Redox-Flow batteries as a solution?

The demand for battery storage systems for renewables is set to grow massively in the future. To meet this anticipated demand, new batteries are being developed. Those that are cheap and more environmentally friendly than conventional lithium-ion models. For example redox flow batteries.

The growing amount of renewable energy can hardly be stored in pumped-storage power plants alone. For this reason, the demand for inexpensive, stationary battery storage systems will continue to see strong growth. Lithium-ion batteries are currently the dominant technology:. Thanks to electromobility, their development has really picked up pace over the past few years. Market researchers at IDTechEx assume that the market for lithium-ion batteries for stationary energy storage will come to be worth over 50 billion US dollars by 2031.

Ecological risks

Despite their popularity, lithium-ion batteries are anything but sustainable. They are made from expensive materials drawn from all over the world. And the extraction of which brings considerable dangers for both people and the environment. Lithium in particular is highly explosive, while other important elements of these batteries are dangerous to handle. For example cobalt, cadmium, and manganese. Moreover, the batteries’ durability remains problematic, while recycling is also difficult and associated with further risks.

Safe, powerful, and sustainable

There is good cause for the EU Commission using its “Green Deal” to propose a change. Batteries brought into circulation in the European Union should remain safe, powerful, and sustainable throughout their entire service life. In other words, the batteries should be made from materials that are extracted in full compliance with human rights, social, and ecological standards. Additionally, with the lowest possible environmental impact. They need to be safe and durable. At the end of their useful life, they should be repurposed, reconditioned, or recycled. Meaning that valuable materials will flow back into the economy.

Going with the flow with redox flow batteries

Redox flow batteries (RFBs for short) could be a solution to this problem. These boast a number of advantages over lithium-ion batteries. Their service life is relatively high and their capacity remains largely the same even after thousands of cycles. Additionally they do not release any substances that are hazardous to the environment. Essentially, very high storage capacities can be accumulated with redox flow batteries. Even though the energy density in their liquid electrolytes is low.

One very attractive storage technology concept has emerged in recent years in the form of flow batteries whose electrolyte tanks contain dissolved vanadium. For example, RedFlow announced the installation of zinc–bromine batteries with a capacity of two megawatt-hours in California at the start of 2021. And VanadiumCorp even wants to collaborate with Conoship International Projects from the Netherlands and Vega Reederei from Germany to develop a vanadium redox flow battery technology to replace diesel propulsion aboard ships.

Redox flow batteries with iron

Nonetheless, redox flow batteries have simply been too expensive for the mass market until now. Yet this should begin to change as other materials arrive on the scene. “Nowadays, iron is by far the most promising candidate for flow batteries. It is an inexpensive metal that is commonly available worldwide, distinguished by low toxicity, and can be used in a variety of molecular environments,” says Dr Birgit Weber, Professor of Inorganic Chemistry at the University of Bayreuth. In a research project focusing on iron, she aims to develop highly efficient, environmentally friendly RFBs.

Landshut University of Applied Sciences is also conducting research into all-iron redox flow battery (IRFB) technology. This is hoped to be at least as powerful as existing power storage systems, yet considerably cheaper to procure and maintain: “Based on initial estimates, savings of over 80 per cent could be achieved,” emphasises Professor Karl-Heinz Pettinger, Scientific Director of Landshut University of Applied Sciences’ Energy Technology Centre. “This is due to the fact that IRFB technology can be manufactured from materials available worldwide and can undercut the cost of lithium-ion batteries by between 50 and 70 per cent.” In addition to this, IRFB technology is very environmentally friendly since the materials required are commonplace all over the world and predominantly recyclable.

Aluminium plus carbon

Researchers at Cornell University are exploring a different avenue under the leadership of Professor Lynden Archer. They have developed an aluminium-based battery that can deliver up to 10,000 faultless charging cycles. “A very interesting feature of this battery is that only two elements are used for the anode and the cathode – aluminium and carbon – both of which are inexpensive and environmentally friendly,” says project participant Jingxu Zheng. “They also have a very long cycle life.” Aluminium’s advantages include its widespread availability in the Earth’s crust, its low weight, and the fact that it can store more energy than many other metals.

Real competition is still lacking

Even though a lot of new material combinations for battery storage systems are being investigated at present, Empa and ETH Zurich researchers Kostiantyn Kravchyk and Maksym Kovalenko remain unconvinced that any one of these technologies could actually replace lithium-ion batteries in the foreseeable future. They have looked closely at potential alternatives and claim that none of the presented technologies can compete with lithium-ion batteries in terms of energy density. In their view, that is very likely to remain the case in the future. Other solutions (e.g., using compressed air, liquid air, or gravity) will therefore also be needed for economical and long-term storage of energy or electricity.

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