A hierarchical micro/mesoporous carbon fiber/sulfur composite for high-performance lithium–sulfur batteries

Abstract

A carbon matrix with an appropriate porous structure plays a vital role in developing high-performance sulfur/carbon cathodes of lithium–sulfur batteries. In this work, a hierarchical porous carbon fiber (HPCF) with a few mesopores and abundant micropores was prepared via electrospinning with a SiO₂ template and subsequent KOH activation. The HPCF with an ultra-high surface area and a large pore volume can construct a loose network structure to promise high sulfur utilization and sufficient sulfur loading. Mesopores can provide pathways for the infiltration of electrolyte to ensure fast transport of lithium ions during electrochemical reactions, whereas micropores can effectively suppress the diffusion of polysulfides by their strong adsorption capability. Due to such advantages, the proposed cathode, with 66 wt% sulfur content, can yield a high reversible capacity of 1070.6 mA h g⁻¹ at 0.5C, and a stable cycle performance with a capacity retention of 88.4% after 100 cycles.

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1. Introduction

Nowadays, with the development of high energy-consuming portable devices and electric vehicles, the demand of energy storage devices with large specific energy is increased dramatically. As a promising next-generation energy storage system, the lithium–sulfur (Li–S) battery has received extensive attention due to its high theoretical energy density of 2600 Wh kg⁻¹, low cost and non-toxicity.^1,2 Nevertheless, its widely practical application has still faced a number of challenging problems, including the low electrical conductivity of sulfur and the shuttling behavior of soluble lithium polysulfides (Li₂S_x, 4 < x < 8) between anode and cathode during cycling.^3,4

In order to overcome these issues, many strategies have been developed, such as modifying electrolyte compositions,^5–7 combining sulfur with host substrates,^8–11 coating a conductive polymer layer^12,13 and adding a interlayer.^14,15 The commonly-used approach is to introduce carbon materials as supporting matrices for the sulfur cathodes due to their favorable characteristics of high electrical conductivity, low cost and large surface area.^8,12,13 Therefore, various carbonaceous materials, such as porous carbons,^16,17 carbon fibers,^18,19 hollow carbon spheres^20,21 and graphenes,^22,23 have been widely used as the sulfur hosts. Recent investigations suggested that hierarchical micro/mesopores porous carbon would be an ideal pore structure to improve the electrochemical performances of Li–S batteries.^24–27 Mesopores can provide pathways for the infiltration of electrolyte to ensure fast transport of lithium ions during the electrochemical reactions, whereas micropores can suppress the diffusion of soluble lithium polysulfides effectively.^28–30 Guo et al.³¹ confirmed that sulfur in the 0.5 nm micropores existed as small chainlike molecules S_2–4 because of the space limitation. Thus, the soluble high-order lithium polysulfides would not be formed during the redox process.³² Moreover, micro/mesopores carbonaceous materials usually possess a large surface area, which is beneficial to improve the utilization of sulfur and provide more sites for the deposition of insoluble low-order Li₂S₂ and Li₂S.³³ Guo et al.³⁴ synthesized a micro/mesoporous carbon sphere with 37% micropores via KOH activation of hydrothermal carbon precursors. With these materials, they showed a long cycle life of 800 cycles at 1C with a retained capacity of 600 mA h g⁻¹. Li et al.³⁵ prepared a highly ordered meso–microporous core–shell carbon, which exhibited a reversible capacity of 837 mA h g⁻¹ at 0.5C after 200 cycles and a capacity retention of 80%. In addition, to achieve a high utilization of sulfur, porous carbon fibers also have aroused much interest because of their high electrical conductivity, large surface area and robust mechanical property.³⁶ Zhang et al.³⁷ reported a porous carbon fiber prepared via electrospinning and demonstrated a stable high capacity of about 1400 mA h g⁻¹ at 0.05C. Yang et al.³⁸ synthesized a novel lotus root-like carbon fiber by alginate fibers and their Li–S battery delivered a high initial capacity of 1477 mA h g⁻¹ at 0.1C. Therefore, hierarchical micro/mesoporous carbon fibers would be an interesting and promising structure for sulfur host to achieve high electrochemical performance.

