Chalmers University of Technology researchers recently unveiled a promising breakthrough for the lithium sulfur battery, using a catholyte with the help of a graphene sponge. Lithium sulfur batteries, which offer a theoretical energy density more than five times that of lithium ion batteries could meet the demands of an electric future when new battery technologies will be essential.

The Chalmers design for a lithium sulfur battery. The highly porous quality of the graphene aerogel allows for high enough soaking of sulfur to make the catholyte concept worthwhile. ‚Äč‚ÄčImage Credit: Yen Strandqvist, Chalmers University. Click image for the largest view.

The researchers’ novel idea is a porous, sponge-like aerogel, made of reduced graphene oxide, that acts as a free-standing electrode in the battery cell and allows for better and higher utilization of sulfur.

The research group’s paper has been published in the Journal of Power Sources.

A traditional battery consists of four parts. 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 previously experimented with combining the cathode and electrolyte into one liquid, a so-called ‘catholyte’. The concept can help save weight in the 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.

Taking a standard coin cell battery case, the researchers first insert a thin layer of the porous graphene aerogel.

Carmen Cavallo of the Department of Physics at Chalmers, and lead researcher on the study said, “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.” Then, a sulfur-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 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,” explained Cavallo.

Some of the catholyte solution is applied to the separator as well, in order for it to fulfill its electrolyte role. This also maximizes the sulfur content of the battery.

Most batteries currently in use, in everything from mobile phones to electric cars, are lithium-ion batteries. But this type of battery is nearing its limits, so new chemistries are becoming essential for applications with higher power requirements. Lithium sulfur batteries offer several advantages, including much higher energy density. 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 sulfur batteries meanwhile, have a theoretical energy density of around 1000-1500 watt-hours per kg.

Aleksandar Matic, Professor at Chalmers Department of Physics, who leads the research group said, “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,”

The problem with lithium sulfur batteries so far has been their instability, and consequent low cycle life. Current versions degenerate fast and have a limited life span with an impractically low number of cycles. But in testing of their new prototype, the Chalmers researchers demonstrated an 85% capacity retention after 350 cycles.

The new design avoids the two main problems with degradation of lithium sulfur batteries – one, that the sulfur dissolves into the electrolyte and is lost, and two, a ‘shuttling effect’, whereby sulfur 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 the 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.

At five times the power for weight is really enticing even now. Some questions coming are sure to involve the ever increasing price of lithium with its share in the manufacturing costs and the prospects for longer cycle life.

This team may have launched a new battery chemistry and technology.


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