Rubber electrolytes key to long-lasting EV batteries

In order for electric vehicles (EVs) to become mainstream, they need cost-effective, safe, long-lasting batteries that won’t explode during use or harm the environment.

Researchers at the Georgia Institute of Technology, in collaboration with the Korea Advanced Institute of Science and Technology (KAIST), have now found a promising alternative to conventional lithium-ion (Li-ion) batteries made from a common material: rubber.

In conventional Li-ion batteries, ions are moved by a liquid electrolyte. However, the battery is inherently unstable: even the slightest damage can leak into the electrolyte, leading to explosion or fire. The safety issues have forced the industry to look at solid-state batteries, which can be made using inorganic ceramic material or organic polymers.

“Most of the industry is focusing on building inorganic solid-state electrolytes, but they are hard to make, expensive and are not environmentally friendly,” said Seung Woo Lee, an associate professor at Georgia Tech.

Solid polymer electrolytes continue to attract great interest because of their low manufacturing cost, non-toxicity and soft nature; however, conventional polymer electrolytes do not have sufficient ionic conductivity and mechanical stability for reliable operation of solid-state batteries. Georgia Tech engineers have now solved the common problems of slow lithium-ion transport and poor mechanical properties using rubber electrolytes, with the results published in the journal Nature.

Elastomers, or synthetic rubbers, are widely used in consumer products and advanced technologies such as wearable electronics and soft robotics because of their useful mechanical properties. The researchers’ breakthrough came when they allowed this material to form a 3D interconnected plastic crystal phase within the robust rubber matrix. This structure resulted in high ionic conductivity, good mechanical properties and electrochemical stability.

Michael Lee, a graduate researcher at Georgia Tech, explained, “Higher ionic conductivity means you can move more ions at the same time. By increasing specific energy and energy density of these batteries, you can increase the mileage of the EV.” The rubber material thus acts as a superhighway for fast lithium-ion transport with superior mechanical toughness, resulting in longer-lasting batteries that can go further.

The rubber electrolyte can be made using a simple polymerisation process at low-temperature conditions, generating robust and smooth interfaces on the surface of electrodes. These characteristics prevent lithium dendrite growth and allow for faster moving ions, enabling reliable operation of solid-state batteries even at room temperature.

“Rubber has been used everywhere because of its high mechanical properties, and it will allow us to make cheap, more reliable and safer batteries,” Assoc Prof Lee said.

Assoc Prof Seung Woo Lee and Michael J Lee have demonstrated a more cost-effective, safer solid polymer electrolyte for all-solid-state batteries. Image credit: Georgia Tech.

The researchers are now looking at ways to improve the battery performance by increasing its cycle time and decreasing the charging time through even better ionic conductivity, and their efforts have already resulted in an improvement in the battery’s performance/cycle time. SK Innovation, a global energy and petrochemical company, is meanwhile funding additional research of the electrolyte material as part of its ongoing collaboration with Georgia Tech to build next-generation solid-state batteries that are safer and more energy dense than conventional Li-ion batteries.

“All-solid-state batteries can dramatically increase the mileage and safety of electric vehicles,” said Kyounghwan Choi, Director of SK Innovation’s next-generation battery research centre. “Fast-growing battery companies, including SK Innovation, believe that commercialising all-solid-state batteries will become a game changer in the electric vehicle market.

“Through the ongoing project in collaboration with SK Innovation and Professor Seung Woo Lee of Georgia Tech, there are high expectations for rapid application and commercialisation of all-solid-state batteries.”

Image caption: A researcher stretches the large-scale rubber material. Image credit: Georgia Tech.

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