A Japanese-Chinese research team has developed a titanium molten salt redox-flow battery (TMSRB) that uses titanium ions as the redox-active material and molten salt as electrolyte.
Intended for use in grid-scale energy storage, the TMSRB is designed to provide higher charge-discharge current density compared to conventional vanadium redox flow battery (VRFB) designs.
The scientists explained that, compared to vanadium, titanium is a far more abundant element, addressing supply and cost limitations. “Titanium is the seventh-most abundant metal in the earth's crust, with a crustal element abundance of 0.56%, 35 times that of vanadium. Thus, there is no concern about the sustainable supply of redox-active materials in TMSRB,” they stressed.
The system operates using titanium ions in multiple oxidation states, with the Ti⁴⁺/Ti³⁺ redox couple at the cathode and the Ti³⁺/Ti²⁺ redox couple at the anode, enabling reversible redox reactions. It also employs molten salt electrolytes, such as lithium chloride–potassium chloride (LiCl–KCl) and sodium chloride–magnesium chloride–potassium chloride (NaCl–MgCl₂–KCl), which the scientists said offer a wide electrochemical stability window and high ionic conductivity, while supporting efficient and high-voltage operation, rapid charge-discharge rates, and stable cycling at temperatures between 300–450 C.
The battery also employs a porous aluminum oxide (Al₂O₃) crucible as a separator, along with carbon and graphite electrodes connected by nickel leads. Titanium tetrachloride (TiCl₄) is carefully introduced into the system, and its evaporation is controlled using lithium fluoride (LiF) additives.
The battery was assembled with the separator being positioned between the positive and negative compartments. The negative electrode consisted of a multilayer carbon net connected to a graphite rod as a current collector and a nickel rod as a lead. A similar configuration, including a hollow carbon cylinder, was used on the opposite side. The molten salt electrolytes containing titanium ions were added into the cell and LiF was introduced to the positive side to suppress evaporation of TiCl₄.
The fully assembled battery was then operated under an argon (Ar) atmosphere in a resistance furnace to evaluate redox behavior and charge–discharge cycling stability. In addition, ab initio molecular dynamics (AIMD) simulations were used to track ion distribution during operation.
The analysis demonstrated the suitability of the multivalent titanium ions as redox-active materials for high-performance batteries. Cyclic and square-wave voltammetry in molten LiCl–KCl at 400 C revealed clear and reversible Ti²⁺/Ti³⁺ and Ti³⁺/Ti⁴⁺ redox reactions, which the scientists said could provide a high theoretical cell voltage of about 1.55 V, extendable to 1.80 V when including Ti/Ti²⁺. Moreover, multiple stable oxidation states and distinct redox transitions were found to further enhance system flexibility and stability.
The researchers also ascertained that the molten salt composition can be tuned to optimize cost, temperature range, and electrochemical performance. Experiments in different electrolytes confirmed consistent redox activity and high voltage across a wide temperature range.
In addition, experimental tests demonstrated high coulombic efficiency of over 97% and “stable” cycling even at high charge–discharge rates. Performance remains strong across various molten salt systems, highlighting robustness and adaptability.
“In summary, the developed TMSRB has great advantages, notably higher operating voltages, extremely high coulombic efficiency, rapid charge–discharge capability, and abundant, low-cost raw materials,” the academics emphasized. “Further engineering optimization—such as advanced cell-stack designs, enhanced thermal management strategies, and more detailed evaluations of system-level performance metrics, including voltage efficiency, energy efficiency, electrolyte tank capacity, and practical volumetric energy density, is currently ongoing.”
The VRFB system was introduced in “A high performance redox-flow battery for grid-scale energy storage,” published in Electrochemistry Communications. The research team included academics from the University of Science and Technology Beijing in China and the Tohoku University in Japan.
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