Effects of iron substitution for nickel on electrochemical properties of LiNi0.8Co0.2O2 for lithium rechargeable batteries

Iron was substituted for Ni in LiNi0.8Co0.2O2 in order to enhance its electrochemical properties such as cycle life and rate capability. LiNi0.8−xFexCo0.2O2 (0≤x≤0.1) was synthesized in flowing oxygen for 12–48 h at 650–800°C, and its structural and electrochemical properties were characterized with X-ray diffraction (XRD), cyclic voltammetry (CV) and charge–discharge tests. The XRD patterns of LiNi0.8−xFexCo0.2O2 showed that the α-NaFeO2 type layered structure was maintained up to x=0.1. As Fe was substituted for Ni up to x=0.05, the rate capability of LiNi0.8−xFexCo0.2O2 was improved. LiNi0.75Fe0.05Co0.2O2 showed better cycle stability than LiNi0.8Co0.2O2. XRD measurements indicated that the lattice of LiNi0.75Fe0.05Co0.2O2 expanded less than that of LiNi0.8Co0.2O2 when the lithium was de-intercalated. CV curves of LiNi0.8−xFexCo0.2O2 showed current peaks, which are related to the phase transitions, diminished with Fe substitution during charging and discharging owing to the decrease of Ni3+. Consequently, the improved cycle stability of LiNi0.75Fe0.05Co0.2O2 may be attributed to the decrease of phase transition.