Power storage technology is an important technology for efficient use over the whole fields of energy such as efficiency of power use, enhancing ability or reliability of power supply systems, expanding introduction of new and renewable energy having large fluctuation depending on time, and energy regeneration of moving bodies, and its potential and demands for social contribution have gradually increased.
Studies on secondary batteries have been actively conducted in order for adjusting the balance of demand and supply in semi-autonomous regional power supply systems such as micro-grid, properly distributing non-uniform outputs of new and renewable energy generation such as wind power or solar power generation, and controlling affects such as voltage and frequency changes occurring from differences with existing power systems, and expectations for the use of secondary batteries have increased in such fields.
Particularly, when examining properties required for secondary batteries used for high capacity power storage, energy storage density needs to be high, and as high capacity and high efficiency secondary batteries suited for such properties, redox flow batteries (RFB) have recently received attention.
Like general secondary batteries, redox flow batteries store electric energy inputted through a charging process after converting to chemical energy, and output the stored chemical energy after converting to electric energy through a discharge process. However, such redox flow batteries are different from general secondary batteries in that electrode active materials storing energy are present in a liquid state instead of a solid state, and therefore, a tank or a storage container storing the electrode active materials is required.
As described above, redox flow batteries have properties such as capable of being manufactured to have large capacity, requiring lower maintenance costs, operable at room temperature, and capable of designing capacity and output each independently, and therefore, considerable studies on redox flow batteries as high capacity secondary batteries have been currently conducted.
Among these, vanadium redox flow batteries using vanadium ions have received attention as a next generation energy storage device, however, there are problems of capacity decrease in the redox flow battery due to phenomena such as separator (or ion-exchange membrane) cross-over of vanadium ions, generation of hydrogen in an anode, and oxidation reaction of vanadium ions when exposed to air, and studies for improving these have been consistently ongoing.
Among these, a separator cross-over phenomenon of vanadium ions is caused by ion imbalance between an anode electrolyte and a cathode electrolyte occurring by the anode electrolyte and the cathode electrolyte having different oxidation numbers, and as a result, a problem of causing deterioration in the battery capacity occurs.
Specifically, V2+ and V3+ ions of the anode electrolyte permeate the membrane in a relatively higher rate compared to V5+ and V4+ ions of the cathode electrolyte, and as a cycle progresses, the vanadium ion concentration rapidly increases in the cathode electrolyte. Accordingly, the concentration of vanadium ions decreases in the anode electrolyte, and as a result, ion balance between the anode and cathode electrolytes is broken causing a problem of cycle deterioration when operating a battery.