Porous carbon nanofiber paper as an effective interlayer for high-performance lithium-sulfur batteries

Porous carbon nanofiber paper as an effective interlayer for high-performance lithium-sulfur batteries

Electrochimica Acta 168 (2015) 271–276 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/elect...

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Electrochimica Acta 168 (2015) 271–276

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Porous carbon nanofiber paper as an effective interlayer for high-performance lithium-sulfur batteries Jiangan Wang a, * , Ying Yang b , Feiyu Kang c a State key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an, 710072, China b Department of Electrical Engineering, Tsinghua University, Beijing 100084, China c Institute of Advanced Materials Research, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 18 March 2015 Received in revised form 6 April 2015 Accepted 9 April 2015 Available online 11 April 2015

Lithium-sulfur (Li-S) battery with new configuration is demonstrated by inserting a flexible activated carbon nanofiber (ACNF) interlayer between the sulfur cathode and the separator. The ACNF with tunable pore structure is fabricated by a combination of electrospinning polyimide and a subsequent activation treatment. The influence of the textual characteristics of ACNFs on the electrochemical performance of Li-S batteries has been studied. The highly porous ACNF not only effectively intercepts/stabilizes the shuttling migration of polysulfides within the cathode region, but also provides reliable ionic/electronic conductivity for fast kinetics. The lightweight ACNF interlayer with higher specific surface area can yield enhanced cell performance at a low mass ratio of ACNF/sulfur (0.4). An initial specific capacity of 1224 mAh g 1 along with high Coulombic efficiency, long cycling stability and good rate capability is achieved in the modified Li-S cell. ã 2015 Elsevier Ltd. All rights reserved.

Keywords: Lithium-sulfur battery Carbon nanofiber Electrospinning Cell configuration

1. Introduction Rechargeable Li-ion batteries are considered to be the most promising technology for large-scale applications of consumer electronics, electric vehicles and smart utility grids [1]. Unfortunately, the currently Li-ion batteries suffer from limited energy density. Lithium-sulfur (Li-S) batteries are one of the excellent alternative energy storage systems. Owing to the ultrahigh capacities of sulfur and Li metal (1675 and 3861 mAh g 1, respectively) operated under a safer cell voltage of about 2.15 V, the Li-S cell can deliver a theoretical energy density of 2567 Wh kg 1 [2,3]. However, to enable this system to be a viable technology, there are many challenging issues required to be urgently addressed [3–7]. The first obstacle is the insulating nature of sulfur and its Li2S product, which leads to poor utilization of active materials. The second limitation is the shuttle effect caused by the soluble polysulfides (Li2Sx, 4 < x  8), resulting in low Coulombic efficiency and serious capacity fading. The last but not the least problem is the huge volume change (80%) during the electrochemical conversion between sulfur and Li2S. Such a

* Corresponding author. Tel. and fax:. +86 029 8846 0361 E-mail address: [email protected] (J. Wang). http://dx.doi.org/10.1016/j.electacta.2015.04.055 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.

structural expansion/contraction would give rise to pulverization of active materials, followed by electrical disconnection to the current collector and thus fast capacity degradation. To solve these critical problems, a great number of studies have devoted significant efforts to develop effective design strategies. The mainstream approach includes inside modification of sulfur cathodes, such as integrating sulfur with various substrates of carbon materials, conducting polymers and metal oxides, etc [3,4]. The heterogeneous substrates are expected to increase the electrical conductivity, suppress the shuttling migration and accommodate the volume change. Despite the effectiveness of these sulfur-based composites, this approach involves complex fabricating processes and reduces the actual sulfur loading in the cathode. Recently, Manthiram's group proposed a novel cell configuration based on a carbon interlayer inserting between the cathode and the separator [4,8,9]. The new cell configuration eliminates the complex material processing concerns compared to the sulfur-based composites. The interlayer serves as bifunctional roles of current collector and polysulfide stockroom, which localizes/stabilizes of the redox reactions within the cathode region. Porous carbon particles, carbon nanotubes (CNTs), carbonized leaf/Kimwipes, reduced graphene oxide (rGO) films, carbon microfiber paper and carbon nanofibers have been demonstrated to be appropriate interlayers for Li-S batteries [8–16]. For example,

