SPring-8, the large synchrotron radiation facility

   A research group led by Yoshiharu Uchimoto (professor) and Yuki Orikasa (assistant professor) of the Graduate School of Human and Environmental Studies, Hiroshi Kageyama (professor) of the Graduate School of Engineering, and Cédric Tassel (program-specific assistant professor) of the Hakubi Center for Advanced Research, Kyoto University, in collaboration with JASRI, succeeded in developing high-energy-density rechargeable magnesium (Mg) batteries that can replace conventional lithium ion batteries. The newly developed rechargeable batteries will be produced at a lower cost because they mainly consist of Mg, iron (Fe), and silicon, which are abundant resources. Also, the thermal stability of the batteries is improved by using Mg metal with a high melting point, resulting in marked improvements in their safety compared with that of conventional lithium ion batteries.

   Their achievements were published in the British online journal Scientific Reports published by Nature Publishing Group on 11 July 2014.

   This study was carried out as part of the “Innovation of s-Block Metal Batteries towards Low-Carbon Societies” project (research director, Professor Yoshiharu Uchimoto of the Graduate School of Human and Environmental Studies, Kyoto University; research period, from October 2008 to March 2014) within the research scope of the “Creation of Innovative Technologies to Control Carbon Dioxide Emissions” project [research supervisor: Itaru Yasui, President of the National Institute of Technology and Evaluation (NITE)] of the Core Research for Evolutional Science and Technology (CREST) supported by the Japan Science and Technology Agency (JST).

   Because of the high theoretical capacity density, abundant resources and high safety, rechargeable Mg batteries^*1 are expected to be put into practical use as rechargeable batteries with performance exceeding that of lithium ion batteries^*2. The problems, however, are the extremely slow electrode reactions due to divalent Mg ions having a stronger ionic interaction and a greater difficulty in diffusing through a solid phase than monovalent lithium ions. Also, no Mg electrolyte solution for stable and safe charge-discharge reactions that enable repeated deposition and dissolution of Mg metal has yet been found. Namely, it is necessary to resolve the issues concerning the cathode and the electrolyte solution in order to fabricate rechargeable Mg batteries.

   The research group reported on a MgFeSiO₄ cathode material, in which the diffusion pathway for Mg ions is maintained by accurately controlling the crystal structure of the cathode material. Using this material, twice the number of Mg ions can be inserted and extracted, compared with conventional cathode materials. Because this material is stabilized by Si-O bonds, it enables repeated charge-discharge reactions over a long period of time. Also, the research group demonstrated the stable operation of the Mg metal anode using an electrolyte composed of Mg bis(trifluoromethylsulfonyl)imide [Mg(TFSI)₂] and triglyme. The mechanism of stable and high-energy-density charge-discharge reactions was clarified using the high-brilliance synchrotron radiation at SPring-8^*3. Combining the above cathode and electrolyte with an anode made of Mg metal, the research group fabricated rechargeable Mg batteries that offer the world’s highest performance.

   The results of this research and development will promote the practical use of rechargeable Mg batteries. The realization of a low-cost and safe electric energy storage medium will enable the storage of renewable energy, the supply of which fluctuates greatly under existing conditions, thus opening new possibilities of stable energy supply.

Publication
“High energy density rechargeable magnesium battery using earth-abundant and non-toxic elements”,
Yuki Orikasa, Titus Masese, Yukinori Koyama, Takuya Mori, Masashi Hattori, Kentaro Yamamoto, Tetsuya Okado, Zhen-Dong Huang, Taketoshi Minato, Cédric Tassel, Jungeun Kim, Yoji Kobayashi, Takeshi Abe, Hiroshi Kageyama, Yoshiharu Uchimoto,
Scientific Reports,
DOI: 10.1038/srep05622