Aqueous flow batteries are highly promising for large-scale energy storage due to their independent energy and power, high safety, and adjustable energy storage scale. However, the solubility limit of active substances in water restricts the energy density of aqueous flow batteries.
Ferrocyanide/ferrocyanide is a common cathode active substance for aqueous flow batteries, which has the advantages of good reversibility, high stability, and low raw material cost, but its solubility is low, so it is urgently needed to break the solubility limit of the active substance and to develop a high energy density electrolyte system.
Based on the principle of hetero-ionic effect, the research team used a mixture of potassium ferrocyanide and sodium ferrocyanide as the anode active substance, and succeeded in dramatically increasing the solubility of ferrocyanide/ferrocyanide to 1.62 moles/liter. Introducing Prussian blue as a solid energy storage material in the positive electrode reservoir, the team further increased the theoretical concentration of the positive electrode active substance to 10 moles/liter by using the redox targeting reaction, and the theoretical energy density of this side of the neutral iron-sulfur liquid flow battery system is as high as 260 watt-hours/liter.
The test results show that the actual energy density of the cathode electrolyte is 92.8 watt-hours/liter, and the maximum power density of the battery reaches 284.7 mW/cm2. Meanwhile, the system has excellent high-temperature performance, and the battery still maintains good cycling stability at 50 degrees Celsius, and the utilization rate of solid energy storage materials increases gradually with the temperature. The Coulombic efficiency of the neutral iron-sulfur liquid current battery small stack is close to 100%.
It was found that the neutral iron-sulfur liquid flow battery based on redox-targeted reactions exhibited an ultra-long cycle life, with a capacity retention rate of 181.8% after 7,000 cycles due to the continuous capacity release of the solid energy storage material.
The neutral iron-sulfur flow battery based on redox-targeted reaction designed in this study has a low electrolyte cost and has a promising application in large-scale energy storage, the researchers said.
