The cubic spinel zinc ferrite (ZnFe₂O₄) is a well-investigated material which shows a comparably small band gap and interesting magnetic properties. This makes it an ideal candidate for a number of different applications ranging from energy generation to data storage. Recently, there have also been attempts to use zinc ferrite as an active material on the negative electrode of lithium-ion batteries due to its high theoretical volumetric capacity of 1142 mA h cm^-3, but this failed due to the poor chemical stability of the material during repeated lithium (de)insertion.

However, a conductivity for zinc ions analogous to the related tetragonal spinel ZnMn₂O₄, which is already successfully used as an active material at the positive electrode in zinc-metal batteries, has only been described in theoretical studies to date [1, 2].

In the present study, part of the Fe³⁺  in zinc ferrite was substituted by Ti⁴⁺ to increase and stabilize zinc vacancy concentration in the structure according to 2TiO2ZnFe2O4 2TiFe°+VZn''+4OOx  in order to raise Zn²⁺ conductivity of the material. Regarding the electronic conductivity, several previous studies on doping with Ti⁴⁺ have produced controversial results, characterizing Ti⁴⁺ either as n-type dopant [3] or as inhibitor for an effective electron transport by small polaron hopping via Fe^2+/3+ ions, leading to a lower electron conductivity [4].

For this study, ceramic pellets with the composition ZnFe_2-xTi_xO₄ and x = 0 to 0.25 were prepared via a Pechini synthesis route and investigated regarding their electrochemical characteristics. The electronic conductivity of ZnFe₂O₄ is reduced by Ti addition (0.3 mS cm^-1 for pure zinc ferrite to 0.01 mS cm^-1 for x = 0.25 at 20 °C) and also the work function decreased slightly, while the optical band gap was not affected, showing that Ti in this case is not a typical n-type dopant. The possible Ti concentration is additionally very limited by formation of secondary phases at concentrations above x = 0.13.

Using Kelvin Probe Force Microscopy (KPFM)-based polarization-relaxation measurements, a diffusion coefficient in the range of 3·10^-12 ± 1·10^-12 cm² s^-1 was determined for pure zinc ferrite. In addition, both pure zinc ferrite and zinc ferrite with x=0.09 were successfully cycled in symmetrical cells with 0.5 M zinc triflate in acetonitrile and in a ZnFe_2-xTi_xO₄|0.5 M zinc triflate in acetonitrile|Sn cell setup.