Lithium-sulfur battery (Li-S battery) successfully developed by Monash University, Australia
Earlier this month, it was announced that Mahdokht Shaibani's research team at Monash University in Australia had successfully developed a lithium sulfur battery (Li-S battery). The original paper published in Science Advance is here.
The research group has applied for a manufacturing process patent (PCT / AU 2019/051239) and appears to have successfully manufactured a prototype cell with the Fraunhofer Institute in Germany. In the test, we succeeded in maintaining 99% charge and discharge efficiency even after 200 cycles.
Lithium-sulfur batteries are reported to have the following possibilities because of their large capacity.
- Smartphone battery lasts 5 days
- Electric vehicle that can travel 1000 km
- Installed on solar grids
Will these futures really be realized with lithium-sulfur batteries? I would like to explain in this article.
Lithium sulfur battery is light
One of the characteristics of lithium-sulfur batteries is that they are "light anyway". Theoretically, the weight energy density is more than 10 times that of current lithium-ion batteries. Although it is not realistically that value, it has already been achieved 2-5 times at this stage. Therefore, lithium-sulfur batteries may be useful in applications where current lithium-ion batteries are too heavy to be used. For example, drones, flying cars, electric airplanes, and the like, the use of lithium-ion batteries is currently limited by weight, so the development of lithium-sulfur batteries could be a breakthrough.
For example, Oxis Energy, a UK company, is working with Bye Aerospace, a small electric airplane developer, to develop a lithium-sulfur battery with a weight energy density of 500 Wh / kg (about twice the current lithium-ion battery).
The use of lithium-sulfur batteries is expected to double the range of small electric airplanes. It is also said that larger aircraft can be electrified.
Possibility of application to EV
Lithium-sulfur batteries have shown potential for breakthroughs in the electrification of flying vehicles as described above, but the most frequently cited press report is their application to electric vehicles (EVs). For example, Mainak Majumder himself at Monash University mentioned above also stated that "the development and implementation of lithium-sulfur batteries will cause a major change in the automotive market." However, in reality, it is subtle to say that even if a lithium sulfur battery is mounted, it will be applied to an EV. The reason for this is that the cost and volumetric energy density are the limiting factors for EV batteries.
First of all, it is subtle whether lithium sulfur batteries are cheaper than current lithium ion batteries. Certainly, the sulfur of the positive electrode is cheaper in resources and can be expected to reduce costs, but the lithium metal used for the negative electrode (lithium ion batteries are graphite) is a bottleneck. In addition, depending on the electrode design, lithium-sulfur batteries have a high energy density per weight but are not much different from lithium-ion batteries per volume. In other words, using a lithium-sulfur battery does not mean that the battery pack can be made compact. Another major challenge is that lithium-sulfur batteries are difficult to output, meaning that it is difficult to respond to rapid charging. The results presented by Monash University are far from being able to charge and discharge quickly. Sulfur is by no means a material with excellent conductivity, so it is considered difficult to apply it to EVs that require quick charging.
Challenges of lithium sulfur batteries①
Regarding such lithium-sulfur batteries, whether they are drones, electric airplanes, flying cars or EVs, there are two major issues in putting them to practical use. The first is that the battery life is short. For example, the above-mentioned Monash University lithium-sulfur battery has about 200 cycles, Oxis Energy has less than 500 cycles, and the lithium-sulfur battery developed in Japan's industry-government-academia project has about 800 cycles. Considering that current lithium-ion batteries have thousands of cycles, it's honestly not enough. The problem of battery life is even more pronounced when used in forms such as sharing or leasing as discussed in previous articles.
The shorter the number of cycles that can be used, the higher the cost of replacement, which is more severe in applications where cost is required.
Challenges of lithium sulfur batteries②
What is rarely reported but cannot be avoided in commercialization is the question of how to ensure safety.
For example, Oxis Energy seems to be taking safety measures into consideration, such as using a flame-retardant electrolyte, but the most essential problem is that lithium metal is used for the negative electrode. The current lithium-ion battery was able to ensure safety (at a level that can be put on the market) because the negative electrode was changed from lithium metal to graphite, which made it less likely that "dendrite" that would cause a short circuit would occur. is. However, it is difficult to completely eliminate the danger of dendrite short-circuiting because lithium-sulfur batteries use lithium metal for the negative electrode. Efforts have been made to reduce the generation of dendrites by using flame-retardant electrolytes.However, especially for batteries that have deteriorated due to use, making batteries that do not completely generate dendrites will vary in quality. That's a very difficult problem.
Conversely, if we can overcome these two issues, battery life and safety, there may be a day when lithium-sulfur batteries will be put to practical use in applications that require particularly light batteries. Just as current lithium-ion batteries have pioneered new applications such as EVs, energy storage systems, and IT equipment, lithium-sulfur batteries are likely to create new applications instead of replacing current lithium-ion batteries.