Effect of Mn Rate on Structural, Optical and Electrical Properties in LiCo1−xMnxO2 (x = 0.5; 0.7) Compounds

The compounds LiCo1−xMnxO2 (x = 0.5, 0.7) were synthesized via the solid-state method and exhibited crystallization in the cubic spinel structure (space group Fd-3m). UV–Vis spectroscopy reveals strong visible-light absorption and a reduction in the indirect optical band gap from 1.85 eV (x = 0.5) to 1.60 eV (x = 0.7) with increasing Mn content, which is consistent with semiconducting behavior. This narrowing arises from Mn3+/Mn4+ mixed valence, which introduces mid-gap states and enhances Co/Mn 3d–O 2p orbital hybridization within the spinel framework. In contrast, the Urbach energy increases from 0.55 eV to 0.65 eV, indicating greater structural and energetic disorder in the Mn-rich composition which is attributed to the Jahn–Teller distortions and valence heterogeneity associated with Mn3+. Impedance and dielectric modulus analyses confirm two distinct non-Debye relaxation processes related to grains and grain boundaries. AC conductivity is governed by the Correlated Barrier Hopping (CBH) model, with bipolaron hopping identified as the dominant conduction mechanism. The x = 0.7 sample displays significantly enhanced conductivity due to increased Mn3+/Mn4+ mixed valence, lattice expansion, efficient 3D electronic connectivity of the spinel lattice, and reduced interfacial resistance. These findings highlight the potential of these two spinels compounds as narrow-gap semiconductors for optoelectronic applications including visible-light photodetectors, photocatalysts, and solar absorber layers extending their utility beyond conventional battery cathodes.