High-Energy-Density Lithium–Sulfur Batteries Based on Blade-Cast

High-Energy-Density Lithium−Sulfur Batteries Based on Blade-Cast Pure Sulfur Electrodes Long Qie 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: A facile pure sulfur electrode strategy is demonstrated to increase the areal loading of blade-cast sulfur electrodes and improve the electrochemical performances of thick sulfur electrodes with consideration for scale-up in practical, commercial applications. Benefiting from the unique pure sulfur electrode and an upper current collector design, the pure sulfur electrode displays low polarization, high sulfur utilization, and promising cycling stability even with an ultrahigh sulfur loading. A high areal capacity of 19.2 mAh cm−2 has been achieved, which is the highest areal capacity for the reported blade-cast sulfur electrodes and 4 times higher than that of available lithium-ion battery electrodes. Furthermore, we demonstrate that the use of large-sized sulfur particles only slightly decreases the rate of conversion from S8 to S42− during the initial discharge and has negligible impact on subsequent electrochemical performance.

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progress made on cycle stability, it should be noted that almost all the long-term cycling performance is achieved with low areal loading sulfur cathodes (usually 4.0 mg cm−2) reported are aluminum-foil-free, and such “free standing” electrodes have a high implementation cost in order to be applicable for industrial production. In this regard, the LIB industry-standard blade-casting technique should be developed for high areal loading sulfur electrode fabrication. However, because of the high carbon content in the sulfur electrodes and the poor electrolyte immersion throughout the thick electrodes, the high areal loading sulfur electrodes prepared with the traditional blade-casting method suffer from fractures and exhibit low sulfur utilization with poor rate performance.40−43

afe, high-energy-density energy storage systems at an affordable cost are in urgent need for storing electricity generated from renewable sources and for electric vehicles.1 To enhance the energy density of rechargeable batteries, researchers have shifted their focus from traditional insertion-compound-based lithium ion batteries (LIBs) to novel conversion-reaction-based systems such as lithium−air (O2), zinc−air, lithium−sulfur (Li−S), and sodium−sulfur (Na−S) batteries, of which the Li−S system is regarded as the most promising.2−6 Based on the conversion reaction between sulfur (S8) and the end discharge product lithium sulfide (Li2S), Li−S batteries could supply a high theoretical energy density of 2600 kW kg−1, which is five times higher than that of LIBs.7 However, the practical use of Li−S technology is still hindered by significant challenges such as low sulfur utilization, low Coulombic efficiency, fast capacity fade, and short cycle life.8,9 These problems mainly arise from the insulating nature of S8 and Li2S, loss of active material, severe consumption of electrolyte, and corrosion of lithium−metal anode during cycling.10 In the last ten years, most of the Li−S battery research has focused on suppressing the diffusion of dissolved long-chain lithium polysulfides (Li2S4−8, LPS) from the cathode to the anode side.11−15 On the cathode side, physical confinement of sulfur with porous conductive matrices; chemical adsorption of LPS by polar hosts such as metal oxides, metal carbides, or doped carbons; and physical blocking of LPS diffusion with a barrier between the sulfur cathode and the separator have been regarded as effective approaches.16−23 With these approaches, many groups have been able to achieve excellent cycling performance up to 500 cycles.24−29 Despite the remarkable © XXXX American Chemical Society

Received: March 26, 2016 Accepted: April 18, 2016

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ACS Energy Letters

Figure 1. (a) Schematic diagram depicting the fabrication of pure sulfur electrodes and (b) photographic images of pure sulfur electrodes with sulfur loadings varying from 2.5 to 16.2 mg cm−2. The diameter for the pure sulfur cathode is 12 mm.

Figure 2. Diagrams depicting the (a) GITT curves of a pure sulfur electrode during the first two discharges and (b) XRD patterns of (i) sulfur and Li2S powder (standard patterns), (ii) pure sulfur electrode, (iii) carbon paper covering pure sulfur electrode, and (iv) carbon paper covering the sulfur electrode after the initial discharge. Here, the sulfur areal loading of the pure sulfur electrodes is 16.0 mg cm−2.

