similar system using zinc, where charging the battery deposits the metal onto electrodes from a basic solution of zincate ions, periodically wiping off the metal which falls through to another chamber where it can be re-oxidised by air to discharge the battery.
Enter the flow
There is also intense activity around redox flow batteries – which use reduction and oxidation reactions but store the redox-active species in two liquid electrolyte solutions. These are pumped through a central stack containing inert electrodes separated by a membrane. The oxidised and reduced products are kept in separate external reservoirs, where they can sit until the battery needs to be charged or discharged. ‘In principle, they are infinitely scalable, because you decouple the storage of the fuels from the electricity generation pod,’ explains Clare Gray, a materials and battery chemist from the University of Cambridge, UK.
Redox flow batteries could be the ideal technology for energy storage. ‘They’re very flexible compared to these things that need salt caverns and big geographical spaces,’ says Kathryn Toghill, an electrochemist at Lancaster University in the UK. Plus they can deliver back electricity very quickly to respond to demand. They have a longer life than lithium-ion batteries because there are no structural changes at the electrodes to interfere with performance. ‘If you have a fading capacity, you can top up the tank, which is clearly less disruptive than replacing a stack,’ says Mezzavilla.
The next step is finding the right chemistry to optimise the energy density of flow batteries. One important element is choosing substances that have high solubility to increase the concentration of active species. In aqueous solution the cell needs to run at a potential where water is stable or it will itself electrolyse. According to Gray, this means that ‘you’ve only got about 1.2V to play with’.
The most commercially advanced redox flow battery technology is the vanadium redox flow cell. Several companies are already supplying these batteries, including Invinity Energy Systems, created in 2020 through the merger of flow battery companies based in the UK and US. In this aqueous system the positive half-cell electrolyte contains VO2+ and VO2+ ions, switching from vanadium v and iv oxidation states, while the electrolyte in the negative half-cells contains V3+ and V2+ ions, cycling between vanadium ii and iii oxidation states. Having vanadium species in both half cells has the added benefit of limiting the impact of ‘crossover’ – when ions diffuse across the membrane. In some systems this can lead to electrolyte poisoning and battery discharge.
Invinity say their battery can provide up to 40MWh of storage, run from 2–12 hours and deliver 3.8 times the lifetime energy throughput of a lithium-ion battery. To date they have supplied units to over 70 sites across 15 countries, including a 5MWh battery for an energy superhub in Oxford, which is expected to cut 25,000 tonnes of CO2 emissions in the city per year by 2032. For grid applications, the largest vanadium flow battery facility is in Dalian in northeast China which has been in operation since 2022. Costing $266m, it has a capacity of 400MWh, with a second phase to double that capacity in the works.
Cost is still the major issues that may limit how extensively flow batteries are used for grid storage. ‘With flow batteries, I can just put a massive tank and then it was discharged for 100 hours, but I also have to pay for the material ,’ says Schmidt. Although more abundant than lithium, vanadium is still expensive and on the EU’s list of critical raw materials, which are of economic importance but have high supply risk.
But there is more vanadium in the earth’s crust than copper, according to Adam Tuck, Invinity’s director of technology, and an increased demand will lead to increased production, as well as recycling of the batteries themselves. ‘The electrolyte doesn’t degrade, so even after the end of the 25 year life of the battery itself, we can just take that and put it right back into 