer exhibits excellent cycle stability and superior rate capability [15]. These carbon materials interlayers are highly conductive and sufficient in adsorption of lithium polysulfides, therefore the interlayer strategy greatly improves the utilization of sulfur and the cycle stability of Li-S battery. However, the capacity fading was still remarkable due to the limited adsorption
ability of carbon materials for lithium polysulfide. Recently, Nazar et al have reported a quite different chemical approach to polysulfide retention in the sulfur cathode, polysulfide can be sufficiently anchored on the surface of ultra-thin MnO2 nanosheets by the transfer mediator named as polythionate complex [16]. Interestingly, some other metal oxides used as additives or complexes in the sulfur cathode, such as La2O3 and Mg0.6Ni0.4O etc. also achieved superior electrochemical performances compared with the carbon-sulfur cathode materials [17,18]. Researches demonstrated that these metal oxides acted as adsorbents and catalysts of the lithium polysulfides, therefore the capacity, cycle stability and rate capability could be improved obviously. In this work, we have for the first time used the composite of ketjen black (a kind of conductive carbon) and MnO nanoparticles as the barrier layer of lithium polysulfides. KB-MnO composite in which MnO nanoparticles were well wrapped by KB was prepared by wet impregnation and then mechanically coated on one side of conventional separator. Electochemical tests and microstructure characterizations suggested that KB-MnO barrier layer have stronger lithium polysulfides adsorption ability than the pure KB barrier layer as well as a catalytic effect. The easy preparation and cheap raw materials made the KB-MnO coated separator a promising solution for the further improvement of Li–S battery.
2. Experimental
2.1. Synthesis of KB-MnO coated separator and the assemble of Li-S battery
The KB-MnO composite was prepared by a wet impregnation method. In a typical fabrication process, 0.8 g KB and 0.5 g manganese acetate [Mn(CH3COO)2] were dispersed in 100 cm3 ethanol with ultrasound at 500 W for more than 2 h. Then the solution was magnetic stirred for 12 h followed by a drying procedure at 80 °C in a drying oven. At last, the dried sample was heat treated under nitrogen atmosphere at 450 °C for 4 h with the heating rate of 5 °C min-1. The slurry of the membrane coating on one side of the commercial separator Celgard 2400 was prepared by mixing
KB-MnO composite, super-P and polyvinylidene fluoride (PVDF) in the weight ratio of 8 : 1 : 1 with N-methyl-e-pyrrolidinone (NMP) as the dispersant. Then the slurry was coated on one side of Celgard 2400 by an automatic coating machine and dried in vacuum oven for 12 hours at 50 °C. The thickness of the coated layer was about 8 μm by the measurement of micrometer as shown in Fig. 1 (a) and (b), the weight was approximately 0.1 - 0.15 mg cm-2. The cathode material of Li-S battery was prepared by melt diffusion method. KB and sulfur was mixed in the weight ratio of 1 : 3 and ball milled for 4 h, then the mixture was sealed in a reactor and heated at 155 °C for 12 h. The slurry of the cathode electrode for Li-S battery was prepared by mixing KB-sulfur composite, super-P and polyvinylidene fluoride (PVDF) in the weight ratio of 8 : 1 : 1 with N-methyl-e-pyrrolidinone (NMP) as the dispersant. Then the slurry was coated on an aluminum foil by an automatic coating machine and dried in vacuum oven for 12 h at 60 °C. The obtained film was cut in to disks with a diameter of 11 mm, the thickness of the cathode was about 80 μm by the measurement of micrometer as shown in Fig. 1 (c) and (d), the average mass of the cathode (include sulfur-KB composite, super-P and PVDF) was estimated to be 1.5 - 2 mg cm-2. Half-cells schematically shown in Fig. 1 (e) were assembled in a glove box (Mikrouna) filled with Ar atomosphere, KB-MnO coated Celgard 2400 was used as the separator, 1,3-dioxolane (DOL) and dimethoxymethane (DME) mixed by the volume ratio of 1:1 with 1 mol dm-3 LiTFSI and 0.1 mol dm-3 LiNO3 addition was used as the electrolyte. For comparison, KB coated Celgard 2400 and Celgard 2400 without coating were used as the separator in reference samples. The three samples were marked as conventional separator sample, KB coated separator sample and KB-MnO coated separator sample, respectively.