Herein, we prepared a high surface area porous carbon fiber (HPCF) with a hierarchical structure of a few mesopores and abundant micropores as the sulfur container via electrospinning technique. The synthesis process was illustrated in Fig. 1. To produce micro/mesoporous structures, SiO₂ was used as a hard template to generate the mesopores, while the micropores were formed by KOH chemical activation. Obviously, the pore size distribution can be easily controlled by adjusting the content of SiO₂ and KOH. After sulfur infiltration into HPCF by heat treatment, the S/HPCF composite was coated onto a commercial porous carbon paper which can accommodate more active materials and retain the electrolyte containing the dissolved polysulfides.^39,40 It is believed that the prepared S/HPCF can not only construct a loose network structure to conduct electron and alleviate the volume variation during cycling, but also facilitate the transport of lithium ions and effectively impede the diffusion of polysulfides by its unique pore structure.

Fig. 1 Schematic illustration for the preparation process of S/HPCF.

2. Experimental

2.1 Materials preparations

The SiO₂/polyacrylonitrile (PAN) fibers were prepared by electrospinning method. N,N-Dimethylformamide (DMF, Aladdin) containing PAN (M_w = 130 000, Aldrich) and hydrophobic silica aerogel (SiO₂, Aladdin) were used as the polymer solution and the pore former, respectively. In a typical synthesis process, 1.1 g PAN was dissolved in 20 ml DMF with stirring for 12 h. Then, hydrophobic silica aerogel was added into the PAN solution and was stirred for another 12 h. The well-mixed SiO₂/PAN solution was poured into a 30 ml syringe with an 18 gauge stainless steel needle and was electrospun by a commercial apparatus (TL-Pro, TLWNT Corporation). The applied voltage, pumping rate of syringe, tip-to-collector distance and collector (aluminum foil) rotating speed were 14 kV, 0.5 ml h⁻¹, 12 cm and 800 rpm, respectively.

The obtained SiO₂/PAN fibers were stabilized at 260 °C for 4 h under air in order to convert PAN from a thermoplastic to a thermoset state, which was critical to keep the fibrous morphology at high temperature. Then, the stabilized fibers were carbonized at 800 °C for 2 h with a heating rate of 5 °C min⁻¹ in Ar atmosphere. Subsequently, the carbonized fibers were immersed into 20 wt% HF aqueous solutions for 12 h, followed by filtration and water washing several times. Finally, the porous carbon fibers (PCF) were obtained after drying at 80 °C for 12 h. To prepare HPCF, 1 g PCF was mixed with 3 g KOH in 50 ml deionized water. The solution was heated at 80 °C with continuous agitation until the water was completely evaporated. The mixture was then transferred into a nickel crucible and heated to 800 °C for 1 h under Ar atmosphere. HPCF was ultimately obtained after filtration, washing and drying.

The as-prepared PCF and HPCF were milled with sublimed sulfur (purity > 99.5%, Aladdin) in a weight ratio of 1 : 2, respectively. The mixtures were then sealed in a closed vacuum glass tube and heated at 155 °C for 12 h. Subsequently, the heating temperature was raised to 300 °C for 2 h to ensure better dispersion of sulfur into the pores of PCF or HPCF. After cooling, S/PCF and S/HPCF were obtained.

2.2 Materials characterizations

The morphologies and element distributions of samples were analyzed by a field emission scanning electron microscopy (FESEM, Hitachi SU-70) equipped with an energy dispersive X-ray spectrometer (EDS) and a transmission electron microscopy (TEM, Tecnai G² Spirit 120 kV). Nitrogen sorption isotherms were measured at −196 °C with a gas sorption instrument (BELL V-Sorb 2800TP). The pore size distributions were calculated by the Barret–Joyner–Halender (BJH) and Horvath–Kawazoe (HK) methods. The structure and composition of samples were examinated by a powder X-ray diffraction (XRD, D8 Advance, Bruker) using Cu Kα radiation (λ = 0.15406 nm) at a scan rate of 4° min⁻¹. The sulfur content was confirmed by a thermogravimetry (TG, STA409PC) with a heating rate of 10 °C min⁻¹ in N₂ atmosphere. X-ray photoelectron spectroscopy (XPS) analysis of S/HPCF and S/PCF were conducted on an auger electron spectroscopy (Thermofisher, Microlab 350).

