Why is lithium battery unstable?

At present, the use of lithium-sulfur batteries is more and more extensive, but sometimes its cycle stability is not satisfactory. Scientists have finally found the answer to this question by conducting research on the composition of their electrolytes.

When you need your mobile phone most, do you worry about it? When you drive an electric car, what do you do when there is no electricity in the car? For these problems, a light lithium-sulfur battery can answer you! Its energy storage capacity is more than twice that of the battery on the supermarket shelf, but there is often no electricity and the service life is not long. Scientists at the Joint Research Center for energy storage Research (JCESR) and the Pacific Northwest National Laboratory have discovered one of the reasons behind this problem.

They found that the salts used in the electrolyte in the battery make a big difference. When a salt called LiTFSI (lithium bis(trifluoromethylsulfonimide)) is made into an electrolyte of a battery, the test battery can be operated for more than 200 times while maintaining the maximum charge and discharge amount. In lithium-sulfur batteries, LiTFSI binds lithium and sulfur atoms on the electrodes but releases them quickly. In contrast, a similar electrolyte has a stronger binding force on lithium and sulfur atoms and is not released at all. The result is a rapid decline in battery performance; there is no energy after running the battery for dozens of times.

One of the concerns of electric vehicles is that drivers do not want to be trapped between charging stations when driving on the highway for a long time. This concern has led consumers to decide to buy low-displacement cars. The results of this study add another important factor to the design guidance of high-energy lithium-sulfur batteries.

To determine the effect of the electrolyte on lithium-sulfur batteries, the research team used LiTFSI and LiFSI for related experiments. LiTFSI and LiFSI are very similar electrolytes, only LiFSI contains less carbon and fluorine than LiTFSI. They used the equipment in the environmental molecular laboratory to continuously test the energy of the battery charge and discharge and finally investigated the electrode.

They found that lithium-sulfur batteries using LiTFSI as an electrolyte, lithium atoms are bound by sulfur atoms, and lithium sulfide (LiSx) is formed on the surface of the electrode. When LiFSI is used as the electrolyte, lithium sulfate (LiSOx) is formed. By calculating the tightness of the combination of the two lithium compounds, they found that the lithium sulfide is easily broken to release lithium. However, lithium sulfate is difficult to separate, so the oxygen element in lithium sulfate is the culprit.

"By combining macroscopic component analysis with simulation, we can see which bonds are easily broken and what happens if the chemical bonds break." Dr. Ji-Guang (Jason) Zhang, who is leading the research at the National Laboratory, said: "This process allows us to identify the behavior of electrolytes, lead us to design better electrolytes, and increase the cycle life of lithium-sulfur batteries."

For researchers, the next step is to study electrolyte additives to form a protective layer on the anode surface of lithium to protect it from corrosion.

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