A Carbon-Cotton Cathode with UltrahighLoading Capability for Statically and Dynamically Stable Lithium−Sulfur Batteries Sheng-Heng Chung, Chi-Hao Chang, and Arumugam Manthiram* Materials Science and Engineering Program and Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *
ABSTRACT: Sulfur exhibits a high theoretical capacity of 1675 mA h g−1 via a distinct conversion reaction, which is different from the insertion reactions in commercial lithium-ion batteries. In consideration of its conversionreaction battery chemistry, a custom design for electrode materials could establish the way for attaining high-loading capability while simultaneously maintaining high electrochemical utilization and stability. In this study, this process is undertaken by introducing carbon cotton as an attractive electrode-containment material for enhancing the dynamic and static stabilities of lithium−sulfur (Li−S) batteries. The carbon cotton possessing a hierarchical macro-/microporous architecture exhibits a high surface area of 805 m2 g−1 and high microporosity with a micropore area of 557 m2 g−1. The macroporous channels allow the carbon cotton to load and stabilize a high amount of active material. The abundant microporous reaction sites spread throughout the carbon cotton facilitate the redox chemistry of the high-loading/content Li−S system. As a result, the high-loading carbon-cotton cathode exhibits (i) enhanced cycle stability with a good dynamic capacity retention of 70% after 100 cycles and (ii) improved cellstorage stability with a high static capacity retention of above 93% and a low time-dependent self-discharge rate of 0.12% per day after storing for a long period of 60 days. These carbon-cotton cathodes with the remarkably highest values reported so far of both sulfur loading (61.4 mg cm−2) and sulfur content (80 wt %) demonstrate enhanced electrochemical utilization with the highest areal, volumetric, and gravimetric capacities simultaneously. KEYWORDS: lithium−sulfur batteries, high-loading electrode, high capacity, self-discharge, porous carbon, electrochemistry and limit the sulfur loading.3,4,8−10 The conversion between sulfur and lithium sulfide involves the formation of polysulfide intermediates (Li2S4−8), which are soluble in the liquid electrolyte used.5−7 The dissolved polysulfides with high electrochemical activity function as catholytes to facilitate the redox reaction.11,12 This implies that electrolyte/catholyte stability should be well designed for boosting the electrochemical-conversion chemistry. However, dissolved polysulfides easily diffuse out from the cathode side and migrate toward the anode side. The polysulfide diffusion leads to active-material loss, lithium−metal corrosion, and insulating Li2S2/Li2S deposition on electrode surface during cell cycling and resting.5,7 Electrode instability leads to both dynamic and static irreversible capacity loss.5−7,13,14 In an effort to minimize these negative effects, a variety of cathode architectures have been put
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ith the ever-increasing demand for energy density, the performance of lithium-ion batteries has been steadily improving.1 As a mature technology, the insertion-compound electrode hosts used in commercial lithium-ion batteries are reaching their maximum chargestorage capacities.1−4 Going beyond lithium-ion chemistry, a high-capacity sulfur cathode coupled with a lithium−metal anode could offer a high-energy density of 400−600 Wh kg−1. This conversion-reaction battery chemistry has the potential to offer 2−3 times higher energy density than the commercial lithium-ion batteries.2−4 In terms of battery chemistry, the lithium−sulfur (Li−S) system is known for its complex chemical and electrochemical processes.5−10 At the cathode, the insulating sulfur and lithium sulfide cause low electrochemical utilization of the overall capacity.3−7 Porous carbons and functional polymers have been developed to encapsulate and bond sulfur to form nanocomposites. Sulfur-based nanocomposites must be well mixed with additional conductive carbons to reduce cathode resistance, while these processes reduce the sulfur content © 2016 American Chemical Society
Received: September 21, 2016 Accepted: October 26, 2016 Published: October 26, 2016 10462
DOI: 10.1021/acsnano.6b06369 ACS Nano 2016, 10, 10462−10470
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Figure 1. Comparative analysis of the cell parameters of the carbon-cotton cathodes with those of the reported Li−S batteries, investigating (a) high-loading cathodes and (b) self-discharge effect. (c) Schematic of the fabrication process of the carbon-cotton cathode. Digital and SEM images of (d) cotton and (e) carbon cotton.