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the modified Li-S batteries using interlayers of microporous carbon paper [8], carbon microfiber paper [14] and carbon nanofiber fabric [16] can deliver a specific capacity over 1000 mAh g 1 after 100 cycles. Although the novel configuration strategy is effective to improve sulfur utilization, it is important to note that the interlayer is electrochemically inactive, which would decrease the overall cell performance when taking all of the components into account. In other words, one should reduce the weight of the interlayer as much as possible. However, decreasing the weight or thickness of interlayer would weaken its unique bifunctional roles [11,16]. Porous structure is demonstrated to be efficacious for localizing/ reutilizing active sulfur-based species. Therefore, to maintain the effectiveness of the interlayer at a reduced thickness (weight), it is expected to create a highly porous structure to compensate the weight limitation. Electrospining technique is a straightforward method to fabricate freestanding carbon nanofiber (CNF) fabrics with tunable porous structure and good electrical conductivity. Recently, Singhal et al. employed polyacrylonitrile (PAN)-based CNF fabrics as interlayers to intercept the migrating polysulfides. However, the best performance is based on a high weight interlayer of 4.2 mg cm 2, which is three times higher than the sulfur loading (1.4 mg cm 2). Owing to the low specific surface area of CNF, the performance is substantially degraded when the weight of the CNF interlayer is decreased to 1.3-1.8 mg cm 2. In order to obtain an improved performance at a lower weight of CNFs, we herein explore a highly porous activated carbon nanofiber (ACNF) fabric as an effective interlayer between the cathode and the separator (Fig. 1(a)). Polyimide (PI) is used as the carbon precursor to produce porous ACNFs, because PI has a high carbon yield of 70 wt.% (vs. 40-

50 wt.% for PAN) [17]. In addition, the PI-based CNFs show much better mechanical strength than the PAN-based CNFs [18], which may be favorable for cycling stability. The areal weight of ACNF is about 1.0 mg cm 2 to couple an areal sulfur loading of 2.5 mg cm 2. ACNFs with different surface area and pore structure have been prepared to study the effect of textual characteristics on the performance of Li-S batteries. The highly porous ACNF is found to offer numerous nano-reservoirs for interception/stabilization of soluble polysulfides and reliable ionic/electronic conductivity for fast kinetics, which results in much improved electrochemical performance of Li-S cells. 2. Experimental 2.1. Material synthesis The polymer precursor of polyamic acid (PAA) for electrospinning was synthesized according to our previous study [17]. Typically, 2.2 g of pyromellitic dianhydride (PMDA) and 2 g of 4,4oxydianilline (ODA) were dissolved in 30.8 g of N,N-dimethyacetamide (DMAc) to form a homogeneous polymer solution of PAA, which was used as a precursor for electrospinning. During the electrospinning process, the polymer precursor was loaded into a syringe pump and a high voltage of 25 kV was provided by a highvoltage power supply. A flow rate of 1 ml h 1 and a needle-tocollector distance of 25 cm were applied to ensure a stable electrospinning. The as-electrospun nanofiber was collected as a flexible fabric, which was subsequently stepwise-treated by imidization and carbonization (900  C/2 h under argon atmosphere). Finally, the CNF fabric was activated at 700  C under

Fig. 1. (a) Schematic illustration of a Li-S cell configuration with an ACNF paper inserting between cathode and separator; (b) A panoramic view of the ACNF paper (inset is the diameter distribution statistic of the nanofibers). (c-d) Magnified SEM images of (c) ACNF3 and (d) ACNF1.

