Lithium-Sulfur Batteries – A Dealbreaker?

STM Logo

Australian scientists claim they have made a breakthrough in the development of a light weight, high capacity Lithium-Sulfur battery.

STM Logo

Apart from being found in nearly every rechargeable, battery powered consumer application, they are the backbone of the often proclaimed e-mobility revolution: batteries.

E-scooters, e-bikes, e-cars, and nearly all other types of electrical powered vehicles need an energy storage, if not directly grid-connected like some inner city metro busses. Inductive roads to quick charge supercapacitors in vehicles are not in use yet. And they are not likely to become reality in the near future. So all hope is set onto battery systems to power our new mobility.

So far the best option is the Lithium-Ion cell. This term describes battery types that use Lithium-compounds in all three phases of the electrochemical cell. They offer an energy density of around 150 Wh/kg, depending on the used materials, which is a fairly high value compared to other types of batteries. But Lithium-Ion cells require a variety of precautions, in order to be safe for use. Regular Lithium-Ion cells often use a combination of Cobalt and Nickel as kathode material. These so called rare earths or conflict minerals are mined under severe impact for the environment, and their supply is limited.

In Lithium-Sulfur cells, the cathode consists of Sulfur, which is readily available. It’s often created as a by-product during other chemical production processes, what makes the usage very ecologically friendly.

The new kid on the block: the Lithium-Sulfur cell

This technology itself is not new. First patent for a Lithium-Sulfur cell was filed in Germany in 1958 and released in 1962.

It describes a battery cell that dissolves Lithium on the anode during usage, which combines with Sulfur on the cathode to Lithium-sulfide. During the recharging process, this connection is resolved, which releases the Sulfur and accumulates Lithium metal on the negative pole of the cell. As a byproduct, different Lithium-sulfide gases are formed.

In theory, a cell that uses Lithium and Sulfur is capable of a maximum energy density of 2,600 Wh/kg. This estimate proclaims the full usage of Lithium and Sulfur mass. In practical use this is not possible, because Sulfur itself does not have electrical conductivity. Therefore it needs to be combined with other materials, such as Carbon for example. This lowers energy density of the cell, leading of a practical value of 350 Wh/kg.

But still, this leads to a value of almost 2.5 times the energy density of currently used Lithium-Ion batteries. That means an equally sized cell contains a much higher amount of capacity. As a result, it’s possible to build lighter cells, which lowers the total weight of the application. In electric cars for example, a significant amount of weight derives from the weight of the battery packs. This might have a huge impact on our e-mobility sector, as companies like Volocopter for example, use heavy weight batteries in their flying taxi. With the use of more light weight battery technology, the effective range can be increased dramatically.

A problem with this type of battery, that has been challenging scientists around the world, is the mechanical stability of the cell itself. During usage, the cathode expands and contracts a significant amount when Lithium is received and released. The resulting stress during charging and discharging leads to microscopic cracks in the cathode, limiting the battery’s lifetime. So far, the existing cells have been limited to around 100 loading-cycles.

A German-Australian scientists team now claims they have developed a new cathode. It consists of Sulfur, Carbon and a special bonding agent. This way, the cathode is able to compensate higher mechanical stress than previous Lithium-Sulfur cells. This decreases capacity and power losses during usage. But not just the combination of material itself is part of the new wonder cell, it’s also the layer-design they are put together in. According to Monash scientist Matthew Hill, the inspiration came from the production process of laundry detergent.

The current cell layout provides impressive results. After 200 loading cycles, the efficiency of transporting electrons stays at 98 per cent of its original value. Unfortunately, the total capacity of this cell has dropped approximately 25 per cent during these 200 loading cycles.

What are the Pros & Cons?

By far the biggest plus for the Lithium-Sulfur cell are the materials used. The rare earths in Lithium-Ion cells are expensive, and post a great burden to many ecosystems around the world. They are mined under major ecological impact in some of the most fragile regions in the world. And the end of their availability is foreseeable. Sulfur is often generated as a byproduct of other chemical production processes. Being able to reuse it is a big factor for the sustainability of this technology. Due to the higher energy density, it’s possible to design lighter battery cells, than with other currently available cells.

However, the Lithium-sulfides that result from usage are poisonous and form the explosive Sulfur-Hydrogen-gas in combination with air. So the cells need to be gas tight in order to prevent an explosion. On the other hand, the same goes for Lithium-Ion cells.

After almost four decades of research, the German-Australian team at Monash University achieved a great leap in the development of a high capacity, eco-friendly battery cell. But there are a lot of unsolved issues with this technology, and until we see a functioning product, many more years of R&D are necessary.