Researchers at Chalmers University of Technology, Sweden, have developed a free-standing reduced graphene oxide (r-GO) aerogel for use as a supporting electrode for the electrochemical redox reaction of sulfur in a catholyte-based lithium-sulfur battery.

A mesoporous matrix is formed by a layers of r-GO, providing sites for electrochemical reactions and a highly conducting pathway for electrons. The highly porous structure is easily infiltrated by a catholyte solution providing a homogeneous distribution of the sulfur active material in the conductive graphene matrix and ensures efficient electrochemical reactions. This is demonstrated by a high capacity—3.4 mAh cm⁻²—at high mass loading—3.2 mg cm⁻² of sulfur in the cathode—and in total the sulfur loading in the Li-S cell is even double (6.4 mg cm⁻²).

An illustration of 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. Credit: Yen Strandqvist/Chalmers University of Technology.

Additionally, the presence of oxygen groups in the r-GO aerogel structure stabilizes the cycling performance and the Li-S cell with the fluorine-free catholyte shows a capacity retention of 85% after 350 cycles. A paper on their work is published in the Journal of Power Sources.

A traditional Li-ion battery consists of four parts: two supporting electrodes coated with an active substance; an electrolyte; and a separator, which acts as a physical barrier, preventing contact between the two electrodes while still allowing the transfer of ions.

The researchers previously experimented with combining the cathode and electrolyte into one liquid—a 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 promising results.

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

—lead researcher Carmen Cavallo of the Department of Physics at Chalmers

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

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 significantly 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.

—Aleksandar Matic, Professor at Chalmers Department of Physics, who leads the research group behind the paper

Resources

• Carmen Cavallo, Marco Agostini, James P. Genders, Muhammad E. Abdelhamid, Aleksandar Matic (2019) “A free-standing reduced graphene oxide aerogel as supporting electrode in a fluorine-free Li₂S₈ catholyte Li-S battery,” Journal of Power Sources, Volume 416, Pages 111-117 doi: 10.1016/j.jpowsour.2019.01.081