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Adsorption amount (cm3 g-1, STP)

(a)

273

350 300 250 200 150 100

ACNF1 ACNF2 ACNF3

50 0

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0 )

dV/dr (cm g-1 nm-1)

(b)

1.0

ACNF1 ACNF2 ACNF3

micropores

0.8

0.10

0.6

0.4

0.05

mesopores

0.2

0.0

0.00

1

2

10

Pore size distribution (nm)

Fig. 3. (a) CV curves in the initial three cycles and galvanostatic charge/discharge profiles at various cycles of the Li-S cells with an ACNF interlayer.

Fig. 2. (a) N2 adsorption/desorption isotherms and (b) pore size distribution of ACNF.

steam/N2 flow (1:3 v/v) for 15, 30 and 45 min, respectively. The as-obtained ACNF samples were labeled as ACNF3, ACNF2 and ACNF1, respectively, for convenient discussion. 2.2. Material characterization, cell assembly and electrochemical evaluation.

employed for cyclic voltammetry (CV) measurements at a scan rate of 0.2 mV s 1 in the potential window of 1.5-3.0 V and electrochemical impedance spectrometry (EIS) tests in the frequency range of 100 kHz-10 mHz after the CV measurements. Galvanostatic charge/discharge tests were carried out on Land Battery Testing system at various rates. 3. Results and discussion

The morphology of the ACNF product was examined using a field-emission scanning electron microscopy (FE-SEM, LEO-1530). N2 adsorption/desorption measurement (Belsorp, Japan) was performed to characterize the textual properties. BrunauerEmmett-Teller (BET) and non-linear density functional theory methods were used to determine specific surface area and the corresponding pore size distribution, respectively. The total pore volume was calculated from the adsorption amount of liquid N2 at a relative pressure of 0.99 atmosphere. Sulfur cathode was fabricated by mixing 70 wt.% of sulfur, 20 wt. % of conductive carbon black and 10 wt.% of polyvinylidene fluoride (PVDF) in an N-methyl-2-pyrrolidinone solution. The slurry was casted onto an Al foil and dried at 60  C in a vacuum oven for 12 h. The sulfur electrode and the flexible ACNF paper were punched into circular discs. The mass loading of the active materials is 2.5  0.1 mg cm 2 and the weight of ACNF is 1.0  0.1 mg cm 2. The electrolyte was 1 M lithium bis(trifluoromethanesulfonyl) imde (LiTFSI) in 1,2-dimethoxyethane and 1,3-dioxolane (DME-DOL 1:1 v/v) containing 0.1 wt.% LiNO3 additive. CR2032 coin cells were assembled in an argon-filled glove box with sulfur cathode, ACNF paper, polypropylene separator and Li metal foil in sequence. An electrochemical workstation (solartron 1260 + 1287) was

Fig. 1(a) displays a schematic of the cell configuration of Li-S batteries, in which the ACNF paper is purposely sandwiched between the sulfur cathode and the separator, aiming to mitigate/ stabilize the polysulfides migrating from the cathode to the anode (i.e. shuttle effect). The flexible ACNF paper was fabricated by a combination of electrospinning technique and steam activation. Fig. 1(b) shows a panoramic morphology of the as-fabricated ACNFs. It is clearly observed that the paper is composed of regular and randomly oriented nanofibers, forming an interconnected and porous network structure. The length of these nanofibers can reach several hundreds of microns. The nanofiber diameter is ranging from 50 to 500 nm with a most probable distribution centering around 250 nm (inset). Notably, these nanofibers possess a highaspect-ratio over 1000, which not only endows the freestanding paper with strong mechanical flexibility, but also facilitates continuous electron transfer for re-utilization of active materials during charge/discharge process. There is no obvious morphology change on the interwoven ACNFs with different activation time, as illustrated by their smooth and clean surfaces in Fig. 2(c) and (d). This is due to that steam activation process generally creates small invisible pores (mainly micropores) in the body of CNFs.

