Interfacial and Hydration‐Associated Lithium Retention During FePO 4 Regeneration from Spent LiFePO 4 Cathodes

ABSTRACT Selective FePO 4 precipitation is used to separate iron and phosphorus from lithium during hydrometallurgical recycling of spent LiFePO 4 , yet measurable lithium can remain in the regenerated FePO 4 precursor. Here, lithium partitioning was examined in a simulated Fe–P system, a commercial LiFePO 4 ‐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 FePO 4 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 FePO 4 . The findings identify precipitation and washing as linked control stages for managing lithium carryover during FePO 4 regeneration.