2.2. Characterization
The crystal structure analysis was carried out by power X-ray diffraction (XRD, D/Mmax 2500PC) with CuKα radiation (λ=0.154178 nm). The morphology of the KB-MnO composite was carried out by field emission scanning electron microscopy
(FE-SEM, JSM-7001F) and transmission electron microscopy (TEM, JEM-2100). The MnO content of KB-MnO composite was tested by the thermal gravimetric analysis (TGA, Netzsch, STA449F3) at the scan rate of 5 °C min-1 from room temperature to 900 °C. Nitrogen adsorption isotherm was measured by a pore size and surface area analyzer (Nova 2000) to analyze the specific surface areas and pore size distribution of KB and KB-MnO composite.
2.3. Electrochemical measurement
Galvanostatic charging/discharging tests were carried out in a potential range of 1.7 V - 2.8 V (vs Li/Li+) to measure specific capacity and cycling stability of the sulfur-KB composite cathode by using the LAND-CT2001A instrument (Wuhan Jinnuo, China). Cyclic voltammetry (CV) was employed on the AUTOLAB electrochemical work station in a potential range of 1.7 V - 2.8 V (vs Li/Li+) to investigate the redox peaks at the scan rate of 0.1 mV s-1. The electrochemical impedance spectrometry (EIS) was carried out in the frequency range between 100 kHz and 100 mHz.
3. Results and Discussion
Fig. 2 is the XRD pattern of KB-MnO composite. The broad peaks around 26 º and 42 º marked by black dots belong to the (0 0 2) plane and (1 0 0) plane of the graphite carbon structure, the other peaks around 34.9 º, 40.6 º, 58.7 º, 70.2 º and 73.8 º marked by inverted triangles are well-indexed to (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes of face-centered cubic phase of MnO [19-21]. None of the other diffraction peaks are observed in the XRD pattern. This result suggests that the as-prepared production is the mixture of carbon and MnO phase.
Fig. 3 (a) is the TEM image of KB-MnO composite, MnO nanoparticles display in the morphology of grey dots and black dots are uniformly distributed throughout
KB. Fig. 3 (b) illustrates the HRTEM image of KB-MnO composite, MnO nanoparticle is well enwrapped by KB, and the lattice fringes of MnO nanoparticle is clearly shown in the figure. Moreover, the size of a single lattice fringe is estimated to be 0.25 nm which agreed well with the interplanar spacing of (1 1 1) planes of MnO phase. The TEM and HRTEM image demonstrate that the KB-MnO composite was successfully synthesized. Fig. 3 (c) is the SEM image of KB-MnO composite and Fig. 3 (d) is the SEM EDS curve of Fig. 3 (c). EDS curve suggests that only elemental C, Mn and O exist in the KB-MnO composite, the peak of Si is caused by the silicon substrate and the peak of Au is caused by metal spraying before the SEM EDS analysis.
The contents of carbon and MnO in KB-MnO composite were checked by the thermal gravimetric analysis at the scan rate of 5 °C min-1 from room temperature to 900 °C in air atmosphere and TG curve is shown in Fig. 4. The contents of carbon and MnO can be estimated by the mass loss of the as-prepared KB-MnO composite. As shown in the figure, KB-MnO composite remains approximately 20 % of the initial mass, therefore, the MnO content in the KB-MnO composite is around 20 % and the carbon content in the KB-MnO composite is around 80 %. However, part of the MnO may be oxidized to Mn2O3, Mn3O4 and MnO2 etc. in the air atmosphere, which will increase the mass of MnO. Thus, the real content of MnO should be a little less than 20 %.
N2 adsorption–desorption isotherms of KB and KB-MnO composite are illustrated in Fig. 5 (a). Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific surface areas. The BET surface areas of KB and KB-MnO composite are 1200 and 1300 m2 g-1, respectively. It indicates that the specific surface area of KB-MnO composite is even a little larger than that of KB, therefore the adsorption ability of KB-MnO composite is probably better than that of KB. BJH pore size distributions of KB and KB-MnO composite are shown in Fig. 5 (b), most of the pores are below 5 nanometers.