Here, for the first time, we report a high areal loading pure sulfur electrode prepared via a blade-casting method using commercial sulfur powder directly as the active material. Using this pure sulfur strategy, an ultrahigh areal loading sulfur electrode could be achieved with low polarization, high sulfur utilization, and long cycle stability. More importantly, no complex porous carbon or nanosized sulfur synthesis or timeconsuming mixing process are required in the fabrication process for pure sulfur electrodes, which might trigger a revolution for the large-scale production of high-performance sulfur electrodes with a low-cost, industry-adopted technology. As shown in Figure 1a, fabrication of the pure sulfur electrode is quite facile. First, commercial sulfur powder (95 wt %) and poly(vinylidene fluoride) (PVDF, 5 wt %) binder were mixed, dispersed well in N-methyl-pyrrolidone (NMP), and then coated onto an Al foil substrate using the doctor blade casting method. By restricting the amount of NMP solvent, pure sulfur electrodes with areal sulfur loadings of 2.5−16.2 mg cm−2 could easily be obtained (Figure 1b). These electrodes demonstrate excellent adhesion even with high sulfur areal loading. As shown in Figure S1 (Supporting Information), with the traditional sulfur@carbon electrodes, the majority of the coating layer fractured and detached from the Al foil after folding. In contrast with the pure sulfur electrode, the sulfur powder adhered firmly to the Al foil after folding, demonstrating its excellent mechanical properties. Prior to being assembled as cathode in a Li−S battery, a sheet of carbon paper was used to cover the pure sulfur electrode. Here, the carbon paper serves as (i) the upper current collector and (ii) the adsorption layer for the electrolyte.

Because of the poor electronic conductivity of sulfur, previous studies have extensively focused on the idea that sulfur should be well mixed and in good contact with conductive hosts. Thus, much research has focused on searching for and implementing nanosized sulfur particles to realize high sulfur utilization.44 For the pure sulfur electrode in this study, commercial sulfur powder was directly used as the active material without any mixing with conductive additives or grinding into smaller particles. With this configuration, there lies the question of if the larger sulfur particles impede the lithiation discharge process or show a decreased sulfur utilization. To determine the electrochemical utilization of larger sulfur particles with our pure sulfur cathode, we used the galvanostatic intermittent titration technique (GITT). As Figure 2a shows, even with an ultrahigh areal loading of sulfur (16.0 mg cm−2), the pure sulfur electrode still exhibits an initial discharge capacity of 1435 mAh g−1 (sulfur utilization ∼85.7%), revealing that most of the pristine sulfur powder was successfully lithiated during the initial discharge process. In the high-voltage region, the changes in voltages on removing the current remain small, demonstrating relatively fast electrode kinetics in this region.45 It should also be noted that slower conversion dynamics from solid S8 to S42− is revealed during the initial discharge. The upper discharge plateau of the first discharge is slightly lower (0.030 V), and the voltage changes on removing the current are slighter higher (0.016 V) than those of the second discharge. This effect might be due to the poor electronic transfer through the larger sulfur particles. However, in the lower discharge plateau region, both the quasiequilibrium potentials and the discharge curves of the 47

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Figure 3. Electrochemical performance of pure sulfur electrodes with a sulfur loading of 13.9 mg cm−2: (a) Discharge−charge curves at different current densities, (b) rate performance, and (c) cycling performance at a C/3 rate. Here, the sulfur content in the cathode side is 55 wt %, and the electrolyte-to-sulfur ratio for the cion cell was fixed to around 15.

