0278-x(s).fmINVITED REVIEW PAPER
†To whom correspondence should be addressed. E-mail: ys93@kier.re.kr Copyright by The Korean Institute of Chemical Engineers.
Carbide-derived carbon/sulfur composite cathode for multi-layer separator assembled Li-S battery
Sun-Hwa Yeon*,†, Wook Ahn*, **, Kyoung-Hee Shin*, Chang-Soo Jin*, Kyu-Nam Jung*,
Jae-Deok Jeon*, Sungnam Lim***, and Youngchul Kim****
*Korea Institute of Energy Research, 102, Gajeong-ro, Yuseong-gu, Daejeon 305-343, Korea **Department of Materials Science & Engineering, Yonsei University, 50, Yonsei-ro, Seodaemun-gu, Seoul 120-749, Korea
***Department of Chemical and Biomolecular Engineering, KAIST, 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea ****Agency for Defense Development, Yuseong-gu, P. O. Box 35-4, Daejeon 305-600, Korea
(Received 26 March 2014 • accepted 21 September 2014)
Abstract−To improve the electrochemical performance of Li-S rechargeable batteries, tunable porous carbon materi- als, which are known as carbide-derived carbons (CDCs), are employed as adsorbents and conductive matrices for the cathodic sulfur materials. A new assembly for Li-S cells was developed by introducing multi-layer membranes as sepa- rators. The use of the multi-layer membranes enables the minimization of the shuttle effect by expanding the distance between the separators and blocking the penetration of the polysulfide. The best discharge capacity and cycle life were obtained with ten layers of PP membrane in a sulfur-CDC@1200 composite cathode, resulting in a discharge capacity of 670 mA h g−1 and a minimal gap in the charge-discharge capacity during cell cycling.
Keywords: Li-S Battery, Carbide-derived Carbon, Multi-layer Membrane
INTRODUCTION
Sulfur (S) has many valuable characteristics, such as a low equiv- alent weight, extremely low cost, and nontoxicity. In a Li-S cell, the potential corresponds to 1/2-2/3 of the potential exhibited by a typ- ical positive electrode for a Li-ion cell (3.2 V-4.2 V). However, this low potential is outweighed by the 1,672mA h g−1 theoretical specific capacity, which is the highest of all known solid cathode materials [1].
Porous carbon possesses a high adsorption capacity when used as a storage medium and is the best active sorbent material for the cathodes in Li-S batteries. Recently developed carbon-sulfur cath- odes in Li-S batteries have achieved enhanced stability by incorpo- rating carbon nanotubes/nanofibers, graphene, mesoporous carbon, microporous carbon, and hierarchical porous carbon [2-7]. Partic- ularly, microporous carbon has been highlighted as the most promis- ing material for the sulfur cathode because it allows the creation of a homogeneous-amorphous composite via the doping of sulfur on microporous carbon. This material displays excellent cyclability because it maintains high porosity, even after the formation of the composite state [8]. However, while microporous carbon has an advantageous high specific surface area (SSA), its numerous pores cause low electrical conductivity. Therefore, control of the struc- ture and porosity are required to develop good electrical conduc- tivity within Li-S batteries. Previous studies revealed a high electro- chemical capacity (680mAh g−1) was enabled with low sulfur loaded- porous MSP carbon (~25 wt% S) as the Li-S cathode [9]. Unlike
commercial microporous carbon (MSP-20), carbide-derived car- bons (CDCs) are a type of carbon with easily tuned proper struc- tures, as well as intrinsic carbon structures that can be controlled via selective etching of crystalline metal carbides and temperature control during synthesis [10]. CDCs are possible candidates for use in Li-S batteries because their pore structures and highly ordered crystalline phases can simultaneously be controlled by modulat- ing the conditions during synthesis.
In Li-S batteries, the dissolution of polysulfides in the electrolyte during the discharge-charge process causes redox shuttling between the anode and cathode, triggering severe loss of usable active mate- rial [11,12]. Once the Li anode is fully coated, Sn
2− reacts with these reduced sulfides to form lower order polysulfides (S2−
n−x ), which be- come concentrated on the anode side. Additionally, after charging, the soluble polysulfide irreversibly deposits as lithium sulfide on the cathode surface to hinder lithium ion migration, deteriorate the electrode through sulfur poisoning, and reduce the electrode’s conductivity. These severe issues degrade the capacity when the system is subjected to a large current and lead to poor cycling sta- bility. To solve these problems, the formation of a sulfur composite framework and physical cell assembly has been attempted in the cathode, electrolyte, and anodes. A non-carbonate-containing sol- vent-based electrolyte [13] with an additive LiNO3 salt form the best electrolyte because the additive salt can prevent the polysul- fide shuttling or reduce the solubility of the polysulfide. The addi- tion of LiNO3 improves the cyclic performance of Li-S cells to some extent [14]. However, in spite of the improvements caused by the additive, the large gap between the discharge and charge capaci- ties during cycling remains an unsolved problem for diverse sul- fur composites at high sulfur loading.
