The Role of Surface Oxygen in Eliminating Fluorine Impurities and Relithiation Towards Direct Cathode Recycling
Authors: Fang Zongtang , Dhindsa Kulwinder , Pan Lei , Atienza Dianne 
Published: August 2024

Recycling materials back to supply chain not only mitigates the shortage of critical materials such as the metals of Li, Ni, Co, etc, but also protects global environment. Direct recycling of lithium nickel manganese cobalt oxides (NMC) mainly relies on a relithiation process based on purified spent NMC cathode materials. How to efficiently get rid of chemically bonded surface impurities on spent NMC and obtain a clean surface is critical for relithiation. One of the unavoidable impurities is fluorine, which exists in the form of either carbon fluoride (CFx) or metal fluoride (M-F) and can be eliminated by either thermal treatment or hydrothermal procedure under alkaline environments depending on the state of the F.1,2 Generally the hydrothermal method needs relative high temperature (> 200 °C ) and long reaction time (> 16 hours) due to the slow kinetics. A redox mediator such as H2O2, ethanol or ethylene glycol has been reported to be effective to lower the temperature and pressure for relithiation of recycled NMC111 and NMC622, which was pretreated with N-Methyl-2-pyrrolidone solvent.3 However the mechanism is still not fully understood.

This work combines material characterizations and electrochemistry and density functional theory (DFT) calculations to study the role of surface oxygen in eliminating fluorine species on recycled NMC532 cathode material. The recycled NMC532 powders were separated from graphite by froth flotation method and there were significant amount of fluorine impurity species on the surface confirmed by XPS. Initial trial of the addition of small amount of H2O2 additive to LiOH aqueous solution during the hydrothermal process was able to decrease the reaction time significantly to fully eliminate both CFx and M-F species compared to the reaction without H2O2. Further calcination steps with the addition of 5% excessive lithium source using LiOH successfully recovered the recycled NMC532 and the half coin cell using the rejuvenated cathode materials shows a discharge capacity of 154 mAh/g at 0.2 C within 2.7 to 4.2 V voltage range. DFT calculations was conducted on a NMC (012) surface and both lithium hydroxide and sodium hydroxide were selected to understand the role of OH- group and surface oxygen species. The kinetics study was conducted with transition state theory4 and the activation energy barriers were calculated by climbing image nudged elastic band (cNEB) method.5,6 The removal of surface F- takes place with the displacement of F- with OH- (*-(M)F(s) +XOH(s) à XF(s) + *-(M)OH(s), X= Li, Na). F- displacement reactions require more than one basic molecule per F- removal with the formation of either LiF or NaF, which is soluble in water and can be removed by a filtration process. Depending on the surface ‘foreign’ oxygen species, the reaction can either go with a hydroxide pathway or a peroxide pathway, with the latter involving the formation of a peroxide intermediate with a ‘foreign’ oxygen atom leading to much smaller barrier energy compared to that in the hydroxide pathway. On a delithiated surface, the barrier energy for F- removal is predicted to be higher than pristine surfaces as the delithiated surface is less reactive to bind ‘foreign’ oxygen. This is consistent with the experimental results in which the additive of H2O2 can provide ‘foreign’ oxygen atoms and favors peroxide pathway leading to shorter time to remove surface fluorine. Additionally, the reaction with NaOH has a lower barrier than that with LiOH for the removal of fluoride species. Future work will investigate alternative additives as well as fluorine surface-bulk transfer which potentially happen if small amount of fluorine residues exist during calcination step.

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Figure 1