wella. 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 we are a worthy investment, from a technology and capability standpoint,’ says Matthew Walz, vice-president of business development at Invinity Energy Systems. ‘It is pretty widely recognised that there is a need for some sort of policy support,’ says Woodward, but it is likely to be through backing and commercial guarantees rather than direct subsidies. One tool that was used for the roll-out of grid interconnector technology is the ‘cap and floor’ mechanism, which provides a minimum floor on returns for investors, under which revenue would be topped up, but coupled to a maximum revenue cap. The UK government is yet to decide on this, but Woodward says ‘more than anything, what people need is clarity… to know what sort of environment they’re going to be operating in and what sorts of support will be available.’
With so many potential solutions in the mix, are we likely to see one eventually becoming dominant? ‘I don’t think we have a clear winner,’ says Gray. Mezzavilla thinks many electrochemical systems are still in their infancy and we can expect large improvements in cost and performance.
Ultimately, Walz says, ‘there is no one size fits all for energy storage’. The grid will need shorter term storage, in the region a few hours, to help smooth the variability of renewables generation all the way through to large scale seasonal storage. Flow batteries like Invinity’s are likely to provide medium-term storage capabilities that allow for charging and discharging several times a day and Walz thinks once we have this capability we have 95% of the solution. Schmidt agrees that flow batteries will be ‘the workhorses’ that will allow us to successfully transition to a predominantly renewable energy infrastructure.
For longer duration storage there is still everything to play for. ‘I think eventually, there will be one [dominant technology],’ says Schmidt. ‘Someone will be the winner here and that’s going to be very interesting to see in the next few years’.
Rachel Brazil is a science writer based in London, UK
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