Ultrafast spectroscopy has revealed that components of the electrolyte play a much more important role in battery performance than previously thought.
Using ultrafast spectroscopic methods, researchers from the Center for Molecular Spectroscopy and Dynamics at the Institute for Basic Science (IBS) have challenged the existing theory on ion diffusion in the widely used lithium rechargeable batteries. Published in Nature Communications, this study reveals the interactions between lithium ions and electrolytes, organic molecules that surround the lithium ions and conduct electricity.
Although most of our electronic devices like mobile phones, laptops and electric vehicles use lithium rechargeable batteries, what is going on inside them is not actually fully understood. In a typical commercial lithium rechargeable battery, lithium ions dissolved in electrolytes move from the positive to the negative pole of the battery when the battery is charging, migrating in the opposite direction when the battery is in use. The lithium ion mobility determines the performance of the lithium rechargeable battery, and determines 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 our 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 so-called ‘solvation shell’ or ‘solvation sheath,’ while DMC and DEC just enhancing the movement of these shells between the batteries’ poles, like lubricants. 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. Like in a motion picture, where a series of still images displayed rapidly one after the other create the effect of movements, IBS scientists took successive shots to analyze the formation and breaking of these bonds.
However, while movies are typically filmed and displayed at 24 still images per seconds, these measurement ‘shots’ were taken at time intervals of just femtoseconds or 1/1,000,000,000,000,000 of a second. Thanks to a tool called two-dimensional infrared spectroscopy, the team measured how lithium ions bind to the oxygen atoms of DEC and found that these bonds break and form in a matter of 2-17 picoseconds.
The timescale is similar for DMC. This means that 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,” explained Professor Cho Minhaeng. “For this reason, revising the existing lithium ion diffusion theory is inevitable.” The research team is working on a follow-up study to establish a new theory of the lithium ion diffusion process and it is building a new ultra-high-speed laser spectroscopy instrument that can observe the chemical reaction as well as film it on top of the rechargeable batteries’ electrodes.
The article can be found at: Lee et al. (2017) Ultrafast Fluxional Exchange Dynamics in Electrolyte Solvation Sheath of Lithium Ion Battery. Read more from Asian Scientist Magazine at: https://www.asianscientist.com/2017/04/tech/lithium-battery-electrolyte-solvation-shell/