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100

1500

80

1200

60

900

40

600

20

300

ACNF1 ACNF3

ACNF2 S

Coulombic efficiency (%)

Specific capacity (mAh g-1)

274

0

0 0

20

40

60

80

100

Cycle number Fig. 4. Cycling performance and Coulombic efficiency of the Li-S cells using and without using ACNF interlayers.

The textural characteristics of ACNF samples were examined by nitrogen adsorption/desorption measurements. As shown in Fig. 2(a), the resulting isotherms can be identified as mixed IUPAC type I+II patterns, in which the main adsorption below P/P0 < 0.1 indicates the existence of a large quantity of micropores whereas the sloped adsorption at P/P0 = 0.1-0.99 reveals a certain amount of mesopores residing in the ACNFs. The corresponding pore size distribution (Fig. 2(b)) presents a clear observation of micropores and mesopores located in the range of 0.9-2 and 2.5 -10 nm, respectively. The mesopores are believed to be beneficial for channeling dissolved polysulfide ions into the micropores. The nanosized fibers also ensure short diffusion distances for an improvement in ionic conductivity. Furthermore, the BET specific surface area increases with the activation time, which can be determined to be 995, 789 and 488 m2 g 1 for ACNF1, ACNF2 and ACNF3, respectively, associated with a corresponding total pore volume of 0.51, 0.39, 0.25 cm3 g 1. The electrochemical behavior of the as-assembled Li-S cell with an ACNF1 interlayer is studied using CV method. The CV curves in the initial three cycles (Fig. 3(a)) are characteristics of the redox chemistry of the elemental sulfur during the charge/discharge processes. During the cathodic scan, two-step reduction peaks are observed at around 2.3 and 1.9 V, which are typically associated with the open-ring reduction of S8 to long-chain lithium polysulfides (Li2Sn, 4  n  8) and a successive decomposition of long-chain Li2Sn into short-chain lithium sulfides (Li2S and/or Li2S2), respectively [19–21]. In the following anodic scan, the distinct sharp peak centering at about 2.55 V corresponds to the oxidation of Li2S/Li2S2 to Li2S8. The redox feature shows no obvious change in the three cycles, indicating highly reactive reversibility and good cyclability of the cell. Fig. 3(b) depicts the charge/ discharge voltage profiles of the cell at 0.1C rate. The two discharge

voltage plateaus appeared at around 2.25 and 2.05 V are assigned to the two-step reduction during the discharge process. The long voltage plateau (2.33 V) on the charge profiles is related to the reverse oxidation process. These observations are consistent with the CV analysis. The specific capacity as a function of cycle number is plotted in Fig. 4. The cells with ACNF interlayers show remarkable cycling improvement compared to the control cell without interlayer. More specifically, all the cells can deliver an initial discharge capacity of about 1200 mAh g 1, which corresponds to a sulfur utilization of 71.6% based on the theoretical value (1675 mAh g 1). With the extended cycles, the capacity decreases rapidly in the first several cycles, however, the capacities of the ACNF1-, ACNF2-, ACNF3-cells stay at 897, 736 and 632 mAh g 1 after 100 cycles, respectively, while their Coulombic efficiency can maintain in the range of 89%-99.2%. In a sharp contrast, the control cell without ACNF interlayer displays a serious capacity degradation during the cycling tests, and a very limited capacity (46 mAh g 1) is preserved after 100 cycles. The enhanced cycling stability is supposed to be benefited from the porous ACNF interlayer configuration, which holds a strong ability to suppress the polysulfide shuttling towards to the anode. Moreover, it is also observed that the Li-S cell using ACNF1 exhibits much better cycling stability than those using ACNF2 and ACNF3. This is, to a large extent, due to that ACNF1 provides higher specific surface area and larger pore volume for more polysulfide species stabilizing within the cathode region through adsorption, and in turn, the stabilized species can be reutilized in a reverse conversion manner. Table 1 summarizes the performance of Li-S batteries using different interlayers. As shown in the Table, the capacity retention of ACNF1-cell (73.3%) is superior/comparable to those using CNTs (66.5%) [9], rGO (71.0%) [12], acetylene black (71.2%) [13], Toray carbon paper (71.0%) [15], carbon microfiber (CMF) monolith (63.7%) [22] and CMF paper (54%) [23]. Although this value is lower than those using carbon black [8], carbonized Kimwipes [11] and PAN-based ACNFs [16], it is particularly important to note that the areal mass ratio of ACNF/ sulfur loading (i.e. 1.0 and 2.5 mg cm 2, respectively) is only 0.4, which is much smaller than that in the PAN-based ACNF-cell (i.e. 3.0). This indicates that our battery can deliver higher specific capacity based on the total components including the ACNF interlayer and the sulfur cathode, etc. In addition, the sulfur content of 60 wt.% based on the ACNF + sulfur is common to many sulfur-carbon composites, but the small capacity fading rate of 0.27% per cycle in the ACNF1-cell is found to be much lower than many reports using sulfur-carbon composite cathodes, such as 0.48% for sulfur-mesoporous carbon [24], 0.66% for sulfurhierarchical ordered porous carbon [20], 0.80% for sulfur-carbon hollow spheres [25], 1.05% for sulfur-peapodlike mesoporous carbon [26], 0.98% for sulfur-hierarchical porous carbon [27] and 0.35% for sulfur-hierarchically porous carbon nanoplates [28]. Given that the synthesis of sulfur-carbon composites requires a

