HMSC type lithium-sulfur full battery performance: (a) Lithium-sulfur button cell (6.9 mg cm-2 S + 6.8 mg cm-2Mo6S8, electrolyte active substance ratio ~ 1.5μLmg-1); (b) Lithium sulfur soft pack Battery (electrolyte active material ratio ~ 1.2 μL mg-1, ~ 2 times the amount of metallic lithium excess); (c) Comparison graph of energy density of full battery for lithium-sulfur battery. Note: Figure c is the estimated energy density of the button battery experimental parameters and the true energy density of the ampere-level soft-pack full battery.
Lithium-sulfur batteries are regarded as one of the ideal choices for the next generation of high-energy density battery systems. They are highly concerned by the scientific research and industrial circles all over the world, and are also one of the key research directions for the future layout of various countries.
However, with the deepening of research, lithium-sulfur batteries also face increasingly severe challenges. At present, the main problem is that the volume energy density of lithium-sulfur batteries is low, which leads to the loss of competitiveness in many important market applications. At the same time, the high electrolyte consumption has also become a bottleneck for the increase of its weight energy density.
The associate researcher of the E01 group of the Clean Energy Key Laboratory of the Institute of Physics of the Chinese Academy of Sciences / Beijing National Research Center for Condensed Matter Physics, Suo Limin, and the Massachusetts Institute of Technology professor Li Ju and Dr. Xue Weijiang cooperated to solve the common problems of lithium-sulfur batteries New ideas have been proposed, which provide new possibilities for the development of new high-energy density lithium-sulfur batteries in the future. Related research results were published in "Nature-Energy".
Low volume and weight energy density
Limit the development of lithium-sulfur batteries
Suo Limin told the China Science News that the next generation of high-energy density battery systems are mainly battery systems based on metal lithium anodes, such as lithium-sulfur and lithium-air batteries.
"Compared to lithium-sulfur batteries, although lithium-air batteries have a higher theoretical energy density, they are still at the basic research stage, and many key issues have not yet been well resolved. Lithium-sulfur batteries have the advantages of low cost and high energy density. After years of unremitting efforts, lithium-sulfur battery technology has matured and is close to commercialization. "
The results of a paper published in Nature-Materials by the Linda Nazar research group of the University of Waterloo in Canada for the first time achieved a reversible capacity close to 80% of the theoretical capacity, which ignited people ’s passion for research on lithium-sulfur batteries. At present, countries around the world pay more attention to lithium-sulfur batteries. Many universities and research institutes conduct basic scientific research. In addition, many companies such as Oxis in the United Kingdom and Sion Power in the United States have been engaged in the commercialization of lithium-sulfur batteries.
Li Ju said that in the past ten years or so, many key technologies of lithium-sulfur batteries, including sulfur cathodes and electrolytes, have made great breakthroughs and progress in the laboratory, but how to try from laboratory technology to commercialization However, it has encountered great technical bottlenecks and barriers, such as high active material loading, electrolyte system, metal lithium anode and soft battery technology.
At present, the main problem is that the volumetric energy density is low, causing it to lose competitiveness in many important market applications. At the same time, the high electrolyte dosage has also become a bottleneck for its weight energy density increase. In addition, the safety and long cycle life of metal lithium anodes have not yet been well resolved.
Break through key technical bottlenecks
According to reports, the reason for the low volume energy density of lithium-sulfur batteries is mainly due to the following two points: from the point of view, the theoretical density of the active materials lithium and sulfur is relatively low, lithium 0.534 g / cm3, sulfur 2.07 g / cm3, and lithium ion The theoretical density of lithium cobaltate and ternary materials in batteries is much higher; from the electrode structure, there is also one of the most important reasons is that sulfur is an electronic and ionic insulator, so sulfur needs to be dispersed to a large number of high specific surface areas The capacity of the conductive carbon can be used, and the problem caused by using a large amount of conductive carbon is that the specific surface area of ​​the entire positive electrode is very high, and the porosity is very high. Generally speaking, the porosity of the traditional carbon-sulfur positive electrode is twice that of the positive electrode of a lithium ion battery. .
