- The battery is expected to be used in next-generation mobile phones, drones, and electric vehicles owing to its high energy density and improved lifespan.
Professor Jong-Sung Yu’s research group in the Department of Energy Science and Engineering at DGIST (President: Young Kuk) developed a technology for a porous silica interlayer by loading sulfur, an active material, in silica. This new approach is expected to be pivotal to the R&D and commercialization of next-generation lithium-sulfur batteries, in which energy density and stability are essential.
With the recent increase in demand for large-capacity energy-storage devices, research on high-energy, low-cost, next-generation secondary batteries that can replace lithium-ion batteries has been actively conducted. Lithium-sulfur batteries, which use sulfur as a cathode material, have an energy density several times higher than that of conventional lithium-ion batteries, which use expensive rare-earth elements as a cathode material. Therefore, it is expected that the sulfur-based battery will be more suitable for high-energy devices such as electric vehicles and drones. In addition, research on lithium-sulfur batteries is widespread because sulfur is inexpensive, abundant, and non-toxic.
On the other hand, sulfur, an active element that produces electrical energy, has low conductivity, and polysulfide generated during charging and discharging of the battery diffuses toward the negative electrode of the battery, resulting in the loss of sulfur through its reaction with lithium. Accordingly, the capacity and lifespan of the battery significantly deteriorate. This issue has been ameliorated by inserting a new layer between the sulfur electrode and separator (middle) that can absorb polysulfide and block diffusion.
Conductive carbon, which is currently used as an interlayer technology to improve the capacity and lifespan of lithium-sulfur batteries, imparts conductivity to the sulfur electrode. However, the diffusion of sulfur cannot be prevented because its affinity with the polar lithium polysulfide is low. On the other hand, if a polar oxide is used as an intermediate layer material, the loss of sulfur is suppressed owing to its strong interaction with lithium polysulfide. However, the utilization of sulfur is lower owing to its low conductivity. In addition, the various interlayer materials studied previously are not ideal because they cannot achieve the energy density and cycle life required for commercialization owing to their thickness and low redox activity.
To address these disadvantages, the research team first implemented a new redox-active porous silica/sulfur interlayer by adding sulfur in the silica after synthesizing the plate-shaped porous silica. They predicted that the capacity and lifetime efficiency of the lithium-sulfur batteries would be maximized owing to the sulfur-induced increase in the capacity per cell area, because additional sulfur was loaded in the intermediate layer, which could also act as an effective lithium polysulfide adsorption site.
To investigate this theory, the silica/sulfur interlayer was applied to a lithium-sulfur battery, which was then charged and discharged 700 times. As a result, the porous silica/sulfur interlayer achieved a much higher long-term stability than the conventional porous carbon/sulfur interlayer after 700 charge/discharge cycles. In particular, the battery exhibited high capacity and durable, long-lasting properties, even at a high sulfur content of 10 mg/cm2 and a low electrolyte:sulfur (E/S) concentration of 4. Therefore, it is near-ready for practical application.
Professor Jong-Sung Yu stated, “Our study is the first to find that sulfur can be loaded into the pores of a porous silica material to serve as an intermediate layer material for lithium-sulfur batteries, improving their capacity and lifespan.” He added, “This result is a new milestone in the development of next-generation high-energy, long-life lithium-sulfur batteries.”
This study was conducted in collaboration with Dr. Amine Khalil's team at Argonne National Laboratory (ANL) in the USA. Dr. Byung-Jun Lee, who received his PhD under the guidance of Professor Jong-Sung Yu of the Department of Energy and Science and Engineering at DGIST, was the first author. This study was published online on August 8th in Nature Communications, a sister journal of Nature, which is recognized as a world-class academic journal.
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Summary of Research Outcomes
Development of high-energy non-aqueous lithium-sulfur batteries via redox-active interlayer strategy
Byong-June Lee, Chen Zhao, Jeong-Hoon Yu, Tong-Hyun Kang, Hyean-Yeol Park, Joonhee Kang, Yongju Jung, Xiang Liu, Tianyi Li, Wenqian Xu, Xiao-Bing Zuo, Gui-Liang Xu*, Khalil Amine* & Jong-Sung Yu*
(Nature Communications, published online August 8th, 2022)
Owing to the global policy of carbon neutrality, electric vehicles have received great attention as eco-friendly alternatives to replace internal-combustion-engine vehicles. In addition, lithium–sulfur batteries are receiving much attention as next-generation batteries for electric vehicles owing to the low theoretical energy density (387 Wh kg–1) of conventional lithium-ion batteries. Lithium–sulfur batteries have a high theoretical energy density (2600 Wh kg–1), and sulfur is an inexpensive and earth-abundant active material. Accordingly, the lithium–sulfur battery is the most promising among the next-generation secondary batteries currently being developed. However, the low electrical conductivity of sulfur and the shuttle effect, which causes the loss of sulfur owing to the dissolution of polar lithium polysulfides generated during battery charging and discharging in the organic electrolyte, reduce the capacity and lifespan of the battery. To solve these shortcomings, an interlayer has been placed between the separator and sulfur electrode to block the shuttle effect of lithium polysulfides. This method has been actively researched and has been found to reduce sulfur loss effectively.
