A new generation of safe, energy-efficient lithium batteries

Researchers from the Department of Mechanical Engineering at the University of Hong Kong have developed a new generation of lithium-ion batteries. The research team, led by Dr Dong-Myeong Shin, discovered a series of anionic network solid electrolytes that can form an integral part of the new battery, which is safer, higher in power density and has a longer lifecycle. The research findings were published in the Chemical Engineering Journal.

Lithium-ion batteries are among the most commonly used batteries with their state-of-the-art energy storage technology. Currently, commercial technology mainly features liquid electrolytes and carbonaceous anodes, which has the drawbacks of safety issues, limited lifetime and insufficient power density. In liquid electrolytes, lithium cations and counter anions move in opposite directions to conduct electricity. Normally, anions move at least four times faster than lithium cations, and thus lithium cation transfer only contributes 20% of the overall ionic current, while excessive anions accumulate at the interface between electrode and electrolyte, causing internal short circuit and capacity fade of the battery.

Liquid electrolytes’ flammability, instability with respect to lithium metal, and low ion selectivity for conduction are driving research towards solid electrolytes that can provide acceptable levels of safety and are compatible with the lithium metal anode, which exhibits the highest theoretical specific power capacity. The single-ion conducting polymer electrolytes designed by Shin’s team found an increase (at least four-fold) in cationic transport. The anionic network polymers designed by the team consist of borate anions bridged by branched ethylene glycol linkers of differing stoichiometric ratios, in which the anions are tethered into the polymer frame, enabling a selective cation transport. The cation conductivity within the polymer was controlled by systematic engineering of segmental mobility, helping to map out the comprehensive design rules for a new class of highly conductive solid electrolytes.

Image caption: Graphs showing the ordinary liquid electrolyte lithium-ion battery (left) and the single-ion conducting polymer electrolyte designed by the research team. Image credit: University of Hong Kong

As the single-ion conducting polymer electrolytes overcome the persistent problems of current solid electrolytes in battery cells, such as low cyclability and high overpotential, a new design rule for ion-selective electrolytes is predicted to facilitate speeding up the realisation of rechargeable Li-metal batteries.

“We believe the single-ion conducting polymer electrolytes would open up the possibility of new battery chemistries that will revolutionise the field of rechargeable batteries, and offer a high level of safety, high power density and long life cycle,” said Jingyi Gao, the first author of the paper and a PhD student of Dr Shin.

The ion-selective electrolytes in batteries can also result in fast charging due to low overpotential, Shin said. “It can allow electric vehicles to be fully charged in just the time needed for drinking a cup of coffee. This remarkable advantage will unlock a new era of a clean energy world,” Shin said.

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