Figure 2. Microstructural inspections of (a) carbon cotton, (b) cycled carbon-cotton cathodes after 100 cycles, and (c) rested carbon-cotton cathodes after 60 days. Electrochemical performances of the carbon-cotton cathodes: (d,e) dynamic battery stability at C/10 and C/5 rates [the rectangles in (d) indicate the activation processes] and (f) static battery stability after resting for 60 days.
forward to retain polysulfides at the cathode side. Polar hosts,15−18 functional separators,19−25 polysulfide traps,26−29 and structural electrodes30−36 have been employed to offer a long cycle life35−40 and a low self-discharge effect.41−43
Compared to the efforts dedicated to improving the cell cyclability, there is less progress on increasing the high-loading capability in the Li−S system.9,14,37−40,44,45 The Li−S battery chemistry involves (i) a solid(sulfur)−liquid(polysulfides)−solid(sulfides) 10463
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reaction sites facilitate the redox chemistry of the high-loading/ content carbon-cotton cathodes.32,45 Dynamic and Static Battery Performances. To adopt the hierarchical macro-/microporous architecture, the carbon cottons were used as the electrode-containment material in a Li−S cell with polysulfide (Li2S6) catholyte as the starting active material.32,46 The carbon-cotton cathodes loaded with a high sulfur loading of 30.7 mg cm−2 have a sulfur content of 80 wt %. Figure 2b shows the morphological and elemental changes in the carbon-cotton cathodes after 100 cycles. On the other hand, Figure 2c shows the microstructural changes in the carbon-cotton cathodes after resting for 60 days. Both cycled and rested carbon-cotton cathodes exhibit a well-maintained coalescing carbon-fiber network and porous channels. No agglomerated active material covers the carbon-cotton cathodes nor blocks their porous catholyte channels, as compared to those observed with the pristine carbon cotton. Thus, the unblocked conductive carbon-fiber network and the macroporous electrolyte channels supply smooth electron and ion pathways to the active material stabilized within the carboncotton cathode.30,32,42 The elemental mapping results display the high intensity of sulfur signals and their uniform dispersion within the carbon-cotton matrix, demonstrating the excellent encapsulation of a high amount of active material within the carbon-cotton cathodes. Under a high-magnification observation (see Figures S4−S6 in the Supporting Information), the cycled and rested carbon-cotton cathodes show that their spiral carbon fibers are characterized by the recrystallization and infiltration of the sulfur-containing species. A close connection between the active material and the conductive skeleton, offering a fast electron pathway, enhances the redox reaction kinetics.14,30−34 Coupled with a highly accessible reaction area and stereoscopic electrolyte channels, the carbon-cotton cathodes facilitate and persevere the high-loading/content active material with excellent redox chemistry during, respectively, dynamic cell cycling and static cell resting.41−43,45−49 The enhanced high-loading Li−S battery chemistries brought about by the carbon-cotton cathodes were studied by examining the dynamic and static battery performances. Figure 2d shows the dynamic battery performances of the carboncotton cathodes with high sulfur loading and content of, respectively, 30.7 mg cm−2 and 80 wt %. The cells have peak discharge capacities of 1173 and 905 mA h g−1 at C/10 and C/ 5 rates. Before reaching the peak capacities, the cells exhibit an increase in charge-storage capacities during the initial 5 and 14 cycles at, respectively, C/10 and C/5 rates, as shown in rectangles in Figure 2d. The initial capacity increase is attributed to the activation of high-loading active material.36 During this process, the active material rearranges to electrochemically favorable positions within the carbon-cotton cathodes so as to improve the reaction kinetics. Therefore, the activated high-loading active-material cores enable the corresponding cells to deliver increasing charge-storage capacities.14,24 After 100 cycles, the reversible capacities remain at 788 and 638 mA h g−1, corresponding to high dynamic capacityretention rates of 67% and 71%. These features provide electrochemical evidence that the carbon-cotton cathodes simultaneously possess (i) high-loading capacity for raising the loading and content of the active material and (ii) excellent electrochemical quality with high utilization and retention.37,40,45 In consideration of the high sulfur loading, the