In this study, we reveal porous S-CDC composite cathodes that
868 S.-H. Yeon et al.
May, 2015
can improve the cyclability of Li/S batteries at high sulfur loading. Additionally, a new Li-S cell assembly is proposed to minimize the polysulfide crossover during the discharge-charge. The multi-lay- ered separators enable the attenuation of the shuttling effect, pre- venting the penetration of the polysulfide during operation. Therefore, the new Li-S cell delivers improved charge-discharge cyclability by inhibiting the precipitation of solid Li2S2 and Li2S.
EXPERIMENTAL
1. Preparation of Cathode CDC powders were produced by chlorinating TiC powder (par-
ticle size 1-4µm). This precursor was placed in a horizontal tube furnace, purged in argon flow, and heated to 1,200 oC under flow- ing chlorine (10-15 cm3 min−1) for 3 h. CDC powders were then annealed at 600 oC for 2 h under flowing hydrogen to remove re- sidual chlorine and chloride trapped in pores. Previous studies showed that this procedure was not necessary at higher hydrogenation tem- peratures [10]. We denoted the prepared CDCs as CDC@1200, ac- cording to chlorination temperature, for example 1,200 oC. The ele- mental S powder was purchased from Sigma Aldrich (100 mesh particle size, reagent grade). The S loading procedures are as fol- lows. S-impregnation (IS) method: S powder was dissolved in a CS2 solution to form a 10 wt% S solution. The 10 wt% S solution was added dropwise to a CDC matrix with the same pore volume as the volume of solution with vigorous mixing. After the addition was complete, the matrix was slightly wet. Immediately afterwards, the resulting precursor was dried for 16h at 60-70 oC. We denoted the S-carbon composites prepared by the IS process as S-CDC@1200 composites, in which the S loadings were 25 wt%-50 wt%.
2. Characterization Gas adsorption analysis was performed using BELSORP-Max
MP (BEL Japan Inc.) with N2 adsorbate at −196 oC for the porous car- bon. Approximately 70 mg of the carbon was evacuated at 5 mTorr at 300 oC for 16 h. The measured pressure (P/P0) was 0.05-0.999. The Brunauer-Emmett-Teller (BET) SSA was calculated using the BET theory based on the adsorption branches of the isotherms. PSDs and pore volumes were determined using the non-local den- sity functional theory (NLDFT) method provided by data reduction software for N2 isotherms collected at −196 oC [15]. The NLDFT model assumes slit-shaped pores with uniformly dense carbon walls; the adsorbate is considered to be a fluid of hard spheres [15]. Sam- ples for transmission electron microscopy (TEM) were prepared by dispersing each sample in ethanol and placing the solution over a copper grid with a carbon film. A TEM study was performed using the Tecnai F20 microscope at 200 kV. X-ray diffraction (XRD) analysis was done using a Rigaku diffractometer with CuKα radi- ation (λ=0.154 nm) operating at 30 mA and 40 kV. XRD patterns were collected using step scans with a step size of 0.01o (2θ) and a count time of 2s per step between 5 (2θ) and 80 (2θ). Samples were analyzed by Raman spectroscopy (Renishaw 1000) using an Ar ion laser (514 nm) at 20 X magnification (~2µm spot size), 30% defo- cus, and 0.6 mW power. X-ray photoelectron spectroscopy (XPS) analysis was performed on a MultiLab 2000 (Thermo) instrument with Al Kα radiation (15 kV, 150 W) under a pressure of 5*10−10
mbar. 3. Electrochemical Evaluation
To evaluate the electrochemical performance of the porous car- bon composites prepared using the two processes, we fabricated 2032-type coin cells (MTI). The prepared sample was pressed onto
Fig. 1. TEM images (a,b,c), XRD (d), and raman (e) of pristine S powder, CDC chlorinated at 1,200 oC, and S-CDC@1200 composite.
