PIM-1-based carbon–sulfur composites for sodium–sulfur batteries that operate without the shuttle effect

Jun Woo Jeon ab, Dong-Min Kim b, Jinyoung Lee a, Jong-Chan Lee b, Yong Seok Kim *ac, Kyu Tae Lee *b and Byoung Gak Kim *ac
aAdvanced Materials Division, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeong-ro, Yuseong-gu, Daejeon, 34114, Republic of Korea. E-mail: bgkim@krict.re.kr; yongskim@krict.re.kr
bSchool of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, 1, Gwanak-ro, Gwanak-gu, Seoul 088026, Republic of Korea. E-mail: ktlee@snu.ac.kr
cDepartment of Chemical Convergence Materials, University of Science and Technology, 217 Gajeong-ro, Yuseoung-gu, Daejeon 34113, Republic of Korea

Received 4th October 2019 , Accepted 9th January 2020

First published on 13th January 2020


Room temperature sodium–sulfur (RT Na–S) batteries have distinct advantages over other next generation batteries because of their use of abundant and inexpensive resources with high theoretical capacities of 1166 and 1675 mA h g−1, namely for sodium and sulfur. However, problematic side reactions, called the shuttle effect, lead to low coulombic efficiency during cycling. Here, we propose a new strategy to fundamentally suppress the shuttle phenomenon by combining two widely used concepts, covalent bonds and physical confinement, through the preparation of a PIM-1-based carbon–sulfur composite. This sulfur–carbon material was prepared through one-step heat treatment of a mixture of sulfur and PIM-1. The resulting sulfur–carbon composites have characteristics of both ∼0.5 nm-sized ultra-micropores and covalent bonding in a single material, which fundamentally obstruct the dissolution of polysulfide into the electrolyte. This strategy led to long cycling stability over 250 cycles, with a capacity of 556 mA h gs−1 and a coulombic efficiency of approximately 100%.


Introduction

The ever-increasing demand for advanced high-capacity energy-storage systems has led to the development of a variety of next-generation batteries, such as lithium–oxygen,1,2 lithium–sulfur,3–5 and sodium–sulfur batteries.6–9 Among these advanced battery systems, sodium–sulfur (Na–S) batteries have gained significant attention as promising candidates, not only because their resources, namely sodium and sulfur, are abundant, inexpensive, and environmentally benign,10–15 but also because sodium and sulfur possess high theoretical specific capacities of 1166 and 1675 mA h g−1, respectively.16 Although commercialized high-temperature sodium–sulfur batteries (HT Na–S batteries) exhibit high energy densities (theoretical energy density of 760 W h kg−1) and efficiencies of almost 100%,17 safety concerns associated with the use of molten sodium and sulfur at high operating temperatures of approximately 300 °C remain. While significant effort has been devoted to solving these problems,18,19 room temperature Na–S (RT Na–S) batteries are considered to be reasonable alternatives that avoid the above-mentioned problems.20

RT Na–S batteries have distinct advantages over their HT counterparts, including low operating costs and improved safety, and the battery still exhibits a high theoretical specific energy of 1274 W h kg−1.12,14 However, some challenging issues, such as the poor electrical conductivity of sulfur, polysulfide dissolution, and rapid capacity fading, must be resolved for practical applications.3,12 Specifically, the intermediates formed in the electrochemical reactions in an RT Na–S battery, namely soluble long-chain sodium polysulfides (Na2Sx, 4 ≤ x ≤ 8), shuttle between the anode and the cathode and react with the Na metal at the anode. This unwanted “shuttle effect” side reaction leads to low coulombic efficiency during cycling.21,22 Various approaches, including the use of interlayers, electrolyte additives, and novel cathode materials, such as sulfurized polymers and nanostructured carbon–sulfur composites, have been explored in an effort to solve the above-mentioned problems.14,20,23–27