Figure 4. (a) Comparison of the electrochemical performances and scale-up ability of the pure sulfur electrode with traditional blade-cast sulfur electrodes. The scale-up ability (1 lowest, 5 highest) is roughly estimated based on the cost of raw materials and the complexity of electrode fabrication. Pouch cell schematics: (b) Nyquist plot of an as-assembled pouch cell (inset shows the photo of pouch cell) and (c) cycling performance at a rate of C/20 of the pure sulfur cathode pouch cell. Here, the sulfur loading is ∼10.4 mg cm−2 and the electrolyte-tosulfur ratio is 8.

initial two cycles demonstrate good overlap, an indication that the large commercial sulfur particles have successfully been converted to dissolved polysulfides in the high-voltage region

and do not interfere with the conversion dynamics between S42− and Li2S. The successful conversion from large commercial sulfur particles to Li2S is further confirmed by X-ray diffraction 48

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areal loadings of sulfur electrodes of up to 14 mg cm−2 have been developed by Zhang and co-workers, but the electrode was designed for Li−S primary batteries and could discharge for only one cycle.57 In comparison with traditional blade-cast sulfur electrodes, the pure sulfur electrode described in this Letter shows the most balanced electrochemical performance with respect to both areal capacity and cycling life. Moreover, no complex porous carbon or nanosized sulfur synthesis, or time-consuming mixing process, are required in the fabrication process for the pure sulfur electrode, which mitigates many of the barriers concerning the large-scale production of Li−S batteries. To further demonstrate the potential of pure sulfur cathodes for practical applications, pouch cells were assembled. Here, the areal loading of sulfur with the pure sulfur electrode was 10.4 mg cm−2, and the area was 12.0 cm−2 (Figure S3a, Supporting Information). As the Nyquist plot shows in Figure 4b, the asassembled pouch cell develops a high slope (low diffusion resistance, W) at low frequency, small semicircle diameter (low charge-transfer resistance, Rct = 40.1 ohm) at medium frequency, and short x-intercept (low interfacial resistance Rs = 0.8 ohm) at high frequency, revealing that the as-assembled pouch cell possesses favorable electrochemical conditions for the lithiation of commercial sulfur powder. When being cycled at a C/20 rate between 2.8 and 1.5 V vs Li+/Li, a high initial discharge capacity of 1160 mAh g−1 was attained (Figure S3b, Supporting Information) with a capacity of 815 mAh g−1 after 15 cycles (Figure 4c). These values, to the best of our knowledge, are the best results reported thus far for the pouchcell configuration Li−S batteries with similar areal loading of sulfur.31 Thus, with the pouch cell, we can conclude that the pure sulfur electrode serves as a promising pathway for practical Li−S battery applications. The superior electrochemical performance of the pure sulfur electrode could be attributed to its unique electrode structure. The highly conductive bottom current collector (Al foil) provides channels for electron transport, while the upper carbon current collector serves as an electrolyte reservoir and physical trap for LPS. The sufficient electronic source combined with a large Li+-ion supply by way of electrolyte ensures a fast conversion between S8 and Li2S. In addition, as the scanning electron microscopy (SEM), XRD, and energydispersive X-ray (EDX) spectroscopy results show in Figures S4−S6, the signals of Li2S collected from the upper current collector facing the cathode side are much stronger than those facing the separator, revealing that the LPS trapping ability of the upper current collector greatly suppress the diffusion of LPS to the anode (Figure S4a), which is critical for long-term cycling stability. In summary, we have demonstrated a facile pure sulfurelectrode strategy to increase the mass loading of blade-cast sulfur electrodes and improve the electrochemical performance of thick sulfur electrodes. Benefiting from the unique pure sulfur and an upper current collector design, the pure sulfur electrode demonstrated low polarization, high sulfur utilization, and promising cycle stability even with ultrahigh sulfur loading. A high areal capacity of 19.2 mAh cm−2 was achieved with a sulfur areal loading of 13.9 mg cm−2. Furthermore, we showed that the use of large sulfur particles only slightly slows the conversion from S8 to S42− in the initial discharge and has negligible impact on subsequent electrochemical performance, which clearly demonstrates the high possibility of using commercial sulfur powder directly as the active material in

