Lithium sulphur battery designs offer a theoretical energy density roughly five times that of lithium ion batteries, and the researchers used a porous, sponge-like aerogel of reduced graphene oxide as a free-standing electrode in the battery cell to get higher utilisation of the sulphur.
The best lithium ion batteries currently on the market operate at about 300 watt-hours per kg, with a theoretical maximum of around 350. Lithium sulphur batteries meanwhile, have a theoretical energy density of around 1000 to 1500 watt-hours per kg. “Sulphur is cheap, highly abundant, and much more environmentally friendly. Lithium sulphur batteries also have the advantage of not needing to contain any environmentally harmful fluorine, as is commonly found in lithium ion batteries,” said Aleksandar Matic, Professor at Chalmers Department of Physics, who leads the research group.
However, so far lithium sulphur battery designs have been unstable with a low lifetime. Current versions degenerate fast and have a limited life span with an impractically low number of cycles, so many research groups have been looking at the technology (as shown in the stories below). In testing of their new prototype, the Chalmers researchers demonstrated an 85% capacity retention after 350 cycles.
There are four parts to a traditional battery. First, there are two supporting electrodes coated with an active substance, which are known as an anode and a cathode. In between them is an electrolyte, generally a liquid, allowing ions to be transferred back and forth. The fourth component is a separator, which acts as a physical barrier, preventing contact between the two electrodes whilst still allowing the transfer of ions.
The researchers had previously combined the cathode and electrolyte into one liquid, a ‘catholyte’. This saves weight in the lithium sulfur battery, as well as offer faster charging and better power capabilities. Now, with the development of the graphene aerogel, the concept has proved viable, offering some very promising results.
Researchers have been using different techniques to create such structures, including microwaving recycled plastic.
“You take the aerogel, which is a long thin tube, and then you slice it – almost like a salami. You take that slice, and compress it, to fit into the battery,” said Carmen Cavallo of the Department of Physics at Chalmers, and lead researcher on the study. Then, a sulphur-rich solution – the catholyte – is added to the battery. The highly porous aerogel acts as the support, soaking up the solution like a sponge.
“The porous structure of the graphene aerogel is key. It soaks up a high amount of the catholyte, giving you high enough sulphur loading to make the catholyte concept worthwhile. This kind of semi-liquid catholyte is really essential here. It allows the sulphur to cycle back and forth without any losses. It is not lost through dissolution – because it is already dissolved into the catholyte solution,” she said.
Some of the catholyte solution is applied to the separator as well, in order for it to fulfil its electrolyte role. This also maximises the sulphur content of the battery. The new design avoids the two main problems with degradation of lithium sulphur batteries – one, that the sulphur dissolves into the electrolyte and is lost, and two, a ‘shuttling effect’, whereby sulphur molecules migrate from the cathode to the anode. In this design, these undesirable issues can be drastically reduced.
The researchers note, however, that there is still a long journey to go before lithium sulfur battery technology can achieve full market potential. “Since these batteries are produced in an alternative way from most normal batteries, new manufacturing processes will need to be developed to make them commercially viable,” said Matic.