In the realm of energy storage and biological processes, the movement of ions is a critical factor. A team of researchers from the University of Science and Technology of China, led by Dr. Yunfei Chen, has recently shed light on a long-standing puzzle in ion transport. Their findings, published in the journal Nature Communications, challenge conventional understanding and offer insights that could inform the development of more efficient electrochemical energy storage devices.
The researchers focused on the apparent violation of the Stokes-Einstein relation, which predicts that smaller ions should move faster than larger ones in a given medium. However, in water, smaller alkali metal ions like lithium (Li+) migrate more slowly than larger ions. This anomaly has typically been attributed to dielectric friction, a drag force resulting from electrostatic interactions between the moving ion and the surrounding solvent.
To investigate this phenomenon, the team combined nanopore transport measurements with molecular dynamics simulations. They subjected ions to electric fields spanning several orders of magnitude and found that the time-averaged electrostatic force on a migrating ion is not a drag force but a net driving force. This discovery contradicts the conventional wisdom about dielectric friction.
By comparing charged ions with neutral particles, the researchers identified that the ionic charge introduces additional peaks in the frequency-dependent friction coefficient. These peaks, which are most pronounced for lithium ions and progressively weaker for sodium (Na+) and potassium (K+) ions, originate from short-range Lennard-Jones (LJ) interactions within the first hydration layer. These interactions represent additional channels for energy dissipation.
The study reveals that electrostatic interactions primarily serve to tighten the local hydration structure, thereby amplifying short-range LJ interactions rather than directly opposing ion motion. This microscopic mechanism provides a unified physical explanation for the breakdown of the Stokes-Einstein relation in aqueous ion transport.
For the energy sector, these findings could have practical implications. Understanding the underlying mechanisms of ion transport can lead to the development of more efficient electrochemical energy storage devices, such as batteries and supercapacitors. By optimizing the design of these devices to minimize energy dissipation due to short-range interactions, researchers may be able to improve their overall performance and efficiency.
This article is based on research available at arXiv.