Among these approaches for suppressing the shuttle effect, a few cathode materials remarkably prevent the dissolution of polysulfides.11,28,29 For example, physical confinement and chemical linking have been considered to be promising strategies for inhibiting polysulfide dissolution. The former method physically confines sulfur in highly conductive microporous carbon.29–32 Xin et al. demonstrated that small metastable sulfur molecules (S2–4) within micropores (∼0.5 nm) delivered a high specific energy of 750 W h kg−1 with a high coulombic efficiency of nearly 100%.17 However, this approach requires several steps to prepare the required multi-walled carbon nanotubes. The other method involves introducing chemical linkages between the intermediate polysulfides and the cathode materials.33–36 As a representative example, polyacrylonitrile–sulfur (PAN–sulfur) based carbon composites contain covalently bound sulfur that effectively obstructs the dissolution of soluble polysulfides.28,33,35,37–40 Hwang et al. reported a PAN–sulfur composite with a capacity of 153 mA h gtotal−1 even after 500 cycles.28 However, as Fanous et al. revealed, PAN–sulfur composites suffer from constant losses from the sulfur cathode through the re-oxidation of elemental sulfur during recharging, which continues to produce problematic soluble polysulfides.39

In this study, we report that, when combining both the micropore and covalent-bond concepts, covalently bound small sulfur molecules (S2–4) exhibit extremely suppressed shuttle phenomena in room-temperature Na–S rechargeable batteries. To realize both concepts in a single material, the intrinsically microporous PIM-1 polymer was used as the carbon precursor due to its micropores and chemically reactive sites (Fig. S1). Specifically, small sulfur molecules (S2–4) can be stored in the large number of micropores of PIMs41–44 and soluble polysulfide (Na2S4) can be covalently bound to reactive sites, such as the two nitrile groups and three benzene rings of PIM,45,46 as shown in Fig. S2. In order to confirm the extremely suppressed shuttle effect, the resulting carbon–sulfur composites were even examined in carbonate-based electrolytes. It is generally accepted that Na–S batteries cannot be operated with carbonate-based electrolytes because polysulfides are highly soluble in these electrolytes and react with sodium metal.47 However, the PIM based carbon–sulfur composite exhibits excellent electrochemical performance, such as a reversible capacity of 556 mA h gs−1, negligible capacity fading over 250 cycles, and a high coulombic efficiency of approximately >99%, despite the fact that the electrochemical performance of the PIM based carbon–sulfur composite was examined with 1.0 M NaClO4 in a mixture of ethylene carbonate (EC) and propylene carbonate (PC) (1[thin space (1/6-em)]:[thin space (1/6-em)]1 v/v). Focusing on characterizing the ∼0.5 nm-sized ultra-micropores and covalent bonding in the resulting PIM-1-based carbon–sulfur composite, we correlated the physical and chemical properties with the electrochemical performance in order to demonstrate the extremely suppressed shuttle phenomenon. We suggest that this strategy effectively inhibits the shuttle phenomenon by obstructing the formation of high-order soluble sodium polysulfides (Na2Sx, 4 < x ≤ 8) during charging and discharging.

Results and discussion

The PIM-based carbon–sulfur composite was synthesized using a simple thermal-treatment method. This process involved melt diffusion of a 1[thin space (1/6-em)]:[thin space (1/6-em)]5 (w/w) mixture of ground PIM-1 powder and elemental sulfur by heating at 155 °C.48,49 After impregnating sulfur into the free volume of the PIM-1 powder, subsequent heat treatment was applied. Based on screening experiments, a temperature of 600 °C was chosen, as the sample heat-treated at this temperature has a high sulfur content and the PIM–sulfur composite is thermally stable (see Table S1). During heating, the sulfur and the polymer matrix were thermally crosslinked and the polymer was carbonized.37,38 In addition, by maintaining a final temperature of 600 °C, the additional sulfur that was physically confined in the micropores but not chemically bound to the carbonized PIM matrix was removed, leaving only covalently bound sulfur in the carbon host, with a sulfur content of about 30% as determined by elemental analysis (EA). This carbon–sulfur composite is referred to as “cPIMS-600” for convenience.

