In recent years, with the electric power shortage becoming more serious, rapid introduction of natural energy, such as wind power generation and photovoltaic power generation, and stabilization of electric power systems (e.g., maintenance of frequencies and voltages) have become issues to be addressed globally. As one of the measures to address the issues, installation of large-capacity storage batteries to achieve, for example, smoothing of output fluctuation, accumulation of surplus power, and load leveling has been receiving attention.
One of large-capacity storage batteries is a redox flow battery (hereinafter, may be referred to as an “RF battery”). The RF battery has characteristics such as (i) ease of capacity increase to a megawatt (MW) level, (ii) a long life, (iii) capability of accurately monitoring the state of charge (SOC) of the battery, and (iv) high design freedom such that battery output and battery capacity can be independently designed, and is expected to be a most suitable storage battery for stabilization of electric power systems.
An RF battery mainly includes a battery cell including a positive electrode to which a positive electrode electrolyte is supplied, a negative electrode to which a negative electrode electrolyte is supplied, and a membrane interposed between the two electrodes. Typically, an RF battery system is constructed, the system including the RF battery and a circulation mechanism for circulating and supplying electrolytes to the RF battery. The circulation mechanism usually includes a positive electrode tank that stores a positive electrode electrolyte, a negative electrode tank that stores a negative electrode electrolyte, and pipes that connect the two electrode tanks to the RF battery.
A solution containing, as an active material, metal ions whose valence is changed by oxidation-reduction is typically used as an electrolyte for each electrode. Typical examples include an Fe—Cr-based RF battery in which iron (Fe) ions are used as a positive electrode active material, and chromium (Cr) ions are used as a negative electrode active material, and a V-based RF battery in which vanadium (V) ions are used as an active material for both electrodes (refer to paragraph 0003 in the description of PTL 1).
PTL 1 discloses a Mn—Ti-based RF battery in which manganese (Mn) ions are used as a positive electrode active material, and titanium (Ti) ions and the like are used as a negative electrode active material. The Mn—Ti-based RF battery is advantageous in that it can generate a higher electromotive force than an existing V-based RF battery and that the material for the positive electrode active material is relatively inexpensive. Furthermore, PTL 1 discloses that by incorporating titanium ions, in addition to manganese ions, into the positive electrode electrolyte, it is possible to suppress generation of manganese dioxide (MnO2), and Mn2+/Mn3+ reactions can be stably carried out.