As we move towards a more energy-efficient society, the need for high-capacity, cost-effective batteries is more critical than ever. Magnesium is a promising material for these solid-state batteries due to its abundance, but its practical application is limited by the low conductivity of magnesium ions (Mg2+) in solids at room temperature. Recently, Japanese researchers have developed a new Mg2+ conductor with superconductivity practically within 10-3 S cm-1, overcoming this decades-long impediment.
The development of highly efficient energy storage devices capable of storing renewable energy is important for a sustainable future. In today’s world, solid-state rechargeable lithium-ion (Li+) batteries are state of the art. But lithium is an unusual earth metal, and society’s dependence on this element is likely to lead to a rapid decline in resources and subsequent price increases.
Magnesium ion (Mg2+) batteries have gained momentum as an alternative to Li+. The earth’s crust contains a lot of magnesium and energy devices based on Mg2+ are said to have high energy densities, high safety and low cost. But the massive software of Mg2+ is limited by its low conductivity in solids at room temperature. Mg2+ has poor solid state conductivity because divalent positive ions (2+) experience strong interactions with their neighboring negative ions in a solid crystal, preventing their migration through the material.
This obstacle was recently overcome by a research team from Tokyo University of Science (TUS). In their new study published online May 4 2022 and May 18 2022 in volume 144 number 19 of the Journal of the American Chemical Culture, they report for the first time, a solid state Mg2+ conductor with a superionic conductivity of 10−3 S cm−1 (the threshold for practical application in solid state batteries solid). This magnitude of conductivity for Mg2+ conductors is the highest reported to date. According to TUS Junior Associate Professor Masaaki Sadakiyo, who led the study, “In this work, we exploited a class of materials called metal-organic frameworks (MOFs). MOFs have very porous crystal structures, which provide space for efficient migration of the embedded ions. Here, we additionally introduced a “guest molecule”, acetonitrile, into the pores of the MOF, which managed to greatly accelerate the conductivity of Mg2+.” The research group further included Mr. Yuto Yoshida, also from TUS, Professor Teppei Yamada from the University of Tokyo, and Assistant Professor Takashi Toyao and Professor Ken-ichi Shimizu from Hokkaido University. The article was posted on May 4 2022 and was published in volume 144 number 19 of the magazine on 10 May 2022
The team used a MOF known as MIL-101 as the main framework, then encapsulated Mg2+ ions in its nanopores. In the resulting MOF-based electrolyte, Mg2+ was loosely packed, allowing migration of divalent Mg2+ ions. To further improve the ionic conductivity, the research team exposed the electrolyte to acetonitrile vapors, which were adsorbed by the MOF as guest molecules.
The team then subjected the prepared samples to an alternating current (AC) impedance test to measure ionic conductivity. They found that the Mg2+ electrolyte had a superionic conductivity of 1.9 × 10−3 S cm−1. This is the highest conductivity ever reported for a crystalline solid containing Mg2+.
To understand the mechanism behind this high conductivity, the researchers carried out spectroscopy measurements infrared and adsorption isotherm on the electrolyte. The tests revealed that the acetonitrile molecules adsorbed in the framework allowed an efficient migration of the Mg2+ ions through the body of the solid electrolyte.2022
The results of this study not only reveal the new MOF-based Mg2+ conductor as a suitable material for battery applications, but also provide essential information on the development of future solid-state batteries. “For a long time people believed that divalent or higher valence ions could not be efficiently transferred through a solid. In this study, we have demonstrated that if the crystal framework and the surrounding environment are well designed, then a high-conductivity solid-state is well within the research,” says Dr. Sadakiyo.
Asked about the future plans of the research group, he reveals: “We hope to contribute more to society by developing a divalent conductor with even higher ionic conductivity.”