With the rapid growth and increasing popularity of electric and hybrid-electric vehicles, worldwide manufacturing capacity of lithium-ion batteries is expected to reach hundreds of gigawatt hours a year in the next five years. Reclaiming end-of-life lithium-ion batteries after five to 10 years of operation when the cathode materials have lost some of their lithium and are less able to move ions in and out means both the lithium concentration and crystal structures need to be restored.
A team led by Zheng Chen, a nanoengineering professor at University of California – San Diego, had previously developed a direct recycling approach to regenerate degraded cathodes. The process involved pressurizing a hot lithium salt solution of cathode particles to around 10 atmospheres and was effective, but the pressurizing step raises costs and requires both extra safety precautions and special equipment.
Chen’s team has just developed a milder process to do the same job at ambient pressure (1 atmosphere) by using eutectic lithium salts—a mixture of two or more salts that melts at temperatures much lower than either of its components. This combination of solid lithium salts produces a solvent-free liquid that researchers can use to dissolve degraded cathode materials and restore lithium ions without adding any extra pressure in the reactors.
The new recycling method involves collecting cathode particles from spent lithium ion batteries and then mixing them with a eutectic lithium salt solution, then heating the mixture in a two-step heating/annealing process. Researchers used the method to regenerate NMC (LiNi0.5Mn0.3Co0.2), a popular cathode containing nickel, manganese and cobalt, which is used in many of today’s electric vehicles.
The regenerated cathodes showed the same capacity and cycle performance as the originals, and the team hopes to fine-tune the process to recycle any type of cathode materials used in lithium-ion and sodium-ion batteries.
“The goal is to make this a universal recycling process for all cathode materials,” Chen said. The team is also working on a process to recycle degraded anodes, such as graphite and other materials.