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

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 Mn⁴⁺ into Mn³⁺, and the activated Mn³⁺/Mn⁴⁺ redox/oxide couple contributed to an increase in the discharge capacity. X-ray photoelectron spectroscopy analyses demonstrated the transformation of Mn⁴⁺ 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.