2.3 Preparations of cathodes and battery assembly

To prepare the sulfur cathode, 80 wt% S/PCF or S/HPCF, 10 wt% acetylene black and 10 wt% binder (LA133) were dissolved in deionized water–isopropanol (3 : 1 in volume) to form a slurry. After stirring for 12 h, the slurry was coated on a porous carbon paper (Toray carbon paper H-060) to form a sulfur cathode. The sulfur loadings in the cathodes were determined to be approximately 2 mg cm⁻² after drying at 60 °C for 24 h in an air circulation oven. The CR2032 coin cells were assembled in a glove box under argon atmosphere, with lithium metal plates as anodes and Celgard 2400 membranes as separators. The electrolyte used in this work was 1 M bis-(tri-fluoromethane) sulfonimide lithium (LiTFSI) in 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) (1 : 1, v/v) mixed solution and the loading of electrolyte in the battery was 100 μl. LiNO₃ was not used as an additive to suppress the shuttle effect due to its decomposition below 1.5 V.

2.4 Electrochemical characterizations

The galvanostatic charge/discharge measurements of S/HPCF were carried out at room temperature by a LAND CT2001A testing system with the cutoff voltages of 1.0 V (vs. Li/Li⁺) for discharge and 3.0 V (vs. Li/Li⁺) for charge. To obtain a stable performance for long cycle measurements of S/PCF, the cutoff voltages were set to 1.7 V (vs. Li/Li⁺) for discharge and 2.8 V (vs. Li/Li⁺) for charge. The cyclic voltammetry (CV) measurements were conducted by a CHI 660D electrochemical workstation from 3.0 to 1.0 V at a scan rate of 0.1 mV s⁻¹. Electrochemical impedance spectra (EIS) were measured from 0.01 to 10⁵ Hz with a voltage amplitude of 5 mV under the open-circuit condition. The specific capacities were calculated based on the mass of sulfur and the current rate of 1C was equal to 1672 mA g⁻¹.

3. Results and discussion

FESEM images of PCF, HPCF, S/PCF and S/HPCF are shown in Fig. 2(a)–(f). The porous fiber structure with connected open macropores is clearly observed in PCF, which is caused by the removal of connected or aggregated SiO₂.⁴¹ In addition, it is found that the morphology of PCF changes with various SiO₂ contents. When the weight ratio of PAN to SiO₂ is 5 : 1, the carbon fibers are relatively smooth without apparent open pores. Increasing the content of SiO₂ to 50 wt% (PAN : SiO₂ = 5 : 5), the fibrous structure is significantly destroyed. When the SiO₂ content is 37.5% (PAN : SiO₂ = 5 : 3), the PCF remains fibrous and a well-developed pore structure with numbers of open macropores is obtained, which are favorable for the permeation of KOH solution to produce micropores and the infiltration of electrolyte to promote the transport of lithium ion. The morphology of PCF (PAN : SiO₂ = 5 : 3) after KOH activation is shown in Fig. 2(d) and it is found that no substantial macroscopic morphological change can be observed. Fig. 2(e) and (f) show the morphologies of PCF and HPCF after the introduction of sulfur. It is seen in Fig. 2(e) that the surface of PCF is covered with sulfur and few open pores can be observed. In contrast, the morphological change of HPCF is less significant: a number of open pores can still be seen, indicating that most of sulfur has penetrated into the micropores of HPCF. To further identify the microstructures of S/PCF and S/HPCF, the morphologies are characterized by TEM and the results are shown in Fig. 2(g) and (h), respectively. It can be seen that sulfur mainly deposits on the surface of PCF and almost wraps around the fiber. On the contrary, only a small amount of sulfur exists as small particles of about 10 nm in the mesopores of S/HPCF, which further indicates that sulfur is mainly distributed in the micropores of S/HPCF. The distribution of sulfur in S/HPCF is examined by EDS and the results are shown in Fig. 3. Clearly, carbon, oxygen and sulfur elements are detected in the selected area of S/HPCF. By combing the elemental mappings of S and C, one can infer that sulfur is distributed homogeneously in the carbon substrate. It is worth mentioning that oxygen element derived from the oxidation process can facilitate the formation of the C–S bond to trap the sulfur, which is believed to be beneficial for improving the cycle performance.^42,43

Fig. 2 FESEM images of PCF obtained with different weight ratios of PAN and SiO₂: 5 : 1 (a), 5 : 5 (b), 5 : 3 (c); FESEM images of HPCF (d), S/PCF (e) and S/HPCF (f); TEM images of S/PCF (g) and S/HPCF (h).