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Korean J. Chem. Eng.(Vol. 32, No. 5)
an aluminium substrate as the working electrode, including binder and conductive additives. Li was used as the counter electrode. The separator used was CelgardR 3501 (microporous monolayer membrane). To inhibit the shuttling effect in the cathodes, multi- layer PP separators were implemented as physical barriers through 2, 4, and 10 stacks of one-layer PP separators. The characteristics of the separator are as follows: thickness of 25µm, porosity of 55%, pore size of 64 nm. The specific capacities were calculated based on only the sulfur mass. The cathode electrode material was pre- pared by adding Super-P (10%) as a conductive additive and poly- vinyl fluoride (PVDF, 10%) as a binder in all cases. The utilized electrolyte was composed of the following: 1 M of Li TFSI in 1,3- dioxolane and 1,2-dimethoxyethane (volume ratio 1 : 1), denoted as 1 M LiTFSI/DOX·DMO.
RESULTS AND DISCUSSION
Fig. 1 provides TEM images, XRD, and Raman results of the pris- tine sulfur powder, CDC@1200, and S-CDC@1200 composite. In Fig. 1(a), the pristine S powder exhibits crystalline structures with d-spacings (2 2 2) of a-S8, corresponding to approximately 0.3 nm. The CDC@1200 formed highly ordered curved sheets of graphite with an interplanar spacing of approximately 0.3-0.4nm. The amor- phous carbons coexisted within the curved graphite. Micropores and mesopores were embedded in the amorphous area surrounding the graphitic region, as illustrated in Fig. 1(b). For the S-CDC@1200 composite, sulfur and CDC@1200 were mixed homogeneously without being able to distinguish the crystalline S phase from the CDC@1200 matrix. A typical XRD pattern of the CDCs chlori- nated at 1,200 oC displays a strong diffraction line corresponding to the graphite (0 0 2) reflection (2θ=26.54o), which is in the pattern of the sample superimposed on a very broad peak from amorphous carbon [10]. The S-CDC@1200 composite exhibited one broad peak from amorphous S in the 2θ range of 20-25o, but there were no S crystalline peaks. The peaks corresponding to orthorhombic sul- fur disappeared completely after the CS2 extraction during the IS method. The Raman spectra (Fig. 1(e)) were normalized to the inten- sity of the G-band at ~1,580cm−1. In the sulfur powder, strong Raman modes were observed at 153, 217, and 471 cm−1, which agrees with the previously reported values of polycrystalline S [16], and these modes were designated as ν1, ν2, and ν8, respectively. This is an ort- horhombic crystal built of S8 molecules, with unit cell dimensions a=1.045 nm, b=1.2884 nm, and C=2.446 nm (Powder Diffraction Card Number 8-247, International Center for Diffraction Data, New- ton Square, PA, USA). The CDC@1200 exhibited a broad D band typical of amorphous disordered carbon [10,17]. S-CDC@1200 com- posites did not display peaks indicative of crystalline S in the range of 400-600 cm−1.
Fig. 2 shows the N2 adsorption isotherms and the estimated PSDs of CDCs obtained from the CDC-1200, determined from the N2
sorption isotherms using the NLDFT model for slit pores. The marked hysteresis observed in the nitrogen adsorption confirms the pres- ence of mesoporosity. High temperature of CDC@1200 led to larger pores, which can be tuned in a range from 0.5 to 4 nm (micropore +mesopore) with BET SSA, ~1,100 m2/g.
X-ray photoelectron spectroscopy (XPS) (MultiLab 2000 Thermo)
was employed to analyze the CDC@1200 and S-CDC@1200 com- posite. The C 1s XPS spectrum of the CDC@1200 (Fig. 3(a)) clearly indicates only the non-oxygenated ring C (284 eV). The carbon C 1s signal was always multicomponent; only the most intense peak was used in this work for charge compensation. The S-CDC@1200 com- posite has a degree of oxidation, with one component that corre- sponds to carbon atoms in a different functional group: the C in the C-O bonds (286 eV). The most intense sulfur line, S 2p, is asym- metric due to spin-orbit splitting. The sulfur S2p spectrum of the S element is composed of two components, one with the S2p3/2 signal at 165.3±0.2 eV (red curves) and the other with the signal at 166.1±0.2 eV (blue plot). The lower binding energy component can be attributed to the sulfur atoms in the S powder [18,19]. The higher binding energy (168-169 eV) is due to the components of sulfate or some other form of oxidized sulfur. A new peak at 164± 0.4 eV in S-CDC@1200 composites progressively appears that could be due to heterocyclic sulfur within an aromatic environment of the isotropic, non-graphitizing carbon from the porous carbon matrix [20].
