Novel Synergistic Strategy for Developing High-Performance Lithium

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Letter

A novel synergistic strategy for developing high-performance lithium sulfur batteries of large areal sulfur loading by SEI modified separator Junling Guo, Shupeng Zhao, Gaohong He, and Fengxiang Zhang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00290 • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 20, 2018

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ACS Applied Energy Materials

A Novel Synergistic Strategy for Developing High-Performance Lithium Sulfur Batteries of Large Areal Sulfur Loading by SEI Modified Separator Junling Guo, Shupeng Zhao, Gaohong He and Fengxiang Zhang∗ J. L. Guo, S. P. Zhao, Prof. G. H. He and Prof. F. X. Zhang State Key Laboratory of Fine Chemicals and School of Petroleum and Chemical Engineering, Dalian University of Technology, 2 Dagong Road, Liaodongwan New District, Panjin 124221, P. R. China. E-mail: [email protected]

Keywords: Li-S battery; Solid electrolyte interface; CNT array; Large areal sulfur loading; Separator

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Abstract As a crucial energy density determining factor, the areal sulfur loading is still low nowadays for practical applications of Li-S batteries. To address this issue, intensive research efforts have been devoted to development of sulfur-hosting materials with ultrahigh specific surface area. However, design and manufacturing process for these materials is complicated. Herein, we report a novel and facile strategy for developing high-performance cathode with high areal sulfur content using conventional sulfur hosts without ultra-high specific area. This strategy features a dense blocking layer of solid electrolyte interface (SEI) on the separator to insure cycle stability of the cathode with a high areal sulfur loading. Meanwhile, a nano-array cathode structure (CNT array on carbon cloth) is adopted to guarantee high sulfur utilization and rate performance. With the synergistic effect of dense blocking layer and CNT array structure, the cathode with 10mg/cm2 sulfur (90.9% in CNT/S composite) shows good rate- and cycle performance. Our strategy may open up a new avenue for the design and construction of superior Li-S batteries with large sulfur loading without complex materials synthesis. Li-S batteries have been regarded as one of the most promising candidates for nextgeneration batteries since their theoretical energy density (~2,600 Wh/kg) is much higher than that of existing lithium-ion batteries (~200 Wh/kg).1,

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However, there are some

drawbacks impeding their popularization, including low sulfur utilization due to its low electrical conductivity and unsatisfactory cycle performance caused by “redox shuttle reactions” of dissolved lithium polysulfides (PS).3, 4 These issues have been addressed in many previous works; however, the areal sulfur loading, as a crucial factor related to energy-density of the batteries, is still low (< 6 mg/cm2) for practical application.5, 6 Areal


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sulfur loading is primarily restricted by the low mass content of sulfur in the cathodes (50%~70%).7-9 This is because, with a low sulfur content, there is no choice but to increase the thickness of the electrode film to obtain a high areal loading.10 However, the thick electrode tends to fracture and delaminate from the current collector, and always exhibits low rate performance, making the sulfur-rich electrode difficult to realize.11, 12 The sulfur content has rarely been successfully improved in previous works, which is likely due to the contradiction between sulfur loading and the surface area of sulfur hosts (carbon or polar materials). High surface area is needed to trap dissolved polysulfide via physical confinement or chemical interactions so that a good cycle stability can be achieved.13-15 High surface area is also needed for immobilizing insoluble Li2S2 or Li2S16,

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to insure a high sulfur utilization and rate performance; without

adequate surface area, a thick Li2S2 or Li2S layer will deposit on the hosting materials, and impede subsequent reactions of PS and reduce the holistic electrical conductivity of electrode18, 19 . However, if the sulfur content is too high, much surface area will be lost and becomes unavailable for polysulfide trapping and Li2S2 or Li2S immobilization. In the past few years, intensive research efforts have been made to develop sulfur hosts with ultra-high specific surface area.20-22 Through these efforts, the sulfur mass content and areal loading can be increased to 90% and > 6 mg/cm2, respectively.23-24 However, complexity involved in the design and manufacturing process for these materials is not compatible with practical application of the battery.11 Therefore, it is highly desired to design and fabricate a cathode of large areal sulfur loading without involvement of complex sulfur hosts synthesis.