Fig. 3 EDS results of S/HPCF.

The N₂ adsorption/desorption isotherm of PCF displays a hysteresis loop in Fig. 4(a), which is the type IV isotherm according to IUPAC's classification.⁴⁴ In contrast, it is seen in Fig. 4(c) that HPCF shows a combination of type I and type II isotherms. At low relative pressures (P/P₀ < 0.1), HPCF exhibits a strong nitrogen adsorption, indicating the existence of abundant micropores. At mediate and high relative pressures (0.1 < P/P₀ < 0.9), a gentle slope is observed for HPCF, implying that the amount of mesopores is much smaller than that of micropores. After loading sulfur, the N₂ adsorbed volumes of PCF and HPCF decrease dramatically as shown in Fig. 4(a) and (c), from which it can be inferred that sulfur have been distributed into both the micropores and mesopores. As shown in Table 1, the surface area of PCF is 60.3 m² g⁻¹ with a total pore volume of 0.16 cm³ g⁻¹, whereas the HPCF exhibits an ultra-high surface area of 2480 m² g⁻¹ and a large pore volume of 1.84 cm³ g⁻¹. This suggests that the interconnected open macropores and mesopores facilitate the infiltration of KOH to generate abundant micropores. The pore size distributions of PCF and HPCF before and after introducing sulfur are shown in Fig. 4(b) and (d), respectively. By comparing these pore size distributions, it can be inferred that sulfur has penetrated into the mesopores (6–20 nm) of PCF and the micropores (0.4–0.8 nm) of HPCF. It is important to note that due to ringlike molecules S₈ can only be accommodated in the pores larger than 0.69 nm, partial sulfur would exist as small chainlike molecules S_2–4 in HPCF.⁴⁵

Fig. 4 N₂ adsorption/desorption isotherms and BJH pore size distributions of PCF (a and b) and HPCF (c and d). The inset is HK pore size distribution of HPCF.

Table 1 Surface areas and pore volumes of PCF, HPCF, S/PCF and S/HPCF

Samples	S (m^2 g^−1)	V (cm^3 g^−1)
PCF	60.3	0.16
HPCF	2480	1.84
S/PCF	3.0	0.01
S/HPCF	36	0.23

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The XRD patterns of HPCF, S/HPCF, S/PCF and sulfur are presented in Fig. 5(a). HPCF shows a low and broad peak, which is the typical feature of amorphous carbon. After the introduction of sulfur into PCF and HPCF, S/PCF exhibits some characteristic peaks of sulfur, whereas no sharp peaks are observed in S/HPCF. This suggests that an appreciable amount of sulfur is covered on the surface of PCF while most of sulfur has penetrated into the micro/mesopores of HPCF. To determine the sulfur contents of S/PCF and S/HPCF, the TG curves obtained in N₂ are given in Fig. 5(b). It is observed that pure sulfur is completely evaporated at around 300 °C. The weight loss curve of S/PCF is similar to that of pure sulfur, suggesting a weak adsorption of sulfur by mesopores in PCF. For S/HPCF, a slower loss rate and a higher ending temperature of sulfur evaporation are observed due to a strong adsorption of sulfur by micropores. Based on the TG curves, the sulfur contents of S/PCF and S/HPCF are determined to be both 66 wt%. To understand the chemical states of sulfur in S/PCF and S/HPCF, XPS tests are performed and the results are shown in Fig. 6. It is seen that PCF exhibits two normal peaks at 163.52 eV and 164.71 eV, which are in good agreement with the characteristic S 2p peaks of S₈.^46,47 Compared with PCF, HPCF shows two additional peaks at higher binding energies, which are caused by the sulfur atoms located at the chain end of S_2–4.^35,48 Based on the above analysis, it is believed that sulfur has diffused into the ultra-fine micropores of HPCF to form small chainlike molecules S_2–4 because of the space confinement.

Fig. 5 XRD patterns of HPCF, S/HPCF, S/PCF and sulfur (a) and thermogravimetric curves for sulfur, S/PCF and S/HPCF (b).