SEM and TEM were used to investigate the morphology of the prepared carbon–sulfur composite, which confirmed the uniform distribution of the sulfur element in the cathode material. The SEM images (Fig. S3) clearly reveal that cPIMS-600 has a massive powdery structure. In addition, the elemental maps obtained by EDS show that sulfur is homogeneously distributed throughout the carbon matrix, with a similar elemental composition to that obtained by EA (Fig. S3). The TEM images show that cPIMS-600 has an amorphous and porous structure. The annular dark field EDS maps also demonstrate that atomic sulfur, carbon, and nitrogen are uniformly distributed at the nanometre scale (Fig. 1). Hence, we confirmed that a carbon–sulfur composite with well-dispersed sulfur was successfully prepared through a simple heating process.


image file: c9ta10939k-f1.tif
Fig. 1 (a) Schematic illustration of the preparation procedure of PIM based carbon–sulfur composites. (b) Annual dark-field STEM image, (c) carbon element map, (d) sulfur element map and (e) high-resolution-TEM image of the cPIMS-600.

To investigate the pore characteristics of the carbon sample, argon–sorption isotherms were acquired at 87 K (Fig. 2(a) and (b) and Table S2). Since the pore characteristics of the sulfur-impregnated carbon are different from those of the carbon devoid of sulfur in terms of free volume and microporosity, sulfur-free PIM-based carbon was prepared in order to investigate the pore characteristics of the PIM-1-based carbon-matrix structure; this material is referred to as “cPIM-600”. As shown in Fig. 2(a), cPIM-600 exhibits a type I isotherm, which indicates that it mainly consists of micropores, with a BET specific surface area of 387 m2 g−1 and a narrow pore size distribution of ∼0.5 nm, as determined by the Horváth–Kawazoe (H–K) method.50 These ultra-micropores (<0.5 nm) greatly influence the chain length of the impregnated sulfur, because small sulfur allotropes (S2–4) form only in the ultra-micropores, while cyclic sulfur allotropes (S5–8) require pores larger than 0.5 nm in size.29 Meanwhile, cPIMS-600 exhibits a lower free volume and surface area (219 m2 g−1) compared to cPIM-600; however, a large number of micropores still remain. It seemed that physically confined sulfur in the micropores was eliminated at 600 °C; hence, only covalently bound sulfur remains in its ultra-micropores. We employed X-ray diffractometry (XRD) and Raman spectroscopy to further investigate the carbon–sulfur composites. Typical XRD peaks of elemental sulfur (S8) are clearly evident at 23.1, 25.9, and 27.8°, which are attributed to the (222), (026), and (040) planes of the orthorhombic structure of elemental sulfur, respectively.51 However, even though the cPIMS-600 composite has a sulfur content of 30%, no elemental sulfur peaks were observed, which suggests that the sulfur molecules in this composite have lost the crystallinity of orthorhombic S8. Furthermore, the sample showed a broad diffraction peak centred at 2θ = 24°, which is attributed to the graphitic (002) plane of the carbon matrix.28 As seen in Fig. 2(b), Raman spectroscopy also supports the absence of elemental sulfur and the existence of covalent bonds. Elemental sulfur is well-known to exhibit three peaks at 150, 217, and 471 cm−1 in its Raman spectrum; however, no strong peaks at these values were observed in the spectrum of cPIMS-600, suggestive of a lack of vibrational motion associated with elemental sulfur. Furthermore, the cPIMS-600 sample exhibited two major peaks at 1380 and 1560 cm−1 that correspond to the D and G bands of carbonaceous materials, respectively.28,37


image file: c9ta10939k-f2.tif
Fig. 2 (a) Ar adsorption/desorption isotherms and (b) pore size distribution plots of the cPIM-600 and cPIMS-600. (c) XRD patterns, (d) Raman spectra and (e) TOF-SIMS spectra of cPIMS-600.