Cyclic voltammograms (CV) of the conventional separator sample, KB coated separator sample and KB-MnO coated separator sample were carried out at the scan rate of 0.1 mV s-1 between 1.7 V and 2.8 V. As shown in the cathodic curve of Fig. 6 (a), (b) and (c), all samples present two reduction peaks at about 2.0 V and 2.3 V, corresponding to the sulfur reduction to soluble lithium polysulfide (Li2Sn, 2 < n < 8), and insoluble short polysulfide (Li2S2/Li2S), respectively. In the anodic scan, one broad oxidation peak at about 2.4 V is discovered which corresponds to the oxidation of short polysulfide (Li2S2/Li2S) to lithium polysulfide and elemental sulfur [22,23]. It should be noticed that at the reduction peaks around 2.0 V, the precise reduction peak (2.04 V) of KB-MnO coated separator sample is a little larger than that of KB coated separator sample and conventional separator sample (2.01 V). The higher reduction cell potential indicates the improved chemical reaction kinetics which implies that the KB-MnO coated layer have a catalytic effect on the sulfur reduction. On account of the KB coated separator sample shows the same cell reduction potential as that of the conventional separator sample, this catalytic effect is most probably induced by the MnO nanoparticles. This conclusion is similar to some previous studies in which metal oxide nanoparticles were found to have a catalytic effect on the sulfur redox reaction, for example La2O3, Mg0.6Ni0.4O and Mg0.8Cu0.2O [17,18,24]. Fig. 7 (a), (b) and (c) represents the charge/discharge curves of Li-S batteries using conventional separator, KB coated separator and KB-MnO coated separator, respectively. All the samples were tested at 1 C and all the charge/discharge capacities were calculated by the mass of elemental sulfur. The two discharge plateaus observed in all samples are consistent with the reduction peaks shown in CV curves, and the charge plateaus observed in all of the curves also matched well with the oxidation peaks shown in CV curves [25].
Fig. 8 (a) displays the the charge/discharge cycle performance of conventional separator sample, KB coated separator sample and KB-MnO coated separator sample at the rate of 1 C, all the charge/discharge capacities were calculated by the mass of elemental sulfur. The initial discharge capacities of conventional separator sample,
KB coated separator sample and KB-MnO coated separator sample are 723 mAh g-1, 907 mAh g-1 and 1059 mAh g-1, respectively. After 200 cycles, the reversible capacities of the three samples are 420 mAh g-1, 663 mAh g-1 and 901 mAh g-1. The capacity retention ratios of the samples are 58 %, 73 % and 85 % respectively. Moreover, the average coulomb efficiency of the three samples is approximately 97 %, 95 % and 90 %, respectively. The remarkably improved cyclic performance and coulomb efficiency of KB-MnO coated separator sample may be attributed to the strong adsorption ability of KB-MnO structure which enhanced the utilization of elemental sulfur and the suppression of shuttle effect. By rough calculation, the discharge capacity of KB-MnO coated separator sample after 200 cycles is more than two times as that of conventional separator sample, but the total mass and thickness in the cathode side increased by the KB-MnO coated layer are no more than 20 %.
After 200 cycles, Li-S batteries were disassembled. KB and KB-MnO composite coated on the separator were ultrasonic washed and centrifugated for several times in acetonitrile and NMP to remove LiTFSI and PVDF respevtively. Then TEM EDS analysis and elemental map test were carried out to further study the lithium polysulfide adsorption of KB and KB-MnO composite. EDS analysis of KB and KB-MnO composite shown in Fig. 9 (a) and (c) suggest that elemental sulfur exist in KB and KB-MnO composite after 200 cycles. It proves that lithium polysulfides were blocked and absorbed by KB or KB-MnO barriers which could suppress the shuttle effect. Elemental F is found in the EDS curve of KB barrier, it may be induced by PVDF which is not cleaned thoroughly. Fig. 9 (b) is the elemental map of KB barrier, carbon map and sulfur map are uniformly distributed and matched well with each other which indicates that lithium polysulfides are uniformly absorbed by the pores of KB. But in the elemental map of KB-MnO composite, the distribution of carbon and sulfur are not matched so well. Interestingly, the elemental map of sulfur is somehow matched with that of Mn. As shown in Fig. 9 (d) In the area where Mn existed, the orange dots represent elemental sulfur are more intensive than the area without Mn which indicates more lithium polysulfides are adsorbed on the spots of MnO
nanoparticles. Consequently, more lithium polysulfides can be absorbed by KB-MnO hybrid structure than that of KB. Thus, KB-MnO coated separator sample displayed larger discharge capacity and more stable cycle stability than that of KB coated separator sample and conventional separator sample.