Table 1 Summary of Li-S cell performances using different carbon interlayers. Interlayer

1st capacity (mAh g 1)

Capacity retention

Cycle number

Test rate

Interlayer/S ratio

Ref.

CNTs rGO Acetylene black Toray carbon paper CMF monolith CMF paper Carbon black Carbonized Kimwipes PAN-based ACNFs PI-based ACNFs

1446 1260 1491 1260 1120 1500 1176 1233 1548 1224

66.5% 71.0% 71.2% 71.0% 63.7% 54.0% 85.0% 84.7% 83.1% 73.3%

50 100 50 100 100 50 100 100 100 100

0.2C 0.1C 0.1C 0.1C 0.2C 0.2C 1.0C 0.2C 0.2C 0.1C

not not not not not not not not 3.0 0.4

[9] [12] [13] [15] [22] [23] [8] [11] [16] this work

given given given given given given given given

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The remarkable enhanced cell performance demonstrates that the insertion of a porous ACNF interlayer between sulfur cathode and separator can effectively increase the specific capacity of the Li-S batteries with high Coulombic efficiency, good rate capability and long cycling stability. It is believed that the porous ACNF interlayer primarily serves functions of (i) a high-surface-area container that confines the shuttle effect within the cathode region through intercepting/stabilizing polysulfide species; and (ii) a supplementary current collector that enhances the sulfur reutilization at high-rate operation. Moreover, the macroporous network structure of ACNF creates sufficient diffusion channels for fast electrolyte permeation into the sulfur cathode, which is different from the rGO paper that requires a long activation time due to its stacking problem [12]. Our results also suggest that the textural characteristics of ACNF exert significant influence on the cell performance. To hold the superiority of Li-S cells in energy density at a cell level over conventional Li-ion batteries, it is important to decrease the ACNF/sulfur mass ratio as low as possible. Noted that the polysulfide interception effect is primarily determined by the physicochemical properties of ACNF, the goal can be achieved by increasing the specific surface area and modulating the pore size distribution of ACNF as well as by introducing the carbon surface with hetero-atoms (e.g., nitrogen) and/or functional groups. As the ACNF in this study still suffers from a limited specific surface area (<1000 m2 g 1), further study is still ongoing to modifying the structure characteristics and surface chemistry of ACNF, for the purpose of better fulfilling its electrochemical potential in Li-S technology. 4. Conclusions