conversion, (ii) active-material rearrangement, and (iii) a cathode-volume change of 80%.2−7 Coupled with polysulfide diffusion, the electrode instability challenges the development of high-loading sulfur cathodes.9,14,37−40 Moreover, the thickness of the conventional sulfur cathodes is usually 100 μm).2,37 A thicker conventional electrode coating for sulfur cathodes has, unfortunately, proved that it is difficult to attain good electrode integrity.36−39 Given that a traditional Li-ion cathode configuration is not ideal for a Li−S battery, advanced electrode materials with stereoscopic architectures and high flexibility have recently been developed for realizing a high sulfur loading of above 10 mg cm−2 and for achieving a high areal capacity.4,36−40 The remaining challenges are how to simultaneously improve the sulfur loading and maintain the necessary dynamic and static cell stabilities via the electrode design in order to take advantage of the high-energy density of Li−S batteries for practical applications.37−45 In this study, a flexible carbon-cotton cathode is designed as an attractive electrode-containment material for enhancing the dynamic and static stabilities of a high-loading cathode system. The dynamically and statically stable battery chemistries exhibit, respectively, an excellent capacity retention of 70% after 100 cycles and a low time-dependent self-discharge rate of 0.12% per day after resting for 60 days. The ultrahigh-loading carbon cotton realizing a good balance between sulfur loading (61.4 mg cm−2) and sulfur content (80 wt %) demonstrates the highest areal capacity (56 mA h cm−2), volumetric capacity (1121 mA h cm−3), and gravimetric capacity (724 mA h g−1). To the best of our knowledge, the Li−S batteries presented here with a carbon-cotton cathode exhibit the highest sulfur loading/content and promising dynamic and static battery performances among the results reported in the literature. The enhanced high-loading capacity is up to 3 times higher than the highest values reported in the literature, as shown in Figure 1a,b (see Tables S1 and S2 in the Supporting Information).30−43
RESULTS AND DISCUSSION Morphology and Microstructure of Carbon-Cotton Cathodes. Figure 1c illustrates the preparation process of the flexible carbon-cotton cathode. Cotton was carbonized at 900 °C for 6 h under an argon atmosphere to form the carbon cotton (Figure 1d,e). The carbon cotton has a well-maintained porous structure and good mechanical strength. The high flexibility and durability allow the carbon cotton to be bent, rolled, and kneaded, still maintaining good integrity (see Figure S1 in the Supporting Information). This suggests that the carbon cotton could accommodate a high amount of active material and tolerate the volume changes.37−45 The microstructure of the carbon cottons was observed with scanning electron microscopy (SEM, Figure 2a and Figure S2 in the Supporting Information). The carbon cotton has spiral carbon fibers woven into a long-range conductive network, which ensures a high electrode integrity and fast electron pathways. The spiral carbon fibers armored with nanosized slit pores provide a high surface area of 805 m2 g−1 and high microporosity. Micropores spread throughout the carbon fibers have an estimated pore size of 1 nm and a high micropore area of 557 m2 g−1 (see Table S3 and Figure S3 in the Supporting Information). The carbon skeleton provides a hierarchical macro-/microporous architecture. The large macropore volume allows the carbon cotton to load with and stabilize a high amount of active material.30−32,45 The abundant microporous 10464
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Figure 3. Battery performances of the carbon-cotton cathodes: (a) rate capability from C/20 to 2C rates and ensuing cycle stability at C/10 rate and (b) high-loading capability testing.
Figure 4. Dynamic and static electrochemical analyses: (a) CV curves, (b) rate-dependent CV curves, (c) DLi+ coefficient, (d,e) QH and QL analysis, and (f) self-discharge and Ks constant.
areal capacities are raised to 36 and 28 mA h cm−2,38,41−43 but so would the electrode thickness. The increasing thickness of the electrodes would influence the volumetric capacity and could remain as a new challenge for high-loading sulfur cathodes. However, not much literature points out this problem in pursuing high-loading cathodes.8,14,37−40 A possible solution to ameliorate the volumetric capacity issue is to fill up the highloading cathode with a high content of sulfur.14,47 With a sulfur content of 80 wt %, Figure 2e displays the improved volumetric capacities of 1201 and 927 mA h cm−3 at C/10 and C/5 rates. As a result of the high sulfur content, the gravimetric capacities attain 930 and 718 mA h g−1.36−39,47 The excellent dynamic battery performances could conclude that the carbon-cotton cathodes exhibit a good balance between the high sulfur loading and content as well as the areal, volumetric, and gravimetric capacities. The static Li−S battery performances with a long-term cell storage have only been investigated by very few studies,5,41−43,48 and corresponding high-loading studies are even rarer. Figure 2f shows the self-discharge effect of the carboncotton cathodes resting at uncycled and fully discharged states for 60 days. At both the cell-storage conditions, the cells display almost constant open-circuit voltage (OCV) values, indicating