To determine how the sulfur is bound to the other atoms, cPIMS-600 was subjected to time-of-flight secondary-ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS). The sulfur atoms in the carbon–sulfur composite can covalently bind to either carbon or nitrogen. As seen in Fig. 2(e), we found direct evidence for covalently bound sulfur in the cPIMS-600 composite, with CNS-fragments (i.e., CNS at m/z = 58, C3NS at m/z = 82, and C5NS at m/z = 106) observed by TOF-SIMS. CS fragments (i.e., CS at m/z = 44 and C2S at m/z = 56) and an SN fragment (SN at m/z = 46) were also observed, together with an oligo-sulfide structure (i.e., S3 at m/z = 96). These signals indicate that sulfur molecules are covalently bound to the carbon matrix through heat treatment of the sulfur/PIM-1 mixture. XPS also revealed the presence of C–S bonds (Fig. S4). The C 1s spectrum of cPIMS-600 exhibits a prominent sp2-hybridised carbon peak at 284.8 eV, and the presence of heteroatoms was confirmed through peaks at 286.0 eV (C–N or C–S), 287.6 eV (C[double bond, length as m-dash]O), and 291.1 eV (O–C[double bond, length as m-dash]O). The S 2p spectrum of cPIMS-600 exhibits a characteristic S 2p3/2/2p1/2 doublet separated by 1.2 eV with an intensity ratio of approximately 2[thin space (1/6-em)]:[thin space (1/6-em)]1, and where the S 2p3/2 peak (163.9 eV) has a lower binding energy than elemental sulfur (164.0 eV), which reconfirms the presence of C–S bonds.51–53

The type of sulfur in cPIMS-600 was determined by TGA, as shown in Fig. S5. Generally, sulfur molecules exist in three forms in the carbon–sulfur composite, and they show different thermal behaviour.54 The first is residual sulfur on the surface of the carbon matrix or in large pores, which is rapidly lost between 220 and 320 °C. The second is sulfur that is physically confined within the micropores, which evaporates in the 300–480 °C range because additional heat energy is required to overcome the stronger capillary forces in the micropores. The third, namely covalently bound sulfur, decomposes at even higher temperatures because of the high dissociation energy of the carbon–sulfur (C–S) bond. In this regard, the cPIMS-600 composite contains only chemically bound sulfur in its matrix because it exhibits only a single decomposition peak at above 550 °C.

The electrochemical performance of cPIMS-600 was examined using a 2032 coin cell composed of sodium metal as the anode, the PIM–sulfur composite as the cathode, and an electrolyte composed of 1.0 M sodium perchlorate (NaClO4) in ethylene carbonate (EC) and propylene carbonate (PC) (1[thin space (1/6-em)]:[thin space (1/6-em)]1 v/v) with 2 wt% fluoroethylene carbonate (FEC). FEC was added to protect the sodium metal against electrolyte decomposition.

Carbonate-based electrolytes are known to not be appropriate for metal–sulfur batteries because polysulfides are highly soluble in them and react irreversibly with them.47 However, cPIMS-600 showed excellent electrochemical performance even with the carbonate-based electrolyte. cPIMS-600 exhibited a high reversible capacity of ca. 550 mA h g s−1 (Fig. 3(a) and (b)). However, a large irreversible capacity was also observed during the first cycle, which originated from the electrolyte decomposition and irreversible sodium storage capacity of the carbon substrate. The sodium storage capacity of PIM-600 was analyzed with similar current density with cPIMS-600. As shown in Fig. S6, the carbon substrate exhibited ca. 175 mA h g−1 in the 1st cycle and large irreversible capacity. This caused irreversible capacity of cPIMS-600 in Fig. 3(a).


image file: c9ta10939k-f3.tif
Fig. 3 Electrochemical properties of cPIMS-600 composites. (a) The voltage profiles of cPIMS-600 for 250 cycles at 0.1C (167.5 mA h gs−1), (b) specific capacity and (c) coulombic efficiencies versus cycle number of cPIMS-600.

The cPIMS-600 also exhibited high coulombic efficiency (ca. >99%), implying that it effectively prevents the shuttle effect (Fig. 3(c)). For comparison, the voltage profile of cPIMS-600 using 1.0 M NaClO4 in the EC[thin space (1/6-em)]:[thin space (1/6-em)]PC (1[thin space (1/6-em)]:[thin space (1/6-em)]1 v/v) electrolyte without 2 wt% FEC is shown in Fig. S7; even without Na-metal protection, cPIMS-600 showed no shuttle effect during cycling.

