Low-temperature plasma process supports direct recycling of battery cathodes

Direct battery-recycling methods, such as re-lithiation — where depleted lithium is replenished directly onto cathodes via a solid-state reaction — hold many benefits over indirect methods, such as those relying on hydrometallurgy or pyrometallurgy, where cathodic structures are destroyed in smelting and grinding processes. However, two major hurdles for large-scale adoption of direct recycling methods are impurity removal and a lack of elucidation on surface-rejuvenation mechanisms. A new low-temperature plasma process developed by a team of researchers from the University of California, Irvine (www.uci.edu), Brookhaven National Laboratory (BNL; Upton, N.Y.; www.bnl.gov), Honda Research Institute USA (Columbus, Ohio; usa.honda-ri.com) and Princeton NuEnergy, Inc. (Monmouth Junction, N.J.; www.pnecycle.com) enables the efficient removal of carbon and fluorine impurities and supports high-performance rejuvenation of spent battery cathodes via re-lithiation. Complete elimination of these C and F impurities is typically quite difficult, but using plasma-generated oxygen radicals, carbon impurities are oxidized to CO₂ and fluorine impurities are converted into volatile fluorinated compounds (such as HF and COF₂) that are easier to remove.

Carbon and fluorine impurities in battery-recycling processes stem from residual carbon black and polyvinylidene fluoride (PVDF) material in cathode powders. They are especially problematic in re-lithiation processes because they hinder the diffusion of lithium and oxygen to the cathode surface. In experimental cycles with NMC (nickel-manganese-cobalt) battery pouch cells, the new low-temperature plasma method was shown to effectively eliminate surface impurities during re-lithiation, resulting in a reported 87% capacity retention for rejuvenated cathode pouch cells after 1,100 cycles. This exceeds current commercial standards, which show 86% retention after 1,000 cycles, according to the research team. Low-temperature operation helps to protect sensitive cathode materials from degradation and also reduces energy consumption compared to other recycling methods.

Furthermore, the research findings shed light on previously little-understood physical mechanisms in re-lithiation reactions, including phenomena related to cation mixing and the restoration of certain crystal-lattice parameters. Details of this research were originally published in a recent issue of Joule.