Graphene Aerogel Shows Promise for Lithium-Sulfur Battery Prototype

April 29, 2019 by Scott McMahan

Lithium-ion batteries are nearing their limits in terms of performance, a team of researchers from Chalmers University in Sweden asserts. So, new chemistries are becoming vital for applications requiring higher power.

The researchers recently demonstrated a promising technology for a Lithium-sulfur battery that employs a catholyte (a cathode and electrolyte combined) and is aided by a graphene sponge. They studied the structure of the graphene aerogel at the Chalmers Materials Analysis Laboratory (CMAL).

Their idea is to use a porous, sponge-like aerogel, made of reduced graphene oxide, that acts as a free-standing electrode in the battery cell. According to the researchers, the aerogel allows better and higher utilization of the sulfur. The researchers published their findings in the Journal of Power Sources.

A conventional battery consists of four parts. First, two supporting electrodes, which are known as an anode and a cathode, are coated with an active substance. An electrolyte, which is generally a liquid, resides in between them, allowing ions to be transferred back and forth. A separator acts as a physical barrier preventing contact between the two electrodes while still allowing the transfer of ions.

The researchers previously attempted combining the cathode and electrolyte into one liquid, a so-called ‘catholyte'. According to the team, the catholyte concept can help save weight in the battery and offer faster charging and better power capabilities. The development of the graphene aerogel makes the catholyte concept viable, offering some very promising results.

Using the battery case of conventional coin cell battery, the researchers first inserted a thin layer of the porous graphene aerogel. "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," said Carmen Cavallo of the Department of Physics at Chalmers, and lead researcher on the study.

Then, a sulfur-rich solution--the catholyte-- was added to the battery. The porous aerogel acts as the support that soaks 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," Carmen Cavallo said.

Also, some of the catholyte solution was applied to the separator to fulfill its electrolyte role. This use on the separator also maximizes the sulfur content of the battery.

Lithium-sulfur batteries can offer several advantages, including much higher energy density. While, the best lithium-ion batteries currently on the market operate at about 300Whr/kg, with a theoretical maximum of around 350Whr/kg. Lithium-sulfur batteries meanwhile have a theoretical energy density of up to 1000Whr/kg to 1500Whr/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," said Aleksandar Matic, Professor at Chalmers Department of Physics, who leads the research group behind the paper.

Thus far, the issue with lithium-sulfur batteries has been their instability and their resulting low cycle life. Current versions quickly reduce their capacity with an impractically low number of cycles and have a limited life span.

However, the new lithium-sulfur prototype demonstrated an 85% capacity retention after 350 cycles.

Lithium-sulfur batteries tend to have two primary problems. First, the sulfur dissolves into the electrolyte and is lost. Second, a ‘shuttling effect', takes place in which sulfur molecules migrate from the cathode to the anode.

The new design greatly reduces the two main problems without degradation of lithium-sulfur batteries.

The researchers note, however, much has to be done 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 said.