In this study, we used a porous silica/sulfur composite as an interlayer material for lithium–sulfur batteries. To construct the composite, we loaded sulfur into the pores of porous silica, which had not been previously attempted owing to its lack of conductivity. In particular, because silica has a short pore length, which facilitates sulfur loading, we attempted to increase its efficiency by synthesizing plate-shaped porous silica with a large surface area. By using active sulfur in the interlayer as well as in the cathode electrode, the total amount of sulfur in the battery increased, thereby increasing the capacity per unit area of the battery. Therefore, this study was based on the idea that using plate-shaped polar silica as lithium-polysulfide adsorption sites could maximize the reaction efficiency of lithium–sulfur batteries, as suggested by our previous research. Compared with the porous carbon interlayer commonly used in lithium-sulfur batteries, the porous silica/sulfur interlayer demonstrated superior long-term performance over more than 700 charging and discharging cycles. It also had a higher capacity per unit area after the charge/discharge cycle compared with the non-loaded porous silica interlayer. In particular, we implemented a high areal and volumetric capacity by loading a high sulfur content of 14.3 mg/cm2, which is approximately seven times higher than the typical sulfur content (2 mg/cm2) reported in previous studies. In addition, excellent long-term stability was achieved owing to the favorable characteristics of polar silica.
Reducing the amount of electrolyte in the battery is also essential to improving the energy density. Even at a low electrolyte:sulfur ratio (E/S = 4), the battery incorporating the silica/sulfur interlayer had excellent properties, making it promising for practical use.
Lithium polysulfide generated during the electrochemical reaction of sulfur was adsorbed by the porous silica/sulfur interlayer, the shuttle effect was alleviated, and the capacity of the battery was increased by loading additional sulfur to the intermediate layer as an active material. Li diffusion from the anode to cathode was found to be easier in the silica/sulfur interlayer than the non-loaded silica interlayer. Therefore, compared with non-loaded silica, the silica/sulfur interlayer significantly contributed to increasing the battery life and capacity per unit area by facilitating lithium-ion diffusion. This low-cost and simply prepared porous-silica-based material represents a turning point in the research toward developing next-generation high-energy, long-life lithium-sulfur batteries. Therefore, the development of practical lithium-sulfur batteries is expected to accelerate hereafter.
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Questions and Answers on Research Results
Q. What is different about this achievement?
Porous carbon, a typical interlayer material for conventional lithium-sulfur batteries, has a high electrical conductivity but low affinity for polar lithium polysulfide owing to its non-polarity. Therefore, it cannot effectively prevent the loss of sulfur. On the other hand, polar materials that possess low electrical conductivity have a low surface area, resulting in low sulfur utilization. The polar plate-shaped porous silica used in this study has a short pore length and large surface area, making it easy to load a large amount of sulfur into the pores. In addition, owing to the strong interaction of the plates with lithium polysulfides, the former effectively block the latter while increasing the battery capacity per unit area, resulting in a high-energy, long-lasting lithium-sulfur battery.
Q. Where can it be used?
A significant milestone in next-generation high-energy, long-life lithium-sulfur batteries has been reached by loading sulfur onto porous silica and using the material as an interlayer. Therefore, the interlayer can be used to develop other secondary metal–sulfur batteries with high durability for next-generation electric vehicles.
Q. What are the time and tasks required to put it into practical use?
The non-conductive or polar porous silica/sulfur interlayer proposed in this study effectively reduced sulfur loss while increasing the amount of sulfur per area. However, sulfur has a very low electrical conductivity. Therefore, it is also important to improve the electrical conductivity of the interlayer in order to enhance the battery's performance for practical use in the future. To accelerate the commercialization of practical high-energy, long-life batteries, materials that satisfy the requirements of high conductivity, polarity, and porosity must be developed.
Q. What motivated you to initate your research?
My research team has previously used porous silica as a sulfur electrode material to obtain lithium-sulfur batteries with significantly improved durability. We initiated this follow-up study because we envisioned the development of a high-capacity battery with an extended lifespan by splitting sulfur into plate-shaped porous silica pores as a redox-active interlayer as well as in the cathode instead of using all the sulfur in the cathode.
