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Energy Storage

Lowering the cost of grid-storage batteries

New type of molten-salt battery could cost a fraction of lithium-ion devices

by Prachi Patel, special to C&EN
September 16, 2020 | APPEARED IN VOLUME 98, ISSUE 36

09836-scicon3-zebra.jpg
Credit: Matter
This 6-cm-tall battery uses low-cost brass powder (yellow) as the cathode and a molten lithium anode (purple). The cathode mixture consists of brass particles (large orange eggs, inset) and lithium chloride (blue, inset).

Researchers have developed a battery for storing energy for the electrical grid that they think could beat lithium-ion batteries in cost by a large margin (Matter 2020, DOI: 10.1016/j.matt.2020.08.022). The new system costs $16 per kilowatt-hour of energy stored, which is one-eighth the rate for standard lithium-ion batteries. “This is a completely new battery chemistry with promising performance and cost,” says Yi Cui, a materials scientist and engineer at Stanford University.

Storing the intermittent energy produced by solar and wind farms is becoming ever more important as the share of renewable energy grows across the globe. But cost is the biggest hurdle in finding the right battery for the grid. An affordable grid battery should cost $100/kWh, according to the US Department of Energy. Lithium-ion batteries, which lead the charge for grid storage, cost $175/kWh.

To develop a less costly option, Cui, Hui Wu of Tsinghua University, and Yang Jin of Zhengzhou University decided to tweak another battery considered promising for grid storage: the Zebra battery. This is a type of molten-salt battery that uses molten sodium as the anode, and a cathode made of nickel along with a molten sodium chloride salt. Nickel is expensive, so this battery technology has been used only for a few small grid installations.

In their new battery, the researchers chose molten lithium as the anode, and replaced nickel with low-cost zinc and combined it with a lithium chloride salt to make the cathode. Past attempts to use zinc at the cathode have brought challenges. When those batteries discharge, zinc chloride turns into metallic zinc, Cui explains, and the metal forms zinc particles that grow uncontrollably, making the electrode unstable.

So instead of using pure zinc, the researchers used particles of brass, a copper-zinc alloy. The brass releases zinc during battery charging, and when the metallic zinc forms during discharge, it alloys with the copper and stabilizes.

The new battery chemistry works at a lower temperature than traditional Zebra batteries, Wu says. Keeping those batteries’ sodium melted requires temperatures of 350 °C, while the new chemistry works at 215 °C, which means researchers can use less-expensive materials to seal and package the battery. The battery cell that the researchers made has a theoretical energy density of 700 Wh/kg. Cui expects the energy density to drop when they scale up the battery in size, but he says, “If we can even get half of that energy density, it would still be as good as lithium-ion or maybe even slightly better.”

Molten sodium and lithium have been used for energy-storage devices for a few decades so this isn’t brand new technology, says Guosheng Li, a senior battery scientist at Pacific Northwest National Laboratory. “But it’s an important chemistry advance and an interesting approach.”

Still, the battery is a long way from being practical. The cell only lasts 100 charge cycles. A grid battery will need to withstand thousands of cycles. Another issue might be the long-term global supply of lithium at the large scales needed to make these batteries, a challenge that lithium-ion devices also face. But, Cui says, “researchers are starting to find ways to extract lithium from non-traditional sources such as different salts and also from the ocean.” That should address concerns about lithium price and availability.

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