ORNL researchers Lu Yu and Yaocai Bai examine vials that contain a chemical solution that causes the cobalt and lithium to separate from a spent battery, followed by a second stage when cobalt precipitates in the bottom. (Image: Carlos Jones/ORNL, U.S. Dept. of Energy)

Used Li-ion batteries are piling up, but options for recycling them remain limited. The current state-of-the-art methods can pose environmental challenges and be difficult to make economical at the industrial scale.

The conventional process recovers few of the battery materials and relies on caustic, inorganic acids and hazardous chemicals that may introduce impurities. It also requires complicated separation and precipitation to recover the critical metals. However, recovering metals such as cobalt and lithium could reduce both pollution and reliance on foreign sources and choked supply chains.

Now, researchers at the Department of Energy’s Oak Ridge National Laboratory have improved on approaches that dissolve the battery in a liquid solution to reduce the amount of hazardous chemicals used in the process. This simple, efficient, and environmentally friendly solution overcomes the main obstacles presented by previous approaches.

The spent battery is soaked in a solution of organic citric acid — which occurs naturally in citrus fruits — dissolved in ethylene glycol, an antifreeze agent commonly used in consumer products like paint and makeup. Citric acid comes from sustainable sources and is much safer to handle than inorganic acids. This green solution produced a strikingly efficient separation and recovery process for the metals from the positively charged electrode of the battery, called the cathode.

“Because the cathode contains the critical materials, it is the most expensive part of any battery, contributing more than 30 percent of the cost,” said research team member Yaocai Bai. “Our approach could reduce the cost of batteries over time.”

The recycling technique developed there leached nearly 100 percent of the cobalt and lithium from the cathode without introducing impurities in the system. It also enabled efficient separation of the metal solution from other residues. Most importantly, it served a second function by recovering over 96 percent of the cobalt in a matter of hours, without the typical addition of more chemicals in what is usually a tricky process of manually balancing acid levels.

“This is the first time one solution system has covered the functions of both leaching and recovery,” said Lead Researcher Lu Yu. “It was exciting to find that the cobalt would precipitate and settle out without further interference. We were not expecting that.”

The leaching performance of citric acid and ethylene glycol has been explored before, but that approach used more acid and a lower temperature, which proved less effective, Bai said.

“We were surprised by how quickly the leaching happened in our solution,” Bai said. “With an organic acid, it usually takes 10-12 hours, but this took only one.” Conventional solutions using inorganic acid are also slower because they include water, which has a boiling point that limits the temperature of the reaction.

Here is an exclusive Tech Briefs interview with Bai, edited for length and clarity.

Tech Briefs: I’m certain there were too many to count, but what was the biggest technical challenge you faced while developing this improved recycling approach?

Bai: During our study, the biggest technical challenge was how to separate the critical metals like cobalt and nickel from the lithium in the leaching solution. Due to the absence of efficient separation and precipitation methods, it is challenging to recover high-value metals, such as cobalt and lithium, from organic solvents.

In our work, we tackled this challenge by tuning the ratio between the solvent (i.e., ethylene glycol) and citric acid so that not only the waste cathode could be dissolved in the solution, but, more importantly, self-coprecipitation could happen without introducing any additional chemicals.

Tech Briefs: Can you explain in simple terms how it works?

Bai: Our approach uses a special liquid made of citric acid and ethylene glycol, instead of the usual strong acids that are used in the traditional recycling method. This liquid acts like a magnet for the valuable metals in the battery cathode, like cobalt and lithium, pulling them out without harming the other materials such as aluminum and graphite. Imagine dipping the shredded batteries into a warm bath of this special liquid.

Once all the critical metals are pulled out from the battery cathode, the cobalt particles start to clump together, or precipitate, on their own, like tiny Lego bricks building a new structure. In this case, the critical metals like cobalt are separated from the lithium. This happens without needing any extra chemicals, making it simpler and cleaner than current approaches.

Tech Briefs: How soon do you estimate could we see this recycling method adopted at a larger scale?

Bai: I would say in two to three years we should see the recycling method move out of the prototype phase. There is still work to do to optimize the process, such as recovery of the lithium and cost reduction.

Tech Briefs: What are your next steps? Any further research planned?

Bai: Our next steps and plans are to (1) introduce the recovered critical materials into a new loop of cell manufacturing by using the precipitated materials to remake battery cathode materials, and (2) recover the lithium from the solution and reintroduce it into the battery supply chain.

Tech Briefs: Any updates you can share?

Bai: We have successfully remade the battery cathode materials by using the recovered critical materials. Their electrochemical performance is currently under investigation.

Tech Briefs: Do you have any advice for engineers aiming to bring their ideas to fruition?

Bai: Careful observation is a powerful tool in any experiment. By actively watching and recording, you can unlock hidden patterns and refine your approach, leading to more successful results.