Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 23094−23102
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Enabling High-Energy-Density Cathode for Lithium−Sulfur Batteries Dongping Lu,* Qiuyan Li, Jian Liu, Jianming Zheng, Yuxing Wang, Seth Ferrara, Jie Xiao, Ji-Guang Zhang, and Jun Liu* Electrochemical Materials & Systems Group, Energy and Environment Directorate, Pacific Northwest National Laboratory (PNNL), Richland, Washington 99354, United States
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S Supporting Information *
ABSTRACT: High-energy lithium−sulfur (Li−S) battery is built on high loading and dense sulfur electrodes. Unfortunately, these electrodes usually suffer from a low sulfur utilization rate and limited cycle life due to the gap in scientific knowledge between the fundamental research and the application at relevant scales. In this work, effects of electrode porosity on the electrode energy density, cell cycling stability, Li anode interface, and electrolyte/sulfur ratio were investigated on the basis of high-loading sulfur electrodes. Using electrodes with sulfur loading of 4 mg cm−2 and thickness at ∼60 μm, a high energy density of over 1300 Wh L−1 has been obtained at electrode level, which provides a decent basis for high-energy Li−S cell development. In addition, Li−S cells with the high-loading and dense electrodes demonstrate promising cycling stability (∼80% capacity retention for 200 cycles). These significant improvements are contributed by the synergistic effects of dense sulfur cathode, improved electrode wetting, and suppressed quick growth of the interphase layer on Li-metal anode. This study sheds light on rational design of sulfur cathode for balanced cell energy density and cycling life. KEYWORDS: lithium−sulfur battery, high loading, electrode porosity, electrochemistry, cycle life
1. INTRODUCTION Lithium−sulfur (Li−S) battery represents an attractive alternative to the state-of-the-art Li-ion batteries to meet the urgent demand of high energy and cost-effective energy storage technologies.1 This is owing to the superior properties of Li−S batteries, such as high energy density, low cost, and natural abundance of sulfur.2 However, the market penetration of Li− S batteries is still limited because of low practically accessible energy density and poor long-term cycle life.1 Recent attempts at improving performance of Li−S battery focused on the development of nanostructured sulfur host materials,3−6 new electrolyte recipes,7−9 binders,10−13 modified separators,14 novel electrode structure designs,15 and fundamental understanding on the performance of these batteries. 16−20 Encouraging progress in terms of either high sulfur utilization rate or stable battery cycling has been made, which attracts extensive attention from both academia and industry to this promising energy storage technology.21 However, most of those studies are based on sulfur electrodes with either low fraction of sulfur in sulfur/carbon composites or low sulfur loading (less than 2 mg cm−2) in the whole electrode that also includes binder and conductive additive.22,23 For practical applications, both high fraction of sulfur in sulfur/carbon composites and high overall sulfur loading are required to improve the energy density of the system. Typically, an −2 to outperform commercial Li-ion batteries.24 Unfortunately, it is quite challenging to improve sulfur mass loading while maintaining a high material utilization rate and cycling stability, as demonstrated in the thin-film counterpart. © 2018 American Chemical Society
This is because the intrinsically low electronic and ionic conductivities of sulfur and Li2S/Li2S2 severely limit the electrochemical reaction kinetics, which is exaggerated remarkably with the increase of sulfur loading.25 A widely used strategy to mitigate this issue in high-mass-loading electrodes is to employ thick and porous current collectors,26,27 sandwich-type cathode structures,28,29 or free-standing carbon nanofiber (CNF)/nanotube papers as sulfur hosts.30−32 These methods are helpful to improve the sulfur utilization rate and conversion kinetics but at the same time sacrifice energy density of the system. High content of carbon materials increases parasitic weight and electrode volume while contributing no capacity at all. A more detrimental issue is that the ultrathick and porous electrodes or additional thick carbon interlayers require excess amount of electrolytes for full electrode wetting, which lowers gravimetric energy density significantly.23,25 So far, minimizing the electrolyte amount, i.e., effective control of the electrolyte/sulfur ratio, without sacrificing cell energy and cycling life represents one of the greatest challenges in Li−S battery research.33 In addition, the complex electrode preparation and processing methods for preparing such interlayers or sandwiched electrode structures may add additional cost to practical application compared with the conventional tape casting techniques. Thus, facile approaches that can enable a high sulfur utilization rate in a Received: March 30, 2018 Accepted: June 7, 2018 Published: June 7, 2018 23094
DOI: 10.1021/acsami.8b05166 ACS Appl. Mater. Interfaces 2018, 10, 23094−23102
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Dependence of electrode porosity and thickness on calendering pressure and dependence of (b) volumetric capacity and (c) volumetric energy density of sulfur cathodes on the electrode thickness with (red) and without (black) 0.15 M Li2S6 as electrolyte additive.
