A novel hydrometallurgical process for recycling LiNi0.5Co0.2Mn0.3O2 cathode materials harvested from spent Li-ion batteries (LIBs) is established in this work. The cathode material LiNi0.5Co0.2Mn0.3O2 is dissolved in a mixed acid containing phosphoric acid (leaching agent) and citric acid (leaching agent and reductant). Using 0.2 M phosphoric acid and 0.4 M citric acid with a solid to liquid (S/L) ratio of 20 g/L at 90 °C for 30 min, the proposed method results in a leaching efficiency of ca. 100% for Li, 93.38% for Ni, 91.63% for Co, and 92.00% for Mn, respectively. Kinetics of the leaching process is well described by the Avrami equation. It is found that the leaching process is controlled by surface chemical reactions, and the apparent activation energies (kJ/mol) are 45.83 for Li, 83.01 for Ni, 81.38 for Co and 92.35 for Mn, respectively. With aids of various advanced characterizations methods, including UV–Vis, FT-IR and TOC, we find that there are a great deal of citrates and a small amount of dihydrogen phosphates in the mixed acid leachate. This leaching method enjoys advantages of low acid consumption, short leaching time and no need to add extra reductant.