Graphene aerogels could pave the way for high-performance lithium-sulfur batteries
With up to five times higher energy density, improved safety, and a lighter weight profile, lithium-sulfur batteries carry a number of advantages over conventional lithium-ion cells and look to be one of the most promising candidates for emerging battery technologies.
Source: Faraday Institution
Although the theoretical energy density of lithium-sulfur batteries is an impressive 2600 Wh/kg, commercialization of these chemistries has been hindered by performance degradation as the cells are cycled. The main culprit is the dissolution of sulfur into the electrolyte and deposition of sulphides onto the lithium-metal anode, reducing the active sulfur available while acting as an insulator to the anode (See figure below).
Source: Science Direct
Carmen Cavallo’s research group at Chalmers Sweden is working to address this problem through the development of batteries that make use of novel graphene oxide aerogels. For those unfamiliar, graphene is a single-atom thick form of graphite with a set of unique properties that make it a potentially game changing material for batteries, most notably its ability to conduct electricity with 100% efficiency (Graphene Info). However, graphene is poor processable. Therefore, the researcher decided to use another form of graphene, called graphene oxide, in a smart and functional shape: an aerogel.
The graphene oxide aerogel is a highly porous, cake-like form of the material, which allows for the electrolyte solutions to be absorbed into the structure.
“The reduced graphene oxide aerogel takes advantage of its lightness, which can adsorb more than 80 times its weight. Like a delicious sponge cake full of polysulfides!” says Carmen.
This conductive sponge permits cell architectures in which the electrolyte and cathode are combined into a single liquid, also known as a ‘catholyte’, rather than having each component of the battery each kept in separate layers. Battery architectures using catholyte solutions have had difficulty proving reliability in the past, however with the inclusion of a graphene aerogel, more sulfur can be loaded into the cell, allowing for higher capacities while maintaining efficient electrochemical reactions and a smaller weight profile. Moreover, the presence of oxygen, due to the nature of the aerogel itself provides added stability to the battery cycling, demonstrating an 85% capacity retention after 350 cycles.
For any battery technology, cost remains the most decisive factor in their success, and with sulfur being one of the most abundant and inexpensive materials available, figuring out a high-performing and cost effective architecture for lithium-sulfur batteries is a massive opportunity. Moreover, these batteries do not demand the inclusion of fluorine in the catholyte solution that is often employed, an additive that is both expensive and environmentally harmful to produce, making them a much more environmentally friendly alternative to fluoride-ion batteries currently in development.
So far the most significant barrier for the commercial success of this technology is likely the availability of graphene. While there is an abundance of graphite necessary for the production of graphene, the processes nor infrastructure have yet been developed for the production of graphene in commercial quantities. That being noted, the unique and often physics defying properties of graphene have shown immense potential in the lab and researchers around the world are working on novel methods to produce it in commercial quantities. In 2018, researchers at the National University of Singapore published a cost-effective method for producing graphene commercially. The slurry uses 50 times less solvent than conventional production methods and the slurry can then be used to 3D print aerogels for batteries and beyond (Science Daily).
While there remain a number of challenges to overcome, lithium-sulfur batteries may have taken an important step toward commercial production.