Valence Effects of Fe Impurity for Recovered LiNi_0.6Co_0.2Mn_0.2O₂Cathode Materials

Iron impurities are generally included in the obtained leaching liquor solution during the hydrometallurgical recycling method of spent lithium-ion batteries (LIBs) due to the usage of iron in battery casings and machinery parts of recycling equipment, which would definitely affect the physical and electrochemical features of the recovered active materials. In this work, the effects of iron impurity with different valence states (Fe2+ and Fe3+) and gradient concentrations (0.2, 1.0, and 5.0 at. %) for the obtained LiNi0.6Co0.2Mn0.2O2 (NCM622) cathodes are fully studied. It is found that Fe3+ impurity could easily lower the tap density and average size of NCM622 particles and even introduce some impurity phases in the NCM622 structure at high concentration (5.0 at. %), leading to much lower specific capacity, worse rate capability, and cycling performance of the Fe3+-based NCM622 cathode. On contrast, with certain concentrations of Fe2+ impurity (0.2 and 1.0 at. %), the NCM622 cathode material exhibits comparable and much better electrochemical properties compared with the virgin NCM622 materials. Based on these results, the valence of Fe impurity should be considered and controlled as well as its concentration during the recycling process design for spent LIBs.