Lithium ion theory to be revised

1 min read

The ultrafast dynamics of lithium ions have been observed in real-time with femtosecond time resolution by researchers from the Centre for Molecular Spectroscopy and Dynamics, within Korea’s Institute for Basic Science.

The study, said to reveal the interactions between lithium ions and electrolytes, concluded that the existing theory on ion diffusion in lithium rechargeable batteries is not correct.

In a typical commercial lithium rechargeable battery, lithium ion mobility determines the performance of the lithium rechargeable battery, and how rapidly they can charge and discharge.

Lithium ions, however, do not migrate alone; they are surrounded by electrolytes that facilitate the journey from one pole to the other. Currently, the electrolytes in lithium rechargeable batteries are typically composed of a mixture of: ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) in equal concentration.

It is believed that lithium ions associate mainly with EC, forming the 'solvation shell' or 'solvation sheath', while DMC and DEC enhance the movement of these shells between the batteries' poles. However, while most of the previous studies focused on the static properties of the bond between electrolytes and lithium ions, this study clarifies the dynamics of the bonding.

The scientists took quick shots at time intervals of femtoseconds to analyse the formation and breaking of these bonds. The team found that the bonds between lithium ions and the oxygen atoms of the DEC break and form in a matter of 2 to 17ps. The timescale is similar for DMC.

According to the researchers, this means DMC and DEC are more than just ‘lubricants’ – they are also part of the solvation shell together with EC and may play an active role in transporting lithium ions to the battery's pole.

"It was believed that EC makes a rigid shell around lithium ions during the migration between electrodes. However, this study shows that the solvent shell is not that rigid, it is constantly restructured during the ion transport," explains Professor CHO Minhaeng. "For this reason, revising the existing lithium ion diffusion theory is inevitable."