Interfacial and Hydration-Associated Lithium Retention During FePO₄ Regeneration from Spent LiFePO₄ Cathodes

Selective FePO₄ precipitation is used to separate iron and phosphorus from lithium during hydrometallurgical recycling of spent LiFePO₄, yet measurable lithium can remain in the regenerated FePO₄ precursor. Here, lithium partitioning was examined in a simulated Fe-P system, a commercial LiFePO₄-derived leachate, and a spent black-mass-derived leachate. Sequential washing resolved the lithium associated with the precipitates into fractions with different accessibility. Under representative conditions, approximately 70% of the retained lithium was washing-removable, whereas approximately 30% remained after three washing cycles. Contact with lithium-free FePO₄ resulted in an apparent 2.13% decrease in dissolved Li concentration, approaching a plateau within 10 min. Thermodynamic analysis indicated that the formation of the considered discrete lithium-containing precipitates was unfavorable, while changes in hydration and local Fe-O-P environments correlated with the washing-retained fraction. These results support a proposed precipitation-associated model involving lithium retention in environments with different accessibility within hydrated amorphous FePO₄. The findings identify precipitation and washing as linked control stages for managing lithium carryover during FePO₄ regeneration.