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Herein, we report a facile and novel synergistic strategy to achieve the above goal. Enlightened by some reported works,25 instead of seeking polysulfide confinement with ultra-high-specific-area sulfur hosts, we explore polysulfide blocking by using a separator modified with a dense solid electrolyte interface (SEI) layer. This SEI modified separator will help confine the dissolved polysulfide between the cathode and the separator, and thus, allow the use of large areal sulfur loading in the cathode. The SEI modified separator differs from literature reported separators decorated with sulfur-hosts such as CNT,26 V2O5,27 graphene28 and so on; these separators, although able to increase the sulfur content in cathode, still work on an adsorption basis, and therefore, the sulfur content (S/hosts in cathode and separator) can hardly support high areal loading of sulfur.26-29 In addition to the use of SEI modified separator, we also adopt a carbon nanotube (CNT) array in the cathode to load insoluble Li2S2 or Li2S and facilitate electron transport, thus insuring high sulfur utilization and rate performance.19, 30 With the synergistic effect of SEI modified separator and CNT cathode structure, our cathode with an areal sulfur loading as high as 10mg/cm2 can give a high rate- and cycle performance. Meanwhile, the mass sulfur content can reach 90.9% in the cathode, and 80.1% when the carbon materials on separator also is counted in. The SEI modification of separator (SC-separator) was achieved simply by charging– discharging a coin cell with the separator that is pre-coated with an active layer of carbon black and PVDF binder (C-separator) at 50 mA/cm2 (~100 mA/g). The compositions of coin cell used for forming SEI is as follows: the carbon layer of C-separator as cathode, the polypropylene membrane of C-separator as separator and Li metal as anode. The electrolyte we used is the normal electrolyte of Li-S battery, which comprised 1 M


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Figure 1. (a) Discharge-charge curves of the C-separator. (b-g) Nyquist plots of the Cseparator at different states: (1) before discharge; (2) after one discharge within 0.3–1.0 V; (3-6) after one-four cycles within 0.3–1.0 V. SEM images of the SC-separator at state 1 (h-j) and at state 5(k-m). lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in a mixture of 1,3-dioxolane (DOL) and dimethoxymethane (DME) (1:1, v/v) and 0.2 M of LiNO3. The corresponding


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charge–discharge curves were recorded in Figure 1a. Since it is well established that a SEI forms on the surface of a carbon electrode in a Li-ion cell at ~0.8 V versus Li/Li+,3134

and in consideration of the polarization of electrode, we chose 0.3-1.0V as the

charge/discharge potential to form SEI on carbon black. Electrochemical impedance spectroscopy (EIS) was utilized to track the formation of SEI layer.35, 36 The EIS curves corresponding to different charge-discharge states (marked in Figure 1a) are shown in Figure 1b-g. Before discharge (state 1), the battery gave an EIS curve featuring a typical semicircle (Figure 1b). After one discharge at 0.3–1.0 V, the EIS curve (Figure 1c; state 2) is significantly different from that in Figure 1b, but a typical twin-semicircle feature indicating SEI formation is not found.19,

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With the following cycles, the twin-

semicircle feature became more and more prominent (Figure 1d-e). In the EIS curve of state 5 (Figure 1f), there are two distinct semicircles indicating a stable SEI layer formation after 3 charge/discharge cycles.35-38 Interestingly, the EIS curve following four cycles (Figure 1g) is similar to that resulting from three cycles; this implies that a cycle number of three is enough to establish a stable SEI layer. Consistent with the above EIS findings, SEM observation also indicates successful formation of a surface SEI layer. As shown in the SEM images corresponding to state 1 (Figure 1h-j) and state 5 (Figure 1km), the gaps between carbon particles in state 1 (before discharge) almost disappeared after three cycles (state 5). The SEI layer is further indicated by the TEM images (Figure S1a-d) and energy-dispersive X-ray spectroscopy (EDX) mapping results (Figure S1eg). From the TEM images and EDX mapping, the thickness of the SEI layer was approximately ~10 nm (Figure S1d), and the distribution of F on the carbon surface was uniform (Figure S1f).


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Figure 2. XPS peaks of different separators used in the batteries: (a) C1s for pure-, Cand SC-separators; (b) deconvoluted C1s for C-separator; (c) deconvoluted C1s for SCseparator; (d) Li1s for pure-, C- and SC-separators; (e) deconvoluted Li1s for SCseparator; (f) F1s for pure-, C- and SC-separators.


