Secondary batteries are rechargeable. There are small types of secondary batteries obtained from nickel-cadmium, nickel-hydrogen, and lithium ion sources. It is known that lead-acid batteries are relatively large. Secondary batteries are useful for automobiles, airplanes, agricultural equipment, electric vehicles, computers, mobile phones and so on. Among them, the lithium ion batteries are mainly used for various applications, and they are manufactured by lithium cobalt oxide (anode), graphite (cathode), and a liquid electrolyte with organic components.1) The lithium ion batteries provide high voltage and energy density, because the lithium ion supplied from the lithium cobalt oxide is a carrier doing the charge/discharge of the battery. A memory effect hardly occurs. A package of the lithium ion batteries can be compact. A further development of a better secondary battery is also in progress toward a low-carbon society as well as energy security.
In order to improve security of the lithium ion batteries, it is expected to use an ionic liquid electrolyte,2) phosphate-based organic solvent,3) organic solid electrolyte, and inorganic solid electrolyte, since an organic electrolyte solution is more or less flammable. An electrolyte solution requires a fluorine-containing flame retardant as an additive.4) A selection of electrolyte is important for input-output characteristics, lifetime, security and voltage of a secondary battery. It is also expected that the electrolyte shows high lithium ion conductivity, electrical and chemical stabilities, and a low environmental load.
A next generation secondary battery with high energy density must be developed well. Among them, we may expect practical use of a secondary battery based on a multivalent ion carrier that can transport multi-electrons. A magnesium-based secondary battery has a theoretically high energy density. Furthermore, it is expected that we can use abundant magnesium for the battery and it is secure to use.5)