Fig. 6 XPS spectrum of S 2p for S/HPCF (a) and S/PCF (b).

In order to evaluate the influences of PCF and HPCF on the electrochemical behaviors of sulfur, a series of electrochemical tests were performed. The CV curves with a scan rate of 0.1 mV s⁻¹ are shown in Fig. 7(a) and (b). As activation phenomena would occur in the first cycles, the second cycles of CV tests are chosen for comparison. The CV curve of S/PCF cathode shows typical electrochemical features in Li–S batteries:⁸ two reduction peaks at approximately 2.3 V and 2.0 V, respectively, and one oxidation peak at about 2.6 V. The reduction peak at a higher potential is associated with the conversion of S₈ to soluble high-order lithium polysulfides (Li₂S_n, 4 ≤ n < 8) while the one at a lower potential is corresponded to formation of insoluble low-order Li₂S₂ and Li₂S.^1,3 According to previous works,³⁷ the oxidation peak at around 2.6 V should be attributed to the formation of high-order lithium polysulfide Li₂S₈ or S₈. For S/HPCF cathode, it is interesting to see that there are three reduction peaks at 2.3 V, 2.0 V and 1.6 V, respectively. The first and second reduction peaks at 2.3 V and 2.0 V, respectively, correspond to the same reduction process of S/PCF cathode. The additional reduction peak at 1.6 V should be related to the reduction of small chainlike molecules S_2–4 in the micropores to Li₂S₂/Li₂S.^35,49 The kinetics of such a solid–solid reaction is rather slow and thus it is accompanied by a large polarization. Another salient difference between the S/PCF and S/HPCF is that two oxidation peaks exist during the oxidation process. Compared with S/PCF cathode, the additional peak at 2.1 V most likely corresponds to the conversion of lithium sulfides to sulfur in micropores of S/HPCF cathode. The galvanostatic charge/discharge profiles of S/PCF and S/HPCF cathodes at 0.2C are shown in Fig. 7(c) and (d). It is found that the discharge plateaus of S/PCF and S/HPCF cathodes are well consistent with the reduction peaks observed in their CV curves. More importantly, it is seen that S/HPCF cathode delivers a much higher discharge capacity than does the S/PCF cathode; the specific capacity of S/HPCF cathode at the first cycle is about 1458 mA h g⁻¹, almost 90% of the theoretical specific capacity of sulfur (1672 mA h g⁻¹). This fact suggests that the presence of abundant micropores in S/HPCF can significantly improve the sulfur utilization to increase the capacity.

Fig. 7 CV curves of S/PCF (a) and S/HPCF (b) at a scan rate of 0.1 mV s⁻¹, galvanostatic charge/discharge profiles of S/PCF (c) and S/HPCF (d) at 0.2C.

To further investigate the electrochemical performance of S/PCF and S/HPCF cathodes, the results of cycling tests at 0.5C are presented in Fig. 8(a) and (b). For S/PCF cathode, the discharge capacity at the first cycle is 808 mA h g⁻¹, and a fast capacity decay is observed during the first few cycles because of the irreversible dissolution of lithium polysulfides into the electrolyte.⁵⁰ After 100 cycles, only a specific capacity of 524.2 mA h g⁻¹ is obtained with a capacity retention of 64.9% and a capacity decay rate of 0.351% per cycle. Compared with the S/PCF cathode, the S/HPCF cathode yields a higher initial specific capacity of 1070.6 mA h g⁻¹; even after 100 cycles, a reversible specific capacity of 946.2 mA h g⁻¹ with an average coulombic efficiency of 98.7% can be still obtained. The capacity retention of S/HPCF cathode is as high as 88.4% and the capacity decay rate decreases considerably to 0.116% per cycle. The stable cycling performance and high coulombic efficiency of S/HPCF cathode indicate that the introduction of micropores efficiently impedes the diffusion of polysulfides and thus increases the sulfur utilization. The rate capabilities of S/PCF and S/HPCF cathodes at various current densities from 0.1C to 2C are also evaluated and the results are shown in Fig. 8(c). For all current densities, the specific capacities of S/HPCF cathode are much larger than those of S/HPCF cathode; the stable specific capacities of S/HPCF cathode at 0.1C, 0.2C, 0.5C, 1C and 2C, are 1318.2 mA h g⁻¹, 1175.6 mA h g⁻¹, 1014.8 mA h g⁻¹, 748.6 mA h g⁻¹ and 626.6 mA h g⁻¹, respectively. When the current density is decreased from 2C to 0.1C, the specific capacity of S/HPCF cathode can be recovered to 1194.4 mA h g⁻¹. These results reveal that the developed structure of HPCF ensures an excellent rate performance.

