Renewable energies such as wind power and solar power are now rapidly introduced worldwide. Energy storage systems, especially large-scale batteries become more necessary than ever for stabilizing the electric power fluctuations caused by renewable energies. Redox flow battery (RFB) is one of the strong candidates for this application, because of its easiness to scaling up and monitoring of state of charge, long cycle life, and safety. To accelerate the commercialization of RFB, reducing the cost of the battery system is one of the most important issues.

   Nowadays, vanadium (V) electrolyte is generally utilized for commercial RFB. Because the electrolyte is one of the key and expensive components in RFB system, many alternative electrolytes are developed to decrease the cost of RFB system. We have investigated various metal ions as active materials that are abundant and common for industrial use. In our research, titanium/manganese (Ti/Mn) RFB system has good potential, which use the redox reactions of Mn²⁺/Mn³⁺ and Ti⁴⁺/Ti³⁺ as positive and negative side, respectively. We have already reported that the cell voltage and the energy density of Ti/Mn system are comparable to those of V system ^[1].

    One of the challenges in Ti/Mn system is to improve the stability of Mn³⁺ ion in the positive electrolyte at charged state. Mn³⁺ ion can easily disproportionate to Mn²⁺ and MnO₂, which leads to degradation of battery performance due to the precipitation of MnO₂ particles. We reported that mixing Ti⁴⁺ ion in the positive electrolyte makes redox reaction of Mn²⁺/Mn³⁺ more reversible and prevents the precipitation of MnO₂ particles. Ti⁴⁺ ion in the positive electrolyte also changes the color of the electrolyte with Mn²⁺ ion and the potential of positive electrolyte during charging, which strongly suggests the Ti⁴⁺ion interacts with Mn ion.

    This time we investigated the effect of Ti⁴⁺ ion in the positive electrolyte with various ratios of Ti to Mn. The effect of the ratio of Ti⁴⁺ ion on the electrochemical properties and MnO₂ morphologies will be closely discussed.

References

[1] Y. R. Dong, et.al., 228^th ECS Meeting, 120 (2015)