Electrochemically inserting Mg2+ in graphite interlayers achieves large discharge capacity

To realize a low-carbon society, effective utilization of natural energy is required as well as storing that generated energy. Lithium-ion batteries currently have the highest energy density but face resource issues such as long-term stable supply. In recent years, there has been a great deal of interest in the development of magnesium (Mg) secondary batteries as one of the next-generation storage battery devices that do not depend on lithium resources. However, most anode material research is limited to Mg metals for charging. (Reaction: Mg deposition / discharge reaction: Mg stripping).

Concerns about Li metal negative electrodes such as internal short circuits and thermal runaway of the battery can be fundamentally avoided since Mg does not grow in a needle-like manner during charging. In addition to this, Mg metal is attracting attention as a promising negative electrode material because it exhibits a high theoretical capacity (2205 mA h g^-1). However, since the Mg metal surface is passivated by the decomposition of the electrolytic solution, it is not easy to repeatedly cause the reversible deposition- stripping reaction of Mg during charging and discharging. Currently, research is concentrated on the study of the composition of electrolyte solvents and salts that can proceed with this reaction with high efficiency and reversibility. If Mg²⁺ can be inserted between graphite layers up to the composition of MgC₆ in the same way as Li⁺ so two electrons move at a time, it is possible to obtain twice the capacity (744 mA h g^-1) of the LIB system. This electrochemical reaction is extremely attractive if it can be realized.

A typical electrochemical test of an active material (ion storage capacity evaluation test) is often performed using a two-electrode cell with a working electrode (a porous electrode with a dried paste of active materials, binders, and conductive additives) and a counter electrode (in this case, metal Mg) that serves as the ion supply. In order to properly grasp the potential information of the working electrode consisting of the active material, a test may be conducted in a three-electrode cell using a reference electrode. Oxidation reaction and reduction reaction always occur in pairs. In other words, for the charging of graphite in the above two-electrode cell, which is the working electrode, Mg metal is required to proceed the stripping (oxidation reaction) of the opposite electrode (reduction reaction / Mg²⁺ insertion reaction).

On the other hand, it has been reported that the deposition and stripping reactions of Mg are accompanied by a large over-potential. Therefore, the research group led by Assistant Professor Masahiro Shimizu and Professor Susumu Arai of the Department of Materials Chemistry, Faculty of Engineering, Shinshu University states that, "we considered the fact that we may have overlooked the possibility that the reason why Mg²⁺ is not inserted between the graphite layers in the test using the two-electrode cell is that the stripping reaction of the opposite electrode Mg metal has not progressed sufficiently. There have been several studies on the electrochemical insertion of Mg²⁺ between graphite layers, albeit in a small amount. However, when the amount of insertion was converted into electric capacity, it was about ~ 30 mA h g^-1. In addition, the evidence that Mg²⁺ was inserted between the layers was not sufficiently shown. By reviewing the test conditions and applying a reductive current to graphite continuously, we can electrochemically insert Mg²⁺ between the graphite layers even if it is not a Mg²⁺ alone but is surrounded by a solvent as a ligand. It was confirmed that the stage structure of graphite changed according to the amount of intercalant and that graphite became blue with the intercalation reaction. Since these things have not been shown so far, it is a notable achievement of this research that we were able to show the possibility of graphite Mg secondary battery negative electrode by reviewing the test conditions."

The solvent-incorporated Mg²⁺ is active in the atmosphere, so it was difficult to track its structural changes and reactions. In addition, since Mg and its electrolyte are undesirably affected by water and this triggers side reactions, it was difficult to thoroughly carry out the test ensuring moisture was removed sufficiently. However, the research group confirmed that Mg²⁺ can be electrochemically inserted between graphite layers even when the solvent surrounds it as a ligand, and the graphite becomes blue with the insertion reaction. They successfully obtained a relatively large reversible capacity (discharge capacity) of ~ 200 mA h g^-1.

Currently, Mg²⁺ electrochemical storage between graphite layers is limited to insertion in the state of solvated Mg²⁺ with ligands, not ion alone. This research confirmed that the intercalation reaction has good reproducibility, but the deintercalation (equivalent to the discharge reaction) reaction from here does not proceed easily. Even if it progresses, it deteriorates in a few cycles. Unless this can be advanced efficiently and reversibly, it will not function as a negative electrode active material. In addition, since the layers (storage space) are limited, the group believes that a higher charge/discharge capacity can be obtained by inserting Mg²⁺ alone instead of the bulky ionic state with a ligand. The research group at Shinshu University aims to solve the above problems.