Recycling the end-of-life lithium-ion batteries including the lithium iron phosphate (LiFePO4) battery has been one of the most important technology areas since electric vehicles are gradually dominating the way of transportation. For the recycling of LiFePO4 batteries, the regeneration of pure iron phosphate dihydrate (FePO4∙2H2O) from spent cathode material is essential but difficult. The challenge is how to effectively synthesize battery-grade iron phosphate from the impurity-bearing acid solution by precipitation since the spent cathode powder usually holds various impurities. To avoid the introduction of impurities into the iron phosphate, a one-step spontaneous precipitation and phase transformation mode without acidity adjustment was preferred and studied in this work. The focus of this research included the process of phase transformation from amorphous to crystalline, and its effects on precipitation and the properties of the precipitate. It was interestingly found that there was an induction period for phase transformation from amorphous FePO4∙xH2O to monoclinic FePO4∙2H2O during the precipitation, which was distinctly shortened with the elevated temperature. Phase transformation led to fast increased precipitation efficiency, morphology evolution, and composition change, which benefited the consistency of iron phosphate. Phase transformation could be kinetically described by the JMA equation, with a rate-limiting step of materials diffusion towards the nucleation point and an activation energy of 20.4 kJ/mol. The critical crystal size for transformation was increasing along with the temperature’s raising, which was ascribed to the changes in the volume energy and the surface energy. The results in this study have built the feasibility of chemically regenerating FePO4∙2H2O precursors from the spent cathode powder by precipitation purification in an acidic medium.