riethanolamine or triisopropanolamine complexes, although she admits solubility is currently too low.
Others are moving towards organic materials, to create more sustainable metal-free flow batteries. But it isn’t easy to find a system that can compete with the 20 plus years of development that have gone into vanadium flow batteries already. ‘Newer materials that have been produced and explored always have some caveats,’ says Toghill, including the poor long-term stability of the molecules or side reactions that will be detrimental to the cells’ energy capacity.
Finding organic species with solubility in the 1–2M range to provide a high charge density is ‘in itself is a challenge’, says Gray. Adding ionic side chains to molecules can increase solubility, but also risks increasing the viscosity too much. She has been working with Michael Aziz, a chemist at Harvard University in the US, on his quinone-based flow battery which pairs an anthraquinone half-cell with the ferro-ferricyanide couple in basic aqueous solution. His best-performing system so far uses a singly substituted 2,2-propionate ether anthraquinone (2-2PEAQ) whose methyl side chain gives it improved solubility making it a good prospect for a grid-scale organic redox flow battery.
It’s a beautiful hybrid between a lithium-based battery and a traditional flow battery
Not everyone has given up on lithium for grid storage: the lithium–sulfur battery has been around since the 1960s and has a theoretical capacity three times that of the lithium-ion battery. But several factors have hampered their developed as traditional batteries, including the dissolution of intermediate lithium polysulfide species in the electrolyte – known as ‘the polysulfide shuttle’ effect – causes rapid capacity loss and huge volume expansion at the cathode as sulfur reacts. But StorTera are finding ways round these problems with their polysulfide redox single liquid battery.
‘It’s a beautiful hybrid between a lithium-based battery and a traditional flow battery,’ says inventor Pallawella. The battery consists of a solid lithium metal anode with an alkali polysulfide cathode remaining in solution. The electrochemical reaction occurring is complex, involving at least 14 electron transfers and various polysulfide ions. To avoid the power losses of traditional lithium–sulfur battery design, the redox active polysulfite species can be constantly refreshed. The clever design has produced a battery smaller than other flow batteries, but with an improved lifetime to conventional lithium-ion batteries.
Pallawella says their energy density hovers around 200Wh per litre, which compares well to the 25–50Wh per litre capacity of vanadium redox flow batteries, and he expects they can improve on this with further development. Costs are also competitive, because sulfur is cheap – he estimates levelised or lifetime costs of 1–1.2p per kWh which is cheap enough to start to enable widespread use alongside renewable energy resources. In 2022 StorTera secured £5 million from the UK government to install a megawatt long-duration demonstrator battery at the Midlothian Innovation Centre near Edinburgh. By 2024 they expect it to be operating for up to eight hours a day, with an expected 30-year lifespan.
Even conventional lithium-ion batteries shouldn’t be completely discounted for longer-term grid storage, says Schmidt, ‘I wouldn’t underestimate the chance that there’s a breakthrough here, which suddenly means lithium-ion [batteries] are suitable for long duration storage,’ although he admits it’s probably a long shot. Lithium-ion batteries are still likely to play an important role in shorter term grid storage – in 2021, for example, the Australian state of Victoria installed a utility-scale facility powered by Tesla’s lithium-ion batteries which has enough capacity to power one million homes for half an hour after a grid failure.
There is no one size fits all for energy storage
Developing grid-storage technologies is expensive and the route to commercialisation is not simple. ‘Now it’s convincing the market that