Q. What is the significance of this study?
In this study, a silica/sulfur composite loaded with sulfur was used as an interlayer in the Li-S battery for the first time. Porous silica alone was not used as an interlayer owing to its low electrical conductivity. This study attempted to overcome the problems associated with battery capacity and sulfur utilization, which are exacerbated because of excessive sulfur loading in the cathode, by using the plate-shaped porous silica interlayer to prevent the loss of sulfur.Therefore, this is the first study to demonstrate that a redox-active silica/sulfur interlayer can improve the performance and lifespan of lithium-sulfur batteries compared with the previously reported highly conductive carbon interlayer.
Q. What goal do you want to achieve?
The results of this study reveal that the strategy of loading sulfur in the interlayer can increase the capacity and lifespan of lithium-sulfur batteries. In addition, a low-cost, high-energy, long-lasting silica-based lithium-sulfur battery was developed, marking a new milestone in lithium-sulfur battery research. This achievement is expected to spur the development of other next-generation secondary batteries. Furthermore, we intend to significantly improve the energy density and durability of the lithium-sulfur battery by developing a material that can more efficiently block the lithium polysulfide shuttle, thus moving the technology one step closer to commercialization.
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[Figure 1] Comparison of the lithium-sulfur battery incorporating the porous silica/sulfur interlayer developed here with a lithium-sulfur battery prepared using a conventional conductive interlayer or polar interlayer.
(a ) Diagram comparing the composition and characteristics of a lithium-sulfur battery comprising a conductive interlayer (left), polar interlayer (middle), and sulfur-loaded polar interlayer (right). When a sulfur-loaded polar interlayer is used, a high-energy, long-lasting lithium-sulfur battery can be realized owing to its high sulfur loading, high redox activity, and excellent polysulfide adsorption capacity.
(b) Comparative study of the electrochemical performance of interlayer-free lithium-sulfur and lithium-sulfur batteries incorporating porous carbon, porous silica, and porous silica/sulfur interlayers. In the porous silica/sulfur interlayer, a significantly higher capacity per unit area was observed, and a much higher capacity per unit area was also observed after 700 cycles of charging and discharging. Initially, the performance of the battery comprising the porous carbon interlayer was superior to that of the battery with the porous silica interlayer . However, the capacity rapidly decreased as the number of cycles increased owing to poor retention of the polar lithium polysulfides. On the other hand, after 380 cycles, the battery with the polar porous silica interlayer demonstrated a significantly higher capacity and stability than that with the porous carbon interlayer by effectively inhibiting the dissolution and diffusion of lithium polysulfides. In addition, excellent long-term performance was achieved, even after more than 2,000 charging and discharging cycles.
[Figure 2] Photographs of the diffusion test, in which lithium polysulfide was electrochemically reacted at a current density of 167.5 mA g–1 in H-type batteries using Digital photographs of the results of the visual electrochemical tests performed in H-type cells at 167.5 mA g−1 for 36 h using the PP (polypropylene) separator film without an interlayer-free or with, porous carbon, porous silica, and or porous silica/sulfur interlayers
The H-type battery is a battery composed of a lithium metal metal anode on the left and sulfur anode cathode on the right. A separator/intermediate layer is placed in the middle of the two electrodes sulfur anode;, and both electrodes are submerged in an electrolyte. Lithium polysulfide is generated at the sulfur anode cathode as the electrochemical reaction proceeds, and the color of the electrolyte at the anode cathode changes to dark brown. The produced lithium polysulfide can diffuse toward the lithium-metal-metal anode through the separator.
(a) Lithium polysulfides diffuse toward lithium metal through the separator. Therefore, the diffusion of lithium polysulfides was not prevented by the polypropylene separator.
(b) The lithium metal electrolyte turned yellow after 36 hours of reaction. Therefore, the porous carbon interlayer did not block the diffusion of polysulfides.
(c, d) When a porous silica interlayer or a porous silica/sulfur interlayer is used, the color of the electrolyte on the lithium-metal-metal side remains transparent even after 36 hours of reaction, suggesting that the interlayer efficiently blocked lithium polysulfide diffusion. In particular, when a porous silica/sulfur interlayer is used, the color of the electrolyte on the sulfur-electrode side changes to dark brown owing to the generation of lithium polysulfide upon higher sulfur loading. However, the electrolyte on the lithium-metal side remains transparent owing to the effective blocking of the porous silica/sulfur interlayer.
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Department of Energy and Science and Engineering
Daegu Gyeongbuk Institute of Science and Technology (DGIST)
Research Paper on Nature Communications
Byong-June Lee, Chen Zhao, Jeong-Hoon Yu, Tong-Hyun Kang, Hyean-Yeol Park, Joonhee Kang, Yongju Jung, Xiang Liu, Tianyi Li, Wenqian Xu, Xiao-Bing Zuo, Gui-Liang Xu*, Khalil Amine* & Jong-Sung Yu*, "Development of high-energy non-aqueous lithium-sulfur batteries via redox-active interlayer strategy", online published on 08.08,2022.