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Effect of Fe incorporation on electrical properties of (NixCoyMn3-x-y)O4 (NMC) spinel compound is investigated for the application to a negative temperature coefficient (NTC) thermistor. Cation distribution of the Fe doped NMC (FNMC) is calculated based on the size of ions located at tetragonal A sites and octahedral B sites in spinel structure, which can be closely related with temperature dependent electrical properties of the FNMC. With change of Fe contents, the ratio of Mn3+/Mn4+ in octahedral B site of FNMC is changed which can determine the temperature sensitivity factor (B-value). Hopping conduction mode relating with activation energy and hopping distance was also discussed depending on Fe contents, based on the small polaron hopping theory.

The reliability of negative temperature coefficient (NTC) thermistors is one of the important factors for the excellent performance of the battery management system in electric vehicles. The electrical properties of the thermistor can be explained by the indirect electron jump between Mn3+ and Mn4+ at the B-sites in a spinel-type thermistor. Additionally, different types of dopants, such as Fe, Cu, Zn, Cr, and Mn, are considered as electrical modifiers that can further improve the electrical properties of the thermistor. For example, Cu is often added to NTC thermistors, occupying the octahedral sites in a spinel-type thermistor, which can play an important role in conduction with the Mn cations. Addition of Fe is also widely used in NTC thermistors owing to its high sensitivity, B constant, and stability. In this study, composition-dependent structural and electrical properties of Cu0.2/Fey-co-doped Ni0.3Mna-x-yCo0.9O4 (NMC) are investigated. Cu/Fe-co-doped NMC shows the R25 and B25/85 constant values of 4490–12730 Ω and 3185–3490 K, respectively, exhibiting a typical ρ–T curve of NTC thermistors. Higher B constant and reliable electrical stability are observed for Cu/Fe-co-doped NMC. Based on the relationship between the cationic oxidation states and electrical properties of Cu/Fe-co-doped NMC, the hopping conduction mechanism is discussed.