A Novel Titanium/Manganese Redox Flow Battery

Toward the realization of a low-carbon society, renewable energies such as wind and solar power have been promoted rapidly around the world. Since such renewable energies have a fluctuating power output due to the variability of weather, the increasing amounts of such energy sources will bring stability problems to conventional power system. Electrical energy storage systems are expected to solve these problems, and large-scale batteries attach more attention than ever. Redox flow (RF) batteries have several advantages comparing to solid batteries, such as easy to capacity scale up, long cycle life, real time state of charge (SOC) monitor, which make them the most suitable for power network stabilization.

RF batteries have been enthusiastically developed worldwide, since its principle was publicized in the 1970s by NASA^[1], all vanadium system as a representative of them has been put in practical applications^[2]. To meet the growing demand for electrolytes, it is generally desired that active metals should be lower cost, higher electrode potential and stable supply than vanadium. Mn(II)/Mn(III) redox couple as a positive active material meets the requirements mentioned above. However, Mn(III) ion is chemically unstable and tends to disproportionate to Mn(II) ion and MnO₂ oxide, which make it difficult to be used in flow batteries.

We found that the precipitation of MnO₂ can be effectively suppressed by containing Mn ions as well as Ti(IV) ions in positive electrolyte. We gained a very good cycle charge-discharge performance in a cell test, even at SOC in a range of more than 100%, in this case SOC is calculated on Mn(II)/Mn(III) one-electron reaction. It considered that Ti(IV) ions exiting in positive electrolyte suppress the Mn(III) disproportion reaction, as well as particle growth of MnO₂. The theoretical energy density of positive Mn and negative Ti electrolytes RF battery is around 26kWh/m³, which is comparable to that in all vanadium RF battery.

References

[1] N. H. Hagedorn, NASA Redox Storage System Development Project Final Report, DOE/NASA/12726-24, NASA TM-83677(1984)

[2] T. Shigematsu, Redox Flow Battery for Energy Storage, SEI Technical Review, Number 73, (October, 2011)