Fig. 10 displays the rate capabilities of conventional separator sample, KB coated separator sample and KB-MnO coated separator sample at various rates. With the charge/discharge rates increased from 0.2 C to 2 C, KB-MnO coated separator sample delivers the discharge capacities of about 1200 mAh g-1 and 950 mAh g-1, respectively. KB coated separator sample delivers the discharge capacities of about 1120 mAh g-1 and 800 mAh g-1, respectively. Conventional separator sample delivers the lowest discharge capacities of about 930 mAh g-1 and 490 mAh g-1, respectively. When the charge/discharge rates decreased to 0.2 C, KB-MnO coated separator sample delivers the discharge capacities of about 1140 mAh g-1, thus the discharge capacities can maintain 95 % of the original value after the high rates charge/discharge processes. KB coated separator sample delivers the discharge capacities of about 1020 mAh g-1, thus the discharge capacities can maintain 91 % of the original value. Conventional separator sample delivers the discharge capacities of about 780 mAh g-1 which maintains only 83 % of the original value. The result of rate performance shows that KB-MnO coated separator sample can recover most of the discharge capacity when the charge/discharge rate was set back to 0.2 C. It suggests that lithium polysulfides are most sufficiently blocked between the cathode and the KB-MnO coated separator [26]. Moreover, KB-MnO coated separator sample shows the largest discharge capacity at the rate of 2 C which implies that it possesses the highest conductivity among the three samples, therefore it can effectively promote the electrochemical reactions in cathode electrode and KB-MnO barrier layer and improve the rate capability of Li-S battery [27].
In order to further demonstrate the improved conductivity of KB-MnO coated separator sample. EIS of the three samples were tested in the frequency region of 100
kHz - 100 mHz. The Nyquist plots of the samples obtained at the original state and after 200 charge/discharge cycles are shown in Fig. 11 (a), (b) and (c). These Nyquist plots are are consisted of an intercept at high frequency on the real axis which represents the resistance of the electrolyte (Rs), a depressed semicircle at high frequency region which corresponds to the charge transfer resistance (Rct) of the sulfur electrode, and an inclined line at low frequency region which reflects the Li+ diffusion into the active mass. As listed in Tab. 1, the Rct values of conventional separator sample, KB coated separator sample and KB-MnO coated separator sample at the original state are 75 Ω, 21 Ω and 14 Ω respectively. The relatively low Rct values of KB coated separator sample and KB-MnO coated separator sample are attributed to the high conductance of KB and KB-MnO coated layer which can act as the upper current collector. After 200 cycles, the Rct values of the samples are approximately 194 Ω, 35 Ω and 24.5 Ω respectively. The considerable charge transfer resistance increasement of conventional separator sample may be induced by the sulfur agglomeration on the surface of cathode during the charge/discharge process. By contrast, KB barrier layer and KB-MnO barrier layer can sufficiently adsorb the lithium polysulfides on the surface of the cathode, therefore the sulfur agglomeration is accommodated and the increase of charge transfer resistance is suppressed. As KB-MnO coated separator sample shows the lowest charge transfer resistance before and after 200cycles, it is supposed to have the fastest charge transportation speed, therefore the electrochemical reactions and rate capability can be dramatically improved [28]. The inclined line is attributed to the diffusion of the lithium ions into the bulk of the electrode material named as Warburg diffusion. The Warburg diffusion coefficient σw can be obtained by Eq. (1) [29,30]: Zre=Rs+Rct+σwω-1/2
(1)
σw are the slopes for the plots of Zre vs. the reciprocal root square of the lower angular frequencies (ω−1/2), which are presented in Fig. 11 (e). The diffusion coefficient values of the lithium ions (D) for their diffusion into the bulk electrode material have been calculated using Eq. (2) and are listed in Tab. 1 too. D=½ (RT/AF2σwC)2
(2)
where R is the gas constant (8.314 J mol−1 K−1), T is the room temperature (298.5 K), A is the area of the electrode surface (0.95 cm2), F is the Faraday’s constant (9.65×104 C mol−1) and C is the molar concentration of Li+ ions (1.1×10-3 mol cm-3). The D value of KB-MnO coated separator sample before and after cycling is about two orders of magnitude larger than that of the conventional separator sample, proving the superior Li+ diffusion mobility which is beneficial for the redox process. The exchange current density is given by Eq. (3): i°= RT/nFRct
(3)
n is the number of electrons transferred in the electrochemical reaction which is 2 for the charge/discharge reaction of S and Li. The i° of KB-MnO coated separator sample before and after cycling is higher than that of the other samples, therefore the redox reactions are also stronger. The double layer capacitance Cdl is determined by Eq. (4) [30]: Cdl = 1/2πfRct
(4)
Where f is the frequency corresponds to the maximum value of vertical axis on the semicircle. It is observed that the Cdl value of KB-MnO coated separator sample is lower than that of the other two samples, indicating the lower polarization during the charge/discharge process.
