Date: Mar 15, 2019

People don’t ask too much from batteries: Deliver energy when it’s needed and for as long as it is wanted, recharge quickly and don’t burst into flames.

A rash of cell phone fires in 2016 jolted consumer confidence in lithium-ion batteries, a technology that helped usher in modern portable electronics but has been plagued by safety concerns since it was introduced in the 1980s. As interest in electric vehicles revs up, researchers and industry insiders are searching for improved rechargeable battery technology that can safely and reliably power cars, autonomous vehicles, robotics and other next-generation devices.

New Cornell research advances the design of solid-state batteries, a technology that is inherently safer and more energy-dense than today’s lithium-ion batteries, which rely on flammable liquid electrolytes for fast transfer of chemical energy stored in molecular bonds to electricity. By starting with liquid electrolytes and then transforming them into solid polymers inside the electrochemical cell, the researchers take advantage of both liquid and solid properties to overcome key limitations in current battery designs.

“Imagine a glass full of ice cubes: Some of the ice will contact the glass, but there are gaps,” said Qing Zhao, a postdoctoral researcher and lead author on the study, “Solid-State Polymer Electrolytes With In-Built Fast Interfacial Transport for Secondary Lithium Batteries,” published March 11 in Nature Energy.

“But if you fill the glass with water and freeze it, the interfaces will be fully coated, and you establish a strong connection between the solid surface of the glass and its liquid contents,” Qing said. “This same general concept in a battery facilitates high rates of ion transfer across the solid surfaces of a battery electrode to an electrolyte without needing a combustible liquid to operate.”

The key insight is the introduction of special molecules capable of initiating polymerization inside the electrochemical cell, without compromising other functions of the cell. If the electrolyte is a cyclic ether, the initiator can be designed to rip open the ring, producing reactive monomer strands that bond together to create long chain-like molecules with essentially the same chemistry as the ether. This now-solid polymer retains the tight connections at the metal interfaces, much like the ice inside a glass.

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