Batteries will be vital to store wind and solar energy on the grid or quickly deliver backup power. Of the many battery technologies available, experts believe flow batteries are the most adaptable, low-cost option for utilities. Researchers have now designed a new type of lithium-based flow battery that could enable safe, high-voltage batteries for grid storage. Their work, reported in Energy & Environmental Science, shows a generic route to making high-performance lithium flow batteries.

Flow batteries work like conventional rechargeable batteries but swap the solid electrodes for liquid electrodes (called anolyte and catholyte) that sit in two large tanks. The solutions are pumped through a reactor where they flow past each other across a membrane. So increasing the battery’s energy storage capacity is as simple as using bigger tanks. They are also safer and longer lasting than lithium-ion batteries.

Currently, flow batteries use electrolytes made of vanadium ions dissolved in an aqueous solution. But vanadium is costly and has low energy density, so the batteries require large tanks to deliver sufficient energy. Scientists have been trying to use lithium instead. The goal of Li flow batteries is to combine the best of both: the high energy density and voltage of conventional lithium-ion batteries with the ease and flexibility of flow batteries.

Lithium flow batteries developed so far contain cathodes made of conventional redox species such as iodine dissolved in aqueous or organic solvents. They typically use metallic lithium as the anode. But metallic lithium can lead to the growth of tiny dendrites on battery membranes, causing dangerous electrical shorts, says Guihua Yu, a materials science and engineering professor at the University of Texas at Austin.

Yu and his colleagues instead turned to a family of compounds called organometallic compounds called metallocenes for the liquid electrodes. These compounds consist of a metal center sandwiched between two cyclopentadienyl (C5H5) rings. The rings are easy to modify, allowing high flexibility to tailor physical and chemical properties, Yu says. Plus, “metallocenes have a stable molecular structure which gives a very reversible redox reaction for making a good battery.”

The researchers dissolved ferrocene (iron atom sandwiched between two cyclopentadienyl (C5H5) rings) in dimethylformamide solvent for the cathode and cobaltocene dissolved in dioxolane for the anode. Aqueous solvents can be more cost-effective, Yu says, but “with large-scale grid storage, we want flexible working conditions such as in cold areas, where aqueous solvents cannot work.”

They constructed a proof-of-concept battery cell using a Li_1+x+3zAl_x(Ti,Ge)_2-xSi_3zP_3-zO₁₂ lithium membrane as a separator. The prototype device had an output voltage of 1.7 V, high enough to power a light-emitting diode. When the researchers added electron-donating methyl groups on the cobaltocene ring, the voltage went up to 2.1 V. In a battery, electrons are pulled from the anode and go through the external circuit to the cathode during discharge. So adding electron-donating functional groups to the anode or electron-withdrawing groups to the cathode improves the voltage.

It should be possible to make an even higher-voltage battery by further tweaking the molecular structure of ferrocene and cobaltocene, Yu says. The researchers also plan to chemically engineer the molecules to be more soluble in organic solvents, which would increase the battery’s energy density.

Such tailoring would be crucial since the major drawback of metallocenes is their low solubility, says Xiaoliang Wei, a staff scientist at the Pacific Northwest National Laboratory. The researchers’ rational use of electrolyte engineering and molecular tailoring are novel strategies for achieving high-voltage batteries, he says. “By introducing metallocenes as electroactive materials, this work presents a promising pathway to achieve high-performance non-aqueous flow battery systems,” Wei says. To make lithium flow batteries practical, researchers still need to find better membrane materials, and address the cost of electroactive materials and flammability of organic electrolytes, he adds.

Read the abstract in Energy & Environmental Science.