Figure 2. (a) Calculated specific energy densities of the Li−S pouch cells at various electrolyte/sulfur ratios and with different sulfur loadings. The weights of all cell components are included in the calculation, and related parameters of the components are listed in Table S1. (b) Dependence of minimum electrolyte amount on electrode porosity based on 4 mg cm−2 electrodes.
composed of low-density integrated Ketjen Black (IKB)/S composite and carbon conductive additives. Increase of the calendering pressure from 4.6 to 23 MPa induces linear decrease of electrode thickness from the original ca. 120 to 60 μm (Figure 1 a). Correspondingly, calculated electrode porosity is reduced significantly from 64 to 29%. A direct benefit of reducing electrode porosity is the significant improvement of electrode volumetric energy density, which is desired for practical applications. Figure 1b and c show the dependencies of electrode volumetric capacity and energy density on their thicknesses, respectively. The measured electrode volumetric capacity and energy density are only 320 mAh cm−3 and 650 Wh L−1, respectively, for the pristine electrode (∼120 μm thick) without calendering, which are far below theoretically projected values of sulfur. However, both of these values were almost doubled if the electrode was compressed to 60 μm thick, reaching 650 mAh cm−3 and 1300 Wh L−1, respectively. Further reduction of electrode porosity leads to sharp drop of both reversible capacity and energy as a result of blocked electrolyte infiltration (Supporting Information Figure S1). These results strongly suggest that appropriate control of electrode porosity is essential for highenergy-density Li−S batteries, which, however, is less discussed in published reports. The evolution trend of energy density (Figure 1c) doesn’t completely follow that of volumetric capacity (Figure 1b), which is caused by increased overvoltage for the electrodes with reduced porosity. The volumetric energy, which is calculated by multiplying volumetric capacity and working voltage, reflects not only the accessible capacity
high-loading and dense sulfur electrode are urgently needed for practical deployment of Li−S batteries. One effective strategy to utilize nanostructured or highly porous materials for high-loading sulfur cathode preparation is to integrate nanosized particles into large secondary ones, which are suitable for thick electrode preparation using conventional slurry casting methods.24 To pursue higher volumetric energy density, it is essential to control the electrode porosity. Lower electrode porosity is not only desirable for high volumetric energy density but also boosts gravimetric energy density contributed by reduced electrolyte uptake. However, this negatively impacts both sulfur utilization and cell cycling life. A clear understanding of such correlation is critical but rarely mentioned in the literature to date. In the present work, both benefits and drawbacks of electrode porosity control over the sulfur utilization rate, cell cycling, and electrode architecture and interface evolution were systematically investigated and discussed on the basis of the electrodes with a mass loading of 4 mg cm−2. This may shed light on the development of dense and high-loading sulfur cathodes and speed up market penetration of the Li−S battery technology.
2. RESULTS AND DISCUSSION Thick sulfur electrodes with mass loading of ca. 4 mg cm−2 were calendered at different pressures to reach different thickness or porosity. Clearly, the electrode thickness shows strong dependence on the calendering pressure. This is attributed to the loose structure of the as-prepared electrodes 23095
DOI: 10.1021/acsami.8b05166 ACS Appl. Mater. Interfaces 2018, 10, 23094−23102
Research Article
ACS Applied Materials & Interfaces
Figure 3. First discharge profiles and corresponding long-term cycling stability of sulfur cathodes compressed to different thicknesses and with electrolytes of 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)/1,3-dioxolane (DOL)/dimethoxyethane (DME) + 0.3 M LiNO3 (a, c) and 1 M LiTFSI/DOL/DME + 0.3 M LiNO3 + 0.15 M Li2S6 (b, d).
whereas only 4.7 μL is needed for the 60 μm compressed electrode. It means the minimum E/S ratio of 2.75 and 1.18 should be met for pristine and compressed electrodes, respectively, before considering polysulfide dissolution and electrolyte consumption at Li anode, which need more electrolyte for long-term cell operation. These results strongly suggest that control of electrode porosity has a notable benefit on the improvement of both system energy density and specific energy. Electrode wetting is a challenge for a thick sulfur electrode due to hydrophobic properties of both carbon and sulfur with ether-based electrolyte. This problem is exaggerated if reducing electrode porosity. Slow electrolyte infiltration and insufficient electrolyte uptake increase electrochemical polarization, which lowers sulfur utilization and cell output voltage plateaus. Figure 3a compares first discharging curves of electrodes at different thicknesses. Similar specific capacities of ca. 1100 mAh g−1 are obtained for all electrodes at 0.1C rate. However, lower discharge plateaus are identified for more dense electrodes. In particular, when the electrode was pressed to 60 μm, a notable voltage drop is observed. Low electrolyte uptake and slow Li+ transport caused by inhibited electrode wetting are direct reasons for the increased electrochemical polarization since better electronic contact is expected for compressed dense electrodes. Subsequent cycling helps to improve electrode wetting to some extent, as proved by the increase of voltage plateaus (Figure S3A), but it doesn’t work for very dense electrodes (e.g., 60 μm) due to the very limited electrolyte uptake in electrodes during the cell assembly process. So, dense electrodes are desirable for high energy density but negatively impact cell performance, as identified experimentally. First, the compact electrode surface blocks quick electrolyte penetration into the electrode during the limited
from the electrode but also the polarization during electrochemical reactions. It is noted that the energy density is at the electrode level with voltage versus Li anode. Control of electrode porosity has a significant effect on the required electrolyte amount for full electrode wetting. Energy density estimation indicates that system specific energy is related extensively to not only the areal loading of sulfur cathode but also the specific electrolyte amount, i.e., the electrolyte/sulfur ratio (E/S) (Figure 2a). This is because electrolyte takes the biggest weight proportion (43.6%) among all cell components (Figure S2 and Table S2). Cell energy estimation indicates that sulfur loading >4 mg cm−1 and E/S
