To optimize the battery, the researchers created a structure that could speed up the reaction process and absorb the unwanted polysulfides. They used a carbon nanotube framework (CNT) and coated it with a layer of TiN-TiO2. The TiN acted as a material absorbing any polysulfides that were created in the process, whereas the TiO2 sped up the conversion from lithium polysulfides to the final products— Li2S2 or Li2S.
Lithium sulfur batteries can store more energy than the lithium ion batteries that are already commercially available. To put this in numbers, an electric vehicle that runs on lithium ion batteries can drive an average of 300km before it needs to be charged. With the improved energy storage provided by lithium sulfur batteries, it should be possible to extend this to 500km.
The main challenge that has prevented lithium sulfur batteries from becoming commercialized is that the intermediate product is susceptible to dissolving. During the construction of the battery, the sulfur will react with the lithium to form a product. There are two stages to this. In the first stage, the product will be lithium polysulfide, which can easily dissolve into polysulfides. If this happens, the polysulfides will impair the performance of the battery, resulting in its lifespan being greatly reduced. To optimize the batteries, the lithium polysulfide needs to transform to the final product, either Li2S2 or Li2S, as quickly as possible. To do this, the researchers utilized two different materials—TiO2, which absorbs the unwanted polysulfides, and TiN, which accelerates the process.
These materials are very sensitive. To maximize the battery’s efficiency, the researchers worked on the scale of nanometers. They found that 10nm of TiN and 5nm of TiO2 created the most efficient product. With the polysulfides being absorbed and the whole process being accelerated, the batteries performance was greatly improved. This translated to a shorter charging time, a longer life between charges, and a greater overall lifespan. To establish this, the researchers ran the battery for 200 cycles and found that its efficiency was almost the same.
Hui Zhang et al, Long-life lithium-sulfur batteries with high areal capacity based on coaxial CNTs@TiN-TiO2 sponge, Nature Communications (2021). DOI: 10.1038/s41467-021-24976-y