The cyclic performance and voltage profiles of S-CDC@1200 electrodes (25 wt% and 40 wt% S) with separators composed of typical mono PP layer are presented in Fig. 4. In previous study, pristine S showed a poor cycling stability with a rapid discharge capacity fading. The discharge capacity showed low capacity of 240 mAh g−1 at 25th cycle [21,22] The discharge capacity and cycle life
Fig. 2. (a) N2 sorption isotherm and (b) PSDs of TiC-CDC@1200.
870 S.-H. Yeon et al.
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obtained with the S-CDC@1200 composite is ~500 mA h g−1 after 50 cycles at a rate of 0.1 C. However, a large gap in the capacity between the charge and discharge is revealed in the S-CDC@1200 composite cathode when the electrolyte 1 M LiTFSI/DOX·DMO is used in this system. The first discharge of the S-CDC@1200 com- posite does not exhibit the typical two-plateau behavior (Fig. 4(b)). While the reduction of elemental sulfur (S8) to S6
2− and S2 2− stages
at 2.4 V and 2.03 V is very short, the conversion stage between S2 2−
to S2− is long. Usually, the first recharge of a Li-S cell does not result in the transformation of polysulfides into elemental sulfur. During the second cycle, the plateau at 2.03 V increases gradually. As illus- trated in the voltage profiles provided in Fig. 4(b), the two cath- odes revealed large gaps between the charge and discharge during the second cycle. This gap is caused by the shuttling effect; the higher polysulfides diffuse to the anode where they are reduced to lower polysulfides, which diffuse back through the electrolyte to the cath- ode where they are reoxidized to generate higher polysulfides. Fur- thermore, the reaction time required for the charge operation is too long relative to the time necessary for the discharge operation. To solve this problem, the shuttle must be suppressed by a physi- cal barrier covering the Li or inhibited by various classes of NOx compounds, like nitrate and nitrides [23]. However, the additive did not influence the reduction of the capacity gap in this study. To inhibit the shuttling effect in the cathodes of Li-S cells with high sulphur loading, multi-layered PP separators are implemented as physical barriers in place of the additives, which obstruct the migra- tion of the produced polysulfides from the cathode to the anode. Fig. 5 depicts the cycle effects from the charge-discharge profiles of the S-CDC@1200 composite (above 50% sulfur loading) on the variations of mono- and multi-layer (2, 4, 10 layers) PP separators. In the mono-layer separator displayed in Fig. 5(a), there is a large
Fig. 4. The cyclic performance (a) and voltage profiles (b) of S-CDC@ 1200 electrodes (25 wt% and 40 wt% S) with separators com- posed of typical mono PP layer.
Fig. 3. The C 1s XPS spectra of (a) CDC@1200, (b) S-CDC@1200 composite, and the S2p XPS spectra of (c) Sulfur powder, (d) S-CDC@1200.
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Korean J. Chem. Eng.(Vol. 32, No. 5)
gap between the charge and discharge capacities, indicating the pres- ence of more serious hysteresis than in the low S-loading cathode shown in Fig. 4. Therefore, the charge-shuttle time is too long, which is indicative of a slow Li plating process for the polysulfides [24]. However, the gap gradually decreased when the number of PP sep- arator layers, which play an important role in hindering the shut- tle effect, increased, as shown in Fig, 5(b)-(d). The best discharge capacity and cycle life were obtained with ten layers of PP separa- tor in an S-CDC@1200 composite with 50 wt% sulfur loading: 670 mA h g−1 after 50 cycles at a 0.1 C rate with a minimal gap in the capacities. Fig. 5(e) and Fig. 5(f) display the before and after ac im- pedance spectra of the Li metal/electrolyte/PP separator (1, 2, and 4)/S-CDC@1200 unit cells at room temperature. The open circuit EIS spectra of the S-CDC@1200 form a semicircle at high frequen- cies and an inclined tail in the low frequency region. In the initial
cells (Fig. 3(e)), the interfacial resistance increased when the num- ber of PP layers was increased before the cycling operation. The presence of additional PP layers caused an increase in the initial internal resistance of the unit cell. However, the increased resis- tance did not demonstrate a lower capacity at the initial stage, as indicated in Fig. 5(a)-(c). After 50 cycles, the ac impedance spectra were measured again, with results summarized in Fig. 5(e). The resistance after 50 cycles decreased more than it did for the initial cells. Additionally, the resistance of the multi-layer (2 or 4) cell was lower than the resistance of the mono-layer cell after 50 cycles. The resistance of the 10-layer cell was larger than the resistance of the 4-layer cell, but overall the resistance greatly decreased after cycling (See Fig. 1S in SI). This result indicates that the charge and discharge performance of the unit cell depends on the properties and dimen- sions of the separator utilized. Inhibition of the polysulfide pene-
Fig. 5. Effect of cycle performance of S-CDC@1200 composite cathode with separators of: 1-10 layers (a)-(d) and impedance spectroscopy response of the cathode varying the number of separator layers before and after 50 cycles (e). Comparison of discharge capacity on the variance of S-loadings and separator layers (f).
