Lithium-sulfur batteries are a type of rechargeable batteries notable for their high specific energy capable of offering a theoretical energy density more than five times that of lithium-ion batteries.
In order to build our electric future, we will need all kinds of new and powerful batteries and lithium-sulfur ones are good candidates, but they're not yet marketable.
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This is because they have two main issues. The first is that the sulfur dissolves into the electrolyte. The second is that sulfur molecules migrate from the cathode to the anode.
Now, researchers at Chalmers University of Technology, Sweden, have revealed a promising breakthrough for lithium-sulfur batteries. The discovery consists of combining the cathode and electrolyte into one liquid, something they call a 'catholyte.'
A traditional battery consists of four parts: two supporting electrodes, an anode, and a cathode, and between them an electrolyte as well as a separator, which acts as a physical barrier, preventing contact between the two electrodes but still allowing the transfer of ions.
Combining the cathode and electrolyte can not only help save weight in the battery, but it also has the potential to offer better charging and improved power.
The team has invented a porous, sponge-like aerogel, made of reduced graphene oxide. The graphene aerogel acts as a free-standing electrode in the battery cell and allows for higher utilization of sulfur.
"You take the aerogel, which is a long thin cylinder, and then you slice it - almost like a salami. You take that slice, and compress it, to fit into the battery," says Carmen Cavallo of the Department of Physics at Chalmers, and lead researcher on the study.
"The porous structure of the graphene aerogel is key. It soaks up a high amount of the catholyte, giving you high enough sulfur loading to make the catholyte concept worthwhile. This kind of semi-liquid catholyte is really essential here. It allows the sulfur to cycle back and forth without any losses. It is not lost through dissolution - because it is already dissolved into the catholyte solution," adds Cavallo.
But that is not the only purpose of the catholyte solution. It is also applied to the separator in order for it to fulfill its electrolyte role and further maximize the sulfur content of the battery.
Higher energy density
Lithium-sulfur batteries are very much desired due to their higher energy density. The best lithium-ion batteries operate at about 300 watt-hours per kg while lithium-sulfur batteries can offer an energy density of around 1000-1500 watt-hours per kg.
"Furthermore, sulfur is cheap, highly abundant, and much more environmentally friendly. Lithium-sulfur batteries also have the advantage of not needing to contain any environmentally harmful fluorine, as is commonly found in lithium-ion batteries," says Aleksandar Matic, Professor at Chalmers Department of Physics, who leads the research group behind the paper.
However, so far lithium-sulfur batteries have been unstable and have, therefore, had low cycle lives. The new prototype though has demonstrated an 85% capacity retention after 350 cycles by avoiding all the issues associated with lithium-sulfur batteries.
The study is published in the Journal of Power Sources.