Lithium Storage Properties of Pristine and (Mg, Cu) Codoped ZnFe₂O₄ Nanoparticles

ZnFe₂O₄ and Mg_<i>x</i>Cu_0.2Zn_{0.82–<i>x</i>}Fe_1.98O₄ (where <i>x</i> = 0.20, 0.25, 0.30, 0.35, and 0.40) nanoparticles were synthesized by sol–gel assisted combustion method. X-ray diffraction (XRD), FTIR spectroscopy, Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Brunauer–Emmett–Teller (BET) surface area studies were used to characterize the synthesized compounds. ZnFe₂O₄ and the doped compounds crystallize in <i>Fd</i>3<i>m</i> space group. The lattice parameter of ZnFe₂O₄ is calculated to be <i>a</i> = 8.448(3) Å, while the doped compounds show a slight decrease in the lattice parameter with an increase in the Mg content. The particle size of all the compositions are in the range of ∼50–80 nm, and the surface area of the compounds are in the range of 11–12 m² g^–1. Cyclic voltammetry (CV), galvanostatic cycling, and electrochemical impedance spectroscopy (EIS) studies were used to investigate the electrochemical properties of the different compositions. The as-synthesized samples at 600 °C show large-capacity fading, while the samples reheated at 800 °C show better cycling stability. ZnFe₂O₄ exhibits a high reversible capacity of 575 mAh g^–1 after 60 cycles at a current density of 100 mA g^–1. Mg_0.2Cu_0.2Zn_0.62Fe_1.98O₄ shows a similar capacity of 576 mAh g^–1 after 60 cycles with better capacity retention