Lithium sulfur battery - high energy density battery

Performance of hMSC lithium sulfur battery: (a) lithium sulfur button battery (6.9 mgcm-2s + 6.8 mgcm-2mo6s8, electrolyte active material ratio ~ 1.5 μ lmg-1); (b) lithium sulfur soft pack battery (electrolyte active material ratio ~ 1.2 μ lmg-1, ~ 2 times excess lithium metal); (c) energy density comparison diagram of lithium sulfur battery. Note: figure C shows the calculated energy density of button battery experimental parameters and the real energy density of ampere hour level soft package battery.
Lithium sulfur battery is regarded as one of the ideal choices for the next generation of high energy density battery system, which is highly concerned by the scientific research and industry circles all over the world. It is also one of the key research directions of the layout of various countries in the future. However, with the deepening of research, lithium sulfur battery is also facing increasingly severe challenges. At present, the main problem is that the volume energy density of lithium sulfur battery is low, which leads to the loss of competitiveness in many important market applications. At the same time, the high amount of electrolyte also becomes the bottleneck to improve the weight energy density. Suo Liumin, associate researcher of E01 group, Key Laboratory of clean energy, Institute of physics, Chinese Academy of Sciences / Beijing National Research Center for condensed matter physics, cooperates with Li Ju and Dr. Xue Weijiang, professors of Massachusetts Institute of technology in the United States, and puts forward new ideas for solving the common problems existing in current lithium sulfur batteries, which provides new possibilities for developing new lithium sulfur batteries with high energy density in the future. The related research results are published in nature energy.
Low volume and weight energy density limit the development of lithium sulfur battery
Suo Liumin told China Science Daily that the next generation of high-energy density battery system is mainly based on metal lithium anode battery system, such as lithium sulfur, lithium air battery, etc. "Compared with lithium sulfur battery, although lithium air battery has higher theoretical energy density, it is still in the stage of basic research, and many key problems have not been well solved. Lithium sulfur battery has the advantages of low cost and high energy density. After years of unremitting efforts, the technology of lithium sulfur battery has become increasingly mature and close to commercialization. " In 2009, the research group of Linda Nazar of University of Waterloo in Canada published a paper in nature materials, which achieved a reversible capacity of nearly 80% of the theoretical capacity for the first time, igniting people's research passion for lithium sulfur battery. At present, countries all over the world pay more attention to lithium sulfur batteries. Many universities and research institutes carry out research on basic scientific issues. In addition, many companies, such as Oxis company in the UK and sion power company in the United States, have been engaged in commercial research on lithium sulfur batteries. Li Ju said that in the past decade, many key technologies of lithium sulfur battery, including sulfur cathode and electrolyte, have made great breakthroughs and progress in the laboratory. However, there are great technical bottlenecks and barriers in the attempt of commercialization from laboratory technology, such as high active material load, electrolyte system, metal lithium anode and soft pack battery technology. At present, the main problem is the low bulk energy density, which leads to the loss of competitiveness in many important market applications. At the same time, the high amount of electrolyte has become the bottleneck to improve the weight energy density. In addition, the safety and long cycle life of metal lithium anode have not been well solved.
Break through the bottleneck of key technology
According to the introduction, there are two main reasons for the low bulk energy density of lithium sulfur battery: from the intrinsic point of view, the theoretical density of active material lithium and sulfur is relatively low, with lithium 0.534 g / cm3 and sulfur 2.07 g / cm3 The theoretical density of lithium cobalt oxide and ternary materials in lithium-ion batteries is much higher than that in lithium-ion batteries. From the perspective of electrode structure, the most important reason is that sulfur is the insulator of electron and ion, so sulfur needs to be dispersed into a large number of conductive carbon with high specific surface area to achieve its capacity. However, the problem caused by using a large number of conductive carbon is that the specific surface area of the whole cathode is very high, The porosity is very high. Generally speaking, the porosity of traditional carbon sulfur cathode is twice that of lithium-ion battery cathode. Therefore, the key technical bottleneck restricting the practical application of lithium sulfur battery is how to achieve low electrolyte consumption, high electrode density and low content of inactive substances under the condition of high active substance load.
In order to solve the problem of low energy density of battery device level, the research team innovatively proposed to use embedded electrode material mo6s8 with high electronic and ionic conductivity to replace non active material carbon to form embedded conversion hybrid electrode, so that the carbon content of sulfur cathode can be reduced to less than 10wt% under the condition of ensuring high active material loading (more than 10mg / cm2), and the ratio of active material to electrolyte can be reduced 2 μ lmg-1, and the porosity of the electrode is less than 55%. Under the condition of ensuring the cycle life of the battery, the energy density of the whole battery can be greatly increased, and the high volume energy density (581 WH / L) and weight energy density (366 WH / kg) can be achieved simultaneously, which provides a new solution and practical commercial technical scheme for the development of new high energy density lithium sulfur battery in the future. According to the introduction, through the comparison and theoretical estimation with the cathode of lithium-ion battery, such as lithium cobalt oxide, the research team believes that the high carbon content in sulfur cathode material is the fundamental reason for the low volume energy density of lithium sulfur battery and the need for a large amount of electrolyte infiltration. Therefore, the idea of using electrochemically active materials to replace non active conductive carbon has emerged. At the same time, the alternative materials must meet the following conditions: first, high electronic and ionic conductance, which plays the role of carbon; second, compatible with lithium sulfur electrolyte, which can stably contribute capacity within the voltage range of lithium sulfur battery, which improves the overall energy output; and high theoretical density, which can obtain higher electrode density after replacing carbon; and third, it has a better compatibility with lithium polysulfide Strong adsorption can alleviate the "shuttle effect" of lithium sulfur battery. "With the above screening principles, we selected mo6s8 with chevrol phase as the mixed electrode. Other people who have tried to use tis2 as a lithium sulfide adsorbent have contributed to the adsorption capacity. However, previous studies have failed to grasp the key of high conductive carbon content, and only focused on solving the problem of "shuttle effect". Few studies have been able to achieve the high energy density of full cell under harsh conditions Suo Liumin explained.
Increase of comprehensive energy density
Xue Weijiang said that the most time-consuming is the preparation of materials and the optimization of battery performance. As the carbon content has been reduced to an unprecedented 10%, how to ensure the sulfur capacity at such a low carbon content is a big challenge. At the same time, the optimization of battery performance is a system engineering, it is not enough to only optimize the cathode. At the same time, a lot of work has been done in the matching of electrolyte and lithium anode. It took nearly a year to solve these problems. Previously, the energy density, especially the bulk energy density, of lithium sulfur battery was rarely reported. The weight energy density of Oxis lithium sulfur soft pack battery can reach more than 400 WH / kg, but the volume energy density is only about 300 WH / L. At present, the energy density of commercial lithium-ion batteries is about 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 have surpassed the above two battery systems in terms of comprehensive energy density. The team said that in the future, it will continue to optimize the material preparation and soft pack battery assembly process, and strive to achieve commercialization as soon as possible in combination with new research achievements in lithium anode and electrolyte. The follow-up research will continue to enrich the research system along with this idea, and will focus on solving the last obstacle to the commercialization of lithium sulfur battery - the problems existing in metal lithium anode.