May be the tech for unifying green energy future and the not-so-often green methods of producing lithium.

Researchers at Princeton have developed a surprisingly efficient and environmentally friendly method of extracting lithium from brines and seawater using specialized string.

While technologies such as direct extraction from lithium-rich geothermal waters is shaping up to be a way to produce the silvery-white critical mineral necessary for rechargeable batteries without unacceptable environmental costs, most of the lithium produced today bars industry from claiming a truly green battery technology for powering the energy transition.

Large swathes of land, excessive water usage, chemical treatments and long waits are usually needed to extract lithium from briny water, particularly in South America, where the mineral is mainly extracted from lithium-rich salt flats. This is done using massive evaporation ponds and can take years to get up and running, and months to produce a high enough concentration of lithium salts.

Princeton's string system has shown the potential to lower costs, increase efficiency, and speed up production at existing lithium facilities, as well as take advantage of sources previously seen as too small or diluted to be cost-effective.

"Our approach is cheap, easy to operate, and requires very little energy," says Zhiyong Jason Ren, a civil and environmental engineering professor at Princeton University and senior author of the research paper published in Nature Water.

An elegant solution

The method relies on basic physics to do the work – water's tendency to move up in small spaces due to adhesion and cohesion. More commonly known as wicking.

"We aimed to leverage the fundamental processes of evaporation and capillary action to concentrate, separate, and harvest lithium," said Ren. "We do not need to apply additional chemicals, as is the case with many other extraction technologies, and the process saves a lot of water compared to traditional evaporation approaches."

The process uses yarn of porous cellulose fibers twisted into strings, which the researchers designed with an absorbent core and water-repelling surface. With the ends immersed in a briny solution, the liquid travels up the strings and quickly evaporates, leaving behind crystals on the strings that can easily be harvested.

Sodium, with low solubility, crystallizes on the lower part of the string, while the highly soluble lithium salts grow near the top, naturally separating the two similar minerals that have traditionally necessitated the use of additional chemicals.

Once the strings dried, the different sections were rinsed with water, producing two separate solutions 39 times more concentrated in lithium than the brine used and 675 times more concentrated than from seawater.

The strings could then be re-used, and since the materials are cheap and the technology does not require chemical treatments to operate, this technology is a strong candidate for widespread adoption.

"Our process is like putting an evaporation pond on a string, allowing us to obtain lithium harvests with a significantly reduced spatial footprint and with more precise control of the process," said co-author Sunxiang Zheng. "If scaled, we may open up new vistas for environmentally friendly lithium extraction."

Next steps

The Princeton team is developing a second generation of the process that will enable greater efficiency, higher production, and more control over the crystallization.

The compact string technique is far more efficient and can begin producing lithium quickly. Although it will take additional work to scale from the lab to industrial application, the researchers estimate it can reduce current operations' necessary acreage by more than 90% as well as accelerate the evaporation process, potentially yielding harvests in less than one month.

This improved efficiency of operations could expand available sources of lithium, such as disused oil and gas wells, and to improve hydrometallurgical recycling. The accelerated evaporation rate could also allow for operations in more humid climates, such as geothermal wells.

This is especially good news for the U.S., which has as-yet untapped lithium deposits across Nevada, North Carolina, and California, estimated to be about 4% of the world's lithium reserves.

Presently, Nevada hosts the only active brine-based lithium extraction operation due to nationwide opposition by local communities against traditional lithium mines' disruption of land and water.

"As a researcher, you know firsthand that many new technologies are too expensive or difficult to scale," said Zheng, who is launching PureLi Inc., a startup to develop the technology further for a wider marketplace. "But we are very excited about this one, and with some additional efficiency improvements, we think it has incredible potential to make a real impact on the world."