Molecular ions move faster than lithium ions inside electrodes, pointing to new secondary battery possibilities — AIST and collaborators

A team from the National Institute of Advanced Industrial Science and Technology (AIST) and their collaborators has revealed that in rechargeable secondary batteries, the molecular ion hexafluorophosphate (PF₆⁻) moves faster inside electrodes than lithium ions (Li⁺). This demonstrates that rapid charging and discharging could be achieved with "molecular ion batteries" in which molecular ions travel between the positive and negative electrodes, potentially leading to the development of new secondary batteries that could stand alongside the already widespread lithium-ion batteries.

Secondary batteries are also called rechargeable batteries, and lithium-ion batteries are widely used in mobile phones, personal computers, electric vehicles, and other devices. The 2019 Nobel Prize in Chemistry was awarded to Akira Yoshino and others for their development of these batteries.

Research Group Leader Masaru Yao at the Research Institute of Electrochemical Energy at AIST, who specializes in molecular chemistry, focused his attention on PF₆⁻ contained in lithium-ion batteries. In 2015, he demonstrated that PF₆⁻ could serve as a charge carrier in secondary batteries. Because it has higher conductivity in solution than Li⁺, which has a smaller ionic radius, he predicted that PF₆⁻ could transport charge faster than Li⁺, but there was no method to directly compare the movement speeds of the two ions-cations and anions-inside electrodes.

Around the same time, Chief Senior Researcher Hikaru Sano, who focuses on analytical chemistry and was conducting different battery-related research, discovered a polymer material consisting of linked 2,6-bis(diphenylamino)anthraquinone that could exchange PF₆⁻ and Li⁺ at separate sites. He proposed to Yao that this material could be used for direct comparison.

When this polymer material was actually used as an electrode, only the movement of either PF₆⁻ or Li⁺ could be measured depending on the voltage and charge conditions. The measurements revealed that the resistance during discharge was lower for PF₆⁻ than for Li⁺, indicating that PF₆⁻ moves faster than Li⁺. Although the monoatomic ion Li⁺ has a smaller ionic radius, it is thought to move more slowly because of its higher surface charge density.

Yao explained: "Lithium ions are like a celebrity walking alone through a crowd-they attract fans and become unable to move. In contrast, the molecular ion with six fluorine atoms surrounding phosphorus is like a celebrity with bodyguards positioned in six directions, clearing the way and enabling smooth movement."

The 2,6-bis(diphenylamino)anthraquinone used as an electrode in this research may be able to withstand repeated charging. Additionally, molecular ion batteries could in principle be composed of materials that do not undergo thermal runaway. Although still at the basic research stage, the development of molecular ion batteries could potentially provide a solution to rare metal shortages and fire accidents caused by the widespread use of lithium-ion batteries.

The research was conducted jointly with Osaka Metropolitan University College of Technology and Ehime University, and was published in the electronic edition of ChemSusChem, a journal of European Chemical Societies, on July 25.