The Role of Surface Oxygen in Eliminating Fluorine Impurities and Relithiation toward Direct Cathode Recycling

Abstract

Hydrothermal relithiation under oxidative conditions has been reported to be an efficient method to rejuvenate cycle-aged cathode materials. However, the role of surface oxygen is not well understood. In this work, hydrothermal relithiation in LiOH solution with H₂O₂ as an oxidative additive at 125 °C followed by calcination was able to fully recover the capacity of a cycle-aged NMC532 cathode material from end-of-life commercial electric vehicle cells with a state-of-health of 75%. The adsorbed surface oxygen species from H₂O₂ act as catalysts to facilitate both the relithiation and removal of surface fluorine impurities on NMC532. Removal of transition metal fluoride in LiOH solution is a displacement reaction with an *–OH group replacing a *–F group. X-ray photoelectron spectroscopy and Raman spectroscopy combined with electronic structure calculations confirm the conversion of transition metal fluoride to lithium fluoride. The activation energy is reduced via the formation of a peroxide with the adsorbed oxygen to provide more reactive *–OH groups coupled with a redox process. A small amount of lithium fluoride does not significantly influence reversible capacity. However, the presence of transition metal fluorides may have a negative effect. The kinetics of relithiation and impurity removal with the hydrothermal method can be optimized by modifying surface oxygen.