Understanding structural, optical, magnetic and electrical performances of Fe- or Co-substituted spinel LiMn_1.5Ni_0.5O₄ cathode materials

Spinel LiMn_1.5Ni_0.5O₄ (LNMO) and LiMn_1.5Ni_0.25M_0.25O₄(M=Co (LNCMO) or Fe LNFMO) have been purposefully tailored using a sol–gel auto-combustion method at low annealing temperature. Well crystalline disordered spinel Fd3m structure was formed. The particles of LNMO sample evinced a regular octahedron shape-like morphology with wide particle size distribution from 42 to 82 nm. However, the Co- and Fe-doped LNMO samples were obtained from larger and flattened particles with lower facets particles and narrower particle size distribution (55–80 nm). EDX spectroscopy confirmed the elemental analysis were uniformly distributed in the samples. The band gap was found to decrease with Co³⁺ and Fe³⁺ ion substitution, indicating a shortening in the distance between the valence and the conduction bands. The magnetic properties indicated weak ferromagnetic performance. EIS spectra evidenced that a typical semicircle was revealed for each cell, suggesting the absence of ionic conductivity contribution. The values of charge transfer resistance (R_ct) were equal to 16.1, 3.4 and 9.3 kΩ for the LNMO, LNCMO and LNFMO cells, respectively. The AC conductivity measurements showed a faster kinetic rate and lower activation energy of conduction for LiNi_0.25Co_0.25Mn_1.5O₄.