In this study, we investigated the phase transition of cobalt spinel (Co₃O₄) nanoparticles into Co_3-xNi_xO₄/CoO heterostructures by introducing varying amounts of nickel (x = 0.0–0.16) and subjecting the particles to high annealing temperatures of 1000 ℃. X-ray diffraction (XRD) analysis confirmed the Co_3-xNi_xO₄CoO structure for all samples. Transmission electron microscopy (TEM) provided further insights into the phase or heterostructure of the samples after annealing, revealing the arrangement of the two phases. Fourier-transform infrared spectroscopy measurements demonstrated a band shift around 537 cm^-1 with increasing Ni content, while ultraviolet-visible (UV-Vis) measurements indicated the energy band (Eg). Significant morphological changes were observed in scanning electron microscope (SEM) measurements at 0.16 Ni, displaying irregular agglomerates. Our findings suggest that introducing Ni into the Co₃O₄ structure and increasing the annealing temperature to 1000 ℃ can lead to the formation of a heterostructured system. Furthermore, our study's significance is highlighted by the streamlined synthesis of NiCo₂O₄/CoO using the sol-gel method followed by calcination. This departure from complex techniques provides an efficient route to acquiring the NiCo₂O₄/CoO system, a promissory material for advancing supercapacitor research.