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Figure 2 presents the XPS spectra of the following three samples, sample 1 being the pristine polypropylene (PP) separator, sample 2 the C-separator and sample 3 the SCseparator (after SEI modification). The C1s spectrum for sample 1 (Figure 2a) displays only one sharp peak at 285.0 eV, which is attributed to C–H and C–C bonds in PP.39 The deconvoluted C1s peaks for sample 2 (Figure 2b) are attributed to PP (285.0 eV), carbon black (284.5 eV),40 and PVDF binder (C-H at 286.4 eV and C-F at 290.9 eV).41, 42 The spectrum for sample 3 includes C1s peaks (Figure 2c) attributable to Li2CO3 (290.4 eV) and RCO2Li (289.6 eV), the most common ingredients of SEI,19,43-46 and a new peak (293.3 eV) assignable to LiTFSI.40, 47 Sample 3 also exhibits a Li1s peak (Figure 2d), which can be deconvoluted into components (Figure 2e) corresponding to LiTFSI (56.6 eV), ROCO2Li (55.0 eV), Li–CO3 (55.5 eV) and LiF (56.0 eV).19, 40 LiF is also a main ingredient component of SEI; its presence is further confirmed by the F1s peak at 685.0 eV in Figure 2f, where the other F1s peaks can be assigned to LiTFSI (689.0 eV) and LiPVDF (688 eV). Note that the C-F binding energy of PVDF in sample 3 is shifted 0.5 eV relative to that of original PVDF in sample 2; this is probably due to Li coordination with F atom.48 The advantage of SC-separator can be illustrated via comparing the electrochemical performances of the Li-S batteries using a pristine separator, a C-separator and an SCseparator, respectively. The cathodes used in these batteries are carbon cloth electrode with three-dimensional CNT array structure and 10 mg/cm2 sulfur (90.9% in the CNT/S composite). The SEM and TEM images of CNT array are shown in Figure S2. The sulfur content in cathode is 48.3% determined by TGA (Figure S3).


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Figure 3 displays the schematic diagram, corresponding SEM images and first charge-discharge curve at 0.2 C (1C=1675 mA/g) of the batteries assembled with


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different separators. As shown in Figure 3a, the the pristine separator can not hinder PS that survived CNT adsorption, and therefore, much PS reaches the Li anode, leading to the fatal redox “shuttle” reactions;16, 49 this battery could not be charged normally due to the severe redox shuttle. The battery using C-separator also failed to be charged to 3 V (Figure 3b), meaning the carbon material (including that in cathode and separator) is not enough to adsorb PS because the sulfur content is high (84.0%). On the contrary, the battery with SC-separator could be charge-discharged normally (Figure 3c) due to the existence of SEI layer. Figure S4 displays the charge/discharge curves of these Li-S batteries within 1.8-3 V to distinguish between the plateau of S and that corresponding to LiNO3 decomposition. The reason for the above different PS impeding behaviors of the C-separator and SC-separator can be clearly found in their cross-sectional SEM images. As Figure S5a-c shows, there are conspicuous holes in the C-separator allowing PS migration to Li-anode; such holes almost disappeared in SC-separator due to the presence of SEI (Figure S5d-f), which can obstruct PS more effectively. The advantage of SC-separator can be more clearly demonstrated by a diffusion test where a pure electrolyte solution was separated with a PS solution by the C- or SC-separator between the two compartments of the diffusion cell (Figure S5g-h). It is seen that the electrolyte in the C-separator cell became yellow just after 0.5 h and its color became darker after 1.5 h (Figure S5g); by contrast, no appreciable change in color can be found for the electrolyte in the SC-separator cell even after 1.5 h (Figure S5h), and the electrolyte solution became yellow after 6 h. The above observation suggests that the SC-separator can confine the dissolved polysulfide between cathode and separator more effectively than the C-separator.


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The contribution of SC separator was further confirmed by examining the Li-anode of the batteries containing different separators. As shown in Figure 3d-f, the sulfur content of anode with SC-separator determined by EDX is significantly lower than that of the anodes with pure separator and C-separator. Meanwhile, yellowish area could barely be found on anode of battery with SC-separator, but could be clearly observed on anode of batteries with the pristine- and C-separator. These results suggest that the SC-separator can suppress polysulfide migration efficiently. It should be noted that the cathode with 10 mg/cm2 sulfur could not be charged normally using conventional strategies where the separator is decorated with other hosting materials. As Figure S6 and S7 shown, the carbon sphere- and Li4Ti5O12- coated separator can’t provide enough surface area to adsorb so much dissolved PS. Then, the advantage of nano-array cathode structure is confirmed. Figure 4a shows the Nyquist plot of the CNT powder electrode before loading sulfur, which indicates a slightly higher charge-transfer resistance (~20 Ω) than that of CNT array electrode (~15 Ω, Figure 4c). This is because the CNT array can provide a straight transport pathway (1D direct line) for electrons (inset, Figure 4c) than that of powder structure (inset, Figure 4a).50 However, the charge-transfer resistance (Rct, ~75 Ω) of the powder CNT electrode after loading sulfur (Figure 4b) is much higher than that of the CNT array/sulfur electrode (~28Ω, Figure 4d). This difference can be attributed to different transport pathways for electrons. For the powder electrode, impedance becomes much higher after sulfur loading than that before loading. The S-coated CNT can not transport electrons effectively, and thus the length of the electron transport pathway becomes


