As an anode, magnesium metal provides two electrons per atom, giving it an attractive volumetric capacity of 3837 mAh·cm⁻³, which is approximately five times higher than that of the conventional graphite anodes in lithium ion batteries (LIBs). In addition to the high capacity, the relatively high negative reduction potential of magnesium metal can provide high energy density. Moreover, the terrestrial abundance and melting point of elemental magnesium by far surpass that of lithium, translating to a cheap and safe battery system. These advantages of magnesium metal anodes have been previously recognized and a rechargeable magnesium battery cell was first proposed in 2000.

… However, the energy density remained rather constrained by the cathode material, and the narrow potential window, corrosion, and safety problems posed by the electrolyte have hampered the commercial realization of these batteries. …Even though extensive research has been performed on cathode materials, breakthroughs are awaited for the development of practically usable rechargeable magnesium batteries. In this study, we have attempted to address the problems related to cathode materials by using an ion-exchanged polyanion cathode (i.e., MgFeSiO₄) and constructed a rechargeable magnesium battery using this high-energy-density cathode material.

—Orikasa et al.

Divalent Mg²⁺ insertion/extraction in host compounds is difficult—apparently due to the stronger ionic interaction and harder charge redistribution of magnesium compared to lithium ions—thereby limiting the choice of cathode materials for magnesium batteries.

In the quest for a viable cathode material, orthosilicates such as olivine-type MgMSiO₄ (M = Fe, Mn, Co) are promising candidates, the researchers explained, because the theoretical capacities of MgMSiO₄ exceed 300 mAh·g⁻¹ and the operating voltages are expected to be higher than that of conventional magnesium battery cathode materials. Within this family of materials, MgFeSiO₄ is expected to be inexpensive because its constituent elements are abundant.

The Kyoto team sought to improve electrode kinetics by preparing a meta-stable phase of MgFeSiO₄ via the electrochemical ion exchange of Li₂FeSiO₄, rather than by the conventional solid state synthesis. The preparation of MgFeSiO₄ involves two electrochemical processes: 2Li⁺ extraction from Li₂FeSiO₄ followed by Mg²⁺ insertion.

Schematic illustration of the ion-exchange methodology for the electrochemical synthesis of MgFeSiO4 from Li2FeSiO4. Two-dimensional (2D) framework of Li2FeSiO4 and three-dimensional (3D) framework of FeSiO4 and MgFeSiO4. The 3D framework can incorporate Mg ions in the interspace (void). Orikasa et al. Click to enlarge.

The complete extraction of Li⁺ from Li₂FeSiO₄ is performed in a Li-ion battery cell, and then Mg²⁺ is inserted into FeSiO₄ after changing the electrolyte to one containing a Mg salt.

Remarkably, MgFeSiO₄ prepared via ion exchange undergoes reversible electrochemical charge-discharge processes. … Although the charge-discharge potential appears slightly shifted, a reversible reaction is attainable with almost one Mg²⁺ insertion and extraction. The achieved discharge capacity of MgFeSiO₄ is approximately twice that of conventional LIB cathodes such as LiCoO₂ and LiFePO₄.

—Orikasa et al.

Prototype of a high energy-density rechargeable Mg battery. Orikasa et al. Click to enlarge.

They then built a prototype magnesium battery system with ion-exchanged MgFeSiO₄ as the cathode, Mg metal as the anode, and Mg(TFSI)₂–triglyme as the electrolyte. Because the Mg(TFSI)₂–triglyme electrolyte does not contain Cl-, Br- or THF-based solvents, this enables the safe operation of rechargeable magnesium batteries without corrosion and low flammability.

The Kyoto team performed charge–discharge measurements for the magnesium rechargeable battery full cell at 100°C; they obtained reversible charge–discharge capacity of 166 mAh·g⁻¹ was obtained, calculated on the basis of the mass of the active material.

This study demonstrates ion-exchanged MgFeSiO₄ as a feasible cathode material for use in rechargeable magnesium batteries. The application of ion-exchanged MgFeSiO₄ polyanion compounds as rechargeable magnesium battery cathode materials provides a capacity of more than 300 mAh·g+ at an average potential of 2.4 V vs. Mg²⁺/Mg, with good retention upon cycling. The electronic and crystal structure of ion-exchanged MgFeSiO₄ changes during the charging and discharging processes, which demonstrates the (de)insertion of magnesium in the host structure. Batteries using a combination of ion-exchanged MgFeSiO₄ and the Mg(TFSI)₂–triglyme electrolyte represent a prototype for a low-cost, high-energy-density rechargeable magnesium battery in which no toxic or explosive components are used.

—Orikasa et al.

Resources

• 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 (2014) “High energy density rechargeable magnesium battery using earth-abundant and non-toxic elements” Scientific Reports 4, Article number: 5622 doi: 10.1038/srep05622