Study on the dissolution behavior and mechanism of LFP in the cryolite molten salt system of NaF-AlF3

The rapid dissolution of lithium iron phosphate (LFP) in fluoride provides a possibility for the recycling of spent lithium battery cathode materials through molten salt electrolysis. In this article, the dissolution behavior and mechanism of LFP in cryolite based molten salts are studied in detail. The dissolution behavior of LFP in NaF-AlF 3 molten salt with a cryolite ratio (CR) of 1.8 was observed and recorded using a custom-made see-through cell. LFP has high solubility, and its tablets rapidly diffuse and dissolve in cryolite, causing the transparent molten salt to display an orange-yellow color indicative of Fe ions. The XRD results of the cooled molten salt indicate that LFP decomposes into AlPO 4 , LiNa 2 AlF 6 , and Fe 2 (PO 4 )F phases after dissolution in fluoride. Due to the instability of LFP, some ferrous elements are easily oxidized by oxygen, forming Li 3 Na 3 Fe 2 F 12 and Fe 2 O 3 . The complex ion cluster structure of LFP at high temperatures was investigated using high-temperature Raman spectroscopy and quantum chemical calculations. Comparative analysis of high-temperature Raman spectra with the addition of LFP, Fe 2 O 3 , Li 3 PO 4 , and blank molten salt suggests that ion clusters containing Fe may include Fe 2 OF 4 , Fe 2 OF 8 4− , and Fe 2 O 2 F 3 − , while ion clusters containing P may include PO 2 F, PO 2 F 3 2− , POF 4 − , and POF 5 2− .