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Figure 4. (a-d) Nyquist plots of different electrodes: (a) CNT powder electrode; (b) CNT/S powder electrode (10 mg/cm2 sulfur loading); (c) CNT array electrode; (d) CNT/S array electrode (10 mg/cm2 sulfur loading). The insets in panels a-d are the schematic illustration of electron transport pathway in corresponding electrode. (e-f) First chargedischarge curves of (e) CNT/S powder- and array- electrode at 0.2 C and (f) CNT/S powder electrode at 0.05C.


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significantly longer (inset, Figure 4b) than that before loading sulfur.30 On the contrary, for the array electrode, CNT was grown directly on the surface of carbon cloth so that electron can travel from carbon cloth, along CNT, to sulfur for electrochemical reaction (inset, Figure 4d).19 Therefore, the sulfur-loaded array electrode has slightly higher interfacial impedance than that without sulfur, suggesting that the array structure can significantly offset the electrical conductivity decrease caused by the high sulfur load. The benefit of array structure can be further confirmed by the first charge-discharge curves of these two different electrodes. As Figure 4e shows, the powder electrode has a significantly higher potential polarization than the array, and therefore, sulfur utilization of the powder electrode (650 mAh/g) is much lower than that of the array electrode (1065 mAh/g). The first charge-discharge curve at 0.05 C in Figure 4f well confirms that the powder

cathode can work properly. The advantage of SC-separator can be further

indicated by studying the cycle performance of CNT/S array electrodes with low sulfur loading (3 mg/cm2). As Figure S8 shown, with SC-separator, the cathode can exhibit ~90% capacity retention after 50 cycles at 0.2 C, which is much better than that with pure- or C-separator. Then, the SEM of C-separator and SC-separator is displayed in Figure S9, which indicates that the SEI is stable during charging-discharging within 1.5 ~ 3 V. Due to the synergistic effect of SC-separator and the nano-array cathode structure, our battery exhibits an impressive performance. As shown in Figure 5a, the charge−discharge curves of the battery using the CNT array cathode (with 10mg/cm2 sulfur), SC-separator and Li-anode at 0.1 ~ 2C exhibit low potential polarization and flat plateaus. This suggests that the array cathode can provide fast transport for electrons even


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with a very large sulfur loading, and therefore, the cathode delivered a good rate performance (Figure 5b): its mass- (and areal-) specific capacity is 1221 mAh/g (12.2 mAh/cm2) at 0.1C, 1096 mAh/g (10.9 mAh/cm2) at 0.2C, 929 mAh/g (9.3 mAh/cm2) at 0.5C, 752 mAh/g (7.5 mAh/cm2) at 1 C and 400 mAh/g (4 mAh/cm2) at 2C, respectively. The cycle performances at 0.5 C is displayed in Figure 5c, which exhibits 88% capacity retention after 100 cycles. The above performance is better than that of the literature reported LSBs with cathode of similar sulfur loading (Table S1).5, 11, 20, 22, 23, 51-53

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Figure 5. Electrochemical performances of the battery using CNT/S array electrode, SCseparator and Li-anode. (a) First charge-discharge curves at 0.1, 0.2, 0.5, 1 and 2 C. (b) Rate performance at progressively increased C rates. (c) Cycle performances at 0.5 C. Although there are some works reporting very high sulfur loading (>15 mg/cm2), the batteries can not be charge-discharged properly when the current density is higher than 0.5 C. Since our synergistic strategy can guarantee high sulfur loading and high rate


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performance simultaneously, the Li-S battery could drive a mini 30-200 rpm rotationmotor (see Video S1) very easily. In summary, we have developed a novel strategy for constructing high-performance cathodes with large areal loading of sulfur. Differing from the conventional methodology of trapping dissolved polysulfide with ultra-high-specific-area hosting materials, we simply decorated the separator with a dense layer of SEI to confine dissolved polysulfide between the cathode and the separator; this guarantees the cycle stability of the cathode with a large sulfur loading. Meanwhile, we adopted an array structure for the cathode to ensure a good rate performance of the cathode with a large sulfur loading. Attributed to the above synergistic strategy, our battery exhibits an ultrahigh areal specific capacity of 12.2 mAh/cm2 , a high rate performance (1221 mAh/g at 0.1C, 1096 mAh/g at 0.2C, 929 mAh/g at 0.5C, 752 mAh/g at 1 C and 400 mAh/g at 2C, respectively) and good cycle stability (88% capacity retention after 100 cycles at 0.5 C). Our work will open up a new avenue for the facile construction of high-performance sulfur cathodes with large areal loading.

