Researchers at the Department of Energy's Oak Ridge National Laboratory have uncovered a path to design superionic polymer electrolytes for solid-state batteries and other energy applications that could help ensure a future of abundant and reliable energy for the United States. The scientists demonstrated that by carefully controlling the chemical composition of a lithium salt-based polymer, they could create a material that enables superfast transport of ions in batteries and many other energy storage and conversion technologies.
"Researchers around the world are focusing on unlocking the potential of polymer electrolytes because they have a lot of advantages over the conventional liquid electrolytes," said Catalin Gainaru, an R&D staff scientist of ORNL's Chemical Sciences Division. "Achieving fast ion transport has always been a major challenge of polymer electrolytes, but our recent research demonstrates that this may no longer be the case."
Batteries are made up of two electrodes - a cathode and an anode - separated by an electrolyte material. As a battery charges or discharges, ions need to have a high mobility within the electrolyte as they move back and forth between electrodes. Traditional batteries use liquid or gel electrolytes, but the demand for safer and more efficient power storage has spurred interest in solid-state batteries in which the electrolyte is solid, yielding a battery that is faster charging, safer, more compact and durable.
The challenge of ion transport in solid-state batteries
Many solid-state concepts use ceramic electrolytes that transport ions so effectively that they are known as superionic ceramics. Unfortunately, these ceramics are prone to break due to brittleness. They are also difficult to roll into thin films and don't adhere well to the electrodes in a battery. The ORNL researchers demonstrated how a polymeric material can achieve a similar superionic state, in which ions can move up to 10 billion times faster than their surroundings, without the shortcomings of liquids and ceramics.
Polymers are materials formed by long molecular chains made up of small, repeating building blocks. Well-known examples include a variety of plastics, which are usually made up of repeating units containing carbon and other atoms. The ORNL polymer electrolyte contains polar segments that favor the inclusion of lithium salts and strongly enhance the mobility of ions.
The research, which was published in Materials Today , was performed as part of the DOE Energy Frontier Research Center (EFRC) known as the Fast and Cooperative Ion Transport in Polymer-Based Materials (FaCT) Center.
"The goal of the FaCT EFRC is to fully understand how to design novel polymers that change the paradigm of ion transport," said Tomonori Saito, an ORNL distinguished researcher in ORNL's Chemical Sciences Division. "We developed a very special polymer in which the segments self-organize to provide a high mobility path for the ions to move through."
A molecular design strategy enables superionic behavior
The key development was the careful tuning of the structure of the polymer by the addition of precise amounts of molecular groups known as zwitterions. These special functional groups carry both positive and negative charges, which increases local polarity but results in a zero charge for the entire macromolecule. By using careful chemical processes, researchers were able to tailor the number of zwitterionic groups attached to the polymer backbone allowing the ions to assemble into pockets.
In these pockets, ions interact much like conversationalists at a dinner party. At first, small pockets of diffuse conversations form, isolated throughout the material. Add more pockets, though, and the discussions eventually lose individuality and evolve into a pleasant and cohesive hum. That's when the ions start to flow like good conversation. But add too many zwitterions, and the cohesive hum devolves into a cacophony and ion transport slows back down.
Researchers found that the sweet spot was achieved by functionalizing around 80 percent of the units of the polymer electrolyte with zwitterionic groups. At this point, the pockets connect into channel-like structures that allow ions to hop back and forth in an orderly fashion with minimal resistance.
The research team plans to build on this promising early-stage research with additional investigations into the fundamental mechanisms that enable the superionic nature of the polymer. Modeling and simulations using ORNL supercomputing resources as well as robotic autonomous chemistry coupled with AI will help understand what drives this fascinating performance, and neutron scattering studies are planned at the Spallation Neutron Source, a DOE Office of Science user facility at ORNL, to observe the interactions at the molecular level.
While solid-state batteries are a clear application for the new electrolyte, many energy technologies also rely on effective ion transport. Flow batteries, fuel cells, grid-level energy storage and many other applications could benefit from these newly developed polymers.
"It's hard to predict all the technologies that could leverage this discovery," Saito said. "Anything that needs an impermeable barrier layer, but let ions move freely across it, is a potential application."
The research was funded by the DOE's Office of Basic Energy Sciences as part of the FaCT EFRC.
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