New polymer material could help lithium batteries heal themselves

Lithium-ion batteries are notorious for frequent internal short circuits that can ignite the battery's liquid electrolyte, causing the battery to explode and start a fire. Now, engineers at the University of Illinois have developed a polymer-based solid electrolyte that can heal itself after damage and can also be recycled without the use of harsh chemicals or high temperatures.

The new research, which could help manufacturers produce recyclable, self-repairable commercial batteries, is published in the Journal of the American Chemical Society.

As lithium-ion batteries go through multiple charge and discharge cycles, they form tiny branches of solid lithium called dendrites, the researchers said. These structures can shorten battery life, cause hot spots and short circuits, and sometimes grow large enough to pierce the battery's internal components, leading to explosive chemical reactions between the electrodes and the electrolyte.

Chemists and engineers have been pushing to replace the liquid electrolytes in lithium-ion batteries with solid materials, such as ceramics or polymers. However, most of these materials are rigid and brittle, resulting in poor contact between the electrolyte and the electrodes and reduced conductivity.

"Solid ion-conducting polymers are one option for developing non-liquid electrolytes," said Brian Jing, a co-author of the paper. "However, the high temperature conditions inside the battery melt most polymers, again leading to dendrites and failure."

Past research has produced solid electrolytes by using networks of polymer chains that cross-link to form a rubbery lithium conductor, an approach that delays the growth of dendrites. However, these materials are complex and cannot be recycled or repaired if damaged.

To address this problem, the researchers developed a network polymer electrolyte in which the crosslinking points can undergo exchange reactions and swap polymer chains. Compared to linear polymers, these networks actually stiffen when heated, which can minimize the dendrite problem, the researchers say. In addition, they can easily break down and resolidify into a network structure after damage, making them recyclable, and because of their self-healing properties, they can restore conductivity after damage.

"This new network polymer also shows remarkable properties, namely that both conductivity and stiffness increase with heating, which are not seen in conventional polymer electrolytes," Jing said.

"Most polymers require strong acids and high temperatures to break down," said Christopher Evans, professor of materials science and engineering and first author of the paper. "Our material is soluble in water at room temperature, making it a very energy-efficient and environmentally friendly process."

The team explored the new material's conductivity and found it has potential as an effective battery electrolyte, but they acknowledge that some progress is needed before it can rival currently used batteries.

“I think this work provides an interesting testing platform for others,” Evans said. “We used a very specific chemistry and a very specific dynamic bond in the polymer, but we think this platform can be reconfigured to work with many other chemistries to be able to tune conductivity and mechanical properties.”