Experimental Methods Materials preparation Carbon black-, carbon sphere- and Li4Ti5O12 coated separators. A homogeneous slurry was prepared by mixing the carbon black or carbon sphere or Li4Ti5O12 with polyvinylidene difluoride binder (8:2 by weight) in the presence of N-methyl-2pyrrolidone. The slurry was then cast on a polypropylene membrane and heated at 60 °C for 12 h under vacuum.


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CNT array on carbon cloth (CC). A piece of CC (2 cm x 2 cm) was immersed into a precursor solution and kept for 1 h. The precursor solution was prepared by dissolving 0.025 mol Nickel nitrate hexahydrate in a 50 mL mixed solution of alcohol and ethylene glycol (1:1, v/v) under stirring. After dried in ambient air, the treated CC was heated in a tube furnace for 1 h at 800 °C under a flowing N2 atmosphere with an 18 mL mixed solution of ethanol and ethylene glycol (1:5, v/v) placed upstream. Li2S8 electrolyte. The Li2S8 electrolyte (0.2 M) was prepared by dissolving stoichiometric

amounts

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Li2S

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lithium

bis(trifluoromethanesulfone) imide (LiTFSI, 1 M) and LiNO3 (2wt %) in a mixture (21 mL) of 1,3-dioxolane/1,2-dimethoxy-ethane (DOL/DME) with a volume ratio of 1:1. Materials characterizations Morphology of the as-prepared separators was studied using an FEI NanoSEM-450 Nova scanning electron microscope with a field emission gun (FEG) source. TGA (SDT, Q600) was used to determine the sulfur content under a standard test condition (a heating rate of 10 °C min-1 from room temperature to 500 °C in air). XPS characterization was implemented to confirm existence of SEI using an ESCALAB™ 250Xi ThermoFisher Xray photoelectron spectrometer. Cell assembly and measurements RC2016 coin cells were assembled in an argon-filled glove box using a CNT array electrode as the cathode and Li-metal circular foil (0.59 mm thick) as the anode; different separators were used for the cell assembly, including pristine polypropylene, carbon black-, carbon sphere-, Li4Ti5O12-coated polypropylene and the SC-separator. 50 µL of


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Li2S8 electrolyte was introduced into the coin cell, corresponding to about 10 mg/cm2 of sulfur mass loading. The electrochemical performance of these batteries was tested by a multi-channel battery tester (Shenzhen Neware Technology Co., Ltd, China) at room temperature. EIS measurements were performed using a CS310 electrochemical workstation with a frequency range from 0.01 Hz to 100 kHz.

Supporting Information Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Fig. S1 displays TEM images of different separator; Fig. S2 shows SEM images of CNT array cathode; TGA curve of CNT/S array on carbon cloth is provided in Fig. S3; Fig. S4 and Fig. S 5 show the SEM and performance of carbon sphere coated separator and Li4Ti5O12 coated separator respectively; Fig. S6 is the cycle performance at 0.2 C of the battery assembled with CNT/S array electrodes (3 mg/cm2 sulfur loading) and different separator; The SEM images of C-separator (a-c) and SCseparator (d-f) after cycled are given in Fig S7; Table S1 is the performance comparison of our battery with literature results. Video S 1 is the video clip for a mini rotation motor (30-200 rpm) driven by a Li-S battery assembled with a 0.25 cm2 cathode of high sulfur loading (10 mg/cm2).

Notes The authors declare no competing financial interest.

Acknowledgments


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This work was supported by grants from the Fund of State Key Laboratory of Fine Chemicals Panjin (JH2014009), the National Natural Science Foundation of China (No. 21276252 and 21776042) and China MOST (Ministry of Science and Technology) innovation team in key area (No. 2016RA4053).

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Graphical Abstract

SEI layer 20

2000

15

10 5

Capacity (mAh/g)

2

Capacity (mAh/cm )

1.0 1500

0.8

0.5 C

1000

0.4 500 2

CNT array with 10 mg/cm 0

0.6

0 0

20

40 60 Cycle Number

80

0.2

Coulombic efficiency

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0 100

The synergistic effect of solid electrolyte interface (SEI) modified separator and CNT array structure produces good rate- and cycle performance for a cathode with 10mg/cm2 sulfur (90.9% in composite).


25

Novel Synergistic Strategy for Developing High-Performance Lithium

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