"Lithium-metal batteries are basically the dream batteries since they provide an extremely high energy density," said Associate Professor Reza Shahbazian-Yassar of mechanical and industrial engineering at the University of Illinois at Chicago. "But we have not been able to build commercially viable lithium-metal batteries with organic liquid electrolytes due to heterogeneous lithium-metal plating that leads to dendrites under extended battery cycling."
The solution is believed to lie in applying the power of supercomputers to understand the core chemistry and physics at work in dendrite formation and to engineer materials that can mitigate dendrite growth.
Professor Balbuena of chemical engineering at Texas A&M believes a graphene oxide nanosheet that can be sprayed onto a glass fibre separator, which is then inserted into the battery, holds the key.
"The idea was to develop a coating material that can protect the lithium metal and make the ion deposition much smoother," said Prof. Balbuena.
According to the team, this graphene oxide nanosheet allows lithium-ions to pass through it, but slows down and controls how the ions combine with electrons from the surface to become neutral atoms. Instead of forming needles, the deposited atoms form smooth, flat surfaces at the bottom of the sheet.
Using computer models and simulations in tandem with physical experiments and microscopic imaging, the team said they were able to reveal how and why the material effectively controls lithium deposition.
According to the researchers, they were able to show that lithium-ions form a thin film on the surface of the graphene oxide, which then diffuse through defect sites before settling below the bottom layer of the graphene oxide. The material is said to act like the pegs in a pachinko game, slowing and directing the metal balls as they fall.
"Our contribution was to conduct molecular dynamics simulations where we follow the trajectory of the electrons and atoms in time and observe what's going on at the atomistic level," Prof. Balbuena said. "We were interested in elucidating how the lithium-ions were diffusing through the system and becoming atoms when the deposition ends in lithium plating."
The graphene-oxide-doped batteries, Prof. Balbuena explained, demonstrated an enhanced cycle life and exhibited stability up to 160 cycles, whereas an unmodified battery rapidly loses its efficiency after 120 cycles. The oxide, she added, can be applied simply and affordably with a spray coating gun.
How the spray was layered on the nanosheets was another focus of the research. "When you do the experiment, it's not clear at the microscopic level where the coating will sit," Prof. Balbuena said. "It's very thin, so locating these coatings with precision is not trivial."
Their computer model explored whether it would be more favourable if the oxide were oriented parallel or perpendicular to the current collector. Both were said to be effective, but if deposited in parallel, the material requires a certain number of defects so ions can slip through.
"The simulations gave our collaborators ideas about the mechanism of ion transfer through the coating," Prof. Balbuena continued. "It's possible that some of the future directions may involve different thickness or chemical composition based on the phenomenon that we observed."
In separate research, Prof. Balbuena and graduate student, Saul Perez Beltran, described a battery design that used graphene sheets to improve the performance of carbon-sulfur cathodes for lithium-sulfur batteries.
Besides sulfur's natural abundance, non-toxicity and low-cost, a sulfur-based cathode is theoretically capable of delivering storage up to 10 times greater than the commonly-used lithium-cobalt oxide cathodes in conventional lithium-ion batteries.
However, chemical reactions in the battery lead to the formation of lithium polysulfides, Prof. Balbuena said. Long-chain polysulfides are soluble in the liquid electrolyte and migrate to the lithium metal anode where they decompose. Whereas, short-chain polysulfides are insoluble and remain at the sulfur-based cathode. As a result, part of their research looked at how the cathode microstructure may affect the chemistry.
Prof. Balbuena explained the problem of uncontrolled polysulfide formation was addressed through the creation of a sulfur/graphene composite material that avoids the formation of the soluble long-chain polysulfides. She claimed that the graphene sheets brought stability to the cathode and improved its ion trapping capabilities
"These are very extensive computations, that's why we need high performance computers," Prof. Balbuena clarified. "The research is a combination of chemistry, physics and engineering, all enabled by computing, this theoretical microscope that can visualise things through theory."
