Membraneless flow battery to smooth renewables' impact

‘For solar and wind power to be used in a significant way, we need a battery made of economical materials that are easy to scale and still efficient,’ said Yi Cui, a Stanford University associate professor of materials science and engineering. ‘We believe our new battery may be the best yet designed to regulate the natural fluctuations of these alternative energies.’

Cui and colleagues from Stanford and the US Department of Energy’s SLAC National Accelerator Laboratory report their research results in Energy & Environmental Science.

According to the university, the electricity grid cannot currently tolerate large and sudden power fluctuations caused by wide swings in sunlight and wind.

As solar and wind’s combined contributions to an electrical grid approach 20 per cent, energy storage systems must be available to smooth out the peaks and valleys of this intermittent power, storing excess energy and discharging when input drops.

Among the most promising batteries for intermittent grid storage today are ‘flow’ batteries, because it’s relatively simple to scale them to the sizes needed to handle large capacities of energy. The new flow battery developed by Cui’s group has a simplified, less expensive design that is claimed to present a potentially viable solution for large-scale production.

Today’s flow batteries pump two different liquids through an interaction chamber where dissolved molecules undergo chemical reactions that store or give up energy.

The chamber contains a membrane that only allows ions not involved in reactions to pass between the liquids while keeping the active ions physically separated.

This battery design is said to have two major drawbacks: the high cost of liquids containing rare materials such as vanadium – especially in the quantities needed for grid storage – and the membrane, which is also very expensive and requires frequent maintenance.

The new Stanford/SLAC battery design uses only one stream of molecules and does not need a membrane at all.

Its molecules mostly consist of the relatively inexpensive elements lithium and sulphur, which interact with a piece of lithium metal coated with a barrier that permits electrons to pass without degrading the metal.

When discharging, the molecules - lithium polysulphides - absorb lithium ions; when charging, they lose them back into the liquid. The entire molecular stream is dissolved in an organic solvent, which doesn’t have the corrosion issues of water-based flow batteries.

‘In initial lab tests, the new battery also retained excellent energy-storage performance through more than 2,000 charges and discharges, equivalent to more than 5.5 years of daily cycles,’ Cui said in a statement.

To demonstrate their concept, the researchers created a miniature system using simple glassware. Adding a lithium polysulphide solution to the flask immediately produces electricity that lights an LED.

A utility version of the new battery would be scaled up to store many megawatt-hours of energy.