Therefore, the current key technical bottleneck restricting the practicality of lithium-sulfur batteries is how to achieve low electrolyte consumption, high electrode density, and low inactive material content under high active material loading conditions.
In response to the problem of low energy density at the battery device level, the research team innovatively proposed the use of an embedded electrode material Mo6S8 with high electron and ion conductivity to replace the inactive material carbon to form an embedded-conversion hybrid electrode, so that the sulfur cathode is guaranteed high activity Under the condition of material loading (more than 10mg / cm2), the carbon content is reduced to less than 10wt%, the electrolyte active material ratio is greatly reduced to 1.2μLmg-1, and the electrode porosity is less than 55%. The ampere-level soft-pack full battery with this new hybrid electrode greatly improves the monomer energy density under the condition of ensuring cycle life, and can achieve high volume energy density (581 Wh / L) and weight energy density (366 Wh / kg), providing a new solution and practical commercial technical solution for the future development of new high-energy density lithium-sulfur batteries.
According to reports, through comparison and theoretical estimation with lithium ion battery cathodes, such as lithium cobalt oxide, the research team believes that the high carbon content of sulfur cathode materials is the fundamental reason for the low volume energy density of lithium-sulfur batteries and the need for a large amount of electrolyte infiltration. Therefore, the idea of ​​replacing the inactive conductive carbon with an electrochemically active substance has arisen.
At the same time, alternative materials must also meet the following conditions: first, high electron and ionic conductivity-play the role of carbon; secondly, it is compatible with lithium-sulfur electrolytes, which can stably contribute capacity in the voltage range of lithium-sulfur batteries-improve the overall Energy output; and high theoretical density-higher electrode density can be obtained after replacing carbon; in addition, it has a strong adsorption effect with lithium polysulfide, which can alleviate the "shuttle effect" of lithium-sulfur batteries.
"After having the above screening principles, we have selected the Mov 6S8 of the Chevrel phase to form a hybrid electrode among many materials. Previous studies have tried to use TiS2 or other sulfides with capacity contributions to be added to the positive electrode as lithium polysulfide. Adsorbent. However, previous studies have not grasped the key of high conductive carbon content, and only stayed on the problem of solving the 'shuttle effect'. Few studies have been able to achieve high energy density under the harsh conditions of the entire battery. "" Suo Limin explained.
Increased overall energy density
Xue Weijiang said that the longest time is spent on the preparation of materials and the optimization of battery performance. Since the carbon content is reduced to an unprecedented 10%, how to ensure the performance of sulfur capacity at such a low carbon content is a major challenge. At the same time, the optimization of battery performance is a systematic project. It is not enough to optimize the positive electrode. At the same time, a lot of work has been done on the matching of the electrolyte and the lithium negative electrode. It took nearly a year to solve these problems.
Previously, the research on lithium-sulfur batteries rarely reported the energy density of the whole battery, especially the volume energy density. The weight energy density of the lithium-sulfur soft pack battery of Oxis in the UK can reach more than 400 Wh / kg, but the volume energy density is only about 300 Wh / L. Currently, the energy density of commercial lithium-ion batteries is around 260 Wh / kg and 700 Wh / L.
The volume energy density (581 Wh / L) and weight energy density (366 Wh / kg) of the soft-pack battery in this study have surpassed the above two battery systems in terms of overall energy density. The team said that in the future, it will continue to optimize the material preparation and assembly process of the soft-pack battery. At the same time, it will combine new research results in lithium anode and electrolyte to strive for commercialization at an early date. Subsequent research will continue to enrich the research system along this line of thought, and will focus on solving the last obstacle in the commercialization of lithium-sulfur batteries-problems in metal lithium anodes. (Reporter Zhang Jingjing)
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