New Fast-Charging Flow Battery Aims to Advance the State-of-the-Art in Energy Storage

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Masdar Institute researchers have engineered a novel non-aqueous low-cost flow battery equipped with fast-charging that is able to charge itself in half the time it would normally take, which they believe may enable cheaper and more efficient large-scale renewable energy storage.

The prototype battery has an open-circuit voltage of 1.2 volts, with an extra voltage of 1.8 volts leveraged for rapid charging. The device is the first such non-aqueous redox flow battery to employ the unique fast-charging design, cutting the amount of time it takes to fully charge by up to 50%.

“Non-aqueous redox flow batteries have the potential to store more energy than aqueous-based systems, because organic solvents have a wider voltage window than that of water; which allows for formulation of battery chemistries with higher open-circuit voltages. But due to other challenges, their performance has not yet reached the levels achieved by their water-based counterparts,” explained Musbaudeen Bamgbopa, a PhD student at MI.

He believes that a major opportunity for non-aqueous flow batteries – which are a type of flow battery that utilize solvents other than water – is for scientists to expand their focus on performance improvement beyond a narrow focus on increasing energy density.

Bamgbopa and his advisor, Dr. Saif Almheiri, Assistant Professor of Mechanical and Materials Engineering, realized that there was another way to capitalize on the wide voltage window of non-aqueous flow batteries.

“What we did in our work is present another advantage of the non-aqueous based redox flow system that scientists have overlooked. Instead of trying to increase our battery’s energy density, we used that extra allowable voltage for fast-charging,” Dr. Almheiri shared.

Flow Batteries for RE Storage

Flow batteries are one of the most promising electrochemical storage technologies for storing intermittent, renewable energy sources, like the sun and wind, and affordably and reliably dispatching that energy as electricity for long durations. Flow batteries are well suited for storing the excess renewable energy that is generated when it is sunny or windy, and dispatching that energy back to the grid when it’s needed. They can be easily scaled up to meet the requirements of large-scale renewable power generation storage, and they can complete up to 10,000 recharge cycles, giving them a very long lifespan of up to 20 years.

A flow battery capable of fast-charging brings an additional value to storing intermittent renewable sources; if there is just a limited opportunity for charging, the battery can quickly recharge itself with the energy generated during a short time.

The MI lab-scale proof-of-concept flow battery consists of a typical redox-flow design: two tanks of electroactive materials dissolved in a solvent that is pumped into the battery’s cell. Here, separated by a membrane to keep the solutions from mixing, they undergo a redox reaction and release energy. But where the MI flow battery differs, is in the tank’s contents.

Flow batteries have traditionally been aqueous, which means that the electroactive materials – usually metals – are dissolved in water. However water has severe limitations. For example, aqueous flow batteries are temperature sensitive; some reactive metals start precipitating in the water-based electrolyte when the temperature of water exceeds 40-60 degrees Celsius. Another limitation is that water-based batteries are limited by the voltage window of stability of water. Water will start to split into its component elements of oxygen and hydrogen if the applied voltage exceeds the stability limit, significantly restricting the energy density of water-based batteries.

The MI flow battery is non-aqueous, employing organic-based acetonitrile instead of water as its solvent. Acetonitrile has a much wider voltage window, up to 6 volts. However, with the reactants the team used – iron acetylacetonate (acac) in the positive electrolyte and chromium acac in the negative electrolyte – the allowable charging voltage became 3 volts. Beyond 3 volts, the metal reactants began undergoing irreversible reactions, which would have prevented the battery from performing a full recharge cycle.

“Iron acac is a metal complex that was first discovered in the 1980s, but it was never considered valuable for a redox flow battery because it only has one redox couple. Normally engineers like to use a metal that has several redox couples in a flow battery. But we found that the one reaction that iron acac undergoes is isolated, which enables it to go beyond its natural charging voltage as it will not undergo any undesirable reactions,” Bamgbopa said.

Their work is the first to use iron acac in a flow battery, and the first to report on the chemistry between iron and chromium in a non-aqueous flow battery.

A Different Approach to the Extra Potential

The drive for higher energy density is what makes non-aqueous redox flow batteries so attractive. But scientists are faced with many challenges when trying to capitalize on the wide non-aqueous flow battery’s voltage window. Finding the right type of metal and solvent combination to undergo the required redox reactions without experiencing undesirable and irreversible reactions is difficult.

Instead of chasing after an increase in the battery’s overall energy density, the MI researchers developed a way to use that extra allowable voltage for fast-charging without compromising the battery’s performance.

They did this by designing their battery to operate at 1.2 volts, while reserving enough room within the voltage window to be able to allow it to take up to 1.8 volts more during charging, which boosts its charging speed. Instead of making the battery with a voltage of 3 volts, which would run the risk of the reactants undergoing undesirable and irreversible reactions that would severely hinder the battery’s performance, they kept its voltage limited to 1.2 volts, but with a unique ability to take in an extra 1.8 volts when charging.

Dr. Almheiri and Bamgbopa will continue working to improve iron acac’s and chromium acac’s solubility, so that they can further increase the battery’s energy density and efficiency.

This work on fast-charging enabled flow batteries takes new directions in non-aqueous flow battery development, and opens the door to greater innovation and exploration of future flow battery applications.