872 S.-H. Yeon et al.
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tration during cycling can be controlled with the separator, which is an important method for reducing the shuttling effect, as shown in the schematic diagram on shuttle effect between separators of Li-S cell (Fig. 6). Fig. 5(f) compares the discharge capacities rela- tive to the variance of S-loadings (25 wt%, 40 wt%, and 50 wt%) and the multiple separator layers in the 50 wt%-loaded composite. As the S-loading in the composite increased from 25wt% to 50wt%, the discharge capacities decreased in the mono-layer separator cell from ~500 mAh g−1 to ~400 mAh g−1, respectively, resulting in in- creased interfacial resistance. However, in the system using 50 wt% S-loading, the multi-layer separator significantly enhanced the dis- charge capacity (670 mAh g−1), which is 40% higher than the dis- charge of the mono-layer separated cell.
CONCLUSIONS
To suppress the erosion of capacity and improve cyclic reten- tion of lithium-sulfur batteries, tailoring the morphology of cath- ode materials and decreasing the shuttle effect between separator and electrolyte are essential. The cell assembly, including the cath- ode materials, should be specifically designed to physically block the dissolution of polysulfides and maintain electronic conductiv- ity in the electrodes. An S-CDC@1200 composite might be appro- priate for use in Li-S batteries because the porous and crystalline structures can coexist. However, high S-loading still interrupts the intrinsic electrochemical properties in the S-CDC@1200 composite. A new assembly for Li-S cells integrates multi-layer PP membranes as separators in high S-loading cathodes with a DOX·DMO elec- trolyte. The multi-layered PP separators enable the minimization of the shuttling effect by expanding the separator distance and block- ing the penetration of polysulfides; therefore, the precipitation of solid Li2S2 and Li2S on the two electrodes can be inhibited, indicat- ing that the gap between the charge and the discharge capacity can be gradually decreased. The best discharge capacity and cycle life were obtained with ten layers of PP separator in an S-CDC@1200 composite (50 wt% S loading), 670 mA h g−1 after 50 cycles at a 0.1 C rate with minimal gap in capacity. The development of these multi-layer separator assemblies with S-CDC@1200 composites in
advanced practical Li-S cells enhances the overall cell performance to exceed the current technological limits without surfactant addi- tives in the electrolyte.
ACKNOWLEDGEMENT
This work was supported by the Next Generation Military Bat- tery Research Center program of The Defense Acquisition Program Administration and Agency for Defense Development.
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Supporting Information
Sun-Hwa Yeon*,†, Wook Ahn*, **, Kyoung-Hee Shin*, Chang-Soo Jin*, Kyu-Nam Jung*,
Jae-Deok Jeon*, Sungnam Lim***, and Youngchul Kim****
*Korea Institute of Energy Research, 102, Gajeong-ro, Yuseong-gu, Daejeon 305-343, Korea **Department of Materials Science & Engineering, Yonsei University, 50, Yonsei-ro, Seodaemun-gu, Seoul 120-749, Korea
***Department of Chemical and Biomolecular Engineering, KAIST, 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea ****Agency for Defense Development, Yuseong-gu, P. O. Box 35-4, Daejeon 305-600, Korea
(Received 26 March 2014 • accepted 21 September 2014)
Fig. S1. Effect of cycle performance of S-CDC@1200 composite cathode with separators of: (a) 1 layer, (b) 2 layers, (c) 3 layers, and (d) 10 layers. All samples were performed at 0.1C rate.
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