Opto-impedance studies on spinel type pristine and Ni-doped LiMn2O4, an important cathode system for Li-ion batteries have revealed the difference in electronic structure arising out of the Ni-substitution. The opto-impedance spectra of Ni-doped LiMn2O4 even under dark condition exhibits a more complex structure(two semicircles with some skewed structure) suggesting the presence of defect structure(s) remaining dormant which upon UV shining get merged to form a single semicircle. Qualitatively this aliovalent substitution modifies the charge transfer state and activates a defect initiated Warburg diffusion process with a time scale of similar to 50 mu s. (C) 2008 Elsevier B.V. All rights reserved.