Effects of Aluminum Doping on Cobalt‐Free Lithium–Iron–Nickel–Manganese–Oxygen Cathode Materials for Lithium‐Ion Batteries

Abstract To decrease the cost of lithium‐ion cathode materials, cobalt‐free Li‐Fe‐Ni‐Mn‐O (FeAl000) was synthesized by a facile sol–gel method by using citric acid as the chelating agent. To enhance the electrochemical performance of FeAl000, low‐costing and environmentally friendly aluminum was added as a dopant. The Al‐doped Li‐Fe‐Al‐Ni‐Mn‐O (FeAl015) material exhibited excellent electrochemical performance. The charge/discharge capacity and corresponding differential capacity curves suggested that Al‐doping activated the transformation of Mn 4+ into Mn 3+ , and the activated Mn 3+ /Mn 4+ redox/oxide couple contributed to an increase in the discharge capacity. X‐ray photoelectron spectroscopy analyses demonstrated the transformation of Mn 4+ into Mn 3+/2+ . The Nyquist and Bode curves obtained by electrochemical impedance spectroscopy indicated that Al doping enhanced electron transfer and somewhat hampered formation of the solid–electrolyte‐interphase layer during cycling.