Fig. 8 Cycle performances (a), galvanostatic charge/discharge profiles (b), rate capacities (c) and Nyquist plots (d) of S/PCF and S/HPCF cathodes. The inset is galvanostatic charge/discharge profiles of S/PCF.

To better understand the performance variation between S/PCF and S/HPCF cathodes, the EIS measurement is used as a diagnostic tool. The EIS results of S/PCF and S/HPCF cathodes before and after 100 cycles are shown in Fig. 8(d). It is seen that the Nyquist curves of S/PCF and S/HPCF cathodes before cycling exhibit depressed semicircles, the diameters of which represent the charge transfer resistances, R_ct Clearly, the R_ct of S/HPCF cathode is much smaller than that of S/PCF cathode, which can be attributed to the good distribution of sulfur in the micro/mesopores of HPCF. After 100 cycles, the charge transfer resistances of S/PCF and S/HPCF cathodes both decrease as a result of the sufficient permeation of electrolyte into the cathodes.¹³ Furthermore, it is obvious that additional depressed semicircles appear at high frequencies in the Nyquist curves of S/PCF and S/HPCF cathodes. Such semicircles represent the interphase contact resistances (R_ic) in the electrode and they are associated with the formation of the insulating reduction products (Li₂S and Li₂S₂) on the surface of carbon matrices.⁵¹ Due to the strong adsorption effect and large pore volume of micropores in S/HPCF, polysulfides are believed to be well restricted within the micropores so that the passivation of Li₂S or Li₂S₂ is less significant. Thus, the R_ic and R_ct of S/HPCF cathode after cycling are lower than those of S/PCF cathode. The morphologies of S/PCF and S/HPCF cathodes before and after 100 cycles at 0.5C are shown in Fig. 9. It is seen in Fig. 9(a) and (c) that no apparent difference between the S/PCF and S/HPCF cathodes is observed before cycling; both cathodes show loose network structures constructed by carbon fibers, which is in favor of electrolyte infiltration. After 100 cycles, some large cracks (∼20 μm) in the S/PCF cathode are presented, leading to the structural destruction of the electrode and thus a poor cycle stability. In contrast, although some uniform small cracks (∼5 μm) are formed on the S/HPCF cathode, its loose network structure is largely maintained. The relatively small change in the physical appearance of S/HPCF cathode probably arises from the decrease in the formation of reduction products outside the pores as considerable sulfur is confined inside micropores. Hence, the volume expansion effect is somewhat alleviated by the S/HPCF cathode, which contributes to a better cycle life.

Fig. 9 SEM images of S/HPCF (a and b) and S/PCF (c and d) cathodes before and after 100 cycles.

4. Conclusion

In summary, we have successfully synthesized a high-performance micro/mesoporous S/HPCF cathode for Li–S batteries via electrospinning. The connected open macropores and mesopores ensure the infiltration of electrolyte to facilitate the transport of lithium ions. The abundant micropores facilitate the adsorption of sulfur to improve the sulfur utilization and inhibit the diffusion of polysulfides efficiently. Moreover, the hierarchical micro/mesoporous carbon fibers can construct a loose network structure to enhance the electron transfer and alleviate the volume expansion effect during cycling. Therefore, the S/HPCF cathode delivers a high reversible specific capacity of 1070.6 mA h g⁻¹ with an excellent capacity retention of 88.4% after 100 cycles at 0.5C. In addition, the preparation of S/HPCF cathode is facile and controllable, which is desirable for the commercial applications in Li–S batteries.

Acknowledgements

This work was supported by National Natural Science Foundation of China (No. 51306125), Shenzhen Science and Technology Fund (JCYJ20140828163634002, KQCX20140519105122378 and JCYJ20150324141711693) and Natural Science Foundation of SZU (No. 827-000015).

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