Image credit: Ben Mills.
The yellow powder in the image above is the element sulfur, which I am sure many people have seen before in school science lessons. Normally in chemistry classes, we are mainly taught about how bad the compounds made from this element smell. Examples are hydrogen sulfide with the distinct smell of rotten eggs and sulfur oxides. But could there be more to this powder? To find out, I have talked to Dr Matthew Lacey at Ångström Advanced Battery Center who is trying to build rechargeable batteries from sulfur.
Lacey is working on lithium-sulfur batteries (Li-S batteries) which consist of sulfur as the positive electrode (cathode) and lithium metal as the negative electrode (anode). When discharging (or using) these batteries, lithium and sulfur react to form the compound lithium sulfide and the process is reversed during charge. The advantage is the light weight of both sulfur and lithium, which leads to a very high energy density by weight. This gravimetric energy density of the batteries themselves could be up to two or three times higher than for conventional lithium-ion batteries according to Lacey. In addition, sulfur is abundant as well as a biproduct of the oil industry and, for these reasons, very cheap.
Nevertheless, Lacey admits that Li-S batteries also still have some drawbacks compared to conventional lithium-ion batteries. During discharge, sulfur dissolves in the electrolyte (the liquid placed between positive and negative electrode), where it can exist as a large number of different compounds called ”polysulfides”. These polysulfides can react with each other or be transported to the lithium foil (negative electrode) and react there. Especially, the reaction between polysulfides lithium results in very short lifespans for Li-S batteries. Strangely, the dissolution of sulfur in the electrolyte is an important part of how Li-S batteries work, but at the same time it is the source of severe problems. Another disadvantage is that, although the energy density per weight is high, the energy density per volume of Li-S batteries is rather low since lithium and sulfur are light, but have low densities.
Lacey is mainly working on a part of the battery called the ”binder”. In order to build a Li-S battery, sulfur and conductive carbon powder have to be attached to a conductive substrate, the current collector. Binders are the glue that stick sulfur and carbon powder to the current collector surface. Together these components form the positive electrode. Lacey is trying to find new binders that can do more than just be the glue of the electrode. The battery scientist wants his binders to interact with the polysulfides in the electrolyte (described above) to extend the lifetime of Li-S batteries. Lacey and his co-workers have been quite successful in this quest during the past years and more about it can be read in this recently published article.
The battery scientist is also proud about a new method called ”intermittent current interruption” that he developed during the past years. This technique makes it possible to track the resistance in batteries over long periods of time and can be used for all kinds of batteries, not only Li-S batteries.
Despite the challenges, Airbus Defence and Space are developing Li-S batteries for the Zephyr “High-Altitude Pseudo Satellite”. Li-S batteries are the only rechargeable batteries with a high enough energy density per weight to run this ”pseudo satellite” when sunlight is not available in the shadow of Earth. In addition, the UK based company OXIS Energy is working towards the commercialization of Li-S batteries.
Nevertheless, Lacey believes that due the low cost of lithium-ion batteries, they will keep dominating most markets such as electric cars and portable electronics in the foreseeable future. But, he also says, that Li-S batteries are still interesting for applications where battery volume and cost are less important. Examples are defence or space where Li-S batteries could power soldier equipment or satellites. Hybrid electric trucks could be another future market for Li-S batteries with battery volume being less of a problem for trucks than for smaller cars.