the static chemical and electrochemical stability. In the first case, the cell resting at uncycled state remained 94% of its original charge-storage capacity, corresponding to a low static capacity-fade rate, the time-dependent self-discharge rate, of 0.10% per day. This indicates the excellent polysulfide-retention capability of the carbon-cotton cathode because the uncycled cells were examined with the existence of a high content of polysulfide catholytes in the cathode.5,48 In the second case, the cell resting at the fully discharged state hold 93% of its original charge-storage capacity with a low time-dependent selfdischarge rate of 0.12% per day. This demonstrates that the redox chemistry was not impacted by the formation of insulating reduction products (Li2S2/Li2S).41−43,48 The stable OCV values, high static charge-storage capacities, and low capacity-fade rate evidence the excellent static cell stability associated with the carbon-cotton cathodes. Benefiting from the improved dynamic electrochemical properties, the high loading/content carbon-cotton cathode exhibits good rate capability from a low C/20 rate, showing the good polysulfide retention, to a high 2C rate, displaying the improved reaction capability (Figure 3a).14,26,40 After completing the 2C rate measurement, the cells display a high reversible capacity of 1022 mA h g−1 at C/20 rate (the 71st cycle). The 10465
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calculation of the Li-ion diffusion coefficient (DLi+).19,25 The DLi+ values of C1, C2, and A1 peaks are, respectively, 8.19 × 10−9, 2.29 × 10−8, and 3.85 × 10−8 cm2 s−1. The experimental DLi+ values are close to the reference values reported in the literature (DLi+ = 2 × 10−8 − 9 × 10−9 cm2 s−1),19,25 demonstrating the smooth Li-ion diffusion. The cell resistance and reaction kinetics were investigated by electrochemical impedance spectroscopy (EIS, see Figure S7 in the Supporting Information).13,25,49 Both uncycled and cycled cells display low resistance values, evidencing that the conductive carbon cotton and the electrochemically active catholyte improve the cathode conductivity.13,49 According to the CV and EIS analyses, the smooth ion and electron transport within the high-loading/ content carbon-cotton cathodes should improve the redox chemistry so that the cells are able to be normally operated and attain good electrochemical performances. The enhanced redox chemistry is subsequently characterized by the discharge/charge curves at C/10 and C/5 rates (see Figure S8 in the Supporting Information). The overlapping discharge/charge curves show no severe capacity or voltage fades, which reconfirms the highly reversible redox reactions.3,22,23 By examining the discharge/charge curves, the voltage hysteresis (ΔE), upper-plateau discharge capacity (QH), and lower-plateau discharge capacity (QL) provide more electrochemical characteristics in supporting the enhanced dynamically stable Li−S battery chemistry.21,23,25,28 In Figure S9 (Supporting Information), the ΔE values represent the voltage separation between the discharge and charge curves. At C/10 and C/5 rates, the ΔE values decrease, respectively, from 0.38 to 0.29 V and from 0.46 to 0.40 V after the initial activation24 and subsequently remain at 0.27 and 0.38 V during the following cycling. Low-voltage hysteresis demonstrates fast redox reaction kinetics brought about by the electrochemically active catholyte and conductive carboncotton electrode.23−25 The QH value indicates the polysulfidediffusion status and reflects the possible polysulfide retention because the upper plateau region involves the formation, dissolution, and migration of polysulfides.3,21,23,27 Despite a high amount of catholyte loading, the utilization rates of QH approach 80% at C/10 rate and 60% at C/5 rate (Figure 4d). After 100 cycles, the QH retention rates attain 63% at C/10 rate and 73% at C/5 rate. Such high QH utilization and retention suggest the reduced polysulfide diffusion.7,23 The QL value illustrates the reduction capability of the cells due to the slow liquid(polysulfides)−solid(sulfides) transformation in the lower plateau region.3,5,21 In consideration of the formation of solid redox products, the tendency of the QL as a function of cycle numbers also reflects the capability of the carbon-cotton cathodes to eliminate the redeposition of the insulating Li2S2/Li2S layer on their surface and to inhibit the formation of agglomerated active-material buildup within their porous spaces.7,23 The long lower discharge plateaus maintain well, such that the carboncotton cathodes exhibit a high retention rate in their QL of 60+ % at various cycling rates after 100 cycles (Figure 4e). The high QL retention represents a thorough reduction of the stabilized active material and excellent electrochemical reversibility.7,23,27 In general, ΔE, QH, and QL analyses and their favorable features explain why the carbon-cotton cathodes could enhance highloading capability and simultaneously maintain the improved dynamic battery performances. The statically stable Li−S battery chemistry was analyzed by a mathematical model at the uncycled and fully discharged states.5,41−43 In Figure 4f, the self-discharge constant (Ks) is
high electrochemical reversibility displays a low capacity fade of
