Electrical Conduction Mechanisms in KMnO2 as a Promising Cathode Material for K-Ion Batteries

Abstract

K-ion batteries (KIB) are considered the future energy storage and conversion technology due to their remarkable performance. In this work, a high-temperature solid-state process was used to effectively synthesize KMnO2, a promising cathode material for KIBs. The materials were examined using X-ray powder diffraction (XRPD), Raman and infrared spectroscopies, electron microscopy analysis, optical, and impedance spectroscopies. Rietveld refinement of X-ray diffraction data confirmed that the compound crystallizes in the monoclinic system with the P-21/m space group. Fourier transform infrared and Raman spectroscopies revealed the vibrational modes of the KMnO2 compound and proved the existence of the octahedral environment MO6 (M = Mn, K), which affirms structural configuration. The morphological distribution and grain size of the titled compound were examined using SEM studies. A direct band gap of around 3.12 eV was found by optical studies using UV–Vis spectroscopy, confirming the semiconducting nature of KMnO2 and indicating its applicability for optoelectronic and energy-related applications. The characteristics of this material were further examined using impedance spectroscopy at temperatures between 343 and 443 K and a frequency range of 10−1 Hz to 106 Hz. The DC conductivity and relaxation time exhibited Arrhenius behavior, with a significant shift in activation energy at 373 K, suggesting a change in the conduction mechanism. The frequency behavior of AC conductivity, σac, was analyzed using the universal Jonscher law. The findings of the charge transportation study on KMnO2 indicate that this material follows a non-overlapping small polaron tunneling (NSPT) for T < 383 K and correlated barrier hopping (CBH) above for T > 383 K. A correlation between the ionic conductivity and the crystal structure was established and discussed.