(XRD). Figure 2b shows the XRD pattern after the initial discharge, in which all the peaks belonging to crystalline sulfur disappear and new Li2S peaks are observed. The electrochemical performance of the pure sulfur electrode was first investigated with an areal sulfur loading of 2.7 mg cm−2 (Figure S2, Supporting Information). To maximize the actual energy density of Li−S batteries, the areal sulfur loading was further increased to 13.9 mg cm−2, which has rarely been achieved with traditional blade-cast sulfur@carbon composite electrodes. The electrochemical performance of the ultrahigh areal loading pure sulfur cathode was first evaluated at different current densities (1C = 23.3 mA cm−2) from 2.8 to 1.5 V vs Li+/Li. As depicted in Figure 3a, the discharge/charge curves show two distinct plateaus, confirming excellent electrochemical reversibility of the pure sulfur electrode with ultrahigh sulfur areal loading. The ultrahigh sulfur loading electrode delivered a stable discharge capacity of 1278 mAh g−1 at 1.2 mA cm−2, which is 76.3% of the theoretical capacity of sulfur (∼1675 mAh g−1). This further supports the idea that high sulfur utilization is possible without mixing the sulfur powder into conductive hosts or nanosized sulfur particles for electrode fabrication. The rate capability gradually decreased with increasing current density, and a reversible capacity of as high as 797 mAh g−1 was achieved at a high current density (14.0 mA cm−2). After cycling at different current densities for 25 cycles, the capacity recovered to 1139 mAh g−1 when the current density was set back to 1.2 mA cm−2 (Figure 3b). The cycling performance of ultrahigh sulfur loading cathode is shown in Figure 3c. The cell was first discharged/charged at 1.2 mA cm−2, and it delivered an initial discharge capacity of 1383 mAh g−1, corresponding to an areal capacity of as high as 19.2 mAh cm−2. This is the highest areal capacity for the reported blade-cast sulfur electrodes and four times higher than that of the available LIB electrodes (4.0 mAh cm−2). An areal capacity of 14.6 mAh cm−2 (1050 mAh g−1) was achieved when the current density was increased to 7.8 mA cm−2 and gradually decreased to 10.3 mAh cm−2 (741 mAh g−1) over 100 cycles. Considering the ultrahigh sulfur loading (13.9 g cm−2) and high current density (7.8 mA cm−2), these cathodes demonstrate extraordinary cycling performance. With numerous elaborate and successful studies demonstrating excellent Li−S performance, most of them have, unfortunately, been accomplished only at the lab bench.46−53 The time has come to focus more seriously on the requirements for practical battery applications, past high specific capacity and long cycle life. Figure 4a depicts a graph that compares the electrochemical performance and scale-up ability of pure sulfur electrodes with traditional blade-cast sulfur@carbon electrodes. For example, with a low sulfur areal loading (0.4−0.6 mg cm−2), the yolk−shell sulfur@TiO2 could maintain a high discharge capacity of 690 mAh g−1 after 1000 cycles at a C/2 rate,54 but the areal capacity is significantly too low for practical applications. When the areal loading of sulfur increases to 4.0 mg cm−2, the discharge capacity left after 200 cycles at a C/5 rate is only around 650 mAh g−1 (2.6 mAh cm−2) for nitrogen-enriched carbon@sulfur composite electrodes.55 Using Ketjen Black (KB) nanoparticles integrated microsized secondary particles as carbon hosts for sulfur, crackfree sulfur electrodes with areal sulfur loadings of 4.7 mg cm−2 had the ability to be cycled for only 90 cycles with a residual capacity of 700 mAh g−1. This is even after the addition of CNT and graphene substrates implemented to improve the wetting problem of the thicker electrode.56 More recently, ultrahigh 49

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Li−S batteries. We believe that our results will open new avenues for the development of high-energy-density Li−S batteries while using low-cost materials and adopting industryadoptable processes.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00033. Experimental methods, digital images, supplemental electrochemical performance, SEM images, and XRD and EDX results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded in part by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award Number DE-AR0000377. The authors thank Dr. Seung-Min Oh for his assistance with the pouch cell fabrication and Pauline Han and Craig Milroy for their helpful discussions with the manuscript preparation.

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DOI: 10.1021/acsenergylett.6b00033 ACS Energy Lett. 2016, 1, 46−51