Fig. 5. (a) Rate capability of the ACNF1/Li-S cell. (b) Comparison of electrochemical impedance spectra of Li-S cells after inserting ACNF1.

complex procedure, the strategy herein may be more fascinating in terms of easy fabrication and low cost. Fig. 5(a) exhibits the rate capability of the ACNF1-cell. Impressively, the novel cell delivers an average discharge capacity of 960, 815, 663, 538 and 416 mAh g 1 at 0.1, 0.2, 0.5, 1 and 2C, respectively. Moreover, the cell sustains good cycling stability at each rate even when suffering from a sudden switch of different rates. After a high-rate test over 50 cycles, the discharge capacity can still remains at 795 mAh g 1 after another 110 cycles at the recovering rate of 0.2C, again validating the excellent cycling stability. The high-rate cycling performance can be attributed to the presence of ACNF1 interlayer on the surface of sulfur cathode, which provides critically favorable electronic/ionic conductivity to improve the reaction kinetics of sulfur. A better understanding of the role of the ACNF1 in the Li-S cells is elucidated by electrochemical impedance analysis. Fig. 5(b) compares the EIS spectra of the cells using and without using the ACNF1 interlayer. The spectra consist of one semi-circle in the high-to-mediate frequency region and a sloped line in the low-frequency region. The semi-circle is associated with the Faradaic charge transfer resistance (Rct), which can be determined by its diameter. The sloped line corresponding to Warburg impedance is a result of frequency dependence of Li-ion diffusion into the electrode/ electrolyte interfaces [29–31]. Obviously, the ACNF1-cell shows a much smaller Rct value (10.9 V) than the control cell (55.7 V). The lower internal impedance indicates that the ACNF interlayer provides electrically conducting pathways for improving the redox chemistry of sulfur, which not only facilitates fast charge collection/transport but also promotes a higher utilization of active materials.

Electrospun ACNF paper is used as a flexible, lightweight and effective interlayer to sandwich the sulfur cathode and separator of Li-S batteries. The ACNF provides a highly porous structure and 3D interwoven nanofiber network to mitigate the shuttle effect of polysulfides, and offers electrically conducting pathways to promote the electrochemical reutilization of active materials. Higher specific surface area is beneficial for a better cycling stability at a low ACNF/sulfur mass ratio. The cell with an ACNF1 interlayer displays an initial discharge capacity of 1224 mAh g 1 with improved cycling stability (73.3% retention over 100 cycles) and good rate capability. The enhanced cell performance demonstrates that the electrospinning method is a scalable and promising technique for engineering porous ACNF paper to advanced Li-S batteries by novel cell configuration design. Acknowledgments The authors acknowledge the financial supports from NSFC (51402236, 51472204 and 51232005), Fundamental Research Funds for the Central Universities (3102014JCQ01020) and Research Fund of the State Key Laboratory of Solidification Processing (NWPU) (83-TZ-2013). We also appreciated the financial support from Guangdong Province Innovation R&D Team Plan. References [1] N.S. Choi, Z. Chen, S.A. Freunberger, X. Ji, Y.K. Sun, K. Amine, G. Yushin, L.F. Nazar, J. Cho, P.G. Bruce, Challenges Facing Lithium Batteries and Electrical Double-Layer Capacitors, Angew. Chem. Int. Ed. 51 (2012) 9994–10024. [2] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.-M. Tarascon, Li-O2 and Li-S batteries with high energy storage, Nat. Mater. 11 (2011) 19–29. [3] Y. Yang, G. Zheng, Y. Cui, Nanostructured sulfur cathodes, Chem. Soc. Rev. 42 (2013) 3018–3032. [4] A. Manthiram, Y. Fu, S.-H. Chung, C. Zu, Y.-S. Su, Rechargeable Lithium-Sulfur Batteries, Chem. Rev. (2014) , doi:http://dx.doi.org/10.1021/cr500062v.

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