4. Conclusions In summary, KB-MnO composite was successively fabricated by wet impregnation and TEM images showed that the Manganese oxide nano particles were well coated by KB. BET results showed that the specific surface area of KB-MnO composite was approximately the same as that of KB. KB-MnO coated separator was prepared by coating the slurry on one side of the conventional separator for Li-S battery. TEM elemental map analysis demonstrated that KB-MnO composite could adsorb more lithium polysulfides than pure KB, consequently, Li-S battery using KB-MnO coated separator displayed higher discharge capacity and coulomb efficiency after 200cycles. Additionally, the highly conductive KB-MnO coated layer also acted as an upper current collector which speed up the electron transportation and
therefore enhanced the rate capability. However, the intrinsic chemical or physical reactions between KB-MnO and lithium polysifides which induced the better polysulfide adsorption ability still remain to be studied. Furthermore, other metal oxides such as transitional metal oxides and rare metal oxides with excellent adsorption ability and catalytic effect are existed to be researched for the application in Li-S battery.
Acknowledgements This work was financially supported by the Start-up Fund of Jiangsu University (Grant No. 14JDG060, 14JDG058), the Postdoctoral Fund of Jiangsu Province (Grant No. 1402196C), open fund of the Laboratory of Solid State Microstructures, Nanjing University (M28035), the National Natural Science Foundation of China (Grant No. 51274106, 51474113, 51474037), the Natural Science Foundation of Jiangsu Provincial Higher Education of China (Grant No. 12KJA430001).
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Fig. 1 (a) thickness of conventional separator, (b) thickness of KB-MnO coated separator, (c) thickness of aluminum foil, (d) thickness of aluminum foil coated with cathode materials, (e) schematic diagram of KB-MnO coated separator
Fig. 2 XRD pattern of KB-MnO composite
Fig. 3 (a) TEM image of KB-MnO composite, (b) HRTEM of a single MnO nanodot, (c) SEM image of KB-MnO composite and (d) SEM EDS curve of KB-MnO composite.
Fig. (a)
Fig. (b)
Fig. (c)
Fig. (d)
Fig. 4 TG curves of KB-MnO composite.
Fig. 5 (a) N2 adsorption–desorption isotherms of KB and KB-MnO composite and (b) BJH pore size distribution of KB and KB-MnO composite.
Fig. 6 CV curves of (a) conventional separator sample, (b) KB coated separator sample and (c) KB-MnO coated separator sample carried out at the scan rate of 0.1 mV s-1.
Fig. 7 charge/discharge curves of Li-S batteries using (a) conventional separator, (b) KB coated separator and (c) KB-MnO coated separator.
Fig. 8 charge/discharge cycle performance of conventional separator sample, KB coated separator sample and KB-MnO coated separator sample.
Fig. 9 (a) EDS curve of KB barrier layer after 200cycles, (b) Elemental map of KB barrier layer after 200cycles, (c) EDS curve of KB-MnO barrier layer after 200cycles, (d) Elemental map of KB-MnO barrier layer after 200cycles.
(a)
(b)
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Fig. 10 Rate capability of conventional separator sample, KB coated separator sample and KB-MnO coated separator sample.
Fig. 11 EIS of (a) conventional separator sample, (b) KB coated separator sample and (c) KB-MnO coated separator sample at original state and after 200 cycles, (d) schematic diagram of equivalent circuit, (e) the relationship of the samples between Zre and ω−1/2 at low frequencies.
(d)
(e)
Table 1 Electrochemical impedance parameters of the samples Rs Rct σw D Cdl samples (Ω) (Ω) (Ω s−1/2) (cm2 s−1) (F) C-original 1.7 75 22.8 6.3×10-11 5.3×10-6 C-200 cycles 5 194 22.08 6.7×10-11 1.39×10-6 KB-original 1.5 21 14.34 1.6×10-10 1.78×10-6 KB-200 cycles 4 35 11.57 2.4×10-10 2.35×10-6 KB-MnO-original 1.7 14 17.8 1.0×10-10 1.21×10-6 KB-MnO-200 cycles 3.5 24.5 3.05 3.5×10-9 1.03×10-6
i° (mA cm-2) 1.7×10-4 6.6×10-5 6.1×10-4 3.7×10-4 9.2×10-4 5.2×10-4