In this study, an experiment is performed to recover the Li in Li2CO3 phase from the cathode active material NMC (LiNiCoMnO2) in waste lithium ion batteries. Firstly, carbonation is performed to convert the LiNiO, LiCoO, and Li2MnO3 phases within the powder to Li2CO3 and NiO, CoO, and MnO. The carbonation for phase separation proceeds at a temperature range of 600oC~800oC in a CO2 gas (300 cc/min) atmosphere. At 600~700oC, Li2CO3 and NiO, CoO, and MnO are not completely separated, while Li and other metallic compounds remain. At 800 oC, we can confirm that LiNiO, LiCoO, and Li2MnO3 phases are separated into Li2CO3 and NiO, CoO, and MnO phases. After completing the phase separation, by using the solubility difference of Li2CO3 and NiO, CoO, and MnO, we set the ratio of solution (distilled water) to powder after carbonation as 30:1. Subsequently, water leaching is carried out. Then, the Li2CO3 within the solution melts and concentrates, while NiO, MnO, and CoO phases remain after filtering. Thus, Li2CO3 can be recovered.