This high coulombic efficiency (ca. >99%) of cPIMS-600 can be attributed to the fact that covalently bound small sulfur molecules of cPIMS-600 did not diffuse into the electrolyte during cycling. As shown in Fig. S2, these covalently bound small sulfur molecules consisted of small metastable sulfur molecules (Sx, x ≤ 4), which were covalently linked with the PIM based carbon matrix. Such structural characteristics can fundamentally obstruct the dissolution of soluble polysulfide (S4) into the electrolyte, which can lead to a shuttle phenomenon, and were confirmed as follows. First of all, in order to confirm the presence of the small metastable sulfur molecules (Sx, x ≤ 4), the reaction mechanism of cPIMS-600 was also analyzed with cyclic voltammetry (CV) as shown in Fig. S8. CV was conducted after 10 pre-cycles to investigate the sodium–sulfur reaction without the irreversible reaction in the 1st cycle from the carbon substrate. It showed two clear reduction peaks at 1.65 V and 0.90 V. This indicates that cPIMS-600 undergoes two kinds of sodium–sulfur reaction. Regarding the reaction mechanism, Na2S4 + 2Na → 2Na2S2 occurs at 1.65 V and Na2S2 + 2Na → 2Na2S occurs at 0.90 V.17,55 Therefore, cPIMS-600 contains small metastable sulfur molecules (Sx, x ≤ 4) in the microporous substrate. Next, these small sulfur molecules are covalently linked with the carbon substrate, which is different from only physically confined small sulfur molecules. As seen in Fig. S9, Raman analysis revealed that the covalent bonds still remained after 50 cycles. Thus, during electrochemical tests, the covalent bonds between sulfur molecules and carbon substrates can prevent linear sulfur allotropes (S4), which can be converted into soluble polysulfide (Na2S4), from dissolving into the electrolyte.

Also, such characteristics of cPIMS-600 can inhibit the re-oxidation of polysulfide into soluble high-order polysulfides that can cause the shuttle effect. Manthiram et al. reported that the reaction mechanism of RT Na–S batteries is divided into four regions: (i) a plateau near 2.2 V for the solid–liquid transition from elemental sulfur (S8) to long-chain sodium sulfide (Na2S8); (ii) a sloping region from 2.2 to 1.65 V for the liquid–liquid transition from Na2S8 to Na2S4; (iii) a plateau near 1.65 V for the liquid–solid transition from Na2S4 to Na2S3, Na2S2, or Na2S; and (iv) a sloping region from 1.65 to 1.2 V for the solid–solid transition to Na2S.20 Considering this phase transition point of view, it was verified that the re-oxidation of polysulfide into high order polysulfide were not observed in the CV graphs and Raman data, which can be attributed to the fact that it was hard to re-oxidize polysulfide into large cyclic sulfur molecules due to spatial constraints in the ultra-micropores with a size of 0.5 nm. These results were distinguished from those of the PAN based carbon sulfur composites without well-defined microporosity which suffered from constant sulfur losses from the cathode through the re-oxidation of elemental sulfur during cycling.39 Consequently, as shown in Fig. 3(a), the covalently bound small sulfur molecules of the PIM-based carbon sulfur composites fundamentally prevent the shuttle effect through not only the elution of sulfur but also the re-oxidation of elemental sulfur by implementing two concepts, micropores and covalent bonds, at the same time.

Conclusions

In summary, a carbon material covalently bound with small sulfur molecules (S2–4) was investigated as a cathode material for RT Na–S batteries. This carbon–sulfur material, cPIMS-600, was prepared from a mixture of sulfur and the microporous PIM-1 polymer through simple heat treatment. PIM-1 is advantageous as a carbon precursor because of its ultra-micropores and chemically reactive sites. The resultant carbon–sulfur composite exhibited the characteristics of both its ∼0.5 nm-sized ultra-micropores and covalent carbon–sulfur bonds. These physical and chemical properties fundamentally inhibit the formation of soluble long-chain sodium polysulfides (Na2Sx, 4 ≤ x ≤ 8) during cycling. As a result, the shuttle effect was not observed in cPIMS-600, leading to excellent electrochemical performance, including negligible capacity fading over 250 cycles and a high coulombic efficiency of approximately >99%.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

J. W. J. and D.-M. K. contributed equally to this work. This work was supported by the Korea Research Institute of Chemical Technology (KRICT) core project (KK1607-C06, KK1961-02, and SI1921-20) and in part by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2018R1A5A1024127).

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Footnotes

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ta10939k
These authors contributed equally to this work.

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